Thermodynamic cycle design of a Brayton–Rankine combined cycle for a pebble bed modular reactor

Abstract

The rapid development in nuclear technology worldwide has created the need for an efficient power conversion unit to extract the energy from the new generation IV reactors. The generation IV reactor currently under investigation in South Africa is the PBMR-DPP (Pebble Bed Modular Reactor Demonstration Power Plant) based on the High temperature Reactor Modul. This reactor produces 200 MW of thermal energy at inlet/outlet temperatures of 250oC/700oC. Due to the reactor layout and accompanying thermal fluid path design outlet temperatures in the order of 900oC would be possible. This dissertation is aimed at the design and optimisation of a Brayton-Rankine combined cycle for use with a PBMR-DPP. The combination of these two cycles improves the thermal efficiency due to the large difference between the maximum and minimum temperatures. The Brayton and Rankine cycles will be developed independently and optimised to ensure that the best possible efficiency is gained from the combined cycle. The heat energy available in the reactor is the input parameter for the Brayton cycle, After the Brayton cycle's pressure ratio has been optimised the heat rejected to the Rankine cycle will be known. The aim of the design is to determine if 50% combined cycle thermal efficiency is achievable. The initial sizing calculation of the cycle parameters has been done in a software package that has been developed for use in the thermo-hydraulics field. Engineering Equation Solver (EES) makes use of an iterative process to simultaneously solve the set of equations. The results obtained from EES were verified by Microsoft Excel with a specialised macro installed for thermo-hydraulics. A very specific methodology was used to solve the Brayton cycle. Traditionally the Brayton cycle is optimised for maximum cycle efficiency to ultimately obtain the best combined cycle efficiency. Very complex cycles such as reheat and multi-shaft Brayton cycles were used. The solution methodology used in this dissertation is to optimise the simple Brayton cycle for the maximum specific work produced in the cycle. The large amount of heat at the turbine outlet is then transferred to the Rankine cycle. The results obtained from the calculations preformed were that a combined cycle efficiency of 52.914% has been achieved at optimum operating conditions. The combined cycle has been shown to operate above 50% efficiency in a wide variety of load-following conditions.