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Introduction to Solar Photovoltaic Energy
Introduction to Energy and Solar Photovoltaic Energy – Part1
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 | …………… |
| Printer | …………… |
| Washing Machine | …………… |
2.2.1 Daily Energy Consumption
After collecting the above two information (power ratings and average daily hours of usage), one can make a table (like Table 2.10) where you can fill up the information regarding the power rating and hours of usage. Also, there should be one more column added for the number of appliances of same king. For example, in a household, there can be several CFL’s and several ceiling fans. Then, for instance, in order to estimate total energy consumed by all fans, energy consumed by one fan needs to be multiplied with the number of fans in the house. This needs to be repeated for all appliances in the house having more than one unit. An example for estimating total daily energy requirement of household using 3 CFL’s, 2 ceiling fans, 1 TV and 1 computer is given in Table 2.10.
Table 2.10. Example Table for estimating Total Daily Energy Requirement of a House
| Name of appliances | Power rating (in watts) | Average daily hours of usage (in hour/day) | Number of Appliances (in numbers) | Daily energy requirement (watt x hour x number) or Wh |
| CFL | 12 | 6 | 3 | 216 |
| Fan | 50 | 8 | 2 | 800 |
| TV (21”) | 150 | 2 | 1 | 300 |
| Computer | 250 | 3 | 1 | 750 |
| – | – | – | – | – |
| – | – | – | – | – |
| Total daily energy requirement of household (Wh / day) | 2066 | |||
The daily energy requirement from Table 2.10 is 2066 is Wh. This daily energy requirement in terms of kWh will be 2.066 kWh. If you are designing a solar PV system, the system must supply this daily requirement of household.
WORKSHEET 2.5 : A household is using a number of electrical appliances for daily purpose. Fill in Table 2.11 on the estimation of daily energy requirement of appliances and total energy requirement of a household.
Table 2.11 Electrical Appliances and Their Daily Consumption
| Name of appliances | Power rating (in watts) | Average daily hours of usage (in hour/day) | Number of Appliances (in numbers) | Daily energy requirement (watt x hour x number) or Wh |
| CFL | 12 | ….. | 5 | ….. |
| Fan | ….. | 8 | 3 | ….. |
| TV | ….. | 2 | ….. | ….. |
| Computer | 50 | ….. | 1 | ….. |
| Washing machine | ….. | ….. | ….. | ….. |
| Total daily energy requirement of household (Wh / day) | ….. | |||
2.2.2 Monthly Energy Consumption and Electricity Bill
Once you know the daily energy requirement for a given application, it is easy to find out monthly energy requirement by the number of days in that month. Therefore, we can write :
Monthly energy requirement = Monthly energy requirement X Number of days in month.
For the above example given in Table 2.10
Monthly energy requirement = 2066 (Wh/day) X 30 (days/month)
= 61980 Wh or 61.98 kWh
Cost of electricity or Monthly Electricity Bill
The ‘one unit of electricity’ is equal to 1000 Wh or kWh of energy. People get their monthly electricity bill based on the number of electricity units they consume in a month. Once you know the monthly energy requirement of your client, and if you know the rate / cost of a unit of electricity (in terms of ₹ per kWh), you can calculate monthly electricity bill for consuming a certain amount of energy. Keep in mind that besides the cost of electricity, the government also adds tax to monthly electricity bill. The government declares the cost of electricity for 1kWh or one unit of electricity. The cost of one unit of electricity in current situation varies between 2 to 8 ₹ / kWh, depending on the sector (domestic or commercial), depending on the state you live in, etc. Based on above discussion, one can estimate monthly electricity bill in the following way:
Monthly electricity bill = Monthly energy requirement X rate per unit of electricity
EXAMPLE 2.4 Let us assume that the cost of electricity is 6 ₹ / kWh, then estimate the monthly electricity bill for energy consumption represented in Table 1.6.
Solution Reading from table 2.10, the daily electricity requirement is = 2066 Wh = 2.066 kWh (or one can say 2.066 units of electricity is consumed every day)
Therefore, monthly energy requirement = monthly energy requirement X number of days in month = 2.066 (kWh/day) X 30 (days/month) = 61.98 kWh per month or 61.98 units of electricity consumed per month.
The rate of electricity given in problem = 6 ₹ / kWh
Therefore, the monthly electricity bill = 61.98 (kWh/month) X 6 (₹ / kWh)
= 371.88 ₹ / month
Problem 2.1 : Based on daily energy estimation done in Worksheet 2.5, estimate the monthly energy requirement , and also, estimate the monthly electricity bill if the cost of electricity is 5 ₹ / kWh.
2.3 Energy from Solar Photovoltaic ( PV) Conversion
A solar cell is a fundamental block of solar photovoltaic (PV) technology. The solar cell is a device that converts sunlight into electricity directly without any other intermediate conversion steps. Input to solar cells is energy in the form of electricity. In this way, a solar cell is radiation energy to electrical energy conversion device as shown in Figure 2.1.
It has been discussed in Section 2.1.4 that the amount of solar energy reaching the Earth surface in the form of solar radiation is extremely large in amount; many thousands times more than our energy consumption from all other resources. Using solar cells, the solar radiation energy can be converted into electrical energy which is the most desirable form of energy. Therefore, if we manufacture a large number of solar cells, a huge amount of electricity can be generated to fulfill all our energy requirements. However, there are several challenges that needs to overcome before solar PV electricity can become our main supply of electricity.
Solar cells are made using different types of materials. A solar cell technology gets its name from the type of material used for solar cell fabrication. The types of materials include mono-crystalline silicon, multi-crystalline silicon, amorphous silicon, cadmium telluride, etc. and therefore, the names of solar cell technologies are mono-crystalline silicon cell, multi-crystalline silicon solar cells, amorphous silicon solar cells, cadmium telluride solar cells, etc. More discussion about solar cells is given in Chapter 3.
2.3.1 Solar PV modules
A solar cell is a small device in terms of area, and the electricity a single solar cell can generate is also small as compared to our electricity needs. For example, a single solar cell can generate daily electricity in range of 6 Wh to 10 Wh while our daily requirements are much higher. Therefore, in order to generate more electricity, many solar cells are connected together in the form of PV module. The number of solar cells to be connected together and the way they are connected together determine current and voltage that we can get from the modules, and average energy it can produce every day. The connection of solar cells in the form of PV module and actual picture of a PV module is given in Figure 2.2.
PV modules are very important part of PV systems. It is very important to understand the characteristics of PV modules. Some important features of the PV modules are listed here :
- A solar cell converts radiation energy into electrical energy when sunlight falls on it. Since PV module is made using solar cells, a PV module generates electricity only when sunlight falls on it. Thus, in non-sunshine hours or in night, the energy output from PV module is zero.
- The amount of electricity generated from a PV module depends on the physical size of the module, larger is the size of the module, higher will be the amount of electricity generated from it.
- PV modules are mainly characterized in terms of their power rating, which is known as ‘peak power’ rating. The unit of power is watt and to emphasize the ‘peak’ power a subscript ‘p’ is added. Therefore, the symbol for ‘peak power’ of a PV modules is ‘Wp’. The Wp rating is maximum power rating that a PV module can provide under best condition, called standard test condition (STC). The PV modules are available in Wp rating starting from 1 Wp to 300 Wp . Commonly available Wp rating of PV modules are 3 Wp, 10 Wp, 18 Wp, 36 Wp, 50 Wp, 75 Wp, 150 Wp, 175 Wp, 220 Wp, and 300 Wp.
- Electricity that is generated from a PV module is DC (direct current) in nature. The conventional electricity supply available to us is AC (alternating current ) in nature. All our appliances like TV, CFL, tube light, refrigerator, washing machine, etc. runs on AC electricity. Therefore, if we want to use solar PV electricity, it must be first converted into AC electricity. An additional device called ‘ inverter’ is used to convert DC electricity into AC electricity. It means we have to use an additional device in the PV system.
- Since the PV modules generate electricity only during day time, therefore for night time applications, the storage of electrical energy in batteries are required. One advantage of DC electricity generated from the PV module is that the batteries store and supply electricity in the form of DC electricity. In addition, there are several devices that can directly be operated using DC electricity like LEDs, DC fan , DC water pumps, etc. Even other devices like mobile phone, computer, refrigerator etc. can also be run directly on DC. So, if we are using DC appliances, the use of inverter can be avoide. But the use of DC appliances is not very common.
- Similar to different types of solar cells, the solar PV modules also get different names based on type of solar cells used in making solar PV modules. The names of commonly available PV modules are : mono –crystalline silicon solar PV module, multi-crystalline silicon solar PV module, amorphous silicon solar PV module, cadmium telluride solar PV module, CIGS solar PV modules, etc. For each of these types of PV modules, there are many manufacturer in the world, but mono-crystalline silicon solar PV module and multi-crystalline silicon solar PV module are the most commonly manufactured and used.
- When large energy generation, larger than what one PV module can produce is required, many PV modules are connected together. This interconnection of PV module is called ‘PV module array’
PV modules are very important part of PV system and a detailed discussion about the solar PV modules is given in Chapter 3.
2.3.2 Solar PV Systems
The function of a solar PV system is to supply reliable electricity to the appliances when required or in case of large PV power plants to supply electricity to the grid. It has been discussed in Section 2.3.1 that the solar PV modules produce electricity only when sunlight shines on modules and for night time applications, electrical energy storage in the form of batteries may be required if there is no other supply. Batteries are also used when the demand of electricity by appliances is more than the generation of electricity in day time. In some cases, the PV modules supply electricity directly to the existing electricity grid in which the use of batteries can be avoided. Also, the electricity produced by PV modules are DC in nature while most of our appliances are AC in nature, and therefore, inversion of DC into AC is required before making use of solar PV electricity. Therefore, in all, as per above discussion, for the generation of solar PV electricity and reliable supply of electricity to the appliances, not only PV modules are required, but also, several other components are required. The other components include the following :
- Battery : For storing electrical energy for night time application and for time when demand of electricity is more than the generation of electricity. Batteries are not required in grid-connected PV systems.
- Inverter : For converting DC electricity to AC electricity, the DC electricity may either come from PV modules or it can come from batteries.
- Charge controller : For protecting the batteries from overcharge and over-discharge conditions which reduce the life of batteries.
- Maximum powered point tracker (MPPT) : For extracting maximum available power from solar PV modules under given solar radiation input. Many times, charge controller or inverter (in case of grid connection) performs the function of charge controller and MPPT.
PV module together with other components that are put connected with PV modules to supply reliable energy to appliances is referred collectively as ‘solar PV system’ A solar PV system can be of several types (discussed in detail in Chapter 10) depending on the way the energy is generated and used. Broadly, PV system is divided in three categories :
- Standalone solar PV systems,
- Grid-connected solar PV system, and
- Hybrid solar PV system.
Standalone PV systems : These systems are self-sufficient in themselves. They do not depend on any other source of energy to supply electricity to planned appliances or load. The example of standalone solar PV systems include a solar lantern, a solar PV home lighting system, a solar PV water pumping system, etc. Since the standalone solar PV systems do not depend on any other energy sources, they invariably use means to store energy, typically in the form of batteries. And since batteries are used, it is important to use to protect electronics. Also, for conversion of DC electricity from PV modules and from battery, inverter will have to be used. A typical standalone solar PV system and the flow of energy in the system (denoted by arrows) is shown in Figure 2.3.
Grid-connected solar PV systems : As the name suggests, a grid-connected solar PV system is connected with nearby available electricity grid. In this way, the generated electricity is feed into the grid. No battery storage is used in this case. But conversion of DC electricity generated by solar PV modules into AC electricity is required before feeding to the grid. A typical arrangement of grid-connected solar PV systems is shown in Figure 2.4. This type of PV system configuration is used in India for large scale (MW level) solar PV power plants. Electricity grid voltage and frequency are well defined and, therefore, the PV electricity can be fed to electricity grid only after proper power conditioning, i.e., converting PV generated electricity to appropriate voltage and frequency level. Therefore, in the case of grid-connected solar PV system, the inverter not only performs the function of DC to AC conversion but also performs the function of grid synchronization which is related to bringing generated PV energy to appropriate voltage and frequency level.
Hybrid solar PV system : In some cases, an auxiliary source of energy like diesel generator is used in addition to solar PV modules and/or grid. This need to be done when solar PV modules are not designed to supply the full required energy by the load (may be due to cost reason). In such case of use of auxiliary source a solar PV system is called ‘ hybrid solar PV system’
2.3.3 Advantage and Challenges of solar Photovoltaic Energy conversion
Like other technologies, the solar PV technology also has its positive and negative aspects or has its advantages and challenges. These are discussed as follows :
Advantage of the solar PV technology is listed below:
- Abundant source : Solar cell uses solar radiation energy as input, which is a renewable energy source. Solar radiation energy is available in huge quantity as it is abundant. We will not run in the shortage of solar radiation energy in future. On the other hand, the world is already facing the shortage of fossil fuels based energy sources.
- Environmentally benign : The conversion of solar radiation energy into electrical energy does not emit any polluting products and, therefore, it does not cause damage to the environment like the smoke from use of diesel, petrol and coal does.
- Decentralized electricity generation : Since solar radiation energy is available everywhere, the solar PV electricity can be generated everywhere in decentralized manner in small quantities as per the need, unlike the coal based power plant where electricity can be generated only in centralized manner in large quantities. Decentralized electricity generation results in less losses occurring due to transmission of electricity. In the case of solar PV technology, one can have small solar lantern, a solar PV system for lightning house, a solar PV system for running water pump, or a solar PV system for lighting whole village or even solar PV system for pumping MW of power into grid.
- Modular implementation : Implementation of solar PV technology can be modular. Size of a solar PV system for electricity generation can be increased as per the increase in need of electricity. In the case of a diesel generator or a coal based power plant, size once fixed cannot be changed. If we need to increase electricity , we need to buy another diesel generator or set up another coal based power plant.
Challenges
In principle, solar PV electricity can be generated to fulfill our entire energy requirement. But there are significant challenges to be overcome in order to make wider use of electricity generated using solar PV technology. These challenges are listed below :
- PV electricity cost : The conventional energy sources have always been the most cost-effective way to supply the large amount of electricity needed for modern life. Producing electricity using solar PV technology is more expensive. However, the cost of solar PV electricity has been decreasing rapidly, there is further need to reduce the cost in order to make solar PV electricity affordable by all.
- Energy fluctuation : In the case of coal based power plants, companies can easily stockpile coal to meet the ever-changing demand for electricity, especially during peak demand hours. While the solar radiation energy can’t be stored to provide energy for future use. Solar radiation is not available in the night time but there is electricity need in night. Also, during the day time peak radiation availability may not match with peak electricity demand. Therefore a mechanism for effective energy storage and efficient recovery is required.
- Location dependency : Fossil fuel power plants can be placed almost anywhere, as long as a railroad or pipeline can reach the site for bringing coal and gas. In contrast, solar PV electricity generation depends on the availability of solar radiation. The availability of solar radiation varies from place to place. At some places it is more and at some other places it is less. Therefore, solar electricity generation is dependent on location where system needs to be installed.
2.4 Other Renewable Energy Technologies
Solar Photovoltaic is one way of converting renewable energy sources into useful energy. There are several other technologies that convert renewable energy sources like solar radiation, wind energy, biomass energy, etc. into useful energy. There are many people who are also working on other renewable energy technologies and there are many installations for energy generation which uses different types of renewable energy sources. A detailed discussion of the other renewable energy technologies is not in the scope of this manual but for the completeness of the information, brief discussions are given in the following paragraphs.
2.4.1 Solar Thermal Energy
There are two categories of solar energy technologies; one is solar PV technology (for which this manual is dedicated) and other is solar thermal technology. In solar thermal technology, the solar radiation energy is converted into heat energy while in solar PV technology, the solar radiation energy is converted into electrical energy directly.
In solar thermal technology, a device called solar collectors is used for converting solar radiation into heat energy. The sunlight is captured by different types of solar radiation into heat energy. The sunlight is captured by different types of solar collectors which provide heat. This heat is then used for various applications, such as heating water for domestic applications (bathing, washing clothes, etc.). Water is heated up to temperatures of about 500 C- 600 C. For applications which require higher temperatures (more than 1000 C ), special type of solar collectors, called concentrator collectors, are used. The concentrator collectors collect light from large area and concentrate in small area. Due to such light concentration, higher temperatures (from few hundred up to 10000 C are obtained. The solar thermal heating could also be used for industrial applications for steam generation which is used in industry for many process applications.
One of the important applications of connector solar thermal energy is for electricity generation. The technology is normally known as Connector Solar Power (CSP) technology The CSP plants for electricity generation is similar to coal based power plants except that in the CSP technology, the steam is generated using solar radiation while in coal based plants steam is generated by burning coal. The rest of the processes used in steam based power generation are same. The steam is used for rotating the turbines which, in turn, rotates the electrical generator in order to generate electricity. Large megawatt scale CSP solar thermal power plants are being built at various places in the world. In 2010, the worldwide installed capacity of solar thermal power plant for electricity generation was above 190 GW (thermal power). Under Jawaharlal National Nehru Solar Mission (JNNSM), launched by ministry of New and Renewable Energy in 2010, there will be large installations of solar thermal power plants in India as well.
2.4.2 Wind Energy
A moving object possesses energy due to its motion; this energy is called the kinetic energy. Similarly flow of air or wind, around the earth possess kinetic energy. The kinetic energy of the blowing wind is called the wind energy. The flow of wind in our atmosphere is mainly caused by uneven heating of the earth’s surface by the sun. Thus wind energy is indirect manifestation of sun’s energy. The wind energy has a good potential to be a source of renewable and pollution free power. About 1 to 3 % of the solar energy falling on earth surface gets converted into wind energy. The solar energy to wind energy conversion is about 50 to 100 times higher than the solar energy to biomass energy conversion through photosynthesis. Though, similar to solar radiation energy, the wind energy is also dilute in nature and its availability varies considerably over a day and cover one season to other season.
The wind energy can be used in many ways. Historically wind energy is used for sailing ships. The energy contained in the wind can be converted to the useful energy through wind mills. Wind energy can be converted into mechanical energy for grinding or water pumping. Alternatively the wind energy can be converted to electrical energy (the most desired form of energy) by converting wind energy into mechanical energy (rotary motion of wind turbine), which is then converted to electrical energy using generators. Some important applications of wind energy are the generation of electricity and to some extent for water pumping.
The conversion of wind energy to electrical energy is one of the most successful renewable energy technologies. It has already proven to be economical in many parts of the world where wind resources are good. All over the world, large amount of wind machines for electrical energy generation have been installed. In 2010 the worldwide installed capacity of wind machine was about 200,000 MW. In India the installed capacity of wind machine is about 13000 MW.
2.4.3 Biomass Energy
Biomass refers to the mass of biological material produced from the living processes. This includes the materials derived from the plants as well as from animals. Chemically, biomass refers to the materials containing hydrogen, carbon and oxygen. We extract biomass from numerous sources like; plants, trees, agricultural crops, raw material from the forest, household waste and wood. The contribution of the biomass to our energy requirement comes in the form of food and fuel. There are several other requirements that are being served by the use of biomass, namely shelter preparation, fodder for animals, nutrient for soil, etc. The earliest inhabitants on the earth burned wood in their campfires for heat and since then it has been a source of energy for meeting human needs.
Biomass grows in the presence of sunlight only. Therefore, the biomass can be considered as ‘ solar energy stored in organic matter.’ As trees and plants grow, the process of photosynthesis uses energy from the sun to convert CO2 of the atmosphere into carbohydrates (sugar, starch and cellulose). The process of the photosynthesis can be written as follows :
6 CO2 (gas) + 6H2O (liquid) + light → C6H12O6 (solid) + 6O2 (gas)
The biomass has been used extensively in the development of societies since the beginning of civilizations. It has played a very important role in the development of societies. Many people in the developing countries still depend on the use of biomass for their daily livelihood. They use it for food, fuel as well as a source of income. It is estimated that biomass contributes to about 14% of the world’s total energy requirement. In many developing countries, the contribution to total energy requirement is as high as 90%. As a renewable energy source , biomass should be able to meet all its requirements in various forms, provided the balance between production and consumption of biomass is maintained.
Energy can be generated from bio degradable items, such as plants, cow dung, leaves etc. These items are rich in hydrocarbons and release energy when burned. The main application of biomass energy is fuel generation which is used for cooking as well as for vehicular transport applications. Biodiesel is one of the most important product of the biomass. Cooking gas from biomass, also called biogas, is also an important source of energy. Using a process, called glasification, biomass can also be converted into fuel gas, which can be used in engine in place of diesel.
2.4.4 Hydro Energy
An object, kept at a height, is said to possess some energy called potential energy, because a falling object can provide some energy. For instance, in hydro power plants, energy of water falling from a certain height is used to generate electrical energy. Water falling from a height has stored potential energy. This energy could then be utilized to rotate the hydro turbines from generation of electricity. Large hydro projects make use of huge tanks or dams to store water in their catchment areas. This water is then allowed a free passage through pipes or channels to water turbines that are placed at the bottom of the tank. The potential energy of water is then used to rotate the turbine for electricity generation. Around the world, large amount of electricity is generated using this method. The total installed capacity of hydro power plant in the world is about 800,000 MW. In India also the total installed capacity is about 40,000 MW.
The energy of running water in small water streams, rivers and canals is called kinetic energy of water (similar to energy of blowing wind). Similar to wind energy technology, kinetic energy of water can also be converted into electricity. A small water turbine is placed in the movement of water. The turbine converts kinetic energy water into electrical energy. There are water turbines of small capacity in the range of few kW to few tens of kW are available. Thus wherever there is moving water some electrical energy can be generated.
2.4.5 Geothermal Energy
The earth is a large sphere. The surface of this sphere is cold but its center is still very hot. Core of the earth is plasma (material in molten form) which is at very high temperatures about 50000C while the earth surface is at temperatures less than 1000C. Due to the temperature gradient, there is always heat conduction from the earth crust to the surface. The heat flow due to conduction could be used for heat and electricity generation. Special types of heat exchangers are used for conversion of the heat generated in the earth to the end applications, such as steam generation for process heat or for electricity generation.
