Geothermal Power Generation

KENG CHOON LEE

Geopower NZ Limited

Mount Roskill, Auckland, New Zealand

tracer A chemical injected into a well to detect the water flow path.

turbine nozzle A wheel of fixed blades to direct steam flow to a wheel of rotating turbine blades. wellhead The area around the casing head flange. wellhead pressure (WHP) Fluid pressure in a well just below the master valve.

Energy can be converted from one form to another. Geothermal power generation is the production of electrical energy from the heat (thermal) energy of the earth (geo). Electricity is a much more con­venient, clean, and useful form of energy than is heat. Geothermal energy has been used directly for its heat and therapeutic effect for thousands of years by the Romans, Chinese, and Turks. Its indirect use was realized in 1904 when Prince Piero Ginori Conti of Italy successfully produced electricity in an experi­mental ‘‘indirect cycle’’ using pure steam produced from a heat exchanger. This led to the first 250-kWe geothermal power station in 1913 at Larderello in Tuscany, Italy. Nowadays, geothermal energy is used directly for many purposes, but it is the generation of electricity that provides humankind with the most benefits and challenges. Whereas Larderello is the first dry steamfield that was developed for power, Wairakei in New Zealand is the first geothermal power station that was developed from a wet field. Energy conversion is subjected to the laws of thermodynamics. Geothermal power generation is similar to the conventional fossil fuel-fired power generation using steam turbines. The main difference is the lack of a manmade boiler in a geothermal power station; hence, steam supply requires more careful scientific assessment. However, geothermal power generation has other advantages. It has environmental benefits due to its low carbon dioxide emissions; it is ‘‘renewable’’ if managed in a sustained manner; it has cultural, social, and economic benefits in terms of tourism and recreational values; and because it is indigenous, it lessens the dependence on
imported oil. Developing countries such as The Philippines, El Salvador, and Kenya have much of their electricity generated by geothermal energy, saving these countries a great deal in terms of oil imports.

1. GEOTHERMAL RESOURCES

The earth is an enormous sphere with more than a 6000-km radius. A solid brittle outer shell approxi­mately 100 km thick, consisting of the continental crust (5-50 km thick) and the outermost part of the mantle, floats on a dense liquid metal core. Between the solid shell and the liquid core is the plastic layer of the mantle. The innermost core is believed to be solid metal due to the enormous pressure there. The continental crust is not one piece; rather, it is made up of many pieces known as tectonic plates. The earth has a heat source that is mainly from within the core due to radioactive materials. Heat is also generated by friction in the continental crust and the outermost mantle as the various tectonic plates move relative to each other. The consequences of this are the often catastrophic earthquakes and volcanic eruptions.

The temperature on the earth’s surface ranges from about —50°C at the poles to about 50°C in the deserts. The earth’s innermost core is believed to be approximately 5000°C ( + 1000°C). However, the earth’s temperature gradient is not linear from the surface to the center of the earth but rather increases very steeply with depth from the earth’s surface to approximately 1000°C at 100 km. To put things in perspective, we are looking at roughly 10 km of the continental crust for geothermal resources.

Geothermal resources can be defined as all of the heat energy stored in the earth’s crust that is accessible locally. The word ‘‘locally’’ is important because a 20°C resource is usable at a location with —20°C ambient temperature but not 20°C (unless a heat pump is used). This is because heat transfer requires a temperature difference between a heat source and a heat sink. Geothermal resources from which energy could be extracted economically in the near future (<100 years) are parts of the useful accessible resource base. Geothermal resources that can be extracted economically today are defined as geothermal reserves. These identified reserves are further divided into possible, probable, and proven reserves depending on how much scientific work (geological, geophysical, geochemical, and drilling) has been carried out on them.

Whether a geothermal resource is technically suitable for geothermal power generation depends very much on the type and quality of the geothermal system and on the available technology. Currently, the geothermal system that is suitable for power genera­tion is the hydrothermal type. This consists of a reservoir that contains hot water under high pressure. When this water is accessed by a drillhole, it ascends and some flashes to steam as the pressure releases.

The thermal water is mainly meteoric, so geother­mal energy can be considered ‘‘sustainable’’ if the quantity of water withdrawn does not exceed recharge and if the heat added to the replenished water is at least equal to that extracted. However, geothermal energy is technically not strictly ‘‘renew­able’’ because it is heat in the rock that heats the water, and this is not renewable. Most geothermal systems have lifetimes of less than 1 million years. Also, in general, heat that is extracted with the water from wells is much higher than that replenished from the reservoir rock because the heat capacity of water is much greater than that of the rock.

Hydrothermal systems are commonly divided into vapor – or steam-dominated and liquid – or water – dominated systems. In a steam-dominated system, the fluid in the reservoir is such that steam is the continuous phase that controls the pressure. When this fluid reaches the surface, the steam produced is normally superheated. In a water-dominated system, water is the continuous pressure-controlling phase. When this fluid reaches the surface, less than 50% by weight is steam. Although much of the world geothermal power generation comes from vapor – dominated systems, they are rare. The most famous are Larderello in Italy and The Geysers in California. Most geothermal systems are water dominated and are one-tenth the size of The Geysers in electrical power capacity. Examples of famous water-domi­nated systems include Wairakei in New Zealand and Cerro Prieto in Mexico.

Geothermal resources are commonly classified according to the average fluid temperatures in their reservoirs. Unfortunately, the demarcation lines are not agreed on (Table I). Exergy is also used to classify geothermal resources according to their specific exergy index,

SE x I = (h — 273s)/1192,

where h is the specific enthalpy in J/g and s is the specific entropy in J/gK. Geothermal resources with SExI>0.5 are high exergy or quality, those with SExI< 0.05 are low quality, and those in between are medium resources (Fig. 1).

TABLE I

Classification of Geothermal Resources by Temperature (°C)

(a)

(b)

(c)

(d)

Low enthalpy

<90

< 125

<100

<150

Intermediate

90-150

125-225

100-200

High enthalpy

>150

> 225

> 200

> 150

Source. Dickson and Fanelli (1990).

Geothermal Power Generation

High exergy: SE x/ >0.5 Medium exergy: 0.5>SEx/ >0.05 Low exergy: SE x/<0.05

3500

FIGURE 1 Classification of geothermal resources by exergy. Reproduced from Lee (2001).

The average earth’s temperature gradient for the top 10 km from the surface is approximately 30°C/km. Places with gradient greater than 40°C/km are designated as thermal areas. Thermal fields are thermal areas with subsurface permeability that allows a fluid to convey deep-seated heat to the surface. Some thermal areas have 90°C/km gradient. The gradient can be higher in active volcanic areas.

Indicators of geothermal resources are surface thermal manifestations. However, a lack of surface manifestations does not imply a lack of geothermal resources (e. g., hot dry rock [HDR] system, geopres – sured system). Surface manifestations can be divided into active and inactive manifestations. Active manifestations are associated with heat and fluid discharges, whereas inactive ones are cold-altered ground and sinter deposits. Examples of active manifestations include hot springs, geysers, fumar – oles, solfataras, mud pools, hot pools, warm ground, steaming ground, hydrothermal eruption craters, and volcanoes. Altered ground has rocks whose minerals have changed chemically due to interactions with thermal fluids. Sinter deposits are chemicals precipi­tated out of the thermal fluids and persist when the thermal fluids stop discharging.

An ideal geothermal system consists of a heat source and a reservoir of geothermal fluid in a confined space accessible by drilling. Heat is trans­ferred from the heat source to the reservoir, which in turn transfers heat to the earth’s surface. The heat source is normally a magmatic intrusion at shallow depths (5-10 km). The reservoir is a volume of hot rock that is permeable so that water can flow in and out of it. Because of the high pressure and tempera­ture in the reservoir, the water tends to absorb gases such as carbon dioxide and hydrogen sulfide and to dissolve solid chemicals such as silica, calcite, and salts. When the geothermal fluid is withdrawn from the reservoir, pressure in the reservoir is likely to decrease, and this will cause more cold water to flow into the reservoir to be heated up again. This is the ideal renewable or sustainable geothermal system.

An HDR geothermal system contains a reservoir of hot rock that is not permeable. Research has been carried out to create permeability in the rock by hydrofracturing with water at very high pressure after drilling into the reservoir. Temperatures of 200°C have been measured in HDR at a depth of 3 km, and HDR systems of up to 10 km deep are being researched.

A geopressured geothermal system consists of a reservoir that is totally confined and contains water at very high pressure. There is no recharge of the reservoir water, which is usually saline. The presence of a geopressured system is most likely in a deep sedimentary basin. Pressures of 400 to 1000 bar abs and temperatures of 100 to 200°C at a depth of approximately 3 km are said to be typical.

A geothermal resource is normally ‘‘discovered’’ by geoscientific exploration. The exploration program normally follows the sequence of reconnaissance, prefeasibility, and feasibility studies. Geologists iden­tify any geothermal surface manifestations and make geological and hydrological studies. These studies are important for all subsequent work, including siting of exploration and production wells. Geochemists study the chemistry of geothermal fluids on the surface to determine the type of geothermal system and estimate the reservoir fluid temperature. Geo­physicists measure physical parameters such as heat flow, temperature, electrical conductivity, gravity, and

Подпись: FIGURE 2 Power cycle for dry steam systems with condenser. B, barometric leg; C, condenser; G, generator; H, hot well; PW, production well; RW, reinjection well; ST, steam turbine. magnetism of the rocks. The information can be used to deduce the shape, size, depth, and capacity of the deep geothermal reservoir. Exploration wells are then drilled to confirm the power potential of the geothermal resource.

In a geothermal power project, the most impor­tant factor is the capacity and life of the resource. Unlike an oil reservoir that is exhausted when the oil is completely extracted, a geothermal reservoir can have its reservoir fluids replaced. This recharge of the geothermal reservoir fluids is an important process that controls the behavior of the reservoir. The study of recharge behavior is by means of geothermal reservoir engineering. Geological and well test data are normally analyzed using reservoir modeling computer software to estimate the capacity and life of the resource.

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