HEAT TRANSFER IN HORIZONTAL TUBES AT SUPERCRITICAL PRESSURES FOR ORGANIC RANKINE CYCLE APPLICATIONS

Today process industry has to deal with a lot of waste heat. Very often this waste heat is dumped to the environment, while it could be efficiently used. Low grade heat is also available in solar thermal systems, geothermal systems, biomass combustion and etc. This heat can be converted into electricity using an organic Rankine cycle (ORC). This thermodynamic cycle is similar to the well-known Rankine steam cycle, but it works with an organic working fluid instead of water/steam. As a consequence, the temperature of the heat source at the evaporator is typically lower compared to the steam cycles. To increase the cycle efficiency, supercritical cycles seem very promising. The advantage of supercritical ORCs is a better thermal match between the heat source and the working fluid temperature profiles in the supercritical heat exchanger. Consequently, the overall system efficiency improves. In the current literature, research work on heat transfer mechanisms under supercritical conditions in ORC is very limited. Therefore, it is an essence to study and investigate the relatively unknown heat transfer phenomena at supercritical working conditions for organic fluids in the temperature and pressure ranges relevant for ORCs. Heat exchanger design and heat transfer coefficients are factors that have great influence on the heat transfer and the overall cycle efficiency. Furthermore, special attention must be drawn to the choice of appropriate working fluid, which is a factor of great importance because the organic fluid properties also affect the overall efficiency of the cycle. In this paper, relevant researches are reviewed regarding Supercritical Organic Rankine Cycle applications, selection of working fluids and description of newly designed test facility is presented