Power generation from diesel generators within remote communities can incur significant fuel, operational and maintenance costs while being harmful to both the local and global environment. Improvements in the power generation process will help to mitigate these problems providing financial, social and environmental benefits to the surrounding population. This report discusses the process of modelling and experimenting with a small scale, low pressure, low temperature, closed Brayton cycle (CBC). The view is to use the CBC as a method of secondary power generation by utilising the exhaust of a diesel generator as the heat source.
In an initial literature review, the history, current and future developments in the use of CBCs has been discussed. Variations to the basic Brayton cycle and the effect that they have on the operation have been reviewed. Each of the four major components has been researched and recommendations have been made regarding the component choice. It was determined that a vane compressor, a hot-side extended surface counterflow heat exchanger and a cold side shell and tube heat exchanger would be the optimal components for their respective duties based on criteria including cost, availability and reliability.
A numerical model of the cycle was developed within Python using the open source program CoolProp as the thermodynamic database. The outcomes of the numerical analysis were compared to results obtained through experimentation with a rig that was designed to demonstrate a method of control of the temperatures and pressures within the cycle. The major components were chosen with cost in mind and were not optimised for power output.
Experimental Analysis of the compressor showed a significant increase in the wall temperature due to frictional heating between the two screws and wall of the compressor. This was causing the fluid outlet temperature to heat beyond the isentropic outlet temperature indicating a very inefficient component. Electric heaters were used to simulate the heat source and maintain a temperature set point at the inlet to the turbine. Results showed that large amounts of heat were being transferred from the fluid to the heaters and piping, thereby reducing its effectiveness. Similarly a large amount of heat was found to be transferred from the expander to its walls reducing the power generation efficiency. The flow rate of water through the shell and tube heat exchanger was fast enough such that the gas outlet temperature was similar to the water outlet temperature. This indicated that the pump was working harder than required to circulate the water through the heat exchanger.
Based on the highly transient nature of the results, the initial steady-state model could not accurately predict the experimental results. This report gives recommendations about future modelling and experimentation to aid in the development and optimisation of a computer model.