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Introduction
Illumination of greenhouses in Northern latitudes requires large amounts of electricity. In the Netherlands, the standard in the early twenties is that a large fraction of the electricity consumption of the illumination is produced by combined heat and power engines. During summer, the illumination is not used. However, as the greenhouse still needs at least some heating on many nights, the average grower with a CHP-engine will run this engine in order to produce electricity for the public grid. Looking on a year round base, many greenhouses with illumination and CHP are more or less neutral in terms of electricity consumption. During winter there is net buying of electricity and in summer there is net selling of electricity.
As a CHP-engine is typically running on natural gas, the CO2-emission associated with the electricity household of an illuminated greenhouse with CHP is substantially higher compared to a non-illuminated greenhouse.
Without CHP, the greenhouse imports a substantial amount of electricity. This electricity is converted to light, but also to heat. Therefore the heating demand of an illuminated greenhouse is in general lower than that of a non-illuminated greenhouse. The decrement is however less than one might expect, which is caused by the fact that a crop requires a higher average greenhouse temperature when there is more light available for growth.
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Conclusions
Using illumination increases both costs and crop production significantly
The scenario with the highest electricity consumption (HPS and no co-generation) has the lowest CO2 emission. This is because it is assumed that the electricity from the grid is all coming from sustainable sources. In case the average CO2 emission of electricity in the public grid of the Netherlands (350 gram per kWh in the early twenties) is assumed to be associated to the bought electricity, the HPS greenhouse without CHP becomes the greenhouse with the highest CO2-emission, although the largest emission is then produced elsewhere. The CO2 emission of option 2 would then be 56.3+233.7*0.35 = 138 kg/(m² yr).
Assigning this 350 gram CO2-emission to public grid electricity would mean that the CO2-emission associated with option 4 would increase with 83.7*0.35 = 29 kg/(m² yr), whereas the CO2-emission associated with option 5 would decrease by 33.8*0.35=11.8 kg/(m² yr).
This shows that LED-lighting is always to be preferred compared to HPS-lighting.
