Methods of Implementation

Conventional geothermal

Conventional geothermal energy production can be done when very hot water or steam is able to get relatively close to the Earth’s surface. With this method, temperatures need to be at least 200° C and relatively close to the surface (Roberts 2020). Here, the steam needed to spin the turbines is taken directly from the ground and put through the system. This steam is found in fractured rocks trapped beneath unfractured rocks. Cooler water is then injected back into the ground to maintain pressures and to replenish the system (Roberts 2020).

This is an excellent, efficient, lower costs system that is easily accessible. However, high quality geothermal reserves are needed to make these systems economically feasible. High quality systems are generally located near tectonic plate boundaries, limiting where they can be used. That means that in the US, commercial geothermal plants are located primarily in California, Nevada, Hawaii and Alaska (Roberts 2020). Conventional systems also need a naturally occurring, and available, source of water. Finding the right mixture of water and high heat is not always possible.

Enhanced Geothermal Systems

Enhanced Geothermal Systems (EGS) is one answer to the problems posed by conventional systems. In an EGS, the high quality thermal reserve is already in place, however the water reservoir is not. Therefore, with these systems, a reservoir is made by fracturing the rock using high pressure water, allowing that water to permeate and heat up to the temperatures required for electricity generation (Roberts 2020).

There are, of course, issues associated with this method as well. Often, as the heat resources get deeper and hotter, the rock gets harder to fracture (Roberts 2020). This process is also known in the oil and gas industry as fracking. And fracking does not have a very good reputation. However, many of the environmental issues surrounding fracking are limited to the oil and gas industry and do not transfer to geothermal energy (Roberts 2020). In EGSs, less pressure is used than in oil and gas, and the fractures are smaller and more controlled. However, there still remains the issue that these systems can only be used when there is enough heat flow relatively close to the surface. This, like with conventional systems, limits the locations where this is a viable option.

Advanced Geothermal Systems

On the horizon are Advanced Geothermal Systems (AGS). These systems build on the EGSs by asking the question “What if we don’t need a reservoir at all?” These systems capitalize on the thermal expansion, and therefore differences in density, of fluids in a closed loop system. In the most promising system, the Eaver Loop, two wells are drilled around 1.5 miles apart and connected via a series of horizontal wells filled with a fluid (Roberts 2020). This fluid heats up at depth, becoming less dense and begins to rise. At the surface, this heat is harvested from the fluid and cooled. This cooled water, being more dense, then sinks to the heat reservoir to be heated up again. As there is no need to power a pump, the heat reservoir does not need to be nearly as hot as the EGS and conventional systems. Temperatures as low as 150⁰C can be used to generate electricity.

By needed far lower temperature ranges, this system can be used in far more places than the other systems. As there is heat at some depth everywhere, these systems do not need to be confined to tectonically active zones. The footprint of the system is also much smaller than other systems as there is no need for a pump (Roberts 2020). Of course there are roadblocks with this system, but they are primarily engineering problems, not physics. For example, while horizontal drilling has been in use for many years in the oil and gas industry, those systems are not necessarily designed for the high heat that geothermal horizontal drilling requires (Roberts 2020). This however, is much more easily solved than creating new high heat reservoirs where there are none.

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