9.3: The Enhanced Geothermal Systems

The basic methods described in the two preceding sections can be applied in areas where there exist conventional hydrothermal resources (CHR. A Web page of the US Department of Energy lists three components that a CHR contains naturally: namely, it requires fluid, heat, and permeability. Further quoting the linked document: A CHR ...can occur in widely diverse geologic settings, sometimes without clear surface manifestations of the underlying resource. In 2008, the U.S. Geological Survey (USGS) esti- mated that 30 GWe of undiscovered geothermal resources exist in the western United States1 – ten times the current installed capacity.

As of December 2017, the installed geothermal nameplate capacity in the US was 3,600 MW (in the world, over 14,000 MW – see the list of the 10 top countries). So, currently about 0.3% of electric power generated in the US comes from geothermal sources, and, according to USGS, by building new power plants at existing CHRs this contribution can be increased to about 3%.

However, there is an MIT document published in 2006, titled The Future of Geothermal Energy, which shows that the contribution of electric power generated by geothermal plants can reach 10% or more of the total usage by the middle of this century. How comes, if, according to the USGS, the capacity of the still undiscovered resources may contribute only about 3%?

The extra capacity needed, as the MIT document claims, can be obtained from Enhanced Geothermal Systems (EGS). What a EGS is is described in great detail in the document, but since it’s 372 pages long, we do not encourage the reader to study it page-by-page – we will provide a concise explanation over here. We will do that by quoting three pieces of graphics from another US Dept. of Energy Web page. So, the first piece of graphics, shown in Fig. \(\PageIndex{1}\), graphically explains what are the three components needed in a Natural Geothermal System:

Figure \(\PageIndex{1}\): The three necessary components of a natural geothermal system: (i) abundant heat found in rocks in depth; (ii) fluid to carry heat from the rocks; and (iii) crevices and cracs to allow water to flow through the hot rocks (source: aop)

One of those components is available almost everywhere, not only at the Western part of the US. As was stated before, at locations even far away from geothermal activity zones, there is a geothermal gradient of 25-30^◦C per kilometer. So, if we need, say, a 150^◦C source for running a binary power plant, we will find it by drilling a well 5-6 km deep. Well, but there is no guarantee that water and permeability will be down there. The situation may appear to be as it is illustrated in Fig. 9.11: there is little or no water and the rock has a monolithic structure. What can be done in such situation?

Figure \(\PageIndex{2}\): One can always find rocks hot enough for the purpose of geothermal electric power generation, but what if one of the other two components are missing, or both are missing? (insufficient water, the rock ”too solid”, or both).

There is a solution – water can be delivered to the hot rocks through a second “injection well”, and the permeability can be created artificially in the same way the miners who exploit natural gas or oil trapped in non-permeable rocks do it.

Figure \(\PageIndex{3}\): The situation described in the preceding figure can be repaired, by artificially producing cracks and crevices in the rock by applying the ”fracking” technique used in oil and natural gas mining, and by delivering water from the surface through an extra ”injection well”.

It’s perhaps a bit disappointing that despite the many potential advan- tages of the EGS technology, not much effort has been made towards its practical implementation. Until now, only some early-stage R&D work has been undertaken. One of the leaders in EGS is Australia who plans to launch a 25 MW demonstration project in Cooper Basin in the near future. A list of ongoing demonstration projects in some other countries is given in this Wikipedia article.

Good news is that China has entered the club of countries interested in EGS research. China has started search for Hot Dry Rocks (HDR) re- sources later than many other countries, but then it started pushing forward with a zeal characteristic for many new high-tech ventures in this country. HDR deposits are exactly what is needed for “mining” energy using the EGS technology. Chinese geologists have so far identified deposits of nearly a quadrillion tons of HDRs. China is now the biggest emitter of CO₂ in the world, but it has declared many time its sincerely commitment to signifi- cantly reduce that emission. A quadrillion ton of HDR contains an amount of energy sufficient for satisfying the total energy consumption of China at the current level for many thousand years. Therefore, one can expect that China will soon undertake energetic actions to start extracting energy from those vast resources.

The U.S. has known HDR resources of comparable magnitude to those in China – but not much has been done yet to start “mining” energy from them. One reason for that is perhaps that the U.S. – as has been mentioned above – is expected to have not yet discovered hydrothermal resources, capable of increasing tenfold the current geothermal generation of 3,600 GWe. And extracting power from a hydrothermal resource is by far easier than from a HDR resource. Anyway, in order to reach the 100 GWe goal set in the aforementioned 2006 MIT report, sooner or later it will be necessary to activate potential existing EGS resources. In fact, first research efforts

focused on HDR energy started as early as in the 1970s, motivated by the oil crisis. The best known of the studies undertaken at that time is the Fenton Project originated in the Los Alamos National laboratory. Over the years, the status of the project has been reported in several papers – here we have chosen two: one from 2012 because it’s “pedagogically written”, and one posted in 2015 which has a number of highly instructive pieces of graphics, and even more importantly, a long list of “clickable” references to earlier reports from the Fenton Project and relevant reports from other projects.

However, neither the Fenton Project nor any other American HDR re- search project has ever generated any commercial activity – everything has been R&D activity. So, the good news is that the U.S. Department of Energy has created a partnership of seven National Labs. Under the leadership of Lawrence Berkeley National Laboratory (Berkeley Lab) they will start a new cycle of vigorous research aimed at removing technical barriers to com- mercialization of enhanced geothermal systems. Let’s then hope that their research efforts will progress smoothly and fruitfully.