Motivation

• 20% of electricity in the US comes from Nuclear Energy
• Pressurized water reactors (PWR) are most widely used, in which pressurized water is used as the reactor coolant to absorb fission energy
• The fission energy is transferred to the secondary side and use steam turbine to generate power
• Despite high cost, steam cycle is considered as the only choice for PWR with a source temperature between 300-330°C

• Studies found the methanol transcritical cycle has comparable efficiency to the steam cycle efficiency
• Methanol was suggested as an alternative working fluid due to its better thermophysical properties (e.g. higher thermal conductivity)
• With pressurized water reactors producing around 160MW of heat and thermodynamic cycle efficiencies of 30%, almost 50MW of energy can be produced from the cycle which would be enough to power 3-4 USU campus’s

Aim & Methods

• This research aims to find the optimal geometrical design for a heat exchanger used in a transcritical cycle with methanol as the working fluid by solving the following optimization problem:

• A penalty function is added to the problems end constraints to ensure that the pressure drop in each channel does not exceed more than 1% of the original pressure.
• Both inlet and outlet conditions of the heat exchanger are known and act as constraints to the optimization problem.

• The heat exchanger was evaluated for a single unit cell where the channels are assumed to have equal widths and heights

• The differential equations are evaluated using a Runge-Kutta 4 method
• They are solved to find the required length for any set of geometry values while still meeting the state points
• The RK-4 method is terminated when the required heat transfer between the fluids is met, and the final length of the heat exchanger is recorded

Results & Discussion

• The optimal geometry of the heat exchanger was found to be a width of 9.988*10-3 m, a height of 5.012*10-4 m, and 5.806*104 channels
• This resulted in a length of 0.22 m and a volume of approximately 0.134 m3, which is at least an order of magnitude smaller than the current heat exchanger
• Current optimal designs were found using the Dittus-Boelter Nusselt Number correlation, which requires further validation
• Different Nusselt number correlations will be further investigated accuracy due to the phase change that occurs to the methanol fluid

References

[1] Khan, M. N., and Tlili, I., 2018, “Innovative Thermodynamic Parametric Investigation of Gas and Steam Bottoming Cycles with Heat Exchanger and Heat Recovery Steam Generator: Energy and Exergy Analysis,” Energy Reports, 4, pp. 497–506.

[2] Lecompte, S., Ntavou, E., Tchanche, B., Kosmadakis, G., Pillai, A., Manolakos, D., and De Paepe, M., 2019, “Review of Experimental Research on Supercritical and Transcritical Thermodynamic Cycles Designed for Heat Recovery Application,” Applied Sciences, 9

[3] Kissick, S. M., and Wang, H., 2021, “A Comparative Study of Alternative Power Cycles for Small Modular Reactors,” Energy Conversion and Management, 247, p. 114734.