Air Source Absorption Heat Pump Research Articles

Thermodynamic performance and heat and mass transfer analysis of air source absorption heat pump for heating

To lower CO2 emission and obtain better heating efficiency compared with conventional gas boilers, this paper proposed a solar air source absorption heat pump (SAAHP) with falling film heat exchanger for district heating. The thermodynamic model is established to analyze the performance of the proposed system. The falling film generator as the key heat exchange component has been designed carefully, and then the numerical models of the falling film generation process which include one and two-dimensional numerical models are developed. According to the thermodynamic analysis, the primary energy efficiency (PEE) and heating capacity of the system are 1.65 and 45.02 kW at the generation temperature of 190 °C and ambient temperature of −7 °C, respectively, which can save 41.69 t of CO2 emissions from heating. The two-dimensional numerical model indicates the main heat and mass transfer occur between the falling film height x = 0 mm and x = 200 mm, and the interface heat transfer decreases from 74.6 kJ/(m2·s) to 40.5 kJ/(m2·s) in a typical vertical falling film generation process. In addition, the energy transfer as the inter-diffusion is corrected in the energy equation which improves the prediction accuracy by about 1.5% of the two-dimensional model. The research provides a reference for the optimization design of the generator, where one-dimensional models can quickly guide the design and selection of equipment, modified two-dimensional numerical models and Computational fluid dynamics (CFD) models can provide references for the refined optimization design, which attributes to the improvement of thermodynamic performance of the solar absorption heat pump system.

Investigating the performance of heat exchangers in absorption heat pump systems using both numerical and experimental methods

To achieve better heating efficiency and lower CO2 emission, this study has proposed an air source absorption heat pump system with a tube-finned evaporator, a vertical falling film absorber, and a generator. To analyzeboth heat and mass transfer performances and optimize the sizes of both the absorber and the generator, a distributed parameter model and a two-dimensional numerical model have been adopted, both validated. To develop an environmentally efficient working fluid pair for absorption heating for cold climate, a calculation method adopting fugacity and activity models was developed. The defrosting control strategy of this system was developed based on a spatial and temporal frost development model, which determines the characteristics of frost distribution, frost growth, and frost inhomogeneity. To evaluate the functionality of this system, a test rig was constructed, with a heating capacity of 36.88 kW, a coefficient of performance of 1.54 under evaporation temperature and supply water temperature of −9.2 °C and 38.4 °C, respectively. Validation results showed a 1.5 % higher prediction accuracy for the two-dimensional model with correction, comparing to the distributed parameter model. R134a-DMF and R161-DMF were recommended at an ambient temperature of −2 °C. This is because with ambient temperature of −7 °C and supply water temperature of 41 °C, the predicted coefficient of performance was 1.04 and 1.06 for R134a-DMF and R161-DMF, respectively. The frost prediction indicated that at the time of 3,600 s, the thickness of the frost layer in the heavy frost area was 0.94 mm, with a total frost mass of 3,995 g. Compared with the initial stage, the sensible and latent heat transfer rates decreased by 22.1 % and 24.2 %, respectively.

Experimental investigations and multi-objective optimization of an air-source absorption heat pump for residential district heating

The traditional single-effect absorption heat pump is an effective district heating measure, while the low ambient temperature will degrade its performance significantly. To overcome this dilemma, a novel air-source absorption heat pump (ASAHP) for district heating (DH) is proposed in this paper. This system can operate at low ambient temperature and recover the waste heat of the flue gas with higher efficiency compared with the conventional gas-fired boilers. The falling film form is adopted in the generator and absorber, which reduces the system mass flow rate and electricity consumption. The thermodynamic performance of the system is analyzed by the lumped parameter model. An experimental rig is established to study the system performance and validate the mathematical model. Results show that the proposed system is an efficient way for DH, especially in cold regions. The heating capacity and the COP of the system are 38.32 kW and 1.39 at the evaporation temperature of −10 °C, respectively. The system can provide 36.21 kW heating capacity and 39.21 kW heating capacityfg (heating capacity of the system with flue gas recovery) with flue gas recovery to heat water from 25 °C to 39.1 °C with the COP of 1.21 and COPfg (COP of the system with flue gas recovery) with flue gas recovery of 1.36. The maximum ratio of COPfg with flue gas recovery to simulation value and the maximum ratio of heating capacityfg with flue gas recovery to simulation value are 92.91% and 92.23%, respectively. Additionally, to obtain the optimal operating condition, the TOPSIS decision-making method and NSGA-II technology is adopted in multi-objective optimization.

Study on the heating performance of absorption-compression hybrid heat pump in severe cold regions

Traditional heating methods are polluted and inefficient, more than, conventional air source absorption heat pump has poor heating performance at low temperature. To overcome these shortcomings, by sharing evaporator and condenser from absorption and compression heat pump, the novel air source absorption-compression hybrid heat pump (ASACHHP) is proposed. The thermodynamic models of ASACHHP are established and verified. Under different internal thermodynamic parameters, the coefficient of performance (COP) and exergy coefficient of performance (ECOP) of ASACHHP are compared. The maximum COP that can be achieved under given research conditions is 1.241–1.420, the maximum ECOP is 0.697–0.756. In comparison with common absorption heating systems, ASACHHP can provide hot water above 60 °C when the ambient temperature is below −10 °C, which shows better low-temperature applicability than single-stage ASAHP. When the ambient temperature is suitable, ASACHHP can be switched to a single-stage absorption cycle. Although the heating range and environmental application range of ASACHHP are slightly reduced compared with the high-pressure compression-assisted absorption system, ASACHHP has the highest COP among the common absorption heating improved systems, the improving rate can be 1.5–13.0%. In general, ASACHHP is a kind of clean and efficient alternative heating technology in severe cold regions.

Configurations of solar air source absorption heat pump and comparisons with conventional solar heating

A solar air source absorption heat pump (ASAHP) is analyzed to explore the suitable configurations of advanced solar heating systems. A single-effect (SE) and a generator-absorber heat exchange (GAX) ASAHP, driven by a flat-plate collector (FPC), evacuated tube collector (ETC), compound parabolic concentrator (CPC) and parabolic-trough collector (PTC) are modeled. A parametric study and comparative analysis of different solar heating systems indicate that the CPC-SE, PTC-SE, and PTC-GAX are suitable configurations. Comparisons with the conventional direct solar heating (DSH) show that the ETC-DSH performs the best among all the DSH systems, only the PTC-SE and PTC-GAX are advantageous over it under a solar radiation of 800 W/m2. At an ambient temperature of 7 °C, the CPC-SE performs better than the ETC-DSH only when the solar radiation is above 1000 W/m2. The PTC-GAX becomes worse than the PTC-SE if the solar radiation is below 650 W/m2 and even becomes inferior to the ETC-DSH if the solar radiation is below 400 W/m2. Besides, the applicability domain of the CPC-SE is very narrow, the efficiency improving rate (EIR) of the CPC-SE is about 15% under a favorable condition. The PTC-SE and PTC-GAX had a much wider applicability domain, and the maximum EIR value is about 24% and 36%, respectively. Reducing air pollution is very important for the applications of the solar ASAHP systems.

Annual performance investigation and economic analysis of heating systems with a compression-assisted air source absorption heat pump

The compression-assisted air source absorption heat pump (CASAHP) is a promising alternative heating system in severe operating conditions. In this research, parameter studies on the annual performance under various compression ratios (CRs) and source temperatures are performed to achieve the maximum energy saving rates (ESRs). Economic analyses of the CASAHP under different CRs and partial-design ratios are conducted to obtain an optimal design that considers both energy savings and economy improvements. The results show that the optimal CR becomes higher in colder regions and with lower heat source temperatures. For a source temperature of 130°C, the optimal CR values in all of the cities are within 2.0. For source temperatures from 100 to 130°C, the maximum ESR is in the range of 17.7–29.2% in the studied cities. The efficiency improvement rate (EIR) caused by compression in a severe source condition can reach 10.0–20.0%. From the viewpoint of economy, the relative investment of CASAHP is reduced to 30–60% with a CR of 2.0–3.0. With a 2–6% sacrifice in ESR, the payback period can be reduced from 12–32 to 5–9years using compression. Partial-design of the CASAHP can further reduce the payback period to 3–6years with a partial-design ratio of 50% and a CR of 2.8. Additionally, CRs and partial-design ratios are designed comprehensively by seeking the maximum ESR for a given acceptable payback period.

Air source absorption heat pump in district heating: Applicability analysis and improvement options

The low-temperature district heating system based on the air source absorption heat pump (ASAHP) was assessed to have great energy saving potential. However, this system may require smaller temperature drop leading to higher pump consumption for long-distance distribution. Therefore, the applicability of ASAHP-based district heating system is analyzed for different primary return temperatures, pipeline distances, pipeline resistances, supplied water temperatures, application regions, and working fluids. The energy saving rate (ESR) under different conditions are calculated, considering both the ASAHP efficiency and the distribution consumption. Results show that ASAHP system is more suitable for short-distance district heating, while for longer-distance heating, lower supplied hot water temperature is preferred. In addition, the advantages of NH3/H2O are inferior to those of NH3/LiNO3, and the advantages for warmer regions and lower pipeline resistance are more obvious. The primary return temperatures are optimized to obtain maximum ESRs, after which the suitable distances under different acceptable ESRs are summarized. To improve the applicability of ASAHP, the integration of cascaded heat exchanger (CHX) and compression-assisted ASAHP (CASAHP) are proposed, which can reduce the primary return temperature. The integration of CHX can effectively improve the applicability of ASAHP under higher supplied water temperatures. As for the utilization of CASAHP, higher compression ratio (CR) is better in longer distance, while lower CR is more advantageous in shorter distance. For a distance of 50km, the maximum ESR is improved from 13.6% to 20.4–25.6% under different CRs.

Techno-economic analysis of air source absorption heat pump: Improving economy from a design perspective

The heat supply system combining a conventional boiler with an air source absorption heat pump (ASAHP) has been evaluated as having great potential in energy saving and emission reduction. However, an economic analysis needs to be performed before its wide applications. To improve the economy of ASAHP from a design perspective, a year-round simulation and techno-economic analysis are conducted. Results show that the energy saving rate (ESR) does not decrease much when the relative design capacity of ASAHP is within 50–100%. At a relative design capacity of 75% in Harbin, 35% in Shenyang, 50% in Beijing, and 50% in Zhengzhou, ESR can reach 21.6%, 24.3%, 26.2% and 26.3%. Given an ASAHP price of 1.5CNY/W and a gas price of 4.0CNY/Nm3, the payback period is 6.8, 6.6, 4.1 and 4.6 years in these four cities, at a 50% relative design capacity. There is a conflict between ESR and the payback period, but a small sacrifice (1.5–6.0%) in ESR can contribute a great reduction (38–48%) in the payback period. The heating period and energy price in the local area, as well as the expected ESR and acceptable payback period should be comprehensively considered for a better energy efficiency and economy.

Performance improvement of ammonia/absorbent air source absorption heat pump in cold regions

Air source absorption heat pump is promising in energy saving and emission reduction of heating and domestic hot water, but performs badly or even cannot work in cold climate. The ammonia/absorbent air source absorption heat pump with low-pressure boosting is proposed to solve the problem. The hybrid air source absorption heat pump + compressor system is modeled and the compression ratio is optimized to obtain maximum primary energy efficiency. The integrated system is simulated with air temperature ranging from −30℃ to 10℃ and hot water temperature from 30℃ to 60℃. Comparative simulations on three working fluids reveal that NH3–LiNO3 system has the lowest compression ratio and the highest primary energy efficiency value. By pressure boosting, the air source absorption heat pump can operate under air temperatures as low as −30℃. Primary energy efficiency of the hybrid system is about 15–50% higher than that of coal boiler, showing great potential for heat supply in cold regions. Practical application: Heating and domestic hot water consumes a large amount of energy every year. Air source absorption heat pump can be a potential alternative to the traditional boiler systems in the point view of primary energy efficiency. However, air source absorption heat pump performs badly or cannot work when the air temperature is low. This paper presents a hybrid air source absorption heat pump with pressure boosting to improve the performance of the air source absorption heat pump heating system, making it operate under lower outdoor air temperatures with higher primary energy efficiency. The novel heat supply system is expected to make contributions to building energy saving as well as pollution reduction.

A new heating system based on coupled air source absorption heat pump for cold regions: Energy saving analysis

Energy consumption for heating and domestic hot water is very high. The heating system based on an air source absorption heat pump (ASAHP) had been assessed to have great energy saving potential. However, the single-stage ASAHP exhibits poor performance when the outdoor air temperature is very low. A double-stage coupled ASAHP is proposed to improve the energy-saving potential of single-stage ASAHP in cold regions. The heating capacity and primary energy efficiency (PEE) of the proposed system operated in both coupled mode and single-stage mode are simulated under various working conditions. The building load and primary energy consumption of different heating systems applied in cold regions are analyzed comparatively to investigate the energy-saving potential of the coupled ASAHP. Results show that the coupled ASAHP exhibits stable PEE and provides high heating capacity in very cold conditions. The energy-saving rate of the coupled ASAHP in all the typical cities is above 20%. In addition, the energy-saving potential of the single-stage ASAHP in severely cold areas can be improved obviously by coupled ASAHP, with an improvement of 7.73% in Harbin.

Crystallization Analysis and Control of Ammonia-Based Air Source Absorption Heat Pump in Cold Regions

Energy consumption of heating and domestic hot water is very high and will keep increasing. Air source absorption heat pump (ASAHP) was proposed to overcome the problems of low energy efficiency and high air pollution existing in boiler systems, as well as the problem of bad performance under low ambient temperatures for electrical heat pumps. In order to investigate the crystallization possibility of ammonia-salt ASAHP, crystallization margin (evaluated by solution mass concentration) at generating temperature ranging from 100 to 150°C, evaporating temperature from −30 to 10°C, and condensing temperature from 30 to 65°C are analyzed. To prevent the NH3–NaSCN solution from crystallizing, ASAHP integrated with pressure booster located between the evaporator and absorber is simulated. Analysis and comparisons show that NH3–NaSCN is easy to crystallize at relatively high generating temperature, low evaporating temperature, and low condensing temperature. But crystallization margin of NH3–LiNO3 can always stay above 5% for most conditions, keeping away from crystallization. Pressure booster can effectively avoid the crystallization problem that will take place in the NH3–NaSCN ASAHP system.

Comparisons of different working pairs and cycles on the performance of absorption heat pump for heating and domestic hot water in cold regions

In order to explore the suitable working pair and cycle style for heating and domestic hot water in cold region, single-stage and double-stage air source absorption heat pump (ASAHP) and ground source absorption heat pump (GSAHP) are modeled in this paper. Based on these models, the performance of absorption heat pump (AHP) at different generating temperatures, evaporating temperatures and condensing temperatures is simulated respectively when using NH3–H2O, NH3–LiNO3 and NH3–NaSCN as working fluid. The results indicate that NH3–LiNO3 requires lower generating temperature, and at the same time can work at lower evaporating temperature and higher condensing temperature than other solutions for the same heat pump cycle. Besides, the double-stage AHP is advantageous over single-stage AHP in utilizing low temperature driving source, operating in very cold climate and producing higher temperature hot water. Finally, combining the simulated performance with required hot water temperature and climate conditions, the applicability of different AHP cycles (single-stage or double-stage), heat sources (air source or ground source) and working pairs (NH3–H2O, NH3–LiNO3 or NH3–NaSCN) is analyzed in different regions respectively.

Energy saving potential of low temperature hot water system based on air source absorption heat pump

Boilers are widely used to generate low temperature hot water for heating and domestic hot water. In order to improve the efficiency and decrease the emission of pollution in the process of producing low temperature hot water, a heating system based on air source absorption heat pump (ASAHP) is proposed. A simplified model of the proposed system is built based on the heat and mass balance. The model is verified by comparing the predicted results with those from a previous reference. The performances of ASAHP using H2O/LiBr and NH3/LiNO3 as working fluids are simulated at different air temperatures. Based on the simulated performance, energy saving potential of the proposed system applied in four typical Chinese cities is analyzed. The results show that the proposed system can provide energy saving rates of 18%, 28.5%, 37% and 42% in Shenyang, Beijing, Shanghai and Guangzhou respectively, showing great potentials in future applications of low temperature hot water.