Energy Management in Large scale Solar Buildings : The Closed Greenhouse Concept
Sammanfattning: Sustainability has been at the centre of global attention for decades. One of the most challenging areas toward sustainability is the agricultural sector. Here, the commercial greenhouse is one of the most effective cultivation methods with a yield per cultivated area up to 10 times higher than for open land farming. However, this improvement comes with a higher energy demand. Therefore, the significance of energy conservation and management in the commercial greenhouse has been emphasized to enable cost efficient crop production. This Doctoral Thesis presents an assessment of energy pathways for improved greenhouse performance by reducing the direct energy inputs and by conserving energy throughout the system.A reference theoretical model for analyzing the energy performance of a greenhouse has been developed using TRNSYS. This model is verified using real data from a conventional greenhouse in Stockholm (Ulriksdal). With this, a number of energy saving opportunities (e.g. double glazing) were assessed one by one with regards to the impact on the annual heating, cooling and electricity demand. Later, a multidimensional energy saving method, the “Closed Greenhouse”, was introduced. The closed greenhouse is an innovative concept with a combination of many energy saving opportunities. In the ideal closed greenhouse configuration, there are no ventilation windows, and the excess heat, in both sensible and latent forms, needs to be stored using a seasonal thermal energy storage. A short term (daily) storage can be used to eliminate the daily mismatch in the heating and cooling demand as well as handling the hourly fluctuations in the demand.The key conclusion form this work is that the innovative concept “closed greenhouse” can be cost-effective, independent of fossil fuel and technically feasible regardless of climate condition. For the Nordic climate case of Sweden, more than 800 GWh can be saved annually, by converting all conventional greenhouses into this concept. Climate change mitigation will follow, as a key impact towards sustainability.In more detail, the results show that the annual heating demand in an ideal closed greenhouse can be reduced to 60 kWhm-2 as compared to 300 kWhm-2 in the conventional greenhouse. However, by considering semi-closed or partly closed greenhouse concepts, practical implementation appears advantageous. The required external energy input for heating purpose can still be reduced by 25% to 75% depending on the fraction of closed area. The payback period time for the investment in a closed greenhouse varies between 5 and 8 years depending on the thermal energy storage design conditions. Thus, the closed greenhouse concept has the potential to be cost effective.Following these results, energy management pathways have been examined based on the proposed thermo-economic assessment. From this, it is clear that the main differences between the suggested scenarios are the type of energy source, as well as the cooling and dehumidification strategies judged feasible, and that these are very much dependent on the climatic conditionsFinally, by proposing the “solar blind” concept as an active system, the surplus solar radiation can be absorbed by PVT panels and stored in thermal energy storage for supplying a portion of the greenhouse heating demand. In this concept, the annual external energy input for heating purpose in a commercial closed greenhouse with solar blind is reduced by 80%, down to 62 kWhm-2 (per unit of greenhouse area), as compared to a conventional configuration. Also the annual total useful heat gain and electricity generation, per unit of greenhouse area, by the solar blind in this concept is around 20 kWhm-2 and 80 kWhm-2, respectively. The generated electricity can be used for supplying the greenhouse power demand for artificial lighting and other devices. Typically, the electricity demand for a commercial greenhouse is about 170 kWhm-2. Here, the effect of “shading” on the crop yield is not considered, and would have to be carefully assessed in each case.
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