Hot stuff

Lou Reade reports on a new type of heat exchanger that could make gas turbines a viable source of power for the home

A radical design of heat exchanger could help to bring gas turbine technology closer to the home.
Developed by Hiflux, a small firm based in West London, the heat exchanger plays a critical role in a new type of ‘micro CHP’ (combined heat and power) system that could eventually replace domestic boilers.
The heat exchanger – which is based on a series of plates and pins, rather than the more traditional ‘fin’ design – has two main advantages over conventional products: it is easier to manufacture; and it will work at much higher temperatures (up to 800ºC).
“To work at maximum efficiency, a small gas turbine needs to operate at 800ºC,” says Tanzi Besant, Hiflux managing director. “Our heat exchanger will allow this.”
In addition to heating homes, small offices and apartment buildings, Micro CHP systems also generate electricity – which could be sold back to the grid, reducing energy bills. The move towards generating electricity at home is another step towards ‘distributed power’, in which power is generated to meet local needs – rather than generated centrally and used at a remote destination, with its inevitable losses.
The new design of turbine, designed by Swedish company ComPower, generates both heat and electricity – but needs to be hot enough to do this.
“Temperature is a critical factor in its electrical efficiency,” says Besant.
The Hiflux heat exchanger transfers heat from an external burner – which could be powered by a number of fuels, such as bio-gas or wood chips – to a turbine generator.
The high temperature does two things: it rotates the compressor; and it drives the generator.
“If you don’t have enough heat and pressure you don’t have enough force to drive the generator – and you don’t get any electricity,” she says.
Besant says that the ComPower system is claiming an electrical efficiency of around 20% – which could rise as high as 23% once it is fully operational. It is made largely from off-the-shelf components and has a single moving part – a rotating turbine charger.

Turbine power
ComPower’s turbine was developed largely under the European Union’s Framework 6 research programme, in a project called Exheat – the other partners were Hiflux and Italian clean energy company Pianeta.
“We will put the system in the field next year,” says Besant. “We will have two running remotely in parallel applications in Italy.”
Although the largest volume application for the turbines would be as a direct replacement for standard condensing boilers, Besant says this market is too tough.
“The market we’re not aiming at is small buildings where you have 1kW units,” says Besant. “Too many people are chasing this.”
Instead, the most likely first market will be in Germany – in apartment blocks of two to four flats, and a 5kW shared boiler in the basement. The installed price is typically 2 or 2.5 times that of the boiler – but has a payback time of two to five years.
“We expect to make our first sales in 2009, then expect to move to volume sales in 2010,” says Besant.

Pin sharp
Besant calls the Hiflex heat exchanger the ‘enabling technology’ for external combustion systems.
Instead of being brazed and welded from thin foil, the heat exchanger is made more simply – from a metal plate and a series of metal pins. The manufacturing process is still being patented, so details are scarce.
However, she explains that two main materials are used in its construction: stainless steel; and nickel alloys, which perform well at high temperatures.
“The nickel alloys cope well with high temperatures and corrosion, as well as having a long creep life,” she says.
At the same time, the manufacturing can easily be scaled or altered depending on customer needs. ‘Cells’ – comprising a plate and pins – can be stacked close together to form an array. This can be combined with different materials – and made into different shapes – to deliver the exact needs of the end customer, she says.
“Cells can be stacked together so that they are not quite touching,” she says. “Each one can then expand individually – and behave differently to its neighbours.”
Another potential advantage of the design is the ability to alter and scale manufacturing. Different spacings, sizings, pin lengths and shapes could all be adopted, says Besant.
“We have an automated method of manufacture, so can easily change things,” she says. “Core building blocks might be made different sizes for different applications.”
Different surface treatments might also be considered, if they are “economically practical”.
While Micro CHP – with capacities of 1-20kWh – is the most likely first use for Hiflux’s technology, the company is also looking at a second immediate application – larger commercial turbines with outputs of 100kWh-1MWh. These would be used to deliver power to larger facilities such as factories or hospitals.
While the heat exchanger can be used to transfer heat from a burner to a turbine, it can also be used as a ‘recuperator’ – taking turbine exhaust gas to preheat cool air entering the compressor, meaning that less fuel input is needed for combustion.
In addition to these applications, Hiflux has other potential markets in mind: cooling electronic circuits; as a heat exchanger for fuel cells; and to recover exhaust heat for vehicle reciprocating engines.
“We can see this being used with electrical components to remove hotspots,” says Besant. “You can imagine that you might also have to do the same in military applications.”

Micro CHP on the rise
The Micro CHP market is still young in the UK – but has already taken off in Germany.
The European market is estimated to grow to around 120,000 units by 2011. A number of systems are already on the market – such as the Senertec Dachs system, which has sold around 17,000 units in Germany. It burns rapeseed oil, and costs around £15,000. Other systems are also available.
Japan is also a confirmed user of Micro CHP, due to the country’s high electricity costs.
A number of companies are due to launch systems onto the UK market next year.

Written by Lou Reade

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