Flat solar thermal panels or evacuated tube?

Can you describe under what input conditions are you claiming your heat pumps to be 400 % efficient ?

With all the B.S. advert. hype and misinformation that only has any credibility due to it being repeated a lot by folks trying to sound like they know something but know squat about the technology, I kind of doubt that high of a C.O.P. is attainable on a yearly basis for most any heat pump water heater that's commonly available for residential use unless someone has managed to violate the 1st and 2d laws of thermodynamics.

As for what Captron seems to be talking about with respect to what reads to me like seasonal storage, if it's thermal seasonal storage, except maybe for super insulated, high thermal mass building applications, seasonal storage of sensible heat for DHW/process heat applications have been put to bed as both impractical and cost ineffective for any near mainstream residential application and most every commercial or large scale application I've seen, read or heard about. Some specialty applications are possible in places like Albuquerque, etc., but most folks reading this don't live in such places.

As for evacuated tube vs. thermal flat plate, while ET has the big advantage of non freezing (provided the piping is done correctly, and with a lot of considerations for piping design and insulation), the reality is many cold climates don't get enough winter or cold weather sun to justify the effort and cost of flat plate DHW much less the extra cost of evacuated tube units. So, with respect to the efficacy of an application, as referred to earlier in this thread, that makes ET units less efficacious in cold climates that are also cloudy (where a lot of people seem to live) than flat plate units which, without a lot of design requirements that make them mostly a PITA, are probably not practical for most folks in a practical, day/day residential application.

I am not heating water, I am heating or cooling the air inside my house. The COP is
certainly not just a fixed number, like I threw out to indicate compensation for low panel
efficiency. It is a multi dimensional graph which depends on things like inside and
outside temps, direction of energy flow, and percentage of system capacity. That graph
has not been released to me, but I believe a COP of 4 (400%) is actually reached or
exceeded in some favorable situations, like a low temp differential between sink and
source. The COP will be higher by about 1 for heating, because the energy used to run
the system is part of the output benefit, instead of fighting the process in cooling. Here
heating is the primary mission, air conditioning is a small secondary benefit.

If the seasonal storage is yet out of reach, then my net metering is still the only option.

And yes PV panels are way easier to deal with and effective where we see so much
weather around 0 deg F. If net metering goes away, it will take me a lot more PV panels
to get through the winter. The first time a mini split heat pump blows up, I will be looking
into the practicality of feeding panel power directly into the rectified DC section, to run it.

Bruce Roe

Have you run the numbers on just adding more PV and a heat pump water heater? When I compared this several years ago PV was SIGNIFICANTLY cheaper and that was when PV was ~3x more than it is now...

Depends on where you are and what manner of solar thermal is used to heat the water. Cost effectiveness in cloudy and cold climates will usually be a challenge pretty much regardless of alternate energy method. In warm and/or sunny climates, just about any method can be made cost effective or close to it.

Also, and as always, using less of a commodity to complete a task is the most effective way of reducing the cost of meeting a duty. That makes conservation the usual, no brainer first method of satisfying a demand.

The only solar thermal that's more cost effective than PV + HP is a $20 black barrel on the roof... and even that is only cheaper if the labor is cheap.

Not trying to separate fly crap from pepper but I've seen and built what are called breadbox water heaters that are more cost effective than PV + HP DHW systems. There were also manufactured integral storage designs that were quite workable for non freezing climates and relatively inexpensive. Most such designs are now confined to the DIY/redneck engineering crowd.

A well designed dealer/vendor installed flat plate residential solar thermal is less cost effective than a PV+ HP combination in the same duty at least partly because of vendor overpricing of the flat plate system.

That cost effectiveness for solar thermal increases for moderate climates where simple bulk storage can be combined with the heat producing surface, and systems become simpler as the freezing restraints remove a lot of the necessary complications that make DIY or at least less dealer involvement possible.

Part, but certainly not all of the bad rap that solar thermal gets is from complicated designs that are inappropriate to the simple and low temp. task of domestic residential water heating - like evacuated tube units which do indeed serve as a partial solution to the freezing problem but do so at a high cost and what's turned out to be a lower reliability than simpler designs.

However, I do agree that a well designed and sized PV/heat pump combination for DHW can be designed that is, in all likelihood, more cost effective as well and perhaps more importantly as being easier to maintain than a well designed solar thermal flat plate system, particularly for climates where the possibility of freezing is even a remote consideration, with freezing being a consideration for a good portion, if not most of the developed world.

Bottom lines:
It's an ongoing challenge to make any solar thermal system cost effective for a residential DHW application in freezing climates, and not easy even for moderate climates.
PV+HP designs have an are easier road to cost effectiveness than solar thermal for DHW applications.
At the current state of nat. gas prices, solar thermal or HP + PV are probably not as cost effective nor as easy to maintain as a simple nat. gas fired tank type water heater for DHW applications.

That's my point. I'm tired of thermal being held up as a cost effective option when for the vast majority of people it doesn't make any sense. I have a friend that spent $7k on a solar thermal system because she didn't do her homework. Most people don't. Even after spending $7k she only reduced her energy use for water heating by ~70% because sometimes there isn't enough sun to heat the water sufficiently and backup is resistive heat; THEN when there's plenty of sun and the water is hot enough the extra energy that could be harvested has no place to go. It's like adding an off-grid system to your house. It's a cute novelty but it doesn't make a lot of sense. A heat pump water heater would have reduced her annual energy use by just as much for 1/5th the cost.

Really if you're spending >$1k on thermal you're spending too much.

For what you call the vast majority of what I assume are residential users, and for reasons that have mostly but not entirely to do with misapplications of the technology being peddled to well meaning but solar ignorant folks, I'd suggest solar thermal never was cost effective. For some, yes. For most other residential applications probably not. Nothing new there.

Spending $7K to heat water for a residential application is somewhat analogous to buying 2X as much as full retail for a dualie pickup truck to haul groceries. Folks do need to be aware of what they're doing, what things cost and avoid dumb moves.

Part of my point was was that, contrary to your statement and implication that the only solar thermal that's cheaper than HP + PV is a black bucket and free labor, some solar thermal in some applications is, or can be, more cost effective than PV + HP, such as in tropical applications or where simple and appropriate technology can be employed. One of the other points I tried to make was that for many or most other residential DHW applications found in the developed world and in moderate to cold or cloudy climates, well established and historical ways of meeting residential DHW requirements applications using fossil fuels such as nat. gas are often and commonly more cost effective than using most forms of alternate energy. PV, solar thermal, wind or other forms.

Because HP + PV may be available and doable does not make it the best or most cost effective way to meet a duty. Saying so no better than saying solar thermal is the best way to heat water or that solar in all its forms can do everything and save the world.

Yes... the 'black barrel + cheap labor' Possibly slightly more sophisticated equipment so long as the total cost is <$1k.

I don't consider fools fuels like gas and propane viable options at any price; The point is to reduce our pathetic addiction with solar not lock it in...

Just to add another variable, my impression of HP water heaters that they use older technology
such as older air conditioners. I have already demonstrated that the recent technology used in
my HPs is on the order of twice that efficiency. Caring about that here, because the thought is
redirect some of my annual KWH surplus toward hot water, and first calculations indicate I
cannot get there by a simple resistive electric heater. Guess some C. O. P. research is needed.
Bruce Roe