Feedback on my DIY solar Thermal mass design.
Last Post 18 Jun 2010 07:43 PM by toddbailey. 23 Replies.
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toddbaileyUser is Offline
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12 Jun 2010 04:37 PM


Hi All,

I'm in the Seattle Wa USA area.  47°36′35″N 122°19′59″W

I currently have 6 3x6 ft copper/aluminium flat plate thermal collectors used to heat a hot tub (apx 500 gallons).
During the summer months I experience over heat conditions.
I wish to add a thermal mass to dump/store excess heat from solar panels for later use.

I am planing to use a big hole in the ground (6x12x8ft already dug) to construct a thermal mass (tm) out of 1/2in pex,  4 or 6 in insulation and a pres treated  2x4 wood frame and using the existing soil,  arctic till (clay + rock) at the storage medium. because of the continual rain fall this will be considered a wet clay/sandy mix.
 
I'll remove the larger rock potato sized or larger and mix with utility sand.
I plan to use several layers (9) of generic 1/2 in pex spaced 9in horiz.  and 6 in vertical 100 ft in length.  The purpose of the tm is 4-fold  dump/store surplus heat from collectors, act as a heat exchanger for hot tub, dhw and radiant heat systems and support the weight of the tub and deck & transfer heat (via convection) back into the hot tub at a slower rate

The tm will be insulated using 4 inches or 6inches alumin faced polyisocyn. foam  on all 6 sides  and buried in ground with 8 inches vertical exposed that hot tub will sit on (hot tub is in-ground style with no bottom or side panels), later during deck expansion / construction over the tm, the Hot tub will sit on top of the exposed tm and be surrounded with insulated panels (r30?) and become part of deck surface.

I plan to alternate hot in and warm out water lines to provide uniform heat transfers.

I also plan to add more thermal collectors to provide for dhw and radiant heating capacity.
Due to cost concerns, I decided to pass on the more conventional water tank method of heat storage, at least for now.

Initially, I'll probably only have one or 2 water lines in service used to dump the surplus heat once hot tub reaches a set temp.

The solar controller is a Steca 0603 and a grundfos 3 gpm stainless water pump.  Once I add dhw and radiant systems more equipment will be required.

Would anyone care to comment on my design?

If I did the math correctly, this size of thermal mass is the equivalent to using a 750 to 1000 gallon water tank.

thanks all


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14 Jun 2010 10:44 AM
Using foil facers or iso below grade won't work. The foil is useless as insulation when in full contact with the cool damp soil (it needs an air or vaccuum gap to have any insulating value), and in soil the iso will eventually become saturated with water, losing most of it's R value. Use either XPS or EPS rated for earth-contact.

Heat storage is not a heat dump- if the solar gain is exceeding what the hot-tub uses by even 10%, your thermal mass will reach peak operating temp well before the end of the summer.

Calculating the actual thermal mass of mixed soil types is not very precise, but I suspect your thermal mass estimate is low by at least 50%. You might want to look into how much it would cost to build a 2000 gallon tank using insulated concrete forms, an EPDM liner, and additional EPS or XPS to complete the thermal envelope. The thermal mass water can be isolated from your loop water using a similar PEX heat exchanger, but the cost of the concrete would likely be less than the extra amount of insulation required for dirt of equivalent thermal mass. (Large septic tanks insulated with closed cell spray foam have also been successfully used in apps like this.) A cube-shaped tank would be a reasonable surface area/volume ratio, for minimizing the insulation requirements & costs. (Spheres are a little harder to do with concrete. :-) )
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14 Jun 2010 01:33 PM
Hello,
Thank you for your insight into my little project.

I briefly considered using a buried 2000 gallon cistern style water tank then adding pex for creating the heat exchanger but costs were excessive, And it would still have to be well insulated. there was also the concern that it wouldn't be able to support the weight on having a 500 gallon tub sitting on it. Same holds true for having a concrete tank build on site. Very expensive. And being unemployed I have to contain costs as much as possible.

I failed to mention that the tm will be surrounded with 8 mil poly as a water barrier. But I expect that the fill will be kept moist due to the frequent rains we get. I just don't think making it water tight is practical.

Many Thanks for the info on the poly iso foam core insulation. I wasn't aware that it wasn't intended for in ground or wet use.

I'll have to verify with my supplier when I go to make my purchase

This leaves me with 2 choices if I purchase from Home Depot either white (styrafoam) or pink sheet insulation (material name unknown) .

according to this posting 1 inch of xps (r10) is about the min. used under radiant heated concrete

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/12/aft/67908/afv/topic/Default.aspx

so 2-4 inches should be ideal, again costs are the concern.

Is R30 or R40 cost effective over R20 in this case?


I may have mixed terminology in error. My intention is not to use the tm solely as a heat dump, but rater to divert hot water to it once the tub has reached a set temp point. Then any surplus heat will be stored underground and if needed and if it hasn't dissipated can be used on demand, pre heated DHW for example, cold in warm or cold out it doesn't matter. The solar controller can easily be reconfigured to manage up to 3 storage tanks. it will simply turn on a valve to redirect the water flow. Granted the fill material composition is imprecise, but when looking at materials from finely crushed rock to dirt, clay soil and sand, their thermal properties are quite close. I basically estimated what a cu ft of material weights, then multiplied by the total volume and ran the btu numbers. The number I reported here are from memory, I forget the actual values I can up with.

But from you comment, it sounds like I can make the tm smaller and still be effective.
Another question I haven't been to find answers for and using water as the reference is what size is the water tank typically used for Solar heated dhw and radiant floor heating in Seattle area. Once I find that answer, then I can properly size the thermal mass and collector array.

As I recall each collector produces about 10k btu, and much less in the winter on overcast/cloudy days. So figure 60K summer and 15K winter.
Obviously I'm going to need to hang a bunch more collectors to get solar heating to work on those cloudy days.
But I digress.


The real question I have is will this design do what I envision it to do, store and transfer heat to pex and to a small opening of 3x3 where the tub will sit in direct contact with the fill material?
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14 Jun 2010 07:26 PM
Home depot was out of stock on the pink foam so I settled for the white stuff.

Sold under the brand name of R-Tech (www.insulfoam.com), this product is approved for under concrete insulation with a 3600 pound per sq ft compressive strength, water proof and bug resistant. I purchased 2 in thick which is advertised as r20 value.
r40 OK or should I go for r60?
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14 Jun 2010 07:36 PM

If you want to simply dump the heat, rather than mess with that whole heat-storage-in-the-sand thing, try running your heat dump PEX through a tub of water that is open to the air.  Evaporation will cause much more heat loss than any other method.  You could even add a fan blowing across the water to amplify the effect. 

For example, it takes 140 BTU's to raise one pound of water from 72 degrees to 212 degrees - but it takes almost 8 times that much to turn the 72 degree water straight into water vapor, and that energy will come from the water itself.  So, if you have 100 gallons of water - about 850 pounds - you'll consume almost a million BTU's to evaporate all of it.  You'll have to keep people and animals away from the hot water, and you'd have to manually or automatically refill the heat dump tub with water, but that sounds like a great DIY project.

The only limiting factor that I can see would be sizing the surface area of the dump tub to accomodate the excess heat that you need to be rid of.  If you need to dump 200,000 BTU's / day (say, in a 15 hour period) then you'd need to accomodate about 13,000 BTU's hour, or about 1.3 gallons per hour, or 20 gallons / day.  I've seen calculations online to determine the surface area to evaporation rates (transpiration tables, perhaps) but I'm sure that this could be relatively easily calculated.  20 gallons is about 2.5 cubic feet, or about 30 square feet of water that's one inch deep.  I bet that you could do it for 15 square feet if the water is hot, even as the evaporation becomes less efficient as the temperature drops.

Or, in a pinch, you could leave the lid off of your normal hot tub.

Jeff

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14 Jun 2010 07:38 PM
My bad, it's rated at R11 per 2inch sheet inch at around 45 degrees, so rephrasing the question, I can install R11, R22, R33 after that cost get in the way. Originally I was planning for R30, but will R22 work or do I want to follow the more is better rule?
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14 Jun 2010 07:55 PM
OK, let me clarify, I am not really interested in just dumping surplus heat but rather redirecting it for better use later.
Bad use of terminology on my part.

Opening the top helps but so far the best way to dump surplus heat quickly is the run the water through the collectors after sun down. I really haven't measured it that closely, But I use this method more frequently that I'd like to reduce the overly hot temp to a more usable and comfortable level. I can eliminate about 3 to 5 degrees per hour for apx 500 gallons of water.

But I feel that the surplus heat can be better utilized for dhw and maybe space heating. I don't expect any miracles for late fall winter and early spring use, there just isn't that much quality sunlight during those 4 months of the year. Once October hit, thermal output was dicey, Once November arrived, thermal output could barely heat the panels to much over 60 degrees sans any water flow.
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15 Jun 2010 01:06 PM
One of the biggest issues I face in this design is trying to get advice on even the simplest questions.
It's like going up against a grand jury.

Examples: questions like whether to use R20, R30 or R40 insulation values,
what is minimum, what is optimal and what is way too much,

What is a good size to build, is 4x6x8 too small or is 5x8x12 too big?
What is a good tube spacing to use, is 6x6 inches too close or too far apart?

Heaven forbid I ask about water flow rates and is serial flow better than parallel flow.

I've found a few clues to my answers, for the PNW, radiant heat in concrete, poured on site typically use R10 for a minimum with R15 typical and R20 and above to be considered "Green" So R20 or R30 sounds like the value to aim/price for.
BTW below grade insulation R10 is $20 for a 4x8 sheet (2inches), so doing the math with size being variable, about $120 plus for each R10 layer.

In concrete, hot water radiant heating uses between 4 and 8 inches for tube spacing, and is approx 2 to 4 inches below the floor's surface, so I'm guessing that a 6 inch radius will suffice. And I can't get a smaller bend radius anyway. So layers spaced 6 inches apart, hot in, cold out, hot in cold out and so on.

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15 Jun 2010 02:53 PM
White EPS like R-tech is only ~R8 in 2" thickness not, R20. XPS is ~R10 @ 2". You'd have to go to something exotic like aergel to get to R20 in a 2" thickness. EPS is likely to be your cheapest viable option.

BTW: "Styrofoam" is trade name Dow applies to it's XPS (usually blue). The pink board down at the box store is Owens Corning's version of XPS. Bead-board (of any color- usually white) is EPS, which runs ~ R4/inch compared to XPS @ ~ R5/inch or polyisocyanurate (iso) @ ~ R6-6.5 inch.

The answers to all of your questions are dependent on what your design goals are, your subsoil temp, and your storage temp. (If you're storing 90F water or dirt in 65F subsoil the answers will be very different than if you're storing 190F water in 40F subsoil.) A radiant slab in the PNW will never exceed ~80F, so using a net-present value calc on how much insulation is cost-effective there is going to be very different than if you're storing it at domestic hot water temps or higher. If you're storing water at 105F hot tub temps, assuming your subsoil is ~50F your standby losses are going to be about twice as much per square foot of insulated area than with a 72-75F radiant slab. (Twice the delta-T=twice the heat loss per unit area at any given R value.) But that might still be OK, or a disaster, depending on your design goals.

Also, the size & complexity of the heat exchanger in the thermal mass depends on how rapidly you need to draw that heat out or pack it in. If all you're doing is trying to maintain the temp of your hot tub, assuming you have an insulated cover for the tub it won't take a heluva lot, in which case it only needs to be a big enough to keep the delta-T on the collector reasonably bounded.

What are your design goals here- to store enough heat heat your house, coasting through an average Seattle mid-winter day or two without sun? Enough heat to keep the hot tub going in summer/fall through several days of clouds? Or are you looking to store the heat for several days/weeks/months (in which case the standby losses to the ground have to be orders of magnitude lower.)

It's possible to design for any of the above, but unless we know what the actual design goals are it's rather like asking "What kind of tire do you need for a blue car?". The amount of thermal mass, insulation, and collector area required etc will vary dramatically with what you're actually trying to accomplish.
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16 Jun 2010 07:04 PM
All valid points. The more I learn about how this stuff works the more I learn that I know very little about how this stuff works.

After careful reconsideration of what I am trying to accomplish, I will try to briefly explain and provide a few answers of what some of the limitations and design assumption are.

1. build a thermal mass to store heat and later extract it on demand for dhw and radiant floor heating.
2. the existing pump system will be utilized that provides up to 3.5 gpm water flow with a 6 ft head, charts indicate that adding an additional 6 ft will drop flow to 3 gpm.
3 I wish to be able to extract 140 degree water for dhw at a flow rate of approx 5 gpm. but for a short time period. I'm not sure what the flow rates for radiant heat. but that will require long flow durations at a lower temp, if radiant heat systems provide cool and hot mixing capability.
4. the average soil temp is approx 50 degrees, but during winter? maybe 40?
5. the hot tub has it's own independent controls and plumbing and is separated from the solar panels & thermal mass plumbing by use of valves.
6. This is Phase 2 of a potentially 3 phase project, phase three includes more panels, pumps, valves and a 1000 to 2500 gallon in ground storage tank, design tbd.
7 heat exchanger will consists of several vertically stacked parallel plumbed loops of 1/2 pex 100 ft each, using radiant heated concrete as an example, 6 inch spacing is the norm. The planned sequence is solar loop, heated loop, solar loop heated loop
and so on for the entire height of the mass, with 6 inch spacing from top, bottom and sides.
8 the max size of the mass can be up to 9ft long 5 ft high and 7ft wide plus 6 in insulation on all sides, or apx R30
Using generic soil weights, I figure 1 cu ft weighs in at 120 pounds, or 315 cu ft or 11 1/2 cu yrds,
with a total weight of approx 37,800 pounds.
The fun begins with me trying to do the math for the heat capacities, btus for heating etc. which I'll savefor a later post.
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17 Jun 2010 12:09 PM
If you're talking about storing summer heat for winter use, fuggedadaboudit the R-values to retain that heat for months on end would be ridiculous. With water you can use much lower volumes==much less surface area to insulate, but even that would be ridiculous. But you can do the math yourself (it's not hard), or start a club with these guys:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/72961/afv/topic/Default.aspx

http://greenbuildingweb.com/Forums/tabid/53/aff/21/aft/75617/afv/Topic/Default.aspx

It's not really hard math, but for the money you'd spend to insulate sufficient amounts of earth-mass as seasonal storage to support a useful fraction of a Seattle heat load with a standard code-complaint house you could retrofit the house to PassiveHouse Institute standards (seriously!) Winter solar uptake in the Puget Sound area is pretty lousy, but average winter temps are modest compared to much of the US. In Seattle you can probably get there with ~R30-40 clear-wall R values rather than the R50-R80 it would take in much of the central & upper midwest.
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17 Jun 2010 01:09 PM
No I realize that with out a specially designed storage module storing heat for much longer than a week is a non starter.
What I'm trying to accomplish is storing daytime heat for night time use, with maybe 2 or more days reserve capacity.
After last winter I am already well aware that more collector capacity will be needed, but I need a place to store the heat before I double or triple the collector area.

From what little information I've been able to obtain on the design of this project, using R30 insulation and using water through pex, in a 6 inch radius should be in the ball park. I basically wanted someone who knows to review my design plans and comment if they think it will work and if I am missing anything major. Since I am limited by size, anything much over r30 is getting costly, At $20 a sheet and the sq footage in play, each layer of r10 will add approx $120 to the project cost.
I need to be applying fund to components that will provide the best bang for the buck. Unless I'm informed other wise, I would like to use 6 inches insulation and no more

The remaining variables are the length of the pex tubing loops, quantity and spacing.

If 100 ft loop will work, then I can plumb them in a parallel, if more than 100 ft per loop is needed then I can plumb them is serial. if 6inch is too close or too far apart I can adjust the spacing, I can add more or use less.

keep in mind, I have a limited flow rate to work from, 3 gpm the max the existing pump will provide. But it can be replaced if more is required, but I'd prefer not.


How this helps


thanks all



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17 Jun 2010 05:45 PM
To store heat from daytime uptake for same-night use @ ~140-150F storage temps R10 would be fine. To store usable heat for 3 days with the same starting temp R30 wouldn't be enough (R100, maybe.)

Have you done a heat-loss calc on the house yet? (That will be a starting point for how big the heat exchangers need to be.) The dirt itself has some R-value, so with a 20F delta-T between the dirt & desired water temp you may have some issues with only a hundred feet of PEX or so. If you can keep your temps low enough, using something like an unglazed swimming-pool solar collector as your buried heat exchanger might give you better performance and lower head than an ever-increasingly larger ball o' PEX.

The size & cost of hydronic solar to support even half the heat load for a standard-construction house is going to be gia-normous, and bang-for-buck going deep-retrofit on the structure is usually a more cost-effective first-step. If you calculate just how many BTUs it takes to get you through a coupla days, and just how much dirt-storage it would take (even with perfect insulation- R-infinity) to get you there with say a 150F starting temp you'll see what I mean. Adding a rigid-foam R20-R25 overcoat and some better windows to the existing structure would be a good start, and likely reduce your heat load by 2/3 or more, at which point the size & expensive of your solar array & storage needs just shrunk by 2/3. Doing it with EPS or iso 4x8 sheeting is one approach, going with a system approach like R-ETRO might be easier:

http://www.quadlock.com/brochures/R-ETRO_Brochure.pdf

Getting your peak heat load down under 10KBTU/hr on design-day makes things like active solar get to be a lot more practical/manageable/affordable. With design-day heat loads that low you'll likely have to use base 55F or base 60F as your heating degree-day base (with typical construction it's ~65F) which means your active heating won't start until the week before Thanksgiving, and will be pretty much done by tax-day most years. You can buy a whole lot of insulation & air sealing for the price of a coupla flat panels.

The only active-solar that can be done on the cheap is thermal-air panels, and they're relatively easy to design & build. Starting out with an arbitrary storage size and and hydronic flow rate based on your hot-tub hack is just the wrong starting point. Find out the size of the load, how many BTUs you need to store (and for how long). Just plugging at it isn't likely to be very effective (or cost-effective), designing it from first-principles ahead of time gives you a better shot at putting the money where it does the most good, and rarely is putting your money into a hole in the ground going to be the first-best most cost effective approach. Putting the first money into air-sealing & insulating the building envelope is where the first hunk of active solar heating money usually goes.
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17 Jun 2010 06:46 PM
All very good points, thank you for the feedback.
I suppose I should have resided the house and replaced windows when I was still working for a living, At today's costs, residing and window replacement just isn't going to happen just yet.

I wouldn't call heating a hot tub with solar a hack, it is working quite well, and has saved a fair bit of $ on heating costs since it's installation last year. Instead of 55 kw per day my consumption has leveled out at around 35 kw per day.
But there are those times when 125 degrees is just too hot for a quick dip in the pool. Diverting that surplus heat into a storage module would be an excellent means to utilize it at a later time.
Buying new solar collectors is one approach if you have the coin to drop, buying used, like I did is a great way to install on the cheap, I picked up 108 sq ft of collector area for far less than one would pay for new panels.
It's not just math and physics but dollar and cents. That's why I don't consider solar electric a viable option, I simply don't have 40 years to wait around to break even.

Since I am unable to find answers to my questions, I'll proceed with my original plans and hope for the best.


Thanks just the same...



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18 Jun 2010 10:03 AM

OK lets think about simple and cheap ideas

1. For a very simple summer heat dump, try a copper radiator salvaged from a very large old car. Think 70's vintage V8. Buy a small 20 watt solar PV panel and use it to power a small dc fan to blow air over the radiator coils. Instant heat dump, works whenever it is sunny. Put this a ways away from the house.

2. Now for the inside of the house in winter, buy a fan coil hydronic heater with a sensor that triggers the fan when the water is hot and place it in your house running off the solar loop. This won’t offer you any storage capability, but it is dirt simple, cheap and works. You could use the car radiator inside also if you wanted

3. If your basement is inside the heated envelope of the house, find a surplus 500-750 gallon stainless milk tank, put it into the center of the basement and run the solar loops into it. Then just let it act as a low temp radiator ie don’t insulate the tank.

There you go, 3 solutions that are all inexpensive and will all help somewhat. Are they going to make you heating bills go to zero? Nope. IF you have a known quantity of panel area you can calculate how much heat you can potentially gain from the system in the winter. If you are talking about 100 ft^2 of panels, optimally tilted for winter heat gain, facing due south, you still likely won’t produce enough heat to require heat storage unless you have a very small or uber well insulated house. For passive solar with High SGHC windows ~ 10:1 floor area: exposed south facing window area is about right if the house is well insulated without adding extra thermal mass. A water based system should be effective at about the same ratio without any heat storage capability.

Cheers,
Eric

Think Energy CT, LLC Comprehensive Home Performance Energy Auditing
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18 Jun 2010 11:07 AM
Well, sorry no basement, and space heating isn't really part of the equation, The amount of solar collector required is well put one way impressive. One engineer told me about a year ago, I'd need to quadruple the area to come close to provide heat for the tub, dhw and space heating. This project is strictly phase III.
I'm still working on phase 2.

Which is the surplus heat into thermal mass/ heat exchanger for a dhw water heat transfer.

2 sets of lines will be used - solar i/o and cold in / dhw out, Ideally I'd like 150 out, but a more realistic value might be 80 to 100. I'm not look for stellar performance, and just a general design guide to get started would be appreciated

Which brings back the original questions

is r30 adequate for 50 degree soil and a 150 degree thermal mass target?
what spacing should I use to heat the mass? Radiant heat in concrete uses 6 inches, is this a good enough ball park?

So two very basic, general and simple questions, no rocket science, no physics,
just a generic what does the industry use in practice under similar situations.
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18 Jun 2010 11:11 AM
Posted By toddbailey on 17 Jun 2010 06:46 PM
All very good points, thank you for the feedback.
I suppose I should have resided the house and replaced windows when I was still working for a living, At today's costs, residing and window replacement just isn't going to happen just yet.

I wouldn't call heating a hot tub with solar a hack, it is working quite well, and has saved a fair bit of $ on heating costs since it's installation last year. Instead of 55 kw per day my consumption has leveled out at around 35 kw per day.
But there are those times when 125 degrees is just too hot for a quick dip in the pool. Diverting that surplus heat into a storage module would be an excellent means to utilize it at a later time.
Buying new solar collectors is one approach if you have the coin to drop, buying used, like I did is a great way to install on the cheap, I picked up 108 sq ft of collector area for far less than one would pay for new panels.
It's not just math and physics but dollar and cents. That's why I don't consider solar electric a viable option, I simply don't have 40 years to wait around to break even.

Since I am unable to find answers to my questions, I'll proceed with my original plans and hope for the best.


Thanks just the same...




Coming from an engineering background, I don't use the term "hack" in any derisive sort of way, but rather to indicate a design that isn't quantitatively designed, to be revised based on the empirical results until it's performance becomes satisfactory.  The fact that it's overheating the load  is an indication of a hack rather than a well-engineered system.  Sure, it works, but does it work optimally?

The fact that it's saving you ~20kwh/day is means you're bagging about as much heat in the high-solar-gain summer solstice period every day as you would net out of burning 1-therm of natural gas in an old-school non-condensing ~70-75% efficiency gas furnace or a better than average (or high volume use) gas-fired tank type HW heater.  In the winter your daily gain will be less than half what you're getting out of the panels right now, and your losses to the ground will be a function of square feet of surface area and the delta-T between the storage temp & surrounding dirt.  To get even 1/4 of your current uptake per panel delivered to a heating or domestic hot water load will take some careful design, but is remotely possible.

A coupla points: 

The higher BTU storage per unit volume of water vs. dirt means dramatically less surface area to insulate to high-R using tanks rather than soil as the thermal mass.

Putting the thermal mass inside of conditioned space means much of the standby loss accrues to the space-heating load rather than getting "wasted" by heating up the soil in the back yard.

It's neither tough nor expensive to build R20-R40 500-1000 gallon tanks that can live in basements or garages that can operate at sub-170F temps, using PEX or polyethylene heat exchangers eg. Another advantage to this approach is that it's REPAIRABLE and ADJUSTABLE after the fact, unlike a buried thermal mass. (Too much standby loss? Insulation can be added later. The heat exchanger develops a kink, clog, or a leak? You can pull it & fix it!)  You can also use lower-cost insulation- built a wall around it, blow it full of cellulose, etc.  Burying a bunch of EPS & PEX in the back yard on an un-designed system is at high risk for becoming a total loss.

Surf around Gary Reysa's site for examples of fairly complete & low cost low-temp solar hydronic designs.  Pay attention to the details- they count, (Gary is also an engineer, with a predisposition for first designing, then verifying performance via multiple metrics.)



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18 Jun 2010 12:01 PM
Like I said in previous posts I have no basement, so any storage module will have to be outside, either above or in ground. I wouldn't exactly call the current system and planed install undesigned, I spent several hours pouring over material properties, heat loss and transfer calculations. The problem with solar is that it simply doesn't work very well during the late fall to early spring months of the year in my geographic location. And in the attempt to get year around service, the systems are typically oversized, But when Summer approaches something needs to be done with the surplus heat. If I design a system optimal for the 6 weeks of summer Seattle has, the system would be undersized for the remaining months of the year. And make to doubt about it I want 75% functionality. Trying to obtain solar heat during the 3 months of rain is that missing 25%. The point of diminishing returns. Even now I need more collector area to obtain the output required to maintain water temps. BUt with a limited budget, the project will take a few years to complete.

Phase III will include a underground water tank probably in the 1500 gallon size for radiant heat, plus enough collectors to properly heat it. But for now, Lets stick to the basics for now. I have sImple questions and would like generic answers.
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18 Jun 2010 01:45 PM
An engineering approach would have provided for heat dumping or intentional undersizing from the get-go, with automated controls for the switchover to keep the tub from becoming dangerously hot.

Crummy uptake during late fall & winter is nothing unique to Seattle- it's the same story all over. This is the very shortcoming that makes solar a lousy choice for space-heating- the output & load peaks are seasonally well out of phase. Although Seattle's maritime overcasts cuts into total uptake, it's not much worse than any other maritime climate- better than some. (I've spent more than 20 years in the Puget Sound area, even did some passive solar-design while living there.) This seasonal phase issue is why solar hot water makes a lot more sense than for space heating- designing to provide 80%+ of the load during the peak solstice period you can still end up supplying more than half the annual hot water without needing to rely on heat dumping to keep temps under control during the summer. But designing for even 50% of the space heating load means a significant BTU overload for a large fraction of the year.

And these ARE the basics: Calculate how many BTUs/hour you need to get into & out of the thermal mass, as well as how many BTUs you need to store, for how long, and at what temperatures. THOSE are the parameters that will determine the size & configuration of the heat exchangers, and how much insulation you need. There ARE no generic answers to how much insulation or tubing, or the spacing of the tubing, flow rates, etc. The size & spacing of tubing in radiant slabs are completely irrelevant to the problem you're trying to solve.

Does the house have a crawlspace? Slab-on-grade? If crawlspace, is it sealed & insulated with foam-board or spray foam to at least R10? If slab on grade, is the slab edge insulated with EPS or XPS (to at least R10) at least down to the frost line? If not, this would be a far better use for your rigid foam than burying it in the back yard for soil based thermal mass project with few known parameters.

How are you currently heating the house (and DHW)? What was your annual energy consumption that supported those loads? Let's put some numbers to this.

But know this: If using buried thermal mass was a cheap & simple way to go about it, it would be the norm rather than the exception. (It's been done going back at least 3 decades, by people with a great deal of solar design experience, but it's never been cheap. It works best/most economically as district heating for dozens of houses, with deep wells and a stable underground aquifer to pump heat into/out of.)

And this: Phase-I of any solar space heating has to be lowering the load, since the size of the array necessary to support even HALF the annual load will be several times the size of the home's physical footprint for a standard-construction home. If your goal is 75%, you'd better have a very big yard. (Domestic hot water, different story- 75% of annual hot water can fit comfortably on most rooftops, unless you have one of those ridiculous showers with 6 side sprays chugging away at 12-20 gallons/ minute or are filling a 100 gallon spa daily, but that doesn't sound like you, eh? ;-) )
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18 Jun 2010 03:19 PM
Todd... build that baby... give a shout back. You will satisfy your curiousity and ours... the best part of life I think... Dana is one sharp cookie... you will know this more as you get into your next phases.
aj

OH... and milk container idea... love that... nothing better than finding materials for close to no cost and putting them back to use. That is real green living to me.
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