Dynamic simulation and techno-economicevaluation of a seasonally insulated hybridgreenhouse concept for subarctic climates

Abstract: The Swedish authority for social protection and preparedness, MSB, believes that the self-sufficiency of the country must be strengthened in order to be prepared for a crisis. Presently, Sweden is only 50% self-sufficientwhich is very low compared to its neighboring countries; and it is worse the further north you go. One solution could be to increase the production of food, for example vegetables, but the number of greenhouse operations in Sweden are already dwindling due to the inhospitable climate, which makes it expensive to keep up all-year cultivations. Norrbotten, the northernmost part of Sweden, has the highest heat demand for greenhouses in the country. One option could be to set up operation in heavily insulated buildings and rely on artificial lighting, in so called plant factories. Though, these are expensive, electricity intensive and have generally lower yields. This report will cover the development, validation, and performance of a dynamic model of a new greenhouse concept, developed for subarctic climates. The greenhouse is meant to utilize the strengths of both glass houses and plant factories to optimize the profits for a year-round operation. This, to find an alternative solution to the self-sufficiency problem in the winter. In addition to this, the optimal glaze for the climate shell had to be determined and what type of lighting technology would be the most viable. The hybrid greenhouse is a gableroof greenhouse with insulated north, west and east walls designed for microgreen cultivation, Lactuca Sativa. The interesting part of the greenhouse is that it has a retractable insulation cover, of mineral wool, inside of the climate shell. This was meant to heavily insulate the greenhouse during the winter seasons, though it would not let any sunlight in. This meant that the hybrid greenhouse must rely on artificial lighting in the winter but will get a reduced heating demand. When the outdoor temperature rises and the sun becomes more visible,the cover can be retracted to utilize sunlight for heating and photosynthesis. The cost between heating and electricity usage and the profits from the amount of yield will therefore vary depending on how long the coveris opened or closed and an optimal cost solution should be found somewhere within that variation. The model was created using Simulink version 10.6, which could simulate the heat demand, the humidity level, CO2-concentration, and the yield of the greenhouse. The models heat demand and yield was validated againsta greenhouse in Nikkala, Sweden, owned by Norrskenstomater. The model produced a standard deviation of 24.6 MWh over three months but it overestimated the yearly yield of Norrskenstomater with about 40%. To make the sure the hybrid greenhouse performed effectively, the amount of leakage must be minimized, as this has a significant impact on the heat demand. The hybrid greenhouse needs lighting alternatives with substantial active cooling (90% of the lamps input power), such as light emitting diodes. High pressure sodium lamps produce too much heat and can not be used at all. For yearly simulations, the cover was set to close for certain amounts of months during the year. The highest yearly profit, highest net present value (4.8 MSEK) and lowest payback time (3.5 years) could be achieved if the cover were closed between October and March. The best glazing material in terms of economic performance was 4 mm glass followed by 16 mm plastic panels in acrylic. The hybrid greenhouse could even outperform a greenhouse that did not cultivate during the winter. The hybrid greenhouse seems to get rid of the negative impacts of winter cultivation and is therefore considereda viable alternative as a cultivation system for subarctic climates.