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Cooling machine with integrated cold storage
lntJ. Refrig. Vol. 21, No. 2, pp. 157-161, 1998 © 1998 Elsevier Science Ltd and IIR All rights reserved. Printed in Great Britain 0140-7007/98/$19.00+00 PII:S0140-7007(97)00040-6
ELSEVIER
Cooling machine with integrated cold storage T. Berlitz Z A E B a y e r n , W a l t h e r - M e i l 3 n e r - S t r . 6, D - 8 5 7 4 8 G a r c h i n g , G e r m a n y
N. Lemke, P. Satzger and F. Ziegler* Department of Physics El9, Technical University of Munich, D-85748 Garching, Germany Revised 3 June 1997; accepted 5 June 1997
The use of thermal solar energy systems in combination with thermal driven sorption chillers for climatisation gains increasing influence. For solar assisted cooling a backup system is necessary for times when no solar energy is available. Absorption chillers driven by a combination of thermal collectors and conventional furnaces, which supply the driving heat in times of no insolation, suffer from an abrupt drop of the system efficiency (COP) during the operation change. This drop in COP can be avoided by installing a combined heat buffer and storage feature. Various possibilities of heat storage features are compared. An experimental setup of a high-efficient absorption chiller which facilitates the supply of a constant load of coldness at constantly high COP in spite of periodically available driving heat is presented. © 1998 Elsevier Science Ltd and IIR. All rights reserved.
(Keywords: refrigerationsystem; absorptionsystem; solarenergy; thermalstorage)
Machine frigorifique ?aaccumulation de froid int6gr6e L'utilisation de syst~mes utilisant l'~nergie solaire et des refroidisseurs fi sorption utilisant une ~nergie thermique pour la climatisation s '~tend de plus en plus. Pour le froid solaire, un systkme de relai est indispensable pour les p~riodes sans soleil. Les refroidisseurs it absorption fonctionnant avec des collecteurs thermiques et des foyers classiques pour fournir la chaleur motrice aux p~riodes non ensoleillges, voient le rendement du systbme (COP) chuter fortement lors du changement du mode de fonctionnement. L'installation d'un systbme d'accumulation interm~diaire peut ~viter cette chute du COP. On compare diff~rentes possibilit~s d'accumulation thermique et l'on pr~sente un refroidisseur experimental it absorption it haut rendement, qui permet d'avoir une charge frigorifique stable, avec un COP toujours ~levg malgr~ l'irr~gularitg de l'apport thermique. © 1998 Elsevier Science Ltd et IIR. All rights reserved. (Mots cl6s: syst~mefrigorifique;syst/~meh absorption;energie solaire;accumulationthermique)
Introduction
collectors as driving heat for cooling systems. The idea is promising as the demanded cooling power is correlated with incident solar radiation intensity. Absorption chillers meet the demand mentioned and associate some more advantages like using working fluids harmless to the environment. To ensure the supply of cooling capacity in times when no solar insolation is available, a backup system is necessary. This could either be an additional gas burner or a storage unit. Absorption chillers driven by a combination of thermal collectors and conventional furnaces, which supply the driving heat in times of no insolation, suffer from an abrupt drop of system efficiency (COP) during the
Most of today's installed thermal solar energy systems are utilised for hot water production and heating purposes. So the main heat demand to be covered is in winter, when insolation is at a minimum. In Southern European countries, where solar insolation favours the utilisation of solar energy, the demand for heating in winter is lower than the demand for cooling in summer. Therefore, an auspicious approach to cover the need for climatisation is using solar energy from thermal * Correspondingauthor.
157
7-. Berlitz e t a I.
158
Nomenclature COP k
coefficient of performance ( - ) heat transfer coefficient (W/m2K)
operation change. If the input of driving heat is not stable their COP will be diminished because conventionally their part load behaviour is not optimised for high COPs. This means that to attain high COPs either a combined buffer and storage between the heat source and the chiller must be installed or the liquid chiller must have an adapted part load behaviour to level out the occurring oscillations. Independently from this fact the chiller must be able to supply the customer with a constant cold production. So in any case some kind of energy storage is required. Moreover there are other forthcoming applications where the supply of driving heat varies strongly, e.g. cascading chillers with discontinuously operating topping stages. In this paper an experimental set-up of an absorption chiller including an internal latent heat storage is presented. Although it was designed as a bottoming stage for discontinuously working solid sorption chillers, it can obtain a constant cooling capacity at a high COP for solar assisted cooling as well. In advance various possibilities of direct and indirect heat storage features will be compared and evaluated.
LMTD logarithmic mean temperature difference (K) SHX solution heat exchanger
realised in two different ways, either storing the driving heat externally or storing the cooling capacity internally inside the chiller. Storing the driving heat in a hot water bath or an oil bath uses only the sensible heat of the material. Further, it takes great effort to run a hot water storage at temperatures above 100°C. Due to the high pressure of a hot water storage an oil bath with a low mass-specific heat capacity must be installed. Another kind of heat storage can be performed by using the latent heat of evaporation of the refrigerant itself and installing the required facilities internally. These imply three containers, two for the weak and the strong solution and one for the refrigerant itself. Compared to the sensible heat storages, a remarkable higher energy density can be attained. In the following, the different storage techniques are compared in more detail by means of an application to a small capacity chiller (see Table I).
insolation
,,//solid sorption /
l
Comparison of storage techniques Basically, cold can be stored directly as cold or indirectly as driving heat for a sorption chiller. In any case, the influence of the COP has to be noted: a COP of 1.2 means that a 1.2 times greater amount of heat has to be stored, if a cold storage instead of a driving heat storage is installed. In Figure 1 some possible storage techniques are illustrated. The direct storage techniques are restricted by the available temperatures. An ice storage linked with a chiller working on lithium-bromide/water is difficult to ran. Although it is possible to approach temperatures below zero in the evaporator by adding salt to the refrigerant, this comprises some drawbacks: as the additional salt in the evaporator reduces the vapour pressure, the solution's concentration attains the crystallisation line. This means that the obtainable temperature lift is reduced and cooling water with a lower temperature than conventional is needed. Further, an additional control for the water inlet into the evaporator is required. To store the produced cold in a cold water tank requires much more volume because the heat capacity amounts to considerable smaller values than the melting heat of ice. Further, there is only a small useable temperature glide restricted by the melting point of the water. The indirect storage techniques have in common that they store the ability of producing cold. This can be
heat storage (indirect)
D
oil hot water
chiller internal storage (indirect)
chemical latent
cold
water ice
storage (direct)
Figure 1 Direct and indirect cold storage techniques Figure 1 Techniques d'accumulation de from directes et indirectes
Table 1 Datafor comparisonof the storages Tableau 1 Conditions retenues pour le comparaison Time without heat input Cooling capacity Cold to be stored Working pair COP Driving heat to be stored
l0 20 12.000 LiBr/water 1.2 10.000
[min] [kW] [kJ] [ ~- 3.3 kwh] [- ] lkJ] [ ~ 2.8 kWh]
Cooling machine with integrated cold storage Referring to Figure 1, three main storage techniques are taken into consideration:
(1) external storage of driving heat as sensible heat by means of thermal oil using the temperature spread 150°C/160°C (required volume: 530 litres); (2) external storage of cold as sensible heat of cold water in the temperature range of 6/12°C (required volume: 464 litres); and (3) internal storage with the latent heat of the refrigerant water and strong lithium-bromide solution (required amount of water: 4.8 litres; required strong lithium-bromide solution: 42 litres, using a concentration difference of 4 wt - %). If only the required amount of storage volume is considered, the internal storage is favourable to a storage using sensible heat. So the limited installation area and the permitted weight per area, e.g. in or on top of a building, can be matched. For a more precise comparison of the different storage techniques the special installation conditions have to be taken into account: the sensible heat storages require a specially stratified tank if no additional tank is be installed. The internal storage requires three additional tanks which, however, are not expensive due to their small size. Some more important properties have to be regarded.
Costs The costs of a storage depend on the storage size, on the required insulation, the storage fluid (water, thermal oil, lithium-bromide solution) and the necessary additional installations. A general valid statement cannot be given, due to the special requests for each application, but a rough estimation reveals that an external storage which runs with thermal oil will always be more expensive than an internal storage or a cold water storage. From these two remaining possibilities each can be cheaper than the other, depending on all relevant conditions.
Temperature gradients and heat exchanger area If an external storage is installed to work between two fixed temperature levels the temperature difference for the heat transport is reduced. So for the same heat flux the heat exchanger area to be installed would be enlarged. An optimisation calculation for a steam driven double-effect chiller with a constant cooling capacity of 20 kW and a COP of 1.2 amounts to a heat exchanger area of 14 m 2. The additional heat exchanger area, when an oil bath is coupled in between the heat source and the generator, amounts to 2,1 m 2, which is an increase of about 15% (k = 800 W/m 2 K; 10 K LMTD). If an internal storage is realised, no additional temperature gradient is required. But in this case the internal storage must include a suitable way to store and buffer the varying driving heat. In the next section an absorption chiller capable of matching these demands will be outlined.
1.59
Experimental chiller with internal storage In a French-German cooperation a cascading tripleeffect system is being realised, consisting of a bottoming double-effect machine working with lithium-bromide/ water and a topping solid reaction machine working with NiC12/ammonia ~'2. A COP of 1.35 is expected in this experiment, equivalent to an improvement of about 13% compared to the industrial standard. The quoted COP of 1.35 can only be attained if the periodic output of the topping stage has no remarkable influence on the COP of the continuously working bottoming chiller. Hence, one of the most important objectives of the project is the experimental investigation of the coupling of the cascading chillers. For this purpose a lithium-bromide/water double-effect chiller with a specially adapted internal storage, capable of meeting the following demands, has been planned. It must cope with a strongly varying availability of the driving heat (Figure 2) at a constantly high COP; and it must include a storage capability for matching a constant cooling demand, even when the driving heat is missing totally for a short time. One point to obtain a high COP is to optimise the part load behaviour of the chiller. Conventionally, a lowering of the available driving power leads to an increased specific solution circuit flow and relatively more heat losses in the solution heat exchangers. An increase of the driving power causes higher temperatures and concentrations in the generator and so can give rise to a crystallisation of lithium-bromide solution in the absorber. To avoid these problems a capacity-controlled regulation of the solution flow is installed. An on-line measurement of the current concentration of the solution leaving the generator automatically adapts the rate of the solution flow to the at present available amount of driving heat. A decrease of the concentration difference between the diluted and the strong solution is compensated by a simultaneous lowering of the flow of the solution. In the same way a rise of the concentration difference is levelled out by increasing the solution flow. This makes it possible to buffer peaks or gaps in the driving heat and to attain constant concentrations and a high COP as well. To provide a cheap and easy way to handle the regulation of such a chiller it would be more favourable to measure the temperature and the pressure instead of the concentration in the generator. But as there is no equilibrium relationship between these three parameters in the generator, first the correlation between them has to be investigated. The second point is to provide a constant cooling demand in spite of the oscillating or periodically missing driving heat. This requires a discoupling of the cold production from the course of the available driving heat and a storage facility to bridge the heat gaps as well. The regulation of the solution flow results in constant concentrations which makes it easy to perform an internal latent heat storage with a separated storing of the refrigerant, the weak and the strong solution.
T. Berlitz et a I.
160
Heat
InputGenerator2 ~ o r
0
10
20
30
40 Timein min
T i
2
l't
| SHX 2
Condenser I SHX 1 Generator 1/ Condenser 2 Storage Tank for Strong Solution
Evaporator
Absorber
Figure 2 Double-effect lithium-bromide chiller with internal storage at the end of the charging phase (marked by a cross in the small chart) Figure 2 Refroidisseur au bromure de lithium ~ double effet, avec accumulation interne en fin de phase de chargement (indiqu~e par une croix dans le petit diagramme; se reporter au texte)
Two of these three required tanks are, due to their small size, integrated into the absorber and the evaporator pool. This means a simplification for the installed hardware, because only one additional storage tank is necessary for the strong solution. Looking at the three pressure levels in the absorption chiller, there is more than one conceivable place for installing an internal storage tank for the solution. The different possibilities shall now be briefly touched on.
generated vapour has to be stored. This requires very high volume at an absolute pressure of e.g. 700 mbar. Besides the weak and the strong solution are stored on a high temperature level which gives rise to heat losses to the surroundings. So it is more reasonable to install the storages for the strong solution and the refrigerant on a lower pressure level.
M i d d l e p r e s s u r e level H i g h p r e s s u r e level
A storing of the solution at the high pressure level creates the problem that due to the double-effect cycle the
Placing the storage tank for the strong solution between generator 1 and the solution heat exchanger 1 consequently enforces the location of the storage for the weak
Cooling machine with integrated cold storage solution between both heat exchangers. Otherwise there would be a mismatch of the flows in the solution heat exchangers. But, in any case, both storage tanks were on a relatively high temperature level compared to the ambient temperature. Low pressure level Storing the strong solution at the low pressure level comprises the advantage that the storage tank doesn't have to be insulated because a lowering of the temperature would support the absorption. The refrigerant is stored in the liquid phase so only a very small volume must be insulated against warming up. The weak solution, which is stored in the absorber pool, must be prevented from cooling down. To sum up, on closer inspection there are two restrictions leading to a storage of strong solution on the low pressure level: the flow of solution on both sides of the solution heat exchangers (SHX) must be nearly equivalent in all operation modes of the machine, otherwise the COP will decrease; and to reduce the losses a storage tank at a low temperature level requires less insulation. Figure 2 is a flow chart of the double-effect lithiumbromide/water chiller: water, circulated by the refrigerant pump, is sprayed over the evaporator tube bundle, evaporates and chills the cold water flowing in the tubes. The evaporated vapour is then absorbed by the lithiumbromide solution in the absorber. So the solution becomes 'weak' and is, passing the solution heat exchangers, sent to the high pressure generator 2. Here the solution is heated up by the external driving heat and ejects vapour. The vapour flows to the high pressure condenser 2, where its condensation heat is used to concentrate the intermediate concentrated solution coming from generator 2. The condensate is fed into condenser 1. The vapour ejected from the intermediate concentrated solution in generator 1 is condensed in the mediate pressure condenser l and then flows down to the evaporator, where it again can be evaporated. The concentrated solution from generator l is called 'strong' solution and is sprayed into the absorber where it again can absorb the vapour coming from the evaporator. The small chart in the upper left corner of Figure 2 shows a schematic course of the available driving heat for the chiller. The X indicates the point in time where it operates at the moment: the charging phase, when driving heat is provided, is nearly at its end and the storages for the refrigerant and the strong solution have been filled up. The absorption chiller has generated cold
161
continuously with the required cooling capacity. Each surplus of driving heat has been used to generate additional refrigerant and strong working fluid, stored separately in the two tanks. The additional amount of weak solution needed to meet this demand has been deposed from the absorber pool. When less driving heat than required to meet the cooling demand is available, the missing driving heat is levelled by removing the appropriate amount of refrigerant and strong working fluid from their storages. In the discharging phase, when no driving heat is available, the evaporator and the absorber provide the cooling capacity supplied by the storages and the other parts of the chiller are shut down. Weak solution is accumulated in the absorber pool.
Conclusion Different methods to provide a storing feature to chillers were compared. If the main aspects are small volume and little weight, respectively of the whole installation (e.g. solar assisted cooling on top of a building), then the internal latent heat storage is also a strong alternative to external storages. The internal latent heat storage enables the chiller to provide a constant demand of cold at a high COP in spite of varying or partly missing driving heat. Further, it does not only improve the performance of cascading chillers but opens up a wider field of applications for thermally driven sorption chillers, e.g. for solar assisted cooling as well. The outline of an experimental setup for a doubleeffect chiller with internal storage feature was presented.
Acknowledgements Part of this work has been sponsored by the German Ministry of Education and Research (BMBF) and the French Ministry for Environment and Energy (ADEME).
References 1. Goetz, V., Elie, F. and Spinner, B., The structure and performance of single effect solid-gas chemical heat pumps. Heat Recovery Systems and CHP, Pergamon Press Ltd, 1993, 13(1), 79-96. 2. Neveu, P., Castaing, J., Mazet, N., Meyer, P., Experimentation sur un pilote de 10 kW de puissance dune thermotransformation (180-220°(2) et dune production de froid (-5°C) a double effet. In Proceedings of the Symposium: Solid Sorption Refrigeration. Paris, 1992.
ELSEVIER
Cooling machine with integrated cold storage T. Berlitz Z A E B a y e r n , W a l t h e r - M e i l 3 n e r - S t r . 6, D - 8 5 7 4 8 G a r c h i n g , G e r m a n y
N. Lemke, P. Satzger and F. Ziegler* Department of Physics El9, Technical University of Munich, D-85748 Garching, Germany Revised 3 June 1997; accepted 5 June 1997
The use of thermal solar energy systems in combination with thermal driven sorption chillers for climatisation gains increasing influence. For solar assisted cooling a backup system is necessary for times when no solar energy is available. Absorption chillers driven by a combination of thermal collectors and conventional furnaces, which supply the driving heat in times of no insolation, suffer from an abrupt drop of the system efficiency (COP) during the operation change. This drop in COP can be avoided by installing a combined heat buffer and storage feature. Various possibilities of heat storage features are compared. An experimental setup of a high-efficient absorption chiller which facilitates the supply of a constant load of coldness at constantly high COP in spite of periodically available driving heat is presented. © 1998 Elsevier Science Ltd and IIR. All rights reserved.
(Keywords: refrigerationsystem; absorptionsystem; solarenergy; thermalstorage)
Machine frigorifique ?aaccumulation de froid int6gr6e L'utilisation de syst~mes utilisant l'~nergie solaire et des refroidisseurs fi sorption utilisant une ~nergie thermique pour la climatisation s '~tend de plus en plus. Pour le froid solaire, un systkme de relai est indispensable pour les p~riodes sans soleil. Les refroidisseurs it absorption fonctionnant avec des collecteurs thermiques et des foyers classiques pour fournir la chaleur motrice aux p~riodes non ensoleillges, voient le rendement du systbme (COP) chuter fortement lors du changement du mode de fonctionnement. L'installation d'un systbme d'accumulation interm~diaire peut ~viter cette chute du COP. On compare diff~rentes possibilit~s d'accumulation thermique et l'on pr~sente un refroidisseur experimental it absorption it haut rendement, qui permet d'avoir une charge frigorifique stable, avec un COP toujours ~levg malgr~ l'irr~gularitg de l'apport thermique. © 1998 Elsevier Science Ltd et IIR. All rights reserved. (Mots cl6s: syst~mefrigorifique;syst/~meh absorption;energie solaire;accumulationthermique)
Introduction
collectors as driving heat for cooling systems. The idea is promising as the demanded cooling power is correlated with incident solar radiation intensity. Absorption chillers meet the demand mentioned and associate some more advantages like using working fluids harmless to the environment. To ensure the supply of cooling capacity in times when no solar insolation is available, a backup system is necessary. This could either be an additional gas burner or a storage unit. Absorption chillers driven by a combination of thermal collectors and conventional furnaces, which supply the driving heat in times of no insolation, suffer from an abrupt drop of system efficiency (COP) during the
Most of today's installed thermal solar energy systems are utilised for hot water production and heating purposes. So the main heat demand to be covered is in winter, when insolation is at a minimum. In Southern European countries, where solar insolation favours the utilisation of solar energy, the demand for heating in winter is lower than the demand for cooling in summer. Therefore, an auspicious approach to cover the need for climatisation is using solar energy from thermal * Correspondingauthor.
157
7-. Berlitz e t a I.
158
Nomenclature COP k
coefficient of performance ( - ) heat transfer coefficient (W/m2K)
operation change. If the input of driving heat is not stable their COP will be diminished because conventionally their part load behaviour is not optimised for high COPs. This means that to attain high COPs either a combined buffer and storage between the heat source and the chiller must be installed or the liquid chiller must have an adapted part load behaviour to level out the occurring oscillations. Independently from this fact the chiller must be able to supply the customer with a constant cold production. So in any case some kind of energy storage is required. Moreover there are other forthcoming applications where the supply of driving heat varies strongly, e.g. cascading chillers with discontinuously operating topping stages. In this paper an experimental set-up of an absorption chiller including an internal latent heat storage is presented. Although it was designed as a bottoming stage for discontinuously working solid sorption chillers, it can obtain a constant cooling capacity at a high COP for solar assisted cooling as well. In advance various possibilities of direct and indirect heat storage features will be compared and evaluated.
LMTD logarithmic mean temperature difference (K) SHX solution heat exchanger
realised in two different ways, either storing the driving heat externally or storing the cooling capacity internally inside the chiller. Storing the driving heat in a hot water bath or an oil bath uses only the sensible heat of the material. Further, it takes great effort to run a hot water storage at temperatures above 100°C. Due to the high pressure of a hot water storage an oil bath with a low mass-specific heat capacity must be installed. Another kind of heat storage can be performed by using the latent heat of evaporation of the refrigerant itself and installing the required facilities internally. These imply three containers, two for the weak and the strong solution and one for the refrigerant itself. Compared to the sensible heat storages, a remarkable higher energy density can be attained. In the following, the different storage techniques are compared in more detail by means of an application to a small capacity chiller (see Table I).
insolation
,,//solid sorption /
l
Comparison of storage techniques Basically, cold can be stored directly as cold or indirectly as driving heat for a sorption chiller. In any case, the influence of the COP has to be noted: a COP of 1.2 means that a 1.2 times greater amount of heat has to be stored, if a cold storage instead of a driving heat storage is installed. In Figure 1 some possible storage techniques are illustrated. The direct storage techniques are restricted by the available temperatures. An ice storage linked with a chiller working on lithium-bromide/water is difficult to ran. Although it is possible to approach temperatures below zero in the evaporator by adding salt to the refrigerant, this comprises some drawbacks: as the additional salt in the evaporator reduces the vapour pressure, the solution's concentration attains the crystallisation line. This means that the obtainable temperature lift is reduced and cooling water with a lower temperature than conventional is needed. Further, an additional control for the water inlet into the evaporator is required. To store the produced cold in a cold water tank requires much more volume because the heat capacity amounts to considerable smaller values than the melting heat of ice. Further, there is only a small useable temperature glide restricted by the melting point of the water. The indirect storage techniques have in common that they store the ability of producing cold. This can be
heat storage (indirect)
D
oil hot water
chiller internal storage (indirect)
chemical latent
cold
water ice
storage (direct)
Figure 1 Direct and indirect cold storage techniques Figure 1 Techniques d'accumulation de from directes et indirectes
Table 1 Datafor comparisonof the storages Tableau 1 Conditions retenues pour le comparaison Time without heat input Cooling capacity Cold to be stored Working pair COP Driving heat to be stored
l0 20 12.000 LiBr/water 1.2 10.000
[min] [kW] [kJ] [ ~- 3.3 kwh] [- ] lkJ] [ ~ 2.8 kWh]
Cooling machine with integrated cold storage Referring to Figure 1, three main storage techniques are taken into consideration:
(1) external storage of driving heat as sensible heat by means of thermal oil using the temperature spread 150°C/160°C (required volume: 530 litres); (2) external storage of cold as sensible heat of cold water in the temperature range of 6/12°C (required volume: 464 litres); and (3) internal storage with the latent heat of the refrigerant water and strong lithium-bromide solution (required amount of water: 4.8 litres; required strong lithium-bromide solution: 42 litres, using a concentration difference of 4 wt - %). If only the required amount of storage volume is considered, the internal storage is favourable to a storage using sensible heat. So the limited installation area and the permitted weight per area, e.g. in or on top of a building, can be matched. For a more precise comparison of the different storage techniques the special installation conditions have to be taken into account: the sensible heat storages require a specially stratified tank if no additional tank is be installed. The internal storage requires three additional tanks which, however, are not expensive due to their small size. Some more important properties have to be regarded.
Costs The costs of a storage depend on the storage size, on the required insulation, the storage fluid (water, thermal oil, lithium-bromide solution) and the necessary additional installations. A general valid statement cannot be given, due to the special requests for each application, but a rough estimation reveals that an external storage which runs with thermal oil will always be more expensive than an internal storage or a cold water storage. From these two remaining possibilities each can be cheaper than the other, depending on all relevant conditions.
Temperature gradients and heat exchanger area If an external storage is installed to work between two fixed temperature levels the temperature difference for the heat transport is reduced. So for the same heat flux the heat exchanger area to be installed would be enlarged. An optimisation calculation for a steam driven double-effect chiller with a constant cooling capacity of 20 kW and a COP of 1.2 amounts to a heat exchanger area of 14 m 2. The additional heat exchanger area, when an oil bath is coupled in between the heat source and the generator, amounts to 2,1 m 2, which is an increase of about 15% (k = 800 W/m 2 K; 10 K LMTD). If an internal storage is realised, no additional temperature gradient is required. But in this case the internal storage must include a suitable way to store and buffer the varying driving heat. In the next section an absorption chiller capable of matching these demands will be outlined.
1.59
Experimental chiller with internal storage In a French-German cooperation a cascading tripleeffect system is being realised, consisting of a bottoming double-effect machine working with lithium-bromide/ water and a topping solid reaction machine working with NiC12/ammonia ~'2. A COP of 1.35 is expected in this experiment, equivalent to an improvement of about 13% compared to the industrial standard. The quoted COP of 1.35 can only be attained if the periodic output of the topping stage has no remarkable influence on the COP of the continuously working bottoming chiller. Hence, one of the most important objectives of the project is the experimental investigation of the coupling of the cascading chillers. For this purpose a lithium-bromide/water double-effect chiller with a specially adapted internal storage, capable of meeting the following demands, has been planned. It must cope with a strongly varying availability of the driving heat (Figure 2) at a constantly high COP; and it must include a storage capability for matching a constant cooling demand, even when the driving heat is missing totally for a short time. One point to obtain a high COP is to optimise the part load behaviour of the chiller. Conventionally, a lowering of the available driving power leads to an increased specific solution circuit flow and relatively more heat losses in the solution heat exchangers. An increase of the driving power causes higher temperatures and concentrations in the generator and so can give rise to a crystallisation of lithium-bromide solution in the absorber. To avoid these problems a capacity-controlled regulation of the solution flow is installed. An on-line measurement of the current concentration of the solution leaving the generator automatically adapts the rate of the solution flow to the at present available amount of driving heat. A decrease of the concentration difference between the diluted and the strong solution is compensated by a simultaneous lowering of the flow of the solution. In the same way a rise of the concentration difference is levelled out by increasing the solution flow. This makes it possible to buffer peaks or gaps in the driving heat and to attain constant concentrations and a high COP as well. To provide a cheap and easy way to handle the regulation of such a chiller it would be more favourable to measure the temperature and the pressure instead of the concentration in the generator. But as there is no equilibrium relationship between these three parameters in the generator, first the correlation between them has to be investigated. The second point is to provide a constant cooling demand in spite of the oscillating or periodically missing driving heat. This requires a discoupling of the cold production from the course of the available driving heat and a storage facility to bridge the heat gaps as well. The regulation of the solution flow results in constant concentrations which makes it easy to perform an internal latent heat storage with a separated storing of the refrigerant, the weak and the strong solution.
T. Berlitz et a I.
160
Heat
InputGenerator2 ~ o r
0
10
20
30
40 Timein min
T i
2
l't
| SHX 2
Condenser I SHX 1 Generator 1/ Condenser 2 Storage Tank for Strong Solution
Evaporator
Absorber
Figure 2 Double-effect lithium-bromide chiller with internal storage at the end of the charging phase (marked by a cross in the small chart) Figure 2 Refroidisseur au bromure de lithium ~ double effet, avec accumulation interne en fin de phase de chargement (indiqu~e par une croix dans le petit diagramme; se reporter au texte)
Two of these three required tanks are, due to their small size, integrated into the absorber and the evaporator pool. This means a simplification for the installed hardware, because only one additional storage tank is necessary for the strong solution. Looking at the three pressure levels in the absorption chiller, there is more than one conceivable place for installing an internal storage tank for the solution. The different possibilities shall now be briefly touched on.
generated vapour has to be stored. This requires very high volume at an absolute pressure of e.g. 700 mbar. Besides the weak and the strong solution are stored on a high temperature level which gives rise to heat losses to the surroundings. So it is more reasonable to install the storages for the strong solution and the refrigerant on a lower pressure level.
M i d d l e p r e s s u r e level H i g h p r e s s u r e level
A storing of the solution at the high pressure level creates the problem that due to the double-effect cycle the
Placing the storage tank for the strong solution between generator 1 and the solution heat exchanger 1 consequently enforces the location of the storage for the weak
Cooling machine with integrated cold storage solution between both heat exchangers. Otherwise there would be a mismatch of the flows in the solution heat exchangers. But, in any case, both storage tanks were on a relatively high temperature level compared to the ambient temperature. Low pressure level Storing the strong solution at the low pressure level comprises the advantage that the storage tank doesn't have to be insulated because a lowering of the temperature would support the absorption. The refrigerant is stored in the liquid phase so only a very small volume must be insulated against warming up. The weak solution, which is stored in the absorber pool, must be prevented from cooling down. To sum up, on closer inspection there are two restrictions leading to a storage of strong solution on the low pressure level: the flow of solution on both sides of the solution heat exchangers (SHX) must be nearly equivalent in all operation modes of the machine, otherwise the COP will decrease; and to reduce the losses a storage tank at a low temperature level requires less insulation. Figure 2 is a flow chart of the double-effect lithiumbromide/water chiller: water, circulated by the refrigerant pump, is sprayed over the evaporator tube bundle, evaporates and chills the cold water flowing in the tubes. The evaporated vapour is then absorbed by the lithiumbromide solution in the absorber. So the solution becomes 'weak' and is, passing the solution heat exchangers, sent to the high pressure generator 2. Here the solution is heated up by the external driving heat and ejects vapour. The vapour flows to the high pressure condenser 2, where its condensation heat is used to concentrate the intermediate concentrated solution coming from generator 2. The condensate is fed into condenser 1. The vapour ejected from the intermediate concentrated solution in generator 1 is condensed in the mediate pressure condenser l and then flows down to the evaporator, where it again can be evaporated. The concentrated solution from generator l is called 'strong' solution and is sprayed into the absorber where it again can absorb the vapour coming from the evaporator. The small chart in the upper left corner of Figure 2 shows a schematic course of the available driving heat for the chiller. The X indicates the point in time where it operates at the moment: the charging phase, when driving heat is provided, is nearly at its end and the storages for the refrigerant and the strong solution have been filled up. The absorption chiller has generated cold
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continuously with the required cooling capacity. Each surplus of driving heat has been used to generate additional refrigerant and strong working fluid, stored separately in the two tanks. The additional amount of weak solution needed to meet this demand has been deposed from the absorber pool. When less driving heat than required to meet the cooling demand is available, the missing driving heat is levelled by removing the appropriate amount of refrigerant and strong working fluid from their storages. In the discharging phase, when no driving heat is available, the evaporator and the absorber provide the cooling capacity supplied by the storages and the other parts of the chiller are shut down. Weak solution is accumulated in the absorber pool.
Conclusion Different methods to provide a storing feature to chillers were compared. If the main aspects are small volume and little weight, respectively of the whole installation (e.g. solar assisted cooling on top of a building), then the internal latent heat storage is also a strong alternative to external storages. The internal latent heat storage enables the chiller to provide a constant demand of cold at a high COP in spite of varying or partly missing driving heat. Further, it does not only improve the performance of cascading chillers but opens up a wider field of applications for thermally driven sorption chillers, e.g. for solar assisted cooling as well. The outline of an experimental setup for a doubleeffect chiller with internal storage feature was presented.
Acknowledgements Part of this work has been sponsored by the German Ministry of Education and Research (BMBF) and the French Ministry for Environment and Energy (ADEME).
References 1. Goetz, V., Elie, F. and Spinner, B., The structure and performance of single effect solid-gas chemical heat pumps. Heat Recovery Systems and CHP, Pergamon Press Ltd, 1993, 13(1), 79-96. 2. Neveu, P., Castaing, J., Mazet, N., Meyer, P., Experimentation sur un pilote de 10 kW de puissance dune thermotransformation (180-220°(2) et dune production de froid (-5°C) a double effet. In Proceedings of the Symposium: Solid Sorption Refrigeration. Paris, 1992.