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Miracle weapon PVT - what are collectors capable of? And what aren't they?

With the rise of photovoltaics in Germany, engineers and scientists have brought an old idea back to the table: the PVT or hybrid collector.

The idea is simple and the promise is great: a PV cell has an efficiency of about 20%. So if an additional 20% of 100% solar radiation is lost due to optical losses (reflection etc.) and only 20% is converted into electricity, the remaining 60% is lost. So why not use this as heat!

 

PVT - The all-rounder

Energy balance on the PVT module
Energy balance on the PVT module

If one uses both heat and electricity, the solar efficiency of PVT collectors can actually be as high as 80-90%. This technology is particularly interesting for heat pump systems:

PVT heat can be used to support the anergy (source) side of the heat pump, while the PVT electricity (at least on balance) drives the heat pump and/or provides household electricity.

Geothermal source systems, such as borehole heat exchangers and ground collectors, benefit from PVT in particular: the regeneration of the subsurface by introducing the PVT heat increases the heat pump efficiency, the operation of the source is more sustainable and the geothermal system can usually also be dimensioned smaller (and thus more economically). A classic win-win situation.

Depending on the design (see below), a PVT collector is not only a solar collector but also an outdoor air heat exchanger and can thus serve as a source for the heat pump or provide night-time cooling energy even without the sun. In the latter case, the collector's heat radiation against the cold night sky is used.

But the PVT collector can also be used on the useful energy side: Just like a solar thermal collector, PVT can directly support the preparation of domestic hot water via a buffer tank.

Not enough yet? If the application temperature is low and the design of the PVT collector is good, the PVT collector even cools the cell and thus increases the electricity yield. Because: A pure PV module reaches up to 65°C (in so-called NOCT) at full irradiation in summer.

Varieties of PVT

Here is the disclaimer.  You've been waiting for this, haven't you?

Is it true that a PVT collector can do everything? The answer: yes and no. Not every PVT collector can do everything equally well. There are different types:

  • If I want to achieve high temperatures (domestic hot water preparation), the collector should be insulated and thus lose little heat.
  • If the PVT collector is to work at minimum temperatures (ice storage/earth probe) and even collect as much ambient heat as possible, it must be only slightly insulated. So rather the opposite.

And if you want both? Well, then the compromise should be somewhere in the middle. Physics is not amenable to negotiation. Sorry.

The bad word stagnation

One type of construction has been commercially successful so far: The uncovered PVT flat-plate collector. In contrast to solar thermal flat-plate collectors, it is only insulated on the back and therefore reaches acceptable peak temperatures even when not in operation without fluid circulation, so that the PV part cannot be damaged. More thermal insulation (e.g. by additional glazing of the top side) would bring more heat yield, but would also increase the so-called "stagnation temperature" (i.e. the maximum standstill temperature).

Heat transfer medium/lamination/ insulation/PV cell/ glazing
Schematic sketch of an uncovered PVT collector [1]

Heat transfer medium/lamination/PV cell/
Schematic sketch of a covered PVT flat-plate collector with thermal insulation on the back and transparent glazing on the front [1].

Thermal connection

However, all PVT collectors have one thing in common: the thermal connection of the PV cell, which captures the sun's rays, to the heat transfer fluid should be ideal. This should be of the highest quality for cell cooling, for the conversion of environmental heat and for the solar thermal yield.

 

A distinction is made here:

  • Direct lamination: The PV cell is laminated directly onto a thermal absorber.
  • Retrofit PVT: A heat exchanger is attached to the back of a PV module.

The following applies to both: The thermal resistance (see above) should be minimal.

Conclusion

PVT collectors are a versatile, highly efficient tool in times of heat transition. But be careful when someone promises everything at once. Except us, of course.... 😉 Because we know what we're doing!

 

 

Quelle:

[1] Pröll, Markus. "Entwicklung eines schwach konzentrierenden CPC PVT Flachkollektors." (2019).

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