Sewage heat recovery doesn't sound very exciting! But it's something most people simply flush down the drain without giving it a thought. After all, it's just waste, right? Well, actually not...it's a pretty valuable energy source / heat sink. Sewage from a building is generally flushed away from toilets, showers, sinks, dishwashers and washing machines at a temperature of about 65° to 75°F (18° to 24°C). That's what is considered a pretty low-grade heat...in fact most people wouldn't think of...
it as a source of heat...it's cooler than most swimming pools. But it happens to be an ideal temperature for water source heat pumps...cool enough to absorb energy from heat pumps efficiently, yet warm enough to add heat to a GHX when the temperature drops during the winter.
The sewage from the building is pushed through a heat exchanger. Heat can be transferred either directly to or from the building loop the heat pumps are connected to, or to and from the GHX, or, if properly connected, it can transfer energy to or from either the building loop or the GHX. If it's connected to the GHX, the temperature of the GHX can be moderated whenever the energy is available from the sewage to meet peak building heating or cooling requirements.
Consideration must be given to water use scheduling in the building. Energy storage, either in the GHX or in a holding tank, allows the system to take full advantage of all the energy available from the sewage. Obviously, with raw sewage, dealing with the dirty water and whatever people dump into the system must be dealt with in the selection of the heat exchanger and how the fluid is dealt with before it hits the heat exchanger.
How much energy is available? It depends on the type of building and water usage patterns in the region the system is located. People in Europe and Asia tend to use less water than people in North America. The average person uses 80-110 gallons (300-400 liters) of water per day. That translates into approximately 300-500 Btu/hr (1.0 - 1.5 kW) per person in waste energy. In a 200 unit apartment building that's about 90,000 to 150,000 Btu/hr (25 to 45 kW) of energy every hour. The important thing to consider, this an average hourly rate...with energy storage the continuous flow of energy can meet a much larger peak demand.
A project built in Tianjin, China has been in operation for several years. International Wastewater Heat Exchange Systems Inc. of Burnaby, BC, supplies a manufactured pump/heat exchanger and provides design support for the integration of waste water heat recovery systems.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.