Download Jean-Louis Canaletti Christian Cristofari Gilles Notton

2011 International Conference on Environment Science and Engineering
IPCBEE vol.8 (2011) © (2011) IACSIT Press, Singapore
Specific enslavement for autonomous solar system
Jean-Louis Canaletti
Christian Cristofari
Gilles Notton
University of Corsica
UMR CNRS 6134
Scientific Research Center of Vignola
Route des Sanguinaires
F20000 AJACCIO – FRANCE
[email protected]
University of Corsica
UMR CNRS 6134
Scientific Research Center of Vignola
Route des Sanguinaires
F20000 AJACCIO – FRANCE
[email protected]
University of Corsica
UMR CNRS 6134
Scientific Research Center of Vignola
Route des Sanguinaires
F20000 AJACCIO – FRANCE
[email protected]
management). The pump is started when Tout > Tint + ΔT1.
It is stopped when Tout < Tint + ΔT2. With ΔT1 ≈ 5K-8K ;
ΔT2 ≈ 1K-3K. The pump is stopped when Tref > 362K
Abstract—The study presents a method for controlling the flow
rate of a fluid by enslavement of the circulator in the stand
alone heat production systems using solar energy (PV/Th). The
electronic control developed using this method allows to adjust
the speed of the circulator to converge to the needs set by the
user and/or maximize the energy exchange (following the sun)
between the solar collectors and the use. It will be presented
here the results obtained with autonomous solar collectors
using heat air fluid.
Solar water
collector
Keywords-solar system; heating; renewable energy; hybrid
collector.
I.
PV
module
Tref
Tout
Tank
Ctrl
Pump
Tin
INTRODUCTION
A new method control has been developed to achieve two
objectives: compensate the disadvantages of conventional
control used in most heat production systems using solar
energy and optimize the energy used to operate the circulator.
For example, in systems of hot water production for
dwellings, an on/off control is not suitable if the energy used
to the pump comes from a photovoltaic module. The
situation is similar for systems producing hot air for heating
assistance [1].
This electronic control uses a new method to adjust in
real time the speed of the circulator to satisfy the desired
temperature set by the user and/or optimize the energy
exchange between the heat collector and the use. A study
was performed on a pair of stand alone solar shutters
equipped with this experimental command control unit.
II. DESCRIPTION OF A STAND ALONE SYSTEM
Figure 1. Stand alone hot water system.
In case of air system, the control is like water system but
the fan is stopped when Tref > 292K
Solar air
collector
PV
module
Tout
Home
Ctrl
Fan
Tref
Tin
Figure 2. Stand alone hot air system.
A conventional heat exchange between heat collector
system and use usually consists of [2]:
• heat collector(s)
• photovoltaic module(s)
• storage
• circulator
• control unit
• heat exchange circuit
The control unit manages the starting and stopping of the
circulator (on/off operating) through the external sensors
(input management) and command transistor (output
If the circulator works with direct current and agrees to
operate in the voltage variation range of the photovoltaic
module, it can be connected directly to the module. With a
proper sizing of all the elements, a natural adjustment occurs
because the density of incident radiation is the same for the
heat collectors and for the PV module [3]. The output
temperature of thermal field is stable if the sun radiation does
not vary abruptly. In this case the command control rarely
operates.
258
PV
module
Ctrl
DC-Fan
Figure 3. DC-Fan control.
If the circulator operates with alternative current, the
previous mode is not possible, because it does not accept too
high change in voltage or variable frequency. In this case the
command control is requested very often. In addition, an
inverter is required.
Inverter
PV
module
Ctrl
AC-Fan
Figure 4.
Figure 5. Experimental wall with solar shutters
AC-Fan control
A. The measurement circuit
The measurement circuit reads in real time the
parameters needed for the command control:
• Outlet collector air temperature, Tout.
• Air temperature inside the housing Tint.
• Air Temperature chosen by the user Tref.
The temperature is measured with an integrated circuit
LM35 [5] (temperature sensor) Thus, for a temperature
ranging between 273 and 313K the voltage will be between
2.73 V and 3.13 V.
This type of control causes some major drawbacks:
• Frequently cycles on/off are hungry for energy and
the circulator wears out prematurely.
• The heat collector's temperature is swinging and
induces many cycles of expansions, which accelerate
the ageing.
• There is no management of inertia. The circulator
responds instantly to a request from the PV module if
it is subjected to an abrupt growth of solar radiation.
The heat exchange is deferred under the heat capacity
of the heat collectors. In a sharp fall of radiation, heat
accumulated in the collector is dissipated to the
outside
• In the case of hot water system, high flow rate
destroys the stratifications of the storage tank. The
quality of water stored is altered in favour of quantity.
T(Kelvin)= V(Volt)×100
(1)
Outdoor temperature Text is not measured but it operates in
the thermal power P produced by heat collector expressed
by the simplified relationship:
III. CONCEPTION OF NEW CONTROL UNIT
P = k ⋅ Qv ⋅ (Tout − Text )
This control unit has been tested on solar shutters
producing low temperature heat in open circuit directly into
the home to be heated. It keeps the advantages of operating
mode "following the sun" while controlling the air flow rate
and its temperature in transient operating conditions (sunset,
sunrise, cloudy, no conform temperatures, etc ...) to enslave
the temperature to a reference required by the user [4].
259
(2)
P, power in W ; Qv, air flow rate in m3.h-1 ; k, constant
B. Supply circuit
We chose to use a switching power supply that can
deliver +5V when the voltage from the photovoltaic module
reaches 3V. So, all Integrated circuit (measurement and
management) are properly supplied with energy before the
start up of the fan. This power supply is built around an
integrated switching circuit µA78S40 [6]. It increases output
voltage (step-up) when the input is less than 5V and
decreases it (step-down) when the input voltage is higher
than +5V [7].
K
=
R2
≈
T1
L1
R2
collector and the air pulsed temperature is never less than
ambient temperature of the house.
• Technical realization :
D2
+
D1
C3
Output
+5V
150mA
+5V
R3
R1
Input
+3-18V
+ C1
C2
16
15
14
13
12
10
9
8
SW
DRV
IPK
Vc
CT
COM
COMP
Vre
R3
1
CAT
2
ANO
3
SW
4
OPV
5
OPVc
6
VE
7
VE
11
GN
LM35
1/4 IC2
V1
+
R2
R3
Figure 8. Amplifier n°1; V1=K.(Vout-Vint).
This amplification allows to obtain, for 10 mV input
difference of (either 1K), a gap of about 0.5 V on the output
amplifier. This gives a voltage V1 which is expressed as
follows:
Figure 6. Switching power supply
Output voltage:
_
Vout
LM35
R4
Vs = 1.25 ⋅
R2
Vint
µA78S40
R5
R1
R1
(R4 + R5)
R5
(3)
C. Command circuit
The main objective is to control the temperature inside
the house and keep it at a value set by the user. It is closely
related to the thermal power injected and its temperature
level. We must act on the air flow rate depending on the
output temperature of the collector and the temperatures
inside the room. We must enslave the fan speed with the
temperatures measured in order to converge, in all
circumstances, the indoor temperature to a temperature set.
V1 = K⋅(Vout – Vint)
(5)
2)
The request
In the previous state, the user has no control over the
indoor temperature because Tout changes randomly above
Tint. He must be able to fix a target temperature where Tint
can converge without exceeding it. Of course, if the radiation
is not sufficient to achieve this target the solar collector must
be able to continue to deliver its maximum power at a
temperature above Tint. That’s why we define the user
request as the difference between the indoor temperature Tint
and a temperature of reference Tref fixed by the user. We
propose, in a second time, to enslave the fan speed to the
temperature difference between inside the house and this
reference so that if the difference is positive, the fan speed is
an increasing function of the gap and if the difference is
negative or null fan speed is null.
1)
The potential
The temperature inside the housing being the key
parameter of the system we wish to control, we define the
solar thermal potential as the difference existing between this
temperature and that of hot air collector. We propose initially
to enslave the fan speed (and thus the flow rate of hot air) to
the temperature difference between hot air collector and
warm air inside the house so that if the difference is positive,
the fan speed is an increasing function of the gap and if the
difference is negative or null fan speed is null.
ΔTr = Tref – Tint
(6)
But this open loop enslavement (figure 9) can converge
to different values:
if Tint<Tout<Tref, indoor temperature converges to Tout.
ΔTp = Tout – Tint
(4)
if Tout<Tint<Tref, indoor temperature converges to Text.
if Tint<Tref<Tout, indoor temperature converges to Tref.
This closed loop enslavement makes converge Tint to
Tout. Indeed, for constant radiation, a decrease in flow rate
causes a temperature increase in the collector and inversely.
Solar collector
Tint
_
Tout - Tint
+
Solar collector
+
Fan
Tint
P
_
Tref - Tint
+
Tout
+
Fan
Tref
Figure 7. Closed loop enslavement of Tint to Tout
Figure 9. Open loop enslavement of Tint to Tref
When enslaved in such a way, the fan speed allows to
make converge Tint to Tout while remaining below Tout.
Therefore, the amount of heat generated in the room is
always optimal compared to the incident energy on the
•
260
Technical realisation :
P
• Technicaal realization :
The voltagee comparator w
with diodes sh
hown on figurre 12
with a gap of 0.6 V, the sm
maller
alllows deliverinng in output, w
off the two voltaages mentionedd above.
+5V
R3
R
R4
R1
R2
Vint
LM35
_
1/4
4 IC2
+
V
Vref
P
V2
K=
R3
≈ 50
R2
V1
R6
R2
+
+5V
R3
R
R5
D2
V3
D3
V2
Fiigure 10. Ampliffier n°2; V2=K.(V
Vref-Vint).
Figgure 12. Voltage comparator with diodes
V1 = K
K⋅(Vref – Vin
nt)
(7)
Indeed, inn the first casee where Tref is greater thann Tout,
increase of thhe difference ΔTr
Δ causes an increase of thhe flow
rate Qv of venntilation, so a decrease of Tout.
T
This onee being
greater than Tint, the thermal power supply leadds in a
T
remainss uncertain but still
convergence of Tint to Tout
T
In the seecond case where
w
Tref is always
higher than Tint.
greater than Tout,
T
the increeasing gap haas the same efffect on
Qv and Tout as before, buut the latter beeing lower thaan Tint,
convergence occurs to Text, temperatture to whichh Tout
t flow rate ddoes not decreease. It is in thhe third
converges if the
case that the desired
d
conveergence is achiieved, because, Tout
being larger thhan Tref, the iinternal tempeerature Tint caan only
converge to Tref.
T
We obtain a voltage V3 w
which is expresssed as follow
ws:
V3 = inff(V1, V2) +0,66
(8)
The circuit thus
t
producedd yields the folllowing result::
if (Vc - Vintt)>(Vref - Vinnt), V3=K⋅(Vref - Vint) + 0,,6
if (Vc - Vintt)<(Vref - Vinnt), V3=K⋅(Vo
out - Vint) + 0,6
We have acchieved an ennslavement wh
hich, whateveer the
syystem state, coonverges to a V
V3 value betw
ween Vref andd Vint.
4)
Fan enslavement
ge V3, a switcching
To enslave the fan speedd to the voltag
off the fan pow
wer supply is made. This switching is built
arround a signall of fixed freqquency and variable
v
duty cycle
thhat will drive the power stage. The duty
y cycle must be a
fuunction of the voltage V3. In
I our case it must
m be equall to 1
whhen V3 is zeroo and equal to 0 when V3 iss max.
The couppling
It thereforre appears thaat the enslaveement of the indoor
temperature (Tint)
(
cannot be achieved by considerinng only
both the poteential or the request. To eliminate divvergent
cases, we musst satisfy two conditions:
c
3)
•
Technicaal realization :
+
_
ΔTp = Tou
ut -Tint
Tint
Inf(ΔTp, ΔTr)
Tout
Sola
ar collector
+
Fan
P
_
ΔTr = Treff -Tint
+
Tref
Figure 13. Relaxation oscilllator and hysteresis comparator
F
Figure
11. Coupliing of potential an
nd request
- If the req
quest ΔTr is greater
g
than thhe potential Δttp, Tref
is greater thaan Tout, it is the potentiall that will driive the
power circuit.. The solar collectors will provide
p
as mucch heat
as possible to
t give by pulsing
p
air att Tout tempeerature,
temperature closest
c
to the reeference Tref..
- If the reequest ΔTr is below potentiial ΔTp, Toutt is less
than Tref, it is
i the request that will drivve the power circuit.
The solar colllectors will provide
p
only the amount of
o heat
needed to meeet the demandd and the indo
oor temperatuure Tint
will convergee to Tref. In otther words, it is the smallestt of the
two temperatuure differences that must drrive the powerr circuit.
261
A relaxatioon oscillator is built arou
und an ampllifier,
deelivering on itts inverting innput a voltagee saw tooth signal
s
V44 and frequenncy fixed equual to 8kHz. The
T choice off this
freequency allow
ws both to limiit the level of noise generateed by
sw
witching and too gain flexibillity to enslave the fan speedd. The
V44 voltage betw
ween 0.6V annd 3V attacks the inverting input
off a comparatorr voltage whille the comparrator gap diodde V3
is applied to its non-invertingg input.
From 8:25am until 8:30am the fluid flow rate is null
because the start threshold of fan is not reached.
From 8:30am until 8:45am the starting threshold is
exceeded by far. But the fan does not work because the
command control measures a negative gap between Tout and
Tint
From 8.45am ΔTr becomes positive and the fan starts.
Between 10am and 11am we see that radiation increases
but the flow rate stabilizes and then slows. This reflects the
difference between the set temperature Tref and the indoor
temperature Tint becomes much lower than 5.28 K. ΔTr
being smaller than ΔTp, the command control increases the
duty ratio of switching signal and decreasing therefore the
fan speed. This has the effect of limiting Tint order not to
exceed the set temperature Tref.
Between 11am and 12pm a cloudy passage leads to a
very large drop in flow rate. This shows that, in normal
conditions, the command control will not affect the operating
'following the sun".
Around 2:15pm the radiation on the photovoltaic module
is not enough sufficient to supply the fan. Then, the flow rate
falls sharply.
V
3V
V3
0,6
V4
125
t (µs)
Vs
Vcc
T2
t (µs)
T
Figure 14. Evolution diagram of Vs according to V3
D. The power circuit
The modulated signal drives a Darlington power circuit
consisting of two transistors T1 and T2 (a NPN and a MOS
N-channel power). It plays the role of switch controlled
between photovoltaic module and fan. The diode D4 protects
the transistor from over voltage due to possible inductance
effects from fan.
R15
IV. PERSPECTIVES
To help implementation and installation, we can improve
this regulation with the establishment of an HF radio link.
Indeed, for various reasons, control can be placed far from
the sensors installation place. The wired connection between
sensor and control can become problematic and lead to
additional costs of installation. A wireless connection would
avoid this extra cost and promote the aesthetic. This
improvement is all the more interesting as it develops a lot in
all domestic applications such as alarms, remote controls for
lighting, etc... This allows for standard modules transceivers
easily exploitable in very low costs. A prototype is being
tested on the solar shutters from modules transmittersreceiver type ZegbeeTM[10].
Fan
D4
Control
Photovoltaic
Module
T2
T1
Figure 15. Power stage
The Vm voltage applied across the fan is the average of
the voltages supplied during the signal period (125 μs).
T⋅Vm = Vpv⋅T2
(9)
E. Results
We can see on figure 16 that the "following the Sun"
operating is satisfied: when Tout>>Tref>>Tint, the air flow
rate is changing with the radiation. During several periods of
the day, the control command affects air flow rate.
-2
(°C)
Qv
100
Tout
Tint
(W.m )
G
900
90
800
80
700
70
600
60
500
50
400
40
300
30
Tref
20
100
10
0
08:25
200
V. CONCLUSION
This specific command control manages both the quality
and quantity of hot air blown into the house in all types of
operation (established or transient) as required by the user.
The prototypes were tested in the laboratory and their
performance has been highlighted by different results.
Moreover, since its installation on prototypes of solar
collector it has shown a good effective management of the
thermal power and the quality of the inside temperature of
the heated space. In addition to its low power consumption
required for this type of applications (photovoltaic energy
source), it is highly reliable. It has an excellent value for
money because it uses known and current components
(30$ max with the accumulator) and has a quiet sufficient
accuracy for this use. Low footprint, it also offers an easy
implementation by the simplicity of his connections.
REFERENCES
0
09:40
10:55
12:10
13:25
14:40
15:55
[1]
Figure 16. Evolution of temperatures and flow rate
262
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May 29, 2008
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