Design - Update On the Thermal Performance of the GeoSolar System

The fenced area in front of the house is the solar collector,
 the downstream air intake for the GeoSolar system

The analysis of the of the house temperatures during 2022, the first partial-year of occupancy, was incomplete and inconsistent as we positioned and repositioned thermometers in an attempt to capture the best data.  Consequently, this report uses data only from the two full years of occupancy --  2023 and 2024 -- that were tracked with identical protocols.   2025 will not be included until the end of the year.

The chart below is a distillation of a more detailed comparison of 2023 and 2024 but it sufficiently summarizes the data in a user-friendly way.  The "feel good" temperatures were calculated as follows:

               Room temperature - Ground temperature x  0.40  =  X

                Ground temperature  +  X   =  Feel Good Temperature

As described in a previous post on the thermal performance of the house, the room temperatures were taken from a thermometer on the wall in the middle of the house.  Ground temperatures were registered by lowering a thermometer to a depth of 5 ft through a pipe protruding from the living room floor.  0.40 is a multiplier that reflects the fact that the temperature of the thermal mass (the soil surrounding the pipe under the house) has a greater effect on the comfort level in the house than does the air temperature -- actually slightly more than twice the effect of air temperature.  As an example, the hottest recording for 2024 was taken on August 1 when the air temperature was 86 and the mass temperature was 74, rendering a tolerable feel good temperature of 76.

The striking feature of the year-to-year comparison is the similarity between them.  In the table, deviations of more than three degrees are highlighted with colored font -- blue when the temperature for 2024 is lower than in 2023 and red when the opposite occurred.  Over half of the year-to-year values deviated less than one degree; five less than two degrees.  If the figures in future years are similarly consistent, it would signal that the GeoSolar system has reached equilibrium.  

The conduits for the system are widely separated under the house and therefore widely separated when they emerged to daylight behind the house.  My thinking was that, by leaving their terminals individually accessible, some could be deactivated if the thermal mass began to overheat.  But, since the passive functioning of the conduits didn't work as intended, they had to be connected above ground and run to a temporary solar chimney so that a fan could be used to pull air through the pipes from the solar collector end.  The temporary system worked so well that it was recently replaced by a submerged system that allowed successive "Y-ing" of the pipes so that only one large pipe entered the solar chimney from each side.  And the ugliness of the exposed pipes was eliminated.  But burying the pipes precludes future changes which, based on bimonthly thermometer readings for two and a half years, seems now to be a non-issue.

The summer feel good temperatures in the upper 70s might be too extreme for some folks who are  accustomed to air conditioning.  As the chart shows, the hottest temperatures occur in the late summer and early fall and last only four to six weeks.  The high temperatures are manageable like they were in the days before air conditioning -- by having high ceilings, keeping the air moving with fans and opening the windows at night.  And it occurs at that time of year when nighttime temperatures are dropping enough to make window opening as effective as it is earlier in the summer.  

Winter feel good temperatures dipping below 70 degrees last longer, actually from the first of the year until late spring.  But hovering in the upper 60s, as they do, is a healthy range and would probably be comfortable for most folks accustomed to conventional HVAC systems.  In our view, any temporary inconvenience, in either winter or summer, is a welcomed trade-off for the savings -- thousands of dollars per year -- in electricity costs. (The details regarding the cold spring temperatures and the warm late summer temperatures, are thoroughly parsed in a previous post.)

 

The photo at right shows the temporary solar chimney and the conduits that originally came to daylight individually.

The photo below shows the permanent solar chimney with the conduits running to it underground.  Like the temporary chimney, it contains a small furnace blower that pulls air from the solar collector in front of the house through the nine conduits under the house.  Despite the PV array in the background being the smallest the vendor would sell, it provides all of the power needed by the house, including for the 240v tools in the garage-workshop.  The electrical requirement for the new chimney blower therefore becomes an inconsequential add-on.

Electric bills divide costs between power consumption and various administrative costs.  Our power consumption for the entire year of 2024 amounted to $160 -- not absolutely passive solar but close.  "At 19 cents per kilowatt-hour, the current price of electricity is the highest on record going back more than forty-five years" according to a July 31, 2025 article in the St Louis Post Dispatch, further validating the wisdom of our project.  What is especially satisfying is that, with reverse metering, the array has already paid for itself.  But the diminutive size of the array is possible only because of the passive solar design of the house.  If its nearly 3,000 sq ft were conventionally-built, it would require a solar array that was significantly larger, more expensive, require a longer pay-off period and, even then, probably never supply the amount of electricity the house would need.