May 15, 2022
According to a 2018 study, experts broadly agree that adopting direct current (DC) power systems in commercial and residential buildings would result in safer, more energy efficient buildings. This begs the questions, why do power grids still distribute AC electricity?
In a previous article we discussed five benefits of distributing direct current (DC) power throughout a building. Essentially, DC power inherently loses less energy when travelling through electrical cables than AC power, and most of our modern devices use direct current. When electrical loads (such as LED lights and HVAC systems with DC motors) are turned on, they must convert the AC electricity they get, into the DC electricity they need. This conversion process is usually quite inefficient, and can waste significant amounts of energy, especially in commercial buildings.
The War of the Currents (that took place in the late 19th century) established AC power as the standard in electrical transmission and distribution systems, but this happened over a century ago. With so many benefits to DC electricity in the 21st century, it's fair to ask why AC electricity is still the standard, and what hurtles technology needs to overcome in order to proliferate the market with DC power.
In this article we'll dive into the three main reasons why AC electricity still runs through your walls. If you want to learn more about the benefits of DC electricity, check out our blog article called 5 Reasons DC Electricity Should Replace AC Electricity in Buildings.
In this section of the article we’ll break down why these 4 challenges currently limit the use of DC power distribution in buildings because they make it more difficult, expensive or inefficient to implement.
First off, let’s take a closer look at the waveforms for AC and DC electricity:
The polarity of AC electricity alternates 50 - 60 times per second (depending where you are in the world), this makes the voltage of AC power alternate between increasing and decreasing. As the voltage alternates, it must pass the threshold of 0V (also known as a zero crossing). Because it alternates so quickly, it’s likely that when a person switches off a light switch, for example, the voltage of the electrical current will be 0V. A voltage of 0V is extremely safe, as you can probably imagine, because it implies no flow of electrical charge. So, when a light is switched off at 0V, no arcs or sparks occur. This alone improves the safety of opening a circuit charged with AC electricity.
But what if you were to shut off a light (thus opening an AC circuit), and the voltage was at its peak?
In actuality, even if a light is at its max voltage when it's switched off, AC power inherently decreases in voltage so quickly that the potential for the circuit to arc or spark is still extremely small. In electrical engineering terms, this is what is meant by AC electricity inherently preventing arc faults.
Electrical arcing is essentially when electricity jumps from one connection to another, through the air. Arcing can cause a fire in a building because arcs can ionize the air and heat it up, and when electricity is uncontained (such as in the case of arcing), it can create a spark and cause ignition. Of course, sparks can grow into fires if arcing occurs close to a flammable material, such as wood. In fact, 33% of all electrical fires in residential buildings are caused by some type of arc or short circuit fault.
On the other hand, direct current (DC) power does not alternate polarity, and therefore remains at a constant, positive voltage (as shown in the above waveform). This means that when you shut off a light switch, there is no possibility of its voltage being at 0V. In fact, its voltage will be at whatever max voltage the circuit is built for. Therefore, when you open a switch for a circuit with DC power flowing through it by flipping off a mechanical light switch, a spark will occur. In order to make switching a DC circuit off safe, it's necessary to "isolate" the electricity before it reaches an opening in the circuit. We'll discuss what this means, and why it's easier to isolate AC electricity in circuits later in this article. If you want to know now, you can skip ahead to "Problem 3". It's also worth noting that DC power can be safely switched on or off with semiconductor devices such as transistors. This because these are not mechanical switches, so electricity is never exposed to air (meaning that it won't ionize and heat up).
A device called a transformer is used to step the voltage of a power supply up or down (increase or decrease voltages), additionally transformers have the added benefit of inherently isolating a power source from a load. The problem is, transformers can only work with AC power. This is the simplified reason as to why its easer to increase or decrease the voltage of AC electricity but, if you read on, you’ll gain a more in-depth understanding of how transformers work and why they can only be used with AC power.
Before we continue, we must make a quick distinction: do not confuse the transformer we're talking about with the transformer that might come to mind when you hear the word.
When higher voltages travel through power lines, more electrical power can be transmitted at a time to where it's needed, and cables can also be slimmer (because high voltages also means lower currents). Generally, power is generated, and flows through a transmission grid to different distribution grids, and then buildings are supplied with the electricity they need. When electricity is generated, it usually needs to be stepped up to higher voltages for transmission. Transformers step up voltages to make this happen. When electricity flows through transmission lines, it needs to be able to travel long distances, so its voltage could range from anywhere between tens to hundreds of thousands of volts. These voltages are much too high for the capacity of distribution lines, so voltages must be stepped down before entering the second phase of the process. This is the second time that transformers are used in power systems. From the transmission grid, power arrives at a power substation, which houses transformers, a "distribution bus", as well as circuit breakers and switches. At the power substation, a transformer is used to step voltages down to distribution levels (typically less then 10,000 volts), then the distribution bus distributes power to local distribution lines. The bus has its own transformers that step down or step up voltages according to local energy needs. At the local level, power lines don't typically carry more then 7,200 volts. According to How Stuff Works:
Lines that carry higher voltage will need to be stepped down further before entering residential buildings and most businesses. This often happens at another substation or in small transformers somewhere down the line. For example, you will often see a large green box (perhaps 6 feet or 1.8 meters on a side) near the entrance to a subdivision. It is performing the step-down function for the subdivision.
Now that you understand why we need to have the ability to increase or decrease the voltage of a power supply, we’ll explain how transformers can accomplish this.
A transformer is not just a cool robot that changes from a human-esque metallic being into a car (as covered previously in the diagram above), it’s also a device that we use everyday without realizing it. Transformers are present in those large blocks on our laptop chargers, the drivers attached to our light fixtures, and much more.
Since about 80% of our modern devices use DC power, and the AC power they get must be converted into DC power, the blocks in our laptops (for example) house, not only transformers, but also rectifiers to convert electricity from AC to DC power. After this conversion process, a DC-DC conversion process takes place where a transformer steps down the voltage level of electricity provided, so as to only provide electrical loads with the power they require. Transformers increase or decrease the voltage level of power supplied to our electrical devices, as well as isolate electricity to improve the safety of our light switches.
Step 1: When AC current flows through a cable, it creates a magnetic field, which grows and shrinks depending on the polarity of the current. As mentioned previously, the polarity of an AC current alternates 50 - 60 times per second.
Step 2: If we wrap the cable into a coil shape, the magnetic field becomes even stronger.
Step 3: If we place a secondary coil beside the first coil, the magnetic field from the first coil will induce a current into the secondary coil.
Step 4: The pulsating of the magnetic field from the first coil will then force the electrons in the second coil to move. This force is called an electromotive force (EMF).
Refer to the diagram below for a visual of steps 1 - 4
EMF does not occur when DC power is passed through a cable. This is because DC power’s polarity remains constant, so its magnetic field does not pulse. In other words, since there is no inherent change in the voltage level of DC power, its magnetic field remains constant, and a magnetic field that remains constant does not pulse. If there is no pulse from a magnetic field, there is no pulse that can induce a current into a secondary coil, let alone force electrons to flow.
To explain further, let’s continue on with our description of how transformers work.
Step 5: In transformers there is also a core made up of ferromagnetic material (such as iron) that is placed in the center of the primary and secondary coils. The purpose of this core is to capture any part of the magnetic field that would’ve been wasted, since not all the power from the magnetic field can reach the secondary coil without the core. Essentially this core makes transformers more energy efficient.
Step 6: In order to transform the voltage level of a power supply with a transformer, it comes down to how many coils the secondary coil contains. If the secondary coil has less coils than the primary coil, the voltage of the power supply decreases and, if it has more, the voltage increases.
Now that you understand the purpose of transformers, why it's essential to step voltages up and down, and why DC electricity does not work with transformers, you can understand why it's easier to step the voltages of AC electricity up or down. It is possible to step the voltage of DC electricity up or down, however, this is typically done by converting electricity from DC to AC, then using a transformer to step the voltage up or down, then the electricity is converted back to DC at the desired voltage. This process is less efficient than the process of changing the voltage levels of AC electricity because it involves two additional conversions. If you think about this from a large-scale perspective, the infrastructure for changing voltages of DC electricity is quite expensive because of these additional conversions that need to take place. To convert high voltage DC power, it's necessary to build rectifier stations. So, although DC electricity inherently loses less electricity inherently when travelling through cables, it's still more expensive to distribute DC electricity because of infrastructure costs.
If you’d like more information on how a transformer works, we would highly recommend this video by The Engineering Mindset.
Above, we discussed how switching AC power on and off is inherently safer than switching DC power on and off. But there’s another reason why it’s easier to optimize the safety of an AC power system: it’s easier to isolate an electrical load from a power supply with AC power systems.
Isolated power supplies provide a clear barrier across which dangerous voltages won’t pass. This provides safety. There’s no need to go into this deeply in this article, but if you’d like to read more about it, we highly recommend reading some more articles about it, such as this one called “What is Power Supply Isolation & How is it Used?”.
It’s easier to isolate an electrical load from a supply in AC power systems essentially because transformers have the inherent benefit of doing this. As we discussed earlier, transformers are not compatible with DC electrical systems, so by default these systems also wouldn’t be able to use a transformer to isolate the power supply from a load.
However, if you have a DC power supply, it's still possible to isolate it from the load by implementing a system called an “Isolated Switch Mode Power Supply”. An Isolated Switch Mode Power Supply converts DC electricity to AC electricity, then a transformer is used to step the voltage level up or down, and then the AC is converted back to DC. Although this solution works for now, it’s not a long term solution because it’s expensive to implement and inherently inefficient.
At Cence we design transformer-less isolation systems for DC power. Get in touch if you'd like to learn more!
Almost all commercial electrical power on Earth is generated with a turbine, driven by wind, water, steam, solar or burning gas. The turbine drives a generator, thus transforming its mechanical energy into electrical energy by electromagnetic induction. In fact, almost all methods of power generation produce AC electricity. It's typically only solar panels and windmills that produce DC electricity. Implementing more solar panels would help to push the advancement of DC electrical systems because, if more DC electricity is being produced, there's less reason to convert that DC electricity into AC electricity for distribution considering the benefits of distributing DC electricity. Additionally, it's a simpler process to convert AC into DC than DC into AC, so there would be even more resistance towards the idea of converting the DC power generated by solar or wind into AC power.
Currently solar panels only produce about 2.76% of the energy needed to power earth, and wind accounts for just 5% to 6% of global electricity production. Additionally, solar panels take up less space (because they can be installed on rooftops), don't make any noise, and have a longer lifetime than windmills, which makes them typically more practical for suburban or urban areas.
Solar panels can take up very little space, making them ideal for suburban or urban areas
So why aren't we generating more power with solar panels (or photovoltaic technology)? The main reason we aren't generating more of the earth's energy with solar panels is because better systems for storing solar energy need to be developed. Solar energy production is relatively unpredictable; it can fluctuate seasonally or even by the hour. So there could be many periods within a day or a month that solar panels are not producing energy. This makes it even more essential to store previously acquired energy for "a rainy day" (both literally and figuratively). The problem is, it's complicated and expensive to store solar energy because of the current technology we have for batteries that store it. The batteries for storing solar energy are very large and expensive. In fact, the more power you need, the larger your battery needs to be. Installing these batteries can be complicated and requires the use of a licensed electrician. Additionally, the fact is that the majority of power lines distribute AC electricity, so there is currently little reason to generate DC electricity for AC power lines, especially because converting DC to AC is a more complicated process than converting AC to DC.
The major benefit of generating power via solar or wind is therefore not that it produces DC electricity, but really that these are renewable energy sources. Solar specifically produces 5x - 10x less carbon emissions per unit of energy relative to coal or natural gas. You might also be surprised to learn that wind is even more environmental friendly than solar with respect to carbon emissions. In fact:
Bernstein Research "determined that wind power has a carbon footprint 99% less than coal-fired power plants, 98% less than natural gas, and a surprising 75% less than solar".
AC has been the standard type of electricity transmitted and distributed via power grids for over a century. In spite of this, DC electricity has many benefits that make its operating costs lower than AC transmission systems, and most of our modern devices are powered by DC electricity. So why do the majority of power grids still distribute AC electricity? The answer is: it's still typically more cost effective for electrical grids to supply AC electricity for 4 reasons. In this article, we covered these reasons and what challenges must be overcome in order for DC electricity to eventually be standardized. As technology continues to develop, there will be more solutions to these problems.
We learned that DC electricity is not as safe to switch off because it's more prone to arcing, and therefore isolation for DC distribution within buildings must be considered before power grids can distribute DC power to them. We also learned that It's more difficult to prevent arcing through electrical isolation in DC circuits because DC electricity isn't inherently compatible with transformers. Not only do transformers isolate a power supply from an electrical load, their primary function is actually to change voltage levels of electrical currents, therefore it's also more difficult to change the voltage levels of DC electricity. Transformers are used at almost every stage of power transmission and distribution and, in order for DC electricity to be transmitted, converter stations need to be built. These stations are so expensive that, up until a so-called "break-even distance", the inherent benefits of DC electricity don't make up for the cost of these converter stations. What is the "break-even distance"? DC electricity is so efficient to distribute, that at a certain distance, the cost of converter stations can be justified by the reduced operating costs of a DC transmission system. For example, it's more cost effective to use a DC transmission line if electricity is travelling 600km through the air, underground for 50km - 100km, or underwater for 20km - 50km.
On the bright side, power grids are evolving. Electrical engineers recognize the benefits of DC electricity, and are working on solutions to isolate high-voltage DC power so that infrastructure costs to transmit it will decrease. At Cence Power, for example, we've developed a transformer-less DC power distribution system for commercial buildings. Once this technology can be upgraded to work with higher voltages, it can be used more often throughout infrastructure in towns and cities.
Contact us today to speak with a DC power specialist, and learn about how we could help you save energy in your commercial building by implementing a DC power distribution system.