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. 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. The design and type of PVT collectors always implies a certain adaption to
operating temperatures, applications, and giving priority to either
heat or
electricity generation. For instance, operating the PVT collector at
low temperatures leads to a cooling effect of
PV cells compared 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). 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
working fluid, typically water,
glycol or
mineral oil circulates in the heat exchanger behind the PV cells. The heat from the PV cells is conducted through the metal and absorbed by the working fluid (presuming that the working fluid is cooler than the
operating temperature 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
HVAC system to deliver
thermal energy. 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. The rear side of the uncovered PVT collector can be equipped with
thermal insulation (e.g.
mineral wool or foam) to reduce
heat losses of the heated fluid. Uninsulated PVT collectors are beneficial for operation near and below
ambient temperatures. Particularly uncovered PVT collectors with increased heat transfer to ambient air are a suitable
heat source for
heat pump systems. 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
cooling by
dissipating heat via the PVT collector to the
ambient air or by utilizing the
radiative cooling effect. In doing so, cold air or water is harnessed, which can be utilized for
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
efficiency. Moreover, covered PVT collectors can reach significantly higher temperatures than
PV modules or
uncovered PVT collectors. The
operating temperatures mostly depend on the temperature of the working fluid. The average fluid temperature can be between 25 °C in swimming pool applications to 90 °C in
solar cooling systems. Covered PVT collectors resemble the form and design of conventional
flat plate collectors or
evacuated vacuum tubes. Yet,
PV cells instead of spectrally-selective absorber
coatings absorb the incident
solar irradiance and generate an
electrical current in addition to
solar heat. The insulating characteristics of the front cover increase the
thermal efficiency and allow for higher operating temperatures. However, the additional optical interfaces increase optical
reflections and thus reduce the generated electrical power. Anti-reflective coatings on the front glazing can reduce the additional optical losses.
PVT concentrator (CPVT) A concentrator system has the advantage to reduce the amount of
PV cells needed. Therefore, it is possible to use more expensive and efficient PV cells, e.g.
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. 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. A limitation of high-concentrator (i.e. HCPV and HCPVT) systems is that they maintain their long-term advantages over conventional
c-Si/
mc-Si collectors only in regions that remain consistently free of atmospheric
aerosol contaminants (e.g. light clouds, smog, etc.). Power production is rapidly degraded because 1) radiation is reflected and scattered outside of the small (often less than 1–2 °)
acceptance angle of the collection optics, and 2)
absorption of specific components of the solar spectrum causes one or more series junctions within the
multi-junction cells 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 ==