Net-zero energy optimization of solar greenhouses in severe cold climate using passive insulation and photovoltaic

To take into account the crop growth of greenhouse and reduce energy consumption, this study investigated to optimize and retrofit a typical solar greenhouse in the severe cold climate of China into a net-zero energy solar greenhouse (NZESG). The envelope passive insulation measure and roof flexible photovoltaic (PV) technique are innovatively combined and some optimization cases were proposed based on simulation and analytical studies. The foamed cement insulation boards with different thicknesses and envelope positions were added as 11 passive insulation retrofit cases. The flexible PV panels in checkerboard (PV1), horizontal (PV2) and longitudinal (PV3) arrangements were positioned on the greenhouse roof and their electrical energy was estimated through PVsyst software. The optimal case was obtained based on the local technical standard for nearly zero energy con-sumption buildings and energy-saving, carbon reduction, payback period and cost multi-objective entropy weight method. The key finding is that passive energy-saving retrofit Case (a) & PV1 has the highest compre-hensive score and is the optimal NZESG case. Its investment cost and payback period of Case (a) & PV1 are 45526.40 CNY and 5.23 years respectively. Case (a) adopts 50 mm foamed cement board with external insu-lation. Although the effect of energy-saving and CO2 reduction is poor, Case (a) has the shortest payback period and the lowest cost. PV1 has the lowest investment and higher power generation. It also shows that the in-vestment payback period and cost weight are high, which are the key parameters for comprehensive evaluation of the retrofit cases; The entropy weight method is objective and scientific. The retrofit measures and optimi-zation method can facilitate the promotion and application of NZESGs.