Conventional Absorption Heat Pump Research Articles

Energy, economic, and environmental evaluation of a GAX-based cross-type absorption-resorption heat pump

Thermally driven absorption heat pump is regarded as one of the most promising methods to efficiently utilize low-grade heat, reduce energy consumption and greenhouse gas emissions. However, there are still limitations when using it at low ambient temperature in cold regions, and its application is limited by their low system performance. In this study, we proposed a novel Generator-absorber heat exchange (GAX) based NH3/H2O cross-type absorption-resorption heat pump driven by waste flue gas, which amalgamates the absorption-resorption and GAX cycles, yielding a dual advantage of diminished heat source temperature and operating pressure requirements. Additionally, it substantively enhances the system's performance by capitalizing on the cross-type solution cycle integration. A mathematical model has been developed and the results have been compared and validated with those reported in the literature. The proposed system is comprehensively studied from the view of energy, exergy, economic and environmental (4E) analysis to determine its advantages and disadvantages. The results reveal that the proposed system achieves a maximum COP of 2.742 under specific operating conditions, which is 82.8% higher than the conventional absorption heat pump (AHP), and 30.57% higher than the conventional GAX-AHP. Besides, the addition of a compressor can significantly enhance the performance of the system at lower driving heat source temperatures. At a PH/PL pair of 1.7/0.47 MPa, the static payback period is 4.113 years, with the heating capacity of 88.51 kW and the supply water temperature of 54.6 °C, showcasing favorable system performance. Furthermore, the total atmospheric pollutant emissions could be reduced by 30,998 kg/year compared to the conventional heating method.

Dynamic response of gravity-driven flow in multi-section absorption heat pump

The multi-section absorption heat pump (AHP) is applied in the novel district heating (DH) system of China. This new type of AHP exhibits improved performance in DH systems, but involves a more complicated structural design and the risk of internal flow instability problems such as reversed pressure difference and liquid accumulation. The flow circulations of the solution and refrigerant consist of several gravity-driven flow processes that are rarely observed in conventional AHPs, and these processes have not been addressed in previous studies. The present work studies the dynamic response of gravity-driven flow inside a multi-section AHP in a DH system. A dynamic model capable of calculating the liquid level, flow rate, and pressure change of various gravity-driven flow processes is developed and validated. The steady-state conditions calculated by the dynamic model are compared with a validated model in the literature. The gap between the temperature and pressure calculations is less than 0.3 K and 0.05 kPa, respectively. The dynamic simulation results are compared with the start-up process test results of a multi-section AHP. The pressure values and trend change fit the test results well. The standard deviation (SD) and root-mean-square error (RMSE) of the pressure calculation errors are less than 5% and 5.5%, respectively. The model is used to study typical transient processes of multi-section AHPs in a DH system. In the start-up process, the pressure of the generator is 1–2 kPa lower than that of the absorber after the solution circulation begins. This reversed pressure difference leads to solution accumulation at the bottom of the generator. The solution flow rate into the absorber is 20% lower than the designed value. After the heat source is input into the generator, the reversed pressure difference disappears. Solution accumulation expedites the flow of the solution into the absorber. The flow rate is 75% higher than that of the designed value, leading to solution accumulation in the absorber. In the heat source sudden drop process, reversed pressure difference and solution accumulation problems also occur. The reasons for flow instability in each process and possible methods for preventing them are discussed.

Performance analysis and optimization for a novel air-source gas-fired absorption heat pump

In district heating technologies, gas fired boilers and conventional heat pumps have poor performance at low ambient temperature. To tackle this issue, this study has proposed a novel air-source gas-fired absorption heat pump for district heating with flue gas recovery. Compared with conventional absorption heat pumps, the proposed solution in this study can absorb heat from both air and flue gas. Additionally, the working fluid works properly when air temperature is below 0 °C, safe and nonflammable. To analyze the economic and the thermodynamic performance of this air-source gas-fired absorption heat pump, a mathematical model, considering energy, exergy, economy and environment, has been developed. According to the simulation results of the model, the proposed air-source gas-fired absorption heat pump system here had good stability and feasibility under various operational conditions. However, as the payback period and the exergy destruction were found to be conflicting with each other, a multi-objective optimization method was established to minimize the system’s payback period and exergy destruction simultaneously. Additionally, the technique for order preference by similarity to an ideal solution decision-making method has been applied to look for the optimal solutions in the Pareto frontier, with optimal solutions of the system under different operational conditions recommended.

Gas-fired absorption heat pump applied for high-temperature water heating: Parametric study and economic analysis

Despite being efficient water heating technologies, conventional absorption heat pumps can hardly produce hot water with temperature over 90 °C, and compressor-based systems have low primary energy efficiency. In order to fill this temperature gap and improve the energy efficiency, a gas-fired ammonia-water absorption heat pump with intermediate process is proposed to supply hot water with large temperature lift for residential and industrial applications. In addition to the ambient heat utilized in the evaporator, the proposed system recovers the exhaust heat in the intermediate evaporator, and the evaporated ammonia vapor is absorbed in the intermediate absorber to further heat the water, which is initially preheated in sequence in the condenser, rectifier and absorber. Simulation results indicate that at the ambient temperature of 20 °C, the proposed system can produce hot water with temperature over 95 °C and the obtained temperature lift is 75 °C, while the primary energy efficiency is 1.38. Obtained results indicate that the intermediate pressure of 0.7 MPa is the optimum pressure from both thermodynamic and economic aspects. Moreover, it is found that the energy saving potential of the proposed system is in range of 36.7% to 43.5%, when the comparison is made with conventional gas-fired boilers, and the average payback period is 3.0 years in three typical cities of south China. It is concluded that the proposed system is an efficient scheme for high-temperature water heating with large temperature lift. This is especially pronounced in warm regions with average ambient temperature over 5 °C.

Double-section absorption heat pump for the deep recovery of low-grade waste heat

Absorption heat pump is an effective option for industrial waste heat recovery. However, significant conflict between low evaporation temperature and high efficiency exists in conventional absorption heat pump, which limits its application in deep recovery of low-grade waste heat recovery. To address this issue, this paper presents the use of double-section absorption heat pump for low-grade waste heat recovery. Both theoretical and experimental researches are carried out to analyze the features of double-section LiBr-water absorption heat pump. Driven by hot water with temperature of 120.1 °C, the experimental prototype was able to extract thermal energy from the waste hot water by reducing its temperature from 45.0 °C to 15.9 °C, realizing a large temperature decrease of 29.1 °C for waste hot water. Meanwhile, the experimental prototype could heat water up to 52.5 °C for domestic heating application with capacity of 8666.4 kW and high coefficient of performance of 1.77. The excellent performance is attributed to the stepped generation/evaporation processes with gradient temperatures, which makes the double-section absorption heat pump achieve both higher efficiency for internal circulation and reduced irreversible loss for external circulation comparing with the conventional single-section absorption heat pump. This makes the double-section absorption heat pump an efficient way to realize the deep recovery of low-grade waste heat.

Working domains of a novel solar-assisted GAX-based two-stage absorption-resorption heat pump with multiple internal heat recovery for space heating

Ammonia-water absorption-resorption heat pumps have great potential in efficiently utilizing low-temperature heat for space heating or hot water supply. However, single-stage absorption-resorption heat pumps have disadvantages of poor low ambient temperature adaptability and low heat supply temperature over conventional absorption heat pumps, which limit their applications in cold regions. In this paper, a novel GAX-based two-stage absorption-resorption heat pump cycle with multiple internal heat recovery is proposed aiming to expand the application scope at lower ambient temperatures, to raise the heat supply temperature and more significantly to utilize solar heat for space heating for longer solar working time with higher contribution. Feasible operation pressure combinations to enable the cycle were investigated. Coefficient of performance, ambient air temperature restrictions, heat source temperature demand, heat supply temperature level, heat inputs/outputs and solution circulation ratios of generators in terms of different operation pressure combinations were revealed by comparing with those of the single-stage cycle. Under given operation conditions, maximum COP value of 1.367 and heat supply temperature of 46.1 °C can be obtained and the cycle could be operated at ambient air temperatures above -15.6 °C. The heat supply temperature is generally above 42 °C, which is suitable for building floor heating or fan coil heating. The cycle can be driven by heat source above 73 °C, highlighting the possibility of integrating it with more economical solar collectors for high efficiency space heating.

Proposal and performance analysis of a novel solar-assisted resorption-subcooled compression hybrid heat pump system for space heating in cold climate condition

Absorption-resorption heat pumps (ARHPs) can efficiently utilize low-temperature solar heat for winter space heating, but are restricted by the relatively weak low ambient temperature adaptability. By combining the different strengthens of vapor compression heat pump (VCHP) and absorption-resorption heat pump (ARHP), a novel solar-assisted resorption-subcooled compression hybrid heat pump system with higher heating capacity under cold climate condition for space heating is proposed. Thermodynamic models for the hybrid system involving two subsystems are developed for performance evaluation under different operation conditions. Feasible high pressure/low pressure (PH/PL) and evaporation temperature/condensation temperature (Te/Ta) ranges for the hybrid system are revealed for component design. Based on that, performance evaluation indices like total coefficient of performance (COPtot), primary energy ratio (PER), primary energy saving ratio (PESR), coefficient from solar irradiation to heating capacity (SCOP) are investigated in terms of different PH/PL and Te/Ta values. The results showed that for the base-case, the above indices are 2.41, 1.66, 0.35 and 1.21, respectively. In addition, heat source temperature demand, heating capacity and ambient air temperature need of the hybrid system are compared with those of the conventional absorption heat pump (AHP), single ARHP and single VCHP under the same operation conditions. The results show that the hybrid system can extend the operation heat source temperature to above 72 °C, highlighting the possibility of integrating with common non-tracking solar collectors, and can extend the feasible ambient air temperature to above −20 °C. Moreover, by recovering the sub-cooling heat of VCHP subsystem for ARHP subsystem operation, the hybrid system can lift the heating capacity by over 50% based on the equivalent VCHP system.

Experimental evaluation of a semi-open membrane-based absorption heat pump system utilizing ionic liquids

While the use of energy efficient absorption heat pumps has been typically limited to the high capacity commercial and industrial applications, the use of a semi-open absorption heat pump for water heating has been demonstrated to be an energy efficient alternative for residential scale applications. A semi-open absorption system uses ambient water vapor as the refrigerant in the absorber where its heat of phase change is transferred to the process water, cooling the solution in the absorber. The solution is pumped to the desorber, where by adding heat, the water vapor is released from the solution and condensed in the condenser. The heat of phase change of water vapor is transferred to process water again in the condenser. This cycle when implemented with a membrane-based absorber in a plate and frame form of heat exchanger using ionic liquids can overcome the challenges related to the system architecture of conventional absorption heat pumps like the lower efficiency at small scale, crystallization/corrosion issues with the desiccants and the high cost of hermetically sealed components. The cycle COP for such a system was previously demonstrated by Chugh et al. for high humidity conditions. In this experimental study, design improvements were made that expand the system’s applicability to more practical and standardized test conditions. With these improvements, the performance of the system was evaluated. The results presented in this study demonstrate the improved system’s viability as a heat pump water heater conforming to standard water heater test conditions. Performance was measured at a cycle thermal COP of 1.2 with a hot water delivery water temperature of 56 °C and ambient air at 19 °C and 49% RH.

Thermodynamic analysis and optimization of a balanced-type single-stage NH3-H2O absorption-resorption heat pump cycle for residential heating application

Ammonia-water absorption-resorption heat pump (ARHP) is a promising technology in efficient utilization of low-temperature heat for residential heating application. This paper proposes a balanced-type single-stage ARHP cycle based on the unbalanced ARHP cycle by eliminating the rectifier and depending on only one solution circulation pump. The proposed cycle can achieve internal mass and species conservation by changing the ammonia-water solution concentration difference at different pressure levels. A mathematical model is developed to obtain feasible high pressure/low pressure (PH/PL) pairs to enable the cycle. The cycle's coefficient of performance (COP) at different PH/PL pair values is investigated. The maximum COP is 1.51 and the corresponding heat supply temperature reaches 43.4 °C when PH/PL pair value is 1.50/0.48 MPa and heat source temperature is 95 °C. The maximum heat supply temperature can reach 53.4 °C and the minimum ambient air temperature to effect the cycle is −5.8 °C. Other parameters like heat output/inputs, solution circulation ratios, and absorption and desorption concentration gradients in the absorbers and generators are studied. Inner pressure (difference), COP and heat source temperature demand have been compared with those of the conventional absorption heat pump (AHP) cycle and the unbalanced ARHP cycle. The results revealed that the proposed cycle could work at lower heat source temperature (above 82 °C) and within wider heat source temperature range than the conventional AHP cycle, indicating that the proposed cycle is potential in efficient utilization of commonly used solar collectors for space heating. In addition, the proposed cycle could be operated at higher COP values and lower inner pressure differences than the unbalanced ARHP cycle.

Waste heat recovery of power plant with large scale serial absorption heat pumps

Large amount of waste heat is dissipated in industries, resulting in energy waste and environment pollution. Waste heat recovery with absorption heat pump is one of the attractive solutions. In this paper, we present the theoretical study and test of a power plant waste heat recovery system with large scale LiBr-water absorption heat pumps for district heating. Waste heat from steam condenser with temperature of ∼35 °C is recovered by the absorption heat pumps driven by steam with pressure of ∼0.25 MPa from the turbine. Heat output from the absorption heat pumps is used to preheat the return water of district heating network from ∼45 °C to ∼80 °C. The return water is further heated by the steam to ∼105 °C for heating supply. Large temperature lift of the return water is achieved by the serial absorption heat pumps. Theoretical analysis of the system is presented, showing advantages compared to the conventional absorption heat pump and original heating supply system. On-site test showed that COP of 1.77 and heating capacity of 63.57 MW were achieved by one serial absorption heat pump, with waste heat inlet/outlet temperature of 34.63/28.33 °C and return water inlet/outlet temperature of 45.94/81.34 °C.

Proposal and thermodynamic analysis of a combined open-cycle absorption heat pump and thermal desalination system driven by high-humidity exhaust gas

A combined system based on an open-cycle absorption heat pump (OAHP) and a low-temperature multi-effect evaporation (LT-MEE) water desalination process is configured, modeled, and analyzed. The system is applicable to be run by high-humidity gases, and the flue gas from the combustion of natural gas is used here. Compared with conventional absorption heat pumps, the OAHP is characterized by a gas-solution direct-contact absorber, where not only the sensible heat but also the latent heat and water vapor in flue gas are partially recovered. The OAHP-MEE system has good internal synergy, as demonstrated by a 36%–74% water production gain over the simple system when driven by flue gas with temperature of 120 °C–250 °C. With the increase of the feed-gas temperature, the combined system becomes increasingly favorable over the simple system in energy utilization and water production. For a specified feed gas condition, an optimal water recovery rate of OAHP and a maximum number of effects of MEE exist, mutually leading to a maximum water output. Using a cooler to cool the strong solution entering the absorber not only has a favorable effect on water production, but also widens the parameters' ranges, thus offering good flexibility for design and operation.

Development of a novel unbalanced ammonia-water absorption-resorption heat pump cycle for space heating

Ammonia-water absorption-resorption heat pump (ARHP) is a promising technology for promoting efficient utilization of low-temperature thermal for space heating. This paper proposes a novel unbalanced ARHP cycle by replacing the condenser and evaporator in the conventional absorption heat pump (AHP) cycle with a high-pressure absorber and a low-pressure generator, respectively. The proposed cycle can work just depending on the concentration difference of ammonia-water solution under different pressure levels. A numerical model has been developed to investigate the feasible high-pressure/low-pressure (PH/PL) values to effect the thermodynamic cycle. The cycle's coefficient of performance (COP) under different PH/PL pair values and other given working conditions are studied. The maximum COP is 1.550 and the corresponding optimum PH/PL pair to effect the cycle is 1.40 MPa/0.38 MPa at a heat source temperature of 95 °C. When PH/PL pair value is 1.40/0.38 MPa, heat supply temperature of 44.5 °C and COP value of 1.331 can be obtained, which can basically meet the temperature demand of building floor heating. In addition, effects of different PH/PL pair values on solution circulation ratios and vapor discharge scopes of generators are discussed. It is concluded that the ideal PH/PL candidates yield large concentration difference between the inlet and outlet of generators and absorbers. Meanwhile, the solution circulation ratios of the two generators at ideal PH/PL pairs are acceptable values between 2.0 and 12.0. The cycle can work when the ambient temperature is above −7.5 °C and the driving heat source temperature is larger than 85 °C, which is potential in efficient utilization of commonly used stationary solar collectors for winter heating.

Experimental study of large temperature lift heat pump (LTLHP) in CHP system

Combined heating and power (CHP) system is one of most promising ways for district heating, however, more than 20% of heat gained in the boiler is discharged to atmosphere directly by the cooling tower. A new absorption heat pump named LTLHP adopting the lithium bromide aqueous solution as working pair is invented for heat recovery at the thermal power plant, and its temperature difference between the evaporator and condenser could reach 50°C without risk of crystallization. The concentration difference of lithium bromide solution is 3.58% for a satisfying COP (coefficient of performance) without the risk of crystallization, which is far lower than that of a conventional absorption heat pump. The LTLHP is also experimented at a thermal power plant with double 330 megawatt steam turbines. The mean COP could reach 1.51 during the whole heating period, and 9.5×10^6kg of coal could be saved annually, and the index of coal consumption for heating at the thermal power plant could be reduced to 14.0kg/GJ, which is far lower than that of a conventional thermal power plant (18.0kg/GJ).

An alternative approach towards absorption heat pump working pair screening

The successful market penetration of modern absorption heat pumps (AHP) today is critically dependent on their thermodynamic performance as well as other key factors like cost, reliability and inherent safety. Conventional AHPs have a proven record in the first two aspects but crucial shortcomings in the last two. For this reason it has been imperative to search for alternative working pairs that could potentially provide comparable performance while also satisfying the rest of the conditions to the best extent possible. As part of a systematic approach towards this direction, a detailed cycle analysis was performed, utilizing an idealized AHP system containing a real working pair, which enabled the identification of five dimensionless parameters and key thermophysical properties that influence the system's thermodynamic efficiency and the circulation ratio. In order to validate those findings, these parameters were calculated and compared between conventional and alternative AHP refrigerants. It turned out that low molecular weight ratios between absorbent and refrigerant have a beneficial effect on both coefficient of performance and the circulation ratio. Furthermore, both the refrigerant acentric factor and the absorbent vaporization enthalpy shall be minimized to obtain better performance.

The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters

Combined heating and power (CHP) system is one of most promising ways for district heating, however, more than 20% of heat gained in the boiler is discharged to atmosphere directly by the cooling tower. A new absorption heat pump named LTLHP adopting the lithium bromide aqueous solution as working pair is invented for heat recovery at the thermal power plant, and its temperature difference between the evaporator and condenser could reach 50 °C without risk of crystallization. The concentration difference of lithium bromide solution is 3.58% for a satisfying COP (coefficient of performance) without the risk of crystallization, which is far lower than that of a conventional absorption heat pump. The LTLHP is also experimented at a thermal power plant with double 330 megawatt steam turbines. The mean COP could reach 1.51 during the whole heating period, and 9.5 × 10^6 kg of coal could be saved annually, and the index of coal consumption for heating at the thermal power plant could be reduced to 14.0 kg/GJ, which is far lower than that of a conventional thermal power plant (18.0 kg/GJ).

Use of membrane transport in an absorption thermal transfer cycle

A thermal transfer cycle utilizing membrane osmotic transport of water against a pressure rise is investigated from the viewpoint of the operation of a conventional absorption heat pump using an aqueous solution as the working fluid. Physical sorption, similar or equivalent to condensation, of water vapor in the membrane material is considered to be an essential step in the overall process of water transport. The thermal nature of this step, during which the heat of sorption similar in amount to the heat of condensation for water vapor must evolve at the evaporator temperature or lower, disqualifies the system in performance as a heat pump. Simple flow relations for the aqueous sugar solution are derived under simplifying assumptions. A set of numerical calculations is given to show that the inferred steps are well within the limits of thermodynamics.