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Low Carbon Heat: Don’t Let Wastewater Be A Wasted Resource

JodyP 16 August 2019
Low Carbon Heat: Don’t Let Wastewater Be A Wasted Resource

It is an underappreciated resource, but there is a continuous stream of heat pouring out of virtually every building in the land. Heat that is bound up within the wastewater that we discharge from our laundries, from baths and showers, pour down the sink after washing and cooking and flush down the toilet. We wouldn’t consider operating a building without insulation yet the vast quantity of heat we pour down the drain goes to waste without a second thought.

And it is a vast quantity. Wastewater carries around 25% of a building’s daily energy consumption straight down the drain, and across North America and the EU, the volume of wastewater is a mind-boggling 330 billion litres every day. If we could capture the heat, we could replace 1.5 billion MWh of natural gas consumption. Discharged wastewater typically reaches around 21ºC, much higher than other regenerative heat sources such as air source, well water or ground source geo-exchange, and is readily available in urban areas where these alternatives are more difficult to access.

The opportunity is now there to recover this heat and use it to provide a high percentage of heating and cooling within buildings or, excitingly, as a significant contributor to district heat networks. SHARC Energy are supplying wastewater heat recovery systems that fully support the new generation of heat network systems (60ºC outflow, 40ºC return) supplying renewable, low carbon heat to businesses and domestic networks all over the world.

In a SHARC system, wastewater is intercepted from both the building being served and also, where appropriate, drawn from the local town sewer and filtered through our patented clog-proof system, allowing the warm effluent to feed our proprietary heat exchanger that transfers heat into a primary heat pump feed. The ‘dirty’ water is, of course, kept completely separate from the clean water and is returned to the sewer. Tests have shown that there is no detrimental effect to the sewer network, no increase in issues from fats, oils and grease. The heat from the wastewater is now captured in the clean water and a heat pump, which acts on the same basic principle as a refrigerator, boosts the water temperature to around 60ºC for distribution to the heat network to be used for space heating and hot water.

The use of heat pumps is highly energy efficient and can typically achieve 350 – 400% efficiency depending on application – in the case of sewage heat recovery this can be increased to 500% – so for every kWh of energy consumed in running the heat pump, 5 kWh’s of thermal energy is generated for use in buildings.

SHARC fully supports the evaluation process conducting a full wastewater energy survey once opportunities are identified, including a review of municipal town sewers in the vicinity of a project, if there is insufficient wastewater available in the building. SHARC operates in partnership with the relevant water utility to access the main sewer and make connections, to enable the sewer to provide the main heat recovery opportunity.

For large scale installations, resorts where a number of buildings are involved or a Heat Network is in place, SHARC will install an Energy Centre on site with the SHARC Equipment managed and maintained by SHARC personnel under a Design, Build and Operate (DBO) arrangement. The first installation by SHARC in the UK was at Borders College near Galashiels and has now been successfully operating since Spring 2016. In partnership with Scottish Water Horizons and Scottish Borders College, SHARC Energy supplied an Energy Centre that makes good use of the 900mm trunk sewer connecting to the treatment works about 500m from the site.

For smaller scale installations, which would include most single-building hotels, the company have developed the PIRANHA system, a fully integrated heat pump wastewater heat recovery solution that connects to a buildings wastewater discharge. PIRANHA has been developed by SHARC in Canada, with the most recent installation at the Lake Louise Inn, a 200-room hotel in the Banff National Park, part of a UNESCO World Heritage Site. The PIRANHA system focuses on the laundry room, intercepting the wastewater before it leaves the premises, the recovered heat then heating incoming water for future laundry loads.

The installation was commissioned in September 2018, and has achieved a daily heat output of 375kWh, with typical COPs of 4.8. Lake Louise Inn’s laundry room is propane heated, and over a short two-week test period the Piranha system reduced their propane requirement by an estimated 1063 litres, the equivalent of 1.64 tonnes of CO2.

Wastewater heat recovery has now proven itself across a wide range of projects. With the opportunities arising from the rise and rise of district heat networks to incorporate renewable energy sources, this is a technology whose time has come.