Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature

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1 See discussions, stats, and author profiles for this publication at: Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature Article in Solar Energy Materials and Solar Cells December 2001 Impact Factor: 5.34 DOI: /S (01) CITATIONS 21 READS 35 3 authors, including: Wilfran Esneider Rivera Universidad Francisco de Paula Santander 81 PUBLICATIONS 1,133 CITATIONS Rosenberg J. Romero University of Morelos' State 71 PUBLICATIONS 518 CITATIONS SEE PROFILE SEE PROFILE Available from: Rosenberg J. Romero Retrieved on: 08 May 2016

2 Solar Energy Materials & Solar Cells 70 (2001) Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond s temperature W. Rivera a, *, M.J. Cardoso b, R.J. Romero a a Centro de Investigaci!on en Energ!ıa-UNAM, P.O. Box 34, 62580Temixco, Mor., Mexico b Instituto de Investigaciones El!ectricas, Temixco, Mor., Mexico Abstract Mathematical models ofsingle-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/carrolt mixtures were developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. Plots of coefficients of performance and gross temperature lifts are shown against the temperatures of the heat supplied by the solar pond. The results showed that the single-stage and the double absorption heat transformer are the most promising configuration to be coupled to solar ponds. With single-stage heat transformers it is possible to increase solar pond s temperature until 501C with coefficients ofperformance ofabout 0.48 and with double absorption heat transformers until 1001C with coefficients ofperformance of0.33. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Solar ponds; Heat transformers; Water/lithium bromide 1. Introduction Solar ponds are a type ofsolar collector which provide inexpensive means for collecting and storing solar energy at temperatures below 1001C. They constitute a low temperature heat source with a large potential for multiple applications. Solar ponds based on the use ofa salt gradient column, were conceived after observing *Corresponding author. Tel.: ; fax: address: wrgf@mazatl.cie.unam.mx (W. Rivera) /01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

3 322 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Nomenclature COP coefficient ofperformance (dimensionless) DT gross temperature lift (1C) EF effectiveness (dimensionless) FR flow ratio (dimensionless) H specific enthalpy (kj/kg) M mass flow rate (kg/s) Q heat load (kw) T temperature (1C) WP pump work (kj) X solution concentration (wt%) Subscripts A actual AB absorber AE absorber/evaporator CO condenser CT Carnot DA double absorption EC economiser ET enthalpic EV evaporator GE generator MAX maximum SS single stage TS two stage WF working fluid that some naturally salt-stratified lakes had higher temperatures at their bottom than their surface. Solar ponds have been studied theoretically and experimentally since the late 1950s. Tabor [1] reports the state ofthe art ofsolar ponds, including basic concepts, discussion oftechnical problems and their solutions, story and experience, applications and costs, as well as an outlook for the future. Since then, many papers have been published about the theoretical and experimental study ofsolar ponds. However, due to their characteristics, solar ponds cannot reach temperatures in general, higher than 901C, therefore limiting their possible applications. To date, the most common application ofsolar ponds have been the generation ofelectric power by means ofa Rankine cycle operating between the high and low temperatures ofthe pond. Gommed and Grossman [2] proposed the use ofabsorption heat transformers to increase the temperature level ofsolar pond to a higher useful levels. They analysed theoretically the feasibility to use heat from solar ponds to generate process steam at relatively high efficiency by the use ofheat transformers.

4 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 1. Schematic diagram ofa solar pond coupled to a heat transformer. Absorption heat transformers are devices for increasing the temperature of moderately warm heat to a more useful temperature level. Typically, up to half of the heat is increased in temperature whilst the rest is discharged at a lower temperature. These systems operating, with the water lithium bromide mixture, have been used in the last years to recover energy in industrial processes [3]. However, they can be used for upgrading the temperature level of energies from sources such as geothermal and solar systems. In order to demonstrate the feasibility of heat transformers for increasing the temperature ofthe heat from a solar pond, which can store large amounts ofenergy at intermediate temperatures, mathematical models ofsingle-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/ Carrol mixtures were developed to simulate the performance of these systems coupled to a solar pond. Carrolt is a new mixture oflibr and ethylene glycol [(CH 2 OH) 2 ] in the ratio 1 : 4.5 (ethylene glycol:libr) by weight. Fig. 1 shows an schematic diagram ofa solar pond coupled to a heat transformer. 2. Description of the systems 2.1. Single stage heat transformers A single stage heat transformer (SSHT) basically consists of an evaporator, a condenser, a generator, an absorber, and an economiser as shown in Fig. 2. A quantity ofwaste heat Q GE is added at an intermediate temperature T GE to the

5 324 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 2. Pressure against temperature diagram for an SSHT. generator to vaporise the working fluid from the weak salt solution. The vaporised working fluid goes to the condenser where it is condensed delivering an amount of heat Q CO at a lower temperature T CO : The liquid leaving the condenser is pumped to the evaporator where it is evaporated by means ofa quantity ofwaste heat Q EV added to the evaporator at an intermediate temperature T EV : After this, the vaporised working fluid goes to the absorber where it is absorbed by the strong salt solution coming from the generator, delivering an amount of heat Q AB at a higher temperature T AB : Finally, the weak salt solution returns to the generator, preheating the strong salt solution in the economiser, starting the cycle again. It can be seen in Fig. 2 that the SSHT operates with two pressure levels and three temperature levels when the same waste heat is supplied to the generator and the evaporator. It can also be seen, that the highest temperature is obtained in the absorber when the heat is supplied to the generator and the evaporator at an intermediate temperature Two stage heat transformers Two stage heat transformers (TSHT) basically consist of two single stage heat transformers (SSHT) which can be coupled in three different ways as shown in Fig. 3. The first way is by connecting the absorber ofthe first stage to the evaporator ofthe second so that the heat delivered by the absorber allows the evaporation ofthe

6 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 3. Schematic diagram ofa TSHT. working fluid in the second stage. The second way is by coupling the absorber ofthe first stage to the generator ofthe second. The third way is by splitting the heat delivered by the absorber between the generator and the evaporator ofthe second stage. The first arrangement is technically simpler than the others and allows the second stage absorber to operate at the maximum possible temperature for any fixed concentration ofthe solution [4]. The supply ofheat to the generator may result in an increase in the coefficient ofperformance (COP). However, as was shown by Rivera [4], for the water/lithium bromide mixture, the increase of the COP was not in general higher than 10%. On the other hand, the third arrangement is technically very complex and results in a second stage with less heat production capacity than the others, causing an increase in the capital costs. Because ofthis, only the first configuration will be analysed in this paper Double absorption heat transformers Double absorption heat transformers (DAHT) basically consist of a generator, a condenser, an evaporator, an absorber, an absorber/evaporator and an economiser

7 326 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 4. Pressure against temperature diagram for a DAHT. as shown in Fig. 4. A heat source is supplied to separate the working fluid in the generator at an intermediate temperature T GE : The vaporised working fluid is condensed in the condenser at a lower temperature T CO : Then the condensed working fluid is split into two streams. One ofthem is pumped into the evaporator, where it is vaporised at an intermediate temperature T EV and pressure P EV : The other one is pumped at a higher pressure P AB and vaporised in the absorber/ evaporator by means ofan amount ofavailable heat Q AE : The vaporised working fluid is absorbed in the absorber, at a higher temperature T AB ; by the strong solution X GE coming from the generator. The weak solution at an intermediate concentration X AB is split into two streams. One goes to the generator preheating the strong solution through the heat exchanger. The other is fed to the absorber/evaporator absorbing the vaporised working fluid coming from the evaporator and delivering an amount ofheat Q AE : Finally, the weak solution at a low concentration X AE leaving the absorber/evaporator goes to the generator starting the cycle again. In Fig. 4 it can be seen that the DAHT operates with three pressure levels and three temperature levels when waste heat is supplied at the same temperature to the generator and the evaporator. It is important to note that there is only one absorber/evaporator (AE), however, the pressures and temperatures are different for the working fluid and for

8 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) the solution circulating inside the component, for that reason they appear as two components in Fig Important parameters The most important design parameters for a heat driven absorption heat transformer are: the flow ratio, the effectiveness of the economiser, the gross temperature lift and the coefficient of performance [5] Flow ratio The flow ratio (FR) is an important parameter since it is directly related to the size and cost ofthe generator, the absorber, the economiser and the pump. It can be defined as the ratio ofthe mass flow rate ofsolution coming from the absorber to the generator (M AB ) to the mass flow rate ofthe working fluid (M WF ). FR ¼ M AB : ð1þ M WF From a mass balance in the absorber or generator, the FR can be rewritten in terms ofsolution concentrations as FR ¼ X GE X WF X GE X AB : ð2þ 3.2. Effectiveness of the economiser The economiser is used to recover heat energy in absorption heat transformers. In particular, the economiser preheats the strong salt solution going from the generator to the absorber, by using the heat supplied from the weak salt solution going from the absorber to the generator. An additional heat exchanger between the generator/ condenser stream and the condenser/evaporator stream could be used, but its effect on the overall efficiency ofthe system would be negligible [6]. The effectiveness ofthe economiser (EF EC ) can be defined as the actual heat recovered to the maximum possible heat that could be recovered. EF EC ¼ Q A Q MAX : ð3þ 3.3. Gross temperature lift The gross temperature lift (DT) is defined as the difference between the absorber temperature and the evaporator temperature. DT ¼ T AB T EV :

9 328 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Coefficient of performance The coefficient ofperformance (COP) is an important parameter since this represents the efficiency ofan absorption heat transformer. It is defined as the heat delivered in the absorber per unit ofheat load supplied to the generator and the evaporator plus the work done by the pumps. Q AB COP ¼ Q GE þ Q EV þwp : ð5þ 4. Mathematical models The following assumptions have been made in the development of the mathematical models for the single stage and advanced absorption heat transformer with reference to Figs (i) There is thermodynamic equilibrium throughout the entire system. (ii) The analysis is made under steady state conditions. (iii) A rectifier is unnecessary since the absorbent does not evaporate in the temperature range under consideration. (iv) The solution is saturated leaving the generator and the absorber, and the working fluid is saturated leaving the condenser and the evaporator. (v) Heat losses and pressure drops in the tubing and the components are considered negligible. (vi) The work done by the pumps is considered negligible. (vii) The flow through the valves is isenthalpic. (viii) Temperatures at the exit ofthe main components, the heat load in the evaporator Q EV ; and the effectiveness ofthe economiser EF EC are all known. The flow ratio, the effectiveness ofthe economiser, the gross temperature lift and the enthalpy coefficient of performance are determined for the different heat transformers by means of the following equations with reference to Figs Single stage heat transformer FR SS ¼ M AB X GE ¼ ; M WF X GE X AB ð6þ EF EC;SS ¼ H 10 H 9 ; ð7þ H 5;10 H 9 where H 5;10 is evaluated at the temperature in point 5 but at the solution concentration in point 10. DT SS ¼ T 5 T 4 ; ð8þ

10 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Q AB H 4 þ ðfr 1ÞH 10 H 5 FR COP ET;SS ¼ ¼ : ð9þ Q GE þ Q EV H 1 þ ðfr 1ÞH 8 H 7 FR þ H 4 H Two stage heat transformer FR TS ¼ M AB X GE ¼ ; M WF X GE X AB EF EC;TS ¼ H 10 H 9 H 5;10 H 9 ; DT TS ¼ T 5 T 4 ; ð10þ ð11þ ð12þ Q AB2 COP ET;TS ¼ Q GE1 þ Q EV1 þ Q GE2 M 1 0H 4 0 þ M 8 0H 10 0 M 5 0H 5 0 ¼ : M 1 ðh 1 þ H 4 H 3 ÞþM 8 H 8 M 5 H 7 M 1 0H 1 0 þ M 8 0H 8 0 M 5 0H 7 0 ð13þ 4.3. Double absorption heat transformer FR DA ¼ M AB X GE ¼ ; M WF X GE X AB EF EC;DA ¼ M 10ðH 12 H 11 Þ ; ð15þ M 16 H 10;16 H 11 ð14þ DT DA ¼ T 13 T 6 ; ð16þ Q AB M 7 H 7 þ M 10 H 12 M 13 H 13 COP ET;DA ¼ ¼ Q GE þ Q EV M 1 H 1 þ M 10 H 10 M 16 H 18 þ M 4 ðh 6 H 4 Þ : ð17þ The physical and thermodynamic properties for the water/lithium bromide mixture were taken from McNelly [7] and for the water/carrol mixtures were taken from the published work proposed for Reimann and Biermann [8]. The physical and thermodynamic properties for water were taken from the data published in Perry s Chemical Engineers Handbook [9]. 5. Theoretical results Fig. 5 shows the theoretical coefficient ofperformance and the gross temperature lift for single stage, two stage and double absorption heat transformers operating the water/carrol mixture at generator and evaporator temperatures of601c, 701C, 801C and 901C simulating the heat supply by the solar pond. The mean values ofthe coefficients ofperformance is reported inside bars. In this figure it can be seen that the highest coefficients ofperformance are obtained with single stage heat transformers, however, the gross temperature lifts reached with these systems are

11 330 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 5. Coefficients of performance and gross temperature lift for the heat transformers at T CO ¼ 301C. the lowest. Also it can be observed that similar COPs and DTs are obtained with two stage and double absorption heat transformers. Fig. 6 is a plot ofthe coefficients ofperformance against the absorber temperature for T GE ¼ T EV ¼ 701C at different condenser temperatures. In this figure it can be seen that the coefficients ofperformance decrease with both the absorber and the condenser temperatures. The theoretical performance is almost the same for the DAHT operating with the two mixtures. Also it can be seen that the highest temperature reached in the absorber is about 1601C (for T CO ¼ 201C), which means that the theoretical gross temperature lift is about 901C. In Fig. 7 can be seen that the highest COPs and absorber temperatures are obtained with the water/carrol mixture. Also it can be seen that the DAHT with the water/lithium bromide mixture cannot operate at T CO ¼ 201C. The highest DT values for the water/carrol mixture are around 1001C. In Fig. 8 it can be seen again that the highest COPs and absorber temperatures are obtained with the water/carrol mixture. Also it can be seen that the DAHT with the water/lithium bromide mixture cannot operate at condenser temperatures of201c and 301C and the water/carrol mixture at condenser temperatures of401c. 6. Conclusions A mathematical model was developed for modelling the performance of single stage, two stage and double absorption heat transformer with an economiser operating with the water/lithium bromide and the water/carrol mixture.

12 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 6. Comparison ofthe COPs and the DTs for a DAHT operating with both mixtures at T GE ¼ 701C. The highest coefficients ofperformance were obtained with single stage heat transformers. However, the gross temperature lifts reached with these systems were the lowest. Comparing the two stage and double absorption heat transformers, it was observed that almost the same COPs and DTs are obtained with both systems at the same operating conditions. The highest gross temperature lifts for single stage heat transformers were about 601C and 1051C for two stage and double absorption heat transformers. Almost the same tendencies and values ofthe coefficient ofperformance were obtained in general for both mixtures. However, theoretically higher absorber

13 332 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 7. Comparison ofthe COPs and the DTs for a DAHT operating with both mixtures at T GE ¼ 801C. temperatures or gross temperature lifts can be obtained with the water/carrol mixture than with the water/libr mixture. Because ofthe higher solubility ofthe water/carrol mixture compared with the water/lithium bromide mixture, double stage and double absorption heat transformers may operate theoretically over a larger range of condenser temperatures. With the results obtained it was demonstrated the feasibility ofthe use ofsingle stage and advanced absorption heat transformers operating with both mixtures to increase the temperature ofthe heat obtained from solar ponds.

14 W. Rivera et al. / Solar Energy Materials & Solar Cells 70 (2001) Fig. 8. Comparison ofthe COPs and the DTs for a DAHT operating with both mixtures at T GE ¼ 901C. References [1] H. Tabor, Solar Energy 27 (1981) 181. [2] K. Gommed, G. Grossman, Solar Energy 41 (1) (1988) 81. [3] K. Mashimo, Proceedings ofthe IEA Heat Pump Conference, Chelseia, Michigan USA, 1987, p [4] W. Rivera, Ph.D. Thesis, University ofsalford, UK, [5] M.A.R. Eisa, R. Best, F.A. Holland, Heat Recovery Syst. 6 (5) (1986) 421. [6] S.I. Pereira Duarte, R. Bugarel, Heat Recovery Syst. CHP 11 (5) (1991) 361. [7] L.A. McNelly, ASHRAE Trans. 85 (1) (1979) 413. [8] R.C. Reimann, W.J. Biermann, Report, Prepared for the USA Department of Energy under contract EG-77-C , Carrier Corporation, [9] R.H. Perry, D. Green, Perry s Chemical Engineers Handbook, Sixth Edition, McGraw Hill, Inc., New York, 1984.