Monday, September 12, 2011

Update on greenhouse construction

South facing side with roof vent

High velocity fan
I have been carefully installing the wet wall inside the greenhouse and have been calculating the optimum rate of air exchange. I purchased a large AEG/Electrolux high velocity metal blade floor fan to use to blow air over the evaporative surface. Unlike most evaporative coolers that force air from outside the greenhouse through media into the greenhouse for cooling, my design is internal. This may sound like a disadvantage given the effect of relative humidity on the efficiency of an evaporative cooler but I have taken this into consideration with air exchange. The evaporative surface area of my wet wall is approximately 10m² and is simple to operate and construct. I ran a length of 50mm PVC pipe overhead along the length of the greenhouse bisecting it into two equal areas. From the ceiling a vertical curtain of shade cloth falls into a gutter where water is recovered and returned to a submerged 200L drum-sump. A self-priming 0.25kw pool pump draws water from the sump and delivers it to the overhead length of pipe which has spray holes drilled into it to form a long spray bar. This spray bar is end-capped. Two spray shields of polycarbonate plastic direct any spray and drops of water evenly onto the top of the curtain. Water runs continuously down the curtain where it covers the entire surface area. Air blown along this surface area is cooled as the water evaporates.
Wet wall sump pump

Wet wall in operation
A float valve in the sump allows for the constant compensation for evaporation automatically. The sump also acts as a thermal mass at night to buffer heat loss. Air exchange is both active and passive. An extractor fan positioned in the apex of the greenhouse roof draws out trapped heated air and provides negative pressure that draws in cool fresh air from the floor side vent. In addition, a manual roof vent can be opened on really warm days if necessary to allow for the escape of hot air. The cooling system fan, pump and extractor fan are all controlled with a temperature controller that automatically turns the system on when the temperature reaches 28°C and turns it off at 26°C.

North facing side 
The North facing roof and wall are covered by shade cloth to assist with cooling and to provide a shaded area for those plants which are more sensitive to bright light. The other side of the greenhouse will be for those plants with higher light intensity requirements.

This past weekend I installed the automatic irrigation system which is coupled to mains pressure and uses a solenoid valve set to a timer to switch on the mist sprinklers overhead at 15 minute intervals. The frequency of watering can be pre-programmed depending on the requirements and the season, keeping in mind that misting will also assist with cooling during our very hot Summer months here on the West coast. Apart from the odd leak which I fixed, everything is running well. If there is one thing I can't stand it's a water leak! I hate wasting water. I am proud to say that my system is both economical and also cheap to operate. I have future plans to use rain water to fill the wet wall sump which will further contribute to a minimal impact.

Automatic irrigation system solenoid

Mist sprayers

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