  This is the most exciting time in my lifetime for geothermal. There are many,
many innovative things happening. To name a few, promising new approaches to
Enhanced Geothermal Systems, geothermal projects in sedimentary and lower
enthalpy formations, new approaches for geothermal exploration, lithium
extraction from produced brines, geothermal energy storage, integrations with
CO2 storage and capture, and new technologies for producing energy from hot
water that is coproduced with oil and gas. However, this post is about a
concept about which I remain skeptical, deep closed-loop heat exchangers. These
designs are sometimes called Advanced Geothermal Systems, AGS. To clarify, this
blog post discusses the use of closed-loop heat exchangers in deep geothermal
wells, 1000s of ft deep. This is a different application than the use of
closed-loop heat exchangers in very shallow wells, 10s of ft deep, for ground
source heat pumps. The fundamental challenge is that closed-loop heat exchanger
designs rely on conduction, and sometimes free convection, to bring energy into
the well. These processes are inherently much slower than forced convection,
which is what drives energy transport into a conventional geothermal well.
Slower energy transport means less revenue, and therefore, worse economics. In
the past few years, there have been many creative new ideas for using
closed-loop heat exchangers for geothermal energy production. Usually, these
proposals recognize the challenge facing closed-loop, but introduce a wrinkle
intended to overcome the problem. Despite these efforts, there are still
daunting technical hurdles to widespread deployment of deep closed-loop
geothermal. In this post, I go through some of the proposed designs and discuss
key issues.
