Dynamic modeling and optimization of an ORC unit equipped with plate heat exchangers and turbomachines

Abstract

Nowadays environmental concerns call for a transition towards an economy based on fossil fuels to a low carbon one. In order to achieve this goal, efficiency optimization of existing energy systems through waste heat to power conversion units based on bottoming Organic Rankine Cycles (ORC) is one of the actions that appears to be suitable and effective both from cost and environmental perspectives. Indeed, these units are able to increase the overall efficiency of production processes, existing facilities and renewable power plants with a limited payback time. However, despite the increasing number of ORC installations at megawatt scale, the waste heat rejected by industrial processes has rather a widespread nature. Hence, ORC units with a power output in the range of kilowatts should be developed to address this opportunity for heat recovery and for business. In the current research activity, a dynamic model of an ORC system was developed in a commercial 1D Computer Aided Engineering software platform. Sub-models of the two plate heat exchangers and of the multi-stage centrifugal pump were developed and calibrated using performance data of industrial components at design and off-design conditions. On the other hand, the R245fa radial turbine design was accomplished using a design procedure that provided geometrical and performance data for the mapping of the device by means of a 1D tool. A steady-state off-design analysis at different operating conditions at the evaporator was further carried out optimizing pump and turbine speeds to maximize the net power output. Furthermore, the thermal inertial effects at the evaporator were assessed with reference to a sample heat load profile of the water hot source and at different time scales.