Open access publications in the SLU publication database

Combined heat and power (CHP) using biofuels is one possible means to reduce emissions of greenhouse gases and increase the use of renewable energy. Cogeneration is also an efficient way to utilize the energy in biofuels. The aim of the project has been to determine if there are economically viable cogeneration technologies for small-scale use. Three case studies were performed, examining facilities using different technologies and fuels. The studied facilities included one small-scale district heating plant using wood chips, one plant producing heat for farm purposes using straw, and one planned biogas facility with dry fermentation.

Steam turbines for cogeneration in larger district heating plants are not economically viable for small-scale use for technological reasons, but there are other possible solutions. Such solutions include combustion engines, stirling engines, ORC technology (similar to steam technologies but using mediums with lower evaporation temperatures), flashbox technology (producing steam through pressure reduction), fuel cells and gas fueled microturbines. Depending on scale, fuel availability and other factors, different technological solutions may be best suited for different applications. Possible biofuels are wood chips, straw, grains, biogas and vegetable oils.

The feasibilities of different cogeneration technologies were assessed in each case study, and the examined plants were designed to meet the needs of power and heat. Power production and fuel requirements were calculated through simulations using heat demands and efficiency factors. By summing up results of hour-based simulations, annual values were obtained. The results were then used in financial calculations, where profitability was estimated from revenues and costs.

The profitability of cogeneration depends on several factors, which include fixed costs for investments, interests, grid connection fees and green certificate management. Fuel purchases, maintenance and potential cooling are examples of variable costs. Revenues are earned from sales of heat, electricity and green certificates. Furthermore, payment is received from the electricity grid owner as compensation for stabilizing the grid and increasing available power.

With the current electricity price it is difficult to achieve profitability for the kinds of small-scale cogeneration that were examined. At the most probable current costs and revenues, there was no profitability in any of the case studies. However, with slightly more optimistic values for service life, investment cost and fuel prices, simulations with current electricity prices achieved profitability in two of the cases. The investment cost was found to be more significant than fuel prices, and if technological development lowers the investment cost, profitability may increase, also if fuel prices rise.

Conditions for cogeneration are more favorable if revenues from heat production are obtained all year round. Additional uses for heat during periods of low load should be investigated, to maximize power production. System properties when operating at partial load are very important when determining appropriate cogeneration technology. A constant development of technologies for small-scale cogeneration makes decreased investment costs probable. Thus, there is every reason for the Swedish agricultural industry to monitor future advances in other Nordic countries and in Europe, to acquire up to date knowledge on the state of technological development in this field.