Free energy under our feet, all you need is water.

Geothermal power generating plants, which make use of the planet's interior heat, are becoming increasingly popular around the world. Research is still being carried out into the most efficient way to extract the heat but commercial plants are already in operation.

The early use of geothermal was to heat buildings with water that was either already at a usable temperature or needed only minimal extra heating. Now, temperatures are being accessed that are high enough to be used in power generation turbines.

Roughly 99 per cent of the Earth's mass is hotter than 1800 E C and, about three miles down, the temperature reaches several hundred degrees. The optimum way of accessing this energy at the moment is Hot Dry Rock (HDR) or Hot Fractured Rock (HFR) technology. These are referred to as Enhanced Geothermal Systems (EGS) because they go beyond the drilling of a simple well.

The HDR system comprises at least two depth drillings and one subterranean heat exchanger. The heat exchanger consists of natural joints in plutonite rock which are fractured and connected to each other with the help of water pressure, known as hydraulic simulation.

This enables the exploration of the Earth's interior heat outside known geothermal provinces. In contrast to a geothermal field in a volcanic or tectonic anomaly, an EGS depends on the artificial stimulation of otherwise tight formations by hydraulic fracturing to create an underground heat exchanger. Fluid is then circulated in a closed circuit.

It has been suggested that there could be sufficient energy to produce hundreds of megawatts of electricity per network and, at these depths, the technology enables geothermal power production virtually anywhere in the world. It is predicted that plants could work over a reservoir for 30 years without experiencing a significant drop in temperature. And this would be available 24 hours a day because it does not rely on variables such as tides, waves, wind or sun. But drilling is an expensive business.

Global production

Karl Gawell of the Geothermal Energy Association says geothermal power is now produced in 24 countries and on all continents except Antarctica. In 2003, geothermal resource supplied 57 000 Gigawatt-hours of electricity. Geothermal energy is today meeting the total electricity needs of some 60 million people worldwide. But there are no power generation projects in the UK.

The United States continues to produce more geothermal electricity than any other country, comprising some 32 per cent of the world total. But this is being challenged, particularly in the Philippines and Indonesia. HDR technology is also expected to produce hundreds of megawatts in Australia.

Societe de Cooperation Miniere et Industrielle (SOCOMINE) has been involved in the development of the Hot Dry Rock (HDR) geothermal technology since 1989. It is the co-ordinator and the main contractor of the European Hot Dry Rock Research Programme funded by the European Union and other public and private organisations.

The site it selected to develop the technology is at Soultz, which is located about 50km north of Strasbourg. Three wells have been drilled, one down to 4km and two at 5km. It is anticipated that some 6MW will be generated when the site comes on full stream.
Germany is a current focus of attention and Egbert Brossmann heads a project some 20 miles north of Berlin. Commissioned by energy giant Vattenfall Europe, he says Germany's first geothermal power plant will supply some 3000 households with electricity.

“We are not as lucky as, let's say, Italy. Says Brossmann. “The volcanic character of the ground there provides working temperatures a few hundred metres below the surface whereas we have to dig down several kilometres.

High temperature

In the south, German utility EnBW is working on an HDR project in the Swabian town of Bad Urach where temperatures reach 170°C at a depth of 4445m. In the initial stage, the plant will have an electrical capacity of about 1250 kW, sufficient to supply about 2000 households. Additional boreholes, an extended joints system, and larger volumes of circulating water will allow capacity to be further increased.

Bad Urach is situated over a geothermal anomaly. In the first 300–400m the geothermal gradient is as high as 11°C per 100m. Below this the temperature increase is 4°C per 100m. At 1600m the bedrock starts and the temperature increase reverts to a normal 3°C per 100m.

Geopower Basel AG was founded in 2004 with the goal of constructing the first geothermal power plant in Basel by 2009. This goes beyond power generation because the exhaust heat released during electricity production will be fed into urban remote heating networks. The energy power plant is expected to start generating electricity and heat in 2009. About 6MW of electricity and 17 megawatts of thermal power are expected.

The first well will penetrate some 5000m into the rock formation targeted for the heat exchanger. The aim is to develop a co-generation power plant with a power production of 3MW and a heat production of 20MW for the local district heating grid. Geothermal Explorers Ltd is responsible for the project development and project management.

Extracting the heat from the underground source requires a heat exchanger and Ecolaire is one company that produces designs in both surface and direct contact geothermal condensers.

Under construction is a 30MW geothermal surface condenser with a six-pass water-side arrangement designed to serve the dual purpose of providing district heating and power for the people of Iceland. In operation it will raise the cooling water inlet temperature by almost 100°F for the purpose of district heating and will also provide a vacuum at the end of a low-pressure turbine-generator for high efficient power production.

Due to the highly corrosive geothermal environment the condenser has been designed with Titanium tubes and tube sheets in addition to internal support structures and an outer body made of 316 stainless steel to counteract the effects of corrosive steam.

The future

Increasing the efficiency of power generation is the subject of a Siemens Industrial Solutions and Services Group project, which plans a geothermal power plant based on the Kalina Cycle. This uses a binary working fluid of water and ammonia instead of water alone. In contrast to pure media with a constant boiling point such as water or pentane, this mixture boils across a larger temperature range at a given pressure.
Kalina Cycle power plants use less energy to heat the working fluid, allowing more of the energy to go directly to generating power and improving the cost effectiveness of the power plant.

The steam power plant now used to make electricity was invented 150 years ago by Scottish engineer William Rankine. It uses a heat source-coal, oil, natural gas, geothermal heat-to produce high-pressure steam that drives a turbine. The excess steam is condensed into water, which is then pumped back to a boiler. But, in a Rankine cycle, only about 35 to 40 per cent of the heat energy released ever becomes electricity, which means an excess depletion of heating resources.

Mixing the water with ammonia, which evaporates at lower temperatures, can raise efficiency at the heat stage of the cycle. But ammonia also condenses less readily, forcing engineers to use smaller turbines and lowering efficiency. Kalina's invention solves that problem, using sophisticated thermodynamics to draw off most of the ammonia before the condensation stage. A Kalina cycle can boost efficiency by as much as 40 per cent.

Siemens has been awarded a contract by HotRock Erdwärmekraftwerk GmbH to develop a geothermal power plant based on the Kalina principle. It is in the municipality of Offenbach ad Queich in the German state of Rhineland-Palatinate, located in the rift valley of the Upper Rhine, geologically the hottest zone in Germany. Here, temperature gradients of 50°C and higher per 1000m are achieved.

The well, which is just under 3km deep, should provide water of at least 150°C. HotRock GmbH is located in nearby Karlsruhe and has designed a 5MW power plant. This corresponds to the energy requirement of around 20 000 households. A coal-fired power plant with the same output would emit around 23 000 metric tons of CO2 every year.

Siemens is responsible for the entire planning of the power plant components located above ground. These include the evaporator-condenser circuit, the steam turbine and generator, and the cooling circuit, as well as the entire automation and control and instrumentation.

Many benefits

Geothermal power generation offers many benefits over other renewable sources of energy. It is constantly available and so is ideal for supplying base load requirements, where reliability of supply is paramount. Wells could be operational for 30 years or more before they cool too far, providing plenty of opportunity for recouping the initial investment.

And the technology of obtaining heat from hot dry rock formations can be applied virtually anywhere in the world. It can also draw on the experience of the oil industry in drilling very deep wells to access energy pools that are well below the earth's surface.

Finally, even modest wells can produce megawatts of electricity, making the technology a very valuable contributor to society's needs.

Via engineerlive.com