Solar is HOT! Sometimes too hot

BY BOB "HOT ROD" ROHR
contributing writer

As the solar thermal industry picks up steam, I mean hot water, some challenges arise. While the solar thermal industry warms, be mindful that the tanks you connect to warm up too.

The sun is an amazing source of thermal energy. It is very accessible in most parts of the planet. The technology to harness the energy and use it to warm our water, homes or processes is fairly simple and straightforward. Lay a garden hose full of water in your yard on a warm sunny day and you have the simplest of collectors. Actually, the early collectors, dating back to the turn of the century, were not much more sophisticated. Early “batch” collectors consisted of a painted steel tank enclosed in a frame with glass over the top.

As time passed, improvements were made. Better collector technology deals with the enclosures and their insulation. Better frame assembly reduces infiltration leaks. Newer insulations eliminate out-gassing and glass fogging. Better coatings on the absorbers capture more and reflect less energy. A specific type of glass is manufactured for solar collectors. Many new technologies are being considered for absorber materials. In some cases, adding additional riser tube in harp style collectors increases performance.

Evacuated tube styles of collectors are back and a growing part of the collector market. This is all good news for the consumers and users of solar thermal — more bang for the bucks, performance-wise.
With increased performance come increased operating temperatures. This too is a benefit, as long as you manage the temperatures appropriately. Solar thermal users quickly realize that, the more energy you can harvest and store, the more you can “park” or store for non-solar days. Storing thermally heated water or heat transfer fluids, is the most common parking medium. Common solar domestic hot water (SDHW) residential systems size around 10 – 20 gallons of hot water per person. I find sizing for 20 gallons per person for the first two occupants and 10 gallons per person for additional occupants works out about right. With today’s low flow faucets and showerheads having flow restrictors, the DHW demand has decreased. Body spray showers are excepted, of course.

So, harvest the Btu while the getting is good and store them in a well insulated solar storage tank for later use. Two options for storing excessive solar harvest would be volume or temperature. Knowing that larger storage tanks equate to additional standby loss, require more space and, of course, cost more, consider higher temperature storage as an option. Installers have the option of increasing the storage temperature with a solar differential control. A typical “factory setting” is 140 F on solar controllers. At 140 F storage, you offer protection from bacteria growth. In some European countries, tanks are required to be elevated to 140 F once a day for a one-hour period to offer “anti-legionella” protection. New solar controllers actually offer an anti-legionella parameter in their programs.

It is not unusual to see tanks operated with a setpoint above 140 F. The hotter the tank temperature, the more useable mixed temperature capacity it can provide. That can be a good thing. It is imperative that you install a quality, listed thermostatic mixing valve if you plan on running elevated temperatures in your solar tanks. Consider that most DHW tanks in the U.S. are shipped with a 120 F setting. You see, it is a balance between scalding potential and potential bacteria growth.

Another DHW generation method that should be of concern is the wood boiler or outdoor wood furnace (OWF) installation. For years, the use of a “long tube” external heat exchanger has been promoted. This copper tube connects to the top and bottom of an existing DHW tank and transfers energy by thermo-siphon principles. Most are installed without proper, if any, temperature regulation. So, the tank may run close to the boiler operating temperature. In some cases, 200 F temperatures are possible in the tank. You really don’t want to push water at that temperature out of a customer’s faucet.

A thermostatic mixing valve is the answer to regulating the tank outlet temperature. When you shop for a thermostatic valve, pay attention to the specifications. The valve will have a Cv rating. A valve with a Cv of 5, for instance, would flow 5 gpm with a 1 psi pressure drop across the valve. Certainly, you can flow in excess of 5 gpm through the mixer, but the pressure drop also increases. Most manufacturers will have a maximum flow rate listed. Look for the correct ASME listing on the valve. Some valves are listed for point of distribution, such as the storage tank. Others have a listing for point of use, under a sink, for example.

The valve will have a minimum flow rate listing and a temperature offset, between hot and cold. Some valves will have a check valve option, and some include inlet screens. At least one brand has a temperature gauge option for the mixed, or outlet, port. This allows you to set the valve properly, and it allows inspectors a means of double checking output temperatures. On larger demand applications, consult with a valve manufacturer. In some cases, you may need a valve with two mixers to handle the largest flow and a smaller valve to cover very low flows, such as a single handsink flow, for example.
Thermostatic mixing valves will require routine maintenance. How frequently depends on how much water flows through them and the condition of the water. Install these valves with isolation valves on all three ports to speed the maintenance. Many plumbers keep an extra cartridge on their trucks to switch out quickly. The cartridge you remove may be cleaned and/or rebuilt as a spare for the next service. If you find a brand you like, stick with it to make service easy. Remember, also, that some states require low lead valves. I understand that these will be a nationwide requirement in the near future.

When you’re hot, you’re hot; when you’re scalding, maybe not. Practice safe DHW services.