Hybrid PV and Thermal Solar Panels
bluesolar
| Posted in Green Products and Materials on
Hi all – I stumbled on this article about hybrid PV-thermal solar collectors. It’s a normal PV panel on top with a thermal collector underneath. Seems pretty slick, but I’m having trouble finding actual products. Have you seen these? Do you know of any manufacturers or vendors?
https://en.wikipedia.org/wiki/Photovoltaic_thermal_hybrid_solar_collector
In addition to providing both electricity and heat (presumably for hot water), they’re supposed to make the PV panels more efficient by keeping them cooler. PV cells are less efficient the warmer they get.
Thanks.
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Replies
There are a lot of interesting product ideas out there that haven't really been developed into a commercial product on a mass scale, unfortunately. I personally have always liked the idea of "solar shingles", but they haven't really caught on (probably due to cost). If solar could come down to a "why not install solar" price point, then there would be more of it out there.
Solar sells do drop in efficiency when they get hot, so there is some advantage to cooling them with a water loop. The downside is a much more complex overall installation, and probably not enough of an efficiency gain to make it worthwhile. You also run into the problem of thermal differentials: as the water in the system warms up, the thermal delta at the solar panel drops so you get both less efficiency gain for the solar cells and also less energy collection with the water loop. Physics is sneaky this way, almost always working against whatever you're trying to achieve.
What you're seeing in that link might be a concept more than a product. For it to be developed into a product, it needs to be both cost effective and have a market. I haven't seen any discussion of such a product and I'm not aware of any commercial vendors, either.
Bill
You will likely appreciate this article about a hybrid system by Albert Nunez: Integrating Solar Electric and Solar Thermal Panels.
BlueSolar,
Bill pretty much sums up the fail factor with such systems - complexity and cost to benefit ratio. Not far from me, there may still stand a lonely tracking solar plus hot water collector system developed by BrightLeaf Power. They went bankrupt four years ago. The system was very cutting edge with optically clever light concentration onto multi-junction photocells mounted on copper blocks that were cooled with water. Between the high efficiency cells and the bonus hot water feature, it looked like a winner. Just not in the US market. It might have been a good match for the island markets where fuel costs are high and the sun plentiful. For whatever reasons that market did not materialize. Perhaps survive-ability issues with a tracking system in hurricane territory. More likely maintenance and support costs in a salt corrosion environment would make using cheap solar panels more appealing to potential customers. For truly remote locations, simplicity is always the better way to go. These were not simple devices.
I was curious to try and estimate the value of the hot water feature in the Nunez article. My lack of engineering background may show, but here goes.
At least one flat plate solar hot water heater spec sheet is a bit squirrely about the actual BTUs collected. A suggested solar input of 2000 BTU/sq. ft./ per DAY translates pretty close to 1ooo watts per square meter if 6 hours of perfect input is used. This means a 40 sq. ft. panel under full sun might "see" 80,000 BTU potential when using the more common 4 hours of perfect solar input, but the chart seems to suggest actual collection of about 30,000 BTU for an inlet temp to exit temp rise of roughly 80F. I say suggest because the chart denotes the temperature differential information as the difference between inlet fluid temp and ambient air temp. Hopefully, the tubes and plate are considerably warmer than ambient air temp. The same chart also claims higher BTU harvest with lower delta T values. I am not quite sure how to interpret that notion.
If one accepts my interpretation of their chart, that would mean a collection rate of 750 BTUs/sq.ft. a day. Roughly 1.5 gallons of water raised 60F per sq. ft. of collector, so maybe 60 gallons per panel. The problem arises with physics and heat transfer just like Bill says. Much of the panel time will be spent at temperatures lower than needed to raise the working fluid above a useful temperature for effective heat transfer to the storage tank. If you have successfully raised the tank temperature to 110F by 2 PM, then the last 15F rise will be looking for at least 135F input from the collector just as the sun is going down during winter. Not a good plan. Heat pump water heaters face the same problem, but the working fluid temperature is designed to be sufficiently high enough that it can still raise the water temperature in a tank albeit with a lowering COP as the tank temp goes up.
The graphic for the referenced article is a bit amusing for the fact that the panel is only a 195w panel getting a presumed 7.5% boost by taking away 435w (1484 BTU)of heat. Typical panels sizes are about 3.25 x 5 ft. or 16.25 sq. ft. so I think the harvest value is 91 BTU sq.ft. per hour just like you measure electric output of the panel. (this is the tricky physics part) I infer from the 435 heat watts that a value of 290watts (990 BTU) of heat energy per square meter is being collected. If solar energy is 1000 watts per square meter tops, that makes the thermal collector an astonishing 29% efficient or higher if the solar cells on top are not re-radiating at least part of the hear away to the air. The poor panel on top is really cooking while only being 14% efficient. Maybe the experimental panel is really outdated.
Just the same collecting 290w/sq meter nets say 990 BTUs per hour at what cost. Natural gas is currently about $1 per 100,000 BTU and is conveniently available 24/7 at the same heat value. Any add-on to a solar panel will cost considerably more than $1 and add failure risk as well as installation complexity. Someone speculated the total cost of the system described was well north of $100K. I kinda suspect this tech will not catch on soon.
This is one of the many great ideas that have been undermined by the relentless decrease in PV costs. When they are so cheap, it's easier to just buy more of them than to try to make them more efficient.
Add to that the fact that for the solar panels, you want the exiting water temperature to be as low as possible, whereas for the DHW application, you want it quite hot. So there's a mismatch there. It could make sense in a scenario where you have a year-round application for mildly warm water, but it's still tough to compete with cheap PV.