Expert talk: the role of direct current in tomorrow’s electricity distribution

Since its early days, our electricity grid has been working on 50 or 60 Hz alternating current (AC). Historically, this used to be the right decision, but as we are more and more consuming, buffering and generating energy locally, researchers have started to question this paradigm. This expert talks looks into the advantages of direct current (DC), specific applications this technology has to offer and the challenges it faces.

WHY DIRECT CURRENT?

The electricity grid as we know it today, has its roots in the late 19^th century, pioneered by Thomas Edison and Nikola Tesla. Thomas Edison started in central Manhattan, on Pearl Street, New York, by constructing the first electricity grid to power his highly desired light bulbs [1]. The technology applied was direct current. Direct current means the voltage remains constant over time, without any noteworthy fluctuations. Today our electricity grid is dominated by alternating current, developed by Nikola Tesla. The voltage periodically increases and decreases at 50 Hz. A hundred times per second the voltage will pass through zero.

Even though, at first sight, alternating current might seem more complex than direct current, it offered and offers some fundamental advantages. The main advantage being that by means of transformers the alternating voltage can be increased or decreased. A higher voltage means less cabling is needed to transfer the same amount of power from point A to B and less power gets lost in the transmission process. Transmitting electricity over dozens of kilometres in an economic way, could only be done at higher voltages which turned alternating current into the appropriate technology. An equivalent transformer for direct current did not exist at the time, which meant only 1 mile could be covered and multiple smaller electricity power plants had to be installed on the way. Please note that nowadays, the installation of multiple smaller generators, distributed over the electricity grid is exactly what the future may bring. Back in the old days this role was fulfilled by generators driven by steam engines, nowadays by solar panels, wind turbines, etc.

In the beginning, electricity was used to supply light bulbs and motors. This is only a limited number of appliances compared to the wide range of appliances that we find today: electronics (television, computers, etc.), domestic appliances, lighting, heat pumps, electric vehicles and battery storage. Typical for all these appliances is that the majority works on direct current, starting from the alternating current which connects them to the grid. Think of the adapters to recharge smartphones and laptops, the electronics in LED-lighting or pumps and compressors with a frequency control to work at variable frequencies. All these applications internally use direct current. Usually the first step is a turning alternating current into direct current. The process going on in the adapters, is called rectification. The word ‘adapter’ even bears this in itself: the grid voltage (AC) being adapted to the appliance (DC).

Also for solar panels, wind turbines, battery storage and fuel cells, adapters are used, better known as inverters or power converters. Their function is to transform the generated direct current into alternating current. Even in wind turbines, the electricity generated will be first converted into direct current, making it able to let the wind turbines operate at variable speeds which results in a higher yield.

All adapters and converters have been enabled by the development of power electronics, electronics which turn electricity from one form into another by means of a semiconductor technology, introduced in the 50’s. Since the early days, power electronics have been embedded in adapters (“switch-mode power supplies”), but only the last 20 years it has become a mass product in the electricity production and use. In 2018, a worldwide capacity of 480 GW installed power of solar power and 564 GW installed power of wind turbines was available [2]. In total this approximately coincides with 75 times the peak power of electricity demand in Belgium. Since all solar panels are connected to the grid by means of inverters, an equivalent power of 480 GW of inverters has equally been installed. Our electricity system thus contains a substantial number of blocks with the sole purpose of turning alternating current into direct current and vice versa. This was definitely not the practice at the end of the 19^th century.

 “Direct current grids are more compatible with the loads and sources used nowadays. Less components, lower costs, higher reliability and less energy losses.”

Why direct current is making a comeback is therefore obvious. Direct current grids are more compatible with the loads and sources we use nowadays. Higher compatibility also means adapters and power converters can be simplified or even entirely omitted. This compatibility advantage means: less components, lower costs, higher reliability and less energy losses. An additional advantage is that more power can be transported with the same cables, i.e. more power with less (expensive) conductor materials (copper).

DIRECT CURRENT IN A BROAD SPECTRUM OF APPLICATIONS

An important role with regard to direct current has been reserved for datacentres. Datacentres store all our data, and in doing so also make use of a substantial amount of electricity. The electric power of a datacentre is typically more than 1 MW. The largest datacentre of Google in Finland consumes 40 MW [2]. In the United States datacenters mounts up to 2% of the total electricity use [3]. Because of the required reliability, datacentres are equipped with an uninterruptible power supply in the form of battery storage, based on direct current. Furthermore also server racks require direct current. By transforming the electricity infrastructure of datacentres into direct current, it becomes possible to save adapters, resulting in 20% less energy losses, to save one third of space and to decrease the time an installation cannot be used because of for example reduced maintenance [4]. A reduction of energy losses also indicates the cooling system can be down-scaled. In short: DC results in a substantial cost reduction.

In the industrial sector power electronics have been on the rise for years. It is estimated that today already 30% of the total power of electric drive trains with a frequency control is equipped to save energy or to make the production process more flexible [5]. Internally each frequency control works on DC. Today, multiple frequency controls are already linked to a common DC voltage. In the futer, such a system will offer the possibility to add a battery to increase the reliability, making production lines less susceptible for faults on the grid and brief or long interruptions. In countries where the power quality is poor, such a system becomes even more profitable.

Additionally, in the future, multiple systems, which are nowadays installed separately, will be able to get linked to each other, decreasing the required amount of battery storage. This industrial DC grid can then be extended with local DC energy generation such as solar panels, fuel cells and wind turbines. Depending on the preferred voltage level, the total amount of cabling will decrease which, again, results in a cost reduction thanks to DC.

In commercial and tertiary buildings, including the EnergyVille building, different DC consumers and sources are present. In the EnergyVille building for example local production (solar panels), charging infrastructure for electric vehicles in the parking lot, LED lights, a server room, elevators, ventilation, cooling and heating systems based on heat pumps. All these appliances are currently connected to an electrical installation on alternating current. Part of the conversion from DC to AC can however be avoided. Electric vehicles for example could be charged directly via DC by means of the solar panels. In practice this is already done in the LVDC test grid in EnergyVille which demonstrates LVDC in practice is indeed possible. In due time we are also planning to exchange energy on direct current between the buildings of Campus EnergyVille. The motivation still being: DC leads to a cost reduction, in this case estimated at 30% of the total cost of the installation.

In mobile applications DC is omnipresent opening up to opportunities for cost reduction. Prototypes of ships exist which demonstrated a reduction of 60% of fuel [6]. Diesel generators are continuously set at an efficient operating point, storage is foreseen on board and propellers are driven by electromotors which efficiently rotate at variable speed. Direct current (DC) is the facilitator to enable this concept with a minimal number of power electronics blocks. This way, the weight and conversion losses are reduced to a minimum.

Last but not least DC solar home kits are rolled out in regions with no or limited access to the electricity grid. This includes 1.1 billion people all around the world [7]. In order to reduce this number, small DC systems of approximately 100W, working at 48V, fed by a solar panel and buffered by a battery are used. This enables its users to supply LED-lighting, a ventilator, a smartphone and domestic appliances. Since all these applications work on DC, alternating current can be entirely omitted. This is an example in which DC can contribute to the improvement of quality of life of an enormous number of people.

CHALLENGES

However, to enable DC electric installations, some important technical challenges still remain. Together with different partners, EnergyVille aims to tackle these issues at both the national and international level.

A first technical issue is to guarantee the voltage stability so that appliances do not unnecessarily disconnect or get damaged by overvoltage or overcurrent. In a system with multiple power electronics converters, in which multiple controllers interact with each other, this is a tough nut to crack. Besides, the nominal voltage level has so far not been established internationally yet. Both 350V, 380V and 750V are debated, all below the low voltage limit of 1500V.

A second technical issue is to guarantee safety for both the users and the applications. An important difference in DC installations compared to AC installations is the absence of a zero crossing in the voltage and current, which mechanically hinders interrupting the current because of the electric arc which originates between the contacts. As a solution, circuit breakers making use of semiconductor technology, are opted for to interrupt DC in time in an arc-free manner. In power converters, semiconductor technologies have proven to be capable of cutting off and redirecting DC power at high frequencies, but remind that a mechanical interruption remains a legal requirement. A second concern is that a DC installation is dominated by power converters that are able to actively limit the current, resulting in lower short-circuit currents. A limited current enables to rate the automatic switches lower, but makes it more difficult to differentiate a short circuit from a temporary overcurrent in normal operation.

Apart from technical issues, also regulatory challenges exist. In Belgium, the General Regulations for Electric Installations are legally binding. In the framework of a feasibility study, it has been analysed whether if GREI is adequate for DC. GREI specifies the safety curves for DC and describes which safety requirements need to be met, for both AC and DC. However, with regard to the required DC residual current protection, uncertainties still remain that undoubtedly have to be tackled sooner or later, since they also hamper the development of electric vehicles and battery storage. In collaboration with the standardization commissions and sector federations, EnergyVille proactively joins the search for solutions.

KEY TAKE-AWAYS

• The distribution of electricity based on Low Voltage DC has taken central stage again thanks to the rise of power electronics.
• LVDC offers a higher compatibility between energy efficient loads, renewable energy sources and battery storage, which results in lower capital and operational expenses
• LVDC has great potential in a wide range of applications: datacenters, industry, electromobility, commercial buildings and electricity in development countries.

REFERENCES

[1]        J. Joness, Empires of Light: Edison, Tesla, Westinghouse, and the Race to Electrify the World. 2003.

[2]        International Renewable Energy Agency, “IRENA Statistics,” 2018. .

[3]        Bergmann, “Data Center Investments in Finland,” 2019.

[4]        A. Shehabi et al., “United States Data Center Energy Usage Report,” 2016.

[5]        G. AlLee and W. Tschudi, “Edison Redux: 380 Vdc Brings Reliability and Efficiency to Sustainable Data Centers,” IEEE Power Energy Mag., vol. 10, no. 6, pp. 50–59, Nov. 2012.

[6]        M. Doppelbauer, “Energy Efficiency And Variable Speed Drives.” p. 27, 2017.

[7]        ABB, “ABB technology helps advanced cable-laying vessel achieve up to 60 percent in fuel savings.” [Online]. Available: https://new.abb.com/marine/articles-and-highlights/abb-technology-helps….

[8]        A. Jhunjhunwala, “The people’s grid,” IEEE Spectr., vol. 54, no. 2, pp. 44–50, Feb. 2017.