Utilization of low temperature heat for environmentally friendly electricity production

The focus on reduction of fossil fuelled electricity generation has increased the attention on exploitation of low grade heat as the energy source for electricity producing power plants. Low grade heat is heat, which isavailable at a low temperature, e.g. from waste heat from marine diesel engines and industrial processes orfrom geothermal and solar heat sources. Utilization of such heat sources makes it possible to produce electricity with no additional burning of fossil fuel, and does therefore represent an environmentally friendly alternative to fossil fuel based electricity production. Utilization of low grade heat is not feasible with conventional steam Rankine cycles (steam engines) due to undesirable properties of steam. Instead the organic Rankine cycle is typically used, since it enables thechoice of a working fluid, e.g. hydrocarbons or refrigerants, with desirable properties. One of the key issues for improving the performance of organic Rankine cycles is to optimize the heat transfer processes for adding and removing heat from the cycle, which can be achieved by employing a working fluid consisting of a mixture of two or a number of pure fluids.This project is aimed at quantifying the benefits of using mixtures compared to pure fluids as working fluids in organic Rankine cycles. In order to do so, thermodynamic and economic analyses are carried out, first on an overall cycle level, and next on component level including detailed modelling of heat exchangers, pumps and expanders involving project collaborators with expertise in these areas. In addition to this, novel innovative cycle layouts are developed with the aim of increasing the economic feasibility of utilizing low temperature heat. As an example, this can be achieved by implementing separators in the power cycle to create optimal variations in mixture composition throughout the cycle (equivalent of combining a power cycle with a distillation process). In collaboration with DTU Chemical Engineering, a search for novel pure fluids and mixtures are initiated in order to develop working fluids that are tailored for maximizing the profitability of the power cycles; both the organic Rankine cycle and the novel power cycles.