Green Dream: A Solar-Powered Water Boiler

Heating your home's water supply without using gas or oil

The Green Dream

John B. Carnett, PopSci's staff photographer, is using the latest green technology to build his dream home. Follow his progress here each month.Peter Bollinger

Stage #2: Solar Water Boiler

Why use natural gas or oil to heat your home's water supply when the sun can do it for free? The big boiler in the sky lays down 100 watts of power across a single square foot. The simple solar-powered hot-water system that I've built from scratch will put these free watts to work on the rooftop of my new eco-conscious home. Once I get it installed, it will generate about 450 gallons of hot water a day, plenty for my family of four, while consuming half the amount of energy of a conventional hot-water system. Plus, it will fuel my home's radiant heating system, a series of polyethylene pipes built into the floor that use hot water to warm the house.

The first step is to build two 150-square-foot solar collectors—tidy sandwiches of glass, copper tubes, aluminum sheets and foam insulation held together by a pair of aluminum frames. I'll position the panels at 65 degrees to the plane of the roof, facing south, like my house, to catch as much sun as possible to heat up the fluid (the antifreeze glycol) flowing through the copper tubes.

Next I'll install a pair of insulated 158-gallon storage tanks in the basement to hold my supply of municipal water. To heat it up, a pump will circulate hot glycol from the collectors through a heat exchanger inside each tank. When sensors in the roof-mounted panels calculate that the panels are warmer than the tanks, a controller automatically turns a valve to start circulating the glycol. One tank distributes water for showers and dishes while the other services the radiant floors.

What happens during the winter when the sun's intensity wanes? That's where heat from my newly drilled geothermal well comes in. More on that project next month.

What's Inside?

Solar collectors for a DIY hot-water systemJohn B. Carnett

    Sold on Solar?

    Four Other Ways to Bring Home the Sun

    1. Robo-mower
    Bury a low-voltage wire around the perimeter of your yard, and the electric Automower mows for you, undocking from its charging station on a programmed schedule to trim the lawn. Unlike other robotic mowers, this hybrid model draws power from a dorsal-mounted photovoltaic array to double its runtime. $3,000; automower.us
    2. Backup Power
    The solar-powered PowerCube 600 generator wraps photovoltaic panels, batteries and a power-management system into one 6-by-3.5-foot box. Just open the lid to expose the cells, and plug in. The base model delivers 3,500 volt-amps of continuous charge, enough to power a small home. $25,000; powercubeenergy.com
    3. Heated Floors
    Most radiant-heat systems require very hot water to heat effectively. Warmboard uses an aluminum surface bonded to plywood that spreads heat throughout each floor panel more evenly and efficiently, allowing it to use 90° to 110°F solar or geothermally heated water. It also prevents hotspots and won't warp wood floors. $15/square ft. (installed); warmboard.com
    4. Battery Booster
    This brick-size solar panel produces only about 2.25 watts of power, but that's enough to keep a car battery charged. Just suction-cup it to a window or the dashboard, and plug it into a power outlet. Or connect it directly to the battery terminals on your weekend-use-only ATV to avoid early-Saturday-morning jump-starts. $50; sunsei.com —Chuck Cage