Friday, August 1, 2014

Timeline - Annualized GeoSolar (Cont'd some more)

This is the third in a series of three posts on Annualized GeoSolar, the passive system for heating and air conditioning that we will use for our energy neutral house. To view the previous posts, go to Annualized GeoSolar AND Annualized GeoSolar (Cont'd)

I became acquainted with the system 7 years ago when stumbling upon an online paper by Don Stephens*.

Insulation-Watershed Umbrella
Carmondy/Sterling and Hiat seemed to have been the first to advocate extending the insulation and waterproofing horizontally below grade, as opposed to attaching it directly to the outside of the roof and walls of the house as was prevalent at the time of their writings.  Hiat called the arrangement the "Insulation-Watershed Umbrella", which is the terminology that I have adopted (as opposed to Stephens' "Moisture-diversion membrane/insulation cape").

Copied from p. 41, Hiat's "Passive Annual Heat Storage" (click on the image for a larger view)
Our iteration of the umbrella will comprise (from top down) the following layers:  (a) plus or minus two feet of backfill; (b) two layers of carpet with the top layer shingled upside down (per Stephens) to protect the umbrella from physical damage from burrowing varmints, garden tools, roots, etc. as well as keeping the carpet out of the waste stream; (c) two layers of 6 mil plastic sheeting; (d) foam board insulation; (e) 6 mil plastic sheeting; (f) with a thin bed of sand between each layer.

Why a 20 Foot Umbrella?
According to Hiat, it takes six months for a unit of heat to travel 20 feet through dry soil. During the summer, the solar collector heats the soil under the house as well as under the umbrella.  Then it takes six months of cold weather for any heat next to and under the house to travel the 20 foot distance to the periphery of the umbrella.  By that time, the solar collector is already recharging the system.  The distance can be shortened by slanting the umbrella downward from the house.

Critical Design Consideration for AGS
For the system to work, however, the soil under and around the house must remain dry for two reasons. Water passing through the soil carries heat away to the water table faster than the collector can manufacture it.  Damp soil, much less wet soil, is more thermal conductive than dry soil and, not only robs heat from the thermal mass, but pulls heat out of the house as well.

Four white pipes are piezometers for monitoring the water table
The umbrella protects against surface water but not from the water table. In wet climates such as ours, several test holes equipped with piezometers** should be used to monitor the water table through several seasons, if possible, to be sure the table remains several feet below the level of the AGS conduits.  If it does not, a French drain system*** must be installed well below the conduits, otherwise heat will be lost to the water table.  Our piezometers (four) told us that we needed a French drain system that, unfortunately, would complicate the excavation and drive up costs for the AGS system.

By having an earth bermed back (north) wall two stories high and half of the west wall, our project will take advantage of AGS without earth sheltering on the roof nor against most of the east wall. The reason is that the floor and tall north wall provide enough earth contact for heat transfer and the house will be super-insulated to retain the heat.   Otherwise, our adaptation of AGS will be pretty much like Hiat and Stephens describe it. The exact design of the all-important north wall and the solar collector is grist for a future posts.
* Although Stephens' paper,, is no longer available, it can be accessed indirectly via Stephens' definitive paper.

**Information on piezometers
***Information on French drains

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Update:  Autumn of 2016
By the autumn of 2016, the AGS system was nearly complete, lacking only the umbrella for the north side of the house and the functioning parts inside the solar collector. Consequently, additional discussion on AGS and the details of construction can be accessed through the following links:

Amount of earth sheltering and configuration of the earth contact north wall
Final decision on the amount of earth sheltering

Design of the insulation/watershed umbrella - first post
Design of the insulation/watershed umbrella - second post

Installation of the French drains - first post
Installation of the French drains - second post
Installation of the French drains - third post

Construction of the solar collector - first post
Construction of the solar collector - second post
Construction of the solar collector - third post

Installation of the insulation/watershed umbrella - first post
Installation of the insulation/watershed umbrella - second post

Installation of the insulation/watershed umbrella - fourth post

Wikipedia's description of AGS  is also a good reference.

Final Cost
With sporadic help from volunteers, I did most of the installation of the AGS system except for digging the hole for the solar collector and trenching for the conduits, so our cost was mostly for materials -- foam insulation (biggest item), concrete blocks (second biggest), sheet plastic, fibercement parging, gravel/sand and piping for the conduits. Without the French drains, the cost for installing the AGS system probably would have been less or about the same as for a conventional HVAC system. However, the professional help with the collector and conduits raised the cost to at least to that of a minimalist HVAC system that would suffice for a home that is south-facing, super-insulated, partially earth sheltered and protected by an insulation/watershed umbrella.  It could be argued that the French drains were necessary anyhow for a partially earth sheltered structure in a site with potential water table problems. If they are factored in, the cost clearly exceeded that of a conventional HVAC system.

However, the story is not complete.  It remains to be seen whether our grand experiment performs as expected.  We will be monitoring and reporting here on the thermal performance for several years after occupancy.  If it performs as expected, the pay-back for the extra cost will accrue over time as we avoid the cost of energy and maintenance on heating and air conditioning equipment.  Also we will be insulated from future spikes in rates, be free of furnace and compressor noises and can take winter vacations without worrying about frozen pipes from furnace failure.  And best of all, the environment will benefit once we begin breaking even with the utility company.

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Update:  Summer of 2021
The summer of 2021 was the first opportunity to check out the efficacy of the AGS system.  We found that spontaneous flow of warm air from the collector through the conduits and exiting from the individual conduits behind the house did not occur.  It was necessary to bring the conduits together into a solar chimney and add a fan to pull the air through them.  Here is a  link to the post that describes in detail the modifications that we made and a thorough critic of our design including suggestions for a similar system without a solar collector. 

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