A system designed to capture waste heat from industrial smokestacks and turn it into electricity could significantly boost the efficiency of power stations, drastically cutting carbon emissions, its inventors claim. It could also reduce the amount of toxic pollution released into the atmosphere. The key to the efficiency of the heat-scavenging system is that it uses propane vapour rather than steam to turn a turbine and drive an electricity generator. This allows it to be driven by low-temperature waste heat. When steam is used to turn a generator, it must be pressurised and raised to around 650°C. Below 450°C, the process no longer operates efficiently because the steam pressure drops too low. This means that the heat in flue gases below 450°C cannot be used to generate electricity, and so is lost to the atmosphere. This is one of the reasons why fossil-fuel-powered generating stations have an overall efficiency of only around 35 per cent. Many other industrial processes, such as chemical plants and oil refineries, also vent waste heat.
Unlike water, propane's properties are much more suited to electricity generation at lower temperatures. After pressurising in its liquid state, propane's lower boiling point means it can be vaporised at much lower temperatures than water. But this propane still contains much useful heat after it passes through the turbine, so a lot of heat is still vented, and the small increase in efficiency usually does not make it worth the investment. Source: New Scientist
