Abstract:
The aim of this investigation is to determine if it would be financially viable to use alternative energy technologies in the heating and cooling of greenhouses that are used to grow temperature-sensitive crops all year round in the central region of South Africa. For the purpose of this study “alternative energy sources/technologies” will refer to technologies that can be used to collect energy directly from the primary source (like the sun, for example). A greenhouse was constructed and equipped with a natural ventilation system as well as a solar heating system that consisted of a flat plate solar water heater, a water storage system and a regulated heat exchange system inside the greenhouse. Cut-roses were grown in raised beds with a heat exchange system installed underneath the enclosed beds. An automatic weather station was used to read and log the climatic variables inside as well as outside the greenhouse. Data loggers were placed underneath the enclosed raised beds and inside the water storage to log the variations in temperatures. Data was collected over two growing seasons with the required alterations being made to the system for the second growing season. Extreme weather conditions were experienced during the experimental growing seasons and were very helpful in determining the applicability of the system. Data obtained from the experiment was plotted on Excel sheets, while theoretical steady-state as well as a transient temperature model were developed to determine the heating requirements during cold winter nights and cooling requirements during hot summer days. From these models the required sizes and efficiencies of the heating and cooling systems could be determined and were ultimately used to develop a financial model that could be used to determine the financial viability of applying these technologies. Results showed that naturally ventilated greenhouses could not be cooled below an internal temperature that exceeded the external temperature by at least 5oC. The efficiency of the constructed solar water heating system was approximately 40%, while the required collector area was approximately 2.5 to 3 times the area of the greenhouse, making it very difficult to ensure the financial viability for the application of the solar water heating system in particular. During moderate climatic conditions a naturally ventilated cooling system can, however, be used effectively to obtained the required climatic growing requirements.
