Energy Conversion- Solar Energy Technlogy.pptx

solar

1. Energy Conversion and Rural Electrification ( ECEg4242) By Mohammed A . Woday Jimma Institute of Technology Jimma University

2. Solar Energy Technology

3. Solar Electricity is the electricity generated directly from sunlight using solar PV cells. Photovoltaic refers “an electric voltage caused by light” The PV power technology uses semi-conductors cells (wafers), generally of several cm 2 Most solar cells are made of a form of silicon, a hard material either blue or red in appearance. Individual solar cells can be compared to batteries used in torches & radios. Each silicon solar cell produces about 0.4 V to 0.7V 3 Introduction :

4. Short lead time to design, install & start up a new plant. Highly modular, hence the plant economy is not a strong function of size. Power output matches very well with peak load demands Static structure, no moving parts, hence no noise. High power capability per unit of weights. Longer life with little maintenance because of no moving parts. Highly mobile & portable because of light weight. 4 Major Advantages of the PV power

5. This is one of category & application of PV system. The standalone systems are self-sufficient, unreachable by state grid but have a battery system for continuous supply. In case of grid connected systems, a major part of the load during the day is supplied from the PV array, and then from the grid when sunlight is not sufficient. A PV hybrid system is installed with a back-up system of diesel generator. Solar street light Home lighting system SPV water pumping system SPV cell for communication equipment in snow-bound areas 5 Standalone PV System

6. The physics of the PV cells is very similar to p-n junction diode. When light is absorbed by the junction, energy of the absorbed photons is transferred to the electron system of the material, resulting in the creation of charge careers that separated at the junction. 6 Principle

7. 7 Principle of Working : 2 important steps

8. Solar cells  Module  Array Rechargeable Batteries – to store electricity Control Units – for switching, protecting batteries & wiring, monitoring the performance of the system, giving warning etc. Distribution of Electricity – if long run, then need of thick cables to avoid drops. Electrical appliances – some appliances can connect directly as it produces dc. 8 Main parts of solar PV system :-

9. The PV cell or solar cell is made of a doped semiconductor material. PV cell materials are of two major types: crystalline and thin films. The types of materials vary in terms of light absorption efficiency, energy conversion efficiency, manufacturing technology, and cost of production. The annual electrical power output of a PV plant depends on the following factors: Solar radiation incident on the installation site Inclination and orientation of the PV module Presence of shading or not Technical performances of plant components, especially modules and inverters Photovoltaic Materials

10. PV Technologies

11. The output of sun is 2.8×10 23 KW. The energy reaching the earth is 1.5×10 18 KWH/year. When light travels from outer space to earth, solar energy is lost because of following reasons: Scattering : The rays collide with particles present in atmosphere Absorption : Because of water vapor there is absorption Cloud cover : The light rays are diffused because of clouds. Reflection : When the light rays hit the mountains present on the earth surface there is reflection. Climate: Latitude of the location, day (time in the year) also affects the amount of solar energy received by the place. 11 Solar Radiation

12. It is a quantity indicating the amount of incident solar power on a unit surface, in units of kW/m2. At the earth’s outer atmosphere, the solar Insolation on a 1 m 2 surface oriented normal to the sun’s rays is called SOLAR CONSTANT and its value is 1.37 kW/m2 . Due to atmospheric effects, the peak solar insolation incident on a terrestrial surface oriented normal to the sun at noon on a clear day is on the order of 1 kW/m2 . A solar insolation level of 1 kW/m2 is often called PEAK SUN . Solar insolation is denoted by ' I '. 12 Insolation:

13. It is an amount of solar energy received on a unit surface expressed in units of kWh/m2 . Solar irradiance is essentially the “ solar insolation (power) integrated with respect to time ” . When solar irradiance data is represented on an average daily basis, the value is often called PEAK SUN HOURS (PSH) and can be thought of as the number of equivalent hours/day that solar insolation is at its peak level of 1kWh/m2. The worldwide average daily value of solar irradiance on optimally oriented surfaces is approximately 5 kWh/m2 or 5 PSH. Solar irradiance is denoted by ' H '. 13 Irradiance

14. A solar module can be seen as a black box that with two connectors, producing a current, I , at a voltage, V having four components: Current Source: This is the source of the photo current, and it is: with the cell area, A, the intensity of incoming light, H, and the response factor ξ in units of A/W. Diode: This non-linear element reflects the dependence on the band gap and losses to recombination. It is characterized by the reverse current, I , and by a quality factor, q . Shunt Resistor Rp : represents losses incurred by conductors. Serial Resistor Rs : also represents losses incurred by non-ideal conductors. Electrical Characteristics of PV Modules The relationship between I and V of a single cell is then expressed by: with thermal voltage with temperature, T (in Kelvin), Boltzmann constant k = 1.38e -23 and the elementary charge e = 1.602e -19 .

15. PV cell characterization involves measuring the cells electrical characteristics to determine the light conversion efficiency and critical equivalent circuit parameters. The performance of PV cells is compared by two simple tests to obtain necessary data for solar cell I-V characterization. These tests are: the forward bias (illuminated) test and the reverse bias (dark) test. A typical voltage versus current characteristic is known as an I/V curve, of a P-N diode without illumination. The applied voltage is in the forward bias direction. The curve shows the turn on and the buildup of the forward bias current in the diode. Theory of I-V Characterization

16. The forward test gives the illuminated I-V curve for a PV cell. In a light test, an I-V sweep is conducted where the voltage is swept upward starting at V=0, while measuring the sinking current. The following values are calculated using the forward bias (illuminated) test: Open circuit voltage (VOC) Short circuit current (ISC) Maximum power (PMAX), current at PMAX (IMP), voltage at PMAX (VMP) Fill factor (FF) Shunt resistance (RSH) Series resistance (RS) Maximum efficiency ( η MAX) Forward Bias (Illuminated) Test

17. The PV cell is tested as a passive diode element by blocking all light to prevent it from exciting a PV cell to determine its breakdown diode properties and internal resistances . The following I-V parameters are obtained via the reverse bias (dark) test. Shunt resistance (RSH) Series resistance (RS) PV cells are modeled as a current source in parallel with a diode. When there is no light present to generate any current, the PV cell behaves like a diode. As the intensity of incident light increases, the current is generated by the PV cell. Reverse Bias (Dark) Test

18. The total current I, in an ideal cell, is equal to the current I generated by the photoelectric effect minus the diode current ID, according to Eq. A more accurate model will include two diode terms. Cont’d Where : I is the diode saturation current, q is the elementary charge1.6x1019 C, k is a constant of value 1.381023 J/K, T is the cell temperature in Kelvin, and V is the cell voltage.

19. The PV equivalent circuit is modified to account for the losses due to series resistance (R S ) and shunt resistance (R SH ) as: R S , is the series resistance that represents the ohmic loss in the front surface of the cell and R SH is the shunt resistance that represents the loss due to diode leakage currents to account for the power dissipation in the internal resistance of the PV cells. Cont’d

20. Short Circuit Current (I SC ) Short circuit current (I SC ) corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0. I (at V=0)= I sc Open Circuit Voltage (V OC ) Open circuit voltage (V OC ) occurs when there is no current passing through the cell. V (at I = 0) = V OC Cont’d

21. The power produced by the cell in watts can be easily calculated along the I-V sweep by the equation P = IV. The power will be zero at I SC and V OC points, and the maximum value for power will occur between the two. P MAX is not constant, but is a function of the intensity of radiation, the temperature, and the type of solar cells in use. In addition with the effect of irradiance on output, there are many other factors which affect the power o/p of the cell such as…. Effect of cell temperature Effect of no. of cells in a module Effect of cell area Effect of type of silicon Maximum Power (PMAX)

22. FF is essentially a measure of the quality of the solar cell. It is calculated by comparing the maximum power to the theoretical power (P T ) that would be output at both the open circuit voltage and short circuit current together. A larger FF is desirable and corresponds to an I-V sweep that is more square-like. Typical FFs range from 0.50 to 0.82. FF is also often represented as a percentage. Fill factor = P max /P T or FF = V mp . I mp =V OC .I SC 22 Fill Factor(FF) Fig - Getting the Fill Factor From the I-V Sweep Figure 4 - Maximum Power for an I-V Sweep

23. The conversion efficiency is the ratio of the electrical power output, Pout, compared to the solar power input, Pin, into the PV cell . Pout can be taken to be P MAX because the solar cell can be operated up to its maximum power output to get the maximum efficiency. η = P out /P in η MAX = P MAX /P in P in is taken as the product of the irradiance of the incident light, measured in W/m2 or in suns (1000 W/m2), with the surface area of the solar cell in m2. Efficiency ( η)

24. The major factors influencing the electrical design of the solar array are as follows: The sun intensity The sun angle The load matching for maximum power The operating temperature 24 Array Design :

25. The magnitude of photocurrent is maximum under full bright sun (1.0 sun or peaksun ). On a partially sunny day , the photocurrent diminishes in direct proportion to the sun intensity. On a cloudy day , therefore, the short circuit current decreases significantly. The photo-conversion efficiency of the cell is insensitive to the solar radiation in the practical working range. 25 Sun Intensity : I.e . the conversion efficiency is the same on a bright sunny day & a cloudy day. We get lower power output on a cloudy day only because of the lower solar energy impinging the cell.

26. The cell output current is given by… I = Io Cos θ , where Io is the current with normal sun (reference), and θ is the angle of the sun-line measured from the normal. This cosine law holds well for sun angles ranging from 0 to 50˚. Beyond 50˚, electrical o/p deviates significantly from the cosine law and the cell generates no power beyond 85˚, although the mathematical cosine law predicts 7.5 % power generation. 26 Sun Angle :

27. The array may consist of many parallel strings of series-connected cells. A large array may get partially shadowed due to a structure interfering with the sun-line. If a cell in a long series strings gets completely shadowed, it will lose the photo-voltage, but still must carry the string current by virtue of its being in series with the other fully operating cells. 27 Shadow effect:

28. Without internally generated voltage, it cannot produce power. Instead, it acts as a load, producing local I 2 R loss & heat . Hence, remaining cells in the string must work at higher voltage to make up the loss of the shadowed cell voltage. Higher voltage in healthy cells means lower string current as per the I-V char. of the string. The current loss is not proportional to the shadowed area, and may go unnoticed for mild shadow on a small area . However, if more cells are shadowed beyond the critical limit , the i-v curve gets below operating voltage of the string, making the string current fall to zero, losing all power of the string. 28 Continued….

29. The commonly used method to eliminate the loss of string due to shadow effect is to subdivided the circuit length in several segments with bypass diodes . The diode across the shadowed segment bypasses only the segment of the string. This causes proportionate loss of the string voltage & current, without losing the whole string power. Some modern PV modules come with such internally embedded bypass diode. 29 Continued…

30. With increasing cell temp., the S.C. current of the cell increases, whereas the O.C. voltage decreases. The effect of temperature on the power is quantitatively evaluated by examining the effects on the current & the voltage separately, 30 Temperature effect:

31. The energy production expected from a PV array is principally a function of its site’s solar radiation . The value for the most critical period of system operation should be chosen as the appropriate value of solar radiation W (Peak Sun Hours/day). It is thus advisable to use the value of the month with the lowest solar radiation. On the basis of solar radiation value thus determined, we can calculate the power generated (Pg) by one module per day . Pg = Ig x W where, Ig is the current generated/module and W is the solar radiation of the site in Peak Sun hours/day (PSH/day) 31 Module Sizing:

32. 3 . On the basis of the total load requirement, we can calculate the power ( Preq ) that should be generated by the solar array/day . Preq = L x C F where, L is the load requirement/day (can obtain from load calculation) C F is the safety factor due to losses, accumulation of dust on the module, to increase the system efficiency. In general C F is around 1.3 4. The equation for Pg and Preq are used to determine the total number of PV module N. N = Preq / Pg  once operating DC voltage has been specified, the sizing procedure gives the array configuration as the number of modules to be connected in series & in Parallel . 32 Continued…

33. 5. Battery sizing Battery capacity (in Ahr ) is determined by using the total load (in Ahr ) required per day and the number of days the system is required to operated without energy generation (days of autonomy), i.e. when there is no sunlight. Normally, the days of autonomy are determined with experience in the field. Generally recommended values are….. Latitude days of autonomy ( Cn ) 0 - 30˚ 5 – 6 30 - 50˚ 10 – 12 50 - 60˚ 15 Battery Capacity = Cn x L 33 Continued…

34. Formula Formula in Units sample P = V x I W = V x A Tape recorder with 12 V & 1.5 A, P = 12 x 1.5 = 18 W E = P x t Wh = W x h Tape recorder is used 2 hrs/day: E = 18 W x 2 hr = 36 Wh I = P / V A = W / V LED-lamp with 1.5 W under current of 6 V: I = 1.5 W / 6 V = 0.25 A P = E / t W = Wh / h LED-lamp needs 6 Wh in 4 hrs: P = 6 Wh / 4 h = 1.5 W E = V x I x t Wh = V x A x h This LED-lamp is used for 4 hrs: E = 6 V x 0.25 A x 4 h = 6 Wh C = E / V Ah = Wh / V Consumption of this LED0-lamps: C = 6 Wh / 6 V = 1 Ah E = C x V Wh = Ah x V Battery with 18 Ah and 12 V: 18 Ah x 12 V = 216 Ah 34 Important Formula:

35. Design a solar system for a user with a following equipment 3pcs of 3W lamps to be on 4 hrs/day Tape recorder drawing 1A in use 4 hrs /day The system voltage is 12 V. The user has a 10 W module, which generates 0.6 A. Calculate the Ah of each load: Lamps ……………….. 3 x (3 W / 12 V) x 4 hrs /day = 3 Ah /day Tape Recorder ……….. 1 A x 4 hrs /day = 4 Ah /day Total Load ………..L = 7 Ah /day 35 Example: 2. Battery Sizing : Battery Capacity (Ah) = Cn x L Since Ethiopia is in the range of 0 to 30˚ ; We assume 6 for the value of Cn ; Therefore, battery capacity = 6 x 7 Ah = 42 Ah

36. 3. Module Sizing : To calculate power generated, take the value of the worst month for solar irradiation (in full sun hours per day) Pg = Ig x W = 0.6 A x 4.01 hrs/day = 2.41 Ah /day Preq = L x CF = 7 Ah/day x 1.3 = 9.1 Ah / day No. Of modules required N = Preq / Pg = 9.1 / 2.41 = 3.78 ≈ 4 4. The current generated by the solar array is 4 x 0.6 = 2.4 A and the current delivered to the load is 1 A for the tape recorder and 0.75 for the 3 lamps, i.e. total of 1.75 A. The charge controller should be able to handle both currents, and as a consequence a charge controller with a value greater than a 3 A is use . 36 Continued….