Sewer Heat Recovery Systems: A Quick and “Dirty” Analysis
At Subterra, our goal is to help developers and property owners find the most cost-effective solutions to reduce the carbon impact of their developments. In the pursuit of this goal, we are constantly being pushed to investigate ever more unconventional places to find new, innovative ways to generate clean, renewable energy. Recently, this search has led us to a “grossly” overlooked untapped renewable energy source – the sewer.
In a sewer heat recovery system, sewer water is partially diverted out of the large city pipe towards the development. The sewer water accumulates in a tank on site and is drawn from the bottom of the tank through a macerator that shreds any solids into small particles. At this point, wastewater and solids are filtered and separated, returning the solid components to the tank and running the wastewater through a simple heat exchanger that is connected to the building loop, and ultimately, a heat pump that either draws or rejects heat to and from the loop.
Our friends at SHARC Energy do a great job explaining this system in the video below:
Sewer heat recovery systems like the SHARC™ System has several advantages when compared to other renewable heating and cooling systems like geothermal. One of the challenges of a geothermal system is that the thermal energy that is drawn from or rejected into the borehole field must be continuously balanced. This problem tends to push one to either:
Oversize a system to allow for more heat dissipation, which is expensive and time consuming for the construction process; or
Use some conventional equipment in a hybrid configuration, which increases operating costs because they require fuel. Conventional systems also produce exhaust and steam, so equipment needs to be placed on roofs or piped through chimney stacks to keep plumes out of sight of the building occupants.
Sewer heat recovery systems offers a superior alternative to both these solutions because it provides the benefit of having a heat sink or source that does not require any balancing. The water used for heat transfer simply flows away down the sewer, while new, consistent temperature water continually arrives from upstream. The amount of heating and cooling capacity of the system is only limited by the flow rate of the sewer, and in an urban setting, this flow rate can be substantial! If the flow is there, boilers and cooling towers can be eliminated from a hybrid system along with all the associated space required and plumes they create.
Apart from tapping into sewer lines to provide thermal energy to district systems, individual buildings can also take advantage of the PIRANHA™ System. This allows buildings to recover thermal energy from their sewage lines to regenerate up to 80% of domestic hot water load in the building. Use of this system would reduce the load imposed on the geothermal field, which is especially useful in already heating dominated buildings.
Operating costs are very low with these systems, requiring only a small amount of input electricity to power the heat and sewer water pumps. In a building that uses heat pumps on the air-side, operating cost performance is similar to a full geothermal system. The key difference is that the source temperature of the sewer is often higher than the ground, which translates to better efficiency in heating mode, and potentially better efficiency in cooling mode depending on how much the geothermal field heats up over the summer months. The installation costs are also significantly lower than a geothermal system on a cost per ton basis, only slightly more than a conventional heating and cooling plant.
The biggest challenge that comes with installing a sewer heat recover system is the
coordination required with municipal governments. Although they are often supportive of these systems, municipal governments are likely to demand that they perform the work to connect the diversion pipe and run it to the property line, adding a layer of complexity to the project. This will probably cost more than bringing in gas or water, because the sewer is usually deeper underground, but all and all we found the costs to be manageable. This is where an experienced consultant in utility management can help reduce the time wasted navigating government bureaucracies. Jamie Amodeo at The Utility Consultants has been a great help to us in the Ontario region in this respect.
“Although they are often supportive of these systems, municipal governments are likely to demand that they perform the work to connect the diversion pipe and run it to the property line, adding a layer of complexity to the project.”
Despite the bureaucratic challenges, sewer heat recovery’s cost-efficient ability to provide a “swing” heating and cooling plant that can backup and support a geothermal loop, combined with its reduced space requirements, and lack of exhaust plumes makes it an obvious choice for many new developments. Sewer heat recovery systems are an exciting technology in the renewable energy mix, which Subterra plans to integrate it in many of its upcoming district energy projects.