Selecting Your Solar Water-Heating System
Choose the one that’s right for your climate, water demand and wallet.
By BARRY L. BUTLER, PH.D.
As utility expenses spiral ever higher, wouldn’t we all like to draw on the sun to heat our household water?
The average 40-square-foot (3.7-square-meter) solar collector system provides 38 gallons (144 liters) of hot water daily, about 60 percent of the daily water needs for a family of four. Virtually every U.S. location has adequate sun for solar water heating.
These systems also benefit from what I like to call the Four E’s: They save energy, create local employment, reduce environmental damage and empower energy security — all necessary for life, liberty and the pursuit of happiness. More than half of your hot-water cost is fixed using solar energy and will not rise with electric, natural gas or heating oil prices. Solar energy is not taxed, and by using less natural gas to heat water, we reduce demand for and hence the price of natural gas. The first cost of a solar heating system may be higher than that of a conventional system, but in the long run solar wins for our society, the U.S. economy and your pocketbook.
In fact, solar water heating makes economic sense by any measure. Compared to the national average price of $1.84 cents per therm for natural gas, unsubsidized solar energy is $1.50 per therm amortized over 35 years.
A solar water-heating system that costs $6,600 installed will produce about 125 therms per year in the Southwest, energy valued at $325 — for an annual return on investment of 5 percent before incentives.
For those who benefit from the 30 percent federal tax credit of $1,980, the cost per therm is just $1.05, with an annual return on investment of 7 percent.
The federal tax credit for solar water heating is capped at $2,000 for residential installations. For a system that replaces an electric water heater, the cost savings is $372 per year, based on the national average retail electricity cost of 10.97 cents per kilowatt-hour, providing a 5.7 percent ROI without federal tax credits, or a 7.6 percent return with federal tax credits.
Depend on a reputable local dealer/installer for advice about what systems and collectors work best in your area.
Here we examine the basic types of solar water-heating systems and thermal collectors, how to find the best match for your location and what to look for in a system and installer.
What System types Can I Choose From?
A solar water-heating system uses a solar collector to heat a working fluid that transfers the sun’s heat to a water-storage tank.
Household occupants use the water from the storage tank to bathe, wash dishes and wash clothes. About 37 percent of the sun’s heat makes it to your hot water faucets. The rest is lost to the air surrounding the collector, piping and water-storage tank.
This efficiency is excellent compared to residential solar electricity-generating systems that may deliver only 12 percent of the sun’s energy.
A 40-square-foot solar water-heating system delivers about 1,400 watts of thermal energy for about eight hours a day. That, as mentioned, is enough to meet about 60 percent of water-heating needs for a family of four.
The most common types are open-loop systems, which directly heat the water that’s used in the house, and closed-loop systems, which heat a fluid that then heats the water used in the home. The most common working fluids are water and water/ antifreeze mixtures.
Systems may use no pumps, a single pump or two pumps. The system consists of a set of solar collecting panels to capture the sun’s heat, a transport fluid to move the heat from the solar collector to the storage tank and a means to move the heated fluid through the system. The general types of systems are shown, in rough order of cost from lowest to highest, assuming that all systems use selective-absorber, single-glazed solar collectors.
All solar manufacturers must submit their solar systems to the Solar Rating and Certification Corp. for performance ratings and listings in order for the systems to be eligible for federal tax credits. The SRCC’s Operating Guideline-300 ratings enable consumers and installers to compare how many therms or kilowatt-hours a solar system will produce per year in any given climate.
Systems are categorized by collector working fluid, means of circulation and system pressure.
Each type of system has been developed for a specific market and climate zone. Some of the systems in this table are no longer manufactured and sold in the United States, such as the open-loop drain-down (4) and the percolator pump Copper Cricket, though they remain in use. All of the other types work well in their particular applications.
Knowing the advantages of each system type, and asking your installer about which systems are preferred in your region, will help you identify the best system for your needs. The schematics show basic elements of the most common system types (details will vary). Let’s look at the pros and cons of each type.
In the integral collector storage system (schematic 1), city water pressure drives the system. When a hot water tap is opened, preheated water from the solar collector moves to the storage tank for final heating, then into the house. There is no pump in the system, so the thermal mass of the water is the only freeze protection.
The thermosiphon system (2) is also pressurized by the city water line, but here cold water settles to the bottom of the solar collector and as it is heated, it rises to the top of the solar collector and then into the hot water tank. The system has no pump. Freeze protection depends on a Dole-type thermostatic valve, which automatically lets warm water run slowly through the collectors and onto the roof during freezing periods.
The integrated collector with a storage tank installed above the collectors (3) is common where hard freezes do not occur. In addition to the thermosiphon configuration shown in schematic 3, where the pressurized water thermosiphons through the solar collector, there are also evacuated-tube heat pipe systems. This configuration uses a freeze-resistant alcohol-water mixture in the heat pipes, with the top of the heat pipe inserted directly into the storage tank. In both cases, an electric heating element in the storage tank prevents freezing. However, a storage tank with 60 gallons (227 liters) of water weighs more than 600 pounds (272 kilograms), so the roof must be strong enough to support a system weighing more than 700 pounds.
Active open-loop drain-down systems (4) are uncommon today. The Heliotrope Corp. sold many of these systems in the 1970s, but it closed its doors when poor sales and a fire at its plant made the business unprofitable.
Many of those early systems are still in use. ACR Solar International has developed a variant that uses city water in the same way but with no drain-down valve. When the system senses freezing conditions, this system uses a Dole-type freeze-protection valve that sends heated water from the tank to the collectors. It also deliberately spills water onto the roof during freezing conditions.
Due to their reliability and low cost, active closed-loop drain-back systems (5) are common where moderate freeze-protection is needed. Among systems that can withstand freezing conditions, they are relatively inexpensive because they use water in the collector loop and a single-wall heat exchanger to the storage tank. They are most common where no hard freezes occur, as in Florida. They drain the collector during nonoperating hours or when freezing is sensed.
Closed-loop, high-pressure antifreeze-filled collector loops (6) are another common option. They are widely used in the Pacific Northwest and other moderate to hard-freeze areas.
Schematic 6 shows a two-tank system, but many systems use a single tank. Because the antifreeze in the collector loop can be nontoxic propylene glycol or toxic ethylene glycol, the heat exchanger must be double-walled, making the system more expensive.
An expansion bladder pressurized with air on one side and filled with fluid on the other allows the heat-transfer fluid to expand when heated and contract when cool, while the pressure stays at about 25 psi, the charge pressure. A 75-psi pressure relief valve prevents overpressure, but may allow the anti- freeze to get to 360oF (182oC) and cause acidification, which corrodes the solar collectors. Antifreeze pH must be measured every few years to maintain a normal level of 8.6. The antifreeze must be replaced if it goes below pH 7.0.
A closed-loop, high-pressure system with a thermosiphon external heat exchanger (7) uses a double-wall heat exchanger outside of the tank, with the tank side of the heat exchanger flow driven by thermal convection.
This avoids the cost of a pump, but the thermal convection does not transfer the heat as well as a two-pump external heat exchanger, shown in schematic 10.
To get a good thermal siphon on the tank side of the heat exchanger, the heat exchanger must be placed low beside or beneath the tank.
These systems are less common today, but widely available.
Closed-loop, low-pressure systems (8), new to the market and proliferating, are similar in operation to type 6 with one major difference: the addition of positive antifreeze over-temperature protection.
The pressure cap-radiator-overflow assembly keeps the antifreeze below 250°F (18°C). greatly reducing the chance of acidification.
This system replaces the bladder expansion tank with an overflow recovery system similar to that used in automobiles. (Full disclosure: This is the type of system my company manufactures.)
The closed-loop, low-pressure, internal-heat-exchanger system (9) is similar to type 7 but overcomes the latter’s convection limits on the tank side of the double-wall heat exchanger.
Because the heat exchanger is immersed in the tank, normal convection and tank stratification are preserved.
Schematic 9 shows the system with the pressure cap-radiator-overflow assembly, but it could be made a high-pressure system using a bladder-lined expansion tank, similar to type (6).
This system uses a single photovoltaic-powered pump and the home’s existing tank, so it is relatively inexpensive among options that can withstand hard-freeze conditions.
Available since 2002, these systems are still new to the market.
Schematic 9a shows a common alternative configuration.
(Again, for full disclosure, my company manufactures this type of system.)
Closed-loop, low-pressure, external-heat-exchanger systems (10) are popular in hard-freeze areas. Since both the collector loop and tank side of the double-wall heat exchanger are pumped, the systems are quite efficient. In schematic 10 the system is shown with the pressure cap-radiator-overflow assembly, but it is more commonly seen with the bladder-lined expansion tank.
What are My Solar Collector Options?
Solar water-heating systems use any of several types of thermal collector: selective-absorber evacuated tubes, selective-absorber single- and double- glazed flat-plate collectors and black-painted single-glazed collectors.
The solar collector you choose depends on where you live. If you live in the Northeast or West, where the skies are cloudy, you may go for the high-performance evacuated-tube solar collectors. They tend to cost more than other options, but they lose less heat to the outside atmosphere.
If you live in the South, flat-plate glazed solar collectors will cost less, and since the climate is warm, a little heat loss is acceptable. Again, your installer can advise on what collectors are best suited to your climate. Or access the SRCC website at solar-rating.org to see how different solar collectors perform in your location.
How Do I Choose?
To select the right system for your needs, narrow the options by asking a few questions.
First consider where you live. Does it have hard-freeze conditions? If the answer is yes, then you need a system that uses antifreeze as a working fluid. There are ways to prevent freezing by sending heat to the collector during freezing weather, but that requires energy, and if the power goes out, the system will freeze. Some recommend drain-back systems, which drain the water out of the collectors if they sense freezing conditions. The problem with these systems is Murphy’s Law. If the sensor fails, the collector freezes. For the lowest risk of freezing, use an antifreeze-filled system. If freezing is not a big issue where you live, then any type of system will do.
Is your region sunny or cloudy? For the sunny South and Southwest, a good flat-plate collector will be fine. For the cloudy Northwest and Northeast, one might consider either a relatively larger area of flat-plate collectors or evacuated-tube collectors. Evacuated tubes work best in cold, cloudy climates, but they cost more. On a heating-per-dollar basis, flat plates and evacuated tubes are comparable. For systems that need to withstand high operating temperatures, you should choose evacuated tubes. Evacuated tubes are also preferable for year-round home-heating and -cooling applications.
Finally, how much hot water do you need? Large families need more hot water. Large homes have more bathrooms and showers to supply. For a family of four, a 50-gallon (189-liter) water tank and 40 square feet of solar collectors is average. If your family is larger
Into Tank, 50 Convected Away Before Use, 370 W/m^2 Delivered To You.
than four people and takes lots of showers and baths, you may want to increase the tank size to 80 gallons (303 liters) and use 60 to 80 square feet (5.6 to 7.4 square meters) of solar collector.
Keep in mind some general rules:
- For most locations, plan on 2 gallons of water storage for each square foot of solar collector area.
- Two-tank systems are more efficient, but they take up more space and cost more than single-tank systems.
- For hard-freeze areas, stick with a glycol-based antifreeze sys- tem.
- Photovoltaic-powered pumps and controllers work when the sun is out, shut off at night and use no grid power to operate;
- AC-powered pumps and their controllers depend on the grid for power. If power is out, your freeze protection may be, too.
- Fewer pumps and fewer parts minimize potential problems.
- Maintenance is inexpensive and should be done every few years.
- Depend on a reputable local dealer/installer for advice about what systems and collectors work best in your area.
Find Available Incentives and Get Some Bids
Once you decide you should not live without solar water heating, access the Database of State Incentives for Renewables and Efficiency, dsireusa.org, to find out what incentives are available in your location. The average installed system cost is $6,600 to $9,000 before any federal, state or local incentives. But depending on where you live, these incentives can offset a large fraction of the installation cost.
To learn who manufactures solar water-heating systems, go to seia.org, the site of the Solar Energy Industries Association. It lists all of the SEIA member companies, including the manufacturers of solar water-heating products.
Next go to the SRCC website, solar-rating.org, and look up the OG-300-rated systems. Manufacturers of these systems are also listed on the site.
With a little research, you should have a list of reputable contractors and offers to install systems well suited to your climate and needs. Listen carefully to what the contractor tells you about systems used in your region. Good installers will know what systems satisfy local customers.
Most systems will last well over 40 years with routine maintenance, and many have built-in indicators that tell when maintenance is needed. Once you’ve made your selection, sit back and enjoy sun-warmed water, knowing that you’ve locked in a fixed water-heating bill while reducing your carbon footprint and creating local jobs.
Barry Butler, Ph.D., owns Butler Sun Solutions Inc., a solar water-heating system manufacturer in San Diego. He is a past president of SEIA and an active member supporting the Solar Thermal Division of SEIA. A recognized expert in solar thermal systems, he also chairs the ASES Solar Thermal Division. Contact Butler at firstname.lastname@example.org.