Author: Susan Petty, AltaRock Energy, Inc.
Published: 2016
Source: Proceedings of the 41st Workshop on Geothermal Reservoir Engineering, Stanford University
Replacing coal with geothermal: a pioneering vision
Long before the Repower Initiative began, engineer Susan Petty of AltaRock Energy was already asking a crucial question:
Could the heat stored deep underground directly replace coal combustion in existing power stations?
Her 2016 study — Transitioning Coal to Geothermal — explored how Engineered Geothermal Systems (EGS) could repower retiring coal plants using the same sites, workers, and grid connections. With over 50 GW of aging coal capacity in the U.S. due for closure by 2030, Petty proposed a technically elegant and socially fair alternative: retrofit the sites with geothermal wells instead of abandoning them.
Key findings
- EGS could unlock up to 1 TW of baseload generation capacity in the U.S., by drilling deep enough to access geothermal heat even where no natural reservoirs exist.
- Co-locating geothermal projects at retired coal sites reuses existing infrastructure — grid links, cooling systems, land, and workforce — while using stored wastewater as a working fluid.
- Early cost models showed geothermal repowering could become competitive with gas or new renewables as drilling costs fall and “learning-by-doing” scales up the technology.
- The most promising sites identified included Colstrip (MT), Mountaineer (WV), Cayuga (NY) and Valmy (NV) — regions with strong geothermal gradients and nearby coal infrastructure.
Why it matters for Repowering today
Petty’s work laid the conceptual groundwork for what Repower promotes globally:
- Reuse of coal assets instead of abandonment,
- Zero-emission baseload energy that supports communities,
- And technological pragmatism — building the clean future from what already exists.
Nearly a decade later, the logic remains powerful: repowering is not only about nuclear. Enhanced geothermal could complement and diversify the clean-heat sources that replace coal.