Keeping in the heat

A new design of heat exchanger could extract energy from low-grade waste water at just 30°C. Julie Bieles reports

Researchers have developed a way of recovering energy from warm waste water.
The Lowheat system uses a heat exchanger to recover energy from the low grade waste water – in the temperature range 25°C-80°C – generated from appliances including washing machines, dishwashers, showers and sinks. Energy captured is then used to heat the premises’ hot water systems.
The system consists of a wireless control system, a probe, a pump and a heat exchanger, and can be installed by a plumber according to UK research company Pera – one of the project partners.
“Lowheat can capture the warm waste water that is discharged down our drains – both in a commercial and domestic setting,” says Pera’s Lowheat project manager Darren Woodcock.
When the system detects water (at 30°C or higher) going down the drain a pump is activated. This diverts water into a compartment in the heat exchanger. Energy collected travels through heat pipes – hollow copper tubes containing distilled water, which turns into steam when one extremity comes into contact with a high temperature. The steam transfers an identical temperature to the other end of the pipe. This is used to reheat the clean water circuits, which pass through the domestic boiler and result in increased heat in the tank now ready for use.
Woodcock says: “The difficulty in using conventional heat exchangers in applications such as showers and dishwashers is that they are inefficient. They are usually associated with a high temperature difference between mediums. Also, there is an increased risk of blockage. Lowheat is designed to be maintenance-free.”
It overcomes another difficulty – the legislation considering the cross-contamination between potable and grey water. In this case, heat is exchanged without any close contact between the potable water and dirty waste water, as there is a physical ‘air gap’ between water vessels.
Woodcock estimates European households use over 25,000 billion litres of water each year. About 60% of this is heated low-grade waste water, which runs into drains without any attempt to recover the associated heat energy.
He says: “Our heat exchanger performs with only a few degrees temperature differential between cold and warm side. Conventional units cannot operate effectively at this level.”
The Lowheat system could reduce utility bills by up to 10%, giving a saving of about £120 per year in an average household. On commercial properties the savings could be far hugher, with an economic return of less than three months, Pera says.
The system is positioned at the base of the drain stack, which exits the building, and, Woodcock says, only requires one drain device and one heat exchanger to operate. The drain device captures a small amount of waste flow water, and contains the electronics and wireless systems. The user can have a wall device to show energy saved, money saved, and shows a real time performance of the heat exchanger.
“The difference between majority of existing heat exchangers and Lowheat is that our system can continue to extract heat energy after the waste water has flowed away. This is due to the waste water capture component. Normally, heat is exchanged simultaneously, as the cold medium side is requested.”
A typical example is that of a 10-minute shower, he says. The Lowheat system would actually be operational for fifteen minutes – 50% additional time than current heat exchangers.
“This enables the system to extract over 1.4MJ from the available energy of 3.8MJ, giving a 36% heat recovery rate,” says Woodcock.
The Lowheat project began in August 2004, and will run until August 2007. Woodcock expects the product to be available commercially at the end of 2008, subject to approvals. He says there is no major difference between commercial and domestic installations, apart from scale – the fact that commercial installations may require a larger waste capture and will recover more energy.
The collaborative project is led by the Institute of Plumbing and Heating Engineering (IPHE), the UK’s plumbing and heating professional and technical body, and backed by the Polish plumbers’ association. It has several international partners.

Tom Shelley

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