Solar Hot Water System Types & Applications: Page 3 of 4

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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

Tanks

Storage tanks range from 40 to 120 gallons for residential systems. Electric water heaters can easily be adapted to serve as solar storage tanks. Specialty tanks come with or without integrated heat exchangers. Most tanks are glass-lined, but can be stainless steel or lined with high-temperature polybutylene plastic.

Drainback (DB) tanks or reservoirs for residential systems usually have a capacity of 8 to 20 gallons. Large DB tanks serve as both a drainback reservoir and storage for the system. The DB tank holds the collector loop water after it has drained from the collector and piping. DB tanks can simply be a small tank or can contain an internal heat exchanger. A DB tank should hold at least twice the volume of the system (piping, collector, and exchanger).

Expansion tanks. Liquids physically expand when they get hot, and closed-loop systems need some way to absorb this expansion. In drainback systems, the DB tank serves this function. In antifreeze systems, expansion tanks are used. They are usually very small—2 to 4 gallons—and are sized depending on system volume and maximum system temperature.

Backup water heater. Many solar storage tanks have an electric element in the top of the tank, which serves as a backup source to heat the water when the SHW system doesn’t produce enough hot water. Single-tank systems have less heat loss and take up less room than double-tank systems.

Two-tank systems use a solar storage tank and a backup gas or electric water heater, which keeps water hot under all conditions. This backup heater can also be a tankless or instantaneous water heater. Double-tank systems add to the overall system storage capability. However, since the storage tank doesn’t have any conventional heating elements, conventional energy cannot preheat the water as it can in a single-tank system. 

Pumps

Pumps are used in forced-circulation systems to move the collector-loop fluid through the collector to the tank or heat exchanger. When an external heat exchanger is used in an indirect system, a pump is also required on the potable or DHW loop. When a pump is used to circulate potable water (open or DHW loops), it must be constructed of bronze or stainless steel because of the corrosive effects of the dissolved oxygen in potable water. 

Drainback systems need a high-head pump large enough to push all the air out of the system on system startup and overcome the height of the collector. Antifreeze systems can use smaller pumps, since there is no air in the piping to overcome and the piping and collectors remain filled with the antifreeze. Because a smaller pump is needed, antifreeze systems more readily use PV-powered DC pumps. The pump industry has a very limited selection of high-head DC pumps suitable for drainback systems.

Controls

Forced-circulation systems must have a controller to energize the pump. Since the water temperature in the tank will increase by 60°F or more on a sunny day, a differential control will maximize the system’s efficiency. The control has a sensor to monitor collector temperature and another to monitor tank temperature. The control turns on the pump when the collector is a few degrees warmer than the tank and shuts the pump off when the collector temperature drops below being able to add heat to the tank water. Many controls have digital displays, multiple differentials for multiple outputs, high-limit controls for storage tanks, and vacation modes to help avoid system overheating when a home is unoccupied.

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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