5 Helpful Tips to Sizing and Selecting Solar Thermal Equipment

Today’s guest post comes from Bob Ramlow, the instructor if our Solar Thermal Boot Camp. Bob Ramlow has more than 30 years of experience with solar-energy systems and is a co-founder and Board member of the Midwest Renewable Energy Association. He’s a solar thermal consultant for the Wisconsin Focus on Energy Program. We adapted this article from his book, Solar Water Heating, a Mother Earth News Book for Wiser Living. Bob teaches HeatSpring’s Solar Thermal Installer Boot Camp.

Click here to sign up for a free product training on real time solar thermal monitoring that is 50% cheaper, and 1/3 less time to install compared with traditional BTU meter based systems. 

Enter Bob Ramlow:

Rarely do we design solar water heaters to provide 100 percent of your hot water. There are just too many cloudy days over the course of a year. Nevertheless, a typical solar water heater will provide between 50- and 75 percent of your annual load. In hot climates, or during the sunniest times of the year, you can expect to get nearly 100 percent of your hot water from solar. And even during the cloudiest periods, you may get as much as 50 percent, depending on your climate.

In cool climates, you should allow 20 square feet of collector and 20 gallons of storage capacity for each person in the household. For large families, you can reduce this by 10 percent for each person over four members in the household. In warm climates, size the system with 15 square feet of collector and 25 gallons of storage for each person in the household, with the same reductions for larger families. These sizing methods will give the best return on investment. Smaller systems will work well, but your savings will be less.

Solar Water Heaters for Warm Climates

When choosing a solar water-heating system, the most important consideration will be your climate. The crucial factor is that when you expose a water-filled pipe to freezing conditions, the water inside the pipe will freeze and the pipe will burst. In any climate that experiences freezing temperatures, you must take some precautions to prevent pipes from freezing.

In climates that never experience freezing conditions, or for systems that operate only in summer, such as those for summer homes and campgrounds, having water in the solar collector at all times presents no problems. The best type of solar water heater for these applications is an Integral Collector Storage (ICS) system, often called a batch heater. ICS systems are simply water tanks exposed to the sun. Commercial ICS collectors have one or more tanks inside an insulated box with glass on one side. Set the glass side to face the sun, and paint the tanks black, or cover them with a special coating that absorbs the sun’s energy. This system has no pumps or controls and you’ll plumb it directly into a home’s water system. It is the simplest, least expensive solar water heater and is popular in all areas surrounding the equator. ICS systems are also the only type that heat domestic water directly. Most water supplies contain dissolved minerals, which will clog the small passages in other kinds of solar collectors, so these systems heat the domestic supply indirectly, using a heat-transfer fluid that you’ll keep separate from the water supply.

Solar Water Heaters for Cooler Climates

In areas that do experience freezing conditions, two kinds of solar water-heating systems are appropriate – closed-loop, antifreeze systems, and drainback systems. The most popular and versatile type of system installed worldwide is a closed-loop antifreeze system. These systems consist of one or more collectors, insulated piping, a circulating pump, an expansion tank, a hot-water-storage tank, a heat exchanger, solar fluid (usually a solution of water and nontoxic propylene-glycol antifreeze), a controller, some valves and some gauges. The piping loops from the collectors to the heat exchanger and back again. You’ll fill this closed loop with the antifreeze solution, which stays inside the collectors and piping at all times. Whenever the sun shines on the collectors, the circulating pump comes on, and the solar fluid circulates within the closed loop. The fluid gets hot inside the collectors and travels through the piping to the heat exchanger. The heat exchanger transfers the heat from the fluid to the water inside the storage tank, which stores the heated water for your use. As the fluid heats, it expands, so be sure to include the expansion tank to absorb the excess pressure in the system. When the sun is not shining, the circulating pump simply turns off, and the fluid stops circulating.

You can power the circulating pump in these systems with either AC or DC current. If AC-powered, the pump gets its energy from your home’s 120-volt electrical power system. In this case, you’ll need a controller to turn the pump on and off at the appropriate times. You’ll connect the controller to temperature sensors in the collectors and on the storage tank. Whenever it is hotter in the collectors than it is in the storage tank, the controller turns the pump on. When it is warmer in the storage tank than in the collectors, the controller turns the pump off.

If the circulating pump is DC-powered, you’ll wire it directly to a small photovoltaic (PV) panel mounted outside. Whenever the sun shines on the PV panel, electricity flows to the circulating pump, and it starts running. When the sun stops shining the pump turns off. DC-powered systems are becoming the most popular kind of closed-loop antifreeze system on the market. There’s no AC connection; the pump runs whenever the sun is out, preventing stagnation and overheating; and it runs at variable-speed, depending on the amount of sunlight, so it automatically matches the collector’s heat output. These pumps have limited power, but they’re strong enough to move the fluid through a piping system, which is always full. The absence of a pump controller (probably the most vulnerable component) makes PV-powered systems quite reliable.

Drainback Systems

Drainback systems are popular in moderate and hot climates. They’re similar to closed-loop antifreeze systems. The big difference is that they include an additional tank (called a drainback tank). When the system is not operating, the fluid stays in the drainback tank and the pipes and collectors are empty. The fluid can be pure water or a weak solution of antifreeze in water.

Drainback systems always use a controller and an AC pump. The pump is a special, high-head pump because it has to lift the fluid to fill the piping and collectors every time the system turns on. This requires more power than an antifreeze system needs.  In those systems, the pump just circulates fluid through piping that’s always full. Otherwise, drainback systems operate similarly to an antifreeze system: When the sun warms the collectors, the high-head pump comes on and circulates the fluid into and throughout the system, and a heat exchanger transfers the heat from the fluid into the storage tank. There is no expansion tank and fewer valves and gauges than you have with an antifreeze system, but you have to install the piping carefully to allow for proper drainage.

Additional Considerations

A few other factors will influence your choice of a solar water heater. If you find a system that is substantially cheaper than others, there’s probably a reason, such as lower-quality components. There is no substitute for quality, and you should never take shortcuts with an investment such as this. You can often expand a solar water-heating system to include space heating as well. You will, however, have to use larger collectors and components. There are numerous approaches to storing and distributing heat in combination systems.

Closed-loop and drainback systems should always have a separate storage tank for solar-heated water and a backup water heater. The same tank can’t do both jobs efficiently. Typically, the output of the storage tank runs to the input of the backup heater. When solar output is sufficient, the backup heater doesn’t come on.

Collectively Speaking

There are two popular kinds of solar collectors:  flat-plate and evacuated-tube. Flat-plate collectors are by far the most popular kind of collector and they work well in all climates. They have been around the longest and are efficient and competitively priced. They are shallow rectangular boxes with glazed tops and insulated back and sides. An absorber plate inside gathers solar heat and transfers it to a network of copper tubing, through which the solar fluid flows. Most flat-plate collectors are made in the United States. They are the only collector that sheds snow and frost well. They also operate precisely within the temperature range needed to heat domestic water — below zero to about 180 degrees — and they rarely overheat.

Evacuated-tube collectors vary significantly from one manufacturer to another. They can overheat more easily than flat-plate collectors, so take care never to oversize the collectors or undersize the storage tank. They tend to cost more than flat-plate collectors for an equivalent heating capacity, but prices for high-quality, evacuated-tube collectors are coming down. Most are made outside the United States. They are also more fragile than flat plate collectors and don’t tend to shed snow or frost very well. On the other hand, they work well when there is a consistent, year-round load on the system. They also produce less wind-loading stress, because there are spaces between the tubes in a collector, unlike the large closed surface of a flat-plate collector.

Solar water heaters can last 40 years or more if the design is appropriate to the climate, and the system incorporates high-quality materials and workmanship. You could even call solar-energy systems patriotic, because a solar investment keeps our energy dollars at home and reduces our dependency on others. You will spend a certain amount of money to heat your hot water in any case, so why not choose to do it with solar energy? Your pocketbook, and the environment, will appreciate it!