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Annualized Geo-Solar Heating

5 min read

Annualized geo-solar heating warms the soil beneath and around a building through summer and lives on that stored warmth in winter, protected by a skirt of insulation. A seasonal temperature wave reaches soil about 4.5 m down roughly 117 days late and at about 13% of its surface strength, so heat added in summer arrives under the building when it is needed; 400 m³ of soil warmed 10 K holds about 1,800 kWh, which supplements rather than replaces other heat. It must be planned before the building is built.

Part of Seasonal Storage

Annualized geo-solar heating uses the earth around a building as a seasonal battery, charged by summer sunlight and discharged by winter heat loss.

What The Parent Page Proposes

Insulate the ground around the foundation 6-10 m out from the walls; route summer solar heat into the ground through buried pipes; the soil beneath the home acts as a seasonal battery; in winter the warm ground keeps the floor and basement warm. The parent page adds that it cannot easily be retrofitted.

Why The Ground Can Hold Summer Until Winter

A temperature cycle of period P penetrates soil to a characteristic depth d = √(α × P ÷ π), where α is the soil’s thermal diffusivity. The amplitude falls as e^(−z ÷ d) at depth z, and the cycle is delayed by z ÷ d radians of its phase (a standard result for periodic conduction; the daily version is derived for walls in Heat Absorption & Release Cycles).

Inputs: α = 5 × 10⁻⁷ m²/s for moist soil (typical values run about 3-7 × 10⁻⁷, recalled); P = 1 year = 3.156 × 10⁷ s.

  1. d = √(5 × 10⁻⁷ × 3.156 × 10⁷ ÷ π) = √5.02 = 2.24 m.
  2. At z = 4.5 m: amplitude = e^(−4.5 ÷ 2.24) = e^(−2.0) = 0.135, about 13% of the surface swing.
  3. Delay = (4.5 ÷ 2.24) radians × (365 ÷ 2π) days per radian = 2.0 × 58.1 = 117 days.

At α = 3 × 10⁻⁷ the delay becomes 151 days; at 7 × 10⁻⁷, 99 days. So summer heat placed several metres down, or reaching there by conduction, arrives in winter. Heat placed by pipes follows the same lag. The ground damps as it delays, so most of the gain comes from deliberately adding heat rather than waiting for the natural wave.

How Far Should The Insulation Skirt Reach?

Heat spreads sideways by the same conduction as it spreads down. A horizontal insulation skirt keeps winter cold from penetrating the stored heat from above and the side, and it should extend a few damping depths: 2 to 4 times d is 2 × 2.24 = 4.5 m to 4 × 2.24 = 9 m. The parent page’s 6-10 m falls in the same range, a reasonable check.

How Much Can It Hold?

Inputs, assumed: a heated block 10 m × 10 m × 4 m = 400 m³; earth capacity 1.66 MJ/m³·K (parent page: 0.92 kJ/kg·K × 1,800 kg/m³); average warming 10 K above undisturbed soil.

  1. Heat = 400 × 1.66 × 10 = 6,640 MJ.
  2. In kilowatt-hours = 6,640 ÷ 3.6 = 1,844 kWh, about 1,800 kWh.

This ignores losses and the heat that escapes sideways. Since a well-insulated house may need several thousand kilowatt-hours over a winter, the ground block typically supplements other heat and does not replace it. Wet soil stores more and conducts better; dry soil insulates better. Drain the site so groundwater does not carry the heat away.

What You Need

  • Insulation for the skirt: dry, covered and drained, extending 6-10 m out (parent page)
  • Buried pipes that carry heat from collectors into the soil, with a pump or a thermosiphon loop
  • Solar collectors (Solar Thermal Collectors)
  • A building design that expects it (Passive Solar Earth-Sheltered)
  • Drainage around the site

Method: Plan It At The Foundation Stage

Step 1: Measure local soil moisture and drainage; avoid wet or flooding ground.

Step 2: Lay the pipes below the future floor before the slab, then the insulation skirt over and around.

Step 3: Connect pipes to the collector loop with isolating valves and a vent.

Step 4: Charge all summer; read ground temperatures under the floor with buried thermometers.

Common Mistakes

  • No drainage. Groundwater cools the store.
  • Skirt too narrow. Cold reaches the stored heat.
  • Retrofitting later. The pipes and skirt can’t be placed under a finished slab.

Safety

Deep excavation and hot buried pipes

Digging a foundation pit deeper than a person is tall risks collapse; shore or slope the sides, and never work alone below ground level. Pipes carrying hot water to the ground can scald if they rupture near the surface, and a closed collector loop feeding them can reach steam temperatures if it stagnates, so vent it (Plumbing Connections).

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