Sewers set to harness heat energy from effluent
R3C’s system can return huge volumes of recovered heat to buildings
Report from – Jo Smit
Heat energy is being poured down the drain – literally. Homes are pouring warm wastewater from baths, showers and washing machines into the sewers, and once wastewater from commercial, office and industrial sites is included it starts to become apparent just how much heat energy is being flushed away. Now a new type of precast concrete sewer is set to turn sewers into suppliers of clean, green heat energy.
The @Source-Energy Pipe, developed by Canadian firm Renewable Resource Recovery Corporation (R3C), has been designed to extract heat from the wastewater discharged to foul sewers and from the ground around them. It will then concentrate it using a heat pump and return it to the building to provide heating or hot water.
The precast concrete pipes incorporate a coil of small diameter pipe encased inside the wall of the sewer. Sections of pipe can be connected in series or parallel. A 30 percent ethanol/water mix circulated through the walls of the pipe absorbs heat both from the effluent running in the pipe and geothermal heat from the ground. A key advantage of the system is that the heat transfer liquid flows within the wall of the sewer pipe and is not in direct contact with the effluent. The ethanol mix transfers the low-grade heat back to a heat pump, which captures the heat energy and concentrates it for delivery through a heat distribution system. The heat transfer liquid is then returned to the concrete sewer to repeat the process.
The amount of heat extracted depends on various factors including the quantity of effluent, the gradient of the sewer, flow velocity, water quality, the temperature of the effluent and the distance over which heat is extracted. Based on trials, R3C estimates that for a 400mm diameter pipe, the amount of heat reclaimed from effluent alone would be between 1.2 and 2.6kW/h per 30m length of sewer pipe flowing one-quarter full.
Geothermal heat – or cooling
As well as reclaiming heat from effluent, the pipe will also take heat from the ground around the sewer. Depending on ground conditions – including the flow of ground water around the pipe – the recovered geothermal heat could be as much as 7.0kW/h per 30m length of 400mm diameter pipe. The system can even be linked to a dedicated ground source heat recovery system to further improve the quantity of harnessed heat.
The system can also run in reverse in summer to provide cooling. In this mode the heat pump will use the ground and effluent as a heat dump to enable heat to be removed from a building.
Another benefit is that there is only a marginal increase in the installed cost since most new developments will require the installation of a sewer. As a result, the cost of a combined sewer/heat recovery system is only a fraction more than the sewer-only installed cost. The precast concrete sewer can meet the requirements for adoption and provide a long term durable and eco-friendly solution.
Stuart Crisp, business development director of the Concrete Pipeline Systems Association (CPSA), recognises the potential for the precast concrete system in the UK. “I’m keen to see this technology used by clients, developers and operators of sewage systems and manufactured by the CPSA’s members,” he says.