The core of the heat exchange system in the greenhouse: the convector with the air duct, on the side of which, at one-metre intervals, fans are placed which take in air from above and blow it out to the side. |
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Under a 40-cm wide walkway grating, there are eight thin tubes with thin iron cooling fins placed at an angle. These are needed for transferring heat or cold. At the same time as the boiler water runs through the tubes, air is constantly blown past the tubes and the cooling fins. This occurs by means of a large number of fans installed under the tables with the plants. One fan is installed every metre, on alternating sides: first one on the left and then one on the right. An air duct has been installed under the walkway grating. The fans take in air from above, blow it down through the walkway grating and the air duct, and then to the left or the right, depending on the side on which the fan is located. The air goes through the plants and back up, to get blown through the grating again. In this way, there is a constant circulation of air in the greenhouse past the plants, through the grating and past the tubes in which the water is flowing. The water transfers heat or cold to the air, and in return the air transfers its temperature to the water. By means of this system, the grower can precisely control the temperature in the greenhouse, while the constant stream of air is beneficial to the growth of the plant. What makes this heat exchange system so extraordinary is the fact that it hardly makes any noise and uses very little energy. This is possible because there is hardly any resistance in the entire air circulation and heat exchange system. The fans circulate the air very evenly over a small three-dimensional space. The important part is the climate at the level of the tables. It doesn’t matter if the air four metres above the tables is warmer or cooler than the air directly around the plants.
Cooling and heating
The cold groundwater is warmed from eleven degrees to eighteen or even twenty degrees, depending on the weather. The sunnier and warmer it is, the cooler the greenhouse must be kept and the more the groundwater must be heated in order to do this. The heated groundwater is sent back underground, into the warm well of course. There are six wells in connected pairs. At the front of the greenhouse are three cool wells and at the back are three warm wells. The combined capacity is sixty cubic metres per well per hour, for a total of 180 cubic metres per hour. If the outside temperature is low, the process works in reverse. In that case, water is pumped from the warm wells in order to heat the greenhouse. This water is between eighteen and twenty degrees. On its own this is not warm enough to heat the finishing greenhouse to 20 degrees. A heat pump has therefore been added to the system for support. This pump has a capacity of 1,500 kW and produces heat and cold simultaneously, just like a refrigerator. This pump is capable of heating the water to a maximum of 50 degrees. This is more than enough to also bring the temperature in the vegetative greenhouse to 28 degrees. On average, 34-degree water is sufficient for the vegetative phase, and for the finishing phase the average required temperature for boiler water is 28 degrees Celsius. What this means is that the warm 20-degree groundwater has to be heated on average an additional eight to fourteen degrees. In this system, the summer sun energy is used as much as possible. In the summer, heat is “harvested” and stored underground in order to be used in the winter. Not only heating the greenhouse, but especially cooling the greenhouse saves energy. Ordinary refrigerators use much more energy than our system. The heat/cold pump produces heat and cold simultaneously. On the warm side, the water is heated to a maximum of 50 degrees, and on the cold side the groundwater is cooled to a minimum of six degrees Celsius
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