Invention to Innovation: New Impulse for DC Grids?
How will the growing use of direct-current power shape tomorrow’s energy grids? Michael Weinhold, CTO of Siemens Energy Management, shares his insights.
column by Michael Weinhold
In November 2007, I was amazed to hear that the US utility Consolidated Edison was converting its last 60 direct-current (DC) customers to alternating current (AC).After all, the “War of the Currents” had been settled more than a century earlier at the 1893 Chicago World’s Fair, when Westinghouse and Tesla demonstrated the superiority of AC. Today, however, Edison’s DC seems set to make a comeback in distribution grids. High-voltage DC transmission has been used since the first half of the 20thcentury – first with mercury arc valves, and since the 1970s, using power electronics.
The pioneering era in the second half of the 19th century was shaped by men like Edison, Tesla, and our own Werner von Siemens. Their achievement was to turn inventions into innovative applications that simplified life. This entrepreneurial spirit is essential. Heron of Alexandria’s steam engine in the first century AD could have revolutionized engineering, but he failed to harness steam power for practical application. Thus, his invention never became an innovation, and the industrial revolution took place 2,000 years later.
The second stage of electrification took place with the expansion of large-scale AC grids during the 20th century. But DC never vanished altogether. It is now being used in an increasing number of applications to link, for point-to-point operation, in hybrid AC and DC transmission systems, and in HVDC lines, where Siemens is a pioneer and market leader.
An era of prosumers
The third stage of electrification belongs to the “prosumers” who produce and consume power at the same time. In the second half of the 20th century, DC got a new boost following the invention of transistors and resulting innovations. We are now surrounded by (low-voltage) DC in consumer electronics such as communications equipment or computers and are using more and more LED lighting. This development is inspiring a range of applications for DC in low-voltage distribution.
A first benefit is an efficiency gain: I can envision a scenario where a central AC/DC converter station in our homes converts the incoming AC power into low-voltage DC power, thereby saving losses and capital expenditure. PV panels could also feed into this home-based DC supply, maybe backed up with batteries in cases where grid power supply is unstable. In developing countries, where off-grid solutions are often the only option, DC combined with batteries is already in widespread use. In those rural areas, cell phones can be charged using solar power, facilitating access to information and business opportunities, and low-voltage DC power is used to refrigerate food and medicine.
It is being discussed whether DC power supply should be used in data centers and server farms to avoid AC/DC conversion losses. Less excess heat means that data centers and server farms could save up to 30 percent power costs if AC is only converted once to DC. Less excess heat also means less air-conditioning costs. In the future, newly constructed homes will feature hybrid systems. Prosumers will power the less energy-intensive part of their DC appliances directly with energy generated by DC sources such as rooftop PV.
For reasons of security and economy, the DC voltage used in homes will be far below 50 volts. The AC grid will cover the energy gap due to the intermittent in-feed and may be supported by home-based batteries at night. Energy-intensive loads, such as electric stoves or heating and air-conditioning, will, of course, still be fed from the AC system.
Instead of a “War of the Currents”, we are debating how and when to use both approaches.Michael Weinhold
Defining optimal uses
In high-voltage systems, we need DC switchgear and circuit breakers, for instance, to connect the North Sea wind parks and terminals with the onshore grid. This means opportunities for business and innovation. I believe there will be applications for DC on both sides, high-voltage and low-load. On the high-voltage side, in power transmission the already existing lines will develop into full-fledged DC grids. On the low-load end of the electricity grid, DC will constitute integral parts of the electricity system operated by the prosumer.
The trajectory of technology development that brought us power electronics, PV panels, LED lighting, and micro-processors is also driving the business case for DC on the consumption side. Edison would be delighted: Today, integration coincides with a proliferation of technology drivers. Instead of a “War of the Currents,” we are debating how and when to use both approaches. And unlike in Heron of Alexandria’s age, global communication and the concept of “open innovation” have created a wealth of new ideas ready for application.
Michael Weinhold, Chief Technology Officer of Siemens Energy Management
Picture credits: Elisabeth Moch, Michael Weinhold
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