Solar Hot Water System Types & Applications

Federal, state, and utility incentives have spurred impressive growth in the U.S. solar hot water (SHW) industry in the last few years—and this demand has attracted many imports from Europe and China. While the basic systems haven’t changed, imported and domestic innovations have altered traditional systems, and have even simplified installations in some instances.

The residential federal tax credits, and most state and utility incentive programs, require that SHW equipment be certified by the Solar Rating and Certification Corporation (SRCC), a private nonprofit organization that certifies solar hot water collectors and systems. Two protocols are specified: Operating Guideline (OG)-100 for collectors, and OG-300 for systems. Even if a collector or system is very simple, as some are, it must carry the SRCC certification to be eligible for those incentives. The SRCC certifications also give relatively new and somewhat longer names to all the systems—for instance, a batch water heater system is now referred to as an integral collector and storage system (ICS). These names are becoming the standard.

System Types

On a sunny day, and when sized correctly, all of the collector types and systems can easily heat a storage tank to domestic hot water (DHW) temperatures. In Hawaii, where it never freezes, the open-loop, direct, forced circulation systems are the most popular. ICS and thermosyphon systems are popular in mild climates in the very southern parts of the United States. Most of the rest of the country uses drainback and antifreeze systems with preferences depending on regions and individual installers.

A quick method of sizing that works well for most residential systems is to have 1 square foot of collector surface area to every 1¹/2 gallons of water in the storage tank. In the Southwest, due to the increased solar resource, collector area is decreased to 1 square foot of collector area to every 2 gallons of storage water.

Batch Water Heaters 

Batch water heaters have been heating water in the lower tier of states for more than a century. Take a cylindrical tank, paint it black, put it in an insulated box with glass on the top and you have a batch water heater. 

To provide some insulation on the glass side of the collector, most manufactured batch water heaters use double glazing. Operation is simple—sunlight shines through the glass, hits the black tank, and heats it and the water it contains. 

ICS systems are considered “passive” heaters since they don’t require any electro-mechanical devices, such as pumps, to operate. Batch water heaters have high overnight winter heat loss because only the glass protects it from cold nighttime temperatures, though some hands-on owners place insulation over the glass at night. ICS units are typically only installed for year-round use in warmer climes like Hawaii, or states that border Mexico or the Gulf of Mexico. 

Thermosyphon 

Thermosyphon systems are also considered passive systems, since they don’t require a pump or control. These systems can have either integrated or separate collectors and tanks, which can be more effective in retaining heat overnight, since the tank can be well-insulated. However, thermosyphon systems are usually more susceptible to freezing than batch water heaters because of the small riser tubes in the collector.