Geothermal System Design Principles Flashcards

What is 'thermal short-circuiting' in a vertical borehole and how is it mitigated?

Answer: Heat transfer between the supply and return pipes within the same bore; mitigated by using thermally enhanced grout and proper pipe spacing

Thermal short-circuiting occurs when heat from the warmer pipe leg transfers directly to the cooler leg inside the bore, reducing efficiency; proper grout and U-tube spacing reduce it.

In the IGSHPA design method, what does the 'run fraction' (PLF) account for?

Answer: Fraction of time the heat pump operates at full load during the design month

The part-load factor (PLF or run fraction) reflects that the heat pump cycles on and off, allowing the ground some thermal recovery between runs.

Which soil/rock type generally yields the highest ground thermal conductivity, allowing shorter bore lengths?

Answer: Granite or crystalline bedrock

Granite and other crystalline rocks have high mineral thermal conductivity (typically 2.5–3.5 W/m·K), reducing required bore length compared to sedimentary or unconsolidated soils.

A Thermal Response Test (TRT) is performed on a pilot borehole. What is the primary output used by the designer?

Answer: In-situ effective ground thermal conductivity and borehole thermal resistance

A TRT measures the temperature rise of circulating fluid under a constant heat injection rate to back-calculate the ground's effective thermal conductivity and borehole resistance.

For a residential open-loop (pump-and-reinjection) geothermal system, what is the minimum recommended well separation between the supply and injection wells?

Answer: 100 ft (30 m) or more depending on aquifer properties

At least 100 ft of separation is commonly required to prevent thermal short-circuiting where cooled/warmed return water re-enters the supply well before recovering.

When calculating the total equivalent length for a vertical bore field, the designer adds a 'penalty temperature' (TP). What does TP represent?

Answer: Long-term ground temperature rise (or drop) due to cumulative annual heat imbalance

The penalty temperature accounts for the multi-year buildup of heat in the ground when annual heating and cooling loads are unbalanced, shifting mean ground temperature over the system's life.

Which ASHRAE publication serves as the primary industry reference for ground-source heat pump system design in the United States?

Answer: ASHRAE Handbook – HVAC Applications, Chapter on Geothermal Energy

The ASHRAE HVAC Applications Handbook chapter on geothermal energy is the principal US reference covering design methods, equipment selection, and loop sizing for GSHP systems.