Day-to-night heat storage in greenhouses: 3 Co-generation of heat and electricity (CHP)

Day-to-night heat storage in water tanks (buffers) is common practice for cold-climate greenhouses, where gas is burned during the day for carbon-dioxide enrichment. In Seginer, I., van Straten, G., van Beveren, P. (2017). Biosystems Engineering, 161, 188-199, an optimal environmental control approach was outlined for conventional greenhouses, the idea being that a virtual value of the stored heat (its 'co-state') could be used to guide instantaneous control decisions. The value of the co-state was heuristically adjusted to minimise the time the buffer was ineffective (being empty or full). Here the same approach is applied to greenhouses with co-generation of heat and electricity (CHP). The parameters-set and weather are characteristic of tomato greenhouses in The Netherlands. The main results are: (1) The heuristic control method is easily adapted to systems with CHP; (2) Buffers are more useful to CO2 enrichment in the summer than to heating in the winter; (3) There is strong synergy between the two production systems - tomatoes and electricity. The tomato crop benefits from the by-products of electricity generation, namely CO2 and heat, sharing this benefit to support low electricity prices; (4) The combined system produces less CO2 pollution than the two production systems operating independently; (5) The contribution of the CHP and buffer to the economic performance of the system is very significant, while that of the thermal screen and boiler is marginal; (6) Flexibility in the system is important. A buffer and/or continuously controlled boiler and CHP are essential to achieving high profitability.