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A unit that recovers heat from waste water and transfers it to clean water entering a heating system could have domestic applications. Siobhan Wagner reports.

Low-grade heat contained in waste water could be recovered and used to supplement domestic boilers with a new heat exchanger.

The Lowheat heat-recovery unit from technical moulders AK Industries Ltd, based in Hereford, is expected to go on the market later this year.

The product was developed under an EU-funded project at Pera research centre in Leicestershire with help from the Chartered Institute of Plumbing and Heating Engineering (CIPHE). CIPHE currently holds the patent for the device.

The system works by recovering heat above 30°C, which is often found in waste water discharged from a variety of domestic appliances. Currently this source of energy is not recovered and it is released into sewers.

With the use of pumps, pipes and sensors, Lowheat reclaims low-grade heat from the waste water of entire buildings, and transfers its energy to clean water entering the heating system.

The system is able to detect when water above 30°C is present in the waste stream with thermocouples. When warm water is present, these temperature sensors signal two low-power pumps to cycle the water into a compartment in the heat exchanger.

The heat exchanger consists of an array of copper-coated heat pipes that are lined with capillary wick and contain distilled water. The application of heat at any point on the pipes’ surface causes the distilled water to turn into steam.

This steam will then transfer heat energy to the other end of the pipe and heat up the clean water entering the heating system. CIPHE said this system can recover more than 40 per cent of what would be wasted heat energy from a home. This, the institute estimated, would reduce the overall energy consumption of domestic dwellings by seven per cent and save more than 1,000kWh per dwelling, per year.

Darren Woodcock, Pera’s Lowheat project manager, said designing such a heat exchanger was a major challenge and not just because the heat involved is low temperature. Also, he added, it was difficult to develop a way to exchange heat from the waste water to the clean water when those two sources must be completely separate.

‘The pipes processing the waste water and the pipes processing the clean water must be physically separated with an air gap,’ said Woodcock. ‘There’s a phrase in the plumbing industry: “Never the twain shall meet”.’

However, he added that his team found a solution to its heat-exchange problem with heat pipes from CRS Engineering Limited in Northumberland. Woodcock said the technology has proven to be a very effective super heat conductor.

He suggested there could be both industrial and domestic uses for Lowheat technology. Woodcock said that industrial laundry providers would see a financial benefit in using it. In domestic applications it would be most likely used for water in boilers.

‘Currently when water comes into a house it’s about 6-7°C,’ he said. ‘The Lowheat system would preheat this water so it would now come into the house at 20°C to 25°C. It is a significant saving because your gas boiler doesn’t have to work so hard.’

One aim of the developers of the technology is to make it user friendly. AK Industries has suggested that the Lowheat system would have wireless capability so that it can be linked to a monitoring device. Homeowners could therefore check their energy recovery in real time on a wall-mounted device.