This study assumes a prototype greenhouse with floor dimensions of 10m x 30m and total glazed area — roof and walls — of approximately 600 m2 made from structured polycarbonate (PP) sheets. This material was chosen due to its thermal isolation properties (U = 3,4 W/m2°C [2]), mechanical stability and weight characteristics. For a AT of 15°C, i. e., considering a minimum inside temperature of 14°C and a minimum outside temperature of -1°C, the thermal loss associated with the greenhouse surface is (equation 1):

Q = U x A x AT = 3,4 x 600 x 15 = 30,6 kW (1)

where Q also represents the power of the required auxiliary heating system that should be installed.

Previous calculation considers that all air inside this greenhouse is heated through convectors with forced circulation. Therefore, all air in contact with the glaze will also be at the temperature required by the crops.

The procedure to determine the design load of a greenhouse heating system (i. e., "sizing" the boiler) is different from the procedure to determine the projected operational heat demand and subsequent fuel consumption during any given day (or entire heating season). The selection of the size of the boiler is primarily dependent upon the greenhouse surface area, insulation properties, and the maximum expected difference of the inside set point air temperature from the minimum expected outside air temperature. This calculation represents the design condition for the worst — case situation. However, the real operating costs to heat the greenhouse are dependent on the difference of the actual inside and outside air temperatures at each and every moment of the heating season, and the length of time that each of those temperature conditions occurs. Thus, the design heating load capacity of the boiler may rarely (or never) be reached, but the boiler must provide heat at some level, which is less than the design capacity, nearly all the time during the heating season.

Unless the air temperature difference is known on an hourly basis through historical weather data or by real time data acquisition system, the Degree Day Procedure can be used to determine the amount of heat that will be needed over a given period [3].

The difference between the minimum acceptable temperature inside the greenhouse (ex. 14°C) and the outside air temperature, multiplied by the number of the hours in a day and by the number of the days in that month, will give us the total Degrees Month to be used in the equation of the thermal losses. This procedure assumes that there is no daily energy gain from solar radiation and that the inside to outside air temperature difference is the same for the entire 24-hour period.

Degrees Month = (Reference temperature — Average month temperature) x 24 hours x nr. of days in that month;

Average monthly consumption on this 300 m2 greenhouse example can thus be estimated. Annual total will be 32.063,9 kWh or, with a 90% efficiency, around 3.281 liters of diesel (1 liter of diesel ~ 10,75 kWh [4])