Abstract:
The South African electricity mix is highly dependent on subcritical coal-fired power stations.
The average thermal efficiency of these power plants is low. Traditional methods to increase
the thermal efficiency of the cycle have been widely studied and implemented. However,
utilising the waste heat at the condenser, which accounts for the biggest heat loss in the cycle,
presents a large potential to increase the thermal efficiency of the cycle. Several methods can
be implemented for the recovery and utilisation of low-grade waste heat.
This theoretical study focuses on replacing the traditional condenser in a fossil fuel power
station with a boiling condenser (BC), which operates in a similar manner to the core of a
boiling water reactor at a nuclear power plant (Sharifpur, 2007). The system was theoretically
tested at the Komati Power Station, South Africa's oldest power station. The power station
presented an average low-grade waste heat source. The BC cycle was theoretically tested with
several working fluids and numerous different configurations. Several of the theoretical
configurations indicated increased thermal efficiency of the cycle. The BC cycle configurations
were also tested in two theoretical scenarios.
Thirty configurations and 103 working fluids were tested in these configurations. The
configuration that indicated the highest increase in thermal efficiency was the BC cycle with
regeneration (three regenerative heat exchangers) from the BC turbine. A 2.4% increase in
thermal efficiency was obtained for the mentioned theoretical implementation of this
configuration. The working fluid tested in this configuration was ethanol. This configuration
also indicated a 7.6 MW generating capacity.
The increased thermal efficiency of the power station presents benefits not only in increasing
the available capacity on South Africa's strained grid, but also environmental benefits. The
mentioned reduction of 7.6 MW in heat released into the atmosphere also indicated a direct environmental benefit. The increase in thermal efficiency could also reduce CO2 emissions
released annually in tons per MW by 5.74%.
The high-level economic analysis conducted, based on the theoretically implemented BC cycle
with the highest increase in thermal efficiency, resulted in a possible saving of R46 million per
annum. This translated to a saving of R19.2 million per annum for each percentage increase
in thermal efficiency brought about by the BC cycle.
The theoretical implementation of the BC, with regeneration (three regenerative heat
exchangers) from the BC turbine and ethanol as a working fluid, not only indicated an increase
in thermal efficiency, but also significant economic and environmental benefits.
