Flat panels or evacuated tubes?

HeatingHelp

Flat panels or evacuated tubes?

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DigitalSolar

There are many options for solar including generating heat by way of Solar PV. Sadly PV energy is only about 17% efficient while solar thermal is closer to 80% efficient. If you could run a heat pump with a real COP of say 3 then it makes more sense – but you can’t. I say that tongue in cheek as solar PV can run a heat pump but due to the start-up load you many more PV panels to make this happen. A rough number is you would need 4X more PV panels as a minimum, and they would need to be be rated 3X more amperage than the HP requires to run.

When it comes to FP vs EvT thermal panels, many responders here are firmly stuck in the past. If it snows where you want to install a solar system then EvT definitely is your best choice. Once panels get below freezing, FP typically lose too much of the captured heat to be useful. An EvT system, either Heat Pipe or U-Tube, can work effectively at -30c. U-Tube is about 8.5% more efficient but you can’t use them in a Drainback systems, which we highly recommend. And yes, EvT panels outperform FP in marginal solar insolation – hand down. On a warm sunny day most decent FP/EvT work as well as each other.

The new EvT quality tubes, typically Borosilicate glass is super strong and resistant but it is still glass. These have zero points of ingress and are a prophylactic tube, meaning it is self-contained and sealed and simply slides on to the active components. They have an easy display at the bottom that changes colour or symmetry if the tube should lose its vacuum. So far, after tens of thousands of tubes installed, only 4 have lost vacuum for no apparent reason. Others were broken in transit or by errant lawnmower ammunition which happens to all glass products.

As for resiliency, a recent massive hailstorm had an unusual effect. We were called out to R&R a solar system to allow roofers to repair a roof and for us to inspect the EvT system. The complete panels were removed and no damage was found to the panels or tubes. Interestingly the roof below the panel was in great shape, while the rest of the roof was totalled and needed 100% re-shingling. I expect the tubes to last in excess of 30years once installed correctly. It takes about 5 minutes to change a tube once up there, and the cost of a replacement tube is often only $30 + labour. I have a side by side evaluation chart for both types of panels. I will see if I can include it here for your edification. If this were a -20c day, the differentiation would increase substantially.

As for price, a quality FP is only marginally less capital cost per SqM but equal when you de-rate them to match the EvT + labour.

As for Data logging: Our new DSH controller, will allow for full data logging as well as Carbon Offset calculations. We have also redesigned the interseasonal heat core storage system to work in Canada’s cold. When it works in Canada you can be sure it will work anywhere solar if effective. This should be ready in early 2018 as we gear up to launch in N America, then the Eu.

The one thing not mentioned which sits at the heart of our solar heating system is interseasonal storage. If you can’t save summers’ heat for use in winter, then a system. sized for use in winter will need to vent vast quantities hard won heat energy, or it will overheat. We store all summer heat in a patented heat storage system under the building, with one system recently in Australia gong from -15c winter to +45c (113f) average daytime temps for over 6 weeks and stored the lot for winter. This was an extreme test of the system.

If there is a way to post a graph of the FP vs EvT performance, let me know.

boilerpower888

Interested in your knowledge!

hot_rod

Comes a point when tubes and flat perform about the same, and a point where unglazed pool collectors out perform both.

I've found that snow sticks on the tubes more often than the FP, and enough snow load can actually damage the This bent racking was an array I spotted last year in Idaho. I guess structural strength isn't part of the certification testing.

One tube manufactures suggests you spray them with hot water when they get snowed in! Take that dollar amount out of the solar production number of course Fossil fueled DHW to melt collectors, really?

All in all over my 30 plus years of solar thermal I have seen and heard of a lot more tube failures than FP. Granted quality has a lot to do with longevity. The US market was flooded with sub-par import tube, many lost vacuum the first year. And many of those manufacturers are long gone. Not much on a FP that the average plumber can't repair.

Granted the tube have an edge in colder ambient, but the % may not be a great as you think if DHW or radiant temperatures are what you are chasing.

Cost to a customer for you to replace a tube, must be more than 30 bucks? I'd hope your service call alone is around 100. Name a brand or two of tubes that have been around for 30 years. US built AET, Sun Earth and Heliodyne have about that long of a track record.

4 tube failures out of tens of thousands? Hard to believe.

There were a few online data loggers running out there comparing side by side tube to plate arrays, one upstate NY installer has one logging for at least 8 years that I know. I'll see if I can still ping that logger.

hot_rod

Some data from Bob's site, near Ithaca, NY. solarishot.net

jumper

Efficiency at what temperature output and at what temperature ambient? With PV I can store heat at 212° with no pressure and at 250° with low pressure. There are electric motor systems nowadays that get around the starting current issue.

The only solar thermal competition to PV now is concentrated (mirror) schemes. For high temperature output the efficiency is around 50%.

For small installations today I recommend used PVs. Inexpensive is more important than nominal efficiency. PV is not as simple as it looks and solar thermal is more complicated than it looks.

hot_rod

jumper said:

Efficiency at what temperature output and at what temperature ambient? With PV I can store heat at 212° with no pressure and at 250° with low pressure. There are electric motor systems nowadays that get around the starting current issue.

The only solar thermal competition to PV now is concentrated (mirror) schemes. For high temperature output the efficiency is around 50%.

For small installations today I recommend used PVs. Inexpensive is more important than nominal efficiency. PV is not as simple as it looks and solar thermal is more complicated than it looks.

I agree, the fluid challenges with typical solar can be a drag. plain water drainback is the way to go, for thermal.

Then again this Sunbandit is fairly straight forward, PV- DHW. No leaks

No if we could only connect the modules to the load wirelessly!

jumper

Why not go all the way? Install a few kilowatts of photovoltaic and store the energy at high temperature? Eventually PUC s will start charging for capacity plus peak.