With the re-authorization and expansion of federal tax credits for solar technologies, 2009 is expected to be a growth year for solar installations. With a much lower initial cost and faster pay-back than photo-voltaics, thermal solar systems (aka solar water-heating) will be of particular interest to architects and homeowners alike.
In the realm of thermal solar systems, many options are available — from drain-back and closed-loop systems to a near infinite number of ways to configure and integrate into the existing space and water-heating infrastructure. Here’s a quick and dirty overview.
Drain-back vs. closed-loop
All thermal solar systems consist of collectors, either flat-plate type or vacuum-tube, usually mounted on the roof, an insulated storage tank, a pumping mechanism to transfer heat collected at the roof and move it to the storage tank, and piping filled with a water/anti-freeze solution used as the heat-carrying medium.
In drain-back systems, the collectors have the ability to drain themselves of their water/anti-freeze solution in order to protect against freezing or over-heating (under certain conditions, collectors can exceed 212 degrees F). A small secondary reservoir tank is required to store the water/anti-freeze solution when not being used in the collectors. In addition, since drain-back systems are “open-loop,” a large, high-head pump is required to lift the solution from the reservoir tank into the collectors.
Closed-loop systems have no such drain-back mechanism and rely on anti-freeze for wintertime protection; and sound engineering and system design to protect against over-heating. Mechanically simpler than drain-back systems, closed-loop configurations require no reservoir tank and only a small low-head pump to move the water/anti-freeze solution through the system.
Our take: Reserve drain-back systems for special situations only (i.e. large commercial systems). For typical residential installations, closed-loop systems, with fewer moving parts, offer a simpler, less expensive and easier to maintain option for homeowners. Properly designed and maintained, closed-loop systems will not freeze and will not cause over-heating problems. Keep it simple!
Thermal solar for space-heating
The Catch-22 of thermal solar is that when you need the most heat, the sun produces the least energy. So, the question is whether it’s worth the additional expense and effort to integrate a thermal solar system with a home’s (hydronic) space-heating infrastructure.
In upstate New York, for example, during the month of October a typical 30-tube collector array collects, on average, between 25,000 and 30,000 btus per day. In a modest home, this represents around 5 percent of the home’s total space-heating needs. In January, on the other hand, the same collector will produce less than 1 percent of the home’s space-heating needs.
Yes, you can add more collectors to capture more heat, but that would require considerable amount of additional storage capacity — I mean considerable. There’s two problems to contend with. First, the collectors only produce heat during the day — when it’s the night you need the most heat. The second problem is the overabundance of heat you’ll be left with in the summer. Unless, you have an Olympic-size swimming pool to heat, you’ll have paid for equipment that heats water you can’t use.
Our take: Integrating a thermal solar system into a home’s heating circuit adds at least $2,500 on the cost of installation, and increases its complexity and long-term maintenance. In the cold Northeast, we say spend the money elsewhere — upgrade insulation or improve the efficiency of the space-heating system.
Simple, cost-effective design
In thermal solar design, less is more. Keeping a system simple will shorten its pay-back and cost less to maintain over the long-term. With good, direct exposure to the sun, we recommend a modest three-panel array connected to a single-coil indirect-fired storage tank serving as a pre-heater to the home’s primary water heater. This configuration will produce approximately 75 percent of a home’s total annual domestic hot water load, uses the least moving parts, is a cinch to trouble-shoot and repair (one pump, one control device), and finds a perfect balance between maximizing the efficiency of the collectors at a relatively low installation cost.
