Solar cooling plants: how to arrange solar collectors, absorption chillers and the load

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1 Solar cooling plants: how to arrange solar collectors, absorption chillers and the load Renato M. Lazzarin Dipartimento di Tecnica e Gestione dei Sistemi industriali, DTG Università di Padova (Italy) renato@gest.unipd.it Abstract The most suitable technology for solar cooling by solar thermal collectors is absorption. The final result does not only depend on the choice of a good machine and efficient thermal collectors. Similar collectors and machines can give rise to completely different seasonal performances of the plant. To begin with the load is seldom in phase with solar energy availability. A hot storage system must be provided. It cannot, however, be sized with solar heating criteria as useful thermal differences are completely different. Moreover it is usually unwise to size the absorption chiller on the solar power input for reason of cost: a cold storage system is often suggested as the cooling capacity control at the chiller level can be very inefficient. Keywords solar cooling; absorption; hot storage; cold storage Nomenclature c p specific heat of the stored fluid (J/kg K) L i average load in the period of time i (W) Q o nominal capacity of the chiller (W) Q s thermal capacity of the cold storage (J) T c temperature of the solar collectors (K) V volume of the storage tank (m 3 ) T load temperature drop of the cold water through the load heat exchangers (K) t i period of time considered (s) r density of the stored fluid (kg/m 3 ) Introduction Many different technologies are available for solar cooling. One can start from compression systems driven by a solar operated direct cycle to ejector systems driven by heat to many types of absorption systems from the simple intermittent cycle to the open cycle, not forgetting solid adsorption systems [1]. However the only technology that has found significant applications is cooling by absorption machines driven by solar thermal collectors [2]. Such a plant includes, of course, solar collectors and an absorption chiller but it is usually also equipped with an auxiliary heat source and a storage unit. This is necessary first of all because the solar availability is seldom in phase with the load and also for other reasons to be described later. How do we arrange and control these elements? To begin with, the auxiliary source can be set in series with the storage towards the absorption chiller or in

2 Solar cooling plants: collectors, absorption chillers and the load 377 Figure 1. Series or parallel settings of auxiliary heat sources. parallel (Fig. 1) [3]. The more suitable configuration would appear to be in series as this allows the use of solar heat even if the storage is not at a temperature adequate to drive the chiller. However, the preferable choice is in parallel as there is a risk in heating up the storage by the auxiliary source as the temperature increment provided by an auxiliary boiler is usually higher than the temperature drop through the absorption chiller generator. Further, is it better to connect the solar collectors directly to the storage or via a heat exchanger? The temperature drop through the heat exchanger allows the use of an anti-freeze mixture in the collectors. Why not connect the solar section directly to the absorption chiller? This could avoid the temperature drop in the heat exchanger, exploiting the best temperature that solar collectors can allow under such conditions without the need of charging the storage before. But this would prevent the possibility of controlling the chiller, more so because it is not suitable to match the absorption chiller capacity with the solar power available from the collectors: this would imply an oversizing of the chiller that is an expensive item of the plant. Moreover a cold storage would be required as the load does not follow solar energy availability. I hope to have given enough clues to testify that the arrangement of the various items in a solar cooling plant is not an easy task and it requires a good knowledge of the various components, beginning with the absorption chiller. Some elements regarding the absorption chiller A thorough description of the absorption principle and of the absorption chiller operations is out of the scope of this paper. Here only the behaviour of low capacity absorption chillers is examined with regards to partial capacity operations. A low capacity chiller means here below 100 kw refrigerating capacity and more typically kw. This means a solar section up to, say, m 2 of high efficiency solar collectors. Of course larger plants can be considered but they are definitely not common. These low capacity chillers are single effect water lithium bromide driven by hot water and not equipped with a capacity control which must be provided if necessary in the frame of the circuit [4].

3 378 R. M. Lazzarin Figure 2. Transient of an absorption chiller. The usual control for these devices is ON-OFF operations. However, this control mode risks being very inefficient [5]. The absorption chiller usually has an important thermal capacity. When it is turned off it cools down and if the following ON period is delayed by some tens of minutes energy is lost and the cooling effect is in turn delayed with a large reduction in COP. Fig. 2 represents the transient of a 25 kw nominal capacity absorption chiller considering the instantaneous capacity and the cumulative COP. The chiller is a widespread model with natural circulation of the solution. After 30 minutes the nominal capacity is attained but the cumulative COP in the meantime is only 0.4 whereas the instantaneous COP is only 0.59 against the claimed 0.7 in steady state [6]. Fig. 3 gives the possible behaviour of the same equipment in ON-OFF operation. Eventual points on the 45 line mean that the capacity reduction is obtained with a similar reduction in COP that is not acceptable at all. The reported experimental points are, however, close enough to this line particularly with frequent cycling (2 cycles/hour). It is possible to reduce capacity to only 10% of nominal but the correspondent COP stays at 30 35% of the nominal (say it is something higher than 0.20!). Two other possible control ways are considered. The first takes advantage of the influence of the temperature of the generator on the capacity of the device. For a widespread model this is represented in Fig. 4 where the capacity is given as a function of the inlet temperature to the generator for different refrigerating temperatures (cooling water at 24 C). At first sight this appears an excellent possibility as it allows a modulation of more than 50%. Unfortunately this strategy also influences the COP as is illustrated in Fig. 5.

4 Solar cooling plants: collectors, absorption chillers and the load 379 Figure 3. Performance of an absorption chiller in ON-OFF operation. Only for a refrigerating temperature higher than 9 C is the penalisation not so strong. For the usual refrigerating temperature of 7 8 C the generator temperature should not go below, say, C. The second possible control way is to modulate the flow rate of hot water to the generator. The effect is not so different from the previous one. A lower rate to the generator gives rise to a higher temperature difference between the generator inlet and outlet with an overall lower mean temperature. Consider in Fig. 6 how the temperature drop through the generator is affected by the relative flow rate. The effect on the whole is less pronounced than before, but it has the advantage of fully using the available temperature level without an inefficient mixing to produce lower temperatures. A summary of this possible operation is given in Fig. 7 where both the strategies are employed with temperatures as low as 80 C (nominal capacity temperature C) and flow rate down to 40%. The capacity control arrives at something more than 40% with a penalty of 20% in COP. It is an almost unavoidable penalisation when the machine modulation is chosen and this is mandatory whenever the absorption chiller directly meets the load requirement without a cold storage. In effect the hot storage is useful only for the connection between the solar section and the absorption chiller [7].

5 380 R. M. Lazzarin Figure 4. Capacity as a function of generator temperature for different refrigerating temperatures. Figure 5. COP as a function of generator temperature for different refrigerating temperatures.

6 Solar cooling plants: collectors, absorption chillers and the load 381 Figure 6. Temperature drop through the generator as a function of hot water fl ow rate. Figure 7. Performance of an absorption chiller modulating temperature and fl ow rate.

7 382 R. M. Lazzarin The hot storage Solar cooling plants in a temperate climate must also provide winter heating. It appears suitable to use the same storage for both operations, summer cooling and winter heating. However the differences are fundamental. Working temperatures are quite different: solar heating usually requires temperatures lower than 50 C and if possible lower than 40 C, whereas in solar cooling the required temperature is higher than 80 C. Moreover the useful temperature drop in heating is from 2 to 4 times that in cooling: this means that to provide a similar thermal capacity the storage should be in the same proportion, larger in cooling than in heating. The stored heat is turned into cooling capacity with a ratio not far from 2 : 1 whereas in heating the energy is used directly. Heating and cooling loads are almost always different as regards both trend and quantity so that one storage size can seldom provide a good behaviour in both operations. Thermal losses of a storage unit inside a building are uncontrolled gains in winter heating and as such they can provide benefits to satisfy the load. In summer cooling, instead, the losses are detrimental, increasing the cooling load, so much so that their amount is higher as thermal levels are higher. Therefore the storage should be inside the building for winter operations and outside for summer. With regards to how to set up the auxiliary source it has been established that it is better to arrange it in parallel with the storage. Another issue concerns the thermal level of the storage. The thermal level should be as high as reasonably possible because the absorption chiller can give a higher cooling capacity even with high values of the cooling water produced by the cooling tower. However, in some periods of the day or the season the cooling load can be smaller so that a lower cooling capacity is needed. Therefore it is suggested to use a double tank storage, where a first tank at a relatively low temperature (say C) drives the chiller in periods of low demand and a second tank is operated at a higher level (say C) useful for periods of high demand [8]. A possible scheme is represented in Fig. 8. A control of the system can be as follows. Under an increasing insolation onto the collectors, their temperature T c also increases until it goes up to the value of the lower temperature storage. Then the pump P is activated with valves V1 and V2 open. If the insolation increases further, the temperature difference between the solar collectors and the first storage also increases. If it exceeds a set value the pump is stopped for a few minutes. If in this period the collector temperature goes over the hotter tank, the pump is again activated but with valves V3 and V4 open (and of course V1 and V2 closed). If, instead, the temperature increase is not enough, the operation starts again with the colder tank recovering practically all the energy collected during the pump stop and stored in the collectors. The control also provides minimum operating periods with each tank (say 15 minutes) to avoid possible frequent switching between the two tanks. As it is known, the cooling load can be highly variable during the day. The absorption chiller must be able to meet even the peaks of demand, driven either by the storage or by the auxiliary. The absorption chiller nominal capacity must be selected. If the thermal level of the generator is low, it is essential to choose a higher nominal

8 Solar cooling plants: collectors, absorption chillers and the load 383 Figure 8. Double tank storage. capacity for the chiller. However, high scheduled levels penalise the solar collector efficiency. The double tank system allows the selection of a chiller that gives the due capacity at the higher level of temperature, without strong penalisation of the solar section as the energy can be collected also at a lower temperature level. Consider the real behaviour of an absorption chiller summarised in Table I. A peak demand over 12 kw can be met with hot water at 96 C (condenser/ absorber cooled at 29.5 C). For cooling water at 27 C a thermal level at the generator of 80 C is enough to produce a 5 kw capacity often enough in some periods of the day. Two types of solar collectors are sometimes suggested, for example with flat plate collectors for the colder tank and evacuated tubular collectors for the hotter tank. Of course the shape of the daily demand and the chiller performance dictates the distribution of capacity between the two tanks. When the cooling demand is evenly high the absorption chiller operates almost continuously with a COP close to nominal. When the demand lowers, an absorption chiller able to meet the peaks of demand, operates intermittently, unless a separate control system is provided (see above). Some tens of ON-OFF operations can occur during a day. The machine cools in about a ten minutes. If the average working period is of the same order the resulting COP can be halved with respect to the nominal. The cold storage To avoid this unfavourable ON-OFF operation a cold storage is sometimes recommended. A cold storage allows long continuous working periods of the machine and

9 384 R. M. Lazzarin Table I. Performance of the absorption chiller ARKLA Solaire 36 as a function of the inlet generator hot water temperature for two different cooling water temperatures. Refrigerated water is produced at 7 C; q o is the cooling capacity T gi Cooling water temperature C 27 C 29.5 C q o (kw) COP q o (kw) COP Figure 9. Load in series with the chiller and the cold storage. eliminates some problems with the hot storage. Moreover a lower capacity chiller can be selected which can operate in more favourable temperature fields. In fact the peaks of demand can be met by the joint contribution of the chiller and of the cold storage. There are many ways to connect the cold storage with the absorption chiller and the load. The first way is represented in Fig. 9. The load is in series with the chiller

10 Solar cooling plants: collectors, absorption chillers and the load 385 Figure 10. Load supplemented either by the cold storage or directly by the chiller. and the cold storage. It is not an advisable configuration as it does not allow for modulation in the chiller. The configuration in Fig. 10 is much better, where the load can be supplemented either by the cold storage or directly by the chiller, whereas the chiller can produce refrigeration for the storage only. The possible stratification in the cold storage must not be undervalued: in fact even a couple of degrees in the refrigerated water temperature can strongly influence both COP and capacity particularly for the lower level of the generator. The more complex system with double tank cold storage is proposed to guarantee the stratification. The absorption chiller takes the water from a first tank and returns it refrigerated to the second tank (Fig. 11). The two tanks are at a variable level and when the chiller operates without load the first tank lowers its level, whereas the level of the second increases. If tank 2 fills up, a connection discharges the surplus in tank 1. Then the control system stops the chiller. Let s suppose a temperature drop for the refrigerating water of 5 C in the chiller evaporator: if the water temperature in tank 1 were 14 C, cold water would be stored at 9 C in tank 2. Going below that temperature level is not advisable due to the poor performance of the chiller. At the same time, for the usual sizing of the plant a thermal return in tank 1 should not be higher than 14 C. Cold water is taken from tank 2 to meet the load demand and returned to tank 1: therefore the tank 2 level lowers and tank 1 increases. When there are demand and chiller operations simultaneously, the tank 2 level is modified according to the flow rate of the two pumps: if these rates are close to one another the level remains almost stable. The cold storage sizing is not a simple task as it must take into account the load distribution during the day and the chiller capacity [9]. One could assign to the cold storage the task of satisfying the whole daily demand at design conditions. Then the

11 386 R. M. Lazzarin minimum chiller capacity must supply a 24 hr operation of the whole daily load whereas the cold storage satisfies the peaks. Of course the lower the chiller capacity the larger the storage must be. On the contrary, when the chiller capacity increases, the storage size can be reduced arriving at zero when the chiller capacity can overcome all the demand peaks. The thermal capacity of the cold storage Q s can be determined once the load distribution is known (load L as a function of time t) together with the nominal capacity of the chiller Q o : n Q = ( L Q ) τ if L > Q s i o i i= 1 i o The day must be divided into n time intervals summing up the imbalances between cooling demand and chiller capacity: these imbalances usually happen during consecutive hours so that the proposed equation does not give an excess of capacity. The value Q s is the necessary thermal capacity. To draw from it the cold storage volume the temperature drop of the cold water through the load heat exchangers ( T load ) must be supposed: Qs V = c ρ T p load Figure 11. Scheme with double tank cold storage. Where c p is the specific heat of the stored fluid and r is its density. The demand distribution greatly influences the storage sizing. Two curves are reported in Fig. 12 that give the possible combinations of storage and chiller capacities for the same daily demand but with different distributions: curve A refers to a residential building and curve B to a commercial one. The two

12 Solar cooling plants: collectors, absorption chillers and the load 387 Figure 12. Possible combinations of storage and chiller capacities for the same daily demand but with different distributions: curve A refers to a residential building, curve B to a commercial one. Figure 13. Demand distribution which Fig. 12 refers to. demand distributions are represented in Fig. 13 where it is easy to notice the higher peaks of the commercial building for an equal daily demand. Of course the commercial building requires high capacities both for the storage and the chiller. All the combinations described in Fig. 13 are suitable. However, low values of the storage capacity can produce intermittent chiller operation, unless a proper control system is provided. Then it is better to select a combination that imposes a minimum chiller working period. For a given chiller capacity the minimum cold storage capacity is then the chiller capacity given in the set working period.

13 388 R. M. Lazzarin Figure 14. Minimum cold storage capacity able to meet the demand, given the minimum chiller operating period for the chiller nominal capacity reported in abscissa. Table II. Hourly demand of a building as a function of time for the proposed example Time of the day Load (MJ/h) hours Overall demand (MJ) Total daily demand (MJ) 519 Therefore every time period produces a line that intersects the previous curves giving the minimum cold storage that is able to meet the demand, giving the due minimum chiller operating period for the chiller nominal capacity reported in abscissa. This is represented in Fig. 14. Of course here the sizing of the chiller with respect to the solar section is not considered. If the chiller capacity is too low a fraction of the solar heat cannot be used without a properly-sized hot storage. An example can better illustrate the procedure. Consider the daily demand listed in Table II. The temperature drop at the load is 6 K and the selected chiller capacity is 7 kw. Determine the storage volume that would allow the chiller to operate continuously for at least one hour. Determine the storage size to supply the demand.

14 Solar cooling plants: collectors, absorption chillers and the load 389 Figure 15. Building daily demand for the proposed example: the hatched area represents the needed cold storage capacity. The selected chiller capacity is of = 25.2 MJh 1 not enough for the peaks. Therefore a cold storage is requested. The cold production of one hour chiller operation can be stored in a volume V: V = = 1m The storage size to meet the demand is given by: 3 ( 30 25) 1+ ( 45 25) 2 + ( 55 25) 1+ ( 45 25) 2 + ( 36 25) 2 + ( 27 25) 2 = 141MJ The correspondent volume V is 5.6 m 3 much larger than that suggested by continuous chiller operation for the fixed period of one hour. The cold storage thermal capacity is represented by the hatched area in Fig. 15 defined by the demand curve and the line of chiller nominal capacity. Alternative storage made either of solution or of refrigerant was proposed in literature. This gives interesting advantages but it requires suitable absorption chillers that are built up and it is beyond the field of this report which concerns only market available items. Conclusions In the design of a solar cooling plant it is not enough to select very efficient solar collectors and good absorption chillers. How the various items of the plant are con-

15 390 R. M. Lazzarin nected is of paramount importance for the seasonal performance of the plant [10]. Not only must excess solar energy be duly stored but also the capacity modulation of the chiller must be carefully considered, as the traditional ON-OFF control capacity mode (which is typical for machines under 100 kw cooling capacity) can be very penalising indeed. Instead of a steady operating COP of , the seasonal COP can be even lower than 0.2! A cold storage properly sized and operated can be of great help to improve the seasonal performance of a solar cooling plant. References [1] R. Lazzarin, L Energia Solare e la Produzione del Freddo (Solar Energy and Refrigeration), PEG Ed., Milano, pp. 284, [2] H. M. Henning, Solar assisted air conditioning of buildings: an overview, HEAT SET 2005, Grenoble, [3] D. S. Ward, C. C. Smith and J. C. Ward, Operational modes of solar heating and cooling systems, Solar Energy, 19 (1977), [4] D. S. Ward, Solar absorption cooling feasibility, Solar Energy, 22 (1979) [5] J. S. Rauch and B. D. Wood, Steady-state and transient performance limitations of the ARKLA Solaire absorption cooling system, Sharing the sun, 3 (1976), [6] R. Lazzarin, Steady and transient behaviour of LiBr absorption chillers of low capacity, Int. J. of Refrigeration., 3 (1980), [7] B. Boldrin and R. Lazzarin, How to control a solar powered absorption chiller, Progress in Refrigeration Sciences and Technology, Proc. XVIth Int. Cong. of Refrg., Venezia, (1980), [8] A. Newton, Control and analysis of solar energized air conditioning systems, Proc. Meeting IIF Commission E1 and E2, , Jérusalem, [9] D. S. Ward, G. O. G. Lof and T. Uesaki, Cooling subsystem design in CSU Solar House III, Solar Energy, 20 (1978), [10] R. Lazzarin, Solar cooling plants: how to arrange solar collectors, absorption chillers and the load, Proc. 61 Congr. ATI, Solar Heating and Cooling International Session, Perugia, 2006.