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I'm very curious, why hasn't anyone done this?
In most places on earth, the temperature gets 25 degrees higher (celsius) for every kilometer in depth. How hard would it be to drill 4-5km, put a U-shaped pipe, and push cold water at one end, and get hot water at the other? It would definitely be cheaper in the long term than all of the other solutions that are in use right now.
Someone has done it. A majority of the electricity I buy comes from geothermal energy †.
Of course, I live in the Pacific Northwest of North America, which puts me close to a tectonic plate boundary. This presents good conditions for extracting geothermal heat (since the earth's crust is thinner at such boundaries ... making shallower wells productive).
In terms of why it isn't more widely used, I don't know for sure, but I would guess that at least some of the following factors apply:
The major impediments which this industry faces include initial financing of large scale projects, resource development risks like failure of drilling wells and insufficient productivity, lack of skilled labor to work on the projects, issues related to drilling rigs and market and network issues like proximity to the market and network capital costs. Also, there are some environmental problems in producing geothermal energy like release of hydrogen sulfide and disposal of toxic geothermal fluids while extraction.
Challenges to this industry are carrying out efficient programs for land leasing where reserves of geothermal energy are located, better drilling assistance in terms of accurate positioning of the site and its viability, and significant financial support needed to establish the plants at the desired locations.
One issue that should no longer be considered a primary factor limiting deployment of geothermal power is cost. From the US Energy Information Association, the pro-rated cost of new power plants (excluding externalities like pollution):
Plant type Total system levelized cost (2011 $/MWh)
---------- ----------------------------------------
Conventional Coal 100.1
Advanced Coal 123.0
Advanced Coal with CCS 135.5
Natural Gas
Conventional Combined Cycle 67.1
Advanced Combined Cycle 65.6
Advanced CC with CCS 93.4
Conventional Combustion Turbine 130.3
Advanced Combustion Turbine 104.6
Advanced Nuclear 108.4
Geothermal 89.6
Biomass 111.0
Wind 86.6
Wind (Offshore) 221.5
Solar PV 144.3
Solar Thermal 261.5
Hydroelectric 90.3
Note that this data is an estimate for cost, including well / plant development, averaged over the electricity produced (kWh). While I can't speak to the metrics used in board rooms, I believe this to be fundamentally better than the $/kW capacity data featured in this answer. That data cannot provide a basis for comparison between sources like wind and geothermal, because geothermal plants can run at 90%+ capacity continually, while wind farms do not (wind is inherently variable).
So, geothermal is already a cost-competitive technology.
I'm certainly a proponent of geothermal energy, and I think its future looks bright.
P.S. I read your question to be about large-scale energy production, but geothermal is also a legitimate small-scale solution, too.
† - of course, as with most green energy utility programs, my electricity is the same as that of my neighboor, who doesn't participate in the program. My utility payments simply subsidize renewable projects (such as Raft River geothermal).
Indeed, hot dry rock geothermal may not be widely utilized, but it may be so not only because of novelty and cost, but also because there are plenty of water-based geothermal resources that are still underutilized, and those could be a lot easier to get to. As an example, let’s look at Idaho — state in north-western US in the Rocky mountain region.
In 2008 the Raft River project began providing power commercially. It was one of the first GT power-generating plants in the Pacific North-West, according to PBS documentary. Built by private company, utilizing the former geothermal site of US Department of Energy (in the early 80s there was built and run 7MW demonstration project, allegedly first commercial-sized binary liquid GT power plant), it produces 13 MW, and is accessing surveyed water source that is theoretically capable of giving out 110 MW of energy. To access the water at the temperatures of 135 to 148 °C they only need to drill 1300 to 1800 m (4500 to 6000 feet) — which is a lot less drilling than HDR geothermal would require to achieve the same temperature difference.
Idaho Department of Water Resources provides overview of geothermal in the state, including definitions (sources of 30 to 100 °C are classified as low temperature geothermal (LTG), and those over 100 °C — as high temperature geothermal (HTG)), and the link to the map of geothermal resources. Map shows quite a few sources surveyed, including a decent number of those with temperatures of over 50 °C, and while the use of those resources is increasing (downtown Boise and recently connected Boise State University campus have many buildings already heated by geothermal systems), there are still many of them untapped, and most of them require a lot less work than HDR geothermal.
Again, it is understandable that specific geological situation on the border of tectonic plates provides for abundance of sources, and those areas deep inside plate boundaries may not have these resources so easily accessible, but there is a lot of potential for the exploitation of the more traditional GT sources in many areas of the world, before the need for the more costly methods arises.
There are several pilot projects (wikipedia) in Australia which have been stopping and starting for a while. They suffer from a shortage of funds - drilling holes 2km or more deep is expensive. Even once they have the hole, they have to pump water down it and see what happens. Other companies are trying to reuse existing fossil fuel wells to avoid that cost.
The Australian Government minerals department (BREE) have a useful summary report. 3.9 Geothermal technology options ... "There are two types of geothermal resource available in Australia: Hot Sedimentary Aquifer (HSA), and Engineered Geothermal System (EGS). A Hot Sedimentary Aquifer (HSA) system is characterised by hydrothermal groundwater resources in a sedimentary basin. This setting is typical of some of the low temperature resources in the USA, particularly Nevada, which were developed in the 1980s."
Capital Costs from that report (AUS$/kW net)
So geothermal is not being used right now because it's significantly more expensive than wind power (which is fairly cheap by renewable energy standards).
The Beyond Zero Emissions (BZE) Stationary Energy Plan looked at geothermal energy because people kept suggesting it. We said "only existing commercial solutions are specified as deployment of the ZCA2020 Stationary Energy Plan needs to start right away. However, if other technologies become commercial during the roll-out at a competitive cost they could also form part of the future energy mix." There are a wide range of cheaper options than deep geothermal, and the most promising (cost-wise) is well re-use as fossil fuel wells fall out of use. But none of those are ready for commercial production yet, while many other technologies have mature markets (ie, multiple suppliers, multiple installations, 10 year grid-connected production records). Geothermal looks promising and if practical would be competitive with hydro in terms of reliablity and flexibility (specifically, it would need to have those properties to justify the somewhat higher cost).
Disclosure: I was one of the researchers who helped write the BZE report.
For simple water heating which can then be used for other purposes it is already quite common. And you don't have to go so far down into the Earth as you suggest.
This video shows a working example in a football stadium in Germany. The video is a mix of English and German ( always subtitled in English ) https://www.youtube.com/watch?v=WsZSTc-rYBw
