Category Ecological Footprints and Energy

Two-Phase (Liquid-Dominated) Systems

Two-phase systems are liquid-dominated systems. In two-phase geothermal systems, the geothermal fluids contain both water and steam at roughly an equal weight ratio (i. e., dryness, x~0.5) at the wellhead. The water phase needs to be removed by a separator, and the steam is piped to the steam turbines (Fig. 3). The separated water can be reinjected directly or, depending on the fluid pressure, can be flashed to produce a lower pressure steam for power generation before being reinjected (Fig. 4).

Two-Phase (Liquid-Dominated) Systems

FIGURE 3 Power cycle for single-pressure hot water or two – phase geothermal systems. B, barometric leg; C, condenser; G, generator; H, hot well; PW, production well; RW, reinjection well; S, separator; ST, steam turbine.

Two-Phase (Liquid-Dominated) Systems

FIGURE 4 Power cycle for dual-pressure hot water or two – phase g...

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2. GEOTHERMAL POWER CYCLES

When a geothermal resource has been deemed suitable for power generation, a decision is required to select a suitable power cycle. A power cycle consists of a series of thermodynamic processes that converts heat energy to electricity. The most common geothermal power cycle is the Rankine cycle, consisting of two isothermal and two adiabatic processes. More than one power cycle can be used for a geothermal resource. Indeed, other direct uses can be combined with power generation.

Geothermal power cycles can be classified into five major types: steam Rankine cycle, organic Rankine cycle (ORC), total flow cycle, Kalina cycle system (KCS), and trilateral flash cycle (TFC)...

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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...

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ECONOMIC CONSIDERATIONS

Geothermal projects require a relatively large initial capital investment, with small annual operating costs thereafter. Thus, a district heating project, including production wells, pipelines, heat exchan­gers, and injection wells, may cost several million dollars. In contrast, the initial investment in a fossil fuel system includes only the cost of a central boiler and distribution lines. The annual operation and

Подпись: in Cooling out water Подпись: * CondenserПодпись: Liquid ammoniaПодпись: Weak ammonia- water solutionECONOMIC CONSIDERATIONSECONOMIC CONSIDERATIONSGeothermal.

In water Out
Generator

ECONOMIC CONSIDERATIONS

Expansion valve

.Evaporator coil

Pump

maintenance costs for the two systems are similar, except that the fossil fuel system may continue to pay for fuel at an ever-increasing rate, whereas the cost of the geothermal fuel is stable...

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Heat Pumps

Ground-coupled and groundwater (often called ground-source or geothermal) heat pump systems are being installed in great numbers in the United States, Sweden, Switzerland, and Germany. Ground­water aquifers and soil temperatures in the range of 5-30°C are being used in these systems. Ground – source heat pumps utilize groundwater in wells or by direct ground coupling with vertical heat exchangers (Fig. 16). Nearly every state in the United States, especially in the midwestern and eastern states,

Heat Pumps

FIGURE 16 Typical ground-source heat pump installation.

utilizes these systems, which are in part subsidized by public and private utilities...

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Piping

The fluid state in transmission lines of direct-use projects can be liquid water, steam vapor, or a two – phase mixture. These pipelines carry fluids from the wellhead to either a site of application or a steam – water separator. Thermal expansion of pipelines heated rapidly from ambient to geothermal fluid temperatures (which may vary from 50 to 200°C) causes stress that must be accommodated by careful engineering design.

The cost of transmission lines and the distribution networks in direct-use projects is significant. This is especially true when the geothermal resource is located a great distance from the main load center; however, transmission distances of up to 60 km have proven economical for hot water (e. g...

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Downhole Pumps

Unless the well is artesian, downhole pumps are needed, especially in large-scale, direct utilization systems. Downhole pumps may be installed not only to lift fluid to the surface but also to prevent the release of gas and the resultant scale formation. The two most common types are lineshaft pump systems and submersible pump systems.

The lineshaft pump system (Fig. 10) consists of a multistage downhole centrifugal pump, a surface – mounted motor, and a long driveshaft assembly extending from the motor to the pump. Most are enclosed, with the shaft rotating within a lubrication column, which is centered in the production tubing. This assembly allow the bearings to be lubricated by oil since hot water may not provide adequate lubrication...

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2. EQUIPMENT

Standard equipment is used in most direct-use projects, provided allowances are made for the nature of geothermal water and steam. Temperature is an important consideration, as is water quality. Corrosion and scaling caused by the sometimes unique chemistry of geothermal fluids may lead to operating problems with equipment components exposed to flowing water and steam. In many instances, fluid problems can be designed out of the system. One such example concerns dissolved oxy­gen, which is absent in most geothermal waters except perhaps the lowest temperature waters. Care should be taken to prevent atmospheric oxygen from entering district heating waters, for example, by proper design of storage tanks...

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