Thermal energy use in three Swedish greenhouses – the outdoor temperature-dependent variation and the influence of wind speed under no-sunlight conditions

Abstract

Thermal energy is used in greenhouses for the temperature and humidity control.  In the North of Europe, energy expenditures have a significant share in the total greenhouse production cost.  A study was performed to quantify and analyze the use of thermal energy in three Swedish commercial greenhouses (two with tomatoes and one with ornamental plants), to test the hypothesis that substantial amounts of heat are used at higher outdoor temperatures, and to investigate the influence of the wind speed on the energy use under no-sunlight conditions.  Hourly climate data were registered by means of Priva control systems.  The use of thermal energy was computed based on the readings from the flowmeters and the temperature of the inlet/outlet water to/from the boilers.  In the study, seven different ranges of outdoor temperatures for studying energy use at various outdoor temperatures were considered.  The use of thermal energy in Greenhouse A amounted to 793 MJ/m² (April-October).  The yearly use of thermal energy in Greenhouse B with tomatoes was 1529 MJ/m² and in Greenhouse C with ornamental plants 873 MJ/m².  The results showed a positive relation between the wind speed and the use of thermal energy.  Under no-sunlight conditions, correlation coefficients between the wind speed and the use of thermal energy obtained for the various outdoor temperature ranges being considered were in the range of 0.017 to 0.470.  Calculations suggested that a reduction of the wind speed, e.g. by a fence or trees, by 50% may result in a 4% to 10% decrease of thermal energy use.  In one of the greenhouses (A) the magnitude of thermal energy use at higher outdoor temperatures (15°C to 20°C) was similar to the use in the same greenhouse at outdoor temperatures of 10°C to 15°C.  It was noted that the use of thermal energy is dependent on more factors than the wind speed and the outdoor temperature and that the importance of energy efficient dehumidification should be studied in future work.

Keywords: Thermal energy use; greenhouses; tomatoes; ornamental plants; wind speed