Solar Water Heating

Published in the Sun October 1, 2011

In my yard is a large black garden hose.  On a sunny day, the water that comes out of that hose is as hot as coffee.  My plants don’t really appreciate the hot water, but it does make you think about the excessive solar energy that has lately been falling on us like a plague, and how it might be put to a useful purpose.

Back in 1891, when people still heated their weekly bath water on a wood stove, Clarence Kemp patented the first commercially available solar water heater.  The Climax Solar Water Heater consisted of a pine box, lined with black felt and covered with a pane of glass.  Inside the box were four iron tanks, painted black, which held a total of 32 gallons of water.  The Climax would be installed on a south facing roof and plumbed into the bathroom.  Mr. Kemp’s advertisement warns that under very sunny conditions the water almost boils.  With this contraption the lucky owner could “go home at night and find hot water for bath ready for you, on tap same as cold water… There is no delay and no expense for fuel.”

Over the next 30 years, many thousands of solar water heaters were installed in California and Florida, but by the 1930s the widespread availability of natural gas (and its subsidized low cost) killed the market for solar hot water.  The idea didn’t go away though.  In 1979, in a show of support for alternative energy, Jimmy Carter installed 32 solar collectors on the roof of the White House to provide hot water for the staff cafeteria.

Only five years later, Jeremy and Linda Gambell began building a home in Georgetown.  The cheapest option for water heating would have been natural gas, but the Gambell’s subdivision did not provide natural gas, so they were faced with a choice between the hassles of propane or the expense of electricity.  As newcomers from the snowy regions of New York, the Gambell’s were impressed by the reliability of Texas sunshine.  Or maybe they were just hippies, but they decided to buck the system and try solar water heating.

The water heater that Jeremy chose had advanced a long way beyond the Climax Solar Water Heater.  There are two 4x8 foot collector panels on his roof.  From the driveway they look almost like skylights.  Circulating through the collectors is a 50% propylene glycol solution, a foodsafe antifreeze.  The glycol is heated by the sun and is then pumped through a heat exchanger where it transfers the heat to the household water, which is stored in a regular 120 gallon hot water tank.  The water can stay hot in the insulated tank for several days.  A thermostat on the tank allows for back-up heating by electricity should the collectors fail to provide enough heat, such as might happen when we have several days in a row of cloudy weather.  It is not necessary for the weather to be hot for the panels to work.  A cold sunny day can heat water, just like the inside of your car can become quite warm even in the winter.

I asked the Gambells if they really have enough hot water.  Linda looked askance at Jeremy and said, “Well, there was that time you forgot to turn the breaker back on in the fall.”  It turns out that every spring, Jeremy turns the back-up electric water heater completely off, and doesn’t turn it on again until about November.  In other words, for most of the year, they don’t even use the back-up electricity at all; the solar collectors supply 100% of their hot water.  I was a bit incredulous.  Surely they must take their showers at night and just didn’t notice that the water was cool in the mornings?  Nope, they both take showers AND they do the laundry in the morning, and all through the hot season they have plenty of hot water.  Linda says she never worries about running out of hot water, but with the drought she does sometimes worry about running out of cold water.

The Gambells paid $3955 for their hot water system in 1985.  Every two years the “Solar Man” comes and changes the glycol solution for $42.50.  I recently bought a similar system and mine cost $4830, including the tank and installation.  There is a 30% federal tax credit, bringing the net cost down to $3381.  Mine won’t need the glycol exchanges, because it uses distilled water in the collectors.  If you would like to estimate your own return on investment in a solar water heater, there is an excellent lifetime cost calculator at www.solar-estimate.org that lets you plug in the rate you pay for electricity and the number of people in your family.  To make a long story short, by reducing electricity costs, a solar water heater will pay for itself in 4-10 years, depending on how much hot water you use and how fast electricity rates go up.

On average in the US, water heating accounts for 14% of total household energy consumption.  In a large family, the annual carbon dioxide emissions of the water heater can equal those of the family automobile.  The sun is going to shine on us anyway; we should put it to work.