Ammonia-water Absorption Heat Pump Research Articles

Comparative analysis of solar-powered heat pump systems for decarbonization of process heating and cooling applications: Case of milk pasteurization

The dairy processing industry is heavily dependent on fossil fuels for heating applications at different temperature levels and contributes to greenhouse gas (GHG) emissions. Besides, process cooling, product refrigeration, and storage are mainly reliant on vapour compression refrigeration technologies. This study investigates the performance of solar photovoltaic (PV) and solar thermal (ST) powered heat pumping technologies for simultaneous heating (at 75 °C) and cooling (at 2 °C) applications in a selected typical thermal process in a dairy processing plant (milk pasteurization). An ammonia/water absorption heat pump (AHP) and a two-stage R717 vapour compression heat pump with an open intercooler (VCHP) are used in the present study. The VCHP and AHP are powered by market-available solar PV modules and ST collector technologies. The electrical COP and exergy COP of the VCHP were about 4.710 and 0.535, respectively. The AHP operates at a thermal COP and exergy COP of 0.979 and 0.301 when the driving hot water and absorber cooling water inlet temperatures are 160 °C and 30 °C. The grid-connected PV-VCHP system and ST-AHP with natural gas-fired auxiliary heater operate at an annual solar fraction of 0.533 and 0.572, respectively, for a typical weather condition in Barcelona (Spain).

Compact Diesel Engine Waste-Heat-Driven Ammonia–Water Absorption Heat Pump Modeling and Performance Maximization Strategies

Abstract An investigation of the best ways to achieve optimal performance from a waste-heat-driven ammonia–water absorption heat pump over a wide range of operating conditions is presented. Waste heat from an 8-kWe diesel engine generator is recovered using an exhaust gas heat exchanger and delivered to the desorber by a heat transfer fluid loop. The absorber and condenser are hydronically coupled in parallel to an ambient heat exchanger for heat rejection. The evaporator provides chilled water for space-conditioning with a baseline cooling capacity of 2 kW. All heat and mass exchangers employ novel microscale geometries. A detailed thermodynamics model is developed to simulate performance and develop strategies to achieve the best performance in both cooling and heating modes over a range of operating conditions. These parametric studies show that improved coefficients of performance can be achieved by adjusting the coupling fluid temperatures in the evaporator and the condenser/absorber as the ambient temperature varies. With the varying return temperatures, the system is able to provide the 2-kW design cooling capacity for a wide range of ambient temperatures.

Transient behavior assessments of a single-effect ammonia-water absorption heat pump system: Development of an efficient experimentally validated numerical framework

In this study, the long-term transient response of a single-effect ammonia-water gas-fired absorption heat pump prototype is investigated both numerically and experimentally. For the numerical model, heat and mass transfer components are divided into four groups; (a) tubular heat exchangers comprising condenser, evaporator, absorber, solution and refrigerant heat exchangers, (b) tray column heat exchanger comprising gas-fired generator, (c) refrigerant and solution tanks, (d) solution pump and restrictors. Components of groups a-c are modelled based on discretized volumes in which the unsteady mass, species and energy conservation equations are imposed, whereas solution pump and restrictors are idealized as algebraic models. The experimental analysis covers a period of over 10 h, during which various step changes are applied to system inputs like gas power, inlet water temperature, solution and refrigerant flow rates, and brine inlet temperature. The results of the numerical model are compared with the experimental data which show good agreement in both transient and near steady state operation. Results show that, during transient operation average deviations are less than 2.94% and near steady-state conditions are less than 1.70%. The accuracy of the numerical model in predicting both the transient response and the steady-state values of the measurable outputs, makes it a valuable tool to gain insight on the internal dynamics of ammonia-water absorption heat pumps and cut down costs on time-consuming optimization experiments, such as the fine-tuning of initial solution charge, the implementation of a suitable concentration control strategy and the performance optimization at full and partial loads.

Thermodynamic analysis and comparative investigation of a new combined heating and power system driving by medium-and-high temperature geothermal water

Geothermal energy is one of the most promising renewable energy. In order to improve the overall utilization degree of medium-and-high temperature geothermal water by flash cycle, a new combined heating and power system is proposed to provide electricity and domestic hot water at the same time. This new system is integrated by single-stage flash cycle and ammonia-water absorption heat pump cycle, of which the heat pump cycle recovers two streams of waste heat of the flash cycle, including the waste heat of the turbine exhaust steam and the geofluid drained from the flasher. The mathematical models of the new system is established at length in this article, and a numerical simulation is conducted to present the preliminary design condition. The simulation results show that the thermal efficiency and exergy efficiency of the system could reach 81.8% and 46.99% respectively under the condition of 170℃ geothermal water. Then a thermodynamic parameter analysis is carried out to explore the impact of five key thermodynamic parameters on the system performance. The results show that an optimal flash pressure happens to make the system exergy efficiency maximal, and there is a similar case for the generator temperature. Within some certain value ranges, a higher rectification column pressure brings about a sharp drop in the exergy efficiency, whereas a higher ammonia concentration of ammonia-strong solution leads to the exergy efficiency rising. Additionally a higher evaporation pressure just has slightly positive effect on the exergy efficiency. Finally an optimization and comparison study is implemented between the proposed system and four flash cycle based combined systems, and the optimization results indicate that the proposed system has a better performance no matter from the viewpoint of the first law or the second law of thermodynamics.

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.

Seasonal performance assessment of three alternative gas-driven absorption heat pump cycles

When designing a gas driven heat pump, the selection of the cycle impacts both on complexity and efficiency. Among absorption cycles, the nominal efficiency usually increases with complexity. However, heat pumps are required to operate over a wide range of conditions, of different temperatures and capacity. In this research, three alternative cycles, i.e single-effect (SE), generator-absorber heat-exchange (GAX) and vapor-exchange (VX) for a gas-driven air-source ammonia-water absorption heat pump are compared, considering small-scale space heating as target application. For each cycle, both full-load under various temperature conditions and seasonal performances are predicted by means of numerical simulations. Small capacity appliances are usually equipped with fixed geometry restrictors, meaning that the solution mass flow rate is driven by the pressure difference across the associated restrictor valve. This feature is included in the modeling assumption as it affects the temperature of the generator and, ultimately, the performance at the various conditions. At full load, the VX cycle resulted to achieve the highest Gas Utilization Efficiency (GUE), followed by the GAX and SE. The differences among the cycles are higher at low thermal lift conditions, where the GAX effect is achieved in both the GAX and VX cycles, while are limited at high thermal lift. Looking at the seasonal performances, calculated on the basis of the method prescribed in the European standard EN 12309, the differences among the cycles became much narrower, with the Seasonal GUE ranging between 1.427 (SE) and 1.493 (VX). The small differences in terms of seasonal performances are explained considering that, in the prescribed working conditions, the GAX effect is only partially exploited.

Theoretical and experimental investigation on a GAX-Based NH3-H2O absorption heat pump driven by parabolic trough solar collector

This research involves a GAX-based ammonia-water absorption heat pump, which utilizes solar energy and natural gas as heat source. The experiments of parabolic tough collector and GAX absorption heat pump were carried out separately. GAX absorption heat pump is operated when the ambient temperature is 4.31–11.07 °C, COP approaches 1.44–1.66. Thermal efficiency of the PTC varies from 0.50 to 0.60. A mathematical model of the coupled solar absorption heat pump is built. The effect of each heat exchanger size on the GAX cycle performance is investigated, results show that the heat exchanger size of solution cooled absorber has the greatest impact on the cycle. Both Solar driven mode and Solar-NG driven mode were studied. Solar driven mode is strict with solar energy, if the direct normal irradiance is below 560 W/m2 the system cannot work. Solar-NG driven mode could work under any solar radiation condition. In this mode, generation temperature could be controlled by changing the flow rate of natural gas. The optimal generation temperature is found to be around 180 °C when Tamb = 7 °C, DNI = 800 W/m2, Twout = 45 °C. Besides, system analysis reveals that the heat pump could perform competitive in low ambient temperature if the solar radiation is abundant, which makes Lhasa a perfect site for the system. Economic analysis in Lhasa show that the addition of solar collector could reduce the operating cost by more than 25%.

Simulation analysis of performance optimization of gas-driven ammonia-water absorption heat pump

The generator-absorber heat exchange ammonia-water absorption heat pump has a relatively high efficiency and the COP is improved by recovering internal heat. In order to resolve the problem that generator-absorber heat exchange effect becomes less obvious or even fails under the working condition of large temperature lift, a novel system is proposed to recover the rectification heat and absorption heat by solution split method. Compared with the method of using total strong solution to recover absorption heat after extracting rectification heat, the method of solution split avoids the problem of high temperature of inlet strong solution of the solution cooled absorber, and improves the internal heat recovery capacity of the system. When using solution split method to recover the rectification heat and absorption heat, the solution split ratio K has an important influence on the system performance. This paper will theoretically study the selection range of the solution split ratio K and the effect of evaporation temperature, cooling water temperature and generation temperature on the optimum split ratio K. Compared to the system that uses the total strong solution to recover the rectification heat and absorption heat, the performance of the novel system is significantly improved, and the novel system performance coefficient can be increased by up to 15.7%.

Experimental investigations of an absorption heat pump prototype with intermediate process for residential district heating

Despite being efficient means of district heating, conventional single-effect absorption heat pumps suffer significant performance deterioration at low ambient temperatures. To solve this problem, an ammonia-water absorption heat pump prototype with intermediate process is designed and built. It utilizes both low-grade heat from the ambient and exhaust heat from natural gas combustion, through the evaporator and intermediate evaporator, respectively. To perform comparison study, the prototype also has the single-effect mode. Experimental results indicate that when the evaporation temperature is 0 °C, the prototype with intermediate process can provide 30 kW heating capacity to heat the water in sequence through the rectifier, condenser and absorber, from 34.24 to 55.09 °C, and the coefficient of performance and primary energy efficiency are 1.66 and 1.28, respectively. When evaporation temperatures reduce to −5, −10 and −15 °C, the coefficient of performances are 1.51, 1.40 and 1.28, respectively, reaching 77%–90% of the simulation values under corresponding working conditions. Compared to the conventional single-effect system, the prototype with intermediate process performs better at lower evaporation temperature, with the coefficient of performance improved by 11%. By replacing the original heat exchangers, the proposed system can be easily applied to the existing heat networks based on gas-fired boilers to improve energy efficiency, and the favorable experimental performance proves that it is a more efficient way of residential district heating, especially in cold regions.

Design and analysis of an ammonia-water absorption heat pump

This study focuses on the improvement of the efficiency of an absorption heat pump for high heating temperatures with ammonia-water as working pair, based on experimental investigations. For this purpose, the absorber and the solution heat exchanger are designed. Furthermore, a new generator assembly, consisting of a plate heat exchanger (PHX), a storage tank and a separator, is implemented in the absorption heat pump. In comparison to a vessel-type generator, the weight of the residential size absorption heat pump with a total heating power of about 20kW in OP1 was decreased from 300kg to 180kg. The ammonia filling quantity was also reduced from 15kg by at least 9.5kg compared to a vessel-type generator. Experimental investigations are carried out at different operating points where each operating point represents a specific loading condition in the building heating system. An efficiency up to COPh=1.42 is achieved. Even at challenging conditions like a low external evaporator inlet temperature of -10°C or a high external condensator outlet temperature of 63°C, a COPh of 1.10 and 1.05 respectively is reached. The solution heat exchanger is working excellently with an efficiency of 0.972<ηSHX<0.996. Furthermore, improvements like an integration of a condensate separator or an absorber with internal heat recovery are presented.

Reduction of the return temperature in district heating systems with an ammonia-water absorption heat pump

In this study, a new setup for the reduction of the return temperature in district heating systems will be presented. For this purpose, an ammonia-water absorption heat pump will be integrated within a district heating substation and theoretically investigated. The idea of this coupling is that the district heating flow is used to power the generator of the heat pump. The district heating return is cooled in the evaporator. In this way, the capacity of the district heating system can be increased or the mass flow rate can be reduced. Additionally, the lower return temperature leads to a higher efficiency of the whole system.The field of possible operating points for such a system is also revealed and discussed.

Experimental investigation of a compact thermal compressor for an ammonia-water absorption heat pump

An experimental evaluation of the thermal compressor of an ammonia-water absorption system is presented. A compact desorber and a rectifier based on diabatic distillation principles that lead to optimized system operation are designed and fabricated. Performance of these compact components as a thermal compressor for small-capacity absorption heat pumps is measured and compared with model predictions. Optimal desorption temperatures for energetic and exergetic performance established in previous modeling work are validated. The results of this study demonstrate the utility of modeling several components of an absorption system as a thermal compressor, which facilitates system design and control.

Development and demonstration of a microscale absorption heat pump water heater

The development and demonstration of a compact ammonia–water absorption heat pump water heater using microchannel heat and mass exchangers is presented. The absorber, condenser and evaporator are hydronically coupled, while the desorber is direct natural gas-fired. A heat pump comprised of discrete microchannel components was first fabricated and tested on a breadboard facility. Component and system performance are experimentally investigated over a range of conditions, and insights from experiments are used to refine designs. The refined designs are used to develop an integrated microchannel monolithic heat and mass exchanger packaged into a prototype absorption heat pump water heater containing all auxiliary support systems to allow for connection to a water storage tank. The system is nominally sized to provide 2.79 kW of heat at a coefficient of performance (COP) of 1.74. Performance is experimentally investigated at the design condition of 32 °C coupling water into the system and an ambient of 20 °C, and over the range of expected water temperatures (10–60 °C) and ambient conditions (10–32 °C).

Modeling of an ammonia–water absorption heat pump water heater for residential applications

An investigation of a direct gas-fired single-effect ammonia–water absorption heat pump water heater for residential applications is presented. Combustion of natural gas provides heat to the desorber where refrigerant is generated. The absorber and condenser are hydronically coupled in parallel to a hot water storage tank, while the evaporator is hydronically coupled to an ambient air heat exchanger that extracts ambient heat. A thermodynamic model is developed and the system configuration is optimized to provide a baseline heating capacity of 2.79 kW at a coefficient of performance of 1.74. A detailed parametric study over a range of water and ambient temperatures is used to understand the variation in system performance as the water is heated from 14.5 to a minimum of 57.0 °C. The performance of the heat pump coupled to a 227-liter storage tank is also modeled for three different scenarios, a cold start response to a 76-liter draw, and response to stand-by losses. The absorption heat pump water heater is found to achieve coefficients of performance better than those of commercially available gas fired heaters and electric heat pump units.

Development and demonstration of a compact ammonia–water absorption heat pump prototype with microscale features for space-conditioning applications

This paper presents the development and demonstration of a compact packaged prototype of a single-effect ammonia–water absorption cooling system with a nominal cooling capacity of 3.5kW. The packaged unit encompasses a combustion module to supply thermal energy to the system where heat is exchanged between combusted gas and a heat transfer fluid. The heat transfer fluid provides heat to the desorber where refrigerant is generated. The heat and mass exchange components of the absorption system are integrated into two monolithic blocks. The sub-components of each monolith utilize microscale heat and mass transfer passages, which have higher heat and mass transfer coefficients compared to conventional designs, resulting in substantial reductions in the required size of the system. Semi-autonomous control algorithms are developed to enable stand-alone operation of the packaged unit. System performance is measured over a range of operating conditions and compared with predictions of the corresponding system model.

Analysis of gas-fired NH3-H2O generator with cross flow gas burner

This paper presents a model to simulate the combined heat and mass transfer processes that occur in a gas-fired generator of an ammonia water absorption heat pump. The model comprises three interconnected components: the gas furnace, the distillation column and the rectifier. The furnace is modeled like a well-stirred single volume. The distillation column and the rectifier are discretized along their axial dimension. Several heat transfer mechanisms are simultaneously solved: radiative and convective heat transfer between combustion products and generator finned surface, flow boiling and pool boiling in the two-phase ammonia water mixture inside the distillation column. The model is applied to operating conditions of interest in real applications. The generated outputs provide insight on the internal gradients, the quantification of gas furnace efficiency, and effectiveness of solution boiling and internal heat recovery. The model can be used for the simulation of absorption heat pumps with modulating gas burners.

Heating Mode of Type I Ammonia-Water Absorption Heat Pump with Parabolic through Collector

Presents the application of the system to a project in Dezhou. Tests the application effect and the COP of absorption heat pump units and the heat pump system. Analyses and evaluates the energy saving property of the system based on the test data, providing reference for other similar application.

Performance characteristics of an ammonia-water absorption heat pump system

SUMMARY The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio Rp on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and Rp that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. Rp has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of Rp on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.

Design, fabrication, and experimental demonstration of a microscale monolithic modular absorption heat pump

The first ever conceptualization, design, fabrication and successful experimental demonstration of a thermally activated microscale absorption heat pump for miniaturized or mobile applications is reported here. Several-fold enhancements in coupled heat and mass transfer possible in microscale passages remove significant hurdles that have hindered the implementation of thermally activated heat pumps. Cooling capacities of 100 W–10 s of kW are possible through minor changes in component geometry. These mass-producible miniaturized systems can be packaged as monolithic full-system packages or as discrete, distributed hydronically coupled components integrated into buildings. A 300 W nominal cooling capacity ammonia–water absorption heat pump with overall dimensions of 200 × 200 × 34 mm and a mass of 7 kg was fabricated and tested over a range of heat sink temperatures from 20 to 35 °C with 500–800 W of desorber heat input to yield cooling duties of 136–300 W.

Local Heat and Mass Transfer in a Counter-current Slug Flow Absorber for Ammonia-water Absorption Heat Pump System

Local Heat and Mass Transfer in a Counter-current Slug Flow Absorber for Ammonia-water Absorption Heat Pump System