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Insulation/heat transfer questions for active solar/radiant/ICF house
Last Post 24 Jul 2007 04:55 PM by metamerman. 12 Replies. |
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metamerman
New Member
Posts:8
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| 22 Jul 2007 07:48 PM |
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I need some advice on insulation and heat transfer for a solar-hot-water storage system. Here are the specs:
4500 square feet ICF/SIP (roof) construction
12 4x8 flat panels mounted due south at 40 degrees (our latitude)
DHW and space heat
5554 Degree days heating (Boulder, CO)
Hydronic heating
HRV
Tankless water heater for backup
Heat storage:
2 80-gallon tanks with heat exchangers for DHW
38x22x4 feet sand bed with muliple loops of PEX tubing (heat in from solar, out to incoming DHW and hydronic)
(We've ruled out the huge-water-tank approach: no room, too expensive, too afraid of what'll happen when it starts leaking, etc.)
Questions:
Any projections as to whether the components are sized correctly? My calculations shows that they ballpark are, although I must admit that the system is designed as much by what'll fit as what's optimal...
We'd like to heat incoming HRV air from the sand using some sort of heat exchanger. The possibilities are:
1) Bury concrete block in the sand, push air through it with a fan
2) Bury pipe (PVC or ribbed ABS septic drain line) and push air through it
3) Make another PEX loop through heat storage and use water/air heat exchanger (requires pump)
I'm thinking 2 would be easiest, 1 would be cheapest (depending on the source of the blocks) and probably have the best thermal transfer, and 3 would be the most flexible (easier temp control). But are any of them worth considering or should we just accept the cold incoming air and rely on the radiant floors to rewarm it? It's all the same heat source after all...
The sand bed heat storage is under the garage floor (don't want it directly under living space because we want a crawl space and because we get 70 degree days here in January). Stem walls (backfilled about 4') and garage walls will be ICF, the ceiling of the garage will be the floor of the rec/bonus room, and we'll put a vapor barrier above and below the sand, but what (if any) insulation should be used above (between it and the slab) and below (between it and the ground) the sand?
Will the sand transfer heat fast enough to the pex tubing running through it, or should I wrap the pex in aluminum plates (like a staple-up hydronic system)?
Do we want to consider putting some sort of wetting system in the sand? It would greatly increase capacity and heat transfer, but would there be negative effects too?
Anyone know of an absorbtion-type water chiller for residential cooling? Evaporative coolers work well here, but I figure as long as we got lots of excess heat in the summer, why not run the cooling with it too?
PS: I realize this post is somewhat off topic, but inexplicably there's no active solar forum on this site.... |
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PanelCrafters
Advanced Member
Posts:680
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| 22 Jul 2007 09:24 PM |
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Posted By metamerman on 07/22/2007 7:48 PM
I need some advice on insulation and heat transfer for a solar-hot-water storage system. I would not rule out a water storage tank! One of the best systems that I've seen so far is here. That should help. For what you are attempting, the main key is: Super Insulating. By doing that, you are reducing the amount of heating and cooling required. It's a simple concept, but many people miss it. I'm talking about R-40 walls, and, not some simple stick framed structure. Use 10.25" SIP walls, ICF walls with an additional R-15 framed wall inside, or maybe double wall construction. They all work fine. R-40 minimum!Here is s page that will help you with your storage requirements. Also, since you are planning on using radiant floor, water is the easiest solution. Good Luck! |
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| ....jc<br>If you're not building with OSB SIPS(or ICF's), why are you building? |
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metamerman
New Member
Posts:8
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| 22 Jul 2007 09:49 PM |
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Coincidentally, we went and looked at that house for the first time just this afternoon ;-) It's a little "solar looking" for our tastes, but that's probably mostly because it's the southern exposure that faces the street (lucky for us, the front of our house will face east). They went the superinsulating route in a big way, and also combine passive and active solar (that sunroom looks even bigger in person than in the photo). But the water tank is way oversize because they wanted to meet code with no backup source at all. We're not that zealous, and given Boulder's weather we really don't have to be (they've designed for 8 consecutive days with no sun, whereas we rarely see more than 2 or 3).
We're also basing the system more on "Solar Water Heating" a book that came out in 2006 which unfortunately specifically recommends against most of the techniques discussed in the tech-report you've recommended (salt systems *always* eventually leak, rock bins get dusty and moldy, and air systems are too inefficient to capture and store the amount of heat we'll need). In particular, we're *not* planning to use water, but rather an antifreeze solution, which unfortunately is even less practical for a large storage-tank system because of the costs involved.
I do agree that superinsulation should be a key component of the system, and plan to direct most of our effort toward that. |
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PanelCrafters
Advanced Member
Posts:680
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| 22 Jul 2007 10:26 PM |
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Posted By metamerman on 07/22/2007 9:49 PM
They went the superinsulating route in a big way, and also combine passive and active solar (that sunroom looks even bigger in person than in the photo). Why do you think that it works?
We're not that zealous, and given Boulder's weather we really don't have to be (they've designed for 8 consecutive days with no sun, whereas we rarely see more than 2 or 3). Do you want to be successful? Or, do you want to just tell people that you tried? You must be committed, or you will fail.
We're also basing the system more on "Solar Water Heating" a book that came out in 2006 which unfortunately specifically recommends against most of the techniques discussed in the tech-report you've recommended (salt systems *always* eventually leak, rock bins get dusty and moldy, and air systems are too inefficient to capture and store the amount of heat we'll need). In particular, we're *not* planning to use water, but rather an antifreeze solution, which unfortunately is even less practical for a large storage-tank system because of the costs involved. The 'tech-report' mearly discussed the methods available. The anti-freeze is only used in the closed loop system to transfer the heat to the water. Me thinks that you should dump the book, and do a bit more research. Maybe it would just be easier to just pay a furtune for natural gas.
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| ....jc<br>If you're not building with OSB SIPS(or ICF's), why are you building? |
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PanelCrafters
Advanced Member
Posts:680
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| 22 Jul 2007 10:26 PM |
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And, I toured a house today that heats via hot air and stores the heat in water. Works well. |
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| ....jc<br>If you're not building with OSB SIPS(or ICF's), why are you building? |
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dmaceld
Veteran Member
Posts:1465
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| 23 Jul 2007 12:12 AM |
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I considered solar panels for the ICF house I'm getting ready to build, but ruled them out because of cost and winter performance. For space heating the main issue, as I see it, is the time of year you need heat the most, mid winter, is the time of year solar panels have the lowest performance. I plan to use a direct exchange heat pump and hydronic heating/cooling.
What I am kicking around for my next house, and maybe for this one, is installing solar panels and then capturing summer heat by running the water through loops in the ground next to the heat pump tubes. The idea would be to store summer heat in the ground for the heat pump to extract during the winter. The ground isn't a particularly efficient heat storage medium, but if done correctly the operating cost to capture summer heat should be pretty low. The panels would help in the winter, but not much.
If poly tubing won't set me back a whole lot, I may go ahead and put it in the ground along with the heat pump tubing just to have it there if I decide to try the solar panel idea.
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| Even a retired engineer can build a house successfully w/ GBT help! |
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dmaceld
Veteran Member
Posts:1465
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| 23 Jul 2007 12:52 AM |
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Posted By metamerman on 07/22/2007 7:48 PM
(We've ruled out the huge-water-tank approach: no room, too expensive, too afraid of what'll happen when it starts leaking, etc.)
Questions:
Any projections as to whether the components are sized correctly? My calculations shows that they ballpark are, although I must admit that the system is designed as much by what'll fit as what's optimal...
We'd like to heat incoming HRV air from the sand using some sort of heat exchanger. The possibilities are:
1) Bury concrete block in the sand, push air through it with a fan
2) Bury pipe (PVC or ribbed ABS septic drain line) and push air through it
3) Make another PEX loop through heat storage and use water/air heat exchanger (requires pump) Are you familiar with the physical property of material called specific
heat? If not, do yourself a huge favor and read up on it. Basically
what it is is a measure of how much heat it takes to raise the
temperature of a material by one degree. The more heat required to
raise a pound of material one degree F, the higher its specific heat. I
thought about putting a stone pile immersed in water under my garage
floor for my hydronic system storage in the house I'm getting ready to
build. The idea was to store the heat in the rocks and transport it in
and out with water. Then I did some research and found that water holds
multiple times as much heat as does a rock! It turns out that water is
one of best heat storage mediums there is, much better than stone,
sand, bricks, concrete, or whatever. This is in spite of the fact that
rocks are much denser than water. In other words, you can store more
heat in a cubic foot of water than you can in a cubic foot of rock! Another
consideration is the cost of pumping. One of the reasons hydronic heat
systems are so efficient is the energy required to deliver a btu of
heat into the living space from the source is much less when that heat
is transported in water instead of air. I don't have specific numbers
at hand, but with some research you can find them. Based on what
I have researched and calculated, transporting and storing heat with
water is much more efficient than transporting with air and storing
with stone. Also, why are you concerned about heating the
incoming HRV air? The purpose of the HRV is to extract the heat, in
winter, from the exhausting air and put it into the incoming fresh air.
Heating the incoming air would defeat the purpose. |
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| Even a retired engineer can build a house successfully w/ GBT help! |
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metamerman
New Member
Posts:8
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| 23 Jul 2007 01:26 PM |
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Here's the specific-heat math: 38x22x4 (space we need to fill under our 3-car garage) = 3344 cubic feet. At 7.5 gallons to the cubic foot, we're looking at around 25,000 gallons of sand. It's true that sand is not nearly as effective for storing heat as water, somewhere around 1/5 as good, actually. So we divide our number by 5, and so our proposed sand bed is equivalent to 5000 gallons of water. The "solar harvest" house has a (used) 6000 gallon tank for which they paid around $7000 including heat exchangers, but apparently not counting insulation. Strutural fill here is $24 a cubic yard, which works out to about $3000 for our bed. Add a few hundred for "heat exchanger" (pex) and we're looking at less than half the price of their tank, not even counting the vastly easier and cheaper installation and maintenance (e.g., the sand will never leak, whereas the tank eventually will). I'm also not counting the fact that they had to give up half their basement for the tank, whereas we give up nothing (since our garage floor needs to be raised anyway). Still investigating whether we need to upgrade to road base or pure quartz sand, but even the latter will be less than $5000 all up.
There are lots of ways to measure success and failure here, and I don't know about you, but getting equivalent functionality for half the cost seems to me to be a measure of success! |
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metamerman
New Member
Posts:8
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| 23 Jul 2007 01:39 PM |
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The HRV issue: a 100% efficient HRV/ERV exactly averages the temperatures of the incoming and outgoing air. Take a wintertime example here: inside temp is 70 and outside temp is -20, our "standard" winter low temp (the record is -24!). So the incoming air from the HRV is going to be at the average of the two or 25 degrees. I don't know about you, but even the slight breeze blowing into the house from an HRV at that temperature is going to be a problem for us, particularly if the county insists on full ducting for it (I'm going to propose just dumping it into the (insulated) crawl space and putting vents from there to the interior of the house, diluting the cold air stream). What I'm proposing is to run that 25 degree air through the sand bed, raising its temperature. Possibly by a good deal. |
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jmagill
Basic Member
Posts:374
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| 23 Jul 2007 01:51 PM |
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"Will the sand transfer heat fast enough to the pex tubing running through it, or should I wrap the pex in aluminum plates (like a staple-up hydronic system)?"
What about building a test case? A small mock-up? I would like to know about the sand transfer rate.
Your building sounds very similar to what we envision. Our biggest concern is cooling for a few months of the year. We don't want airconditioning but do not like the heat. Where we live now we get cool nights and can drop the temp in the house
to prepare for the next day. Our new location does not have the same cool nights.
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metamerman
New Member
Posts:8
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| 23 Jul 2007 02:02 PM |
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Troll alert: I know I'm going against conventional wisdom here (though I'd rather call it a "fad" than "conventional wisdom") but IMHO ground-source heat-pumps (often misidenified as "geothermal" systems) are not only not a economical solution for reducing an individual home's carbon footprint, if adopted on a wide scale they'd end up *preventing* our solving the country's greenhouse gas emission problem. The root of the problem is exactly as you describe: right when you need the most energy to run the system, the available input is trailing off. It's not just a wintertime problem with heat, the same problem exists in the summertime with cooling: temperatures in most places peak at between 3 and 5 pm, by which time solar input is 50 to 75% off its peak. The wintertime performance is worse, however, at least relative to thermal solar which run at 4 times the efficiency as PV Claimed zero-energy homes that use GSHP and PV panels are just cheats IMHO: basically they use the grid for power when demand is at its peak (cloudy days and nights in winter, late afternoon in summer), and then pay back in hours (and months) when demand is much lower. This is no way to design an energy infrastructure. Active solar with large heat storage is IMHO the only viable solution to heating needs. For about half the country some variant of evaporative cooling is the solution for summer time, but for the rest, they need to join me in my search for a reasonably priced absorption chiller. |
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dmaceld
Veteran Member
Posts:1465
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| 24 Jul 2007 01:57 AM |
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Posted By metamerman on 07/23/2007 1:39 PM
The HRV issue: a 100% efficient HRV/ERV exactly averages the temperatures of the incoming and outgoing air. Take a wintertime example here: inside temp is 70 and outside temp is -20, our "standard" winter low temp (the record is -24!). So the incoming air from the HRV is going to be at the average of the two or 25 degrees. Do you have a link that explains this? Everything I find indicates HRV efficiencies are on the order of 80 - 85% of the exhaust air energy being recovered. This tells me if the indoor exhausted air is 70F, and the outdoor air is 0F, the incoming air gets warmed up to around 55 - 60F, not just 35F as you say. I want to know so I can select the proper HRV and duct it properly for the ICF house I'm getting ready to build. In a theoretically 100% efficient counterflow heat exchanger the exit temperature of each fluid will equal the entering temperature of the other fluid. In a parallel flow heat exchanger the exit temperature of both fluids will be the average of the incoming fluid temperatures. From what I find on the web sites HRVs are more like counterflow heat exhangers.
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| Even a retired engineer can build a house successfully w/ GBT help! |
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metamerman
New Member
Posts:8
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| 24 Jul 2007 04:55 PM |
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I think you're right: My description is of a 50% efficient system, not 100%. Unfortunately in cold climates that's apparently a more typical number: Mostly I'm seeing manufacturer claims of 70%, and more but the efficiency goes down in colder weather because the unit must keep from freezing by heating the incoming stream or periodically closing and recirculating to melt the accumulating ice. Many manufacturers are also apparently not properly accounting for things like leakage and fan energy use when quoting efficiency...
In any case, around here it definitely *is* possible or even likely that the even after passing through the ERV/HRV the air will be below freezing. But I'm starting to think that a better solution to that problem is preheating by running the air through a ground tube (as is done in the "Solar Harvest" house) rather than postheating via the sand bed... |
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