Energy management in protected cropping: management of CO2 enrichment
CO2 enrichment is vital to the cost-effective production of many glasshouse crops. However, it can have a high energy cost.
In this factsheet, Farm Energy Centre's Chris Plackett, and Steve Adams and Allen Langton from Warwick Horticultural Research International (HRI), discuss how to keep the energy expense of CO2 enrichment in check.
- For some crops, CO2 enrichment can greatly increase annual energy use. It is important to ensure, therefore, that the benefits outweigh the costs.
- Flue gases from natural gas boilers and CHP units can be used for CO2 enrichment. CO2 from natural gas boilers is essentially a by-product of glasshouse heating, but should be costed at around 13 pence/kg (for a gas price of 2.5 pence/kWh) if the boiler is being used solely for the purposes of CO2 enrichment. CHP units (including microturbines) produce more CO2 per unit of heat output than a boiler, but for reciprocating engines, a catalytic converter is needed to remove harmful pollutants. On economic grounds, the installation of a CHP unit cannot be justified on the basis of CO2 production alone. The flue gases of combusted biomass cannot currently be used as a source of CO2.
- CO2 enrichment using flue gases frequently generates surplus heat and this should be stored as hot water in well insulated storage tanks for later use. A heat storage capacity in the range of 150–200 m3/ha is typically recommended.
- A permanent minimum pipe temperature setting can be used to generate CO2 in the absence of a heat store or if this is full. However, this practice is energy intensive and minimum pipe temperatures greater than 40°C are unlikely to be economic.
- CO2 can also be generated by LPG or kerosene burners within the glasshouse. However, CO2 generated in this way can be expensive, and dealing with the associated products of combustion (water vapour, heat and, possibly, aerial pollutants) can be problematic. However, associated heat production in winter will offset glasshouse heating costs
- Enrichment can also be carried out using pure, liquefied CO2. This is a by-product of industrial processes and its use for enrichment does not increase overall energy use. It has the advantage over flue gases in that it can be used in summer without associated heat production. However, a certified bulk storage tank is required. Liquefied CO2 currently costs around 11 pence/kg, but additional surcharges can just about double this.
- CO2 needs to be distributed around the glasshouse to achieve uniformity of supply and this is achieved either by integral fans in the case of burners, or by the use of a fan and duct system with perforated supply lines. The supply lines are best sited within the crop canopy or, in the case of benched crops, on or under the benches. Accurate CO2 measurement is important to optimise enrichment practices, and all CO2 sensors should be calibrated regularly.
- Raising the vent temperature for part or all of the day will reduce ventilation losses and increase the potential for CO2 enrichment.
- It is best to spread the use of available CO2 over the whole course of the day to avoid canopy depletion. Ideally, the highest gas burn rates should be around noon in spring and autumn, but in the morning in summer.