Knowledge

Dehumidification with heat recovery: how it works

In a swimming pool, a greenhouse or a drying room, a lot of water ends up in the air every day. That moisture has to go. The only question is: do you throw away the heat contained in that moisture, or do you use it again? This page explains how that works.

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Why does indoor air become humid?

Water evaporates wherever it gets the chance: from a swimming pool, from wet floors, from the leaves of plants and from products that are drying. That water vapour goes into the air.

Warm air can hold more water vapour than cold air. But even warm air fills up at some point. How full the air is, is given by the relative humidity, as a percentage. At 100% nothing more fits in.

• Swimming pool: the pool water evaporates all day long.
• Greenhouse: a growing crop transpires water through its leaves.
• Vertical farm: the plants transpire water and the LED lights give off heat.
• Industry: products dry and release their moisture into the air.

What is latent heat?

Evaporating water takes heat. That heat is then held in the water vapour. You do not feel it as temperature, which is why it is called latent (hidden) heat. For each litre of water, it comes to about 0.68 kWh.

The heat is only released again when the vapour condenses, in other words when it turns back into water. That happens on any surface that is colder than the dew point of the air. On a cold window that is a nuisance. In a heat pump it is exactly what you want.

Why ventilating moisture away costs energy

The simplest way to get rid of moisture is to ventilate: humid air out, drier outdoor air in. That works. But the air you blow outside is warm and full of latent heat. And the air that comes in has to be heated again.

A heat exchanger helps. It passes heat from the exhaust air to the fresh air. The Dutch knowledge centre Stimular writes that in a swimming pool, depending on the type of installation, 40 to 80% of the heat can be retained. Such a heat exchanger mainly transfers sensible heat. Part of the latent heat still leaves with the water vapour.

The same applies in a greenhouse. According to research by Wageningen University & Research, dehumidifying by ventilation in a tomato crop easily costs 8 to 10 m³ of natural gas per m² per year.

How a heat pump recovers the moisture and the heat

1. Humid air flows past the cold evaporator of the heat pump.
2. The air cools below its dew point. The water vapour condenses and drains away as water.
3. As it condenses, the latent heat is released. The heat pump takes it up, together with sensible heat from the air.
4. The heat pump raises the heat to a higher temperature and releases it where you need it: to the dry air, to the pool water or to a process.

So the moisture disappears and the heat stays inside. What you put in is mainly the electricity for the compressor and the fans. A swimming pool or a greenhouse also always needs some fresh outdoor air. You can recover the heat from that air as well.

How is this different from an ordinary air handling unit?

An air handling unit refreshes, filters and heats or cools air. With an integrated heat pump, the same unit can also actively dehumidify and recover the latent heat. Read more on the page about the air handling unit with an integrated heat pump.

Where is this used?

• Swimming pools: the vapour from the pool goes out, the heat goes back into the pool water. Background: indoor pool humidity.
• Greenhouses: less venting, and still a dry crop. Background: greenhouse dehumidification.
• Vertical farms: cooling and dehumidification in a single unit.
• Industry: drying with waste heat.

What the X-Cross does

The X-Cross is our unit for this approach. It recovers heat from air that you have to dehumidify anyway, and it heats, cools and dehumidifies in one unit. We build it together with Wagenaar Refrigeration.

Which unit suits you follows from a calculation using your own figures. The report shows the latent and the sensible load separately, with the standards and the assumptions.