Combined heat and power generation from low grade heat:Thermodynamic investigation using fluid mixtures as working mediums
The importance of the recovery of low grade heat has been growing for the last 15 years,due to the increasing concern over energy shortage and global warming.Power plants based on organic Rankine cycles(ORC)are being used to utilize the low grade heat available from geothermal resources,and as waste from exhaust gases from turbines and engines as well as from industrial plants to produce electricity.The selection of working fluid in designing an ORC process is crucial.Different working fluids,generally pure,have been investigated for this purpose while mixtures are less investigated.Among others the efficiency of the process depends on the extent of losses from irreversible heat transfer from the source and to the sink.The losses depend on the levels of matching of T-? curves of heat source,working fluid and the heat sink.This can be achieved by matching the corresponding T-? curves.One way to tune the properties is to use a suitable mixture.Different classes of compounds(alkanes,aromatic hydrocarbons,water,siloxanes)and their mixtures were investigated as working fluids at various condenser pressures.The attention is not laid on the cycle efficiency alone,but also on the total exergy usage from the enthalpy stream.Calculations have been done for the cases where the waste heat is available at 523 K or 623 K.The present study reports the results for different cases specified by the temperature of heat carrier at the turbine inlet and the condenser outlet.The boiler pressure was varied from 5 to 20 bar and the condenser pressure suitable for co-generation.The efficiency varies with the condensation pressure and is higher at lower condenser pressures.The exergy losses occurring at various stages were also investigated.In general,the mixtures perform better than the pure components.For example the second law efficiency for heptane+dodecane mixture is 10%or 30%higher than that of pure substances if the heat source is at 523 K or 623 K.It was found that under certain conditions when a pure fluid can′t be used its mixture with other component offers the solution.The saturation curve is found to be the most important characteristic of working fluid.The results for the process efficiency,second law efficiency and the surface area of the heat exchanger have been discussed.Recommendations of working fluids are made for best performances under defined conditions.
ORC Exergy analysis Thermal efficiency T-H Diagrams
M.Aslam Siddiqi Burak Atakan
Thermodynamics,IVG,Faculty of Engineering,University of Duisburg Essen,Lotharstr.1,47048 Duisburg,Germany
国际会议
桂林
英文
1-16
2013-07-16（万方平台首次上网日期，不代表论文的发表时间）