COMBINED DESALINATION AND REFRIGERATION SYSTEMS DRIVEN BY LOW-GRADE HEAT

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1 Proceedings of IMECE8 8 ASME International Mechanical Engineering Congress and Exposition November -6, 8, Boston, Proceedings Massachusetts, of IMECE8 USA 8 ASME International Mechanical Engineering Congress and Exposition October 3-November 6, 8, Boston, Massachusetts, USA IMECE8-679 COMBINED DESALINATION AND REFRIGERATION SYSTEMS DRIVEN BY LOW-GRADE HEAT Yongqing Wang College of Mechanical Engineering, Jimei University, Xiamen, 36, P. R. China Tel: ; Fax: ; yongqing@jmu.edu.cn Noam Lior Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA , USA Tel: ; Fax: ; lior@seas.upenn.edu ABSTRACT There is often a need for both water desalination and cooling (refrigeration/air-conditioning). The cooling can be used to significantly raise system efficiency by compressor inlet cooling in a dual-purpose power-generation and desalination system using gas turbines, or simply to supply refrigeration or air conditioning beside fresh water. Motivated by the good synergetic potential of energy/exergy utilization through the combination of the LiBr-H O refrigeration unit, LiBr-H O heat pump, and low-temperature multi-effect evaporation desalter, two combined refrigeration and water systems, ARHP-MEE (Absorption Refrigeration Heat Pump and Multi-Effect Evaporation desalter) system and ARHP-AHP-MEE (Absorption Refrigeration Heat Pump + Absorption Heat Pump + Multi-Effect Evaporation desalter) system, driven by lowgrade heat were configured, modeled and analyzed in detail in the paper. Typically, driving steam with saturation pressure of MPa and correspondingly saturation temperature of is applicable to run the systems. The main results are: () the combined systems have good synergy, with an energy saving rate of 4% in a case study of ARHP-MEE; () the refrigeration-heat cogenerated ARHP subsystem is the main reason for the synergy, where the coefficient of performance is around.6 and exergy efficiency above 6% when driven by.5 MPa saturated steam; (3) at the cost of a more complex configuration, the ARHP-AHP-MEE system has the ability of varying its outputs in very wide range, offering good flexibility on design and operation; (4) the ARHP-MEE system is predicted to have good economics, and its outputs can be varied in a wide range but not independently because their ratio remains almost constant. A parametric analysis was also performed for the ARHP-MEE, further improving the understanding of the system performance. Keywords: Integrated refrigeration and desalination system; Combined cycles; Refrigeration; Absorption refrigeration/heat pump; Multi-effect evaporation water desalination NOMENCLATURE BPE Boiling point elevation [ ] COP Coefficient of performance E Exergy [kw] h Specific enthalpy [kj/kg] m Mass flow rate [kg/s] p Pressure [kpa] [MPa] Q Energy [kw] ESR Energy saving rate of combined system to separate single-product systems [%] RWR Refrigeration-water ratio [kj/kg] s Specific entropy [kj/kg K] T Temperature [ ] [K] W max Maximum work produced in an ideal mixing process [kw] X Mass concentration of LiBr solution [%] x v Mass fraction of vapor ε Exergy efficiency [%] ξ Dimensionless exergy loss [%] Subscripts A Absorber D Desalination G Generator in Input Copyright 8 by ASME

2 L Loss R Refrigeration SH Solution heat exchanger T Thermal W Water Base case, ambient,, States on the system flow sheet. INTRODUCTION Driven mainly by low-grade extraction and/or exhaust heat from power generation plants, thermal desalination units have been widely used as their bottoming system (sometimes called dual-purpose plants). This synergy between power and water production, well-understood from second-law (exergy) considerations, resulted in lower specific fuel demand and overall cost of the produced water and electricity []. Most of the power-water systems operating in the world are the combination of steam turbines with thermal desalination units, but in recent years, there has been an obvious interest in moving to gas turbines based systems [,3] because of the higher overall energy/exergy efficiency of these systems. One of the main disadvantages of gas turbines is that their generated power and efficiency decrease significantly with the increase of ambient temperature. For every ºC of increase in ambient temperature, the power output decreases by 6% to more than %, depending on the type of the gas turbine, and at the same time, the thermal efficiency decreases by.5% to more than 4% [4]. For a gas-turbine-based power-water system, both the power output and water production are consequently the lowest in the hot season when the need for them is the highest. Compressor intake-air cooling is an effective way for maintaining the performance of gas turbines in hot weather [4, 5]. Among the various methods, pre-cooling inlet air by exhaust-heat absorption refrigeration has several advantages: ability of keeping a nearly constant temperature of compressor intake air, consuming negligible power, and saving fuel by using low-grade heat [4, 5]. In a combined power and thermal desalination system, some of the power plant output low-grade heat is used to produce the fresh water. If part of it is used to produce refrigeration for inlet air cooling, the water production will decrease because of decreased amount of heat for driving the desalination plant. It is thus necessary to investigate ways of producing the refrigeration while minimizing the negative impact on water production for given heat source conditions, which is the main purpose of this paper. This study also points to efficient ways of producing fresh water and refrigeration simultaneously when both of them are needed and low-grade heat is available. Two desalination and refrigeration cogeneration systems driven by low-grade heat are configured and modeled, and the thermal performance analyzed in detail in this paper. LiBr-H O absorption refrigeration and heat pump systems are used to pursue good performance, and a low-temperature multi-effect evaporation (MEE) desalination unit is employed because of its advantages of low corrosion rate, power consumption and capital cost over the other commonly used thermal desalination system, the multi-stage flash desalination plant [6].. SYSTEM CONFIGURATIONS FOR THE ANALYSIS The fact that absorption refrigeration, absorption heat pump and thermal desalination are all run by thermal energy with overlapping operating temperature regions can be used in a synergistic way. Driven by low-grade heat, LiBr-H O absorption refrigeration unit produces refrigeration by evaporating refrigerant water at around 5, and releases waste heat to the ambient. Also driven by low-grade heat, LiBr-H O absorption heat pumps (AHP) absorbs heat from the ambient (air or water), and produce heat with temperatures between the driving heat source and the ambient. At the same time, the top brine temperature of MEE typical desalters is limited to 7 to reduce scaling and corrosion, and the driving steam top condensation temperature is thus about 7, just within the temperature range of AHP output. High-efficiency cogeneration system can therefore be configured by combining the three systems by cascade utilization of energy sources and sinks according to their temperature level, as proposed and analyzed below. An example of such cascading use for combined power and refrigeration system is shown in [7]. The proposed two combined refrigeration and water systems, ARHP-MEE (Absorption Refrigeration Heat Pump and Multi-Effect Evaporation desalter) and ARHP-AHP-MEE (Absorption Refrigeration Heat Pump + Absorption Heat Pump + Multi-Effect Evaporation desalter) system, are schematically shown in Figs. and, respectively. The ARHP-MEE system is composed of two subsystems: a single-effect LiBr-H O absorption refrigeration/heat pump (ARHP), and an MEE desalter. The driving steam () heats the LiBr-H O mixture in the generator G and boils off the water in it. This steam (9) generated in G is routed into the evaporator, ED, of the first effect of MEE, providing energy for seawater evaporation by releasing its sensible and latent heat. Its condensate () is subcooled by the ambient seawater, throttled and then introduced into the ARHP evaporator ER to produce refrigeration. The refrigerant vapor (3) from ER enters the absorber A, and the absorption heat is taken away by the cooling seawater (4). It is clear that the two subsystems are linked by ED, which is both the condenser of the ARHP and the evaporator of the MEE. In a typical absorption refrigeration unit, the condensation temperature (at ) is usually around 4 ºC, while in this paper, it is raised to above 6 ºC by regulating the operating parameters of the absorber A and the generator G, to produce the temperature required for MEE desalination. Producing refrigeration (in ER) and heat (in ED ) simultaneously, the ARHP unit works as both a refrigeration unit and a heat pump. Detailed description of the working process of MEE can be found in [8]. Copyright 8 by ASME

3 Motive steam 4 G 5 6 SH A ER Heating steam H H H n- Feed seawater ED ED ED n- ED n C SC V Cooling seawater F F n- F n Brine Cooling seawater Fresh water Seawater Saline water Steam Distillate LiBr-H O solution A Absorber C Condenser ED Evaporator for desalination ER Evaporator for refrigeration F Flashing box G Generator H Seawater preheater SC Subcooler SH Solution heat exchanger V Throttling valve Motive steam G SH SH V A A 3 ER Fig. Schematic diagram of the ARHP-MEE combined system. 7 Heating steam V SC 5 H H H n- Cooling seawater Feed seawater ED ED ED n- ED n C F Entrained steam F F n- F n Brine Cooling seawater Fresh water Seawater Saline water Steam Distillate LiBr-H O solution A Absorber C Condenser ED Evaporator for desalination ER Evaporator for refrigeration F Flashing box G Generator H Seawater preheater SC Subcooler SH Solution heat exchanger V Throttling valve Fig. Schematic diagram of the ARHP-AHP-MEE combined system The ARHP-AHP-MEE system is the integration of an ARHP, an AHP, and an MEE desalter. The ARHP and AHP use the common generator G and the common condenser ED. The flow process of ARHP and its interconnection with MEE are the same with that in the ARHP-MEE system. In the AHP subsystem, part of the vapor (5) produced in the last effect of MEE is entrained by the absorber A and the absorption heat is used to heat and vaporize part of the condensate (7) from ED. The vapor (6) formed in the generator together with that (8) from A serves as the heat source for the MEE. Obviously, the ARHP-AHP-MEE system is the coupling of the ARHP-MEE cogeneration system and the AHP-MEE water-only system. Similar configurations of AHP-MEE water-only systems have been studied by a few researchers [9-], and the results indicate a competitive thermal performance (the economics were not addressed). For instance, Mandani et al. [] performed a thermal analysis of a single-effect evaporation desalination process combined with a single-effect LiBr-H O AHP, and claimed performance ratios of.4-.8, 5%-7% higher than the single-effect thermal vapor compression (TVC) systems driven by the same heat source. Su et al. [] studied a water-production-only system composed of a double-effect LiBr-H O AHP and a 9-effect MEE, obtaining a performance ratio of 7.5, much higher than the.5 of a TVC-MEE 3 Copyright 8 by ASME

4 system. Our study shows that ARHP-AHP-MEE system, configured by combining the ARHP-MEE and AHP-MEE systems, has a much wider control range of the refrigeration and water outputs than the ARHP-MEE system, as discusses in more detail in Section 5 below. 3. CALCULATION CONDITIONS AND PERFORMANCE CRITERIA The main assumptions for the base-case calculation of the two systems are summarized in Table. After referring to the operating conditions of an existing MEE unit [3], a six-effect MEE was chosen and the performance simulated. The vapor produced in each effect of MEE and the generator was considered to be salt-free, and, in accordance with industrial practice, each evaporator of the MEE had the same heat transfer area [8]. The analyzed systems have two useful outputs: fresh water and refrigeration, and performance criteria definition is not straightforward because the products, fresh water and refrigeration, do not have the same physical units. Water is not energy, so the commonly defined energy efficiency is not suitable here. The exergy efficiency, ε, typically defined as Wmax + ER mw wmax + m[ h h3 T ( s s3 )] ε = = () E m [ h h T ( s s )] in where m W is the water production rate and E in are the exergy of produced refrigeration and of the heat input into the entire system, respectively, and W max is the maximal work that could be obtained by mixing the produced fresh water and the rejected concentrated seawater in an ideal way, which is also the minimum work consumed in an ideal separation process of the saline feedwater. Examining the meaning of such an ε, we note the exergy efficiency of a thermal desalination unit is very low, say, about 4% for a common MSF plant run by 99 saturated steam [5], while the exergy efficiency of a single-effect absorption refrigeration system is much higher, about 3% in a case study reported in [6]. This means that, about 3.3 kw of driving thermal exergy is needed to produce kw cold exergy by absorption refrigeration, while about 5 kw thermal exergy is needed to produce kw power capacity by thermal desalination. It is thus clear that the exergy efficiency defined in Eq. () unreasonably weights water production as a very trivial contribution, and cannot reflect the performance of the waterrefrigeration cogeneration systems suitably. Although the energy and exergy efficiencies are thus not applicable to the refrigeration-water combined system, they are applicable to the ARHP and ARHP-AHP subsystems, because the two are refrigeration and heat cogenerators. It is interesting to analyze the performance of ARHP and ARHP-AHP, which determine the performance of the whole system when the performance of the MEE unit is specified. The coefficient of performance and the exergy efficiency of the refrigeration-heat ARHP and ARHP-AHP subsystems are defined as Table Main assumptions for the base-case calculation Ambient conditions Temperature 3 Pressure atm Salinity of seawater 35, ppm Generator Pressure of motive steam (saturated), p.5 MPa Generator approach temperature, T -6, T -6 Mass concentration difference between strong and weak solutions, X 5% Absorber Absorber approach temperature in ARHP subsystem 3 Absorber approach temperature in AHP subsystem 5 Absorbed vapor pressure minus absorber operation pressure 4 Pa Solution heat exchanger Temperature difference at the cold side Minimum temperature difference between outlet strong solution and crystallization point 5 Evaporator for refrigeration Evaporation temperature of the refrigerant 6 MEE unit Number of effects 6 Salinity of the discharge brine 7, ppm Temperature rise of seawater in preheater 4 Condensation temperature of heating steam in the st effect, T 65 Temperature difference at the hot side of end condenser C 4 Operation temperature in the last effect 43 Mechanical work consumption per kg produced fresh water 7. kj [4] 4 Copyright 8 by ASME

5 QR + QT m ( h h3 ) + m9 ( h9 h ) COPRT = = Qin m ( h h ) ( ) ER ET ε RT = ε R + εt = + E E m[ h = h 3 in T ( s s3 )] + m9[ h9 h m [ h h T ( s s )] in T ( s 9 s where is the produced refrigeration, Q in is the thermal energy input to the system, Q T and E T are the thermal energy and thermal exergy provided for MEE, and ε R and ε T are the exergy efficiency of producing E R and E T, respectively. A dimensionless exergy loss parameter, ξ, is used to evaluate the process irreversibility of each component: EL ξ = (4) Ein where E L represents the process exergy loss. For system performance evaluation we define the Energy Saving Ratio of the water-refrigeration system, which is the ratio of the amount of the defined-property steam used to produce the same amount of water and refrigeration by using two separate single-product units, one of them a conventional )] (3) single-effect LiBr-H O refrigeration unit with seawater as cooling water that produces just refrigeration, and the other an AHP-MEE unit producing just fresh water, and the amount of steam used by the combined system: mr + md ESR = (5) m where m R and m D are the motive steam mass flows consumed by the single-purpose refrigeration and desalination systems, respectively, and m is the flow rate of the motive steam, with the same thermodynamic properties for m R and m D, used by the combined system producing the same amount of refrigeration and water. We also use the Refrigeration-Water Ratio (RWR), defined as QR RWR = [ kj/kg] (6) m W 4. PERFORMANCE ANALYSIS OF THE ARHP-MEE COMBINED SYSTEM 4. Base-case performance of the ARHP-MEE combined system Table The main parameters of the base-case of the ARHP-MEE system ARHP subsystem T ( ) p (kpa) m (kg/s) X(% LiBr) x v Motive steam Strong solution from generator G Strong solution from solution heat exchanger SH Weak solution from absorber A Weak solution from solution heat exchanger SH Steam produced in generator G Refrigerant before throttling valve V Refrigerant entering evaporator ER MEE subsystem Effect number Feed seawater T ( ) m (kg/s) Brine T ( ) m (kg/s) BPE ( ) Produced vapor T ( ) p (kpa) m (kg/s) Condensate T ( ) m (kg/s) System production Produced refrigeration, Produced fresh water, m W Refrigeration-water ratio, RWR 65 kw 4.6 kg/s 4.4 kj/kg 5 Copyright 8 by ASME

6 The simulation was carried out using the Engineering Equation Solver (EES) software [7]. The properties of LiBr- H O solution were taken from [8]; the properties of seawater and brine, the boiling point elevation of brine, as well as the non-equilibrium allowance of flashing evaporation in the flashing box were taken from [8] and []. The computerized models were validated by () checking the relative errors of mass and energy balance of each component and the entire system where they were found to be < -8, () making sure that the calculation results satisfy the system exergetic equation, that is, the sum of the exergy output of the system and that lost or destructed in the system equals to that input into the system, which is not included in the equation systems solved by EES, and (3) comparing the simulation results of the absorption refrigeration unit and the MEE unit separately with those in [9] and [] under the same conditions where they show good agreement (for example, the relative error of the coefficient of performance of the refrigeration unit is around %, and that of the performance ratio of MEE, defined as the mass ratio of the produced water and the heating steam for MEE, is within 3%). The calculations are performed for a flow of kg/s motive steam for the combined system. The main parameters of the base case of ARHP-MEE are shown in Table. The performance comparison between the refrigeration-water combined system and the separate single-product systems are shown in Table 3. The energy saving of the combined system (compared with the single-product systems) is significant, about 4% for the base case. It is the ARHP subsystem that contributes to this substantial improvement. Tables 4 and 5 show the energy and exergy utilization of the ARHP. The output cold and thermal energy account for 75.7% and 78% of the input thermal energy for ARHP, respectively, resulting in a total COP RT of.54, which is considerably higher than the COP of.77 of the refrigeration-only unit running under the same conditions. The refrigeration exergy and the thermal exergy produced are 6.8% and 33.9% of the total input exergy, leading to a total exergy efficiency of 6.7%, much higher than of the 7.% exergy efficiency of the refrigeration-only unit. So, raising the condensation temperature of the generator produced vapor at 65 ºC in this case (higher than the 4 ºC in a conventional Table 3 Comparison between the combined system and the separate refrigeration and water systems ARHP-MEE combined system AHP-MEE water-only system Operation pressure of generator, kpa Operation pressure of absorber, kpa Condensation temperature of generator-produced steam, Temperature of absorber outlet weak solution, Mass concentration of weak solution, % LiBr Mass concentration of strong solution, % LiBr Cooling capacity, kw Produced fresh water, kg/s Mass flow of motive steam, kg/s Total mass flow of motive steam, kg/s.4 Absorption refrigeration system Table 4 Energy utilization of the ARHP subsystem for the base-case Components Generator G 8 Absorber A 45 Condenser ED 7 Evaporator ER 65 Solution heat exchanger SH 56 Subcooler SC 87.7 Heat load (kw) Unit: kw Percentage Thermal energy input to ARHP, Q in 8 Cold energy produced, Thermal energy output for MEE, Q T COP RT.54 Table 5 Exergy utilization of the ARHP subsystem for the base-case Components or streams Exergy loss (kw) Generator G Absorber A Solution heat exchanger SH Subcooler SC 5.3. Cooling seawater Others Dimensionless exergy loss (%) Unit: kw Percentage Thermal exergy input to ARHP, E in 53.5 Exergy of produced refrigeration Thermal exergy output for MEE, E T Exergy efficiency, ε RT 6.7 % 6 Copyright 8 by ASME

7 refrigeration-only unit), a temperature high enough to makes the condensation heat suitable for desalination, leading to an additional gain of 33.9% of thermal exergy or 78% of thermal energy, at the cost of only.4% decrease of produced cold exergy or.3% decrease of cold energy. 4. Parametric analysis of the ARHP-MEE combined system Under the specified ambient conditions, the main factors influencing the performance of the ARHP-MEE system are: generator approach temperature T -6, LiBr-H O strong-andweak solution concentration difference X, motive steam pressure p, and the heating steam condensation temperature T in ED (or the generator operation pressure). The performance of the MEE unit certainly has great influence on the whole system, with the discussions not included in this paper. Detailed information on MEE unit can be found in many publications (cf.[8, ]). 4.. Influence of the generator approach temperature T -6 Figure 3 shows the influence of T -6, with the other conditions kept constant at the base-case values shown in Table. To exhibit more clearly the sensitivity of water and refrigeration production in the combined system, m W, and E R are normalized by their base-case values shown in Table. / /E R, m W /m W...99 / / E R m W / m W RWR T -6 ( o C) Fig. 3 Effect of the generator approach temperature T -6 The m W and (E R ) of ARHP-MEE increase with T -6 (Fig. 3), and reach the highest value under the maximum T -6 allowed. Increasing T -6 from 5 to.5 produces about % more refrigeration and water. The increase of T -6, causes the operation temperature of the generator and then that of the absorber to decrease. The temperature of the cooling medium used in the absorber determines the lowest absorber outlet temperature, and then the maximum T -6. We thus draw a conclusion that improvement of thermal performance can be achieved by raising T -6 as highly as allowed by the cooling medium used in the absorber. RWR (kj / kg) Figure 3 also reveals that water and refrigeration production in the ARHP-MEE system have the same trend with the variation of T -6, and that the refrigeration-water ratio RWR remains almost constant. The reason is that it is the same stream of working fluid, i.e. the vapor produced in the generator, that produces both the desalination heat (in ED ) and the refrigeration (in ER), so when the mass flow of the vapor, m 9, increases with T -6, both (or E R ) and Q T (or E T ) increase at almost the same rate with m 9, resulting in an almost constant RWR. 4.. Influence of LiBr-H O strong-and-weak solution concentration difference X Figure 4 shows the influence of the concentration difference, X, between the strong and weak LiBr-H O solutions. The lines 3, 4 and 5 are for T -6 =, and lines, and 6 for the maximum T -6 allowed as discussed above. It is revealed that increasing X leads to distinct improvements of water and refrigeration production. When X is increased from 3% to 6%, the two outputs both increase by over 6% for T -6 =. The reason is the same as in a conventional absorption refrigeration system [9]. The increase of X is limited by the point at which crystallization of the strong solution at the SH outlet commences. Fig. 4 also shows that RWR depends only slightly on X, and has almost the same value as that shown in Fig. 3, for the same reasons given in Section 4... / / E R, m W / m W : / / E R ( T -6 = T -6, max ) : m W / m W ( T -6 = T -6, max ) 3: / / E R ( T -6 = o C) 4: m W / m W ( T -6 = o C) 3 4 5: RWR ( T -6 = o C) 6: RWR ( T -6 = T -6, max ) X (%) Fig. 4 Effect of the strong-and-weak solution concentration difference X 4..3 Influence of the condensation temperature T of the heating steam in ED Since ED is both the condenser of ARHP and the evaporator of MEE, i.e. the interface between the refrigeration and water production subsystems, the condensation temperature, T, of the heating steam in ED has a great RWR (kj / kg) 7 Copyright 8 by ASME

8 influence on the performance of each of these two subsystems and thus on the performance of the ARHP-MEE combined system. Figure 5 shows the refrigeration and heat production, and Fig. 6 shows the exergy utilization, of the ARHP unit, for different T. With the increase of T, the outputs and E R as well as Q T drop slightly, while the thermal exergy E T for MEE rises significantly. The strong increase of E T is mainly contributed to the decreased exergy loss in the generator (Fig. 6) where the heat-transfer temperature difference has a distinct decrease with increasing T (Fig. 7). Higher T broadens the operation temperate range of the MEE unit, implying the possibility of running an MEE with more effects than the six chosen for this study. More effects lead to a much higher performance ratio [8]. It is thus clear that for the specified motive heat source, raising T would cause a minor decrease of refrigeration production but a great potential for producing more fresh water. For instance, increasing T from 65 to 68.4 would decrease the cooling capacity by.%, but increase the water production by 5% when the number of effects of the MEE is changed from 6 to 7, without almost any change of the specific heat-transfer area (per kg/s produced fresh water) of the MEE for the two situations Q / Q, E / E....9 T -6 = T -6, max X = 6% E T / E T / E R / E R Q T / Q T T ( o C) Fig. 5 Effect of the condensation temperature of the MEE heating steam, Τ 5 ε, ξ (%) T = 57 T = 6 T = 65 ε R ε T ξ G ξ A ξ SH ξ others Fig. 6 Exergy utilization of ARHP subsystem for different Τ T ( o C) Motive heat source Solution (T = 65 o C) Solution (T = 57 o C) X = 6%, T -6 = T -6, max Q / Q G (%) Fig. 7 The generator T-Q diagram for different T 4..4 Influence of the motive steam pressure p The motive steam is assumed to be saturated, and Figure 8 shows the influence of its pressure p. Increasing p was found to reduce both water and refrigeration production. Typically the refrigeration capacity of real absorption refrigeration units increases with p, primarily because the motive steam mass flow m increases with p too. For instance, increasing p from.4 MPa to.6 MPa causes an increase of m from 575 kg/h to 795 kg/h in one example [9] that also gives further explanation. Different from real units, m is kept constant at kg/s in our analysis. From thermodynamics, based on kg/s motive steam, the thermal energy input to the ARHP-MEE system decreases when p is increased because of the decreased condensation latent heat of the motive steam, while the input thermal exergy increases, with the increase of p. The exergy loss in the generator has a significant rise with p (Fig. 9) because of the consequent enlarged heat-transfer temperature difference in the generator (Fig. ), resulting in decreased E R and E T, and a decreased E T leads to a decreased m W (Figs. 8). The above discussions on p are performed based on a constant T. A higher p can allow the raising of the heating steam (9) pressure and correspondingly a higher T. This, in turn, can allow a higher m W by adding effects to the MEE as discussed in Section For instance, with the maximum T -6 allowed and the other conditions kept constant at the basecase values (Table ), saturated motive steam of.5 MPa can produce heating steam with T = 58 C at the highest, which is suitable to run a four-effect MEE unit, while motive steam of.5 MPa has the ability to produce heating steam with T =7.9, suitable to run a seven-effect MEE unit. Different calculation conditions lead to different T. Generally, for the typical range of T from 58 to 7, the ARHP-MEE system proposed in this paper, which is based on a single-effect absorption refrigeration/heat pump, is applicable to be run by motive steam with p from.5 MPa to.35 MPa (saturation temperature T from.4 to 38.9 ). Further study is 8 Copyright 8 by ASME

9 needed to find more favorable ways of using higher pressure steam / / E R, m W / m W / E R / E R m W / m W T -6 = T -6, max X = 6% p (MPa) ε, ξ (%) Fig. 8 Effect of the motive steam pressure p Fig. 9 Exergy utilization of the ARHP unit for different motive steam pressures p 5 p =.5 MPa p =.3 MPa p =.35 MPa ε R ε T ξ G ξ A ξ SH ξ others 5. PERFORMANCE OF THE ARHP-AHP-MEE COMBINED SYSTEM AND DISCUSSIONS From the discussions in Section 4, we can draw a conclusion that, for a specified ARHP-MEE plant (T and the MEE number of effects are fixed), water and refrigeration production can be regulated in a wide range by changing the operating parameters, but the ratio between the two, RWR, keeps almost constant, which means that it is almost impossible to regulate RWR, indicating a relatively narrow working condition of a real ARHP-MEE plant. To meet the requirement of the situations where RWR needs to change, we proposed the ARHP-AHP-MEE system. We focus only on the main performance characteristics of the ARHP-AHP-MEE system, and do not perform its parametric analysis. Compared with ARHP-MEE, the main advantage of the ARHP-AHP-MEE system is that the water and refrigeration production as well as the refrigeration-water ratio can be changed in a wide range, as shown in Fig. which illustrates the variation of m W,, RWR and ESR under base-case calculation conditions. When all the motive steam is used to run the AHP unit, the ARHP-AHP-MEE system works as a wateronly AHP-MEE system, and 9.4 kg/s of fresh water can be produced with RWR and ESR both having the value of zero (Point A in Fig. ). When all the motive steam is used to run the ARHP unit, the ARHP-AHP-MEE works as an ARHP-MEE system, and 4.6 kg/s of fresh water and 65 kw of refrigeration can be obtained, with RWR = 4 kj/kg and ESR = 4% (intersection points between Line B-B and the lines showing ESR, and RWR in Fig. ). It is clear that, the ARHP-AHP-MEE system should have a performance that is between ARHP-MEE and AHP-MEE. Theoretically, RWR can be changed from to 4 kj/kg. By regulating the mass flow of the refrigerant entering ER (m in Fig. ) which determines the proportion of the motive steam allocated to ARHP and AHP in ARHP-AHP-MEE, m W and can be varied in a very wide range, indicating greater flexibility for design and operation. 4 Motive heat source ( p =.35 MPa) 8 B 4 T ( o C) 3 Motive heat source ( p =.5 MPa) Solution (p =.35 MPa, p =.5 MPa) T -6 = T -6, max X = 6 % Q / Q G (%) Fig. The generator T-Q diagram for different motive steam pressures p (kw), RWR (kj / kg) B ESR RWR A m W (kg / s) Fig. m W,, RWR and ESR of the ARHP-AHP-MEE combined system ESR (%) 9 Copyright 8 by ASME

10 Comparing the two combined systems, the ARHP-AHP- MEE is more favorable for higher m W and lower RWR, and has wider products variation range, but its configuration is more complex; the ARHP-MEE is simpler, more efficient, but with a narrow products variation range. While a detailed economic analysis was not performed, some basic important observations can be made. The two combined systems, especially ARHP-MEE, have much fewer components than those needed for the sum of the separate single-purpose systems. For instance, the ARHP subsystem works as a refrigeration unit and a heat pump, and all the components, including the generator, absorber, solution heat exchanger, evaporator, condenser, pumps and valves, are common parts for both refrigeration and heat production and thus do not need to be duplicated as in the separate systems. Obviously, it is not just the number of the components but also their size that determines the capital cost of the ARHP, indicating the necessity for a detailed study of the heat-transfer processes in the components. Another economic advantage is that the higher energy utilization rate of the ARHP-MEE system means that the energy/operating cost is lower too. 6. CONCLUSIONS There is often a need for both water desalination and refrigeration. The latter can be used to significantly raise system efficiency by compressor inlet cooling in a dual-purpose powergeneration and desalination system using gas turbines, or simply to supply refrigeration or air conditioning beside fresh water. Motivated by the good synergetic potential of energy/exergy utilization through the combination of the LiBr-H O refrigeration unit, LiBr-H O heat pump, and low-temperature MEE, two combined refrigeration and water systems, the ARHP-MEE system and ARHP-AHP-MEE system, driven by low-grade heat were configured, modeled and analyzed in detail in this paper. Good synergy, reflected by energy saving, is accomplished by combining an absorption refrigeration/heat pump with lowtemperature thermal desalination. In a case study of the ARHP- MEE system, the Energy Saving Rate is 4%, compared with the individual refrigeration-only and water-only systems. Driven by.5 MPa saturated steam, the coefficient of performance of the ARHP is around.6 and the exergy efficiency above 6%. A parametric sensitivity analysis of the ARHP-MEE system shows that a higher generator approach temperature and a higher concentration difference between the strong and weak solution raise water and refrigeration production simultaneously. Using driving steam of higher pressure allows increasing the number of effects of the MEE unit and thus produces more water. Comparing the two combined refrigeration-water systems, the ARHP-AHP-MEE is more suitable for higher water production rate and a lower refrigeration-water ratio, and has a much wider products variation range, indicating greater flexibility of design and operation, while the configuration is more complex. The ARHP-MEE is simpler, more efficient and economic, but with a relatively narrow products variation range. 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