Converting Waste Hot Water to Energy

Wednesday, November 30, 2016 @ 01:11 PM gHale

There is a new engine that can generate electricity from waste hot water which could reduce energy consumption and carbon emissions for thousands of different businesses.

Heat lost in waste hot water from industrial processes amounts to around twice the energy in Saudi Arabia’s annual oil and gas output, said researchers at Exergyn, the Dublin, Ireland-based firm that created the engine.

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“There’s just so much waste hot water in the world,” said Exergyn Chief Executive Alan Healy. “In most cases [companies] are actually spending energy to cool it.”

Cargo ships, for example, typically pump waste hot water from the engine around the vessel to cool it down. And in data centers, electricity-hungry fans help dissipate heat generated by rows of servers. Finding an efficient way to capture and use this wasted energy would reduce costs and cut carbon emissions.

The Exergyn Drive uses the quirky properties of an alloy of nickel and titanium called nitinol. You can bend nitinol out of shape, but when heated it undergoes a phase transition and reverts to its original crystal lattice structure. This “shape memory” property makes nitinol desirable in a wide range of applications, including medical devices, unbreakable sunglasses and NASA’s Mars rovers.

It also has another unusual quality. Unlike most materials, nitinol expands when cooled, rather like water does when it turns to ice (think of the mess in your freezer when you leave a bottle of beer to cool in there too long).

“There aren’t many materials in the universe that do that,” said Mike Langan, Exergyn’s head of product management.

These two properties drive the Exergyn engine. Inside the device, a bundle of meter-long nitinol wires attach to a piston. Hot and cold water is alternately flushed over the wires every 10 seconds, which causes them to rapidly expand and contract by 4 centimeters, driving the piston up and down. A hydraulic system converts that forceful linear motion into rotary motion, which in turn drives a generator. The engine produces 10 kilowatts of electricity from around 200 kW of thermal energy in the waste hot water.

That might not be hugely efficient, but this is “free” energy that would otherwise be wasted. And often, money and energy would be spent actively cooling down the waste water.

The company has spent three years perfecting the design and modifying the material so it will keep working for millions of cycles. It earned €2.5 million from the European Commission’s Horizon 2020 fund last year to help bring the technology to market and is now planning three industrial tests in 2017, at Dublin Airport and two landfill sites.

In all three cases, the Exergyn technology will use warm water at 90 °C or less – from a gas engine at the airport and from biogas generators at the landfill sites – to produce electricity on-site.

In addition to harnessing waste heat from industry, the company hopes the engine could expand the geothermal energy market. At the moment, generating electricity from geothermal sources in a cost-effective manner requires very hot water at high flow rates. That typically means digging very deep wells with a wide diameter, which increases drilling costs. Langdon said Exergyn’s technology makes a broader range of geothermal sites viable, as it works with water at a lower temperature and flow.

John Blowes, a past president of the Institution of Diesel and Gas Turbine Engineers, who has seen the technology but has no stake in the company, agrees there is a “massive” range of applications. But he said only a small percentage of these will be viable unless the company can produce the technology cheaply. “It comes down at the end of the day, for me, to commercial viability.”

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