Hello, Anyone using their Garn with a solar hot water panels?  We are considering adding panels to our system at some point, but currently the cost of the panels vs performance is prohibitive. Right now it makes more sense to buy or harvest solar energy in solid form (ie, get some more cords of wood).  I've been reading NoFossil's site (www.nofossil.org) on their experiments with solar panels and daydreaming about a roof full of panels. Anyone's experience with this would be helpful. Thanks! 

I'm not, but as a solar installer, There's no reason it wouldn't work.

 That's the simple.  the complicated answer is, of course, complicated.   

The efficiency of Flatplate solar hot water panels drops off as the temperature gets higher than ambient temps.  of course, to do anything useful with them they need to be above ambient temps.  The real drop off comes when the fluid temps in the panel get above about 90degF above ambient temps.  let's say your winter temp is 30 degrees out, and your garn is 140 degrees, that differential is 110 degrees.  your performance is going to be non impressive to say the least. 

 This is one of (in my opinion) one of the good uses for evacuated tube collectors.  a collector such as the one from Apricus uses heat pipecollectors, and the efficiency stays high even when the storage temps (the GARN) is higher.  the efficiency drops a little, but not much. 

 This has the double whammy of removing the problem of having the collectors overheat in the summertime (a drawback of evacuated tube collectors)  with a minimum of 1500 gallons, the garn will absorb all the heat your tubes will produce (assuming some reasonable number, like a couple hundred tubes, and give you heat for domestic hot water as well.  getting a little too hot?  just open the lid of the unit, and make sure to keep it full of water.  or leave the door open.  that'll move heat out. 

I'd like to hear other ideas.  

 

 Karl

 

 Karl has some excellent points.  However, for colder weather use, I know I will not be worried about solar input for my GARN storage.  By the time temps are in the mid-low 40s (F), I will be firing my unit and putting heat in the old fashioned way.  What I think flat plate collectors will do great for is summertime DHW supply, and during "shoulder seasons" to pre-heat my cold potable water supply to some degree prior to it entering the indirect water heater.  If I can bring my well water from 45 deg to 75 or 85 degrees, I have substantially reduced the heat load on the GARN, and thereby extend the time between burns.

 

I plan on building a closed solar system with large panels and use a flat plate HX to put heat into the GARN (prop-glycol in the panels and piping).  My DHW pre-heat will use a seperate, smaller panel and a flate plate HX between it and the indirect WH.  I also plan to build my own flat plate solar collectors, rather than buy them or the rather pricey evac tubes.

 

Karl, please feel free to rebut and cross examine! 

 Hi Jim,

 unless you're going to have a serious area dedicated to panels, (several hundred square feet at least) I'd not dump them into the garn tank.  I'd use a second indirect tank in the standard fashion of solar hot water systems, or use a HX to dump directly to your indirect tank.   If you use your hot water regularly, and will have something close to 1 gallon per square foot of collector, that's going to give you plenty of hot water.  my wife and I have 150 square feet of panel due south and an 80 gallon tank with a HX coil in the bottom and an electric element in the top.  we turned off the electric element in late march, and didn't turn it on till October.   One square foot of collector will heat one gallon of water from 50 degF to 120 degF per sunny day.  nice and easy to remember.  

 diluting that relatively unconcentrated heat source in 2000 gallons of water will either result in you getting less temperature, why preheat to 80 when you can preheat to 120, and avoid running your indirect at all?  

incidentally, I'll recommend Gary Yresa's site www.builditsolar.com for homebrew solar projects.   gary does some nice homebrew projects.  I'm going to try building a bunch of his homebrew PEX collectors (experimental!!!!!!)   

 I'm on vacation in Hawaii right now, and the beach and Mai Tai's are calling.  

 More after surfing tomorrow. 

Karl.

 

 Thanks for the feedback Karl.  I have some questions/clarifications, but do NOT waste a minute while you are on vacation in such a wonderful place! 

 I'm fried, so sitting in front of the computer for a bit is fine with me.  I have to keep up on email/posting or I get buried after a week gone. 

 I realized that my previous post was a bit disjointed (I'll chalk it up to jet lag) , so I'll make a better description of my suggested solar water heating/GARN indirect system.  

 for the solar/indirect tank, use a tank like a SSC-119-SB from Heat transfer products.  it's a 120 gallon tank with 2 coils in it.  the top is designed for boiler indirect fired, and a bottom coil for solar.  but you would have no backup.  in the winter, running 150+ deg water thru that top all the time will give you about 50 gallons of 150 deg water.  just fine for all  but the biggest loads.  in the summer, you get solar heated water, and for my wife and I, 80 gallons will  get us thru a cloudy 3-4 days.  

 or maybe better would be to run the upper coil off the boiler loop, the way I'd expect it runs now, and use the solar to heat the tank in the summer.  a simple switch will allow you to enable/disable the boiler loop from wherever (including the bathroom, or top of the basement steps.)  if you're using a Taco zone control, they have a indirect water heater priority option if I recall right.  one of your posts mentioned a boiler upgrade to LP soon, and some of those will do it automatically.   

quickly on panel sizing for DHW, best bang for the buck (and to avoid overheating) is typically 1 square foot of collector and a little more than 1 gallon of storage per gallon of hot water use per day.  panels at about latitude.  42 degrees or so for PA? 

 

There are solar hot water systems EVERYWHERE!! here.  I'd say easily 1/3 of the roofs I can see a system on.   that is the ones I can see a south face of the roof from the road.  but here electricity is $0.25 per KWH, and LP is expensive too.  nice rebate program here.  

but life would be too easy here.  need winters to maintain proper levels of character.  

back to things with umbrellas in them!

 

 

 

 

 

 

 

 

 Thanks for the follow-up Karl.  I thought the output of the collectors was a little higher, but you did not mention what Delta T for the 1 gallon/1 foot sq. rule of thumb.  I was thinking of a smaller panel with higher output dumping into the GARN, but if you are only getting a couple thousand BTu/hr,/100 sqaure feet of panel, that is not enough to  make a dent in 1800+ gallons of water.  I'll still build something, but I was hoping to build 2 or 3 25-30 square foot panels for the GARN and a couple of 10-20 sqaure foot panels for the indirect WH.

 

Enjoy the rest of your stay! 

 

 

 Hi Jim,

I promise I had a great, witty and informative response to your reply(it even had calculations in it).  when I clicked "post" I was informed that I would have to log in.  fine.  but the post was gone. I'll try to replicate.  

the shorter version is that in heating applications, we can harvest between 600 and 700 BTU per square foot of solar hot water panel per day.Longer explanation follows.

   a standard solar domestic hot water setup for the midwest is 2 4x8'panels and an 80 gallon tank.

Panels are rated in 5 temperature categories, based on the difference between the working fluid temp and the ambient temp, I'll use a solarskies 32 square foot panel as an example, because I know the numbers by heart.  (these numbers are avaliable for any commercially available panel at www.solar-rating.org)

A: panel is 9 deg less than ambient.  extremely efficient, say pool heating (85 degrees) in a hot climate (say 94 degrees)  not gonna happen too often in the midwest.   ss-32 panel produces about 42,000 btu per clear day.  or about 1312 btu/sf

B: Panel is 9 degrees warmer than ambient, say pool heating in a mild climate (  85 panel and 77 deg ambient) still pretty efficient.ss-32 produces about 38,000 btu per day, or 1187 btu per SF.

C: Panel is 36 degrees warmer than ambient (say heating domestic water in a warm climate, 120 panel temp and 84 ambient)  Panel producses 32,000 btu per day, or 1000 btu/sf

D panel is 90 deg above ambient (say water heating/space heating, 120 deg on a 30 degree day) panel produces 19,000 btu per day, or 593 btu per SF

and E, panel is 144 deg above ambient ( say water heating/space heating in a really cold climate, or absorbtion air conditioning, panel produces 7000 btu per day, or 218 btu per day per SF.  

 you can see the efficiency drop off steeply as the panel temps rise.  we try to keep solar loop temps under 120 deg in the winter to keep those losses lower. 

 so a domestic hot water system, with 64 SF of panel area will collect half of it's heat (in the mild summers we have in WI), at  category C, and the other half will be closer to category D. 

64 sf of panel will give you 32,000 btu for half a day's collection(in cat C), and 19,000 btu for half a day's collecton in category D (as the tank heats up, the solar loop temp increases, and the efficiency decreases) so we can get something like 51,000 btu into that 80 gallon tank. 

assuming that all the previous day's production was used up overnight and in the morning, and a starting tank temp of say, 50 degF (or whatever your ground water temp is), that 51,000 btu will get you an 81 degree rise.  but there'll be some losses in the heat exchanger, say 10%, so that gets you about 73 degree rise, or 123 degrees.  presto, the same temp your water heater was going to heat it to anyway.  now you don't need to heat it up any more, and your water heater just sits there idle.  nothing wrong with that.

whew.  . 

 by those calculations, dumping that same SF of panel into your 70 degree GARN unit (I'm guessing 70 degrees might be a sensible summer ambient temp?) you'll be in the 0 degrees above ambient temp for your drainback system, so at halfway between -9 and 9 from ambient would get you 40,000 btu per day per panel (and no HX losses).  so you'll be dumping 80,000 btu per day into the GARN, and 1850 gallons of water, you'd get about a 5.5 deg rise.  and your water heater makes up the difference.  that 51,000 btu in the smaller storage tank offsets about 1 gallon of propane at 60% efficiency. (91,000 btu per gallon LP * .60= 54,600 btu)

and in the winter, you're heating that GARN up to 160-180 regularly, and putting your panels into the Cat D range or worse yet, the cat E range (180 deg minus 144= 36 deg ambinent) you'd be getting 218 BTU per SF per day.  

this is why we solar installers like Hydronic radiant heat so much.  it works at temps down to 85-90 degrees in the best cases, and more like 120 deg in less than optimal cases.  

 this is where evacuated tube collectors come in.  they have a shallower efficiency vs deltaT slope than flatplate collectors.  and as far as I can tell they're all made in China.  I can get flatplate collectors from less than 300 miles away.  I also don't have any good info on long term reliabilty.  Sure I'm going to put some up on my place, but I know the flatplates I'm using will last 30 years ( I know people with the same panels that are 30 years old, and working fine)

 if you dive into the SRCC (www.solar-rating.org) OG100 info, make sure you're comparing net apeture especially for the evacuated tube collectors.  

 end of mini lesson.  now I need to learn more about GARN performance. 

 ciao,
Karl

 

I'm just scanning over the items on the Forum and it triggered a thought... and that could be dangerous. But back when solar was going to transform the planet.... in the 70's... it seems to me someone I worked with was dealing with the whole cost vs efficiency vs seasonal use.... etc, etc. and he proposed to just put the collector plate on the surface and forget about the insulation and glazing. The thinking was if you are only using it in the warmer months... it would be significantly less expensive and would still yield some benefit. We all know about the garden hose laying in the yard. I think he was proposing the inexpensive black plastic sheets with all the channels in it. Sounds a little "Mother Earthy" .... but for what it's worth. Another trick I learned as a bachelor living along, was to shut the gas water heater off. Just the pilot light was enough to re-heat the tank in 24 hours. I used only hot water and it came out at the perfect temp for a 5 minute shower.... and that water heater was sitting in a 45 degree space. Ahh.... the good ol' days....

 

I like the pilot light idea. as I don't have a gas tank style WH anymore, I'll leave it to others.

There are collectors with no isulation and no glazing available, they're plastic pool collectors.  they work great at near ambient temps, but their performance drops off dramatically as the temp of your working fluid goes up.  that's fine when you're working with something that stays at a relatively steady temp (like a pool, always about 82 deg or whatever) but when the temp goes up beyond that, your efficiency goes down dramatically.  the SRCC only lists glazed panels up to the class C range, or 36 deg above ambient.   wind also increases the heat loss dramatically. 

 realistically, in cash terms, solar water heating makes prefect sense.  (if you're ofsetting nat gas, LP or electric)  solar space heating with hot water panels isn't as good a move on financial terms. (I will temper that statement by saying that it makes good sense where you can use hot air panels, or where you can do a High Mass space heating system)  in a typical space heating system you're using 2 panels output all year, and only using the other 3 or more panels' worth in the winter when they're going to do you the least good. 

high mass systems use a 2'deep sandbox under the living space in the house, with lots of PEX tubing in it, pumping directly from the panels (panel area something like 25% of the floor area)  it uses extremely low temp fluid (less than 100 deg in most cases) and no heat exchangers, so the losses are lower.   we can ( I think) compete with geothermal space heating systems in the right scenario, at a similar cost and NO operating expense.  and they're usually completely independant of grid power. 

 Karl

 

 

Northwind:

 Hi Jim,

I promise I had a great, witty and informative response to your reply(it even had calculations in it).  when I clicked "post" I was informed that I would have to log in.  fine.  but the post was gone. I'll try to replicate.  
Karl

 

 

 

Karl - thank you for the information (sorry you had to type it twice   )  You have given me a good understanding of the limitations that I'll be working with.  Given the (relatively) small harvest of energy, it makes no sense to dilute it in the huge volume of the GARN.  If I do build some panels, it will be for input into my DHW storage tank. 

 

I have a good friend that likes to keep things . . . "basic".  He has been using a coil of 1" black poly pipe on his roof for DHW in the summer.  That is all he needs to meet his demands for his family (three kids) from May until September.  I was thinking about making something similar but a little more sophisticated with a series of black poly coils in a glazed, insulated collector, but the temps would apparently get too high for the poly tubing. 

 

I think some prototype panels might get built for fun anyway. 

 

 I'll recommend looking at builditsolar.com and look at his $1000solar water heating project.  it's really pretty slick for the home tinkerer.  and if some part doesn't last 20 years, you can probably replace it for $50.  

 I'm going to try building some of his projects, and use them on my in-laws.  

 

Karl