study of geothermal power plant electrical and control system with ...

GEOTHERMAL TRAINING PROGRAMME Orkustofnun, Grensásvegur 9, IS-108 Reykjavík, Iceland

Reports 2002 Number 7

STUDY OF GEOTHERMAL POWER PLANT ELECTRICAL AND CONTROL SYSTEM WITH EMPHASIS ON RELIABILITY ASPECTS Haile Kebede Ethiopian Electric Power Corporation P.O. Box 1233 Addis Ababa ETHIOPIA

ABSTRACT The main aim when constructing and installing electrical and control systems for a power plant is to have continuous and trouble free operation of the plant. Geothermal power plants are very different from hydro power plants. It is difficult to restart a geothermal power plant after shutdown and it is exposed to H2S corrosion. This report emphasizes plants using the binary process. The electrical system of a geothermal power plant is described, including generator, power transformer, different alternatives for switchgear configuration, protective relaying, auxiliary power supply and instrumentation. The control system configuration is described along with its effect on reliability. Operation and maintenance aspects are discussed, and both the reliability and H2S corrosion prevention aspects are discussed. Finally, the electrical and control systems in the Aluto-Langano geothermal pilot power plant are included as an example.

1. INTRODUCTION Geothermal energy is a proven resource for direct heat and power generation. In over 30 countries geothermal resources provide directly used heat capacity of 16,000 MWt (Lund and Freeston, 2000) and electric power generation capacity of over 8,000 MWe (Huttrer, 2000). It meets a significant portion of the electrical power demand in several developing countries. The geothermal resources can be divided into low- and high-temperature systems depending on the temperature of the reservoir. The hightemperature geothermal resource can be utilized for electricity production. The utilization is mainly achieved by direct-steam or flashed steam installation. On the other hand, from low-temperature geothermal resources it is also possible to produce electricity by applying non-conventional methods of geothermal power generation, such as binary cycle generation. In this report, the control and electrical system of a geothermal power plant is discussed, with emphasis on a binary cycle plant, like the one already in operation in Ethiopia (Bronicky, 2000). The binary cycle consists of a separate closed cycle, which uses an organic compound as a working fluid (Organic Rankine Cycle-ORC), receives the input heat from geothermal brine and releases output heat into a suitable heat sink. The geothermal brine is passed through a heat exchanger to heat up the organic compound which, 77

Kebebe

78

Report 7

having a low-boiling point, is vaporized before entering a turbine. Ethiopia is one of the African countries that has a large amount of waterfalls in the high mountains but has also considerable geothermal resources. The electricity supply is mostly from hydro power generation and in some parts of the country from thermal energy. The rivers are season dependent and the price of oil is rising. Thus, an alternative energy source is important to compensates for the shortage of electrical power (Aquater, 1979). Geothermal exploration in Ethiopia has been in progress since 1970 and to date. Prospects have been identified with an estimated 700 MWe potential. The three most intensely explored areas are Aluto-North, Langano and Corbetti in the Lake District, and Tendaho in the central Afar region. At Aluto-Langano, a geothermal pilot power plant was constructed by Ormat under a turnkey contract of Genz (EPC) and has been operational since 1998. It is the first geothermal power plant in Ethiopia with two units that are operating by combined steam and binary fluid. The electrical system of a geothermal power plant needs much attention during the design process. The control system automation have a wide span of techniques and technologies. For reliable operation of the geothermal power plants, nowadays the control system is important. The vast development of PLC systems for plant control makes both local and remote mode of operation possible.

2. ELECTRICITY GENERATION FROM GEOTHERMAL ENERGY In geothermal power plants, steam or hot water from geothermal reservoirs provides the force that drives (rotates) the turbine and generator to produce electricity. The used geothermal water is then returned down an injection well into the reservoir to be reheated, to maintain pressure, and to sustain the reservoir or can be used for district heating. There are three kinds of geothermal power plants, depending on the temperatures and pressures of the reservoir (Kestin et al., 1980). •

•

•

Dry steam reservoirs produce steam and very little water. The steam is piped directly into the power plant to provide the force to spin the turbine, which is coupled to drive the generator in order to produce electricity. The temperature and pressure of the reservoir should be high enough to boil the fluid at the wellhead. These kinds of reservoirs are not common. A geothermal reservoir that produces mostly hot water is called a hot water reservoir and is used for a flash power plant. Hot water is brought up to the surface through the production well where, upon being released from the pressure of the deep reservoir, some of the water flashes into steam. The steam is then separated from the water in the separator, and powers the turbines and generator for electricity production. A reservoir with low and medium temperatures is not hot enough to flash enough steam but can still be used to produce electricity in a binary power plant. In a binary system the geothermal water is passed through a heat exchanger, where its heat is transferred into a second (binary) working fluid, such as isopentane, that boils at a lower temperature than water. When heated, the binary liquid flashes to vapour, which, like steam, expands across and spins the turbine blades. Then, the working fluid in the vapour state is recondensed to a liquid and reused repeatedly. In this closed loop cycle, there are no emissions to the air (Ormat, 2002).

3. THE BINARY PLANT PROCESS For moderate temperature water, binary cycle technology is generally most cost effective. In these systems, the hot geothermal fluid vaporizes a secondary or working fluid, which then drives a turbine and generator. Selection of a working fluid is an important task in designing a binary plant; usually the working fluids are isobutene or pentane. The following points are important when selecting the working fluid (Sagun, 1992):

Report 7 • • • • •

79

Kebede

The thermal characteristics required for the binary system are satisfied; Adverse effects on environments are negligible; Adverse effects on human body are negligible; Handling is easy such as non-inflammable and non-explosive; Properties are stable.

A binary power plant is a cycle designed for utilizing moderate temperature geothermal resources. The binary plant is like the steam turbine, but uses a working organic fluid, which has a lower boiling point than the geothermal fluid. It is based on the Rankine power cycle (Figure 1). The cycle starts by heating the organic fluid followed by vaporization of the fluid to steam FIGURE 1: Schematic diagram of binary cycle with in order to drive the turbine. The geothermal fluid as heat source (Sagun, 1992) Rankine-cycle fluid, usually a hydrocarbon chosen according to heat source, is heated and vaporized in a heat exchanger by heat transfer from the geothermal water. The pressurised vapour expands through the turbine blades from which mechanical energy is produced to rotate the generator shaft in order to produce electrical energy (Keio and Shojirou, 2000). The exhaust vapour from the turbine is passed to a condenser where it is cooled and condensed to a liquid by cold water or natural atmospheric air or by cooling fans. The organic fluid is condensed and pumped back to the vaporizer by the feed pump to recycle the working fluid. The cycle will continue as long as there is no leakage, thus leakage detectors are important in this system. 3.1 The ideal Rankine cycle The basic Rankine cycle is presented schematically in a Ts diagram in Figure 2. The ideal simple Rankine power cycle consists of the following: • • • •

Isentropic compression in a pump; Constant-pressure heat addition in a boiler; Isentropic expansion in a turbine; Constant pressure heat removal in a condenser.

The heat added in process 2-3 is from low-temperature geothermal fluid or any other lowtemperature source such as fossil fuel. In the ideal case, all losses are negligible; the heat transfer to the working fluid is presented on a Ts

FIGURE 2: Schematic flow diagram and a Ts diagram of an ideal Rankine cycle (Wark, 1988)

Kebebe

80

Report 7

diagram by the area enclosed by states 2s-2’-3-b-a-2s. The area enclosed by the states 1-4s-b-a-1 represents the heat removed from the working fluid inside the condenser. For an ideal case, the first law for an open cyclic process indicates that the heat energy lost by the working fluid inside the turbine is the work done. Thus, the net work done is represented by the difference in the areas for the net input and heat rejection, i.e., the area under 1-2s-2’-3-4s-1. The thermal efficiency 0th the cycle is defined by Equation 1 (for definition of variables see Nomenclature) as

η th =

Wnet

(1)

Qinput

Efficiency is a parameter used to measure the effectiveness of any cyclic heat-work converter. Expressions for the work and heat interactions in the ideal cycle are found by applying the steady flow energy equation. The basic equation for each process considering an ideal case, reduces to

q + w = hout − hinlet

(2)

winput , pump = h2 − h1

(3)

The isentropic pump work is given by

The pump work is also frequently determined within the desired accuracy from the relation

winput , pump = ν f ( P2 − P1 );

s1 = s2

(4)

where