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.
Photoelectric Effect
The photoelectric effect is the phenomenon that allows the PV modules to convert light into electricity. It was first discovered by Heinrich Hertz in 1887. It was not explained or understood, however, until 1905 by Albert Einstein. This phenomenon refers to electrons being released from or within a material when it absorbs electromagnetic radiation, specifically, light.
PV modules are made from silicon, which is a semiconductor. In an atom, the positively-charged protons are clumped together along with the neutrons in the nucleus. The electrons are arranged in shells around this nucleus, like concentric spheres.
The innermost shell contains the lowest energy electrons are while the outermost shell contains the highest-energy ones. The outermost shell is called the valence shell, while the electrons in this shell are called valence electrons. These valence electrons have the highest energy level that an electron can have while still being bound to its parent atom. If they receive more energy, they will become free electrons which are not anymore bound to its parent atom. When light shines on a solar cell made from a semiconductor material, the valence electrons absorb the incoming light and they become free electrons. These free electrons are then collected at the terminals of the solar cell to produce electric voltage. When this illuminated solar cell is then connected to an electric load, these free electrons flow as electric current, transferring the energy absorbed from the sun.
Residential, Commercial and Utility-Scale Solar PV Systems
We have already discussed about solar PV having the advantage of being a distributed energy source. This is because solar PV is modular in nature. The system size can be easily scaled up or down by increasing or decreasing the number of PV modules. In reality, residential, commercial and utility-scale solar PV systems do not have much of a difference in terms of system design other than the number of modules and the size and type of inverter used.
Residential Solar PV Systems
Residential solar PV systems usually have system sizes of 1-10kW, depending on the electric consumption of the house and the available roof space. String and microinverters are used for this type of system.
For on-grid solar PV systems, the homeowner can apply to be under a net metering scheme. With a net meter, excess energy produced by the solar PV system can be sold back to the grid for an agreed price which is called the feed-in-tariff. This gives the homeowner an additional incentive for excess energy that should normally have been unused.
Commercial Solar PV Systems
Commercial solar PV systems have system sizes from 10kW up to the megawatt range, depending on the size of the roof. String inverters are more commonly used for this type of solar PV system, but just recently, microinverters have also been gaining more popularity.
Commercial solar PV systems require a much bigger investment, so it usually has a more complex monitoring system than residential systems. Protection systems like lightning arresters are also commonly used to protect this investment.
Utility-Scale Solar PV Systems
Utility-scale solar PV systems are more popularly known as solar farms. Instead of being mounted on roofs, they are usually ground-mounted. They have sizes in the megawatt range, the largest of which is the 1,574MW Tengger Solar Park in Zhongwei, Ningxia, China. This solar farm covers 43 square kilometers of land and used a total of 5.2 million PV modules.
Central inverters are more commonly used for this type of system although string inverters may also be used for the smaller ones. Unlike residential and commercial systems that just tap or connect on the main supply of the house or building, solar farms tap on the utility grid itself. Because of this, they also have to have their own substations, complete with switches, transformers and protection systems.
Types of Solar PV Systems
On-Grid Systems
On-grid solar PV systems are connected in parallel with the utility grid acting as a backup supply. When the system produces energy in the morning, all of this energy is consumed by the house or building although it may not meet 100% of its energy demand. The deficit is taken from the utility grid.
During noontime, the system may produce more energy than the demand. Under a net metering scheme, this excess energy is exported back to the grid. The utility pays for this exported electricity at a fixed price per kWh called the feed-in-tariff. The payment is usually just deducted from that household’s monthly electric bill.
During the afternoon, the same thing happens with that in the morning. The house or building consumes the energy that the system produces and imports the deficit from the grid to meet the total demand. In the evening, the system is not producing any energy, so the house or building imports all of its energy demand from the grid.
Off-Grid Systems
Off-grid solar PV systems are, as its name suggests, off the grid. This means that the house’s or building’s only power supply will be the solar PV system. This is possible through the use of batteries. The system is designed in a way such that in the morning, it produces more than the household’s consumption. The excess energy is used to charge the batteries, which will later be used to meet the nighttime energy demand.
Off-grid systems are required to be oversized to account for cloudy/rainy days and for the winter months. This usually makes it, for most of the time, less feasible for sites with an already existing utility supply. Its main advantage, however, is that it can be used to provide electricity on locations that are too far away for the utility grid to reach.
Hybrid Systems
Hybrid solar PV systems are a combination of on-grid and off-grid systems. This means that they are still connected to the utility grid, but they also have a battery backup which can be used in case of a power outage. This type of system can also use the utility grid to charge the batteries in case of low solar PV production. Also, it can help the user save from their electricity bill by storing energy from the grid during off-peak hours and using this during peak hours, where the price of electricity is more expensive.
Parts of a Solar PV System
On-grid Solar PV Systems
PV Array – this is composed of the PV modules which are grouped into strings. It acts as the power generating unit of the system.
DC Roof Isolator – depending on the standards followed, a DC roof isolator may be required for fire safety, especially for commercial solar PV systems. It is used to isolate the PV array from the rest of the system.
Inverter – it converts the DC electricity produced by the PV modules to alternating current (AC) and performs many other functions.
AC Circuit Breaker – used to disconnect the AC utility supply to the inverter and vice versa.
Monitoring Device – some solar PV systems use a separate device to monitor the system’s parameters while some inverters have the monitoring system built-in.
Off-grid Solar PV Systems
Off-grid Inverter – off-grid solar PV systems require a different kind of inverter than on-grid systems.
Batteries – used to store unused energy and supply it when needed.
Charge Controller – older off-grid solar PV systems use a charge controller. It controls the current going in and out of the battery allowing it to work optimally.
Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.