The rise of direct current using devices and direct current generation have some rethinking the use of alternating current in the grid. Let’s start from history of electric power transmission:
In the late 19th century both alternating current (AC) and direct current (DC) were both used to power devices like motors and light bulbs. Over 100 years ago, Edison and Westinghouse were on each side of the DC versus AC debate. Those systems were not interchangeable and they competed for dominance. Thomas Edison developed the first power transmission systems using DC. Meanwhile, AC was pushed by George Westinghouse and several European companies that used Nikola Tesla’s inventions. Finally AC won the game because it made easier to transmit power over long distances using thinner and cheaper wires (voltage could be changed with a simple transformer). Besides problems on going long distance DC had also issues like arcing and connector corrosion.
Has Thomas Edison ultimately won the DC vs AC power transmission controversy against Tesla? Nowadays there are several application fields where DC is making comeback:
These homes that live off the grid (more than 25 percent of the world) easily could accept and benefit from a DC power source, such as solar panels. There might not be any need for DC to AC conversion that wastes some power. How do we get to a DC-powered home? article speculates that the DC-powered future home most likely will need two DC buses. The “other,” electronics, and lighting categories could use a low-power DC bus, for example 12V to 48V. However, the heating, cooling, and appliances categories would benefit from a much higher voltage, for example 380V to 400V. DC will likely be used for new communities that rely on renewable power sources. There are also visions that Wind, solar could provide 99.9% of ALL POWER by 2030.
I would say that design of the DC outlets would need special care (especially at higher voltages): if you unplug a DC device while it’s running, the electricity could arc through the air. This can also cause corrosion and pitting in the metal components. Besides the connectors, there are needs for some redesign on power switching components (light switches, relays etc.) and protection components (fuses, breakers, fault detectors etc..). Edison’s Revenge: Will Direct Current Make a Comeback in the U.S.? article says that DC power system will become cheaper over time, and there are envisions of buildings with both AC and DC power outlets. The biggest issue is going to be the transition.
How do we get to a DC-powered home? article tells that combined, the electronics and “other” categories account for 15-20 percent of the average home total energy use. It is predicted that by 2020 the number can reach 50 percent. More and more of those devices in modern home are DC-powered inside. DC devices such as cell phones, LED lights, and computers typically contain their own rectifier either as separate “wall charger” or built inside device. Most of those devices could be designed in such way that they could accept AC or DC power (very many switched mode AC power supplies can run on DC but not all).
Many small electronics gadgets nowadays can be charged from USB connector (5V power source). There is USB charging specification and EU standards for common mobile phone charger. USB power seems to be becoming more and more commonly available DC power source. There are nowadays mains power extension cords with both normal AC and USB power outputs. And there are even wall Outlet with USB.
AC vs DC power in data center is talked about lately. The preferred DC system is based on a conceptual 380 V DC distribution system (consensus in the literature as a preferred standard) supplying IT equipment that has been modified to accept DC power. Going to DC can help the efficiency compared to USA standard 120/208 system (typically DC systems are 5 to 8 percent more efficient than AC), but best AC power distribution systems today already achieve essentially the same efficiency as hypothetical future DC systems at least then powered from AC source. If the power comes from DC source in the beginning, the situation can be different.
High-voltage direct current transmission (HVDC) is a solution to make more efficient long distance power transmission than what is possible with AC systems. Especially on systems that use cables, the cable capacitance on the high voltage cables causes considerable losses. Transmission losses are lower for DC than for AC voltage, especially in transmission systems that run on several hundred thousand volts. HVDC transmission has typically 30-50% less transmission loss than comparable alternating current overhead lines. High voltage cable links longer than approximately 80 km are only possible with HVDC transmission because the cable capacities absorb the usable electricity at longer runs. The conversion equipment on the both ends of the cable are expensive, which means that HVDC is usually economically feasible on longer than 50 km cable runs and 600 km overhead lines.
DC distribution power systems and energy storages in urban areas is starting to gain interest. DC network built using modern components can be more energy efficient than traditional power distribution (10/20 kV medium voltage and 230/400V low voltage). DC distribution power systems are capable to provide an uninterruptible delivery of current. Feeding house with DC to feed more power for longer distance using existing low voltage cables. There are systems where the power goes to house through existing cables as DC (there are no standards and systems like 900V DC and +-750V DC have been proposed) and is converted to conventional mains AC (230V in Europe) at the house with power electronics. DC in Urban Areas Distribution Power Systems and Microgrids paper says that if the investment costs of the DC power systems can be reduced it could be a strong competitor to conventional AC power systems. Here are some links to papers on this topic (those are in Finnish):
- Tasasähköjakelu ja kiinteistöjen tasasähköverkot
- Tasajännite taajaman sähkönjakelussa ja mikroverkoissa
- TASASÄHKÖNJAKELUN KÄYTTÖPOTENTIAALIN MÄÄRITTÄMINEN
- Verkkovisio 2030: Jakelu- ja alueverkkojen teknologiavisio
- Tehoelektroniikka sähkönjakelussa – Pienjännitteinen tasasähkönjakelu
Tekniikka ja Talous magazine article “Tasasähkö tulee takaisin” says that in USA and Asia there are plans to use DC power distribution inside very high buildings which would normally need very expensive medium voltage distribution inside the building. Siemens, working together with European partners, is studying if and how direct current (DC) could be used inside buildings in addition to the usual alternating current (AC). This addition could save energy when used in certain applications, such as in office buildings. The project, known as DC Components and Grid (DCC+G), is funded by a number of European research ministries and will run until spring 2015.
It is expected that a DC power network within a building would enable the innumerable decentralized power supplies to be replaced by several large rectifiers. Such centralization would boost efficiency for the IT sector, for example, because the power supply units of laptops and computer clusters suffer relatively high losses.