Debates about the pros and cons of geothermal energy focus on environmental concerns. While there is some environmental cost, geothermal could prove to be the most benign energy resource of all, and the most abundant.
Before we get into the arguments, let’s take a quick look at just what geothermal energy is.
Using Department of Energy Geothermal Data , we see that if you could dig 4,000 miles below the surface of the Earth, you would reach the core of our planet. It is estimated to be 7,200 degrees F or higher at that depth.
The heat exists in part because of the heat created when billions of tons of hot mass collided and collapsed under continuing bombardment from yet more hot mass that fell when our world was in the process of creation.
By most accounts, that was over 4 billion years ago and the core would have cooled down by now unless something else contributed to the heat.
It turns out that the largest contributor to the heat below our feet is the decay of radioactive particles. We stand on nothing less than a planet-sized nuclear reactor that promises to continue generating heat for billions of years in the future.
Getting closer to the surface of our planet, if you drill a hole 50 to 60 miles deep, you hit hot molten rock of 1200 to 2200 F.
And about 3 to 4 miles below the surface of the Earth you find hot dry rock ranging in temperature from about 300F to 400F.
You can drill a hole just about anywhere to get this abundant, high temperature geothermal energy, but the hole would be miles deep, and dry. It is possible to use that heat resource, but terribly expensive, and drilling is just the beginning of the expensive engineering problems of utilizing extremely deep geothermal energy resources.
Within a few hundred feet of the surface, and at the surface in some locations, we find areas that, because of geological faults, blend deep earth heat with underground water to produce either hot water or steam. Using current technologies, these are the primary locations that enable economical use of geothermal energy resources.
These geothermal hot water and steam (hydrothermal) sites are used in a number of ways. Some are used as heat sources for heat related processes used in modern industry. A large number of them are used to warm greenhouses and aquaculture farms. The heat can be used to warm homes and businesses. And some sites use the hydrothermal energy to turn power turbines on electrical generators.
Like every other energy resource, however, geothermal energy utilization brings with it a number of issues. The six main areas of discussion about the pros and cons of geothermal energy are:
– Environmental Friendliness
We’ll look at the pros and cons of geothermal energy environmental concerns first.
PROS AND CONS OF GEOTHERMAL ENERGY ENVIRONMENTAL FRIENDLINESS
The Geothermal Energy Association has a wealth of information about geothermal energy. Here is a short environmental summary from the site.
Geothermal energy use does entail some environmental impact, but it is safe to say that the environmental benefits far outweigh the costs.
Most geothermal facilities operate virtually emission free. Some even reduce sulfur emissions that would have occurred from natural venting if these sites had been left untapped.
A few do produce some silica and sulfur dioxide, both of which are largely removed from the vapors and either returned to the hydrothermal well or processed and sold for industrial uses.
Almost 100% of the visible, airborne effluent seen rising from geothermal plants is water vapor.
Geothermal energy has the smallest land use of any major power generation technology. A typical geothermal facility occupies about the same space as a gas fired plant of the same capacity. But the geothermal facility does not require miles of buried pipeline to carry fuel to keep it running.
Geothermal facilities also have no coal or nuclear fuel to mine and transport, no radioactive wastes or ash wastes to deal with, and no emissions of carbon dioxide, particulates, or other combustion byproducts.
In other words, geothermal energy is one resource that can be universally celebrated for its contribution to a cleaner, safer environment.
PROS AND CONS OF GEOTHERMAL ENERGY RELIABILITY
Once in operation, geothermal plants may be the most reliable of all energy production methods. Since they are fundamentally simpler than most other power systems, there is less to go wrong.
Other factors that contribute to the impressive reliability of geothermal systems:
They require no purchase or transport of fuel. They require no waste disposal.
They can be used either as base load systems or swing with demand.
They have no intermittency or dispatchability problems. (A discussion of these problems can be found on the Pros and Cons of Wind Power page.)
It’s noteworthy that every geothermal energy facility that has been built in the last 100 years is still in production.
PROS AND CONS OF GEOTHERMAL ENERGY COST
In geologically suitable areas, geothermal energy is currently cost competitive with conventional generation methods.
Geothermal power plants do have high initial costs to drill and construct new facilities. But relatively high construction costs are paid back because there are no fuel costs.
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As shown in this Department of Energy Geothermal Report geothermal power costs are currently competitive with coal power plants, making them among the cheapest power providers around and getting cheaper with every project.
But current cost figures are based on projects that are located at the best geothermal sites.
The key to economically exploiting geothermal resources using current technologies is finding the best producing hydrothermal wells.
PROS AND CONS OF GEOTHERMAL ENERGY AVAILABILITY
Using current technologies, geothermal power is primarily available where hot magma finds its way close to the surface and heats ground water to usable temperatures above 212F.
These hydrothermal hot spots don’t occur everywhere. In the U.S. they are located in the Western States, Alaska, and Hawaii.
To be both usable and economical a site must have an adequate volume of hot water or steam that is not too impure to use, a surface water source to cool generating equipment, and close proximity to power transmission lines. So, even in promising areas, economically usable sites are few and they are difficult to locate.
The U.S. Geological Survey Circular 790 estimates a hydrothermal resource base in the U.S. of between 95,000 and 150,000 MW (Megawatts), of which 25,000 MW are known resources.
While that is a significant amount, it’s roughly one fourth of our current electrical need, less if we start plugging our cars into the electric grid. And most of that geothermal resource is way out west. That’s good for the West Coast, Hawaii, and the few people who live in Alaska, but it falls far short of being able to serve our Nation or the world.
The big key to universal use of geothermal resources is the development of deep, hot dry rock resources. The key to that is drilling technology. Due to the hot, often corrosive, environment of geothermal resource areas, drilling for geothermal resources is far more expensive than any other kind of drilling.
Even in relatively shallow hydrothermal wells, drilling costs can amount to half of the price of a new generating plant.
But drilling technology is improving. Drilling costs, on average, have dropped by a fourth in the last two decades. As drilling technology improves, geothermal power plants could become universally available.
PROS AND CONS OF GEOTHERMAL ENERGY AESTHETICS
Geothermal power plants have less aesthetics problems than other power plants.
Since they occupy the smallest space per kilowatt generated, it follows that they have the least visual impact of any power generation technology available.
If river water is used as a cooling medium, the entire power plant could be built underground or within structures already built for other uses. That would end all debate about the pros and cons of geothermal energy aesthetic concerns.
PROS AND CONS OF GEOTHERMAL ENERGY SUSTAINABILITY
Geothermal resources may outlast the sun. There is enough thermal nuclear energy in our Earth to fuel the engines of civilization for billions of years.
Beneath our feet we have more usable geothermal energy resources than oil, coal, gas, and mineable nuclear fuels combined.
Geothermal Resource Council Data shown below compares global continental (not including deep ocean resources)geothermal energy resources to global oil reserves.
Units: Billions of barrels of oil equivalent
*Annual Global Energy Consumption—-70
*A Stanford University Wind Power Study shows total annual global Btu consumption from all sources of about 50 to 70 billion barrels of oil equivalent.
Thermal aquifers are the primary source of geothermal electrical power using current technology. That 130 billion barrels of oil equivalent masks the fact that geothermal resources are a form of nuclear power and can continue to provide those energy supplies year after year, decade after decade, century after century.
Fossil fuel reserves, on the other hand, may be gone in a few decades.
Energy from thermal aquifers (hydrothermal) has a couple of problems, though. Earlier in this discussion of the pros and cons of geothermal energy we saw that most of these sites are area specific and often located far away from population centers. In addition, many of these resources aren’t hot enough (a minimum of 100C or 212F) to economically use in electrical power generation.
Far more exciting is the energy resource available in crustal heat. Technologies currently under development promise to economically harness this incredible resource within a few years.
We aren’t talking about Star Wars gizmos, either. All that’s needed is a hole, a really deep hole about 4 miles down, water, and generating equipment.
Drill a couple of holes, fracture the rock between the holes, pour water down one hole, and harvest steam from the other to drive turbines for electrical generators. That’s it.
The technology already works, it’s only a matter of improving costs to the extent that deep geothermal energy becomes competitive with existing power generation technologies.
We are close to doing just that. And when we do, we can move forward with the very reasonable expectation that geothermal energy resources can permanently and economically provide a million times more usable energy than we consume.