Industrial waste heat or residual heat as it is also called is a resource that is not fully utilized. Residual heat involves the utilization of losses that would otherwise not have been utilized, and no alternative use of the heat is expected to exist. The purpose of this feasibility study is to investigate the possibilities and barriers to establishing a greenhouse for vegetable production in Borlänge, Sweden that is heated with waste heat.
A literature search among Swedish-language literature on energy statistics, heating technologies and energy efficiency for greenhouses has been carried out. Based on data on U-values for various cover materials, a calculation is made of the annual energy requirement for a greenhouse of 10,000 m2 in Borlänge and the dimensioning heat rate is calculated. Based on hourly weather data, the heat demand is also calculated considering the solar radiation and the heat demand per hour is calculated and summed monthly.
Measurement points in the district heating network of return temperature and the greenhouse's sizing power requirements are used to calculate the design heat flow rate that is compared to available flows in the district heating network.
It is found that existing capacity in the flue gas condenser at Kvarnsveden's paper mill can cover the entire power needs of the greenhouse by lowering the return temperature. During the four coldest months of the year, the flow rate through the flue gas condenser amounts to between 250 kg/s and 350 kg/s, which is about 4 to 6 times higher than the design heat water flow rate for a greenhouse of 10,000 m2. The condenser power can be increased by lowering the return temperature, while maintaining the water flow constant. It is the simplest solution for utilizing residual heat in Borlänge, but it requires that the greenhouses can be heated with the district heating return temperature which reaches as low as 42 ° C during the winter months. However, the literature study has not been able to detect any greenhouses that use low temperature heating, but an ongoing project are investigating how low-temperature excess heat from data centers can be used for greenhouse heating.
The heating need for continuous cultivation in a greenhouse of 10,000 m2 in Borlänge amounts to approximately 5 to 10 GWh / year. Lighting for year-round cultivation of tomatoes is estimated to be in the order of half the heating requirement and since it is used mainly during the darkest period, the lighting element will replace energy for heating to a corresponding degree.
The yield from year-round cultivation of tomatoes is reported to be as high as 66 kg/m2, but it also entails high energy consumption, corresponding to about 10 kWh/kg electricity and 10 kWh/kg heat. The heating cost for the district heating return amounts to about 220 SEK/m2 or 3.3 SEK/kg of tomato. Electricity costs for plant lighting and other operating costs for year-round cultivation are likely to exceed the heating costs.
Pipe heating systems, fan convectors and airborne heat are the dominant techniques for heat distribution and heating in greenhouses. The heat-emitting surface of a water-borne low-temperature heating system that uses return heat in Borlänge needs to be about 2 m2 per m2 of floor area, which is 3.5 times the surface area when using an 80/60 pipe heating system. The electric power requirement for circulating heating water in the greenhouse amounts to about 3 kW compared to the demand for an airborne heating system that is about 150 kW. System design and costs for heating systems for low temperature heating must be further investigated. Aspects such as the greenhouse's insulating performance in relation to the size of the heating plant, the length of the growing season, the need for lighting and the required yield must be considered.