Photovoltaic thermal hybrid solar collector

aluminum, copper or polymers)
7 - Thermal insulation (e.g. mineral wool, polyurethane

)

Photovoltaic thermal collectors, typically abbreviated as PVT collectors and also known as hybrid solar collectors, photovoltaic thermal solar collectors, PV/T collectors or solar

Significant research has gone into developing a diverse range of PVT technologies since the 1970s.[3] The different PVT collector technologies differ substantially in their collector design and heat transfer fluid and address different applications ranging from low temperature heat below ambient up to high temperature heat above 100 °C.^[4]

PVT markets

PVT collectors generate

]

The electricity demand in buildings and industry is expected to grow further due to ongoing

water power

.

The market for renewable heat and electricity is therefore vast, illustrating the market potential of PVT collectors.

The report "Solar Heat Worldwide"[9] assessed the global market of PVT collectors in 2019. According to the authors, the total area of installed collectors amounted to 1.16 million square meters. Uncovered water collectors had the largest market share (55%), followed by air collectors (43%) and covered water collectors (2%). The country with the largest installed capacity was France (42%), followed by South Korea (24%), China (11%) and Germany (10%).

PVT collector technology

PVT collectors combine the generation of solar electricity and heat in a single component, and thus achieve a higher overall efficiency and better utilization of the solar spectrum than conventional PV modules.

Photovoltaic cells typically reach an electrical efficiency between 15% and 20%, while the largest share of the solar spectrum (65% - 70%) is converted into heat, increasing the temperature of PV modules. PVT collectors, on the contrary, are engineered to transfer heat from the PV cells to a fluid, thereby cooling the cells and thus improving their efficiency.^[10] In this way, this excess heat is made useful and can be utilized to heat water or as a low temperature source for heat pumps, for example. Thus, PVT collectors make better use of the solar spectrum.^[2]

Most photovoltaic cells (e.g. silicon based) suffer from a drop in efficiency with increased cell temperatures. Each Kelvin of increased cell temperature reduces the efficiency by 0.2 – 0.5%.^[4] Therefore, heat removal from the PV cells can lower their temperature and thus increase the cells' efficiency. Improved PV cell lifetimes are another benefit of lower operation temperatures.

This is an effective method to maximize total system efficiency and reliability, but causes the thermal component to under-perform as compared to that achievable with a pure

collector. That is to say, the maximum operating temperatures for most PVT system are limited to less than the maximum cell temperature (typically below 100 °C). Nevertheless, two or more units of heat energy are still generated for each unit of electrical energy, depending on cell efficiency and system design.

Types of PVT collectors

There are a multitude of technical possibilities to combine

:

• PVT liquid collector
• PVT air collector

In addition to the classification by

:

• Uncovered PVT collector (WISC PVT)
• Covered PVT collector
• Concentrating PVT collector (CPVT)

Moreover, PVT collectors can be classified according to their design, such as

The design and type of PVT collectors always implies a certain adaption to

PV modules

and therefore results in an increase of electric power. However, the heat also has to be utilized at low temperatures.

The maximum operating temperatures for most PV modules are limited to less than the maximum certified operation temperatures (typically 85 °C).[12] Nevertheless, two or more units of thermal energy are generated for each unit of electrical energy, depending on cell efficiency and system design.

PVT liquid collector

The basic water-cooled design uses channels to direct fluid flow using piping attached directly or indirectly to the back of a PV module. In a standard fluid-based system, a

of the cells).

PVT air collector

The basic air-cooled design uses either a hollow, conductive housing to mount the photovoltaic panels or a controlled flow of air to the rear face of the PV panel. PVT air collectors either draw in fresh outside air or use air as a circulating heat transfer medium in a closed loop. Heat is radiated from the panels into the enclosed space, where the air is either circulated into a building HVAC system to recapture heat energy, or rises and is vented from the top of the structure. The heat transfer capability of air is lower than that of typically used liquids and therefore requires a proportionally higher mass flow rate than an equivalent PVT liquid collector. The advantage is that the infrastructure required has lower cost and complexity.

The heated air is circulated into a building

. Excess heat generated can be simply vented to the atmosphere. Some versions of the PVT air collector can be operated in a way to cool the PV panels to generate more electricity and assist with reducing thermal effects on lifetime performance degradation.

A number of different configurations of PVT air collectors exist, which vary in engineering sophistication. PVT air collector configurations range from a basic enclosed shallow metal box with an intake and exhaust up to optimized heat transfer surfaces that achieve uniform panel heat transfer across a wide range of process and ambient conditions.

PVT air collectors can be carried out as uncovered or covered designs.^[2]

Uncovered PVT collector (WISC)

Uncovered PVT collectors, also denoted as unglazed or wind and/or infrared sensitive PVT collectors (WISC), typically consist of a PV module with a heat exchanger structure attached to the back of the PV module. Despite their name, the solar cells are generally attached to the back side of a front glass and thus covered by it, but without an air gap. While most PVT collectors are prefabricated units, some products are offered as heat exchangers to be retrofitted to off-the-shelf PV modules. In both cases, a good and longtime durable thermal contact with a high heat transfer coefficient between the PV cells and the fluid is essential.^[13]

The rear side of the uncovered PVT collector can be equipped with

. When the temperature in the heat pump's source is lower than the ambient, the fluid can be heated up to ambient temperature even in periods without sunshine.

Accordingly, uncovered PVT collectors can be categorized into:

• Uncovered PVT collector with increased heat transfer to ambient air
• Uncovered PVT collector without rear insulation
• Uncovered PVT collector with rear insulation

Uncovered PVT collectors are also used to provide renewable

HVAC

applications.

Covered PVT collector

Covered, or glazed PVT collectors, feature an additional glazing, which encloses an insulating air layer between the PV module and the secondary glazing. This reduces heat losses and increases the thermal

solar cooling

systems.

Covered PVT collectors resemble the form and design of conventional

.

The insulating characteristics of the front cover increase the

PVT concentrator (CPVT)

Main article:

Concentrated photovoltaics § Concentrated photovoltaics and thermal

A concentrator system has the advantage to reduce the amount of

multi-junction photovoltaic cell

. The concentration of sunlight also reduces the amount of hot PV-absorber area and therefore reduces heat losses to the ambient, which improves significantly the efficiency for higher application temperatures.

Concentrator systems also often require reliable control systems to accurately track the Sun and to protect the PV cells from damaging over-temperature conditions. However, there are also stationery PVT collector types that use nonimaging reflectors, such as the Compound Parabolic Concentrator (CPC), and do not have to track the Sun.

Under ideal conditions, about 75% of the Sun's power directly incident upon such systems can be gathered as electricity and heat at temperatures up to 160 °C.^[15] CPVT units that are coupled with thermal energy storage and organic Rankine cycle generators can provide on-demand recovery of up to 70% of their instantaneous electricity generation, and may thus be a fairly efficient alternative to the types of electrical storage which are joined with traditional PV systems.^[16][17]

A limitation of high-concentrator (i.e. HCPV and HCPVT) systems is that they maintain their long-term advantages over conventional

to under-perform. The short-term impacts of such power generation irregularities can be reduced to some degree with inclusion of electrical and thermal storage in the system.

PVT applications

The range of applications of PVT collectors, and in general solar thermal collectors, can be divided according to their temperature levels:^[18]

• low temperature applications up to 50 °C
• medium temperature applications up to 80 °C
• high temperature applications above 80 °C

Accordingly, PVT collector technologies can be clustered with respect to their temperature levels: the suitability per temperature range depends on the PVT collector design and technology. Therefore, each PVT collector technology features different optimal temperature ranges. The operating temperature ultimately defines which type of PVT collector is suitable for which application.

Low temperature applications include

Uncovered PVT collectors with enhanced air-to-water heat exchange can even be the only source of a heat pump system. In combination with a system architecture allowing to store cold produced with WISC or air collectors also air conditioning is possible.

Low and medium temperature applications for

of the heat demand (e.g. hot water pre-heating), thus reducing operating temperatures of the PVT collector.

with concentrating PVT collectors).

Depending on the type of heat transfer fluid, PVT collector technologies are suited for several applications:^[19]

• PVT air collector: space heating systems, agricultural processes (e.g. drying crops);
• PVT liquid collector: Space heating (domestic, industrial), water heating systems, water desalination, space cooling, food processing systems.

PVT technologies can bring a valuable contribution to the world's

.

See also

• Solar air conditioning
• Photovoltaic system
• Solar Energy

References