Small-scale biogas and greenhouse system

Sammanfattning: Greenhouse cultivation is a growing industry, especially in mild climates, much due to the ability toadjust the growing conditions and increased water utilization efficiency. The most important aspect onthe cultivation is the indoor temperature. The variation in temperature is due to the Swedish climatewhere the highest and lowest outdoor temperature respectively varies greatly during the year. To enableoptimal indoor climate additional heating is required during colder periods. Presently, most of theexisting greenhouses utilizes combustion of fossil fuels for heating, which contributes to the climatechange through the emissions of greenhouse gases. One way to circumvent this impact is to exchangethe fossil fuels with biogas. Combining biogas production and greenhouse cultivation enables synergiesand a more closed cycle of material flow can be achieved. However, this combination is rather immaturedue to lacking previous research, giving this report its main purpose, to examine the synergies andsustainability of combining a greenhouse with small-scale biogas production. Initially, an extensive literature study was carried out followed by a simulation based on the obtainedknowledge. The simulation was comprised of two greenhouses with different geometries, one with theshape of an arch with polyethylene-film cladding and the other with a sawtooth roof with glass cladding,both with two layers. The other properties such as internal area and volume are more or less the samefor the simulated greenhouses. Useful data such as outdoor temperature, rainfall and solar irradiationetc. was obtained for the city of Enköping, Sweden. The calculations for the models were carried out inthe program Microsoft Excel. In order to evaluate the feasibility of these models a reference greenhousewas studied, which had similar properties and conditions. The optimal temperature for tomato cultivation is 20° C, and to maintain this level over the entire yearit was found that the heat requirements were 89 500 kWh for the arched greenhouse and 94 400 kWhfor the sawtooth greenhouse. In comparison with the reference greenhouse, the heat requirement wasaround 200 kWh per m2 and year less in the simulated greenhouses. Furthermore, it was found thataround 31 800 kWh of cooling is required over the year (249 kWh per m2 and year) for the archedgreenhouse and 30 900 kWh per year (241 kWh per m2 and year) for the sawtooth greenhouse, to keepthe indoor temperature at 20 °C. Moreover, two to three possible harvests annually gives the yield of 3456-5184 kg tomatoes per year. Both the simulated greenhouses are feasible concepts, however thesawtooth greenhouse is a better option due to its increased longevity and lower contribution ofgreenhouse gas emissions over time. Furthermore, more research needs to obtain a fully closed cycle.