Does thermal solar DHW make any economic sense for a 2-4 person home today?

Teemok

PV panels are cheap, efficient and there's heat pump water heating. I know thermal makes more sense the more daily hot water needs you have. I think the solar collection space is better given to PV. Obviously owner builders and hobbyists enthusiasts are a different case. When inherent risks, maintenance and cradle to grave carbon are accounted for isn't PV the better choice for the typical modern small dwelling?

jesmed1

Are you planning an install, or is this just a general question?

Hot_water_fan

https://www.greenbuildingadvisor.com/article/solar-thermal-is-really-really-dead

Yup it’s toast. There are some being installed still, but it’s a tiny market share.

Teemok

I might install one because I can, already own many of the parts and I don't call on techs.
I'm asked regularly by customers "should I install or big fix a thermal solar system." I'm just verifying that what I'm telling them is really good advice. That is, no... you should not, most of the time. Daily use pattern and already installed unpaid for valve are big factors.

I was thinking almost all the newer installs I see are con jobs with proud victims. Unless I'm missing something. I wanted to be sure that is truly the case. I'd never say that to some well meaning person but I'll note the contractor for my own info as a suspected wasteful con job. You have to be careful with that labeling.

Thanks for the article. That dating lines up with when I learned it and my current memory fade. Just wanted to re-verify.

Jamie Hall

Let's start with the basics. Solar thermal -- whether for domestic space heating or domestic hot water -- can, if well designed, capture over 90 percent of the incoming solar radiation. PV can, because of the basic physics involved, only capture at most 20 percent.

Ss there is a huge amount of power available from a given collector area which PV can't ever capture. Even the best heat pump technology can't overcome that difference.

Now. That said, the reason -- in my view -- why solar thermal has lost favour is that unhappily it is a little harder to do right and there aren't any government programs to pay for it. Further, each job is different and requires thought and skill. PV doesn't. Slap up the panels and hook up the wires and you're done.
Little training, little skill, lots of profit. Therefore the solar bandwagon at the moment is all PV.

OK. Like some other technologies, such as heat pumps, solar thermal works best in applications where the target collected heat is needed at relatively low temperatures. Thus the best possible use is passive solar space heating. Next up is active solar space heating, using radiant floors and either water or air as the heat transfer medium; if the collector panels are to be roof mounted (most common), it will usually be water (takes less space to move). These applications may involve independent storage, but they don't have to. Solar thermal domestic hot water (or high temperature space heating -- think baseboards or radiators) is the least efficient, but quite doable, and does require storage.

Hot_water_fan

@Jamie Hall IIRC solar thermal is eligible for incentives. Or was until very recently at least.

Teemok

A skill problem yes, I agree. That is easily overcome if the other numbers made sense. Site, system reliability, cost, load coverage, use patterns and maintenance are all factors. It is antithetical to aim of efficiency and reducing carbon to build a system that can produce lots of hot water when you only use a fraction of it and then it is condemned early. Capturing 90% of available heat is mostly fantasy. Under very specific conditions high efficiencies are possible. As you stated heating water from 120F to 200F is not wher it's at but that is often the role of solar with an all in one tank resistance back up design. Space heating is off topic. My question was about 2-4 person use pattern residential DHW. If the design, physics, materials, system life cycle were superior we would see that in practice. The cost of a sq.ft of water storage area and the container is becoming a bigger problem.

Jamie Hall

As you imply, the biggest single problem with solar power -- whether for space heating, hot water, or electricity -- is not collecting it, but storing it. For passive space heating -- which I agree is off topic -- the problem is trivial; the first all-passive solar house I participated in was built in 1965, so the technology is available and amply proven (if largely ignored). Domestic hot water requires that the storage maintain a minimum temperature of 140 F for safety, and while this is routinely done for indirect water heaters powered by boilers, that application does not require nor usually provide more than 24 hours of storage. Longer storage -- in New England I'd not be happy with less than 72 hours -- the major problem is adequate insulation, although the volumes aren't outrageous (240 gallons total storage should suffice for a two to four family use).

This problem is not unique, however, to domestic hot water, but applies to any stand-alone solar power installation, whether for space heating, domestic hot water, or electricity and, except for passive space heating, is a rather daunting obstacle. For electricity, the problem is mostly financial at the moment. Satisfactory long term storage for heat is, so far, a good deal more problematic.

You mention " produce lots of hot water when you only use a fraction of it and then it is condemned early". Forgive me for being blunt, but if your system is doing that your engineering is poor.

Teemok

Systems designed for families or multi family that served design loads for 5 years or so then with new owners or kids moving out, things change and 160 gallons of storage is very under utilized till something breaks and no one can be found to fix it. A kid plumber isolates it and it eventually gets removed. Payback never achieved. There is a tendency for solar designers to want all the fruit instead of just low hanging. They tend to over build as to never run out of hot water (perceived weakness) and much of the capacity is never used. Blunt is fine, I'm not engineering these things. I'm not saying there is never a good design use match up that lasts beyond payback. It's just not typical.

Jamie Hall

Oh I couldn't agree with you more!

jumper

PV can be less expensive for resistance heating water. No inverter.
Storage is cheaper than thermal because you can achieve higher temperatures. Good application for solar thermal is pool heating.

Jamie Hall

jumper said:

PV can be less expensive for resistance heating water. No inverter.
Storage is cheaper than thermal because you can achieve higher temperatures. Good application for solar thermal is pool heating.

PV is good for water heating in that sense... yes. Provided you have enough collector area (and either battery or thermal storage, unless you like cold showers at the end of three days of rain). Let's see here. Suppose you are looking at 80 gallons of hot water per day. OK, that's somewhere in the neighbourhood of 50,000 BTU, or 14 KWh. Let's further assume, quite reasonably, that you have an average of 3 hours of effective sunshine per day, so you need an array to produce 5 KW. This is not impossible, of course -- the array at Cedri's home will be a 15 KW array. But is it cost effective solely for water heating? No, sorry, it's not (if your array is large enough to be on the order of your average electrical usage over the year, and you have true net metering in your area, a PV array will just about break even over a 10 year period, by the way).

I will agree that a nice big solar array like that (a 5 KW array will be on the order of 250 square feet) is a nice talking point (though not, if it is roof mounted, a good selling point).

Solar heated swimming pools would be nice. There's only one family in my parish which can afford a swimming pool, period, never mind a heated one...

Larry Weingarten

Hi, Please read this:https://www.heatinghelp.com/systems-help-center/another-solar-myth-bites-the-dust/ from the HH Systems Help Center. It's a game changer.

Yours, Larry

hot_rod

The key to solar thermal is a good consistent load. Low temperatures ideally.
I've had ST on all of my homes, because I can do it very cheaply. Often times with take-off parts.

The age of the roof is always a question, the panels should last 25- 30 years or more.

It takes 3 things to make ST work for average homers. Incentives that cover 50% or more of the cost. High energy casts. An awareness driven by whoever is in the White House.

I've been through 3 ST "revolutions. The 70's, the 90's and mid 2000's. I'm not sure if I'll see another.

There is still a good market for ST in states like Hawaii and some commercial applications. I keep in touch with folks at the US brands Heliodyne, Sun Earth, and AET. They all stay busy some how.

I just picked up the glass for this 28' collector I' m building for my shop radiant. Maybe get it on the roof this weekend.

If you go ST I have some parts around. Controllers and pump stations. Find a 4x8 collector and a 50- 60 gallon tank. That should get you a 50% or more solar fraction in Colorado.

Teemok

High energy costs. Yup, that's a real mover. Ground up design mind set helps, spin and grants are temporary stimulants. I have a high mass radiant slab. When I built, I thought I'd have the sun heating a chunk of the load by now, 21 years later. Thanks for the parts offer but I have pile of stuff I've collected already. The space for the storage tanks is the hold up. My Polaris water heater was another contractors troublesome igniter eating reject, a free 3 year old transplant. I converted it to Nat. and crossed my fingers. It still works flawlessly. I mean ignition is smoother than any I've worked on. It's 24 years old and has had just a single igniter replacement, shakes head, shrugs and looks off to the horizon. When it does eventually go, I'd like to go air to water heat pump with thermal solar assist. Maybe twin Sanco2's with spoofed sensor relays. Maybe a custom made 4 ton Mitsubishi franken-air to water.

Teemok

@Larry Weingarten I really enjoyed that article. Thanks. Finding it is partly why I posted the question. I love to learn about exceptions, work a rounds and simplicity is always a favorite.

Teemok

Thermal collector product currently unavailable... Hmmm. New company owner. The un-glazed version is $391 per unit. I wonder if HDPE leaches anything at 170F? Did the glazing fail?
The DHW storage use percentage pattern flirts with legionnaires disease. It's a good growth environment sometimes.

I don't know where Zak got his 105gal. Rheem tank but a quick price search puts the tank well over the $3k mark. Maybe that's bogus internet pricing. My local supply house has the 85gal @$2,408 The 4 coils are more than $1,600 right there with no glazing. Pump, fittings, pipe, insulation and Goldline control puts this stories cost numbers in question. Real cost installed? Guessing $8-10 depending on labor cost, skill and would be profit.

What is Zaks daily use volume? Maybe there's 2 peoples worth for most of the year- 40 gallons of 120f a day available at the right times. The times it can't is resistance heat and maybe waiting for HW $.34/kwh guess avg. over 25yrs. Each 10% of the year that is done with resistance heating with no losses included is 6.36kwh/day x 36.5days x $.34= $79 x 25yrs=$1975 could be more + the $8-10 installed is $10-12K cost for consistent DHW for 2 people for 25yrs?

A sanco2 uses about 1kwh to make 15gals of 140F water that mixes down to 20gal of 120F the average daily DHW use per person. Average installed cost per watt for PV in CA is $2.73 1kw of PV that produces 2.2kwhs average daily in CA, enough for 2 peoples DHW. That's $2,730 in pv investment... no, say $5k for pv degradation and HP service. We're assuming a larger pv system is being installed to cover other loads. The Sanco2 80 gallon is maybe $10k installed and is capable of more production if needed. Say you have guests or kids. Under $15k investment for consistent HP+PV DHW for 2 people for 25yrs?

Resistance only heat daily 2 persons or 40 gallons use 55F to 120F rise is 21,684 btu/3,410 =6.36kwh x $.34 (variable rate, not going down)=$789 yearly or $19,728 for 25 yrs. Alternatively 3kw of PV fixed @ $8.2k + normal WH costs. 2@ $4k= $8k

25yrs is a long time for any tank, HP, glazing or tubing in the sun but I though I'd extend it all out to the full expected life of a PV panel for comparison. Napkin coffee talk calculations. I know it's not a reality model.

FilthyTeaSnob

Hi Teemok, Zak here and I was just catching up on this thread. I was preparing to write a long reply to all of your queries about usage volume performance of the system I have been using for the last decade but I thought I would ask some questions first to help narrow down what you are looking for.

You opened by asking for thoughts on the state of solar thermal with some arguments by folks on both sides and correctly moved on to pointing out that it really is a question of costs. The first task therefore is to ballpark your existing costs (of what you are using) and then ballpark what a PV or thermal system would run. I haven't met anyone (other than Larry and Gary Klein) who could ballpark the cost of their domestic hot water or their shower for year.

For reference, prior to my solar thermal system I was using a 20gallon GE resistance heater that used around 3,000-3,200 kWh per year. Back then the PG&E pricing ranged from $0.18 - $0.25 per kWh meaning that I used around $700 of electricity per year (2010-2014). I have my PG&E statements and pricing in a massive excel sheet and those numbers today would run more like $1,000 per year. The system cost me ~$4,500 and my time so you could say my payback was a bit less than 7 years. But I use 4x as much hot water because there are 2 other people and everyone takes longer showers because there is no penalty... do I count that into my ROI? Anyhow....

After you figure out your existing annual costs and have a rough number to work with, there is an even bigger question that really determines what you are going to build regardless.

"what are you actually after?"

You probably want:

-An energy efficient system
-An inexpensive system to buy upfront
-An inexpensive system to run annually (not the same thing as the above)
-A reliable system
-tax credit eligible system
-A system that provide adequate hot water

but you can't have all of those be #1 on the list. Much like the old manufacturing quote, you want it:

-cheap
-reliable
-quick

Pick 2 you can't have everything. So what do you want?

I have a neighbor who built a lovely 20kW PV array in his field to power his 5000sq ft house full of incandescent lights and propane appliances... He got a MASSIVE tax break. I'm not condemning him, he did build a system to largely offset his massive usage, but his goal was not to reduce usage or improve his resilience.

Upfront costs are usually the thing that people I have talked to about this are ultimately driven by, not lifecycle costs or annual costs. Even if this thermal system provided more hot water and costs less to run per year than a PV version, if the PV gets a $5000 in credits, that's what they go after.

Anyhow, just posing some thoughts. If you would like the details of my usage, in gallons, kWh and costs measurements I can give you the particulars and answer any other questions about how the system has worked over the years.

jumper

I have experience, albeit out of date, with solar thermal for off grid cottage. Owner spent plenty but ended up trucking in LPG and kerosene.
Now that PV is available; one can use electricity to recover heat. For example when washing dishes or showering when sun shines you can recover wastewater's thermal energy with heat pump.

Jamie Hall

Not that surprised at your experience. While solar thermal -- even full passive space heating -- really isn't all that hard to do right and have good results, it's astonishingly easy to do it wrong, and there are a lot of "experts" out there who will happily tell you how.

Teemok

I started with the question not because I personally want anything but knowledge. I'm asked about these things regularly as a hydronics contractor and I have a story I tell about where I think things are. I like to challenge my stories. You wisely present the real question. What is it that we want? The cheapest hot water? The most carbon neutral cradle to grave hot water? The most reliable. Production volume? I'm a sucker for good design. The tax credits are a hand on the scale no doubt. There's a big difference between what I would for my own needs and what I would recommend for my average customer. Being a good adviser is what I'm after.

FilthyTeaSnob

Makes perfect sense. So perhaps the best way for me to classify this solar thermal system that Larry and I built is to list in order of importance the key assets of what I required and what I expect and what I have (so far) received from this system

1. It MUST provide than my previous water system provided. I had a 20gallon tank; I would NOT accept less, only more... how much we can fight about later.
   
2. It MUST AT LEAST be the same level of maintenance over its lifespan as the original. It SHOULD be less maintenance but I accept that if the old system only required a checkup every 3 years... ok I will live with the same level of work... but NOT more.
   
3. It MUST consume less energy than the original system that it is replacing. How much we can fight about but I would like to think that 25% or greater is a starting point.
   
4. It MUST costs less to own over its full lifespan than the original. I am being very careful with my words. Lifetime. NOT first time costs or installation. If it costs $500 to put a crappy electric heater in and I spend $1000 a year powering it and I replace the anode every 3 years that means that its about $11,000 to have hot water each decade. If I can build a thermal system that costs me $6,000 right now and then costs $100 to run each year... ok I can do basic math $7,000 is less than $11,000.
   
5. The lifespan of the system should be far greater than the original system. Ties right into the above lifetime costs and maintenance. I don't care if the system costs $50 to install; If I have to replace it every 6 months, I will be over that crap in less than 3 years. I got other things to do like drink tea... or sleep.
   

Although not listed, I think it's worth mentioning that it is presumed by myself, that I am not building a system that is unhealthy. It was mentioned above that thermal systems can bring the water temps into microbe growth ranges. I felt confidant that this system would not have that issue but I have taken the time every 2 years to drain the bottom of the tank, where the coldest water and sediment build up and to have that sent to the lab for analysis. They have yet to test positive for any pathogens. Is that proof? No but it's continuous historical data for your consideration.

(look at that nasty sediment)

Teemok

I've lived in two passive homes in Maine winters. They excel in some situations and suffer in others but over all I would design, build and live in one if I had the option. We can build tight well designed and insulated homes, reducing the heating loads to small mini split levels. It's proven to be a reliable and efficient choice.
8.34 x gallons x delta T is the problem. How we do it has trade offs. Most people don't want to know about them at all. They just want the best solution for them. We all have ideas about the hierarchy of design priorities and cost is often chief. Solar thermal has challenges that make it hard to recommend in many cases. Zaks storage solution involves a Polybutene liner. I think this a renaming of the chlorine catastrophe Polybutylene or PB-1. Warranty is 10 years. Maybe it's better than stainless. PV has the set it and forget it factor. Zaks simple system leans in that direction but you can't use the excess heat to power your 120v ac refrigerator or stereo. While the tank is satisfied or the water is not used the more efficient thermal collector is just taking up roof space that could be spinning the meter backwards. It's hard to beat the economics, simplicity and flexibility of resistance heating with PV. As PV costs go down, maybe it becomes "better" to face three times the panels to the sun than go the heat pump route. Lifetime carbon accounting is a next level factor for determining better.

Jamie Hall

You mention lifetime carbon accounting. At some risk of derailing this thread, if one is truly interested in overall environmental quality and impact, one has to get beyond an excessive -- indeed narrow minded -- focus on carbon (or more accurately, carbon dioxide). At this particular moment in history, all the focus -- and all the noise both civilised and idiotic -- is focused on that one thing, but what of other environmental and social aspects? What are the tradeoffs? Where are the metals for your alternative energy projects coming from? How much damage does mining them do? How about the social aspects of the miners and refiners? What really is the impact of offshore wind turbines on whales and other sea life (hint: no one knows). How about onshore ones on birds. Are truly huge solar panel arrays really benign? Or are they? I could go on and on. Some scientists are trying to get a handle on these tradeoffs, but I'm sorry to say that they don't make headlines, unlike gluing one's self to a highway or destroying a priceless monument.

Teemok

Zak good work on the water testing. Heatgeek did a good write up about the real risks. Residential water heaters with good turn over are very low risk. Apparently LD is all around around us at low levels. I mentioned it because it's always brought up and Murphy's law and all. Open loop radiant gets smacked down for it a lot. I have known some daring or foolish souls who had systems the work great with daily pump exercising.
I have a few pieces of glass from old plate style panels. I wonder if putting twin coils under them would be too hot for the poly when stagnant vs the acrylic. Maybe one rectangular elongated spiral. I'd have to wind proof the glass of course. Is the plexi in contact with tubing?

Teemok

Jamie sounds like you know lifetime carbon is just one added level of info and not necessarily the most important. Whales, birds, the elements, rest assured, humans are doing it wrong. Correct beyond a certain population and consumption level is very mythical. The timing and areas of consequence and the degrees of severity will be debated till there's no point to it. Even if we had conclusive answers to what is best, comfort and profit are very large ships to turn around.

Jamie Hall

After some four decades of banging on the climate drum, @Teemok , I've come to one major conclusion. Each and every person and family has to do the best they can to live in as environmentally conscious way as they can. Now this doesn't mean a hair shirt,. Nor does it mean going out with a megaphone and making, bluntly, an **** of yourself (which only annoys people). Nor does it mean one size fits all government mandates set out by single focus bureaucrats.

It certainly doesn't mean lecturing others -- whether your neighbours or another state or another country -- on how they should do things. You don't and can't know what will work for them.

It does mean becoming as knowledgeable as possible about all the impacts -- good, bad, or neutral -- about what you are doing, whether environmental, social, or even aesthetic -- and acting on your knowledge. Hard work.

The really sad part is that mankind has the tools and technology to provide the energy for all mankind to thrive, and for all mankind to eat and eat well, and all without ruining the environment -- and has had, for at least 80 years now. But we refuse to use it.

Teemok

Agreed. The best we can witout being a jerk is all we have. I guess I'm a bit pessimistic about what "thrive" and "well" means for the long term. We exist at a singular time of explosive growth, consumption and change. A critical human flaw is the inability to really understand exponential growth. Doubling times go unnoticed till.....it's too late. The finite and growth are at odds. Limits exist but are not part of our self conception. It doesn't take much mental capacity to conceive that this type of existence might be limited somehow. It's hard to envision humans ever in balance with the Goldilocks balance of systems that have afforded us this far. I'm not a member of the we'll figure something out cult. Yup, real fun at parties. That said, I'm happy to help bail water out of the boat, attempt to consider the unborn and enjoy what seems to be the great privilege of now.

ethicalpaul

Which monument?

Jamie Hall

ethicalpaul said:

Which monument?

Brandenburg Gate. Never mind various works of art, statures, what have you. Sorry, but vandalism is vandalism; the end does not ever justify the means.

SlamDunk

My employer is pushing the envelope and figuring it out. Quietly.

This is the fifth net zero commercial building they have built. As far as we know, this one is the first of its kind.

Another, larger building is coming soon and I hear it will be carbon zero. I am thoroughly impressed with the engineering that I have seen. It all works. And, I am obsolete.

DHW comes from high efficiency electric water heaters (heat pump) and the building has a drive in cold room to store product at 5c(40f); it is @25000sqft, with a 35ft high ceilng.

https://youtu.be/6xw2qXUXT-Y?si=y9eEs9snwabXQh9m

https://wraltechwire.com/2023/06/14/united-therapeutics-opens-a-net-zero-energy-center-in-rtp/

Larry Weingarten

Hi @SlamDunk , That's impressive! You would only be obsolete if you weren't learning from this. Looks like you get to be cutting edge 😉

Yours, Larry

Jamie Hall

Elegant, @SlamDunk . Illustrates rather nicely that there really is no excuse for a new build not to be net zero -- whether it is a plant like yours, or a residence. Except, that is, for layers and layers of building codes and red tape and sheer inertia! The real problem is in the existing built infrastructure...

SlamDunk

I'm glad to see it before the end of my career @Larry Weingarten . Another 8 years or so, -if my wife lets me retire.
There arent many moving parts to these buildings. A few pumps and computer controlled power grid. Maintenance of the different pieces will be contracted out.

@Jamie Hall , Noone in my company involved with the design/ build complained about red tape. There were questions to be answered and concerns to be addressed but as far as I could tell, we had a lot of cooperation from all involved.

Around here, net zero residential new builds and "near net zero" remodelling of older homes are becoming popular. It does make the homes more expensive. I couldnt afford it!

eclecticmn

A little OT but some guy in Ukraine has some clever ST prototypes.
https://www.youtube.com/@sergiyyurko8668

Teemok

You would think if thermal was the move it would really make sense at that scale and budget. Geo is 24 hours a day. It looks like all PV. I've seen some insulated sub foundation cisterns combining Geo and thermal. Big money. Nice to see the build. Drugs leading the way.

WillyP

To me the big advantage of solar heat as opposed to PV powered electric is storage. A well designed system can store enough hot water to run a radiant floor heater all night.

ethicalpaul

Jamie Hall said:

ethicalpaul said:

Which monument?

Brandenburg Gate. Never mind various works of art, statures, what have you. Sorry, but vandalism is vandalism; the end does not ever justify the means.

I haven't seen it in person, but it still seems to be standing? Do you mean the destruction of the Berlin Wall?

https://www.visitberlin.de/en/brandenburg-gate

jumper

WillyP said:

To me the big advantage of solar heat as opposed to PV powered electric is storage. A well designed system can store enough hot water to run a radiant floor heater all night.

Forgive me for repeating:
PV can be less expensive for resistance heating water. No inverter.
Storage is cheaper than thermal because you can achieve higher temperatures. Good application for solar thermal is pool heating.

The higher temperature really kicks in in cold weather.