Optimal selection and sizing of heat exchangers for organic rankine cycles (ORC) based on thermo-economics

Organic Rankine cycles (ORCs) are recognized as a viable technology to convert waste heat to electricity. While several implementations already exist, there is an extensive potential for thermo-economic optimization. Because heat exchangers take a large portion of the investment cost of an ORC installation, it is advisable to optimize these first. In this view, the heat exchangers are optimized as part of a system. Several design issues need to be taken into account. Firstly, pressure drops in the heat exchanger will decrement the power output of the system. Secondly, the heat transfer rate of the evaporator is crucial. A too small evaporator will not result in fully evaporated refrigerant at the evaporator outlet, which might cause turbine damage. In contrast, a too large evaporator results in a marginal performance improvement in relation to the added cost. Similar issues arise at the condenser side. An appropriate heat exchanger selection and sizing is thus beneficial for both the investment and operational cost. In this work, a thermodynamic model of the ORC is coupled to a thermo-hydraulic design methodology for the heat exchangers (plate heat exchanger). With this model a thermo-economic optimization is performed and the Pareto fronts of net power output versus investment cost and net power output versus heat exchange area are determined. A case is specified where waste heat is available from water of 100 °C. The results are valuable as a guideline for new ORC installations