HYBRID PV-THERMAL PANELS– PV-T

Only 15-22% of the solar energy incident on a PV panel is converted into electrical energy. Almost all the rest is converted into heat, and the panel gets warm or even hot. And as it gets warm, electrical output drops. (We saw in my October article, 'PV Systems', that electrical output falls by about 0.5% for each 1°C rise in panel temperature.)

Why not capture the unused heat to warm up water for Domestic Hot Water, or possibly for UnderFloor Heating? Moreover, if heat is transported away, the PV cells are kept cooler, and thereby electrical output is increased a little. (A further incidentalbenefit of cooler PV cells is that the gradual degradation of their efficiency over the years is reduced.)

Hybrid solar collector have been available from the Continent for many years, andnow there are British-made systems, too.

Solimpeks hybrid collectors

The Turkish company, Solimpeks, produces two varieties of PV-T collectors:

• Volther PowerVolt, for which the emphasis is on producing PV power.The panel has an area of 1.4 m2 with a peak output of 190 W (electrical) and 460 W (thermal).

• Volther PowerTherm, With a similar area this has a peak output of 170 W (electrical) and 610 W (thermal).

A copper plate is incorporated into the underside of these PV-T panels, and the heat is transported away by a pumped fluid – as for a conventional solar thermal panel.

The panels can be installed in-roof or on-roof.

Anaf Solar PVT panels

These panels are made in Italy by Anaf, and sold in the UK by various companies. The H-NRG hybrid panel incorporates an aluminium sub-panel at the back for heating the fluid. Anaf make the modest claim that this hybrid panel generates doublethe energy of a conventional PV panel (and its 'Return On Investment' is better).

Anaf suggest two different ways of using the panels:

• Simple system, as for a conventional thermal panelPass the solar heated water through a heat exchanger coil in the base of the hot water cylinder. You can expect to obtain hot water at 56°C in summer, and warm water at 28° in winter. (If required, the water temperature can be raised further by an immersion heater, or whatever.)

• Incorporate a buffer tank and heat pumpThe energy output from the panel is maximised if it produces a large volume of water at 26°C – the ideal application for this would be to heat your swimming pool. But if you don't have a swimming pool, the output can be used to heat water in a buffer tank. A small heat pump can then be used to

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transfer heat at a higher temperature into the hot water cylinder and/or into anUnderFloor Heating system – a system for the techies, maybe.

Configuration for both Domestic Hot Water and space heating

Note: The buffer tank (1st stage buffer) and the dual output heat pump.

Anaf suggest that the system can provide thermal energy even during the night. (Don't forget that a heat pump can extract heat even from cold water.)

Solar Angel

This hybrid PV-T panel has been developed by Natural Technology Developments in County Durham. Their claim is that it delivers four times more usable energy than standard PV, and that it is competitively priced.

In addition, the company produce a PV panel that looks the same as the PV-T panel. You can have a mix of the two types of panel on your roof, all looking the same. (Viridian – see below – also offer matching PV and thermal panels.)

Solar Angel suggest several ways of utilising the heat from the PV-T panels:

• Heat the Domestic Hot Water cylinder. And they also suggest using PV electricity to power the immersion heater when the PV electricity is in surplus. (See 'Diversion devices' in last month's article.)

• Pre-warm water in a buffer tank. The pre-warmed water is fed into the hot water cylinder where it is fully heated by an Air Source Heat Pump, for example.

• If a Ground Source Heat Pump is present: When the PV-T panels generate a surfeit of warm water in the summertime, the warm water can be piped into the ground loop to recharge the ground, raising its temperature in readiness for the following winter.

PowerHybrid panels

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Minimise Generation produce hybrid panels which incorporate Panasonic HIT cells. These cells incorporate wafers of monocrystalline silicon, and so have high efficiencies. But what makes the Panasonic HIT cells special is that the wafers have abacking of thin film amorphous silicon, which boosts their efficiencies. (This combination of two types of silicon results in a hetero-junction – hetero means 'other'. HIT is said to stand for 'Heterostructure with Intrinsic Thin film', but I suggestyou park that nugget in the portion of your brain headed 'Useless information'. Nonetheless, Panasonic can claim exceptionally high efficiencies for their HIT PV panels.)

For the PowerHybrid panels, the module efficiency is 19% (and the cell efficiencyis 21.6%). The electrical output is 240 W(peak). This is less than the 250 Wp of many panels, but standard PV panels are wider – 1 metre, cf the 0.8 m of this hybrid panel. The thermal output is an impressive 675W(peak) – presumably under STC conditions.

PowerHybrid 240 panel

(Approximate size: 1.6 x 0.8 m. Number of cells: 72. Aluminium strip heat exchanger)

FIT and RHI

At present, PV-T systems are eligible only for payments under the Feed In Tariff scheme, and not under the Renewable Heat Initiative. (As we saw in my December article about Feed-In Tariffs for PV, the government has recently cut FIT payments drastically for new installations.) It may possibly be that in the future some PV-T systems could be eligible for both schemes – but see the next paragraph.

Currently, the RHI payment for solar thermal panels is 20p per kWh, for seven years. But since March, the whole RHI scheme has been under review, and the government has proposed to remove solar thermal from the RHI scheme altogether. Will the solar thermal industry manage to persuade the government to keep solar thermal in the scheme, albeit with a less generous tariff?

Is PV-T useful for a selfbuild?

The benefits of a PV-T panel seem obvious at first sight: both electrical and thermal energy is generated from the same panel. And moreover, it is said that because heat isbeing extracted from the panel, the PV cells stay cooler, thereby increasing electrical output. So why aren't they more widely used?

In most applications, solar thermal energy is required only to heat domestic hot water, not for UFH. Only two or three panels are required for DHW. Other panels

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will be plain PV panels. With two types of panel (PV and PV-T), the electrical system becomes more complex with separate strings for the two types. It is usually more sensible to use instead a couple of conventional thermal panels plus conventional PV panels.

The PV-T panel only remains cool (with increased electrical output) while the water flowing through it is cool. As the panel fulfils its purpose and heats up the hot water cylinder, the cooling benefits are lost. On a hot sunny day, the hot water cylinder will soon be fully charged with heat – and after that there will be no cooling of the PV-T panels.

However, PV-T can be useful in two scenarios:

• Limited roof space.A PV-T panel gives about the same electrical output as a conventional PV panel, and in addition it gives about 80% of the thermal output of a conventional thermal panel of similar area. If only a small roof space is available, you may wish to maximise the electrical and thermal output from the space available by installing PV-T panels.

• Ground source heat pump.The excess thermal energy that is collected in summertime by PV-T can be pumped along the GSHP ground loops to warm up the ground. The ground becomes 'super-charged' for the winter.

Viridian

Possibly with tongue-in-cheek, Viridian also offer a 'PV-T' system. In this case, the system is composed of one or more conventional thermal panels alongside conventional PV panels. The two types of matching Clearline panel integrate well to look good together on a roof.

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Viridian's Introduction to Solar Energy

In addition to the FIT payments for the PV panels, the thermal panel(s) are currently eligible for RHI payments. But for how much longer? (See 'FIT and RHI', above.)

Scepticism about hybrid panels

The founder of Viridian Solar, Stewart Elmes, is the Solar Blogger, and on his website he has an article entitled 'Hybrid PV-Thermal Solar Panels – a Good Idea?' He is sceptical of the claim that adding solar thermal technology results in a cooler PV panel (and thus higher PV efficiency). He says that for much of the time a PV-T panel will, in practice, be hotter than a PV panel.

The web article has a couple of images to demonstrate why.

Temperature of a thermal panel over a year

Temperature of a PV panel over the same year

The diagrams show panel temperature for each day throughout a particular year, starting with January 1 on the left, and ending with December 31 on the right. The temperature is shown along a straight line as it changes each day, from 7am (at the top) to 6-30 pm (at the bottom). The colours range from blue (for temperatures 0 - 10 °C) to rust (70 - 80 °C).

It can easily be seen that the thermal panel is generally hotter than the PV panel, so combining the two technologies is rarely going to result in cooler PV cells.

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There may be good reasons for using PV-T technology, but, in the context of conventional selfbuild, 'cooling' the PV cells by heating Domestic Hot water is not one of them.

Postscript: Don't believe everything your read, or see

On the webpage for their Hybrid PV-T range the Minimise Group publish the diagram below.

Did a nincompoop concocted this chart? Was it designed in ignorance, or with the intent to deceive?

There are no figures or scale on the diagram, but the length of the bars show that the electricity output of the PowerHybrid 240 is twice as large as that of Conventional PV.

Can that be possible?The efficiency of most PV panels is in the range 15% to 19%. So let's say that

'Conventional PV' has an efficiency of 15%, at the low end of the range. To get double its electricity output from unit area, a panel would have to have an efficiency of 30%. No PV panel gets anywhere near that efficiency. Indeed the company's own figure elsewhere for the electrical efficiency of the PowerHybrid is 19% – a good figure, but well short of the 30% that would be required for the above diagram to make sense.

The blue therm output is even more ludicrous. It invites comparison with the electricity output – though we don't know whether the 'electricity output' is a measureof power (kW) or energy (kWh). Anyway, the therm output is for energy (kWh). Visually, the chart implies that the thermal output of the PowerHybrid is a third of its electrical output. Which is nonsense. The company's own figures show that the thermal power output (under full irradiation) is nearly three times as great as the electrical power output.

Let's hope that the technical people in this company are more clued up than its marketing people.

FURTHER INFO

Solimpeks Turkish manufacturer of the Volther panels: PowerVolt and PowerTherm.www.solimpeks.com.

Anaf SolarThe first European PV-T producer.www.anafsolar.eu/eng.

Solar AngelThe first British PV-T producer.

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www.solarangel.com.

Minimise Generation (A division of the Minimise Group)PowerHybrid 240. Two other variants: PowerVolt 200, and PowerTherm 180.www.minimisegroup.com.ViridianMatching PV and solar thermal panels.Their website has an informative 'Guide to Solar Energy', covering both solar thermaland PV technologies.www.viridiansolar.co.uk.

Solar Bloggerwww.solarblogger.net.

NEWSFLASH – SAP2016

In November, BRE belatedly published the draft of a revised Standard Assessment Procedure (SAP2016), open for consultation until January 27. (See www.bre.co.uk/sap2016.)

A most welcome innovation is a web version of the complex Worksheet that formsthe basis of the consultation version of SAP2016. This free app is called cSAP, and isavailable from its own website: www.isap.org.uk.

Presumably, cSAP will disappear once the final version of SAP2016 is published. Whether BRE intend to publish an app for the final version of SAP2016 remains to be seen. So if you want to carry out your own informal SAP2016 calculations for your future house – or even for your present house – make use of cSAP while it is remains available.

Note that SAP2016 is an updated/improved version of SAP. Assuming the Conservative government has not given up completely on progressing towards zero carbon/energy for new housing, the updating of those parts of Part L of the Building Regulations that use SAP will come after SAP2016 has been finalised.

Words 2152.

© Copyright article by Robert Matthews in SelfBuild & Design magazine, February, 2017.

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