Articles
SHELTER AND THE EFFECT OF WIND ON THE HEAT LOSS FROM GREENHOUSES
The rate of heat loss can be reduced by the provision of shelter.
Correctly designed and positioned windbreaks will give an overall reduction in windspeed of 30% resulting in an annual fuel saving of 10%. Loss of light from shelter is inevitable but unlikely to exceed 0.75%.
Under average conditions in a well constructed heated greenhouse with light winds, heat is lost partly by radiation and convection to the outside (80%), partly by air leakage (12%), and partly by conduction through the soil (8%). The rate of heat loss is governed by two main factors: the temperature differential between the greenhouse and the outside and the external windspeed.
As windspeed increases the rate of heat loss increases partly due to an increase in the temperature gradient across the envelope by the scouring action of the wind and partly to increased air leakage.
The value of the heat transfer coefficient ‘U’ is as follows:-
| Glasshouses Single skin plastic houses Inflated roof houses with two skin | U = 4.04 + 0.65V U = 4.76 + 0.52V U = 4.06 + 0.25V | |||
| U = w/m2°C | V = windspeed in m/sec | |
In glasshouses the rate of heat loss more than doubles with a rise in windspeed from zero to 7m/s.
Structures covered with a single sheet of plastic are slightly less affected by windspeed and inflated roof houses with a double skin much less affected by windspeed.
For a given condition the rate of heat loss relative to glass = 100 are 98 for single skin plastic and 78 for double skin plastic.
In Britain the maximum rates of heat loss occur at the highest windspeeds and not at the lowest temperatures.
The relative importance of wind and temperature is shown below using data from computer print-outs of relative wind and temperature frequencies produced by the British Meteorological Office:
| Total 100 | Frosty, light winds 10.5 | Wind frosts 7.0 | No frost light winds 37.1 | Windy, no frost 45.4 |
The figures show the apportionment of fuel usage according to weather when this is categorised into four types, for the month of January, maintaining a glasshouse temperature of 21°C on the South Coast of England.
It is possible by correctly designed and positioned windbreaks or