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by Klaas Althof, Horti-Cultura:

"The New Heating": saving energy in your greenhouse"

The best way to save on energy is not to procure it. Just make sure the heat already in your greenhouse doesn’t get out, and is evenly distributed. That will save you money right then and there. In a series on "The New Heating", Klaas Althof of Horti-Cultura gives advice on how this can be achieved. This time it’s about screening, and its impact on horizontal temperature distribution.

Why screening?
With the placement of a 1st or 2nd screen, climate conditions will change. This allows you to lower the heating, but the climate balance between greenhouse and roof must be examined, in particular with different screens. The energy savings of a screen is determined under laboratory conditions. Research indicates that a screen with 35% energy savings, actually only saves 10% because of small cracks and holes ranging from 0 to 7 cm.

Physical principle

Humid air (eg. 80% RH) has a higher energy content than dry air (eg. 50% RH) with the same temperature. Moist air is lighter than dry air and thus rises. So high levels of humidity may enhance the effects of air currents and energy loss.

Horizontal temperature differences and installing the screen

Air flows in the greenhouse are in many cases the cause of horizontal temperature differences. Air flow can be caused by the slope of the greenhouse (the cold air sinks to the lowest point), an open screen or simply bad screening. A broken window or poor closing vents can also be a cause, though with most modern greenhouses, draft is usually not the cause. However, the connection of the screen between the glass and the front pillar is a point where much energy can often be saved. The warm air rises and escapes through a crack in the screen.

What to do?
First, take measurements under different conditions in order to accurately record temperature variations. Subsequently, it’s advisable to make air flows visible using a smoke test. Based on these results, targeted measures can be taken.

Ensure proper ventilation
The best way to avoid horizontal temperature differences, is to install a tight-fitting screen and to use dry air to dispel humidity. There are several systems available on the market specifically designed for your crop.

Interrupt airflow
If the placement of tight screens isn’t an option, it’s possible to prevent the air from cooling by making sure the distance travelled by the air is shorter. To do this, you need to divide the space above the screen into segments and thus create multiple short flow loops instead of one long one. This can be done by placing transparent synthetic triangles above the grating. The horizontal temperature difference and the permissible temperature difference determine the number of screens.

Care and maintenance

In order to prevent a large part of the energy losses described above, regular maintenance is essential, allowing for timely detection of problems. This includes: sealing of cracks between screen and glass, sealing of cracks in wall screen on the concrete base, sealing of pipes, ducts, etc.

More information:
Klaas Althof
Nijverheidsweg 20p
2821 AW Stolwijk
+31 (0)182 604 871
+31 (0)6 12 14 19 14

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