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Solar PV Basic Concepts

Updated: Jun 24, 2020

We will start discussing the basic concepts regarding solar PV. These are the most basic concepts that will serve as the foundation for all the following chapters. We will be discussing about the solar resource, how to optimize the available solar resource through tilt and orientation, how the PV modules convert sunlight into electricity and lastly, the different types of solar PV systems: on-grid, off-grid & hybrid systems and their applications: residential, commercial and utility-scale.


Solar Resource


The sun is a sphere of hot gas and plasma that produces energy through nuclear fusion at its core. The sun fuses 620 million metric tons of Hydrogen gas per second to form Helium. This process converts some of the mass to energy which is radiated outward from the core.


The solar radiation emitted by the sun gets spread out in space, which means that the solar radiation received by an object decreases as it gets farther from the sun. Fortunately, the Earth’s distance from the sun is just inside the Goldilocks’ Zone. This zone is the region around a star where the received radiation is not too much or too less, allowing liquid water, and therefore, also life, to exist.


In 14 and a half seconds, the Sun provides as much energy to the Earth as humanity uses in a day. This just shows the huge potential of solar energy as an energy source. In 2011, the annual global energy consumption is at 17 TWy (TeraWatt-year, which is equivalent to 8.766 x 1012 kiloWatt-hours). The total energy potential of all the energy sources available on Earth is shown below:

The solar irradiance or the power per unit area received from the sun outside the Earth’s atmosphere is equal to 1.36kW/m2. This value is called the solar constant because, as the name suggests, it is a fairly constant value that does not change significantly. The radiation at the Earth’s surface, however, varies greatly due to:


  • atmospheric effects, including absorption and scattering

  • local variations in the atmosphere, such as water vapor, clouds, and pollution

  • latitude of the location

  • the season of the year and the time of day


As solar radiation passes through the atmosphere, it is subjected through absorption and scattering. Absorption reduces the amount of radiation that reaches the surface of the Earth while scattering, on the other hand, is responsible for making light appear to be coming from all regions in the sky. The scattered light created by this effect is called diffuse radiation.


Solar irradiance can be measured hourly for every location and can be used to check system performance at any given time. For system design, however, the average daily solar insolation is used. The solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of kWh/(m2day). The average daily solar insolation on a specific place is most often referred to using “peak sun hours”. This term refers to the solar insolation that a particular location would receive if the sun were shining at its “maximum value” of 1kW/m2 for a certain number of hours. I have put maximum value in quotes because, in reality, the sun can shine to give more than 1kW/m2during noontime. The “maximum value” is conveniently chosen to be 1kW/m2because PV modules are rated at an input of this value, 1kW/m2. A location that is said to have 5 peak sun hours receives 5kWh/m2/day. This is equivalent to having an irradiance of 1kW/m2 for 5 hours.


The peak sun hour data for every location is readily available from NASA’s web mapping application named: Projection of Worldwide Energy Resources or POWER. It contains geospatially enabled solar, meteorology, and cloud-related parameters formulated for assessing and designing renewable energy systems. It has a resolution of ½ by ½ degree datasets (latitude/longitude) by a single point. This means that one set of data describes every ½ degree latitude by ½ degree longitude square of space on the map.


This can be accessed on the internet from https://power.larc.nasa.gov/. When you go to this website you will see their homepage that looks like this:

You will need to scroll down to the Multiple Data Access Options and click on Power Data Access Viewer.

You will be directed to a map with the POWER Single Point Data Access window open.

  • On 1. Choose a User Community, choose SSE-Renewable Energy.

  • On 2. Choose a Temporal Average, choose Climatology.

  • On 3. Enter Lat/Long or Add a Point to Map, you can enter the latitude and longitude of the site or you can manually click it on the map using the pin button above the Clear button.

  • The Select Time Extent on number 4 will not be needed anymore.

  • On 5. Select Output File Formats, choose ASCII.

  • On 6. Select Parameters, double click on the last folder (Tilted Solar Panels) and click on the first checkbox (Solar Irradiance for Equator Facing Tilted Surfaces).

  • Scroll down on the bottom part of the window and click Submit.

The window will change to the next page that looks like this:

Click on ASCII. It will open a new tab with the results that we want. It will look like this:

The peak sun hours for the location is the one on the rightmost column, ANN, which means annual average and on the second row, named SI_EF_TILTED_SURFACE_0, which pertains to the irradiation for a horizontal surface. For this example, the peak sun hours is 4.75.


The peak sun hours is very important in solar PV system design because it can quickly give you an idea of how much energy a solar PV system will produce for a given location. A very simple and quick, but for most cases, accurate enough calculation of solar PV production is given below:


Daily Energy Production = Solar PV System Size (kW) * Peak Sun Hours * Estimated System Efficiency


An estimate of 80% for the system efficiency would, for this purpose, be reasonably accurate.


Optimum Tilt and Orientation


Tilt

The power density of the sunlight hitting the PV modules will be at its maximum when the sun is perpendicular to the PV modules as shown by the image below:

Since most solar PV systems have fixed-tilt modules, we have to choose a tilt angle that will allow our PV modules to absorb the most amount of sunlight over the whole year. During the summer months where the sun is higher in the sky, lower tilt angles allow maximum sunlight. During the winter months, the sun is lower in the sky and larger tilt angles are optimal. To get the maximum total sunlight over the whole year, however, the tilt angle should be equal to the latitude of the site.


In matching solar PV production to the load of a building, it is important to remember that we have the option to increase or decrease the tilt angle with respect to the optimum for high winter or high summer consumptions. We will discuss about the importance of matching solar PV production to the site’s consumption in the next chapters.


Orientation

On the northern hemisphere, the ideal orientation for PV modules is to point them due South and vice versa for those in the southern hemisphere. However, it is also important to note that there is a difference between true north or South and magnetic north or South. The true north or South poles are where the Earth’s rotation axis intercepts its surface. The Earth’s magnetic north and South poles do not directly coincide with the true north and South. The difference in the angle between two compasses pointing on both the true and magnetic north or South is called the magnetic declination.


To adjust a compass reading (which points to magnetic north and South) to get true north or South, you must know the magnetic declination angle for your location. A magnetic declination angle of -10 degrees means that you must adjust your compass reading by 10 degrees due west (for negative, east for positive) to get the true north or South reading. You can easily get the magnetic declination angle for a certain location on this website: http://www.magnetic-declination.com/.

Just enter your location and country on the left side of the screen and it will already give you the magnetic declination angle for your location.