The supply of energy from primary sources is not constant and rarely matches the pattern of demand from consumers. Electricity is also difficult to store in significant quantities. Therefore, secondary storage of energy is essential to increase generation capacity efficiency and to allow more substantial use of renewable energy sources that only provide energy intermittently. Lack of effective storage has often been cited as a major hurdle to substantial introduction of renewable energy sources into the electricity supply network. The author presents here a comprehensive guide to the different types of storage available. He not only shows how the use of the various types of storage can benefit the management of a power supply system, but also considers more substantial possibilities that arise from integrating a combination of different storage devices into a system. This book will be important to those seeking to develop environmentally sound energy resources.
Inspec keywords: renewable energy sources; energy storage; power system management
Other keywords: power supply system management; renewable energy sources; electricity supply network; secondary storage; generation capacity efficiency; energy storage
Subjects: Other energy storage; Power system management, operation and economics
In this chapter trends in power system development are presented. Section 1.1 and 1.2 discusses demand side characteristics of the power system and supply side characteristics. In section 1.3 power generation expansion planning is discussed.
The diversity of applications of electricity and particularly the fact that some of its uses, such as lighting and space heating, are subject to substantial seasonal variation makes the economic ideal of supply for constant consumption throughout the year unrealistic. There should be an intermediate unit between producer and customer that can coordinate them. This intermediate unit therefore has to be able to separate partly or completely the processes of energy generation and consumption in the power system. Secondary energy storage in a power system is any installation or method, usually subject to independent control, with the help of which it is possible to store energy, generated in the power system, keep it stored and use it in the power system when necessary.
In an electricity power system based on thermal, nuclear, hydro and renewable generation, storage will find a wide field of application and may perform various duties, which must be taken into consideration in order to gain the largest possible advantage in the supply side optimisation. An energy storage unit can be connected to the transmission, subtransmission or distribution system in a manner similar to customer-owned conventional or renewable generation facilities such as gas or wind turbines. These dispersed sources are able to change the character of a typical electricity power system completely.
This chapter discusses the following topics for thermal energy storage: general considerations; storage media; containment; power extraction; thermal energy storage in power plant; and economic evaluation.
Storing energy in the form of mechanical kinetic energy (for comparatively short periods of time) in flywheels has been known for centuries, and is now being considered again for a much wider field of utilisation, competing with electro chemical batteries. In inertial energy storage systems, energy is stored in the rotating mass of a fly wheel. In ancient potteries, a kick at the lower wheel of the rotating table was the energy input to maintain rotation. The rotating mass stored the short energy input so that rotation could be maintained at a fairly constant rate. Flywheels have been applied in steam and combustion engines for the same purpose since the time of their invention. The application of flywheels for longer storage times is much more recent and has been made possible by developments in materials science and bearing technology. The energy capacity of flywheels, with respect to their weight and cost, has to date been very low, and their utilisation was mainly linked to the unique possibility of being able to deliver very high power for very short periods (mainly for special machine tools).
Pumped hydro storage is the only large energy storage technique widely used in power systems. For decades, utilities have used pumped hydro storage as an economical way to utilise off-peak energy, by pumping water to a reservoir at a higher level. During peak load periods the stored water is discharged through the reversible pump-turbines to generate electricity to meet the peak demand. Thus, the main idea is conceptually simple. Energy is stored as hydraulic potential energy by pumping water from a lower level to a higher level reservoir. When discharge of the energy is required, the water is returned to the lower reservoir through turbines that drive electricity generators.
Citywide compressed air energy systems have been built since 1870. Cities such as Paris, Birmingham, Offenbach, Dresden in Germany and Buenos Aires in Argentina installed such systems. Victor Popp constructed the first systems to power clocks by sending a pulse of air every minute to change the pointer. They quickly evolved to deliver power to homes and industry. As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured in metres. The systems were the main source of house-delivered energy in these days and also powered the machines of dentists, seamstresses, printing facilities and bakeries. The application of elastic energy storage in the form of compressed air storage for feeding gas turbines has long been proposed for power utilities; a compressed air storage system with an underground air storage cavern was patented by Stal Laval in 1949. Since that time, only two commercial plants have been commissioned; Huntorf CAES, Germany, and Mcintosh CAES, Alabama, USA.The compressed air energy storage (CAES) concept involves a thermodynamic process in which the major energy flows are of work and heat, with virtually no energy stored in the compressed air itself. The performance of a CAES plant depends on the precise details of both the compression process and the expansion process.
Synthetic fuels are considered to be substitutes for natural gas or oil and are made from biomass, waste, coal or water. Production of these fuels demands energy, which can be obtained from base-load power plants during off-peak hours. Therefore, synthetic fuels are a type of energy storage since it is possible to use them instead of oil or gas for peak energy generation. The fuels themselves are only a type of medium (hydrogen, e.g. is simply a method to store and transmit energy); as with any other storage concept, a power transformation system and central store are also required. Storage media have to be produced during off-peak hours in a chemical reactor or electrolyser this has to be considered as a part of a power transformation system used during the charge regime.
The most traditional of all energy storage devices for power systems is electro chemical energy storage (EES), which can be classified into three categories: primary batteries, secondary batteries and fuel cells. The common feature of these devices is primarily that stored chemical energy is converted to electrical energy. The main attraction of the process is that its efficiency is not Carnot-limited, unlike thermal processes. Primary and secondary batteries utilise the chemical components built into them, whereas fuel cells have chemically bound energy supplied from the outside in the form of synthetic fuel (hydrogen, methanol or hydrazine). Unlike secondary batteries, primary batteries cannot be recharged when the built-in active chemicals have been used, and therefore strictly they cannot be considered as genuine energy storage. The term 'batteries', therefore, will only be applied for secondary batteries in this chapter.
The power transformation (extraction) system for capacitor bank storage is practically the same as for chemical storage. It uses a thyristor-based AC/DC convenor and all the relevant devices: AC transformer, reactive power sources, etc. A circuit schematic for a PTS is shown in the chapter. The main requirement for a capacitor bank energy storage PTS is the necessity to change the polarity of the central store when changing working modes from charge to discharge. This requirement doubles the PTS size, and therefore cost, compared with that for a magnetic energy storage system.
This chapter discusses superconducting magnetic energy storage.
A power system has an ability to act as a capacitor, magnetic, flywheel or thermal energy storage device without additional investment; generators play the role of power transformation systems, while thermal equipment, rotating machinery and transmission lines play the role of a central store. The capacities of these stores are limited, however, and therefore the power system's built-in storage can only accommodate short time fluctuations in load demand. However, this fundamental power system property allows small quantities of intermittent renewable sources to be accommodated on an interconnected grid without any technical problems.
In summarising the information given in the preceding chapters, it is necessary to review the status of large-scale electrical energy storage and to make a comparison of storage plants with very different characteristics, as well as considering con ventional alternatives.
Energy storage is necessary because the demand side in a power utility is characterised by hourly, daily and seasonal variations, whereas the installed capacity of the supply side is fixed. To facilitate this varying demand at minimum cost and acceptable reliability, the utilities plan and operate their generation resources to match the load characteristics. During the decision-making process of planning, information regarding the effect of an energy storage unit on power system reliability and economics is required before it can be introduced as a decision variable in the power system model. The main objectives of introducing energy storage to a power utility are to improve the system load factor, achieve peak shaving, provide system reserve and effectively minimise the overall cost of energy production. Constraints of various systems must also be satisfied for both charge and discharge storage regimes.
Any power utility suffers from regimes in which power swings take place, the so called transient regimes. In these regimes undesirable oscillations of frequency and voltage take place, decreasing or removing the utility's ability to transmit generated power to consumers. The problem arises of how to damp these oscillations.
This chapter is devoted to energy storage in power systems. There are three possibilities for the use of storage in the power system: compulsory regime; optimal regime; and reserve regime. A compulsory regime arises when the planned load curve coincides with the rated load curve; there is a necessity for the energy storage unit to ensure power balance in the system. It may arise in two different ways: (i) During the trough period, when load demand is less than the technical minimum of the total installed generation equipment. The energy storage is charged so that stored energy can then be used during peak demand, (ii) During the peak period, when load demand exceeds the total generation. In this case, the energy stored has to be discharged at its rated power capacity. The necessary energy was accumulated by the ES during the previous load trough. Unused or spare energy storage capacity may also be used as spinning reserve. Use of the reserve regime provides fuel saving whenever there is a need for spinning reserve. Table 16.1 shows how constant loading decreases the fuel consumption necessary to generate a given amount of energy.
Renewable energy sources tidal, solar and wind particularly will play a significant role in the supply side structure of future power systems. Their intermittent nature may be smoothed partly by reserve capacity of the power system, partly by finding a special place for them in the load demand curve and partly by using secondary storage. Wind and tidal sources are expected to participate in base generation while solar sources are only suited to the intermediate zone of generation curve.
The problem of energy storage integration into a power system is one of the most interesting ones facing power utilities today. In any scenario of power system expansion, there needs to be efficient storage of generated electricity. It is equally essential both for nuclear or coal-fired power plants and for large-scale exploitation of intermittent renewable sources.