Hot water solar panels
Solar hot water heater panels and important specifications
Hot water solar panels, or collectors, are the heart and soul of any solar water heater. With that said, the panels' orientation, performance specifications as well as physical specifications can determine the panels' efficiency and application.
Panel or collector orientation
With either an integrated collector storage unit , a flat plate collector as well as an evacuated tube collector, it will be important to maximize a site's solar resource. The position the hot water solar panels are installed in is commonly referred to as the collector orientation. The collector (or panel) orientation will determine if the sun's radiant heat is being efficiently maximized or not.
Panel or collector mount
The panel or collector mount is simply the physical from of installation chosen by the professional solar installer. With all hot water solar panels there are three collector mounting options to consider; a roof mount, a ground mount and an awning mount.
- The most common mount used in residential applications.
- Horizontal brackets are used for positioning.
- The surface area and weight of the solar water panel must be considered.
- Attaches the collector to the top of an outside wall.
- Horizontal brackets are used for positioning.
- Overall surface area and weight of the panel is somewhat of a concern.
- Installation is usually as simple as placing support poles in the ground.
- In most instances, brackets can be used for positioning.
- The hot water solar panels' size and weight is much less a concern.
Panel or collector tilt
The panel (or collector) tilt is simply the angle at which the collector is installed. A proper tilt, or angle, has to be pre determined in order to maximize a site's solar resource. The tilt can be influenced by the collector mount. A particular mount can (to some extent) limit the solar water panels' tilt.
Both the cold and warm climate seasons' must be factored in when determining an advantageous year round tilt or angle. Professional solar panel installers strongly recommend using an angle close or equal to your site's global latitude.
For all locations in the Northern Hemisphere, the solar water panel should face south or southwest at an angle close or equal to the locations' global latitude.
Panel or collector headers
The headers are the large tubes that are bonded to the risers of hot water solar panels. The headers circulate inlet fluid through the collector and circulate heated fluid from the collector as well. The position of the panels' headers have important installation ramifications.
Internal headers are positioned on the inside of the hot water solar panels' frame or plate. Hot water solar panels with internal headers have two separate inlet points and two separate outlet points.
The purpose of having two sets of inlets as well as outlets is to connect two or more hot water solar panels to one another. In fact, these panels are designed for applications that require two or more collectors for greater hot water output.
External headers are positioned inside of the solar water panels' frame or plate. These headers have a single inlet and a single outlet, one at the top and one at the bottom. Hot water solar panels with external headers are designed for those heating systems that only require a single panel or collector.
Hot water solar panels with external headers can be installed together in larger applications, if it's required, but additional plumbing parts will be needed.
Important physical specifications
The panel (or collector) surface area is the total size of the panel, including the outer frame. The overall size of the panel is important because the surface area in some cases can determine if a certain model or mount is unfit for a site.
Net Aperture Area
The net aperture area is the surface area of the collector that actually absorbs the sun's thermal radiation. The collector net aperture area is an important physical specification to consider when sizing a solar water heating panel.
Absorber Plate Material
The absorber plate is responsible for capturing the sun's radiant heat. So the material it is made out of is important. Copper is generally the most trusted absorber plate material, but other metals are commonly used.
Coatings are used to assist the absorber plate in collecting solar heat. Black paint is often used, as well as more complex coatings referred to as selective surfaces. Both are effective at absorbing heat but selective surfaces generally retain heat better than black paint, especially low E selective surfaces.
Manufacturers use selective surface absorber plate coatings with descriptive names such as black chrome, metallic oxide and titanium oxide, and some just simply identify that a selective surface is used to coat the absorber plate.
In flat plate collectors a sheet of polyisocyanurate foam is often used to further insulate the frame. Polyisocyanurate foam is very effective at reducing heat loss. Some models use fiberglass insulation instead, and fiberglass is effective, but not as effective as polyisocyanurate foam.
Panel or Collector Glazing
The glass or plastic material used to cover the panels' frame is important as well. With flat plate collectors, low iron tempered glass is quickly becoming the new glazing standard. Low iron tempered glass is essentially window glass that's chemically altered to reduce the iron content.
This allows about 90% of the sun's rays to penetrate through the glass. The low iron glass is then tempered, or toughened. This is important because tempering the glass enables flat plate collectors to withstand the toughest hailstorms.
Some less expensive flat plate hot water solar panels use special plastic glazing. The plastics used generally allow just about the same amount of solar radiation inward, but these plastics don't insulated as well as low iron tempered glass.
In evacuated tube solar collectors, more times than not, the glass tubes are made up of either borosilicate glass or soda lime glass. While both are very durable, borosilicate glass is more resistant to extreme thermal temperatures.
Evacuated tube collectors with borosilicate glass tubes are generally slightly more expensive than models with soda lime glass tubes.
Important performance specifications
Thousands of BTU
Thousands of BTU, or thousands of British thermal units, is the unit the industry uses to measure the thermal output of hot water solar panels. This performance specification is always found on the collector thermal performance rating label of SRCC certified and rated hot water solar panels.
Knowing how much solar thermal output a particular water heating system will require is an important part of sizing a solar water panel.
Y - Intercept
This rating specification is found on the thermal performance rating label of all SRCC certified and rated hot water solar panels also. It's a measurement of the effectiveness of a panel when the circulating fluid temperature is equal to the outside climate temperature. Y - intercept is usually provided in the form of a decimal or a percentage.
More on solar hot water panels
More on solar hot water heaters