www.elsevier.com/locate/apthermeng

Applied Thermal Engineering 27 (2007) 2205–2212

Development of an adsorption chiller and heat pumpfor domestic heating and air-conditioning applications

Tomas Nunez a,*, Walter Mittelbach b, Hans-Martin Henning a

a Fraunhofer Institut for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg, Germanyb SorTech AG, Weinbergweg 23, 06120 Halle a.d. Saale, Germany

Received 24 February 2005; accepted 25 July 2005Available online 13 September 2005

Abstract

The scope of this paper is to present the development of a prototype of a small adsorption heat pump working on the adsorptionpair silica gel–water. The development of this prototype with remarkable high power densities has been carried out during the lastyear and is a result of continued joint work on adsorption heat transformation systems carried out at SorTech AG and the Fraun-hofer Institute.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Adsorption; Chiller; Heat pump; Silica gel; Solar cooling

1. Introduction

With the aim to reduce the amount of primary energyused for domestic heating purposes the introduction ofheat driven heat pumps can provide a significantimprovement in fossil fuel utilisation. Furthermore, ifthe design of the heat pump allows the additional useof its cooling properties, year round operation becomespossible using waste or solar heat in summer in order toprovide cooling for air-conditioning applications. Com-bining such a thermally driven heat pump/chiller with acombined heat and power system a tri-generation systemfor heat, cold and power is made available. But up tonow no small power heat pumping system with suchcharacteristics is available on the market although an in-creased demand is currently observed.

The results presented in this contribution are resultsobtained with the first and second prototype that hasbeen constructed and tested in the last six months.

1359-4311/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2005.07.024

* Corresponding author. Tel.: +49 761 4588 5134.E-mail address: nunez@ise.fhg.de (T. Nunez).

Therefore they should be considered as preliminary re-sults. The present machine has still a high potential forimprovement and optimisation in the near future.

2. The developed heat pump prototype

2.1. Description of the heat pump

The developed system consists of two identical mod-ules. Each module contains a heat exchanger for theadsorption material and a second heat exchanger forevaporation and condensation of the process water.Both heat exchangers are assembled into one single vac-uum tight container forming a sealed unit that is con-nected to the surroundings only by hydraulic piping.At the present stage of development the adsorption heatexchanger is filled with silica gel as adsorbent. But thedesign of the reactor is such, that also other sorptionmaterials could be used.

Both modules are interconnected through a hydraulicswitching unit. This hydraulic unit connects both

Nomenclature

Thigh driving temperature for the heat pump (hightemperature source), inlet temperature tothe heat pump.

Tmedium re-cooling (cooling application) or heatingtemperature (heating application). Inlet tem-perature to the heat pump, temperature ofthe medium temperature sink.

Tlow cooling temperature. Outlet temperature ofthe heat pump, low temperature level.

Treduced reduced temperaturePhigh mean driving (desorption) powerPmedium mean power at the medium temperature level.

Plow mean cooling power.tcycle cycle duration (phases 1–4)COPcooling coefficient of performance for coolingCOPheating coefficient of performance for heatingA1 adsorber 1A2 adsorber 2VK1 evaporator/condenser 1VK2 evaporator/condenser 2RK re-cooling circuitVL inlet temperatureRL outlet temperature

2206 T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212

modules to the heat source and sink and allows theiroperation in a quasi-continuous mode. An internal con-trol unit ensures proper operation of each module pro-viding the switching signals for the valves in thehydraulic unit switching between the different internalphases of the heat pump. In Fig. 1 a schematic of theheat pump is presented.

The dimensions of both modules together without thehydraulic switching unit are 355 mm · 520 mm ·1360 mm. The total weight of both modules is 258 kg.Each module is filled with about 35 kg of silica gel.

2.2. Operation

The two modules are designed in order to be operatedwith hot water at temperatures of 75–95 �C. Design

internal control

Module 1

Module 2

hydr

auli

c sw

itchi

ngun

it

high temperaturecircuit

mediumtemperature

circuit

low temperaturecircuit

adsorption machine

Fig. 1. Schematic of the heat pump. The main components are: twomodules, the hydraulic switching unit and the control unit.

Table 1Sources for the three temperature levels for heating and cooling application

Hydraulic circuit Cooling application

High temperature heat source (Thigh) Driving heat source; eMedium temperature heat sink (Tmedium) Heat rejection; e.g. dry

tower; ground coupledLow temperature heat source (Tlow) Useful cooling, chilled

heating temperatures of 35–40 �C which are deliveredin the heating mode are suitable for low temperatureheating systems such as wall or floor heating installa-tions. For cooling application this is the temperature le-vel of the heat rejection. In the cooling operation chilledwater of 10–15 �C is produced in an almost continuousmode. Further on these three temperature levels aredenoted Thigh, Tmedium and Tlow.

According to the type of operation the heat pump canbe connected to different heat sources and sinks. Table 1gives some possible connections.

The two modules of the heat pump are operated in aperiodic and phase-shifted mode. While one module is inthe adsorption phase the other module is being des-orbed. This results in four consecutive operation phasesof the four heat exchangers. The four phases are summa-rised in Table 2. The duration of each phase and there-fore the duration of the whole cycle depends on therequired heating or cooling power. For the results pre-sented here it is in the order of 15–30 min. During thephases of internal heat recovery no heating or coolingpower is provided to the external circuits. These phaseshave a duration of about 30–60 s.

2.3. Measurements

The first tests of the system were carried out at thetest facilities for heat driven heat pumps and chillers in-stalled at SorTech AG. Different values of the drivingtemperature, the medium temperature and the low tem-

s of the sorption system

Heating application

.g. solar system Driving heat source, e.g. gas furnaceor wet coolingheat exchanger

Useful heat, heating system

water circuit Low temperature heat source;e.g. ground heat exchanger

Table 2Operation phases of the heat exchangers in the modules of the heat pump

Phase Ads. 1 Ads. 2 Evap./Cond. 1 Evap./Cond. 2

1 Desorption Adsorption Condensation Evaporation2 Heat transfer to Ads. 2 Heat exchange with Ads. 1 Heat transfer to Evap./Cond. 2 Heat exchange with Evap./Cond. 13 Adsorption Desorption Evaporation Condensation4 Heat exchange with Ads. 2 Heat transfer to Ads. 1 Heat exchange with Evap./Cond. 2 Heat transfer to Evap./Cond. 1

T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212 2207

perature were tested and the performance characteristicswere obtained.

Measurements for the following operation conditionswere carried out:

• Driving temperature Thigh for desorption: 75 �C,85 �C and 95 �C. Thigh is the inlet temperature ofthe reactor during the desorption phase.

• Medium temperature Tmedium: 25 �C, 30 �C and35 �C. Tmedium is the inlet temperature to the reactorduring the adsorption phase. For condensation it isthe inlet temperature to the condenser.

• Low temperature: Tlow from 10 to 20 �C. Tlow is theoutlet temperature of the evaporator.

All temperatures are measured in the pipes of the heatexchangers, i.e. at inlet or outlet position to the heatpump.

In Table 3 the nominal volume flows used during thetests are presented. These values are preliminary valuesand further tests with different volume flows will be car-ried out.

Table 3Volume flows in the three hydraulic circuits

Circuit Volume flow [l/h]

Low temperature 1600Medium temperature 3200High temperature 1600

Temperature of a

20.00

30.00

40.00

50.00

60.00

70.00

80.00

7000 7500 8000 8500

Experime

Tem

pera

ture

[°C

]

Fig. 2. Diagram of the inlet (VL) and outlet (RL) temperatu

With these volume flows cooling powers Plow from 3to 7 kW and heating powers Pmedium from 8 to 22 kWare achieved. During one cycle the instant power ofthe three circuits is not constant as it can be seen inthe temperature curves presented in Figs 2–4. Thereforethe mean power over the whole cycle has to be calcu-lated. It is obtained by the exchanged heat at each tem-perature level during the cycle divided by the duration ofthe complete cycle:

P high ¼Qhigh temperature

tcycle

P medium ¼Qmedium temperature

tcycle

P low ¼Qlow temperature

tcycle

The duration of the cycle tcycle is the time necessary tocomplete the whole cycle consisting of the phases 1–4of Table 2.

The measured powers Plow were limited by the avail-able power of the low temperature heat source. In theexperiments presented in this contribution the powerPlow was set to a nominal value and the resulting powersof the other circuits were measured. The volume flowswere kept fixed in all circuits. Lower volume flows asin Table 3 are possible, but it has still to be studiedhow the reduced volume flows affect the power of theheat pump.

For the evaluation of the heat pump the cooling andheating COP have been calculated. The COPcooling of the

dsorber circuits

9000 9500 10000 10500

nt time [s]

T_A1_VLT_A1_RLT_A2_VLT_A2_RL

res of the two reactors in the four phases of the cycle.

Temperature of evaporator / condenser circuits

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

7000 7500 8000 8500 9000 9500 10000 10500

Experiment time [s]

Tem

pera

ture

[°C

]

T_VK1_VLT_VK1_RLT_VK2_VLT_VK2_RL

Fig. 3. Diagram of the inlet (VL) and outlet (RL) temperature of the two evaporators/condensers in the four phases of the cycle.

Temperature of re-cooling circuit

20.00

25.00

30.00

35.00

40.00

45.00

7000 7500 8000 8500 9000 9500 10000 10500Experiment time [s]

Tem

pera

ture

[°C

]

T_RK_VLT_RK_RL

Fig. 4. Diagram of the inlet (VL) and outlet (RL) temperatures in the medium temperature circuit during one whole cycle (cooling of adsorber andcondenser).

2208 T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212

machine is determined by the mean power in the hightemperature and low temperature hydraulic circuits

COPcooling ¼P low

P high

The COPheating for heating applications is determined bythe mean power in the medium temperature and thehigh temperature hydraulic circuits:

COPheating ¼P medium

P high

In order to take into account all the different tempera-tures that are involved in the operation conditions ofthe machine a new parameter, namely the reduced tem-perature Treduced is defined. The reduced temperatureTreduced is the fraction between the temperature differ-ence between reactor and evaporator/condenser duringthe adsorption and the desorption phase. It gives an ideaof the ratio of the �gained� temperature difference (‘‘tem-perature lift’’) to the �driving� temperature difference(‘‘temperature drive’’).

T reduced ¼T medium � T low

T high � T medium

Figs. 2–4 show an example of the temperatures in thehydraulic circuits.

In Fig. 2 a diagram of the temperatures in the inletand outlet pipes of the reactors during the different cyclephases are presented. The value T_A1_VL is the inlettemperatures of the reactor 1, the value T_A1_RL isthe outlet temperature. Corresponding temperatures ofthe reactor 2 are labelled with A2. In Fig. 3 the corre-sponding temperatures of the evaporator/condensersare shown. The two evaporator/condenser are labelledVK1 and VK2, respectively. In the application these val-ues correspond to the temperature of the low tempera-ture heat source in the heating mode and to thetemperature of the produced cold water in the chillingmode. In Fig. 4 the temperatures in the medium temper-ature circuit are plotted. In the cooling mode this is thetemperature of heat rejection and in the heating modethe temperature of the heating system.

T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212 2209

In Fig. 2 it can be seen that each adsorber is goingthrough a desorption and an adsorption phase. Betweenthese phases a short heat recovery phase in which heatfrom the desorbed reactor is transferred to the adsorbedreactor can be observed. The evaporator/condenser partin each module switches from condensing mode to evap-oration mode during one cycle. The diagram of the med-ium temperature circuit shows the final result at themedium temperature level: in this circuit the heat fromthe adsorber in the adsorption phase and the heat fromthe condenser during condensation is combined andrejected together to the medium temperature heat sink.

In Fig. 5 the resulting heating and cooling powers forthese operation conditions are presented. During mostof the cycle the cooling power Plow is constant at about3.5 kW. The heating power Pmedium shows stronger vari-ations between peak values above 25 kW at the begin-ning and about 5 kW at the end of each half-cycle.The reason for a constant cooling and non-constantheating power is to be found in the operation mode:the machine was operated with a constant cooling de-mand and the cycle evolved according to this constraint.

Heating and co

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

7000 7500 8000 8500

Experimen

Pow

er [k

W]

Fig. 5. Heating and cooling power during one typical cycle. The

COP for Heating

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0.20 0.25 0.30 0.

Reduced temp

CO

P [-]

Fig. 6. Heating and cooling COP as a fu

The duration of the adsorption and desorption phaseis an important optimisation parameter and is defined inthe control procedure of the machine. The longer theadsorption phase is extended, the more water is adsorbedin the reactor. The adsorbent gets closer to the equilib-rium loading at the given heat rejection temperature.This measure increases the COP of the machine. Onthe other hand it reduces the average power over the cy-cle as at the end of each adsorption phase, the processgets increasingly ineffective and only low powers can beextracted. Therefore choosing the best cycle time is oneof the important parameters for the control of the unit.

2.4. Results for COP and power

In the following the results for the COP and thepower are presented as function of the reduced temper-ature. All the results were obtained for a nominal cool-ing power of 3.5 kW. In Fig. 6 the heating and coolingCOP for different high, medium and low temperaturesis presented. In Fig. 7 the corresponding results for themean heating and cooling power are shown.

oling power

9000 9500 10000 10500

t time [s]

P heatingP cooling

machine was operated with a constant cooling demand.

and Cooling

35 0.40 0.45 0.50

erature Tred [-]

COP heatingCOP cooling

nction of the reduced temperature.

Heating and Cooling Power

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0.20 0.25 0.30 0.35 0.40 0.45 0.50

Reduced temperature Tred [-]

Pow

er [k

W]

P heatingP cooling

Fig. 7. Mean heating and cooling power of the adsorption chiller as a function of the reduced temperature.

2210 T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212

With the presented operation conditions of Figs 6and 7 an average COPcooling of around 0.5 and COPheat-

ing of 1.5 can be expected. At a constant cooling powerPlow of 3.5 kW average heating powers Pmedium of be-tween 8 and 16 kW have been achieved. The exact re-sults depend on the particular operation temperaturesof the machine, but stay remarkably constant over awide range of values of the reduced temperature.

In Fig. 8 the improvements achieved during the devel-opment of the second prototype can be observed. In thediagram the results for the power density of the first pro-totype (labelled SWP2) at two nominal cooling powersand the results for the second prototype (labelledSWP3) at a nominal power of 3.5 kW is presented. Itcan be seen, that at the same cooling power output of3.5 kW the new prototype achieves higher values ofthe reduced temperature. Higher reduced temperaturesmean, that higher ‘‘temperature lifts’’ are achieved withlower ‘‘temperature drives’’. This indicates a better per-formance. The same is the case for the COP as can beseen in Fig. 9. The design of both prototypes is identicalwith only small improvements in their construction. The

SorTech A

10.0

15.0

20.0

25.0

30.0

35.0

0.10 0.15 0.20 0.25 0

Reduced te

Pow

er d

ensi

ty [k

W/m

3 ]

Fig. 8. Improvement in the reduced temperature for two SorTech prototypesprototype.

improvements are mainly due to an optimised control ofthe machine.

Finally a comparison of the COP values and powerdensities of the SorTech machine with the manufacturerdata of some comparable commercially available ma-chines is presented.

The Nishiyodo NAK 20/70 machine is an adsorptionchiller working with the adsorption pair silica gel–waterand is available with cooling powers starting from70 kW. It has two periodic working adsorbers withone condenser and one evaporator connected to thereactors via self-activated vacuum valves. The presenteddata are manufacturers data [1].

The Yasaki WFC-SC10 machine is a small, 36 kWabsorption chiller working with the adsorption pairLiBr–Water. The presented data are manufacturers data[2].

In Fig. 10 the comparison of the cooling COPcooling,and in Fig. 11 the comparison of the power densityis presented. The power density is calculated bydividing the nominal power by the volume of themachine.

G Machine

.30 0.35 0.40 0.45 0.50

mperature Tred [-]

SorTech AG SWP2 - 3.5 kWSortech AG SWP2 - 6 kWSorTech AG SWP3 - 3.5 kW

at the same cooling power output. SWP2 is the first, SWP3 the second

Improvement in COPcooling

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50Reduced temperature Tred [-]

CO

P [-]

SorTech AG SWP2 - 3.5 kWSorTech AG SWP3 - 3.5 kW

Fig. 9. COPcooling for the two tested Sortech prototypes. The second prototype SWP3 shows the same range of COP values as SWP2 but at higherreduced temperatures.

Comparison of COPs

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Reduced temperature Tred [-]

CO

P co

olin

g [-]

SorTech AG SWP3 - 3.5 kWNishiyodo NAK 20/70Yazaki WFC-SC10

Fig. 10. COPcooling of the small power SorTech adsorption chiller compared to the results of commercially available chillers.

Comparison of cooling power density

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0.20 0.30 0.40 0.50 0.60

Reduced temperature Tred [-]

Pow

er d

ensi

ty [k

W/m

3 ]

SorTech AG SWP3 - 3.5 kWNishiyodo NAK 20/70Yazaki WFC-SC10

Fig. 11. Cooling power density for the Sortech, Nishiyodo and Yazaki machines over the reduced temperature.

T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212 2211

It can be seen, that the SorTech machine in its presentdevelopment and optimization stage, shows COPs thatare close to the values of the commercial machines. Nev-ertheless, the COPs are much more scattered than in the

two other machines and the reached reduced tempera-tures are somewhat lower. These results are due to thefact, that in the SorTech machine the evaporator andcondenser is integrated into one unit reducing therefore

2212 T. Nunez et al. / Applied Thermal Engineering 27 (2007) 2205–2212

the COPcooling value. On the other hand, with this designthe machine gains compactness and simplicity as nointernal valves are necessary. The results for the powerdensity supports this fact. Here, the SorTech machineshows higher values than the NAK 20/70 adsorptionmachine. Nevertheless, both adsorption machines havelower densities than the Yazaki absorption chiller.

3. Conclusions

The preliminary results of the first two prototypes ofan adsorption chiller and heat pump shows a promisingdevelopment. With the tested prototypes heating coeffi-cient of performance of more than 1.5 and cooling COPsfor air-conditioning purposes (12–15 �C) of 0.5 havebeen achieved. The machine is being developed for themarket of small capacity chillers with nominal coolingpowers in the range of 3–8 kW and nominal heatingpowers of up to 16 kW.

Using silica gel as adsorbent driving temperatures of75 �C are sufficient to operate the machine, but the

developed design also allows the use of other adsorbentslike zeolites if higher driving temperatures are available.A reliably working control procedure has been devel-oped and implemented in the machine. Work will becontinued in order to improve the operation and reli-ability of the prototype.

Acknowledgements

This work was supported by the German ministry ofeconomy and work (BMWA) through the Contractnumber 0327278B.

References

[1] Nishiyodo Manufacturers data sheet, GBUmbH, 1998, availablefrom: <http://www.gbunet.de/outgoing/nak-prospect.pdf>.

[2] Yazaki Manufacturers Data sheet, Yazaki Energy Systems Inc.,available from: <http://www.yazakienergy.com/docs/YazakiWate_dChillHeatBroch.pdf>.