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2.5 Storage & Load

In the next two subsections delineate both different methods of energy storage and load growth as well as the basic concepts behind energy storage and load growth.

2.5a    Energy Storage

Electric energy production requires the conversion of energy into electricity. However, conversion processes such as solar, wind, and hydro rely on a fluctuating fuel source. In these cases, the power system must have some energy storage capability to overcome the fluctuations in the energy supply. In other cases energy storage provides a means for harnesses excess energy production, for example utilities produce more excess electricity at night. Energy storage normally occurs through a conversion process from electrical energy to another form of potential energy. Options for large scale energy storage include batteries, superconducting coils, flywheels, and pumped storage.

As previously described, batteries provide a general solution to the conversion problem in remote systems in conjunction with solar and wind sources, or as a backup in case of utility failure. Current research includes using battery systems in the utility grid for bulk power storage. Applications where batteries are being considered for utility power systems include load levelling; voltage, VAR, and frequency control; and spinning reserve. Batteries provide quick response time: response to load changes occurs in about 20 milliseconds.They are also quiet and nonpolluting, making them ideal for installation in suburban areas, close to load centers.

A type of energy storage to consider is Superconducting Magnetic Energy Storage (SMES). This type of energy storage involves converting off-peak power direct current and feeding it to a doughnut shaped coil of superconducting wire. The coil is installed in a trench and kept at superconductive temperature by a refrigeration system. With this process the unit can store and discharge energy at an efficiency of greater than 90%, and charge in less than 20 milliseconds. However, this system is very expensive, and some engineering problems related to superconductors must be solved. In addition to these obstacles, SMES contains unknown health effects due to the large magnetic field.

Flywheels are rotating wheels used to store kinetic energy, much like a spinning top. Electricity is used to "wind" the wheel up through a system of gears. The flywheel then delivers rotational energy to power an electric generator until friction dissipates it. The sum of the kinetic energy of the individual mass elements that comprise the flywheel equals the energy stored. The kinetic energy of a flywheel is given by

where I is the moment of inertia (the ability of an object to resist changes in its rotational velocity), and w is the rotational velocity in rpm. The moment of inertia is defined as

where M is the mass, R is the radius, and k is the inertial constant. The inertial constant depends on the shape of the object. Some common inertial constants are found below, in Table 2.4.

Table 2.4: Inertial constants


Inertial Constant k

Wheel loaded at rim


Solid disk of uniform thickness


Solid sphere


Spherical shell


Thin rectangular rod



To optimize the energy to mass ratio the flywheel needs to spin at the maximum possible speed. Kinetic energy only increases linearly with mass but increases at the square of the rotational speed, see the preceeding formula. However, centrifugal forces can rip apart rapidly rotating objects. The centrifugal force for a rotating object is proportional to its density. Therefore, while dense material can store more energy it is also subject to higher centrifugal force and fails at lower rotational speeds than do low density material. This effectively means that tensile strength is more important than density of material. For effective storage of energy, long rundown times are required. Using frictionless bearings and a vacuum to minimize air resistance can result in rundown times of 6 months. Flywheels provide about 80 percent efficiency. Figure 2-12 shows a schematic of a flywheel energy storage system.

Figure 2-12: Flywheel Energy Storage System

Compressed-Air Energy Storage (CAES) plants use off-peak electricity to compress air into an underground reservoir. When electricity is needed, the air is withdrawn, heated with gas or oil, and run through expansion turbines to drive a generator. These plants burn about one-third of the fuel of a conventional combustion turbine, and producing about one-third the pollutants. Approximately three-fourths of the United States has the geologic potential for underground air storage. Since this process uses an electromechanical converter to produce electricity, the machinery is commercially available.

As described in the Conversion Section, pumped storage is a special use of hydroelectric energy. Excess off-peak power is used to pump water to an elevated reservoir. When electricity is needed, the potential energy of the water is released to flow through hydroelectric turbines, exactly like a hydroelectric dam. Pumped storage plants require a large area with suitable topography for the upper and lower reservoirs, limiting the number of desirable sites, and leading to opposition from environmental groups. Pumped storage plants also have to be large (1000-2000 MW capacity) to be economical, resulting in long lead times and high capital costs.

Some estimated costs for energy storage technologies are shown in Table 2.5.

Table 2.5

Estimated costs for Energy Storage Technologies



Power-related cost ($/kW)

Energy-related cost ($/kW)

Hours of Storage

Total Cost ($/kW)


Small module (25-50 MW)





Large module (110-220 MW)






Conventional (500-1500 MW)





Underground (2000 MW)






Lead-acid (target)(10 MW)





Advanced (target)(10MW)






(Target)(1000 MW)






2.5a    Load Growth

An electric load (or demand) is the power requirement of any device or equipment that converts electric energy into light, heat, or mechanical energy. The total of all such loads connected to the system constitutes the power system load. As such, the load varies daily, weekly, monthly, and yearly with loads addition or subtraction from the power system. The minimum system load for a given period is called the base load. The maximum system load for a given period is known as the peak load or peak demand. The peak demand is usually quite short in duration. The operation of generation plants must be closely coordinated with the load demands to ensure that enough generation capacity is on-line. Since peak loads are generally only a few hours long, economical fast-starting generators like pumped storage hydro are used. On weekdays, the base load generally begins increasing at about 6:00 a.m., and hits peak load around 5:00 p.m. Maximum yearly peak loads generally occur during the summer in the south, and during the winter for the north.

As more people and businesses connect to an electric system, the amount of load on the system increases. Load forecasting is performed to ensure that power system generating capacity will be adequate to meet these future load demands. Power stations take years to build and require advance planning, like load forecasting. One important part of load forecasting is the idea of the load growth rate. This describes the estimated rate at which load on the power system increases generally based on historical data. The growth rate of the system load L is mathematically represented by

where a is the constant of proportionality, also known as the per-unit growth rate. The solution to this equation is written as

where L0 is the value of L at t = 0. At any two values of time, t1 and t2 , the ratio of the corresponding L1 and L2 is

This equation may be used to determine the time tk such that L2 = kL1 and t2-t1= td , given by

When talking about the growth rate of a quantity, the term "doubling time" is often used. This term referes to the period necessary to double the initial value of load L , given a constant value of a .


Doubling time is used to describe how long it will take, at a constant growth rate, to use twice what is currently used. For example, assuming a present peak energy demand of 700 GW, and a 5 percent growth rate in the peak demand, the doubling time for energy demand is 14 years. This means that peak energy demand in 14 years will equal 1400 GW, or twice the current peak demand. Obviously, setting a steady growth rate for the use of any quantity is unrealistic, as the growth rate depends on many factors. Load growth forecsting is still an important feature of planning and constructing power plants and it involves more than simple linear growth calculations.

The next section describes several of the environmental impacts of power production. These are explored for both DG and non-DG technologies.




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