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 degrees C. Below 450 degrees C, the process no longer operates efficiently because the steam pressure drops too low. This means that the heat in flue gases below 450 degrees 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. But now Daniel Stinger, a turbine engineer, and Farouk Mian, a petroleum engineer, have developed a surprisingly simple way to harness almost all this waste heat. They calculate that a second turbine, driven by the waste heat from the first, would capture almost all the remaining energy. The first turbine's waste heat would vaporise and pressurise still more propane to drive the second (see Diagram). The pair calculate that flue gases will then emerge at a relatively cool 55 degrees C. They have set up a company, called Wow Energy (www.wowenergies.com), based in Sugar Land, Texas, to license the technology to industry once a pending patent is granted. Wow's concept should allow industry to make use of heat sources below 450 degrees C - which includes most industrial waste heat. The company's calculations suggest that power stations adopting dual turbines should be able to boost their efficiency from 35 per cent to potentially as much as 60 per cent. BP and Chevron Texaco have told New Scientist they are interested in adopting the systems to harness waste heat in their industrial plant. If even 20 per cent of industrial waste heat, say, could be converted to electricity in this way, Stinger estimates the US alone could add over 200 gigawatts of generating capacity- almost 20 per cent of its power needs. No one is pretending it would be cheap: it would produce electricity at about the same cost per megawatt as electricity from conventional steam turbines. But more power from the same fuel means less CO2 emissions. Promising as it sounds, Wow Energy's scheme, called a cascading closed loop cycle (CCLC), remains untested. But engineers who have studied it say it makes sense. "It certainly looks very feasible, and the numbers seem to pan out," says James Prochaska, an engineer with turbine maker GE Aero Energy in Houston, Texas. If CCLC can be shown to work, says Joseph Roop, an economist at the US Department of Energy's Pacific Northwest National Laboratory in Richland, Washington, it "opens a vista of possibilities for capturing low-grade heat that we don't currently try to exploit at all." CCLC also has another potential advantage. Because it cools smokestack emissions to about 55 degrees C, many pollutants that enter the atmosphere today, such as mercury oxide and cadmium oxide, would instead condense inside the stack, from where they could be disposed of safely through chemical treatment.
Written by BOB HOLMES
New Scientist issue: 29 May 2004
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