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Solar PV-T systems - what are the pros and cons?

Posted by Alex Barrett on 10 March 2017 at 10:45 am

There are two main ways to collect energy from sunlight. Both have their advantages and both are increasingly being used by households and industry across the world. A hybrid or PV-T system combines the two approaches into the same panel.

Solar PV

Solar Photovoltaics (Solar PV) are possibly the most recognisable type of solar panels. They uses the energy from sunlight to generate electricity through the photovoltaic effect. More information on how solar PV panels work can be found in this blog. Solar PV is pretty much a ‘plug in and play’ option and requires very little maintenance.

Solar Thermal

Solar thermal panels are less hi-tech than solar PV. They use the heat of sunlight to warm up water, which can then be used inside the building. Solar thermal systems aren’t just useful in warm climates. Even a small increase in water temperature dramatically reduces the amount of energy that a boiler must use to get it to heat it for taps and appliances. The solar thermal panels give the heating systems a head start, meaning that it uses less energy overall.

Solar thermal systems consist of a panel of heat absorbing material, through which a mix of water and antifreeze is pumped to collect heat and use it to heat up incoming water. Solar thermal needs to integrate with your existing hot water system, so you may need to replace your old hot water cylinder with a larger one with several coils.

Solar PV-T pros

These two ways of collecting solar energy work in very different ways, but they can be combined to good effect. Some buildings will have multiple arrays on different parts of their roof, generating electricity with one system and heat with the other. Another option today is a hybrid or PV-T system which combines the two technologies into the same panel.

Heat and electricity are used for very different purposes. Electricity costs around four times as much as gas for heating so should be avoided in heating applications.

Solar PV-T systems have a couple of potential advantages over independent solar arrays. The first is of course space. Even the most efficient Solar PV cells don’t absorb all of the energy they receive, particularly not the infrared photons which causes heating [1, 2]. All of the roof surface receives the full spectrum both light and heat but if you have two separate systems then you are wasting potentially collected energy.

Energy that heats up a solar PV panel isn’t just wasted, it is actually a disadvantage. Solar PV output decreases by around half a percent for every degree of temperature rise above 25°C [3] on average. The exact figure relates to specific panel and some systems can be as bad as 0.8% drop.

A solar thermal array is designed to collect heat and channel it out of the panel, so serves as a cooling system for the solar PV system. It has been claimed that hybrid panels can have an efficiency as high as 85% and “generate four times the energy produced from the same surface area for only a 25% increase in cost.” [3]

Solar PV-T cons

Solar PV-T owner Kit Knowles sounds a cautionary note. Kit has refurbished his SuperHome for a carbon saving of 78%. A few of the technologies he has embraced, like solar PV-T, were installed when the product was new to market.

Kit now runs his own business Ecospheric, advising others on how to eco retrofit their home. He believes solar PV-T is well suited for heating a swimming pool and can suit a carefully planned new build or deep refurbishment where contributions to space heating is made possible.

However, he says that “it is not a plug in and play” option and setting it up with the right level of control as part of a retrofit project may be beyond the average installer. “Set-up and control is difficult and this system will only pre-heat your water, so you’ll still use your boiler to bring the water to temperature.”

Kit found that his PV-T system produces loads of surplus warm water in the peak summer season. This is to be expected as in most cases you’ll be giving more roof surface area over to the solar thermal component than would normally be the case. A home might normally only fit 4sqm of solar thermal on its own whereas up to 20sqm of PV-T might be recommended for the same property – so you are increasing the solar thermal provision beyond what can be easily stored.

But Kit says he faced “double the complexity” when it came to controlling his PV-T system in conjunction with his existing household heating controls. In fact, he has yet to get it running to his satisfaction.

The problem with a huge surplus of lukewarm water (only delivered at 35-40oC with PV-T) is where to store it. A very large storage vessel, like a swimming pool, allows you to make the most of high volumes of warm water, but if you can’t use or store the water and keep it circulating away from your PV-T panels, then it is unlikely to have the cooling effect required for the solar PV in the panel to run at optimal efficiency.

One of the disadvantages of this system is also the cost - they are quite expensive to install. And hybrid systems are also limited in what subsidies they can benefit from. They only qualify for the feed in tariff, not the renewable heat incentive [2]. If two systems had been installed separately then it is possible that the building would be able to benefit from both.

The future of Solar PV-T

Hopefully as this technology becomes more standardised the cost will come down. At the moment Solar PV-T comes in a flat plate approach, but systems using evacuated tubes are in development.

Solar PV-T works best if it is combined with batteries to store the electricity and a very large thermal storage system to preserve heat. You’ll need the right storage technology and controls in place and suitably sized system. However, if you plan to retrofit an older home and install solar panels on your roof, solar PV-T may introduce unwanted complexity.

Finally Kit Knowles suggests that anyone planning to generate electricity in their home should install an energy management system (eg. batteries) to really make the most of it.

To find out more about Kit Knowles’s solar PV-T, see Kit’s SuperHome online or visit one of his free open days.

References

  1. Minimise group
  2. The Green Age
  3. Renewable Energy Focus

Image credit: Cross sections of PV-T panels courtesy of Solimpeks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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