Solar storage tank – necessary?
I’m getting educated on solar space heating for my house in Montana. One of the articles i’ve read recently in a Mother Earth News magazine says:
“Closed-loop and drainback systems should always have a separate storage tank for solar-heated water as well as a backup water heater. The same tank can’t do both jobs efficiently. Typically, the output of the storage tank runs to the input of the backup heater. When solar output is sufficient, the backup heater doesn’t come on.”
I don’t understand why i need two tanks for storage since they are plumbed in series. On the face of it, it seems that more standby losses would occur with two tanks given the greater surface area over volume compared to one (heatable) tank.
Since the article didn’t explain why “The same tank can’t do both jobs efficiently”, i’ll guess this is a simple principle that i’m just not understanding. Would someone explain it to me, please?
Replies
I am not sure of the context of your quote but it sounds like they are talking about domestic hot water in addition to the space heating. You want to be able to heat water for domestic use when the solars are not putting out.
This is the schematic that went along with that part of the article. I know a back-up electric or gas water heater is needed, i just don't see why an aux tank is also required, per the article. It seems like the hot water from the exchanger could dump right into the water heater, "cutting out the middle man".
Typically the back up tank has an electric element in it or a side arm boiler that keeps the water hot when the usage has over matched the solar input. the solar tank is allowed to get as cold as the incoming water from the well (or the radiant return flow if you have that and your differential temperature controller senses that the outflow from the radiant is cooler than the stored water in the solar tank)
It's harder to heat water that is already hot so it's good to allow your solar tank to drop to a cool temp to optimize the heat collection from your panels.
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"You cannot work hard enough to make up for a sloppy estimate."
<<so it's good to allow your solar tank to drop to a cool temp to optimize the heat collection from your panels>>Ah...that makes sense. Thanks.
Not only that, but your solar storage tank (well-insulated and large) is like your battery bank with solar PV. You need someplace to store the energy when it's not being consumed. If you have passive solar heating (south windows), for example, the house may not need much if any additional heat when the sun is shining, but it would sure be nice to be able to use the daytime solar heat later that night. For this, sufficient diurnal storage is required.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Nice to see you posting again, Riversong.I can understand the storage tank/battery principle, but that makes more sense to me with an on-demand water heater/boiler as the source for bumping up the temp to usable range. When using an off-the-shelf tank water heater for that source, an appealingly inexpensive idea, i had a notion that stand-by losses might overrun the gain with yet more storage battery to maintain. I can figure the heat from stand-by losses stays in the house, but it wouldn't be very useful down in my basement utility room.Clearly i have a lot more reading to do.
http://www.builditsolar.com/
This site will tell you what you need to know.
He's not far from you.
Joe H
That fellow at BuildItSolar, Gary Reysa, wrote a couple of the articles in the Mother Earth News magazine i picked up just recently, a sort of primer on DIY solar. He lives near Bozeman, very comparable climate, so his numbers were exciting. I'm accepting i'm not selling any time in the near future, so i thought i'd also take a whack at recovering at least pre-heating warmth from the sun. That guy makes it seem a lot less fangled and expensive than the solar store i visited to start collecting information.
He's pretty easy to understand, nothing he builds is too much for an average DIY type to handle.
He's into to cheap too. Cheap is good.
Joe H
I'm putting up a new shop this spring. I got all the materials for framing it for $400 from a truss company nearby me that i originally hit up for scrap firewood. I saw these trusses set aside and turning gray...struck a deal for those and a couple bunks of grayed 2x6s for $400. It's an ideal chance to site it and experiment with DIY solar. Notchman might have me talked into a chicken ranch if i didn't have to pay to heat them in winter. <G>
Sounds like a perfect place to do his "Solar shed" - there's a link there to the whole thing.
Any free siding in sight?
Joe H
In the magazine i picked up, he wrote about a solar shed that heats water in a buried tank, then sends it to his house's RFH. Also another article on using a couple layers of black window screen to make a collector plate of sorts for an air-heating scheme in his shop. I'm still gathering info to see whether retrofitting RFH makes sense in my house, but the air heating scheme is very simple to accomplish. I'll spend some time on his website tonight.No free siding yet, but i still have 19 pieces of Hardieplank left from doing the house; 'tis a start. There's a recycled house parts outfit in Missoula where a person can get all the lovely windows, doors, etc. dirt cheap. They get so many commercial lighting fixtures dropped off that they have a "free" sign on that shed. Same with paint. They have shelves and shelves full of Simpson fasteners...it's the Christmas store! <G>
If there is enough composting "litter" (heh heh) in the henhouse, they will stay warm enough. You need chickens made for cold, like feathered feet and small combs type.
colleen..... first i assume it's a drain-back system....
so what is the size of your storage...... for dhw we used to use a 100 gal plastic tank
with a heat exchanger in it
the back up was our regular 40 gal
we always used Gold Line controls .. which back then used to Independent Energy
Drain-back seems unsuited for my situation. From what i've read, that kind of system assumes electric controls and we are on rural electricity here that goes out occasionally and always during the worst weather. I'm considering something similar to the schematic from Mother Earth News attached to Post #5, though i'd opt for the anti-freeze fluid for floors and collector both. That would avoid calamity and since not all rooms need to be heated all the time, i could "zone" at the manifold just by turning the loop off w/o worrying about that floor freezing. I have an Aquastar for potable water; the water for that could be preheated via an exchanger.The article i read recommended 20 s.f. of collector and 20 gal. storage for each person. That didn't seem a useful equation in my situation of 3000 s.f. that doesn't all have to be heated all the time, and one person. I put in a Seisco space-heating water heater for a friend a year and a half ago, sort of experimental in these parts, but i'd read good stuff on BT and on the Net. It's working amazingly well for her: she was paying $300-400/mo in propane before with a badly designed system. I got design help, replumbed it all, and her elec bill for the whole house, including heat, last month was $130. However, the Seisco was $1300 compared to about $500 for an 80-gallon electric hot water tank, and the tankless requires a LOT of room in an electrical panel while a tank heater . My preliminary thought is to boost the 80 gal tank with solar. The tank elements could be shut off entirely in the warm months - no standby losses. In the heating season, it seems like all the heat would be pulled off the solar collector all the time, so a large storage capacity wouldn't be needed anyway. I'd have to bypass the collectors when it got too cold, of course.Feels like i'm in first grade again...
all that is true... but drain-back "drains back" if you have no electricity... so it doesn't matter , as long as the system is designed for fail-safeMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
I'll keep reading. One of the other positive notes about a closed-loop anti-freeze system that i read was that it uses a smaller pump (could possibly use a PV-powered DC pump that only circulates when the sun shines) bec the pump is only circulating fluid as opposed to a high-head pump required for a drain-back system. I'm just parroting stuff now. <G>
While it is true that with a 2nd tank you will have standby losses in it.But if you look at any of the discussion that we have had in the last year or so about tankless vs tank WH you will find that the modern tanks have fairly low standby losses.Specially the electric ones.If you wanted to get fancy you have some valve to bypass the electric tanks during the summer. But that you probably require flushing at the start of the "winter" season.Or you could get fancier and a recir pump and monitor the temp in the WH and the storage tank and when the WH temp drop and if the solar tank is warmr to circulate some from the solar tank through the WH..
William the Geezer, the sequel to Billy the Kid - Shoe
I want to keep my tankless as my DHW heater and i want antifreeze in my floors, so i won't need to do any flushing for sanitation purposes, if that's what you meant. If i pull heat from the solar panels to preheat the water for the tankless, i'll do it with an exchanger.I could run the tankless at a higher temp while the electric tank heater puts along in a lower range, making the elements last longer...or such is the state of my education tonight, lol. <<when the WH temp drop and if the solar tank is warmr to circulate some from the solar tank through the WH>>In all the examples i've seen, the water heater and aux. storage tank are plumbed in series, so mixing seems continual under use; that's why i was wondering why not use one BA water heater to both store solar heat and generate heat via electricity. The consensus seems to be that no one HW tank is big enough to store enough BTUs that the solar panels would produce...depending on how many s.f of solar panels i install, of course.
"I want to keep my tankless as my DHW heater and i want antifreeze in my floors, so i won't need to do any flushing for sanitation purposes, if that's what you meant."This seems to be for a different function.You where talking about the solar storage tank and an supplemental electric tank for domestic HW. And you mentioned in the summer that did not think that you would need the supplemental heat. But with your setup you would still need to run the electric WH to count for standby losses.My suggestion was to use valving to simply bypass that tank (and turn the power off). But then you have a tank of watter just setting there all summer. So you want to flush it before putting it back on line.
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William the Geezer, the sequel to Billy the Kid - Shoe
<<You where talking about the solar storage tank and an supplemental electric tank for domestic HW.>>I might not have been clear enough. My intention was that the two systems, space heating and DHW, would be separate; treated water in the floor and plain water in the tankless. The only connection i was considering is pulling pre-heated water via an exchanger from the solar-heated antifreeze-filled system for the tankless. (My Aquastar is actually propane-powered, which made sense at the time i installed it, but electric prices are much lower here now compared to propane.)One reason for separate systems is for prevention of freezing, but the other is that our water is corrosive; copper-piped systems make green stains in the fixtures. The supply company i dealt with when i installed baseboard radiator/boiler system in another house suggested i put in a conditioner because of our highly alkaline water. I figured i'd have to do this for a RFH heating system, too, but i don't think that needs to be flushed then.
The 20 sf/20 gal spec is for domestic hot water. You're talking DHW & space heating, which is an entirely different beast.
You have to get some solar availability data for your area (avg. BTU/sf-day), figure the BTU heat load of the house, and calculate how many SF of solar collector at what efficiency at what angle and azimuth will be sufficient.
Then you have to have a very large insulated storage tank to hold the daytime BTU output of the system so it's available for 24 hours (8.34 BTU/gal per °F).
Then you need a back-up heater to boost both the DHW and the hydronic heat to appropriate temperatures.
You cannot "zone" a radiant heating system by shutting off individual loops at the manifold - you need a separate manifold for each zone, with all loops within a zone balanced (equal length). By closing off a single loop, you increase the velocity in the remaining loops and won't have sufficient dwell time for heat transfer and temperature drop of at least 20°.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
<<The 20 sf/20 gal spec is for domestic hot water.>>Thanks for the clarification. The article discussed both space and DHW heating, so it wasn't clear that that metric applied only to DHW, but that makes sense. I've already used Slant-Fin's heat-loss spread sheet to calculate the heat loss for my house. Latitude is 46° 30′28″and HDD is 3494-7523, depending on selected base temp from 50-65º. I frankly haven't a clue at this point how to juggle those variables with any skill, however.<<Then you have to have a very large insulated storage tank to hold the daytime BTU output of the system so it's available for 24 hours>>This is the principle i'm stuck on, not understanding how i can pull BTUs 24 hours a day when any system i could afford to install will most likely not keep up with the SH needs of a conventional house in MT, but need to be supplemented with fossil fuels, at least from Nov.-March. <<By closing off a single loop, you increase the velocity in the remaining loops...>>I read a bit last year on variable-speed pumps that can respond to this, sometimes resulting in substantially lowered energy requirements if used properly - big "caveat emptor" there. At that point, i was considering using Euro-style panel heaters (with a high-temp heater like the Seisco, not a tank water heater) that are closed down at the panel itself, though they can use a bypass. It didn't make sense to me to circulate water if one isn't actively pulling heat from it. I'm just smart enough to know a little knowledge is dangerous, but none is even worse!
The storage tank that is just for storage is a big heat sink. You want the biggest heat sink you can afford because the sun doen't shine all the time. So a big tank without a heater shouldn't set you back to much. You want volume.
Then you want a tank sized about like you would for domestic hot water if you didn't have solar. This tank will make sure you always have hot water.
One size never fits all. Multi-purpose things do several things poorly. Tese are your starting points.
The solar system does not, generally, heat water to 'water heater' temperatures. Even a few degrees of heating provides some heat you can 'harvest.'
If you heated the solar tank to water-heater temperatures, much of the time the solar panels would function as radiators, cooling the water that passed through. Plus, a big tank would need a big heater. Do you really want a, say, 500 gallon water heater?
Plus, solar tanks are often not insulated, instead transferring their heat to stones, or other 'thermal mass.' This heat is recovered when air is passed through those stones.
However, the solar heated water can be used to 'pre heat' water for the water heater. The simplest way is what you call 'piping in series.' This method ... water is removed from the solar tank, and transferred to the water heater ... is simplest.
A more efficient, more versatile method is to keep the two water supplies separate, using a heat exchanger to pre-heat the water for the water heater.
<<much of the time the solar panels would function as radiators, cooling the water that passed through>>Ah, there's the flaw in an argument for a straight shot from the collector to the floor. OK, i can't heat the whole outdoors with my electric hot water heater... <G>
So i put in an exchanger and a little DC-powered pump run by a small PV panel that just moves the anti-freeze through the collector to an exchanger when the sun shines. Another pump would push the antifreeze through the other side of the exchanger through the floor tubing, bumped up by a tank heater, similar to the schematic i posted with the tank in series. Unless i have a surplus of BTUs to store in the storage tank until harvested by the water heater, the storage tanks is still superfluous. Maybe i'm way underestimating how much a [afordable] solar collector would add to my BTU needs.Thanks for the input. I'll keep thinking/reading until i become more fluent in the principles.
the base 50 ... base 65 i assume refers to indoor air temp
so you can calculate delta -T .. i've always used 70 and in our coastal climate i use 10 for exterior design temp
so our delta-T is ( 70 - 10 = 60 )
yours might be ( 70 - ( -10 ) = 80) don't know... but the weather data link that rond budgell gave is a good place to figure your climate data
and .... you may have a micro climate ( like we do for coastal RI as opposed to inland RI ( hah, hah, hah,... 20 miles ! but what a difference )
you also have to estimate winter wind temperature ... and look at your edge loss.. ( door crack, window crack = LF )
then look at all of the composites of your structure
foundation
floor ( band joist too )
gross wall - ( windows & doors ) = net walls
ceiling / attic / cathedral ceilings
peculiar situations .....like fireplaces
Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
<<the base 50 ... base 65 i assume refers to indoor air temp>>I pulled the information from here: http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?mt7894It wasn't clear to me why the base 50 figure was used. I could understand 65 as a comfortable, conservative temp to aim for for air temp indoors. I didn't understand the other bases, except perhaps as a measure of what might be ECONOMICALLY achievable with solar, as a help to figure how much auxiliary fossil or elec heat would be needed on top of the solar. If that's not it, i'm just reachin'...I did a pretty detailed heat loss calculation using Slant Fin's room-by-room program. (http://www.slantfin.com/heat-loss-software.html) I'm constantly upgrading the insulation, weather-stripping, storm windows, etc. as i continue working on the house. It's very obviously taking less to be comfortable this year than it did last year. I assumed last year i would just have to buy a boiler to heat the house adequately for resell purposes, but i'm hopeful now that i can do it with an off-the-shelf water heater and put the saved money towards solar. (After the cost of propane doubled from last year to this, that is no longer one of the options!)
<<the base 50 ... base 65 i assume refers to indoor air temp>>I finally found why 65 is commonly used for both HDD and CDD. I know that most people would keep it warmmer when cooling.What I found is that it is based on if you have a closed house then people and activites will keep the house warm and if the temps are above that you need to run AC. Of course if you are cheap like me you just open the windows..
William the Geezer, the sequel to Billy the Kid - Shoe
I can live without A/C, but i have another hare-brained idear for running well water through a truck radiator with a fan blowing or pulling air through it. Yet another experiment, LOL...
One way to approach this is to go for the easier and most straightforward (and probably most cost effective) phase one: Solar DHW. It's a year-round system bringing benefits 12 months, it is easier to engineer, and then you can decide if you want to pursue an "active" solar space heating system or invest instead in the upgrades to the building that should be done 1st anyway. I have found over the years that folks want to "Go Solar" when in reality they can reduce the btu heat loss cheaper than they can produce it with solar. And, the bonus is a much more comfortable house. Do you have the best air-sealing, insulating and ventilation systems? Do you heat with wood? Are your windows the best you can afford? Do you have a good solar exposure for your windows on the south side? Have you minimized the north-facing glass? All these are steps to take BEFORE laying out some serious cash for panels, plumbing, pumps and controls.Then if you feel that the cost of space heating is still worth working on, then do a good heat-loss calculation and model the efficiency of various sized arrays to determine the best system size, storage volume, roof mounts, and pumping requirements.By the way, the antifreeze in the floor is usually a bad idea and unnecessary expense. Use water and have a good air-eliminator in the loop.These are just my opinions =-)
i arrived at the same conclusions as flathead
i do have some observations about solar in general
1.... you can run glycol , water .. or some other liquid thru your collectors
but the easiest is water
and it doesn't take much to treat the water in your system so it won't eat the collector... as long as it's a closed system ( besides make-up water ) you can get the ph just where you want it
drain back to storage is the easiest and most efficient with the heat exchanger immersed in the storage
pumping heads are not a big deal even with drain back systems
2....... hard to beat copper collectors
3.... for glazing use only low iron glass.. single glazed.... don't use polycarbonates.. don't use fiberglass ( like Kalwall )...... don't use double glazing
glazing is a big cost...... but if you use low iron single glazed , it will probably last forever... as long as it isn't hit by foreign objects
the other systems will not stand the test of time.. they will have to be reglazed before the collectors wear outMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Advice noted and i will attend to it. I haven't read as positive of comments on drain back systems in my climate, but i'll make that a feature of local investigation.What kind of heat exchanger do you use? I've come across coil and flat-plate kinds so far that seem to be the primary ones use in fluids, but googling the topic shows that's the very tip of the heat-exchanger iceberg.It seems that evacuated tubes are kinda expensive risky for this climate, though i liked that they weren't subject to wind lift like the flat collectors are. Did you solder your own collectors? I'll check the low-Fe glass - i hadn't heard of that yet.Well! I've got quite a page of notes to follow up. Thanks!
yes... when we were doing hw collectors ...we soldered our own from soft k and 16 oz plate
most of our heating collectors were 4 x 14... but they used double Kalwall
today.... if i were doing hw heat i'd have an array of 4x10's.... say 10 x 40....or 10 x 36
but really.... if i were back into solar heat , i'd be doing hot air with 4x10's
only of course...what i'd really do is (2) 4x10 hw for dhw and superinsulate the house so i could heat it with the heat from 2 passionate people...
my dad always said you should be able to heat it with 20 couples dancing... but i like my solution betterMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
I like the part about two passionate people, and i think it would work out especially well with a fresh one every winter. If you cut 'em loose in April, there aren't any standby losses. ;^)I've filled my ceiling joists with cells. I want to put in a floor, though, so no more depth. I was thinking of dense filling the rafters as well, making a buffered-temp room up there, cooler than the house, but warmer than outside, OK for things that don't need a narrow temp range, but insulating the top floor even better - or so i imagine. I'm thinking the attic area would be perfect for a couple hundred jars of canned goods. Want to take a guess on where the dewpoint would end up? I can amuse myself in DMV lines thinking that over.
Good questions and input; thanks. I've done much to stop losses with blowing cells in walls (dense) and ceiling (loose), another layer of foam insulation against thermal bridging, replace single-pane double hungs with thermal panes and storms, etc. The reason i'm mostly interested in space-heating is that's far and away my major expense, far more than DHW. In the summer using my Aquastar, i barely make a dent in my 500 gal. propane tank. When i was using a small, direct-vent, propane heater (30K BTU, 85% efficient) for just a portion of my house warmth, i could easily spend a couple hundred bucks a month in winter and not be that comfy unless i ran an electric heater on my feet, in the bathroom, etc. I don't expect solar heating to do it all for me, but i'd really like to defray my costs and increase the use of the rest of my house without becoming an indentured servant to the utilities.I also have a woodstove, but i've had to buy wood until this year when i got hooked up with a truss plant that lets me salvage their drop. As long as i'm home, it's a fine way to keep my out-of-pocket costs low, but it's inherently limiting. My setting is such that i live rurally, get lots of sun w/o shade most of the year, and i have the time/skills/interest/tools to do all the work myself once i develop a plan. I'm in the process of becoming more fluent in solar so that i can model the pros and cons of various systems and trade offs as you suggest. I must admit one of the pros is just to learn the subject better and experiment, kind of like i did with my hoophouse last summer (Photo Gallery).I'm curious bec of your screen name...are you located in the Flathead Valley to the north of me?
Hello,
My handle derives from what I drive, not where I live:http://picasaweb.google.com/flathead50/Flathead50Ford#The Flathead Valley is gorgeous. I travelled through there a few times and was there once visiting Kalispell for a conference for Electric utilities personnel. I taught a seminar on building energy-efficient homes.Your climate lends itself well to solar. Sometimes I'm envious of Montana folks with sunshine. Then I remember the heating degree days and the hot summers; and then I really appreciate my beach shack here in Pugetropolis :-)
Oh, she's just beautiful! My favorite color! I got to thinking that you might be paying homage to a Harley, but the Ford is unexpected.The next time you come here to lecture on energy-efficient homes, let me know and i'll grab a name tag and impersonate someone vastly more important so i can alleviate a bit more of my ignorance.The MT climate this summer was just delightful. No super hot spells, no forest fires. The eight inches of snow since last night i could do without...trying to think of it as "insulation".