AC/DC is not just the name of a popular band. AC (alternating current) and DC (direct current) power are actually the two different types of electricity. The vast majority of power grids distribute AC electricity, but it's been over 100 years since AC became the standard. Since then, much has changed. For example, a growing fraction of the electricity consumed in modern buildings is either "consumed as DC or passes through a transient DC state on its way to being consumed" (according to Physics World). Because of this, and other inherent benefits of DC power, many experts agree that adopting direct current into commercial and residential power systems could result in safer, more comfortable, and more energy efficient buildings.

When was AC electricity chosen as the standard?

DC electricity was developed by Thomas Edison. Edison invented an incandescent light bulb that could last for 14.5 hours in 1879 and used DC electricity to power it. AC electricity came on the scene a little later when Nikola Tesla (working for Edison at the time) popularized it with his many inventions. Tesla first demonstrated his AC (alternating current) electricity at the World Columbian Exposition in Chicago in 1893. Tesla ultimately won the so-called "War of the Currents", and AC electricity has been used as the standard ever since. But technology, devices, and infrastructure have changed dramatically since the 19th century, and this begs the question, should AC electricity still be the standard? 

In this article we'll explain briefly what the differences are between AC and DC electricity, and what these differences mean for building managers and designers who are considering powering their building systems (like lighting and HVAC) with DC power.

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What's the Difference Between AC and DC Electricity?

Essentially DC electricity travels in a straight line (directly) on a graph of voltage vs. time, meaning that it does not have a frequency, and its voltage remains constant. On the other hand, AC electricity alternates polarity 50 - 60 times per second (depending where you are in the world). This gives it a frequency, and also means that its voltage is not constant over time (it increases and decreases).

Frequency: The number of complete alternations per second of an alternating current.

These graphs visualize how the two types of current would appear if they were being viewed on an oscilloscope (a device for viewing voltage changes over time): 

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Benefits of DC Power Distribution in Buildings

1. Eliminate the Need for Inefficient Power Converters
2. Essential for Smart Buildings to Work Efficiently
3. DC Electricity is Safer to Handle
4. Many DC Powered Devices are Intrinsically Efficient
5. Get certified with the LEED program

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1. Eliminate the Need for Inefficient Power Converters

An increasing number of modern devices use DC electricity, including LED lights, HVAC systems, laptops, microwave ovens and more. In fact, DC consumption currently makes up about 32% of total energy loads, and could climb as high as 74% in buildings that have electric vehicles and HVAC equipment with DC motors. Currently, power grids distribute AC electricity to homes and buildings, meaning these devices must convert the AC power they get into the DC power they need.

In order to do this, these devices are equipped with drivers and converters, which can be efficient (up to 96%) when cost is no concern, but efficiency is the first thing to be sacrificed when manufacturers cut back on production cost. This means that manufacturers will often buy cheaper drivers to produce a cheaper product. At Cence, we conducted some primary research (lab testing) to determine just how inefficient some LED drivers are, and how common it is for drivers in LED lights to be inefficient.

An inefficient driver essentially burns energy in the form of heat during the conversion process of AC to DC power. In our own in-lab testing at Cence, we’ve done comparison tests between different LED light bulbs and found that most drivers for LED lights are inefficient (80% on average), especially in residential lighting. In fact, on average about 20% of energy is lost during conversion. Ultimately, by providing these systems and devices directly with DC power, the need for these inefficient drivers is eliminated.

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Why AC Electricity is the Standard in Transmission Systems

Knowing that we mostly use DC power today, It may seem intuitive to simply distribute DC power directly to buildings and DC powered devices, and there are a few different methods of doing so that exist. Most systems that distribute isolated DC power do so via the use of a transformer. Transformers have two main purposes: 

1. They step-up or down voltages
2. They isolate the electricity, improving the safety of the circuit, and reducing the risk of electrical fires

The problem is that transformers only work for AC electricity, which is one of the benefits of AC, and essentially why AC is standardly used in high voltage transmission systems.

So how can DC distribution and transmission systems use transformers when DC power isn't compatible with transformers? Great question.

If DC electricity was compatible with transformers, DC electricity would be the cheaper and more efficient option in distribution and transmission systems. This is because, in contrast to AC power, DC power is entirely made up of active power, meaning that there are almost no losses due to the capacitance of wires when DC power travels long distances. In fact, high voltage AC transmission systems have losses of 7% to 15% with aboveground transmission.

Unfortunately, however, the reality is that DC power is not compatible with transformers, but voltages still need to be stepped up and down when DC power is distributed. Even with this limitation, there are still dozens of HVDC transmission systems in the world today, and that's possible because engineers developed a work-around that is used most of the time in DC power distribution. If AC electricity was being transmitted into a building, for example, a transformer would be used to step-down the voltage to the desired level, then power would be converted from AC to DC, and DC electricity would be distributed throughout the building. However, if the voltage needed to be stepped down further, an inverter would be used to convert the electricity back to AC, then it would be stepped down with a transformer, and converted back to DC. This process is a little cumbersome, and expensive when it's being done at a large scale for transmission systems. So when is it worth it to distribute DC power throughout a building or via a transmission system when this inefficient work-around needs to be used? The answer to this question depends.

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It's worth it to distribute DC electricity throughout a building if:  

Energy saved by distributing DC power directly to devices > Energy wasted by inefficient drivers in the conversion process of AC to DC power

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It's worth it to distribute high voltage DC electricity via transmission lines if: 

Power is to be transmitted underwater, underground, over 600km on land, or across country borders. In these cases, enough energy would be lost in the process of transmitting AC power, that the cost of rectifier stations can be justified.

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Why is DC power not compatible with Transformers in the first place?

The answer to this question has a lot to do with how electricity works, which is out of the scope of this article. If you'd like to learn more about it, we recommend this video by The Engineering Mindset: 

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DC electricity will become the standard in power systems throughout the world when a better solution is devised to step up, step down, and isolate high voltage DC power, and make the transmission of it safe. Once this advancement in technology is made, the benefits of DC power transmission will far outweigh the benefits of AC power transmission. In turn, once high voltage DC transmission systems are standardized, buildings will receive DC power, and the need for inefficient conversions from AC to DC power will be eliminated. In fact, a study featured on ScienceDirect highlights how distributing DC electricity has been proven as an effective way to reduce electrical consumption in commercial buildings through the reduction of power conversions and facilitation of a transition to efficient DC appliances. In the study, they also show that residential buildings experienced savings of up to 25% when solar PV was distributed to all home appliances, and when battery storage for excess solar energy was considered (solar power provides DC power, and batteries store DC power). These results are promising, but the best way for buildings to receive DC power is for power grids to supply it. This will happen when a technological solution is devised that will replace the need for, or decrease the cost of expensive rectifier stations in HVDC transmission systems.

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2. Essential for Smart Buildings to Work Efficiently

As mentioned above, most modern devices and systems require DC electricity, and this is no different for smart buildings or homes. Sensors, cameras, LED lights, and other devices necessary for smart buildings, are all powered with DC electricity.

Smart buildings are designed to be more energy efficient. For example, they can usually collect data on environmental quality and occupancy of a space in order to enable the optimization of energy use. But if high consumption devices, like LED lighting and HVAC, still have to convert AC to DC electricity, energy is still being wasted. To sum up this point, Brad Koerner said it best at the Smart Building Conference (2020): “we need a revolution in just basic electricity before a lot of our smart building technologies will actually be implemented”. 

3. DC Electricity is Safer to Handle

Regardless of whether you’re working with AC or DC electricity, it’s never perfectly safe. So always take precautions. Alternatively, if you’re not sure of what you’re doing, it’s safer to get in touch with a professional. That being said, while both are dangerous, AC electricity is more dangerous to work with due to these reasons: 

• The human body has a higher impedance to DC currents than AC; humans can withstand higher voltages of DC electricity than AC. To explain the technical reason for this in simple terms, AC electricity creates an alternating magnetic field, which can penetrate insulators. Our skin is an insulator (albeit a thin one), so the magnetic field created by AC electricity can surpass our skin, react with our nervous system, and this is what causes us to feel the pain induced by an electric shock. Conversely, DC electricity has no frequency, so no alternating magnetic field is created, and therefore it's much more difficult for DC electricity to penetrate our skin and react with our nervous system.
• Experiments have demonstrated that it’s easier to let go of live parts of a DC circuit than observed in AC circuits. Naturally, this makes it easier to limit exposure to harmful voltages because you can simply let go of the source of shock.
• Even if DC electricity were to penetrate our skin, the penetration of AC electricity would still be more harmful. This is because the alternating behavior in the nature of AC causes the heart's pacemaker neurons into atrial fibrillation, whereas DC electricity might instead cause cardiac standstill (due to ventricular fibrillation) in case of electric shock. Both sound scary. However, there is a better chance for a “frozen heart” (caused by DC) to get back to normal, in comparison to a fibrillating heart (caused by AC). So DC electricity is also technically safer for this reason. 

4. Many DC Powered Devices are Intrinsically Efficient 

LED lights are a great example of an efficient DC powered device; they use 75% less energy than AC-powered incandescent lighting. If a building is powered by DC electricity, the building manager would have more of an incentive to incorporate energy efficient DC powered devices into their building systems. For example, if a building had an HVAC system with an AC motor, they might decide to upgrade their HVAC system to one with a more efficient DC motor. DC HVAC motors operate at least 50% more efficiently than AC motors, so this switch alone saves a significant amount of energy. Integrating more DC power distribution systems into our way of life creates an incentive for more DC-powered technologies capable of increasing efficiency within buildings. 

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5. Get Certified with the LEED Program

According to the US Green Building Council’s (USGBC) website, LEED (Leadership in Energy and Environmental Design) is the most widely used green building rating system in the world. For a building to be LEED certified, means that it’s been globally recognized as a symbol of sustainability, achievement and leadership. There are four possible levels of certification: certified, silver, gold and platinum. The more points a building has towards a LEED certification, the higher the level they can be eligible for. The LEED program does, in fact, provide points for DC powered buildings, legitimizing the case for the proliferation of DC powered buildings in the market.

But why care about being LEED certified? The benefits of being LEED certified include: 

• Gaining a Competitive Edge: The USGBC website mentions that “61% of corporate leaders believe that sustainability leads to market differentiation and improved financial performance”. Additionally, green buildings attract potential employees who care about sustainability, so optimizing energy use in a workplace can help to make a business a desirable workplace.

• Become a Net Zero Energy Building (NZEB): The recent IPCC report made it clear that we need to reduce our energy consumption in order to fight climate change. In response, more businesses than ever are prioritizing decreasing their carbon emissions. In fact, more than 100 businesses and organizations have signed on to the World Green Building Council's Net Zero Carbon Buildings Commitment, which seeks to decarbonize the buildings sector by 2050. Additionally, more than 200 businesses have signed on to the Climate Pledge, launched in 2019 by Amazon and Global Optimism, agreeing to decarbonize by 2040 a decade ahead of the goals of the Paris Agreement.

• Attract Higher Quality Tenants: According to the USGBC website, LEED certified buildings command the highest rent, and vacancy rates are estimated to be 4% lower in green buildings than non-green properties 

• Boosted Ability to Manage Performance of Buildings: The LEED program defines a framework that decision makers for buildings can follow in order to optimize their building's energy consumption. With the increased ability to optimize energy consumption in buildings, often comes the increased ability to control and automate building systems. When you can control building systems, such as lighting and HVAC, not only can you optimize the energy consumption of these systems, but you can also optimize indoor spaces for comfort and air quality.

• A Data-Backed Framework: LEED takes the guesswork out of designing a green building, or retrofitting an existing building to be green. 

• Green Buildings are Cost Effective: Although optimizing a building for energy efficiency can be an initially expensive investment, it is just that: an investment. Often the cost of retrofitting a building to be sustainable, or designing a building with sustainability in mind, is covered through the energy savings in the first few years of operation.

1. Take a look at this screenshot from the USGBC’s website page about the LEED program: 

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The USGBC website also mentions that additional benefits to designing a green building with the LEED framework include: health benefits for occupants and employees, reduced pollution, reduced energy use and carbon emissions, water conservation, and a reduction in waste. 

The LEED program provides 18 points for DC powered buildings as a way of influencing the market to improve energy efficiency, resilience and reliability of electrical systems in buildings. The LEED program’s DC power credit also complements LEED’s Renewable Energy and Grid Harmonization credits because solar photovoltaic (PV) power systems (like solar panels) provide DC power. This means that the DC power credit additionally encourages people to invest in renewable energy sources.

At Cence, we can help your building gain points towards a LEED certification by answering questions such as: “how do I power major systems, such as lighting and HVAC, with DC electricity?”.     

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In Conclusion

As it becomes more beneficial than ever for buildings to reduce their energy consumption, it's time to reconsider whether AC electricity should remain the standard type of electricity transmitted throughout the world (or at least in buildings). AC electricity was chosen as the standard in the late 19th century when Nikola Tesla won the War of the Currents. But that was over a century ago. At the time, Tesla's advancements in AC power distribution were chosen as the standard way to transmit electricity over long distances because the infrastructure for it was cheaper. AC electricity was cheaper to transmit over long distances because it is compatible with transformers. As technology advanced, the first high voltage DC transmission system was implemented in the 1950s via the development of rectifier stations or mercury arc values. Rectifier stations convert DC power to AC in order to step up or step down voltages, and then they convert AC electricity back to DC electricity for transmission or distribution. As we discussed in this article, these stations can be relatively inefficient, and are significantly expensive. In the future, when this technology is developed further, and infrastructure costs for DC transmission systems lower, DC electricity can be distributed directly to buildings. This would save our many DC powered devices a significant amount of energy by eliminating the need for inefficient power conversions at the load level.

Although there are some DC transmission systems in the world, chances are, your commercial building is not connected to one. Therefore, the only way to reap the benefits of distributing DC power to your building systems is by implementing a DC power distribution system at the local level. At Cence, that's exactly what we provide. If you're looking for an easy to install DC power distribution system, we invite you to review how our system works. Our system does not use a transformer, and our patented technology safely and efficiently distributes power to all DC powered devices.

If you're interested in learning more about the Cence DC power distribution system, talk to a DC power specialist, or request access to view our whitepapers, spec sheets and more reach out to us through our contact form.