CLIMATE CONTROL TO REDUCE ENERGY INPUTS
Energy-saving techniques are classified as passive or active types and as static or dynamic types. As indicated by the long argument over "which is better as a covering material, PVC or PE?", selection of covering and thermal screen materials is one of the important aspects of climatic control that can reduce energy inputs to greenhouses in a static sense. Double-layer panels and air-inflated or air-supported houses are new approaches to the same purpose. Introducing mirrors, reflective blinds and Fresnel prism panels increase solar energy in the greenhouse by collecting more energy on larger collecting areas and concentrating it on the greenhouse floor. Ways of modifying coverings in a dynamic way include the introduction of movable screens and pellet houses.
Dynamic control is another aspect of reducing energy inputs. Adaptive control logic might be the best way to achieve the purpose for hot water heating systems. Reducing set points based on a moving average is also promising. The use of control logic to open and close thermal screens at the right time has been studied; control based on a solar clock is sufficient at this moment. Humidity control, especially dehumidification is essential for energy-saving systems. Control mechanisms ranging from heat pumps to silicagel systems are being tested. A combination of feed-forward and -back control is recommended for floor heating systems which have large heating and cooling masses. Heat pumps which can be used not only for heating but also for cooling and dehumidifying, and cogeneration systems which generate electricity and heat are beginning to be used.
Solar energy is widely used to heat greenhouses. It is stored in rockbeds, soil layers, water tanks or PCM materials. Groundwater and waste warm water from power plants are used as economical energy sources. Air from old mines and tunnels can be a conditioned throughout the year economically, but restricted location and capacities of mines make their wide use impractical.
Renewable energies such as wind, and agricultural and industrial wastes are also used, but their locality and availability would be problems.