SOLAR POWER
FOR DUMMIES

As in any science or discipline, there is a lot of terminology that is used to describe whatever it contains. Solar power systems is no exception. So to make things more clear this section is dedicated to sort out some of the lingo in this field, and describe some of the integral parts of a solar power system..

The different systems

Monocrystaline, thin-film, silicone, hybrid inverter… So many buzz words! But what do they actually mean? In this section we start by giving a short description to the main types of photovoltaic type (what most people usually mean when they say solar power, the type that uses electro-chemical reactions in materials to transform the power of photons into electrical power) solar modules. Then we continue with a description of the different types of systems (Island, Hybrid and Grid-connected) that these can be used in depending on the needs of the end-customer.

Different Types of PV Technologies

Photo Voltaics is divided into different technologies. However, they all use the so called photovoltaic effect, a both chemical and physical phenomenon that describes the creation of an electric potential difference and a current in a semiconductor, when it is exposed to a source of light. This can then be used to generate electric power. The higher the irradiance of the source, the higher the current will be. Explained in an easy fashion, the light causes the electrons located in the valence band of the semiconductor to absorb energy. That was easy, wasn’t it?

With this increase in energy they jump to the conduction band and in doing so become free and start conducting. Modern type solar cells use semiconductor materials with doped PN-junctions for higher efficiency.

The different types of Photo Voltaic solar cells can also be divided in different groups. The big majority of cells and modules can be placed in either one of the two largest groups; crystalline silicone type and thin film type. Here we will only give a very short description of the two types and  their characteristics. 

For a full and detailed explanation of their physical and chemical properties we recommend the reader to aim for a PhD in photovoltaics and semi-conductor physics.

Thin Film Type Solar Cells

The first types of thin film solar cell technologies appeared already in the wake of the energy crisis in the 1970ies. Despite the relatively long time in development it is first in the past two decades that the thin film cells have begun to reach adequate efficiencies. The advantage of the thin film technologies is that they do not require silicone, the price of which can fluctuate wildly on the global commodity market and thus affect the price of the cells. Also the energy required to produce the thin film type cells is considerably lower than for silicone type cells. The thin structure of the cells also opens up for making flexible modules, something that could revolutionize the installation of solar panels. However right now, the use of Cadmium in some of the manufacturing processes is a big environmental drawback. Also, the struggles to reach higher efficiencies and the fact that the average silicone type solar modules still have a better efficiency has slowed down the development and deployment of the thin film modules. In 2014 the global production was around 9% and had been declining the last five years [Photovoltaics Report 2015,” Fraunhofer Institute for Solar Energy Systems, ISE, Freiburg, 2015-11-17].

Monocrystalline and Polycrystalline Silicon Type Solar Cells

The crystalline silicone type is the most common type of solar cell in the world today. In 2014 they accounted for roughly 90% of the solar cells being produced [Photovoltaics Report 2015,” Fraunhofer Institute for Solar Energy Systems, ISE, Freiburg, 2015-11-17]. Therefore they are also considered the most conventional type of solar cell and they are also the most deployed and available worldwide. Crystalline silicone solar cells can also themselves be divided in two main groups; Monocrystalline and Polycrystalline silicone solar cells. As the name implies they both use silicone as the Photo Voltaic semiconductor, but the crystal structure of the silicone is different between the two. The Monocrystalline cells overall have a higher efficiency, but also sells at a higher price. The Polycrystalline cells have a lower efficiency than the Monocrystalline cells, but the lower price and easier manufacturing process make them the most widely manufactured and deployed type of PV-solar cells. The silicone type solar cell technologies can be regarded as relatively mature and they have an average efficiency in commercial samples of just over 16% for Polycrystalline cells, and up to 21% for the best commercial types of Monocrystalline (depending on the technology their efficiency vary between 17-21%)  [Photovoltaics Report 2015,” Fraunhofer Institute for Solar Energy Systems, ISE, Freiburg, 2015-11-17].

Different types of systems 

Island Type Solar Power Systems

When there is no possibility (or desire for that matter) to connect to a national or regional electric grid, the Island system is what is used. At minimum, the systems should consist of a battery bank (usually consisting of an array of batteries that sum up to the system voltage of 12V or 24V), a solar charge controller, and the solar module or modules themselves. When the sunlight hits the modules, they start generating a current at a voltage which is determined by the solar charge controller. For charging 12V battery systems a voltage of around 14.4V is usually recommended and for 24V battery the charge voltage should
be 28.8V.

 Whatever electricity consuming appliances should then be connected to the charge controller so that all current drawn from the batteries is directed through the charge controller. This to ensure that the batteries are never over-discharged or even completely discharged something that damages the batteries. Also the charge controller keeps the voltage constant at the charging voltage as long as the sun is shining, and when the batteries are fully charged it disconnects them so they cannot be over-charged. It would be possible to have a system without the solar charge controller, and just let the solar modules directly charge the batteries. This would however mean that the voltage would most likely be too high, something which would damage the batteries and that might even cause an explosion, because of the hydrogen production that occurs when the voltage and charge current gets too high. So a solution without the charge controller is never recommended.

In order to use this setup in a household, a 12/24V to 230V step-up power inverter is usually required for using most all of standard everyday electronic items that is design to run on grid electricity.
A generic Island Type Solar Power System can be seen in
Figure 1.

Figure 1: : Graphical representation with electrical wiring of a generic Island-type solar power system, including Solar Modules, Solar Charge Controller, Battery Bank and an optional Power Inverter.

Hybrid Type Solar Power Systems

The Hybrid Type Solar Power System is, as the name implies, a hybrid between the Island and the Grid Connected System. This is the most popular type when there is an electric main grid available, but it is not considered reliable and suffers from shorter and/or longer power cuts. It can also be used for more complex systems; where the price of electricity is constantly monitored, and the system can then buy energy from the power company whenever it is cheap, and only use already stored energy from the battery bank or even sell electricity to the power company when the price is high.

It has the solar modules connected to a battery bank via a solar charge controller, just as the island system. But the battery bank is also connected to the grid via an inverter capable of converting 230VAC from the grid to 12/24VDC and vice versa. This means that when the electric grid is available and enabled, the inverter will charge the battery bank with the power from the grid at the same time as supplying the household with the grid power, as in a normal grid-connected household. When the batteries become fully charged the grid will then only feed the household.

When the sun is shining and the grid is available, the inverter system will first and foremost use the power from the battery bank which is being charged by the solar modules. If more power is needed, this will be taken from the grid. If no grid is available (if there is a black-out), then the inverter system will simply use the energy stored in the battery bank and convert it to 230VAC to supply the household. If there is power from the solar modules, they will meanwhile charge the batteries via the solar charge controller.

As mentioned earlier, some hybrid-inverters also have the ability to feed/sell electricity back to the electric grid. If for example the batteries are fully charged and the solar modules are generating more power than the household is currently using, the excess can be fed to the grid and sold to the electric supplier company to a price arranged in a prior agreement. Even more complex systems have the ability to monitor the price of electricity in real time, and can then be programmed to buy and sell energy as to optimize the total cost or even profit for the household.
An illustration of this setup is displayed  in Figure 2.

Figure 2: Graphical representation with electrical wiring of a generic Hybrid-type solar power system, including Solar Modules, Solar Charge Controller, Battery Bank, Power Inverter and Electric Grid.

Grid Connected Type Solar Power Systems

The Grid Connected Type Solar Power System is a system that is simple in the sense that it only connects a solar power system to the household’s 230VAC system and/or the available grid. No battery bank is used and the inverters designed for use with pure grid connected systems are also simple in the matter that they only convert the DC power from the solar modules to 230VAC and feed it directly to the household and/or to the grid. Usually they then also have a built in controller for setting and monitoring the operating voltage of the solar modules, to maximize their power output from at any given time during the conditions at that time. So no solar charge controller is needed. As in the case with the hybrid systems, depending on the electric infrastructure and capacity of the electric grid there is sometimes a possibility to feed power back to the grid. Since these systems have no battery bank to store energy, one then simply use the power from the solar system first if it is available, and then get the rest from the grid. If the solar system is generating more power than is being consumed by the household this can then be fed to the grid and sold to the electric supply company, as the case with the hybrid system.
An illustration of this setup is displayed below in Figure 3.

Figure 3: Graphical representation with electrical wiring of a generic pure Grid-type solar power system, including Solar Modules, Power Inverter and Electric Grid.

 solar module basics

The solar modules are, for obvious reasons, the central part in every solar power system. Most all of conventional modules are built from the same basic principle and work in a similar way, even if they have different efficiencies and characteristics depending on the type of materials used in the construction, and how they are composed.

One solar module is built from smaller elements called solar cells which are organized in an array. It is the cells that contain the semi-conductor PN-junctions that generate power when exposed to sunlight and put into an electric circuit. The cells are effectively diodes in the sense that they become forward biased when the light hits the cells, and they consist of an anode and a cathode. The incoming light then creates a potential difference between the anode and the cathode of the cells, the forward bias voltage. If the circuit is then closed, a flow of electrons (in other words a current) can pass from the anode to the cathode.

By arranging multiple cells in a series configuration (connecting the cathode of the first cell to the anode of the second, the cathode of the second to the anode of the third, and so on) one can then construct a module with a known open circuit voltage, by simply adding all of the voltages of each cell. 

By then adding multiple of these configurations in parallel the manufacturer can increase the current of the module and thus also the power.In order to monitor and evaluate the performance and health of a solar power system, there is a number of different parameters that are important.

Again, the incoming light itself is not enough to generate a current. This only puts the cells in forward bias mode, but if the circuit is open then no current can flow. This is when the modules have their highest voltage between the positive and negative terminals. This voltage is then what is called the “Open Circuit Voltage”, VOC of the modules. The difference in the type of chemical elements used for the PN-junction, in itself leading to different forward bias voltages, means that the VOC of modules will vary greatly between different types of solar module technologies. The irradiance-current-voltage relationship of cells composed from different materials also vary, which is one reason why different technologies result in different efficiencies. Since even modules of the same technology have variations due to the arrangement of the cells made by the manufacturer (how many cells in series, how many in parallel) something is needed to describe the physical behaviour of a solar module. That something is IV-curves.

 

iv-curves

The power output of a PV-based solar power system is very much linked to the so called IV-curves.

All solar modules tend to follow roughly the same behaviour. They have their highest voltage when the individual cells of the module are forward biased due to incident light and no current is flowing, in other words the VOC. Then when the circuit is closed and current is flowing, the voltage will drop. The current will follow an almost linear behaviour until a point when it rapidly goes down. By plotting the voltage-current relationship one get what is called an IV-curve, something which in a good way describes the characteristics of a certain module.

Because of the current-voltage relationship of the modules displayed in the IV-curves, the power will vary depending on the current produced at certain voltages. Below are two different generic I-V curves, Figure 4 and  Figure 5.

As can be seen the current at a specific voltage depends on the irradiance, something which is rather trivial for a solar panel. The higher the power of the incoming light (in other words the higher the irradiance), the higher the power produced by the panel will be. As we know, the power equation (1) can be used to calculate the power from a given current and voltage.

 

(1) 𝑃 = 𝐼 ∗ 𝑈 The power equation

Using (1) for all the given voltages and currents along the curve we can calculate the power for each given point, and thus find the voltage which will yield the highest power. This is called the Maximum Power Point. In Figure 4, we can see that the IV-curve has also been complemented with a PV-curve, a Power-Voltage curve that describes how the power change with the voltage. This makes it trivial to find the Maximum Power Point, and we can see that it is just before the sharp decline of current at the higher voltages.

The behaviour of solar modules is also affected by the temperature of the module, something which is also described in. As we can see in Figure 5, a lower temperature will “push” the whole curve to the right, meaning that the sharp decline in current will occur for a higher voltage and thus increasing the maximum power that one can get from the module. So the higher the temperature, the lower the maximum possible power from a module. Because of this the peak solar production of northern countries, like Sweden, usually happens in spring, instead of in summer as one might think. This is due to the relatively high irradiance during the spring, combined with a relatively low temperature.

other impotrant part of the system

The String

When several modules are connected together in series this is called a String. In the same way as the voltage of the individual cells is added to get the total voltage of a module, the voltages of the serially connected modules in the string is added to get the total voltage of the string. If the solar charge controller and the voltage of the battery bank, or the inverter if using a purely grid-connected system, is designed for higher voltages than can be supplied by one module (which is most often the case), several modules can be organized in a string to better match the set-up and optimize the performance.

The Array

Several strings can also be placed in parallel, and the result is then called an array. The number of modules should be the same in each string and only modules of the same effect class and type should be used to ensure that the total VOC of each string is similar. However, sometimes even modules of the same sort from the same manufacturer can vary somewhat in VOC and their other IV-characteristics. For that reason matching of modules with different voltages is sometimes used to try and get roughly the same voltage for all the strings. This because the highest possible voltage of the system will be that of the string with the lowest VOC because of their parallel connection. So if the modules are poorly matched the overall performance of the system will be degraded. The total current of the system will be that of the sum of the currents from all the strings. Again, it is the inverter or solar charge controller that sets the limit as to how many strings can be connected in parallel, depending on its power/current rating.

An illustration of the solar cell, module, string and array . Note that this is based on monocrystalline silicon type modules, so modules of other types will look different, but the principle is the same.

So now that you know everything, it’s time to get back to the The Nerdy stuff