Research On Thermodynamic Performance And Design Method Of A Combined System Of Solar Water Heating And ORC Power Generation

Solar energy is a reliable energy source,and solar water heating technology is a very important in solar energy applications.Especially in the qinghai-tibet plateau,where solar energy resources are abundant.However,there are some problems with solar water heating technology,such as the problem that the heat collecting system is in idle during the non-heating season and the equipment utilization rate is low.Moreover,due to the underutilization of the solar water heating system and the lack of operation and maintenance,the solar water heating system has a low service life and poor reliability.This paper proposes a novel combined system of solar water heating and ORC power generation?CSO?,which can run all year round.Compared with the solar water heating system,the CSO system can not only reduce the energy consumption and carbon emissions,but also improve the utilization of system equipment and enhance the reliability of the system.It provides a new idea for the construction of building energy and building environment in the qinghai-tibet plateau.In this study,system configuration,operation modes,thermodynamic characteristics,evaluation index and design method of the CSO system based on the building heating load are analyzed.Three types of solar collectors are considered,including flat plat collectors?FPC?,evacuated tube collectors?ETC?and parabolic trough collectors?PTC?.Firstly,system configuration and operation modes of the CSO system were studied based on the building heating load through theoretical analysis.There is only one configuration of the CSO system,but there are six operation modes for PTC or ETC/FPC.The main difference is that CSO system operates in co-generation mode in the heating season for PTC.But for ETC/FPC,CSO system operates in co-generation mode only when The daily heating load of the building is less than the heat collection of the collector in the heating season and in solar heating mode when the heating load of the building is larger than the heat collection.Secondly,the thermodynamic characteristics of the CSO system are analyzed by the energy and exergy analysis method.Through the formula derivation,the calculatingformulas of total thermal coefficient,thermal coefficient,power generation efficiency,total exergy efficiency,thermal exergy efficiency and electrical exergy efficiency of CSO system under different operating modes are obtained.Several key factors affecting the total thermal coefficient and total exergy efficiency of the system are obtained,including inlet temperature of solar collectors(t_col,in),effective radiation(G_eff),Q_n?d?/Q_use?d?,ambient temperature?t_a?and return water temperature of the heating water(t_n,h).After the further calculation,it was obtained that the exergy efficiency of the system increases with G_effff and t_n,h,h but decreases with t_a.In addition,the exergy efficiency of the system increases with t_col,inol,in at first until the maximum value,then it decreases.Moreover,it was also found that the collector area of the solar collector subsystem has an optimum value for the CSO system from the relationbetween exergy efficiency and Q_n?d?/Q_use?d?.Then,a multi-objective optimization configuration model of mixed integer nonlinear programming for the CSO system is established based on the superstructure theory.The solution method of the optimized configuration model is given based on the improved fruit fly optimization algorithm.The evaluation index“Solar Conversion Index”?SCI?applicable to the CSO system is proposed and the corresponding evaluation system is given.This paper also presents a design method of the CSO system based on SCI?y?.Furthermore,an actual engineering project in Lhasa was studied using the design method based on SCI?y?and the multi-objective optimization configuration method.The equipment capacity of the CSO system was obtained by the two methods and results are compared and analyzed.The relative error of equipment capacity given by the two methods is less than 10%,which indicates that the design method based on SCI?y?of the CSO system is feasible.Research in this paper shows that the CSO system is more economical,energy-saving and environmentally friendly compared with the solar water heating system.It can be operated all year round,which is beneficial to improve the reliability of the system.