The energy consumption is a key environmental pressure for greenhouse horticulture in both cold and warm climates.
This can be minimised by using renewable energy sources and better insulation of greenhouses (Vox et al., 2010). This is relevant for all regions of Europe. Indeed, although the Mediterranean Basin and the northern European areas have significant climatic differences, they share the same objectives in greenhouse management.
During autumn and winter, the radiation entering should be maximized and energy loss should be minimised. During summer and spring, high temperatures must be avoided. Indicative figures for annual energy use in horticulture greenhouses are in the order of 1,900 MJ/m2 (measured for Scandinavia) for northern European climatic conditions, and 500-1,600 MJ/m2 in the Mediterranean Basin. However, this energy use is increasing due to more use of heating to achieve earlier production, and to the use of cooling systems.
Average energy use accounts for 10-30% of the total production costs depending on the region (FAO, 2013). Having said so, the different climate conditions between Northern and Southern Europe determine to a large extent the most appropriate climate control technology. In northern Europe, heating is a primary issue and the geothermal potential for heating the greenhouse should be investigated. In southern Europe, instead, it is the cooling systems that are usually more important.
This BEMP describes a full set of techniques to reduce the energy demand of greenhouses and meet it with on-site renewable energy generation:
- Dynamic control of climatic parameters
- Geothermal heating system for greenhouses in Northern Europe
- Cooling system in Southern Europe
Even before considering them, the initial step in the design of an energy efficient greenhouse is the characterisation of the climate of the area.
Parameters like temperature, humidity, sun, wind and snow must be determined in order to select structural materials, orientation, window-positioning and eventually the proper heating system technology (Panagiotou, 1996).
Dynamic control of climatic parameters The management of climatic conditions within the greenhouse is strongly linked to the growth of the crop and eventually to production (Vox et al., 2010). Each crop species requires different climatic conditions, while for a given species optimal temperatures for different phenological stages can differ.
The usual temperature range for crop species is from 10 to 24oC. Under sunny conditions, crops are usually grown with day-time temperatures that are 8-10oC higher than night-time temperature (Nelson, 2002; Vox et al., 2010). Atmospheric humidity within the greenhouse must be also controlled.
High levels of humidity increase the possibility of condensation on leaves which favours the development of fungal diseases. High humidity is reduced by applying effective passive ventilation through window, forced ventilation within the greenhouse or by heating (according to the current weather conditions) without opening windows. Forced ventilation can be combined with heating.
The main advantage of using forced ventilation is that it provides more uniform distribution of temperature and atmospheric humidity. In particular, the use of forced ventilation means that outside dry air gets into the greenhouse where it is heated and thus reduces the internal relative humidity (de Gelder et al., 2012).
Dynamic climate control systems allow setting a control strategy that is based on adapting the conditions in the greenhouse within certain limits to the external weather conditions in order to reduce energy use.
A commonly-used approach is to increase the temperature set point to initiate ventilation during the day to enable relatively high average day-time temperature and to lower the set point during night in order to reduce heat needs. The system can also take into account other external weather parameters like wind speed (Vox et al., 2010).