A quasi-steady state model for predicting the heating requirements of conventional greenhouses in cold regions

A time-dependent,quasi-steady state thermal model(GREENHEAT)based on the lumped estimation of heat transfer parameters of greenhouses has been developed to predict the hourly heating requirements of conventional greenhouses.The model was designed to predict the hourly heating requirements based on the input of greenhouse indoor environmental control parameters,physical and thermal properties of crops and construction materials,and hourly weather data including temperature,relative humidity,wind speed,and cloud cover.The model includes all of the heat transfer parameters in greenhouses including the heat loss for plant evapotranspiration,and the heat gain from environmental control systems.Results show that the predicted solar radiation data from the solar radiation sub-model are a reasonable fit with the data from the National Solar Radiation Database(NSRDB).Thermal analysis indicates environmental control systems could reduce 13–56%of the total heating requirements over the course of a year in the study greenhouse.During the winter season,the highest amount of greenhouse heat is lost due to conduction and convection,and the heat used for evapotranspiration is dominant in the summer.Finally,the model was validated with actual heating data collected from a commercial greenhouse located in Saskatoon,and the results show that the model satisfactorily predicts the greenhouse heating requirements.

Heating demand and economic feasibility analysis for year-round vegetable production in Canadian Prairies greenhouses

Greenhouse vegetable production in Canadian Prairies is important for creating a sustainable regional food economy,especially in northern communities.This study included the estimation of heating demand for year-round production and evaluation of the economic feasibility of greenhouse vegetable production(tomato,cucumber,and pepper)in a conceptually designed greenhouse(0.6 ha)located in remote northern communities in Saskatchewan,Canada.The heating simulation was based on a greenhouse heating simulation model(GREENHEAT)developed by the authors recently.The simulation results showed that the annual heating requirement for the production of tomato,cucumber,and pepper are 1486 MJ m^2,1657 MJ m^2,and 1754 MJ m^2,respectively.The economic analysis indicates the net return(NR)from the production of tomato,cucumber,and pepper,are C$69.2/m^2(in Canadian dollar,CAD),C$41.5/m^2,and C$43.8/m^2,respectively,based on the market price C$3.5/kg,C$2.7/kg,and C$8.0/kg,and yields of 55.0 kg m^-2,65.0 kg m^-2,and 23.0 kg m^-2.The net present value(NPV)for the tomato,cucumber,and pepper production are C$1.9 M,C$1.2 M,and C$1.1 M,respectively,and the benefit-cost ratio(BCR)are 1.38,1.21,and 1.21.The economic feasibility analysis indicates the year-round production of vegetables in a greenhouse at remote northern Saskatchewan would be economically profitable.