Types of Solar Hot Water Systems

Both Closed-Loop and Open-Loop systems may use Flat Plate Collectors or Evacuated Tube Collectors

Drainback Solar Water Heating System

Drain Back Systems use distilled water as the heat-transfer fluid (HTF) in the collector loop. A pump circulates the water through the collectors. The water drains by gravity to the storage tank and heat exchanger; there are no valves to fail and the system is completely non-pressurized. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.

Since water is used as a heat transfer fluid, it never needs to be changed like pressurized antifreeze systems. Most plumbing codes do not require double wall heat exchangers for drainback systems using distilled water.

Advantages of this System

  • The system has no check valves, no air vents, no pressure gauges, and no expansion tanks.
  • Cannot reverse thermosyphon at night.
  • Drainback systems may out-heat antifreeze systems by up to 8%.
  • Larger piping (3/4″ copper pipe) and insulation must be used.
  • The drainback system components cost about 15- 20% more than an active open direct loop solar water heating system for residential water heating.
Active Open Loop Solar Water Heating System

It is the simplest and typically the least expensive active system to install. There is no heat exchanger, which allows efficient heat transfer directly to the water. The system operates at standard line pressure.

A differential controller compares the temperature of a sensor, located at the solar collector with the temperature of a sensor located in the bottom of the hot water storage tank (where your colder water is). When the solar collector is warmer than the water in the bottom of the tank by 4 or 5 degrees, the differential control activates a small circulating pump, which draws cold water from the bottom of the solar hot water storage tank and circulates it through the solar collector. Solar heated water is returned to the top of the tank.

Another version of this system uses a small photovoltaic (solar electricity) panel to operate a direct current (DC) circulating pump.

Open Loop Direct Systems are suitable for mild and moderate climates where freezing is minimal.

How Freeze Protection Works:

The differential controller uses the circulating pump to circulate warm water from the storage tank through collectors and piping in the rare evenings when temperatures approach freezing.

There is an additional back-up dole freeze valve located at the collector that will activate by trickling water from the collector onto the roof when temperatures approach freezing.

Advantages of this System

A direct system produces the highest operating performance, because there is no nighttime heat losses from hot water stored on the roof as in a passive system nor is any efficiency lost through a heat exchange process as in a closed loop system.

Potable water from the hot water storage tank is circulated directly through the collector.

The result is water heated between 140 and 160 degrees, as a result, by turning on the faucet, you are mixing (adding) more cold water with less hot water sustaining more free solar heated water awaiting use. There is also an anti scald valve, which prevents anyone from getting shocked or hurt by only turning on the hot water.

Passive Solar Water Heating Systems

Integral Collectors Storage (ICS)

Thermosyphon Flow

Passive systems can be divided into two types:  Thermosyphon and Integral Collectors Storage (ICS).

Passive solar water heating systems are popular because of their inherent simplicity and reliability, no pumps or controllers or wiring.

The storage tank is located on the roof and cold water from the city or well flows directly to the collector on the roof, where it is heated and then flows to your conventional 40 or 60 gallon water heater located on the ground level, and you have storage of hot water in your tank overnight. If you have large morning draws of hot water (ie: showers or baths) most of the collected heat energy returning to your tank is lost overnight, thereby saving you 50-60% of your long term hot water energy costs, compared with close to 85% savings with an Active Open Loop Solar Water Heating System

In the case of a thermosyphon system, an insulated tank on the roof, located above the collector will lower the loss of stored heat during the night, but is not aesthetically appealing. These types are more popular in the islands.

Active Closed Loop Solar Water Heating System

Pumps circulate a non-toxic, heat-transfer fluid (HTF) through the collectors and a heat exchanger. They are popular in climates prone to consistent freezing temperatures.

These systems pump heat-transfer fluids (usually an FDA approved glycol fluid) through collectors. Heat exchangers transfer the heat from the fluid to the household water stored in the tanks. Closed-loop glycol systems offer good freeze protection. However, the glycol must be checked each year and changed every 3 to 10 years, depending on glycol quality and system temperatures.  They are generally more complicated than an open direct loop system, because either a tank with a heat exchange coil or an external heat exchanger is required. As a heat exchanger is required the collector loop will run at slightly higher temperatures than an open loop system. The collector loop needs to be pressurized(8 -12 psi).

The antifreeze may need to be recharged on a 3 to 5 year basis.

Evacuated-tube solar collectors

They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy but inhibits radiant heat loss. These collectors are used more frequently on commercial applications.

Are evacuated tubes better for solar water heating?

We don’t highly recommend roof-mounted evacuated tube solar collector systems for wind zones like Florida. While these systems can be more efficient than flat plate solar collectors, only during overcast weather conditions, an evacuated tube collector system resembles a bank of florescent light bulbs on your roof. Naturally, an evacuated tube cylinder will have minimal wind resistance, which is good. But it will have very poor impact resistance if struck by a flying object. Modern flat plate collectors have tempered glass cover plates that can sustain direct strikes by such wind-driven objects as a piece of lumber.

Evacuated tube collectors are more popular in Europe, where they can be more effective in frequent overcast weather conditions with lower average air temperatures than Florida.

Tankless Water Heating System

Does a tankless water heater save more money than a solar water heater?

Savings of 20% on tankless versus up to 85% savings on a solar water heating, is the reality behind your electric bill savings, however the convenience factor of instant hot water is the overriding benefit of a tankless water heating system.  Here is why:

The water you use still has to be heated: The amount of energy required to heat a gallon of water to a specific temperature does not change just because it is being heated at a faster rate.

A tankless water heater only eliminates energy costs for maintaining the temperature of water that has already been heated and is sitting in the storage tank awaiting use. Standby losses of (approx 5 minutes out of an hour)

So a tankless water heater is saving about 15 to 20% percent of water heating costs compared with solar at 85% savings over a conventional tank type electric or gas water heater.

And tankless water heaters do have drawbacks. According to the U.S. Department of Energy, “Sometimes … even the largest, gas-fired model cannot supply enough hot water for simultaneous, multiple uses … taking a shower and running the dishwasher at the same time can stretch a (tankless) water heater to its limit.”

Naturally, you can overcome this problem by installing multiple tankless water heaters. On the other hand, the total installation cost for more than one tankless water heater easily approaches the cost of a single solar water heating system.  Additionally, Solar water heating systems useful life is over 30 years.