Solar cooling for Mediterranean Countries Integration of solar concentrating systems and absorption cycle technology

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Solar cooling for Mediterranean
CountriesIntegration of solar concentrating
systems and absorption cycle technology
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per le Energie Alternative e Rinnovabili
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Whats 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 doesnt 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
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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
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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
temperature that affects the cooling of the
machine
Data from Robur SPA
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Main Components
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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
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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
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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 350C). 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
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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
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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
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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
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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
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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 1x
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
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Meterological Input Data
Meterological Output Data
_Daily horizontal global radiation
H
_ Daily horizontal diffuse radiation
H
d
Geometrical location data
Temperature information
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The model for the plant simulation
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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
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A plant has been dimensioned on a real site,
Hyrghada (Egypt) (2714N3349E)

• 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
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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.
The secondary load can be an additional power to
hot water production, that is already feasible as
cogeneration production from the standard plant.
?p 21.5
En.outR kWh 1.44e4
En.2Load kWh 3.34e3
CO2 saved kg 2764
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Multiple PLCs arrays results Yearly Performance
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6 PLCs and 8 ACs
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 COP3
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Multiple PLCs arrays
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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, d185
and load peak fit to chiller output
peak) Pload1014 kWh Pout, plan 702
kWh Solar Fraction69
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Daily Performance 19th July
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BlueExternal Temperature
Green Power collected by PLCs
Red Power exploited by ACs
Cyan Cooling Power
Number of ACs active
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Daily Performance - 1st November
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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
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-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
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per le Energie Alternative e Rinnovabili
-Italian Ministero dellAmbiente 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!