Dependency of production planning on availability of thermal energy in commercial greenhouses – A case study in Germany

Abstract

Commercial greenhouses are often referred to as the optimum heat dump for very low grade waste heat from a variety of sources. However availability, temperature levels and fluctuations in the availability necessitate a very different approach towards production planning as compared to traditional systems where energy supply is adapted to production demand and not the converse.In this study, a commercial ornamental plant nursery that had switched its heat supply from natural gas to utilizing the waste heat of a commercial CHP system in 2007 was analysed. The differences between production planning and temperature regimes before and after switching to waste heat were compared. Furthermore, the degree of utilization of the waste heat available was evaluated.The study undertaken showed three main results: Firstly, greenhouses present a good opportunity for the use of the low grade waste heat. However production needs to be planned very carefully to facilitate the production of high quality plants, for which traditional cultivation planning is unsuitable. Secondly, when planning on utilizing the heat (7.1 GWhth/a) of a CHP system which is sized based on electricity output (6.7 GWhel/a), to the full capacity, additional users need to be found and ideally heat storage integrated. Currently heat utilization amounts to less than 45% (3.1 GWhth) in total. Especially in summer most of the heat is discarded. Thirdly, several scenarios for utilization optimization were considered. The scenarios described allow for an increase of heat utilization to 62% or 4.4 GWhth using some of the heat to run absorption chillers to provide cooling in summer and storing excess heat in long term heat stores (300 MWhth) to reduce the additional gas demand. The results of the economic evaluation show that the integration of a cold store, supplied by the coolth by using an absorption chiller is financially attractive with a resulting payback period of 2.9 years, whilst consideration of integration of a PCM heat store is far from being viable with a payback period of almost 300 years.