The Untapped Energy in Water Vapour

Last week, Heeren XVII, the biosystems engineers’ society at Wageningen University, organised an evening featuring three companies in the greenhouse horticulture industry. The first of these was Ammerlaan Construction (known as Maurice Kassenbouw in The Netherlands and Belgium), presented by Ronald Thijssen.

Courtesy Heeren XVII.

Ammerlaan builds greenhouses all over the world, tailored to all sorts of contexts – from Scandinavia, where 75 kilos of snow per square metre could be on your roof; to the booming cannabis industry in North America. This requires broad knowledge.

More specifically though, this article will be about Ammerlaan’s Air & Energy system and how it saves both energy and water.

First, a quick history of the state-of-the-art in Dutch greenhouse construction:

  • 1970s: greenhouses were only 2.5 metres tall, had no energy-saving screens, and would leak like glass colanders. This resulted in 80 m³ of natural gas being consumed per square metre per year.
  • 2000: greenhouses grew taller, to about 5 metres. Energy-saving screens. Less leakage. Polycarbonate gables instead of glass. The result? Now only 43 m³ of natural gas per square metre per year.
  • 2010: the first semi-closed greenhouse. These have no windows, ventilating instead by constantly creating an over-pressure through forced ventilation. All of this got natural gas consumption down to 30 m³, but led to an increase in electricity consumption.

Of all the energy going into a greenhouse, only 4% ends up in biomass. The other 96% heats up the greenhouse. A side-effect of this is that water starts to evaporate, driving relative humidity up. A high relative humidity puts crops at risk of disease.

Conventionally, to get rid of excess humidity, growers would open all their windows and put the heating on full blast (known as ‘droogstoken’, or ‘dry stoking’ literally translated). Obviously this is a huge waste of energy.

Energy-saving screens are great, but they have to be closed most of the time. If they are opened by as little as 3% (that’s not a typo), you lose most of the benefits anyway. But by keeping them closed, your relative humidity goes up, because there is less opportunity for all that moisture to condense on the glass panes.

Energy screens in a greenhouse. Courtesy AP Holland.

What is Ammerlaan’s solution to this? How can we save energy whilst keeping humidity in check?

The main thing to understand is this: water vapour contains a lot of energy. Evaporating water takes as much energy as it does to heat water from zero to fifty-five degrees. Why not kill two birds with one stone by capturing that moisture, recovering its energy and decreasing humidity?

This is the whole idea behind Air & Energy. Air & Energy is a dehumidification system that works through air-to-air heat exchange. It’s actually not a new idea and has been used in hotels for a while.

Humid air gets pushed up by blowing dry air through hoses below the plants. Humid air has a lower density than dry air, so it rises quite easily. Once it gets up to the screen, it gets sent to the heat exchangers (of which there are many).

One of Ammerlaan’s greenhouses, built for Multigrow, a Dutch cucumber grower. Those boxes are all Air & Energy heat exchangers.

Inside the heat exchangers, condensation happens, just as it would on the glass panes in older greenhouses. Through condensation, all the energy locked in that vapour gets released back into the air. 

The result? A 30-40% energy saving compared to before, and even a 70% energy saving in places like Scandinavia.

There is one downside. Greenhouses using Air & Energy don’t have an overpressure like those semi-closed greenhouses. This means that nets are required to keep insects out.

But there are other benefits other than energy savings, since energy and humidity go hand in hand. Crops get fewer fungal diseases as a result of the lower humidity levels, meaning pesticides are only required in the last few months of the crop cycle. All of this leads to a 5-10% higher yield as well.

Is it worth the cost, though? All in all, with these benefits, this system has a payback time of about 3-5 years. Not bad I’d say.

What about the other companies at this evening? I hope to be writing articles on some of the ideas in their presentations too. Stay tuned.

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