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Active
Solar Heating Systems
Heating domestic water with solar water heaters
Active solar heating system is mainly used for domestic hot water heating in colder regions, where sun's heat energy is collected inside the solar collectors and transferred through the heat exchanger to heat the water stored inside the storage tank.
Active solar heating is applicable in colder regions, but to eliminate the risk of freezing you have to use the antifreeze as the thermal fluid. As the required heat or temperature is usually not met with the solar heating system, so backup heating has to be provided (electrical heating elements for example).
The efficiency of the active solar water heater and its performance depends on the system design, quality of material used, how well the system is installed and maintained and the amount of time the system is exposed to the sun.
Active solar energy system for water heating can use controllers with sensors to control the pump or Photovoltaic solar collector where the pump runs only when sun heat energy is available.
Types of the 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:
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 are using solar collectors where 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 a case of the solar panels which are built from the 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 solar fluid remains within a 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 where it transfers the heat to domestic water.Indirect systems are using mainly popular solar panels or flat plate collectors, and also evacuated tubes and concentrated collectors.
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 the open-loop system, often seen in the warmer regions, where
freezing never occurs. The main components are solar collector, pump,
controllers, sensors (i.e. temperature limit) and valves (i.e.
tempering valve, isolation valves, air vent).In the open-loop systems, solar collector is attached through the pipeline to the storage tank. 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.
Pressurized glycol system or Antifreeze Systems
Antifreeze pressurized system is the example of the closed-loop active solar heating systems, where water inside the storage tank is heated by the fluid that absorbs the sun 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. 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 lower temperature environment, closed-loop with 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 solutions. 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 in order to add or change the fluid.
Drainback system
Drainback system
is another example of the closed loop systems 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 really hot (but not above the maximum as it is controlled by the temperature limit sensor), 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 water from collectors and piping, when system is not collecting sun's heat. Provide sufficient continuous slope of the solar panel and that will keep the pipes and collector from freezing.
Distilled water is used to transfer the heat from the solar collector to the domestic water, and it is stored in the drainback reservoir. Distilled water is used due to its capability to increase the heat transfer and prevent the 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 choices, but they can also use evacuated tubes.
Drainback active heating systems are reliable and long lasting. If this active solar heating system is designed properly (i.e. collector and the pump size) there won't be any problem, and it will work perfect 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 cause the decrease of the efficiency on solar collectors. When the system is drained, thin film of the antifreeze stays on the tube walls, dries and leaves the small deposits which can buildup over time affecting the water flow and heat transfer. If only distilled water is used the fluid rarely needs to be changed.
Photovoltaic
Photovoltaic (PV) operated active solar heating is using the pump only when the sun is shining. The pump is DC (direct current) and is operated by the PV panel that converts the sunlight into direct current electricity. The PV power can be used for both direct and indirect system. PV systems are reliable and efficient, providing the hot water when other systems are shut down by power outages.Which one from the above active solar heating systems is right for you?
It depends on the location you live. Other important factors are the quality of materials used for collectors, strength, reliability... is the system properly installed 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. Even with higher initial cost expect the payback in 5-10 years.
Related articles
Pictures are courtesy of zenrenewables.com and wikipedia.org
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