Research On The LPG Vaporization System Utilizing Solar Assisted Heat Pump

Well known as a clean energy and an inexhaustible energy, respectively, LPG and solar energy have been paid more attention to in China due to the shortage of the normal fossil energy and the pollution of the environment. As the supplement of the natural gas pipe network, piped LPG supply systems have been applied widely in China. The solar assisted heat pump technology, as a mature means of solar energy utilization, has attained great development in recent decades.The conventional vaporization way of LPG which is supplied to users by pipes consumes more energy and is not energy efficient. However, the solar assisted heat pump technology can provide medium-high temperature water effectively. It makes the solar assisted heat pump may work as the heat source for the vaporization of LPG. In order to reduce the energy consumption used to vaporize LPG and broaden the application fields of solar-assisted heat pump technology, a novel LPG vaporization system utilizing direct-expansion solar assisted heat pump (named as DX-SAHPV system) has been presented. The DX-SAHPV system mainly consists of a direct-expansion solar assisted heat pump (DX-SAHP), a thermal storage water tank, an auxiliary heat source and a vaporizer. It makes the solar assisted heat pump technology be applied in the field of urban gas and improves the annual utilization of solar assisted heat pumps.Based on the law of conservation of heat, the law of conservation of mass, the vapor-liquid equilibrium principle and the SRK equation of state, a mathematical model of LPG spontaneous vaporization in a cylinder has been introduced for studying the dynamic behavior of LPG spontaneous vaporization. The vapor LPG and the liquid LPG are considered as real gas and real solution, respectively, in the model. The influence of composition variation caused by the characters of non azeotropic mixtures on the physical properties of the vapor and liquid LPG, such as temperature, pressure, latent heat of vaporization and density, has also been taken into account. Numerical simulation of LPG spontaneous vaporization in the ysp-50 type cylinder was performed with initial compositions of liquid LPG and some external parameters. Some existing experimental results were used to verify the model. The results show that the compositions of both vapor LPG and liquid LPG vary instantly during the process of spontaneous vaporization. The LPG temperature approximately maintains a constant value after a significant temperature decrease. The LPG pressure also drops due to dual influences of reduced concentration of low boiling point components in the vapor LPG and fall of LPG temperature. Simulation results show better agreement with the existing experimental data and the practical phenomena of LPG spontaneous vaporization, so that the numerical model has an acceptable accuracy. Based on the numerical simulation, the capcaities of spontaneous vaporization under conditions of constant or variational gas consumption can be calculated. The gas consumption of residential customers and the volatility of gas load are also calculated and analyzed. These works gives a objective support for the reasonable design and the performance study of DX-SAHPV system.In order to make DX-SAHP have good thermo-economy, a thermodynamic model of DX-SAHP based on the finite-time thermodynamic theory has been performed and the heating load per unit total cost has been chosen as the thermoeconomic objective function for optimization. Analytical expressions are derived for the optimal thermodynamic performances, as well as for the optimum design parameters, such as the refrigerant temperatures and the ratios of heat exchange area. The influences of various parameters (such as environmental parameters, performance parameters of heat exchangers and economical parameters) on the optimal performance characteristics and the optimal design parameters of the system are analyzed in detail. The results show that the increase of the specific heating load will be negative for both the COP and the thermoeconomic objective function when the specific heating load is larger than the optimum specific heating load at the maximum thermoeconomic objective function. Internal irreversibility parameter, investment cost per unit heat exchange area, investment and operation cost per unit power, solar radiation intensity on the collector surface, product of absorptivity and transmissivity, and hot water temperature have more effects on the maximum thermoeconomic performance of DX-SAHP. If all other parameters are kept constant, the 10% variation in any one parameter will lead to more than 2% variation in the maximum thermoeconomic objective function.Six operation modes and corresponding principles of the DX-SAHPV system have been introduced based on the character of LPG spontaneous vaporization and the fluctuations both of gas load and of meteorological parameters. A mathematical model for multi-objective optimization design of the DX-SAHPV system has been proposed from the prerequisite of thermo economic optimization of DX-SAHP. In this model, annual cost, primary energy savings and global wanning index are taken as optimization goals and gas supply requirements for the DX-SAHPV system are also considered. The optimal load sharing between DX-SAHP and auxiliary heat source, the optimal design parameters of each component and the optimum form of auxiliary heat source have been analyzed by use of the model. This will help designers in the preliminary optimization of the DX-SAHPV system.Dynamic mathematical simulation models for solar collector/evaporator, condenser, compressor, expansion valve, thermal storage water tank, LPG vaporizer, auxiliary thermal source and the whole DX-SAHPV system have been proposed. After setting up start/stop control regulations of DX-SAHP, auxiliary thermal source and LPG vaporizer, control strategies of the system among six operation modes have been generated. And then, the specific expressions of the model for water tank have been inferred for different operation conditions. On the basis of LPG spontaneous vaporization model, multi-objective optimization design of the DX-SAHPV system, simulation model and control strategies of the system, the yearly operating performance of the system being used to supply gas for a community with 1000 families has been investigated in Beijing. Meanwhile, the use of DX-SAHPV system in some typical regions in China has been predicated under different wether conditions.