Introduction
A large portion of energy input inindustry is lost as waste heat in forms of exhausted gas, coolingwater, hot products, and etc. The percentage of waste heat to totalenergy input varies between 15% and 50% in different countries [1,2]. This causes both energy waste and extra CO2 emission. Waste heat reuse is an effective solution which can bringboth environmental and economic benefits. That is why waste heat reuseattracts researchers’ attention in recent decades [3–7]. Typical options for waste heat reuse include economizer[2,4], compression heat pump [8], absorption heat pump [2,8], power generation [3], etc. Absorption heat pump is one of the best choices due to itslow driven temperature and low electricity consumption.
The absorption heat pump has differentconfigurations for different aims. Usually, the term “absorptionheat pump” not only refers to the general concept of absorptionheat pump, but also refers specifically to the absorption heat pumpfor heat amplification while the term “absorption heat transformer”refers to the absorption heat pump for temperature upgrading. In theabsorption heat pump as shown in Fig. 1 (a), high temperature heatis input to the generator, and medium temperature heat is output fromthe condenser and the absorber. The medium temperature heat outputhas a larger power than the high temperature heat input, and the heatpower is amplified. In the absorption heat transformer as shown inFig. 1 (b), medium temperature heat is input to the generator andthe evaporator, and high temperature heat is output from the absorber.The temperature of heat input is upgraded.
Except for the basic single stagecycle and commonly used working pairs of water-LiBr and ammonia-water,advanced absorption heat pump cycles [9–10] and various working pairs [11] are proposed to fulfill different demands. These absorption heatpumps are widely used for different applications such as districtheating, water desalination, drying. To get a clear opinion aboutthe future development of waste heat reuse with the absorption heatpump, it is necessary to review the current states of this technology.In this paper, three most important aspects of absorption heat pump,i.e., the absorption cycles, the working pairs, and the applicationsare emphasized on.
Absorption heat pump cycles
The absorption heat pump can achieveheat amplification or temperature upgrading with type-I and type-IIconfiguration respectively. Type-I and type-II absorption heat pumpsare usually referred to as absorption heat pump and absorption heattransformer in literature. Except for the basic single effect or singlestage cycle, different advanced cycles for both absorption heat pumpand absorption heat transformer are proposed for higher efficiencyor larger temperature lift.
Absorption heat pump for heat amplification
The basic configuration of the absorptionheat pump for heat amplification is the single effect absorption heatpump. It is usually used for residential heating [12–14], hot water production [15] and drying [16]. As is illustrated in Fig. 2, the single effect absorption heatpump has components such as the generator, the absorber, the condenser,the evaporator, the solution heat exchanger, the solution pump, andthrottling valves. The high temperature heat from the heat sourceis input to the generator (G), the low temperature heat is absorbedby the evaporator (E), and the medium temperature heat output is deliveredfrom both the condenser (C) and the absorber (A). The high temperatureheat source could be hot water, exhaust gas, or steam. Solution flowsbetween the generator and the absorber with the aid of the pump andthe valve, and sensible heat recovery of solution is done in the solutionheat exchanger. If the boiling temperatures of the refrigerant andthe absorbent are close (such as ammonia-water solution), a rectifieris needed to purify the vapor generated from the generator. Sinceheat power is amplified, the coefficients of performance (COP), definedas the heat output divided by the heat input is always larger than1.
The efficiency of the single effectabsorption heat pump is decided by both the working temperature andthe working pair. Abrahamsson et al. [16] theoretically studied the performance of single effectabsorption heat pump for paper drying at generation temperature, condensationtemperature, absorption temperature, and evaporation temperature of139°C, 80°C, 80°C and 50°C respectively. COPs of 1.71,1.80 and 1.66 were theoretically obtained with working pairs of NaOH-water,LiBr-water, and LiBr-CH3OH respectively. Inreal operation, the COP will be lower than the calculated value dueto internal irreversibility. Engler et al. [17] theoretically studied the performanceof a single effect water-ammonia absorption heat pump at evaporationtemperatures of −40°C–18.3°C and an output temperatureof 42.2°C. COPs of 1.23–1.62 were obtained for the singleeffect absorption heat pump without internal heat recovery. When arefrigerant pre-cooler was used, the COP was increased to 1.25–1.66.When a solution heat exchanger was also used, COPs of 1.3–1.9were obtained. Jian et al. [18] theoretically studied the performance of a single effect absorptionheat pump with LiBr-water and LiBr-LiNO3-water.COPs of 1.68–1.73 were obtained at generator inlet temperaturesof 145°C–167°C and evaporator outlet temperatures of20°C–30°C. The system with LiBr-LiNO3-water was found to have less crystallization risk. Qu et al. [19] studied a heat recovery absorptionheat pump for the efficiency improvement of a natural gas boiler.The experiment showed that the absorption heat pump was able to heatthe water from 45°C to 57.5°C by using the hot water of 98°Cas the heat source. A COP of 1.4 was achieved. A simulation studyof exhaust type, hot water, and direct fired absorption heat pumpswas conducted. Under the given condition, the three types of absorptionheat pumps obtained COPs of 1.73, 1.60 and 1.75 and simple paybackyears of 1.7, 3.1 and 3.6 respectively.
Advanced absorption heat pumps
The configurations of the absorptionheat pumps are the same as those of the absorption refrigeration cycles,except that the desired outputs are different. Theoretically, allabsorption refrigeration cycles can also work as absorption heat pumpcycles. The absorption cycles with higher temperature lifts than thesingle effect absorption cycle include the double stage absorptioncycle, the 3/4 effect absorption cycle, the single effect/double liftabsorption cycle, and so on [20,21]. The absorptioncycles with a higher COP than the single effect absorption cycle includethe double effect absorption cycle, the GAX (Generator Absorber heateXchange) absorption cycle, the vapor exchange GAX absorption cycle,the triple effect absorption cycle, the quadruple effect absorptioncycle, and even the seven effect absorption cycle [20,21]. However, only some of the cycles are studied forthe heat amplification purpose. Other cycles are either complicatedor only suitable for cooling purpose. In this case, only the practicalchoices for heat amplification will be introduced in detail.
Double effect absorption heat pump
The double effect absorption heatpump has a higher COP than the single effect absorption heat pump.It is used for high heat source temperature or low output temperature.The schematic of the double effect absorption heat pump with seriesflow configuration is given in Fig. 3. The double effect absorptionheat pump consists of a high pressure generator (HPG), a high pressurecondenser (HPC), a low pressure generator (LPG), a low pressure condenser(LPC), an absorber (A), and an evaporator (E). The condensation heatfrom the HPC is used to heat the LPG, and extra refrigerant vaporcan be obtained without external heat input. This increases both theCOP and the driven temperature. Except for the series configurationshown in Fig. 3, the parallel flow configuration and the reverse flowconfiguration can be adopted. Alarcón-Padilla et al. [22] experimentally studied a doubleeffect LiBr-water absorption heat pump designed for multi-effect distillation.The absorption heat pump was designed to work at heat source temperaturesof 180°C, 60°C, and 35°C. COPs of 2.14–2.2 at loadratios of 30%–100% were obtained. Alarcón-Padilla etal. [23] also connectedthe double effect LiBr-water absorption heat pump with the multi-effectdistillation system later. The average efficiency of the absorptionheat pump was found to be only about 1.67 due to unstable operation.
Double stage absorption heat pump
The double stage absorption heatpump has a higher temperature lift than the single effect absorptionheat pump. It is used for low heat source temperature or high outputtemperature. The schematic of the double effect absorption heat pumpis presented in Fig. 4. The double stage absorption heat pump hasa high pressure generator (HPG), a high pressure absorber (HPA), alow pressure generator (LPG), a low pressure absorber (LPA), a condenser(C), and an evaporator (E). Since two solution cycles are includedin the double stage absorption heat pump, the pressure differencebetween the condenser and the evaporator is larger than that of thesingle effect absorption heat pump. A larger temperature lift is achieved.However, the efficiency will be lower because both the HPG and theLPG need heat input.
Researches about double stage absorptionheat pump are not very popular. Compared with the single effect absorptionheat pump, the double stage absorption heat pump needs the extra generator,the absorber, the solution pump, and the throttling valve, but onlyabout 35% extra heat can be obtained which is not very cost competitive.However, it is still an option for certain circumstances. Le Lostecet al. [24] studied adouble stage LiBr-water absorption heat pump to achieve a high outputtemperature in wood chip drying. Under the given condition, the singleeffect absorption heat pump was only interesting when the requiredtemperature of drying air was below 60°C. The double stage absorptionheat pump was necessary for a higher drying air temperature. Witha generator temperature of 152.9°C and an evaporator temperatureof 27.9°C, the double stage absorption heat pump was able to deliverheat output with temperatures of 127.4°C, 107.9°C and 82.2°Cfrom the condenser, the high pressure absorber, and the low pressureabsorber, respectively. A theoretical COP of 1.34 was achieved.
GAX absorption heat pump
In an ammonia-water absorption system,there is no crystallization risk and the workable concentration rageof ammonia-water solution is large. The large concentration glidein the generator and the absorber ensures the possibility of GAX heatrecovery. Figure 5 is the schematic of a GAX absorption heat pump.There are two parts in the generator: one part of the generator (G)is boiled by the external heat input, the other part of the generator(G) is boiled by the absorption heat released by the absorber (A).Since only part of the generation needs external heat input, the efficiencyof the GAX absorption heat pump is higher than that of the singleeffect absorption heat pump. Except for the GAX heat recovery, therectifier (R), the refrigerant pre-cooler, and the solution preheatingin the absorber (A) can also be used to increase the efficiency.
The GAX absorption heat pump is agood choice for space heating due to its high efficiency. Since water-ammoniais used in the GAX absorption heat pump, sub-zero condition and highoutput temperature can be considered without frozen and crystallizationrisks. Engler et al. [17] theoretically studied the ammonia-water GAX absorption heat pump.COPs of 1.6–2.2 were obtained at evaporation temperatures of−6.7°C–18.3°C and an output temperature of 42.2°C.Garimella et al. [25]studied the performance of the GAX absorption heat pump using a modularsteady-state simulation program. A COP of 1.40 was obtained with anambient temperature of −5.6°C. This implied that the systemwas able to capture a large amount of heat from outside air even ata low ambient temperature. When the ambient temperature was increasedto 16.7°C, the COP was increased to about 1.53. When the ambienttemperature was further decreased, the GAX heat recovery was eliminated,and the system worked as a single effect absorption heat pump. Atan ambient temperature of −30°C, a COP of 1.0 could be maintainedby single effect mode. Kang and Kashiwagi [26] developed an ammonia GAX absorptionheat pump cycle for panel heating which was named PGAX cycle. Typically,a coolant temperature of 45°C was high enough for spacing heating.However, a coolant temperature of 65°C was required for panelheating. Heat pump COPs of 1.6–1.8 were obtained for the studiedparameter configurations. Kang et al. [27] also developed four heat pump cycles combining bothvapor absorption and vapor compression for performance enhancement.A maximum hot water temperature of 106°C was achieved. Phillips[28] studied differentgas-fired absorption heat pump cycles for residential and small-commercialapplications. Target heating COP was 1.78 at an ambient temperatureof 8.3°C. The ammonia-water GAX absorption cycle was found tohave a better performance.
Open-cycle absorption heat pump
The absorption heat pump cycles mentionedabove are all closed loop cycle. The solution and refrigerant do notcontact with the ambient. Different from these absorption heat pumps,the open absorption heat pump is able to capture the thermal energyin the moist gas through direct contact and the absorption process.Figure 6 shows the schematic of an open-cycle heat pump. Moist airenters the absorber (A). Concentrated solution in the absorber capturesthe moisture, becomes diluted and releases heat output. The dilutedsolution is pumped into the generator (G) and boiled by the externalheat source. The vapor from the generator condenses in the condenser(C) and releases heat output.
Compared to the single effect absorptionheat pump, the open-cycle absorption heat pump has a simpler configuration.Besides, the direct contact between solution and moist gas is alsomore efficient than the heat exchange through heat exchanger. Lazzarinet al. [29] proposed anopen-cycle absorption heat pump for building heating. An internaltemperature of 20°C and a relative humidity of 50% were considered.The system was simulated for a heating output of 12 kW and an airflow rate of 0.3 kg/s. The primary energy ratio defined as the ratiobetween the output and chemical energy input from natural gas wasfound to be higher than 1.55. Westerlund et al. [30] studied an open-cycle absorptionheat pump coupled with a biomass boiler. The waste heat of the fluegas from the boiler was recovered. Besides, the particles in the fluegas were captured by the solution in the heat pump, and therefore,less particles were released to the environment. Particle reductionsof 33%–44% and a heat production increment of 40% were achieved.Ye et al. [31] proposedan open-cycle absorption heat pump with both two stage mode and singlestage mode. Driving temperature ranges of the two stage mode and singlestage mode were 130°C–160°C and 160°C–175°Crespectively. When the heat source temperature varied from 135°Cto 175°C, the system was able to produce saturated steam of 100°Cwith COPs of 1.52–1.97.
Absorption heat transformer for temperature upgrading
An absorption heat transformer isused to increase the temperature of heat input. It can be used forseawater distillation and different industrial applications. The basicsingle effect stage absorption heat transformer is depicted in Fig.7. The system includes an absorber (A), a generator (G), an evaporator(R),a condenser (C), a solution pump, a solution heat exchanger, a refrigerantpump, and a throttling valve. The generator and the evaporator requireexternal heat input with a medium temperature. The condenser releasesheat to the low temperature ambient. The absorber delivers high temperatureheat output. Since heat input is required for both evaporation andgeneration, the COP of an absorption heat transformer is always lessthan 1.
Various simulation and experimentalresearches were conducted for the single effect absorption heat transformer.Romero and Rodríguez-Martínez [32] theoretically studied the performanceof a single stage LiBr-water absorption heat transformer for waterpurification. The generator and the evaporator were set to have differenttemperatures due to their different equilibrium conditions. An absorbertemperature of higher than 105°C was required to vaporize thewater. The simulation showed that the absorption heat transformerwas able to deliver heat output with temperatures of 105°C–115°Cat evaporation temperatures of 60°C–75°C, generatortemperatures of 60°C–80°C, and condensation temperaturesof 25°C–30°C. COPs of 0.3–0.49 were obtained.Rivera et al. [33] theoreticallystudied the absorption heat transformer to recover the energy in adistillation column of butane and pentane. The calculation was performedat an evaporator temperature of 74.1°C, a generator temperatureof 74.1°C, and condenser temperatures of 20°C–30°C.Absorber temperatures of 121°C–127°C were obtained withCOPs of 0.3–0.5. Mashimo [34] reported an experiment of the operation of ten absorption heattransformers with LiBr-water. The absorption heat transformers weremainly installed in distillation plants and synthetic rubber factories.A temperature lift of about 40°C was obtained. Riesch et al. [35] reported an experiment of the operationof an absorption heat transformer with LiBr-water. Heat outputs of2–8 kW were obtained with COPs of 0.42–0.48. Rivera etal. [36] experimentallyevaluated an absorption heat transformer working with Carrol-watermixture. The COP was found to decrease with both the temperature liftand the absorber temperature, but to increase with the evaporatorand generator temperatures. COPs of 0.1–0.2 were obtained. Thehighest temperature lift of 52°C was achieved with heat outputtemperatures of 70°C–90°C. Ibarra-Bahena et al. [37] experimentally studied a singlestage absorption heat transformer with Carrol-water mixture. The prototypetested had a heat output power of 0.99–1.35 kW from the absorber.Gross temperature lifts of 18.5°C–22.2°C was obtainedwith COP’s of 0.30–0.35. Ma et al. [38] built a single stage LiBr-waterabsorption heat transformer to recover the waste heat from a syntheticrubber plant. The waste heat with a temperature of 98°C was usedas the heat source. Water was heated from 95°C to 110°C bythe absorption heat transformer. A plant with a capacity of 5000 kWwas built and operated. The results showed an average COP of 0.47and a maximum temperature lift of 25°C. Sekar and Saravanan [39] experimentally studied a LiBr-waterdouble absorption heat transformer coupled with a distillation system.A maximum COP of 0.38 and a maximum temperature lift of 20°C wereachieved at a heat source temperature of 60°C.The experiment alsoshowed that the water distillation coupled with an absorption heattransformer was able to produce drinkable water. Abrahamsson et al.[40] designed an absorptionheat transformer with self-circulation configuration. The self-circulationwas obtained from thermosiphon effect. A reference absorption heattransformer plant with capacity of 100 kW was built with the NaOH-waterworking pair. The plant was able to produce steam at 123°C witha steam input of 100°C at a condenser temperature of around 40°C.
Advanced absorption heat transformers
To get larger temperature lift orhigher efficiency, different advanced absorption heat transformerswere proposed. These advanced configurations mainly include the doubleabsorption heat transformer, double stage absorption heat transformer,double effect absorption heat transformer, triple absorption heattransformer, absorption-compression heat pump [41,42], and absorption-demixing heat transformer [43,44]. In these different options, the double absorptionheat pump and the double stage absorption heat transformer attractmore attention. The double effect absorption heat transformer is onlysuitable for limited circumstances. The triple absorption heat transformeris limited by its complicity. The absorption-compression heat pumprequires electricity consumption, which should be classified intocompression heat pump. The absorption-demixing heat transformer islimited by its small temperature lift [45], which was only 8°C in experiment [46]. In this case, the absorption-compressionabsorption heat pump and the absorption-demixing heat transformerwill not be introduced in detail.
Double stage absorption heat transformer
The absorption heat transformer is used to increase the temperatureof heat source. A larger temperature lift is always the pursuit ofdifferent researches. The double stage absorption heat transformeris effective in achieving a large temperature lift. It also ensuresa small crystallization risk for slat solution working pair [47]. Figure 8 demonstrates one possibleconfiguration of the double stage absorption heat transformer whichincludes two single stage absorption heat transformers. The heat outputof one absorption heat transformer is input to the evaporator of theother absorption heat transformer.
Except for the absorption-evaporationcascading exhibited in Fig. 8, the absorption process in the firststage can also be coupled with the generation, or both the evaporationand the generation in the second stage. All the cascaded configurationsare able to achieve a large temperature lift. However, the absorption-evaporation/generationcoupling is technically very complex with high fixing costs [48]. To compare the three couplingconfigurations, Wang et al. [49] carried out a theoretical analysis of the double stage heat transformer.When LiBr-water was used for both of the two stages, the absorption-generationcoupling configuration, the absorption-evaporation coupling configuration,and the absorption-generation/evaporation coupling configuration obtainedCOPs of 0.32, 0.3–0.28, 0.23–0.22, and gross temperaturelifts of 26°C–61°C, 30°C–63°C, and 42°C–96°C,respectively. The absorption-generation coupling configuration hadthe highest COP and the absorption-generation/evaporation couplingconfiguration had the highest temperature lift. However, the absorption-evaporationwas more favorable due to its relatively high temperature lift andless technical complexity. Since LiBr-water was suitable for low operatingtemperatures and NMP-TFE was suitable for high operating temperatures,they also considered the double stage absorption transformer withLiBr-water used in the first stage and NMP-TFE used in the secondstage. The combined LiBr-water and NMP-TFE configuration achieveda higher output temperature but a lower COP than the LiBr-water configuration.Romero et al. [47] theoreticallystudied the performance of a double stage absorption heat transformerwith Carrol-water mixture. The absorption-evaporation coupling wasselected because of its lower crystallization risk than the absorption-generationcoupling. For the heat output of 140°C from a heat input of 70°C,COPs of 0.289 and 0.277 were obtained at a first stage absorber temperatureof 100°C and 95°C. Ji and Ishida [50] studied the double stage LiBr-water absorption heattransformer by energy-utilization diagrams. The absorption-evaporationcoupling configuration was considered and a multi-compartment generatorand absorber were used. The calculation showed that the multi-compartmentsystem was able to increase the output temperature of the single-compartmentsystem from 121.2°C to 131.9°C. However, the COP was decreasedfrom 0.314 to 0.293. Ciambelli and Tufano [48] studied a double stage absorptionheat transformer using sulfuric acid. The absorption-evaporation couplingconfiguration was considered. The COP was optimized with differentfirst stage absorber temperatures. At a condensation temperature of15°C, a heat input temperature of 80°C, and heat output temperaturesof 125°C–200°C, the system obtained COP of about 0.33.
Double absorption heat transformer
The double stage absorption heattransformer has too many components to achieve a large temperaturelift. The double absorption heat transformer is able to get a similarperformance with a simpler construction, which is cost-saving. Inthis case, the double absorption heat transformer is a popular choicefor a large temperature lift. Figure 9 shows the schematic of a doubleabsorption heat transformer. The system is composed of a high pressureabsorber (HPA), a high pressure evaporator (HPE), a low pressure absorber(LPA), a low pressure evaporator (LPE), a generator (G), a condenser(C), a solution heat exchanger, throttling valves, refrigerant pumps,and a solution pump. The LPA, the LPE, the generator, and the condenseractually form a single stage absorption heat transformer. Externalheat input is delivered to the LPE and the generator, and then theheat output is delivered from the LPA. To further increase the temperatureof heat output, the heat output from the LPA is used to heat the HPE,and finally the heat is delivered from the HPA. The LPA and the HPEare integrated to one component in real operation. It is similar toan absorption-evaporation coupling double stage absorption heat transformer.
The double absorption heat transformeris both effective and simple for a large temperature lift, which attractsthe attention of a lot of researchers. Researches mainly focus onthermodynamic analysis, simulation, prototype, and working pair. Zhaoet al. [51] studied differentflow configurations of the double absorption heat transformer. Inthe first configuration, the solution from the generator was pumpedinto the HPA. The solution from the HPA flowed into both the generatorand the LPA. The second configuration was a parallel configuration,in which the solution from the generator was pumped to the HPA andthe LPA at the same time. The last configuration was a series configuration,in which the solution flowed through the generator, the HPA, the LPAand back to the generator. For evaporator/generation temperature of70°C, a condensation temperature of 20°C and output temperaturesof 130°C–170°C, the first and the second configurationsobtained COPs of 0.27–0.32. For evaporation/generation temperatureof 70°C, condensation temperatures of 20°C–35°C,and output temperatures of 120°C–200°C, the third configurationobtained COPs of 0.24–0.32. Rivera et al. [33] simulated the LiBr-water doubleabsorption heat transformer for waste heat recovery. The calculationwas done at an evaporation temperature and a generation temperatureof 74.1°C, a LPA temperature of 96°C, and output temperaturesof 123°C–135°C. COPs of 0.23–0.33 were obtained.Martínez and Rivera [52] analyzed the double absorption heat transformer from the firstand second law of thermodynamics. The results showed that the generatorcontributed about 40% of the total exergy destruction which was thecomponent with the highest irreversibility. Saito et al. [53] experimentally evaluated the doubleabsorption heat transformer which could generate steam at 180°Cfrom waste hot water at 90°C. A 14 kW plant was designed and built.The production of 180°C steam from 88°C hot water and 25°Ccooling water was achieved with a COP of 0.302. A 200 kW plant wasalso built for the practical test. The production of 179.8°C steamfrom 88.3°C hot water and 25°C cooling water was achievedwith a COP of 0.278. Barragán et al. [54] studied the double absorption heattransformer with the CaCl2-water working pair.The CaCl2-water working pair was used due toits low cost. The modeling was performed based on the property ofCaCl2-water. The results showed that a temperaturelift of 40°C could be achieved with a COP of 0.3.
To get a better option, the performanceof the double absorption heat transformer was also compared with thesingle stage absorption heat transformer and the double stage absorptionheat transformer. Horuz and Kurt [55] compared the single stage absorption heat transformerand the double absorption heat transformer both in series and parallelconfiguration. The LiBr-water solution was used as the working pair.For the single stage absorption heat transformer working at a generator/evaporationtemperature of 80°C and a condensation temperature of 25°C,a heat output of 130°C could be achieved with a COP of 0.482.For the double absorption heat transformer working at a generationtemperature of 70°C, an evaporation temperature of 80°C, anda condensation temperature of 25°C, a heat output of 160°Ccould be achieved with COPs of 0.405–0.377. The double absorptionheat transformer had a higher temperature lift of 30°C but lowerCOP.
Double effect absorption heat transformer
To get higher efficiency than thesingle stage absorption heat transformer, a double effect absorptionheat transformer can be used. Figure 10 shows the schematic of a doubleeffect absorption heat transformer which includes an absorber (A),an evaporator (E), a high pressure generator (HPG), a high pressurecondenser (PHC), a low pressure generator (LPG), a low pressure condenser(LPC), a solution heat exchanger, a solution pump, a refrigerant pump,and throttling valves. External heat input is required for the evaporatorand the HPG. The condensation heat of HPC and LPC is released to theLPG and ambient respectively. Heat output is delivered from the absorber.Since extra generation can be achieved in the LPG without externalheat input, the system has a higher efficiency than the single stageabsorption heat transformer.
Actually, the research about doubleeffect absorption heat transformer is not popular because only smallimprovement can be achieved with complex configuration. Zhao et al.[56] studied a doubleeffect absorption heat transformer with the E181-TFE working pair.The simulation results showed that a gross temperature lift of 30°Ccould be achieved with a COP of 0.58 when the temperature of the generatorexceeded 100°C. This was 20% larger than the COP of 0.48 for singlestage absorption heat transformer with TFE- E181, but still smallerthan the COP of 0.64 for single stage absorption heat transformerwith LiBr-water. Since the range of gross temperature lift suitablefor the double effect absorption heat transformer was narrower thanthat for the single effect absorption heat transformer, the doubleeffect absorption heat transformer was more suitable for the circumstancewhere a high temperature heat source was available but a large temperaturelift was not required.
Triple absorption heat transformer
The single stage absorption heattransformer and the double stage absorption heat transformer are ableto achieve gross temperature lifts of 50°C and 80°C with COPsof about 0.5 and 0.35, respectively. If a larger temperature liftis required, the triple absorption heat transformer can be adopted.Similar to the double absorption heat transformer, the thermal outputreleased from the lower stage is used to heat the evaporation of thehigher stage. Figure 11 shows the schematic of a triple absorptionheat transformer. The first stage is composed of a condenser (C),a generator (G), a low pressure absorber (LPA), and a low pressureevaporator (LPE). External heat input is delivered to the LPE andgenerator. The heat output from the LPA is delivered to the MPE. Thesecond stage is composed of a condenser (C), a generator (G), a mediumpressure absorber (MPA), and a medium pressure evaporator (MPE). Theheat output from the MPA is delivered to the HPE. The third stageis composed of a condenser (C), a generator (G), a high pressure absorber(HPA), and a high pressure evaporator (HPE). Heat output is finallydelivered from the HPA.
Zhuo and Machielsen [57] studied the triple absorption heattransformer with KNO3-water. The triple absorptionheat transformer with series flow configuration was able to deliverheat output of 250°C from a heat input of 105°C and a condensationtemperature of 50°C. A COP of 0.206 was obtained. They also studieda two stage absorption heat transformer with absorption-evaporation/generationcoupling. This configuration could achieve a similar performance withthe triple absorption heat transformer and was named triple-lift absorptionheat transformer. This triple-lift absorption heat transformer wasalso able to deliver a heat output of 250°C from a heat inputof 120°C and a condensation temperature of 75°C. However,a temperature lift of 130°C and a COP of 0.162 were both smallerthan those of the triple absorption heat transformer. A comparisonbetween KNO3-water and LiBr-water system wasmade. It was found that the KNO3-water systemhad the same or a better performance than the LiBr-water system underidentical working conditions. Donnellan et al. [58] evaluated the LiBr-water tripleabsorption heat transformer from the first and second law. The resultsshowed that the pinch heat transfer gradient and condensation temperaturehad the biggest impact on the COP. A case study at a condensationtemperature of 30°C, an evaporation temperature of 85°C, apinch heat transfer gradient of 10°C, and a gross temperaturelift of 140°C was conducted. A COP of 0.205 was obtained. Becauseof the crystallization risk, the triple absorption heat transformerwas advised to be used only when the required gross temperature liftwas larger than the gross temperature lift of the single stage anddouble stage absorption heat transformer. Khamooshi et al. [59] simulated six different configurationsof triple heat transformer with LiBr-water as working pair. Calculationsbased on a condensation temperature of 20°C and heat input temperaturesof 80°C–90°C were performed. The configuration of thethree solution heat exchangers shown in Fig. 11 could deliver a heatoutput of 180°C with COPs of 0.221–0.231. The configurationof the five solution and refrigerant heat exchangers could delivera heat output of 215°C–220°C with COPs of 0.245–0.256.The results showed that proper assembling of additional heat exchangerwas able to increase both the temperature lift and the COP.
Tab.1 Summary of absorptionheat pump cycles |
| Aim | Configuration | Features |
|---|---|---|
| Heat amplification | Single effect cycle | Basic cycle, widely used, COP= 1.5–1.8[16,18,19] |
| Double effect cycle | High COP, seldom researched, COPth = 2.14–2.2, COPexp =1.67 [22,23] | |
| Double lift cycle | Large temperature lift, seldom researched,COPth = 1.34 [24] | |
| GAX cycle | High COP, feasible with ammonia-water,COP= 1.6–2.2 [17,25–28] | |
| Open cycle | Simple system, work with moist gas,COP= 1.55–1.97 [29–31] | |
| Temperature lift | Single stage cycle | Basic cycle, widely used, COP= 0.3–0.5[32–40] |
| Double stage cycle | Three coupling configurations, absorption-evaporationcoupling has a better performance, COP= 0.22–0.33 [47–50] | |
| Double absorption cycle | Large temperature lift, simple, widelyused, COP= 0.2–0.33 [33, 51–55] | |
| Double effect cycle | High COP, small temperature lift,seldom used, COPexp = 0.58 [56] | |
| Triple absorption cycle | Large temperature lift, complicated,COP= 0.21–0.26 [57–59] | |
| Absorption-demixing cycle | No generation, high theoretical COP,small temperature lift [43–46] |
Summary Table 1 summarizes the absorption heat pump cycles reviewed in thissection. The aim, configuration, features, and COP are included. Itcan be seen that the single effect absorption heat pump, the GAX absorptionheat pump, the single stage absorption heat transformer, and the doubleabsorption heat transformer attract more attention. The open cycleabsorption heat pump and triple absorption heat transformer are alsopromising due to their simple system and large temperature lift respectively.
Working pairs for absorption heat pump
The working pair of absorption heatpump includes the refrigerant and absorbent. There are several importantcriteria for refrigerant and absorbent: ① boiling temperature differencesbetween the refrigerant and the absorbent should be large enough toavoid rectification process; ② the refrigerant should have a largelatent heat to ensure a high efficiency and a small plant volume;③ the pressure of refrigerant should be in a reasonable range; ④ boththe refrigerant and the absorbent should have good heat and mass transferproperties; ⑤ both the refrigerant and the absorbent should be costsaving; ⑥ both the refrigerant and the absorbent should be non-toxic,non-corrosive, inflammable, chemically stable, and environmentallyfriendly. Actually, there is no perfect working pairs satisfying allthese criteria, which is the reason why researchers have studied variousabsorption working pairs. These absorption working pairs can be roughlyclassified based on the refrigerant used.
Water based working pairs
Water is a cost-saving and environmentallyfriendly refrigerant with high latent heat and thermal conductivity.The water based working pair of LiBr-water is one of the most popularchoices [22–24, 32,38,39], which has been widely used inboth researches and commercial plants. In the widely used LiBr-waterworking pair, water ensures large latent heat, and the big differencebetween LiBr and water ensures no rectification. However it stillhas problems including corrosion at a high temperature and crystallizationrisk. Except for LiBr-water, there are mainly the following categoriesof water based working pairs.
LiBr-water solution with additive
To improve the LiBr-water workingpair, different additives have been used. Ethylene glycol is one ofthe successful additives. Carrol invented by Carrier Corp. is a mixtureof LiBr and ethylene glycol in a weight ratio of 1:4.5. Carrol-waterhas almost the same characteristics with LiBr-water, but it increasesthe solubility of LiBr from 70% to about 80%, which is widely usedin many researches [35–37,46,60,61]. Riveraet al. [60] compared theperformance of a single stage and advanced absorption heat transformerwith LiBr-water and Carrol-water. The results showed that the useof Carrol-water could achieve a higher absorber temperature and agross temperature lift thanks to its higher solubility. The COPs ofCarrol-water systems were very close to that of LiBr-water systemsat a low absorber temperature. However, the COP of LiBr-water systemsdecreased faster when the absorber temperature increased. Rivera etal. [62–63] also studied the single stage absorptionheat transformer using LiBr-water with additives of 1-octanol and2-ethyl-1-hexanol. The experimental results showed that the 1-octanoladditive slightly increased the absorber temperature and the coefficientof performance, and the 2-ethyl-l-hexanol additive increased considerablythe performance of the system. The absorber temperature was increasedby 7°C and the COP was increased by 40% by the additive.
Binary salt-water solution
Except for the LiBr-water workingpair, other combinations of water and salt are also researched. Theseworking pairs include CaCl2-water, LiCl-water,LiI-water, KNO3-water, and LiNO3-water. Barragán et al. [54] theoretically evaluated the double absorption heat transformerwith CaCl2-water, and a temperature lift of40°C was achieved with a COP of 0.3. CaCl2-water was selected due to its low cost. Barragán et al. [64] also built an absorption heat transformerprototype with CaCl2-water. An absorber temperatureof 84°C was achieved with a gross temperature lift of 19°Cand a COP of 0.45. Grover et al. [65] presented the proper working range and thermodynamicdesign data of an absorption heat transformer with CaCl2-water and LiCl-water. Reyes et al. [66] theoretically studied a singlestage heat transformer with CaCl2-water andLiCl-water working pairs. The systems with both working pairs delivereda thermal output of 80°C–100°C. A better performancewas achieved by the LiCl-water system. Patil et al. [67] studied an absorption heat transformerwith LiI-water. A COP of around 0.5 was achieved with an absorptiontemperature of 70°C–80°C, a condensation temperatureof 20°C, and an evaporation temperature of 60°C. Patil etal. [68] theoreticallystudied an absorption heat pump with LiI-water. COPs of 1.75–1.8were achieved with a generation temperature of 100°C, an absorptiontemperature of 50°C, and temperature lifts of 15°C–30°C.Zhuo and Machielsen [57] theoretically investigated a single stage, double lift and triplelift absorption heat transformer using KNO3-water. The results showed that KNO3-waterwas useful for high temperature operation. Heat output at 260°Ccould be achieved. The COP of the KNO3-watersystem was the same as or higher than that of the LiBr-water system.However, a condensation temperature of higher than 50°C was recommendedfor the solubility issue.
Multi salt-water solution
The aforementioned salt water workingpairs are all binary mixture. Different salt water combinations withmultiple salts are also researched. Jian et al. [18] theoretically studied a singleeffect absorption heat pump with the LiBr+ LiNO3-water working pair. The mole ratio between LiBr and LiNO3 was 4. This ternary working pair was found to increasethe heat pump COP by 5% compared to LiBr-water. A higher output temperaturecould be achieved with less corrosivity. Bourouis et al. [69] studied an absorption heat transformerwith the LiBr+ LiI+ LiNO3+ LiC-water workingpair. This working pair achieved a better performance in the absorptionheat pump than LiBr-water. The solubility range was also enlarged.Barragan et al. [70] experimentallystudied an absorption heat transformer with LiCl-ZnCl2-water and CaCl2-ZnCl2-water. The results showed that the temperature lift was increasedcompared with the binary solution. LiCl-ZnCl2-water showed a generally better performance than CaCl2–ZnCl2-water. The highestgross temperature lift of 37.5°C was achieved with an absorbertemperature of 96°C.
Acid and lye
Acid and lye can also be used asabsorption working pairs. Typically, strong acid [48,71,72] andlye [16,40,73] are used due to their higher affinity to water. Abrahamssonet al. [16] studied absorptionheat transformers with NaOH-water, LiBr-water and LiBr-CH3OH in simulation. For a condenser temperature of 80°C,an evaporator temperature of 50°C, and a generator temperatureof 140°C, the NaOH-water, LiBr-water, and LiBr-CH3OH systems obtained COPs of 1.71, 1.80 and 1.60, respectively. Romeroet al. [74] compared anabsorption heat pump with LiBr-water and NaOH-KOH-CsOH-water. A similarheat pump COP was obtained with the two working pairs. The systemwith NaOH-KOH-CsOH-water showed higher possible condenser and absorbertemperatures. Ciambelli and Tufano [48,72] studieda double stage absorption heat transformer and a double absorptionheat transformer using sulfuric acid working pair. The performanceof the absorption heat transformer was calculated using the lumpedparameter mathematical model. The results showed that the two stageabsorption heat transformer achieved an optimal COP of about 0.33with a condensation temperature of 15°C, a heat input temperatureof 80°C, and absorption temperatures of 125°C–200°C.The optimal gross temperature lift of the double absorption heat transformerwas found to be lower than that of the double stage system.
Ammonia based working pairs
Water-ammonia is another popularabsorption working pair. Compared with LiBr-water, water-ammonia hasa larger concentration range but no crystallization risk, and it canalso work under sub-zero condition. In this case, a lot of researchesabout the heat pump have been carried out with water-ammonia [17,25–27,75]. However, water-ammoniaalso has a major drawback: the boiling temperatures of water and ammoniaare close. This requires rectification and increases the investmentfor the absorption heat pump. A lot of ammonia based working pairsare researched to achieve a better performance or to avoid rectification.
Salt-water-ammonia solution
McLinden and Radermacher [76] compared absorption heat pumpswith water-ammonia and with LiBr-water-ammonia. The mass ratio betweenLiBr and water was 0.48:0.52 for the ternary working pair. The COPof the absorption heat pump with ternary working pair was 0.05 lowerthan that of the binary working pair on average. However, refrigerantvapor entering the rectifier had a significant lower concentrationof water in the system with ternary working pair. Cacciola et al.[77] studied the absorptionheat pump with the working pair of KOH-water-ammonia. The ternaryworking pair was used to improve the high pressure and rectificationof ammonia.
Salt-ammonia solution
Li et al. [15] simulated air source absorptionheat pumps with LiNO3-ammonia and LiBr-waterworking pairs at different air temperatures. Energy saving ratiosof 18%–42% were achieved in different cities. Wu et al. [78] compared absorption heat pumpswith water-ammonia, iNO3-ammonia, and NaSCN-ammoniafor heating supply in cold regions. Single stage, double stage andground source absorption heat pumps were modeled. The results showedthat LiNO3-ammonia required a lower generationtemperature and could work under a lower evaporating temperature anda higher condensation temperature.
Organic working pairs
Similar to the vapor compressionsystem, organic refrigerants can also be used in vapor absorptionsystems. Compared with the LiBr-water and ammonia-water, organic workingpairs are usually more stable and non-corrosive at high working temperature.This can be helpful for absorption heat pumps since a higher workingtemperature is always the pursuit. Here, two major categories of absorptionworking pair based on organic refrigerant are reviewed.
Alcohol based working pairs
Alcohol refrigerants have a highthermal stability at high working temperature. However, most of therefrigerants are toxic as ammonia [11]. Trifluorethanol (TFE) is one of the commonly usedalcohol refrigerant in absorption working pairs. Genssle and Stephan[79] built a single-effectabsorption heat transformer with E181-TFE. The temperature was liftedfrom 348.2 K to 366.2 K with a COP of 0.42. The COP decreased from0.42 to 0 when the temperature lift was increased from 18 K to 65K. Systems with E181-TFE, water-ammonia, and LiBr-water were comparedthrough calculation. The system with E181-TFE was found to have aCOP higher than the system with water-ammonia, but lower than thesystem with LiBr-water. Zhao et al. [56] studied a double effect absorption heat transformerwith E181-TFE. As mentioned in Sub-subsection 2.4.3, the results showedthat a temperature lift of 30°C could be achieved with a COP of0.58 when the temperature of the generator exceeded 100°C. Zhangand Hu [80] compared thesingle stage absorption heat transformer with LiBr-water, [EMIM][DMP]-water,and E181-TFE. The calculation was carried out under generation, evaporation,condensation, and absorption temperatures of 90°C, 90°C, 35°Cand 130°C, respectively. COPs of 0.494, 0.481, and 0.458 wereobtained for the system with LiBr-water, [EMIM] [DMP]-water, and E181-TFE,respectively. Coronas et al. [81] studied absorption heat pumps with TEGDME-TFE and TEGDME-TFE–water.For the TEGDME-TFE system, a maximum COP of 1.6 was achieved at anevaporation, condensation, and generation temperature of 80°C,120°C, and 195°C, respectively. Although the working pairof TEGDME-TFE was non-corrosive, completely miscible, and thermallystable up to 250°C, the low thermal conductivity and latent heatof TFE limited its application. This could be improved by using combinedwater and TFE refrigerants. For the TEGDME-TFE–water systemunder the same condition, the COP was improved to 1.65 and the solutionflow ratio was increased from 7.2 to 10.7. Zhuo and Machielsen [82,83] simulated an absorption heat transformer with TFE-Pyr.The results showed that the system with LiBr-water was superior tothe system with TFE-Pyr for an output temperature of 150°C. However,TFE- Pyr allows higher working temperature without corrosion. Yinet al. [84] compared anabsorption heat transformer with LiBr-water, NMP-TFE, E181-TFE, andTFE-Pyr. Similar results with previous works were obtained that LiBr-waterwas suitable for lower working temperatures while TFE based workingpairs were suitable for higher working temperatures.
Except for TFE, other alcohol refrigerantslike methanol and ethanol are also researched. Abrahamsson et al.[16] studied an absorptionheat pump with LiBr-CH3OH working pair. Theperformance was calculated at a generation temperature of 139°C,a condensation/absorption temperature of 80°C, and an evaporationtemperature of 50°C. The LiBr-CH3OH singleeffect absorption heat pump obtained a COP of 1.66 which was lowerthan that of systems with NaOH-water and LiBr-water. A double liftabsorption heat transformer with LiBr-CH3OHwas also investigated. A temperature lift of 30°C was achievedwith a COP of 0.3. Iyoki et al. [85] studied an absorption heat pump and an absorption heat transformerwith different alcohol based working pairs. These working pairs includedLiBr-CH3OH, ZnBr2-CH3OH, LiBr-ZnCl2-CH3OH, LiI-ZnBr2-CH3OH,LiBr-ZnBr2-CH3OH, LiBr-CH3OH-C2H6O2, and LiI-C2H5OH. The single stage absorption heat pump, double effectabsorption heat pump, single stage absorption heat transformer anddouble stage absorption heat transformer were evaluated with theseworking pairs.
Halogenated hydrocarbon based working pairs
Kripalani et al. [86] presented a comparative study ofsingle stage absorption heat transformers with LiBr-water, DMF-R21,DMF-R22 and DMETEG-R22. The results showed that the systems with LiBr-waterand DMF-R21 performed well with a temperature lift of 30°C. Takeshitaet al. [12] and Ando andTakeshita [87] built anabsorption heat pump prototype using R22 as refrigerant. The prototypehad a heating capacity of 8000 kcal/h. A COP of 1.25 was achieved.The solvents of DEGDME and TEGDME were used. The results showed thatthe DEGDME solvent ensured a better performance than the TEGDME solvent.The properties of R-124 based working pairs [88], DMF-HFC32 [89], R134a/R32 [90] based working pairs, and many otheroptions were also investigated. However, only thermodynamic propertiesor cooling application were considered.
Summary Table 2 summarizes the working pairs for absorption heat pumps.Water based working pairs are most frequently researched. This isdue to the large latent heat and high thermal conductivity of water.However, crystallization and corrosion at high working temperaturesstill limit their applications. Organic based working pairs also havetheir advantages for high temperature application. Ammonia based workingpairs are mainly used for the absorption heat pump with a low ambienttemperature.
Tab.2 Summary of absorptionheat pump working pairs |
| Refrigerant | Classification | Working pair | Feature |
|---|---|---|---|
| Water | LiBr-water | LiBr-water [22–24,32,38,39] | Widely used, crystallization risk,corrosive |
| Carrol-water [35–37,47,60,61] | Solubility of LiBr is increased to80% | ||
| Additive 1-octanol [62–63] | Slightly better COP than LiBr-water | ||
| Additive 2-ethyl-1-hexanol [62–63] | Better COP than LiBr-water | ||
| Binary salt-water | CaCl2-water[54,64–66] | Low cost, acceptable COP | |
| LiCl-water [65–66] | Better performance than CaCl2-water | ||
| LiI-water [67] | Low temperature lift, good COP | ||
| KNO3-water[57] | Higher working temperature than LiBr-water | ||
| Multi salt-water | LiBr+ LiNO3-water [18] | 5% higher COP than LiBr-water, lesscorrosive | |
| LiBr+ LiI+ LiNO3 + LiC-water [69] | Better performance, lager solubility | ||
| CaCl2-ZnCl2-water [70] | Larger temperature lift than binarysolution | ||
| LiCl-ZnCl2-water[70] | Better performance than CaCl2–ZnCl2-water | ||
| Acid and lye | NaOH-water [16,40,73] | Smaller COP than LiBr-water, corrosive | |
| NaOH-KOH-CsOH-water [74] | High output temperature for heat pump | ||
| H2SO4-water [48,71,72] | High COP, highly corrosive | ||
| Ammonia | Ammonia-water | Ammonia-water [17,25-27,75] | Widely used, need of ratification |
| Salt-water-ammonia | LiBr-water-ammonia [76] | COP 0.05 lower than binary mixture | |
| KOH-water-ammonia [77] | Less rectification | ||
| Salt-ammonia | NaSCN-ammonia [78] | No rectification, possible crystallization | |
| LiNO3-ammonia[15,78] | Lower driven temperature than NaSCN-NH3 | ||
| Organics | Alcohol based | E181/Pyr/NMP-TFE [49,79-84] | Non-corrosive,low thermal conductivity,suitable for high working temperature |
| LiBr-CH3OH[16] | Sub-zero condition, low COP | ||
| Ternary CH3OH working pairs [85] | Sub-zero condition, low COP | ||
| Halogenated hydrocarbon based | DMF-R21, DMF-R22 [86] | DMF-R21 has better performance | |
| DEGDME-R22, TEGDME-R22 [12,87] | COP of 1.25 for absorption heat pump,DEGDME-R22 has better performance | ||
| R134a/R32. DMF-HFC32, R124 based working pair [88–90] | Only property of working pair hasbeen studied |
Applications of absorption heat pump for waste heat reuse
The absorption heat pump can be drivenby various heat sources including wasted water, exhaust gas, gas burner,solar thermal power, etc. The absorption heat pump technology forwaste heat reuse is of particular interest due to the large amountof waste heat. The heat output from waste heat driven absorption heatpump can be used for spacing heating, hot water production, drying,industrial preheating, and distillation in different industries. Inthis section, applications of both absorption heat pump and absorptionheat transformer are reviewed.
Waste heat reuse with absorption heat pump
Waste heat reuse for power plant
The IEA Heat Pump Centre [8] reported a waste heat recovery casewith an absorption heat pump, as shown in Fig. 12. A biomass powerplant was installed in SchweighoferFiber GmbH in Austria, which hada steam generator to supply the in-house steam grid and cover theenergy demand of the company. The plant was fired by 77% of externalwood and 23% of in-house remnants, and delivered an electricity capacityof 5 MW and a heat output of 30 MW. A LiBr-water absorption heat pumpwith a heating capacity of 7.5 MW was installed for waste heat recoveryof flue gas. The absorption heat pump was driven by the steam fromthe biomass heating plant with a temperature of 165°C. For anevaporation temperature lower than 50°C, the flue gas got subcooledbelow the dew point temperature. The latent heat of flue gas can berecovered by the evaporator of the absorption heat pump. A heat outputof 95°C was used for district heating. An average COP of 1.6 wasachieved for a total operation hour of 37000 h. The investment wasabout 215000 EUR and the annual reduction of energy cost was about39620 EUR, which resulted in a payback period of 5.4 years.
Waste heat reuse for boiler
Zhu et al. [14] proposed a gas boiler waste heatrecovery system using an absorption heat pump and a direct-contactheat exchanger. As is shown in Fig. 13, the boiler and absorptionheat pump were heated by natural gas. Exhaust gas from the gas boilerand the absorption heat pump entered the direct-contact heat exchanger,released the thermal energy to cooling water, and the thermal energywas finally recovered by the evaporator of the absorption heat pump.The return water was heated by the absorption heat pump and the gasboiler in sequence. In this system, waste heat from the exhaust gaswas recovered by the absorption heat pump, and more heat output canbe delivered from the absorption heat pump compared with the gas boiler.Field test and experiment analysis showed a stable performance underdifferent conditions. The total heat capacity was increased by 12%.
Qu et al. [19] proposed a heat recovery absorptionheat pump (HRAHP) for natural gas boilers to improve boiler efficiency.As is shown in Fig. 14, the system consists of a LiBr-water absorptionheat pump and two heat exchangers, HX-1 and HX-2. The flue gas fromthe boiler entered HX-1 and HX-2, released heat to the hot water andchilled the water of HRAHP. The hot water returned was heated by theHRAHP, the HX-1, and the boiler in sequence. The HRAHP could be drivenby hot water supply from the boiler or the exhaust gas from the boileror natural gas burner. Three types of HRAHP driven by hot water of98°C, flue gas of 150°C, and flue gas of 250°C–350°Cwere modeled. The results showed that the three types of HRAHP wereable to achieve COPs of 1.60, 1.73, and 1.75, and improved the efficiencyof the boiler by 10.0%, 5.5% and 10.0%, respectively. Experiment validationand economic analysis were also performed. The experimental data indicatedthe absorption heat pump driven by hot water of 98°C was ableto heat the hot water from 45°C to 57.5°C. The simple paybackyears for the exhaust gas type, hot water type, and direct-fired typesystem were 1.6, 3.1, and 3.6, respectively.
Waste heat reuse for waste processing plant
Keil et al. [91] reported a waste heat recoverysystem with a gas-fired absorption heat pump. The system was builtfor a municipal waste processing plant in Warngau, Germany. As isshown in Fig. 15, a single effect LiBr-water absorption heat pumpwas installed for recovery of waste heat from the rotting processof the organic waste. In the rotting process, compost heaps with themost recent base material had a temperature of about 65°C. Airventilation was used to ensure appropriate oxygen supply. The exhaustmoist air from the rooting process caused huge amount of energy loss.In this case, the heat of moist air from the rooting process was collectedby the cooling water loop, and then supplied to the evaporator ofthe absorption heat pump with a temperature of 33°C/42°C.The gas-fired absorption heat pump was able to lift the waste heatto 82°C. The heat output was used for local heating network ofa commercial area. The absorption heat pump and burner had an efficiencyof 1.6–1.65 and 0.88–0.85 respectively, resulting in aprimary energy ratio of 1.45. The operation showed good accordancewith the thermal design.
Waste heat reuse for industrial processes
Le Lostec et al. [24] modeled a waste heat driven LiBr-waterabsorption heat pump for wood chip drying process. As is shown inFig. 16, the wood chips were dried in the convection dryer where hotair was injected. The exhaust air from the dryer was used to heatthe evaporator. A wood burning furnace was used to boil the generator.Ambient air was heated by the absorber and condenser, and deliveredto the dryer. The two stage absorption heat pump was able to heatthe air to 50°C–100°C with COPs of 1.40–1.34.A single effect absorption heat pump was able to heat the air to 50°C–60°Cwith COPs of 1.73–1.68.
Abrahamsson et al. [16] theoretically studied a waste heatdriven absorption heat pump for paper drying. The pulp and paper millinvestigated in this work produced thermo-mechanical pulp, newsprintand lightweight coated paper. In the paper drying process, there wasa mixing-pit before the paper machine, where the pulp was mixed withwater. Purge stream was withdrawn from the paper machine, heated to80°C and recycled back to the mixing-pit. This stream heatingprocess required an external heat input. On the other hand, wet exhaustair streams leaving the dryer had a temperature of 54°C. The absorptionheat pump was used to increase the wasted heat of 54°C from wetexhaust air to useful heat output 80°C for the stream heating.Single effect absorption heat pumps driven by the steam boiler wereable to achieve the required temperature lift. COPs of 1.71, 1.80,and 1.66 were obtained from single effect absorption heat pumps withNaOH-water, LiBr-water and LiBr-CH3OH respectively.A double stage absorption heat transformer with LiBr-CH3OH was also able to achieve this temperature lift. Alower COP of 0.3 was obtained.
The IEA Heat Pump Centre [8] reported a waste heat recovery systemusing an absorption heat pump for metal processing industry. Fivenatural gas driven absorption heat pumps with ammonia-water were installedin 2007. Each absorption heat pump delivered a heating output of 34kW and a cooling output of 16 kW. The cooling output was suppliedto laser cutting machines, welding machines, edging machines, andproduction halls with a temperature of 20°C. In another word,the waste heat from these machines was used as the low temperatureheat input to the absorption heat pumps. A heat output of 60°Cwas used for the washing process, the drying process, and space heating.The payback period of 4 years was achieved. Compared to the old systemwithout an absorption heat pump, 40% of CO2 emission could be reduced.
Waste heat reuse with absorption heat transformer
Waste heat reuse for paper industry
Costa et al. [92] theoretically studied the integrationof an absorption heat transformer in a Kraft pulp process, as shownin Fig. 17. Before the absorption heat transformer was used, blowtanks released 46t/h of contaminated steam at 96°C to the directcondenser. The cooling water in the direct condenser was heated from55°C to 95°C and used in the preheating of process steam.However, the initial system did not make full use of the wasted heat,and a double lift absorption heat transformer with LiBr-water wasused to recover the waste heat from contaminated steam. Low pressuresteam with a flow rate of 17 t/h could be produced with a COP of 0.35.The heat released by the condenser of the absorption heat transformercould be used to pre-heat 373t/h of fresh water to 60°C. Afterthe recovery of latent heat, the contaminated steam became hot waterand could be used for the preheating of process steam.
Cortés and Rivera [93] studied the optimization of a cogenerationsystem including a paper mill plant and an absorption heat transformer.The optimization was conducted based on exergy, exergoeconomics, thermoeconomics,and pinch analysis. A two stage absorption heat transformer was usedfor waste heat recovery. Evaporates with temperatures of 70°C–80°Cfrom the evaporators were used as the heat source of the absorptionheat transformer. The temperature of the demineralized water was increasedfrom 20°C to 25°C to 90°C–120°C by the absorptionheat transformer. The demineralized water was then supplied to thecogeneration plant and force plant boilers. A natural gas consumptionof 25% was reduced by the waste heat recovery.
Waste heat reuse for oil industry
Zhang et al. [94] analyzed a waste heat driven openabsorption heat transformer for water distillation. The system wasdesigned to distill waste water with the SiO2 from heavy oil production. The distilled water could then be usedto produce high quality steam for heavy oil recovery in oil reservoirs.In this open absorption heat transformer, there were two parts ofdistilled water production, as shown in Fig. 18. In the first part,wasted heat was used to boil the generator and distilled water canbe obtained in the condenser. In the second part, the heat outputfrom absorber was used to vaporize the wasted water, and distilledwater could be obtained by condensing the vapor. The condensing heatwas consumed by the evaporator. The results showed that the systemwas able to elevate the waste heat of 70°C to 125°C, thusproducing steam at a temperature of 120°C. This temperature washigh enough to drive the four-effect distillation to produce distilledwater.
Rivera et al. [33] presented a theoretical analysisof a single stage and a double stage absorption heat transformer coupledto a butane and pentane distillation column in a refinery. In thedistillation process, the steam of 155°C was used to heat thebottom of distillation column. Products leaving the distillation columntop entered the condenser and dissipated thermal energy to the ambientwith a temperature of 82°C. The condensing heat was used as theheat input to the absorption het transformer, and the heat outputof the absorption heat transformer was used to preheat the steam intothe distillation column. Energy consumption for steam generation couldbe saved. Both the single effect and the double stage absorption heattransformer with LiBr-water were investigated. The results showedthat the energy consumption of the reboiler could be saved by 43%and 33% with the single effect and the double stage absorption heattransformer respectively.
Waste heat reuse for chemical industry
The IEA Heat Pump Centre [8] reported the application of an absorptionheat transformer in an ethylene amine plant. The system was installedin 1985 which was the first one installed in Netherlands. Saturatedsteam at 100°C was used as the heat source, and the absorptionheat transformer was able to produce saturated steam at 145°Cwith 4.6 bar. The measured heating capacity and waste heat input were6.7 MW and 13.7 MW, respectively. A COP of 0.49 was obtained. Thetotal electricity consumption was 53 kW which was less than 1% ofthe total output. The payback period of two years was achieved. Thesystem was taken out of operation due to serious internal corrosionproblems caused by the LiBr-solution and air-leakage.
Aly et al. [95] studied the application of an absorptionheat transformer for oleochemical industry. The oleochemical plantstudied was used for production of fatty acids and refined glycerol.Saturated water vapor with a temperature of 100°C was initiallyproduced from flash vessels used to depressurize condensate streamsemerging from different processing units in the plant. The water vaporwas condensed and discharged. The absorption heat transformer wasused to recover the waste heat from the water vapor, and produce steamat 3 bar which would be used in the plant. According to the calculation,74 kg of steam at 134°C could be produced by the absorption heattransformer from 147 kg of waste vapor at 100°C. A payoff periodof 18 months was obtained based on the specific economic parametersand a COP of 0.45.
Currie and Pritchard [96] studied an open-cycle absorptionheat transformer for spray drying in chemical industry. Spray dryingwas widely used in chemical industry which needed hot and dry airstream with a temperature of up to 550°C. Exhaust moist air wasvented to the ambient with a lot of energy waste. An open-cycle LiBr-waterabsorption heat transformer was used to recover both the sensibleand latent heat of the exhaust moist air, and then to deliver heatoutput for inlet air preheating. The open-cycle absorption heat transformerincluded a direct contact absorber, an indirect contact absorber,a generator, and a condenser. The latent heat in the exhaust air wasrecovered by the direct contact absorber. The dehumidified air fromthe direct contact absorber was further heated by the mixing of solutionand external input steam in the indirect contact absorber. The resultsshowed that an exit air temperature of 160°C was achieved witha temperature lift of 63°C.
Waste heat reuse for rubber industry
Ma et al. [38] reported the first industrial applicationof an absorption heat transformer in China. Figure 19 shows the installationof the absorption heat transformer. The system was built to recoverthe wasted heat of 98°C released by steam and organic vapor mixturefrom the coacervation section of a synthetic rubber plant. Hot waterwas heated from 95°C to 110°C by the absorption heat transformer,and fed back to the coagulator. The absorption heat transformer usedLiBr-water as working pair and had a heat flow of 5000 kW. A meanCOP of 0.47 was achieved with a gross temperature lift of 25°C.The payback period was about 2 years.
Waste heat reuse for textile industry
Horuz and Kurt [97] studied the application of an absorptionheat transformer in a textile company. The aim was to produce hotprocess water with the hot water from the cogeneration system. Thecompany had four different units which produced hot water of about90°C, which was not effectively used. The company had some pressurizeddying machine which needed hot water of 120°C. The single effectabsorption heat transformer was able to recover the waste heat at90°C and produce a heat output of 130°C with a COP of 0.482.About 50% of the wasted heat could be recovered by the absorptionheat transformer. An extra heat recovery process was added to thebasic absorption heat transformer. A COP improvement of 14.1% couldbe achieved.
Waste heat reuse for CO2 capture system
Zhang et al. [98] proposed a CO2 capture system with an absorptionheat transformer and a flash evaporator. In the amine absorption CO2 capture system, the condensate left the reboiler witha temperature of 128°C. To reduce water consumption and avoidcondensate contamination, the condensate needed to be cooled to 40°Cand recycled back to the steam turbine. A large amount of thermalenergy was wasted in this process. In the proposed system shown inFig. 20, the absorption heat transformer was used to recover the hightemperature condensation heat. The heat output from the absorptionheat transformer was used for steam production through flash evaporation.In this case, the cooling system for condensate from the reboilerwas no longer needed. Besides, the energy consumption for steam heatingwas also saved. The absorption heat transformer was able to deliverhot water with a temperature of 152°C from condensate of 128°Cand cooling water of 32°C. COP of 0.5 was achieved. The overallanalysis showed that the energy consumption of the CO2 capture system was reduced by 2.62%. The payback period was approximately2.4 years.
Tab.3 Application ofabsorption heat pump for waste heat reuse |
| Aim | Application | Features |
|---|---|---|
| Heat amplification | Power plant | Flue gas from biomass boiler was reused,heat output at 90°C was used for district heating, COP= 1.6 for37000 h of operation, payback period was 5.4 years [8] |
| Gas boiler | Flue gas from boiler was reused, heatoutput was used for preheating of water, COP= 1.6–1.75, boilerefficiency was increased by 5.5%–12% [14,19] | |
| Waste processing | Exhaust moist air from rotting processwas reused, heat output at 82°C was used for heating network,COP= 1.6–1.65 [91] | |
| Drying process | Exhaust air from dryer was reused,heat output was used to preheat the ambient air into dryer, air washeated to 50°C–100°C by two stage cycle with COP of1.4–1.34, air was heated to 50°C–60°C by singleeffect cycle with COP of 1.73–1.68 [24] | |
| Metal processing | Heat from cutting machine, weldingmachine and other machines was reused, heat output was reused fordrying, washing and heating, payback period was 4 years, and 40% CO2 emission was reduced [16] | |
| Temperature lift | Paper industry | Contaminated steam from Kraft pulpprocess at 96°C was reused by double lift cycle, clean steam wasproduced with COP of 0.35 [92,93] |
| Oil industry | (1) Waste hot water from heavy oilproduction was recovered by open absorption heat transformer, wasteheat was elevated from 70°C to 125°C, and steam at 120°Cwas produced for oil reservoir [94] (2) Condensation heat from top ofdistillation column at 82°C was reused, steam at 155°C wasproduced for the bottom of distillation column, and 43% – 33%energy consumption was saved [33] | |
| Chemical industry | (1) Steam at 100°C from ethyleneamine plant was reused, steam at 145°C was produced, measuredheat capacity was 6.7 MW, COP= 0.49, payback period was two years,internal corrosion happened [8] (2) Steam at 100°C from oleochemicalplant was reused, steam at 134°C was produced, COP= 0.45, paybackperiod was 18 months [95] | |
| Rubber industry | Steam and organic vapor mixture at98°C from coacervation section was reused, 110°C hot waterwas produced, COP= 0.47 for temperature lift of 25°C, heat capacitywas 5000 kW, payback period was 2 years [38] | |
| Textile industry | Hot water at 90°C from cogenerationplant was reused, heat output at 130°C was used for water heating,COP= 0.428 [97] | |
| CO2 capture | Condensate at 128°C from reboilerwas reused, heat output at 152°C was used to preheat the waterinto flash evaporator, COP= 0.5, overall energy consumption was reducedby 2.62%, payback period was 2.4 years [98] |
Summary Table 3 summarizes the literature review in this section. The applicationsof both the absorption heat pump and the absorption heat transformerfor waste heat reuse are included. Since the output temperature ofthe absorption heat pump for heat amplification is lower, it is usedfor different systems including power plant, natural gas boiler, wasteprocessing plant, and some industrial processes. The output temperatureof the absorption heat transformer is higher, and it is mainly usedfor industrial application where high temperature output is preferred.From the limited cases listed in Table 3, it can be found that theabsorption heat transformers for waste heat reuse have attractiveeconomic performances with short payback periods.
Summary and perspective
A large portion of heat input inindustry is dissipated in the forms of hot water, steam, flue gas,etc. An absorption heat pump is able to recover the waste heat anddeliver useful heat output at different temperatures. This reducesboth the energy consumption and CO2 emission.Economic benefit could also be gained from the waste heat recoverysystem. That is why the absorption heat pump for waste heat reuseattracts the attention of different researchers. In this paper, thecurrent states of the absorption heat pump for waste heat reuse havebeen reviewed as follows.
1) Absorption heat pump cycles includean absorption heat pump for heat amplification and an absorption heattransformer for temperature upgrading. Different advanced cycles havebeen proposed for higher efficiency or larger temperature lift. Theadvanced absorption heat pumps include the double effect cycle, thedouble stage cycle, the GAX cycle, and the open cycle. The advancedabsorption heat transformers include the double stage cycle, the doubleabsorption cycle, the double effect cycle, the triple absorption cycle,and the absorption-demixing cycle.
2) The working pairs for the absorptionheat pump include the water based working pair, the ammonia basedworking pair, and the organic based working pair. Working pairs witha large latent heat, a high solubility, a good stability at high temperatureand less corrosivity are preferred.
3) The absorption heat pump has beenapplied to different scenarios for waste heat reuse. The absorptionheat pump for heat amplification usually has a lower output temperature.It is widely applied to the waste heat reuse for power plant, boiler,waste processing plant and some industrial processes for heating,drying, or improvement of system efficiency. The absorption heat transformerusually has a higher output temperature. It is widely applied to differentindustries including paper industry, oil industry, chemical industry,rubber industry, and textile industry for preheating, process hotwater or steam generation.
The review of literature also demonstratesthat the following work should be done in the future.
1) The traditional single effectand multi effect configurations for absorption heat pumps and absorptionheat transformers have been well developed. Triple absorption heattransformers with large temperature lift and open-cycle absorptionheat transformers with direct contact to exhaust gas should be furtherresearched. Besides, new concept transformers like the absorption-demixingheat transformer should also be further explored.
2) The water based working pair andammonia based working pair work well at low working temperatures.However, the corrosion problem still limits their application at highworking temperatures. Organic working pairs have better performancesand almost no corrosivity at high temperatures. However, their latentheat and thermal conductivity are their limitations. Working pairswith a high stability, no corrosivity and good thermophysical featuresat high working temperatures are still needed.
3) Waste heat reuse by absorptionheat pumps has been widely used in different scenarios. However, theexperimental research or industrial application report are still fewand far between. Long-term robustness of the absorption heat pumpis one of the concerns. Besides, the concept of waste heat reuse stillneeds to be promoted and accepted by more people in different industries.Large scale demonstration and long-term operational monitoring ofwaste heat reuse by absorption heat pumps might be helpful.