Non-conventional Power Generation

Introduction:

The whole world is already familiar with the conventional power generating resources like hydal, thermal

and nuclear resources etc. The sources of energy other than the conventional sources are called the non-

conventional energy sources. The non-conventional sources may be renewable or non-renewable. The

hydroelectric power plant is the only conventional power plant which is renewable. In all the conventional

systems the potential or thermal energy is first converted into mechanical energy and then this

mechanical energy is converted into electrical energy. The conversion of potential energy into mechanical

energy is significantly high i.e. 70-80 % but conversion of thermal energy into mechanical energy is

significantly poor i.e.40-45 %. This requires huge capital cost as well as maintenance cost.

All across the world researches are trying to convert thermal energy directly into electrical energy by

eradicating the mechanical process involved in energy conversions which have significant energy losses.

Research is now focusing its efforts on conversion process that do not involve mechanical energy

conversion step. In the absence of moving mechanical part may allow in achieving the operating

temperature much higher than the typical conventional processes to attain effective power generating

systems. These processes are known as direct conversion systems in which primary or secondary

energy is directly converted into electrical energy without passing through the stage of mechanical

energy. Some of the direct conversion methods are described below:

Magnetohydrodynamics generation (MHD)

Photovoltaic generation system (solar cells)

Electrochemical energy conversion (Fuel cells)

Thermoelectric power generation

The reason for using new and direct energy conversion methods is to overcome the flaws in the

conventional energy generating systems. The possibility of using new sources of energy seems enhanced

by the development of new direct energy converters. There are many methods of converting direct

thermal energy to electrical energy.

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Direct Energy Conversion Stages

MHD Power Generation:

The Magnetohydrodynamics (MHD) power generation is a direct energy conversion system which

converts the heat energy directly into electrical energy.

Working Principle of MHD Power Generators:

The principle of Magnetohydrodynamics (MHD) power generation is Faraday’s law of electromagnetic

induction which states that when a conductor and a magnetic field move relative to each other, an electric

voltage is induced in the conductor which produces an electric current. As the name implies MHD

generator is concerned with the flow of a conducting fluid in the presence of magnetic and electric fields.

In conventional generator or alternator the conductor consist of copper windings or strips while in MHD

generator the hot ionized gas or conducting fluid replaces the solid conductor. A pressurized electrically

conducting fluid flows through a transverse magnetic field in a channel or duct. Pair of electrodes is

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located on the channel walls at right angle to the magnetic field and connected through an external circuit

through an external circuit to deliver power to load connected to it. Electrodes in MHD generator perform

the same function as brushes in a conventional dc generator. The MHD generator develops dc power and

the conversion to ac is done by an inverter.

Basic principle

According to the Faradays Law of Electro Magnetic Induction “An electric conductor moving through a

magnetic field experiences a retarding force as well as an induced electric field and current.”

The power generated per unit length by MHD generator is

P=σuB2/ ρ……………………… ..(1)

Where u is the fluid velocity, B is the magnetic flux density, σ is the electrical conductivity fluid and ρ is

the density of fluid.

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Faraday MHD generator

An ionized gas is employed as the conducting fluid. Ionization is produced either by thermal means i.e. by

an elevated temperature or by seeding with substance. The fluid is heated at high temperature and alkali

materials like Potassium, salt, cesium having low ionization potential are injected in the hot fluid to ensure

reasonable conductivity of the fluid. The injected material is termed seed material. Actually seed material

acts as conductor and flowing fluid acts as a carrier. Only small amount of seed material is required and is

typically 1% of the total mass flow.

To achieve the higher velocity, the fluid is passed through the nozzle and the fluid should have higher

pressure prior to the nozzle. In MHD duct the K.E and internal energy of hot fluid decreases due to the

effect of strong magnetic field and the consequent Lorentz force. To accommodate the flow a due to a

reduction in velocity along the length of duct, the duct is made of diffuser. The fluid looses its ionization

potential in MHD duct with reduction in internal energy or temperature and its state the fluid in which is

still at higher temperature is exhausted from the duct. The thermal energy of the hot fluid leaving the MHD

duct is further utilized in a heat power cycle.

Types of MHD Power Generation:

Following are the two distinct approaches to retrofitting a thermal power plant with MHD power system.

1. Open Cycle MHD Power System

2. Closed Cycle MHD Power System

Seeded Insert Gas System

Liquid Metal System

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Open Cycle MHD Power System:

In an open cycle MHD power system air is used as fluid for the MHD duct. First atmospheric air is

compressed in a compressor and the air is preheated in heat exchanger by using the heat of air or gas

leaving the MHD duct. The air is preheated so that high temperature is possible in combustion chamber.

For this oxygen enriched air is also used when the preheated air temperature is not high enough. The

temperature of preheated in between 2300 to 2700C either by directly burning pulverized coal in

combustor or in a separate heat exchanger. Since a very high temperature is required to ionize a gas

(thermal ionization) which cannot be endured by the material available, so the hot air or gas is seeded

with cesium or potassium (K2CO3, KOH) to make conductor at low temperature in the range of 2300 to

2700C.

The pressurized hot air from combustion chamber is passed through the nozzle and the seed material is

injected before the gas enters the MHD duct. In MHD duct, K.E and internal energy of the conducting gas

is converted in electricity. The gas looses its ionization potential when the temperature falls about 1700C.

This high temperature exhaust gas from the MHD duct is then used by in preheating of compressed air

which is to be heated in the combustion chamber. The heat of exhaust gas is used by the steam

generator next from air preheater to generate steam. Before leaving the exhaust gas into atmosphere, the

seed material is recovered in an electrostatic precipitator (ESP) which can be used again in MHD duct.

The use of ESP helps in the abatement of atmospheric pollution since the oxides or hydroxides of the

seeding element cause severe air pollution.

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Closed Cycle MHD Power System:

Seeded Insert Gas System

Liquid Metal System

In closed cycle operation, noble gases and liquid metal are considered as fluid. In closed cycle fluid is

heated in a heat exchanger by using the heat energy of any combustion source. A high temperature

nuclear reactor probably helium cooled is also a feasible source for MHD and in the early stages of

development. In closed cycle scheme, helium (or argon) gas seeded with cesium is heated in a nuclear

reactor, passed into the MHD duct then into the steam generating system.

Difference between Open and Closed Cycle MHD Power Generation:

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Thermoelectric Power Generation:

The term thermoelectric is combination of two words thermo and electric and as its name indicates

thermal means heat energy and electric means electrical energy. Thermoelectric power generators are

the devices which used to convert temperature difference between two junctions (hot and cold) into

electrical energy. This is also a type of direct power conversion.

Working Principle of Thermoelectric:

A working of thermoelectric generator is based on Seebeck effect. It converts heat energy directly into

electrical energy. According to which, a loop of two dissimilar materials developed an emf (V) when the

two junctions are kept at different temperature also known as Seebeck Power Generation. A thermo-

electrical generator basically consists of heat source (hot junction), which is kept at high temperature and

a heat sink (cold junction), which is maintained at a temperature less than the heat source. The

temperature difference between heat source and heat sink causes direct current (DC) to flow through the

load. The power generated due to Seebeck power generation is single phase DC and given by I2RL or VI

where RL is the load resistance.

Thermoelectric generator

The output voltage and output power are increased either by increasing the temperature difference

between the hot and cold ends or by connecting several thermoelectric power generators in series. The

current will continue to flow as long as heat is supplied to the hot junction and removed from the cold

junction. In a thermoelectric generator large current and low voltage is developed. A typical thermoelectric

generator operating at hot and cold junction temperatures of 600C and 200C can develop 0.1V and 2A.

This current produced by thermoelectric or Seebeck power generation is DC in nature and can be

converted into ac by using invertors and its voltage level can be further step up by using transformers.

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The energy conversion through thermoelectric or Seebeck power generation is reversible process i.e the

direction of energy flow can be reversed. If we remove load and supply DC power across the terminals

where load was connected, the heat can be easily drawn from the thermoelectric power generator.

The voltage of this generator is given by V = α ΔT where α is Seebeck coefficient and ΔT is the

temperature difference between hot and cold junction. Let R is the internal resistance of the

thermoelectric power generator then the current flowing through the external resistance RL is given by

Efficiency of Thermoelectric Generator:

The efficiency of thermoelectric power generator is defined as the ratio of power developed, PL across

load resistance, RL to the heat flow, Q from the source.

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Materials Suitable for Thermoelectric Power Generators:

The most commonly used material for this generator is lead telluride. Lead telluride is a compound made

of lead and tellurium having small amount of bismuth or sodium. Other compounds used for making

thermoelectric power generators are bismuth telluride, bismuth sulphide, germanium telluride, zinc

antimonite, tin telluride and indium arsenide etc.

Application of Thermoelectric Generators:

For increasing the fuel efficiency of cars, thermoelectric generators are used. These generator

use heat produced when the vehicle is running.

Seebeck Power Generation is used to supply power to spacecrafts.

These generators are used to supply power to remote stations like weather stations, relay

stations etc.

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