Operational Performance And Optimization Analysis Of The Solar Photovoltaic And Photothermal Hybrid Energy Supply System Applied To Residential Buildings

The carbon emission of the whole life cycle of buildings in China accounts for over 40% of the total carbon emissions,and the carbon emission from building operation accounts for over 50% of the total carbon emission.Reducing the carbon emission from building operation is of great significance for reducing the total carbon emission and achieving carbon peaking and carbon neutrality goals on schedule.The use of renewable energy,such as solar energy,is an important measure to reduce building energy consumption.The western and plateau regions of China are rich in solar radiation resources and have large heating demand.The photothermal(PT)system is widely used in local residential buildings,but this system has the higher utilization rate in winter and the opposite in summer,resulting in the prominent seasonal contradiction between energy supply and demand,leading to problems such as overheating of the solar collector and serious waste of solar energy resources.When using photovoltaic(PV)system for energy supply,the PV efficiency is relatively low,and it is difficult to meet the heating load with high intensity and long duration in winter.The unsynchronized changes in system energy supply and building energy consumption can lead to the contradiction between supply and demand.Resolving the contradiction is the core of making full and efficient use of solar energy and achieving lower carbon emissions in building operation.This topic combines PV system and PT system to supply energy for residential buildings,in order to solve the current problems of mismatched solar energy supply system in buildings and low energy efficiency.However,in the design of solar photovoltaic and photothermal hybrid energy supply system(PV-PTHS),the building thermal load is jointly met by PV system and PT system,and the electricity required by the auxiliary heat source of the PT system is provided by the PV system.The electrical load borne by the PV system also changes with the capacity of the PT system.Therefore,the design method of individual PV system and PT system is not applicable to the PVPTHS.The capacity allocation design method of the PV-PTHS needs to be studied.Therefore,the design method for the PV-PTHS based on the above problems is proposed in this study.The specific contents are as follows:Firstly,the mathematical model for theoretical analysis of the PV-PTHS is established.On the basis,typical cities in the western and plateau regions of China are selected to explore the impact of equipment capacity on solar system fraction and system exergy efficiency.The operation performance of the PV-PTHS is analyzed and compared with individual PV system and individual PT system.Secondly,the optimization model is established with the minimum levelized average energy cost(LCOE)as the objective function and PV area,PT area,volume of heat storage tank and rated power of battery as the decision variables.The optimization and comparative analysis are carried out for the two modes of PV residual power on-grid and off-grid.The sensitivity of economic parameters to the optimization results is analyzed.Finally,based on the economic optimization model,the impact of energy conservation as the constraint on the optimization results is added,and the impact of heat loss and exergy efficiency as the constraint on the optimization results are analyzed.The fitting relationship between the system exergy efficiency and the solar system fraction is obtained.Based on the system exergy efficiency,the solar system fraction is calculated to obtain the design method of the PV-PTHS.The design method is further elaborated with reference to engineering cases.The main conclusions are as follows:(1)The analysis of the energy flow and exergy flow of the PV-PTHS shows that the energy loss of the PV cells is the largest,followed by the solar collector.On the contrary,the exergy efficiency of the PV cells is greater than that of solar heat collectors,while the low exergy efficiency of solar heat collectors makes the exergy efficiency of the PT system lower.(2)When optimizing simply with the goal of optimal economic performance,the capacity obtained in the mode of residual power off-grid connection mode will result in greater heat loss and smaller electrical loss.The optimized capacity of the residual power on-grid mode results in significant power loss for the PV system,but it is mainly due to the limited energy conversion efficiency of the equipment,which is difficult to avoid by adjusting the equipment capacity.(3)The designed system exergy efficiency is divided into three categories: better economy,better energy conservation,and equivalent system performance.The recommended range of exergy efficiency for the corresponding categories is provided.The overall range for the exergy efficiency of Nagqu is 6.90% ～9.50%.(4)Based on the system exergy efficiency,the solar system fraction is further designed to obtain the design method of PV-PTHS.Compared with simulation result,the design method has the higher LCOE.In typical engineering cases,the LCOE is 0.57CNY/k Wh,increased by approximately 3.27%.In the western and plateau regions of China,the PV-PTHS is adopted to provide energy for residential buildings throughout the year,solving the current problems of mismatched solar system energy supply and building energy consumption,and low energy efficiency.The design method for the PV-PTHS with the goal of optimal economic performance and the constraint of energy conservation is proposed,providing certain reference value for the design of solar energy supply system in engineering.