Optimizing the thermal energy contribution of a solar energy storage system in a greenhouse
A mathematical model developed by Ntinas et al. (2014) describes the thermal performance of a hybrid solar energy saving system in a heated greenhouse. According to this model, a large amount of the heat released from the solar system, by thermal radiation, is either transferred to the greenhouse cover and lost to the outer atmosphere, or it is transferred to the white ground cover, when the system is placed on the ground of the greenhouse. The purpose of the present study is to examine two test cases which can optimize the operation of the solar energy storage system and its contribution to the greenhouse thermal needs. The two cases are a) installing the solar system in a higher position from the ground and b) installing thermal-insulating screens under the greenhouse cover. The first case will make a higher amount of heat available to the greenhouse crop during night-time; since the heat transferred from the sleeves to the white ground cover can reach 8% of the sleeves released thermal energy. Installing the solar sleeves on benches, this amount can be available for the plants and the surrounding air. The second case deals with the reduction of heat losses from the greenhouse cover. According to the model results, heat losses by thermal radiation from the greenhouse cover can reach more than 50% of the heat released from the solar sleeves. By installing thermal screens under the greenhouse cover this percentage will change in the benefit of the crop, since the temperature of the screens will be higher than the greenhouse cover which tends to be very close to the external air. Therefore, the heat transfer by thermal radiation from the solar sleeves to the plants can be doubled.
Ntinas, G.K., Fragos, V.P. and Nikita-Martzopoulou, C. 2017. Optimizing the thermal energy contribution of a solar energy storage system in a greenhouse. Acta Hort. (ISHS) 1170:875-882
solar sleeves, insulation screens, tomatoes, thermal radiation, overall heat transfer coefficient