UNIT 3 INTRODUCTION TO NON-
CONVENTIONAL ENERGY SYSTEM
Structure
3.1 Introduction
Objectives
3.2 Non-Conventional Energy Resources
3.2.1 Energy Sources 3.2.2 Solar Energy 3.2.3 Wind Energy 3.2.4 Biomass Energy 3.2.5 Geothermal Energy 3.2.6 Tidal Energy
3.2.7 Ocean – Thermal Energy 3.2.8 Magnetohydrodynamics
3.3 Photovoltaic Systems
3.3.1 Photovoltaic System for Power Generation 3.3.2 Photovoltaic System for Lighting 3.3.3 Photovoltaic System for Water Pumping
3.4 Solar – Thermal Systems
3.4.1 Solar Thermal Systems to Produce Hot Water 3.4.2 Solar Thermal Systems for Generation of Steam 3.4.3 Solar Thermal System for Cooking
3.4.4 Solar Thermal System for Drying
3.4.5 Solar Still for Production of Potable Drinking Water
3.5 Wind – Energy Systems
3.5.1 Wind Electric Power Generating System 3.5.2 Wind Energy System for Lifting Water
3.6 Bio – Energy Systems
3.6.1 Biogas Generating System 3.6.2 Biomass Gasification System 3.6.3 Alcoholic Fermentation System
3.7 Geothermal Energy Systems 3.8 Summary
3.9 Key Words 3.10 Answers to SAQS
3.1 INTRODUCTION
With the ever increasing per capita consumption of energy and exponentially rising population, scientists and technologists all over the world, see the end of earth's non-replenishable fuel resources, like - coal, oil, natural gas, etc. At a present rate of consumption, it is estimated that oil and gases are not expected to last beyond 50 years and many countries will face serious shortage of coal after 2200 A. D., while nuclear fuels may carry as well beyond the middle of the next century. The fossil fuels consumption in most developed countries has already reached a level which this planet cannot afford and it has caused a grave threat to the very existence of plant and human life due to undesirable of fuel of pollutants. The entire world is facing a severe energy crisis with increased cost of fuels, fast depletion of
conventional energy resources and tremendous increase in environmental pollution. The situation warrants active research and development activities in utilizing alternate, non-conventional pollution free energy resources.
Government of India has created a separate Ministry, called Ministry of Non- Conventional Energy Sources (MNES) for this purpose. There has been tremendous boost for the use of non-conventional energy resources like Solar, Wind, Biomass, MHD, Hydrogen, etc.
Objectives
After studying this unit, you should be able to
describe the characteristics of the different non-conventional energy resources, and their potential for harnessing.
elaborate the application of the non-conventional energy resources, and
appropriate the nature and characteristics of the various types of systems which make use of non - conventional energy resources.
3.2NON-CONVENTIONAL ENERGY RESOURCES
3.2.1 Energy Sources
Before we consider in detail non-conventional energy sources, let us define three main types of energy sources. These are:
a) Primary Energy Sources
Primary energy sources, also called conventional energy sources or commercial energy sources, provide a net supply of energy. The examples include coal, oil and gas etc. The energy produced by these sources by conversion processes is much higher than the energy used in obtaining them. The energy yield ratio, defined as the energy released by the source to the energy received from the environment, is very high and that is why they are referred to as commercial energy sources.
b) Secondary Energy Sources
The secondary energy sources produce no net energy. These sources are also called non - conventional, renewable etc. and include solar, wind, biomass, geothermal, tidal, ocean - thermal energy etc.
c) Supplimentary Sources of Energy
For supplimentary sources of energy, the net energy yield is zero.
They require high investment in terms of energy and an example include thermal insulation.
A detailed discussions of these sources is beyond the scope of this unit. We will now discuss in somewhat more detail the non- conventional energy sources.
Non-conventional energy resources are eco-friendly. Some of the non- conventional energy resources like-solar, wind, etc. which are also called renewable energy resources, are free of cost, easily available and non - exhaustable. Suitable technologies and devices are required to be developed to harness them more effectively and efficiently. Emphasis should be given to trap these energy resources more and more to conserve conventional energy resources so that they last long. In fact, non-conventional energy resources play important roles to save not only our conventional energy resources, but also to control environmental pollution. It is the interaction of three Es i.e., Energy, Economy and Environment, which is important. The use of solar energy does not create environmental pollution but helps in saving conventional energy resources mentioned above.
Let us consider the case of heating water. Water can be heated by several ways. Some of them widely used are:
(a) by making use of electricity which is a high quality source of energy produced by burning primary sources (coil, oil, gas) at rather low efficiency causing environmental pollution (ash, waste harmful gases, etc.).
(b) by making use of biomass (wood, agrow-waste etc.) which also leads to the environmental pollution.
(c) by using solar water heaters which makes use of solar energy.
The non - conventional energy resources whose nature and characteristics will be discussed in the following sections are listed below:
(i) Solar energy (ii) Wind energy (iii) Biomass energy (iv) Geothermal energy
(v) Tidal energy
(vi) Ocean-thermal energy, and (vii) Magneto hydrodynamics energy.
3.2.2 Solar Energy
Amongst the non-conventional sources of energy, solar energy appears to be the most promising. It has the advantages of being free of cost, non-
exhaustable and completely pollution free. On the other hand, it has several drawbacks: energy density per unit area is conditions greatly reduce the energy received. Therefore, in harnessing solar energy to be very low, it is available for only a part of the day, and cloudy and hazy atmospheric utilized for various purposes, challenging technological problems exist, the most important being of the collection and concentration of solar energy and its conversion to the other forms through efficient and comparatively economical means.
On the basis of the nature of energy conversion, the solar energy systems can be classified mainly into two categories. These are photovoltaic systems and solar thermal systems. There are various applications of both photovoltaic and solar - thermal systems which will discussed later.
In India and other tropical countries, solar energy holds out a great promise for the future. With the limited reserves of conventional fuels and slow progress of nuclear energy development programmes, a strong need exists in India to develop solar energy devices for both electrical and non-electrical use. The atmospheric conditions in the country are quite favourable to solar energy reception except during the rainy season. India has a land area of 3.28 x 10¹¹ square meters, with an average of 5 kilowatt-hour per square meter per day solar energy being available for over 300 days per annum with intensity varying from place to place.
Nature of Solar Radiation
Two types of solar radiations are received on the earth. These are (i) Direct or Beam Radiation
It is the radiation received by the earth without any
interruption while passing through the atmosphere as shown in Figure 3.1. In this case there is no change in the direction of the sun rays.
(ii) Diffused Radiation
It is the radiation received by the earth from all sides as shown in Figure 3.2. In this case the sun rays change their directions after scattered by clouds, dust particles, etc, while passing through the atmosphere.
Collection of Solar Energy
A simplest method to collect solar energy is to heat water using plate collectors which captures both direct as well as diffused radiations.
The solar radiations can also be concentrated.
Two ways of developing concentrated solar radiations have been explored using and refraction collectors, viz, mirrors These devices concentrate the solar rays to a focal point or focal plane which is characterized by high degree of heat which can be utilized for different purposes, The reflector is the better of the two methods due to the convenience with which it can be manufactured in different shape and sizes. If the arrangement is provided to turn the
concentrator with sun so that the rays can constantly concentrate at the focal point or plane, a continuous supply of heat is made available during the hours of the day. A solar tracking system can be used for the above purpose.
Measurement of Solar Energy
For the measurement of solar radiation, two instruments, which are generally used, are pyrheliometer and pyranometer. Pyrheliometer measures beam radiation whereas pyranometer measures the total radiation.
3.2.3 Wind Energy
Wind is essentially created by solar heating of the atmosphere. Wind as a power source is attractive because it is plentiful, inexhustable and non- polluting. Further, it does not impose extra burden on the environment.
Unfortunately, it is non-steady and undependable. Several attempt has been made since 1940 to develop various types of wind mills and wind turbines to harness wind energy for generation of electricity, pumping water, etc. and the
development is still going on. However, techno-economic feasibility has yet to be established.
India has vast coasted, hilly and desert areas where the wind energy potential is quite high. It is estimated that India has 20,000 MW wind power potential.
In India, wind power plants are functioning in Rajasthan, Karnataka, and parts of Gujarat, Madhya Pradesh, Andhra Pradesh, etc. where wind blows at an average speed of 30 km/hr.
When large movement of air takes place wind energy can be used for getting power from wind by using wind mills. They can be used for pumping water or making flour. Additional equipments like wind turbine generator help in producing electricity. Large differences in solar flux falling on the earth surface leads to different air temperature. In regions with solar radiation which is strong atmosphere air gets heated and expands to give rise to a high pressure region. At a places where radiation is less air gets cooler to give rise to a low pressure region. This difference causes acceleration of air particles and is termed as wind.
Since the power extracted by the rotor of the wind machine is proportional to the cube of the wind velocity, so for installing a wind mill plant we may consider first the average value of wind velocity per year by using formula
𝑉₁ = ∫
( ) …(3.1) where, 𝑉₁ = Daily average wind velocity,
t = Time, and t₂ - t₁ = Duration per one year.
These are three factors which determine the output from a wind energy converter. The first, and the most important is the wind speed; second, the cross-section of wind swept by the rotor and third, the overall conversion efficiency of the rotor, transmission system and generator. Making use of equation for kinetic energy, the available wind power, Pw, may be expressed as follows:
Pw = ρ π D2𝑉 …(3.2) Where, ρ = density of air (kg/m2), and
D = diameter of the intercept area (rotor diameter), m.
Wind Climatology of India the India
The large amount of climatology data at the National Climatological Data centre of the Meteorological Department may be used to prepare climatological maps wind speed for the country.
In the winter months, the mean wind speed is less than 5 km/hr over the greater part of north, east-central and north-east India. Over most of the peninsula speeds are higher, between 5 to 10 km/hr, while along the west coast the wind speeds exceeds 10 km/hr.
The monsoon is the season of strong winds throughout the country and wind speeds of 20 to 30 km/hr winds over Saurastra-Kutch and 15 to 20 km/hr winds over Western India, South Tamil Nadu and Coastal Bengal.
Strong winds also occur during the months of May to
September/October, when depressions and cyclonic storms form in the Bay of Bengal, and move generally in a west-north westernly direction. Hurrican winds then occur often along the east coast and less frequently on the west coast during these months.
By March, land and sea breezes also become more conspicuous over the coastal areas. Thunderstorms and dust storms occur during April and May with increasing frequency and violence, over most of north and central India.
Measurement of Wind Speed
To measure the speed of the wind throughout the year, several types of anemometers are used.
3.2.4 Biomass Energy
Among the non-conventional and renewable forms of energy, Bio-energy offers the most potential scope due to a wide spectrum of biomass available in different agroclimatic regions of the world. Technologically, there are many options. Two important conversion technologies with various processes are as follows:
(i) Chemical conversion technology
(a) Direction combustion for production of steam and the generation of electricity
(b) Pyrolysis (c) Liquification (d) Gasification
(ii) Biological conversion technology (a) Anaerobic digestion
(b) Fermentation
The choice of a particular technology depends upon a number of factors like - chemical composition and availability of the feedstocks, cost of processing energy, input-output analysis, employment generation potential,
environmental impact and social acceptability of the end product. An integrated view of R & D in all major section of biomass, viz, its impact and social acceptability production, conversion, utilization and conversion may help accelerating the pace of transition from non-renewable to renewable sources of energy to meet our growing needs on decentralized basis.
3.2.5 Geothermal Energy
Geothermal energy refers to the heat contained in the inner core of the earth.
This heat, deep inside the earth, acts as a source of power. The temperature at the inner core of the earth varies from 3000 ° C to 10000 ° C. As a result of this difference between earth's crust and the inner portion heat flow in the earth's surface is equal to 63 kW / km².
Two ways of electric power production from geothermal energy have here been suggested. In one of these heat energy is transferred to a working fluid which operates the power cycle. This may be particularly useful at places of fresh volcanic activity where the molten interior mass of earth vents to the surface through fissures, and substantially high temperatures, such as
between480K to 590k can be found. By embedding coil of pipes and sending water through them steam can be raised. In the other, the hot geothermal water and/or system is used to operate the turbine directly. With the use of binary cycle, a well producing 6750 1pm of 149°C water and steam could produce a net power of about 3400kW.
At present geothermal power generation in the world amounts to a total of about 800 MW of this, nearly 50% in Italy; 10% in Mexico and about 35% in USA. The imperial valley area of South California in USA has one of the greatest geothermal energy potentials of the world.
At present only steam coming out of the ground is used to generate electricity, the hot water is discarded because it contains as much as 30%
dissolved salts and minerals, and these cause serious rust damage to the turbine. The water, however, contains more than 1/3rd of the available geothermal energy. Research is being carried out to build turbines which can withstand the corrosive effects of hot water coming out of wells.
In India, feasibility studies of a 1 MW station at Pugga valley in Ladakh is being carried out. Another geothermal field has been located at Chumantang.
There is a number of hot springs in India, but the total exploitable energy potential seems to be very little.
3.2.6 Tidal Energy
Due to the gravitational pull of the moon and the earth, the sea water level shows changes. Their level is often upto 1 metre and cause tides. Sometimes in coastal regions they reach a height of 20 metres also causing a havoc.
Tidal energy has been vigorously investigated as a source of power for generating electricity in some countries like France, Germany, U.K, Canada and USA and a tidal power plant on the French coast and another in U.K.
have been commissioned for commercial power production.
In India, the tidal power potential is quite small less than 1000 MW. Some of the major sites under investigation are: Bhavnagar, Kutch, Diamond Harbour and Ganga Sagar.
3.2.7 Ocean Thermal Energy
Ocean can store a huge amount of thermal energy since it has a large water surface area to receive solar radiation. Due to continuous heating of the upper surface of the ocean by solar radiation, the solar energy is converted into thermal energy and it is stored in the water. But different layers of the ocean water will have different temperatures. This difference in temperature of any two layers of water can be exploited to trap thermal energy using heat exchangers. The amount of heat that can be extracted from ocean water depends on the temperatures of the water layers and the efficiency of the heat exchanger. Heat contained between any two layers of ocean water can be expressed mathematically as
qw = mw Cw (T2 – T1) …(3.3)
where, mw = Mass of water between two layers, kg.
Cw = Specific heat of water, J/kg°C, T₂ = Temperature of upper layer, °C, and T₁ = Temperature of lower layer, °C.
This ocean thermal energy can be utilized to vaporize some organic fluids having low boiling point using heat exchangers and then this vapour at high pressure can drive a turbine to produce mechanical power and subsequently electrical power when the same turbine is connected to a generator. The whole system works on a thermodynamics cycle called 'Binary Vapour Cycle'. R & D work is going on to harness ocean thermal energy be used for different purposes.
3.2.8 Magnetohydrodynamics (MHD)
Magnetohydrodynamics is a direct energy conversion process. It is an
important mode of non-conventional energy generation in which great deal of development work has been done in several countries.
MHD power generation works on the principle that when a hot, partially ionised and compressed gas is expanded in a duct, and forced through a strong magnetic field, electrical potential is generated in the gas. Electrode placed on the sides of the duct pick up potential generated in the gas. High temperatures in excess of 2780°C are needed to produce necessary ionisation so that it has good electrical conductivity.
The conducting gas can be obtained by burning a fuel and injecting seed material such as potassium in the product of combustion. Seeding the gas helps in ionization and reduces temperature requirements somewhat. The exhaust from MHD generator is at a temperature of about 2200 C and can be used as the heating medium for steam raising in a conventional boiler, thus suggesting use of a combined cycle. It is believed that the present efficiency of about 38% of the thermal power plants can be raised to over 50% with addition of MHD generation in the conventional plants. A MHD generator of 6 m long with magnetic field intensity of 100 kilogauss may produce about 360,000 kW of d. c. power at 5000 volts.
Though the technological feasibility of MHD generation has been established, its economic feasibility is yet to be demonstrated. India had started research and development project in collaboration with Russia and installed a pilot MHD plant based on coal and generating 2 MW power in Trichy, to study its feasibility. In Russia, a 25 MW MHD Plant which uses natural gas as a fuel has been in operation for some years.
SAQ 1
(i) Mention the different methods by which solar energy can be collected.
(ii) Name the instruments used for solar energy measurement.
(iii) Name the instrument used for wind speed measurement.
(iv) Mention the suitable locations in India where wind energy can be exploited.
(v) What is the source of geothermal energy?
(vi) What is the difference between tidal energy and ocean thermal energy?
(vii) Mention two main technologies by which bio-energy can be harnessed.
(viii) What is the scope of power generation using magnetohydrodynamics principle in India?
3.3PHOTOVOLTAIC SYSTEMS
A photovoltaic system is used to generate power by converting solar energy directly into electricity by means of a device called photovoltaic cell or solar cell.
Solar Cell
It has a sandwich construction consisting of a metal base plate, a semi-conductor material and a thin transparent metallic layer shown in Figure 3.3.
The transparent layer may be in the form of a sprayed, conduct semi- conductor materials are used for the solar cell. They are CU2S, PbS, Pb Tc, GaAs etc. When the light strikes the barrier between the transparent metal layer and the semi-conductor material, a voltage is generated as shown.
There are four basic processes taking place in a solar cell. These are:
(i) carrier (electron-hole pair) generation by optical absorption.
(ii) carrier recombination,
(iii) carrier separation by build-in field, and
(iv) carrier collection to generate photovoltage and photo current.
Components of the Photovoltaic System
A photovoltaic system has mainly three components as shown in Figure 3.4. These are:
(i) Solar Cell Panel
The solar cells are combined in a series or parallel manner in module to generate higher voltage or current. These modules are then
connected form a panel to generate the required power. The complete system with solar cell panel and supporting structure is called an array.
(ii) Batteries
These are mainly used for storage of power generated by the solar cell panel. Batteries are particularly important for night applications.
(iii) Solar Tracking System
The function of the tracking system is to focus the solar panel directly toward sun to receive maximum intensity of solar radiation. The tracking systems may be automatic or manually operated.
Efficiency of a Solar Cell
The efficiency, ηc, of a solar cell is defined as the ratio of the output power to the input power. Thus
ηc = …(3.4) The energy produced, Sp (I), per unit of active area of photovoltaic system, Ap, is
Sp (I) = ηc . RB (I) for cell temperature Tc≤ 25 °C
= [ηc – 0.005 (Tc – 25)] RB (I) Ap for Tc≥ 25 °C …(3.5) where, RB (I) = monthly average global radiation for Ith period, W/m2. Eq. (3.5) shows that at higher temperatures, the output of a solar cell goes down.
Example 3.1
On a particular day a solar cell having a surface area of 30 cm2, produces a power of 0.216 Watt with the intensity of solar radiation of 600 W/m². Find the efficiency of the solar cell.
Solution
The efficiency of solar cell (Eq. 3.4) is given by ηc =
Input power = Intensity of solar radiation × Area of the cell = 600 × 30 × 10-4
= 1.8 watts
Output power = 0.216 watt (given) ηc = .
.
= 0.12 = 12 %
The dream of converting solar radiation directly into electricity for small and large scale applications is increasingly becoming reality. This has been brought about by an enormous improvement, from both cost and performance point of view, in photovoltaic generation systems. Several such systems, with power generation capacity ranging from a few watt to megawatt, for a diverse range of applications, are operational in different parts of the world.
The main applications of photovoltaic systems to be discussed in the following sections are:
(i) Power generation, (ii) Street lighting, and
(iii) Water pumping for irrigation.
3.3.1 Photovoltaic System for Power Generation
Photovoltaic power plants are generally stand - alone systems and cater to the energy requirements of isolated villages. Photovoltaic power plants can be used for supplying power to D. C. load or A. C. load.
Power Packs for D. C. Loads
These power packs are widely used for powering various standard D.
C. operated loads such as T. V, radio, telephone sets for
communication. Many other D. C. loads like aviation obstruction warning lamps, very low power transmitter of Doordarshan, cathodic protection for oil pipe lines, telemetry and gas detection equipment for ONGC off-shore platforms. Vaccine refrigerators used for
immunization, communication equipment, etc. are powered through similar power packs.
Photovoltaic power plant for D. C. loads consists of a photovoltaic array, a voltage regulator and a battery as shown in Figure 3.5.
Power Packs for A. C. Loads
Inverter is the essential part of the photovoltaic power plants for powering the A.C. operated loads as shown in Figure 3.6. Its design depends on the type of the load special used. An ordinary invertorscan be used to power normal lights and fans, however special invetors are required for driving induction loads such mortor-pump sets used. Till recently, such invertors were not available in the country. With the advent of such invertors photovoltaic power packs can be used for powering petrol pumps, conventional vaccine refrigerators, or any other loads driven through A. C. induction motors.
3.3.2 Photovoltaic System for Lighting
Photovoltaic lighting systems can cater to both indoor and outdoor
illumination requirements. The most popular indoor systems are with one or
two photovoltaic modules, one to four 9 W compact fluorescent lamps, one battery, one charge controller and other accessories.
The lamps can be operated for 3 to 6 consecutive sunless days operation.
These systems are most suited for all domestic lighting requirements in rural areas as also for community sunless days operation.
The second category of lighting system is the portable photovoltaic lanterns which consists of a lantern assembly (with 8/7 W compact fluorescent lamp, a battery and electronics housed in a lantern - like enclosure) and a
photovoltaic module. The lantern can be operated for about 3 hours/day. The module can be connected to the lantern assembly for charging during day and can be detached during night. The lantern can be carried to the place of use.
The third category of lighting system makes use of photovoltaic system for street lighting for remote villages. A 20W, 12V (or 24 V) tubelight is used for these applications. The tubelight is mounted on a stand alone pole, alongwith two modules and a battery. A lighting sensing device switches the tube light on in the night and after a pre - determined time (3 to 9 hours), it is switched off by a timer. Thousands of such systems have been installed in different villages all over the country.
3.3.3 Photovoltaic System for Water Pumping
It has three different models depending on suction head and water requirements:
Model I
This system consists of a D. C. motor centrifugal pump (mono block) set run by 300 W solar cell panel. The peak water output of the pump is 1.5 to 2.0 litres/sec. and average water output is 30,000 to 40,000 litres/day at 5 metres total head on a clear sunny day.
Model II
It consists of an photovoltaic array of 600 W and a permanent magnet D. C. motor pump set suitable for suction head of upto 7 metres. It delivers about 75,000 litres of water against a total head of 6 metres and 50,000 litres against a total head of 10 metres on a clear sunny day.
Model III
It consists of a photovoltaic array of 900 W and a permanent magnet D. C. motor water against a total head of 6 metres and 75,000 litres against a total head of pump set suitable for suction head upto 7 metres . It delivers about 110,000 litres of 10 metres on sunny day.
The rated water output in all these models in achieved by positioning photovoltaic panels thrice daily to face the sun optimally.
Example 3.2
A photovoltaic array of 0.6 m² is designed to pump 10 m3 of water daily from a depth of 30 m. If the daily radiation is 0.6 kWh/m2, determine the system's efficiency.
Solution
We will use Eq. (3.4) for calculating daily efficiency.
ηc =
= × , × .
× × × .
= 0.227 or 22.7 %
3.4SOLAR THERMAL SYSTEMS
A simple method to use solar energy is to beat water or air with the help of flat plate collector. The temperature of energy collection, for actual use, by this method is limited to about 85 °C, thereby limiting the scope of their applicability.
Solar energy collection-high temperature can be achieved with the help of a concentrating solar collector which focusses the solar energy collected over a large area to a smaller area where it is absorbed.
There are wide applications of solar thermal systems. These systems are used to convert solar energy into thermal energy. Some of the important solar thermal systems which can be used to produce hot water/steam; for cooking and drying crops or to generate portable drinking water will be discussed in the following sections.
3.4.1 Solar Thermal Systems to Produce Hot Water
Amongst various thermal applications of solar energy, solar water heaters have only commercial application which is economically viable. To produce hot water at low temperature range (upto 85 °C) following solar water heaters are used.
(i) Collection-cum-storage solar water heater.
(ii) Natural circulation type solar water heater, and (iii) Forced circulation type solar water heater.
(i) Collection - cum - storage Solar Water Heater
It has both collection and storage provision in the same unit and therefore it eliminates the requirement of a separate insulated tank for storage of hot water. Owing to this feature, this type of solar water heater is attractive for domestic uses. Different designs of collector - cum - storage solar water heaters have been investigated by several scientists. Out of these the built-in storage and the shallow pond water heaters are well known designs. The design and the working principle of each of them are discussed below.
Built - in - Storage Solar Water Heater
This type of solar water heater is compact in design and it is characterised by advantages like low cost, easy installation, good collection efficiency and satisfactory over night thermal storage. A schematic diagram of a built-in-storage water heater is shown in Figure 3.7. It consists of a rectangular box covered with 5 cm thick insulation around the bottom and sides.
The top surface of the box is covered with one or two glass (called glazing) keeping air gap of nearly 1 cm, between the absorbing surface and glass cover. The box is filled with water in the morning, the water heats up throughout day and
withdrawn for use in the evening.
This system works on the principle that the glass cover works as a glazing that allows short wave solar radiations to pass through it and not allow to thermal energy (long wave radiations) to come out through it. So, solar energy
converted into thermal energy is trapped inside the box and stored in the water Insulation is used to reduce heat losses from the box.
Shallow Solar Water Heater
It is a simple and cost effective device for heating water using solar energy.
It consists of a blackened tray having some water within it, the depth being very small typically only a few centimeters.
A transparent plastic film covers the water in such a way that the film coming in contact with the top surface of the water. A number of designs has been proposed for the shallow solar pond heater. The simplest of them consists of essentially a large plastic made water pillow with black bottom film and clear upper film. The water depth in the bag ranges between 4 to 15 cm.
The whole assembly is placed in an enclosure. The typical peak temperature achieved by a shallow solar pond water heater between 60°C in summer and 40°C in winter.
A typical shallow solar pond type water heater is shown in Figure 3.8.
A solar assisted swimming pool works on the same principle of above solar water heater.
(ii) Natural Circulation Type Solar Water Heater
A solar water heater operating under thermosyphon flow is referred to as thermosyphon type or natural circulation type solar water heater. It is used for domestic purposes as well as for remote places where electricity is beyond the reach of the people. A natural circulation type solar water heater basically consists of separate heat collector, i.e., flat plate collector and a storage tank units integrated by means of
insulated connecting pipes. The storage tank is placed at a place above the level of collector. As the water of the collector is heated by solar energy its density goes down and it flows naturally to the top of the storage tank and its place is occupied by high density cold water from the bottom of the storage tank. Thus the circulation of water takes place automatically until the temperature of collector water is equal to that of the storage tank. Whenever, hot water from the tank is
removed for use then cold waterautomatically enters at the bottom.
For this system, hot water in the temperature range of 50 to 60°C is obtained. A schematic diagram of a natural circulation type solar water heater is shown in Figure 3.9.
Collector Performance
The useful gain, Qu, from a solar collector may be written as Qu = m Cp (To – Ti) …(3.6) where, m = Fluid (water) flow rate, kg/s.
Cp, = Fluid specific heat, J/kg.°C, To = Outlet fluid temperature, °C, and Ti = Inlet fluid temperature, °C.
Under steady state conditions, the useful heat delivered by a solar collector is equal to the energy absorbed in the metal surface minus the heat loss from the surface to the surroundings. Thus
Qu = Ac [ I(𝜏𝛼) – UL (𝑡 – Ta) ] …(3.7) which can be written as
Qu = FR Ac [ I(𝜏𝛼) – UL (Ti – Ta) ] …(3.8) where, AC = Total collector area, m² ,
I= Solar energy received on the upper surface of the collector, W/m²,
τ = Transmissivity of glazing,
α = Absorptivity of the absorber surface,
UL= Overall heat loss coefficient of the collector, W/m², 𝑡̅p = Average plate temperature, °C, and
FR = Heat removal factor, having a value less than 1.0.
The thermal efficiency, 𝜂 ,of a solar collector is then given by 𝜂 =
Thus dividing Eq. (3.8) by I, we get
𝜂 = FR (𝜏𝛼) − 𝑈 …(3.9)
Eq.(3.9) is known as Hottel-Whillier-Bliss equation, named after three pioneers in the field of solar engineering.
Example 3.3
Determine the thermal efficiency of a solar collector having following parameters.
FR = 0.84, τα = 0.84, UL = 4 W/m².°C, I= 800 W/m² and Ti = 30°C, and Ta = 25 °C.
Solution
The thermal efficiency is given by T = FR (𝜏𝛼) − ( ) = 0.84 0.84 − ( ) = 0.6846% or 64.46%
(iii) Forced Circulation Type Solar Water Heater
This type of solar water heater is used when a large amount of hot water is required. Here, water circulations is made by means of a pump and instead of one collector, many collectors are connected in series as per the requirement of the amount of water. Main advantage of this type of system is that it is free from restriction of placing the storage tank above the collector. But this system requires electricity for the pump. A typical forced circulation type solar water heater is shown in Figure 3.10.
3.4.2 Solar Thermal Systems for Generation of Steam
For generation of steam solar thermal systems make use of concentrators which collect solar radiations at very high temperatures. There are various types of solar concentrator which can be used in solar thermal systems to generate steam. Some of such solar concentrators are - Dish type
concentrator, parabolic concentrator, Fresnel lens, Linear Fresnel reflector, parabolic trough, segmented mirror, Hemi - spherical bowl, etc. A typical solar steam generating system using linear solar concentrator is shown in Figure 3.11.
It consists of a parabolic trough, a cylindrical receiver placed at the focal plane of the collector, a steam collector and a pump for circulating water.
Solar concentrators are the collectors which are designed to condense the large amount of solar radiation upon a relatively small absorber area. Not only high temperature of energy collection is possible by using solar collector but heat loss from the absorber is also reduced considerably due to smaller area of the receiver. Since, a solar concentrator is anoptical system, it is necessary that it faces the sun at all times in order to receive maximum solar flux on the absorber surface. A solar tracking system can help to obtain this result.
3.4.3 Solar Thermal System for Cooking
Solar cooker is the most popular among all solar devices developed due to its simplicity of handling, operation and use. Box type solar cookers are
generally used for cooking various types of foods. Wide scale application of solar cooker can help in substantial fuel saving.
A box type solar cooker consists of a rectangular wooden box. It is painted black. It is covered with thick insulation around the bottom and sides. The top surface is covered with a glazing i.e., glass and another glazing (reflector) is placed making an appropriate angle to receive solar radiations and to direct them to the bottom of the cooker as shown in t Figure 3.12. Small boxes, painted black and containing food to be cooked, are placed inside cooker. The whole system is kept in a open space to receive clear sunshine. Frequent shifting and adjustment of glazing are required to track the system so that it always focussed towards sun to receive maximum intensity of solar radiation.
3.4.4 Solar Thermal System for Drying
Drying is one of the most important steps of post - harvest handling of crops.
The traditional method of drying employed in developing countries are open air sun drying or natural drying (in shade). Since, there is a little control over the drying rates in either method, the dried product is very often underdried or overdried. Underdrying results in deterioration of food due to fungi or bacteria, whereas overdrying may result in hardening followed by bursting and spoilage of the food. In addition to this, the quality of the produce is deteriorated due to dust and strong winds etc.
Solar driers are the most viable option for most of the developing countries, especially those within the belt of good solar radiations. Experimental results for the development of solar driers indicate large quantities of grain can be successfully dried to produce high quality products at nearly competitive cost.
Solar energy driers usually employ higher air flow rates at a low temperature over a long drying period, in comparison of fossil fuel fired systems which use high temperatures and low air flow rates for rapid drying.
Some of the useful solar driers are as follows:
(i) Solar Cabinet Drier
This type of drier consists of a small wooden box, with its length about 3 times of the width, to minimize the shading effect of the side panels. The box is glazed at the top and provide with insulation at the bottom and the sides. The interior surfaces of the box are painted black to absorb solar radiation transmitted through the transparent cover. This causes a rise in temperature of air in the box. The product to be dried is placed in simple trays having perforation / wire mesh at the bottom. The trays are then placed in the box. Holes are provided at the base and the top of the rear vertical side (a few cm below the cover) to permit air movement by natural convection.
(ii) Glass Roof Solar Drier
This type of drier is identical to a greenhouse structure and its working is based on the same principle of cabinet drier. The drying unit consists of two parallel rows of drying platforms with slanted glass roofs, aligned lengthwise along a north-south axis. The inside surfaces of the drier are painted black and openings are provided on the eastern and western walls, above and below level of the platforms containing the product. Airpasses through the wire mesh of the platform is heated and picks up the moisture from the product spread over it. Such driers have mainly been used to dry cocoa.
3.4.5 Solar Still for Production of Potable Drinking Water
Solar stills are also called solar distillators. One of the main advantages of distillation process is that it requires heat only upto 120°C which can be supplied from solar energy of other cheap fuels.
As a result of large interest in solar distillation, several types of solar stills are evolved. These are single effect solar basin stills, inclined or stepped stills, multi - flash distillation stills, single or multiple Wick stills, solar flim covered stills, etc. Out of these, only the basin stills using single effect distillation have been used for the supply of large quantities of water for isolated communities or small supplies of water such as for battery charging etc.
The distillated output from the solar still depends on many parameters like climate parameters such as solar insolation, ambient air temperature, wind speed, atmospheric humidity, sky condition, etc. and design parameters such as thermo - physical properties of the material used in its construction, orientation of the still, tilt angle of the cover, spacing between cover and water surface, insulation of the base, depth of the water etc.
A basic type solar still is shown in Figure 3.13. It consists of tray painted black. Its upper surface is covered with a glazing at tilted position. It has a channel to receive distillate. The solar still is filled with impure water for distillation.
In operation, solar radiation, after transmission through the transparent pane is absorbed in the water and basin and therefore, water temperature becomes high compared to the cover. The water losses heat by evaporation, convection and radiation to the cover and by conduction through the base. The distillate is collected in the channel.
The efficiency of evaporation of still is given as ηa = Qe / HT... (3.10)
where, Qe = Amount of heat used by solar still ( in W / m² ) for evaporation, and
HT = Intensity of solar radiation in horizontal surface in (W / m²).
Example 3.4
A basic solar still having evaporation efficiency of 70 % is used to produce potable drinking water on clear sunny day with intensity of
solar radiation (average) of 800 W / m². The latent heat of evaporation of water is 2.43 × 106 J / kg.
It is required to find the amount of heat used for evaporation of water and the amount of distillate produced.
Solution (i) 0.70 =
→ 𝑄 = 560 W/m2 (ii) me = =
. × kg /m2 sec.
Let total sunshine hours/day = 8 Hence, me= × ×
. ×
= 6.63 kg / m2 day SAQ 2
(a) Describe the working principle of a solar cell.
(b) State the function of each part of a photovoltaic system.
(c) What is the difference between natural circulation type and forced circulation type solar water heaters?
(d) What is the function of a solar concentrator?
(e) Explain the working principle of a solar cooker.
(f) What is the difference between a solar pond and solar still?
(g) What are the advantages of solar driers?
3.5WIND ENERGY SYSTEMS
Wind as a power source can be utilized for various purposes like generation of electrical power, pumping water and grinding crops. Two important systems which make use of wind power for our discussion will be:
(i) Wind electric power generating system, and (ii) Wind energy system for water pumping.
3.5.1 Wind Electric Power Generating System
Wind electric power generating systems have been given due importance for the last many years. Since wind source is non-exhaustable and non-polluting, so the systems making use of can help in not only to conserve fossil fuels, etc.
but also to control environmental pollution.
For generation of electrical power, savonius type rotor is generally used.
A typical wind electric power generating system is shown in Figure 3.14. It consists of mainly a horizontal axis rotor, a gearing arrangement, generator and a tower.
Using aerodynamic power balance equation, the power extracted by rotor from the wind is given by
Pr = 2 π r2 a(1 – a)2𝑉 …(3.12) where, r = Radius of rotor blade,
a = Axial interference factor, and Vf = Wind speed.
The electrical power generated by the system, is given by Pe = Pr . ηtηg
= η ηg 2 π r2 a (1 – a)2𝑉 …(3.13) 3.5.2Wind Energy System for Lifting
One of the important applications of wind energy system is to lift water from wells. In this system the power of wind is converted to mechanical energy.
This type of system are widely used in the countries like, Egypt, Mexico, Indonesia, etc.
A typical wind mill used for lifting water for irrigation is shown in Figure 3.15.
It consist of rotor, a crank-shaft arrangement and a pump, etc.
The capacity of wind mill to lift water depends on the velocity of wind, water level of source etc.
3.6BIO-ENERGY SYSTEMS
There are various methods of extracting energy from biomass. The biomass is an important resource of obtaining energy in solid, liquid and gaseous forms.
Some of the important methods of obtaining valuable product from biomass and the systems used for them will be discussed in the following sections.
These are:
(i) Anaerobic digestion of biomass for obtaining cooking gas, (ii) Gasification of biomass using indirect combustion process to
obtain various useful gaseous products, and
(iii) Liquefication of biomass by using pyrolysis method.
3.6.1 Biogas Generating System
Anaerobic digestion of organic wastes to generate biogas has attracted much in the recent years due to depleting reserve of fossil fuels. A biogas unit helps in eliminating the age - old practice of burning cow dung, agricultural wastes etc. Biogas production is microbial process. The process is called anaerobic digestion.
For generation of biogas, feed stock like - animal and human wastes, such as cowdung. urine, poultry dropping free from litter, horse dung , stock excreta , night soil, etc. are used.
There are three tested and field - worthy designs of biogas units. The names of these models are:
(i) Floating gas holder type (Gobar gas plant), (ii) Fixed dome type (Janata biogas plant), and (iii) Ganesh model.
A typical fixed dome type biogas plant is shown in Figure 3.16. It consists of mainly a mixing tank to prepare the slurry of waste materials, a digester and piping system to remove biogas.
Biogas normally contains by volume 50-70 % of methane, 25-50 % of carbon dioxide with small amount of other gases such as hydrogen sulphide and carbon monoxide.
There are several applications of biogas. It is an ideal gas for cooking, and lighting. A biogas engine can be used to drive pumps for irrigation. It is an excellent and economic fuel for petrol and diesel engines.
Manure obtained as by - product from the biogas plant can be used for improving crops production.
3.6.2 Biomass Gasification System
Biomass gasification systems make use of thermo - chemical process for the gasification of various types of biomass. The nature of the products obtained in this process depends on the temperature, supply of steam and air. The gasification is done in the temperatures ranges of 600 ° C to 1000 °C. Mainly methane , carbon dioxide , hydrogen etc. are produced . A typical biomass gasification systems is shown in Figure 3.17. It consists of mainly a gasifier, heat exchanger, wet scrubber, filter, etc.
The product gas mixture from the biomass gasification system can be used for generating process steam in industries, and electricity in gas turbine power plant, etc. With the available technology, one can meet rural energy needs by a combination of biomass, biogas and photovoltaic generating systems.
3.6.3 Alcoholic Fermentation System
Alcohol is an important liquid fuel. It can be obtained by fermentation of different types of biomass. But only molasses is used now - a - days for production of ethyl alcohol (C2H5OH). Bacteria acts as an catalyst for alcohol production. For example production of ethyl alcohol yeast is used. The different steps of obtaining ethyl alcohol is given below
Biomass ⎯⎯⎯⎯⎯⎯⎯⎯ Cellulose ⎯⎯⎯⎯ Glucose ⎯⎯⎯⎯ Ethyl alcohol (Yeast)
3.7GEOTHERMAL ENERGY SYSTEMS
The various ways of using geothermal energy and the systems that make use of it are given below:
(i) Dry Steam Systems
Dry steam which can be extracted from geothermal reservoir or which comes out directly may have temperature up to 250°C and pressure of 35 bars. It can be used to run a turbine for producing electricity.
(ii) Wet Steam Systems
Geothermal reservoir which can remain under pressure can emit a water steam mixture with temperatures between 180°C - 370°C. After separation from water steam can be used to generate electricity or process steam heat applications and the remaining water can be used for heating and air - conditioning.
(iii) Hot Water Resources
They contain hot water under normal pressure and temperature between 50 - 80 °C. This can be used to boil liquids like freon and butene through a heat exchanger and can be used to generate electricity.
(iv) Hot Rock System
When temperature gradient is above average in which the water or water - vapour is available , the heat can be extracted directly by boiling and letting water flow through hot rock region for further applications as in dry steam and wet steam systems.
SAQ 3
(a) What are the applications of wind energy systems?
(b) What are the valuable products that can be extracted from biomass?
(c) Explain the role of biogas generating system for controlling environmental pollution.
(d) Show the steps by which ethyl alcohol can be obtained from biomass.
(e) What will be the expected useful applications of geothermal energy systems in future?
3.8SUMMARY
Non - conventional energy resources are environment friendly.
Amongst various non - conventional energy resources, solar energy appears to be most promising.
Solar energy can be harnessed for the following purposes:
-- To generate electricity using photovoltaic systems. Again, photovoltaic systems can be used as power packs in rural and remote place to supply power to D. C. and A. C. loads; pumping water for irrigation and street lighting etc.
-- To produce thermal power for producing hot water / steam / drinking water, cooking foods, drying crops,
Wind energy can be utilized for generating electrical power and lifting water for irrigation.
Energy can be extracted from the biomass to generate valuable products like - solid, liquid and gaseous fuels and chemicals.
Geothermal energy is yet to be exploited effectively to produce steam, electricity etc.
3.9KEY WORDS
Non - conventional Source:It is the energy source which can be revived during the life span of human being.
Photovoltaic System: It is used to convert solar energy directly into electrical energy.
Solar Thermal System: It is used to convert solar energy into thermal energy.
Bio - energy System: It is a system used for extraction of energy from biomass.
Wind Energy System: It is used to harness wind power.
Solar Concentrator: It is used to collect solar radiation at high temperature.
Solar Radiation: It is the radiation received from the sun.
Geothermal Energy: It is the heat content in the inner core of the earth.
Biogas: It is the gaseous fuel generated by anaerobic digestion of organic materials.
3.10ANSWERS TO SAQ
s SAQ 1(a) (i) Heating of water using flat - plate collector
(ii) Generation of hot water (at high temperature) and steam (iii) Solar cooking, drying etc.
(iv) Photovoltaic power generation.
(b) (i) Pyranometer (ii) Pyraheliometer.
(c) Anemometer.
(d) Coastal and desert areas of India.
(e) Heat content in the interior core of earth is the source of geothermal energy.
(f) Tidal energy is the Kinetic energy offered by the waves of water during their movement in tides, whereas ocean thermal energy refers to the thermal energy stored by the different layers of ocean water at different temperatures.
(g) (i) Chemical conversion technology (ii) Biological conversion technology.
(h) The scope of power generation in India, using magnetohydrodynamics principle is limited.
SAQ 2
(a) A solar cell works on the principle that when light energy strikes the junction of the metal plate and semi - conductor, electrons and holes are generated. These carriers after the processes of recombination and separation, procedures photo - current and photo - voltage.
(b) (i) Solar Cell: It converts solar energy into electrical energy.
(ii) Battery: It is used to store the power generated by solar cell.
(iii) Solar Tracking System: To move the panel with the position of the sun to receive maximum intensity of solar radiation.
(c) Natural circulation type solar water heater makes use of syphonic principle to circulate water, whereas a pump is used to circulate water in forced circulation type solar water heater.
(d) A solar concentrator is used to collect solar radiation at high temperature.
(e) Solar cooker works on the principle that the solar energy transmitted through the glazing is absorbed by the inner surfaces of it and
converted into thermal energy. This thermal energy is utilized to cook the food.
(f) A solar pond is used to collect solar energy and to store thermal energy whereas solar still is used to purify dirty water to produce potable water by evaporation method.
(g) A large quantity of grains can be successfully dried to produce high quality products at nearly competitive cost. It also employs higher air flow rate at low temperature for drying.
SAQ 3
(a) Wind energy systems can be applied for (i) generating electrical power
(ii) pumping water, and (iii) grinding grains.
(b) (i) Solid Product: Charcoal, Briquettee
(ii) Liquid Fuels and Chemicals: Ethyl alcohol , oils etc.
(iii) Gaseous Product: Biogas, etc.
(c) Biogas generating system makes use of pollution making waste materials. These waste materials are converted into biogas in the digester. Biogas does not cause pollution when it is burnt.
(d) Biomass ⎯⎯⎯⎯⎯⎯⎯⎯ Cellulose Cellulose ⎯⎯⎯⎯ Glucose Glucose ⎯⎯⎯⎯ Ethyl alcohol
(e) Efficient system that can trap geothermal energy can be used for the generation of process steam in future.
