Category Ecological Footprints and Energy

ENVIRONMENTAL IMPACTS AND OTHER TECHNICAL PROBLEMS

Geothermal power stations have their own unique developmental, operational, technical, and environ­mental problems. Many geothermal resources occur in or near national parks or remote uninhabitable locations because they are normally associated with volcanoes and have beautiful surface manifestations and so become public reserves and tourism hot spots. Most of these geothermal resources do not get developed for power generation. Examples include Yellowstone Park in the United States and Orakei Korako Geyserland in New Zealand. However, geothermal resources that exhibit little surface manifestation in national parks do get developed. Examples include Hatchobaru and Otake in Japan.

Volcanoes, and hence geothermal resources, are also associated with mythical power and beliefs...

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GEOTHERMAL POWER STATIONS

The total installed geothermal power generating capacity in the world is approximately 9000 MWe from 21 countries, with the United States leading at nearly 3000 MWe and The Philippines with nearly 2000 MWe (Table II). Other major countries are Italy, Mexico, Indonesia, Japan, and New Zealand, with between 400 and 800 MWe each.

Geothermal power stations have very high avail­ability, load, and capacity factors (>90%). They are most suitable as base load power stations. Liquid – dominated reservoirs are not suitable for peak load generation because two-phase flow and the separa­tion process make their operations and control at changing flows difficult.

A geothermal power station that uses steam turbines to generate electricity is similar to a conventional fossil fuel-fired power station...

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The Silencer

A geothermal silencer is like a separator operating at atmospheric pressure. It is normally connected directly to a well and a separation plant for bypass or emergency discharge. The silencer reduces the high – frequency noise discharging from an open pipe to a low-frequency rumbling noise that is more bearable to human ears. The earlier Wairakei silencer design consisted of twin barrels, and the body was made of timber. The modern silencer design consists of a single barrel made of corrosion-resistant steel, sometimes internally lined with concrete (Fig. 13). Fiberglass barrels worked well for silencers not in constant use. Some new silencer designs have a cone fitted at the top of the barrel to increase the steam velocity so that it is dispersed farther away from the silencer area...

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The Flasher/Separator

Although sometimes a flasher is differentiated from a separator, the two are the same pressure vessel in practice. A flasher allows high-pressure geothermal water to flash at a lower pressure. In practice, the lower pressure is achieved by a throttling orifice or valve upstream of the flasher/separator. The vessel then separates the two phases by centrifugal and gravity effects.

The modern separators differ in many aspects from those first used at Wairakei during the 1950s. They differ in size, location, body arrangement, separated water vessel, two-phase inlet nozzle, water outlet nozzle, steam outlet nozzle, and efficiency. The original Wairakei separators were wellhead separa­tors. Each wellhead has its own small separator handling the capacity of a single well...

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The Wells

The use of a geothermal resource depends entirely on the output from the wells. A well’s output can vary significantly from that of a typical well (Fig. 10). An example is the cycling wells that have fluctuating mass output with a cycle time as short as a few minutes. This type of well is not suitable for power production unless it can be balanced by stable output wells. Another type of problem well is one that continuously discharges solids with the well fluids. Sometimes this problem can be solved by throttling, but generally a dirty well is not used for production.

3.1 The Wellhead

A typical wellhead consists of several valves to provide service functions required (Fig. 11). The master valve is for completely shutting off the well...

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GEOTHERMAL STEAM PRODUCTION FIELDS

Geothermal wells need to undergo a test program before they are used. This is so that the baseline conditions of both the wells and the geothermal aquifers that they tap can be determined. This baseline data are critical because all future informa­tion is compared against them.

After drilling has been completed and before the initial discharge, the well downhole conditions are measured. The temperature and pressure are mea­sured by using a clockwork Kuster gauge or electronic logging tools. Standard practice is to initially do an injection (or completion) test, that is, sometime at a series of flows, where the temperature, pressure, and possibly flows using a spinner tool are

Подпись: 100GEOTHERMAL STEAM PRODUCTION FIELDSПодпись: о % 0 2 5 о Li ra га га Подпись: 50Подпись: CD —) measured at intervals down the open hole section of the well...

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GEOTHERMAL DRILLING

Geothermal drilling technology is adapted from the oil industry. Most of the geothermal wells are drilled with a rotary-type drilling rig. Because of the hard, abrasive, and high-temperature volcanic rocks of geothermal systems, the drilling bits require hard abrasive resistant teeth such as diamond or tungsten carbide. Hence, geothermal drilling is an expensive activity in geothermal development. Most geother­mal wells are drilled 1 to 2 km deep to intersect faults and fractures so as to increase the success rate of production from permeable zones. This is enhanced by directional or deviated wells as opposed to vertical wells of earlier development. Directional drilling also allows drilling of more than one well on a single well pad, especially in difficult terrains...

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Hot Water (Liquid-Dominated) Systems

Hot water systems are the most common geothermal systems suitable for power generation. In a hot water system, both the liquid and steam phases appear at the wellhead because the water flashes as it ascends the wells. The dryness fraction at the wellheads is normally less than 0.3. Depending on the wellhead pressures, the power cycle selected can be a single­pressure steam system (Fig. 3) or a dual-pressure one (Fig. 4). It is common to flash the water phase to produce more steam in a low-pressure flasher or separator. In practice, there is no real distinction between a flasher and a separator. Both are pressure

Hot Water (Liquid-Dominated) Systems

FIGURE 5 Power cycle for hot water or two-phase geothermal systems with binary ORC...

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