Energy Saving: from Engineering to Crop Management

In greenhouse horticulture, energy costs form an increasingly larger part of the total production costs. Energy is primarily used for temperature control, reduction of air humidity, increase of light intensity and CO2 supply. Use of fossil energy can be reduced by limiting the energy demand of the system and decreasing energy losses, by intelligent climate control, by increasing the energy efficiency of the crop and by replacing fossil energy sources by sustainable ones. Energy requirement of the greenhouse can be lowered up to 20-30% by using greenhouse covers with higher insulating values and the use of energy screens. A prerequisite is that these materials should not involve considerable light loss, since this would result in a loss of production. In energy efficient greenhouse concepts, durable energy sources should be included. In (semi-)closed greenhouses, the excess of solar energy in summer is collected and stored in aquifers to be reused in winter to heat the greenhouse. Ventilation windows are closed, with specific benefits to the crop: high CO2 levels can be maintained, and temperature and humidity can be controlled to the needs of the crop. Development of new greenhouse concepts is ongoing. Current examples are greenhouse systems which convert natural energy sources such as solar energy into high-value energy such as electricity. Given a certain technical infrastructure of the greenhouse, energy consumption can be further reduced by energy efficient climate control and crop management. Essential elements are to allow fluctuating temperatures, lower crop transpiration, allow higher humidities, make efficient use of light and create fluent transitions in set points. Consequences for plant growth are related to rate of development, photosynthesis, assimilate distribution, transpiration and the occurrence of diseases or disorders. Since processes involved are complex, knowledge exchange between researchers and growers is essential to realize the goals set to reduce the energy consumption. INTRODUCTION In current greenhouse horticulture, next to high production levels, quality and timeliness of production are important. This can be reached by optimal control of greenhouse climate for which energy is of major importance. The need for (energy) cost reduction has become higher, since with increasing prices of natural gas in the last decade, energy forms a substantial fraction of the total production costs. The liberalisation of the energy market for growers since 2002 has increased the growers’ awareness of the energy consumption of their cropping systems. This free market implies that growers do not pay a fixed price per unit of natural gas anymore, but that prices are greatly determined by the maximum supply capacity of the gas contract. Therefore, it is important to reduce peaks in energy use. In view of the Kyoto protocol several governments have set goals for energy use and CO2 emission. In the Netherlands, the horticultural sector and government have agreed to improve the energy efficiency (production per unit of energy) by 65% in 2010 compared to 1980 and to increase the contribution of sustainable energy to 4%. Over the period 1980-2005, energy efficiency in the Dutch greenhouse industry has more than doubled. However, total energy use per a [email protected] Proc. IS on High Technology for Greenhouse Systems GreenSys2009