• Energy Research

Abstract 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.

Abstract The absorption refrigeration system driven by low grade heat sources, especially the waste heat sources, becomes more and more attractive in recent decades. However, most traditional absorption systems cannot achieve a high utilization rate of the waste heat with limited heat capacity. These systems are usually designed to obtain heat in the generator, which means that the waste heat sources cannot be utilized to the temperature lower than the generator temperature. This paper proposed a new structure heated by heat conduction oil in the generator and electric heating rings around the stripping section. This structure can simulate the temperature-distributed heat sources when the electric heating rings work. It can also simulate a traditional generator when the electric heating rings do not work. Influences of different heat distributions are analyzed in detail in this paper. The results show that the heat sources utilization rate will increase with the increase of the heat in the stripping section, while the coefficient of performance will be negatively affected by the increasing heat in the stripping section. By optimizing the heating structure, the coefficient of performance can be similar to that of a traditional system when the heat is just added in the middle and lower part of stripping section. The optimum utilization rate of heat sources in this test model can reach 1.8 times to that of a traditional system. Under this heating model, the lowest temperature required in the heating section is 82 °C when the heat conduction oil inlet temperature is 169 °C. It is much lower than the temperature inside the generator, which is 137.3 °C.

Abstract Heat capacity and temperature matching between the heat sources and energy system is of vital importance to improve energy efficiency, especially in occasions with multiple temperature-distributed heat sources. In this paper, an improved ammonia-water absorption refrigeration system with additional intermediate-pressure generator and absorber is proposed, aiming at recovering two kinds of heat sources with different grades. To match the heat capacity of two sources, solution distribution to the high- and intermediate-pressure generator can be adjusted. Moreover, heating areas of the two generators are extended from the reboilers to the stripping sections, in order to improve the heat utilization ratio. Simulation study shows that as the solution split ratio to the intermediate-pressure generator increases, the capacity for recovering low-grade heat sources is improved. The coefficient of performance of the proposed system is 5% higher than that of the modified vapor exchange system within a large range of the intermediate pressure. Under the baseline working condition with evaporation temperature of -15°C and condensation temperature of 31°C, a significant increment in exergy efficiency is achieved by the proposed system, which is 32.6% and 56.5% higher than that of the conventional single-effect and conventional vapor exchange system, respectively.

Abstract An ammonia–water absorption refrigeration system with a novel generator structure driven by temperature-distributed heat sources to increase the utilization rate of waste heat is presented. The stripping section of the distillation column and generator were replaced by a generator with temperature-distributed heat sources. The heat sources were distributed throughout the entire process in the generator, so the mass and heat transfer were coupled at different temperatures, which increased the waste heat utilization at low temperatures. Compared with a traditional system, temperature–distributed heat sources increased the utilization temperature range. In the simulation of the system, the condition at cold end kept unchanged while the heating distributions and quantity inside the generator were changed. The performance of the new generator driven by temperature-distributed heat sources was analyzed while compared with a traditional system through the coefficient of performance, separation efficiency and waste heat utilization.

Abstract In a variety of waste heat utilization technologies, absorption refrigeration can be driven by low-grade heat to provide cooling capacity, which is suitable for situations with both waste heat supply and refrigeration demand. In this paper, an ammonia-water absorption refrigeration prototype with a new generator structure is experimentally studied. Its economic analysis is carried out for pre-cooling in a liquefied natural gas plant. It is investigated that 1.8 kW cooling capacity at −10 °C is obtained, COP maintains between 0.29 and 0.35, and waste heat recovery ratio is improved by 150%. The proposed system is applied to a 30,000 Nm3 d−1 liquefied natural gas plant to replace the conventional compression refrigerator as the pre-cooling stage, and waste heat from exhaust gas and jacket coolant is recovered in the reboiler and stripping section, respectively, in order to provide cooling capacity for pre-cooling of both natural gas and refrigerant. Economic analysis shows that specific power consumption of the modified system is reduced by 30%, from 0.40 to 0.28 kWh Nm−3. Consequently, the annual natural gas consumption and operating costs are reduced by 17.3% and 9.4%, respectively, and the payback period is 2.2 years compared to conventional systems with compressor-based pre-cooling stages.

Abstract Among district heating technologies, gas-fired boiler has difficulty in recovering flue gas waste heat and conventional heat pump has poor performance at low ambient temperature. In the present work, a novel gas-fired absorption heat pump is proposed. High-grade sensible heat and low-grade latent heat of the flue gas is recovered in the solution preheater and intermediate evaporator successively. Moreover intermediate evaporation and absorption processes are introduced, and the intermediate pressure is adjusted according to the ambient temperature, to improve the system adaptability in cold regions. Simulation results show that the coefficient of performance (COP), primary energy efficiency and exergy efficiency are 1.69, 1.39 and 54.5% when the evaporation, supply and return water temperature are −15, 55 and 35 °C, respectively. The COP of the proposed system has 13% and 20% improvement compared to those of the single-effect absorption system with and without waste heat recovery. Furthermore, the proposed system has energy saving potential of 11.7% and 39.6%, and payback time of 2.9 and 2.5 years compared to the single-effect absorption heat pump and the gas-fired boiler, respectively. The proposed system is suitable to provide 50 kW close range district heating for a typical urban residential building, especially in cold regions.