A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may also refer to large power generating installations such as solar parabolic troughs and solar towers, or to non-water-heating devices such as solar cookers or solar air heaters.
Solar thermal collectors are either non-concentrating or concentrating. In non-concentrating collectors, the aperture area (i.e., the area that receives the solar radiation) is roughly the same as the absorber area (i.e., the area absorbing the radiation). A common example of such a system is a metal plate that is painted a dark color to maximize the absorption of sunlight. The energy is then collected by cooling the plate with a working fluid, often water or glycol running in pipes attached to the plate.
Concentrating collectors have a much larger aperture than the absorber area. The aperture is typically in the form of a mirror that is focussed on the absorber, which in most cases are the pipes carrying the working fluid. Since the sun moves through the sky during the day, concentrating collectors often require some form of solar tracking system, and are sometimes referred to as "active" collectors for this reason.
Non-concentrating collectors are typically used in residential, industrial and commercial buildings for space heating, while concentrating collectors in concentrated solar power plants generate electricity by heating a heat-transfer fluid to drive a turbine connected to an electrical generator.
Heating water Flat-plate and evacuated-tube solar collectors are mainly used to collect heat for space heating, domestic hot water, or cooling with an absorption chiller. In contrast to solar hot water panels, they use a circulating fluid to displace heat to a separated reservoir. The first solar thermal collector designed for building roofs was patented by William H. Goettl and called the "Solar heat collector and radiator for building roof".
Evacuated flat-plate solar collectors are a more recent innovation and can be used for Solar Heat for Industrial Cooling (SHIC) and Solar Air Conditioning (SAC), where temperature in excess of are required. These non-concentrating collectors harvest both diffuse and direct light and can make use of steam instead of water as fluid.
Flat plate collectors
Flat-plate collectors are the most common solar thermal technology in Europe. They consist of (1) an enclosure containing (2) a dark-colored absorber plate with fluid circulation passageways, and (3) a transparent cover to allow transmission of solar energy into the enclosure. The sides and back of the enclosure are typically insulated to reduce heat loss to their surroundings. A heat transfer fluid is circulated through the absorber's fluid passageways to remove heat from the solar collector. The circulation fluid in tropical and sub-tropical climates is typically water. In climates where freezing is likely, a heat transfer fluid similar to an automotive antifreeze solution may be used instead of water, or in a mixture with water. If a heat transfer fluid is used, a heat exchanger is typically employed to transfer heat from the solar collector fluid to a hot water storage tank. The most common absorber design consists of copper tubing joined to a high conductivity metal sheet (copper or aluminum). A dark coating is applied to the sun-facing side of the absorber assembly to increase its absorption of solar energy. A common absorber coating is black enamel paint.
In higher performance solar collector designs, the transparent cover is tempered soda-lime glass having reduced iron oxide content same as for photovoltaic solar panels. The glass may also have a stippling pattern and one or two anti-reflective coatings to further enhance transparency. The absorber coating is typically a selective coating, where selective stands for having the special optical property to combine high absorption in the visible part of the electromagnetic spectrum coupled to low emittance in the infrared one. This creates a selective surface, which reduces black body energy emission from the absorber and improves performance. Piping can be laser or ultrasound welded to the absorber sheet to reduce damage to the selective coating, which is typically applied prior to joining to large coils in a roll-to-roll process.
Absorber piping configurations include:
harp: traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems; serpentine: one continuous S-shaped pipe that maximises temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role); flooded: consisting of two sheets of metal molded to produce a wide circulation zone that improves heat transfer; boundary layer: consisting of several layers of transparent and opaque sheets that enable absorption in a boundary layer. Because the energy is absorbed in the boundary layer, heat conversion may be more efficient than for collectors where absorbed heat is conducted through a material before being accumulated in the circulating liquid.
A flat plate collector making use of a honeycomb structure to reduce heat loss also at the glass side too has also been made available commercially. Most flat plate collectors have a life expectancy of over 25 years..
Evacuated tube collectors
Evacuated tube collectors are the most common solar thermal technology in the world. They make use of a glass tube to surround the absorber with high vacuum and effectively resist atmospheric pressure. The vacuum that surrounds the absorber greatly reduces convection and conduction heat loss, therefore achieving greater energy conversion efficiency. The absorber can be either metallic as in the case of flat plate collectors or being a second concentric glass tube ("Sydney Tube"). Heat transfer fluid can flow in and out of each tube or being in contact with a heat pipe reaching inside the tube. For the latter, heat pipes transfer heat to the fluid in a heat exchanger called a "manifold" placed transversely with respect to the tubes. The manifold is wrapped in insulation (glass wool) and covered by a protective metal or plastic case also used for fixing to supports.
Glass-metal evacuated tubes are made with flat or curved metal absorber sheets same as those of flat plates. These sheets are joined to pipes or heat pipes to make "fins" and placed inside a single borosilicate glass tube. An anti-reflective coating can be deposited on the inner and outer surfaces of such tubes to improve transparency. Both selective and anti-reflective coating (inner tube surface) will not degrade until the vacuum is lost. A high vacuum-tight glass-metal seal is however required at one or both sides of each evacuated tube. This seal is cycled between ambient and fluid temperature each day of collector operation and might lead to failures in time.
Glass-glass evacuated tubes are made with two borosilicate glass tubes fused together at one or both ends (similar a vacuum bottle or dewar flask). The absorber fin is placed inside the inner tube at atmospheric pressure. Glass-glass tubes have a very reliable seal, but the two layers of glass reduce the amount of sunlight that reaches the absorber. The selective coating can be deposited on the inner borosilicate tube (high vacuum side) to avoid this, but heat has then to flow through the poorly conducting glass thickness of the inner tube in this case. Moreover, moisture may enter the non-evacuated area inside the inner tube and cause absorber corrosion in particular when made from dissimilar materials (galvanic corrosion).
A Barium flash getter pump is commonly evaporated inside the high vacuum gap in between tubes to keep the internal pressure stable through time.
The high temperatures that can occur inside evacuated tubes may require special design to prevent thermal shock and overheating. Some evacuated tube collectors work as a thermal one-way valve due to their heat pipes. This gives them an inherent maximum operating temperature that acts as a safety feature. Evacuated tubes collectors can also be provided with low concentrating reflectors at the back of the tubes realising a compound parabolic concentrator (CPC) collector.
Comparisons of flat plate and evacuated tube collectors A longstanding argument exists between proponents of these two technologies. Some of this can be related to the structure of evacuated tube collectors which have a discontinuous absorbance area. An array of evacuated tubes collectors on a roof has space between the individual tubes and a vacuum gap between each tube and its absorber inside, covering only a fraction of the installation area on a roof. If evacuated tubes are compared with flat-plate collectors on the basis of the area of roof occupied (gross area), a different conclusion might be reached than if the absorber or aperture areas were compared. The recent revision of the ISO 9806 standardISO 9806:2017. Solar energy – Solar thermal collectors – Test methods International Organization for Standardization, Geneva, Switzerland states that the efficiency of solar thermal collectors should be measured in terms of gross area and this might favour flat plates in respect to evacuated tube collectors in direct comparisons.
Flat-plate collectors usually lose more heat to the environment than evacuated tubes because there is no insulation at the glass side. Evacuated tube collectors intrinsically have a lower absorber to gross area ratio (typically 60–80% less) than flat plates because tubes have to be spaced apart. Although several European companies manufacture evacuated tube collectors (mainly glass-metal type), the evacuated tube market is dominated by manufacturers in China, with some companies having track records of 15–30 years or more. There is no unambiguous evidence that the two designs differ in long-term reliability. However, evacuated tube technology (especially for newer variants with glass-metal seals and heat pipes) still needs to demonstrate competitive lifetimes. The modularity of evacuated tubes can be advantageous in terms of extensibility and maintenance, for example, if the vacuum in one heat pipe tube is lost it can be easily be replaced with minimal effort.
In most climates, flat plate collectors will generally be more cost-effective than evacuated tubes. However, evacuated tube collectors are well-suited to cold ambient temperatures and work well in situations of low solar irradiance, providing heat more consistently throughout the year. Unglazed flat plate collectors are the preferred devices for heating swimming pool water. Unglazed collectors may be suitable in tropical or subtropical environments if domestic hot water needs to be heated by less than over ambient temperature. Evacuated tube collectors have less aerodynamic drag, which may allow for a simpler installation on roofs in windy locations. The gaps between the tubes may allow for snow to fall through the collector, minimizing the loss of production in some snowy conditions, though the lack of radiated heat from the tubes can also prevent effective shedding of accumulated snow. Flat plate collectors might be easier to clean. Other properties, such as appearance and ease of installation are more subjective and difficult to compare.
Evacuated flat plate collectors