Simple Solar DHW

VeraCruz

So I am pretty commited to off grid solar, no lines or pipes come into this place, kind of remote. So I built a simple DHW system, 4 each 4x8 flat plate collectors, a 250 gallon tank with submerged heat exchangers. ( similar to the one in Build it Solar) And the topmost panel tops out at 18' above the tank level. I went with a TACO 009. But too many 90s involved I reckon, since I only get a trickle return. And I think I already trashed one cartridge by over working it as the impeller came off. I need to up my pump. Should I order another 009 and series them, or is there a better alternative? Also can one bolt Tacos flange to flange for series? Thanks folks.

hot_rod

Sounds like you have a drainback system?

In a closed loop system the pump doesn't lift the fluid those 18" the fill pressure does, the circuit just moves the fluid.

In a drainback the pump needs to actually lift to that highest point UNTIL the siphon is established, then it just circulates.

What size tubing did you use to build all this? got a drawing of how you built it?

Good info here on how the various systems pipe and pump.


https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_3_0.pdf

Larry Weingarten

Hi @VeraCruz , Can you post a diagram of what you have? Maybe photos? Answers to Bob's questions would be very useful. Also, what climate are you in? Freeze protection will be different in Florida than in Maine I'm in an off-grid solar powered house as well, so might have done some similar thinking.

Yours, Larry

VeraCruz

Thank you Bob, yes this is a drain back, pure water in the Southern mountains of New Mexico. Yes we have lots of freezing weather and such. We also have some pretty good sun, and I have had a system like this, in principle, working ( inside a passive solar greenhouse which does not freeze) for 10 years. So when I built the new house, ( ongoing) the simplicity appealed. Understand all, we are kind of " off the reservation" out here. I at least do not have any of the city things like electric/gas water heaters. And the inspector does not call me if I do not call the inspector. That said I get it about the value of code, and good practice. I respect accumulated wisdom,
Physics does work out here.
I did a quick sketch to answer your question. Against my spec ( while I was working out of state) the Pex to/from the collectors wound up being 3/4" But the manafolds in the 4 collectors are 1" and the verticals are 1/2" That's all copper.

VeraCruz

Yes Larry, I just went out and took a south side pic. The flat plates are to the right. Everything else about this build is underground. Yes we freeze about 6 months of the year.

VeraCruz

If interested, these sites are my basis. http://www.solarhotwater-systems.com/advantages-drainback-solar-hot-water-systems/ and especially https://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Larry Weingarten

Hi, I like what I see Do you have any vents or vacuum breakers on the solar piping? I didn't see any. A thought to reduce effort for the pump is to put a vacuum breaker just downstream of the pump. When the pump runs, it will close the vacuum valve and allow gravity to help you as Bob described. This assumes there is no vent or vacuum breaker at the top of the collectors. When the pump shuts down, air will enter and let both sides of the collector and piping drain back to your tank. You may want a valve in the return from the collectors to be able to slow down that water as it might want to fall faster than the pump can handle. This way, you only need a pump capable of producing enough head to get water to the top and whatever gpm you want for the collectors once gravity kicks in and falling water helps with flow. Hope that all makes some sense!

Yours, Larry

hot_rod

How much 3/4 Pex? You could have a bit of pressure drop in the Piping. Collectors reverse return?
The common digital controls like Resol have a variable speed function. Pumps runs at 100% for a few minutes until siphon is established, then varies based on delta T
I have never used vacuum breakers, if all the piping is indeed sloped air will rise up and break the siphon when the circ shuts down

What altitude.? At high altitudes and high fluid temperatures it can be tough to maintain a siphon, 18’ should be okay. I prefer pressurized drainbacks, the pressure helps suppress the potential to flash when the fluid first hits a hot hot absorber plate

VeraCruz

Larry, that kind of makes sense about the vacuum breaker. No there is no vent at the top. I had wondered, but read somewhere that this would somehow break the siphon back to the tank, I guess it is pretty easy to confuse me. I am fair at electricity, suck at water flow dynamics but I can build a rocket ship out of the parts of a tree. I do not get the valve in the return suggestion, seems that would only be 6/8 gallons from the 4 panels.

VeraCruz

Hot rod, yes we are at about 7500 feet. Clear air and 2 cases of Covid 19 in our county. Which is the size of some states.
Yes all the piping is sloped about 1/4" per foot. This statement mystifies me..." I prefer pressurized drainbacks, the pressure helps suppress the potential to flash when the fluid first hits a hot hot absorber plate"

Larry Weingarten

Hi @VeraCruz , I think the Idronics book that Bob linked to shows pressurized drainback. Efficient collectors can reach 400 degrees in sun with no flow. When flow starts, it's going to get interesting quickly! If the system is under pressure, the amount of steam produced will be far less. Often sensors can be placed to prevent the pump from coming on if the collectors are too hot.
My comment about putting a throttling valve on the return is because I've seen water falling so fast that it pulls in air and gurgles steadily, which people don't appreciate. A question; is the return pipe from the collectors above the water line in the tank or is it submerged? I'm looking to see where air will come from to allow the system to drain down.

Yours, Larry

hot_rod

Water boils at 198F at your altitude, adding pressure raises the boiling point. When the water hits that copper, often over 250F, the pressure prevents steam flash. The hammering sound that drain backs often make😯

VeraCruz

Yes, I reckon all that is true, in a pressurized system. I do not have one of those. I should say except for atmospheric pressure. The return is open air. Now you are making me question physics. And gravity and ****. ( I hate that) Even though I have a system like this working for a decade in another application. The differential is set at 140F. Unless there is some return stoppage who cares what temp a dry copper pipe sees? In an open system, 60 degree water brings a 2-3-4 hundred degree dry collector to 65 degrees in a nano. Yes I have steam blown solar DHW stuff up in the past, I know how fun that can be.

VeraCruz

I mean it is really this simple. I was just questioning about adding more pumping. But does this design not work?

hot_rod

Maybe that loop over the top from tank to tank gets air locked? Seems every time it drains down there is an air bubble at that high point?

From your numbers and drawing you have plenty of pump as far as lift head, but it is not an ideal pump in that application..

I always considered buying a hot water rated sump pump and throwing it right into the HX tank🤭 In those underground tank systems. Sump pumps are designed to lift from a tank or pit, circulators, not really.

A circulator pump really needs to have a few psi pressure at all times called NPSH to work properly. You have a basic open, unpressurized system, high head circulators don’t work well in that application.

I’m guessing you want 4-6 gpm, with 4 collectors, so your 3/4 down size should be adequate, you want 2 FPS velocity so water coming down can push the air in front of it, or else you won’t fill the array completely.

Summertime you should be able to go beyond 140F, can the tank take 150- 160F?


https://www.libertypumps.com/Product/HT40-Series

VeraCruz

Hot rod, I think you are on to something. The 009 lost its prime several times, I think that is what killed the impeller, replacement cartridge may do the same. The sump pump idea has crossed my mind, and now that you suggest it also, it would solve lots of the issues at one time. And the pump you link should do the trick. I am indebted to you. Thanks.

VeraCruz

Hot Rod, yes the tank could handle increased temperatures, I just don't need them and 140 seemed a safe level. I need tpo do some redesign of the tank to accommodate the submersible and allow service access. But your idea gives me a path to a solution.

Zman

I recently noticed this in the "00" manual. More pump will only make the problem worse.

I suspect your problem has more to do with tubing size than pump size. Do the panels have output ratings? The piping size should be designed based on those ratings.

Also keep in mind that pex has a smaller ID than copper. It also has a faily low melting point which makes it less than ideal for solar.

hot_rod

Zman said:

I recently noticed this in the "00" manual. More pump will only make the problem worse.

I suspect your problem has more to do with tubing size than pump size. Do the panels have output ratings? The piping size should be designed based on those ratings.

Also keep in mind that pex has a smaller ID than copper. It also has a faily low melting point which makes it less than ideal for solar.

He mentioned 4X8 collectors, 1" header, 1/2 risers. Typically you flow a 4X8 around 1 gpm, so 4 or so gpm totally. The length of the 3/4 run of pex would be a place to look also for excessive pressure drop, but 4 gpm should be workable.

It's more of a temperature AND pressure relationship to prevent flashing and cavitation, and of course the altitude when looking at vapor pressure.

Grundfos looks more at the temperature side of the equation, it seems.

PPI has a nice calculator at their site.

I spitballed 120' of 3/4 and some fittings, looks like this.

http://www.plasticpipecalculator.com/PressureDropHeadLoss.aspx

Zman

It is unfortunate that wet rotor circ manufacturers do not publish NPSH data.
Given the low suction side pressure, using generously sized, straight pipe on the suction side would be advisable.

joseph annon

I have installed a high temperature (180*F was the temperature rating) the sump pump in an open drain back system. I had to do a significant bypass as the smallest pump I could get was many more GPM than 4 ea 4X10 panels needed. I ended up installing a restriction valve on the return also to help flow rate down in the collectors. The system was piped in 1" copper to 1" headers on the collectors. Elevation change was about 12' to top of collectors from top of water tank.

It has been several years since I did this so I don't remember what the model of the pump was. First pump ran for about 5 years,