Steady and transient behaviour of LiBr absorption chillers of low capacity

Steady and transient behaviour of LiBr absorption chillers of low capacity R. L a z z a r i n

Comportement en r6gime permanent et en r6g me transitoire des refroidisseurs absorption de LiBr de faible puissance On a essay# deux refroidisseurs ~ absorption de LiBr de faib/e puissance (4.5 et 25 kW) con cus spdcia/e-

ment en vue du refroidissement so/aire. On a vdrifi~ /es courbes de performance du constructeur, puis on a dtendu/a recherche ~ diffdrentes valeurs des principaux param#tres. On a exam/n#/e comportement en r#gime transitoire. Pour/e fonctionnement en refroidissement so/aire on a trouv# que/a rdgu/ation par simp/e cyc/e de tout ou rien est inacceptab/e en raison des fortes pertes d'dnergie qui en rdsu/tent. On propose deux strat#gies diffgrentes de r#gu/ation ainsi que des syst#mes d'insta//ation appropri#s.

Two, low capacity, LiBr, absorption chillers (4.5 and 25 kW) expressly designed for solar cooling have been tested. The manufacturer's performance curves have been checked, then the investigation was extended to different values of the main parameters. Transient behaviour has been examined.

For solar cooling duties it was found that control by simple on-off cycling is unacceptable owing to the large resulting energy loss. Two alternative control strategies are suggested together with the appropriate plant schemes.

Once a small building has been equipped with solar collectors to supply space and household water heating, it is easy to obtain solar refrigeration too. In practice, only LiBr absorption machines have been employed as they are commercially available and can work at the low temperature level allowed by flat-plate collectors.

The purpose of this study is to analyse steady performances of a tested chiller and then to examine transient operations, referring particularly to control requirements.

Recentlyl low capacity absorption units have been built for solar cooling purposes. The capacity range varies from 5 to 35 kW and the required temperature level at the generator is claimed to be well below 1 00°C. Manufacturers usually supply estimates of performance. Nevertheless it is advisable to check these data experimentally. Moreover these performance curves refer, as a rule, to steady conditions, whereas transient behaviour is an inherent feature of a solar cooling system. Transient performance is very difficult to estimate because these machines are not usually equipped with an electrical, solution pump. Instead the solution is pumped by a thermally driven, vapour lift pump, whose performance at different loads strongly influences the behaviour of the whole conditioner 1,2.

The author is at the Universita di Padova. Istituto di Fisica Tecnica via Marzolo, 9, Padua, Italy and also the Laboratorio per la Tecnica del Freddo. CNR, Padua, Italy. The work was supported by the CNR of Italy through the Solar Energy Project. Thanks to the Industrie Zanussi Spa and to the Laboratorio per la Tecnica #el Freddo CNR, for performing the tests. Paper received 18 January 1980

Volume 3 Number 4 July 1980

The m a c h i n e Two, absorption chillers of different capacity (Yazaki WFC-400S 4.5 kW and Yazaki WFC-2300S 25 kW) were tested. A plan of the machine is represented in Fig. 1. The generator and absorber operated at a pressure of 6.6661 x 103 to 7.9993 x 103 Nm -2 (5060 mm Hg) whilst the absorber and evaporator operated at 6.6661 x 102 to 1.3332 x 103 Nm -2 (510 mm Hg). The effectiveness of an absorption cycle is evaluated through a coefficient of performance (COP) defined as the ratio between cooling capacity and heating input. The COP is always less than one in a single effect absorption cycle because the cooling capacity of one unit mass evaporating water can be produced only if one unit mass of refrigerant is boiled out in the generator. An experimental study concerning COP and cooling capacity was carried out in a testing circuit whose features are reported elsewhere 3.

Steady performance Many parameters influence the performance of an absorption chiller. The most important ones are, for 0140-7007/80/04021 3-O6502.00 © 1980 IPC Business Press Ltd and IIR

213

given flow rates, the temperatures of heating, cooling and chilled fluids. Therefore COP and cooling capacity are reported in Figs 2 and 3 as a function of chilled water outlet temperature for various pairs of heating and cooling inlet water temperatures. Figs 2 and 3 refer to the smaller unit, Figs 4 and 5 refer to the higher capacity machine. For a given value of the cooling water temperature, every heating temperature allows a minimum, chilled fluid temperature: no cooling capacity is produced below it. This is because vapour production is possible only when the saturation temperature of the solution is exceeded in the generator and a sufficient absorbing ability is available. Therefore a lower evaporator temperature is compatible with a higher generator and/or a lower condenser-absorber temperature, and vice versa 4.

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Chilled water

Increasing the chilled water temperature increases both COP and cooling capacity. Higher evaporator temperature mean higher evaporation pressures. This results in an increased amount of refrigerant evaporated; a more dilute solution returns to the generator and a higher refrigerant production is possible. All the performance curves are characterized by an elbow after which the behaviour is insensitive to evaporator temperature. This zone of low sensitivity arises because the cooling of absorber and condenser is no longer adequate when the refrigerant production is too high. The elbow position is very important in the design of a plant because, once the desired working temperature of the collectors is given, the elbow determines the lowest suitable chilled water temperature and this value must be used to size fan coils, radiant ceilings or other load, heat exchangers. It is significant that the elbow position is at about 12-14°C for the permissible temperatures of solar collectors, iea value somewhat higher than the usual level in residential cooling plants. The elbow moves further to the right for decreasing heating temperatures. The machine can work at a temperature even less than 80°C, but the chilled water temperature does not fall below 1618°C. Therefore only temperatures higher than 80°C were fully investigated. Figs 2 and 3 also show the differences between experimental and manufacturer's curves as regards COP and cooling capacity for the smaller unit. A similar result was obtained for the higher capacity machine. A slight difference is revealed. Cooling capacity is higher than the claimed one, whereas COP curves show a general displacement of the elbow to the right. The considered results were referred till now to nominal flow rates, ie rates suggested by the manufacturer, of heating, cooling and chilled fluids. The investigation was then extended to different heating fluid rates to obtain information for control purposes. As a matter of fact, the variation of water rates to the generator should be an efficient method of control because less heat supplied to the generator reduces water vapour production, ie cooling capacity.

214

Heating water

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Revue Internationale du Froid

The results with flow rates of 40% and 60% (the nominal one being 100%) are pictured in Figs 6 and 7 for the small machine. Again the larger machine presents a similar behaviour.

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This control mode has proved unsatisfactory because the resulting average COP is very low 5. These poor performances are easily explained by observing the transient behaviour of the machine. Time is required for going from start to steady conditions; this time is longer for 'bubble' pump machines than for the electrical pump ones. Fig. 9 gives the time variation of cooling capacity and cumulative COP (ratio between integrated capacity and integrated heat input from start to time, t). More than 30 min are needed to approach steady values of cooling capacity (cooling capacity reaches 25.8 kW and instantaneous COP 0.59, still below the due steady value of 0.7). The cumulative COP reveals the poor average performance. As a matter of fact, first, the heating fluid must heat the solution in the generator; the temperature rises, but the vapour lift pump starts to circulate solution only when the refrigerant begins to boil. Only then does the absorber resume absorbing water vapour and the cooling effect occurs. If the start is followed by too long a stop period, the heat supplied to reach steady conditions is completely lost.

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behaviour

These small machines are not provided with capacity control systems; only some safety switches (anti-freeze, time delay and anti-overheating) are present. Control is usually obtained by simple on-off cycling. The machine starts to meet a cooling demand and runs until this load is satisfied.

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Whereas a flow rate lower than nominal can result in a reduction of the cooling duty the converse is not true, iea higher flow rate has a negligable influence on cooling capacity. The reason is that the greater amount of refrigerant from the generator overloads the absorber and condenser.

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Volume 3 Num6ro 4 Juillet 1980

16

Some performance results in on-off cycling are summarized in Table 1 for different on-off fractions and cycle frequencies. In the table are reported the ratios between average COP and capacity and the steady values at the same input conditions (heating fluid at 90°C, cooling fluid at 29.5°C, chilled fluid at 13.5°C) for the lower capacity machine, The steady COP was 0.60 and the cooling capacity 5.8 kW. As previously stated the efficiency may be strongly penalized. For example, for equal length of stopstart (50%-50%) and one cycle per hour, ie on-off

215

Table

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Cycle frequency, h

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lengths of 30 min, the cooling capacity is reduced more than 60%, but at the expense of a COP reduction of 36%. Thus, this control mode is not advisable and should be actuated only when sure that few starts-up happen every day.

The variation in heating temperature is not so penalizing. Let us consider a smooth, sinusoidal variation from a minimum of 80°C to a maximum of 95°C for a six hour period (Fig. 10). This fluctuation is intended to simulate the direct input from solar

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International

Journal

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collectors. During the first hours, there is a rapid increase in cooling capacity and heat supplied. After the temperature has reached its maximum, the heat delivered to the generator decreases faster than cooling capacity, thus increasing the COP. Considering only the four, central hours the related average COP is 0.64, slightly lower than the 0.66 COP obtained in steady conditions for the same average generator temperature.

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strategies

The analysis of the experimental results shows that on-off cycling control should not be used. It is better to use a progressive reduction of either the IOC

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Fig. 10 Performance trans/toire sous /'effet de variations smusoTda/es de/a tempdrature de/'eau de chauffage ~ /'entr#e

Volume 3 Number 4 July 1980 i.J.R. 3/4--c

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Fig. 1 2a, b Relative variations of COP and cooling capacity for f l o w rates at 60%, 40% and heating fluid temperature at 80°C, 85°C

Fig. 12 a, b Variations re/atives du COP et de/a puissance fr/LTorifique pour des ddbits de 60 et 40% et des tempgratures du flu/de de chauffage de 80 et 85°C

217

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F/~7. 13 Organigramme d'une /nsta//at/on frigor/fique sola/ke

heating fluid temperature or f l o w rate or a combination of both.

reliable, and required little maintenance, although transient w o r k i n g conditions were particularly severe. Short crises occurred occasionally due to crystallizationl but the machine resumed regular w o r k i n g by itself in f e w minutes.

Fig, 12 shows relative variations of COP and cooling capacity for f l o w rates at 60% and 40% and heating fluid temperature at 80°C and 85°C; the reference is nominal f l o w rate and heating temperature at 90°C. The cooling capacity can be halved at the expense of a reduction in COP of 15%. This control mode can be achieved in a Solar cooling plant such as the one represented in Fig. 13. The variation in the heating fluid f l o w rate to the generator may be controlled through the three-way valve V 3, whereas the heating fluid temperature may be modulated through v a l v e V 1. The set point of valve V 1 is raised or lowered according to the load. The control, through variation in the f l o w rate o n l y , is operated for low cooling loads. When the cooling demand increases the variation of the heating temperature is also actuated. The machine is powered by an auxiliary source or from storage w h e n solar heat is not available. The only drawback of this regulation mode is that a temperature level of at least 90°C must be available. The only alternative is a more complicated plant equipped w i t h cold storage. The chiller can run w h e n e v e r there is enough solar heat available and the control is accomplished downstream of the cold store 6.7.

Conclusions

The machines -tested, w h i c h were expressly designed for solar use, worked, well even if at moderate generator temperatures. During the tests they proved

218

These new machines are certainly suitable for solar cooling of small, residential houses equipped w i t h double glazed black, or single, selective collectors. The cost of the chiller is rather high (~-S5000 in 1 979), but it may' be reduced by mass production. Moreover the machine's use can be extended by using it as heat pump 8. Particular attention must be paid to the control system.

References 1 Rauch, J. S., Wood, B. D. Steady-state and transient

performance limitations of the ARKLA Solair absorption cooling system. Sharing the sun 3 Winnipeg (1 976) 387-405 2 Judd, T. R. Gas air conditioning using the absorption system Austr Refr (1 976) 11-22 3 Lazzarin, R., Boldrin, B, Experimental investigation On control modes for an absorption chiller of low capacity. SUN II ISES Silver Jubilee Congr 1 Pergamon (1979) 710-714 4

Lazzarin, R., Rizzon, E., Sovrano, M., Boldrin, B., Scalabrin, G. Performance predictions of a LiBr, absorption

air conditioner utilizing solar energy. Sun, mankind's future source of energy (1978) 1572-1580 5 Ward, D. S., Weiss, T. A., L6f, G. O. Preliminary performance of CSU solar house, I: heating and cooling system So/ar Energy 18 (1 976) 541-548 6 Lazzarin, R. Control problems in solar cooling plants. Permanent School on Solar Energy Processes, Urbino (1978) 7 Ward, D. S. Solar absorption cooling feasibility So/ar Energy 22 (1 979) 259-268 8 Lazzarin, R., Sovrano, M., Camporese, R., Grinzato, E.

Absorption heat pumps as heating systems. Presented at XV Int Congr of Refr, Comm El, Venezia (1979)

Revue Internationale du Froid