In greenhouses, fossil CO 2 is commonly used to increase crop yield. Regardless of future availability, which is expected to decrease, fossil CO 2 enrichment is unsustainable. However, without CO 2 enrichment, yields per unit area may decrease, increasing the energy intensity of produce. Non-fossil (‘short-cycle’) CO 2 may prevent this, but its provision also requires energy. Here, CO 2 sources are quantified from an energy perspective, using a new methodology applied to three sources: (1) biogenic CO 2 , (2) direct air capture (DAC), and (3) ambient CO 2 from ventilation. The energy demand of CO 2 has two aspects: (1) a ‘fixed’ aspect, from generating or supplying the CO 2 , and (2) a weather-dependent aspect, from heating displaced air in the greenhouse. Both affect the environmental impact of CO 2 enrichment, depending on the carbon intensity of the energy source. Approximately 1% volumetric CO 2 concentration is sufficient to make weather-dependent energy demand negligible, which makes greenhouses an interesting destination for biogenic CO 2 . The implications of non-fossil CO 2 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 CO 2 introduced into the greenhouse. This study consistently showed the following: the role of ambient ventilation in CO 2 provision is minor, the energy efficiency of DAC should be prioritised rather than high outgoing CO 2 concentrations (>1%), and biogenic CO 2 is an energetically favourable option to be explored depending on locally available sources. • An approach to compare non-fossil CO 2 sources from an energy perspective is presented. • Non-fossil CO 2 can raise yields without increasing energy demand per kg yield. • Ventilating outdoor CO 2 uses the most energy, as high volumes of air must be heated. • Biogenic sources are promising, as 1% CO 2 by source volume is enough for greenhouses. • For direct air capture machines, lowering energy use per kg CO 2 must be prioritised.
Tuyll et al. (Mon,) studied this question.