With today’s high fuel costs, tax incentives and rebates, rapidly increasing product options, and growing consumer demand, there has never been a better time to add a solar water heater to a new or existing home. The U.S. Department of Energy estimates that a home’s domestic water heating costs should drop by 50% to 80% with the installation of a properly sized solar thermal system. Installed costs for typical residential solar water heaters begin at around $5,000, and they qualify for the 30% federal tax credit currently available, as well as for state and local incentives, where available.
Given these offsets, manufacturers claim that a system can recoup its initial cost in as little as three years, but the payback period varies widely depending on the home’s average DHW consumption, local energy prices, geographic location, and other factors, such as whether natural gas, propane (LP), oil, or electricity is used to heat the home’s water.
TYPES OF SOLAR HOT WATER
There are two basic types of solar thermal systems:
“Passive” systems require no pumps or electricity to operate, relying instead on natural thermo-siphon action driven by the temperature differential between the solar collectors and the water inside the storage tank. Because these systems circulate the home’s actual domestic water, they are mainly used in areas where temperatures stay well above freezing.
“Active” solar thermal systems use a differentially controlled pump to circulate freeze-protected fluid, such as glycol, up to the collectors and back to the holding tank through closed-loop heat-exchange tubing, which transfers the fluid’s heat to the home’s domestic water. For maximum energy efficiency, some systems use a small photovoltaic panel mounted with the collectors to power their pumps.
Sun-heated water is not difficult to produce, and the solar collectors available today have changed little in form and function from those developed over the past 50 years. The most common, and typically least expensive, type are flat-plate collectors, which circulate water or fluid through serpentine rows of copper tubing arrayed against a heat-absorbing surface. Average efficiency ratings—the ability to convert sunlight to usable heat energy—for flat-plate solar collectors can reach 80%.
In recent years, manufacturers have increased the efficiency of flat-plate collectors by adding better insulation, heat-retaining glazing, heat-absorbing coatings on the collector plates, and other high-performance features. The low-profile collectors introduced by Velux in 2008, for example, have a complex, computer-designed pattern stamped into the absorber plate surface that refracts solar radiation to increase heat absorption and reduce heat loss from reflectivity.
Evacuated—vacuum—tube solar thermal collectors, the main alternative to flat-plate collectors, collect and transfer heat energy through a series of insulated glass tubes purged to a near-perfect vacuum. Evacuated tubes are growing in popularity because they offer higher heat transfer—manufacturers claim up to 99% efficiency—and are better at capturing solar radiation where site conditions do not allow placing the collector units at an optimum angle toward the sun. This is especially important in cold-weather locations. However, evacuated-tube systems have a higher initial cost, are more susceptible to snow and ice accumulation that limits performance, are at risk for vacuum loss, and are bulkier in design, which many homeowners and communities dislike.
Comparing solar product performance is difficult because it is based on not only the equipment itself but also on the geographic and site conditions where it is installed. The best comparative tool available would probably be the test ratings published by the Solar Rating and Certification Corp. (SRCC) for solar water heater components and systems. These ratings are the basis for obtaining points toward green home certification under the National Green Building Standard.
INSTALLATION CONSIDERATIONS
Homeowners who expect to install a solar system, whether during new construction or at a future date, should design and orient the home to provide at least one roof section with an un-shaded, south-facing roof slope suitable for a solar array.
According to Coley Fudge of Alteris Renewables, a Vermont-based company that installs solar equipment from a number of manufacturers, roof pitch is another important factor, but it doesn’t limit a builder’s or homeowner’s options. In the Northeast, he explains, “a 45-degree roof angle is ideal, but in summer it doesn’t really matter—you’re going to get 100% [of solar radiation] from a 30- to 40-degree slope. In summer, the ideal angle is only 32 degrees.” Alternatively, roof mounts may be used to improve the collector position on less-than-optimum roof pitches, and solar arrays can be installed on ground-mounted racks when necessary.
Although the sun rises every day, it is not reliable as the sole source of hot water in most geographic locations. Building and plumbing codes recognize this and dictate that a fuel-fired heater be provided to support 100% of a home’s hot water needs, regardless of whether a solar-heating system will be added. As a result, solar thermal systems are sized to supplement, not replace, a home’s own water heater. Solar manufacturers are just beginning to adapt their products to satisfy the code demands. One recently introduced system, Caleffi’s Solarie Solar Heat Pump, eliminates the need for a separate water heater by including a backup gas or electric heating element in its storage tank to provide hot water when the sun cannot.
Buyers should beware that, in order to qualify for federal and most state incentives, the key components of all solar systems—the solar collectors—must meet industry certification standards. The most widely accepted certification is bestowed by the SRCC, which tests and approves solar equipment and systems based on ASHRAE standards. Another third-party organization, IAPMO R&T, certifies equipment according to ANSI standards.
Installing solar water heaters qualifies homes for points under most green certification programs. LEED for Homes offers two points for backup systems accommodating more than 40% of the annual load and three points for systems that account for 60% or more. The ANSI National Green Building Standard awards points (from eight to 20) toward certification for solar water heaters on a scale tied to their SRCC Solar Energy Factor rating.
Michael Morris is a freelance writer for Ecohome Magazine.