a novel retrofittable solar cooler/heater based on ...ptes-ises.itc.pw.edu.pl/art/2009_8.pdf ·...
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A NOVEL RETROFITTABLE SOLAR COOLER/HEATER BASED ON
ADSORPTION CYCLE FOR DOMESTIC APPLICATION
L. Crema1,*
and A. Bozzoli1, G. Cicolini
1, A. Zanetti
1
1. Renewable Energies and Environmental Technologies unit (REET), Fondazione Bruno Kessler (FBK)
Povo Trento, Italy;
*Corresponding author, email: [email protected]
ABSTRACT
Solar driven cooling machines have been employed until recent years only in niche markets due to a series of reasons: dimensions of
the machines, cooling power storage, production costs, and others. At present times there's a high request for a reliable technology developed for domestic applications at distributed level. The solar heating is a rapidly growing market and the available technologies are almost consolidated, but a strong limitation to the application of solar collectors is due to the waste heat available in summer periods, which limit the sizing of solar domestic installations, limit the capacity to provide not only support to hot sanitary water, but even to indoor space heating. The objective of the present work is to suggest a possible solution and try to give a contribution to the described problem. The R&D performed at FBK-REET labs has produced a new concept of solar driven cooling/heating machine based on a double adsorption/desorption cycle acting between two small tanks. The machine, working cyclically between desorption and adsorption, is provided of an heat storage to separate the cooling energy availability from the solar radiation. Thus the system is
able to provide not only cooling power in hot periods, but even heating in colder times or for hot sanitary water production. The system may be scaled up or down in cooling capacity, sizing properly the porous adsorbing material volumes, and in cooling power, changing the air mass flow through the system. A prototype has been built, provided of a cooling capacity of about 25 kWhth and a
cooling power retrofittable until a max of 4 kWth in the range of cold temperatures of about 8¸12°C. The overall COP is in the range
of 0,6¸0,7. The system is patent pending (Publication number: WO2008099262).
INTRODUCTION
REET unit (Renewable Energies and Environmental
Technologies), part of the Centre for Materials and
Microsystems at FBK (Fondazione Bruno Kessler), has
worked on a proposal for a layout of a Solar Cooling
machine based on Adsorption / Desorption reactions on
a porous material.
In the first prototypal realization it will be tested a Silica Gel material, in a second time, by the end of this
year, a research will start in cooperation with the Earth
Science Department of the University of Ferrara to find
out a more advanced material (zeolites, molecular sieves
or other).
Silica Gel performs a surface physical-chemical
reaction, by which it creates variations, under the
thermodynamic point of view, between the conditions of
the input air and the output air of the material itself.
The variations are of a different kind secondly to two
possible reactions: adsorption and desorption. The two reactions happen alternatively in function of the
input air on the tank and depending on the internal
conditions in the tank itself. The chemical specie that
could be adsorbed may vary from a porous material
to another. In the described paper it has been used
hydrophilic materials, with a chemical potential to react
specifically with the water molecules present in the air.
In the case of adsorption reactions, the input mass of
the air has got an isenthalpic heating process. In case of
desorption reaction the same air has got an opposite
isenthalpic cooling process.
The energy required by the thermodynamic
variations comes from the creation (chemical
adsorption) or rupture (chemical desorption) of the
chemical bonds on the surface of the porous material.
The described reactions are reversible in time,
so that in a cyclic circuit, a mass of air with analogue
thermodynamic conditions regenerates the cycle
while adsorbing the water vapour.
These first considerations have taken to the development of a machine layout provided of
a double cycle that could be able to solve a certain
number of problems not yet completely solved in the
field of the cooling technologies based on solar
energy, such as:
· the necessity for a simple system, with
a simple maintenance required and possibly
without complex problems due to applied
materials or working conditions;
· the necessity for a system able to provide thermal
power in conditions of no solar energy availability, when the heating and cooling
necessity may occur in an indoor environment. It
has outlined a solution provided of a heat storage
of sizeable capacity;
· the possibility, by inverting the cycle, to have not
only cooling power in the summer season, but
even heating power in the winter time;
· the possibility to have an adaptable system in the
full cooling capacity (kWh) and cooling power
(kW), acting by independent selection
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on a different dimension for the silica gel tanks and
on the air flow on the adsorption cycle.
The actual work is more focused on the application of a solar cooling/heating machine for a domestic
application in the range between 2 – 5 kW of cooling
power and with a variable cooling capacity of 25 – 50
kWh. The below description are more focused on the
cooling cycle and the cooling effect of the developed
solution, but may be changed to an heating effect by
acting on the air cycle itself.
ADSORPTION MATERIAL AND
THERMODYNAMIC EQUILIBRIUM
Adsorption is a process in which fluid molecules are
trapped on the surface and within the pores of a solid. It's
a reaction caused by a surface potential or a chemical
affinity between different species. In the first case the
solid adsorption process is called physical adsorption and
it's a result of weak Van der Waals' forces of attraction.
It's a predominant reaction at low temperatures and it's
characterized by low energy of adsorption. The second
reaction is named chemical adsorption and is
characterized by higher energy of adsorption. The pore structures of an adsorptive solid material, in the case of
cooling application, are defined micropores (radii < 15Å)
The adsorption cycle and the material used have been
widely studied and described (Cacciola and Restuccia,
1994). The performances have been either theoretically
or experimentally assessed. The adsorption reaction
equilibrium, for a given adsorbent/adsorbate system can
be express independent of temperature by using the
adsorption potential ε, according to Polanyi theory (Dubinin and Asthakov, 1971):
(1)
Where Ps is the saturated pressure of liquid adsorbate
at the adsorption temperature T, and P is the pressure of
the adsorbate vapor in equilibrium with the adsorbed
liquid film, and R is the universal gas constant.
There are a lot of studies that describe the
thermodynamic equilibrium of adsorption, derived from
theoretical, semi-theoretical and empirical models
(Freundlich, 1926), (Polany, 1970), (Sadoka and Sazuki,
1986), (Langmuir, 1918), (Brunauer et al., 1938), (Dubinin and Radushkevich, 1937), (Dubinin and
Astakhov, 1970), (Jing and Exell, 1993), (Leite, 1998).
The adsorption material is the heart of the Solar
Cooler /Heater system.
It’s the storage of energy that will be converted into
the cooling or the thermal power. Many efforts have
been made in different directions to obtain the idea cycle
with an output energy continuously released and a small
range of temperature variation. These cycles are called
"regenerative" and usually are based on a double tank,
operating one as the energy provider, the other in a
regenerative cycle. Some difficulties are still present not
only due to the intermittent availability of the solar
energy, but more due to problems of thermal
conductivity on the adsorbent. In the present work it has been worked
on this last obstacle in order to increase the COP and
the efficiency of the cooling cycle, for example:
· working on a heated air flow passing through the
silica gel to regenerate the porous material;
· sizing properly the silica gel tank;
· sizing properly the solar thermal collector's area;
· working on a solution with a reduced need for
energy to make the system work.
A series of characteristics of the material itself are
essential for the proper behavior of the machine: the physical properties, the applied fluid dynamic to the
adsorption /desorption cycles where the silica gel is,
the thermal power and the temperatures transferred
from the solar thermal collectors to the thermal fluid
(air) which act in the desorption circuit.
The silica gel has been widely studied and
described too. Numerous experimental studies and
technical notes demonstrate the capacity for
a saturated silica gel material to desorb water
at temperatures of about 40°C. As evidenced in the
formula (1), the most important parameter that act on the desorption reaction is the relative humidity of
the air, so the ratio between the vapor partial pressure
and the saturated pressure at a certain temperature.
A factor of correction is the temperature of the input
air. Hotter the air in the material at the same relative
humidity, less would be the water content adsorbed
on the silica gel, with an increased desorption effect.
Another parameter with a higher weight is the air
velocity through the silica gel. A higher kinetic
energy causes the rupture of an higher amount bonds
between the water molecules and the micropores of the silica gel. This fact reduces the time required for
the desorption reaction and the quantity of water that
remains adsorbed on the silica gel. Some
experimental data have been analyzed and integrated
in the following tables to provide a full prospect
about the silica gel behavior (Sukhmeet and Parm
Pal, 1998), (Wanga et al, 2005). Graphs in Fig. 1
represent the desorption reaction and the desiccant
time in function of the air velocity through the silica
gel at different temperatures. It’s evidenced the
increasing of the desorption times as the air velocity
through the material decreases. In the prototype realization a flow of about 350 m3/h passes the
silica gel in a 1 m2 of area of the tank with a porosity
of about 0,25%. The approximate velocity of the air
would be about 0,37 m/s.
Graphs in Fig.2 shows a strong dependence of the
water content on the silica gel from the relative
humidity more than the temperature of the input air.
The air relative humidity conditions derives from the
values of the ambient air heated by the thermal fluid
of the solar panel in a A/W heat exchanger. At about
90°, the relative humidity is near a value of 3%, and in no case with values above 25% at 50°C.
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Fig. 1. Bed-Air velocity vs Drying time of the Silica Gel at different temperatures (42, 52, 62 and 72°C)
Fig. 2. Moisture content in the silica gel vs relative humidity at two temperatures, 0 and 80°C
.
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Fig. 3. Water content adsorbed on the silica gel vs the relative humidity of the input air
Table 1. Water Desorption hypothesis in relation to the air temperatures
The main curve that drive the phenomena is
reported below. It’s the ratio between relative
temperature on the silica gel starts decreasing, the
process would have been arrived to a saturation and an electrical command would close the locking valve on
the silica gel tank humidity of the air which passes
through the silica gel and the maximum water content
adsorbed on the silica gel. The curve works
bidirectional and could have a small variation for
higher or smaller temperatures and air velocities.
The above data are in agreement with experimental
trials performed by the producer of the same material
used in the prototype, Levosil company, and provided
during the pre-engineering of the prototype.
As reported on table 1, it has been derived the
desorption rate at different temperatures and it has been simulated an hypothesis of drying rate for a total
amount of about 30 kg of water for a total amount of
220 kg of silica gel. Monitoring the desorption process
by the use of thermocouples, as the
LAYOUT OF THE MACHINE
The developed machine has got a thermodynamic
cycle that is composed of some essential modules:
· 2 tanks for the porous material;
· evaporator module;
· air/water heat exchangers between the
adsorption/desorption cycles and the
cooling/heating hydraulic circuits;
volumetric blowers for the circuit air;
· air/air central heat exchanger on the adsorption
cycle;
· input air filter on the desorption cycle. It has been developed a layout based on two cycles,
one for the desorption reaction and the regeneration of
the cooling capacity, the other for the adsorption cycle
and the use of cooling power.
The double cycle changes between one tank and the
other each 24 hours, partially depending on the daily solar
energy availability.
In the first 24 hours the desorption circuit, by the
thermal power provided from the solar thermal collectors,
will regenerate the porous material, breaking the chemical
bonds between the water molecules on the surface of the
material itself. At the end of the desorption process, the tank will
be hermetically closed by pneumatic valves
electronically controlled. Contemporarily, through the
adsorption process, the thermal energy obtained from
the heated material (the chemical attraction of water on
the porous material and the adsorption reaction cause
the transfer of thermal energy to the material itself and
its heating) will be used in a specific cycle that could
use such energy to generate cooling power and/or
heating for hot sanitary water.
The separate and interchangeable double cycle allows, in such way, the regeneration of an energy
storage in a time basis of 24 hours. This would be
closed inside the tank, with the possibility of
a distributed use, depending on the real user needs,
during the subsequent 24 hours.
Air Flow
Temperature
Drying rate
(100kg silica gel)
Drying rate
(220kg silica gel)
Desorbed water Hypothesis
°C kg/h kg/h kg h/gg
40 1.82 4.00
50 2.18 4.80 4.8 1h @ 50°C
60 2.90 6.38 12.76 2h @ 60 °C
70 4.20 9.24 12.44 (saturation) 3h @ 70°C
80 6.33 13.93
90 9.64 21.21
TOTAL 30
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Fig. 4. Schematic layout of the heating / cooling machine based on the double adsorption / desorption cycle.
Adsorption on Tank ZEO1 and Desorption on Tank ZEO2.
This has been a core topic, due to the fact that the
real user energy demand has been disassociated from
the energy availability provided by the sun,
at an advantage of the first.
The tanks dimension will determinate the thermal
capacity of the machine. The higher the volume of
the tanks, the higher the duration of the heat storage,
the higher the time and the energy required for the
regeneration process. The correct sizing of the
machine must take count of such considerations.
In the prototype realization, the silica gel will
cover a volume of 250 Lt, with a total weight of 220
kg each tank.
On the layout of the machine it has been
developed a solution to properly monitor the process,
to automate the process through the use of proper
actuators and to make a selection of the most suitable
materials and elements.
Fig. 5. Desorption cycle on the Psychometric Chart
Fig. 6. Adsorption cycle on the Psychometric Chart
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Fig. 11. Adsorption reaction on Tank ZEO2 and Desorption reaction on Tank ZEO1.
Fig. 7. Simulation of the power of the system in function
of the temperature difference on the hot reservoir
Fig. 8. Simulation of the power of the system
in function of the temperature difference on the cold
reservoir
Fig. 9. Power of the system in function of the Input
Temperature from the hot reservoir, at different volume
flow and A/A heat exchanger efficiencies
Fig. 10. Power of the system in function of the cold
reservoir for different volume flows and at two different
temperature for the hot reservoir (30, 35°C)
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MODELLIZATION
A software has been developed for the simulation of the whole thermodynamic cycles. The software has some
input data from the cycle temperatures, water content,
flows and controls a system of thermodynamic reactions
based on the real data of each single component obtained
from the producers (e.g. A/A and A/W heat exchangers,
evaporator panel, silica gel tanks, solar thermal
collectors). The software monitor the process since the
starting conditions and through the transient until it
confirms the steady-state condition. At the steady-state
the energy and mass balances are verified and the full
cycle is represented on the psychometric chart. A second software module perform a modellization
of the cycle and give back the curves describing the
behavior of the machine on different cycle and/or
external conditions. Results are presented in Fig.6-17
EXPERIMENTAL SETUP
The full system is actually under testing and
monitoring of its performances. The solar cooler /
heater will be provided of energy from a double
interchangeable solar thermal plant, of 6 m2 one and 8
m2 the second.
The prototype has been fully pre-engineered, in all its
components, hydraulic and pneumatic circuits,
electrical circuits, sensors and actuators, comprising the
control and monitoring electric boards. Below there's a picture represent the pre-
engineering of the full system and some photos about
the works on the real prototype.
Fig. 12. Power of the system in function of the hot
reservoir temperature, for different cold reservoir
temperatures and a volume flow of 200 m3/h.
Fig. 13. Power of the system in function of the hot
reservoir temperature, for different volume flows and for
different temperature at the cold reservoir.
Fig. 14. Relative humidity at the silica gel input
in function of the hot reservoir temperature, at different
temperatures on the cold reservoir and for a fixed
volume flow (200 m3/h).
Fig. 15. Relative humidity at the silica gel input
in function of the cold reservoir temperature, at different
temperature on the hot reservoir and for a fixed volume
flow (200 m3/h).
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Fig. 16. Typical day in Trento on the summer 2008
(meteorological data from www.iasma.it/meteo), max, min and average temperatures
Fig. 17. Cooling system behavior at three different
volume flows (150, 250 and 350 m3/h).
Fig. 18. pre-engineering of the full cooling / heating system
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Fig. 19. Double silica gel
tank Fig. 20. Evaporator
module Fig. 21. Central heat exchanger
The prototype is connected with a server room where
provide the cooling power for a proper indoor
environment. In addition it’s connected with the solar
plant for the thermal heat in input of the regeneration
circuit and to a third heat exchanger for the waste heat
dissipation. This last step will be evaluated for
a heat recovery useful for domestic purposes, such as
hot sanitary water or space heating in case of floor
heating infrastructure at low temperature.
Fig. 22. Schematic about the installation of the prototype on the roof of the FBK north building
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Fig. 23. The Solar Cooling unit monitoring system, some of the installed sensors and actuators
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All the modules will be provided of a thermal insulation with the external and a remote monitoring software
will control and automatic transfer of the data to a control system.
The full prototype is monitored by more than 65 sensors and 15 retrofitting controls. It has fourteen
thermocouples monitoring the thermal cooling capacity stored on the tanks or its regeneration process.
EFFICIENCY OF THE SYSTEM AND CONCLUSIONS
The total pressure drop has been valued, equal to about 350 Pa on the Adsorption cycle and to 250 Pa on the Desorption cycle. From the above values it has been calculated the energy and power consumption:
· blower for the adsorption cycle (Q = 150 m3/h): about 80 Wh
· blower for the desorption cycle (Q = 200 m3/h): about 100 Wh
· monitoring and control system: 5 W (approximate calculation in excess, including all the cycles
actuators)
· water pumps (3): < 100 W
It derives a total power consumption less than 300 W.
The total energy at the output of the desorption cycle (cooling) is, for the above volume flow values, equal to
36519 kJ.
The total energy at the input of the desorption cycle plus the energy required from the whole system is equal
to 46257 + 5988 kJ = 52245 kJ. The efficiency of the cycle is about 0,70.
The work needs to be concluded with the experimental phase. The full prototype, completed with monitoring
and control system, is actually at the start up activity in FBK, on the roof of the institute north building.
The prototype will reproduce exactly the above described machine and will be connected to
an already working solar thermal plant located on the roof of the north FBK building itself, made of two solar
thermal subsystems: one from Kloben collectors (aperture area of about 8 m2) and the other from MIG Solar
Solution collectors (aperture area of about 6 m2).
The paper has been written after more than two years of work on the system and a patent pending for
international extension (Publication number: WO2008099262) . We would like to thank Kloben, MIG Solar
Solution GmbH, PHILIPPINE GmbH and AERMEC SpA for the support reserved.
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