Study On Transportation Of Low-Grade Thermal Energy Over Long Distance By Ammonia-Water Absorption Technology

Transportation of low grade thermal energy over long distance is a newly developed research subject since the end of last century. It has gained more and more attention in the utilization of the waste heat from nuclear power generation and industrial processes, and it has been considered as a possible direction to improve the energy utilization efficiency. Several methods have been developed but none of them is competitent for overcoming the long distance problem to utilize the low grade thermal energy which locates far away. These methods include reversible chemical reactions, heat storage materials, hydrogen absoring alloys, solid gas adsorption and liquid gas absorption, etc, in which liquid gas absorption is a novel way which has been introduced in recent years. The rearching work on it has been rarely reported. In this thesis, the transportation of low grade thermal energy over long distance by ammonia water absorption technology was deeply studied. The following main points are involved.(1) A series of cycles were analyzed to generally investigate the feasibility of the transportation of low grade thermal energy over long distance by ammonia wate absorption technology. These cycles included single stage ammonia water absorption cycle, double effect cycle, simple cascade cycle, typical two stage cycle, two stage cycle with expanding process of the weak solution from generator, compression absorption hybrid cycle, ejection absorption hybrid cycle, GAX cycle and diffusion absorption cycle. The comparison showed that the single stage cycle was the best choice, and the simple cascade cycle, the two stage cycle with expanding process and the compression absorption hybrid cycle were also suitable for the transportation of low grade thermal energy over long distance.(2) The thermodynamic and hydraulic analyses were performed on the single stage ammonia water absorption cycle to transport low grade thermal energy over long distance. The results showed that it was capable of producing heating or cooling for residential use. When a heat source at 120°C is available, in summer the single stage cycle can output cooling above 5°C at the user site for air conditioning use, or output heating below 60°C for floor radiation heating or hot water supply. When a heat source at 130°C is available, the cooling and the heating thermal COP can both reach as high as 0.5 and the electrical COP can both reach as high as 50.(3) A new cycle was proposed in order to extend the output heating temperature range at the user site. This cycle took the advantages of the two stage cycle, double effect cycle and the TYPE 2 absorption heat pump. It can self support a second heat source at the user site and lift the output heating temperature without extra transporting burden. The simulation showed that the new cycle was very suitable for producing high temperature heating over long distance in cold areas. When the output heating temperature is as high as the heat source temperature, the coeffient of performance reaches above 0.3. Based on the new cycle, two types of simplified cycles were introduced. The TYPE 1 simplified cycle is suitable for producing high temperature heating at high ambient temperatures, while the TYPE 2 is suitable for producing low temperature cooling at high ambient temperatures. The new cycle with its two types of simplified forms and the single stage cycle compose an all the year around solution for the transportation of low grade thermal energy over long distance. This solution can adapt a wide output temperature range from 30 to 140°C and a wide ambient temperature range from 20 to 40°C if a heat source at 120°C is available.(4) A compact small scale experimental prototype was designed and constructed in order to verifier the thermodynamic feasibility of the single stage ammonia water absorption cycle for the transportation of low grade thermal energy over long distance. The annual experimental data concerning producing cooling in summer and procucing heating in the others seaons have been gotten for evaluating the thermal performance of the prototype. It can be considered that the thermal feasibility of the single stage ammonia water absorption cycle for the transportation of low grade thermal energy over long distance has been basically verified.(5) The economical analysis was performed by a calculation example. It was shown that the payback period for constructing the pipeline between the source site and the user site was about 15 months. The filling amount of the working fluid in the pipeline varies in different seasons, which leads to a great inconvenience for all the year around running. A pracitical suggestion was introduced in order to overcome this problem, i.e, the concentration of solutions can be kept constant all through the year, but the heat source temperatures and the user output temperatures have to submit the environment temperatures.