If you’re aiming to reduce your carbon footprint, you’ve probably thought about installing a solar hot water system. Here’s the good news: if you have an unshaded south-facing roof, you can install a solar hot water system that will meet about half your annual hot water needs.
The bad news: the typical solar hot water system costs between $6,000 and $10,000.
ICS, Drainback, or Antifreeze?
There are three main types of solar hot water systems (also known as solar thermal systems):
- Integrated collector-storage systems;
- Drainback systems; and
- Antifreeze systems.
Integrated collector-storage (ICS) systems, also known as “batch” systems, store hot water in a roof-mounted tank above the collector. Such systems are common in Israel, Hawaii, and other warm climates where freeze damage is unlikely.
Those of us who live in frosty climates need a freeze-resistant system. To protect against freeze damage, a solar hot water system either drains all the fluid from the collector when it’s cold — the drainback approach — or circulates an antifreeze solution through the collectors.
The collectors in a drainback system are usually dry. When temperature sensors indicate that the sun is shining, a control activates a pump which circulates water through the collector pipes. Later, when the control senses that the sun has gone away, all of the water in the collectors is automatically drained to a “drainback tank” — a special tank, separate from the home’s hot-water storage tank, that holds the collector water. The next day, when the sun comes out again, the water from the drainback tank is again circulated through the collectors. Whenever the pump is operating, hot water from the collectors circulates through a heat exchanger to raise the temperature of the water in the main storage tank.
Propylene Glycol Antifreeze
The second way to make a solar hot water system freeze-resistant is to use an antifreeze solution in the collector loop. When the sun shines, a pump circulates the antifreeze solution; after being warmed in the solar collectors, the fluid passes through a heat exchanger and gives up some of its heat to the water in the hot-water storage tank.
Regardless of which method of freeze-protection you choose, you’ll have to make two other important equipment decisions:
- Do you want flat-plate collectors or evacuated-tube collectors?
- Do you want an AC pump or a DC pump?
Copper Plates or Evacuated Glass Cylinders?
Flat-plate collectors are glazed, insulated boxes containing copper tubing mounted on black-painted copper absorber plates.
Evacuated-tube collectors consist of an assembly of glass cylinders, each enclosing a partial vacuum. When the sun shines, metal elements inside the evacuated tubes get hot; the heat is conducted to a fluid circulating in a manifold at the top of the collector.
Flat-Plate Collectors Are Simple and Robust
I’m a fan of flat-plate collectors:
- The technology is simple and time-tested.
- They cost less than evacuated-tube collectors.
- On sunny days, they perform better than evacuated-tube collectors.
- On an annual basis, they provide more energy per square foot of collector and more energy per dollar invested than do evacuated-tube collectors.
- Snow melts faster from flat-plate collectors than from evacuated-tube collectors.
Evacuated-tube fans like to point out that flat-plate collectors experience more heat loss at cold ambient temperatures than evacuated-tube collectors. (True enough — if the snow ever slides off the evacuated tubes). Moreover, evacuated-tube collectors perform better during cloudy weather, and begin collecting heat earlier in the day, than flat-plate collectors.
Remember, though, that you’re not going to be collecting much useful energy on cloudy days or very cold days anyway. As explained by the author of a technical bulletin from EnerWorks, a Canadian manufacturer of solar equipment, “On overcast days the evacuated-tube collector will perform better than a flat-plate collector. Of course, if there’s not much sun to begin with, doubling your efficiency is not a big advantage. The question is, do you want a collector that will perform better when there is plenty of sun to be captured, or one that will perform better when there is not much sun to start with?”
PV-Powered Pumps Are More Dependable
There are two types of pumps used in solar hot water systems: AC pumps and DC pumps. AC pumps use line-voltage electricity; they are usually activated by a controller responding to temperature sensors. DC pumps, on the other hand, are usually wired directly to a small photovoltaic (PV) module.
You should choose a DC pump — ideally, the El Sid pump from Ivan Labs in Jupiter, Florida — powered by a 10-watt or 20-watt PV module. Such a pump will be much more dependable than an AC pump activated by a differential-temperature controller and sensors — especially since AC pumps don’t operate during a power failure.
Don’t Forget the Downside
Let’s say you’re a fan of solar power, perhaps because you’re old enough to remember when Jimmy Carter installed solar collectors on the White House roof. (Ronald Reagan did not appreciate the symbolism of a solar-powered White House; he had the collectors taken down soon after he became President.)
You’re ready to shell out $9,000 for your own solar hot water system. (Since you’re probably eligible for a 30% Federal tax credit, your actual out-of-pocket expenses will only be $6,300.) You’d like to have two solar collectors, a 120-gallon solar storage tank, and a lot of insulated pipes filled with a glycol solution.
Before you sign the contract, remember:
- Glycol antifreeze solutions degrade over time. These fluids are quickly damaged by “stagnation” during sunny weather; stagnation occurs when a pump fails. That’s why pump dependability is so important.
- As part of normal maintenance, it’s a good idea to check the pH of your antifreeze solution every year or two. If the solution has become acid, it’s time to replace the antifreeze.
- When it’s time to re-roof your house, your solar collectors will have to be temporarily removed at significant expense.
- Unless you live in Hawaii, don’t expect your solar hot water system to save you any money any time soon. A 2006 study by Steven Winter Associates calculated a 58-year payback period for a system installed in Massachusetts, while a Wisconsin system had a payback period of 76 years. The longer payback period in Wisconsin was due in large part to the system’s AC pump, a significant parasitic energy load. Because the Massachusetts system had a PV-powered pump, it had a quicker payback.
- The more expensive the energy you use for your conventional water heater, the sunnier your climate, and the higher the available rebates or tax credits, the faster your payback will be. Investing in a solar hot water system makes much more sense for an Arizona homeowner with an existing electric water heater than for a Minnesotan with a natural-gas water heater.
I Installed One Anyway
Three years ago, ignoring the bleak payback calculations, I installed a solar hot water system on my roof. My system includes two 4′ by 8′ collectors, an 80-gallon stainless-steel tank with an integral heat exchanger (although the tank was affordable, it’s smaller than ideal), an El Sid pump connected to a 20-watt PV module, and an expansion tank. (The hot water in the storage tank is connected to a stainless-steel coil in my woodstove; the water in the wood-stove loop circulates by means of a passive thermosyphon.)
Here’s what I learned:
- The best book on designing and installing a solar hot water system is Solar Hot Water Systems by Tom Lane.
- If you’re willing to do all the work yourself, you can install such a system for about $3,000 or $4,000.
- Solar collectors are big and awkward — much bigger and more awkward than PV modules.
- It’s important to insulate your pipes well. Protect all rooftop pipe insulation with foil-faced butyl flashing tape.
- Be meticulous when soldering and tightening threaded joints. You don’t want any leaks.
- For me, the most challenging tasks were pressurizing and bleeding the air from the antifreeze loop.
- It’s fun to watch the temperature gauges rise on a sunny day.
Conservation Is Always More Important Than Energy Production
Before concluding that you need to produce more energy, it’s always best to try to lower your energy consumption. Be sure that you have an efficient washing machine, a low-flow showerhead, an efficient plumbing layout, and a conserving lifestyle. All of these steps will yield a better return than a solar hot water system.
Last week’s blog: “Can Foam Insulation Be Too Thick?”