Solar Hot water... for heat...

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couchsachraga
couchsachraga Solar Expert Posts: 84 ✭✭
As promised in another thread, here is a little information on a system I've been working on for the past few months, thankfully with the assistance of some very capable professionals (who think this is a great project as it is "out there" a bit).

Completely off grid system (1kw of PV, 4 T1275's batteries, Midnite Classic 200, small SureSine inverter for a little AC), a 3 season structure I'm trying to push to 4 season use in upstate NY. Roof is well insulated (R32+), as are walls (well, sort of.. R32 as well mostly, but problems with infiltration I'm working on (the foam panels should have been sealed in and were not)).

In the past for occasional use during ski season (upstate NY remember... closer to Montreal than NYC... in the Adirondacks) I have a small (35k BTU) vented propane heater. Works, but 100 lbs of propane (23 gal or so) lasts about 6 days usually. Hauling the 100 lb ers in and out gets old...

I wound up with an extra dual coil 80 gallon tank from another solar thermal project, so I thought, why not set up a system here? Well, my wife didn't want the evacuated tubes (or PV) on the roof.. so I built a free standing structure for them. And since the tubes are below the tank, they sort of system we're used to here won't work (active system with a pump, when tank reaches full pressure the pump stops and the manifold (top of the system here) can "steam back" and push the glycol down (in to the expansion tank). When it cools it goes back up.... Apparently that won't work when the tank (and expansion tank so it won't freeze) is at the top as the pressure / heat of those cycles would degrade the glycol within a few years.

So, we have a passive system (thermosiphon), and the racks the tubes are on are at an angle. So far works VERY well (270 degree glycol up, 80 or 110 back down (or X, whatever it is while it heats the tank of water up).

So the lower coil heats the domestic hot water, or "heat bank" as I think of it as. The upper coil (dual coil tank) runs to 2 small baseboards (8 feet total), again a glycol loop to not worry about freezing. Setting the control units up was interesting - I had to set it up as a "cooling" zone as if the upper part of the TANK is above 70 I want it to be on and pumping (with a manual "off" switch for summer of course). Below 70 I do not want it to pump, as the rest of the place may still get below freezing and I don't want to worry about 80 gallons of water freezing (and splitting the tank...) when I'm not there in the event it is cold and cloudy (like right now... lows below zero F at night, windy, highs in the single digits... but no sun. Have to love upstate NY!).

So what happens when the water gets really, really hot? Well two safeties - one set to open a "dump valve" at the top of the (domestic) tank when it reaches 190 or so, and then of course the standard T n P valve which is set at 210 degrees. At the moment i have no worry of it getting that hot ("heating" loop running anyway), but in the summer I'll need to keep water pressure on the tank to replace any dumped water. My plan is to make shades for the panels (they are ground mount remember) for the summer as I'd prefer to not have water going up there when not needed (2nd floor of a structure, I drain all the water (except that in the 80 gal tank) when not there as a precaution. I have it set so in 15 min or less I"m drained).

It has been an interesting project, and I've learned a lot.

Right now I'm working on finally setting up an "opportunity load" for the PV - it is full charged most of the time, so when it is in that state the Midnite will power up a heating element in the tank (12v / 25 amp). My hope is to also wire it up as an "emergency" heat unit as well - if the bottom sensor in the tank gets down to 35 or so that even if there isn't sun out the controller will turn the solenoid on and heat things up a bit (even just 35 to 37, then off again).

Ask questions if you have them, and I'll do my best to answer them.

If you've done something similar I'd love to hear about it!!

Comments

  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
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    Re: Solar Hot water... for heat...
    ... a 3 season structure I'm trying to push to 4 season use in upstate NY. Roof is well insulated (R32+), as are walls (well, sort of.. R32 as well mostly, but problems with infiltration I'm working on (the foam panels should have been sealed in and were not)).

    There are always learning curves :)

    I built our current house by myself. Having no previous experience in home-building, there are a few things that I should have done differently. But life is a journey.


    ... In the past for occasional use during ski season (upstate NY remember... closer to Montreal than NYC... in the Adirondacks) I have a small (35k BTU) vented propane heater. Works, but 100 lbs of propane (23 gal or so) lasts about 6 days usually. Hauling the 100 lb ers in and out gets old...

    Besides propane is not cheap.

    I live in 'southern' Maine, a bit North of Bangor [45N]. Our home has radiant heated floors. The water circulates through a thermal-bank, and through the floor PEX loop. When I first built it, our primary heat source was a propane water-heater. It was our plan to later rig-up a woodstove to heat water. That first winter, using propane was very expensive as a heat source.

    Now years later, with the woodstove going to heat the water, we go through between 3 and 4 cords of firewood each year. Which is a lot more economic.

    Our plan is to add-on a solar-thermal array this summer, to take on the role of primary heat source. [At least during the shoulders]


    ... I wound up with an extra dual coil 80 gallon tank from another solar thermal project, so I thought, why not set up a system here? Well, my wife didn't want the evacuated tubes (or PV) on the roof.. so I built a free standing structure for them. And since the tubes are below the tank, they sort of system we're used to here won't work (active system with a pump, when tank reaches full pressure the pump stops and the manifold (top of the system here) can "steam back" and push the glycol down (in to the expansion tank). When it cools it goes back up.... Apparently that won't work when the tank (and expansion tank so it won't freeze) is at the top as the pressure / heat of those cycles would degrade the glycol within a few years.

    So, we have a passive system (thermosiphon), and the racks the tubes are on are at an angle. So far works VERY well (270 degree glycol up, 80 or 110 back down (or X, whatever it is while it heats the tank of water up).

    That sounds good. Most systems I see around here, design the system for natural convection flow. And also include pumps, in case the fluid does not want to convect.

    I served 20 years on FBM submarines. Our nuclear reactors do that too [designed for natural convection, and they also have feedpumps as a secondary method of flowage]


    ... So the lower coil heats the domestic hot water, or "heat bank" as I think of it as. The upper coil (dual coil tank) runs to 2 small baseboards (8 feet total), again a glycol loop to not worry about freezing. Setting the control units up was interesting - I had to set it up as a "cooling" zone as if the upper part of the TANK is above 70 I want it to be on and pumping (with a manual "off" switch for summer of course). Below 70 I do not want it to pump, as the rest of the place may still get below freezing and I don't want to worry about 80 gallons of water freezing (and splitting the tank...) when I'm not there in the event it is cold and cloudy (like right now... lows below zero F at night, windy, highs in the single digits... but no sun. Have to love upstate NY!).

    So what happens when the water gets really, really hot? Well two safeties - one set to open a "dump valve" at the top of the (domestic) tank when it reaches 190 or so, and then of course the standard T n P valve which is set at 210 degrees. At the moment i have no worry of it getting that hot ("heating" loop running anyway), but in the summer I'll need to keep water pressure on the tank to replace any dumped water. My plan is to make shades for the panels (they are ground mount remember) for the summer as I'd prefer to not have water going up there when not needed (2nd floor of a structure, I drain all the water (except that in the 80 gal tank) when not there as a precaution. I have it set so in 15 min or less I"m drained).

    It should be an easy task of covering each panel with tarp. Such is my plan.



    Every year at a local fair [called 'Common Ground', it focuses on organics, off-grid sustainable lifestyles and draws 30,000 each day], they normally have a dozen vendors of solar-thermal panels.

    I have asked repeatedly for any UL estimates of BTU ratings for the panels they sell. Last year I was finally able to get an estimate from one manufacturer that said around 30k BTU from a 3' by 6' panel.

    I have read many articles which seem to agree that for any given size of panel, the output of heat is really about the same. That the individual manufacturer, or construction methods tend to have only a very small variation in actual heat output.

    Given this, it is my plan to make 10 panels, each panel using a triple-pane sliding glass door and a wooden coffin with 2" of foam on back and sides.

    Then splice this manifold into my existing thermal-bank.



    I want to avoid every turning our propane back on again; and to cut our firewood consumption in half.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Solar Hot water... for heat...

    Infiltration of outside/inside air is a big issue with my wife.

    She likes it. :D

    To her, our home is very stuffy feeling when everything is closed up (80 year old home, but new windows and doors, stripped sheet rock to insulate walls/ceilings/floors with fiberglass bats, etc.).

    And when I look at heat recovery ventilators--Their energy usage (running a fan 12-24 hours per day) is significant.

    Not sure what the solution is other than to allow some more heat loss to keep home ventilated.

    Fortinatually, we live in a moderate climate--So we are probably talking about $10 of dollars per month energy waste--Not $100s or more.

    -Bill "happy wife, happy home B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
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    Re: Solar Hot water... for heat...

    Your high glycol temps is a sign of poor flow of your system. Not very suprising as most thermosiphon setups suffer from this. Much greater btu collection could happen with better flow. If you need btus is the question. Easiest fix would be a pv driven solar pump, with thermosiphon as backup. They are other ways to increase flow rate but somewhat more involved to implement.
    In thermosiphon system:
    Increase flow force comes from 2 factors. Delta t and height differential. Minimizing delta t(to maximize efficiency ) while keeping max flow force involves greater height differential between tank and solar panel. Flow force and height differential are directly proportional. Double height = double flow force at any set differential t.

    Another way to increase flow is reduce flow resistance. Which is usually accomplished by parallel flow collectors and larger diameter piping to and from storage tank. Also better hx with respect to flow characteristics.

    In your case i would assume your coil in your tank maybe your biggest flow restriction. This is why adding pv driven pump by say a 20w panel may help you maximize btu collection. Do you need more btus?
    I am a big fan of thermosiphon. But careful design and implementation is needed to maximize btu. Although many are happy wjth less optimized systems that just work, and dont worry to much about efficiency
    Matthew
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
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    Re: Solar Hot water... for heat...

    Last note. Evacuated tube systems do much better with low flow and higher temps.
    For those interested look at:http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm
    There is a nice graph off efficiency of collectors at different delta t.
    One can judge how much more energy they can expect to collect at lower.delta t.

    Based on thiz graph dropping glycol temps from 270f to say 150f may increase effiency from say roughly 30 percent to something like 45 percent. A 50% increase in btus.

    Matthew
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
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    Re: Solar Hot water... for heat...

    On note about triple glazed doors i would recommend against it unless you have them for free. Double pane will do better. It will let more light in your collector. It will also let more heat escape but overall greater results for double pane. Cheaper and lighter as well.
    Triple pane is nice on a house as it works 24/7 to keep heat in or out. But in solar thermal collector it is only working several hours a day.

    The only way i would say triple pane may be beneficial is if panels are mounted vertical against a wall of the house. This would increase the r value of the wall and again you could benefit. 24/7 from increase r value. Mounted on house vertically also allows insulation of the panel to also increase r value of the wall.
    Makes adding insulation to an outside wall a bit easier.

    Matthew. sorry for any typos written from phone.
  • couchsachraga
    couchsachraga Solar Expert Posts: 84 ✭✭
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    Re: Solar Hot water... for heat...

    Matthew,

    That is an interesting graph you link to, but if I understand it correctly it is looking at efficiency looking at the Delta T of the collector temp vs. ambient air temp - not hot temp. vs. return temp.

    I've read many, many statements that "passive solar thermal is less efficient than those with a pump", and I wholeheartedly agree that is very likely that case - but I've yet to find anyone who put numbers to that statement. If you know of a study or site that has (with data to back it up) I'd be very interested to see it!! I'm sure it's out there somewhere....

    We may ad a 12v pump, as long as we're certain it isn't restricting flow further... IF we need it. The question I'm trying to answer first is does what we have work well enough (there is a value to keeping things simple, even if it is less efficient). And that is a question I don't have an answer to yet!

    Thank you all very much for your thoughts - it has been interesting, and I've learned a lot (and I'm sure I'll continue to do so!).

    We have pretty good height differential I think - 20' or so over 30' run (one-way).(this is after the evacuated tubes, so add in another 16' or so there, 20" drop over that distance IIRC).
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
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    Re: Solar Hot water... for heat...

    That graph is of ambient vs collector temps. If you have 270f glycol your collector is atleast that hot. Ambient temps are what 20 to 30f. That is a delta t of 240f. That is not even on the charts but you could extend the lines to get there. Also the cold return will keep a part of the collector somewhat cooler.

    All that said i definately accept lower performance in exchange for simplicity .

    Your points are very valid. And you have a healthy height differential.

    If someone is considering a similar system vaccum tube collectors are of great benifit, as can be seen in the graph. They can operate at higher delta t with not a terrible lose in efficiency. system with a pump i would probably go flat plate collector but that is just personal choice.

    Also anyone interested in solar hot water heating should probably check out builditsolar.com
    Matthew
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
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    Re: Solar Hot water... for heat...
    I've read many, many statements that "passive solar thermal is less efficient than those with a pump", and I wholeheartedly agree that is very likely that case - but I've yet to find anyone who put numbers to that statement. If you know of a study or site that has (with data to back it up) I'd be very interested to see it!! I'm sure it's out there somewhere....

    You can't compare them with hard numbers, any more than you can compare apples and oranges with hard numbers. If you know the power draw of the pump, and for the thermosiphon, know its elevation, its pipe resistance, and the energy input to the system, then you can say which is more efficient.

    The solar energy input is an important parameter... there is a threshold below which a thermosiphon doesn't work and has zero efficiency. On the other hand, a pump system can have negative efficiency if the energy to run the pump is more than the solar energy gain.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • catamount
    catamount Registered Users Posts: 12
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    Re: Solar Hot water... for heat...
    Ask questions if you have them, and I'll do my best to answer them.

    I'm interested in the dump valve and the expansion tank. Can you tell more about them or maybe put a picture?
    There are always learning curves :)...

    How's it going FBK? I'd know that avatar anywhere ;)

    - sarco2000
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
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    Re: Solar Hot water... for heat...
    catamount wrote: »
    ... How's it going FBK? I'd know that avatar anywhere ;)

    - sarco2000

    I am doing okay here. Glad to see you made it over to this site. :)
  • couchsachraga
    couchsachraga Solar Expert Posts: 84 ✭✭
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    Re: Solar Hot water... for heat...

    Right now I have two expansion tanks - one for the solar thermal (evacuated tube) glycol loop, the other for the heating (again glycol) loop inside. What I need to add is one for the hot water tank - much of the time I'll likely have the rest of the place drained (in case it does get too cold in there), and just water in the hot water tank. Right now as temp / pressure increase it will blow off via the Temperature and Pressure valve on the top (standard hot water tank type).

    The dump valve is set up for summer time when it has domestic water going to it all the time - when it gets to hot at the outlet pipe (say, 190F), the valve opens up and the water pressure pushes all that hot water out... replaced by nice icy cold ground water. Valve closes, cycle repeats. I think of this an emergency option though, and my plan is to cover said tubes when I'm not there in the summer so I'm not constantly wasting water.

    Animatt has me thinking pumps, I have to admit. I may toss a 12v El Sid in there (doesn't consume much power). It might be easiest to put it outside (along with it's own panel and run it direct), but I'll likely put it inside (on the "return" side heading out to the tubes to keep it cooler) and run it with a normal controller (I'd prefer not to be pumping cold glycol through my hot tank!).

    Whether I do this during this winter (which is certainly hanging on) or next fall... we'll see.

    Thanks for the thoughts / comparisons! It's been quite a project. I've learned a lot about relays as well...
    Control panel has 110AC to 24 AC to 3 Johnson Crontrol units (reads the temp in the tank, can be set to turn on / off at temps as needed)
    1) When top of tank is 70+ turns the heat loop on (cooling the tank... turns off below 70 so in theory even with several days of no sun and cold the tank won't freeze and burst...)
    2) Reads domestic outlet temp to control dump valve
    3) (almost hooked up...) reads bottom of tank... set to trip relay / solenoid to turn electric heating element on at 35 (again, to keep it from freezing)

    Then a 12v panel I just finished that the Midnite Classic controls, Aux 1 used to control a diversion load of the same electric element. I have another relay, and another solenoid that at some point I may set up as a low voltage disconnect. Or I might not. Still pondering that one and watching my use.

    The biggest problem so far is cold - it still gets colder inside than I'd like, and the midnite classic temp-corrects it's charging. Which is great and all, except at cold enough temps it charges at 15.6 volts... and the SureSine inverter trips off at 15.5...and when it does that means the whole control panel goes down (and stops the heating...which of course would help warm the batteries!).