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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 50 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 References [1] “War of Currents,” [Online]. Available: http://en.wikipedia.org/wiki/War_of_Currents. [2] K. 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