AC vs DC voltage

Perhaps Thomas Edison was right after all. As new technology develops, it's time to ask the question: should we be using direct current (DC) instead of alternating current (AC) power?

Let’s review how we ended up in an AC-powered world. The preference for AC stemmed from the desire to transport large amounts of power from central station power plants to distant load centers over transmission lines at very high voltages to reduce ohmic losses. Even though DC has lower ohmic losses than AC, the transformers necessary to convert relatively low voltage generation to high voltage transmission require alternating current. In contrast, DC power electronics components to accomplish the same task were first unavailable, then prohibitively expensive.

Also, the fact that AC current is zero twice every cycle made it much easier to interrupt normal and fault-induced current flow. Nowadays, thanks to the relentlessness of Moore’s Law and technological advances, the relative advantages and disadvantages of AC versus DC are being reversed.

For instance, solar photovoltaic (PV) panels, fuel cells and batteries produce DC power and residential, commercial and industrial electronics loads use DC. But if a power source is to be used “on the grid” it must first be inverted to AC at the source and transmitted to the load, then rectified back to DC at the end use device. This arrangement adds expense, increases losses (including “vampire load”), degrades AC power quality and reduces power transfer capability due to reactance.

If homes and businesses could use solar PV, fuel cells and batteries for DC power directly for their electronics needs, including lighting, entertainment and computing, they could not only increase efficiency but also cut grid reliance. Many electronics devices such as LED lights actually work better on DC. They last longer because they're not going through thermal cycling, have lower losses because there is no AC skin effect and provide higher quality light (i.e., no 60 Hz flicker).

Lower ohmic losses is just one of many advantages for DC power transmission. DC lines don’t introduce reactance or susceptance, meaning greater power transfer capacity and less voltage variation. DC power is not subject to frequency variation or leading/lagging power factor. And electrical engineering students and professionals everywhere will rejoice in the elimination of the complexities of symmetrical components and multiphase analysis for planning and operations.

It’s really quite amazing how technology has changed the electric utility landscape. The revival of DC power is just one of several trends that’s going to happen because of the revolution in technology.

It would be extremely difficult and expensive to retrofit today’s bulk transmission system for DC. However, bypassing the AC grid entirely for some or all of an individual customer’s loads (a “nanogrid”) or several customers combined (a “microgrid”) is technically and economically feasible. And converting short segments of bulk transmission lines to AC-DC-AC links could make sense to help make the grid more controllable. Think in terms of the Pacific Northwest-Southwest Inter-tie. One portion of that is already a DC tie in part because it’s possible to have some control over the power that flows over the DC tie. Changing the phase angle on an inverter rectifier – they’re called “uniform power controllers” – creates a valve-like function whereby a certain amount of kilowatts can be forced through the line. Using this capability requires real-time monitoring, analysis and control of the combined AC/DC system, but, again, steady, dramatic improvements in electronics and information technologies is making this possible.

Developing countries and their emerging economies may leapfrog our century of AC infrastructure and go directly to DC power just as they moved directly to wireless telecommunications without building out the copper wire infrastructure for basic telephone service as we did in the United States. The drive to create and spread basic electricity service in emerging economies will likely push the envelope much farther and faster than we will in the U.S. Our motivation to innovate quickly beyond the limitations of our existing infrastructure, while seemingly great, is trivial in a global context. We use about as much electricity per capita now as we ever will. The past century of 5 to 10 percent compounded annual growth in consumption in the U.S. is over. Expert projections for U.S. electricity consumption in the current century range from less than 2 percent per year to negative 2 percent per year.

Just as circumstances have evolved to reverse the AC/DC value proposition, so evolving circumstances have implications for U.S. electricity consumption over time.  

U.S. citizens comprise only 4 percent of the world’s population, yet we use more than 25 percent of all the energy in the world. We use more than 20 percent of all the kilowatt-hours (kWh) produced in the world. On average, that’s about 14,000 kWh per capita per year. Only four countries in the world use more than we do – including Liechtenstein and Iceland – and they're tiny countries in cold climes where electricity provides heat.

In contrast, China uses essentially 10 percent as many kilowatt hours per person per year as we do. India uses about 6 percent as many kilowatt hours per person per year. The combined energy consumption of “Chindia” will grow exponentially for the next few decades as those two massive countries seek the quality of life and economic productivity that electric power and energy have provided for developed countries. And these emerging economies will produce and consume electricity with the latest, best technologies. They do not have to preserve incumbent infrastructure, corporations or regulations, which present hurdles in the U.S.

That presents developed nations with big risks. Unless we similarly exploit the newest and best technologies, including the revolutionary ones that will severely disrupt the industry, then the 80 percent of the world that now uses only 30 percent of the world’s electricity will blow past us in technology and, thus, competitiveness. This potential trend carries economic and environmental risks as well. Suppose U.S. citizens cut their per capita energy consumption in half, while the rest of the world grows to use even half as much as that amount. The impacts could drive up the price of coal, natural gas, uranium and other fossil fuels, which in turn drives up the cost raw materials, finished goods, even technology. This scenario is widely cited as potentially driving new sources of economic and even military conflict.  

The upshot is that if the U.S. seeks to maintain its global leadership politically, economically and militarily and the preserve the lifestyle and productivity we seem to take for granted, we’ll need to get serious about developing and deploying the very best energy production, transportation and end use efficiency technologies. That includes adopting the best models for our power infrastructure and the regulatory institutions that govern the grid and the “edge.” The alternative is to cling to the obsolescence of centralized power generation and AC transmission that we inevitably will experience.

Meanwhile, developing nations have every incentive to embrace innovation to produce economic productivity and thus political stability, generating future opportunities while we bemoan the lack of our own. Those developing nations don’t attach any stigma to, say, harnessing renewable energy or empowering end users – they will pursue every common sense measure to surpass us if they can. They may well utilize solar PV’s DC output and tie it directly to DC-consuming electronics, raising efficiencies, reducing losses and unleashing initiative and innovation.

In the face of this highly probably scenario, the U.S cannot afford to be content with an incremental evolution of its grid. The stakes are too high. We must be willing to capitalize on technology in the most effective ways possible to maintain and improve economy, reliability, safety, security, sustainability and customer service. And DC power is just one of many means to accomplish this goal.

We can and should have more robust discussions about emerging technologies for electric and thermal energy storage, transactive energy markets, distributed energy monitoring built into end use devices, analysis and control, machine-to-machine communications via the Internet of Things and microgrids. In short, we need to shake ourselves from our torpor and our press our advantage in our ability to innovate and adapt to a changing world.