Introduction to Energy and Solar Photovoltaic Energy
In this modern world Energy has become an integral part of our daily life. One cannot think of living a single day without the use of energy in one form or other. We use energy in cooking our food, cooling our spaces, travelling from one point to other, transportation of goods, watching TV, using our mobile phones, running machines in an industry, water pumping, and so on.
The energy we use must come from somewhere. Normally, the energy we use is supplied to us in the form of diesel, petrol, coal, LPG, CNG and electricity (mostly derived from other fuels like coal and petroleum). These sources of energy are finite in nature and cause environmental pollution. In India, every citizen does not get sufficient amount of energy that he / she requires. There is a huge shortage of energy supply. There are 5,90,000 villages in India and 700 million people live in rural India. Most households in rural India do not get sufficient electricity, which hinders the growth of rural India both at social and economic front. There is either lack of sufficient infrastructure to supply energy to all or sufficient fuel is not available at reasonable cost. Therefore, there are efforts to use infinite or renewable energy sources such as solar radiation, wind and biomass energy. These energy sources are also available in distributed manner which means that the required energy can be generated where there is a need.
Solar photovoltaic (PV ) technology converts sunlight into electricity directly without any other additional energy conversion step. India is blessed with a large amount of sunlight. We receive solar radiation in a range of 4 to 7 kWh/m2/day. Such amount of radiation is good enough to generate electricity to fulfill our entire electricity requirement using solar PV technology. Importantly, the energy can be generated in any area, where there is need, by installing the solar PV modules. Considering the importance of the solar PV technology in needful energy supply, this training manual is focused on solar PV technology only.
In this chapter, idea of ‘energy’ is explored in detail which includes various forms, energy units and their conversion. An estimation of energy need is given in the following section. Practical examples are given for estimation of energy is given. At the end brief discussion is presented about other renewable energy technology including solar thermal, wind and biomass energy technologies.
2.1 Basic Concept about Energy and Its Use.
In our daily life we use energy for many activities throughout the day. It has become an integral part of our daily life, and it is difficult to even think of a day without consuming any energy. Due to this reason we should have good idea about; what is energy ? From where it comes ? What are the different forms of energy ? Etc. This section deals with basic concepts about energy and related to its use.
2.1.1 What is Energy ?
Energy is a concept, which in simple term, can be described as “ an ability of an object to do work”. Work is done by our body when we move, work is done by fan when it runs, work is done by a vehicle when it moves, work is done by stove when it heats water, work is done by a horse when it carries a person, work is done by a bulb when it illuminates a room and so on. Thus, all the objects which have ability to work are said to process energy and by doing work the objects transfer energy. For instance, a piece of wood can be burned, which can boil water and can generate steam. The generated steam can be used to move an object (as in steam engine). It implies that the piece of wood posses some energy.
We use energy to get work done. There are always numerous example around us wherein we get the work done by using energy. Use of light bulb, heating of water, cooking of food, driving a vehicle, running a fan, etc. are examples of the use of energy. The use of energy to get our work done has become one of the necessity of our modern life. Without using energy, it is difficult to live even a single day. Energy in the form of the food is a fundamental need of a living organism. We need energy for our body to maintain it, to do the physical work and to do the mental thinking. The food, our energy source is provided to us by our mother nature. But apart from the food energy, we use energy to get several other works done. Some of these works are required for running our lives, for example cooking of food while many other works are required to provide comfort to our body such as the use of air conditioner, cooler, etc. Overall, energy is required for a wide range of applications like transportation, industrial application, agricultural application, household requirements, office applications etc.
2.1.2 Forms of Energy
The energy can have many forms like heat energy, electrical energy, light energy etc. Heat energy is used for cooking, drying and heating applications. Electrical energy is used for cooking, drying and heating applications. Electrical energy is used for running fan, TV, cooler, water pumps etc. The light energy is used for illumination of our rooms. The other forms of energy include chemical energy, nuclear energy, radiation energy ( for example solar energy), gravitational energy, kinetic energy, potential energy, etc. Each of these energy forms are used either directly to get our work done or we convert energy from one form to other form before utilizing it to get work done. For instance, when we drive motor vehicle, chemical energy of the petrol (or any fuel) is converted into mechanical energy to provide air flow. Thus energy gets converted from one form to other and in this process of energy conversion we get work done of our choice.
In pre-industrial era, fuel wood was the major source of energy. After the discovery of steam engine, coal has become a choice of energy source. The discovery of internal combustion engine (used in motor cycle and cars)resulted in the use of energy of the petroleum products (petrol, diesel, natural gas) to fulfill our energy requirements. The energy of the fossil fuels like coal, oil and gas are used directly for heat by burning them. The fossil fuels are also get converted into electricity in power plants and then electricity is used to get our work done in industries, agriculture and at houses.
Electrical energy is one of the most convenient forms of energy. Almost all equipments around us can work on electrical energy. Running of fan, TV, computers, bulbs, trains, machines in industry, water pumps, etc. are the examples of the use of electrical energy. Also, cooking and heating can be done using electrical energy and even cars can run on electrical energy. Thus, electrical energy is very versatile and commonly used form in our daily life. This versatility comes from the fact that the flow of electrical energy can easily be controlled, many times just by switching a simple ‘on’ and ‘off’. Also, the transmission and distribution of electrical energy is simple. The transmission and distribution of other forms of energy like coal, petrol, wood, etc. is quite cumbersome.
2.1.3 Renewable Energy and Non-renewable Energy Sources
The energy sources can be divided in two broad categories ; Renewable and Non-renewable energy sources. Both of them are derived from the nature but they are different from the perspective of availability.
The natural energy sources, such as coal, petroleum, oil and natural gas take thousands of years to form naturally, meaning their rate of production is low. In the present world, the rate of consumption of these resources is quite high as compared to their production. These fuels cannot be produced as fast as they are being consumed. Therefore, in practical terms, we can assume that the fossil fuels are available in limited amount and continues use of these fuels will result in their depletion from the earth. Thus, due to limited availability, the fossil fuels are considered non-renewable energy sources.
In contrast, the natural energy sources which are called renewable energy sources are continuously produced by natural processes and forces occurring in our environment. These renewable energy sources include solar radiation, wind , biomass, hydro, etc. These sources are available intermittently in cycles and can be harnessed during any number of cycles. For instance, solar radiation energy is available in cycle of 24 hours of day-night cycle. Any amount of solar energy can be harnessed without affecting the availability of solar energy for the next day, and therefore, it is termed renewable energy source. Similarly, wind energy (movement of wind) and hydro energy (movement of water) are renewable energy sources, can be harnessed in any amount and cannot be depleted. If the balance is made between consumption of biomass energy (plants energy) and growth of biomass then biomass energy can also be considered renewable energy.
2.1.4 Amount of Available Solar Radiation Energy
The sun is the main source of energy at the earth . The energy from the sun reaches to the earth in the form of electromagnetic radiation. On the earth, the solar radiation energy gets converted in various other forms of renewable energy. On reaching some of the radiation energy is reflected back, some energy gets absorbed in the atmosphere, some part reaches to the earth’s surface without any conversion, some part is converted into wind energy, some part is converted into biomass energy, and some part of energy is used in water evaporation causing rain and becomes available in the form of hydro energy.
The amount of energy that reaches to the earth is very large as compared to what we are using from fossil fuels. This can be seen from all possible sources including electricity, coal, gas, diesel, petrol, biomass, etc. was 580 Exa joule (1 Exa Joule = 1 000 000 000 000 000 000 joule) and the total electricity consumption was about 70 Exa joules. The availability of annual solar energy sources is 3,850,000 Exa jouls which is many thousand times more than what we are consuming annually. Thus, in principle, solar energy alone can fulfill all the energy requirements of the world if harvested in cost-effective manner. Solar photovoltaic technology is one such means of harvesting solar radiation energy and converting into electricity. Solar thermal technology harvests solar energy in the form of heat energy.
TABLE 2.1 Annual Available Renewable Energy and Annual World Energy Requirements
|
Annual available renewable energy (in Exa joules = 1018 J) |
|
|
Solar Energy |
3,850,000 |
|
Wind Energy |
2250 |
|
Biomass Energy |
3000 |
|
World’s annual energy consumption (in Exa joules) |
|
|
Total energy consumption (including biomass, coal, petrol, electricity etc.) |
580 |
|
Electricity Consumption |
70 |
2.1.5 Energy and Its Units
‘Energy’ as quantity can be represented in several units. One of the basic units of energy is called ‘joule’ and it is abbreviated as ‘J’. One joule of energy is equal to energy expended (or work done) in applying a force of one newton through a distance of one meter. In terms of electrical energy, one joule energy is equal to energy expended in 1 watt of power running for 1 second. One joule represents a very small amount of energy. For instance, energy consumed by a 100 watt bulb in one hour is 360000 joules. The energy content of the food that a normal person eats daily is about 10000 joules.
Other than joules, there are many other units of energy that we usually hear in our daily context. The other energy units normally represent higher amount of energy. For instance, energy content of food is given in terms of calorie and one calorie represents 4.182 joule of energy. Our monthly electricity bill is given in terms of number of electrical energy units consumed by us. One ‘electrical energy unit’ is equal to 1 kilo-watt-hour (or kWh) and 1 kWh represents 3,600,000 joules of energy. The energy content of a metric ton of crude oil is given in term of Tons of Oil equivalent (ToE).
Unit conversion factors
The different energy units are related with each other through different constants. Table 2.2 gives relationship between different energy units. Normally, in order to represent larger units, prefix are added to the unit. For instance, joule is a small amount of energy in term of joule only prefix ‘kilo’ which represents 1000 is added to make 1000 joules or 1 kJ. This is similar to write 1000 grams of weight as 1 kg. Similarly , prefix ‘mega’, which represents 1000,000 (or 1 million) is added to make it 1000000 joules or 1 MJ. Also, if we multiply 1 MJ with 1000, we will get 1000 MJ. In brief, 1000 MJ is written as 1 giga joule or 1 GJ. Here, giga represents 1,000,000,000 or 1000 million.
One can notice from the above discussion that 1 kJ is 1000 times larger energy than 1 J, 1 MJ is 1000 times larger energy than 1 MJ. This can be represented in the following way :
1 kJ = 1000 J
1 MJ = 100 kJ
1 GJ = 1000 MJ
This conversion factors are not used only in application to energy units, but they are also used in the application to other units like unit of power (watt or W). Applying these unit conversion factors to power unit we will get ; W, kW, MW, and GW.
Some commonly used prefix and conversion among them is given in Table 2.2.
Table 2.2 Commonly used Prefix, Their value and Symbols for Representing Large Values
|
Prefix |
Value of Prefix |
Alternate way of writing prefix |
Symbol |
Example |
|
Kilo |
1000 |
103 |
K |
1 kg = 1000 grams |
|
1 kJ = 1000 joule |
||||
|
Mega |
1,000,000 |
106 |
M |
1 MJ = 1,000,000 J |
|
1 MJ = 1,000 kJ |
||||
|
Giga |
1,000,000,000 |
109 |
G |
1 GJ = 1000 MJ |
|
1 GJ = 1,000,000,000 J |
||||
|
Tera |
1,000,000,000,000 |
1012 |
T |
1 GJ = 1,000,000,000,000 J |
|
Peta |
1,000,000,000,000,000 |
1015 |
P |
1 PJ = 1,000,000,000,000,000 J |
|
Exa |
1,000,000,000,000,000,000 |
1018 |
E |
1 EJ = 1,000,000,000,000,000,000 J |
Various Units of Electrical Energy
In this training manual, we are mainly concerned with electrical energy. One joule of electrical energy is equal to energy expended in 1 watt of power in duration of 1 second. From the above discussion, we can write the following expression for energy :
Energy (joule) = Power (watt) X Time (Second)
Or 1 J = 1 W X 1 s
Thus, energy in joule is obtained if we multiply power (in watt) by time (in second). Alternatively, power can be in kilowatt (kW) and time can be in hour (h). In this way, kilowatt (kW) X hour (h) also represent energy unit.
We know that
1 kW = 1000 watt
And 1 hour (h) = 3600 seconds
Thus, 1 kW X h = 1000 W X 3600 s = 3,600,000 Ws
From Eq. (2.1), we know that 1 Ws = 1 J
Therefore, 1 kWh = 3,600,000 Ws = 3,600,000 J = 3,600 kJ
Or, using prefix kilo for 1000, we can write as follows :
1 kWh = 3,600,000 J = 3,600 kJ
In this way, energy unit ‘J’ can be converted into kWh or vice versa. Both of these units are commonly used to represent electrical energy as well as solar radiation energy. These units and their conversion from one unit to other units are important and one should understand these units carefully and thoroughly.
The commonly used electrical energy units and their conversion for each other are presented in Table 2.3.
Table 2.3 Energy units and Their Conversion
|
Energy Unit |
Equivalent Energy unit |
|
1 Joule |
= 1 Ws (watt-second) |
|
1 Wh |
= 3,600 Ws |
|
= 3,600 J |
|
|
1 kWh (kilowatt-hour) |
= 3,600 kJ |
|
= 3,600,000 J |
|
|
1 kilo Joule (kJ) |
= 1,000 J |
|
1 Mega Joule (MJ) |
= 1,000,000 J |
|
1 Mega Joule (MJ) |
= 278 kWh |
|
1 Giga Joule (GJ) |
= 1000 MJ |
EXAMPLE 2.1 A 200-watt fan runs for 12 hours every day. How much electrical energy it consumes in one day ? Give your answer in kWh.
Solution It is given that the power of the fan = 200 watt
The number of hours of usage per day = 12 hour
Now, electrical energy can be obtained by multiplying watt by hours.
Therefore, Electrical energy = watt X hours = 200 X 12 = 2400 watt-hour or Wh and therefore, we should divide the answer by 1000 as the value of prefix ‘kilo’ is 1000.
Hence, electrical energy consumed = 2400/1000 = 2.4 kWh.
Thus, the answer is 2.4 kWh.
EXAMPLE 2.2 A household in Mumbai received the monthly electricity bill of 130 units (or 130 kWh). Calculate the electricity bill in terms of joules.
Solution Monthly bill of household is 130 units = 130 kWh
From Table 2.3, we can use the conversion between kWh and joules as follows :
1 kWh = 3,600,000 J
Therefore, 1300 kWh = 130 X 3,600,000 J = 468,000,000 J
WORKSHEET 2.1 : Fill the following table (Table 2.4) on energy units and their conversion from one unit to other unit :
Table 2.4 Energy Units and Their Conversion
|
1 MJ |
= …………………..kJ |
|
10 kWh |
= …………………..J |
|
1000 J |
= ………………… kWh |
|
1 kWh |
= …………………Wh |
|
……………kWh |
= 10,000 kJ |
|
1 MWh |
= …………………kWh |
|
……………kJ |
= …………………MJ |
|
……………kWh |
= 5000 Wh |
|
10 kWh |
= ……………….. units of electricity |
WORKSHEET 2.2 : Fill Table 2.5 on estimation of electrical energy consumed by electrical appliances.
Table 2.5 Estimation of Electrical Energy Consumed
|
Type of appliance |
Power of the appliance |
Daily duration of usage of appliance |
Electrical energy consumed |
|
Tube light |
40W |
4 hours |
= ..…..Wh |
|
Tube light |
40W |
….hours |
= 400 Wh |
|
Fan 1 |
60W |
12 hours |
= ..…..Wh |
|
Fan 2 |
30W |
12 hours |
= ..…..kWh |
|
TV |
150W |
2 hours |
= ..…..Wh |
|
Cooler |
200W |
10 hours |
= ..…..kWh |
|
Computer |
……W |
2 hours |
= 400 Wh |
|
LED Light |
……W |
….hours |
= 20 Wh |
|
AC |
1.5 kW |
10 hours |
= ..…..kWh |
|
AC |
1.5 kW |
……hours |
= 7.5 kWh |
|
Unknown appliance |
…..W |
10 hours |
= 500 Wh |
|
Unknown appliance |
…..W |
5 hours |
= 10 kWh |
When we multiply the wattage of appliances with hours of usage in a day, we get energy used by appliance in a day.
2.1.6 Power and Its Units
Power is not same as energy. Many times, there are misconceptions and people tend to believe that energy and power are same and they use these two different terms for same meaning. One good place to identify the difference between these two terms is our own house. We pay electricity bill for the energy that we have consumed during the month. But when we talk about our appliances in terms of power; 10 watt bulb, 50-watt bulb, 1000 watt water heater, etc.
Power is the rate at which energy is used. The unit of power is watt and it is abbreviated as ‘W’. When one joule of energy is consumed in one second, it is referred as one watt of power consumption. The definition of the power can be presented in terms of the following equation :
Let us take an example of two CFL’s ; a 20-watt CFL and a 10-watt CFL. Since the power consumption of a 20-watt CFL consumes energy twice as fast as a 10-watt CFL. Thus, if both CFL’s are used for 1 hour, a 20-watt CFL will consume double energy ( 20 watt x 1 hour = 20 watt-hour) as compared to energy consumed by a 10 watt CFL (10 watt X 1 hour = 10 watt-hour). In this way, when we multiply watt (power unit) by hour (time unit), we get energy unit or when we divide energy unit by time we get power unit.
In practice, power plants capacities are mentioned in terms of MW (Megawatt = 106 watt) and the energy contents of the fuels like petrol, diesel, coal, etc. are mentioned in terms of MJ or kWh). The electricity bill is made in terms of kWh and the ratings of our appliances are given in terms of watt.
Table 2.6 Different Power Units and Their Equivalent Units
|
Power unit |
Equivalent unit |
|
1 watt |
= 1 joule – second = 1 W |
|
1 kilowatt (kW) |
= 1000 watt or 1000 W |
|
1 megawatt (MW) |
= 1,000,000 W |
|
1 Gigawatt (GW) |
= 1,000,000,000 W |
EXAMPLE 2.3 A tube light consumes 320 watt-hours of electrical energy when used for 8 hours. Estimate the power rating of the tube light.
Solution Given, energy consumption of tube light = 320 watt-hour
Time duration of usage of tube light = 8 hours
WORKSHEET 2.3 : Fill the following table (Table 2.7) on power units and their conversion from one unit to other unit.
Table 2.7 Power Units and Their Conversion
|
1 kW = ……. W |
|
1 MW = ……. kW |
|
2.4 kW = …….W |
|
200 W = ……. kW |
|
0.5 kW = …… W |
|
….. kW = 5000 W |
|
…… W = 0.3 kW |
2.2 Estimating energy Requirement.
One of the skills that a trainee should develop while dealing with solar PV system design and installation is to be able to estimate the energy requirement of client. The client may need small amount of energy for household applications or he/she may need energy for large industrial applications. In any case, the estimation of energy required is the first and important step for stand alone PV system. In the case of grid connected PV systems (typically in range of megawatt or MW), estimation of annual energy potentially generated by the power plant is made.
Based on the above discussion, it should be easy now for anybody to estimate the energy requirement for a given application, for a given period of time. The energy requirement can be estimated on daily basis, monthly basis or yearly basis.
It can be seen from discussion in section 2.1.5 that the energy consumed by an appliance is the product of its power rating (in watt) and duration of usage (in hour). Energy is then presented in the units of watt-hour or Wh or kWh. Therefore, in order to estimate the energy requirement, one needs to collect the information about the power ratings of various appliances that are used in a given premises and number of hours of operation or use of those appliances.
Power ratings of appliances used at home and in industry are always mentioned on the product. Typical power rating (in watt) of some of the appliances are given in Table 2.8. But one should try to get the actual power rating or what is called the ‘name-plate rating’ of the appliances.
Table 2.8 Typical Power Ratings of Electrical Appliances
|
Name of the appliances |
Range of available power rating (in watts) |
|
Incandescent light (bulb) |
5 to 100 |
|
Tube light |
30 to 50 |
|
Compact fluorescent lamp (CFL) |
3 to 30 |
|
Ceiling fan |
30 to 70 |
|
Air conditioner (room) |
1000 to 1500 |
|
Air conditioner (central) |
2000 to 5000 |
|
CD player |
15 to 30 |
|
TV |
60 to 300 |
|
Laptop Computer |
50 to 75 |
|
Desktop Computer |
80 to 200 |
|
Printer |
100 to 250 |
|
Washing Machine |
500 to 1000 |
|
Refrigerator |
50 to 300 |
The next step is to find out the duration of usage of each appliance for which you are designing a PV system. Let us say that we are trying to estimate daily energy requirement for a person. There may be daily variation in hours of usage for an appliance. A TV may be used for more number of hours on Sunday than any other day of the week. Therefore, one should try to estimate the average of daily hours of usage. There may be seasonal variation in the daily usage of an appliance as well. For instance, a cooler will be used mainly in summer but not in winter. Such seasonal variation should also be considered while estimating annual energy consumption.
WORKSHEET 2.4 : Fill the typical daily duration of usage of appliances listed in Table 2.9.
Table 2.9 Appliances and Their Daily Consumption
|
Name of the Appliance |
Typical daily duration of usage (in hours) |
|
Incandescent light (bulb) |
…………… |
|
Tube light |
…………… |
|
Compact fluorescent lamp (CFL) |
…………… |
|
Ceiling fan |
…………… |
|
Air conditioner |
…………… |
|
CD player |
…………… |
|
TV |
…………… |
|
Laptop Computer |
…………… |
|
Desktop Computer |
