Geothermal Energy

What is Geothermal Energy?

Geo (Greek) – earth

Thermal - relating to, using, producing, or caused by heat.

Sources of Earth’s Internal Energy

•70% comes from the decay of radioactive nuclei with long half lives that are embedded within the Earth

•Some energy is from residual heat left over from Earths formation.

•Friction of tectonic plates

•The rest of the energy comes from meteorite impacts.

The Earth

•Heat can be ejected as steam or hot water.

•Hydrothermal reservoirs, water and hot porous rock. (aquifer)

It is around 4000-6000oCelsius at centre of the Earth

In general, the temperature rises one degree Celsius for every 36 metres you go down.

Where is Geothermal Energy?

• Heat generated by natural processes occurring within the earth

• Hot springs, geysers and mud pots are natural phenomena that result from geothermal activity

Different Geothermal Energy SourcesHot Water Reservoirs: As the name implies these are reservoirs of hot underground water. There is a large amount of them in the US, but they are more suited for space heating than for electricity production.

Natural Steam Reservoirs: In this case the underground water changes to steam.

Hot Dry Rock: This type of condition exists in 5% of the US. It is similar to Normal Geothermal Gradient, but the gradient is 400C/km dug underground.

Molten Magma: No technology exists to tap into the heat reserves stored in magma. The best sources for this in the US are in Alaska and Hawaii.

Where Can Geothermal Energy be Harnessed?

• Technology today allows for small scale harnessing everywhere – Heat pumps (for shallow

geothermal energy)• Different areas have different

thermal gradients and thus different utilization potentials

• Higher thermal gradients correspond to areas containing more geothermal energy

High Temperature Systems

• These areas are associated with the “Ring of Fire” volcanic activities around the Pacific Rim Basin

High Temperature Surface Geothermal Systems

There are three different types of surface geothermal system designs:

1. Dry-steam Power Plant2. Flash-steam Power Plant3. Binary-cycle Power Plant

Direct Use of Steam

Conventional geothermal plants capture hot water from geysers or steam from vents to spin

turbines.

Dry-steam Geothermal Power Plant

• Steam passes through turbine• 1050 -1220 degrees F

These were the first type of plants created. They use underground steam to directly turn the turbines. Generation of Electricity is appropriate for sources >150oC .

Vapor dominated resources where steam production is not contaminated Steam is 1050°F - 1220° F Steam passes through turbine Steam expands Blades and shaft rotate and generate power Cooling towers generate waste heat Most common and most commercially attractive (Godfrey Boyle) Used in areas where geysers do not exist Need water to inject down into rock Well is deep Takes more time to inject water in well

Dry-steam Geothermal Power Plant

Flash-steam Geothermal Power Plant

• Hot, High pressure water • Turbines generate electricity

• Costs 4-6 cents per Kwh.

These are the most common plants. These systems pull deep, high pressured hot water that reaches temperatures of 3600F or more to the surface. This water is transported to low pressure chambers, and the resulting steam drives the turbines. The remaining water and steam are then injected back into the source from which they were taken. Use very hot (more than 300° F) steam and hot water resourcesSteam either comes directly from the resource, or the very hot, high-pressure water is depressurized ("flashed") to produce steam. Steam then turns turbines, which drive generators that generate electricity. Only significant emission from these plants is steam (water vapor). Minute amounts of carbon dioxide, nitric oxide, and sulfur are emitted, but almost 50 times less than at traditional, fossil-fuel power plants. The flash steam power plant uses hot water reservoirs as a source of power. When the hot water comes up from the earth into the flash tank, there is a drop in pressure which causes some of the water to turn into steam. This steam is then used to spin the turbine much like in the Dry Steam power plant. The water is then returned to the earth to be used again later. This is the most used type of geothermic power plant since there are a lot of hot water reservoirs.

Flash-steam Geothermal Power Plant

Binary-cycle Geothermal Power Plant

• Hot water (100 – 300 deg F) • Heat Exchanger • Binary liquid lower specific heat

(vaporizes)

This system passes moderately hot geothermal water past a liquid, usually an organic fluid, that has a lower boiling point. The resulting steam from the organic liquid drives the turbines. This process does not produce any emissions and the water temperature needed for the water is lower than that needed in the Flash Steam Plants (2500F – 3600F).

Uses lower-temperatures, but much more common, hot water resources (100° F – 300° F). Hot water is passed through a heat exchanger in conjunction with a secondary (hence, "binary plant") fluid with a lower boiling point (usually a hydrocarbon such as isobutane or isopentane). Secondary fluid vaporizes, which turns the turbines, which drive the generators. Remaining secondary fluid is simply recycled through the heat exchanger. Geothermal fluid is condensed and returned to the reservoir. Binary plants use a self-contained cycle, nothing is emitted. Energy produced by binary plants currently costs about 5 to 8 cents per kWh. Lower-temperature reservoirs are far more common, which makes binary plants more prevalent.

Binary-cycle Geothermal Power Plant

Limitations of Surface Geothermal Power Plant

• Dependent on location. • Most viable sites have been tapped.• Not as efficient as coal fired power plant.

The simplest models have one injection well and two production wells. Pressurized cold water is sent down the injection well where the hot rocks heat the water up. Then pressurized water of temperatures greater than 2000F is brought to the surface and passed near a liquid with a lower boiling temperature, such as an organic liquid like butane. The ensuing steam turns the turbines. Then, the cool water is again injected to be heated. This system does not produce any emissions. US geothermal industries are making plans to commercialize this new technology.

Geothermal Power from Hot Dry Rocks

How Energy is extracted from Hot Dry Rocks

•Direct Sources function by sending water down a well to be heated by the Earth’s warmth.

•Then a heat pump is used to take the heat from the underground water to the substance

• Then after the water it is cooled is injected back into the Earth.

Overview of Geothermal systems

Flash Systems:1) Single‐Flash vs. Dual‐Flash Systems2) Combined Cycle Systems ≥ 180 °C

Binary Systems:1) Organic Rankine cycle technology2) Kalina Cycle technology≥ 87 °C

• Hot Dry Rock/Enhanced Geothermal Systems

Dry Steam Systems:≥ 200 °C

1900’s 1960’s 1980’s

• Other Technology Directions like direct use

The Kalina cycle uses water and ammonia at various ratios and a registered thermodynamics facility to reduce  thermodynamic irreversibility and therefore increase overall thermodynamic efficiency. There are multiple variants of the Kalina cycle systems available specifically applicable to increase the efficiencies of different types of heat source that would have previously been lost as waste thermal energy.

Low Temperature Heat Extraction/Rejection

• The classic use of earth/water is as a heat sink or source for air conditioning or heating

• Pipes embedded in the earth carry refrigerant or water and conduct heat from the hotter to cooler substance

• Since the earth (or water) has a high specific heat in comparison with air, there is good thermal transfer

• In winter, heat is extracted from the earth by the chilled refrigerant, while in the summer, the hot refrigerant conducts heat to the earth

• Houses have been built partially underground to moderate the winter and summer temperatures

•Geothermal Heat Pumps:

- produces 4 times the energy that they consume.

-initially costs more to install, but its maintenance cost is 1/3 of the cost for a typical conventional heating system and it decreases

electric bill. This means that geothermal space heating will save the consumer money.

-can be installed with the help of special programs that offer low interest rate loans.

Heat pumps (for shallow geothermal energy), At a lower thermal level, an air conditioner can extract heat from the ground for winter heating or insert energy into the ground to gain a more efficient cooling sink

Geothermal’s Positive AttributesGeothermal production of energy is 3rd highest among renewable energies. It is behind hydro and biomass, but before solar and wind.•Useful minerals, such as zinc and silica, can be extracted from underground water.•Geothermal energy is “homegrown.” This will create jobs, a better global trading position and less reliance on oil producing countries.•In large plants the cost is 4-8 cents per kilowatt hour. This cost is almost competitive with conventional energy sources.•Geothermal plants can be online 100%-90% of the time. Coal plants can only be online 75% of the time and nuclear plants can only be online 65% of the time.•Flash and Dry Steam Power Plants emit 1000x to 2000x less carbon dioxide than fossil fuel plants, no nitrogen oxides and little SO2. •Geothermal electric plants production in 13.380 g of Carbon dioxide per kWh, whereas the CO2 emissions are 453 g/kWh for natural gas, 906g g/kWh for oil and 1042 g/kWh for coal.•Binary and Hot Dry Rock plants have no gaseous emission at all. •Geothermal plants do not require a lot of land, 400m2 can produce a gigawatt of energy over 30 years.

Availability of Geothermal Energy • On average, the Earth emits 1/16

W/m2. However, this number can be much higher in areas such as regions near volcanoes, hot springs and fumaroles.

• As a rough rule, 1 km3 of hot rock cooled by 1000C will yield 30 MW of electricity over thirty years.

• It is estimated that the world could produce 600,000 EJ over 5 million years.

• There is believed to be enough heat radiating from the center of the Earth to fulfill human energy demands for the remainder of the biosphere’s lifetime.

Geothermal’s Harmful Effects Brine can salinate soil if the water is not injected back into the reserve after

the heat is extracted.• Extracting large amounts of water can cause land subsidence, and this can

lead to an increase in seismic activity. To prevented this the cooled water must be injected back into the reserve in order to keep the water pressure constant underground.

• Power plants that do not inject the cooled water back into the ground can release H2S, the “rotten eggs” gas. This gas can cause problems if large quantities escape because inhaling too much is fatal.

• One well “blew its top” 10 years after it was built, and this threw hundreds of tons of rock, mud and steam into the atmosphere.

• There is the fear of noise pollution during the drilling of wells. • Geyser-temperature steam is contaminated with salts that cause corrosion of

turbines or engines • Geophysical/geological data availability• Access to finances• Technologies – drilling (for EGS)

Direct uses of geothermal energy is appropriate for sources below 1500C

• space heating• air conditioning• industrial processes• drying • Greenhouses• Aguaculture• hot water• resorts and pools• melting snow

Geothermal technologiesEXPLORATION ENERGY SYSTEMS

Enhanced geothermal systems• demo facility• system operation

Drilling• mud-pumping technology• better drill bits for hard rocks and higher temperatures

Geothermal heat pumps