Solar Hot Water 101 - Continued

Since they don't involve any pumps or controllers, thermosiphon systems are simple and extremely reliable. But, because the tank is outside, they have low flow rates and high storage losses, making them less efficient than pumped systems. Also, the tank in these systems is typically mounted on the roof, which means there are aesthetic and structural issues to deal with, too (Figure 7).

Figure 7. Because it's located in a warm climate, this thermosiphon unit contains potable water. Closed-loop thermosiphon systems containing glycol are also available for areas where freezing temperatures are common. Open-Loop Recirculation

In an open-loop recirculation system, pressurized potable water is actively pumped between the collectors mounted on the roof and a storage tank installed inside the house (Figure 8). Heat sensors wired to an electronic controller activate the electric recirculating pump — typically whenever the collectors are 5°F warmer than the tank. This "differential" control causes the pump to run continuously as long as the sun is out (Figure 9).

Figure 8.In an open-loop recirculating system, a sensor-activated pump moves water between the collector and the storage tank whenever the collector's temperature is warmer than the tank's. When the temperature drops, the sensor activates the pump to bring warm water from the tank back into the collector to protect against freezing.

Figure 9.An the installer inserts a heat sensor into a flat-plate collector (top left); this sensor and another one on the storage tank connect to an electronic controller (top right) that activates the pump (bottom) whenever the collector is 5°F hotter than the tank.

If the weather gets cold enough, the collector could freeze and burst, so when the controller senses an imminent freeze the pump comes on and brings warm water up from the indoor tank. It shuts off once the collectors reach 40°F. While this is a simple method of freeze protection, it's not particularly energy-efficient, and there are several ways it might fail: Power may go out, the pump can stop working, or a sensor or controller might malfunction. So, again, we always install a freeze drip valve just in case.

Although more expensive than such passive systems as thermosiphon and ICS, open-loop recirculation costs less than other types of pumped systems.

Closed-Loop Antifreeze System

A closed-loop antifreeze system is designed for areas with moderate to frequent freezing. These systems resemble pumped open-loop systems, except they have additional components like a heat exchanger, two independent sets of pipes, and sometimes a second circulating pump. One pump circulates antifreeze between the collectors and a heat exchanger, while the other circulates potable water between the heat exchanger and the storage tank (Figure 10).

Figure 10.Designed for cold climates, closed-loop antifreeze systems use glycol to protect the collector. This requires a heat exchanger to transfer heat to the potable water, and a second pump to circulate domestic water between the heat exchanger and storage tank.

A typical heat exchanger consists of a pair of concentric copper pipes; liquid from the collectors flows through one pipe and potable water flows through the other. The liquids don't mix, but heat transfers easily though the conductive wall of the inner pipe. It's also possible to exchange heat by running heated fluid through a coil inside the storage tank or backup heater, but an external heat exchanger is usually less expensive and easier to repair.

Because the liquid in the collectors contains a mixture of propylene glycol and water, it won't freeze. Unlike the ethylene glycol used in automobile radiators, this antifreeze is a nontoxic food-grade additive, so if a leak in the heat exchanger did occur, the worst that would happen to the homeowner is that the water might taste sweet. Good-quality antifreeze in a well-designed system should last at least 10 years. But because antifreeze can degrade and become acidic enough to damage the system, it should be periodically replaced.

This type of system is virtually immune to freezing, but the heat exchanger, additional pump, and antifreeze increase the cost of the system.

Drain-Back System

A drain-back system is a closed-loop system that relies on a pump to lift distilled water from a nonpressurized indoor reservoir and move it through the collector. When the outdoor temperature is high enough and the collector is warmer than the reservoir, the pump comes on and circulates water between the reservoir and collector. When the pump is off, gravity causes the water to drain out of the collectors and into the reservoir below. The controller won't activate the pump when the outdoor temperature is close to freezing; this keeps water out of the collector, which protects the system (Figure 11).

Figure 11.A drain-back system uses distilled water as the collector fluid, pumped from a nonpressurized indoor holding tank. The circulator pump runs continuously while heating conditions are good, then shuts off when the temperature drops, allowing the water in the collector to drain back to the tank, thereby preventing freeze damage.

Solar-heated water is stored in the reservoir and transferred to the potable water with an internal or external heat exchanger. In some designs, a second pump moves water between the heat exchanger and the storage tank. In others, the reservoir is the tank, so there's no need for a second pump.

Drain-back systems provide trouble-free, reliable freeze protection because the closed side of the loop contains distilled water, which, unlike glycol, doesn't require periodic replacement. On the other hand, drain-back systems require greater pump power to lift fluid to the collectors.

Designing the System

Because there are bound to be periods when the sun doesn't shine for several days in a row, there's no point in trying to design a solar hot-water heating system that provides 100 percent of the total yearly hot-water demand. We typically aim for 60 percent to 80 percent capacity, with the backup heater providing the rest.

As a rule of thumb, we assume that each person in a household uses 20 gallons of hot water per day, so a family of four would need an 80-gallon storage tank. In our mild San Francisco Bay-area climate, 1 square foot of collector will produce about 1.5 gallons of hot water per day, so a system with an 80-gallon tank requires 53 square feet of collector. Since collectors aren't available in that size, we would install two 4-by-8-foot collectors (Figure 12).

Figure 12.A pumped system typically contains more than one flat-plate collector (left). To allow for easy installation and repair, the author joins the collectors with unions (right).

The relationship between collector and tank varies by climate. In the Sun Belt, the rule of thumb is 1 square foot of collector per 2 gallons of tank capacity (daily use). In the Southeast and Mountain states, this ratio is 1-to-1.5, in the Midwest and Atlantic states it's 1-to-1, and in the Northeast and Northwest it's 1-to-.75

Orientation. It's generally best to face the collectors due south, though in some cases it's wise to account for local weather patterns. For example, in the San Francisco Bay area there are a lot of overcast mornings, so we prefer to orient collectors slightly more to the west.

For optimal annual collection, collectors should not face straight up, but should be tilted above horizontal to an angle 5 to 10 degrees higher than the latitude at which they are located. Our latitude is 38 degrees, so ideally the collectors would be tilted 43 to 48 degrees. The steeper angle makes for better wintertime solar collection, when the sun is lower in the sky. In cases where aesthetic concerns trump efficiency, we'll install the collectors at the same pitch as the roof.

Temperature rise. When an actively pumped system has been properly sized, each exchange of water will increase the temperature in the storage tank 10°F. On an average day, there might be eight exchanges, creating a total temperature rise of 80°F; in hot, sunny weather it could be more. Our systems routinely reach 180°F in the summer, especially when water usage is low. This water would be too hot to use safely, so to prevent scalding we install a tempering valve downstream from the backup heater.

Excessive pressure can build up in the collectors if they get too hot, so as a matter of course we install a pressure-relief valve on the pipe where fluid exits the collector or group of collectors. A closed-loop system will have a pressure-relief valve on the roof and, if the loop contains glycol, an expansion tank in the building (Figure 13).

Figure 13.When this system is up and running, the gauges (top) will show how much heat the water gains as it passes through the collectors. Because the water may become too hot to safely use, the author always installs a tempering valve to prevent scalding (bottom left). A pressure-relief valve (bottom right) opens if the collector itself gets too hot; the cylindrical valve at the top automatically bleeds air from the system.

Solar Orphans In the early 1980s, hefty tax credits and high energy prices led to a boom in the installation of solar water heaters. A lot of people entered the business and installed all kinds of equipment, then went under after the tax credits expired and energy prices fell in 1986. Whereas some of these systems were quite good, others were experimental, and with so many solar companies out of business, there were few qualified people around to maintain and repair them. As a result, many of the older systems failed and gave a black eye to a legitimate technology. Our company runs into these orphaned systems all the time; some are still going strong while others have been "broken" for many years. Bad advice. When homeowners move into a house with a nonfunctioning system, they're almost always advised to tear it out. Unfortunately, most of the people giving this advice — plumbers, roofers, and GCs — don't know anything about solar water heating. Old solar systems, like the one on this original wood roof, may no longer be operable but can often be put back into service for a reasonable cost. An experienced solar hot-water installer can tell you which systems should be torn out and which can be repaired. If the system was built with high-quality components and the collectors have never frozen, there's a reasonable chance it can be saved. Inexpensive repairs. Our repair crews have revived any number of systems by making a few inexpensive repairs. Sometimes it's a matter of spending $450 (including labor) to replace a pump. A leaking storage tank can be replaced for just over $1,000, which may seem like a lot, but it's a small price to pay to repair a system that would cost $6,000 new. The most common problem with a pumped system is a failed sensor or loose wire. These repairs may cost only $100, but most plumbers don't know how to make them. Sometimes the problem is simply that the homeowner doesn't know how to turn on the system.

Installation

The lines to the roof are usually 3/4-inch copper. We don't use PEX because in California it's illegal to use it for potable water — plus the high temperatures found in the closed loop of a glycol system could easily be too hot for it.

On new work, we run the lines up through the house. Because we work in a mild climate, on retrofits we usually run pipes down the exterior of the house. We insulate all the pipes that carry hot or recirculated liquid with 3/4-inch neoprene, which handles high temperatures better than plastic foam insulation does. Without UV protection, the sun will destroy this insulation in less than five years, so we jacket it with aluminum (Figure 14).

Figure 14.The author's crew insulates every pipe that contains hot or warm water. Here, an installer protects the neoprene insulation with an aluminum jacket.

Another option is to protect the insulation with a painted coating, but a metal jacket looks better.

Structural issues. To install the collectors, we use the same mounting hardware we use to install the roof-mounted portions of a photovoltaic system (see "Installing Solar Electric Power," 3/05). The best approach is to install post mounts before the roofing material goes on, but it's also possible to retrofit various mounting brackets over the shingles.

Weight is rarely a concern with flat-plate collectors, the largest of which weigh less than 175 pounds even when full of water. But a full ICS unit might weigh 500 pounds, and the system might require more than one unit. In such a case, it's important to find out if the roof can carry the load.

Power needs. Most pumps will run on less than one amp of electricity, so inspectors often allow us to tie into an existing circuit or share a circuit with another load in new construction. A few inspectors require us to install a separate circuit. In some jurisdictions, it's legal to plug pumps and controllers into wall receptacles, which we do whenever possible to reduce wiring costs.

Gary Gerberis the owner of Sun Light & Power in Berkeley, Calif. He has been in the solar business since 1975.