Day-to-night heat storage in greenhouses: 4. Changing the environmental bounds

Controlling the greenhouse environment usually involves bounds (restrictions) on the indoor conditions. In model-based control, these bounds are meant to keep the plant environment away from high risk zones, the effects of which are not sufficiently well described by the model. The objective is to estimate the potential energy saving and gain in profit resulting from relaxing the bounds. The calculations employed a previously developed simulation-optimization program in conjunction with a new, solar-driven evapotranspiration model. Spanning a whole year, the simulations were carried out for a typical Dutch tomato-greenhouse configuration, utilising a gas-fired boiler for both heat and CO2 production, and a water tank for day-to-night heat storage. The main findings are as follows: Provided that the crop is not damaged by the change, the expected gain from increasing the permissible humidity is about 0.74 € m−2y−1 per one percent relative humidity, and from reducing the minimum temperature − about 0.87 € m−2y−1 per degree. Roughly 2% of the energy is saved by a 1K reduction of temperature or a 1% increase of the relative humidity. Adding a heat buffer has no noticeable effect on the total amount of gas used. It does, however, increase the effectiveness of CO2 enrichment, thus increasing the yield and the economic gain (by 3.4 €m−2y−1). Replacing the profit goal by energy-use-minimisation goal, results in a substantial loss (−11.5 € m−2y−1).