Solar panels produce electricity upon taking the electromagnetic energy radiated by the sun. The sun emits photons that travel a large distance to the Earth and hit the PV arrays, which process and transform that radiation into electricity.
AC electrical current requires an electromagnetic field induced by a system of symmetrically placed coils rotating at a certain frequency (60 or 50Hz), phenomenon that does not occur in solar modules. Solar panels generate in DC using a different physical process called the photovoltaic effect in which photons displace electrons from silicon semiconductor structure and thus generate a direct current.
This article is intended to guide the reader through a comparative analysis between AC and DC power and its relationship with photovoltaic solar energy, considering the theoretical and historical background.
Getting to Know DC Vs AC: What’s The Difference?
During the late 1870s and early 1880s, took place what’s commonly known as “War of the currents”, wherein Thomas Edison, American businessman and inventor, and Nikolai Tesla, Serbian American engineer and inventor, carried on a war to dispute which would become the dominant system of power supply. In this regard, while Edison strongly supported direct current (DC), Tesla was convinced that alternating current (AC) was the best solution.
As seen on figure 1, the difference between them is basic: direct current power runs continually in one direction of electric charge at a same value in time. On the other hand, alternating current, as you might expect because of the name, alternates or reverses its direction periodically (a certain number of times per second — 60 in the U.S.), continuously changing its magnitude in time.
During the early years of electricity, direct current was the standard in the U.S., however, there were several issues with its operation. Firstly, it was not easily converted to higher or lower voltages, meanwhile, this feature was achievable with alternating current using a transformer. In addition, transporting DC power across long distances produced high electricity losses which increased costs.
Based on the above, from those days, alternating current took the lead in most applications of electrical power supply across the globe, such as generation, transmission, and distribution as well, and left behind DC power in a very limited number of low voltage applications.
Today, our electricity is still predominantly powered by AC. Although, many ongoing projects are once again embracing DC power applications, such as solar photovoltaic energy, and others that will be mentioned later in this article. So, do you believe that the war of currents is over?
Why Does Solar Generation Produce Only DC?
Conventional power generation is based on electromagnetic induction. The most widely used type of electricity generators across the globe take advantage of this phenomenon. They convert mechanical energy produced by rotation into electrical energy, which can be used as AC electricity, for instance, hydroelectric and thermoelectric plants, wind turbines, among others.
Unlike the above, not one of the many different types of solar panels is driven by mechanical motion, but instead rely on another type of energy conversion. Solar panels directly transform the energy of electromagnetic radiation (expressed as photons travelling from the sun) into DC electricity by displacing electrons from the atomic structure of semiconducting materials such as silicon turning their passive state into an active conduction mode. This process is known as the photovoltaic (PV) effect.
How Do I Get AC From My Solar Power System?
To obtain AC electricity from PV arrays, the use of an inverter is essential. Today, most devices are designed to run on AC, for example, motors, home appliances, electronic devices, and so on. That’s why it’s likely you’ll want an available source of AC coming from your solar cells, ultimately requiring that the adequate equipment be attached to it.
When it comes to PV installations, there are many alternatives for wiring and putting into operation your system. Nevertheless, a basic but often used diagram is shown in figure 3, which is made up of:
- Solar panels
- AC loads for power consumption
- DC-to-AC converter device – Pure sine wave or grid-tie inverter
- Utility meter for connection to the grid
- Energy storage device for backup power (optional for grid-tied systems)
However, the scope in this section isn’t regarded to detail PV installations but focused on the mechanism of DC-to-AC conversion.
DC to AC Conversion: The Inverter
As mentioned, an inverter is a crucial device in solar PV generation, it belongs to an important class of devices which are regularly used in today’s utility grids to control and regulate the flow of electrical power, called power electronics.
The main function of the inverter is to convert the direct current obtained from the panels into usable AC power to energize electrical equipment, process that can be achieved by switching the direction of the DC input signal back and forth obtaining an AC output signal at a desired frequency and voltage.
The first inverters were created in the 19th century and were mechanical. Today electrical switches are composed out of transistors and solid-state electronics with no moving parts, which are made up of semiconductor materials like silicon and gallium arsenide, characterized by having the electrical property to control the flow of electricity in response to certain operation conditions.
Filters and other electronics can be used to clean the signal and shape it into a pure sine wave pattern, increasing the quality of electrical service and lowering maintenance and operating costs.
Modern inverters can perform additional functions like intelligent monitoring of the system that allow bidirectional flow of energy and information between the supply and demand sides. This provides channels for communication with utility network operators, where within, critical grid operating conditions will take control to regulate the operation of the PV system and strengthen the interaction between users and the electrical network.
Is It Possible To Take Advantage Of The DC Power Produced By Solar Panels?
Regardless of an extensive use of AC equipment in energy consumption during many years after electricity was created, new technological tendencies are rising progressively, and DC applications are rapidly appearing in the scene.
Modern DC applications include power electronics, electric vehicles, renewable energies, energy storage, and high voltage transmission systems (HVDC), just to name a few. In the same order of ideas, many of them have resulted efficient and less costly than AC applications, which is why they are currently being promoted.
In terms of solar energy, DC development is highly relevant since the PV output is in direct current, which wouldn’t need to be converted to AC if the loads were been compatible with direct current. At the same time, this would avoid the losses of the conversion process.
Smart grids are collecting all this modern DC applications and establishing a solid presence in the industry looking forward to developing alternative, efficient, economic, and environmentally friendly technologies on power supply.
The development of these fields will be vital in the coming years, consolidating both AC and DC technologies with PV solar systems.
Though AC applications have ruled the electrical field for many years after Tesla took advantage in the war of the currents, there has been exhaustive research, testing, and implementation of DC developments to solve different issues that undoubtedly have gained popularity among modern technologies.
When it comes to photovoltaic energy, there is no such thing as an absolute answer about whether AC or DC is the best to implement. What is clear is that both systems have their own applications within different branches.
Nowadays, electrical power systems are getting more efficient due to the combination of both, DC and AC applications, giving rise to hybrid systems, where renewable energies like PV solar energy play a crucial role and its development will be decisive in the coming years to define how electricity is going to be generated, transported, controlled and distributed.
One of the most significant achievements that boosted up AC applications in power supply was the licensing of Tesla´s AC induction polyphase motor patent by Westinghouse Electric company, which afterwards was implemented to obtain electricity from the Niagara Falls and powered Buffalo in 1896.
There´s one type of solar panel that has been increasing its popularity throughout the solar industry, called AC modules.
These solar panels produce AC power right after its output rather than generating DC and travelling from the panels to a central inverter to convert to AC, feature that is possible by using a microinverter attached independently to the back of each panel.
The main benefit of using these panels is that simplifies the PV installation by removing the central inverter, minimizing wiring, and reducing overall labor costs. However, it may require a higher initial investment.
Yes. However, to power DC loads with solar panels, it is required to connect the modules to a solar charge controller which will regulate the voltage fluctuations coming from the panels, allowing a safe and stable DC output (generally 5V, 12V, 24V) to plug DC loads, such as LEDs, electronic devices, and others.
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