A Heated Exchange

Technology transfers sewer energy to heat air and water

• By L. K. Williams
• Mar 31, 2009

A nine-year-old company based in Zurich, Switzerland, has been busy persuading property owners to take a second look at their wastewater. As it leaves the building through the sewers, wastewater contains enough organic material to produce energy. Rabtherm Energy Systems has developed a way to capture that power using a patented heat exchanger technology.

The company said it has installed the Rabtherm system at 25 buildings thus far and about 120 projects are pending.

Urs Studer is chief executive officer of the firm, which has offices in Germany, British Columbia, and the Ukraine. Studer is scheduled to speak about the system at the Clean Technology Conference & Expo 2009 in Houston from May 3-7.

Last year, Rabtherm engaged the help of strategic partners in France and Korea to grow its business. The technology is offered under the name Degres blues by Lyonnaise des Eaux and Saunier in France. The system is represented by the brand HSE (Hudigm Sewage Energy) by Hudigm Architects and Engineers Co. Ltd. in Korea.

A classic decentralized power concept, the Rabtherm System takes advantage of energy where it exists. Research has shown that wastewater temperature is 15 degrees Celsius in the sewer on an annual average basis, varying between 18 to 22 degrees in the summer and 10 to 12 degrees in the winter. According to the company, sewer heat can warm air and water all year round, and for some additional costs, it can produce chilled water for air conditioning.

In existing or new sewers, heat exchangers can be installed and connected to a heat pump. Heat exchanger elements, made from high-grade stainless-steel alloy, are hollow, sandwich-type elements measuring approximately 14-mm thick.

"In working with the University of Zurich, we are constantly trying to improve the properties of the heat exchanger, and we have recently improved its performance substantially," said Roland Schober, director of international marketing and based in the firm's Seattle, office. "By changing the heat conductivity, we can now withdraw approximately 6 to 9 kilowatts (KW) per square meter of heat exchanger," he explained. Previously, the company extracted only 2 to 3 KW per cubic meter of sewage.

Realizing that sewage soils pipes and thereby reduces heat transfer, Rabtherm developed an anti-fouling system that uses strips of copper between the sections of the exchanger elements. The company says the heat exchanger has a service life of at least 50 years.

A small circulating pump sends water (the medium) from the heat exchanger to the heat pump in the building where the extracted energy is used.

The company's Web sites notes that not every building is a good candidate for this renewable heat capture system. Criteria for optimal use of the system include:

• 400- to 800-mm minimum sewer diameter
• 12 liters per second average minimum wastewater flow
• Between 9 and 200 meters of heat exchanger lengths
• Maximum of 200 meters (built) and 300 meters (undeveloped) of medium plastic pipe that connects the heat exchanger to the building
• Maximum building heating temperature of 70 degrees C.
Among the Rabtherm System installations is a medical center building in Leverkusen, Germany, that employs about 300 people. At this site, the heat exchangers were set in concrete elements in the new sewer, which was completed in 2002. The system includes 120 meters of heat exchanger elements that extract 170 KW of energy. Rabtherm estimates the wastewater provides about 70 percent of the total heating and cooling for the building.

When the medical center system started operating in the fall of 2003, the heat exchanger technology produced 981 MW per hour per year for heat and 545 MWh/a for cooling.

Total cost of the system was € 480,000 (roughly US $650,000). Since the system was implemented, Rabtherm estimates that the company saves 65 percent of the costs it spent on oil energy. The system also reduced carbon dioxide emissions by 200 tons per year, Schober said.

In general, the cost of the heat exchanger ranges between 8 and 15 percent of the total system value, without installation factored in. The cost of operation depends on the type of heating system, but the company claims many property owners can save up to 50 percent on their annual heating costs. Schober said only the heat pumps require some maintenance and this is not very costly.

Several European governments offer tax incentives (deductions of portions of the systems or of finance charges, for example) for the installation of sustainable energy sources, including the Rabtherm System, Schober explained. "Even without this help, the return on investment is between five and six years," he added.