A NEW METHOD FOR MODELLING OFF-DESIGN PERFORMANCE OF sCO2 HEAT EXCHANGERS WITHOUT SPECIFYING DETAILED GEOMETRY

The advantage to using supercritical carbon dioxide (sCO2) in a power cycle (whether Rankine or Brayton) over other fluids is justified on the basis of cycle performance simulations. These simulations are influenced by the assumptions and resolution of turbomachinery and heat exchanger models. For turbomachinery, isentropic or polytropic efficiencies can be used for design conditions whereas generalized performance maps are used for off-design analysis. For heat exchangers, either the logarithmic mean temperature approach or the ε-NTU methodology can be used to obtain a first-order estimate of the required global heat transfer coefficient at the cycle design operating point for unknown heat exchanger geometry. However, neither of these methods allow the straight forward calculation of offdesign heat exchanger performance. The work described in this paper discusses the development of a tool to evaluate off-design heat exchanger performance without specifying heat exchanger geometry. Special attention is given to the application of this tool to heat exchangers found in sCO2 power cycles. To that end, common assumptions to the treatment of overall heat transfer coefficient during heat exchanger design will be discussed using quantitative examples for sCO2 applications. In the examples,