Photovoltaic thermal hybrid solar collector
Photovoltaic thermal hybrid solar collectors, also known as hybrid PV/T (PVT) or solar cogeneration systems, are power generation technologies that convert solar radiation into usable thermal and electrical energy. Such systems combine a solar cell, which converts sunlight into electricity, with a solar thermal collector, which captures the remaining energy and removes waste heat from the PV module. These technologies can be more energy efficient overall than solar photovoltaic (PV) or solar thermal alone.
Significant research has gone into developing a diverse range of PV/T technologies since the 1970s.. While many collector types have been developed for small-scale residential or specific process use, powerful and highly-efficient systems for industrial and utility-scale applications are also beginning to enter the market. Collectors co-generating up to 250kW electricity plus 400kW heat, and operating with 80% conversion efficiency, are commercially available as of 2017.
PV/T system engineering
Photovoltaic cells suffer from a drop in efficiency with the rise in temperature due to increased resistance. PV/T systems are engineered to actively capture and carry heat away from the PV cells, thereby cooling the cells and thus improving their efficiency by lowering resistance. Cell lifetimes also improve at lower temperatures, and the thermal capture can minimize the urban heat island contribution from the generator.
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 solar thermal collector. That is to say, the maximum operating temperatures for most PV/T 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.
A number of PV/T collectors in different categories are commercially available and can be divided into the following categories:
- PV/T liquid collector
- PV/T air collector
- PV/Ta Liquid and air collector
- PV/T concentrator (CPVT)
PV/T liquid collector
The basic water-cooled design uses a channel to direct fluid flow using piping of various materials or plates attached to the back of a PV module. The fluid flow arrangement through the cooling element will determine which systems the panels are most suited to.
In a standard fluid-based system, a working fluid, typically water, glycol or mineral oil is then piped through these pipes or plate chillers. 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). In closed-loop systems this heat is either exhausted (to cool it), or transferred at a heat exchanger, where it flows to its application. In open-loop systems, this heat is used, or exhausted before the fluid returns to the PV cells. It is also possible to disperse nanoparticles in the liquid to create a liquid filter for PV/T applications. The basic advantage of this type of split configuration is that the thermal collector and the photovoltaic collector can operate at different temperatures.
PV/T air collector
The basic air-cooled design uses a hollow, conductive metal housing to mount the photo-voltaic (PV) panels. 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.
While energy transfer to air is not as efficient as a liquid collector, the infrastructure required has lower cost and complexity; basically a shallow metal box. Placement of PV panels can be vertical or angled.
PV/T concentrator (CPVT)
A concentrator system has the advantage to reduce the amount of photovoltaic (PV) cells needed, such that somewhat more expensive and efficient multi-junction photovoltaic cells can be used that will maximize the ratio of produced high-value electrical power versus lower-value thermal power. Concentrator systems also often require reliable control systems to accurately track the sun and to protect the PV cells from damaging over-temperature conditions. Under ideal conditions, about 75% of the sun's power directly incident upon such systems can be gathered as electricity and heat. For more details, see the discussion of CPVT within the article for concentrated photovoltaics.
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 MJ 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.
- Ahmad Mojiri, Robert A. Taylor, Elizabeth Thomsen, Gary Rosengarten, Spectral beam splitting for efficient conversion of solar energy — A review. In: Renewable and Sustainable Energy Reviews 28, December 2013, Pages 654–663, doi:10.1016/j.rser.2013.08.026
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- S.A. Kalogirou, Y. Tripanagnostopoulos (30 January 2006). These systems are most often used for domestic hot water (DHW) and electricity production
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- "PV Thermal". Solarwall. Retrieved 15 February 2017.