From waste heat to tethered tornados

Each day, excess heat billows from the chimneys of manufacturing plants across the world. Companies burn large amounts of fuel to smelt steel and make silicon, and then send most of it up the chimney to get rid of it. There are a lot of thermal watts in that heat. Why are we wasting it?

The idea of recovering energy from heat isn't new. Power plants do it all the time. Utilities operate their plants by taking the heat used by natural gas, coal-fired and nuclear plants, and then use it to turn a turbine, which in turn generates power.

The problem is that the turbine can only use a certain percentage of the heat energy. The rest gets expelled as waste heat. More modern plants use combined cycle generators, which take the excess heat and use it to make steam, which then drives a second turbine. That way, they get to harvest at least some of the excess energy from the wasted heat.

But if you can do it in a power station, why not do it in industrial plants? The idea of generating electricity locally to offset increasingly expensive power provided via the grid is appealing, but much of the debate in this field has so far centred on solar thermal projects.

Cutting costs

Several companies are now involved in projects that they say could save industrial customers substantial portions of their energy bills. Perhaps not surprisingly, at least one of them comes from the geothermal area. Raser Technologies cut its teeth creating geothermal heating systems that would draw heat from wells deep in the ground. It was only a short step towards producing systems that would recover energy from smokestack heat, rather than natural heat under the ground.

The company is entering that field now, says VP of heat recovery Paul Stephan, who plans to retrofit industrial plants by putting pipes into their chimneys. The pipes, which would take up to 20 per cent of the chimney's height, are part of a closed loop system that cycles a chemical. As the chemical hits the part of the pipe coming into contact with the heat, it turns to a hot vapour which is used to turn a turbine. It then condenses and the process continues.

"A smaller plant might cost as much as $2,500 per kW," says Stephan. "But as you're building a plant that's able to produce 8MW, then the cost could be closer to $1,200 to $1,500 per kW,"

The commercial advantage for industrial customers that convert some of their waste heat to energy are huge, argues Tom Casten, chair of Recycled Energy Development (RED). RED specialises in combined heat and power plants, which use some of the excess heat to generate electricity, and then pipe the rest of the heat back into the plant to help fulfill its commercial requirements.

"Industry tends to use two fires: one to make electricity and throw away the heat, and then another to make the heat and throw away electricity," he says. " Instead of cooling the steam as much as possible and dumping it into a cooling tower, you can pull it away just before it's spent and power an industrial process. You can then double the amount of useful work, or cut the fuel that you use in half."

RED says that it has saved some customers up to $100m per year using this Combined Heat and Power (CHP) approach. "They have no fuel costs because they start with energy that was being thrown away," says Casten, citing the example of the Mittal Steel plant in Northern Indiana, which he was involved in while working as chair of energy specialist Primary Energy. The installed system recycles hot exhaust from the plant to generate 95MW of electricity, and 900,000lbs of steam that can be reused in other areas. "That project produces as much clean energy every year as was produced all of the grid-connected solar connectors in the world in 2004, " he adds.

The company provides its own financing to make such projects – which can cost hundreds of millions of dollars – more viable for customers. It then sells the energy that it harvests from its own system back to the customer at rates substantially lower than grid electricity. RED claims that it hopes for a six-year return on each contract, which entails a high level of due diligence on the contracts it carries out.

By selling customers energy and steam from its own local power plant, RED essentially sets itself up as a mini-utility selling watts and BTUs harvested from the customer's own processes. It's perhaps not surprising, then, that Casten riles against the larger utilities' production model. Putting a large power plant out in the middle of nowhere is counterproductive, he says. They may use Rankine or Kalina cycles to harvest more of the thermal energy produced by burning fuels and turn it into electricity, but up to two thirds of the energy is still going up in smoke (or water vapour, to be precise). Because steam loses its thermal energy the further it travels, you can't heat houses in a city using the thermal energy from a plant too far away – but then again who wants a coal or nuke plant across the street?

Stormy weather

Casten would like to sell the energy generated at customers' industrial plants back to the grid, but says he would get a raw deal. He contends that energy produced locally at an industrial plant is worth more, because it can be transferred to another industrial customer in the same area without losing efficiency by being transmitted over long distances. A percentage of energy produced at a remote plant will be lost during that transmission, which makes that energy less valuable.

"The [big utilities] don't credit you for wires and line losses, and you're having to compete with other, older plants that don't have to control their pollution," he complains. "The [CHP] industry in North America just didn't develop to be really good at this. It's not strong enough to get in there and lobby." Countries like Denmark, on the other hand, were early pioneers in this type of local cogeneration, he says.

While Casten battles up a storm with the utilities, others are proposing ways to create storms of their own. Researchers at the University of Western Ontario (UWO) want to use waste heat from industrial plants to create artificial tornados that would reach up to 12km into the air. Waste heat would enter a cylindrical structure at an angle that would cause it to spin. The spin, combined with the convection, would create a tornado which would be tethered to the structure at the bottom. Inside the structure would be turbines that would be spun by the tornado.

"I'm not aware of him making any calculation of the efficiency of those things, or the exact amount of energy, but at this point he's at the level of concept, and it looks like it works," says Horia Hangan, a professor at UWO and one of the directors of the wind tunnel laboratory, of the researcher currently working on the concept. "We just hopefully he'll get into a phase 2, in which he will do some numbers to support the concept."

However, Hangan admits the idea may face staunch opposition from the ever vocal nimby lobby. "It would be an interesting campaign convincing people that it's fine to have a tornado nearby," he observes

It's the kind of scientific spectacle that pioneering scientists such as Tesla and Marconi, with their focus on grandiose projects, would have been proud of. But today's industrial plant owners don't need to control the weather to make a profit on their waste heat — there are simpler ways to put a new twist on energy harvesting.

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