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Introduction

Geothermal electricity is one of the alternative sources of power used in the modern era by nations where hydro sources do not seem to satisfy the users. Geothermal power comes from the energy generated underground through steam (Aksoy, 2014). Currently, there are 24 countries that have incorporated geothermal power into their grid, while 70 are using geothermal heating. There are three principal types of geothermal power generation plants based on the technology applied as well as on the ground conditions. Flash steam, dry steam, and binary cycle are the three types of power generation systems which are defined by the ground temperature, the depth of the sources of steam, and the quality of water in the reservoir (Ghasemi et al., 2014).

A cooling system for the geothermal power system is essential since it prevents the power turbines from getting overheated as well as ensures that the system runs for long without breaking down due to extreme heat conditions. There are several types of cooling systems that use water and air to maintain the levels of temperature at the acceptable maximum for efficiency. The most popular cooling system is the one that uses water. The system is also known as an evaporative one. Geothermal power is one of the best sustainable sources of energy that is renewable and cheaper to generate and maintain (Ahiska & Mamur, 2013). The main reason geothermal power is considered environment-friendly is the fact that the amount of heat extracted from the steam is petite if compared to the amount of heat that the earth generates. This means that it is possible to keep producing geothermal power without tampering with the heat stability underground (Cheng, Li, Nian, & Wang, 2013).

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System Description and Key Equipment Function

Direct Dry Steam

The steam plants use steam which goes to the turbine and propels a generator for the power to be produced. This process eliminates the need for burning fossil fuels while running the turbines and reduces the costs associated with purchase and storage of such fuels. The direct dry steam process is the oldest of the three systems whose history dates back to 1904. The first plant to be established was built in Lardarello in Italy, 1904 (Aksoy, 2014). Today, such plants are used in many other countries. In northern California, this type of system is used in The Geysers, which is the biggest geothermal plant in the world. Due to the elimination of fossil fuels in the process, this system emits a petite amount of gasses since most of the gases going out in the air are steam (Ghasemi et al., 2014).

Flash and Double Flash Cycle

The current technology allows fluids of hydraulic nature to be used at the temperatures above 182 degrees Celsius in the production of electric energy (Lund, Freeston, & Boyd, 2005). The hydraulic liquid is sprayed into a container which is set at very low pressure making some of it to vaporize immediately through the process called flashing. The vapor collected is then pushed into a chamber where the turbines are, driving them to generate electric energy (Bertani, 2016). If after the process there is some liquid remaining in the tank, the process can take a second flashing dimension where the remaining substance is passed to the second tank until it vaporized and drove the turbines. This is what makes the process get the name “double flashing.”

Binary Cycle

Because there are some geothermal plants located in places where underground water temperatures go below the 182 degrees, the binary cycles system is used to extract energy when the other two have failed (Huttrer, 2001). In this type of a plant, the underground water is combined with a binary liquid with boiling point lower than the one of water. The other liquid passed through a system of the heat exchanger where it is extremely heated by the geothermal power beyond its boiling point in order to vaporize and flash driving the turbines (Ghasemi et al., 2014). Since the process is closed due to the presence of the binary liquid, there are no emissions to the air. It avoids contamination of the atmosphere with the vapor from the fluid. This is the most common system of geothermal power generation in the world since most of the underground water do not reach the required temperature. Depending on the levels of the underground temperatures, the binary liquid is carefully selected to ensure that it can be vaporized faster and consistently. The range of the boiling points required for the binary fluids is between 85°C to 170°C (Cheng, Li, Nian, & Wang, 2013).

All the geothermal power generation types discussed above use steam as the primary component, and steam turbines are common in all of them. Their approach to power generation is similar since the rotation of turbines by steam is what generates the geothermal energy. The approach to the driving of turbines is very similar to other sources of energy like hydro and wind systems. Once the turbines are rotated, they generate some energy through the process scientifically known as electromagnetic induction. This energy is first formed in the kind of heat/thermal power; but after the induction process, it is converted to electricity (Aksoy, 2014). Since the heating of water takes place by natural means, there are no boilers in the geothermal power system. In fact, heat is replaced by cooling, since the natural means of heating water goes beyond the manageable limits and has to be cooled to the levels of use.

The earth is capable of heating water beyond 200 degrees which might be difficult to do using human efforts (Ahiska & Mamur, 2013). This is what makes a cooling system extremely vital in order to prevent it from overheating and melting the metallic materials like pipes and turbines. Production wells are the channels through which the hot water from the ground gets to the power generating system. These are deep vertical tunnels drilled into the ground to allow water to come up and get into the plant when it is still hot. The liquid is heated thousands of miles down the earth rocks, but due to the pressure generated by the weight of rocks, it can be sent up to the ground through cracks. These cracks are the ones that are drilled as standard pipe tunnels for collect water without allowing it to leak outside and lose pressure (Blodgett, 2014).

Rock catchers are placed between the heat exchange chamber and the turbine chamber to ensure that only the hot fluids have their access to the turbines. They are supposed to stop any object that could be obtained underground (like rocks) from getting into the chambers and destroying the turbines. The pressure that pushes water from the ground into the system is capable of plucking stones from the rock layers and sending them into the turbines which can be a real danger if not prevented (Huttrer, 2001).

Injection wells are supposed to ensure that the water that leaves the turbine chamber is returned to the underground reservoirs for reheating. Hot water is not supposed to be allowed to flow over-ground after leaving the system since it can be a cause of damage to the environment and people. Another reason for returning it back to the source is to ensure that the reserve does not suffer reduction in the amount of water being heated (Blodgett, 2014).

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Cooling system. Typically, heat has been produced by the rocks at the center of the earth for billions of years without any instances of breaks. This means that the water had been hot all these years, and people discovered a commercial use for it only some hundred years ago. It is approximately 4000 miles underground where the heat is generated at extremely high rates. This is where water may be heated beyond its boiling point (Cheng, Li, Nian, & Wang, 2013). Scientifically, it is believed that the center of the earth shares almost the same temperatures with the sun at 5500°C, which is many times higher that the boiling point of water. Scientists go ahead to give an estimate that there could be 42 million megawatts of power circulating underground through conduction due to the extreme conditions existing there. This gives assurance that there is an opportunity of making electricity from the earth conditions if only the right materials are discovered and put to the right application (Ghasemi et al., 2014).

Due to the enormous levels of heat coming from the underground water reservoirs, it is necessary to have an efficient cooling mechanism to ensure that the system does not burn down. There are three different mechanisms of cooling the system using water or air; they are explained below.

Once-through cooling. This system gets cold water from the rivers or wells nearby and distribute it through pipes tapping all the excessive heat. This method was initially the most popular since it is cost effective. However, as the plants get bigger and more complex, the process gets difficult to use, hence the need for an alternative. Water is passed over the steam pipes ensuring that the metal pipes do not get overheated to a point of breaking or melting down if temperatures from the underground go extreme (Lund, Freeston, & Boyd, 2005). Once the water has been used to cool the system, it is discharged back to its source and the system does not re-use it, hence the name once-through system (Blodgett, 2014).

Wet-recirculating or closed-loop cooling. This system uses water just like the once-through, but this one does not release the water back to the source after using it for the first time. This cooling system has openings where the water is allowed to get in contact with air where some of it evaporate and is replaced by fresh liquid from the source, but it is never released back to the rivers or dams (Thain & Carey, 2009).

Dry cooling. Unlike the other two that use water to cool the system, this one uses air instead. This type of a cooling system is considered water effective since it reduces its consumption by 90 percent compared to the other two. However, it is regarded as a pollutant since it tampers with the air in the process of cooling the systems. However, it is popular in the plants where water sources are far from the power generation unit. Cold, dry air is trapped from the atmosphere and pumped through the system just like an air conditioner cools a living room (Lund, Freeston, & Boyd, 2005).

System Block Diagram

In the diagram, it is evident that there are different points where the liquid is turned from one physical state to another as well as other things taking place for the generation of electric power. The hot rock is where the hottest water comes from while the ocean surface gives moderately heated water. The heat exchanger is where two differently heated fluids get into the system to meet with the binary liquid which is heated to vaporization. The turbine chamber is the place where the vapor is directed to drive the system for the production of energy which is then converted to electric power by synchronous generator (Bertani, 2016). The condenser is where the cooling system works most to ensure that the water that condenses from the turbine chamber is converted entirely into liquid form and sent back to the heat exchanger in a way that is possible to drain it back to the ground. In a situation like double flashing, the vapor is sent back to the turbines and flashed again to ensure that turbines keep rotating as more hot water is tapped from the ground.

Operation Principle

In the above diagram, hot water is tapped from the ground through pipes and directed to the heat exchanger where the binary liquid is also supplied through different entry (Maehlum, 2013). Immediately, the binary liquid comes in, the temperatures from the ground water heats up the liquid to vaporization point, and the vapor is directed through the heat exchanger to the turbines. Before the vapor gets to the turbines, it passes through a control mechanism which regulates the speed to ensure consistency (Aksoy, 2014). Since the process is fast and regular, the liquid in the turbine chamber cools down in the process. This pushed the liquid out of the chamber back to the heat exchanger for vaporization, and later it is taken back to the turbines. This vapor drives the turbines and generate electric power in the synchronous generator in a form of alternating current (AC) (Thain & Carey, 2009).

The work of the condenser is to ensure that everything that leaves the turbine chamber is returned back to the heat chamber in liquid form (Maehlum, 2013). Cold water is then released from the heat chamber into the ground for re-heating and replaced by fresh and hot water from the underground. The process is continuous and regular, thus producing a constant flow of power from the generator. Hot water is collected from two distinct sources with the difference in temperature. One source is hot underground rocks which generate the hottest fluid while the other one is the ocean surface giving moderately warm water. The geothermal system is divided into two parts based on the form of liquid that goes to that side. The first is the liquid circuit which includes the collection point for hot water and the flow back to the ground, while the other one is the vapor circuit where steam drives turbines to generate power (Ahiska & Mamur, 2013).

Once the power is in AC form, it is then transferred to transformers which regulate the flow as well as bringing down the units to consumable limits for the sake of different households and industrial requirements. In most cases, households do not consume power that is higher than 415V, which means that there have to be mechanisms for lowering down the voltage to those limits. On the other hand, industries require higher voltage, and they have to be supplied accordingly, thus needing transformers (Lund, Freeston, & Boyd, 2005). The first lowering down of voltage is done immediately after the power leaves the synchronous generator in AC form since it can be risky to transmit it in higher voltage due to chances of burning down the cables which would cause a fire.

Key Design Factors

During the construction of a power generating plant, several factors determine the design. Most of those factors are cost-related since it is only cost that determines the size and capacity of the plant, hence influencing everything else within the design list.

Cost

The cost of constructing a geothermal power plant is a significant determining factor since it directly affects the design of the structures and the quality of the materials to be installed. Cost goes ahead to determine the size of the plant. Thus, it influences the capacity resources the plant can be able to handle at any particular operation stage (Bertani, 2005). The cost of constructing a power plant and managing its operation is the combination of the materials needed for building permanent structures like offices and machine housing among others. It also includes the cost of buying such machinery as generators and pipes as well as other system equipment. All these elements combined are the cost-related factors which have to be considered during the determination of design (IEA, 2010).

Underground Temperature

Underground temperature is the determinant of the steam temperature that is tapped for geothermal power generation. The levels of temperature dictate the type of plant to be established between direct streams and binary cycles, which are critical factors for design. If the underground water reaches temperatures of above 182 degrees, the design of the plant will require a direct steam setup. However, if the temperatures go below the required 182 degrees, the design of the plant will take the binary dimension. If the binary system is adopted, the design of the plant will be directly and significantly influenced (GTP, 2008).

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Size of Underground Water Deposits

In the development and establishment of a geothermal power plant, the size of the underground activity has to be considered as a determining factor for the design. This should be always considered because geothermal power production is primarily dependent on the activities that take place underground generating heat that vaporizes water. If the size of the geysers is small, the design of the plant will be realized in the form of a small power station with the small capacity system.

Application & Example Cases in the World

Due to the improvement in technology, the distribution of geothermal power plants has increased over the years with the former ones getting improved and expanded to accommodate more capacity.

The Geysers Geothermal Complex

The biggest single plant in the world is known as The Geysers Geothermal Complex located in California. This power plant houses 18 different sub-plants making it the biggest in size and power generation capacity. The plant has a capacity of 1,517MW with 900MWactive production. 15 of its sub-plants are owned by Calpine while Northern California Power Agency owns two others jointly. The 15 Calpine sub-plants have a total of 725MW (Lund, Freeston, & Boyd, 2005). US Renewables Group owns the last sub-plant known as Bottle Rock Power Plant. Currently, Ram Power is in the process of constructing another plant with a capacity of 26MW in the same complex to make a total of 19 sub-plants.

Larderello Geothermal Complex

This is the world’s second largest geothermal electricity generation plant. The plant is sistuated in Italy in a place called Tuscany (central Italy) with 34 sub-plants within the complex and a total capacity of 769MW. It is believed that this plant produces 10 percent of all geothermal electric power consumed globally (Lund, Freeston, & Boyd, 2005). The plant can give power to 26.5 percent of all the energy-consuming clients within the region. This complex is owned by the company called Enel Green Power, serving an estimated clientele of over 2 million families, 25 hectares of greenhouses and 8700 residential and corporate clients. Surprisingly, this is the oldest geothermal plant to have been established, having its history dating back to 1904.

Cerro Prieto Geothermal Power Station

Cerro Prieto Geothermal Power Station is located in Mexico. It is considered the third largest geothermal plant of energy production with a capacity of 720MW. The plant is owned and operated under the authority of Comisión Federal de Electricidad (CFE) and features four plants within the complex. These plants are sub-divided into 13 small units. The first commissioning of a unit in this complex was deployed in 1973 with the fourth taking place in 2000. The plant has four double-flash units with 112MW capacity, four single flash units of 37.5MW, four single flash units of 25MW and one single flash unit of 30MW capacity. Currently, there is a fifth plant there being under construction with two turbines of 50MW capacity (Maehlum, 2013).

Makban Geothermal Power Complex

This plant is located in the Philippines in a place called Batangas province. It is the fourth largest plant globally. The capacity of the plant stands at 485MW, although it is on its way of improving the operations. The complex has five units of 3MW, one binary plant, and one unit of 0.73MW capacity. The plant opened its operations in 1979 with Mitsubishi Heavy Industries supplying the first turbines to the complex.

CalEnergy Generation’s Salton Sea Geothermal Plants

This plant is in the United States. It is located in a place called Calipata in South California. The plant holds a production capacity of 340MW and is ranked the fifth in the world’s top list of geothermal production. 50 percent of the facilities are owned by CalEnergy Generation, while MidAmerican Geothermal owns the other half. In 1982, the first unit was completed with a capacity of 10MW, while the tenth one was ready in 2000. Currently, three modern projects are going on for the expansion of the complex with 50MW units being installed (Lund et al., 2003).

Hellisheidi Geothermal Power Plant

This plant is a flash steam system with a combined power and heat power. The plant is located at Mount Hengill, which is about 20km away from the Reykjavik, the capital city of Iceland. The plant has a capacity of producing 400MW of thermal and 303MW of electric energy. It is the sixth biggest plant in the world, entirely owned by Orkuveita Reykjavikur. The construction of the plant was done by Verkís Engineering and Mannvit Engineering (Bertani, 2005).

Tiwi Geothermal Complex

This complex is located 300km southeast of the city of Manila in the Philippines. The capacity of the power plant is 289MW, which makes it be ranked seventh globally. This power generation complex is owned by the firm known as Aboitiz Power’s subsidiary AP Renewables. This complex is made up of three sub-plants that consist of two small units each. The commencement of the construction of this plant was in 1972, but the power generation officially started in 1979 (Bertani, 2016).

Advantages of Geothermal Energy

Geothermal power generation is an activity that is dependent on the nature dynamics. Hence, it has its advantages as well as disadvantages to the people, to the economy, and to the environment as explained below.

Geothermal Energy Is Renewable

Due to the cost of power generation and the effects on the environment, it is recommended that the power plan is a renewable one for the sake of sustainability. Geothermal energy is renewable since the process of generation is a cycle that never stops, and the ingredients of generating the power are the substances that occur freely in the environment like water and heat energy from underground rocks. The water used is later drained into the underground reservoirs where it is heated naturally and tapped again for continuous power generation (GTP, 2008).

Geothermal Power Is Environmental Friendly System Description and Key Equipment Function

Direct Dry Steam

Since the whole process uses natural resources which are freely available in the environment, it does not pollute or over exploit those resources. The main component of the geothermal power generation used is water heated naturally from the ground and later taken back for reheating. Thus, there are no dangerous gasses released to the environment, residual dumping, or cutting down trees for heating the water. Even in the cases of fossil fuels being used by generators, the pollution limits are negligible, hence making this process environmentally friendly (Bertani, 2005).

No by-Products

Geothermal power system does not produce by-products in the process of generating electric and thermal energy. The only product released at the end of the process is power, and all other components of the system go back to the ground for re-heating. This makes it a clean process that does not interfere with the environment as well as the people working in it (Thain & Carey, 2009).

Geothermal Energy Can Be Used Directly

The process of geothermal power generation is short and easy, and anybody with the knowledge of how to set up the system can be able to make something out of the available resources. The process allows people to use the power directly without having to seek for assistance from experts and having complicated mechanisms installed for power distribution. This is an advantage since after the power has been converted to AC form, it is usually ready to use for domestic and commercial purposes (GTP, 2008).

Low Maintenance Cost

The cost of installing and maintaining a geothermal power plant are lower than those of hydraulic power plant. This is due to fewer machines required as well as a shorter process. Since the process is a cycle, it is less complicated with few systems being installed at lower cost. Much of what is done in the process requires hot water which is heated naturally by the underground activities, hence having no need for extra heating costs (Atrens, Gurgenci, & Rudolph, 2009).

Little Space Requirement

The geothermal power plant does not involve a lot of construction which would consume a lot of space. A lot of the systems used require over-ground pipes from the source of steam to the heating chambers which can be used together with other activities. Compared to other power generation systems, the geothermal one consumes less space, and other activities may take place on the same portion of land where the pipes pass through. This makes it less destructive to the environment (Bayer, Rybach, & Brauchler, 2013).

Geothermal Power Is Independent Of Weather

Alternative sources of energy tend to be dependent on the weather conditions which affect productivity when climate changes with seasons. For instance, solar energy depends on sunlight, and when it is dark, rainy, or foggy, the system is halted only to recover when the light comes back. Wind energy depends primarily on wind power, and when the weather gets calm, the power generation is blocked. However, there has never been an instance when the underground heat gets affected by weather conditions, meaning the geothermal power generation is constant and continuous regardless of the current weather situation, thus being more reliable than other alternative forms (Lund et al., 2003).

Disadvantages of Geothermal Energy

Few Sites with Geothermal Power Potential

In the world, there only a couple of places with the potential for producing geothermal energy, and this makes it unreliable. There are some countries without a single geothermal power plant, while others only have a single plant with a small capacity, meaning that it has to work together with other sources of power like solar or hydro. Power requirements are massive due to the constant modernization of activities. Thus, geothermal power cannot be enough to serve alone the whole country (Atrens, Gurgenci, & Rudolph, 2009).

Geothermal Sites Are Far from Energy Consumption Points

Most of the geothermal power production points are located hundreds of miles from the cities where power is consumed in large scale. This makes it costly to distribute the power from production to consumption. Even the cost of construction is increased by the distance engineers have to transport equipment from the cities where it was purchased.

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Low Production of Power

The total global production of geothermal power is petite compared to the total power requirement by industries and households. This means that there is no way geothermal power can be cost-effective if there is a need to serve corporate clients with that source only. Each plant can produce very little amount of power compared to the other sources of energy production (Thain & Carey, 2009).

Risk of Volcano Eruption

Since geothermal power generation works with underground heat and fluids, this process is always under a constant risk of triggering a volcano eruption. Underground heat is massive, and the cycles of fluid tapping and release might be a trigger to volcano if something disturbs the hot rocks to produce excessive temperatures (Bayer, Rybach, & Brauchler, 2013).

Conclusion

The current state of industrialization has come with a massive need for power since the technology that employed power is intensive. This has led to people coming up with different ideas of power generation, for exapmle geothermal energy tapping. Geothermal power generation is not very new, but it has been a dormant process where very little progress has been witnessed globally over the decades. However, it is a sustainable and environmentally friendly process since it does not tamper with the state of nature. Several countries, especially those within the American continent, are in the process of expanding their already operational plants in a bid to increase the power output that those plants offer to the national grid. The first idea of power generation from the thermal capacity of the underground rocks was put to the test in Italy in 1904, when Lardarello power plant was established. Since then, other countries have been discovering their geothermal sites and establishing plants, with the USA having the biggest single site in California knows as The Geysers Geothermal Complex. However, this type of power generation has its shortcomings, especially since the potential sites are fewer than the required energy amounts.