The cost of generation and distribution as well as the cost of the fuel are the key factors that must be considered. It is usual in tomato production, for example, to employ CO2 enrichment using flue gas from natural gas boilers and, because this is essentially a ‘by-product’, it is widely considered to be ‘free’. However, there would be little enrichment in the summer months if reliance were placed solely on greenhouse heating to supply the CO2 and, in practice, the boiler is frequently run with the sole aim of generating CO2. This generates surplus heating energy and the surplus heat should be stored for later use as hot water in heat storage tanks. Modern nursery installations can burn gas at rates in the region of 200 m3/hr/ha, and a heat storage capacity of around 150–200 m3/ha is typically recommended. The heat store should be suitably insulated to ensure that heat is not wasted. This clearly has a major impact on energy consumption, and simulations have shown, for example, that maintaining a minimum daytime CO2 concentration of 600 ppm could almost double energy use. Practical information on managing boiler set points for CO2 dosing can be found on the GrowSave web pages. Results from PE 003 (2011) CO2 enrichment in the future: a technical and economic analysis of alternative CO2 sources provides some more information on the options. Table 1 gives an idea of CO2 costs from fossil fuel burning.


Table 1. The cost of CO2 derived from the combustion of fossil fuels 


Fuel required

to produce 1 kg CO2

Fuel cost*


CO2 cost


Cost (£) to enrich a 1 ha greenhouse for 12 hours a day at a rate of 100kg/ha/hr

Natural gas

5.41 kWh

1.30 pence/kWh

9 pence/kg



0.40 litres

30 pence/litre

12 pence/kg


LPG (propane)

0.66 litres

25 pence/litre

17 pence/kg


* typical costs at the time of writing (March 2016)


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