- Energy Saving
Energy management in protected cropping: humidity control
Controlling humidity can be expensive in energy terms, yet it is essential for the control of fungal disease and to ensure active plant growth. Humidity control also needs to be carefully targeted so as not to negate the energy savings from measures such as temperature integration (TI) and thermal screens.
In this factsheet, Farm Energy Centre's Chris Plackett, and Steve Adams and Allen Langton from Warwick Horticultural Research International (HRI) discuss the twin requirements of effective humidity control and energy saving.
- Effective humidity control is essential to keep fungal disease in check and to promote active growth. Humidity deficit (HD) and relative humidity (RH) are both used routinely to monitor humidity, but RH may be the better indicator of disease risk.
- An effective humidity control strategy is to prevent the humidity rising above 90% whilst, at the same time, preventing condensation occurring on the plants. Condensation poses a particularly serious disease risk, and occurs when plant temperature is at or below the dew point temperature of the air.
- Humidity control is expensive in heating energy terms and, in tomato production, can account for around 20% of energy usage when control is set at 85% RH. However, relaxing control from 85% to 90% RH can be expected to reduce overall energy use by around 12%, demonstrating that large energy savings can be made by adopting less aggressive (but effective) control strategies.
- Air humidity can be accurately measured using a traditional wet/dry bulb measuring box, but regular maintenance is essential. Because of this, electronic sensors are probably more reliable. Positioning of sensors relative to the crop is very important.
- The most energy-efficient way of controlling humidity is to vent first (Figure 1, previous page), then re-heat to maintain temperature. This can be done by having a humidity influence on the vent set-point and/or by introducing a minimum vent which is dependent on the humidity. Heating is then used to maintain temperature.
- Setting a permanent, minimum pipe temperature is wasteful in energy terms, since much of the pipe heat introduced into the greenhouse has to be vented away to avoid excess temperatures. However, the use of a minimum pipe operating with a humidity influence can be useful in preventing undesirable temperature fluctuations.
- Plant temperature measurements (or estimates) will help identify condensation risk periods, and RH values based on plant temperature rather than air temperature (plant humidities) can be especially useful when introduced into the control strategy.
- Thermal screens can make humidity control more problematic, but good operational practices can minimise their impact. For example, effective humidity control can often be achieved by controlled screen gapping to enable cold, dryer air from above the screen to mix with the moisture-laden air beneath.
There is potential to reduce humidity levels and energy use associated with edible vine crops by reducing their leaf area and, within limits, this can be done without affecting yield.