Cost of Heating Greenhouses
Cost of heating
Estimating the cost of heating a greenhouse is always open to some conjecture, although it is possible to arrive at a reasonably accurate figure. It is obvious that the prevailing outside temperature patterns for any country will greatly affect heating costs. Some reconciliation of the regional implications can be reached by checking on the degree day tables published by the Meteorological Office. These tables give estimates for average months, but variations are bound to occur seasonally and locally.
A ‘degree hour’ is the sum of the temperature difference between the inside and outside temperatures of the greenhouse (when it is cooler outside) multiplied by the time in hours which each temperature difference lasts. To keep the figures to a reasonable size they are divided by 24, and called ‘degree days’. This tends to be a little complicated but should offer no difficulties to the competent mathematician. A more basic approach is to consult a table of costs which takes into account the varying operational efficiency of different equipment and different fuels.
It is thus relatively easy to calculate the running cost on a 24 hour basis assuming a 50-60% demand overall, although it should be pointed out that this is assumption and nothing more and demand depends on exposure, region, tightness of greenhouse and other issues. If, for example, one has a greenhouse with a calculated heat loss of 2.3kW (8,000Btu), working on a 13°C (55°F) minimum, then using a 2-1/2kW (8,530Btu) fan heater, the cost of heating will be 50-60% of the actual cost per hour, which is the cost of about 1-1/4 – 1-1/2 units per hour, as electricity is 100% efficient, there being no combustion heat loss.
When calculating the cost of electrical heating it is usual to discount the primary unit rate, which tends to be used up in the home, and simply calculate on the basis of a flat average rate unless there is a separate meter to give “off peak” power. Where frost protection only is desired, ie 4-7°C (40-45°F) maximum, then there will obviously be a much lower demand than 50-60%, as low as 10-12% in some cases. Relating this to solid fuel with a calorific value of 32MJ/kg (12,500Btu/lb) and 60% boiler efficiency, you only get 60% of 32MJ (12,500Btu) which is 19.2MJ (7,500Btu) for every kg/lb of fuel you burn, so by relating this to fuel cost you can roughly calculate what your fuel bill is likely to be. It is of course again necessary to assume a figure for percentage loss of efficiency on combustion. In the absence of a properly carried out efficiency test, help is given in this direction by taking into account the calorific output of the boiler. With small boilers of around 4.3kW (15,000Btu) capacity it is fair to assume an operational efficiency of about 60%, when it will require 1kg (2.2lb) of coal per hour on rapid burning, which would be 21kg (48lb) per 24 hours. Rapid burning is seldom a constant necessity, however, and 9kg (20lb) per day or 63kg (140lb) per week is a fair average.
With an oil-fired boiler or oil stove the same calculations can be made assuming 75% efficiency and 50-60% demand.
It can perhaps now be appreciated that while it is possible to estimate running costs, so much depends on the temperature range desired in the greenhouse and, more important, on the outside temperature.
Where linked systems are involved there is no basic difference in calculation, it being a case of working out the heat loss for the greenhouse and, provided this is efficiently linked by small bore pipes or ducts, it should be possible to calculate the heating costs in the same way as for an independent system. A distinct advantage is likely to be the higher operational efficiency of the more expensive and sophisticated type of domestic heating units invariably used. In certain instances it may of course be possible to site the domestic boiler inside the greenhouse or conservatory, the greenhouse then being kept heated by the radiant heat from the boiler, which could be completely lost if the boiler is sited out of doors. While there could be temperature control problems, these are not likely to be any greater than the control problems which can arise with linked systems.