Solar Panels for DHW
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Tips when selecting and building solar panels for home water heating. See the main differences between the flat solar collectors and other types, including the advantages and disadvantages.

Solar panels or flat plate collectors are designed to absorb the solar heat energy and transfer the heat to the heating fluid, which passes through the device and further to the storage tank where it heats water. They are the most used type of solar collectors in domestic solar water and space heating.

Advantages

The flat plate collectors are considered as the standard type, widely used, inexpensive, very effective and long-lasting. They collect both direct and diffuse solar energy radiation, so they don't have to track the sun. According to some reviews and studies, the solar collectors are more efficient than evacuated tubes, since the larger collector area is exposed to absorb the solar heat. That means that fewer solar panels are used for the same performance.

Disadvantages

Since the solar panels use the small riser tubes, it is important to know that they are prone to clogging if the water is hard. According to the studies, they have the largest heat absorbing area and the highest heat loss of all types of collectors. They are also susceptible to freezing as they have a large exposed area and many small passages.

Things to consider

It is expected from solar panels to work in a wide temperature range, which can go from below freezing to about 180 F even higher. All elements should be made of the high-quality materials and capable to withstand higher temperatures during the long-lasting sun exposure. Most panels last 20 years or more.

It is important to know that panels do not store any energy, only a small amount of the heating fluid is inside the riser tubes, so most of the energy is kept inside the solar tank, for later use. The solar panels can be used either in the passive systems such as the thermosyphon or active solar heating systems with the circulating pump and controllers.

Frame

The frame in solar collectors should be strong so it can withstand high winds, heavy rain, snow, hail and other conditions without braking or losing its functionality. Most of the time the shape is rectangular and can come in different sizes, where the most used are 4 x 8 or 10 feet and 4-6 ft. deep.

The recommended material is the aluminum, lightweight but strong so it can be installed on the roofs without damaging it. The aluminum channels are better solution as they come with the designed flanges for easy and convenient installation. To attach the frame on the rooftops, you can either buy the mounting hardware kit or separate elements, but be careful to buy the long-lasting, such as the stainless steel.

Glazing

The front of the collector, glazing, is usually made of the low-iron, tampered glass or strong plastic, and should have the characteristics such as the high transmission, non-degradability, strength and durability. They can either use the rubber gaskets or silicone caulk when installing into the frame. Glass is a good solution as the cover due to its high transmittance and better thermal insulation than plastic, while plastic covers can have the larger area as they are lighter. Also, the window glass is unsuitable as it has the high iron content.

Absorber plate

The absorber plate is installed right below the glazing (1" gap is recommended), and on its ends it usually has manifolds, made of the 3/4" or 1" copper pipes to collect the fluid from the riser tubes. Each riser tube is attached with the copper or aluminum flat fins for the effective transfer of the collected heat. It is recommended to make the fin-tube connection snug by using the soldering or welding.

The most important factor when considering the absorber plates is the coating or "selective surface"; it should have a high absorptance and low emittance. They can be made of steel, aluminum or most of the time copper; coated in the black paint, black chrome or copper oxide. Together, they provide a strong, corrosion-resistant assembly with the high thermal conductivity.

There are several types of absorber plate/riser tube assemblies, and the selection depends on the particular application:

  • Tube in flat plate
  • Tubes fastened to upper or lower flat plate
  • Tubes fitted in grooved plate
  • Corrugate sheet on a flat plate or riveted together

Riser tubes

The riser tubes are 1/2" diameter copper pipes where the heating fluid circulates. These small pipes can be made with the manifolds at its ends or shaped as the serpentine, running continuously from one side of the collector to the other. They are installed a few inches apart (3-6 inches is recommended) and they can freely expand or contract when heated or cooled, since they are not attached to the frame.

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Insulation

In order to keep the captured heat longer and reduce the heat loss from the absorber plate and pipes, collectors are insulated in the back and on the sides. The recommended insulations are mineral wool and PUR because of its lower thermal conductivity.

Conclusion

The solar panels are easy to construct and are inexpensive to build. Most of the big hardware stores, such as Home Depot and Lowe's carry all the materials you need to construct panels, or if you prefer plug-and-play systems, you can buy a kit.


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