Student Project Stops Heat and Money from Disappearing Down the Drain

January 25, 2008

By Jennifer Ouellette
ISNS Contributor

Have you ever thought about what happens to all the hot water you use during your daily morning shower after it disappears down the drain? The teenaged members of Issaquah High School’s Student Energy Conservation Group in Issaquah, Washington, pondered the problem of wasted hot water, and came up with an ingenious solution to recycle all that heat energy: a holding tank that holds hot waste water inside a wall until the heat has dissipated throughout the house. Their system holds such promise that it was presented at this week’s national meeting of the American Association of Physics Teachers, in Baltimore, Maryland.

IHS physics teacher Thomas Haff first presented the problem to his students a year ago, but he’s been thinking about it for quite a bit longer. Ten years ago, he spent a year living in rural Japan, where barely 50% of the population had flush toilets in their homes. Instead, they had built-in indoor outhouses: essentially, holding tanks equipped with fans to vent the methane and any excess heat. Once a month, a truck would drive through the neighborhood and pump out all the accumulated waste.

The Japanese scheme gave Haff an idea after he returned to the US and moved into a house that had no heat in the bathroom. “When my kids were little, they’d take a bath and I’ve leave the hot water in the tub until it reached ambient room temperature, and use that to heat the bathroom,” he said. “Then I thought, why don’t we literally collect the water from a hot shower and use that to heat the bathroom? People tend to forget that heat is energy.” He figured it would be cheaper than a portable space heater. The greater Seattle area already recycles its waste water, but there is no system to recapture wasted heat.

Haff’s students first had to determine their parameters. For instance, how long does it take hot water to travel from showerhead to the drain? The answer is between five and 15 seconds. Within one minute, the water has left the household system and has entered the municipal waste stream. They also had to determine values for how much water is used during the average shower, the surface area and volume of the tank, and starting and ending temperatures of the water collected in the holding tank. As with Haff’s previous use of bathtub water for heat, the “end” temperature would be room temperature.

Using these parameters, the students devised a mathematical model and then designed experiments to test the model. High school senior Chris Matson borrowed an old physics textbook on heat transfer from his father, an engineer, and found an equation for how long it takes hot water in a pot to cool. That allowed them to determine the appropriate heat constant for their project. They tested their model by filling a tank with hot water and tracking the changing temperature twice a minute, for eight hours. And they achieved the best possible result: “Our mathematical model and our data matched perfectly,” says Haff.

The apparatus itself is simple in design: a large galvanized steel holding tank that fits neatly between the walls of a building. Instead of flowing directly out of the house, hot water from the shower drains into the holding tank. A thermometer monitors the temperature, and when it hits a certain level, the system opens a solenoid valve - similar to those used in dishwashers and sprinkler systems - which allows the heat from the water to dissipate throughout the house. This basic passive system can be augmented further with additional vents and blowers to speed the rate of dissipation, but Haff cautions that this would reduce the overall efficiency of his system, since those add-ons would require extra energy to operate.

The benefits of recycling the heat energy in waste water from our daily shower are considerable: it lowers both total household energy consumption and your heating bill. It also lowers greenhouse gas emissions. Nor is Haff limiting the scope to individual homes. He hopes to see similar scaled-up systems installed in university gymnasiums, for example. Haff says he envisions a system that would cost about $1,000 for an average-sized house, and would capture heat efficiently enough to pay for itself in about five years.

But perhaps the greatest benefit of the project is the impact it has had on the students, many of whom had no prior experience with building experimental apparatus or applying the lessons learned in physics class to a real world problem. “The most exciting part for me was going from concept, and abstract equations and ideas, and taking it all the way to this tangible physical object that you can test and see if it works,” says Rachel Chin, also a senior at IHS. “And it did! It was a lot more challenging than just sitting in class solving textbook problems. It showed me that physics is real.”

ISNS contributor Jennifer Ouellette is the author of The Physics of the Buffyverse and Black Bodies and Quantum Cats. She lives in Los Angeles, CA.

***This story is provided for media use by the Inside Science News Service, which is supported by the American Institute of Physics, a not-for-profit publisher of scientific journals. Contact: Jim Dawson, news editor, at jdawson@aip.org.