Energy intensity of non-fossil CO2 enrichment in greenhouse horticulture

Abstract

In greenhouses, fossil CO2 is commonly used to increase crop yield. Regardless of future availability, which is expected to decrease, fossil CO2 enrichment is unsustainable. However, without CO2 enrichment, yields per unit area may decrease, increasing the energy intensity of produce. Non-fossil (‘short-cycle’) CO2 may prevent this, but its provision also requires energy. Here, CO2 sources are quantified from an energy perspective, using a new methodology applied to three sources: (1) biogenic CO2, (2) direct air capture (DAC), and (3) ambient CO2 from ventilation. The energy demand of CO2 has two aspects: (1) a ‘fixed’ aspect, from generating or supplying the CO2, and (2) a weather-dependent aspect, from heating displaced air in the greenhouse. Both affect the environmental impact of CO2 enrichment, depending on the carbon intensity of the energy source. Approximately 1% volumetric CO2 concentration is sufficient to make weather-dependent energy demand negligible, which makes greenhouses an interesting destination for biogenic CO2. The implications of non-fossil CO2 are calculated using year-long simulations, comparing the three sources' effect on yield and energy intensity of produce. Efficiency and the potential to increase it are investigated with Lorenz curves, revealing that most additional yield comes from a small minority of CO2 introduced into the greenhouse. This study consistently showed the following: the role of ambient ventilation in CO2 provision is minor, the energy efficiency of DAC should be prioritised rather than high outgoing CO2 concentrations (>1%), and biogenic CO2 is an energetically favourable option to be explored depending on locally available sources.