Solar Energy PV Systems. Ass. Prof. Hristo Hristov Technical University of Gabrovo

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1 Solar Energy PV Systems Ass. Prof. Hristo Hristov Technical University of Gabrovo

2 THE SOLAR RESOURCE To design and analyze solar systems, we need to know how much sunlight is available The source of insolation /Solar Radiation/ is, of course, the sun that gigantic, 1.4 million kilometer diameter, thermonuclear furnace fusing hydrogen atoms into helium. The resulting loss of mass is converted into about MW of electromagnetic energy that radiates outward from the surface into space. While the interior of the sun is estimated to have a temperature of around 15 million kelvins, the radiation that emanates from the sun s surface has a spectral distribution that closely matches that predicted by Planck s law for a 5800 K blackbody. Figure shows the close match between the actual solar spectrum and that of a 5800 K blackbody. The total area under the blackbody curve has been scaled to equal 1.37 kw/m2, which is the solar insolation just outside the earth s atmosphere. Also shown are the areas under the actual solar spectrum that corresponds to wavelengths within the ultraviolet UV (7%), visible (47%), and infrared IR (46%) portions of the spectrum. The visible spectrum, which lies between the UV and IR, ranges from 0.38 μm (violet) to 0.78 μm (red).

3 THE SOLAR SPECTRUM The extraterrestrial solar spectrum compared with a 5800 K blackbody As solar radiation makes its way toward the earth s surface, some of it is absorbed by various constituents in the atmosphere, giving the terrestrial spectrum an irregular, bumpy shape. The terrestrial spectrum also depends on how much atmosphere the radiation has to pass through to reach the surface.

4 Air mass ratio The air mass ratio m is a measure of the amount of atmosphere the sun s rays must pass through to reach the earth s surface. For the sun directly overhead, m = 1. Thus, an air mass ratio of 1 (designated AM1 ) means that the sun is directly overhead. By convention, AM0 means no atmosphere; that is, it is the extraterrestrial solar spectrum. Often, an air mass ratio of 1.5 is assumed for an average solar spectrum at the earth s surface. With AM1.5, 2% of the incoming solar energy is in the UV portion of the spectrum, 54% is in the visible, and 44% is in the infrared.

5 The impact of the atmosphere on incoming solar radiation for various air mass ratios Solar spectrum for extraterrestrial (m = 0), for sun directly overhead (m = 1), and at the surface with the sun low in the sky, m = 5.

6 THE EARTH S ORBIT The earth revolves around the sun in an elliptical orbit, making one revolution every days. The eccentricity of the ellipse is small and the orbit is, in fact, quite nearly circular. The point at which the earth is nearest the sun, the perihelion, occurs on January 2, at which point it is a little over 147 million kilometers away. At the other extreme, the aphelion, which occurs on July 3, the earth is about 152 million kilometers from the sun. The earth s spin axis is currently tilted with respect to the ecliptic plane and that tilt is, of course, what causes our seasons.

7 Altitude angle A key solar angle, namely the altitude angle βn of the sun at solar noon. The altitude angle is the angle between the sun and the local horizon directly beneath the sun.

8 CLEAR SKY DIRECT-BEAM RADIATION Solar flux striking a collector will be a combination of direct-beam radiation that passes in a straight line through the atmosphere to the receiver, diffuse radiation that has been scattered by molecules and aerosols in the atmosphere, and reflected radiation that has bounced off the ground or other surface in front of the collector

9 Direct-Beam Radiation Illustrating the collector azimuth angle φc and tilt angle along with the solar azimuth angle φs and altitude angle β. Azimuth angles are positive in the southeast direction and are negative in the southwest.

10 Diffuse Radiation Diffuse radiation can be scattered by atmospheric particles and moisture or reflected from clouds. Multiple scatterings are possible Diffuse radiation on a collector assumed to be proportional to the fraction of the sky that the collector sees

11 Reflected Radiation The ground is assumed to reflect radiation with equal intensity in all directions

12 Tracking Systems Trackers are described as being either two-axis trackers, which track the sun both in azimuth and altitude angles so the collectors are always pointing directly at the sun, or single-axis trackers, which track only one angle or the other. A single-axis tracking mount with east west tracking. A polar mount has the axis of rotation facing south and tilted at an angle equal to the latitude. Two-axis tracking angular relationships.

13 Annual insolation, assuming all clear days, for collectors with varying azimuth and tilt angles. Annual amounts vary only slightly over quite a range of collector tilt and azimuth angles.

14 Daily clear-sky insolation on south-facing collectors with varying tilt angles. Even though they all yield roughly the same annual energy, the monthly distribution is very different.

15 Clear sky insolation on a fixed panel compared with a one-axis, polar mount tracker and a two-axis tracker.

16 SOLAR RADIATION MEASUREMENTS There are two principal types of devices used to measure solar radiation. The most widely used instrument, called a pyranometer, measures the total radiation arriving from all directions, including both direct and diffuse components. That is, it measures all of the radiation that is of potential use to a solar collecting system. The other device, called a pyrheliometer, looks at the sun through a narrow collimating tube, so it measures only the direct beam radiation. Data collected by pyrheliometers are especially important for focusing collectors since their solar resource is pretty much restricted to just the beam portion of incident radiation.

17 Example of a pyrheliometer on a solar tracker which keeps the instrument pointed at the sun. A black shadow band keeps the pyranometer shaded, so that it measures diffuse radiation only. The global solar radiation is then calculated from direct and diffuse radiation.

18 Alexandre-Edmond Becquerel ( ) was a French physicist who discovered the photovoltaic effect, the physics behind the solar cell in One of his primary interests was in the study of light, investigating the photochemical effects of solar radiation Edward Weston ( ) was an English chemist and was an early competitor of Thomas Edison in the early days of electricity generation and distribution. He was awarded a patent for the solar cell in 1882.

19 Best laboratory PV cell efficiencies for various technologies

20 The Solar Spectrum We know the band gap for silicon is 1.12 ev, corresponding to a wavelength of 1.11 μm, which means that any energy in the solar spectrum with wavelengths longer than 1.11 μm cannot send an electron into the conduction band. And, any photons with wavelength less than 1.11 μm waste their extra energy. Photons with wavelengths above 1.11 μm don t have the 1.12 ev needed to excite an electron, and this energy is lost. Photons with shorter wavelengths have more than enough energy, but any energy above 1.12 ev is wasted as well.

21 A GENERIC PHOTOVOLTAIC CELL Electrons flow from the n-side contact, through the load, and back to the p-side where they recombine with holes. Conventional current I is in the opposite direction. A simple equivalent circuit for a photovoltaic cell consists of a current source driven by sunlight in parallel with a real diode. Two important parameters for photovoltaics are the short-circuit current ISC and the open-circuit voltage VOC.

22 Photovoltaic current voltage relationship for dark (no sunlight) and light (an illuminated cell). The dark curve is just the diode curve turned upsidedown. The light curve is the dark curve plus ISC

23 Photoelectric Effect

24 Photovoltaic Solar Cells Generate electricity directly from sunlight 2 Main types: Single-crystal silicon (traditional) Widespread Expensive to manufacture Dye-sensitized ( nano ) Newer, less proven Inexpensive to manufacture Flexible

25 How a Silicon-Based Solar Cell Works Light with energy greater than the band gap energy of Si is absorbed Energy is given to an electron in the crystal lattice The energy excites the electron; it is free to move This separation of electrons and holes creates a voltage and a current A positive hole is left in the electron s place

26 How a Dye-Sensitized Cell Works Light with high enough energy excites electrons in dye molecules Excited electrons infused into semiconducting TiO 2, transported out of cell Positive holes left in dye molecules Separation of excited electrons and holes creates a voltage

27 The PV conversion efficiency The PV conversion efficiency (η) is calculated from the measured maximum or peak PV power (Pmax ), device area (A), and total incident irradiance (Etot ): P E max tot 100 A Parameters which directly influence the measurement or calculation of the efficiency, and therefore must be well controlled and well-defined, are the area of the device, the spectrum and intensity of incident radiation which determine Etot, and the temperature of the device.

28 FROM CELLS TO MODULES TO ARRAYS Since an individual cell produces only about 0.5 V, it is a rare application for which just a single cell is of any use. Instead, the basic building block for PV applications is a module consisting of a number of pre-wired cells in series, all encased in tough, weather-resistant packages. A typical module has 36 cells in series and is often designated as a 12-V module even though it is capable of delivering much higher voltages than that. Some 12-V modules have only 33 cells, which, as will be seen later may, be desirable in certain very simple battery charging systems. Large 72-cell modules are now quite common, some of which have all of the cells wired in series, in which case they are referred to as 24-V modules. Some 72-cell modules can be field-wired to act either as 24-V modules with all 72 cells in series or as 12-V modules with two parallel strings having 36 series cells in each. Multiple modules, in turn, can be wired in series to increase voltage and in parallel to increase current, the product of which is power. An important element in PV system design is deciding how many modules should be connected in series and how many in parallel to deliver whatever energy is needed. Such combinations of modules are referred to as an array.

29 The I V curve and power output for a PV module. At the maximum power point (MPP) the module delivers the most power that it can under the conditions of sunlight and temperature for which the I V curve has been drawn.

30 Bypass Diodes for Shade Mitigation Showing the ability of bypass diodes to mitigate shading when modules are charging a 65 V battery. Without bypass diodes, a partially shaded module constricts the current delivered to the load (b). With bypass diodes, current is diverted around the shaded module.

31 Photovoltaic technologies (1) single crystal, the dominant silicon technology; (2) multicrystalline, in which the cell is made up of a number of relatively large areas of single crystal grains, each on the order of 1 mm to 10 cm in size, including multicrystalline silicon (mc-si); (3) polycrystalline, with many grains having dimensions on the order of 1 μm to 1 mm, as is the case for cadmium telluride (CdTe) cells, copper indium diselenide (CuInSe2,) and polycrystalline, thin-film silicon; (4) microcrystalline cells with grain sizes less than 1 μm; and (5) amorphous, in which there are no single-crystal regions, as in amorphous silicon (a-si).

32 Small Off-grid DC System DC systems include applications that directly use the DC energy produced by PV modules to supply power to DC loads. These include space-based systems, portable solar devices and small consumer products, very small residential systems, water pumping, and other small applications that are typically less than 1 kw in size and for which all of the loads only require DC electricity to function.

33 Off-grid AC System Off-grid systems are those in which the PV energy is converted to AC power, but there is no utility grid available. The loads in this type of system run from AC power. The inverter in this type of system acts to regulate the AC voltage to all of the loads. Energy storage (i.e. usually batteries) is typically included in off-grid systems to allow for the required power balance between the intermittent PV energy source and the load requirements (i.e. lighting at night). A common example of this is a remote home with a system composed of solar modules on the roof or on a pole nearby, an off-grid inverter, and a battery bank for storage.

34 On-grid Systems Utility grid-connected systems are those in which the energy produced by PV modules is converted to AC electricity and either used on-site or injected into the utility grid. To accomplish this, the DC PV output must be converted to AC current by an inverter. Like any power plant, this AC current must be synchronized with the utility grid that it is being interconnected with. This includes the AC voltage and frequency. As the excess PV generated energy is passed into the utility grid which is also being energized by other power generating sources, it is distributed to all of the loads connected to the grid and not to specific equipment. On-grid Systems; residential, commercial, and utility-scale systems.

35 Residential on-grid system

36 Denver, Colorado 300kWP commercial grid-tied rooftop ballasted system

37 Perovo Capacity: 100 MW Country: Ukraine Owner: Activ Solar Developer: Activ Solar Module Maker: Module type: c-si Grid connection: 2011 Sarnia Capacity: 92 MW Country: Canada Owner: Enbridge Developer: First Solar Module Maker: First Solar Module type: CdTe Grid connection: 2010

38 Hybrid PV Systems The term hybrid can be broadly used to denote a PV system that is used in conjunction with one or more auxiliary sources of power. Traditionally this has meant a second source such as wind or hydroelectric turbines, however, many modern PV systems employ auxiliary dispatchable (on-demand) sources such as on-site fossilfueled generators or the utility grid. The term multimode is often reserved to describe the special case of a hybrid system which operates either in parallel with an external AC source (on-grid) or as a stand-alone AC source (off-grid) when grid power is unavailable.

39 Simplified grid-connected PV system Simplified diagram of the first of these systems a grid connected or utility interactive (UI) system in which PVs are supplying power to a building. The photovoltaics in a grid-connected system deliver dc power to a power conditioning unit (PCU) that converts dc to ac and sends power to the building. If the PVs supply less than the immediate demand of the building, the PCU draws supplementary power from the utility grid, so demand is always satisfied. If, at any moment, the PVs supply more power than is needed, the excess is sent back onto the grid, potentially spinning the electric meter backwards. The system is relatively simple since failure-prone batteries are not needed for back-up power, although sometimes they may be included if utility outages are problematic. The power-conditioning unit also helps keep the PVs operating at the most efficient point on their I V curves as conditions change.

40 On-grid Inverters The basic function of the inverter is to convert the DC power produced by the PV modules to AC power for system electrical loads. This is accomplished through the use of transistor-based power electronics circuitry. The power transistors are switched on and off at a high frequency in a manner that draws power from the PV modules at their maximum power point and passes the power to the AC grid (for grid-tied systems) or to the local loads (for off-grid systems).

41 Basic structures of grid-connected PV systems The basic structure of a typical grid connected PV system is shown in Figure It consists of a PV generator directly coupled to the input side of an inverter without any means of storage. The output side of the inverter is coupled to the grid via safety devices, which typically are integrated into the inverter. A meter measures the energy fed into the public grid. Based on this fundamental structure, three typical layouts of grid connected systems have emerged: 1. Central inverter/master slave systems. 2. String concept. 3. Module integrated power conditioning units.

42 Orientation and Tilt The orientation and tilt of a system impacts how much of the available irradiance the system can collect. There are general rules of thumb to follow when orienting a fixed tilt system. Theoretically, the optimal orientation, or surface azimuth, is true south/north (not magnetic south) and the optimal tilt is equal to the latitude. However, empirically, it is generally preferable to have the system facing the equator and tilted at approximately less than the local latitude. This is principally a consequence of poor weather being concentrated in the winter months. Other factors that influence the optimal orientation and tilt are: (1) convenience (an existing slope is often less expensive to install upon); (2) local obstructions (shading due to trees and surrounding buildings); (3) asymmetrical microclimates (consistent morning fog or afternoon showers); (4) Sensitivity to time-of-delivery generation.

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44 Solar Flight's Sunseeker NASA Helios Prototype

45 PV on the International Space Station

46 During the day, excess power from the array is sold to the utility; at night, the deficit is purchased from the utility.

47 References: Patel, Mukund R.; Wind and Solar Power Systems; CRC Press LLC; Handbook of Photovoltaic Science and Engineering, Second Edition; Edited by Antonio Luque and Steven Hegedus; John Wiley & Sons, Masters Gilbert M., Renewable and Efficient Electric Power Systems; John Wiley & Sons, Inc., Hoboken, New Jersey, Khaligh, Alireza, Omer C. Onar., ENERGY HARVESTING Solar, Wind, and Ocean Energy Conversion Systems; by Taylor and Francis Group, LLC; Pimentel, David, Editor, Biofuels, Solar and Wind as Renewable Energy Systems, Benefits and Risks, Springer Science+Business Media B.V Kalogirou, Soteris. Solar energy engineering: processes and systems; Elsevier Inc. 2009,