By Steve Blankinship,
Water has historically been the medium of choice to drive steam turbines. There are other substances far better suited as a heat transfer medium for power generation at lower temperatures. Propane is an extremely efficient one. After being pressurized in its liquid state, propane’s low boiling point means it can be vaporized at much lower temperatures than water. It is about seven times more efficient at capturing heat than water, and after passing through a turbine, propane still contains a significant amount of useful heat.
When steam is used to turn a generator, it must be pressurized and raised to around 1,200 F or above. Below 800 F, the process no longer operates efficiently because the steam pressure drops too low. This means that the heat in flue gases below 800 F cannot be used to generate electricity, and so is lost to the atmosphere.
A surprisingly simple way to harness almost all this waste heat has been developed and is being commercialized by Houston-based WOW Energies. Called the Cascading Closed Loop Cycle (CCLC), the heat of the process utilizes a turbine — specifically a turboexpander driven by waste heat from another turboexpander — to capture waste heat. The key to the efficiency of the heat-recovery system is the use of propane vapor in a closed loop rather than steam to spin a multiple turbine arrangement. This allows it to be driven by low-temperature waste heat.
The first turboexpander’s waste heat vaporizes and pressurizes propane that drives a second turboexpander. Developers calculate that a second turbine, driven by the waste heat from the first turboexpander, would capture almost all the remaining energy. The first turbine’s waste heat would vaporize and pressurize still more propane to drive the second. The WOW concept should allow industry to make use of heat sources below 800 F, which includes most industrial waste heat.
Engineers from the U.S. Department of Energy and a growing number of original turbine equipment manufacturers are intrigued by CCLC’s possibilities. The technology’s developers have been talking with BP, ChevronTexaco, ConocoPhillips, Equistar Chemical, Southern Co., AEP, Bechtel, GE, and others regarding a wide range of applications.
When the DOE asked the company about the potential market, WOW used DOE estimates of waste heat in the U.S. from electricity production, petrochemicals, refineries, incinerators, pulp and paper plants, foundries and cement kilns to show that if only 20% of that waste heat could be converted to electricity, it would equal about 207 GW of generating capacity. By contrast, renewable energy capacity — including hydro, geothermal, municipal waste, biomass, solar and wind — totals about 82 GW. The company’s calculations suggest that power stations adopting the technology should be able to boost their efficiency from 35% to potentially as much as 60%.
“Basically there’s very little that’s new to the process, other than the cascading arrangement,” says Martin Brau, CFO of WOW Energies. CCLC uses readily available components and does not require development of any kind of new equipment. Key to the process is the use of turboexpanders, used for years in the petrochemical industry where there is a need to reduce high pressure gases to lower pressures so it can be bottled. In order to do it, gases are run through turboexpanders producing electricity. GE builds turboexpanders as large as 50 MW.
“A turboexpander is a very simple piece of equipment with only one moving part,” says Brau. Brau adds that it is also possible to add a third turboexpander to capture the condensate heat from a steam turbine, thus eliminating the need for low-pressure steam. “That reduces corrosion because you are no longer running the steam turbine at a vacuum at the end of the cycle, and also you are not condensing the steam to air, but rather putting it through a heat exchanger and condensing it using the propane to extract the heat. In that configuration, the water loop in the steam turbine becomes closed, which means an enormous saving of water. And power generation is the second greatest user of water after agriculture.”
Although the technology’s pollution reduction capability is one of its most attractive features, it was a bonus that the developers had not originally expected. Because heat from the stack is extracted at near ambient temperatures, a significant amount of pollutant precipitates out because it cannot exist in a vaporized state. “It turns out that when you take flue gases from high temperatures down to near ambient temperatures, it condenses out a lot of pollutants, including heavy metals,” says Brau. “That includes mercury.”
Rather than going into the atmosphere, those pollutants are precipitated and collected for safe disposal. “So you are getting additional electricity at zero fuel cost because you’re using waste heat and at the same time dramatically reducing pollutants to the atmosphere,” he says.
Although turboexpanders are cheaper than steam turbines, the substantial cost of heat exchangers required by the process offsets the savings. As a result, the cost of installing CCLC is comparable to that of a steam turbine system, but has the advantage of producing electricity at zero fuel costs. And CCLC’s ability to make more power from the same amount of fuel means less CO2 emissions.
Because the propane is used in a closed-loop process, it is not consumed, but rather re-used. “Propane is only a small component of the overall cost of CCLC since it is not being consumed,” he says. In terms of fuel savings, he estimates a 10 MW unit might save $3 and $4 million a year in operating costs when used to capture waste heat.”
Illustration Courtesy of WOW Energies.
“Until the development of this technology, there was no method to efficiently convert waste heat between the temperatures of 300 F and 800 F to power,” says Daniel Stinger, who co-developed the technology with Farouk Mian. “This is the temperature range where industry generates the majority of waste heat.”
Stinger and Mian want to get the technology designated as a renewable energy and introduce it to state legislatures for tax credits when used in connection with state environmental implementation plans, although Stinger says it’s so inexpensive it doesn’t really require tax credits. “This technology is low-cost and produces revenue in the form of additional energy while reducing pollution at no additional cost,” he says.
And the source of the name for Stinger and Mian’s company? “In virtually every meeting we’ve had, they are making wisecracks at first,” says Stinger. “Then they get quiet. And then, invariably, every one of them says ‘Wow! Why didn’t we think of that?'”