greenhouses 1Greenhouses are used globally for the production of high value produce such as tomatoes, lettuce and other vegetables.  We have recently carried out work in the Middle East conducting a trial growing vegetables in desert conditions.

Problem

Greenhouses are by nature incredibly warm places, especially in areas of the world that are very hot. Vegetables and fruit cannot withstand being too warm or too humid and ideally the environment should be in the region of 24 – 27 °C temperature and 60 – 70 % relative humidity maximum.  Maximum productivity will be achieved if this can be maintained.

Solution

When used where the ambient temperatures are high but humidity relatively low, we have supplied cooling with Humidair evaporative cooling. This is a very cost effective way to cool large volumes of air at low cost by the use of water and a small amount of electrical power. This will only work when the air being dried is well below the saturation point on the wet bulb temperature. If this is the case the maximum cooling effect will take place.

greenhouses 2In areas of the world where the temperature and humidity are both high and evaporative cooling cannot be utilised effectively, then the solution would be to implement desiccant cooling in the formation of a two wheeled air treatment unit.  This has many combinations and depending on the ambient climate and desired off conditions will determine this configuration.  

Recently we have supplied such desiccant cooling machines to the Middle East for a trial to produce fruit and vegetables locally rather importing them. 

In this combination, the plants were grown in artificial light provided from LED bulbs and the greenhouse was not glass but totally enclosed and very much insulated to prevent heat gains from the ambient. The growing method was to grow the plants in nutrient film, hydroponics, no soil required. 

A combination of an enthalpy wheel, sensible heat wheel and latent cooling coil was used to provide the environments with a supply airflow of 20 000m³/hr.  The savings gained using the desiccant cooling over conventional systems to achieve the same results were 769 kW per greenhouse.

In addition to this there were other benefits. The condensate coming off the latent cooling coil, 600 litres per hour was at a temperature of 11.0°C.  This was put into heat exchangers that cooled the nutrient film liquid.  The benefits of keeping the roots of the plants at 18.0 °C or as close to this increases the production and growth rate meaning the produce off the system is lower in cost and profits are higher.

Related information

  • Please download from our website the PDF on evaporative cooling / humidifiers
  • Please download from our website the PDF on desiccant cooling.