Design of a greenhouse energy model including energy exchange of internal plants and its application for energy loads estimation

Abstract

The greenhouse cultivation in South Korea has increased dramatically in the past few decades and most of the greenhouses use fossil fuel as an energy source. As interest in new energy sources that can replace fossil fuel has increased globally in recent years, the greenhouse industry in South Korea has also attempted to utilize renewable energy for heating and cooling systems. Moreover, it is important to analyze energy use in greenhouses by crops species and growing stage to determine how the new energy source will be utilized. However, greenhouse energy loads have been calculated by assuming that the crops are not in cultivation or without considering the characteristics of each crop using prior research, in spite of that a large portion of radiation which penetrates the cladding use for crop transpiration. As energy exchange by crops take a large part in greenhouse energy balance, there should be a quite difference in energy loads whether the energy exchange by crops is considered. In this thesis, the dynamic energy exchange model of greenhouse was designed to calculate greenhouse energy loads hourly for the essential prerequisite to apply new energy source in the greenhouse. Literature reviews were conducted for a calculation method to build energy consumption, an energy balance equation to simulate the greenhouse environment, and a model for energy exchange by crop. These were used as methodology for the model design. As a result, a dynamic analysis method was used to design the greenhouse energy model and the empirical equation by Stanghellini (1987) was used to realize the energy exchange between crops and ambient air in the greenhouse. The target was an eight-span plastic-covered greenhouse, which applied thermal effluent from a nearby thermal power plant on a trial basis as a heat source of the heat pump. Field experiments were conducted to collect the greenhouse structural characteristics, working schedule of the heating, ventilating and air conditioning (HVAC) system, and crops characteristics for the design and validation of the dynamic energy exchange model of greenhouse. The greenhouse energy model was designed by using one of the commercial building energy simulation programs TRNSYS (Transient Systems Simulation Program). The entire greenhouse energy model was designed in three parts: greenhouse structure modeling, crops energy exchange modeling, and thermal effluent – heat pump modeling based on measured data. To realize the energy exchange by crops, the average leaf area index (LAI) of cultivated crops in the greenhouse was measured. Then, the regression equation of the stomatal resistance was derived. The micro-climate data inside and outside the greenhouse, which include solar radiation, and air temperature, were also used to validate the greenhouse energy model. The crop transpiration rate was used to validate the energy exchange model of the crop while the greenhouse internal air temperature was used to validate the entire greenhouse energy model. As a result, the designed crops energy exchange model provides a good estimate of the transpiration rate throughout the experimental period (R2 = 0.96, d = 0.99) and the designed dynamic energy exchange model of greenhouse provides a good estimate of the greenhouse internal air temperature (R2 = 0.97, d = 0.99). The validated dynamic energy exchange model of greenhouse was used to calculate the hourly greenhouse energy loads based on 10-years of weather data. The periodic energy loads were calculated by the sum of the hourly data based on the Irwin mango growth period and the maximum energy loads found for each period. The periodic energy loads were used to analyze the energy cost by two energy sources: the kerosene boiler and the thermal effluent – heat pump system. The average energy cost when using the thermal effluent – heat pump system was analyzed to be 68.21% lower than the kerosene boiler. The proper performance of heat pump was calculated according to the design standard which was provided by National Institute of Agricultural Engineering (NIAE) of the Rural Development Administration (RDA) of South Korea. It suggests that the proper performance of heat pump in greenhouse should be 70% of the maximum cooling and heating loads of last five years. The maximum cooling and heating loads of the last five years were 518,703 and 469,872 kJ/hr, respectively, therefore the proper performance of heat pump was calculated as 363,092 kJ/hr in the cooling capacity and 328,910 kJ/hr in the heating capacity. The results were compared with the performance of heat pump installed in the target greenhouse. The rated cooling and heating capacity were designed to exceed about 28.7% and 20.8% of the proper performance, respectively, due to the different method used to calculate the greenhouse energy loads. The effect of crop energy exchange to energy loads was analyzed by comparing the energy loads dependent on the existence of crops. The annual and maximum cooling loads were increased on average by 23.23% and 14.25%, respectively, and the heating loads were decreased on average by 11.02% and 7.56%, respectively, when assuming the crops are not in cultivation. Therefore, the energy exchange between crops and ambient air in the greenhouse should be considered when calculate the energy loads of greenhouse.