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Small Hybrid Solar Power System: First Field Test Results

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This paper presents field tests of an original concept of a small hybrid solar power plant integrating three technologies: hermetic volumetric scroll expander-generators installed in two superposed Organic Rankine Cycles (ORC), a (bio-)Diesel engine with heat recovery exchangers and a solar field made of two rows of sun following flat plate concentrators with vacuumed isolated collector tubes. The basic idea of the concept is to exploit the synergy between equipment, use cheap and maintenance free expander- generators, guaranty power availability at all time and improve the efficiency of the engine when it has to operate alone at night time by converting the waste heat with the solar ORC. This type of hybrid power plant is intended for rural electrification purposes in developing countries or cogeneration in applications like heated swimming pools in other countries. Pressurized hot water is used at this time as a thermal fluid in the collectors with HCFC123 in the topping cycle and HFC134a in the bottoming cycle. The field tests have been performed during the summer 2001 in Lausanne (Switzerland) and the pland proved operationally reliable. However performance results (with exergetic efficiencies up to 45%) did not meet the expectations but measures to improve the concept have been identified.
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HEFAT 2002
1st International Conference on Heat Transfer, Fluid Mechanics, and Thermodynamics
8-10 April 2002, Kruger Park, South Africa
TJ2
SMALL HYBRID SOLAR POWER SYSTEM:
FIRST FIELD TEST RESULTS
Samuel Martin, Malick Kane and Daniel Favrat
Laboratory for Industrial Energy Systems
Swiss Federal Institute of Technology of Lausanne
CH-1015 Lausanne, Switzerland
E-mail: daniel.favrat@epfl.ch
ABSTRACT
This paper presents field tests of an original concept of a
small hybrid solar power plant integrating three
technologies: hermetic volumetric scroll expander-
generators installed in two superposed Organic Rankine
Cycles (ORC), a (bio-)Diesel engine with heat recovery
exchangers and a solar field made of two rows of sun
following flat plate concentrators with vacuumed isolated
collector tubes.
The basic idea of the concept is to exploit the synergy
between equipment, use cheap and maintenance free
expander-generators, guaranty power availability at all
time and improve the efficiency of the engine when it has
to operate alone at night time by converting the waste heat
with the solar ORC. This type of hybrid power plant is
intended for rural electrification purposes in developing
countries or cogeneration in applications like heated
swimming pools in other countries.
Pressurized hot water is used at this time as a thermal
fluid in the collectors with HCFC123 in the topping cycle
and HFC134a in the bottoming cycle.
The field tests have been performed during the summer
2001 in Lausanne (Switzerland) and the plant proved
operationally reliable. However performance results (
with exergetic efficiencies up to 45%) did not meet the
expectations but measures to improve the concept have
been identified.
INTRODUCTION
In most developing countries, electricity grids are
available mainly, and often only, in urbanized areas. At
the beginning of this XXIst century, numerous rural areas
throughout the world still do not have access to a reliable
source of electricity, fact which impairs their
development. Even if development involves a complex
problematic which cannot be solved only by access to
electricity, its availability together with a rational use of it
can have very positive effects. On the other hand, poor
areas are most of the time isolated and decentralized
electricity production seems to be the most rational
solution, especially in large countries, like South-Africa
for example. Moreover, this production mode avoids grid
losses and transportation costs, and is suitable for waste
heat utilization (co-generation).
Different concepts of small decentralized power plants
currently exist, but for sustainability reasons solar energy
is often considered especially in countries with a
significant average solar radiation. However, the
drawbacks of solutions relying only on solar supply are
well-known and include, among others, the low density of
incident energy requiring large collector areas and
therefore high investment costs. Another problem is the
production dependence on the meteorological conditions
with the associated reduced power availability unless
bulky and expensive storage systems are introduced.
When using photovoltaics the latter drawback is often
avoided by adding a fossil fuel engine, generally Diesel,
resulting in two technologies in parallel with only a weak
exploitation of the synergies between these equipments.
However, technological developments integrating solar
thermal power plant technologies with similar
technologies fired by easily storable fuels open new and
more interesting perspectives. The general idea in so-
called Integrated Solar Fossil Cycle Systems (ISFCS) is to
provide heat both from solar collectors and from a
cogeneration power plant or engine, reducing then the
above mentioned drawbacks and offering a way to
gradually substitute fossil fuel by solar (Favrat, 1995;
Allani et al., 1996). The most often mentioned ISFCS
concepts deal with high power production (up to several
hundreds of MWe) and are reviewed in (Buck et al., 1998
or Kane et al. 2000). Earlier work is also reported in
(Koai, Lior and Yeh, 1984).
However to meet the needs of rural areas in emerging
countries , mini- ISFCS are required and one such concept
has been designed at the Laboratory for Industrial Energy
Systems (LENI) in the frame of a project called Solar
Power System (SPS).
SPS Concept
The prototype designed in Lausanne is composed by two
rows of linear solar concentrators, two superposed ORC
both equipped with hermetic volumetric scroll expander-
generators and a Diesel cogeneration engine (15 kWe)
with heat recovery on its exhaust gases as well as on its
cooling water circuit. Therefore, heat sources at different
temperature levels are available. One, at approximately
150°C, is pressurized hot water heated first by the solar
collectors and then further heated by the exhaust gases of
the engine. A second one is the engine cooling water at
about 80°C (at present with a potential to go up to 95°C).
In the single solar mode, the second source as well as the
heat from the exhaust gases are no more available and the
pressurized hot water is heated by the sun only .
At present the solar collectors are designed to heat water
up to 170°C, but higher temperatures using thermal oil
instead of water are envisaged in order to increase the
efficiency of the ORC. Pressurized water flows in vacuum
insolated tubes which are at the focusing line of the
concentrators. These are made of series of thin plate
mirrors (CEP) of different width and fixed at calculated
angle on linear supports in order to concentrate the solar
radiation on the insulated tubes. These can be assimilated
to Fresnel mirrors. The two rows of collector are oriented
North-South with a tracking system from East to West. As
mentioned above pressurized water has first been chosen
for simplicity reasons. When total reliability of the
concept will be proven, switch to thermal oil (allowing
much higher temperatures at moderate pressure and
avoiding freezing problems) is planned.
One option taken from the start has been to rely on fully
hermetic cycles with hermetic expander-generators to
avoid any shaft seals which could induce undesired
leakage and long term maintenance problems.
Furthermore cost reasons with the interest of using cheap
large production components oriented the choice of
turbine towards hermetic scroll compressors modified into
expanders . This latter option had been proven to work
successfully in previous studies (Zanelli, Favrat 1994,
Favrat, 1995; Kane et al., 1999). However those
volumetric expanders have a range of efficient expansion
ratios which is limited which constrains the cycle design.
Accounting for those constraints and to maximize the
efficiency of the plant and allow future extension to
higher driving temperatures , it has been decided to
implement two superposed ORC each with its own
working fluid. In the present setup the High Temperature
(HT) cycle uses HCFC123 and the Low Temperature
(LT) cycle HFC134a. After having been pumped by a
variable speed membrane piston pump into a plate
evaporator, the topping cycle fluid is heated, evaporated
and superheated (with pressurized water heat) before
being expanded1 in the HT scroll. Discharged vapor is
then cooled, condensed and sub-cooled in a condenser-
evaporator plate heat exchanger. Heat recovery is
transferred to the bottoming cycle fluid for heating,
evaporation and superheating. In hybrid mode, heat is also
supplied to the HFC134a by the engine cooling water.
Therefore, an additional plate heat exchanger, called
preheater, is placed in series upstream of the evaporator of
the bottoming cycle. The same type of pump and
expander technologies are used for the bottoming cycle.
The fluid is condensed in a plate condenser with cold
water (°7C). Accounting for the heat recovered from the
engine cooling network, the LT expander is oversized
compared to the topping cycle one (8kWe versus 5kWe).
The variable speed control of the pumps facilitate
operation at part load.
To lubricate the bearings of the expanders and to avoid
additional oil pumps, oil circulates with refrigerant from
turbine outlet to the condenser, pump and evaporator at
the outlet of which a separator is placed. The latter
recovers oil to be injected within the hollow expander
shaft, using the pressure ratio available. The efficiency of
the separator doesn't need to be very high, as some
amount of oil is desirable at the expander inlet to seal the
inner gaps during expansion.
Pressure and temperature sensors are installed before and
after each components of the plant and the values can be
directly checked on a computer, with a special designed
LabView Vi, allowing post-processing calculation of all
the operation parameters. Figure 1 shows a schematic
1 During warm-up, vapor is by-passed
view of the plant (screen shot).
The cogeneration group showed in Figure 2 is composed
of a Diesel engine, a shell-in-tube high temperature heat
recovery exchanger for exhaust gases (leaving the engine
at temperatures up to 650°C), and a preheater plate
exchanger for the cooling water network. Regarding this
cooling network, temperature at the engine outlet is kept
constant at 82.5°C with a thermostatic valve regulating
the water flow rate. The Diesel engine is a Lombardini
LW 903 designed for industrial purposes. Its 3 cylinders
in lines have a total capacity of 913 cm3 and the engine is
coupled with a three-phase asynchronous generator.
Figure 1. Schematic representation of the SPS power
plant and typical parameters values (LabView screen
shot).
Experimental results
ORC, solar concentrators and engine have been tested
separately or only partially integrated during the year
2000 and detailed results are presented in (Larrain et al.,
2000; Kane et al., 2001) and (Thély, 2000). During the
summer 2001, the aim has then been to proceed with the
integration on the field of the three technologies. The
experimental analysis included the study of the cycles
with a variable temperature heat source and in the various
modes allowed by the integration of the engine.
In 2000, heat was supplied to the cycles by an oil heater at
a constant (adjustable) temperature. This is no more
possible with solar concentrators, available heat at
evaporator being given by the direct solar radiation
(varying along the day) and the concentrator efficiency
(sensitive to solar radiation). To facilitate the startup of
the cycles, the first approach tested was to allow time for
the preheating of the heat source, typically up to 150 to
160°C2. However for such temperatures, the required heat
supply to heat, evaporate and superheat the HCFC123 is
about 55 to 65 kWth (based on last year measurements).
Such values turned out to be hard to achieve in the field
because of a mismatch between the design nominal values
of the power plant and of the solar field3. In such a
starting mode the power demand being higher4 than the
supply, the water inlet temperature decreases with time,
before reaching an equilibrium.
2 With a direct solar radiation of 800 W/m2, the average rising
temperature rate in closed loop from 25°C is about 2°C/min.
3 This is due to a reduction in budget which did not allow the
construction of the complete solar field as initially planned (100 m2
instead of 160 m2)
4 Although the speed of rotation of the volumetric scroll expanders could
potentially be varied within a range from 50 to 110%, the choice was
made here, for simplicity reasons to operate them at constant speed
(direct connection to the grid).
Figure 2. The cogeneration unit
This transition period duration as well as the equilibrium
temperature depend on the direct solar radiation and the
working mode (solar or hybrid); For the different tests
cited in this paper the allowed stabilization temperature is
in the range of 115 to 135°C.
Typical working days
The direct solar radiation varied along the summer and the
best values were measured during the month of June with
more than 850 W/m2. The output electrical power of the
engine is around 12 kWe5 and approximately 11 kWt,
respectively 20 kWt are recovered from the exhaust
gases, respectively from the engine cooling network. The
main values obtained for two typical days are presented in
the table 1 below6.
5 This value could be easily increased by a better ventilation of the
engine.
26.06.2001 26.09.2001
Direct Solar Radiation (Average) [w/m2] 740 597
Direct Sol ar Radiation (Pic) [w/m 2] 783 654
Wo rkin g M ode Sola r Hy brid
Working time [hours] 7.9 3.9
To tal El e ctri cit y pr oduct ion [kWh ] 33 .3 67 .5
Turbines Electricity production [kWh] 33.3 19.3
Diesel Cosumpt ion (l) 0 18.8
ORC Efficiency (Average) [%] 10.2 7.93
ORC Efficiency (Best) [%] 16 8.5
ORC Exergetic Efficiency (Average) [%] 44.0 43.1
ORC Exergetic Efficiency (Best) [%] 61.0 47.02
Plant Globa l Efficiency (Average) [%] 4.9 15.59
Plant Globa l Efficiency (Best) [%] 7.3 16.29
Fossil Efficiency (Average) [%] - 35.3
Fossil Global Efficiency (Best) [%] - 36.7
Table1. Main values of the ORC cycles for 2 typical days
The cycles efficiencies are quite low, with averages of
10% in solar mode and around 8% in hybrid mode. First
of all, it is important to notice that ORC are working at
low temperature (heat source at about 130°C) and at that
temperature the maximum theoretical efficiency (Carnot)
is 25%. Moreover, the lowest efficiency value for the
hybrid working mode is easily explained by the fact that a
large part of heat power given to the cycles is supplied at
low temperature (80°C) and only for the bottom cycle. In
these conditions, the exergy efficiency is much more
representative and reaches about 44%.
Important sources of losses are most of the heat
exchangers and in particular the condenser-evaporator
linking the two cycles. In fact, as it can be noticed in
figure 3 and 4, the temperature difference between
condensation and evaporation stage in this exchanger is
about 20°C which is much too high. This can partly be
explained by the large amount of oil mixed with the
evaporating working fluid (134a), whose boiling
temperature strongly increases in the dryout region of the
evaporator (non linearity not represented in Figure 4).
One solution would be to introduce a falling film shell in
tube evaporator instead of the plate evaporator with an
6 All the efficiency given are calculated in subtracting pump powers to
electricity production. The exact expressions are given at the end of the
present paper
accurate and fine control of the liquid pump, the topping
fluid 123 condensing inside the tubes.
0
20
40
60
80
100
120
140
0102030405060
Hot and Cold supply
HT Cycle
LT Cycle
Figure 3. Heat-Temperature diagram of the ORC in
hybrid mode
It can also be pointed out that the ORC are over-
dimensioned for the existing heat supply. Indeed, as
shown in figure 5, both expanders are used at very part
load, especially the LT one, in solar mode. The latter
turbine was in fact designed to work with heat recovered
from the engine cooling network. A compromise has to be
found and alternatives include either a variable speed LT
expander or the introduction of two LT expanders in
parallel to better adjust the load.
A last remark regarding the cogeneration group and its
role on the power plant, is that the preheater works in
parallel-flow mode (as it can be noticed on Figure 4). This
was done to facilitate the regulation of the temperatures of
the cooling network at the engine outlet. The valve keeps
a constant water temperature (82.5°C) whereas the heat
exchanger being oversized results in a very small pinch
(about 1°C). In counter-flow mode, the pinch would move
at the ORC working fluid inlet, with potentially too high
temperature variations, not ideal for the engine.
0
20
40
60
80
100
120
140
0102030405060
Heat [kWt]
Hot and Cold supply
HT Cycle
LT Cycle
Figure 4. Heat-Temperature diagram of the ORC in solar
mode
Another problem appeared during the test period and
concerns the condenser. Indeed, probably due to variable
cold water flow rate, the condensation pressure of LT
cycle could not be maintained at 5 bars as it was first
planned and realized during the laboratory test. This is
also an important point to notice in order to improve the
efficiency concept.
0
10
20
30
40
50
60
70
Partial load HT
Partial load LT
With Engine
Without Engine
Figure 5: Part load average of the two turbines for 6
typical days
As it has been said, exergetic efficiency of the cycles is
fair for this size of plant and with a significant potential
for improvement. Figure 6 shows that in hybrid mode, this
efficiency increases with the solar share.( ratio of the part
of solar versus engine heat recovered).
This latter figure also illustrates the heat exergy limit
(about 37 kWth and 115°C evaporator temperature) at
which one of the turbine has to be disconnected. For
lower values, the plant can still be maintained in operation
but with the HT cycle only (and at very part load). The
LT expander is then bypassed and the bottoming cycle
plays only a heat transfer role to the condenser.
0
1
2
3
4
5
6
7
8
3 5 7 9 11 13 15
Exerge tic Heat supplied to the cycle [kW]
Turbines Production [kWe]
Low Solar share
HighSolar share
HT Cycle alone
Without Engine With Engine
Figure 6: Turbines electricity production vs heat exergy
provided to the cycles
100
200
300
400
500
600
0
28.5.2001
- 5.00 hours
30.5.2001
- 6.87 hours
5.6.2001
- 6.75 hours
26.06.2001
- 7.9 hours
28.08.2001
- 3.77 hours
26.09.01
- 3.87 hours
Day - hours working
Energy [kWh]
Total kWh solar received Total kWh produced (turbines)
Total kWh fuel consumed Total kWh produced (engine)
Figure 7: Total energy received and produced for 6
typical days
The latter figure summarizes the balances of energy in
both working mode.
Conclusion
A new concept of mini hybrid solar power plant designed
at LENI was field tested in Lausanne (Switzerland) during
the summer 2001. The plant integrating three technologies
(Linear Fresnel concentrators, Organic Rankine cycles
and a Diesel engine) operated in a satisfactory way in
various modes (from pure solar to hybrid with engine at
full load). The hermetic scroll expander-generators
equipped with a special in-shaft oil injection system
performed adequately and reliably in spite of the strong
variations of thermodynamic conditions. Fluctuations of
solar radiation can be coped with the adaptation of the
mass flow rates of the ORC working fluids and a
reasonable range of vapor superheating can be
maintained. Of course, the variation of solar radiation
makes the cycles work at part load, which is detrimental
for the efficiency.
Low ORC and global efficiencies (exergetic efficiencies
below 45%) have been obtained and can mainly be
explained by the mismatched nominal design between the
ORC expanders and the solar field. This resulted in too
low heat supply inducing too low evaporation
temperatures of the topping cycle. Another reason is the
low efficiency of the condenser-evaporator heat
exchanger. A new design of the evaporator-condenser is
being studied and new laboratory tests with a higher heat
rate thermal oil source are being planned for the winter
2001-2002. . Other improvement possibilities include
better and less expensive pumps, variable speed
expanders, direct evaporation in the collectors, higher
temperatures of the topping cycle, etc.
The concept of ORC with hermetic scroll expander-
generators can be applied to a whole range of heat
recovery applications. Two single cycles are presently
being installed to increase the efficiency of biogas engines
in a green waste fermentation plant in Geneva, converting
only the engine cooling energy in a first approach.
The field experience gained is being used to improve the
automatic control of such plants which is currently
underway. Tests replacing Diesel fossil fuel by biodiesel
can also be imagined, resulting in a fully renewable
hybrid power plant.
Acknowledgments
The authors would like to acknowledge the financial
support provided by the Swiss Federal Office of Energy
and the contribution of the Swiss company COGENER
which is responsible for the equipment of the solar field .
Equations
Different efficiencies given in table 1 are calculated with
the following expressions:
εORC =E
elecORC E
ORCpump
M
pv (hinpw houtpw )+M
cw (hincw houtcw)
ηORC =EelecORC EORCpump
M
pv (kinpw koutpw )+M
cw (kincw koutcw )
• •
εplant =EelecORC EORCpump +Eelecengine
Q
directsolar +M
fuel LHV
• •
εfossil =EelecORC EORCpump +Eelecengine
M
fuel LHV
With:
εORC: ORC efficiency [%]
ηORC: ORC Exergetic Efficiency [%]
εplant: Plant Global Efficiency [%]
εfossil: Fossil Global Efficiency [%]
Symbols:
E
: Power [W]
M
: Mass flow rate [kg/s]
h: Specific enthalpy [J/kg]
k
=h: Massflow specific exergy[J/kg] Tas
s: Specific entropy [J/kg°K]
Ta
: Atmospheric temperature [°K]
Q: Heating power [W]
LHV : Low Heating Value [J/kg]
Subscripts
pw: Pressurized water
cw: Engine cooling water
in: Heat exchanger inlet
out: Heat exchanger outlet
directsolar: Direct solar radiation
elecORC: Cumulated for both expanders
pumpORC: Cumulated for both pumps
elecengine: Engine electricity
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Buck R., Goebel O., Koehne R., Tamme R., Trieb F.,
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Favrat D., Concept de centrale électrothermosolaire
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Kane M, Favrat D et al. Thermoeconomic analysis of
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Kane M., Brand F., Favrat D., SPS: Projet d'une
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Koai K., Lior N., Yeh H., Performance analysis of a
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Larrain D., Kane M., Favrat D., SPS: Projet pilote
d'une mini-centrale électro-thermo-solaire, Final
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Zanelli R., Favrat D., Experimental Investigation of a
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Purdue, July 1994.
... When used, the power range of il\e engine varied between 11 and 13 kWe, due in particular to variations in the air aemperaflue, and gave a heat recovery of the order of 20 kWth on the engine block and 7 kWth on the exhaust gases. For all tests covering a cumulated duration of 110 hours, the power plant produced about 800 rThe approach is part of iglobal methodology developped in a recent thesis (Kane, 2002) for the systemic optimisation of hybrid thermal solar power plants in general. ...
... The tests of the SPS prototype allowed the identiFrcation and model validation of the most significant operational patameters (Kane, 2002). However the actual size of the different components (motor, turbine) as well as the type and number of installed turbines ar9 not optimal when considering the yearly operation. ...
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