Design a ground-source heat pump (GSHP) that uses the stable 8-15 degC ground temperature as its source. Change building heating/cooling loads, ground type, ground temperature and number of boreholes, and the simulator returns the annual COP, total borehole length, length per bore, annual electricity and CO2 savings to size a vertical-loop heat exchanger.
Parameters
Heating load Q_h
kW
Cooling load Q_c
kW
Ground type
Sets thermal conductivity k and diffusivity alpha
Ground temperature T_g
degC
Near-constant below 10 m of depth
Number of boreholes
bores
Borehole diameter D
mm
Grout
Thermal-enhanced grout lowers R_b
Operation hours
hr/yr
Split 60% heating, 40% cooling
Results
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Heating COP
—
Cooling EER
—
Total R (K·m/W)
—
Total bore length (m)
—
Length per bore (m)
—
Annual CO2 saved (kg)
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Ground / Borehole / Heat Pump — Flow Animation
Heat flows from building -> heat pump -> U-tube -> ground. Particle colour shifts with ground temperature T_g and grout conductivity.
R_b: in-bore resistance; R_g: ground resistance (Infinite Line Source). Fo = alpha*t / r^2 is the Fourier number, evaluated at 25 years of operation.
Ground Source Heat Pump (GSHP) — Borehole Length and COP
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A "geothermal heat pump" — is that like a hot spring? How is it different from a normal split AC (air-source heat pump)?
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Not that hot. Below about 10 m of depth, the ground sits at a near-constant 8-15 degC year-round. Even when the outside air swings to -10 or +35 degC, a GSHP (Ground Source Heat Pump) can still exchange heat with that "lukewarm" ground. An air-source heat pump (ASHP) has to pull heat from a cold winter atmosphere, so its COP drops; a GSHP keeps pulling from 8-15 degC and runs at COP 4-5 instead of 2-3. Roughly 1.5-2x better all year.
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So the "borehole" is the hole that fetches that heat? With the defaults I see 6 holes x 98 m. Why so long?
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You spotted it. Heat extraction per metre is surprisingly thin — typically 30-90 W/m: ~40 W/m in clay, ~90 W/m in granite. For a 30 kW heating load, L = Q_h*(1 - 1/COP)/q_per_m gives (30000*0.83)/42 ≈ 590 m total. Split into 6 bores, that is ~100 m each. You can trade more bores for shorter holes when land is available; deep+few is common in dense housing, shallow+many in commercial sites with parking lots above.
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When I switch ground type from clay to granite, the length per bore drops sharply. Why?
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Because thermal conductivity k_ground differs: clay 1.4, granite 3.4 — a 2.4x ratio. The heat you can "squeeze out" of the ground scales with k and the driving temperature difference, so granite delivers more than twice the extraction per metre. That is why Sweden, Finland and mountain regions are the natural home for GSHPs, while the deep clay of the Kanto plain in Japan needs longer bores, more bores, or a real TRT (Thermal Response Test) to size properly.
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Switching grout to "thermal-enhanced" changes R but the COP number stays the same. Why doesn't COP move?
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COP in our formula depends on the evaporator vs condenser temperature gap, so changing grout alone does not move it by definition. In reality, a smaller R lets the evaporator approach the ground temperature, so COP creeps up too. Read "thermal-enhanced grout (k 0.6 -> 1.5)" as "same heat with a 10-20% shorter bore". Drilling savings dwarf the grout premium, so enhanced grout is now standard in commercial GSHP work.
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Finally, "10,000 kg/yr of CO2 saved" looks huge. Is that for real?
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That is versus a gas boiler (0.18 kg/kWh) plus chiller (0.32 kg/kWh equivalent). For 45,000 kWh of heating + 25,000 kWh of cooling, ~10 t/yr is realistic. A typical Japanese household emits ~4 t/yr, so this is 2.5 households' worth — which is exactly why the US IRA 2022 30% Geothermal Tax Credit and EU REPowerEU subsidies push the technology so hard. Dandelion Energy (Alphabet), Bosch, Waterfurnace and ClimateMaster are leading vendors; Stockholm Royal Seaport, Cornell University, and 1000+ Wal-Mart stores are landmark installations.
Frequently Asked Questions
A GSHP exchanges heat with the ground, which stays at a roughly constant 8-15 degC year-round below 10 m of depth. Even when outdoor air swings to -10 degC or +35 degC, the evaporator-to-condenser temperature lift stays small. Because Carnot efficiency COP_Carnot = T_cond / (T_cond - T_evap) grows as the temperature lift shrinks, a GSHP typically reaches an annual COP of 3.5-5.5, about 1.5-2x higher than an air-source heat pump (ASHP) at 1.5-3.5. This tool uses 55% of Carnot as a realistic heating COP.
Simplified IGSHPA-style sizing uses L = Q_h*(1 - 1/COP_h) / q_per_m. Typical heat extraction rates per metre are 30-90 W/m: about 30-45 W/m for clay, 80-100 W/m for granite. This tool estimates q_per_m from ground thermal conductivity k and grout conductivity. For real projects a Thermal Response Test (TRT, 24-72 hours) on a pilot bore is used to measure k_ground.
R_b is the in-bore thermal resistance between the U-tube fluid and the borehole wall, dominated by grout conductivity (standard bentonite ~0.6 W/m.K vs thermal-enhanced ~1.5 W/m.K, a 2-3x difference). R_g is the soil thermal resistance outside the bore, computed by Eskilson g-functions or the Infinite Line Source (ILS) solution. Over 25 years of operation, Fo grows large and R_g keeps slowly rising, so always size for long-term operation.
Drilling represents 50-60% of total cost. A US single-family GSHP runs 15,000-30,000 USD installed; a commercial 150 m bore costs 5,000-8,000 USD. The IRA 2022 Geothermal Tax Credit (30%) makes payback against gas or oil boilers 5-10 years in the US. Japan's Ministry of the Environment also subsidises ground-source heat installations, and commercial sites report 30-50% reduction in annual electricity cost vs ASHP-only systems.
Real-World Applications
University campuses and public facilities: Cornell University's "Earth Source Heat" project is building 70+ boreholes at ~600 m depth, targeting completion before 2030; Stockholm Royal Seaport's 1.3 MW central GSHP serves ~6,000 homes. Long-life, multi-decade civic projects amortise the high drilling cost into very low operating cost — a hallmark GSHP use case.
Single-family homes and small commercial: In the US, Dandelion Energy (Alphabet spin-off) sells compact 1-2 boreholes x 150 m residential systems as monthly subscription. In Japan, the Ministry of the Environment subsidises GSHP installations and ZEH (Net-Zero-Energy Houses) increasingly include ground-source heating and DHW. Bosch, Waterfurnace and ClimateMaster cover the residential range.
Big-box retail and logistics: Wal-Mart has installed GSHPs at 1000+ stores, cutting electricity by 20-30% per site. Wide parking lots provide free heat-exchange real estate. Hybrid layouts that use shallow horizontal loops under parking, combined with deep vertical bores, are increasingly common in big-box deployment.
Nordic and temperate housing standard: In Sweden, Finland and Germany, 20-40% of new single-family homes ship with GSHP. The geology (k = 3.0-3.8) and easy access to several hundred metres of bedrock are ideal. In central Japan, alluvium k = 1.4-2.0 forces more bores or a hybrid layout where a gas boiler supplies the peak load.
Common Pitfalls and Caveats
The biggest pitfall is treating the ground temperature as a constant. This tool reports a 25-year-averaged result, but in reality long heating seasons can drag the local ground temperature down by 2-5 degC, with the cooling season recovering it. In cold climates with heavily heating-dominated loads, the ground stays "over-extracted" year-round and COP degrades over 10-20 years. Hybrid systems (Solar-Assisted GSHP, or a small cooling tower to balance the seasonal load) are then required.
Second, skipping a Thermal Response Test (TRT). The k_ground values here are geology averages; the same "clay" can range from 0.8 to 1.8 W/m.K depending on water content and compaction. Without a TRT on a pilot bore (24-72 hours of heated injection while logging the loop temperature), sizing can be off by 20-30% — leading to undersized bore fields and emergency drilling. On any project above ~100,000 USD, the 5,000-10,000 USD TRT cost is a mandatory investment.
Third, spacing the boreholes too tightly. The thermal influence radius of one borehole reaches 3-5 m after 25 years of operation, so bore spacing must be at least 6 m (ideally 8-10 m) to avoid adjacent bores stealing each other's heat. This tool computes a single-bore resistance only; mutual interference via g-functions is not modelled. For real designs, use GLD, EED or GLHEPRO to simulate the long-term behaviour of the full array.
How to Use
Enter heating demand (qhNum in kW) and seasonal range variation (qhRange in %) to define winter load profile
Specify cooling demand (qcNum in kW) and cooling range (qcRange in %) for summer operation
Set ground temperature (groundTempC: typically 8–15°C for temperate climates) and number of vertical boreholes (numBoreholes: 2–8 standard)
Simulator calculates Heating COP (typically 3.5–4.5 for GSHP), Cooling EER, total borehole thermal resistance (R-value in K·m/W), and required bore length per hole at 150–200 m depth
Review Annual CO2 saved (kg) based on grid displacement versus air-source alternatives
Worked Example
Office building in Hampshire: qhNum=45 kW, qhRange=35%, qcNum=38 kW, qcRange=25%, groundTempC=11°C, numBoreholes=6. With U-tube polypipe in granite (R-value ~0.08 K·m/W), simulator yields Heating COP=4.1, Cooling EER=18.2, total bore length=900 m (150 m per borehole), Annual CO2 saved=127 tonnes versus gas boiler + window AC. Ground loop fluid (30% glycol/water) maintains exit temperature 2–3°C below undisturbed ground during peak heating.
Practical Notes
Thermal conductivity varies by geology: limestone ~2.5 W/m·K, clay ~1.8 W/m·K, saturated sand ~2.0 W/m·K—conduct site thermal response testing (TRT) to refine R-value input
COP degrades 3–5% per °C of ground temperature drop over 25-year operation; undersizing boreholes risks 15–20 year thermal drift
Intermittent cooling loads (offices, retail) maintain higher COPs than continuous loads; peak heating COP drops when ground temp falls below 5°C, triggering auxiliary electric resistance
Borehole spacing minimum 5 m center-to-center for residential; commercial clusters require 6–8 m to minimize thermal overlap