Friday 19 October 2012

Hybrid PV-Thermal Solar Panels - A Good Idea?

The following article is a summary of a more technical paper, which can be downloaded here:

Solar Thermal Panel Operating Temperature During a Calendar Year

Working for a manufacturer of both solar thermal and solar photovoltaic (PV) panels, I am often asked why we didn't combine both technologies into a single panel.  Then we wouldn't have had to spend so much time and effort making our PV and themal panels match so they look great together on the roof.  And what's more, (they continue), a hybrid PV-Thermal (PV-T) panel is better because the solar thermal removes heat and cools the PV cells to a lower temperature where they work more efficiently.

I'm going to share with you some of the work we did that informed our decision to keep solar thermal panels and solar pv panels separate. 
You see, the notion that the solar thermal part of a PV-T panel keeps the PV part of the panel cooler ignores the whole point of a solar heating system - for it to be useful it must increase the temperature of something.
The most common "something" to be increased in temperature is a tank of water for use washing and bathing in a residential building.  For sure this starts the day at a cool temperature, but if the 'T' in your PV-T panel is to be of any use, then it will finish the day at a warmer temperature.
The image at the top of the page shows the operating temperature of a solar thermal panel heating domestic hot water in three minute time slices for a whole year.

As each day starts (top of the image), the panel is working at low temperatures (blue/green colours), but for days with good light levels, the hot water cylinder heats up and the panel has to work at higher and higher temperatures (red/orange colours) to keep on adding heat to the hot water cylinder.  The higher temperatures reached as we move from January (left of the image) to the longer days of the summer (middle of the image) are also evident.

Also of great relevance is that the thing you are heating often has a maximum allowable temperature.  For example, when heating a hot water store it is common to stop at 60-65C to avoid scalding risks.  Once this happens, the solar thermal system is not circulating the "coolant" to the panel and the insulation that makes it thermally efficient means the panel gets very, very hot - temperatures around 220C are common.  Aside from the effect of these super-high temperatures reducing the efficiency of the PV cells in a hybrid panel, the effect of such temperature cycling of the PV cells and solder connections seems unlikely to be beneficial to the panel life expectancy .
By contrast, here's the temperature for a south facing PV panel for the same year of weather data using the same temperature colour map:

Temperature of a PV panel over the same calendar year of weather data

Like the thermal panel the PV panel starts cooler in the morning (top of the image) - at ambient temperature, then raises to the afternoon as more and more light falls upon it, before falling again as we progress to the evening (bottom of the image).

It's clear just comparing the two images visually that a solar thermal panel runs hotter than a PV panel, but here is a plot comparing the temperature of the two panels over the year:

Plot showing when a thermal panel would be cooling (blue/purple)
or heating (orange/red) a PV-T hybrid panel

Blue and purple pixels show times when the solar thermal panel is operating at a temperature below the PV panel, and would therefore be cooling a PV-T hybrid panel.  Red and orange pixels indicate that the thermal panel is operating at a temperature above the PV panel and would therefore be actually heating a hybrid panel and diminishing its electrical output compared to a stand-alone PV panel.
White pixels are where there is little difference.  Black areas are where the solar thermal system is not circulating, either because the panel is cooler than the hot water cylinder or because the hot water cylinder is at its maximum temperature (65C in this case).

Note that the red pixels are where the 'T' is heating the 'PV' to a temperature more than 10C higher than it would be in a standard PV panel.  This corresponds to a reduction of electrical power output of more than 5%.

It is possible to imagine a solar heating application which would keep the temperature lower for more of the day.  For example PV-T panels heating water in a swimming pool would operate at 30-40 degrees all the time. 
For the majority of applications though, the intuitively appealing idea that the sum of the whole is greater than its constituent parts turns out to be a mirage.  A heat-haze if you like.
This article is a summary of a more technical paper, which can be downloaded here: