[MUSIC] Hello here we start the model about solar reserves. The aim of the sequence is to list and understand the terms and units to quantify solar radiation received by a PV model. What are the global diffuse direct and ground reflected components of solar radiation. And how do we sum up all these components to have the total radiation that is received by a PV model? To do this, we'll go through the definitions of solar irradiance which relates to power per square meter and solar irradiation, which is the energy per square meter. So imagine that we are at noon, on a sunny day, in summer and we have this panel. The incident solar power at every square meter is called solar irradiance. Let's say that it receives 1000 watt per square meter at this time. If at the point, there's a big thick dark cloud that arrives and covers the sun, the solid irradiance will suddenly decrease in this case, down to 200 what per square meter. Let's see the whole day now through the solar irradiance early curve. The irradiance will be always zero at night time. It will start increasing from sunrise and for a sunny day it will arrive to its maximum around noon, then it will decrease to become zero again after the sunset. Thus the radiance tells us the power per square meter that would be available from the sun at every moment. If we now want to know the amount of energy that would be available for this given day. We can do it with the sum of all irradiance values over the whole period. This give us the value of solar irradiation. Solar irradiation is usually expressed in watt hour per square meter or even more commonly in kilowatt hour per square meter. Remember that one kilowatt hour per square meter is the equivalent energy of 1000 watt per square meter of irradiance during one hour. For our particular example, the daily solar radiation was 5.8 kilowatt hour per square meter represented as the field area in the figure. Solar irradiation thus indicates the amount of energy per square meter and can be calculated for any time interval one day, one month, one year. In Paris region solar irradiation in one year is about 1200 kilowatt hour per square meter on a horizontal plane. But if we tilt it about 35 degrees towards the South, then the yearly irradiation will be higher. About 1400 kilowatt hour per square meter, which makes a daily mean irradiation of 3.8 kilowatt hour per square meter per day. Why tilting the collecting surface makes this difference? Is this difference always the same? Well, the answers will be found if we introduce the components of solar radiation. This panel when it is upside exposed to sunlight, receives the radiation from three sources, the direct component, which is the amount of radiation that comes directly from the solar disk to the panel. This company is at its highest level, if the panel is facing towards the sun, the diffuse component, which is the contribution of solid radiation that comes from all other directions of the sky that are visible to the panel. So not the part of the sky that is behind the panel and the ground reflected component which is the amount of solid education that is reflected by the ground towards the panel. The maximum direct component is achieved when the sun is visible and the surface is facing the direction of the sun. In this case, we say that the panel received the direct normal irradiance, also known as DNI. If the panel is facing any other direction, the angle between this direction and some direction known as the angle of incidence. AOI needs to be known to calculate the direct component as the projection of the DNI on the panel. The calculation of the angle of incidence involves four angles two concerning the sun position and two concerning the panel. Let's start with the solar angles. The solar zenith angle is found from the vertical, there's any direction to the direction of the sun, it's complimentary, that is the elevation angle, the angle from the horizontal plane to the direction of the sun is also widely used. The solar azimuth angle indicates the orientation of the sun from a reference direction. The azimuth is then minus 90° when the sun is at the East in the morning plus 90 degrees when, the sun is at the West in the evening and it's zero when the sun is the South. As for the two angles concerning the panel, the tilt toward slope angle is here when the panel is on the horizontal plane and it's 90 when it is at a vertical position. And the orientation or azimuth angle is zero when it's facing the South it's minus 90 when it's facing the East and plus 90 when it's facing to the West. With these angles which are shown in the figure, we can derive the cosine of the angle of incidence with the shown trigonometric expression that comes from the scalar product of the vector pointing to the sun to the vector normal to the panel surface. A particular case of the angle of incidence is when the panel is on the horizontal plane. In this case, the angle of incidence is equal to the solar zenith angle. Let's now understand the diffuse component received by our panel. When the panel is horizontal, the tilt is zero. The diffuse irradiance is then called diffuse horizontal irradiance or DHI. As I tilt the panel more, it will see the small portion of the sky. So the diffuse component will decrease. If we assume that diffuse radiation is isotropic in the sky that is that all the directions send the same amount of radiation, then the following multiplication factor would be applied. This factor is close to one for his more tilt angles and it gets 0.5 one half if the panel is vertically tilt as a show so that the beta is 90° and as the panel sees only half of the sky. What about the ground reflected component? Imagine that the ground in front of the panels is flat and horizontal. Let's consider the total radiation incident to this surface. Normally referred to as global horizontal irradiance or GHI. Part of the radiation will be reflected by the surface back to the sky, and this fraction is given by the so called albedo. The ground albedo is around 0.2 for grass, meaning that 20% of the incoming radiation is reflected back and it can get close to one for fresh snow. Finally, we need to know which part of this reflected radiation will reach our panel. And here is a simple factor that accounts for that. This factor assumes that the surface is Lambertian, which means that the ground reflects radiation in the same way in all directions. When the panel is vertical at the 90 degrees, the factor is 0.5, as the panel gets half of the reflected irradiance by the ground. But if the final is horizontal, the tilt is zero, the factor becomes zero as the panel does not see the ground at all. The sum of all these contributions, direct plus diffuse plus ground reflected give the total solar irradiance that is received by our photovoltaic panel for any position of the sun, and for any panel tilt and orientation angles. The sum is known as global tilt irradiance or more commonly as plane of array irradiance. For our particular summer day of the example, with the panel at 35 degrees still towards the South. This was the daily curve that we had for the POA, and let's see how the three Iranians components contributed to it. The direct component was very important. The largest contribution all in the morning as it fell down to zero when the sun was blocked by the clouds from noon on. The diffuse was much lower than the direct in the morning but in the afternoon it was almost the only contribution to the POA. The ground reflected company was very small all through the day with the contribution of only one percent to the daily irradiation. This ground reflecting company is usually this small as the panels in general are not very much tilt. We could actually address the question of what are the optimal tilt angles. The answer would then depend on many considerations related to the specific photovoltaic project such as, the available solar reserves and the panel mounting systems considered, fix panels when access tracking to access tracking. So this is an interesting discussion for future sequence. Let's finish with a particular interesting case when the tilt angle is zero. In this case the POA equation gets highly simplified. The direct component becomes the DNI, projected with the coastline of the solar zenith angle. The diffuse becomes the and the ground reflected contribution is zero and the becomes actually equal to the GHI. At this point, it is important to highlight that the GHI, DHI and DNI are three standard variables of the world meteorological organization. They are related through this equation and they are very well used in the solar energy field. That is why they were introduced here and actually the GHI, DHI and DNI will be often used during the solar research module and they will be further introduced in the future sequences. in particular, these three variables are interesting as they can be directly measured by dedicated instruments. We will cover all the subjects in future sequences. [MUSIC]
