Solar Cooling for Mediterranean Countries

C.R.E.A.R.
Centro interdipartimentale di Ricerca
per le Energie Alternative e Rinnovabili
Solar cooling for Mediterranean Countries
Integration of solar concentrating systems and
absorption cycle technology
What’s Solar Cooling?
The core idea is to use the solar energy directly to produce chilled water.
The high temperature required by absorption chillers is provided by solar troughs.
The system doesn’t require “strategic” materials (like in PV systems) and has peak
production in the moment of peak demand.
Chilled water
Heat
Transfer
Fluid
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Main Components
Absorption Chiller (AC)
An absorption chiller is a device that uses a heat source to separate a volatile substance from
the liquid substance in which is dissolved. The vapor is condensed outside and then the liquid
evaporates in an exchanger where the water to be chilled flows. The vapor is then dissolved
again in the main liquid substance. The chiller output is a cooled liquid at a temperature of -5°.
The use of a pump to increase the moisture pressure leads to low electricity consumption.
In solar cooling the heat for the separation is provided by a Heat Transfer Fluid (HTF), for
example in this simulation it has been assumed diathermic oil.
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Absorption Chillers
Cooling Power
The cooling power of Water-Ammonia Absorption Chiller is influenced
by the mass flow and the temperature of the oil and by the external
Data from Robur SPA
temperature that affects the cooling of the machine
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Main Components
Parabolic Linear Collectors (PLC)
A parabolic mirror concentrates the sun on a dark painted pipe placed in the focus of the
parabola. The insulation may provided by a front glass that protects the reflecting surface and
by a circular tube around the pipe, that allows vacuum insulation between them. The
temperature upper limit is imposed by piping materials.
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Parabolic Linear Collectors
Power delivered (1 array-4 rows-8 PLCs- 54m2)
The power outpiut of the PLC is mainly influenced by the heat losses, so
a higher difference between HTF temperature and external temperature
increases the heat losses and affects the efficiency.
Data from SHAP srl
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Parabolic Linear Collectors
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Other Components
Oil Tank – it works as expansion vessel and it is possible to use it for
heat storage. In this case the dimensions of the tank and the quantity of
HTF are critical. Other systems, using phase-changing mixtures, are
under study.
Burner – it can provide the heat when the request is
higher than the power provided by the sun. It allows: a
fast startup at sunrise, working of ACs during sunset
hours or even night hours, fast activation to maintain
the power feeding in case of clouds’ shading.
Pumps – variable flow pumps for hot HTF (upper limit 350°C). With
an inverter it allows constant head with variable flow and so a fine
reglation of the circuit without energy waste.
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Configuration of the collectors array
For the solar collector array, two orientations are possible:
-Axis on N-S direction and “daily” tracking
-Axis on E-W direction and “seasonal” tracking
The choice is driven by two
considerations:
-The different energetic behaviour
-The availability of free room for
installation
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Configuration of the collectors array
N-S axis configuration has a higher yield but the winter season yield is lower and it
emphatizes the energy peaks in summertime
E-W axis configuration has a smoother behaviour during the year
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The control strategy
The major issue of this kind of plant is the control. In fact the driving
parameters (user request, solar power and external temperature) are
uncontrollable and not completely predictable. Beside this, each component
has different reaction to input parameter changes (HTF temperature, mass flow,
external temperature) and so a working point that optimizes the plant reacting
to the oscillations in driving parameters is hard to maintain!
A program to optimize the plant layout and the
control strategy is needed.
A precise simulation of the plant and a model for simulating the driving
parameters has been developed.
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The Model setup
The model for each component will be implemented in Simulink® according to
the data available from the manufacturers. Rough models are already available
for most of them.
Once the blocks are ready, they can be assembled in various configurations of
plant in order to simulate the production and the behavior of the plant.
The simulation step is one hour, the simulation period is one year.
Lower steps are available in order to investigate the transient behavior of the
plant components.
A Meteorogical block provides radiation and external air temperature.
The results with different regulation strategy can be compared.
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The Meteorological data NASA- SSE
The Meteorological data comes from NASA-SSE (Surface meteorology
and Solar Energy) data set. They are monthly averaged data on a ten
years period on a 1° x 1° grid.
The data are assumed constant in the cell
of 1°x 1° (111 km x 111 km), this
means that they have to be integrated
using ground measured data,where they
are available.
Available data of temperature, total and
diffuse radiation have been used, wind
speed at 10m on flat terrain will be used
in future for structural calculations and
heat exchange with external air.
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The Meteorological model
Meterological Input Data
Meterological Output Data
H _Daily horizontal global radiation
H ph _Hourly incident radiation on collector aperture
H d _ Daily horizontal diffuse radiation
Geometrical location data
Text _Hourly external temperature
Temperature information
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The model for the plant simulation
Model description
-Steady state simulation on time step
-Possibility to change time step size form 1 hour to few minutes
-Possibility to run model to
simulate year or daily behavior
Input:
-Devices parameters
-Weather parameters
Output:
-Temperature history in
critical points
-Energy yield
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Layouts for the plant simulation
A plant has been dimensioned
on a real site,
Hyrghada (Egypt)
(27°14’N;33°49’E)
Two layouts have been investigated:
-Single collector array and single absorption chiller unit
-Multiple collector array and several absorption chiller units with possibility to
switch on only some according to the available solar power or user request.
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Single PLCs array results
ηp
21.5
En.outR [kWh]
1.44e4
En.2°Load [kWh]
3.34e3
CO2 saved [kg]
2764
Second load
4,0
Second load activity
[h]
In summer days peak hours the power
provided to the plant by PLCs may exceed the
power consumption of ACs. This may lead to
overheating. In order to avoid overheating,
the protection system is based on defocusing
of some rows of collector field or using a
second load to absorb the PLC power output
peaks.
3,5
3,0
2,5
2,0
1,5
1,0
120
140
160
180
200
220
240
day
The secondary load can be an
additional power to hot water
production, that is already
feasible as cogeneration
production from the standard
plant.
260
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Multiple PLCs arrays results: Yearly Performance
6 PLCs and 8 ACs
Eta_plant
En_in_Sun
En_Out_SC
Eta_SC
En_R_out
En_El_in_R
En_Burner
CO2_saved
E_oil_pump
E_H2O_pump
Ecoo/Efossil
%
[kWh]
[kWh]
%
[kWh]
[kWh]
[kWh]
[kg]
[kWh]
[kWh]
25
4,85E+05
2,16E+05
44,5
1,22E+05
1,91E+03
1,31E+03
2,39E+04
2,54E+03
1,60E+03
6,3
The cycle is activated as
the solar radiation exceed
the defined thresholds.
The CO2 saved has been obtained considering that the electric power is obtained on the
site by diesel engines (assumed efficiency 0.3) and comparing it with the option of the
compression chillers with COP=3
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Multiple PLCs arrays
The optimization of this plant on an energetic, environmental and economic
point of view has led to a solution with 6 PLC array (6x54m2) and 8 AC
(8x17kW); 2-4-6 or 8 ACs are turned on according to the availability of energy.
Power demand and plant output along
a typical day (result obtained using
NASA-SSE data set, d=185 and load
peak fit to chiller output peak)
Pload=1014 [kWh]
Pout, plan =702 [kWh]
Solar Fraction=69 %
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Daily Performance –
th
19
July
Blue:External Temperature
Green: Power collected by PLCs
Red: Power exploited by ACs
Cyan: Cooling Power
Number of ACs active
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Daily Performance -
st
1
November
Blue: External Temperature
Green: Power collected by PLCs
Red: Power exploited by ACs
Cyan: Cooling Power
Number of ACs active
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Conclusions
-Solar Cooling main components have different optimisation
point for HTF temperature and mass flow rate. A control strategy
to keep the plant at optimisation point despite of oscillation in
driving parameters is needed.
-For sites with peak irradiation around 1000 kW/m2 a good
matching can be obtained with a little water-ammonia absorption
chiller and a 54 m2 PLC array.
-The best matching can be obtained with some PLC arrays and a
number of ACs slightly higher but above all having the
possibility to switch on only some ACs, as few ACs at full load
have a higher yield than more ACs at partial load.
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Acknowledgments
-Italian Ministero dell’Ambiente for supporting and financing
the project
-Shap srl for the data about solar collectors
-Robur spa for the data from the experimental measurements
they did after our request
-NASA for the availability of meteo data for academic
institutions
-You all for the attention!