Flat solar thermal panels or evacuated tube?

[bcroe]

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

[nwdiver]

such as in tropical applications or where simple and appropriate technology can be employed.

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...

[J.P.M.]

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.

[nwdiver]

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.

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.

[J.P.M.]

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.

[nwdiver]

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.

[J.P.M.]

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.

[nwdiver]

I have a question regarding the number of water collector panels on my house. I have six 7’ and one 9’ panels. I have one 119 gallon solar hot water tank. I have a two story house with full basement. I have five bathrooms. The solar company to installed my solar panels on my roof told me I will only need one 7’ panel on my roof to take care of my hot water. I believe this is wrong and I believe there should be four. I am traveling to Florida this weekend to pick up another 119 gallon tank. I am going to sell the collectors and hot water tank I do not use.

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...

[bcroe]

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

[J.P.M.]

I see this type of system as the only currently available technology that could compete
with my net metering system, with season to season storage. Yes my panels may be
only 20% efficient, but to compensate my heat pumps are 400% efficient. Bruce Roe

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.

[bcroe]

From my other post:
There are so many crazy claims and counter claims about ST that I find it hard to talk to people that already have their minds made up. Here is reality for 2020.

To be clear I am in the business of Solar Thermal and have been for almost 2 decades. We have a new system preferably for new buildings that uses interseasonal storage. We can store MWt in this area and its huge, so this is the best way to prevent overheating, hands down, and it extends the solar heating into dark times and winter.


Efficiency: An EvT collector typically exceeds 80c efficiency. A PV panel rarely exceeds 20% efficiency.

I see this type of system as the only currently available technology that could compete
with my net metering system, with season to season storage. Yes my panels may be
only 20% efficient, but to compensate my heat pumps are 400% efficient. Bruce Roe

[Mike90250]

We have an efficient house, off grid, and a single rooftop solar collector. About a 4'x4' panel with a 40 gal storage tank on top ( glycol in the panel and heat exchanger loop ).
With 2 adults, showers, dishes and washing machine - we have 130F water from that all summer long. That's preheat for our tankless. Summers, the tankless does not do much, Winter we feed it preheated water from a 80gal tank that is plumbed into our masonry heater. That preheat water is 80-90F and the tankless just loafs along.

So depending on your usage and timing, one panel might be enough

[paul.ketcham]

I have a question regarding the number of water collector panels on my house. I have six 7’ and one 9’ panels. I have one 119 gallon solar hot water tank. I have a two story house with full basement. I have five bathrooms. The solar company to installed my solar panels on my roof told me I will only need one 7’ panel on my roof to take care of my hot water. I believe this is wrong and I believe there should be four. I am traveling to Florida this weekend to pick up another 119 gallon tank. I am going to sell the collectors and hot water tank I do not use.

[Captron]

From my other post:
There are so many crazy claims and counter claims about ST that I find it hard to talk to people that already have their minds made up. Here is reality for 2020.

To be clear I am in the business of Solar Thermal and have been for almost 2 decades. We have a new system preferably for new buildings that uses interseasonal storage. We can store MWt in this area and its huge, so this is the best way to prevent overheating, hands down, and it extends the solar heating into dark times and winter.

Retrofitting in low caloric heating systems (ie radian heat) is possible. If you have an existing hydronic heating system, and enough room adjacent, the storage system can be installed there and integrated.

No moving on to solar collectors (not panels) for hot water that overheats. Yes you Can cover the collectors, but it inhibits their performance on poor days of insolation. So how do you keep them cool?
The easiest solution is to divert fluid when it reaches sub 100C let’s say 93C (200F) and run it through a cooler like a baseboard pipe with fins behind the header box on EvT collectors or anywhere behind flat plate, or run to a pool or some other heat dissipating location. Then you can walk away and not worry about overheating. There are several ways to do this, so I won’t go into them here.

Efficiency: An EvT collector typically exceeds 80c efficiency. A PV panel rarely exceeds 20% efficiency.

EvT vs Flat Plate: In cold areas EvT collectors are the option of choice, they even run fairly well even at -30c, so for heating and even hot water they rule. In locations where winters are rarely colder than say -6c (20f), there a new flat plate collectors that will do, if less costly. In really warm winter locations it doesn’t matter. Therefore it all depends on where you are as to which is best. If it freezes near you or you have lots of overcast conditions - you need EvT. See attached.

EvT issues: If your solar EvT tubes are older than 5 years old there can be design issues for some EVT units. There are 3 main types of EvT’s:
Basic Heat Pipe collectors, the typical heat pipe installation: Typically double wall glass with vacuum between and no vacuum in the center. Older collectors had issues with cold weather. The interior of the heat pipe is a narrow heat absorbing pipe typically copper, where a moderate high vacuum is pulled and a water fluid injected. The new fluids have a copper content to stop them freezing up and warping the pipe. They work even in very cold weather. Heat pipe EvT’s are great for Drainback systems as the header allows the Drainback. If you have old Heat Pipes, the good news is you can USUALLY replace them, fairly inexpensively.
High temp heat pipes: These operate as above but are sealed through the glass, usually single glass with vacuum inside. Not recommended for residential use.
U tube EvT’s: Less common but These are the most efficient, often as high as 90%, but lack any control of heating except by increasing heat transfer fluid flow. They don’t drainback due to the network of headers and pipes. Only the glass double envelope can slide off during service.

So there is a primer on these collectors.

[J.P.M.]

In our area, Toronto, an installed thermal system is around $8k CDN or $6.5k US and there are no rebates of any kind. Gas is cheap here so people don't have a natural desire to install anything except for PV and that is only for the last 5-6 years. The PV thermal systems I use have a DC-DC converter to run a 6kw element and usually has a similar performance to a thermal system for the same or lesser cost and of course there is no PITA maintanence issues. Also, the power can switch over to batteries or go to the grid so that is gravy on top. I think a heat pump with PV might be a better use of panels but the jury is out on that.

Understood. Thanx. A Heat pump for DHW in your area will likely be less cost effective than in a milder climate.