Castle Geyser, Yellowstone National Park, USA

Going underground

By using the energy from hot water trapped several kilometres below the Earth's surface it could be possible to satisfy much of the world's energy needs. But, as E&T discovers, there is much work to be done before this dream becomes a reality.

Simply by mining the huge amounts of heat stored as thermal energy in the Earth's hard rock crust we could supply a substantial portion of the electricity we will need in the future, probably at competitive prices and with minimal environmental impact.

In the current climate, with nations around the globe throwing money at developing green energy technologies, that seems to be too good to be true.

But according to landmark research from the Massachusetts Institute of Technology (MIT) whose report 'The Future of Geothermal Energy' launched back in 2007 reinvigorated the interest for what is a well established, but under-utilised energy source.

Although geothermal energy is produced commercially today, and the United States is the world's biggest producer, existing US plants have focused on the high-grade geothermal systems primarily located in isolated regions of the west.

The MIT study took a more ambitious look at this resource and evaluates its potential for much larger-scale deployment.

The study says that drilling several wells to reach hot rock and connecting them to a fractured rock region that has been stimulated to let water flow through it creates a heat-exchanger that can produce large amounts of hot water or steam to run electric generators at the surface.

Unlike conventional fossil-fuel power plants that burn coal, natural gas or oil, no fuel would be required. And, unlike wind and solar systems, a geothermal plant works night and day, offering a non-interruptible source of electric power.

Even in the most promising areas, however, drilling must reach depths of 5,000ft, and much deeper in less promising regions.

"What people don't understand about geothermal is that it is a resource - it is not a power plant," says Karl Gawell, executive director of the Washington DC-based Geothermal Energy Association. "The difficult part of geothermal is the subsurface.

"If you look at a thermal map of the United States, you can see that at a depth of 6km the temperature is above 100°C. This is deep but not incredibly deep. Oil wells are already drilled to two or three times that depth.

"Obviously the deeper it is the harder it is and the more expensive, but the question is, can you use the heat in the rock to create artificial systems where you don't have the ideal natural system? What we are finding is that you can but it is expensive, but we are learning how to do it better."

Gawell likens the current state of geothermal with the oil industry at the start of the last century. "Originally, we saw oil coming out of the ground and we drilled there. On the front cover of National Geographic magazine in 1917 there was a front page story 'The world is running out of oil'. Why? Because we had drilled all of the places where we could see it coming out of the ground and we were running out of oil.

"Since then, we have invented most of the modern geophysical techniques that helped us explore and find oil. And guess what? We found most of the oil. Well right there, that's where geothermal is.

"It is in this early stage where conventional plans are to produce where we can find them, but the estimates are that 70-80 per cent of all conventional geothermal is hidden and we don't know how to find it."

Geothermal: The basics

Heat emanating from the Earth's interior and crust generates magma (molten rock). Because magma is less dense than surrounding rock, it rises but generally does not reach the surface, heating the water contained in rock pores and fractures.

Wells are drilled into this natural collection of hot water or steam, called a geothermal reservoir, in order to bring it to the surface and use it for electricity production.

The three basic types of geothermal electrical generation facilities are binary, dry steam, and flash steam. Electricity production from each type depends on reservoir temperatures and pressures, and each type produces somewhat different environmental impacts.

Recent advances in geothermal technology have made possible the economic production of electricity from lower temperature geothermal resources, at 100°C  to 150°C. Known as binary geothermal plants, these facilities reduce geothermal energy's already low emission rate to near zero.

In the binary process, the geothermal water heats another liquid, such as isobutane, that boils at a lower temperature than water. The two liquids are kept completely separate through the use of a heat exchanger used to transfer the heat energy from the geothermal water to the working-fluid.

The secondary fluid vaporises into gaseous vapour and, like steam, the force of the expanding vapour turns the turbines that power the generators. If the power plant uses air cooling the geothermal fluids never make contact with the atmosphere before they are pumped back into the underground geothermal reservoir, effectively making the plant emission free. Developed in the 1980s, this technology is already in use in geothermal power plants throughout the world in areas that have lower resource temperatures.

The ability to use lower temperature resources increases the number of geothermal reservoirs that can be used for power production.

Cooling systems

A cooling system is essential for the operation of any modern geothermal power plant.

Cooling towers prevent turbines from overheating and prolong facility life. Most power plants, including most geothermal plants, use water cooling systems. Water-cooled systems generally require less land than air cooled systems, and are considered overall to be effective and efficient cooling systems. The evaporative cooling used in water-cooled systems, however, requires a continuous supply of cooling water and creates vapour plumes. Usually, some of the spent steam from the turbine (for flash- and steam-type plants) can be condensed for this purpose.

Air-cooled systems, in contrast to the relative stability of water-cooled systems, can be extremely efficient in the winter months, but are less efficient in hotter seasons when the contrast between air and water temperature is reduced, so that air does not effectively cool the organic fluid. Air-cooled systems are beneficial in areas where extremely low emissions are desired, or in arid regions where water resources are limited, since no fluid needs to be evaporated for the cooling process.

Air-cooled systems are preferred in areas where the viewshed is particularly sensitive to the effects of vapour plumes, as vapour plumes are only emitted into the air by wet cooling towers and not air cooling towers. Most geothermal air cooling is used in binary facilities.

A combination of flash and binary technology, known as the flash/binary combined cycle, has been used effectively to take advantage of the benefits of both technologies.

In this type of plant, the flashed steam is first converted to electricity with a backpressure steam turbine, and the low-pressure steam exiting the backpressure turbine is condensed in a binary system.

This allows for the effective use of air cooling towers with flash applications and takes advantage of the binary process. The flash/binary system has a higher efficiency where the well-field produces high pressure steam, while the elimination of vacuum pumping of non-condensable gases allows for 100 per cent injection.

Enhanced Geothermal Energy

The focus right now for the US Department of Energy (DoE) for geothermal energy is on something they call Enhanced Geothermal Energy, or more accurately engineered geothermal systems.

These are systems where you are either improving fluids circulation in an existing hydro thermal system by some sort of engineering technology, or you are actually creating sub-surface permeability.

"A geothermal resource requires a confluence of three variables in the sub surface," explains Burton Mack Kennedy, who heads up the US government geothermal programme at the Lawrence Berkeley National Laboratory.

"One obviously is high temperature, the other is that you have to be able to flow water through the hot rocks so you need permeability. And then you need some kind of fluid that you are using to extract the heat from the rocks coming up to the surface and converted to electricity either by a binary power plant or a flash steam plant or a combination of the two.

"In the DoE's Enhanced Geothermal System programme, if we have a geothermal source that is a natural hydrothermal system where the permeability of the system isn't quite adequate for obtaining maximum efficiency of energy withdrawal from the rocks we would want to drill a hole down and somehow physically stimulate the hole and create a bigger fracture network.

"That would be something that I would call enhancing an existing geothermal system. So you might do that on the edge of an existing field or near one and try to connect into the field somehow.

"The other way of looking at this is when you don't have the permeability, you have got to do both. If there is no natural system there, you are just going down and drilling into hot rock. You have to figure out a way to create a natural reservoir that you can extract heat from. In this instance you are starting from scratch and I call them engineered geothermal systems, that are actually building a geothermal resource."

At the levels that Kennedy is talking about the rocks are naturally stressed because of tectonic forces and so the aim is to take advantage of that natural stress and pressurise the well with water. The effect of this is to add stress to the system that is slightly bigger than the weaker stress of the rocks and thereby fracturing it along that stress pattern.

Another option would be to use a chemical to stimulate the fracture, manipulate the chemistry that dissolves minerals entering pre-existing fracture systems and provide pathways for fluids. Typically that has not been tried very successfully because you cannot get very far out into the reservoir with the chemistry before the fluids weaken.

The research development programmes are both in the field and in the lab and they cover a lot of different areas. Some areas are related to engineering issues like having pumps at the bottom of 3km holes that can withstand 250°C for two years and pump enough water - there are none of these available at present.

Future development

New tools are being developed for measuring the geophysical properties. "We are working in the dark with very few technologies to see what we are doing," Kennedy says. "The one that we use the most right now is acoustic emission and we are working on developing other physical and geochemical technologies to try and visualise what is going on in the sub surface when we are trying to stimulate one of these things.

"It's like a lot of the problems we have in other geoscience issues like environmental cleanup or waste management or fresh water in the sub surface. We really don't have a lot of technologies to image fluids in the sub-surface, I don't care what depth you are at, particularly below a kilometre.

"So a lot of research is being done across all of the DoE and even the International Science Foundation to develop technologies to help us image and manipulate fluids in the sub surface in some advantageous way."

The latest available annual figures show that 16,010GWh of electricity was produced in the US by geothermal energy, just 0.36 per cent of their annual consumption. So the technology clearly has a long way to go, but with a huge prize at the end of the road, development is moving at a heady rate.

"The toughest thing with Enhanced Geothermal Energy is going to be, and the governments have always had trouble with this, trying things that fail," Gawell adds. "The government likes to say, you have failed once so shut it down. I think they should be trying all the major geologic systems, multiple systems because I think that some of them will work, but we are only going to know by doing it and having some failures."

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