click to return to the Solar Utilities Network home page Solar Utilities Network projects

Low Hanging Fruit:

by Michael Potts

Solar hot water is easily the most cost effective energy project. We americans are so funny about the way we hoard hot water, keeping a few dozen gallons in the closet in case the next ice age sneaks up on us in our unawares. Everyone else on the planet gifted with domestic hot water heats only what they need. (In a later article I will write more about instantaneous hot water heaters, the only acceptable fossil-fueled hot water source.)

Keeping a tankful of hot water in the closet is just as sensible as leaving the car running in the garage in case you want to go somewhere!

In gentle climates like coastal California, where freezing is a rare and transient phenomenon, harvesting hot water from the sun is easy. Even in hard-freezing climates along the northern tier and into Canada, solar hot water PRE-HEATERS pay for themselves in two or three years.

The idea is simple: water usually comes to us, whether from the well, spring, or water company, at a chilly sixty °F (degrees Fahrenheit). Just letting water sit in a tank indoors can allow it to warm up to room temperature, but setting such a tank in the sun in a passive solar room of its own may in summer supply enough heat for washing dishes or a shower. Even in winter, solar hot water heaters can warm the water by ten or twenty degrees in the harshest climates. Every degree gained for free from the sun reduces the energy required to heat the water to our liking.

batch hot water heater diagram Our hot water system (shown in the diagram) sends water from the well, which reaches 67°F in the summer, through a solar batch hot water heater on the roof, where the temperature may rise as high as 140°F on a sunny day. On the first Sunday in September, a slight cloud cover coupled with a fit of cleanliness by the occupants reduced the gain to a mere 50°F (!).

Our system is the simplest type there is - no pumps, sensors, thermostats, heat exchangers, or relays to go wrong. A black tank is enclosed by a well-insulated box that keeps the heat in at night. Sun shining through the double insulated plastic window on the front of the enclosure heats the black tank and the water within, which stratifies, the hottest rising to the top. Baffles in the tank keep the cold make-up water from mixing with the hottest water drawn from the top of the tank when someone turns on a hot water tap. The water pressure comes from the supply.

There are several other types of solar hot water heaters, but all share the idea that they are PRE-heating. In all but the sunniest places, periods of cloudy weather and patterns of water use - early morning showers! - demand a secondary source of heat. Veteran solar hot water harvesters are proud to tell you that they can turn the secondary source off during a few months of the year. We have valves and a timer which allow us to take a wholly solar bath, and to insure that there is always hot water for morning washing.

Electric back-up heating is the easiest to control, but is by far the least kind to the planet. Heating ANYTHING with electricity is wasteful and inefficient, because there are so many energy conversions involved - from original fuel to steam to spin to electricity - followed by the tortuous and wasteful transmission from generator to point of use. Natural gas, propane, and kerosene are non-renewable fossil fuels, and typically do only a fair job of converting heat content to hot water, nor are they easy to control. In a conventional american home where all the hot water is made with fossil fuels, they are cheaper, and utilities across the land have shifted consumers to them. Biomass hot water heating, especially as a by-product of a wood-fired gasifier, is probably the only sustainable technique, and this works well only in places where the fire is kept burning 24 hours a day 12 months a year.

Freeze protection, the major problem with solar hot water, is simple, too. There's enough mass in the heated water, and the box is well enough insulated, that the water never gets near freezing, but even if it did, there is always a pillow of air in the tanks, which the water would compress if it froze. The pipes connecting the batch tank to the heated interior of the house are kept as short as possible.

Batch hot water heaters work very well anywhere the sun shines and night-time temperatures seldom drop below 20°F. Some manufacturers of batch tanks claim that their systems have been tested in, and survived, days at fifty below!

Local conditions determine which solar hot water strategy will work for you, but Einstein's rule of appropriate simplicity must be applied:

Everything must be as simple as possible, but no simpler.

During the orgy of energy retrofitting which followed the 1973 OPEC Oil Embargo, thousands of solar hot water heaters were installed to garner tax breaks. Since subsidization and sense seldom coexist, these systems violated Einstein's precept, and failed due to their own over-complexity. Sadly, this gave solar hot water an undeservedly bad name. Happily, new technology is simpler, lessons were learned, and more appropriate and reliable systems can now be gotten from reputable local installers: they are your best source of information about what will work best for you. Look in the yellow pages for plumbers who claim solar expertise. As with any journeyman, it would be prudent to ask for, and interview, references. As explained below, all but the most complicated solar hot water systems can be installed confidently by a competent do-it-yourselfer.

In the table which follows you will find some of the conditions and salient features of the different types of solar hot water systems available. Generally, each successive system includes the attributes of the one above it, so the Drain Back system may include a small PV-powered pump.

Black containertropics, camping Unpressurized container in the sun above the point of use. $
Batch (Breadbox)mild (fewer than 6 hours at a time below 20°F) Black pressure tank in well-insulated box with translucent front. Heavy on the roof. $$
In-lineaverage (as many as 6 days a year with 8 hours below 20°F) Solar collectors separate from storage tank, small PV powered pump for circulation, drip-valve freeze protection. Lighter on the roof. $$$
Drain Backhard freezes common Solar collectors (may be filled with antifreeze; Vodka works well) drain back into tank or heat exchanger when pump is not running. Hot water and collector fluid stored indoors. Fairly simple active pumping. $$$
Drain Downsevere Pressurized antifreeze in collectors pumped indoors when cold out; complex sensor / valve system, usually two pumps, three pressurized tanks, heat exchanger. $$$$

If you are a reasonably handy do-it-yourselfer, you can manage to install anything but the complicated Drain Down systems, unless you have an advanced degree in fluid hydrodynamics. Most reputable solar suppliers - see the SUN links page - will be able to get all the necessary parts.

If you mean to have a system installed, break-even will not come quite so quickly, but you will spare yourself the anguish (or delight) of making the technology work. Prepare yourself for a certain amount of "upselling" from vendors, who always prefer to install a little more hardware than is required. While information may be solicited from any plumber who includes "solar" in his advertisement in the yellow pages, you must insist on a good track record, longevity in the community, and a can-do attitude from your installer.

Solar hot water technology runs from the used oilcan in the sun on the roof - the really fancy ones are painted black - to sophisticated drain-back systems costing many thousands of dollars. Surprisingly, all of these systems are outstandingly productive and cost effective if wisely chosen and well installed. Heating hot water with the sun is a great deal!

How great a deal? We at the Solar Utilities Network can no more resist counting the money we save than we can resist the juicy alliteration. In the example given above, a semi-sunny September Sunday, we drew two 20-gallon baths and washed ten gallons worth of dishes, for a total consumption of 50 gallons of hot water. Our electric hot water heater would take an hour to heat that much water from 67°F to 117°F and consumes 3.8 kilowatts while doing so. We pay 13.2¢ a kilowatt-hour for electricity, so that's a savings of about 40¢. We are frugal in our hot water use, and our utility tells us that a typical family may pay about $1 a day for hot water. Our batch hot water system cost just over $900 counting money for the young man we hired to help with the heavy lifting, and we installed it ...I can't believe it! Eight years ago! At our average savings of 40¢ a day it paid itself off about two years ago, and will continue to shave $146 a year off our electric bill (assuming the rates stay the same - fat chance!) A typical family might expect a typical $2,000 solar hot water system to pay off in about 5½ years. A good solar hot water pre-heating system should last 15 years, so after 5½ years, the savings can be considered interest on the original investment.

But that's really not the point, is it? Why waste dead dinosaurs on things we can get freely from the sun?

Heating water with the sun makes great economic sense, but what about heating your home with the sun. In the next article, Solar Space Heating, we explain how to do it, and why it makes sense ... and cents!

If you haven't already audited your energy use, please read about Energy Auditing your home in the previous article.

 click for the Solarnet home page    click to the previous article TopB.gif - 904 Bytes  click for Projects list click to the next article click to the next topic     click for Solarnet Opinions  click for the Solarnet Bookshelf click for relevant Links

Peace is Patriotic -- click to the originators of this lovely logo
Regime change begins at home. Participate!
Michael Potts, webster
updated 1 January 2005 : 16:37 Caspar (Pacific) time
this site generated with 100% recycled electrons!
send website feedback to the Solarnet webster

copyright © 1993-2005 Solar Utilities Network. All Rights Reserved.