ENERGY GENERATION BY HOT ROCK ENERGY (HDR) TECHNOLOGY

Hot Rock Energy is a vast, environmentally friendly, economically attractive energy source.

The concept is very simple.

Water is injected into a borehole and circulated through a “heat exchanger” of hot cracked rock several kilometres below the surface. The water is heated through contact with the rock and is then returned to the surface through another borehole where it is used to generate electricity. The water is then re-injected into the first borehole to be reheated and used again.

 

Hot Dry Rock Geothermal Energy

HDR geothermal energy relies on existing technologies and engineering processes, and is the only known source of renewable energy with a capacity to carry large base loads. The concept behind HDR geothermal energy is relatively simple. Heat is generated by special high heat producing granites located 3km or more below the Earth’s surface. The heat inside these granites is trapped by overlying rocks which act as an insulating blanket. The heat is extracted from these granites by circulating water through them in an engineered, artificial reservoir or underground heat exchanger.

 

HDR geothermal energy relies on existing technologies and engineering processes such as drilling and hydraulic fracturing, techniques established by the oil and gas industry. Standard geothermal power stations convert the extracted heat into electricity. HDR geothermal energy is environmentally clean and does not produce greenhouse gases.

It has been classified as renewable by National and International authorities.

 

The Hot Rock Energy system works with two closed circulation loops

 

The subsurface loop

This loop circulates water down an injection borehole where it passes through the

underground “heat exchanger” and is heated. The superheated water is then recovered by one or more production boreholes which return it under pressure to the surface. By keeping the water under pressure and preventing it turning to steam, any materials dissolved from the underground rock mass (such as silica or carbonates) are kept in solution and can be returned to the ground.

 

At the surface, the superheated water is passed through a metal heat exchanger where most of the heat is removed. The now cooled water is then returned to the injection borehole where it is sent down again to recover more heat.

 

The power station loop

 

At the surface a second closed loop fluid system is used to transfer the heat into the power station and generate the electricity in a turbine. The fluid used in the power station loop can be water, but more usually a lower boiling point fluid is used. Organic fluids such as refrigerants and iso-pentane are often used.

 

The HDR Energy Extration Process

All modern HDR development work is based on the relatively simple concept described in a US Patent issued to Los Alamos when HDR technology was more theory than reality. A well is first drilled into hot, crystalline rock. Water is then injected at pressures high enough to open the natural joints in the rock, thereby creating an engineered geothermal reservoir.

 

The reservoir consists of a relatively small amount of water dispersed in a very large volume of hot rock. The relative dimensions and orientation of the reservoir are determined by the local geologic and stress conditions, while its ultimate volume is a function of the duration of the hydraulic fracturing operation and the fracturing pressures applied.

 

Seismic techniques are used to follow the growth of the reservoir, to assess its location, and to determine its approximate dimensions. Using the seismic data as a guide, one or more production wells are subsequently drilled into the engineered

reservoir at some distance from the first well. In a properly engineered HDR reservoir, there are a number of fluid-flow pathways between the injection and production wellbores.

 

Operation of an HDR heat mine is extremely simple. A high-pressure injection pump

provides the sole motive force to circulate water through the engineered reservoir and deliver it to a power plant on the surface. The hydraulic pressure applied via the injection pump also serves to keep the joints within the reservoir propped open. The operating parameters applied to the injection pump thus greatly affect both the flow rate through the reservoir and its instantaneous fluid capacity. By using a  Combination of injection and production control measures, an almost limitless variety

of operating scenarios may be employed to mine the heat.

 

The Major Components of a HDR System

1. One, or more, hot dry rock reservoirs are created artificially by hydraulically fracturing a deep well drilled into hot, impermeable, crystalline basement rock. The hydraulic fracturing, achieved by pumping water into the well at high pressure, forces open tiny pre-existing fractures in the rock, creating a system or “cloud” of fractures that extends for tens of meters around the well. The body of rock containing the fracture system is the reservoir of heat. The fracture system provides for the heat transport medium, water, to contact a large area of the rock surface in order to absorb the heat and bring it to the surface. More than one reservoir could supply hot water to a single power plant.

 

2. Deep wells are meant for production and injection of water. The wells are drilled with conventional rotary drilling technology similar to that used for drilling deep oil and gas wells. The total number of wells and the ratio of production wells to injection wells may vary. Experimental HDR systems to date have typically involved one injection well and one production well. The earliest commercial HDR systems will likely include a “triplet,” two production wells for each injection well. A triplet of deep wells will support about 5 MW of power plant capacity, assuming adequate flow rates and fluid temperature. It is possible that other well configurations, such as a

quadruplet (3 production wells per injection well) or a quintuplet (4 production wells per injection well) could be used.

 

However, the cost effectiveness of using a quadruplet or quintuplet has not been established. Also, the ellipsoidal, rather than spherical, shape of the fracture pattern at Fenton Hill suggests that one production well on each side of the injection well, on the long axis of the reservoir, is the logical configuration. For these reasons, this analysis is limited to a ratio of two production wells per injection well, with earlier commercial systems limited to three wells total, and later systems using multiple triplets of wells. The original well, from which the fracture system is created, is

used for injection. Two additional nearby wells are drilled directionally to intersect the fracture system and are used as production wells. Operation of the system involves pumping water into the fracture system through the injection well, forcing it through the fracture system where it becomes heated, and recovering it through the production wells.

 

3. A system of microseismic instruments in shallow holes around the well that is being fractured is used. During the fracturing operation, this system gathers seismic data, which is used to determine the extent and the orientation of the hydraulically created fracture system. This information is then used to guide the drilling of the production wells so that they intersect the fracture system at depth. Although the HDR system, once it is completed, can operate without it, the microseismic system is included here because itis an integral part of creating the HDR reservoir and because it may be left in place to gather additional information which could be useful later in

the life of the HDR system.

4. A shallow water well to provide water (or other source of fresh water).

5. Surface piping, or gathering system, to transport water between the wells and power plant.

6. A binary power system is used to convert the heat in the water to electricity. This system is comprised of the following major components:

a. One or more turbines connected to one or more electric generators.

b. A heat exchange vessel to transfer heat from the hot water to a secondary working fluid with a low boiling temperature.

c. A heat rejection system to transfer waste heat to the atmosphere and condense the vapor exiting the turbine. A wet, or dry, cooling system can be used. The capital cost of a wet cooling system is only marginally less expensive than for a dry cooling system.

However, this cost advantage is largely offset by the higher operating cost of the wet

cooling system. For this reason, and since HDR sites in the U.S. are likely to be in arid areas with limited water supplies, this technology characterization is limited to a dry cooling system.

d. Injection pump(s) circulate the water through the HDR reservoir.

e. Pumps repressure the working fluid after it condenses and a vessel storing the working fluid.

f. Electrical controls and power conditioning equipment.

 

Development of an Engineered Artificial Reservoir

Granites have an internal fabric of cooling joints and fractures, a result of cooling down from a melt (like molten glass) to a solid that we see today. Developing an underground heat exchanger involves increasing the hydraulic pressure at the bottom of a deep drill hole (approx. 4- 5km) until the existing fractures and joints are slightly opened.

HOW IS ELECTRICITY GENERATED USING GEOTHERMAL ENERGY?

In geothermal power plants steam, heat or hot water from geothermal reservoirs provides the force that spins the turbine generators and produces electricity. The used geothermal water is then returned down an injection well into the reservoir to be reheated, to maintain pressure, and to sustain the reservoir.

There are three kinds of geothermal power plants. The kind we build depends on the temperatures and pressures of a reservoir.

1.      A “dry’” steam reservoir produces steam but very little water. The steam is piped directly into a “dry” steam power plant to provide the force to spin the turbine generator. The largest dry steam field in the world is The Geysers, about 90 miles north of San Francisco. Production of electricity started at The Geysers in 1960, at what has become the most successful alternative energy project in history.

2.      A geothermal reservoir that produces mostly hot water is called a “hot water reservoir” and is used in a “flash” power plant. Water ranging in temperature from 300 – 700 degrees F is brought up to the surface through the production well where, upon being released from the pressure of the deep reservoir, some of the water flashes into steam in a ’separator.’ The steam then powers the turbines.

3.      A reservoir with temperatures between 250 – 360 degrees F is not hot enough to flash enough steam but can still be used to produce electricity in a “binary” power plant. In a binary system the geothermal water is passed through a heat exchanger, where its heat is transferred into a second (binary) liquid, such as isopentane, that boils at a lower temperature than water. When heated, the binary liquid flashes to vapor, which, like steam, expands across and spins the turbine blades. The vapor is then recondensed to a liquid and is reused repeatedly. In this closed loop cycle, there are no emissions to the air.

WHAT ARE SOME OF THE ADVANTAGES OF USING GEOTHERMAL ENERGY TO GENERATE ELECTRICITY?

• Clean. Geothermal power plants, like wind and solar power plants, do not have to burn fuels to manufacture steam to turn the turbines. Generating electricity with geothermal energy helps to conserve nonrenewable fossil fuels, and by decreasing the use of these fuels, we reduce emissions that harm our atmosphere. There is no smoky air around geothermal power plants — in fact some are built in the middle of farm crops and forests, and share land with cattle and local wildlife.

  For ten years, Lake County California, home to five geothermal electric power plants, has been the first and only county to meet the most stringent governmental air quality standards in the U.S.

• Easy on the land. The land area required for geothermal power plants is smaller per megawatt than for almost every other type of power plant. Geothermal installations don’t require damming of rivers or harvesting of forests — and there are no mine shafts, tunnels, open pits, waste heaps or oil spills.
• Reliable. Geothermal power plants are designed to run 24 hours a day, all year. A geothermal power plant sits right on top of its fuel source. It is resistant to interruptions of power generation due to weather, natural disasters or political rifts that can interrupt transportation of fuels.
• Flexible. Geothermal power plants can have modular designs, with additional units installed in increments when needed to fit growing demand for electricity.
• Keeps Dollars at Home. Money does not have to be exported to import fuel for geothermal power plants. Geothermal “fuel’” – like the sun and the wind – is always where the power plant is; economic benefits remain in the region and there are no fuel price shocks.
• Helps Developing Countries Grow. Geothermal projects can offer all of the above benefits to help developing countries grow without pollution. And installations in remote locations can raise the standard of living and quality of life by bringing electricity to people far from “electrified” population centers.

HOW MUCH ELECTRICITY IS FROM GEOTHERMAL ENERGY?

Since the first geothermally-generated electricity in the world was produced at Larderello, Italy, in 1904 the use of geothermal energy for electricity has grown worldwide to about 7,000 megawatts in twenty-one countries around the world. The United States alone produces 2700 megawatts of electricity from geothermal energy, electricity comparable to burning sixty million barrels of oil each year.

WHAT ARE SOME NON-ELECTRIC WAYS WE CAN USE GEOTHERMAL ENERGY?

Geothermal water is used around the world, even when it is not hot enough to generate electricity. Anytime geothermal water or heat are used directly, less electricity is used. Using geothermal water ‘directly’ conserves energy and replaces the use of polluting energy resources with clean ones. The main non-electric ways we use geothermal energy are DIRECT USES and GEOTHERMAL HEAT PUMPS.

DIRECT USES Geothermal waters ranging from 50 degrees F to over 300 degrees F, are used directly from the earth:

• ‘to soothe aching muscles in hot springs, and health spas (balneology);
• to help grow flowers, vegetables, and other crops in greenhouses while snow-drifts pile up outside (agriculture);
• to shorten the time needed for growing fish, shrimp, abalone and alligators to maturity (aquaculture);
• to pasteurize milk, to dry onions and lumber and to wash wool (industrial uses);
• Space heating of individual buildings and of entire districts, is – besides hot spring bathing – the most common and the oldest direct use of nature’s hot water. Geothermal district heating systems pump geothermal water through a heat exchanger, where it transfers its heat to clean city water that is piped to buildings in the district. There, a second heat exchanger transfers the heat to the building’s heating system. The geothermal water is injected down a well back into the reservoir to be heated and used again. The first modern district heating system was developed in Boise, Idaho. (In the western U.S. there are 271 communities with geothermal resources available for this use.) Modern district heating systems also serve homes in Russia, China, France, Sweden, Hungary, Romania, and Japan. The world’s largest district heating system is in Reykjavik, Iceland. Since it started using geothermal energy as its main source of heat Reykjavik, once very polluted, has become one of the cleanest cities in the world.
• Geothermal heat is being used in some creative ways; its use is limited only by our ingenuity. For example, in Klamath Falls, Oregon, which has one of the largest district heating systems in the U.S., geothermal water is also piped under roads and sidewalks to keep them from icing over in freezing weather. The cost of using any other method to keep hot water running continuously through cold pipes would be prohibitive. And in New Mexico and other places rows of pipes carrying geothermal water have been installed under soil, where flowers or vegetables are growing. This ensures that the ground does not freeze, providing a longer growing season and overall faster growth of agricultural products that are not protected by the shelter and warmth of a greenhouse.

Present Trends in Geothermal Resources Development in India

The geothermal resources in India are mostly aligned along the sub-Himalayan belt, Son Narmada lineament and the West Coast. As many of these hot springs are intermediate enthalpy resources, the activity at present is confined to utilizing them for direct uses or their chemical content. Because of the ready availability of coal resources for generating thermal power and the low priority assigned by the State to geothermal power generation, the development of geothermal energy in India continues to take a back seat after practically three decades of geoscientific studies. The latter led the Geological Survey of India to select prospects for pilot-scale power generation from proven shallow geothermal reservoirs.

The geothermal waters are a storehouse of many rare elements. Metals such as Cs, As, Cd, Rb and Li are expensive and have wide applications in electronics, space research and chemical technology. The on-going research projects are mainly directed at the exploitation of these rare elements.

At Puga, the most promising geothermal field in India, the shallow reservoir has an estimated electricity generation potential of 1.7 MWe using binary cycle power plants. A project is currently under way in this field for the cascade use of the effluent water from the proposed plant and to design equipment for greenhouse and space heating. The hot water from the Puga springs has the following metal contents: Rb ~1 ppm, Li ~ 7 ppm and Cs ~ 9 ppm, (i.e., Cs > Li). The thermal water is enriched in cesium because of the rather high volatility of this element compared to lithium. The concentration of rare alkalies in thermal waters may be attributed to the absorption of the vapors from the magmatic source by circulating geothermal fluids.

Studies are also being conducted at Puga on extracting the rare alkalies from the thermal discharge and soils. Laboratory experiments have been carried out on extracting cesium from the thermal water by “ammonium 12 molybdophosphate” within the pH range of 6 – 7, using Al+3 as a catalyst. Recovery of seventy percent cesium, from the initial content of 10 µg/ml, is possible by this method At Chuza, in the Spiti Valley area, the Geological Survey of India, in collaboration with the National Metallurgical Laboratory, is studying the economic exploitation of cesium, using the high rate of evaporation of cesium and an eutectic process. A study of rare element content, directed at an eventual utilization of cesium, rubidium and lithium, is now underway in the Sohna Valley area, Rajasthan. In the Parbati Valley area, an attempt is being made to define the relationship between geothermal activity and mineral deposition.

Geochemical surveys are still being carried out around hot springs and drill-sites to assess elements, such as arsenic, that are pathfinders to gold mineralization. The arsenic content is of considerable importance as this element is an indicator of gold-bearing epithermal systems. Chemical analysis of deposits formed near Ramshila, Kulu, has shown the following metal content: arsenic ~ 1500 ppm, Hg ~ 810 ppb, Pb ~ 100 ppm, besides the base metal minerals.

The base metal content from thermal water in the Beas Valley area and at Kasol in the Parbati Valley area, suggests that base metal mineralization might be present at deeper levels. Monitoring of thermal water from wells planned for use in direct heat applications, including irrigation, is continuing in Pittorgarh area and at Tapoban, Uttar Pradesh. The Geological Survey of India has taken on a project to study the feasibility of bottling thermal waters (29 – 36°C) from wells at Gaziabad.

The Himalayan foothills are a major earthquake-prone belt. Geothermal studies in the earthquake-affected areas have revealed the influence of seismicity on geothermal parameters. Monitoring of physical and chemical changes in the post-earthquake period in the Bhagirathi Valley area has revealed a drying-up and an emergence of hot springs, which may be attributed to changes in fracture patterns resulting from seismic activity.

As a further follow-up to the proposed 300 kWe binary cycle pilot power plant at Tatapani, Surguja district, technical and economic feasibility studies, including an MT survey to establish the configuration of the high temperature deeper reservoir, well testing to measure temperature and pressure gradients and monitoring of water quality, are being pursued to realize the goal of a full scale (18 MWe) power generation project.
Hot springs are a major tourist attraction. A major impulse is being given to the development of hot spring areas as tourist attractions, in the West Coast area. The main activity in the West Coast area consists of a survey of the heat requirements of a number of industrial units in this region and the preparation of a project document on the commercial utilization of hot springs.

GEOTHERMAL ENERGY

“Geothermal” comes from the Greek words geo (earth) and thermal (heat). So, geothermal means earth heat.Our earth’s interior – like the sun – provides heat energy from nature. This heat – geothermal energy – yields warmth and power that we can use without polluting the environment.Geothermal heat originates from Earth’s fiery consolidation of dust and gas over 4 billion years ago. At earth’s core – 4,000 miles deep – temperatures may reach over 9,000 degrees F.

HOW DOES GEOTHERMAL HEAT GET UP TO EARTH’S SURFACE?

The heat from the earth’s core continuously flows outward. It transfers (conducts) to the surrounding layer of rock, the mantle. When temperatures and pressures become high enough, some mantle rock melts, becoming magma. Then, because it is lighter (less dense) than the surrounding rock, the magma rises (convects), moving slowly up toward the earth’s crust, carrying the heat from below.

Sometimes the hot magma reaches all the way to the surface, where we know it as lava. But most often the magma remains below earth’s crust, heating nearby rock and water (rainwater that has seeped deep into the earth) – sometimes as hot as 700 degrees F. Some of this hot geothermal water travels back up through faults and cracks and reaches the earth’s surface as hot springs or geysers, but most of it stays deep underground, trapped in cracks and porous rock. This natural collection of hot water is called a geothermal reservoir.

HOW HAVE PEOPLE USED GEOTHERMAL ENERGY IN THE PAST?

From earliest times, people have used geothermal water that flowed freely from the earth’s surface as hot springs. The oldest and most common use was, of course, just relaxing in the comforting warm waters. But eventually, this “magic water” was used (and still is) in other creative ways. The Romans, for example, used geothermal water to treat eye and skin disease and, at Pompeii, to heat buildings. As early as 10,000 years ago, Native Americans used hot springs water for cooking and medicine. For centuries the Maoris of New Zealand have cooked “geothermally,” and, since the 1960s, France has been heating up to 200,000 homes using geothermal water.

HOW DO WE USE GEOTHERMAL ENERGY TODAY?

Today we drill wells into the geothermal reservoirs to bring the hot water to the surface. Geologists, geochemists, drillers and engineers do a lot of exploring and testing to locate underground areas that contain this geothermal water, so we’ll know where to drill geothermal production wells. Then, once the hot water and/or steam travels up the wells to the surface, they can be used to generate electricity in geothermal power plants or for energy saving non-electrical purposes