PHILADELPHIA -- Among the many renewable energy sources -- wind, solar, hydroelectric, biofuels -- there is one to which we all contribute that has not yet managed to attract the romantic advocates who have embraced other forms of green energy.
We're speaking about the gray river of warmth flowing right beneath our feet: sewage.
A Philadelphia company, NovaThermal Energy LLC, wants to heat and cool buildings by tapping into the constant, guaranteed heat contained in wastewater. The process is called sewage geothermal.
"It's just like geothermal energy, but we're using a different well source, so to speak," said Elinor Haider, NovaThermal's chief executive.
Public officials cut the ribbon last week on NovaThermal's first project, a pilot plant at Philadelphia's Southeast Wastewater Treatment Plant, near the Walt Whitman Bridge. NovaThermal is planning to install a second, commercial-sized project later this year at a sewage-treatment plant in Camden, N.J.
Haider said the initial projects are located at treatment plants only because they are public buildings with abundant sources of wastewater. But, she added, the company plans to market its patented Chinese technology to any large building located near a major sewer trunk line that contains a steady flow of wastewater still warm from its previous use.
In China, where the technology was pioneered, several large buildings have successfully employed the technology for heating and air conditioning, including a hotel and a 1 million-square-foot train station in Beijing and a 450,000-square-foot high-rise apartment building in Tianjin, China's third-largest city.
Wastewater picks up heat from a number of sources, including dishwashers, showers and industrial processes, said Jimmy W. Wang, NovaThermal's chief engineer. There's also the "biomatter" that still contains heat, Wang said during a visit to the project last week.
During the winter, sewage is about 60 degrees, and in summer it can exceed 75 degrees. That's plenty of energy that can be extracted with a conventional heat pump.
Haider said the technology is more efficient and cost-effective than traditional geothermal systems, in which deep water wells are drilled into the bedrock to capture heat from the earth.
The Philadelphia pilot project, which is funded by a $150,000 federal stimulus grant, is expected to reduce energy costs 40 percent, she said. It is expected to pay itself off in eight years, though full-scale projects would have a quicker return on investment.
NovaThermal is not the only company marketing a sewage-geothermal process. A Swiss company, Rabtherm Energy Systems AG, is promoting a patented idea that involves piping water through a network of embedded tubes inside concrete sewer mains. The water captures heat from passing wastewater.
Rabtherm's process involves replacing public sewer mains. NovaThermal's technology taps into existing sewer lines and diverts some flow into a heat exchanger.