by Gary Gerber

My company has been designing and installing alternative energy systems since 1975; to date, we've installed more than 1,000 solar water heating systems. Lately, as energy prices rise at an ever-accelerating rate, we've seen renewed interest in solar hot water.

A simple solar hot-water system can be installed for as little as $4,000, while a larger and more complicated freeze-protected system can cost as much as $10,000. Thanks to the recently signed energy bill, the customer's cost is about to fall; the bill grants a 30 percent federal tax credit to property owners who install solar energy systems in 2006 and 2007 (see "Incentives Sweeten Energy Bill for Builders and Homeowners," In the News, in this issue). Homeowners can claim a credit of up to $2,000; for business owners, there is no cap on the credit.

In areas where natural-gas water heaters are the norm, the payback period for a residential solar water heating system will be 9 to 12 years if the value of the credit is included. Because a well-designed system will last 20 to 30 years, it should pay for itself two to three times over. If the system saves $250 worth of natural gas the first year and gas prices escalate 5 percent per year, then it will save almost $17,000 over 30 years. If the homeowner is currently using an electric water heater, the savings may top $40,000.

On the environmental side, heating water with the sun will typically reduce a home's greenhouse gas emissions by 18 tons over the life of the system. With an extra 60 to 120 gallons of hot water at their disposal, homeowners can take long, guilt-free showers. We often equip our solar hot-water systems with valves that allow the customer to completely shut off the backup heater and use only solar-heated water in the summer.

Solar Water Heating Basics

All solar water heating systems contain collection, storage, and transfer components; some systems combine all three into a single element. Most systems are designed to preheat water that goes to a backup heater — typically a conventional gas or electric water heater. A tankless heater will also work as a backup, as long as it is designed to accept hot-water input (not all of them are).

Although there are some systems in which the preheated water flows directly to the backup, it's more common for the preheated water to be stored in a separate storage tank upstream from the backup. In hot, sunny weather, the backup is rarely if ever needed, but during cloudy periods it may have to provide virtually all the domestic hot water.

Collection. The most visible part of any solar hot-water system is the collector. There are three main types of collectors, but all basically consist of a black collecting surface that transfers heat to a fluid. The collecting surface is typically enclosed in an insulated aluminum box with clear glazing to trap the heat.

A flat-plate collector — which is about 3 inches thick — contains a grid of copper tubing attached to an aluminum or copper plate (see Figure 1). Both components have a black surface coating; when sunlight hits the plate, heat is conducted to the fluid inside the tubing. Sensors measure the temperature in the collector, and when it's hotter than the fluid in the system's storage tank, an electronic controller activates a pump to move the heated fluid to the tank. The uninstalled cost of a 4-by-8-foot flat-plate collector is approximately $750.

Flat-plate collectors range in size from 3 by 6 feet to 4 by 12 feet and are light enough to be carried by hand (left). Because the tubing inside is of small diameter, even a large collector might contain only a gallon or two of fluid.

An evacuated tube collector is similar to a flat-plate collector except that the heat-absorbing tubes are housed in a series of evacuated glass cylinders. The vacuum insulates against heat loss in the same way that a thermos bottle does. Evacuated tube collectors are extremely efficient, but cost about twice as much as conventional flat-plate collectors.

In an integral collector storage — or ICS — unit, water is heated and stored in a series of interconnecting tubes in a roof-mounted box. Sometimes called batch heaters, ICS systems are simple and inexpensive because they require no pumps or controls. However, since they store water in an exposed location, they are subject to high heat loss and freezing (Figure 2). A 42-gallon ICS unit costs about $2,100 uninstalled.

Figure 2.Integral collector storage — or ICS — units contain 4-inch-diameter pipes in which water is heated and stored (top left). Because of their weight, they are typically craned into place (top right). Potable water enters through a pipe at the lower end, is heated by the sun, and exits through the upper end (bottom) when a hot-water tap is turned on.

Storage. Solar energy is available only for the six to 10 hours that the sun is out, so heated water must be stored for later use. While ICS systems store hot water right in the collector, most other systems keep it in a separate storage tank located upstream from the backup heater. Because the tank has to hold an entire day's worth of hot water, it is larger than a conventional heater.

There are some systems that send solar-heated water directly to the backup, but I'm not a fan of doing this with a conventional backup heater. Because it's too small to hold an entire day's worth of water, this kind of heater will short-cycle and heat the water before the sun has a chance to do its job.

In pumped systems, storage takes place in a pressurized steel tank that resembles an electric hot-water heater. Usually located near the backup heater, this storage tank connects to the collectors with copper pipes (Figure 3).

Figure 3.An installer plumbs an 80-gallon storage tank for a pumped system (left). The tank has taps for supply and return lines to and from the collector, plus a cold-water supply inlet and a hot-water outlet to the home (right).

ICS and thermosiphon systems are pumpless. Water moves through the ICS unit but does not circulate within it. Within thermosiphon systems, which rely on the principle that hot water rises, water circulates between the collector and a tank above.

Transfer. Solar hot-water systems can be categorized according to their method of heat transfer and freeze protection. In open-loop — or direct — systems, potable water flows through the collector and is heated there. In closed-loop — or indirect — systems, the liquid in the collector is isolated from the potable water and transfers heat to it with a heat exchanger next to or inside the storage tank.

Closed-loop systems provide the best freeze protection because the liquid in the collector is chemically or mechanically protected from freezing. Open-loop systems, on the other hand, are subject to freezing because the collector contains potable water. While it's possible to provide some freeze-protection to open-loop systems, I don't recommend installing them in climate zones where there are hard freezes more than once every five years.

On the following pages, I'll describe the most common system designs, ranging from simpler passive systems to more complex active systems.

Integral Collector Storage

In an ICS system, potable cold water is piped into a roof-mounted unit and preheated by the sun on its way to the backup heater. Water moves through this system only when a hot-water tap is opened (Figure 4). An ICS system is simple and relatively inexpensive, but a lot of heat can be lost through the glass, so the backup has to run if the client wants hot water first thing in the morning.

Figure 4.Since ICS heaters both heat and store water in the rooftop collector, they can be quite heavy, requiring reinforced roof framing. They are not well-suited for cold climates.

Early manufacturers of ICS systems simply placed a single bulk storage tank within a glass-covered insulated enclosure aimed at the sun. Newer designs typically consist of an interconnected series of 4-inch-diameter copper tubes in an 8-inch-deep insulated box with glazing on top.

With a capacity ranging from 20 to 50 gallons, these collectors can be quite heavy. They are also subject to freezing, because the water is stored on the roof.

Thermosiphon Systems

Like an ICS system, a thermosiphon system has no pump, but it's more efficient because it separates heating and storage functions. When sunlight hits the collector, the liquid inside heats up and becomes buoyant, then flows up to the storage tank, which is located above the collector. It's replaced by cooler liquid that flows down from a separate line on the bottom of the storage tank (Figure 5). While a pump would certainly speed up the recirculation process, the convective flow is more than adequate to move the entire contents of the tank through the collector several times per day in sunny weather.

Figure 5.Thermosiphon units rely on convection to move hot water from the collector to the storage tank, which is mounted right above the collector. As hot water rises into the storage tank, cool replacement water enters at the bottom of the collector.

Thermosiphon systems are available in both open-loop and closed-loop configurations. In the open-loop version, the collector contains potable water, whereas the closed-loop version contains a glycol mix that flows to a heat exchanger surrounding the tank.

Two mechanisms provide freeze protection in an open-loop thermosiphon system. Water gets lighter just before it turns to ice, creating a "reverse thermosiphon" that pulls warm water down from the tank. I don't rely on this phenomenon alone, however; we also install a freeze drip valve, which opens when the collector temperature reaches 35°F (Figure 6). This bleeds water from the collector and brings warm replacement water from the tank. Normally, the freeze valve won't open unless the primary protection fails.

Figure 6.The author's crew always installs a freeze drip valve on the outlet side of the collectors on open-loop systems. If the temperature drops below 35°F, the valve drains enough water from the collectors to bring warm replacement water up from the house.