Download Comparison of cost and efficiency of DC versus AC in

Report for:
EP/I031707/1 - Transformation of the Top and Tail of Energy Networks;
Work Package 2.1: Starting with demand;
Activity 2.1.2: Demand-driven energy conversion
Comparison of cost and efficiency of DC versus AC
in office buildings
Giuseppe A. Laudani, PhD Student, Imperial College London
Paul D. Mitcheson, Imperial College London
Acknowledgements
The authors of this report gratefully acknowledge EPSRC Grand Challenge Programme for the
financial support for Mr Laudani, under grant EP/I031707/1
2
Contents
Comparison of cost and efficiency of DC versus AC in office buildings................................................................... 1
Acknowledgements ................................................................................................................................................... 2
Contents .................................................................................................................................................................... 3
Executive summary.................................................................................................................................................... 4
Introduction ............................................................................................................................................................... 6
1.
Background ................................................................................................................................................... 8
a)
Transmission lines and electrical cables ................................................................................................... 8
b)
Power losses ........................................................................................................................................... 10
c)
IR (voltage) drops .................................................................................................................................... 13
d)
Input rectifier .......................................................................................................................................... 15
2.
Low voltage DC and extra low voltage DC .................................................................................................. 16
a)
Low voltage DC: pros and cons ............................................................................................................... 16
b)
AC versus DC losses: a first comparison.................................................................................................. 20
c)
AC versus DC losses: case study .............................................................................................................. 22
3.
Converter efficiencies; AC vs DC system efficiencies .................................................................................. 27
a)
Converter efficiencies ............................................................................................................................. 27
b)
Efficiency comparison of AC versus DC supplied offices......................................................................... 33
4.
Five case scenarios of AC vs DC system comparison................................................................................... 36
a)
AC vs DC supply: conventional electrical sources, old-technology appliances ....................................... 36
b) AC vs DC supply: conventional electrical sources, old-technology appliances, 95% efficiency of AC/DC
bulk conversions .............................................................................................................................................. 38
5.
c)
Hybrid AC-DC vs DC supply: conventional electrical sources, old-technology appliances ..................... 39
d)
AC vs DC supply: conventional electrical sources, new-technology appliances ..................................... 40
e)
AC-DC vs DC supply: distributed energy resources (DERs), new-technology appliances ....................... 42
f)
Summary results ..................................................................................................................................... 43
Cost comparison of AC versus DC supplied offices ..................................................................................... 44
Conclusions .............................................................................................................................................................. 51
References ............................................................................................................................................................... 52
3
Executive summary
It has been known for a long time that direct current (DC) is potentially more convenient, in terms
of efficiency, in the transmission of electrical energy than three-phase current and, a fortiori, singlephase current. Losses that occur in cables are potentially lower for DC than AC, because the cable
can be used to its highest insulation rating. However, AC possesses some unique characteristics that
have made it the main form of current used for electrical transmission. In fact, AC voltage can be
easily raised in magnitude, thanks to transformers, and power carried over long distances with a
small amount of loss. Further, the development of low-cost, rugged and efficient AC machines, to
dominate the electrical machines panorama paved the way towards the international adoption of
AC for electrical transmission. Nonetheless, the increasing penetration of power electronics in the
electrical, electronic and electromechanical engineering is at the base of the potential switch
towards the use of DC for the same objective. In fact, developments in many sectors of power
electronics such as circuit topologies, control techniques, power semiconductor devices,
development of wide-bandgap (WBG) semiconductors and carbon-based materials, CAD software
pave the way towards the increase of efficiency of power electronic based systems. Power
electronics plays a key-role posing challenges on the use of electrical energy in DC form, for both
technical (in addition to conversion efficiencies) and economic aspects. This happens in both
transmission (HVDC systems) and distribution (LVDC systems). The increasing use of renewable
sources for energy generation as well as increasing use of more efficient electrical loads based on
power electronics are the main forces pushing towards the re-evaluation of the DC distribution
capabilities and the potential energy savings.
The aim of this document, therefore, has been to analyse and compare the impact of power losses
in cables and converters on the efficiency and cost of DC versus AC supplying systems for office
buildings. The work starts with an introductory overview of losses in cables and their dependence
on voltage level, cable cross-section area (CSA), and transmitted power. Some potential advantages
and disadvantages are shown especially when evaluating the possibility of using DC to reach extralow voltage (ELV) DC levels which is attractive for safety reasons. Then a case study showing that
main losses in building internal networks generally occur in power converters is presented. This
leads to the investigation of efficiencies in power electronic converters as a function of power rating.
The analysis of manufacturers’ data for the efficiency of converters versus power rating along with
some assumptions are made to gain an understanding of the total supplying system efficiency in
4
several scenarios. A simple cost analysis is presented at the end of the document. After the
examination of four case scenarios, it was found that DC shows itself as a better solution than AC
for office supplying in terms of system efficiency, and in spite of higher capital costs more
economically convenient in the long term, due to lower operating costs. This is going to become
always more true by the trend towards more “green” and distributed energy generation to comply
with targets imposed from national and international authorities.
5
Introduction
It’s well known that DC current internally supplies most electronic appliances that we usually find
inside an office environment. However, to connect them to a standard 230 V AC outlet, a conversion
from AC to DC is required. Effort is expended in tightly regulating the AC power quality whilst few
of the connected appliances use the energy in that form. Nevertheless, using a low voltage direct
current to supply offices has always been discarded because the power demand per unit area is too
high for the distance over which the power must be transferred. DC distribution1 is becoming more
attractive mostly because of continuous improvements in the field of power electronics, the
introduction of new renewable energy sources naturally providing DC voltage, and appliances
naturally suited for DC supply. The increasing reduction of power demand, due to improved
technologies, especially in lighting and IT, make possible thinking about a shift from the standard
single-phase AC system towards a low-voltage DC one. However challenges remain, mostly related
to the efficiency of power conversions, to the squared dependence of power losses with the power
required by electrical appliances, and for safety reasons intrinsically related to the nature of the DC
current. The concept of DC current for electricity transmission has been known for a very long time,
but it was not used in modern systems until the great development of HVDC systems in the 1960s.
It’s famous in this regard in the “War of currents” between George Westinghouse, supporter of the
development of AC distribution and Thomas Edison, supporter of DC distribution. As it is well known,
Westinghouse finally succeeded, mainly because of the invention of transformer that allowed
distribution of power over long distances with small power and voltage losses and of the invention
of AC motors for energy generation [1]. Nowadays, the expanding field of renewable energy sources
like solar, wind and fuel cells in buildings jointly with the continuous reduction of use of fossil fuels
make reasonable to reinvestigate the role of DC for building supply. In order to carry out this
investigation, a preliminary work is required on the efficiency and costs of using DC for offices, and
a comparison with the standard AC. This has been the objective of this work and the followed
approach can be summarised as follows:
1. Background. In this section some basic information regarding issues concerning cables and
power converters for a AC and DC networks are given. There are four subsections:
a. Transmission lines and electrical cables
1
The word distribution is used to indicate both outside and inside-building distribution. The meaning will be clarified
from the context
6
b. Power losses
c. Voltage drops
d. Input rectifers
2. Low voltage DC and Extra low voltage DC. In this section, a first comparison of the advantages
and disadvantages of AC vs DC systems is made. There are three subsections:
a. AC vs DC: a first comparison
b. AC vs DC losses: a case study
c. Low voltage DC: pros and cons
3. Converter efficiencies; AC vs DC system efficiencies. In this section, a representation of the curves
relating converter efficiencies with power ratings is made. The results are then used to compare
AC distribution networks (DNs) with DC DNs. There are two subsections:
a. Converter efficiencies
b. Efficiency comparison of AC versus DC supplied offices
4. Cost comparison of AC versus DC supplied offices. In this last section a summary cost analysis is
done. The main components of the capital and operating costs are taken into account for
economic comparison of AC vs DC DNs.
7
1. Background
a) Transmission lines and electrical cables
Before embarking in the analysis of losses in distribution systems, we briefly summarise the results
of the theory of the transmission lines studied in the basic courses of Circuit theory and/or
Electromagnetic Fields.
As any other electrical means used to transmit electrical energy or information, electric cables can
be modelled as distributed circuits, constituted by an infinite cascade of the elementary lumped
circuit shown in Figure 1.
I
R
L
I+dI
V
G
C
V+dV
dx
Figure 1 – Transmission line modelling
In this model, , , ,
take account of physical characteristics of the line and of its operating mode.
As it is well known, two partial differential equations, called telegrapher’s equations, describe the
functioning of transmission lines:
∂V
= ( R + jω L ) ⋅ I
∂x
∂I
= ( G + jωC ) ⋅V
∂x
(1)
∂ 2V
= ( R + jω L )( G + jω C ) ⋅ V
∂x 2
∂2I
= ( R + jω L )( G + jω C ) ⋅ I
∂x 2
(2)
or
8
Usually the quantity
is called
, square of the propagation constant , while
is the angular pulsation. In the transmission lines theory, it is customary to introduce another
quantity, called characteristic impedance, given by
Z0 =
R + jω L
G + jωC
(3)
For electric power cables, as transmission lines, we therefore have to consider four parameters
, , , . Nonetheless, in distribution networks, two of the parameters,
and
(transversal
parameters) can be generally neglected considering the line as constituted by a cascade of many
series
branches. Besides, in the particular case of DC supplying the line shows only a resistive
behaviour since the reactive part is zero because of zero frequency. Usually, other two phenomena,
taking place in cables, can be considered and they are the skin-effect (AC supply) and the proximity
effect. Both of those cause an increase in the magnitude of the resistance shown by cables.
However, in our treatment of DC low-voltage networks, we can neglect both of them because their
small influence in computations. The following picture shows, as an example, the distribution of
current, due to the skin effect, in a six mm2 circular copper cable when a 10 current is distributed
respectively in DC or AC. From the figure, it can be noticed a larger current density in the “skin” of
the conductor that determines a small increase of the cable resistance for the AC case respect to
the DC one. However, as mentioned, at a frequency 50 Hz, this increase of resistance is very small
and in many cases negligible.
Figure 2 – Skin-effect in cables: current density in a 6 mm2 circular copper cable when a 10 A current is transmitted in
DC (left) or 50-Hz AC (right). The increase of resistance in AC 50 Hz is practically negligible; in both cases the resistance
per unit meter is approximately equal to the DC value of 2.9 mΩ/m
9
b) Power losses
Power losses in cables are almost solely due to the non-zero resistance. In the AC distribution case,
losses occur also in the dielectric, but they are neglected in our treatment of low-voltage networks.
In order to make comparisons of AC and DC systems we have to consider the rms value for AC
quantities. So, we have, for example, the same transmitted power for the AC 230 V (rms value)
source and a DC 230 V one for a resistive load. Taking into account a unipolar DC system, power
losses are simply given by the following formula:
∆PDC = 2 ⋅ R ⋅
where
is the transmitted power,
P2
VDC 2
(4)
the resistance per core, and
the voltage level. Instead, in
single-phase and three-phase AC systems, we have to take account of the power factor and the
following formulas hold:
∆P1φ = 2 ⋅ R ⋅
P2
Vrms 2 ⋅ cos 2φ
P2
∆P3φ = R ⋅
3 ⋅ V ph 2 ⋅ cos 2φ
where P is in both cases the total transmitted power,
(5)
is the rms phase voltage of a symmetric
three-phase source. In (5), the DC value for the resistance is considered (skin effect neglected). We
also assume negligible cable reactance for LV networks.
So the ratios between (4) and (5) become:
∆PDC Vrms 2
=
cos 2φ
2
∆P1φ VDC
(6)
2
∆PDC 2 ⋅ V ph
=
cos 2φ
∆P3φ 3 ⋅ VDC 2
(7)
It can be seen that the DC-to-AC power losses ratios change depending on the chosen DC level. In
(6), a DC value equal to the rms AC value would guarantee an equal transmitted power for the same
load, but unequal stress on the dielectric. In fact, equal maximum stress on the dielectric occurs
when the DC value is taken equal to the peak value of the AC waveform. Doing so we get:
∆PDC 1
= cos 2 φ
∆P1φ
2
10
(8)
Taking temporarily into account equation (8), it can be seen that in this case, the DC distribution
would be advantageous over the single-phase AC one. In fact, power losses will be at most a half of
the AC case ones (Figure 3). That is, by taking a DC voltage level equal to the peak of the AC
waveform, we have halved power losses, at the same conditions of transmitted power, cable length,
and stress on the dielectric. In addition, many of the existing electrical appliances could theoretically
work without the use of an input rectifier [2], reducing losses in it, improving the quality of the
supplied voltage, and using already installed cables. If we use a DC value equal to the rms AC value,
we will have less stress on the dielectric than the AC case at the same conditions of transmitted
power and cable length. As before, many appliances may still work, and we can continue to use the
already installed cables. In any case, a relation between the maximum allowable electrical power
and cable length at different levels of DC voltage and cross-section areas (CSA) holds. Figure 4 shows
some curves regarding the maximum allowable electrical power vs. cable length in several cases of
DC voltage level and CSA. The maximum transmitted power determined by the most constraining
of the following two conditions: maximum current-carrying capability (ampacity) of the cable and
voltage drops to guarantee a proper functioning of the network (5%, 3%, or 1%) (Figure 4). An
analysis, comparing two three-phase systems with three DC bipolar (with neutral) and with equalstress on the conductor-earth insulator, leads to the following results:
= √2
∆
∆
=
11
1
√2
Figure 3 – Power losses ratio: DC to single-phase AC (VDC=VAC,max, red line), DC to three-phase AC (VDC=VAC,max, blue
line)
Figure 4 – Ampacity and voltage drop (5%) limitations on the transmitted power for several DC levels
12
c) IR (voltage) drops
Another issue related to the transmission of electricity is voltage drops along cables. They are also
called IR drops, because they are determined by the product of the non-zero cable resistance (and
reactance in AC) and the current flowing in it. To analyse them, let us consider the following circuit:
Z
V2
V1
Figure 5 – Line modelling for IR drop analysis
The following equation holds
Vɶ1 = Vɶ2 + Zɶ ⋅ Iɶ
(9)
where the symbol ~ is used to indicate complex quantities. Let’s suppose that we are supplying a
load with a delay angle , then we can explicit the right side and write:
Vɶ1 = Vɶ2 + ( R + jX ) ⋅ I ⋅ ( cosφ + j ⋅ sinφ )
Vɶ1 = (Vɶ2 + RIcosφ − XIsinφ ) + j ( XIcosφ + RIsinφ )
(10)
Taking the squared-modulus of both sides of (10) we have:
V12 = V22 + ( RI ) + ( XI ) + 2V2 ( RIcosφ + XIsinφ )
2
2
( RI ) + ( XI )
(V1 − V2 ) =
(V1 + V2 )
2
2
+
2V2
⋅ ( RIcosφ + XIsinφ )
(V1 + V2 )
(11)
Supposing
V1 + V2 ≈ 2 ⋅V2
(12)
it results
( RI ) + ( XI )
(V1 − V2 ) =
( 2 ⋅V2 )
2
2
+ ( RIcosφ + XIsinφ )
The first piece of the right side is usually small, and thus neglected, so (13) becomes
13
(13)
(V1 − V2 ) = RIcosφ + XIsinφ
Multiplicating and dividing both sides of (14) by
(V1 − V2 ) =
(V1 − V2 )
V2
where
(14)
, it results
RP + XQ
V2
RP + XQ
( % ) = 100 ⋅
V22
(15)
and ! are the active and reactive power respectively. If it is possible to neglect the
reactance the equation (14) will give:
(V1 − V2 ) = R ⋅ I ⋅ cosφ
(16)
∆VDC = R ⋅ I
(17)
In particular, in the DC case it results:
Taking account of bipolar cables
∆VDC = 2 ⋅ R
P
VDC
(18)
while, generally, in the single-phase AC case:
∆V1φ = 2 ⋅
P
( R + X ⋅ tanφ )
Vrms
(19)
The ratio between (18) and (19) is:
2⋅R
P
VDC
∆VDC
=
P
∆V1φ
2⋅
( R + X ⋅ tanφ )
Vrms
(20)
Taking VDC = 2 ⋅ Vrms , we get:
∆VDC
R
=
∆V1φ
2 ⋅ ( R + X ⋅ tanφ )
(21)
We can notice from (21) that the ratio of DC IR drops to AC IR drops is less than 1 because " ∙
$%& ≥ 0 ( ) 0 ≤
< ,⁄2 and ohmic-inductive loads. Therefore, we also have, in the case of DC
level equal to the maximum AC voltage, lower voltage drops than the single-phase AC case, in
addition to lower power losses. The following figure shows, as an example, the profile of voltage
14
drop along the main bus of the floor of a building with a regular rectangular structure made of 20
offices, all of the same size and absorbing the same amount of power, P=500 W. The section of the
main-bus cable is 50 mm2, when standard AC 230 V or DC 326 V is used as the main bus voltage. The
voltage drop along lateral buses is neglected. The total length of the floor is 200 m.
100
99.5
Voltage (%)
99
98.5
98
97.5
97
96.5
0
20
40
60
80
100
120
140
160
180
200
Length (m)
DC
Single-phase AC
3% limit
Figure 6 – Voltage drop profile for DC and single-phase AC
d) Input rectifier
As already mentioned, one of the advantages of using DC respect to AC is the possibility to remove
the input rectifier in appliances with possible improvements in the system efficiency [2] [3]. In fact,
as it’s well known, diodes present conduction and switching power losses. Conduction losses are
given by
PL = D ⋅ V f ⋅ I f
(22)
supposing zero turn-off and turn-on times of the diode, because they are much smaller than the
switching period. D is the duty cycle,
.
is the forward voltage and /. is the forward current.
Conduction losses occurs because of the built-in potential and non-zero on-state resistance. In (22)
the forward voltage keeps account of both of them. Explicating them, (22) can be also written as
PL = D ⋅ (Vbi + Ron ⋅ I f ) ⋅ I f = D ⋅ Vbi ⋅ I f + D ⋅ Ron ⋅ I 2f
15
(23)
Power losses in the off state are often negligible. So are switching losses.
Besides avoiding losses, removing the input rectifier gives another advantage. As it can be seen in
Figure 7, two waveforms are represented: the blue one represents the sinusoidal input 230 V AC,
the green one is the voltage that develops at the anode of the diode D1 (PCC, Point of Common
Connection) respect to ground (cathode of D2). The green waveform is distorted, due to current
harmonics that flow through the series of the input resistance and inductance. It can be seen that
the voltage wouldn’t have been distorted, if we hadn’t had the input inductance, but in any case,
the input current would. Therefore, avoiding the input rectifier (and PFC2), we don’t have to worry
about voltage or current harmonics in the supplying network.
Figure 7 – Single-phase diode bridge rectifier (left); Input voltage waveform and distorted waveform at PCC in
presence of line inductance (right) (Ls=1 mH, Rs=1mΩ, Vf=0.8V, Cd=1mF, Rload=20Ω)
2. Low voltage DC and extra low voltage DC
a) Low voltage DC: pros and cons
To take advantage of the ELV regulations3, a value below DC 60 V (ripple-free) should be chosen.
Nonetheless, such a low value has not traditionally being recommendable because of very high
power losses and IR drops (Figure 8-Figure 9).
2
3
In the presence of a PFC stage, there are not problems with harmonics
IEC 60364 Part 4-41: Protection for safety – Protection against electric shock
16
Figure 8 – Power losses trend with voltage (V) (squared) and cross-section area (mm2) (linear)
Figure 9 – IR drops trend with voltage (V) (linear) and cross-section area (mm2) (linear)
To mitigate this problem, two options are possible: use of large CSA cables, with increased costs, or
use of lower power, more expensive appliances. The hypothesis of replacing AC with DC current, to
supply offices in particular, has become an attractive idea because of the trend in lowering the
power demand by lighting and IT products. The use of very-high efficient appliances, which are DC
suitable, as well as the use of distributed energy resources (DER), etc. are a few of the key-enabling
elements for the achievement of concepts like the Zero-Net Energy (ZNE) building, DC microgrids
etc. all aimed to improve the efficiency of buildings and more generally of the entire electric power
17
system. For offices in particular, new technologies such as LED lighting, increasing of the use of
variable speed drives (VSDs), as well as innovations in the IT sector, are leading towards a lower
power demand for offices and an increase of the share of the intrinsically DC load. It is therefore
reasonable to think of shifting from AC to LVDC for supplying an office. In more technical details,
taking the ratio of (4) to (5):
∆PDC PDC 2 ⋅ Vrms 2
=
cos 2φ
∆P1φ
P1φ 2 ⋅VDC 2
(24)
If the power factory is unitary, (24) becomes:
∆PDC PDC 2 ⋅Vrms 2
=
∆P1φ
P1φ 2 ⋅VDC 2
(25)
If we want equal or less power losses than the AC case then it results
PDC 2 ⋅ Vrms 2
=1
P1φ 2 ⋅ VDC 2
(26)
PDC 2 Vrms 2
=
P1φ 2 VDC 2
(27)
PDC Vrms
=
P1φ VDC
(28)
and thus
(28) shows that there is an inverse proportionality between the delivered power and the voltage
level. That means if we want to pass, for example, from the standard 230 V AC to a DC level of 48 V
and keep the same amount of power losses for the 48 V-line as for the 230 V AC case, we should be
able to reduce the power demand of:
=
123
=
0
230
48
≅ 4.79
that is, almost five times less transmitted power than the AC case, at the same amount of power
losses. However, the relative losses will be more than the AC case. So if we are currently using an
average power value of 200 W (including only low power appliances like lighting and IT products)
per office, for example, that should be reduced to 40 W. Due to the great technology improvements
18
above mentioned that value is not so far from being reached. It is to notice, as already mentioned,
that besides reducing the power demand of appliances, the requirements of cable ampacity and
voltage drop should be always respected. That means that the power demand must be such to not
overcome the maximum transmitted power per length (Figure 4). In order to illustrate the impact
of the cable losses on the choice of the network layout, a simple example is presented below. The
following Figure 10-Figure 11 show two potential strategies to implement an Extra Low Voltage
(ELV) DC supplying system. The first figure represents a LV DC radial distribution network with DC/DC
conversions, to obtain 48 V DC at the output, carried out at “office-level”. Lower ELV DC levels could
be obtained by means of “socket-level” or Point-Of-Load (POL) DC/DC converters, to supply lowpower devices. The second figure represents an ELV DC distribution network where the ELV DC is
already available at the output of the upstream supplying rectifier. As it can be understood, the
supplying scheme of Figure 10 with a main DC bus of 326 V, distributing electricity along the floor
of the building, and DC/DC conversions at office-level to provide ELV DC inside offices is potentially
feasible, while, on the contrary, the scheme of Figure 11 is not. In fact, even if in Figure 11, the
conversion at the beginning of each floor could be more efficient because it is a bulk conversion 4,
losses along lines become very high and the system less reliable. Differently from the AC
distribution, some issues such as fault management, arc voltaic extinction, system reliability etc.
need more attention in the DC distribution. They lie outside the purpose of this document and will
be considered in future work.
Figure 10 – LVDC building distribution network with “office-level” DC/DC conversion to provide ELV DC supply (main
bus voltage equal to 326 V)
4
Bulk conversions are generally more efficient as shown later
19
Figure 11 – LVDC building distribution network with “floor-level” DC/DC conversion to provide ELV DC supply (main
bus voltage equal to 48 V)
b) AC versus DC losses: a first comparison
A first comparison in terms of power losses and IR drops in cables, between single-phase AC and DC
for low-voltage supplying networks can be made. In fact, from (7), it’s possible to see what happens
for different choices of the DC level. The following three choices of DC level are of particular
importance, and require a more detailed treatment:
1. DC value equal to the maximum AC value (i.e. DC 326 V, AC 230 V (rms value)): if a DC level of
326 V is chosen, that is equal to the peak of the standard 230 V AC voltage, then at the same
conditions of transmitted power and stress on the dielectric, power losses will be lower, ranging
from 0.5 times the AC case ones (power factor equal to 1) or less (power factor less than 1). So
briefly, power losses will be 50% (or more) lower than the AC case, IR drops 29.3% lower than
the AC case. It’s also possible to use already installed cables and remove the input rectifier in
many appliances. Many old appliances would continue to work without modifications
2. DC value equal to the rms AC value (i.e. DC 230 V, AC 230 V (rms value)): in this case, at the same
conditions of transmitted power, power losses and IR drops for the DC case will be equal (in the
highly unlikely case of
= 1) or less than the AC case (
< 1). Dielectric stress for the
DC case will be less than the AC one, it’s also possible to use already installed cables and remove
20
the input rectifier. Some of the already existing appliances could be operated without
modifications.
3. DC value less than the rms AC value: in this case, in general, power losses will be more than the
AC case. So will be voltage drops. This is the most critical case, and measures need to be taken
to manage for increased losses by either reducing the power demand of appliances or using very
large CSA cables.
DC voltage level
VDC=326 V (>VAC,rms)
Comparing with AC 230 V
•
Less power losses at the same transmission conditions (-50% or more)
•
Less IR drops (-29.3% or more) at the same transmission conditions
•
Same stress in the dielectric
•
No skin effect
•
Unitary line power factor
•
Removing of the input rectifier in many appliances
•
Possibility of using the already installed cables
•
Possibility of using many of the already existing appliances without
modifications
•
Potential improvements of the voltage quality and EMC characteristics
due to the absence of harmonics
•
Equal or less power losses at the same transmission conditions
•
Less IR drops at the same transmission conditions
•
Less stress in the dielectric
•
No skin effect
VDC=230 V
•
Unitary line power factor
(=VAC,rms)
•
Potential removing of the input rectifier in many appliances
•
Possibility of using the already installed cables
•
Possibility of using some already existing appliances
•
Potential improvements of the voltage quality and EMC
characteristics due to the absence of harmonics
VDC< VAC,rms
•
Generally more power losses at the same transmission conditions
•
Generally more IR drops at the same transmission conditions
•
Less stress in the dielectric
•
No skin effect
•
Unitary line power factor
•
Potential removing of the input rectifier in many appliances
21
•
Replacement of already installed cables may be needed
•
Possibility of using low voltage appliances without power conversion
•
Better electrical safety and compliance with Extra-Low Voltage (ELV)
regulations (VDC≤60V)
Table 1 – Comparison table between AC and DC for three specific cases
The third case is the one that needs more attention. In fact, decreasing the DC level, power losses
and IR drops on cables will increase. The main reason that motivates decreasing the DC level is
electrical safety: for a DC level less than 60 V, no protection against direct contacts is required (IEC
Class III). In this regard, using ELV DC to supply office appliances might be appealing, provided
measures to manage for increased losses are taken.
c) AC versus DC losses: case study
In the preceding paragraphs, the impact in terms of power losses and voltage drops on cables,
deriving from the distribution of electricity in either DC or AC was shown. The description of power
losses and voltage drops on cables was made along with a general comparison of DC with AC. At this
stage, however, it’s worth it to emphasise that power losses in LV distribution networks mainly occur
in power conversion stages. In case DC supply is used, power conversions occur in the passage from
the MV usually outside buildings (AC), to the LV DC supply inside buildings. Power conversions also
occur inside building and appliances. Further, electrical motors inside some appliances convert
energy from electrical to mechanical type. As already mentioned, losses in all these energy
conversions play the main role in the efficiency of the distribution system, making losses occurring
in cables generally negligible. Although electromechanical energy conversions play an important
role in the efficiency of appliances using electrical motors, it’s not the main purpose of the
document, so the focus will be on the efficiency of electronic power converters. Therefore, the
objective of this paragraph will be to show that electronic power converters play the main role in
the efficiency of a DC distribution system and this will be done by means of an illustrative example
referring to the distribution network shown in Figure 12. By referring to Figure 12, it’s possible to
make a comparison of power losses when single-phase AC or DC is used as the main bus voltage.
Although the idea of using a DC or single-phase main bus is not strictly realistic, in fact, a three phase
DN with single-phase rings system may be used in place; it will be used in the following example for
illustrative purposes. Generally, for the case of a three-phase DN with regularly and cyclically-shared
single-phase loads, it can be said that power losses and voltage drops are higher than a three-phase
DN with regularly shared three-phase loads. In fact, in the first case the neutral current will be higher
22
than the second one. For the case of a three-phase DN with a number of cyclically-shared singlephase loads greater than 20, the error that is made considering it as composed by 20 balanced threephase loads is less than 5%. This last case can be theoretically analysed using the per-phase
equivalent network, therefore, the focus will be on the comparison of single-phase AC with DC
distribution networks. This is done, in the following example of Figure 12.
In the DC case, supposing that each one of the offices in Figure 12 is absorbing an amount of power
(output power), /<= , equal for all, the total power losses (31) of the DN are given by losses along
the lines (29) and losses in the converters (30):
Figure 12 – LVDC building distribution network with “office-level” DC/DC conversion to provide ELV DC supply (main
bus voltage equal to 326 V)
2
Pcables
where

 NO 2 + 3 ⋅ N O + 2 
P
 ρ 
=  2⋅ 
⋅
2
l
⋅



6
 A   η ⋅ NO ⋅VDC 


(29)
1 
PDC / DC =  −1 ⋅ P
η 
(30)
Ptotal _ losses = Pcable + PDC / DC
(31)
is the total power absorbed by the loads,
is the cross-section area of the cable, under the
assumption of equal cross-section area for all cables,
is the DC voltage level, > is total length of
the “corridor” starting from the first two offices, <= is the number of offices and ? the efficiency of
converters, supposed to be equal for all converters. Just to give an idea in numerical terms, it will
be assumed that each office is effectively absorbing 100@ of active power and we have a number
23
40 offices. Let’s also suppose that the DC main bus voltage is constant along the line and equal to
326 , the CSA is equal to 60 BB , the cable temperature is equal to 70 °C5, the efficiency of the
DC/DC converters is equal to 0.85 and the total length > is equal to 200 B. Cable losses are given
by (29):
 2 ⋅10 −8  
 402 + 3 ⋅ 40 + 2 
4000

Pcables =  2 ⋅
⋅
⋅
4
0
0
⋅

 ≅ 9.97 W

−6  
60
⋅
10
0.85
⋅
40
⋅
3
26
6






2
(32)
Losses in converters are given by (30):
PDC / DC ≅ ( 0.1764 ⋅ 4000 )W ≅ 705.6 W
So total losses are:
Ptotal ≅ 716W
The overall efficiency of the system is
η sys =
4000
≅ 0.85
4000 + 715.57
As it can be see, total losses are massively dominated by the converter losses making cable losses
negligible in this case. Therefore, the system efficiency is practically related to the efficiency of
power converters. The voltage drop profile along the bus is shown in the following Figure 13, for a
main bus DC value of 326 V. The same is done for a main bus DC value of 48 V in Figure 14. In this
case, it’s visible that the main bus voltage falls below the 5% voltage drop limit of the nominal value,
at about 120 m from the upstream bulk converter.
5
This value of temperature represents the maximum steady-state temperature allowed for PVC-like insulated cables
24
Voltage drops @Vbus_nom=326 V
326
325.95
325.9
Voltage (V)
325.85
325.8
325.75
325.7
325.65
325.6
325.55
325.5
0
20
40
60
80
100
120
140
160
180
200
180
200
Length (m)
Bus voltage
Figure 13 - Voltage drops along cables for a DC main-bus nominal value of 326 V
Voltage drop @Vbus_nom=48 V
49
48
Voltage (V)
47
46
45
44
43
42
41
0
20
40
60
80
100
120
140
160
Length (m)
Bus voltage
5% drop limit
Figure 14 – Voltage drops along cables for a DC main-bus nominal value of 48 V. The orange line indicate the 5%
voltage drop limit on the nominal value according to electricity regulations for DNs.
By comparison, if a high voltage DC value of 326 V is used for the main bus, losses will be around 10
W and there will be no reliability issues for the network. If a low voltage DC value of 48 V is used,
25
losses will become unacceptable and that means that an upstream DC/DC converter with a very
high efficiency will be needed in order to have the same system-efficiency of the preceding case.
In the AC case, supposing that AC/DC converters are drawn in place of DC/DC converters in Figure
12, assuming that their efficiency is 0.85 and that the power factor of the entire network is equal to
one, because of wide installations of PFC equipment, loss in cables will be given by:
 2 ⋅10−8  
 40 2 + 3 ⋅ 40 + 2 
4000

Pcables =  2 ⋅
⋅
⋅
4
00
⋅

 ≅ 20.02W

−6  
60
⋅
10
0.85
⋅
40
⋅
230
6






2
(33)
In (33) the same formula used for DC voltage (32), with a voltage value equal to 230 V, has been
used. This because of the assumption of unity power factor and equal efficiency of AC/DC and DC/DC
converters. A power factor less than 1 will make these losses higher and so will do an efficiency less
than 0.85 for the conversion. In any case, we can see that power losses in cables for the AC case are
about twice the DC case. Therefore, it is possible to affirm that passing from the existing AC system
to a DC system, with a main bus equal to the peak of the AC value will halve losses in cables.
Coming back to the DC case it can be said that the preceding considerations suggest rejecting the
idea of using a low value, such as 48 V, as main bus voltage and therefore the solution with “floorlevel” DC/ELV DC conversions. Nonetheless, the idea of using bulk conversions is a useful one,
because as will be shown later they are generally more efficient than dedicated ones. Based on this
idea, a possible implementation strategy for DC supplying systems that take advantage of the
greater efficiency of bulk conversions could be the following one:
•
A first bulk DC/DC conversion from DC 326 V to DC 230 V (or the nominal rms AC voltage of
appliances to be retrofitted). This conversion is needed because appliances that currently work
at the rms value of the AC voltage need to keep the value of their nominal rms voltage
unchanged when passing to DC, otherwise they may operate improperly with consequences in
terms of reliability and performance. Generally, these kind of appliances are the one that
present a “resistive” behaviour, or some others that embed an induction motor for their
operation;
•
A second bulk conversion from 326 V to an ELV DC value of 48 V, to supply many low-power
appliances such as telecom appliances, which mostly work at this voltage level.
•
Further ELV DC levels such as 5 V, 12 V, 24 V can be derived from highly-efficient POL
conversions, carried out in proximity of sockets. In any case, it’s recommendable using a
26
“centralised” or “bulk” conversion approach when feasible and convenient to get higher
conversion efficiency.
This approach would give great reliability to the entire system along with maximising the power
density. Another feasible solution could be the adoption of a hybrid AC-DC system.
In order to give figures of system efficiency for AC vs DC supplying system, a detailed analysis of the
efficiency of all power conversions that occur in the entire electrical supplying chain is needed.
Therefore, in the following paragraph, figures of efficiency for the power converters will be derived
and a system-level efficiency estimation will be made. In any case, the example presented before
shows that the role played by power converters in the total power losses computation is
predominant. This means that small improvements in the efficiency of converters would have a
great impact on the efficiency of the entire supplying system and therefore on the overall energy
consumption.
3. Converter efficiencies; AC vs DC system efficiencies
a) Converter efficiencies
As pointed out in the preceding section, the biggest impact of losses in “inside-building” distribution
networks is due to power converter losses. It’s thus of primary importance to focus on the efficiency
of converters to better understand whether it’s possible to save energy with DC distribution and
what conditions. A brief survey of data from four AC/DC power supply manufactures is shown in the
following Figure 15. As it can be seen, the efficiency of AC/DC converters increases with the output
power. The efficiency changes with loading conditions, in particular at low-load conditions it can be
very low, wasting a large amount of energy that goes through the converter as heat (Figure 15).
Regarding to the scaling of converters, we can say that an analysis done by the Lawrence Berkeley
National Laboratory (LBNL) [4], estimated that, on average, the efficiency of individual AC/DC
adapters for appliances is 68%, while for bulk rectifiers a figure of efficiency of 90% is averagely
representative. Hammerstrom [3] instead considers that each converter stage loses, as a first order
assessment, about 2.5% of the energy absorbed by the load. Differently from [3], it’s necessary to
take account, in the efficiency analysis, of the difference from dedicated AC/DC power supplies to
bulk ones. The shift towards a “centralised” conversion is also advantageous because it eliminates
excessive dedicated conversions, simplifies and make possible potential improvements of the supply
system (elimination of “cord spaghetti” issues, plug & play modularity, energy storage etc.) [5].
27
AC/DC converters: Efficiency vs Power rating
100
95
90
85
Efficiency (%)
80
75
70
65
60
55
50
45
40
0
200
400
600
800
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000
Power (W)
Power (20% load)
Power (40% load)
Power (80% load)
Power (100% load)
Power (60% load)
Figure 15 – AC/DC power supplies: Efficiency versus Power rating at different loading conditions
In Figure 15 a trend of curves of efficiency for various loading conditions is shown. These curves
have been deduced by fitting, according to the least squares algorithm, data from four
manufacturers of AC/DC and about 500 different types of DC power supplies. These data are
referred to converters having ELV DC output voltages and can be considered representative of the
AC/DC converters present on the market. It can be noticed that generally there is an increase of the
efficiency with the nominal power of the converter. An increase of efficiency is also noted by moving
towards full-load operating conditions. The equations of the plotted curves are:
28
D.DFGH
, for the 20% load curve
?GD% = 68.151 ∙
D.D I
, for the 40% load curve
•
?JD% = 71.998 ∙
D.D
F
, for the 60% load curve
•
?ID% = 74.923 ∙
D.D JH
, for the 80% load curve
•
?0DD% = 76.502 ∙
•
?
•
D%
= 57.984 ∙
D.D
, for the 100% load curve
For the efficiency analysis, the AC/DC power supplies with a power rating under 100-150 W can be
considered as dedicated power supplies, and the AC/DC power supplies rated above 1000-1500 W
as bulk power supplies. Then we have the following figures of efficiency for AC/DC power supplies:
Dedicated AC/DC power supplies
Bulk AC/DC power supplies
MAX efficiency
MIN efficiency
20%
76.3
84.7
40%
82.5
88.7
60%
84.7
90.1
80%
85.7
90.2
100%
85.9
89.8
Loading condition
Table 2 – AC/DC converter efficiencies
Considering that appliances do not always work at their rated power but a lower one, it is possible
to introduce a factor that keeps account of this smaller power consumption and that is called load
factor Kl . In case of an appliance supplied by an adapter, the load factor corresponds to the loading
condition of the dedicated adapter. In order to deduce the loading conditions of dedicated and bulk
converters, some preliminary considerations need to be made. Firstly, for example in a building
there are many categories of appliances and generally within each category different load factors
for different appliances. That means, for example, that in general, a lighting fixture works with a
different load factor than a laptop, and two different lamps or laptops generally work with different
load factors themselves. Therefore, it’s necessary to consider an average value of the load factor,
Kl , representative of all load factors of all appliances. Secondly, for each category, not all appliances
present in the building work at the same time. So, it’s necessary to introduce another factor, called
factor of simultaneity, Ks that takes account of that. Even in this case it’s necessary to consider an
average value of the factor of simultaneity, Ks . Overall, the total power absorbed by the edifice is
given by the total nominal power of all loads, multiplied by a factor, K ad , that is the product of the
average load factor and the average factor of simultaneity, and that for office buildings ranges,
29
approximately, in the interval 0.6 K 0.8 [6]. For this work, the lowest value of 0.6 was chosen. That
means
Kad = Kl ⋅ Ks = 0.6
(34)
K ad can be seen as the loading condition of bulk power supplies. To estimate the average value of
the load factor, Kl that can be seen as the loading condition of dedicated power supplies, the
following approach can be followed. The factor K ad , derives considering the so-called “load after
diversity”, that is the load that is seen from the main panel (or main bulk converter) due to many
individual electrical loads. By referring to the following picture, that illustrates schematically the
floor of an office building, it can be seen that the ELV DC distribution network is constituted by two
(or more, according to the power density) stages of conversions from a relatively high value of DC
voltage, say 326 V to an ELV DC voltage of 48 V or less.
Figure 16 – Two stage conversion system ELV DC supplying system
The “load after diversity” that is seen from the upstream rectifier is therefore constituted by the
total nominal load of all offices (represented by rectangles in the picture) multiplied by the factor
of simultaneity, that is established according to the number of offices supplied by the power
rectifier. At its time, the “load after diversity” seen from each “office level” DC/DC power supply is
multiplied by the load factor of the office that is established taking account of the number and type
of loads inside the office, and so on until the individual loads with their load factors are considered.
At this stage, in order to derive conclusive results, some simplifications could be made. First of all,
it will be assumed that between the power input and the load only two stage of conversions occur.
This is a reasonable assumption, in fact a feasible and effective way to supply a DC low-voltage tries
30
to minimise the number of conversion stages, so it’s reasonable to assume that only one more
DC/DC conversion stage is present to supply ELV DC, after the LV DC (e.g. 326 V) value at the output
of the rectifier. Therefore, it can be assumed that from the AC bus to the load there are, at most,
two conversions (AC->DC->DC). Secondly, it will be assumed that all offices have the same
electrical/electronic equipment, same office-load factors. This assumption is reasonable since
usually in the same building, offices for the same final scope are approximately equipped with the
same electrical furniture. If the total load of an office, that can be called Po , is equal to
Po = Klo ⋅ ( P1 + P2 + ... + Pn )
(35)
because of the assumption of equal office-load factors, (35) is valid for whichever office and K lo can
be replaced by Kl , the average load factor of all appliances of all offices. Now coming back to the
first assumption of, at most, two conversion stages in the supplying network, it follows that
Kad = Ks ⋅ Kl = 0.6
(36)
and because the factor of simultaneity has a value according to the following table [7]:
Number of downstream consumers
2 to 4
5 to 9
10 to 14
15 to 19
20 to 24
25 to 29
30 to 34
35 to 39
40 to 49
50 and more
Factor of simultaneity (ks)
1
0.78
0.63
0.53
0.49
0.46
0.44
0.42
0.41
0.40
Table 3 – Factor of simultaneity in relation to the number of downstream consumers
then
Kl =
K ad 0.6
=
Ks
Ks
(37)
Another assumption is needed to derive the value for Kl . Because we are considering bulk
converters with power rating of 1000-1500 W (see page 26) and dedicated converters with power
31
rating of 100-150 W then the ratio of number of dedicated converters to bulk converters is less or
equal to 10:1. According to
Table 3 the value of Ks should be taken 0.63, 0.78 or 1. The value 0.63 for the factor of simultaneity
is to choose when the number of downstream customers is according to the table between 10÷14.
However, if there are “high power” DC loads supplied from the same bulk converter then it’s much
more likely that the ratio is less than 10 and therefore the factor of simultaneity is to be taken equal
to 0.78, and thus:
Kl =
0.6
= 0.77
0.78
(38)
This means that the loading condition for dedicate converters can be considered equal
approximately to 80%. Summarising the loading condition for bulk converters is to be taken equal
to 60% and for dedicated converters equal to 80%. Doing so, according to Table 2, the value of the
efficiency of dedicated AC/DC power supplies is 85.7% and the efficiency of bulk AC/DC converters
is 90.1%. For DC/DC converters, with the same input and output voltage range of the preceding
AC/DC converters, due to the lack of enough manufacturers’ data, we can make a simplification and
assume that the removal of the first conversion stage makes them more efficient than AC/DC
converters of 2.5% [3]. So a dedicate DC/DC power supply is assumed 88.2% efficient while a bulk
one is 92.6%. For DC/AC inverters we still make a simplification, of 2.5% power conversion loss, so
that they are both (bulk and dedicated) 97.5% efficient. This last assumption, even if it could be not
verified in terms of absolute values, doesn’t invalidate our results because on the two cases
happens. In the first case, energy is derived from conventional sources. In this case, DC/AC
inversions occur inside appliances like CFLs or air conditioners. This means that whether the main
bus voltage is in AC or in DC, because conversions occur internally to the appliance, they don’t affect
the comparison of efficiency. In fact, if the bus is in AC and it’s supplying a CF lamp then inside the
CF lamp two conversions occur (AC->DC->AC). If the bus is in DC occur there will be only the first
conversion will not occur but the second one will as before. So the DC/AC conversion is present in
both case and thus irrelevant in the comparison of efficiency of AC vs. DC. In the second case, energy
is derived from distributed energy resources (DERs). In this case, DC/AC inversions occur in feeding
the AC grid from naturally-DC generating sources like solar panels or fuel cells. In this case, the
presence of DC/AC inverters is relevant in the comparison. Therefore, in this case, for simplicity
reasons, we’ll make the assumption that the overall DC/AC conversion, from solar panels to the AC
32
main bus for example, is as efficient as the overall DC/DC conversion from solar panels to the DC
main bus. In this case, as suggested from a brief survey on manufacturers’ data, we can consider
their efficiencies very high and therefore use, for both of them and only for this specific power
conversion, the assumption of overall conversion loss equal to 2.5%6. The other DC/DC conversions,
e.g. from the main bus to loads, are still considered 88.2% and 92.6% efficient as before indicated.
In the following, we will separate explicitly the various power conversions, when they are meant as
split stages, so for example we’ll refer to the figure of efficiency of the AC/DC (controlled output)
conversion as representing the total conversion efficiency, even if the actual power conversion
occurs in two stages.
b) Efficiency comparison of AC versus DC supplied offices
The following Figure 17 splits by end use the amount of 8245 ktoe7 (~96 TWh) of electrical energy
consumed for the service sector in the year 2011, in the UK. The energy consumed in the service
sector takes account of about 31% of the overall (excluding the transport sector) consumption of
electrical energy (26510 ktoe, ~308 TWh) in the UK.
UK electric energy consumption in Service sector,
2011
14%
14%
3%
14%
40%
6%
9%
Space heating
Water heating
Cooking catering
Computing
Ventilation
Lighting
Other
Figure 17 – UK electricity consumption in the service sector by end use for year 2011
It can be noticed that lighting dominates the electricity consumption in the service sector with a
share of about 40%. It’s clear that even with the present AC supplying system, small improvements
6
This corresponds to assume the overall conversions DC/AC and DC/DC from PV panels to AC and DC main bus
respectively, equally efficient. This is actually a simplification penalising the DC distribution case.
7
ktoe=thousand tonnes of oil equivalent
33
in the efficiency of power conversions that occur in many office appliances will be greatly beneficial
for the efficiency of the entire electrical system, with a large impact on the overall energy
consumption and CO2 emissions in the environment. As said before, losses occur in converters and
cables, with the first term dominating the second one by large. For what regards losses in cables, an
estimation of losses for cables was done before with the conclusive result that it’s possible to affirm
that passing from the existing AC system to a DC system, with a main bus equal to the peak of the
AC value will halve losses in cables. For what regards power losses in converters, it can be generally
said the power losses are supposed to be higher in the AC rather than the DC due to the one or
more conversion stages when using AC voltage to supply DC loads. In fact, for many appliances
functioning internally in DC, it’s necessary to convert from AC to DC voltage. A DC/DC conversion
usually follows the first AC/DC conversion8 and the overall result is a global AC/DC conversion with
a smaller figure of efficiency than a single DC/DC conversion, that is just needed when supplying the
appliance directly from DC [3]. Things will become worse, when electrical energy is derived from a
DC source such as a PV panel, a fuel cell, a micro-turbine or battery storage. In this case, a first
conversion is needed to feed the AC grid because those sources naturally provide energy in DC9
form. An internal AC-DC conversion, constituted of a cascade of an AC/DC (uncontrolled) and a
DC/DC (controlled) conversion, usually follows. Therefore, if energy is derived from distributed
energy resources, naturally generating energy in DC form, two conversions can be avoided by using
DC as main bus supply. In order to proceed with the study of the impact of power conversion losses
on the system efficiency, the figures of converter efficiencies found in the preceding section will be
taken into account. Under these premises, to estimate the efficiency of a DC supplying system for
offices and compare it with the efficiency of an AC one, the data regarding the UK electricity
consumption in the service sector, split by end use, for the year 2011, are reported in the following
Table 4:
8
9
Electrical energy end use
Average consumption (%)
Lighting
40
Cooking catering
14
Space heating
14
Water heating
3
Computing
6
Ventilation
9
A PFC stage is also present after the input stage.
A preceding DC/DC converter is needed for PV panels.
34
Other
14
Total
100
Table 4 - UK electricity consumption in the service sector by end use for year 2011
Following the approach outlined by Hammerstrom [3], the power absorbed by each appliance will
be considered as a constant. Taking also account of [2], it can be reasonably assumed that each AC
designed load has a DC counterpart. Savings that can derive from improvements in technology itself
such as:
•
Use of VSDs instead of traditional ones can reduce the energy consumption of the appliance of
as much as 30% [8].
•
Improvements in the LED technology can reduce the power consumption of lights even further
than compact fluorescent lights (CFLs)
•
Introduction of electric vehicles (EVs) or plug-in hybrid electric vehicles (PHEVs) for
transportation
•
Induction cooking in place of electrical resistance-based cooking (-12% energy consumption [9])
•
Introduction of VSDs based heat pumps for space and water heating
will not be considered. Distributed energy resources (DERs) as constituting the total energy sources
of the supplying system will be taken into account. As first case, the case where AC electrical current
is derived from conventional generation/transmission will be examined. Before embarking in the
analysis, it’s supposed that a bus constituted by a group of three-phase AC voltage is present and
either single-phase AC is derived from it or bulk DC voltage (with auxiliary ELV DC voltage from
office-level or point-of-load DC/DC converters) from a three-phase bulk rectifier in order to supply
goods Figure 18. We can therefore list, for each appliance type, the number and type of conversions
carried out to supply it with single-phase AC or bulk DC respectively.
3-ϕ AC
Input: 3phase AC
Output: 1ϕ AC or
bulk DC
Office or
POL DC/DC
converter
Figure 18 – AC or DC supply of office appliances
35
Office
Appliance
4. Five case scenarios of AC vs DC system comparison
In order to facilitate the comprehension of the analysis five case scenarios will be presented in the
next pages, the following table summarises the characteristics in terms of electrical sources,
electrical loads, and type of supply that characterise each one of them.
Case
Electrical sources
Electrical loads
1
Conventional sources
Old-technology
appliances
2
Conventional sources
Old technology
appliances
3
Conventional sources
Old technology
appliances
4
Conventional resources
New technology
appliances
5
Distributed energy resources
(DERs)
New technology
appliances
Comparison
Standard AC
VS.
DC supply
Standard
AC vs DC supply
(95% efficiency of bulk
AC/DC conversion)
Hybrid standard AC-DC
VS.
DC supply
Standard AC
VS.
DC supply
Standard AC
VS.
DC supply
Table 5 – Five case scenarios of AC vs DC comparison analysed in the following pages
a) AC vs DC supply: conventional electrical sources, old-technology appliances
Appliance
AC supply
DC supply
End use
Technology
Number of bulk
conversions
Number of
dedicated
conversions
Number of bulk
conversions
Number of
dedicated
conversions
Lighting
CFLs
0
2 (AC/DC, DC/AC)
1 (AC/DC)
1 (DC/AC)
Cooking,
catering
Resistance
based
0
0
1 (AC/DC)
0
Space
heating
Resistance
based
0
0
1 (AC/DC)
0
Water
heating
Resistance
based
0
0
1 (AC/DC)
0
Computing
Digital
electronics
0
1 (AC/DC)
1 (AC/DC)
0
Ventilation
VSD based
2 (AC/DC,
Other
-
2 (AC/DC,
0
DC/AC)
0
DC/AC)
-
-
-
-
Table 6 – Number of power conversions for each appliance type with conventional AC and DC supply respectively
36
Data from Table 6 represent the number of conversion that can be attributed to each appliance type
to supply it respectively from single-phase AC or DC voltage. It can be noticed that in the case of DC
supply, the bulk rectifier, present at the beginning, always represents one conversion. For
resistance-based loads we assume that there is a first conversion a DC voltage equal to the rms AC
value. For computing, we assumed that a bulk conversion from AC to a low-value DC is present. This
last condition could not be always true, in fact for high power-densities of computing, it could be
necessary to carry out a dedicated (POL) conversion rather than a bulk one. For ventilation, the
DC/AC conversion has been considered of bulk type. Another aspect to point out is that we are
assuming for cooking, water and space heating that the old resistance-based technology is
dominant. This is not exactly true now and will become always less in the future with the increasing
development of induction cooking equipment and VSD-based systems. This will be, of course, all to
the advantage of the DC distribution. Using data from Table 4 as “weights” for the data of Table 6,
and the preceding mentioned values for efficiencies of converters, it results:
Appliance
AC supply
DC supply
End use
Technology
Average
consumption (%)
Efficiency
(%)
Average
consumption (%)
Efficiency
(%)
Lighting
CFLs
40
83.6
40
87.8
Cooking,
catering
Resistance
based
14
100
14
90.1
Space heating
Resistance
based
14
100
14
90.1
Water heating
Resistance
based
3
100
3
90.1
Computing
Digital
electronics
6
85.7
6
90.1
Ventilation
VSD based
9
87.8
9
87.8
Other
-
14
100
14
100
Weighted
average
-
-
91.5
-
90.4
Table 7 – Efficiency comparison for offices using appliances as of Table 6
The item “Other” has been left unchanged, because there are no precise data of what it includes,
so it is supposed that its effects on the system efficiency are the same in both AC and DC supply. It
can be noticed that there is a small “advantage”, in terms of efficiency, of the AC supply (91.5%)
37
against DC supply (90.4%) in this case. Of course, system efficiency figures will change if different
values for the efficiency of converters are assumed, but only to a small extent. Consequently,
fractions of percentage of difference will be meaningless to justify a replacing of AC with DC, if we
are still using old-technology appliances and conventional electricity generation/transmission.
b) AC vs DC supply: conventional electrical sources, old-technology appliances, 95%
efficiency of AC/DC bulk conversions
It’s also important to stress that the situation of case a) is not a strictly realistic one for several
reasons. Firstly, the efficiency of the conversion AC to high-output DC has been considered 90.1%
efficient, while there are already converters on the market more than 95%-efficient even at lowload conditions. In fact, as it’s shown in the next table, by using the value of 95% for the efficiency
of bulk AC/DC conversions, even with old-technology appliances we get a higher system-efficiency
for the DC case (94.5% ) rather than the standard AC one (91.5%). Nonetheless, the previous case
has been shown because, even if there are already very high efficient converters (?>95%) on the
market, usually their cost is much higher than standard ones, so they not representative of the massscale converters on the market.
Appliance
AC supply
DC supply
End use
Technology
Average
consumption (%)
Efficiency
(%)
Average
consumption (%)
Efficiency
(%)
Lighting
CFLs
40
83.6
40
92.6
Cooking,
catering
Resistance
based
14
100
14
95
Space heating
Resistance
based
14
100
14
95
Water heating
Resistance
based
3
100
3
95
Computing
Digital
electronics
6
85.7
6
95
Ventilation
VSD based
9
92.6
9
92.6
Other
-
14
100
14
100
Weighted
average
-
-
91.5
-
94.5
Table 8 - Same efficiency comparison of the preceding case with the bulk AC/DC conversion efficiency taken equal
to 95%
38
Secondly, we might think of a hybrid system (next subsection) where AC and DC coexist, so that we
can still supply some appliances, best suitable for AC, with AC and others, best suitable for DC, with
DC. Of course, this can have consequences on other characteristics of the power system like stability
[3], but we neglect those for now. Further, in future, we could have electrical current distributed
(distribution from substations to outside of buildings) in the DC form, rather than AC, so, in this case,
the aforementioned bulk AC/DC power conversion for those appliances becomes unnecessary. This
is also the case where there is DC energy generation from distributed resources (this case will be
analysed later). Finally, many appliances for lighting, cooking, space and water heating are already
using newer technologies that besides being more efficient by themselves require less conversion
stages for DC distribution rather than AC one. This will become always more true in the near future
due to the increasing diffusion of LED lamps, induction cooking equipment and VSD appliances.
c) Hybrid AC-DC vs DC supply: conventional electrical sources, old-technology appliances
Before repeating the analysis of a) with the same shares of energy consumption, but with
technologically newer appliances, we would like to show that a hybrid system is more efficient than
a standard AC one when using old-technology appliances for lighting, cooking, space and water
heating:
Appliance
AC supply
Hybrid AC/ DC supply
End use
Technology
Number of bulk
conversions
Number of
dedicated
conversions
Number of bulk
conversions
Number of
dedicated
conversions
Lighting
CFLs
0
2 (AC/DC, DC/AC)
1 (AC/DC)
1 (DC/AC)
Cooking,
catering
Resistance
based
0
0
0
0
Space
heating
Resistance
based
0
0
0
0
Water
heating
Resistance
based
0
0
0
0
Computing
Digital
electronics
0
1 (AC/DC)
1 (AC/DC)
0
Ventilation
VSD based
2 (AC/DC,
Other
-
2 (AC/DC,
0
DC/AC)
0
DC/AC)
-
-
-
-
Table 9 - Number of power conversions for each appliance type with conventional AC and hybrid AC-DC supply
respectively
39
Appliance
Hybrid standard AC
and DC supply
AC supply
End use
Technology
Average
consumption (%)
Efficiency (%)
Average
consumption (%)
Efficiency
(%)
Lighting
CFLs
40
83.6
40
87.8
Cooking,
catering
Resistance
based
14
100
14
100
Space heating
Resistance
based
14
100
14
100
Water heating
Resistance
based
3
100
3
100
Computing
Digital
electronics
6
85.7
6
100
Ventilation
VSD based
9
87.8
9
87.8
Other
-
14
100
14
100
Weighted
average
-
-
91.5
-
94
Table 10 – Efficiency comparison for offices using appliances as of Table 9
We would save about 2.5% of energy by simply using a hybrid system rather than the AC, in case of
old-technology appliances. It is to point out that a hybrid system is convenient when using oldtechnology appliances and conventional energy sources. In the next two cases presented, one with
new-technology appliances and conventional energy sources and the other with new-technology
appliances and distributed energy resources, there is no advantage in using a hybrid system. In fact,
in both cases conversion losses will equal or greater than the standard AC system.
d) AC vs DC supply: conventional electrical sources, new-technology appliances
Appliance
AC supply
DC supply
End use
Technology
Number of bulk
conversions
Number of
dedicated
conversions
Number of bulk
conversions
Number of
dedicated
conversions
Lighting
LEDs
0
1 (AC/DC)
1 (AC/DC10)
0
Cooking,
catering
Induction
cooking
2 (AC/DC11,
2 (AC/DC,
0
DC/AC)
0
DC/AC)
10
By assuming of having a bulk conversion from AC to a low value DC (e.g. 24 V) we are supposing of having already
managed for cable losses issues, as before mentioned.
11
The AC/DC conversion is assumed of bulk type here since it is a high-power load
40
2 (AC/DC,
Space
heating
VSD based
Water
heating
VSD based
Computing
Digital
electronics
2 (AC/DC,
0
0
DC/AC)
DC/AC)
2 (AC/DC,
2 (AC/DC,
0
0
DC/AC)
DC/AC)
0
1 (AC/DC)
2 (AC/DC,
Ventilation
Other
VSD based
0
2 (AC/DC,
0
0
DC/AC)
DC/AC)
-
-
1 (AC/DC)
-
-
-
Table 11 - Number of power conversions for technologically newer appliances with conventional AC and DC supply
respectively
Appliance
AC supply
DC supply
End use
Technology
Average
consumption (%)
Efficiency
(%)
Average
consumption (%)
Efficiency
(%)
Lighting
LEDs
40
85.7
40
90.1
Cooking,
catering
Induction
cooking
14
87.8
14
87.8
Space heating
VSD heat
pumps
14
87.8
14
87.8
Water heating
VSD heat
pumps
3
87.8
3
87.8
Computing
Digital
electronics
6
85.7
6
90.1
Ventilation
VSD heat
pumps
9
87.8
9
87.8
Other
-
14
100
14
100
Weighted
average
-
-
88.5
-
90.6
Table 12 - Efficiency comparison for offices using appliances as of Table 11
The advantages of DC distribution will become more evident, assuming that electrical energy is
entirely generated using distributed energy resources (DERs), naturally providing DC voltage. These
could be fuel cells, micro-turbines or PV panels. In this case, the advantages of DC distribution will
show up because of another avoided conversion respect to the AC supply. In fact, to supply an
appliance working internally in DC directly from PV energy we need at least three or four
conversions in a standard AC system: one DC/DC conversion to implement the MPPT algorithm, a
41
DC/AC conversion to feed the AC bus and one or two conversions (AC/DC – DC/DC) to supply the
appliance itself. These could be reduced to two conversions by using DC supply.
e) AC-DC vs DC supply: distributed energy resources (DERs), new-technology appliances
As shown in Table 14, there is a large saving of 13.1% when using DC distribution with energy
generation from DERs and technologically efficient appliances. This adds up to savings deriving from
technological improvements in the efficiency of the appliances themselves. The figures of systemefficiency of 13.1% can differ and be lower in practice due to inaccuracies in the figures of efficiency
for all types of converters considered here. In any case, it remains the presence of a large gap
between AC and DC supply when deriving the electrical energy from DERs.
Appliance
AC supply
DC supply
End use
Technology
Number of bulk
conversions
Number of
dedicated
conversions
Lighting
LEDs
1 (DC/AC)
1 (AC/DC)
Number of bulk
conversions
Number of
dedicated
conversions
2 (DC/DC,
Cooking,
catering
Induction
cooking
Space
heating
VSD based
Water
heating
VSD based
Computing
Digital
electronics
3 (DC/AC,
2 (DC/DC,
0
0
AC/DC, DC/AC)
DC/AC)
3 (DC/AC,
2 (DC/DC,
0
0
AC/DC, DC/AC)
DC/AC)
3 (DC/AC,
2 (DC/DC,
0
0
AC/DC, DC/AC)
DC/AC)
2 (DC/DC,
1(DC/AC)
1 (AC/DC)
Other
VSD based
2 (DC/DC,
0
0
AC/DC, DC/AC)
DC/AC)
-
-
0
DC/DC)
3 (DC/AC,
Ventilation
0
DC/DC)
-
-
-
Table 13 - Number of power conversions for technologically newer appliances and distributed energy resources
(DERs) with AC and DC supply respectively
Appliance
AC supply
DC supply
End use
Technology
Average
consumption (%)
Efficiency
(%)
Average
consumption (%)
Efficiency
(%)
Lighting
LEDs
40
83.6
40
90.3
Cooking,
catering
VSD based
14
72.9
14
97.5
42
Space heating
VSD based
14
72.9
14
97.5
Water heating
VSD based
3
72.9
3
97.5
Computing
Digital
electronics
6
83.6
6
92.6
Ventilation
VSD based
9
72.9
9
97.5
Other
-
14
100
14
100
Weighted
average
-
-
81.6
-
94.7
Table 14 - Efficiency comparison for offices using appliances as of Table 13
f) Summary results
Just to summarise the results of the five case scenarios examined, the following scheme compares
the efficiency of AC and DC supplying systems in each one of them, and the savings in terms of
efficiency:
Changes respect
AC supply system
DC supply system
Savings (DC
to the first case
efficiency
efficiency
versus AC)
-
91.5%
90.4%
-1.1%
95% bulk AC/DC
conversion
efficiency
91.5%
94.5%
3%
Hybrid AC-DC
supply instead of
simple DC
91.5%
Hybrid AC-DC 94%
2.5%
New-technology
appliances
88.5%
90.6%
2.1%
ELECTRICAL ENERGY
SOURCES:
conventional
generation
ELECTRICAL LOADS:
old-technology
appliances
ELECTRICAL ENERGY
SOURCES:
conventional
generation
ELECTRICAL LOADS:
old-technology
pliances
ELECTRICAL ENERGY
SOURCES:
conventional
generation
ELECTRICAL LOADS:
old-technology
appliances generation
ELECTRICAL ENERGY
SOURCES:
conventional
generation
43
ELECTRICAL LOADS:
new-technology
appliances generation
ELECTRICAL ENERGY
SOURCES:
DERs generation
ELECTRICAL LOADS:
new-technology
appliances generation
Energy generation
from distributed
energy resources
81.6%
94.7%
13.1%
Table 15 – Scheme summarising the efficiency comparison, AC versus DC, in the examined cases
From Table 15 it can be noticed that, apart from the first “outdated” case, we always have a positive
sign in the saving of energy when using DC to supply office appliances. In fact, savings due to the
adoption of DC voltage would range, in any case, between 2% and 13% approximately. We stress,
once again, that what really matters is the large gap of efficiency between AC and DC supply when
deriving energy from DERs and using new-technology appliances (13.1%). This is very important
since the trend is towards pushing the use of more efficient DC-suited appliances and generation of
the energy needed to supply buildings entirely from DERs [5].
5. Cost comparison of AC versus DC supplied offices
A formula for the evaluation of the total costs in the supplying electrical system of Figure 12 is
written below. It gives the cost of the system for both the cases of AC and DC supply. In the following
formula (39) five terms can be distinguished: the first two, representing costs due to cables (DC
voltage or rms AC value, power factor=1) and converter power losses are operating costs, while the
last-but-two and penultimate terms take account of capital costs. In particular, they represent the
cost of copper wires and the cost of all power converters present in the system. The last term takes
account of additional costs.
2
CTotal
  NO 2 + 3 ⋅ NO + 2 
1 
P
 ρ 
= CE  2 ⋅  
 2l 
 ⋅ T + CE  − 1 P ⋅ T + Cwire + Cconverters + Cextra
6
 A   η ⋅ NO ⋅VDC  
η 

(39)
In order to make a comparison in terms of costs between AC and DC the cost of converters, the cost
of copper and the cost of the energy must be known (Figure 19).
44
DC/AC converters: Price vs Power rating
1000
1000
800
800
Price (£)
Price (£)
AC/DC converters: Price vs Power rating
600
400
200
600
400
200
0
0
0
1000
2000
3000
0
1000
Power (W)
Copper wires: Unit price vs CSA
1000
12
10
8
6
4
2
0
Cost per meter (£)
DC/DC converters: Price vs Power rating
800
Price (£)
2000
600
400
200
0
0
1000
2000
3000
Power (W)
3000
0
Power (W)
10
20
30
CSA (mm2)
Figure 19 – Cost of converters versus power rating and copper versus cross-section area
It is clear that the economic analysis will change with the variations in price that happen in time and
with many other factors: for example, the price of a power converter can change depending on
power rating, technology, topology, design characteristics and many other features that
characterise it. Copper cables can differ in cost, depending on the cross-section area, conductor
material, dielectric material and so on. Nonetheless, in the preceding figure, the curves of price
against power rating and cross-section area for converters and wires, respectively, have been
plotted, because of their main role in determining the price. Therefore, we have to take the results
that follow from the analysis cautiously and keep them into account just to have a numerical
estimation of costs when using AC and DC supply respectively. We also have to point out that
economic savings derive, for sure, from technologically improved appliances. These are, anyway,
“common” factors in the comparison of AC and DC so, for the purpose of this document, we have
not considered them. Nonetheless, they certainly contribute to the adoption of the DC technology
for supplying, since these appliances are better DC-suited rather than AC, as well as the use of DERs
for energy generation.
To make the numerical comparison, the costs of converters, copper, and energy need to be known.
Just to keep things simple, copper losses in the main bus (output of the bulk converter) will be
neglected, supposing we managed to keep them low along with voltage drops. At this stage, the
45
terms taking account of the copper cost and additional costs will be also neglected, since they don’t
make any difference in the relative comparison of costs. Economic inflation will be also neglected.
Under these conditions, converter losses we will dominate the power losses amount in the system.
Let’s write the equations relative to the curves price-power of converters:
•
M /
= 1.4005 ∙
D.N DH
•
/
= 5.9761 ∙
D.FNN0
•
/M
= 0.8153 ∙
D.IJGH
, for the cost of AC/DC converters
, for the cost of DC/DC converters
, for the cost of DC/AC converters
The cost of energy has been assumed equal to 0.15 £/kWh. As before, we can consider the power
converters with a power rating under 100-150 W as dedicated power supplies, and the power
converters rated above 1000-1500 W as bulk power supplies. The cost computation will be based
on considering again the data regarding the UK electrical energy consumption in the service sector
for the year 2011 (Table 4) reported below (Table 17).
Cost of
energy,
CE
(£/kWh)
0.15
Cost of
dedicated
AC/DC
conversion
(150 W) (£)
51.88
Cost of bulk
Cost of
Cost of bulk
AC/DC
dedicated
DC/DC
conversion
DC/DC
conversion
(1000 W)
conversion
(1000 W)
(£)
(150 W) (£)
(£)
203.7
107.71
321.9
Cost of
dedicated
DC/AC
conversion
(150 W) (£)
62.15
Cost of bulk
DC/AC
conversion
(1000 W)
(£)
320.64
Table 16 – Cost of electrical energy and electronic power converters
Electrical energy end use
Average consumption (%)
Lighting
40
Cooking catering
14
Space heating
14
Water heating
3
Computing
6
Ventilation
9
Other
14
Total
100
Table 17 - UK electricity consumption in the service sector by end use for year 2011, in percentage
To go further the formula of (39) needs to be applied, supposing appliances of each end-use absorb
an average amount of power according to Table 17. Clearly each one of the appliance listed above
46
requires a different output voltage for supplying. We’ll make the simplification that the same
converter can provide as many output voltages as we need to supply the appliances of Table 17 and
that the we have taken account the trade-off power demand versus cable length for each one of
them. Explicating the amount of electricity absorbed by each appliance and referring to the scenario
of using new-technology based appliances and DERs generation, the scheme of Table 18, regarding
the conversions respectively for the AC and DC supply, holds:
Appliance
AC supply
DC supply
End use
Technology
Number of
bulk
conversions
Number of
dedicated
conversions
Lighting
LEDs
1 (DC/AC)
1 (AC/DC)
Number of
bulk
conversions
2 (DC/DC,
Number of
dedicated
conversions
0
DC/DC)
3 (DC/AC,
Cooking,
catering
Induction
cooking
2 (DC/DC,
AC/DC,
0
0
DC/AC)
DC/AC)
3 (DC/AC,
Space
heating
2 (DC/DC,
VSD based
AC/DC,
0
0
DC/AC)
DC/AC)
3 (DC/AC,
Water
heating
2 (DC/DC,
VSD based
AC/DC,
0
0
DC/AC)
DC/AC)
Computing
Digital
electronics
2 (DC/DC,
1 (DC/AC)
1 (AC/DC)
0
DC/DC)
3 (DC/AC,
2 (DC/DC,
Ventilation
VSD based
AC/DC,
0
0
DC/AC)
DC/AC)
Other
-
-
-
-
-
Table 18 – Number and type of power electronic conversions with new-technology appliances and DERs generation
in the AC and DC supply cases respectively
By using Table 16-Table 18 the cost of converters can be computed. In any case, supposing of
implementing the system from scratch, installation and operating costs will be present for both
cases. Installation costs are due to unitary costs of converters and not to costs of appliances
47
themselves or others. Clearly, a bulk converter can supply a greater number of devices since its
greater power rating. The number of ELV DC appliances that can be supplied by a bulk converter is
theoretically given by the ratio of power rating of a bulk converter to the power rating of a dedicated
converter. For example, if the power rating of a bulk converter is 1000 W and the one of a dedicated
converter is 100 W, theoretically ten 100 W appliances can be supplied by the bulk converter.
Nonetheless, if one these 100 W devices, is, for example, supplied with a DC voltage of 24 V and far
located from the bulk converter, the 5% voltage drop-limit might be overcome. Then, it will become
necessary to supply “high-power” (~100÷500 W) devices by using Point-Of-Load (POL) converters.
This is the case of very high power-demanding data centres. In data centres, the value of 380 V DC
has been standardised for supplying very high-power data servers by using dedicated conversions.
Therefore, it is necessary to take account that the number of appliances that are supplied by a bulk
converter is practically less than the ratio between the power rating of a bulk converter (considered
1000 W in this work) and the power rating of the appliance. Therefore, a “corrective” factor that
takes account of this reduction should be considered and will be called k . Therefore, a “corrected”
ratio follows and its expression is:
1 P
bulk loading factor
r = ⋅ bulk ⋅
k Pdedicated dedicated loading factor
(40)
where the loading conditions of bulk converters (60%) and dedicated converters (80%) have been
considered, and where k is the factor mentioned before and included in the range [0,1]. To
estimate the value of k an analysis of the voltage drop along lines as long as a system reliability
analysis have to be made. The second factor is equal to the ratio “bulk power rating” to “dedicated
power rating” (1000 W/150 W). At this stage, an estimation for the value to give to the factor k
has not been made, and will be made in successive work. In order to continue the cost analysis, a
value of k equal to 0.9 has been assumed. Conceptually this means using a number of dedicated
converters that is 1.1 bigger than the number of converters strictly needed to supply all appliances.
Using more dedicated converters will increase the cost relative to them, but will also improve the
reliability of the system and lower cable and voltage drops along lines. So using k equal to 0.9 it
follows from (40):
r=
1 1000 0.6
⋅
⋅
≅ 5.56
0.9 150 0.8
48
(41)
In making the cost comparison, this means that the price of a dedicated converter, that follows a
bulk conversion, needs to be multiplied by a factor 5.56. Therefore, by assuming that the entire UK
energy consumption in the energy sector for the year 2011, is due to a constant average absorbing
of power equal to:
Pavg =
96 TWh
= 10, 958, 904,109 W ( ∼ 11 GW )
8760 h
(42)
And, assuming that 86% of appliances use power electronics (no power electronics for the item
“Other” of Table 18), we get the results shown in the following table:
AC supply
DC supply
Cost of bulk
converters (£)
Cost of dedicated
converters (£)
Cost of bulk
converters (£)
Cost of dedicated
converters (£)
5,320,399,000
1,454,118,000
6,062,291,000
0
£ 6,774,517,000
£ 6,062,291,000
Table 19 – Comparison of capital costs for bulk and dedicated converters with AC and DC supply respectively
from the preceding analysis, we see that the AC solution would cost more than the DC one, in terms
of capital costs if they were implemented from scratch. These figures are just to be taken as a simple
indication that a AC supplying system is susceptible of costing more, in terms of capital costs, than
DC supply one and not to strongly rely on them for cost analyses. In fact, in time and case-by-case
we can get different values for the costs of converters needed to implement a DC supplying system
or an AC one. In other words, it will not be surprising if a more detailed analysis could give different
results of the capital costs that we have just found, because, as said before, many factors will affect
these results. They should just be taken to get an idea on the order of magnitude of converter costs
in both cases respectively. It is also important to examine the operating costs. As already said, these
are due to losses in cables and losses in converters. The first ones must be kept low for safe and
reliable operation, so we assume of having already done so and neglect them. The other running
losses are due to losses in power converters. Each generic conversion with nominal commutated
power
3
and efficiency ?3 wastes an amount of energy that costs in an interval of time O:
1

C E  − 1 Ps ⋅ T
 ηs

49
(43)
This formula is still an approximation, because the cost of energy will change with many factors (e.g.
time, inflation rate, interest rate, energy sources, etc.), so we still call for prudence in using these
data. Anyway, we would like to estimates the operating (or running) costs of the energy lost in
conversions, for the 96 TWh of energy consumed in the service sector in the year 2011 (Table 18)
for AC and DC supply.
AC supply
DC supply
Cost of bulk
conversion losses (£)
Cost of dedicated
conversion losses (£)
Cost of bulk
conversion losses (£)
Cost of dedicated
conversion losses (£)
1,108,225,000
1,123,809,000
1,008,602,000
0
£ 2,232,034,000
£ 1,008,602,000
Table 20 - Comparison of operating costs for bulk and dedicated converters with AC and DC supply respectively in the
UK service sector for the year 2011
It is clear from Table 19-Table 20 that, theoretically, the total savings of implementing a DC supplying
system (from scratch) over an AC one, for offices in the service sector, in the UK, will be about 1.94
billion pounds for the first year. This figure includes, in the first year, the savings from capital costs
and savings from the operating costs. They will become 1.22 billion pounds per year for the following
years (Table 21).
Total cost AC supply
Total cost DC supply
Savings per year
1st year
£ 9,006,551,000
£ 7,070,893,000
£ 1,935,658,000
Successive years
£ 2,232,034,000
£ 1,008,602,000
£ 1,223,432,000
Table 21 – Comparison of total costs for AC and DC supply respectively and savings for the first and successive years.
In both cases, the capital costs are accounted in the costs for the first year. These data are referred to the total electric
energy consumption in the UK service sector for the year 2011
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Conclusions
In this work, the potential use of DC and extra-low voltage (ELV) DC to supply offices has been
shown. The feasibility of a DC supplying system has been analysed from the point of view of losses
in cables and converters. Power losses in cables increase with a squared dependence when
decreasing the voltage level. They decrease with the cable cross–section area in a linear fashion.
Although power losses in cables play a role in the efficiency of any electrical system, they don’t
represent the main cause of losses in in-building distribution networks. In fact, losses that occur in
power electronic conversions play the biggest role. The efficiency of power converters has been
analysed, along with some simplifications, to gain an understanding about the order of magnitude
of converter losses. The results from the analyses of different scenarios show that the efficiency of
a DC supplying system is, in almost all examined cases, greater than the standard AC one. In
particular, the case, where energy is deduced entirely from distributed energy resources (DERs) and
new-technology efficient appliances are used, shows to be very promising. In fact, from the
efficiency analysis, if we replace AC with DC we will potentially save about 13% of electrical energy
consumed in offices in the last years. This figure is only representative of the bare replacement of
AC with DC, because other savings derive from the replacement of old appliances with new ones
that are naturally DC-suited. The same thing can be said for the increasing use of DERs, they can
help too in the ultimate mission of reducing the “CO2 footprint” deriving from the use of fossil fuels
for conventional electrical energy generation. This also translates in cost savings after a short
interval of time, as shown from the cost analysis carried out. This analysis would change if we had
higher power densities. This happens, for example, if we are supplying an amount of appliances that
increase the “bulk-load” amount or increasing the “loading condition” of bulk converters with more
“power-hungry” ELV DC devices. In the worst case, the operating costs will prevail over the capital
ones after a short period of time.
51
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