Simultaneous Optimization of Power Cycle & Heat Recovery Heat Exchanger Parameters

by Trond Andresen, Yves Ladam, & Petter Nekså
SINTEF Energy Research

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Utilizing low-grade waste heat for electricity production has lately attracted much attention due to its potential in reducing the fossil fuel consumption, and due to the versatility of this form of energy. This is a fast growing business with well established commercial actors. Still, power production from medium to low temperature heat sources is today impeded by high investment cost and poor efficiency. Because of this, cycle optimization is very important.

One of the most common waste heat sources is hot exhaust. These are sensible heat sources, in which the temperature decrease as heat is recovered. To increase the amount of heat recovered, the area of the Heat Recovery Heat Exchanger (HRHE) has to be increased. As the conversion efficiency from heat to power decreases with the heat source temperature, increase of power production will come to the expanse of a large increase of HRHE area and therefore will lead to a non linear increase of plant cost.

An in-house power cycle simulator based on physical heat exchanger geometry has been implemented. It allows for simultaneous optimization of power cycle parameters (heat uptake pressure and working fluid mass flow) and HRHE geometry such that for a given maximum weight of the HRHE, a maximum net power output is found. External constraints such as maximum allowed cooling of the heat source can be taken into account in the optimization procedure.

The procedure was tested for gas turbine bottoming cycle applications. Subcritical pentane cycle and supercritical carbon dioxide cycle are compared. For this particular application and for the simplest cycle configuration, the method illustrates challenges for the CO2 technology.