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Saving energy without technology
To save energy without using technology, you need to think outside the box. And there’s plenty of scope for that when it comes to fruiting vegetables. A good proportion of the energy used in a greenhouse is spent on controlling humidity levels. Dehumidifying with heat can account for anything from 10 to 25% of the annual gas bill. The source of humidity in the greenhouse is the crop. There are two things you can do here: stop the crop from transpiring so much or push the boundaries in terms of how much humidity you can allow.
Growers who follow the principles of Next Generation Growing (NGG) are not keen to use minimum pipe in the mornings because it encourages the crop to transpire more. From spring onwards, solar reflective or diffuse coatings are a good way to keep the crop cool and therefore prevent excessive transpiration.
There is another method you can try in winter: limiting the amount of transpiring foliage in the greenhouse. Wageningen University & Research has been looking into this in some depth. Back in 2005 they discovered that sweet pepper, a crop that is particularly foliage-heavy, will transpire 10% less over the year if the lower leaves are removed. This cuts energy consumption by 5% and doesn’t affect production: in fact, production rises by 0.5%. This could be canceled out by the higher wage bill for removing the leaves, of course. After August, the top 40% of the crop intercepts 90% of the light. The bottom 60% does almost nothing with the light but contributes 40% of the transpiration. A lot of this humidity has to be eliminated by ventilating or heating more, which uses more energy.
Advantages of removing leaves
In 2015, a new series of trials was launched looking at the advantages of removing leaves. A ‘bare’ tomato plant sends a relatively high proportion of assimilates to the fruits in winter and transpires less. So production increases slightly while gas consumption falls. A follow-on trial involving removing leaves in cucumber in the summer was less successful. The trials are prompting discussion on reducing leaf mass in the winter in other crops, including ornamentals: during the winter, maintenance respiration from ‘superfluous’ leaves continues as normal while at the same time adding to the humidity in the greenhouse.
Trials involving maintaining a higher humidity level have been carried out in the past. Growers using NGG are seeing how far they can go by screening for longer. But in order to maintain higher humidity, the temperature needs to be evenly distributed throughout the greenhouse. In cold spots the crop temperature can drop below dew point, causing condensation and putting the crop at risk of a fungal attack.
‘Bare’ tomato plants in a trial at Wageningen UR sent more sugars to the fruits in winter and reduced gas consumption.
Another unorthodox strategy is to plant later. Heating the greenhouse in winter is expensive, yet plants without assimilation lighting only grow slowly. That’s as good a reason as any to look at planting unlit fruiting vegetable crops later and stopping them early. WUR research carried out in 2007 revealed that this had virtually no impact on production in tomato and sweet pepper but clocked up considerable savings in gas consumption, although the results weren’t so good in cucumber. Nonetheless, you’d be hard pressed to find a tomato or sweet pepper grower willing to leave their greenhouse unused for any length of time, even though that could also be a good option in terms of disease control.
The same applies to temperature integration. Most crops react more to the average temperature than to its actual level at any given moment. This is what the principle of temperature integration is based on: a day with low temperatures can be compensated for by a day with higher temperatures. This method enables growers to follow the outdoor temperature more closely, which can help them save energy. The concept of temperature integration could be applied much more in practice than is currently the case.
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