Use of accumulated solar thermal energy in greenhouse for extending vegetation period of crop plants

The consumption of thermal energy for growing vegetables in greenhouses depending on the type of energy resource can make up to 40–60% of the production cost structure, so solutions to reduce it are pressing. Research on possibilities to accumulate thermal energy and extend the growing period was conducted in an experimental greenhouse with created gravel thermal mass for thermal energy accumulation in the hot periods and its recovery in the cool periods. The air delivery to the thermal mass and the recovery of thermal energy from it are provided by a fan, regulated by an automated control unit. A sensor system records the outdoor temperature, the temperature in the greenhouse and the thermal mass. The purpose of the research is to find out how much solar energy can be accumulated in the created thermal mass, as well as to study the possibilities of extending the vegetation period of the plants using the accumulated solar thermal energy. Experiments in the greenhouse were carried out on 01.06.2022 to 05.12.2022. At the beginning of the period the average temperature in the thermal mass was 15 °C, in the middle of June it rose to 20 °C, and from then, until 29.08.2022, it kept fluctuating within the range of 20–22 °C. Despite the very high outdoor and greenhouse air temperatures, the thermal mass showed a positive effect by stabilizing the greenhouse and soil temperatures, thus improving the growing conditions. After 01.09.2022 the temperature in the thermal mass gradually decreased, and on 14.10.2022 it reached the initial 15 °C. In the period of 136 days 4175 kWh of energy was accumulated in the thermal mass. From then until 13.11.2022 the temperature in the thermal mass remained stable at 12 °C, which is the minimum, allowed for the plant growth in greenhouses, then a sharp drop followed, and vegetation ended. Under these meteorological conditions the use of the thermal mass allowed extending the vegetation period in the greenhouse by approximately 20 days.