Theoretical Model And Benefit Evaluation Of The Complementary Coupling Energy Supply System Of Solar Energy And Above-ground Biogas Digester

The abundant solar energy and biomass resources in Northwest China provide favorable conditions and guarantee for the integration of solar low-temperature collector and biomass anaerobic fermentation technology.The construction of a multi-energy complementary coupled with energy supply system can tackle the shortcomings of the single-energy technology utilization.The shortcomings of low density,fluctuating discontinuity,and instability of solar energy flow result in a low guarantee rate in the process of solar thermal utilization,which needs to be supplemented by other energy sources;the low winter temperature in rural areas of Northwest China leads to the low and even no gas production in biogas digesters,so it is essential to maintain a constant temperature of biogas digesters for fermentation in order to increase gas production.Solar energy collects heat to maintain the constant temperature required for anaerobic fermentation and reduce temperature fluctuations by providing customers with stable,continuous cooking gas and domestic hot water.Based on the establishment of the mathematical models of each subsystem of the complementary coupled energy supply system of solar energy and above-ground biogas digester,and combined with the hourly dynamic load in rural areas,the whole process mathematical model of energy supply,productivity and utilization is established in this paper.The operating performance of each subsystem and the heat contribution rate of solar heat collection subsystem and biogas production subsystem to the biogas digester and domestic hot water were studied by simulation and theoretical analysis.The 3E multi-attribute evaluation method,which takes energy benefit,environmental benefit and economic benefit as evaluation indexes,is used to comprehensively evaluate the system,which provides a theoretical basis for the design and application of the complementary coupling energy supply system of solar energy and biogas digester.The main research contents and conclusions are as follows:（1）The typical new rural residential buildings in Minqin,Gansu,were chosen as the research object in accordance with the Technical Standard of Solar Heating and Heating Engineering（GB50495-2019）,and the heat balance relationship of the biogas digester was used to determine the annual heat load of the biogas digester in the Minqin area.The investigation and analysis results obtained from users’hot water habits and the calculation of the hourly domestic heating load in four seasons are in accordance with GB50555 Design Standard for Water Saving of Civil Buildings.The annual biogas load of household cooking was obtained according to the non-uniformity coefficient and correlation analysis method.（2）A complementary coupled above-ground biogas digester and solar energy supply test system has been built and its feasibility and actual operation performance were measured.Analysis of the parameters for the inlet and outlet temperatures of the solar collector,the storage tank temperature,the temperature of the biogas tank,and the production of gas from the biogas tank revealed}that the storage tank temperature fluctuated significantly during the test period,with an average value of 56.07°C.The biogas digester’s temperature remained about 28°C during the period of stable gas production stage.The average daily output of biogas during the period of the test was 1.09 m3,with an average methane content of 54.8%.（3）The characteristics of nonlinear modules such as solar collector,biomass anaerobic fermentation and biogas boiler in the coupled energy supply system of solar energy and above-ground biogas digester were systematically considered.The whole process mathematical model of energy supply,productivity and energy use was established.Experimental data were used to verify the model’s accuracy,and each subsystem’s operational performance was analyzed.Meanwhile,the heat contribution rate of the solar heat collection and biogas production subsystems to the biogas digester and domestic hot water has been studied.During the entire period of heating time,the heat contribution rates of the solar heat collection subsystem and the biogas production subsystem to the biogas digester were 10%and 9.29%,The domestic hot water heat contribution rate is 90%and 90.71%,respectively.The total heat demand of the system is 4636.22MJ,and the total heat supply is 4728.96MJ,which can fully meet the energy demand of users.（4）The results demonstrated that the primary energy utilization rate of both the traditional energy supply mode and the complementary coupled energy supply system are low,at 35%for the traditional energy supply mode and about 8%for the complementary coupled energy supply system.However,the environmental benefits of the complementary coupled energy supply system are obvious,and the CO₂ 410.26kg can be reduced in the whole heating season,and the CO₂ emission reduction rate is more than 40%,with the maximum value of86.7%,The NO_x emission reduction rate is greater than 90%,and the average value is98.13%.