Heat recovery in greenhouses: a practical solution

In recent years, passive infiltration of air into greenhouses has been reduced drastically. However, very low air exchange rates can lead to abnormally high levels of humidity which can damage harvests. Hence, farmers have to ventilate. In an effort aimed at reducing heating costs related to ventilation, a heat exchanger was designed to be used as a dehumidifier. A CDN$ 2000 air-to-air multi-pipe counterflow heat exchanger unit was installed in a greenhouse used for the experimental cultivation of hydroponic tomatoes and cucumbers during the winter of 1996. The first series of tests, carried out between March and May 1996 in a 576 m3 enclosure, demonstrated that average efficiencies of η=84% and 78% were obtainable with air volumetric exchanges rates of 0.5 and 0.9 change/h, respectively. Latent heat was found to play a major role in the overall heat transfer, contributing about 40% of the total energy exchanged in some situations. The unit made of plastic is durable and rot and rust resistant. Its efficiency, mainly because of its very low level of compactness, was found to be very good in the presence of frost and ice. A commercial implementation is now considered as well as experiments in broiler houses.