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Chapter 3
Chapter 4
Chapter 5
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2.4 Conversion and Converters

The following sections outline the basic concepts behind energy conversion and specific converters. There are two general types of energy conversion, electromechanical and direct conversion. In the proceding, the energy source for conversion provides a logical way to group the converter types discussed.

2.4a Conversion.

2.4a.1    Electromechanical Energy Conversion

The most common method for bulk power generation is by rotary generators located in electric power stations. These generators are electromechanical energy converters, also known as electric machines. In practice, a mechanical prime mover coupled to the generator rotates the electrical conductors constituting the generator windings in a magnetic field, inducing a voltage in the generator windings. These windings supply electrical load on the generator. Conversely, if a current carrying conductor is placed in a magnetic field, the conductor experiences a force according to Ampere's law. In general, electric machines are reversible, capable of operating both as generators and motors. There are three major types of rotating electric machines: dc commutator, induction, and synchronous machines.


Figure 2-1. Prime Mover Driving an Electric Generator


The source of mechanical energy for a rotating electric generator is known as the prime mover. The prime mover is directly coupled to the generator. Energy sources for prime movers are thermal, hydro, and wind. The prime movers normally are turbines, but some thermal units use internal-combustion engines. A turbine is mechanical device that is forced to rotate by the pressure of a gas (such as steam for thermal units or air for wind units) or fluid (such as water for hydro units).

An electromechanical energy converter converts mechanical energy into electrical energy, and vice versa. A generator converts energy from mechanical to electrical form, and modulates in response to an electric signal. A motor converts energy from electrical to mechanical form, and modulates in response to an electrical signal. Rotating machines, if lossless, operate on the principle of electromechanical power equivalence as given by

Pmech =Twm = vi=Pelectrical

where T is mechanical torque (N-m), wm is mechanical angular velocity (rad/s), v is instantaneous electrical volts (volts), and i is instantaneous electrical current (amperes). Electric generators are governed by Faraday's law of electromagnetic induction: an electromotive force (emf) is induced in a conductor "cutting" magnetic lines of flux. Specifically if a conductor of length l (m) moves with a velocity u in a uniform magnetic field B (tesla), such that l , u , and B are mutually perpendicular, then the induced emf in the conductor is given by

E = Blu

Figure 2-2.  Relative directions of E , u , and B

The following sections cover the basic types of electromechanical energy converters.

Sample Problems

2.1. An ideal energy converter develops 500 N-m of torque while running at 3600 rpm. If the input voltage is 1200 volts, determine the input current for this generator.

2.2. Calculate the power output of the ideal energy converter.

2.4a.2    Direct Energy Conversion

Direct energy conversion devices convert solar, thermal, chemical, and nuclear energies into electricity without involving a rotating or reciprocating mechanical prime mover. Various direct energy converters include:

•  Fuel cells and batteries

•  Photovoltaic, photoelectric, and electrostatic generators

•  Thermionic, thermoelectric, and ferroelectric generators

•  Magnetohydrodynamic generators

•  Piezoelectric generators

It is projected that, of all the direct energy converters, only solar power and fuel cells will contribute to the production of any significant amount of electrical power in the near future. During the decade 1981-1990, in the United States, considering wind, solar, geothermal, cogeneration, solid waste, wood waste, and fuel cells as alternatives to conventional methods of generating electrical power, no more than 12 percent has been generated from solar and fuel cells in terms of installed capacity. Of these methods, only a fuel cell accomplishes energy conversion directly.

Understanding how direct energy conversion devices operate requires knowledge of the laws of thermodynamics. The first law of thermodynamics states that energy is conserved:

"The increase in the internal energy of a thermodynamic system is equal to the amount of heat energy added to the system minus the work done by the sytem on the surroundings."

If heat ΔQ is added to a system to increase its energy by an amount ΔU (where U is the internal energy of the system) resulting in mechanical output ΔW (where W is the work done by the system on the surroundings), then


Figure 2-3.  Relative directions of E , u , and B

Now, considering a steady-flow system, such as the one shown in Figure 2-3, where there is a change in the pressure p , and volume V , of the system, then this equation becomes

ΔQ = Δ U + Δ W + Δ ( pV )

From Figure 2-3 , ΔQ = Q1 - Q2 , ΔU = U1 - U2 , and Δ ( pV ) = p1V1 - p2V2 . The quantity Δ( pV ) reflects the difference in work required to move a unit mass into and out of the system. Both ΔU and Δ( pV ) are unique forms of stored energy in that they depend on the state variables p , V , and T ( T is the temperature - recall pV=NRT, the ideal gas law, where N and R are constants) of the system. Usually, the sum ( U+pV ) is considered as a single property H , known as the enthalpy of the system.

H = U + pV

The second law of thermodynamics states that it is impossible to completely convert heat into work (Lord Kelvin). According to the second law of thermodynamics, continuous conversion of heat into work by a device receiving heat from a source at a temperature T1 is possible only when part of the heat is rejected into a sink at a lower temperature, T2 (T2 < T1). As a consequence, an energy conversion process involving heat must have an efficiency of less than 100 percent, even without loss. This ideal or maximum efficiency is called the Carnot efficiency, ηc , and is given by

where T1 and T2 are absolute temperatures for a reversible cycle. The Carnot efficiency of a modern large fuel-fired steam-turbine generating unit is about 61 percent. In an electric power station the mechanical energy of the turbine is converted to electrical energy by an electric generator, and the overall efficiency from fuel to electrical power can scarcely exceed 40 percent. Such efficiency limitations exist on all closed-cycle heat engines. On the other hand, a fuel cell, which converts chemical energy into electrical energy, is not a heat engine and is not subject to the Carnot efficiency limitation. Rather, the efficiency of a fuel cell is limited by the thermodynamic quantities related to the fuel oxidation reaction. Fuel cells have efficiencies in the range of 80 to 99 percent.

Heat flow is a function of temperature difference. If a quantity of heat is divided by its absolute temperature, the quotient is called the entropy of the system. Entropy is a measure of degradation of energy through usage. If heat ΔQ enters a system at an absolute temperature T , then the change in entropy of the system for a reversible process is given by

where ΔS is entropy difference.

Note. The second law of thermodynamics can also be stated as: In an isolated system, a process can occur only if it increases the total entropy of the system ( Rudolf Clausius).

In a reversible process, the state variables p , V , T , and U are such that if the process is reversed, these state variables follow the same values in reverse order. Thus reversibility is measured by entropy S . The second law of thermodynamics can be expressed as

where ">" is for a reversible process. For a reversible cycle the net change in entropy is zero.

Consider the thermodynamic limitation of energy conversion in an oxidation reaction, such as that in a fuel cell. Gibbs free energy, G , is defined by

G = H - TS

where H is the enthalpy of the system at an absolute temperature T and entropy S. The change in the Gibbs free energy, ΔG , for an oxidation reaction is equal to the maximum amount of electrical energy obtainable from the reaction. For an isothermal reaction,

ΔG = ΔH - T ΔS

where T ΔS is a measure of the heat absorbed by the system during a reversible change, and is the unavailable energy.

2.4b   Energy Conversion Types

2.4b.1 Electromechanical Energy Converters


Thermal prime movers are the most common prime movers used to generate electricity. These include steam turbines, gas turbines, gasoline engines, and diesel engines. In all cases, the prime mover is a stationary device that rotates electrical conductors through a magnetic field, as discussed in Section 2.4. To produce the steam necessary to drive steam turbines, coal, gas, or oil is burned in boilers. Or for nuclear power plants heat resulting from nuclear fission is used to produce the steam. Steam at maximum possible pressure and temperature is used to ensure maximum efficiency of operation of the turbine. Turbine units with a rating of 500 MW and above are common, as large turbine sizes result in lower per MW capital costs.

Gas turbines are essentially jet engines, driven by hot gases. These gases are produced in a combustion chamber, in which a continuous combustion of injected fuel oil occurs in the presence of compressed air simultaneously delivered to the chamber. In this regard, a gas turbine is an internal-combustion engine, much like an automobile engine. The main advantage of a gas turbine is its capability to quickly start and take up load. Such features are required to meet sudden power system peak loads. Under normal continuous operating conditions, a gas turbine is less economical than a steam turbine of the same rating.

Of the other internal-combustion engines, gasoline engines are seldom used to drive large (50 kW or more) electric generators, and diesel engines are usually used only for isolated units rather than in central stations. The diesel engine differs from the gasoline engine mainly in the mechanism of combustion. In a diesel engine heat is generated in the cylinder to ignite the fuel. In a gasoline engine electric spark ignites the air-fuel mixture in the cylinder. Consequently, a diesel engine has a higher compression and is bulkier, heavier, and more expensive than a gasoline engine, but is relatively less expensive to operate.


Nuclear prime movers, as discussed above, are really only thermal prime movers. The heat developed as a byproduct of the nuclear reaction is used to produce steam to drive a turbine. The two basic forms of nuclear energy are fission and fusion. The fission reaction involves the splitting of the nuclei of a heavy element, while the fusion reaction involves combing the nuclei of light atoms. At the present time, only the fission reaction is used to produce electricity. Uranium 235 (U 235 ) is the most suitable element for fission from an environmental standpoint. However, fuel supplies are limited, since only 0.7% of all naturally occurring uranium is the U 235 isotope. Breeder reactors transform uranium 238 into fissionable plutonium 239 by absorbing neutrons, or transform thorium 232 into fissionable uranium 233. In a breeder reactor, 1g of uranium produces 8.1 x 1010 J of heat, which approximates the heat produced by 2.7 metric tons of coal. Figure 2-4 shows a simple diagram of a fission reaction.

Figure 2-4.  Fission Reaction

Fusion is the process that takes place on the sun, and it is a scientifically feasible and attractive method to generate electricity. The main advantage is that the fusion reaction can use deuterium as a fuel. Deuterium is an abundant (one atom of deuterium per 6,700 atoms of hydrogen) isotope of hydrogen, and has a low cost to separate. Many engineering problems related to the fusion process remain to solve. For fusion to occur, the nuclei must form plasma (atoms heated to such a high temperature that they are stripped of their electrons). The plasma must be contained or confined in a region of space such that the plasma density is high. Furthermore, the plasma must be contained long enough for the fusion process to take place. Heating and plasma confinement are major engineering problems for a fusion power generation. Therefore, no fusion power plants exist, and implementation of a practical fusion power generating station does not seem likely in the near future.


Wind turbines extract the kinetic energy present in the wind, and convert it to rotary shaft motion. The shaft motion transmits power to generators by gearboxes, belts and pulleys, roller chains, or by hydraulic transmissions. The power that can be extracted from the wind is given by

where r is the density of air (1.2 kg/m3 ), A is the wind turbine blade area in m2 , and v is the wind velocity in m/sec. The velocity of the wind, and the size of the turbine blade, determine the amount of power available for a specific wind turbine. Since the velocity of wind increases with height above the ground, the height of the wind turbine has an important effect on the power extracted. The effectiveness of a wind turbine is measured by the power coefficient, Cp. C p is defined as

The maximum value of Cp is 59.3%. Wind tunnel tests have shown that the ideal horizontal axis two bladed wind turbine normally has a power coefficient of 42.0%. Two major factors characterizing the design of a particular wind turbine are tip-speed ratio and blade pitch angle. The tip-speed ratio, b, is the ratio between the blade tip speed and the wind speed. Modern wind turbine generators have a b of 5.0 or more. A low b is associated with low efficiency, but an ability to start under load. A higher b reflects higher efficiency, but inability to start under load. The relationship between blade pitch angle and Cp is not straightforward, as it is controlled by lift and drag forces, choice of the aerofoil section, the air flow velocity over the blade, and the blade width. Note that Cp is not the same as the efficiency of the wind turbine, as rotational losses are not considered in Cp.

Figure 2-5.  Possible positions of horizontal axis wind turbines

Wind turbines can be classified into two major groups: horizontal axis wind turbines and vertical axis wind turbines. Horizontal axis wind turbines, the most common type, uses blades with a horizontal axis of rotation. Under steady state conditions each blade section experiences a constant angle of attack during one revolution. (The angle of attack is the angle between the direction of wind flow and the direction of a line drawn parallel to the blade). Therefore it is extremely important that the wind turbine yaw (or turn) maintain alignment with the direction of the wind. Also, by turning the face of the blade away from or into the wind, the lift on the blade (and thereby the velocity of the blade) can be increased or decreased. The horizontal axis wind turbines may be either upwind or downwind types, as shown in Figure 2-5 . Small generators use upwind horizontal axis wind turbines because the tail vane keeps the blades pointed into the wind. The protection system in case of high winds is easily designed. Downwind turbines are used for large horizontal axis wind turbines. Small downwind turbines have a natural tendency to turn and align with wind flow, while large ones are steered by pilot wind vanes. The blades of downwind turbines are designed to cone for protection against high winds. However, the tower acts as a barrier to the windstream, forcing the blade to undergo stress transients each time it passes the tower. Advantages of horizontal axis wind turbines include a high hub height, due to the tower, and their ability to self-start. A disadvantage is that the generator and gearbox are difficult to access, since they are mounted on the tower.

Vertical axis wind turbines, as shown in Figure 2-6, use blades with a vertical axis of rotation. Therefore, the angle of attack experienced by each blade varies continuously through one revolution. This means the vertical axis wind turbine has no need to yaw with changing wind directions. Other advantages of the vertical axis wind turbine include a higher operating speed than the horizontal axis wind turbine, and also that the generator and gearbox are located on the ground. However, the vertical axis wind turbine is not self-starting, requiring an auxiliary means of starting such as an electric motor.

Much experimenting has shown that large wind turbines (>2.5 MW) are impractical, due to the cost of a tower large enough and inability to make reliable blades large enough. Whereas small wind turbines may be practical for small standalone applications, they are not useful to replace bulk power on the utility grid. For bulk power, many wind turbines are installed in groupings known as "wind farms", usually having a total capacity of 500 MW or less, effectively turning a collection of wind turbines into a large power station. Since the wind is a fluctuating energy source, standalone wind turbines must have an energy supply during low wind periods; batteries are often used to store electricity for this purpose. Popular home systems consist of a roof-mounted wind turbine, a solar panel, and a battery storage system. Grid connected wind systems cannot typically be used in "capacity planning" for serving peak loads because of this since there is no guarantee that peak will not coincide with a low-wind period.

Figure 2-6. Vertical Wind Turbines


Hydroelectric power plants and tidal power plants both use hydraulic turbines as prime movers, which convert the potential energy of an elevated body of water to rotating kinetic energy. There are two basic types of hydroelectric power plants: run-of-the-river plants and reservoir plants. As its name implies, run-of-the-river plants are built so that the turbine blades are simply turned by the water as it flows in the river.

Reservoir plants

But most hydro plants are reservoir type plants, which means there must be a dam to regulate the water flow and add height to the source of the water. The powerhouse contains the hydraulic-mechanical works consisting of turbines, the upstream waterways (penstock) carrying water from the reservoir to the turbine, and the downstream discharge channels. It also contains the electric generators. The height the water falls through the penstock is called the head. The power available at the turbine is

P = gHQ kW

where g is the acceleration due to gravity, H is the head of the water in meters, and Q is the flow rate of water through the turbine in cubic meters per second. Since a dam converts potential energy into kinetic energy


where v is the velocity of water in meters per second through the penstock. The velocity of the water through the penstock can be converted to flow rate in cubic meters per second through

The power produced is proportional to the head and the flow rate. Dams are roughly classified into low head (6 to 30 m), medium head (30 to 200 m), and high head (above 220 m). To transmit water downstream, low-head plants utilize dams, and high-head plants use penstocks.

There are three types of hydraulic turbines available. Kaplan turbines are used for heads up to 60 m; Francis turbines are used for heads from 30 to 300 m, and Pelton wheels, for heads larger than 90 m. Maximum efficiencies of hydraulic turbines are between 85 and 95 percent. Francis turbines of ratings exceeding 500 MW have been built and Pelton wheels of ratings of 40 MW are in use. Hydraulic turbines can be started almost instantaneously from rest, and they have the obvious advantage that no losses are incurred when at a standstill. Thus, working in parallel with thermal power stations, hydroelectric plants can meet peak loads at minimum operating cost.


The pump-storage unit represents a novel use of hydroelectric facilities where the plant may act in either a generating mode or a pumping mode. In the generating mode, the water flows from the upper reservoir to a lower reservoir, and the synchronous machine operates as a generator. In the pumping mode, the synchronous machine operates as a motor, and water is pumped from the lower reservoir to the upper reservoir. Such a system of moving water back and forth between reservoirs seems strange, since there exist losses in such a system. The reason it is done is to take advantage of the variation in the demand of the interconnected power system. When the load is very heavy, the pump-storage facility is operated in the generating mode to help supply the demand. But when the load is very light, the facility utilizes the least expensive plant in the interconnection as a supply and is operated in the pumping mode. In this way, the cost of operating the unit in the generating mode is equal to the cost of the least expensive unit in the interconnection plus the cost of losses. This cost must be less than the cost of the most expensive plant in the interconnection in order for pump-storage to be a good option.

Tidal power

Tidal power is essentially a specific form of hydro power, and therefore uses basically the same equipment as a regular tidal station. The difference is in the available power to extract from the tide. A reversible hydraulic turbine is used so the inflow and outflow of the tide can generate electricity. The theoretical extractable energy for each tidal period is given by

where r is the density of water (1000 kg/m3), g is the acceleration due to gravity, A is the area of the tidal pool, and R is the range of the tide. Average power for a tidal period is calculated by dividing the extracted energy by the tidal period of 12h 25min, resulting in


As discussed previously, solar radiation may be connected directly to electricity, or it may be converted to another form of energy to convert to electricity. This second method generally uses solar radiation to heat a fluid. This heated fluid (which may be stored) is then run through a heat exchanger to produce steam, which is converted to rotating kinetic energy in a steam turbine. The turbine runs an electric generator. The fluid used may be water, or in some cases is a molten salt such as nitrate salts or sodium. A central receiver system uses a field of sun-tracking mirrors to concentrate sunlight onto a tower-mounted receiver. A parabolic dish system employs a point-focusing reflector to concentrate sunlight at the focal point where a heat engine or a heat transfer device is placed. The heat engine produces electricity directly, while the heat transfer device produces a heated fluid.

2.4b.2 Direct Energy Converters

Fuel Cells

A fuel cell directly converts a hydrogen-rich fuel into electricity using a highly efficient electrochemical process. The fuel cell uses the reverse electrolysis process to produce water and electricity from hydrogen and oxygen. The hydrogen is normally produced from natural gas or methanol. Chemical energy is stored outside the fuel cell, and the reaction products are commonly rejected from the cells.

Figure 2-7. Fuel Cell

A fuel cell is shown schematically in Figure 2-7. The reverse electrolysis process is performed by controlled continuous chemical reactions at the anode and cathode of the fuel cell. In a hydrogen-oxygen fuel cell, hydrogen is the fuel and oxygen is the oxidant. The electrodes are porous and are connected to the load. At the anode, the hydrogen fuel splits into positive hydrogen ions and electrons as expressed in the following reaction:

After flowing through the load circuit, the electrons and the hydrogen ions combine with oxygen at the cathode, as expressed in the following reaction:

The overall reaction in the fuel cell is given by

The oxidation product is water, which is removed from the cell. The energy change theoretically available from the fuel cell is ΔG , the Gibbs free energy change. The ideal electromotive force (emf) E of a fuel cell is given by

where the Gibbs free energy is in kilocalories per gram mole, and n is the number of electrons transferred per molecule of fuel oxidized. The net amount of energy liberated is the change in enthalpy ΔH , or the difference between the enthalpy of the reactants and the enthalpy of the products.

For the standard fuel cell reaction, the value for the Gibbs free energy is ΔG= -56.7 kcal at 25°C and the value for the enthalpy is ΔH= -68.3kcal. The maximum possible emf for a fuel cell can be calculated by

Electrode resistance, polarization of the electrolyte, and depletion of the electrolyte limit the actual voltage produced. The ideal efficiency is the useful work per unit enthalpy change at a constant temperature and pressure. Since the useful work is the Gibbs free energy, which means that 17 percent of the energy is lost as heat.

Fuel cell power plants consist of more than just the fuel cell itself. A fuel processing section converts natural gas or other hydrocarbon fuels into a hydrogen rich fuel. This is normally accomplished through a steam catalytic reforming process. This fuel is then fed to the power section, where it reacts with oxygen in fuel cells to produce direct current (dc) electricity, and by-product heat as hot water or steam. Individual fuel cells are connected in series in a fuel cell "stack" to produce usable amounts of electricity. The power conditioning section converts dc to utility grade alternating current.

Fuel cells are classified by the type of electroltye used, and by their operating temperature. Low temperature fuel cells operate in the range of 60°C to 200°C, and include phosphoric acid fuel cells, alkaline fuel cells, and solid polymer fuel cells. High temperature fuel cells operate in the range of 650°C to 1000°C, and include molten carbonate fuel cells and solid oxide fuel cells.

Fuel cells are currently used for special-purpose applications. For example, phosphoric acid fuel cells were used on the Apollo spacecraft to provide electrical power, and alkaline fuel cells are being tested to power cars and buses. Fuel cells have a lower environmental impact than normal generating plants, since there are no emissions from the fuel cell itself, other than heat. Large fuel cells are well suited for industrial cogeneration, because they produce steam as a by-product. However, fuel cell plants are expensive to build, as they cost about $2000-$4000/kW. An 11 MW molten carbonate fuel cell power plant went into operation in California in 1996, which is the first operating fuel cell power plant in the United States.


Batteries are similar to fuel cells. They directly produce electricity as an output from a chemical reaction. Unlike a fuel cell, where the reactants are continuously supplied, a battery stores the reactants. The reactants are generated by electrical recharging of the battery.

The ability to recharge batteries with electricity makes them the most common form of energy storage. Batteries are currently used in electric utility systems to provide backup power for control and relay systems in substations. If the substation itself loses power, the battery system ensures that the control systems in the substation continue to operate. Batteries are also used as to provide reserve power for special cases such as standalone solar units and wind applications. During times when the energy converter is operating, these units supply their load and recharge a battery. When the units are not operating (such as nighttime for a solar unit), the battery supplies the load. A major new use for batteries is as the power source for electric vehicles. The potential growth of electric vehicles is leading to much research into battery technology.

The most common type of battery is the sealed lead-acid battery. Sealed lead-acid batteries are very reliable. They also "recuperate," meaning the terminal voltage and energy density increase slightly over a small time interval following a discharge characteristic. This means a lead-acid battery will give a greater discharge capacity for a series of intermittent discharge cycles than for a discharge at constant current of magnitude equal to the average of the intermittent currents. This is a highly desired feature for electric vehicle applications. The lead-acid battery has an average life of 400 cycles.

The nickel-iron battery is used where reliability and long-lived operations are needed. However, it has a high operating temperature rise, high water consumption, and poor charge-acceptance in comparison to lead-acid batteries.

The nickel-cadmium battery is widely used in vented or sealed form. It has a very high power and energy density compared to the lead-acid battery, and can operate in a wide range of temperatures. However, uneven cell voltages during charging can result in excessive gasing, cell voltage reversal, and cell damage. The initial cost of this battery is very high, and requires special manufacturing techniques for cadmium.

The nickel-zinc battery is a new type of battery still being developed. It is attractive due to its potentially high energy density. This battery has an unusual charging problem due to the instability of the zinc electrode, and current prototypes require careful hand re-charging.

The zinc-bromide battery has an energy density of 60 to 70 Wh/kg with a 70% energy efficiency. Bromine spills pose a safety problem, so this battery is being developed for bulk energy storage in electric utility networks. The battery requires a complete discharge every 5 to 10 cycles to strip the zinc off the negative electrode.

The sodium-sulfur battery operates at around 300°C, and individual cells have had lifetimes of up to 6000 cycles. The sodium-sulfur battery uses a solid electrolyte made of thousands of tiny hollow sodium-ion-conduction glass fibers giving it an energy efficiency in excess of 80%. This electrolyte is made very inexpensively in large quantities, and it is highly resistant to impact shock and vibration breakage. Energy densities of 125-150 Wh/kg are expected from this battery. However, fires or explosions are possible if sodium leakage (due to battery damage) occurs.

Table 2.3 gives a summary of performance and expectations for the batteries discussed above.

Table 2.3

Performance of Various High-performance New Type Batteries


Energy Density (Wh/kg)

Output Density (W/kg)

Life (Cycle)


Current -> Future

Current -> Future

Current -> Future

Sealed Type Lead Acid
























Photovoltaic Energy Conversion

Photovoltaic systems, also known as solar cells, directly convert sunlight to electricity. In an energy and environment conscious society the clean and seemingly inexhaustible source of energy from photovoltaics provide an attractive option. For remote lighting and communications, photovoltaics with battery backup provide the most cost-effective source of electricity.

Figure 2-8. Photovoltaic Effect

The photovoltaic effect is most prominent in various semiconductors. Most commercial solar cells are made of crystalline and amorphous silicon materials. When sunlight strikes the solar cell, part of the light spectrum imparts enough energy to create electron-hole pairs in the semiconductor material. A potential barrier in the cell is set up by forming a junction between dissimilarly-doped semiconductor layers. This separates the light-generated carriers (i.e. electrons and holes) resulting in an induced voltage of about 1/2 volt. The available current is a function of cell area and light intensity. The electricity is collected and transported by metallic contacts placed on both surfaces of the cell. Photovoltaic cells are formed into modules by connecting them in series and parallel in order to get more current and voltage. For even greater power, modules can be interconnected in larger groups to form arrays. The dc electricity generated by the solar cell array is usually passed through a power conditioner for voltage and power regulation, and conversion to alternating current.

Commercially available photovoltaic modules can convert sunlight into electricity with efficiencies ranging from 5% to 15%. The cost of photovoltaic cells has dropped from $1000 per peak watt in the 1950's to under $5 per peak watt, so that they can produce electricity for as little as 25 to 30 cents per kilowatt-hour.

Thermionic Energy Conversion

In a thermionic converter, heat energy is converted to electrical energy by thermionic emission, whereby electrons are emitted from the surface of certain metals when the metals are sufficiently heated. The three major components of a basic thermionic energy converter are the thermionic emitter, the collector, and the working fluid, which may be an electron gas or a partially ionized plasma. A thermionic energy converter is shown schematically in Figure 2-9. The input heat, Qin, heats the emitter, and electrons are emitted. The cold collector receives some of these electrons at an output heat, Qout. The difference (Qin -Qout) is the energy that drives the electrons through the external circuit, and appears as electrical energy. The collector is cooled to remove the output heat.

Figure 2-9: Thermionic Energy Converter

Vacuum converters, which have electron gas as their operating fluid, operate in the range 1200°K to 1400°K. They typically produce 1 W/cm2 at an efficiency of 5 percent. Low-pressure converters produce 10 W/cm2 at an efficiency of 10 percent, operating at emitter temperatures up to 2300°K. High-pressure converters deliver 40 W/cm 2 at an efficiency of 20 percent, operating at emitter temperatures up to 2200°K.

Principal applications of thermionic energy converters are in regions not easily accessible, such as outer space, undersea, and polar regions. The two important heat sources are the sun and nuclear reactors.


Thermoelectric Energy Converters

Thermoelectric energy converters, or solar thermal systems, convert sunlight into heat for various forms of end-use. One of three different conversion cycles may be used in a thermoelectric system. The Seebeck effect results when the junctions of a loop made of two different materials are at different temperatures such that an electromotive force (emf), and consequently, a current is produced in the loop. If Th is the hot junction temperature, Tc the cold junction temperature, then δV12, the open circuit voltage, is related to the temperature difference by the Seebeck coefficienct a s represented by

When a current is passed through a loop of two different materials, one junction becomes hot and the other junction becomes cold. This is the Peltier effect. The Peltier coefficient a P represents the ratio of heat change at the junction to the current flow as shown by

When a current flows through a conductor in which a temperature gradient exists, heat is either liberated or absorbed, depending on the direction of current flow. This is known as the Thomson effect. The Thomson coefficient a T is the ratio of heat change per unit of current flow to the local temperature, as shown by

These three effects are reversible and are interrelated.

Figure 2-10: Flat Plate Solar Collector

Solar collectors make use of one of these effects. There are two types of collectors - flat plate and focusing. Flat plate collectors, illustrated in Fig. 2-10, do not use any device for concentrating the sun's rays. These collectors still function when clouds cut off direct sunlight. This advantage, along with their favorable cost, allows flat plate collectors to be used for low temperature heating of up to 100°C. Simpler flat plate collectors hold all the water that is to be heated, while more advanced ones heat only a little water at a time. The heated water is kept in a separate storage tank, to cut down heat losses from the fluid. Common uses of flat plate collectors are for solar water heating, solar space heating, and solar cooling.

Focusing solar collectors, illustrated in Fig. 2-11, allow the use of much higher temperatures than available from the best flat plate collectors. A focusing collector comprises a concentrator and a receiver. The concentrator is the optical system that directs the sun's rays to the receiver. This could be a parabolic dish with a point focus or a parabolic trough with a linear focus. Focusing solar collectors are often used in systems that produce bulk electricity. They first concentrate the incoming sunlight, convert it into heat, and convert the heat into electricity.

Figure 2-11. Focusing Solar Collector


Now, that you have been exposed to these key concepts (energy sources, efficiency, and conversion/converters), you are ready for the next sections on energy storage and load. Energy storage is an essential feature of electrical systems. Energy storage must be coordinated with the load on the system. These concepts will be further explained in the next section.

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