Week 10 Power Electronics Applications to Photovoltaic Power Generation

Transcription

1 ECE1750, Spring 2017 Week 10 Power Electronics Applications to Photovoltaic Power Generation 1

2 Photovoltaic modules Photovoltaic (PV) modules are made by connecting several PV cells. PV arrays are made by connecting several PV modules. Although the sun will eventually die as a white dwarf star in about 4.5 Billion years, solar power can be considered a renewable source of energy because we can expect that for the next couple of billion years the sun will still radiate power without making the Earth inhabitable. Solar power is radiated through space. Solar power is generated by nuclear fusion. Photons are created at the center or the Sun. It takes an average of 10 million years for the photons to emerge (they collide many times in the Sun interior). Then it takes 8 minutes for a photon to reach the Earth. Once they reach earth some photons are scattered and absorbed in the atmosphere.

3 Photons Journey into Electricity Finally, the photons reach ground on Earth A. Kwasinski, 2017 US Solar Insolation Map: NREL

4 Photons Journey into Electricity The incident power has 3 components depending on the final photons path. Diffuse radiation Direct-beam radiation Reflected radiation A. Kwasinski, 2017

5 Photons Journey into Electricity Sun s location terms A. Kwasinski, 2017

6 Photons Journey into Electricity Magnetic vs. celestial poles: Magnetic poles: Caused by Earth s magnetic field Can be located with a compass They move along Earth s surface! Celestial poles: Caused by Earth s rotation. They are two imaginary stationary points in the sky. Important for PV system applications. Geological Survey of Canada A. Kwasinski, 2017

7 Sun Moves Throughout out the Year June 21 December 21 7

8 Photons Journey into Electricity Impact of the sun s position for the calculation of the direct-beam radiation with respect to the incidence angle and the air mass ratio Edge of Austin s Latitude: 30 o 30 o PV module (for incidence angle calculation) June 21 Tropic of Cancer Latitude o Equator o o March 21 September 21 Tropic of Capricorn Latitude o A. Kwasinski, 2017 Earth s surface (for air mass ratio calculation) December 21

9 Photons Journey into Electricity Sun s position in the sky throughout the day and during an entire year. Solar Zenith versus Azimuth at Austin 22nd Day of Jun, Jly, Aug, Sep, Oct, Nov, Dec (Sun hrs/day. Jun=13.9,Jly=13.6,Aug=12.8,Sep=12.0,Oct=11.0,Nov=10.3,Dec=10.0) 0 10 Azimuth (South = 180) Jun NOON 1 PM Zenith (Degrees from Vertical) Sep Dec 3PM A. Kwasinski, 2017

10 Photons Journey into Electricity The direct-beam insolation I BC depends on the PV module orientation with respect to the sun. If the PV module is fixed, this insolation will change in a deterministic way throughout the day and the year: if the incident angle θ is given by cos cos cos( )sinsin cos S C Then, the direct-beam insolation is I I cos BC B A. Kwasinski, 2017

11 Panel Orientation is Important t tilt panel Line perpendicular to horizontal plane Line perpendicular to panel surface tilt panel Edge of panel Horizontal plane Figure 6. Panel Tilt Angle Best all-year tilt = Latitude Best winter tilt = Latitude + 15 Best summer tilt = Latitude 15 11

12 Atom s energy model: Photons Journey into Electricity Conduction band (partially filled) Conduction band (Empty at T = 0K) Electron Energy E g Gap Forbidden band Filled band Filled band Electron Energy E g Gap Forbidden band Filled band Metals semiconductors Photons energy is quantized. The energy of a photon with a wavelength of λ (or a frequency of υ) is E h hc where h is Planck s constant A. Kwasinski, 2017

13 Photons Journey into Electricity After a long journey, photons are converted into electricity in semiconductors: Whenever a photon with enough energy hits an atom, an electron may jump the energy gap into the conduction band. Once in the conduction band, the electron is free to move in an electric circuit. If the circuit is open or if the load requires less current (charge per time) than the one being gp produced, the free electrons will eventually decay again. Since it is assumed a continuous slow varying incident solar energy, electrons are freed at a constant rate (direct current). Hence, a constant voltage is produced.

14 PV Cells Technologies Uni-Solar solar shingle BP SX170B Polycrystalline BP SX170B Monocrystalline Uni-Solar Laminate PVL-136 Amorphous A. Kwasinski, 2017 Various types of PV Modules Mitsubishi PV-TD 190MF5 Multicrystalline

15 PV Applications More conventional applications (not all necessarily for microgrids) A. Kwasinski, 2017

16 PV Applications Less conventional applications (not all necessarily for microgrids) A. Kwasinski, 2017

17 The p-n junction diode n-type substrate p-type substrate I d Id I e qv kt Bias voltage 0 1 d V d is the diode voltage I 0 is the reverse saturation current caused by thermally generated carriers At 25 C: Id I e V d Ideal diode Real diode I 0 A. Kwasinski, 2017

18 PV Cells physics I SC The current source shifts the reversed diode curve upwards V OC Same curve The bias source p-n junction is (voltage source) I SC equivalent to is replaced by a a diode current source Reverse v-i powered dby the curve for the photons diode

19 PV Cell steady state characteristic From Kirchoff s current law: qv d kt IPV ISC Id ISC I0 e 1 The open circuit voltage is V OC Maximum power point kt I SC V( IPV 0) ln 1 q I Power P I V PV PV 0 Power P max 0.7 V oc I sc V OC Current I SC A. Kwasinski, 2017

20 PV Cell steady state characteristic Dependence on temperature and insolation: A. Kwasinski, 2017

21 PV Cell steady state characteristic More on the dependence on temperature and insolation: A. Kwasinski, 2017

22 36 Cells in Series Make a 12V-Class Panel (Voc 19V) 9 cells x 4 cells is a common configuration Two 12V-Class Panels in Series Make a 24V-Class Array (Voc 38V) 22

23 I-V Curve I( V ) I sc V e PV Station 13, Bright Sun, Dec. 6, 2002 I sc 6 P max at approx. 30V 5 4 P max 0.7 V oc I sc I - amps V(panel) - volts V oc 23

24 The Maximum Power Point PV Station 13, Bright Sun, Dec. 6, Pmax P(panel) - watt ts P=0 at short circuit P=0 at open circuit V(panel) - volts 24

25 Grid tied inverters Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex, and others): CONNECTION BOX (NO ELECTRONICS) Two main issues need to be addressed: Operate PV modules at their maximum power. Control inverter s power output. 25

26 Maximum Power Point Tracking Maximum power point P max Power R load V L I L I L Current VL P L Vmax I max V max When we connect a resistance RL directly to a PV panel the power output is PL. In general P Ideally, we would like to have the PV panel always operating at Pmax regardless of the load. Also, take into account that Pmax changes due to various factors. PL max R max I max

27 Impedance matching I in I out = I in / D Source + V in DC DC Buck Converter + V out = DV in R load V I out out I in + V in Equivalent from source perspective R equiv R equiv Vout V in V R D out load I I D 2 2 I D D in out out So, the buck converter makes the load resistance look larger to the source 27

28 Example of drawing maximum power from solar panel PV Station 13, Bright Sun, Dec. 6, 2002 I sc 6 5 P max is approx. 130W (occurs at 29V, 4.5A) I - amps V(panel) - volts V oc I-V characteristic of 6.44Ω resistor For max power from panels at this solar intensity level, attach R load 29V A But as the sun conditions change, the max power resistance must also change. Also, the load changes based on the user needs.

29 Example of a directed connected load different to that yielding maximum power 6 5 PV Station 13, Bright Sun, Dec. 6, W 130W 4 I - amps V(panel) - volts Consider that the user wants to connect a 2 Ohm load. If it is connected directly it consumes 55 W. To draw maximum power (130W), connect a buck converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferred R equiv R load 2 D, D R R load equiv D is adjusted automatically by a maximum power point tracker (MPPT) so R equiv R max

30 Impedance matching I 1 D I in out in I Source + V in DC DC Boost Converter V out + V in 1 D Rload V out Iout I in + V in Equivalent from source perspective R equiv Vin D Vout V R 1 equiv Iin Iout I 1 out 1 D 2 out 2 1 D 1 D R load 30

31 Example of drawing maximum power from solar panel PV Station 13, Bright Sun, Dec. 6, 2002 I sc 6 5 P max is approx. 130W (occurs at 29V, 4.5A) I - amps For max power from panels, attach R load 29V V A V(panel) - volts V oc I-V characteristic of 6.44Ω resistor But as the sun conditions change, the max power resistance must also change. 31 Also, the load changes based on the user needs.

32 Example of a directed connected load different to that yielding maximum power PV Station 13, Bright Sun, Dec. 6, W 5 So, the boost converter reflects a high load resistance to a low resistance on the source side I - amps W V(panel) - volts Consider that t the user wants to connect a 100 Ohm load. If it is connected directly it consumes 14 W. To draw maximum power (130W), connect a boost converter between the panel and the load resistor, and use D to modify the equivalent load resistance seen by the source so that maximum power is transferred D is adjusted 2 Requiv 6.44 automatically by a MPPT Requiv 1 D Rload, D Rload 100 controller so Requiv R max

33 Impedance matching I D I in I in 1 out D Source + V in DC DC SEPIC V out + DV in 1 D R load V I out out I in + V in Equivalent from source perspective R equiv R equiv V I in in 1 D V D DIout 1 D out 1 D D 2 V I out out 1 D D 2 R load 33

34 Impedance matching R equiv V I in in out 1 D V D DIout 1 D 1 D D 2 V I out out 1 D D 2 R load For any R load, as D 0, then R equiv (i.e., an open circuit) For any R load, as D 1, then R equiv 0 (i.e., a short circuit) Thus, the SEPIC can sweep the entire I-V curve of a solar panel in order to achieve the MPP regardless of actual load used or received solar energy 34

35 Grid-tied inverter control As it has been explained, we can place a dc-dc converter between the PV array and the inverter so the PV array output is controlled so that it provides its maximum power output regardless of the actual load or other condition. The output of the dc-dc converter is the power input of the inverter. Output voltage of the inverter can then be controlled to synchronize with the grid. But what if we want to control the power output of the inverter even without a dc-dc converter at the output of the PV array? The, consider the following. Vdc Equivalent Circuit A+ B+ Z inv Mot A B V inv + cos( t ) Mot At 60 Hz, inverter impedance Zinv V inv V dc 2 m a Z inv is mostly resistive unless a large inductor is added

36 The Electrical Circuit Model Z inv Z grid I, S, P, Q V inv + cos( t ) + V grid cos( t) Inverter Grid I is the phasor current S (complex power) = P + jq P is the active power Q is the reactive power Impedances Z can be expressed as R + jx Zinv depends on circuit parameters and on the inverter controller action 36

37 The Electrical Circuit Model, cont. For typical 60Hz systems, the circuit resistance R s are much higher than the inductive reactance X s (otherwise the inverter can be controlled so it is a mostly resistive impedance). Also, voltage angle δ is zero because the inverter control signal is assumed to be a replica of the grid voltage (also, current standards require that the power factor of grid tied inverters is 1). Then, it can be found that Active power Reactive power Vgrid P V inv Vgrid Q 0 R R tot Thus, we control the direction and amount of P by adjusting this difference 37

38 Effect of real and reactive power from PV inverters 38

39 Effect of real and reactive power from PV inverters With grid tied inverters, when PV power increases, the real power provided by the grid is reduced by the reactive power provided by the grid is not significantly changed. Hence, power factor from the grid is reduced. 39

40 Grid Tied Inverters Traditional architecture (SMA Sunny Boy, PV Powered, Fronius, Xantrex, and others): PV Powered Fronius 40

41 Islanding inverters Alternative to traditional architecture to provide backup and operation without grid connection (SMA Sunny Boy Sunny Island 5048U). 41