2 Non-conventional Power Generation – Generation and Utilization of Electrical Energy

Chapter 2

Non-conventional Power Generation

Objectives

After reading this chapter, you should be able to:

  • know various non-conventional energy sources
  • generate electric power by utilizing non-conventional energy
2.1 Introduction

A plenty of energy is needed for industrial growth and agricultural production. The world's fossil fuels or the conventional sources of energy such as coal, oil uranium, petroleum, and natural gas are not adequate for future increasing energy demands and may be depleted and exhausted in few hundred years until we exploit other sources of energy. Consequently, non-conventional and renewable sources have to be developed by the scientists for future energy requirements.

2.2 Generation of Electrical Power by Non-Conventional Methods

The various non-conventional energy sources are:

  1. solar energy,
  2. wind energy,
  3. tidal energy,
  4. geothermal energy,
  5. magneto-hydrodynamics (MHD) generator,
  6. thermionic converter,
  7. energy from biogas and biomass,
  8. ocean thermal energy conversions,
  9. hydrogen energy,
  10. fuel cells, and
  11. thermo-electric power.

The percentage use of various sources for the total energy consumption in the world is given in Table 2.1.

 

Table 2.1 Energy consumption in the world

Coal 32.5
92%
Oil 38.3
Gas 19
Uranium 0.13
Water 2
Wood 6.6
8%
Dung 1.2
Wastage 0.3

Referring to Table 2.1, the world's energy supplied from commercial or conventional energy such as coal, oil, gas, uranium, and water up to 92%. In many developing countries, non-conventional energy such as wood, animal dung, and agricultural wastage would serve 8% of total energy used in the world.

Main advantages of non-conventional energy sources are:

  • Atmospheric pollution is less.
  • These sources are available in large scale at free of cost.
  • These sources are well suited for decentralized use.
  • Maintenance is less.
2.3 Solar Energy

Solar energy is very large and inexhaustible source of energy. It comes from the sun to the earth. This energy is cheap and free from pollution. The earth receives nearly 4,000 trillions kWh of energy from the sun. Normally, solar power at the atmosphere around the sun is 1017 W but solar power at the atmosphere around the earth is 106 W. Now, total power world requires for all needs of civilization is only 1013 W, i.e., sun gives nearly 1,000 times more than energy what we actually need. If we use only 5% of this energy, it is sufficient for the worldwide energy requirement.

Solar radiation, which is not absorbed or scattered by the atmosphere, reaches the ground directly from the sun is known as direct radiation. The radiation received after scattering is called diffuse radiation. The diffuse radiation comes to earth from all parts of the sky. The total solar radiation received at any point on the earth's surface is the sum of total direct radiation and diffuse radiation. Figure 2.1 shows the solar energy storage.

 

 

Fig. 2.1 Solar energy storage

2.3.1 Solar Energy Collector

Solar energy collectors are used to collect and absorb the solar energy radiated from the sun. The solar energy collectors are essential devices for the system of converting the solar energy into the desired form such as heat or electricity.

Generally used solar energy collectors are of two types. They are:

  1. Non-concentrating or flat plate type solar collector.
  2. Concentrating or focusing type solar collector.

(i) Non-concentrating or flat type solar collectors

Non-concentrating or flat type solar collectors are solar energy collectors which may collect and absorb both direct and scattered solar radiation. These collectors are made in the form of rectangular panels with an area of about 1.7–2.9 sq. m. Construction of such flat plate collector is quite simple and is shown in Fig. 2.2.

 

 

Fig. 2.2 Flat plate collector

 

The absorbing surface of the solar flat plate collector is made up of copper, aluminum, or steel coated with carbon, which absorbs solar energy. The solar collectors are associated with the water-circulating tubes; these tubes are coated with insulating materials (such as fiber glass) to prevent from heat loss. Solar energy collected by the flat plate collectors is converted into heat energy and water flowing through the tubes gets heated. The operating temperature of the flat plate collector is at about 90°C. At low temperatures, water is not converted to steam to run the prime mover. Some organic fluids such as freon-14 and 150 butane are added to the water. These fluids will absorb heat from the hot water and vaporizes at low temperature. The vapors thus formed can be used to run the prime mover to generate electric power. These flat collectors are also known as low-temperature collectors and they have a collection efficiency of about 30%–50%. In non-concentrated type collectors, the collector area is same as to the absorber area.

(ii) Concentrating collectors

Concentrating collectors are also known as focusing collectors. These focusing collectors collect solar energy on the absorbing surface with high intensity. Such collectors are associated with the reflectors or refractors can generate temperature of about 500°C. These are also known as high-temperature collectors. The main difference between focusing and non-focusing collectors is the former one collects radiation coming from any particular direction. Normally, the focusing collectors are classified into two types:

  1. Line-focusing collectors.
  2. Point-focusing collectors.

(a) Line-focusing collectors

Line-focusing collectors collect radiation on the absorber surface coming from a particular direction. Such radiation is concentrated at the focus point ‘F’ on the parabolic trough collectors shown in Fig. 2.3. Usually, in most cases, cylindrical parabolic concentrators are used in which absorber is placed along the focusing axis as shown in Fig. 2.4.

 

 

Fig. 2.3 Parabolic trough collector

 

 

 

Fig. 2.4 Cylindrical parabolic concentrator

 

The length of the reflector unit is about 3–5 m and width is about 1.5–2.4 m. Parabolic reflector is usually made up of polished aluminum, silvered glass, etc.

(b) Point-focusing collectors

A point-focusing collector is in the form of a paraboloidal shape. A paraboidal dish reflector concentrates solar radiation at a focus point shown in Fig. 2.5. A paraboloidal dish is made with 200-curved mirror segments and each of them is known as heliostat. The dish diameter is about 6.5 m. The absorber is a cavity and is made up of zirconium–copper alloy and is coated with black chrome, which is located at the focus point.

 

 

Fig. 2.5 Point focusing collector

 

In these collectors, the heat transferred into and out of the absorber cavity through pipes bonded to the interior dish structure. The dish can be moved in any direction thereby focusing the sun rays on the absorber properly.

2.4 Point-Focusing Collector

Concentrating collectors have many advantages over flat collectors

  • The structure of reflecting surface is less.
  • Collecting system cost is less.
  • The generating temperature of the concentrating collectors is higher than the flat collectors.
  • The absorber area of the concentrator system is smaller than the flat plate system. Thus, the intensity of sun radiation will be more.
  • The concentrated collectors have more efficiency.

Disadvantages

  • The initial cost of concentrated collectors is high.
  • Flux distribution over the absorber area is non-uniform but whereas the flat collectors flux distribution is uniform.
  • Reflector system to track the sun is costlier.

2.4.1 Photovoltaic cells or solar cells

The solar cell is the basic unit of the photovoltaic generator. The solar cell is the device that transforms the sun's rays or photons directly into electricity. There are various models of solar cells made with different technologies available in the market today.

These models have varying electrical and physical characteristics depending on the manufacturer. The element most commonly used in the fabrication of solar cells is silicon. In this research, we will not elaborate on the various fabrication procedure processes or techniques. This subject is covered in great detail in any text dealing with solid-state electronics.

2.4.2 Solar cell characteristics

A solar cell is simply a diode of large area forward bias with a photovoltage. The photovoltage is created from the dissociation of electron–hole pairs created by incident photons within built-in field of the junction or diode. The operating current of a solar cell is given by:

 

 

where Iph is the photocurrent in amperes, ID is the diode current in amperes, Io is the saturation current in amperes, q is the electronic charge in coulombs, KB is the Boltzmann constant in joules per kelvin, T is the junction temperature in kelvin, Rs is the series resistance in ohms, Rsh is the shunt resistance in ohms, and A is the ideality factor.

Under the darkness, the solar cell is not an active device. It functions primarily as a diode. Externally, the solar cell is an energy receiver that produces neither a current nor a voltage. Under this condition, if the solar cell is connected to an external supply, theory shows that the voltage and current are related by the diode equation given by:

 

 

Since the ultimate photovoltaic generator will be composed of N cells in series and M cells in parallel, the IV characteristics of the whole generator can be derived by scaling the IV characteristics of one cell with a factor of N in voltage and M in current. This approach is correct only when the cells are identical.

Electrical characteristics of solar cells

The graph of current as a function of voltage (I = f(V )) for a solar cell passes through three significant as illustrated in Fig. 2.6.

 

 

Fig. 2.6 Solar cell V–I characteristics

(a) Short-circuit current

The short-circuit current, Isc, occurs on a point of the curve, where the voltage is zero. At this point, the power output of the solar cell is zero.

(b) Open-circuit voltage

The open-circuit voltage, Voc, occurs on a point of the curve, where the current is zero. At this point, the power output of the solar cell is zero.

(c) Operation at maximum power

The maximum power output occurs at point A on the curve. The point A is usually referred to as the ‘knee’ of the VI curve. The electrical characteristics of the solar cells are based on their VI curves. The VI curve is based on the cell being under the standard conditions of sunlight and cell temperature, and assumes there is no shading in the cell. Standard sunlight conditions on a clear day are assumed to be 1,000 W of solar energy per square meter (1,000 W-m–2 or 1 kW-m–2). This condition is sometimes called ‘one sun’ or ‘peak sun’ when the cell is operating in conditions less than one sun, the current output of the cell is reduced as shown in Fig. 2.7. Since PV cells are electrical semiconductors, partial shading may cause the cell to heat up. Under this condition, the cells act as an inefficient conductor rather than an electrical generator. Partial shading may run shaded cells and also affect the power output of the cell. Figure 2.8 shows the VI characteristics of shaded and unshaded cell.

 

 

Fig. 2.7 Solar cell V–I characteristics at one sun and one half suns

 

 

 

Fig. 2.8 V–I characteristics of a shaded and unshaded solar cell

2.4.3 Solar power generation

Solar power generation plant is shown in Fig. 2.9. Solar power generation plant employs different power cycles depending upon the temperature of working fluid, as low-, medium-, and high-temperature cycles. A low-temperature cycle uses flat plat collectors to collect solar energy. So, the maximum temperature of the fluid is limited to 100°C. Medium-temperature and high-temperature cycles use the concentrating collectors to collect solar energy, so the maximum temperature of the fluid is limited from 150°C to 300°C for the medium-temperature cycles and above 300°C for the high-temperature cycles.

 

 

Fig. 2.9 Solar power generation plant

 

Thermodynamic cycles preferred for low and medium temperature are the rankine cycles; for high temperatures, Brayton and Stirling cycles are also used.

In a solar power generation plant, solar energy is collected by the solar pond and flat plate collectors. The solar energy collected by the flat plate collector is utilized to raise the temperature of fluid. The fluid from pond may be directly used for various cycles such as rankine or Brayton or passed through the heat exchanger; there organic fluids are heated and converted into vapor or steam. The vapor or steam is fed to the turbine blades used to rotate the shaft of electric generator coupled the turbine.

The vapor from the turbine is fed to the condenser, where cold water from the cooling tower condenses the vapor into liquid and is again fed back to the boiler, where fluid is reheated to convert it into steam then pumped to the turbine, and the cycle is repeated.

2.4.4 Advantages and disadvantages of solar power

Some of the advantages of converting solar energy into electric power are:

  • Solar power conversion system has no moving parts.
  • Absence of pollution.
  • Highly reliable.
  • Less maintenance cost.
  • The average life of photovoltaic cells is high.
  • The efficiency of conversion system is high because of the absence of moving parts.
  • Solar energy is available at free of cost, thus there is no consumption of fuel.
  • The power-handling capability of system is very large.

The main disadvantage of the solar power generation system is high initial cost; this is mainly due to the absence of the sun light during night time, so that additional equipment such as batteries are used to store the energy.

2.4.5 Applications of solar energy

Solar energy has wide applications such as:

  • Water pumping for drinking water supply.
  • Irrigation purpose in rural areas.
  • Street lighting.
  • Battery charging and weather monitoring.
  • Railway signaling equipment.
2.5 Wind Energy

Wind results from air in motion. Air in motion arises from the pressure gradient. The wind is basically caused by solar energy radiating the earth. The useful work done for the conversion of kinetic energy of the wind into mechanical energy can be utilized to generate the electricity. Most of the machines for converting wind energy into mechanical energy consist of number of sails, vanes, or blades radiating air from the hub or the central axis. When wind blows against the vanes or the blades, they rotate about the axis and the rotational motion can be used to perform the useful work.

Wind energy conversion devices are known as wind turbines, because they convert wind stream into energy of rotation because the wind turbine produces rotational motion. Wind energy is readily converted into electrical energy by connecting the turbine to an electric generator.

2.5.1 Basic principle of wind energy conversion

Wind possesses energy by virtue of its motion. Any energy conversion device can extract this and convert it into useful work depending on:

  1. the wind speed,
  2. the cross-section of wind swept by the rotor, and
  3. the overall efficiency of the rotor and generator efficiency.

The power in the extracted wind can be found out by kinetics concept. The amount of air passing in unit time through an area ‘A’ with velocity ‘V ’ is A × V.

And, mass is given by

 

M = pAV,

 

where ‘p’ is the density of the air; kinetic energy of the particle is given by:

 

 

 

 

Equation (2.4) gives maximum wind energy available and is proportional to the cube of the wind speed. Hence, it is observed that small increase in wind speed can have noticeable effect on the power in the wind.

Since power available is proportional to density, it may vary 10–15%, because of pressure and temperature change. It is also shown that by doubling the velocity, the power available increases by eightfold. As power available is directly proportional to the cross-sectional area, it decides the diameter of the vanes for the required power. Since the area is normally circular of diameter ‘D’ in horizontal axis aero turbines, then:

 

 

Available wind power,

 

 

Strictly noted that it is not possible to convert all the wind energy into any other form of energy because the load would reduce the wind speed to zero.

2.5.2 Basic components of wind energy conversion plant

The block diagram representation of the wind energy conversion system is shown in Fig. 2.10.

 

 

Fig. 2.10 Block diagram of wind energy conversion system

 

The main components of the wind energy conversion system are:

  1. Aero turbine: Aero turbines convert wind energy into rotary mechanical energy. This block requires pitch and yaw, i.e., direction of wind flow control for proper operation.
  2. Mechanical interface (coupling & gearing): A suitable mechanical gear should be provided to transmit mechanical energy into electric generator.
  3. Electric generator: Generator that converts mechanical energy from the aero turbine into electrical energy and is connected to the load and or power grid.
  4. Controller: Controller that senses wind speed, wind direction, and shaft speeds. The output power from the generator and temperature is sensed by the controller and if necessary controller will send appropriate signal to the wind energy input to protect the system from abnormal conditions.

2.5.3 Types of wind mills

A wind mill is machine, which plays major role in wind energy conversion. Wind turbine that converts the kinetic energy of the wind motion to the mechanical energy transferred to an electric generator through the shaft. Electric generator converts mechanical energy into electrical energy.

Normally, based upon the axis of rotation of turbine, wind mills are classified into two types. They are:

  1. Horizontal axis wind mill.
  2. Vertical axis wind mill.

In horizontal axis, wind mill uses motional wind energy for the rotation of shaft, in which the axis of rotation of the shaft is along horizontal axis and the aero turbine plane is vertically facing to the wind. In vertical axis wind mill type, the axis of rotation of the shaft is along the vertical axis and the aero turbine plane is horizontally facing the wind.

Horizontal axis type wind mills are further classified into various types such as single-bladed, double-bladed, multibladed, and bicycle multibladed type.

Vertical type wind mills are further classified as savonius or ‘s’ type rotor mill and darrieus type rotor mill. Vertical axis type wind mills are having simple structure and easier to design compared to horizontal axis type wind mills.

2.5.4 Site selection for wind energy conversion plant

Various factors on site selection that are need to be considered while erecting a wind energy conversion plant is:

  • Site for the wind plant should be nearer to the consumers of the generated electrical energy.
  • It must be convenient for transportation facility.
  • Plant should be erected in the place, where winds are strong and persistence.
  • Plant must be installed at higher attitudes, where the motion of wind energy is available with higher velocity.
  • The land cost should be low.
  • It is better to choose the site nearer to the sea coast, mountains, etc. for the wind.
  • Energy conversion plant.

2.5.5 Wind power generation

A basic wind power-generating plant converts motional wind energy into electrical energy. The schematic representation of the wind power-generating system is shown in Fig. 2.11.

 

 

Fig. 2.11 Wind power generating plant

 

In wind energy-generation system, wind turbine converts kinetic energy of wind motion into mechanical energy with the help of blades. The direction of wind flow control, i.e., pitches and yaw control is required for the proper operation. A suitable mechanical transmission gear is provided to transmit the mechanical energy from the wind turbine to electrical generator.

An electric generator converts mechanical energy into electrical energy and is fed to the rectifier thereby converting fixed AC to variable DC supply. Further DC is fed to an inverter, which converts DC into variable AC supply, transmitted to grid system for utility purpose.

A diesel engine is used to drive a synchronous machine when there is no wind energy as input to the aero turbine.

2.5.6 Advantages and disadvantages of wind power

Advantages

  • Wind is renewable source of energy.
  • There is no need of using fuel for wind energy conversion system.
  • There is no need of transportation facility.
  • It is pollution free.
  • The maintenance cost of wind energy conversion system is less for low power generation.

Disadvantages

  • The availability of wind energy is fluctuating in nature.
  • The auxiliary storage devices such as battery must be provided for wind energy conversion system because of the fluctuation of the wind in nature.
  • Wind energy conversion systems are noisy.
  • More space should be needed for wind power generation.
  • The structure of wind power conversion system is complex and the weight of system is also high due to the construction of high towers.

2.5.7 Applications of wind energy

  • Wind machines can generate low power for space heating and cooling of homes.
  • The electric energy generated from the wind stations can be adoptable for domestic appliances.
  • Low power wind energy conversion systems have been used for corrosion protection of buried metal pipelines.
  • Wind power turbines up to 50 kW can be used for irrigation pumps, navigational signals, remote communications, etc.
2.6 Tidal Power

The periodic and continuous raise and fall of water on the surface of the sea is known as tide. These tides are caused mainly due to the gravitational force of the moon and sun on the water of the oceans. Mainly 70% of the gravitational force to produce a tide exists between the moon and the surface of the seawater and only 30% of the force to produce a tide is due to the force of the sun on the water of the ocean. Thus, the moon plays a major role in the formation of tide on the ocean surface.

On the surface of the ocean, most of the water is pulled away from the solid earth surface or toward the moon and at the same time, the earth is moving away from the water in the opposite direction, so that high tides occur at these two areas and low tides will occur at the center of these two water and earth.

Normally, over the surface of seawater, two high tides and two low tides will be produced within a span of 24 hr 50 min, such types of tides are known as ‘semidiurnal tides’. Usually, these tides are sinusoidal in nature as shown in Fig. 2.12.

 

 

Fig. 2.12 Nature of tide

 

In Fig. 2.12, P and Q indicate high- and low-tide points, respectively. The difference between these two points is known as tide range.

2.6.1 Components of tidal power plant

The tidal power plant has the following main parts:

  1. Dam or dyke: A dam or dyke is nothing but a barrier that exists between sea level and a basin or between a basin and the other in case of a multiple basin.
  2. Power house: The tidal power plant equipment such as turbine, electric generator, and other auxiliary devices are placed in the power house.
  3. Sluice-ways: It is nothing but a gate-controlled way either to fill the basin during high-tide period or it will keep empty during low tide.

2.6.2 Site selection of tidal power plant

Various factors that are needed to be considered for the location of tidal power plant are:

  • The location of the plant must be nearer to the ocean.
  • Site selection for the plant should be in such a way that the tidal range of ocean is large.
  • The geographic features of the plant must be encloses of large areas with short dams.
  • The sluice gates of dam should allow water to or from the basins.

2.6.3 Tidal power generation

The electrical energy generated by the generator in a tidal power plant mainly depends upon the raising and falling level of water above the surface of the sea. A simple single-basin tidal power plant is shown in Fig. 2.13. In this arrangement, both the basin and the sea are separated by a dam or dyke at which power house, houses turbine, or generator to generate electric energy.

 

 

Fig. 2.13 Simple single basin tidal power plant

 

During flood-tide period, sluice gates get opened and water is allowed into the basin on the other side of the dam, through the turbine. Then, it will rotate and is coupled to the generator thereby generating electrical energy.

The turbine causes to generate electric power only during the high-tide period and begins to drop. During the low-tide period, water head of the sea will gradually fall down and is not sufficient to generate electric power to meet the no load losses.

2.6.4 Advantages and disadvantages of tidal power

Advantages

  1. Tidal power is free from pollution.
  2. Tidal power generation is not affected by the rain.
  3. The land cost of the tidal plant is less because such plants are located at seashore.
  4. The plant does not require large space.

Disadvantages

  1. The power out of the plant will fluctuate continuously, because it depends on tidal range.
  2. The construction of a tidal plant in a sea is complex.
  3. The transmission cost of tidal power is costlier because such plants are located far away from the load center.
  4. The initial cost of the plant is high.
  5. plant equipment will be subjected to corrosion due to seawater.
  6. The efficiency of plant will be affected due to the variable tide range.
2.7 Geothermal Power

It is a renewable source of energy in the form of heat from high-pressurized steam coming from the earth. This heat energy obtained from the earth when its temperature increases rapidly up to 180°C with increasing depth below the surface. The average temperature of the earth at a depth of 10 km is about 200°C.

2.7.1 Geothermal resources

Various geothermal resources are:

  1. hydrothermal convective systems:
    1. dry steam fields or vapor-dominated fields,
    2. wet steam fields or liquid-dominated fields, and
    3. hot water fields.
  2. geopressure resources,
  3. hot dry rocks (HDR),
  4. magma resources, and
  5. volcanoes.

Nowadays, hydrothermal convective systems and hard rocks are being used as geothermal resources for energy generation.

2.7.2 Geothermal power generation

Normally, hydrothermal convective systems are widely used as geothermal resources to generate electric energy.

Hydrothermal convective systems are broadly classified into the following three categories. By utilizing these resources, geothermal power will be generated. Some methods of generating geothermal power are explained below.

(i) Dry steam or vapor-dominated fields

Figure 2.14 shows the dry steam geothermal power generation. In vapor-dominated systems, geothermal zone has well, which delivers steam with little or without water at a temperature of 150–250°C. These fields are the most attractive geothermal resources. The dry steam supplied by well is delivered to the steam turbine, which drives an electric generator, generates electric energy.

 

 

Fig. 2.14 Dry steam geothermal power generation

 

In this scheme of generation, the main difference between system and conventional steam turbine is only geothermal steam that is supplied from nuclear or fossil fuels at low temperature and pressure.

(ii) Wet steam or liquid-dominated fields

Figure 2.15 shows the wet steam geothermal power generation. In this method of power generation, liquid-dominated fields are the geothermal resources. These wet steam fields are available 20 times more than the dry steam fields. In the wet steam reservoir, the water temperature is above the boiling point of 100°C but it is under pressure, thus water does not boil and remains in the liquid state.

 

 

Fig. 2.15 Wet steam geothermal power generation

 

When the water comes to the surface of the earth, the pressure decreases and then the liquid subjected to rapid heat and flashes into a mixture of water and steam. Now, the mixture of water and steam supplied by well is delivered to the flash separator to separate steam and hot water then steam is fed to steam turbine to drive generator thereby generating electricity. Steam from the turbine is pumped into the condenser, then condensate steam and hot water from flash chamber are fed to the reservoir.

2.7.3 Advantages and disadvantages of geothermal power

Advantages

  1. Geothermal energy is quite cheaper.
  2. Less pollution.
  3. Geothermal energy can be utilized for various purposes from a single resource.
  4. Geothermal resources delivers net energy compared to other resources.
  5. It is versatile.

Disadvantages

  1. The efficiency of the power generation is less about 1.5% compared to the other systems of generation.
  2. Noisy operation.
  3. Large area is required for the geothermal power generation.

2.7.4 Applications of geothermal energy

Main applications of geothermal energy are:

  1. Electric power generation.
  2. The heating of buildings.
  3. Industrial heating purpose such as drying timber, wool washing, and crop drying.
2.8 Biomass and Biogas

Biomass is the natural source of energy such as animal waste, wood, agricultural residues, dung, vegetable waste, and plant waste. Biogas is produced by decomposing the biomass. The conversion of biomass into biogas takes place through the process of digestion, pyrolysis, or hydro-gasification.

Energy from the biomass is obtained from the following ways.

  1. The biomass such as wood, dung, and agricultural residues is burnt directly to obtain energy.
  2. The biomass is converted to fuels such as ethanol and methanol, which can be used as liquid fuels in engines.
  3. The biomass is subjected to fermentation process to obtain a gaseous fuel called biogas.

2.8.1 Biogas generation

Biogas is produced from the decomposition of the biomass. It is a mixture of 55–65% of methane, 30–40% carbon dioxide, and some impurities such as H2, H2S, and nitrogen.

Biogas can be produced from the biomass through various processes such as digestion, pyrolysis, or gasification.

Digestion is the process of decomposition of the biomass in the absence oxygen and in the presence of anaerobic organisms at ambient temperature of 35–70°C. The device or container used for digestion process is known as digester.

Biomass gasification is the process of converting a solid or liquid into a gaseous without leaving a carbon residue.

Equipment used for gasifying biomass such as agricultural waste and wood waste is known as gasifier.

A simple biogas plant is shown in Fig. 2.16.

 

 

Fig. 2.16 Biogas plant

 

A simple biogas plant comprises of the following parts.

  1. foundation,
  2. digester,
  3. dome,
  4. inlet chamber,
  5. outlet chamber,
  6. mixing fans, and
  7. gas outlet pipe.
  1. Foundation: The foundation is nothing but the base of digester; it is made up of cement, concrete, and bricks ballast. The base should be water proof to avoid the water leakage.
  2. Digester: It is a container made up of bricks, sand, and cement. In this digester, fermentation of biomass such as dung, animal waste takes place, thus it is also known as fermentation tank.
  3. Dome: It is the roof of the digester; after the decomposition of biomass, gas gets collected in the space of the dome over the slurry (mixture of water, dung, animal waste, etc.) in the digester.
  4. Inlet chamber: An inlet chamber is made with bricks, cement, and sand. It is of bell mouth shape. It is the opening valve to admit slurry into the digester.
  5. Outlet chamber: It is the part of the plant of rectangular cross-section through which the final slurry moves out of the digester, after the digestion process.
  6. Mixing tank: It is a tank placed on the top of the inlet chamber in which dung and water are mixed properly to make slurry and then admitted into the digester through the inlet chamber.
  7. Gas outlet pipe: It is an outlet pipe fitted on the top of the dome of the digester to take away the gas for the utility purpose. A valve is provided to control the flow of gas to usage.

Plant operation

Initially, slurry is prepared by mixing the cow dung and water properly in the ratio of 1:1 and then the digester is completed filled with the slurry up to the dome level.

The fermentation of slurry takes place in the digester; gas will be generated due to the fermentation process and is accumulated along the dome.

The gas accumulated along dome exerts pressure on the slurry and displaces into the inlet and outlet chambers.

The surface level of slurry falls down continuously till the slurry level reaches the upper edges inlet and outlet chambers.

The gas accumulated along the dome is conveyed to the usable points through the outlet pipe attached on the top of the dome.

The quantity of gas generated can be estimated by calculating the increase in slurry volume in the inlet and outlet chambers.

2.8.2 Site selection of biogas plant

The factors needed to be considered while selecting a site for the biogas plant are listed below:

  1. The distance between the plant and the gas-consuming point must be less for economy, and to minimize the leakage of gas, usually distance is not more than 10 m for a plant of capacity 2 m3.
  2. Site for a plant location should be in such a way that falling of the sun rays will raise the temperature of the slurry from 15°C to 30°C for gas generation.
  3. The plant should be located far away nearly 15 m from the well. This is because fermented slurry may pollute the well water.
  4. The distance between the biogas plant and the cow dung available place should be less to minimize the transportation cost.
  5. The plant should be located in underground so that slurry can be filled and removed easily.

2.8.3 Advantages and disadvantages of biogas

Advantages

  1. The initial cost of the biogas plant is low.
  2. The byproducts of the biogas plant can be used again for biogas generation.
  3. It is pollution free.
  4. Biogas can be conveyed to consumer point through GI pipes.
  5. Biogas can be easily stored in any container and can be transported to the consumers.

Disadvantages

  1. The land required for the biogas plant is relatively large so land cost is high.
  2. Various nutrients must be added to the slurry for developing the bacteria.
  3. The cost of producing energy is high.
  4. Sometimes, the addition of fertilizer will reduce the gas production.
2.9 MHD Generations

MHD generation is one of the methods of generating electrical energy, which is highly efficient and low pollution one. In advanced countries, MHD generators are widely used but in developing countries, those are still under construction.

MHD generators are devices, which convert heat energy of a fuel into electric energy. The principle of the MHD generator is electromagnetic induction, ‘when an electric conductor is passed through a magnetic field some voltage is induced’. This principle is the same as the conventional generator, the only difference being that a solid electrical conductor is used.

2.9.1 MHD generation

An MHD generator is a simple device to convert heat energy into electrical energy. Generally used methods of generating MHD power are:

  1. open-cycle generations and
  2. closed-cycle generations.

In open-cycle generation of MHD power, the working fluid after the generation of electrical energy is released to the atmosphere but in closed-cycle generation, the working fluid is continuously recirculated.

(i) Open-cycle MHD generation

Figure 2.17 shows the schematic arrangement of open-cycle MHD generator.

 

 

Fig. 2.17 Open-cycle MHD generator

 

An open-cycle MHD generator generates electric power. In this generator, fuel is admitted into the combustion. Initially, atmospheric air is fed to preheater then hot air is passed through the combustion chamber, which helps to burn the fuel. Hot gases in combustion chamber are mixed with ionized alkali metals such as cerium and potassium to increase the electrical conductivity of the hot gas. Thus, seeded material potassium ionizes by the hot combustion gas. The operating temperature of the combustion chamber is 2,300°C to 2,700°C. The hot gases from the combustion chamber are then passed to the magnetic field created by the permanent magnets. Thus, the MHD generator is able produce direct current and then is converted to AC power with the use an inverter.

The hot gases passed away from the generator are then heated again in air preheater to increase the temperature. These hot gases are converted to steam in a steam generator and then passed to the steam turbine to drive a synchronous generator thereby generating electrical energy. Remaining steam from the steam generator passed to the atmosphere through stacks.

(ii) Closed-cycle MHD generation

Figure 2.18 shows the schematic arrangement of closed-cycle MHD generation.

 

 

Fig. 2.18 Closed-cycle MHD generation

 

In this scheme of MHD power generation, liquid potassium is used as working fluid. Fluid from the breaker is passed through the nozzle to increase the speed of fluid. The working fluid is then passed through the MHD generator thereby generating energy fluid coming out from the MHD generator is passed through the heat exchanger converting into steam to run the steam turbine as well as generator to generate electric power, and remaining is pumped back to the reactor.

2.9.2 Advantages and disadvantages of MHD power generation

Various advantages of MHD power generating system are:

  • Large amount of electric power generation is possible
  • It is highly reliable, as the system is having no moving parts.
  • Closed-cycle system of MHD power generation is pollution free.
  • The size of the power plant is small.
  • The efficiency of the plant is high about 50% compared to other systems of generation.
  • It is possible to run the standby power plant in conjunction with MHD power generation scheme.
Key Notes
  • Solar energy collectors are used to collect and absorb the solar energy radiated from the sun. The solar energy collectors are essential devices for the system of converting solar energy into the desired form such as heat or electricity.
  • The solar energy collectors are of two types.
  • Non-concentrating or flat plate type solar collector.
  • Concentrating or focusing type solar collector.
  • Non-concentrating or flat type solar collectors are solar energy collectors that may collect and absorb both direct and scattered solar radiation. These are focusing collectors.
  • Concentrating collectors are also known as focusing collectors. These focusing collectors collect solar energy on the absorbing surface with high intensity.
  • Focusing collectors are classified into two types. They are:
    1. Line-focusing collectors.
    2. Point-focusing collectors.
  • The main difference between the focusing and non-focusing collectors is former one collects radiation coming from any particular direction.
  • Solar cell is the device that transforms the sun's rays or photons directly into electricity. The element is most commonly used in the fabrication of solar cells is silicon.
  • A basic wind power generating plant converts motional wind energy into electrical energy.
  • Geothermal power is a renewable source of energy in the form of heat from high-pressurized steam coming from the earth.
  • Biogas is produced from the decomposition of biomass. It is a mixture of 55–65% of methane, 30–40% carbon dioxide, and some impurities such as H2, H2S, and nitrogen.
  • Biogas can be produced from the biomass through various processes such as digestion, paralyses, or gasification.
  • Biomass gasification is the process of converting a solid or liquid into a gas without leaving a carbon residue.
  • MHD power generation methods are:
    1. Open-cycle generations.
    2. Closed-cycle generations.
Short Questions and Answers
  1. List out some of the non-conventional energy sources.

    Non-conventional energy sources are:

    • solar energy,
    • wind energy,
    • tidal energy, and
    • geothermal energy.
  2. What are solar energy collectors?

    Solar energy collectors are collecting plates used to collect and absorb the solar energy radiated from the sun.

  3. What are the generally used solar energy collectors?

    The Generally used solar energy collectors are of two types.

    1. Non-concentrating or flat plate type solar collectors.
    2. Concentrating or focusing type solar collectors.
  4. What is non-concentrating or flat type solar collector?

    Non-concentrating or flat type solar collector is solar energy collectors that may collect and absorb both direct and scattered solar radiation.

  5. What are concentrating collectors or focusing collectors?

    Concentrating collectors are also known as focusing collectors. These focusing collectors collect solar energy on the absorbing surface with high intensity.

  6. What are the types of focusing collectors?

    Focusing collectors are classified into two types.

    1. Line-focusing collectors.
    2. Point-focusing collectors.
  7. What are the advantages of focusing collectors?
    1. The structure of reflecting surface is less.
    2. The cost of collecting system is less.
  8. What are the main components of wind energy conversion system?
    1. Aero turbine.
    2. Mechanical interface (coupling and gearing).
    3. Electric generator.
  9. What is the function of wind turbine?

    Wind turbine converts kinetic energy of wind s motion to mechanically energy transferred to an electric generator through the shaft.

  10. What are the types of wind turbines based on rotation of shaft?

    Based upon the axis of rotation of turbine, wind mills are classified into two types.

    1. Horizontal axis wind mill.
    2. Vertical axis wind mill.
  11. What is meant by geothermal power?

    It is a renewable source of energy in the form of heat from high pressurized steam coming from the earth. This heat energy obtained from the earth when its temperature increases rapidly up to 180°C with increasing depth below the surface.

  12. What is meant by biomass?

    Biomass is the natural source of energy such as animal waste, wood and agricultural residues, dung, vegetable waste, and plant waste.

  13. What is meant by biogas?

    The gas produced by decomposing biomass. The conversion of biomass into biogas takes place through the process of digestion, paralysis, or hydro-gasification.

Multiple-Choice Questions
  1. The energy obtained directly from the sun is called:
    1. Nuclear energy.
    2. Solar energy.
    3. Thermal energy.
    4. Hydroenergy.
  2. Which of the following is unconventional source of electrical power?
    1. Coal.
    2. Diesel.
    3. Geothermal.
    4. Nuclear.
  3. The main daily solar radiation at many places in India is about:
    1. 100 kwh m-2.
    2. 20 kwh m-2.
    3. 5 kwh m-2.
    4. 1 kwh m-2.
  4. The ocean thermal energy is larger than:
    1. Wave energy.
    2. Tidal energy.
    3. and tidal energies.
    4. None.
  5. Ocean thermal energy is:
    1. Low-quality heat.
    2. High-quality heat.
    3. Median quality heat.
    4. None.
  6. The instrument used to measure the solar radiation is:
    1. Thermometer.
    2. Thermocouple.
    3. Monometer.
    4. Pyrheliometer.
  7. Winds are caused due to:
    1. The absorption of solar energy by the earth and the atmosphere.
    2. The rotation of the earth about its axis and around the sun.
    3. Both (a) and (b).
    4. None.
  8. Much of wind energy utilization is closed to the ground level within:
    1. 1 m.
    2. 5 m.
    3. 50 m.
    4. 500 m.
  9. From wind energy viewpoint, wind measurements were conducted since:
    1. 1904.
    2. 1968.
    3. 1986.
    4. 1995.
  10. The main cost component in the wind farm project is:
    1. The post.
    2. Generator.
    3. Exciter.
    4. Wind turbine.
  11. The approximate life time of wind turbine is:
    1. 1 year.
    2. 2 years.
    3. 20 years.
    4. 50 years.
  12. The ocean power plants are existing at:
    1. Kodaikanal.
    2. Kothagudem.
    3. Ramagundam.
    4. None.
  13. The source of power for satellite is:
    1. Wind energy.
    2. Thermionic converter.
    3. Solar cells.
    4. Microwave energy reflector.
  14. The method of generating power from seawater is more advantageous is:
    1. Ocean currents.
    2. Tidal power.
    3. Wave power.
    4. None.
  15. In fuel cell, the electrical energy is generated from:
    1. Mechanical energy.
    2. Heat.
    3. Sound.
    4. Chemical.
  16. Tidal power plant is being installed in India in:
    1. Tarapur.
    2. Vijayawada.
    3. Vijjeshwaram.
    4. Gujarat.
  17. The conductor used in MHD generator is:
    1. Gold.
    2. Silver.
    3. Copper.
    4. Gas.
  18. Wind energy is:
    1. Generated as a supplement to other power.
    2. Developing power proportional to wind power.
    3. Clean, free, and domestically produced.
    4. All the above.
  19. The power constant in a wind mill depends on:
    1. Wind speed.
    2. The shape of rotor blades.
    3. The type of rotor blades.
    4. All the above.
  20. The secondary source of energy are:
    1. Coal, oil, and uranium.
    2. Wind, tide, and the sun.
    3. Hydrogen, oxygen, and water.
    4. None.
  21. In a fuel cell, the electrical energy is obtained from:
    1. Chemical energy.
    2. Mechanical energy.
    3. Electrical energy.
    4. Heat energy.
  22. The sun gives:
    1. Heat.
    2. Light.
    3. Both (a) and (b).
    4. None.
  23. Wind energy is first converted into:
    1. Electrical energy.
    2. Mechanical energy.
    3. Chemical energy.
    4. None.
  24. The current developed by MHD generator is:
    1. AC.
    2. DC.
    3. Either AC or DC.
    4. None.
Review Questions
  1. List out the various non-conventional energy sources and their availability.
  2. What are solar energy collectors and also explain the use of them.
  3. Write short notes on photovoltaic cells.
  4. Give the applications of solar energy.
  5. Discuss the function of basic components of wind energy conversion plant.
  6. With the help of neat sketch explain the function of wind power generation system.
  7. Give the advantages and disadvantages of wind power generation system.
  8. Discuss in detail about the components of tidal power plant.
  9. What is the significance of geothermal power and list out the resources.
  10. Write short notes on biogas and biomass.
  11. Draw a neat sketch and explain the function of biogas plant.
  12. short notes on MHD power generation.
Answers
1. b 7. c 13. c 19. a
2. c 8. c 14. b 20. b
3. c 9. c 15. d 21. a
4. c 10. d 16. d 22. c
5. a 11. c 17. d 23. b
6. d 12. d 18. d 24. b