UNIT-4

Direct energy conversion (DEC) devices.

The devices which convert naturally available energy into electricity, without an intermediate conversion,
into mechanical energy; energy source may be thermal, solar or chemical are called Direct energy
conversion (DEC) devices.
Until now, their use has been confined to small scale, special purpose applications since the voltage
output available with them is rather small and no inexpensive device that is reliable like a turbine or
alternator has been built.
Under this topic the following devices will be discussed:
1. Thermoelectric generator.
2. Thermoionic generator.
THERMOELECTRIC EFFECTS
The quest for a reliable, silent, energy converter with no moving parts that transforms heat to electrical
power has led engineers to reconsider a set of phenomena called the “Thermoelectric effects“. These
effects, known for over a hundred years, have permitted the development of small, self contained
electrical power sources.
The various thermoelectric effects, in relation to the thermodynamic conversion of heat to work, are as
follows:
1. Seebeck effect.
2. Peltier effect. (Reversible processes))
3. Thomson effect. (Reversible processes))
4. Joule effect.
Seebeck Effect states:
If two dissimilar
materials are joined
to form a loop and
two junctions

maintained at different temperatures, an e.m.f. will be set up around the loop. The magnitude of current
depends on the materials .
Peltier effect states:
“When an electric current flows across an isothermal junction of two dissimilar materials, their is either
an evolution or absorption of heat at the junction”. Peltier’s coefficient a1-2 is defined as the heat evolved
or absorbed as the junction per unit current flow, per unit time.
Thomson effect states:
“Any current carrying conductor with a temperature difference between two points will either absorb or
emit heat, depending upon the material“. The ‘Thomson’s coefficient‘ is defined as the heat absorbed (or
evolved) per unit time per unit electric current and per unit temperature gradient.dQt dx

Joule Effect
Joule effect states:
“In a closed electric circuit if the current I flows through a resistance R, the heat generated (Q) by the
resistance is equal to I2R”:
Mathematically, Q = I2R
THERMOELECTRIC GENERATOR
Construction and Working
shows a schematic diagram of a thermoelectric power generator. It uses seebeck effect to produce
electrical energy directly from the available heat input. Its thermal efficiency is very low, of the order of
1 to 3%. In any heat engine the efficiency of thermoelectric generator depends upon the temperatures of
hot and cold junctions.

Theory of working. Consider a metal bar where one side is kept at a higher temperature than the other.
If the free electrons in the metal are considered to behave as a gas, the kinetic theory of gases predicts
that the free electrons in the hot side of the bar will be on higher kinetic energy and will be moving at
greater speed than those in the cold side of the bar. As the fast moving electrons flow from the hot side
to the cold side of the bar, it results in an accumulation of negative charge at the cold side, preventing
further build up until circuit is closed. In a closed circuit the current will flow to reduce the charge built
up and will continue as long as the temperature is maintained.

The source of heat for a thermoelectric generator may be a small oil or gas burner, a radio- isotope or
direct solar radiation. A typical couple operating with hot and cold junction temperatures of 600°C and
200° C could be designed to give about 0.1 V and 2 A i.e., about 0.5 W, so that a 1kW device could
require about 5000 couples in series.
Note. A thermoelectric converter is a form of heat engine which takes up heat at an upper temperature
(hot junction) converts it partly into electrical energy and discharges the remaining part at a lower
temperature (cold junction). The efficiency of a thermocouple, as is the case with other heat engines,
increases by increasing the upper temperature and decreasing the lower temperature. Since the lower
temperature is usually that of environment the efficiency of a thermocouple, practically, depends upon
the hot junction temperature.
where Z is an index used in rating thermoelectirc converters. It depends on the properties of
thermoelectric materials used. A high value of Z (Figure of merit) is obtained by using materials of:(i)
Large seebeck co-efficient (ii) Small thermal conductivity(iii) Small electrical resistivity. l In recent
times, the most commonly used material for thermoelectric converters is lead telluride [a compound of
lead and tellurim, containing small amounts of either
bismuth (N-type)] or sodium (P-type)]. The efficiency of such a thermoelectric converter is, however,
only about 5 to 7 per cent.
Advantages and Disadvantages of Thermoelectric Power Generator
Following are the advantages and disadvantages of thermoelectric power generator.
Advantages:
1. Highly reliable.
2. Free from noise due to the absence of parts.
3. Compact and durable.
4. Minimum maintenance.
5. Portable (can be used at any location).
6. Uses low grade thermal energy.
Disadvantages:
1. Low output.
2. Low efficiency.
3. High cost.

THERMIONIC GENERATOR/CONVERTER
Introduction
A Thermionic converter (or generator) converts heat energy directly to electrical energy by utilising
thermionic emission effect. In this device, electrons act as the working fluid in place of a vapour (or gas)
and electrons are emitted from the surface of the heated metal. A thermionic converter can be analysed
from at least three different points of view:
1. In terms of thermodynamics, it may be viewed as a heat-engine that uses an electron gas as a working
substance.
2. In terms of electronics, it may be viewed as a diode that transforms heat to electricity by the law of
thermionic emission.
3. In terms of thermoelectricity, it may be viewed as a thermocouple in which an evacuated space or a
plasma has been substituted for one of the conductors.
electrons from the metal when it is heated.

Work function (f):

It is defined as the energy required to extract an electron from the metal. It is measured in electron volts.
The value of work function varies with the nature of the metal and its surface condition.
A thermionic converter, in principle, consists of two metals or electrodes with different work functions
sealed into an evacuated vessel. The electrode with a large work function is maintained at a higher
temperature than one with the smaller work function.
Thermionic Generator

Construction and working:

— A thermionic converter/generator comprises a heated cathode (electron emitter) and an anode (electron
collector) separated by ‘vacuum‘, the electrical output circuit being connected between the two as shown
in the Fig.
— The heat which is supplied to the cathode raises the energy of its electrons to such a level that it
enables than, to escape from the surface and flow to anode. At the anode‘ the energy of electrons appears
partially as heat, removed by cooling and partially as electrical energy delivered to the circuit.
Although the distance between anode and cathode is only about 1mm. The negative space charge with
such an arrangement hinders the passage of the electrons and must be reduced, this can be achieved by
introducing positive ions into the inter electrode space, ‘cesium vapour‘ being valuable source of such
ions.
. Desirable of Properties of Thermionic Converter Materials.
enerators.
Following properties are desirable in materials suitable for converters:
Emitter:
A good emitter should have the following properties:
1. High-electron emission capability coupled with a low rate of deterioration.
2. Low emissivity, to reduce heat transfer by radiation from the emitter.
3. High mechanical strength.
4. High melting point.
5. It should be such that in the event some of it vapourizes and subsequently condenses on the collector
it will not poison the collector (that is, change the collector properties, thereby making it less effective).
Collector:
The main criteria for choosing a ‘collector material’ is that it should have as low a work
function as possible.
— Because the collector temperature is held below any temperature that will cause significant electron
emission, its actual emission characteristics are of no consequence. The lower the collector work function
(fc), however, the less energy the electron will have to give up as it enters the collector surface.
— In practice the lowest value of fc that can be maintained stably is about 1.5 eV.
— For applications in which it is desirable to maintain the collector at elevated temperatures (greater
than 900°K) such as space applications, an optimum value of f may cbe determined.
— Molybdenum has been widely used as a collector; it is frequently assumed to have a work function of
1.7 eV.
Advantages, Disadvantages/Limitations and Applications of Thermionic Converter
Following are the advantages, disadvantages/limitations and applications of thermionic
converter:
Advantages:
1. Compact device.
2. High conversion efficiency.
3. Quiet in operation and durable.
4. Low cost and less maintenance required.
5. Absence of rotating parts.
6. Operates at high temperatures.
7. Can be developed for very low power to very high power generation.
8. Can operate in remote areas and harsh environments.
Disadvantages/Limitations :
1. Metal is costly as it has to withstand high temperatures.
2. Needs high operating temperatures at anode.
3. Needs special seal to protect the cathode from corrosive gases.
4. Needs cesium vapour in the tube to reduce the space charge.
Applications:
1. Power generation in both centralised and distributed systems.
2. Residential and Commercial purposes.
3. Automobiles, marine and air vehicles.
4. Electronics and telecommunications.
5. Aerospace and military systems.
WIND ENERGY
INTRODUCTION
Wind is air set in motion by small amount of insolation reaching the upper atmosphere of
earth. It contains kinetic energy (K.E.) which can easily be converted to electrical energy.
Nature generates about 1.67 × 105 kWh of wind energy annually over land area of earth
and 10 times this figure over the entire globe.
l This wind energy, which is an indirect source of energy, can be used to run a wind
mill which in turn drives a generator to produce electricity.
2.Although wind mills have been used for more than a dozen centuries for grinding grain
and pumping water, interest in large scale power generation has developed. over the past
50 years. A largest wind generator built in the past was 800 kW unit operated in France
from 1958-60. The flexible 3 blades propeller was about 35 m in diameter and produced
the rated power in a 60 km/hour wind with a rotation speed of 47 r.p.m.

UTILISATION ASPECTS OF WIND ENERGY

Utilisation aspects of wind energy fall into the following three broad categories:
1. Isolated continuous duty systems which need suitable energy storage and reconversion
systems.
2. Fuel-supplement systems in conjunction with power grid or isolated conventional
generating units. This utilisation aspect of wind energy is the most predominant in use as
it saves fuel and is fast growing particularly in energy deficient grids.
3. Small rural systems which can use energy when wind is available.This category has
application in developing countries with large isolated rural areas.
CHARACTERISTICS OF WIND
The main characteristics of wind are:
Wind speed increases roughly asone by seven power of height. Typical tower heights are
about 20–30 m.
Energy-pattern factor. It is the ratio of the actual energy in varying wind to energy
calculated from the cube of mean wind speed. This factor is always greater than unity
which means the energy estimates based on mean (hourly) speed are pessimistic.
ADVANTAGES AND DISADVANTAGES OF WIND ENERGY
Following are the advantages and disadvantages of wind energy:
Advantages:
1. It is a renewable energy source.
2. Wind power systems being non-polluting have no adverse effect on the environment.
3. Fuel provision and transport are not required in wind energy conversion systems.
4. Economically competitive.
5. Ideal choice for rural and remote areas and areas which lack other energy sources.
Disadvantages:
1. Owing to its irregularity, the wind energy needs storage.
2.Availability of energy is fluctuating in nature.
3.The overall weight of a wind power system is relatively high.
4.Wind energy conversion systems are noisy in operation.
5.Large areas are required for installation/operation of wind energy systems.
6.Present systems are neither maintenance free, nor practically reliable.
7.Low energy density.
8.Favourable winds are available only in a few geographical locations, away from cities,
forests.
9. Wind turbine design, manufacture and installation have proved to be most complex due
to several variables and extreme stresses.
10. Requires energy storage batteries and/or stand by diesel generators for supply of
continuous power to load.
11. Wind farms require flat, vacant land free from forests.
12. Only in kW and a few MW range; it does not meet the energy needs of large cities and
industry.
ENVIRONMENT IMPACTS OF WIND ENERGY
The possible environment impacts of wind energy are:
1. Wind energy creates noise pollution because of mechanical (gear box) aerodynamic
noise.
2. The wind turbine produces electromagnetic interference when placed between radio,
television etc. stations, as it reflects some electromagnetic radiations.
3. It produces visual shining because of reflection and refraction which depends upon
turbine size, number of turbines in wind farm, design etc.
4. Safety consideration for life because of accidental braking of blade.
5. Fatal collisions of birds caused by rotating turbine blades.

SOURCES/ORIGINS OF WIND
Following are the two sources/origins of wind (a natural phenomenon):
1. Local winds.
2. Planetary winds.
1. Local winds. These winds are caused by unequal heating and cooling of ground surfaces
and ocean/lake surfaces during day and night. During the day warmer air over land rises
upwards and colder air from lakes, ocean, forest areas, and shadow areas flows
towards warmer zones.
2. Planetary winds. These winds are caused by daily rotation of earth around its polar axis
and unequal temperature between polar regions and equatorial regions. The strength and
direction of these planetary winds change with the seasons as the solar input varies.

WIND AVAILABILITY AND MEASUREMENT

in-situ measurements.The World Meteorological Organisation (WMO) has accepted the
following four methods of wind recording:
(i) Human observation and log book.
(ii) Mechanical cup-counter anemometers.
(iii) Data logger.
(iv) Continuous record of velocity and direction.
Characteristics of a good wind power site:
A good wind power site should have the following characteristics:
1. High annual wind speed.
2. An open plain or an open shore line.
3. A mountain gap.
4. The top of a smooth, well rounded hill with gentle slopes lying on a flat plain or located
on an island in a lake or sea.
5. There should be no full obstructions within a radius of 3 km.
WIND ENERGY PATTERN FACTOR (EPF)
The energy pattern factor (EPF) is the ratio of power from speed distribution to the power
from coverage speed of the turbine blades.
i.e. EPF =Power from speed distribution/Power from average speed
Generally, EPF lies between 2 to 5.

BASIC COMPONENTS OF WIND ENERGY CONVERSION SYSTEM (WECS)

shows the block diagram of basic components of a wind energy conversion
systems.
— Wind turbines (Aeroturbines) convert the energy of moving air into rotary mechanical
energy. These turbines requires pitch and yaw controls for proper operation.
— A mechanical interface consisting of a step up gear and a suitable coupling transmits
the rotary mechanical to an electrical generator. The output of this generator is connected
to the road or power grid as the application demands.
— A controller serves purposes of sensing: (i) Wind speed, (ii) Wind direction, shafts
speed and torques at one or more points, (iii) Output power and generator temperature as
necessary, (iv) Appropriate control signals for matching the electrical output to the wind
energy input, and (v) Protect the system from extreme conditions brought about by strong
winds, electrical faults etc.

ADVANTAGES AND DISADVANTAGES OF WIND ENERGY CONVERSION

SYSTEMS (WECS)
The advantages and disadvantages of wind energy conversion systems as follows:
Advantages:
1. Wind energy, a renewable energy source, can be tapped free of fuel cost.
2. The wind turbine generation (WTG) produces electricity which is environmentally
friendly.
3. Wind power generation is cost effective.
4. It is economically competitive with other modes of power generation.
5. Quite reliable.
6. Electric power can be supplied to remote inaccessible areas.
Disadvantages:
1. As the wind speed is variable, wind energy is irregular, unsteady and erratic.
2. Wind turbine design is complex.
3. Wind energy systems require storage batteries which contribute to environmental
pollution.
4. Wind energy systems are capital intensive and need government support.
5. Wind energy has low energy density and normally available at only selected
geographical locations away from cities and load centers.
6. For wind farms (which are located in open areas away from load centres), the
connection to state grid is necessary.
7. ‘Large units’ have less cost per kWh, but require capital intensive technology. In
contrast ‘small units’ are more reliable but have higher capital cost per kWh.

CONSIDERATIONS FOR SELECTION OF SITE FOR WIND ENERGY

CONVERSION SYSTEMS (WECS)
Following factors should be given due considerations while selecting the site for WECS:
1. Availability of anemometry data.
2. High annual average wind speed.
3. Availability of wind curve at the proposed site.
4. Wind structure at the proposed site.
5. Altitude of the proposed site.
6. Terrain and its aerodynamic.
7. Local ecology.
8. Distance to roads or railways.
9. Nearness of site to local centre/users.
10. Favourable land cost.
11. Nature of ground.
TERMS AND DEFINITIONS
1. Aerodynamics. It is the branch of science which deals with air and gases in motion
and their mechanical effects.
2. Airfoil or aerofoil. A streamlined air surface designed for air to flow around it in order
to produce low drag and high lift forces.
3. Angle of attack. It is the angle between the relative air flow and the closed of the air
foil (Fig 5.3).

4.5.6.7.8.9.10.11.Blade. An important part of a wind turbine that extracts wind energy.

Leading edge. It is the front edge of the blade that faces towards the direction of flow
(Fig. 5.4).
Trailing edge. It is the rear edge of the blade that faces away from the direction of wind
flow (Fig. 5.4).
Mean line. A line that is equidistant from the upper and lower surfaces of the air foil (Fig.
5.4).
Camber. It is the maximum distance between the mean line and the chord line, which
measures the curvature of the airfoil.
Rotor. It is the primary part of the wind turbine that extracts energy from the wind.
It constitutes the blade-and-hub assembly.
Hubs. Blades are fixed to a hubs which is a central solid part of the turbine.
Pitch angle. It is the angle between the direction of wind and the direction perpendicular
to the planes of blades.
12. Pitch control. It is the control of pitch angle by turning the blades or blade tips [Fig.
5.5 (a)].
13. Yaw control. It is the control for orienting (steering) the axis of wind turbine in the
direction of wind [Fig. 5.5 (b)].
14. Teethering. It is see-saw like swinging motion with hesitation between two
alternatives. The plane of wind turbine wheel is swung in inclined position at higher
wind speeds by teethering control [Fig. 5.5 (b)].

LIFT AND DRAG–THE BASIS FOR WIND ENERGY CONVERSION

The extraction of power, and hence energy, from the wind depends on creating certain
forces and applying them to rotate (or to translate) a mechanism.
Following are the two primary mechanisms for producing forces from the wind: Refer to
Fig. 5.6
(i) Lift force (FL), (ii) Drag force (FD).

Lift force. The component of force at right angles to the direction of air stream on the
airfoil is called the lift force (FL) .
Drag force. The component of force in the direction of stream is called drag force (FD).
— When air stream approaches the airfoil along the axis of symmetry, the force acting on
the body is only the drag force, in the direction of flow and there is no
lift force. The production of lift force requires asymmetry of flow while drag force exists
always. It is possible to create drag without lift but impossible to create lift without drag.
CLASSIFICATION AND DESCRIPTION OF WIND MILLS/MACHINES
Classification of Wind Mills/Machines
The wind mills machines are classified as follows:
1. Based on the type of rotor:
(i) Propeller type (horizontal axis)
(ii) Multiblade type (horizontal axis)
(iii) Savonius type (vertical axis)
(iv) Darrieus type (vertical axis).
2. Based on orientation of the axis of rotor:
(i) Horizontal axis
(ii) Vertical axis.

Description of Wind Mills/Machines

1. Propeller type wind mill: Refer to Fig. 5.7
These are most commonly used wind mills. Such a wind mill has two or three blades for
economical reasons. Though the two blade design is most efficient, yet it faces the
difficulty of vibrations during orientation to wind direction called ‘Yaw control’. These
machines are rated from 1 to 3 MW.
2. Multiblade type wind mill: Refer to Figs. 5.8 and 5.9. The multiblade wind turbines
are high solidity turbines used for pumping the water because of high starting torque
characteristics.

Advantages:
1. Low cost.
2. Operation at low wind velocity.
3. No need of yaw and pitch control.
4. Generator can be mounted at the ground level.
Applications. It is useful for grinding grains, pumping water etc.

Parameters to be Considered While Selecting a Wind Mill

The following parameters should be considered while selecting a wind mill/wind
generator.
1. Low land cost.
2. The area should be open and away from cities.
3. Flat open area should be selected, as the wind velocities are high in flat open area.
4. The proposed altitude is to selected by taking average wind speed data.
5. Minimum wind speed should be available throughout the year.
6. Ground surface should be stable and have high soil strength.
7. It should be atleast 5 km away from the cities to reduce the effect of sound pollution.
8. The wind power should be near the customers, so that the transmission losses are
minimised.
9. Approach road should be available upto site.
Design Considerations for Wind Turbine
The wind turbine must be able to meet the following design considerations/criteria.
1. It should be small in size and suitable for roof mounting in urban area.
2. No risk for its neighborhood.
3. The efficiency should be good.
4. Insensitive to turbulence.
5. Suitable for mass production for low price.
Question 5.2. The following data relate to a multiblade wind turbine: Wind speed = 35
km/h; Rate of pumping water = 7 m3/h with a lift of 6.5 m; Water pump efficiency = 45
percent; Efficiency of rotor to pump = 78 percent; Co-efficient of performance = 0.3; Tip
speed ratio (TSR) = 1.0; Air density = 1.2 kg/m3; Water density = 1000 kg/m3; g = 9.8
m/s2. Determine: (i) Rotor radius; (ii) Angular velocity of the rotor.