Increased traffic congestion, diminished open spaces, suburban expansion and decaying urban areas have caused many people to rethink how we develop our land. Strategies for maintaining economic growth, while also curbing costly development on green-field sites and preserving quality of life, are gaining higher priority in metropolitan areas. With societal attitudes moving toward conservation and preservation of energy and natural resources, greenhouses may well become the preferred venue for growing vegetables, fruit and flowers.

But to control environmental temperatures, greenhouses must overcome several challenges. These include rapid and widely varying changes in solar radiation; preheating delay of large water heating systems; adjusting to unknown factors that affect heat loss, such as shading and equipment malfunction; and evaporative cooling effects of plant transpiration.

Because of a greenhouse's solar gain/heat loss characteristics, temperature conditions can change very quickly--more quickly than heating and cooling systems can react. For example, if sunlight is bright and suddenly disappears, the effect on the greenhouse temperature within the zone can be very rapid and drastic. Therefore, there is a need to anticipate heating and cooling loads within the zone before conditions change. This requires precise controller programming to ensure that the heating system reacts to falling temperatures. When the system anticipates the added heat load, it immediately and automatically raises the water as the temperature is dropping. The result is that proper temperature is maintained in the zone. This requires growers to use the features to their capacity. The heating system designer and the installing contractor must orchestrate all of this.

Precise Temperature Control

Paul Boers Ltd., located in Vineland, ON, a leader in total growing systems, has been successfully using Danfoss VLT adjustable frequency drives to control and maintain temperature levels in its greenhouses. The drive controls the pumps, which in turn deliver the water to a boiler and a piping system throughout the greenhouse. In this closed-loop heating system, water temperature is relative to water pressure.

As the temperature rises inside the boiler, the pressure is also increasing. When water circulates throughout the greenhouse, the water temperature decreases. The challenge is to maintain the same temperature levels throughout the cycle. "Danfoss worked closely with Paul Boers to devise a solution in which the drives and pressure transmitters are combined into a single system," explained Paul Lamarre of Danfoss. The pressure transmitters mounted on the cold water return and the hot water pipes send signals to the drive as the pressure changes. The Danfoss VLT compares the signals, adjusts the pump speed and maintains the pressure within 1/2 psi differential inside the boiler. As a result, the temperature of the water is uniform throughout the greenhouse, and the critical temperature is achieved for desired growth.

There are many benefits to this heating system. Crop quality is improved because the plants are exposed to very similar, predictable temperature conditions throughout the production cycle, and from season to season. Average daily temperatures and differences between day and night temperatures are maintained within tighter bounds. Improved energy efficiency is achieved by eliminating overheating and overcooling that are often caused by a tendency of some control programs to overcompensate after the system catches up. There is less wear and tear on equipment since the program tends to find the ideal stage or target right away, and makes adjustments only as conditions warrant. The system eliminates lag times on water heating systems. And it minimizes thermal shock to boilers by adding head load more gradually, because the program immediately anticipates heat requirements.

"Working closely with leading growers, Danfoss was able to successfully reduce energy consumption," says Rudy Boccinfusso of Danfoss. This approach improved comfort and environmental conservation as well.