Thermostatic mixing valve recommendation - 180 degrees

Have you done this type of work in the past ?

Interesting first response. No, never have done this before but have been planning and researching for over two years. This has included getting professional advice from an HVAC engineering firm, published solar heating expert, and plumber. Now I'm getting down to the finer details and the mechanical work will start very soon. The new garage/barn with the heat storage mass under the slab is fully built.

Why do you ask?

Just curious.

I'd root around on the net. The big plumbing houses have them. Honeywell, Watts, Armstrong. 1 1/4" is sort of a bastard size in the industrial piping world. I'd not expect too much in the ability to fine tune the temp. to the degree you may be looking for however. Put in bypass lines for if/when they need service.

Also, Why so big ? Six decent size solar thermal panels may not need that much flow.

What's your background in solar thermal energy ?

To do any part of what you described it does seem much more than 6 panels would be required.

I like the concept.

The reason for the size is the massive heat storage sand bed. This is approx 110 tons of sand in a two foot deep insulated chamber under my 1400 sq ft barn/garage floor slab. I need a lot of BTUs to heat up that sand in the late summer, then try to maintain most of the heat through the winter. I read an article about this technique in a Fine Homebuilding article several years ago and was intrigued, especially knowing I'd be building my dream garage when I retired and need it warm and dry for vintage cars. I contacted the author, bought his books, and used him as a consultant. This whole system (except for heating the sand) also ties into my house boiler (gets complicated) just in case the roof/sand is't enough all winter, so I had the HVAC engineering firm that did my house system several years ago also consult. Also the new house boiler back then was sized to carry the future barn. So I'm quite sure I have the specs right, and happily both these guys are really psyched/interested in what I'm doing so are great partners in this endeavor when I need them.

My background in solar energy before I started on this fantasy is exactly zero! I do have 11 kw of PV on my roof already (nice to have a big barn) but that was bought/installed turnkey. I'm an engineer by education and an uber-handyman as a hobby and have done plenty of copper, PVC, pex and systems work in various houses over the decades to be very comfortable with it all, but certainly using experts to back me up. If anything this whole system will be over engineered and over built because I'm doing it myself with unlimited time no need to skimp on budget.

So this project is somewhere between really exciting and way overly ambitious!

I don't think its a good idea to install a mixing valve on the outlet of your panels. Installing the valve here would greatly lower your panels efficiency. Panel efficiency is highest at lower water temperatures. When you need the heat the most in winter you also won't have to worry about getting 180 degree water from your panels.
Instead install the valve on the outlet of your storage tank (even if it's only a 10 gallon tank)going to the sand bed.
My second thought is you are going to need a lot more panels than six.
The flat plates that I use only have 1" copper stubs and the manufacturer recommends 5 panels per pump. 1 1/4" is not required and will reduce your water velocity through your panels affecting efficiency.
The system that you are considering will reguire more extensive engineering.

Look back through my posts, I had posted a simulation for a radiant heating application you will get an idea of how many panels are required, and the total amount of BTU generated in the winter month's.

We went through a few iterations of panels a year ago and I just pulled out the final drawing. The exact spec is 8 panels that are 6'6" x 4' each. These have 1" stubs. I'll mount them in two groups of four in series, then the 1" pipe from each group will tie to 1 1/4" main supply/return lines. The pump will be variable speed so I'll have some adjustment for maximizing flow/efficiency. Certainly if I only need 1" pipe that would make me happy as it's easier and cheaper.

The sand bed has close to 2000' of 1/2" pex in 8 loops. I can give it more heat than you would a radiant floor. It's the optimal flow through this grid that everything else should be built to. The idea is to "charge" it up in Aug/Sep when lots of BTUs are available and then add whatever the panels will give it through the winter. Being a garage it's ok if it's 80 degrees in Oct, and down to 50 in April (it's a very well insulated and tight structure, except the unavoidable garage doors). The cars won't really care as long as it stays warm enough, and especially dry (it's a damp location usually). I also have supplemental Modine heaters on the walls fed from the house boiler if I need to or want to make it a little warmer.

With enough capacity for the sand bed I'll easily have enough to tap for DHW when needed. Also we heat the pool in the opposite seasons of heating the garage - so that's what the panels will do in the spring/summer. Nonetheless we've designed in a "shunt loop" with 300 feet of pex under the lawn to take excess heat out of the panels when nothing else wants it. You can see this gets complex.

I'll take a look at your posts. Naturally all input is welcome. Everyone's experience here can help my lack of experience a lot.

This leads to one question - what is the highest temp a panel might give you? What's the average? Am I being overly cautious to have a mixing valve at all if the max temp never really proposes a threat? My thought is that if I keep all temps and pressures in the same range of a typical boiler system then all the valves, pumps, etc., will be fine.

Thanks

JR

As an engineer with time and money, buy/acquire a copy of Duffie & Beckman - "Solar Engineering of Thermal Processes". You will not regret it.

Seasonal storage as you are considering has been around as a concept for a long time. I looked at it in some depth in the early 80's. One big consideration among others, was how to handle the heat loss.

High temperature storage is difficult - notice how solar thermal plants are so limited.

Low temperature storage is very, very difficult to make go. I considered it for my house then discarded the thought as too limited and too expensive.

The consensus back in the day seemed to be that unless the storage is VERY large (approaching municipal scale or larger), and the heat loss VERY well contained (with some, at that time, yet uninvented ways to accomplish the low heat loss necessary to accommodate/accomplish that low heat loss rate), seasonal heat storage remained a future hope. I kinda' doubt it, but perhaps things have changed.

Using a sand bed for storage is the key concept for my needs. Sand's flaw, in general, is that it absorbs heat slowly. But that also means it gives it back up slowly. It's also cheap. So the concept to charge it up slowly when BTUs are abundant makes it an advantage at that time. Also you don't want to contain the heat (except with insulated bottom/sides), you want it to bleed up and into the slab. A big disadvantage of this concept in a residential application is there is no way to regulate the temperature except to open a window. In a garage this doesn't matter much at all comfort-wise. So in my application the disadvantages work in my favor.

I'll admit that if I did some deep economic analysis this probably doesn't make sense. Might need 30+ years to pay back. At least I plan to live that much longer! For me this is a fun project that will keep my brain and body busy on stuff I like to learn and do. It's also very cool to talk about with visitors who have any interest at all, especially starting with the post and beam barn I designed and built (with help) already.

I'll buy the book. Thanks for the reference.

Can anyone share their actual experience with water temps on their hottest/sunniest summer days?

95+ degrees C

72 sq ft of flat plate panels at 45 degrees inclination facing SE the highest water temp recorded has been 160 degrees F
Jan. Temps run around 90 F degrees on sunny days.
I have run numerus simulations for ST space heating, just can't justify the cost and the BTU'S produced in the winter in the northeast US.
Check out the the SRCC web site, check the ratings there for your panels.
You can get away with running 70 F water in a concrete slab and still maintain comfort in a garage.
If you can locate a copy of TSOL you can run your own simulation with all of your criteria and get an accurate prediction of what your proposed system will produce.