Small scale combined heat and power units using external combustion

Abstract

Combined heat and power plants are of increasing interest due to the rising concern over global warming as they can lower emissions, by having very higher efficiencies than traditional power plants. Implemented on a small scale units and with external combustion they allow for great flexibility in implementation and fuel, and can allow for remote locations to serve their own heat and power needs. This thesis investigates in the first part the available technologies for such plants on a small scale and compares them on criteria which are important for efficiency, economy, implementation and operation in a remote area. Organic- and steam Rankine cycles are evaluated as well as gas turbines, Stirling engines and thermoelectric generators. The first part concludes with the choice of gas turbines as the best technology for small scale power and heat generation based on defined criteria. In the second different concepts for the basic layout and components of such a plant is evaluated. A commercially available gas turbine is chosen as a base and a solution where the gas turbine feeds its outlet air into the combustion chamber is selected. Two alternative layouts are simulated using the open source DNA, and compared along with a base case. While both layouts compared have advantages and drawbacks, only one has an efficiency comparable to larger plants, and should therefore the preferred concept under most circumstances.