Photovoltaic thermal hybrid solar collector

Photovoltaic thermal hybrid solar collectors, sometimes known as hybrid PV/T systems or PVT, are systems that convert solar radiation into thermal and electrical energy. These 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. and thus be more overall energy efficient than solar photovoltaic (PV) or solar thermal alone. A significant amount of research has gone into developing PVT technology since the 1970s.

Photovoltaic cells suffer from a drop in efficiency with the rise in temperature due to increased resistance. Such systems can be engineered to carry heat away from the PV cells thereby cooling the cells and thus improving their efficiency by lowering resistance. Although this is an effective method, it causes the thermal component to under-perform compared to a solar thermal collector.

Principles
Photovoltaic elements (PV cells, typically doped silicon) transform solar light (visible range) into potential difference and electric current, while the thermal sensor part (absorber or “concentrator”…) recovers caloric energy sent by the sun (in particular the infra-red radiation usually lost in the form of heat dispersed by the panel) via a coolant (air or water / glycol, injected by a pump whose operation is powered by electricity).

The electricity produced can be used locally immediately or after storage (battery), or be injected into the electricity grid (resale / buyout).

The heat produced can be connected to any conventional thermal installation, used for heating or preheating air or domestic water (domestic hot water, swimming pool,…), a drying unit,…

System types
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. A major limitation of high-concentrator (i.e. HCPV and HCPVT) systems is that they maintain their advantage over conventional c-Si/mc-Si collectors only in regions that remain consistently free of atmospheric aerosol contaminants (e.g. light clouds, smog, etc.). Concentrator system performance is especially 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 underperform.

Concentrator systems also 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 suns 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.

Structure of a PVT collector
As mentioned, a PVT collector is an association of a photovoltaic collector and a heat exchanger. The photovoltaic collector is almost always of the glazed type, to reduce heat loss.

Collectors with a front air chamber
They exploit the greenhouse effect. They are used almost exclusively for heat exchange with air.

Collectors without inner tube
The most common type. Here the heat exchange is carried out on the back of the photovoltaic collector; it is an obligatory structure in the case of liquid cooling, since the exchanger would mask the photovoltaic cells, and in any case has the advantage of a rear location of the fluid supply and extraction pipes, which would otherwise pose shading problems.

Liquid manifolds
Compared to a normal PV collector, in a liquid manifold there is the addition of a heat exchanger and its insulation. This exchanger can be of various shapes; in the most frequent cases it is made up of adherent copper pipes, with various technologies, to the backsheet or, more effectively, it consists of an aluminum roll-bond exchanger, which allows a better heat transmission. The heat exchange with the liquid collector is very effective for cooling the photovoltaic cells, increasing their yield.

Concentration collectors
By abandoning the use of silicon cells and introducing thin film technology, it is possible to design a hybrid panel that sees the use of solar concentration. An interesting application sees the presence of a CPC concentrator (from the English Compound Parabolic concentrator ) in the fire of which a tube is placed on whose side surface a film of thin-film cells (for example CIS or CIGS ) is placed. This configuration allows to achieve higher yields of photovoltaic cells (thanks to the concentration) but at the same time a more effective heat removal (since the whole cell is in contact with the heat transfer fluid).

Installation
The installation of PV-T panels includes:

like any solar panel, fixing panels (usually, on the roof)
like any PV panel, the installation of electrical cables and equipment downstream (inverter,…),
as any thermal panel, a ventilation circuit, if air cooled, or a hydraulic circuit with a hot water accumulator (if the PV-T panel is directly connected to a hot water system available at the tap (and not for the only heating), it is obligatory to connect, at the outlet of the stock, a thermostatic mixing valve of safety, so that one can not be burned with too hot water.

Trading system
Several French manufacturers offer hybrid panels: DualSun, Sillia (with a copper absorber), ABCD Intl, as well as Cogen’air (air cooling) and Systovi.

Advantages
The overall energy efficiency is significantly higher than that of the photovoltaic panels (12-20%), mainly due to the thermal component (which also values the non-exploited IR irradiation in PV alone – 46% of the total).

In addition, thermal capture has two favorable effects on electricity production:

Photovoltaic cells work better. Indeed their dark color because they heat up in the sun, or their power generation efficiency decreases with heat, especially above 45° C. In a PV-T panel, the heat collector captures the solar calories, which cools the PV cells and increases their production especially during peaks of insolation. The heat is injected into an accumulator (closed circuit water / glycol in general) thanks to the current produced by the photovoltaic cells; this significantly improves electricity production (around 15% in the Paris region according to manufacturers).

The permanent cooling of the panels improves their service life and their efficiency (increase of the COP of the heat pumps when a heat pump is associated).

These advantages are particularly evident for panels in a central position on a roof.

The equal cost of installing a hybrid panel is reduced compared to that of a PV solar panel and a solar thermal panel.

A hybrid PV-T panel can work faster in the event of snow (or frost or mist) that obscures it: this can be eliminated by circulating the heat transfer fluid in the opposite direction. The PV-T panel also participates during hot weather to reduce the heat in the attic of the home, by cooling the roof.

Source from Wikipedia