Have you been thinking about installing a solar hot-water system for your domestic hot water, but are still a little unsure of just what incarnation you would like that system to take? If indecision likes company as much as misery does, you should be consoled by the knowledge that you are hardly alone. Everyone would like to shave a few dollars off their energy bills, and most of us perceive the installation of a solar hot water system as a viable candidate for achieving that end.
The Ewings 80-gallon solar storage tank with pump assembly (shown on the left).
Photo by LaVonne Ewing
But what type of system should you install? There are practically as many variations on the basic systems as there are people out there installing them. When LaVonne and I decided to add solar hot water to our off-grid home last fall, we quickly discovered that nothing is cut and dried with solar hot water—plumbing solar style, it seems, leaves plenty of latitude for the creative spirit to play. Still, there are just a few basic system types to choose from, and that was the best place to start.
Passive systems for warm climates
Batch heaters and thermosiphon systems
A solar batch heater can best be compared to a big solar camping shower plumbed into your house. It is the simplest and cheapest way to heat water with the sun. A handy homeowner can install one with a small investment and a lot of spare parts and elbow grease.
How does it work? Picture the copper pipe that runs from your well, or municipal water supply, to the cold-water supply side of your water heater. Now imagine that same pipe taking a detour through a glass-covered enclosure on your roof or the south side of your house. To complete this mental exercise, imagine cutting the pipe and installing a tank at the exact place where the pipe encounters direct sunlight
The Ewings finished 30-tube assembly set at 55 degrees (the best angle for them in winter).
Photo by Rex Ewing
beneath the glass. The tank—either a commercially available batch tank or something as simple as the liner from a discarded water heater—is filled with water and exposed to the sun through the glass covering the enclosure. Cold water enters the bottom of the tank; hot water is drawn off the top. No pumps, no heat exchangers, no electronic controllers; it’s a truly passive system. The pressure to move the water is provided by your home’s own water system, and the water heated in the batch tank is the same water that runs through your shower head and your faucets.
It’s an attractive idea that works for many, but it just wouldn’t work for us. The problem was that the piping and the batch tank have to be protected from freezing, something not easy to do in the midst of a Colorado winter. Batch heaters also suffer from the problem of losing heat at night, which is not very useful if you like to shower in the morning.
Thermosiphon systems are similar to batch heaters. The main difference is that water is kept in an insulated tank directly above a solar collector (such as a flat-plate panel). It’s a bit more complicated, and suffers the same problem of being impractical in a freezing environment, but thermosiphon systems at least have the advantage of being able to preserve more heat through the nighttime hours.
Active systems for cold climates
Closed-loop drainback systems
A popular active system for heating domestic water in cold climates is the closed-loop drainback system. As the name implies, the system uses a heat-transfer fluid (usually distilled water) to ferry heat in a closed loop (i.e., a sealed loop that does not come into direct contact with the domestic water supply) from the solar collector(s) through the heat exchanger in a solar storage tank. Potable water moves through the solar storage tank, where it is warmed by the heat exchanger before flowing to the cold-water inlet of the backup water heater.
The pump receives its instructions from a differential thermostat that measures the difference in the heat-transfer fluid temperature between the solar tank and the collector(s). Whenever the tank is hotter than the collector, the pump kicks off and all the fluid drains by gravity flow from the collector into a small drainback reservoir.
Rex Ewing installing copper pipe for their solar water heating system. Photo by LaVonne Ewing
These are tried-and-true systems that work well as long as nothing goes haywire with the pump or the controller (which is highly unlikely). Drainback systems must be plumbed with care, of course, since a continuous gravitational gradient is required to ensure that water always drains into the drainback reservoir when the system is inactive, but that can usually be done with a little forethought.
The Achilles heel of this system—from our viewpoint, at least—was the size of the pump that would be needed to lift water from our garage to the roof, a distance of over 20 feet. Since we live off the power grid and rely on photovoltaic panels and a wind turbine for our electricity, a large, ravenous pump would be too big of a drain on a system that already has enough to do. This left us with one viable choice.
Pressurized glycol systems
Pressurized glycol systems are similar to closed-loop drainback systems in that the heat-transfer fluid (freeze-proof glycol, in this case) circulates through a closed loop, heating water in a solar storage tank via a heat exchanger. The only real difference is that the glycol is pressurized, and thus remains in the collectors at all times. This is both good and bad: bad because glycol tends to break down after a few years of service, so it has to be periodically replaced and the system re-pressurized, but good because a smaller pump (ours draws a mere 38 watts from our off-grid system) is sufficient to keep the glycol circulating.
It was the perfect fit for our solar- and wind-powered home.
Having settled on a pressurized glycol system, it came time to decide which type of solar collectors to use. Considering that we’d recently picked up about a dozen Carter-era flat plate collectors from a friend who was no longer using them, it at first seemed like our decision had been made for us. When you get something for free, you don’t go out and spend a lot of money on something else that does the same thing, do you? Sometimes, it seems, you do. But I’m getting ahead of myself.
Flat-plate collectors have been around since before any of us were born. They are basically flat boxes (usually 4 x 8 feet) covered with tempered glass, through which serpentine loops of black copper pipes are set against an absorber plate. They’re simple, efficient—and heavy. One of our collectors is all two strong men can handle on flat ground (though newer ones are, thankfully, somewhat lighter).
Evacuated tubes are the new kids on the block. Each individual unit consists of two glass tubes—one inside the other, separated by a vacuum—that enclose a heat pipe attached to a black copper absorber plate. The heat pipe is filled with a liquid that is converted by sunlight into steam. The steam wicks to the top of the tube where it gives up its heat to a heat-conducting manifold with water or glycol running through it.
Three flat-plate solar collectors.
Photo by Mile Hi Solar.
Which technology is best? It really depends on the application and how much you’re willing to spend. The beauty of evacuated tubes is that they take up less space than flat plate collectors; they’re far lighter, and they’re hardly affected by cold or wind. Evacuated tubes perform better in cloudy conditions and when the difference between the (ambient) air and (hot) water temperature is high. For winter heating in cold climates, evacuated tubes can’t be beat. On the downside, they are more expensive, so in warm or moderate climates you might do just as well or better with less expensive flat plate collectors.
In the end, we went with evacuated tubes. It was a matter of both space and convenience. To mount two or three of our monster flat plates on a steep two-story roof would have required the use of a boom truck. By contrast, LaVonne and I easily mounted the rack, manifold and 30 tubes in less than a day, with no heavy equipment. As for space, the entire collector assembly fit nicely within the one optimal area of the roof where it could get good sunlight unimpeded by the south-side dormer.
How well does it work? Our 30-tube array can easily heat the water in an 80-gallon solar tank to 160 degrees on a sunny, windy day when the wind chill hangs below zero. Our propane usage has dropped to a fraction of what it was last year, which means we now only take delivery once a year, when the price is lowest.
And the old flat plates? We’ll save them for a hot tub.
|Don’t forget the tempering valve!
One problem with all solar hot-water systems is that the water temperature can vary greatly. The water flowing into the cold-water inlet of your backup heater can easily be 180 degrees or hotter—enough to cause a world of misery on bare skin. But with an adjustable tempering valve (also called a mixing valve) between the backup heater and the water tap, it’s like having a foolproof thermostat; no matter how hot the incoming water, the tempering valve will automatically mix it with just the right amount of cold water to bring it to a (predictable) preset temperature. Don’t jump in the shower without one.
Annual costs of heating water
Although costs vary considerably, depending on where you live and what type of energy (electricity, natural gas, etc.) you use to heat your domestic water, one statistic remains fairly constant: you can figure that 15 to 20 percent of your home’s annual energy costs will go toward heating water. A properly sized solar water-heating system should be able to supply at least 70 percent of your domestic hot water, thus reducing, for example, a $650 annual water heating bill to less than $200.
State and federal incentives
Although the U.S. Congress, in its infinite fickleness, last year failed to renew many of the federal tax credits available to homeowners in the Energy Policy Act of 2005, the 30-percent tax credit for solar hot-water systems extends until the end of 2008. In addition, there are plenty of state and local incentives out there to be had. To find out which ones apply to you, go to the Database of State Incentives for Renewables & Efficiency website at: www.dsireusa.org. Do-it-yourselfers should take the time to determine which installations and components qualify and which do not.
Who does the installation?
If you’re handy with a pipe wrench and a plumber’s torch, chances are you could install a basic batch system without much assistance. But once you get into active systems the level of difficulty rises considerably. Every component in the system must be sized properly, and a number of safeguards will need to be built into it. If nothing else, a consulting fee paid to a seasoned installer could save you a lot of trouble in the long run.
Rex Ewing is the author of several renewable energy books, including Power With Nature, Got Sun? Go Solar, and the newly released Crafting Log Homes Solar Style. He lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind in the foothills of Colorado. His books can be purchased at the Countryside Bookstore.