Geothermal & evaporative cooling

A cooling system is only as efficient as the place it dumps heat. Outdoor air on a hot day is the worst option. Give the cycle a cooler sink — a ground loop or an evaporative tower — and the same duty costs far less electricity. This compares them head-to-head.

Duty required20 kW

Indoor setpoint24 °C

The space or product temperature.

Outdoor conditions

Outdoor air (dry-bulb)42 °C

Relative humidity35 %

Wet-bulb ≈ 29 °C — the floor for evaporative cooling.

Ground / water loop temp22 °C

Stable year-round; near the local mean annual temperature.

Refrigerant

Efficiency by heat source/sink

Best: Geothermal (ground loop)

Air-sourceCOP 4.75 · 4.6 kW

Condenser/coil exchanges with 42 °C outdoor air. → evaporator 16 °C, condenser 54 °C.

Geothermal (ground loop)COP 17.10 · 1.3 kW

Buried/water loop sitting at 22 °C year-round. → evaporator 16 °C, condenser 28 °C.

Evaporative towerCOP 8.72 · 2.5 kW

Wets a surface, approaching the 29 °C wet-bulb. → evaporator 16 °C, condenser 38 °C.

Geothermal vs air-source

260%

higher COP for the best loop than air-source, saving 3.3 kW of continuous power at this duty.

Why it works

A cooler sink lowers the condensing temperature, shrinking the pressure lift the compressor has to overcome. Smaller lift → less work → higher COP. The ground barely changes temperature across seasons, so geothermal keeps winning when air-source struggles most.

Approach temperatures: air coils need a wider air-to-refrigerant gap than water-coupled loops, which is modeled here (air ≈ 10–12 K, ground/water ≈ 5–7 K). Evaporative performance is bounded by the wet-bulb, so it fades in humid climates and shines in dry ones.