A principle for dynamic optimization of artificial lighting in greenhouses is presented, where the optimization criterion is maximization of the term <economic yield minus related costs>. To this purpose it is important to consider the break-even point for artificial lighting, which can be defined as the natural light intensity in a greenhouse at which the costs of using supplementary light equal the additional yield (in economic terms) caused by that supplementary light. Based on a mechanistic model of crop photosynthesis, break-even points are calculated and their dependency on CO2 concentration in the greenhouse, product price, leaf area index (LAI), price of electricity and crop conversion efficiency (g fresh weight of harvestable product per g crop gross CO2 assimilation) is shown. At given CO2 concentration and LAI the economic value of 1 g gross CO2 assimilation and the price of electricity are the 2 major parameters determining the break-even point for artificial lighting. Economic value of 1 g CO2 assimilated for any specific situation depends on the fraction of dry weight diverted to harvestable products (Fso) and percentage dry matter and price of the harvestable product. In the present paper a method to obtain this value for a given situation is worked out and the relation with break-even points for artificial lighting is graphically summarized. The same graph can also be used to get a first impression of the feasibility of the use of artificial light in a specific situation by considering the marginal profit. According to the present calculations, use of artificial lighting is not profitable for Chinese cabbage in The Netherlands nowadays. Artificial lighting might be feasible with radish (high Fso), egg plant and sweet pepper (high auction prices). For cucumber, lettuce and tomato use of artificial lighting is unprofitable during a large part of the year.
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