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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: crema@fbk.eu

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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