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Mobile Master Tech 05-21-12 11:31 AM

Passive Annualized Heat Storage/Annualized Geo Solar-Don't Waste That Heat!
I have always been bothered by the fact that many alternative energy sources, especially solar thermal, have the least output when you need it most. Most solar and process heat recovery systems have innovative and elaborate systems to throw away surplus heat that can't be used or stored when it is generated. Even large storage tanks, such as the one I describe in the solar storage tank thread, can't store a large amount of heat or for a long time. Phase change materials help, but have drawbacks (including $$$!). The problem is the sheer number of BTU's and how much deltaT there is between storage and environment. Concentrate the heat in one place at high enough of a temp to make the storage size feasible, and most of it has leaked out by the time you need it, plus you lose tons of efficiency getting it to that high of a temperature.

I have been inspired to design a cost-effective way of getting that heat back after researching the work of Don Stephens, who pioneered Passive Annualized Heat Storage (PAHS) back in the 80's, as well as John Hait and others.

My plan is to drill "reverse geothermal ground loops" under my basement slab to "inject" waste heat into the soil below the house, eventually raising all of the soil from its' natural 62F (in northern Georgia) to 70-73F, plus a hotter center section whose greater heat diffuses to the surface around 6 months after going in. Sources differ about exactly how far the heat diffuses in six months, but the lowest I have seen is 9ft, and AC Hacker seems to have found a source saying it is 16 ft. I have done the math from government logs detailing the time lag of underground temperature peaks at various depths compared to the peaks at other depths as well as aboveground temperature peaks. I came up with 10-11ft per 6 months for most soil types. This backs up what I gleaned while researching PAHS.

More info to come, but I wanted to get the ball rolling and get others in the dialogue! Google the Drake Landing Solar Community in Canada, as well as PAHS, ground heat storage, etc.


Piwoslaw 05-21-12 03:46 PM

I've heard of buildings, and even communities, which store surplus solar heat in the ground to use during the winter. The up side is that it's cheap: The ground loop for the heat pump is used to "dump" heat when it is in excess. The down side is that alot of that heat is lost, especially with flowing underground water.

More efficient, and maybe not more expensive, is storing the heat in an insulated tank. How much heat you will lose over 4-8 months depends on four things:
  1. Average ground temperature at the depth of the tank,
  2. Temperature inside the tank,
  3. Surface area of the tank,
  4. Amount (effectiveness) of insulation.
1) There is not much you can do about your ground's average temperature, you might gain maybe 1-2 degrees by burying deeper.
2) By lowering the temperature inside the tank, you reduce the Delta T. This can be done by increasing the tank's size: Putting the same amount of energy (heat) into more water (mass) reduces the temperature.
3) The shape with the greatest volume to surface area ratio is a sphere, but this usually isn't easy to construct. A cylinder is next best, and is a good compromise between efficiency and technical limitations. Of course, some cylinders are better than others, but what you end up with is a trade-off between what you build/buy and what you can fit.
4) Always use the best, and the most, insulation that you can afford.

Summing up, it is possible to minimize the amount of heat that is lost though the tank's walls, so you retrieve a larger percent of the heat you put in. This in turn means that you can invest less in solar collectors, etc.
If you are already doing lots of digging, and can get a relatively cheap tank, then it may turn out to be cheaper to bury it than to drill and/or lay hundreds of meters of ground loop for a GSHP. Make the tank as large as possible, insulate it, and use it as you would use the ground loop of a heat pump.

This has been successfully done: An insulated milk tanker holding 20 tons of water at lower temperature (10-90C) feeds a heat pump to charge a smaller indoor tank with high temperature (50-95C) water for household use.
Milk Tanker Thermal Store with Heat Pump

Iirc user Osolemio has a similar setup, though on a smaller scale.

AC_Hacker 05-21-12 09:38 PM


Originally Posted by Piwoslaw (Post 22066)
I've heard of buildings, and even communities, which store surplus solar heat in the ground to use during the winter. The up side is that it's cheap: The ground loop for the heat pump is used to "dump" heat when it is in excess. The down side is that alot of that heat is lost, especially with flowing underground water.


You are correct in this...

However, part of undertaking PAHS is understanding the soil conditions, especially water table and water transmigration through the soil. To not understand the role of water transmigration in PAHS would be exactly equivalent to not understanding the role that infiltration plays in house insulation.

So site selection of for PAHS implementation is step number 1.

If the water table is low in MMT's area, and if his house is sited so that there is not water flow through the PAHS site it when it rains, burying milk tankers would probably be unnecessary.


Mobile Master Tech 05-22-12 12:29 AM

Piwoslaw, that's a great idea about the milk tanker and the links you provided! Unfortunately that is a lot of digging & buying and it would be impossible on my site due to trees, space, underground storm drains, etc. I figured I only need maybe $500 in extra materials plus some boreholes angled out from a small access area cut from the middle of my basement slab. I'll post more on how to accomplish those holes in an existing house shortly. In any case, I want my storage to be within the building envelope so any losses are still useful at least half the year.

Since I am almost at a high point of terrain which slopes down well beyond my house, I think I won't have too much worry about the groundwater carrying my heat away. I figure anyone that can go down around 20' below the slab to be heated before hitting the water table will be able keep enough heat in the soil to be wothwhile, allowing at least 9' depth so the heat returns around 6 months later, another 9' for the heated area, and a bit of buffer room. I'll confirm that my water table is low enough when I dig the boreholes for my GSHP. I hope to lose a lot less heat than usual because of the pattern I am placing them in, shown by the red line:

The boreholes will be spaced 10-12' out from the house and around 10' apart. Since they are in a u-shaped line, they won't be competing with each other for the same heat and it will form a "cage" around any heat trying to escape from under the house, raising the COP of the GSHP. Also check out that new Energystar rated roof (another post) with lots of flat SSE facing area that has only one code-required roof obstacle!:thumbup:

Clay soil weighs around 110lb/cuft and has a specific heat around 0.3btu/lb per degree F. My house footprint is almost exactly 48' square. Assuming at least 20' useful Georgia red clay depth, I think its reasonable that the outer 5' of slab and area under it (9,000cf/990,000lb) could be maintained about 5F above my average 62F soil temp, taking around 1.5 million btu to bring up to temp. The remaining 43' square (37,000cf/4,070,000lb)could easily be maintained 10F warmer at a perfect 72F (warm enough to add winter heat but below air conditioning temps in summer), taking 12.2 million btu to bring up to temp.

Insulating a 3' perimeter around the house with buried 3" thick polyiso then working inward 20' (per Don Stephens) from there to make sure the high temp heat doesn't get away, that leaves a chunk of soil in the center 13' square by 20' deep, or 3,380cuft/372,000lbs to seasonally store heat I am likely to retreive almost all of through the slab or the GSHP field as it cools back down. Taking that soil from 72F to say 150F (very doable with evacuated tube solar and an A/C desuperheater in the summer), that center soil can store 8.7 million btu, plus extra btu as that center heat diffuses towards the edge of the storage area over the course of the "injection season". There will obviously be a temperature gradient that someone better at math than I am might be able to describe more accurately.

The demand will be lowered because the basement is now a net btu adder sitting in that large 72F area instead of in a 62F heat sink (See the details of my calculated demand load in the solar storage tank build thread). I expect I will have a surplus of 27 million btu (4.5 million/month) during the hottest 6 months from the 60 evacuated tubes and the A/C desuperheater, plus maybe half that in the shoulder seasons. Even if I lose 50% of this heat it will be worthwhile.

Piwoslaw 05-22-12 01:18 AM

If you're on top of clay (I'll remember the red clay from the south-eastern states for a looong time!) without water problems then that should greatly raise the efficiency of your setup:thumbup:

Just a thought: In Georgia you probably need more A/C than heating, or a similar amount, then what about dumping your A/C's waste heat into the ground. With the recieving end much cooler the unit's COP would be much higher, killing a second bird with the same stone (ground loop). Solar would just be an addition, so it wouldn't need to be too big.

BTW: The picture you attached got lost.

Mobile Master Tech 05-22-12 11:08 AM

I'm 30 mi northeast of Atlanta, a mixed climate with roughly 50-50 heating/cooling. I actually will be using the A/C heat, as I will have a desuperheater on the compressor discharge where outlet temps will be around 150F, absorbing the superheat and probably beginning the condensing process. This heat WILL be dumped into the ground-under the slab!:)

I got a Nordyne (Tappan branded in my case-check out!) IQ inverter drive 22 seer outdoor unit with a matched ECM blower air handler. It has a HUGE condenser coil and the indoor unit uses a communicating EXV. The upper 2/3 of the condenser has multiple flow paths, but the bottom third is single pass-I believe they are expecting most of the refrigerant to be condensed by then, getting additional subcooling very close to ambient before it leaves. I plan to put vinyl downspout channels around the bottom third holding a synthetic evaporative (swamp) cooler pad, which will then get sprinkled in the upper channel by the condensate water when it is pumped. This water will dribble down the pad and whatever doesn't evaporate will collect in the lower channel, wetting at least the bottom several inches by capillary action. I will adjust the refrigerant charge so that the final refrigerant condensation happens in the lower third, so the compressor will "see" a condensing temperature that is below ambient.

What happens next depends on ground temps. If the ground loops are significantly lower than that subcooled discharge, I may run the refrigerant through a second HX, but this only helps much if there is still condensing to be done. Instead, I may use groundloop water as a precooler to incoming ERV air/return air through another HX. I don't want to boost fan power draw by too many restrictions though-I will arlready have MERV 4 prefilters and a MERV 14 bag filter, as well as the coil.

Some may ask why didn't I just get an ASHP instead of a straight cool system? The efficiency is 20 SEER/approx 13 EER for the ASHP version instead of 22SEER/14 EER for the straight cool. When cooling, you don't get the heat of compressor work back like you do when running in heating mode. Since I already have efficient heating in the works, maximizing cooling efficiency is paramount. I may even look into an evacuated tube superheater like SednaAire offers. They even use my identical Nordyne unit as their base system! I'm not yet sold that it would help the cause more than using that solar collection space elsewhere. Anyone else have any insights?

The Google Earth pic shows up for me-anyone else having trouble seeing it?


AC_Hacker 05-22-12 11:50 AM

1 Attachment(s)

Originally Posted by Mobile Master Tech (Post 22079)
They even use my identical Nordyne unit as their base system! I'm not yet sold that it would help the cause more than using that solar collection space elsewhere. Anyone else have any insights?

This sounds like a really good project and well thought out, but as a minor aside, there is passive solar and there is active solar... the difference being that passive solar uses no pumps or control system. The best example of Passive Annual Heat Storage that I know of is John Hyatt's house which uses no mechanicals, and is truly passive.

Wikipedia has a very interesting entry on Seasonal Thermal Storage which has links to studies of projects that are very similar to yours.

* * *

By the way, the link button is on the editing toolbar...

... and the in-line image button is three buttons to it's right. I saw in your DIY radiant floors post that you found the inline image button... there may be other readers that didn't know.

It's even possible to make hyper-linked images by combining the buttons, but I haven't found that to be very useful.

* * *

Lastly, I put in some PEX plumbing and it got pretty noisy whenever I turned on the hot water. I solved the problem by getting some thin-walled polyethylene irrigation pipe and cutting and slitting short sections of it, which I slipped over the PEX and slid to the place where the PEX was rubbing... problem solved.



Mobile Master Tech 05-22-12 01:57 PM

Thanks for the props and the help, AC!

I included the PAHS and the AGS descriptors in the title thread to cover both bases, but they are often used interchangeably to describe earth heat storage. I guess we should get out of that habit.

Using no mechanicals is ideal, but I would rather have the flexibility that pumps using 10-60w each provide without much energy cost to move the heat like I described here.

I got the links to work-hooray!!

Administrators, maybe we should have a sticky tutorial on links like there is on pics. If there is one, I didn't spot it easily so others probably wont either. Thoughts?

Mobile Master Tech 06-10-13 11:26 PM

Well, AGS works with flat plate collectors up north! Drake landing has had solar storage cover 98% of their thermal energy this year, and more than 90% for 5 years running AS DETAILED HERE.

I installed my Nordyne AC, which is rated for 23SEER instead of the original 22 with new thermostat control programming. It is working beautifully, and I can't believe I can turn my 3 story home AND the garage into an ice cube for less than 15kwh/day! The old units would usually take more than 60kwh/day!

I have left it slightly undercharged to allow some extra condensing in the lowest 1/3 of the mammoth condenser, which lowers the high side pressures and the work the compressor has to do to achieve them. I ensured I have enough subcooling to ensure the expansion valve always sees liquid, preventing "sneezing", by soldering the liquid and vapor lines next to each other for 3 feet, creating a zero-added-restriction heat exchanger, reducing unwanted heat gain in the vapor line in the process.

I have decided to wait on installing a desuperheater on the AC to see if I get enough thermal energy from heat collectors I will make on the back of PV panels. Sundrum Collectors use the same idea. Anyone else ever "roll their own" heat collectors for commercial PV panels?

Boreholes, hacked GSHP, collectors and storage tank details to come!


jeff5may 06-12-13 01:12 AM

Sedna Aire and their knockoffs are a bogus add-on device. The solar collector acts as the opposite of a desuperheater(sort of like running steam through a desuperheater). They have a habit of making your heat pump explode on a hot, sunny day in mid-summer. Not a bad idea for a heating-only unit, but definitely no good in cooling mode.

Either way, an evacuated tube collector would perform much better. The Sedna units' glass tubes are filled with soybean oil "buffer liquid". They claim the oil retains heat collected while the heat pump isn't running...

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