Active Solar Heating Guide: Types, Advantages, and Disadvantages

Active Solar Water HeaterActive Solar Water Heater

Compare different types of active solar heating systems, including advantages and disadvantages, to install the most cost-effective solar-powered water heater. See if heating domestic water with the active systems such as glycol pressurized and drainback is better than with the passive units?

Active solar heating systems are mainly used for domestic hot water heating in colder regions, where the sun's heat energy is collected inside the solar collectors and transferred through the heat exchanger, heating water stored inside the storage tank.

All active solar water heaters are designed to use a pump to circulate the solar thermal fluid through the system, also utilizing sensors and controllers, all working together to provide higher efficiency and better performance than the passive systems.

Active solar heating is applicable in colder regions, but to eliminate the risk of freezing, you have to use antifreeze as the thermal fluid. As the required heat or temperature is usually not met with the solar heating system, backup heating has to be provided (electrical heating elements, for example).

The efficiency and performance depend on the system design, quality of the materials, how well the system is installed and maintained, and the amount of time the system is exposed to the sun.

The system can use controllers with sensors to control the pump or photovoltaic solar collector, where the pump runs only when solar heat energy is available.

Types of active solar heating systems

Active solar heating systems are using liquid-to-water and air-to-water heat transfer to heat the water. Two active water heating systems are the most popular:

  • Active solar heating that is using antifreeze or pressurized glycol
  • Drainback system

The above examples of active solar water heating systems can use photovoltaic panels to run the pump and operate as:

  • Direct or in the open-loop systems
  • Indirect or in the closed-loop systems

Direct or open-loop system

Direct active solar energy systems utilize solar collectors where the household water is heated directly, which is the reason for being more efficient than indirect systems.

On the other side, the main disadvantage is that they require more maintenance due to the mineral buildup problem. In the case of solar panels, which are built from small size tubes, scaling might clog the pipe, reduce the water flow and decrease the collector efficiency.

Indirect active system

Indirect active solar water heater systems are closed-loop systems where the solar fluid remains within the single circuit. Solar thermal fluid, glycol, for example, after it is heated in the collector by the sun, is moved by the pump to the heat exchanger and where it transfers the heat to the domestic water.

Indirect systems mainly use popular solar panels or flat plate collectors and also evacuated tubes and concentrated collectors.

A good example of the closed-loop glycol design is the Rheem Solpak active solar system.

Active solar heating - Open-loop system

The simplest active solar water heating system is an open-loop system, often seen in warmer regions, where freezing never occurs. The main components are the solar collector, pump, controllers, sensors (i.e., temperature limit), and valves (i.e., tempering valve, isolation valves, and air vent).

In the open-loop systems, the solar collector is attached to the storage tank with the pipes. Once the water is heated inside the collectors, the pump circulates the heated water, and when it stops, the circulation stops as well.

The most popular and used collectors for this active system are solar panels or flat-plate collectors and vacuum tubes.

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Pressurized glycol system or Antifreeze systems

An antifreeze pressurized system is an example of a closed-loop active solar heating system, where the water inside the storage tank is heated by the fluid that absorbs the sun's heat inside the solar collectors and runs through the heat exchanger.

The heating fluid is usually a 50/50 propylene glycol and water solution that works as the antifreeze. A double-walled heat exchanger must be used if the heat transfer fluid is considered toxic and single-walled if the fluid is non-toxic.

Due to its ability to work in a lower temperature environment, closed-loop, with the antifreeze active solar heating, is the only reliable solution in colder regions.

Pressurized glycol systems are the most versatile systems of all solar water heating systems. They can be installed in any direction and far from the heat exchanger. Fill and drain valves must be installed as the system is pressurized - to add or change the fluid.

Drainback systems

A drainback system is another example of a closed-loop system where the circulating fluid is distilled water. The main advantage of the drainback system is its ability to empty the solar collector when the pump is turned off. Drainback active solar water heating systems can be installed in both colder and warmer areas.

The main components of the drainback system are the collector, pump, large storage tank (size depends on the demand, i.e., 80 gallons), small drainback tank (for example, 10-gallon), sensors, controllers, and valves.

In warmer areas, when the storage tank gets hot (but not above the maximum as the temperature limit sensor controls it), the solar fluid drains out of the collectors preventing the system failure.

In colder areas, drainback active systems are subjected to freeze-ups unless you put the antifreeze solution and remove all the water from the collectors and piping, and when the system is not collecting the sun's heat. Provide sufficient continuous slope of the solar panel, keeping the pipes and collector protected from freezing.

Distilled water is used to transfer the heat from the solar collector to domestic water, and it is stored in the drainback reservoir. Distilled water is used, due to its capability, to increase heat transfer and prevent mineral buildup.

To allow complete drainage, a slight tilt of the collectors is required. This is why the collector has to be installed higher than the storage tank. Flat-plate panels are the most popular choice, but they can also use evacuated tubes.

Drainback active heating systems are reliable and long-lasting. If this active solar heating system is designed correctly(i.e., collector and the pump size), there won't be any problem, and it will work perfectly for a long time and in any climate. Drainback kits are also available for DIY projects.

Note: Adding the antifreeze to the drainback system can decrease the efficiency of the solar collectors. When the system is drained, the thin film of the antifreeze stays on the tube walls, dries, and leaves small deposits, which can build up over time, affecting the water flow and heat transfer. If only distilled water is used, the fluid rarely needs to be changed.

Photovoltaic systems

Photovoltaic (PV) operated active solar heating is using the pump only when the sun is shining. The pump is DC (direct current) and is performed by the PV panel that converts the sunlight into direct current electricity. The PV power can be used for both direct and indirect systems. PV systems are reliable and efficient, providing hot water when power outages shut down other systems.

So, which active solar heating system is right for you?

Selecting the correct active solar heating system often depends on the location of your home.

Other important factors are the quality of materials used for collectors, strength and reliability, and whether the system is installed correctly and oriented.

The above active systems allow greater flexibility than the passive systems as the solar storage water tank doesn't have to be above or even close to the collectors. They can be used year-round, which is not the case with passive systems. Even with the higher initial cost, they can pay off in 5-10 years.

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