Overall Energy Analysis of (Semi) Closed Greenhouses

Abstract

Natural ventilation to discharge excess heat and vapour from the greenhouse environment has serious drawbacks. Pests and diseases find their way through the openings; carbon dioxide fertilisation becomes inefficient and the inescapable coupling of heat and vapour release results often in sub-optimal conditions for either temperature or humidity. The present trend, therefore, is to reduce ventilation as much as possible, also in Mediterranean conditions. This relies obviously on improved means for diminishing the heat load and proper use of cooling equipment. Especially the latter can combine the benefits of cooling the greenhouse air with serious energy conservation. However, opposite to the clear benefits there are also serious investments associated with active cooling of greenhouse. Therefore, there is a growing demand for some computational tool that enables quantitive comparisons between the vast number of alternatives with respect to the different components of (semi) closed greenhouse systems. The benefits in terms of improved production (quality, ornamental value and quantity) are quite difficult to quantify, due to the complexity of the biological processes involved. On the energy side of the balance, however, since the physics of greenhouses, climate controllers and horticultural hardware can be described very well, it is quite possible to develop such a tool for predicting the energy consumption of a (semi) closed greenhouse for a wide range of horticultural and outside climate conditions. This paper gives an outline of such a tool and discusses some results. Just as an illustration, a number of quantitative effects are shown of changing the fraction of closed greenhouse surface in a 1 hectare enterprise that consists of closed and non-closed compartments. This analysis is made for both a Dutch climate situation and a Mediterranean weather data set.

Developing a new energy-saving, photosynthesis-promoting environmental control system for greenhouse production based on a heat pump with a heat storage system

“…The use of cooling to limit the ventilation time has been studied in the Netherlands (Heuvelink et al, 2008; de Zwart, 2008;Qian et al, 2011). Minimizing the ventilation time extended the time during which CO 2 concentration and high humidity could be maintained and thus increased yield (Yasuba et al, 2011).…”

Monitoring of Climate Variables in Semi-Closed Greenhouses

“…The semi-closed greenhouse concept claims to save energy by reduction of losses and by a -partly diurnal and partly seasonal -phase shift of heat (cold) usage and heat (cold) storage. Additional technical equipments (humidifiers, air conditioners, heat exchangers, fans, cold-and heat storage in sub-soil aquifers, double screens) are used to keep the air temperature and humidity within acceptable limits, even when the windows are closed Campen, 2006;de Zwart, 2008). Not only allow these techniques a shift of the climate to a different "spot in the Mollier diagram", they also allow higher levels of CO 2 (≥1000 ppm) to be maintained (closed windows) at high global radiation (≥600 W/m 2 ).…”

Crop growth and development in closed and semi-closed greenhouses

“…However, at least 2000 MJ m -2 y -1 energy is lost through window ventilation in an open greenhouse. This consists of 1500 MJ m -2 y -1 energy loss during sunny days for greenhouse cooling and 500 MJ m -2 y -1 during dull days for dehumidification (de Zwart 2008). The concept of closed and semi-closed greenhouses (reducing window ventilation, using aquifer for heat and cold storage, and air treatment unit (ATU) for temperature and humidity control) enables harvesting heat that would otherwise be lost by window ventilation.…”

“…It means that the harvested energy is more than enough for heating. However, energy saving by a closed greenhouse (reduction in % energy consumption compared to an open greenhouse) is not 100% because primary energy is needed for running the pumps, the ventilators of the ATU, and the heat pump (Bakker et al 2006;De Zwart 2008;. De Zwart mentioned that energy saving is less than 50% and De Gelder et al (2012a) mentioned in their review a saving of 30-40% (heat pumps are needed to bridge the gap between the water temperature from the aquifer (18°C) and required heating water temperature (above 40°C) in the ATU (Bot et al 2005;de Zwart 2012).…”

“…A major reason for the differences in harvested energy between the two experimental years was that dehumidification was completely realized by window ventilation and that the ATU was not used for this purpose in the semi-closed greenhouses in the second year. By dehumidification about 500 MJ m -2 y -1 could have been harvested (De Zwart 2008).…”