Solar Hot Water System Types & Applications

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Inside this Article

Luke Frazer with BOS components
Luke Frazer of The Solar Collection in Talent, Oregon, with a closed-loop, forced-circulation, drainback solar hot water system.
Batch SHW
Simple batch-style solar water heaters are relatively inexpensive and great for climates with little chance of freezing temperatures.
Flat Plate Collector
Flat-plate collectors are a proven and simple technology for reliably making hot water.
Evacuated Tube Collector
Evacuated-tube collectors are touted as having better performance in overcast weather, but at additional complexity and cost.
Solar tank, showing exchanger
Some storage tanks have an integrated heat exchanger, as this cutaway shows, increasing efficiency and simplifying system plumbing.
A small electric water heater is used as a drainback tank, allowing heat- transfer fluid to drain from the collector when the system is not operating.
Circulation Pump
A circulation pump in a drainback system has to be able to overcome the head from the tank to the collector.
Collector with PV module to power pump
In a pressurized closed-loop system, one PV module can control a small DC pump.
SHW system controller
A differential pump controller uses thermal sensors to determine when the collector temperature is high enough to add heat to the SHW tank.
Flow meter
Optional flow meters show the flow rate of heat-transfer fluid through the collector loop. When placed at the top of the DB tank, they can also serve as a sight gauge to monitor liquid level in the tank.
Water Heater Relief Valve
Temperature and pressure-relief valves are mandatory safety features.
Mixing, or Anti-Scald Valve
Antiscald valves are important, since high system temperatures are possible.
Temperature Gauge
Temperature gauges can help determine the state of a system at a glance.
Solar Pump Station
Pump stations combine many system components into a single, easier-to-install package.
Luke Frazer with BOS components
Batch SHW
Flat Plate Collector
Evacuated Tube Collector
Solar tank, showing exchanger
Circulation Pump
Collector with PV module to power pump
SHW system controller
Flow meter
Water Heater Relief Valve
Mixing, or Anti-Scald Valve
Temperature Gauge
Solar Pump Station

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. 

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Comments (1)

Fred Golden's picture

For those with a SHW heating system, and missing out on sunny days of summer, but still have a wood stove, why not add a second loop to the system.

By installing a 50' loop of 3/4" pipe behind the stove, in a area where it will warm to above 100F, you can heat that coil of water, activate the pump, and pump that water into the water heater. You would need to buy another temperature sensor for the indoor loop, and have a switch to let the controller know what sensor to pay attention to. If the SHW system is open flat plate type, then you would need it to still pump at 4F for anti-freeze protection, so a second pump and controller would be required.

The $150 - $200 in piping, insulation and $200 pump cost can be saved at $30- $50 per month (depending on water usage and fuel costs) in about 3-4 winter's use.

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