Heat And Mass Transfer In Solid-gas Reaction Heat Transformer And System Performance

With the development of world economy and increment of energy demand,'energy saving and environment protection'has become the common concern around the world. Solid-gas reaction heat transformer was the system based on reversible solid-gas reaction and natural material, and was able to effectively recycle and utilize low-temperature waste heat sources. Comparing with conventional vapor-compressed heat pump system, solid-gas reaction heat transformer system has the following advantages: 1) less consumption of electric power; 2) no reciprocate-motion device; and 3) less noise. As to solid-gas reaction heat transformer system, the working pair of metallic chloride and ammonia has large reaction heat and was the best choice available for recycle of wild-range waste heat. But there are several problems in the application of solid-gas reaction heat transformer system. Firstly, thermal conductivity of metallic chloride is very low (e.g. the thermal conductivity of CaCl₂ is only 0.1-0.3W·m^-1K^-1), so heat generation or consumption in solid-gas reaction can not be effective transferred, leading to low system power (SHP). Secondly, serious agglomeration phenomenon was happened in repeating synthesis/decomposition process, and it significantly deteriorated mass transfer and reaction performance. Thirdly, low system performance (i.e. SHP, COP and COP_ex) led to minimal economic value. For solving all of above problems, investigation on'heat and mass transfer in solid-gas reaction heat transformer and system performance'was conducted in this thesis. Based on this work, the main contents in this paper include as follows:(1) Two types of basic solid-gas reaction heat transformer system (i.e. E/C + R and R + R) were compared, with the concern of temperature upliftΔT, system COP and COP_ex, as well as option of working pairs. As a result, R + R system was taken as the subject of investigation in this paper. Equilibrium temperature drop and incomplete heat recovery were found as two major factors leading to thermodynamic irreversibility of solid-gas reaction heat transformer system.(2) Effect of gas volume in solid-gas reaction heat transformer system on reaction rate balance and system performance, mainly as system SHP, COP and COP_ex was discussed in this paper. It was concluded that gas volume has balance effect on reaction process of low-temperature salt and high-temperature salt in solid-gas reaction heat transformer system. In the case of small gas volume, system power SHP, system COP and COP_ex were improved.(3) Physical parameters, e.g. thermal conductivity and gas permeability, of composite reactive block based on expanded graphite matrix were investigated. The theoretical model took consideration of the physical structure of composite reactive block and the physical and chemical processes suffered. Theoretical results were compared with those experimental data from references; good agreement was concluded. The influences of heat and mass transfer performance and system structure on system performance were also conducted in this paper. It was concluded that system performance was mainly confined by heat transfer process. Optimistic thermal conductivityλand heat transfer coefficient U were also proposed.(4) Experimental set-up of single-stage solid-gas reaction heat transformer system was developed and its performance measured. The main system performance was:ΔT = 30℃, SHP = 254W·kg^-1, COP = 0.19 and COP_ex = 0.23. With the experimental set-up, feasibility of single-stage solid-gas reaction heat transformer system was proven; the effects of multi-step reaction and system operation mode on system performance were also investigated. It was confirmed that closed protocol on gas valve control was favored for system performance improvement, i.e. system power SHP, system COP and COP_ex.(5) To improve temperature uplift, novel two-stage solid-gas reaction heat transformer system was proposed and the experimental set-up was developed. Theoretical comparison between novel system and other two-stage solid-gas reaction heat transformer systems in references suggested that reliability, operation safety and system performances were better in the novel system. Preliminary experiments were conducted and system performance was measured, with significant improvement of temperature uplift, i.e. maximum temperature upliftΔTmax was approaching 70℃.