Theoretical analysis of an integrated thermoelectric-absorption cooling system

Theoretical analysis of an integrated thermoelectric-absorption cooling system R. Boukhanouf (corresponding author) and A. Supasuteekul School of the Built Environment University of Nottingham, Nottingham, NG7 2RD, UK E-mail: Rabah.boukhanouf@nottingham.ac.uk Abstract This paper presents the mathematical analysis of a single stage absorption cycle using heat rejected from thermoelectric modules to drive the cycle. The absorption cycle uses LiBr/H 2 O solution as the working pair with thermoelectric modules sandwiched between the generator and absorber. The results of the analysis show that thermoelectric modules would provide the heat required to drive the absorption cycle and the overall coefficient of performance (COP) of the integrated system could be higher than unity. Keywords absorption; thermoelectric; cooling; COP; LiBr Nomenclature a Seebeck coefficient of TE module (V/K) h Specific enthalpy (kj/kg) ṁ Mass flow rate (kg/s) I Electrical current into the TE module (A) K t TE module thermal conductance (W/K) N Number of thermoelectric module R Electrical resistance of the TE module (Ω) P Electric power supplied to the TE module (W) Q. Power (W) T Temperature ( C) T M Average temperature ( C), (T H + T C )/2 V Voltage applied to TE module (V) X Solution concentration (%) Z Figure of merit of the TE module (K 1 ) COP Coefficient of performance Subscripts A Absorber C Condenser E Evaporator G Generator H Hot junction L Cold junction ABS Absorption refrigeration system NET TE/absorption cooling system TEC Thermoelectric cooling system

Theoretical analysis of an integrated thermoelectric-absorption cooling system 53 1. Introduction Recently, there has been increased interest in using thermoelectric (TE) for cooling and heating applications in the aerospace, medical, electronics and telecommunication sectors [1, 2, and 3]. In refrigeration and heat pump applications, TE systems present a distinct advantage in that they do not have moving parts or CFC-based refrigerant to transfer heat, and emit no noise or vibration. In addition, TE can be installed in any orientation and are ideal for applications where size and weight are concerned. Furthermore, growing international concern about global warming and ozone depletion has encouraged more research and development of energy efficient and environmental friendly refrigeration and air-conditioning systems to be undertaken [4 and 5]. However, use of TE in domestic refrigeration and air-conditioning systems has not made the anticipated progress. This is mainly due to the high cost of the TE manufacturing process and low COP of typically 0.1 to 0.4 when operated in a cooling mode. This has restrained the application of a TE cooling system to specific applications such as low cooling capacity of electronic components. Despite the low COP of TE cooling systems compared to conventional vapour compression units, efforts have been made to develop thermoelectric air-conditioners and refrigerators for domestic application to exploit the advantages associated with solid-state energy conversion technology [6]. Currently, the highest cooling capacity for a TE air-conditioner available in the market is about 730 W and is marketed by EIC Solutions Inc. [7] and a prototype thermoelectric 250 litre refrigerator is under study by Hydrocool Pty Ltd [8]. Other research work on the application of thermoelectric investigated the use of TE to assist conventional working cycles to improve performance. Vian et al. [9] have demonstrated a dehumidifier prototype based on the thermoelectric cooling technology with a COP comparable to that offered by vapour-compression devices. Similarly, Gordon et al. [10] have presented the study of a miniature chiller, called electro-adsorption chiller, which combines adsorption and thermoelectric cooling devices. The overall COP of the system was reported to be higher than in conventional adsorption chillers and the system can offer significantly lower temperatures at high a COP. However, the performance is limited by poor heat transfer in solid sorption beds. In this work, the investigation is focused on using heat generated from TE modules to drive a single effect absorption cycle in small scale cooling systems. By combining the two systems, not only heat that would normally be rejected to the environment from the TE cooling system would serve as a heat source to drive the absorption cycle, but also recover a considerable amount of low grade heat rejected from the absorber and condenser. In the conventional absorption system, a good heat transfer rate in the generator section is usually difficult to achieve, as heat must be supplied at near the boiling temperature of the refrigerant-absorbent pair, which is in turn fixed by the operating pressure in the generator. Using TE to supply heat to the generator will eliminate this constraint, allowing accurate temperature control of the working pair as low as 0.01 C [11], fast thermal response to changes in operating pressure, and reduction of the effect of the thermodynamic irreversibility in the absorption cycle. Two main absorbent/absorbate working pairs were considered in

54 R. Boukhanouf and A. Supasuteekul this work: LiBr/H 2 O and H 2 O/NH 3 and the former working pair was chosen for its good thermodynamic and cycle performance properties. 2. Description of the system A schematic diagram of the integrated thermoelectric-absorption cycle is shown in Fig. 1. The thermoelectric module, used as a heat source to drive the absorption cycle, is sandwiched between the generator and absorber cylinder end plates. Power supply to TE modules appears as heat at the TE/generator interface. The single stage adsorption cycle is made of four main components: a generator, a condenser, an evaporator and an absorber. As shown in Fig. 1, The TE module is used to heat the LiBr/water working pair mixture in the generator causing water vapour to flow at high pressure to the condenser while the strong concentration LiBr solution is returned to the absorber via the pressure regulator valve. In the condenser, the water vapour is cooled and condensed to its liquid state. The liquid refrigerant is then throttled through an expansion valve and enters a low pressure evaporator where it evaporates by absorbing heat from the surroundings. After that, low pressure water vapour is mixed in the absorber with the high concentration solution returning from the generator hence releasing large amounts of heat in the process. The TE module is then used to absorb this heat and transfer it to the hot side which is part integrated with the generator. The diluted solution in the absorber is then pumped to the generator for regeneration and maintaining its concentration potential, thus completing the cycle. In addition, in order to improve the performance of 7 1 8 Generator Condenser Solution Heat Exchanger 6 9 10 TE module 4 2 Absorber Evaporator 5 3 Figure 1. Schematic diagram of TE/absorption cooling system.

Theoretical analysis of an integrated thermoelectric-absorption cooling system 55 Ceramic plates (Electrical insulator) Heat rejected (Hot side) P N P N P N P N Copper tab (Electrical conductor) Negative ( ) Positive (+) P-type semiconductor N-type semiconductor Heat obsorbed (Cold side) Figure 2. Schematic diagram of thermoelectric module. the system a solution heat exchanger was used to recover heat from the solution returning the generator to the absorber. The thermoelectric module is composed of a large number of solid state thermoelements made of n-type and p-type bismuth telluride semiconductor materials. In a typical TE module, the thermo-elements are connected electrically in series and thermally aligned in parallel between two ceramics plates, as shown in Fig. 2. By applying a DC Voltage from an electric supply source, electric current flows through the TE causing heat to be transferred from one end of the n-p junction to the opposite junction, creating a static heat pump operating according to Peltier s effect principle. A single-stage high temperature TE can operate at temperature differences of up to 70 C and transfer heat at a rate of 125 W [12]. Higher temperature differences (up to 130 C) can also be achieved using multistage cascaded modules. For this application, high temperature TE modules with a maximum temperature difference T of 89 C and a hot junction temperature of 125 C [12] were selected as suitable and which are commercially available from a number of manufacturers. 3. Theoretical analysis 3.1 Absorption refrigeration cycle A mathematical model was developed to determine the performance of the absorption cycle using equations of energy and mass conservation for each component composing the cycle. The thermodynamic properties of LiBr/water working pair used in the model were determined using equations as derived in ASHRAE [13] and Irvine [14]. The model was based on the following assumptions:

56 R. Boukhanouf and A. Supasuteekul The system operates under steady state conditions. Power consumption of circulation pumps is negligible. The generator and condenser operate at equal high pressure. The evaporator and absorber operate at equal low pressure. The expansion valves operation follows isentropic process. The computer model was used to compute the mass balance at each component s boundary interface using equations 1 and 2. m 5 = m 10 + m 4 (1) mx 5 5 = m 10 X10 + m 4 (2) The energy balance equations were also developed for each component of the cycle and in which heat input and output to the cycle were computed by solving the following equations 3 to 6: Q E = m 4( h4 h3) (3) Q A = m 10h10 + m 4h4 m 5h5 (4) Q G = m 1h1 + m 8h8 m 7h7 (5) Q C = m 1( h1 h2) (6) Since the work supplied through the pump is assumed to be negligible, the COP of the single-effect absorption refrigeration system can be expressed as: QE COPABS = (7) Q G 3.2 Thermoelectric system The basics of TE cooling can be found in many text books and published papers [15]. In this analysis the external irreversibilities were neglected and only internal effects (i.e., Ohmic and Peltier effects) that characterise the TE module material were considered. Hence the heat source and heat sink temperatures are considered to be equal to the hot and cold junction temperatures respectively. The governing equations of the TEC module cooling power and the COP can be expressed as follows [16, 17]: 1 2 Q L = aitl I R Kt( TH TL) (8) 2 where I is the current that flows through the TE module when a voltage, V, is applied at its terminals and is given by: V a( TH TL) I = R and a is an important physical property for the TE module known as figure of merit, Z, which is defined as:

Theoretical analysis of an integrated thermoelectric-absorption cooling system 57 Z = a 2 KR t The power supplied to the TE module needed to generate the cooling power Q. L and overcome Ohmic losses associated with the TE materials can also be computed from: 2 P= ai( T T )+ I R H L (9) Finally, the COP of TE when operated in cooling mode can be written as: COP TEC 1 2 IT I R K T T Q a L t( H L) L = = 2 2 P ait ( T)+ IR H L (10) 3.3 The integrated TE-absorption cooling system The overall COP of the integrated TE-absorption system can be expressed as a function of the individual performances of the absorption cycle and thermoelectric module by considering overall energy inputs and outputs to the system. The energy input to the system is represented by the power, P, supplied to the TE module and the cooling power absorbed in the evaporator, Q. E whereas energy outputs are represented by the heat rejected in the condenser and absorber and not recovered by the TE module. By applying the first law of thermodynamics to the integrated system, the total power supplied to the generator to drive the absorption cycle is given by: Q = P+ Q G L The overall COP of the system can be defined as the cooling power obtained in the evaporator divided by the total electric power input to the TE module, and can be written as follows: QE COPNET = (12) P Hence substituting equation 7, 10 and 11 into equation 12, the COP of the TEabsorption cooling system can be re-written as follows: COP = COP ( 1+ COP ) NET ABS TEC Equation 13 shows that the COP of the integrated TE-absorption system, COP NET, is strongly dependent on individual performances of the absorption cycle and the TE module. The mathematical analysis of performance of the integrated cooling system was evaluated under a range of operating conditions for the generator, absorber, condenser and evaporator. The generator and absorber temperature was varied from 90 to 100 C and from 25 to 47 C respectively, resulting in a maximum temperature difference across the TE module of 75 C. The control input parameters and data used in the analysis are presented in Table 1. (11) (13)

58 R. Boukhanouf and A. Supasuteekul Table 1. Design parameters Absorption system: Cooling capacity, Q E (W) 200 T C ( C) 40 T A ( C) 35 T E ( C) 5 Solution heat exchanger effectiveness, e 0.6 Thermoelectric specifications: Manufacturer Melcor Corporation Model HT8-12-40 Dimensions (mm) 40 40 3.3 Number of thermocouples 127 Area/Length of TE element, A/L (cm) 0.171 I max *(A) 8.3 Q max *(W) 88 V max *(V) 20.5 T max *( C) 89 * Specifications for T H = 125 C, Q max rated value at T = 0 C 4. Simulation results and discussion The steady state operating temperature range in the generator that needs to be maintained by the TE was chosen to span the practical range for the single-effect absorption application and so avoid problems of crystallisation of LiBr. Fig. 3 shows the performance of the absorption cycle for a generator temperature ranging from 90 to 100 C and condenser temperature, T C, and evaporator temperature, T E, maintained at 40 C and 5 C, respectively. It can be seen that its coefficient of performance (COP ABS ) increases with an increase in generator temperature while decreases markedly at a high absorber temperature. At lower absorber temperatures, the effect of the generator temperature on the performance of the cycle is however reduced and the modelling results show that a maximum COP ABS of about 0.82 could be achieved. Fig. 4 shows the performance of the TEC module for absorber (heat source) temperatures ranging from 25 to 50 C. Unlike the absorption cycle, it can be seen that the coefficient of performance of the TE module, COP TEC, increases linearly with the increasing absorber temperature. However the performance of the TE module is reduced with the increasing generator (heat sink) temperature as the amount of obtained cooling load is proportionally affected by temperature gradient between the absorber and generator, as described by Equation (8). It can be seen that over the range of the operating absorber temperatures, COP TEC increases from 0.18 for T G = 100 C to 0.48 for T G = 90 C. A 3D plot of the overall performance of the integrated TE-absorption cooling system is shown in Fig. 5. It can be seen that under operating temperatures of the generator and absorber ranging from 90 to 100 C and from 25 to 50 C respectively, the coefficient of performance of the integrated TE-Absorption system, COP NET, reaches an optimum value. For a specific operating condition, the maximum COP NET

Theoretical analysis of an integrated thermoelectric-absorption cooling system 59 0.85 0.8 COP ABS 0.75 0.7 0.65 0.6 0.55 100 C 98 C 96 C 94 C 92 C 0.5 0.45 90 C 0.4 25 27 29 31 33 35 37 39 41 43 45 47 Absorber Temperature ( C) Figure 3. COP of single stage absorption cycle. COP TEC 0.6 0.5 0.4 0.3 100 98 96 94 92 90 0.2 0.1 0 25 27 29 31 33 35 37 39 41 43 45 47 Absorber Temperature ( C) Figure 4. COP of TE cooling system. value can be obtained from Equation (13). For example, for a constant generator temperature of 90 C and absorber temperature of 35 C, a maximum COP NET of 1.06 was obtained. The performance of the integrated system is also influenced by the number of the TE modules employed, as illustrated in Fig. 6. It can be seen that for a given operating temperature in the condenser, evaporator and absorber, increasing

Figure 5. Overall performance of integrated TE/absorption cooling system. 1.1 1.08 1.06 1.04 COP NET 1.02 1 0.98 0.96 N = 3 N = 2 N = 4 0.94 0.92 0.9 Figure 6. 90 92 94 96 98 100 Generator Temperature ( C) Effect of number of TE modules on COP NET (T E = 5 C, T C = 40 C, and T A = 35 C).

Theoretical analysis of an integrated thermoelectric-absorption cooling system 61 35 32.5 Heat Recovery (%) 30 27.5 25 22.5 20 17.5 N = 3 N = 2 N = 4 15 Figure 7. 90 92 94 96 98 100 Generator Temperature ( C) Effect of number of TE modules on heat recovered from the absorber (T E = 5 C, T C = 40 C, and T A = 35 C). the number of TEC modules has a marginal improvement on the system s overall performance and COP NET has a linearly decreasing trend as the generator temperature increases. This is due to increased heat generation by the Joule s effect in TEC modules that needs to be conducted to the hot side (generator) and offsets the benefit of generating the additional cooling effect as described in Equations (8) to (10). Likewise, Fig. 7 demonstrates that increasing the number of TEC modules does not necessarily improve substantially the amount of heat that can be recovered from the absorber and supplied into the generator. It can be seen that under favourable operating conditions, increasing the number of TEC modules from 2 to 4 has increased COP NET by only 5% while the amount of heat rejected in the absorber that can be recovered is limited to about 30%. Finally the performance of the integrated system was analysed to determine the effect of the evaporator and condenser temperatures. Figures 8 and 9 show a similar trend in that for a constant condenser and evaporator temperature, the overall performance of the system, COP NET, decreases as the generator temperature increases. Also for a given generator temperature, COP NET increases with increasing evaporator temperature and decreasing condenser temperature. From the discussed computer modelling results, the optimum design conditions of the system were taken to occur for an absorber temperature of 35 C, a generator temperature of 90 C and using 3 TE modules (N = 3). A full listing of the computer modelling results of the absorption cycle, TEC modules and the integrated TEabsorption system is given in Table 2.

62 R. Boukhanouf and A. Supasuteekul 1.2 1 T e = 10 C T e = 5 C 0.8 T e = 10 C T e = 5 C COP 0.6 0.4 0.2 T e = 10 C T e = 5 C 0 90 92 94 96 98 100 Generator Temperature ( C) COPTEC COPabs COPnet Figure 8. Effect of generator and evaporator temperature on COP NET (T C = 40 C and T A = 35 C). 1.2 COP 1 0.8 0.6 T c = 30 C T c = 40 C T c = 30 C T c = 40 C 0.4 0.2 T c = 30 C T c = 40 C 0 90 92 94 96 98 100 Generator Temperature ( C) COPtec COPabs COPnet Figure 9. Effect of absorber and condenser temperature on COP NET (T E = 5 C and T A = 35 C).

Theoretical analysis of an integrated thermoelectric-absorption cooling system 63 Table 2. Operating conditions and modelling results of the TE/absorption system Absorption operating parameters Q E (W) Q A (W) Q C (W) Q G (W) 200 249.355 213.595 262.95 Point on the cycle h (kj/kg) m (kg/s) P (kpa) T ( C) X (%) 1 2669.68 8.53 10 5 7.33 90 0 2 166.683 8.53 10 5 7.33 40 0 3 166.683 8.53 10 5 0.863 40 0 4 2510.37 8.53 10 5 0.863 5 0 5 85.879 7.77 10 4 0.863 35 55.698 6 85.879 7.77 10 4 7.33 35 55.698 7 153.536 7.77 10 4 7.33 68 55.698 8 223.249 6.919 10 4 7.33 90 62.567 9 147.247 6.919 10 4 7.33 49.06 62.567 10 147.247 6.919 10 4 0.863 49.06 62.567 Thermoelectric operating parameters a(v/k) R (Ω) k (W/K) N ( ) P (W) I (A) V (V) Q L (W) 0.0529 1.789 0.665 3 187.168 15.443 12.12 74.91 Overall performance COP TEC COP ABS COP NET 0.4 0.761 1.065 5. Conclusion A theoretical analysis and a computer model were conducted to optimise the performance of an integrated TE module and a single stage absorption cycle. The effect on the performance of the integrated system of various operating conditions including generator, absorber, condenser, evaporator temperature as well as number of TE modules was studied. It was found that an improved overall efficiency can be achieved when integrating the TE and absorption cycle into a single cooling system. The modelling was carried out first to determine individual performances of TE modules and absorption cycle and then the integrated system. The modelling results showed that the optimum number of TE modules required to attain the design heat capacity for the generator was N = 3 and a maximum of 30% of the heat that would normally be rejected from the absorber to the environment could be recovered. It was also found that maximum overall COP NET can be achieved by operating the generator at the lower end of its operating temperature range with the absorber being maintained at temperatures lower that 40 C. The integrated thermoelectricabsorption system could achieve a COP higher than unity at an evaporator temperature of 5 C and produce a cooling capacity of 200 Watts.

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