Low Temperature Waste Heat to Power Generation

点击翻译按钮获取中文摘要

Most of the waste heat (more than 60%) from industrial processes and institutional buildings, is low grade heat which makes its direct use within the facility difficult and also reduces the potential for power generation using Rankine or Organic Rankine Cycles (ORCs). ORCs also require that adequate pressure and consequently temperature differential is maintained between the heat source and heat sink to provide sufficient expansion energy for the turbine. This limits the temperature that the heat source can be cooled down to and the heat recovery and power generation potential. The Main aim of this project is to overcome the limitations of conventional technologies by developing an innovative low temperature heat to power conversion system that can recover energy from low temperature waste heat streams. The project addresses the energy trilemma through: i) Reduction of Emissions: by generating electricity from low temperature waste heat and displacing fossil fuel use. ii) Security of supply: the generation of electricity from a waste heat will reduce the amount of primary fuel required and provides an alternative source of energy supply. iii) Cost savings: using waste heat will result in significant cost savings for the users. The system will be optimised for maximum efficiency at heat source temperatures 75 C to 90 C to maximise the range of applications and potential markets. It will operate on what we term 'Controlled Phase Cycle (CPC)'. The CPC is a variant of the Trilateral Flash Cycle (TFC), which is a 3-leg power cycle in which the working fluid expands from saturated liquid state at high pressure to a two-phase state at lower pressure in an expander before rejecting heat and condensing in the condenser. The resulting liquid is then compressed by a feed pump to the higher pressure level before is heated up by the heat source (waste heat stream) and re-expanded. The 'wet' expansion increases significantly the power output compared to the ORC. The TFC has not been commercialised as yet, due to the low efficiency of available expanders and high parasitic losses, particularly pumping power. These shortcomings will be overcome by innovations in this project which include: i) Control of the quality of the fluid before expansion to a high wetness fraction to increase expansion efficiency;ii) Thermal instead of mechanical pumping of the liquid from the condenser to the expansion pressure; iii) Innovative heat exchanger design and controls to maximise heat transfer from the waste heat stream and minimise pressure losses;iv) Improved screw expander rotor profile to further increase efficiency. The project will lead to a proof of concept system to be installed for demonstration and evaluation in a dairy facility.