A Technical And Economic Study Of The Hot Water Cycling Assistant System Using Condensing Heat Recovery From A Vapor Compression Heat Pump Unit

With the improvement of people’s living standard, the centralized air-conditioning systems are widely used in the building region. Meanwhile, the energy consumption for the execution of these air-conditioning systems grows increasingly. Hence it is necessary to apply the energy conservation technology to the air-conditioning systems. It is well known that a chiller in a centralized air-conditioning system under the refrigeration condition in summer can not only provide the cooling load of an air-conditioning system in the whole building, but also expel a lot of condensing heat generated from the condenser to the atmosphere environment in order to keep the stability of the corresponding condensing pressure. The value of condensing heat is1.15～1.3times to the one of refrigerating capacity. In general, the condensing heat is directly discharged to the atmosphere environment by the cooling water system, which causes energy waste, results in a rise of the ambient temperature, and enhances the urban heat island effect to some degree. As a result, the research on new energy conservation technology to recover the condensing heat has become the hot issue and received more and more attention at present.The traditional condensing heat recovery technology mostly focuses on the summer cooling condition of a vapor compression heat pump unit but rarely to its winter heating condition. On the basis of the thermodynamics modeling principle and the heat recovery technology, a hot water cycling system that is compatible with the corresponding vapor compression heat pump unit is proposed in this paper. It can completely recover the condensing heat from the running process of a vapor compression heat pump unit under cooling condition in summer to produce sanitary hot water with suitable temperature, and then hot water is stored in a hot water tank covered by thermal insulators in order to satisfy the sanitary and washing requirements of building users. Under cooling condition in winter, this hot water cycling system absorbs a part of condensing heat and then releases the relevant heat to make liquid refrigerant evaporated in the evaporator such that the corresponding vapor compression heat pump unit can run normally, which can replace the underground water source or soil source subsystem that is compatible with the vapor compression heat pump unit.A vapor compression heat pump unit in a hotel in Lanzhou city is selected as a research plant in this paper, and the above hot water cycling system is designed for this plant. The performance analysis of the proposed hot water cycling system is verified by thermodynamics modeling and simulation. The main results in this research are as follows:1. Aimed at the research plant, the corresponding thermodynamics model is established for this hot water cycling system. The important parts and the different operating mode of this hot water cycling system are principally analyzed.2. According to the research objective, the selection and calculation of main equipments of this hot water cycling system are executed.3. The variation of critical parameters of this hot water cycling system, such as the hot water temperature of the outlet and the inlet of the hot water tank and the hot water temperature in the hot water tank, is numerically simulated and the corresponding curves of these parameters are hourly obtained. Simulation results show that the proposed hot water cycling system is feasible in theory and technology, and the critical parameters that can characterize this hot water cycling system meet the requirements of heating and refrigerating for the corresponding vapor compression heat pump unit. In addition, this hot water cycling system can absorb the condensing heat to produce and sanitary hot water, which reduces the waste of energy and economic cost.4. On the basis of the annual cost method and the static payback period of investment indexes, an economic comparison of this hot water circulation system with the traditional underground water source heat pump units is carried out. The results indicate that the former one is superior to the other ones in terms of the initial investment, the annual cost, and investment payback period.5. The hot water circulation system can recycle condensing heat to produce sanitary hot water in summer, which not only reduces the emissions of condensing heat, but also do not need to consume the fossil fuels for heating sanitary hot water. As a result, it directly reduces the emissions of CO2and has the significant role of environmental protection and energy saving.