The growing need for precision DC energy metering - Embedded.com

The growing need for precision DC energy metering

Driven by the development of efficient power conversion technology based on wide band gap semiconductors, many applications now see benefits in switching to dc energy exchange. In this first article of a two-part series, we discuss opportunities for dc metering in electric vehicle charging stations, renewable energy generation, server farms, microgrids, and peer-to-peer energy sharing.

In the 21st century, world governments are working on action plans to tackle complex and long-term challenges in reducing CO2 emissions. CO2 emissions have been proven responsible for the devastating effects of climate change, and the needs of new efficient energy conversion technology and improved battery chemistry are rapidly growing.

Including both renewable and non-renewable energy sources, the world population consumed nearly 18 trillion kWh last year alone and demand keeps growing. In fact, more than half of the energy ever generated has been consumed in the last 15 years, and our electrical grids and power generators are constantly expanding. The need for more efficient and environmentally friendly power has never been greater.

Because it was easier to use, early grid developers worked with alternating current (ac) to feed power to the world, but in many areas, direct current (dc) can dramatically improve efficiency. Driven by the development of efficient and economic power conversion technology based on wide band gap semiconductors, such as GaN and SiC devices, many applications now see benefits in switching to dc energy exchange. As a consequence of that, precision dc energy metering is becoming relevant, especially where energy billing is involved.

In this first article of a two-part series, we discuss opportunities for dc metering in electric vehicle charging stations, renewable energy generation, server farms, microgrids, and peer-to-peer energy sharing. The second article will address challenges to dc metering and offer a proposal for a dc energy meter design.

DC Energy Metering Applications

DC Electric Vehicle Charging Stations

The growth rate of plug-in electric vehicles (EVs) is estimated at +70% CAGR as of 20181 and projected to grow +25% CAGR year by year from 2017 to 2024.2 The charging station market will follow at 41.8% CAGR from 2018 to 2023.3 However, to accelerate the reduction of the CO2 footprint caused by private transportation, EVs need to become the first choice for the automotive market.

In recent years great effort went into improving the capacity and lifetime of batteries, but a widespread EV charging network is also a fundamental condition to allow long trips without concerns about range or charging time. Many energy providers and private companies are deploying fast chargers up to 150 kW, and there is strong interest in ultrafast chargers with power up to 500 kW per charging pile. Considering ultrafast charging stations with localized charging peak power up to megawatts and associated fast-charge energy premium rates, EV charging will become a massive energy exchange market, with the consequent need of accurate energy billing.

Currently, standard EV chargers are metered on the ac side with the drawback of no measurement of the energy lost in the ac-to-dc conversion and, consequently, billing is inaccurate for the end customer. Since 2019, new EU regulations are forcing energy providers to bill the customer only for the energy transferred to the EV, making the power conversion and distribution losses borne by the energy supplier.

While state-of-the-art SiC EV converters can reach efficiency above 97%, there is a clear need to enable accurate billing on the dc side for fast and ultrafast chargers, where energy is transferred in dc when directly connected to the battery of the vehicle. In addition to public EV charging metering interests, private and residential peer-to-peer EV charging schemes might have even more incentive for precise energy billing on the dc side.

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Figure 1. DC energy metering in the EV fuel station of the future. (Source: Analog Devices)

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Figure 2. DC energy metering in a sustainable microgrid infrastructure. (Source: Analog Devices)

DC Distribution—Microgrids

What is a microgrid? In essence, a microgrid is a smaller version of a utility power system. As such, safe, reliable, and efficient power is required. Examples of microgrids can be found in hospitals, on military bases, and even as part of the utility systems where renewable generation, fuel generators, and energy storage are working together to make a reliable energy distribution system.

Other examples of microgrids can be found in buildings. With the wide deployment of renewable energy generators, buildings can even become self-sufficient, with rooftop solar panels and small-scale wind turbines generating as much energy as is used, independent but backed up by the grid.

Moreover, as much as 50% of a building’s electric loads run on dc. Currently each electronic device must convert ac to dc power, and up to 20% of energy is lost in the process, with a total savings estimated up to 28% vs. traditional ac distribution.4

In a dc building, energy consumption can be decreased by converting ac to dc all at once and feeding dc directly to the appliances that need it, such as LED lights and computers.

Interest in dc microgrids is rapidly growing, as is the need for standardization.

IEC 62053-41 is a pending standard that indicates requirements and nominal levels for residential dc systems and enclosed type meters similar to the ac equivalent for dc energy metering.

The dc microgrid segment is valued at around $7 billion as of 20175 and will see further growth from the emerging dc distribution trend.

DC Data Center

Data center operators are actively considering different technologies and solutions to improve the power efficiency of their facilities, as power is one of their largest costs.

Data center operators see relevant benefits in dc distribution as the minimum number of conversions required between ac and dc decreases, and the integration with renewable energy is easier and more efficient. The reduction of conversion stages is estimated as:

  • 5% to 25% energy savings: increase in transmission and conversion efficiencies, and less heat generation
  • 2× reliability and availability
  • 33% floor space reduction

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Figure 3. Fewer components are required in a dc supply for data centers, and there are lower losses than with traditional ac distribution. (Source: Analog Devices)

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Figure 4. Renewable energy integration in a dc data center. (Source: Analog Devices)

Distribution bus voltages range up to around 380 VDC, and accurate dc energy metering is gaining interest since many operators are switching to the more measurable approach of charging the colocation customer by power use.

The two most popular ways to charge colocation customers for power usage are:

  • Per whip (flat fee for each outlet)
  • Consumed energy (metered outlet—power charged for each kWh consumed)

With a view toward encouraging power efficiency, the metered output approach is gaining popularity and customer pricing can be described as:

Recurring cost = space fee + (meter reading for IT equipment × PUE)

  • Space fee: fixed, includes security and all the building operational costs
  • Meter reading for IT equipment: the number of kWh consumed by the IT equipment multiplied by the cost of energy
  • Power usage effectiveness (PUE): takes into account the efficiency of the infrastructure behind IT, such as cooling

A typical modern rack consumes up to 40 kW of dc power. Therefore, currents up to 100 A are required to be monitored with billing-grade dc meters.

References

1 Tom Turrentine, Scott Hardman, and Dahlia Garas. “Steering the Electric Vehicle Transition to Sustainability.” National Center for Sustainable Transportation, UC Davis, July 2018.

2 “Global Electric Vehicle Market Report by Type (Battery Electric Vehicle, Hybrid Electric Vehicle, and Plug-In Hybrid Electric Vehicles), by Vehicle Type (Two Wheeler, Passenger Car, and Commercial Vehicles), and by Regions—Industry Trends, Size, Share, Growth, Estimation, and Forecast, 2017-2024.” Value Market Research.

3 Electric Vehicle Charging Stations Market by Charging Station (AC Charging Station, DC Charging Station), Installation Type (Residential, Commercial), and Region (North America, Europe, Asia Pacific, and Row)—Global Forecast to 2023. Research and Markets, April 2018.

4 Venkata Anand Prabhala, Bhanu Prashant Baddipadiga, Poria Fajri, and Mehdi Ferdowsi. “An Overview of Direct Current Distribution System Architectures and Benefits.” MDPI, September 2018.

5 “Global Microgrid Market by Type (AC Microgrid, DC Microgrid, Hybrid), Connectivity (Grid Connected, Remote/Island), Offering (Hardware, Services, Software), Power Source (Natural Gas, Solar, Fuel Cells, Combined Heat and Power, Diesel, and Others), Application (Healthcare, Industrial, Military, Electric Utility, and Educational Institutions), Region (North America, Europe, Asia Pacific, South America, and Middle East and Africa), Global Industry Analysis, Market Size, Share, Growth, Trends, and Forecast, 2018-2025.” Researchstore.biz.


Luca Martini received an M.Eng. degree in electronics and telecommunication engineering for energy from the University of Bologna, Italy, in 2016. As part of his M.Eng. degree, he spent seven months at Fraunhofer IIS, Nuremberg, Germany, developing a precision real-time control system for the characterization of piezoelectric energy harvesters. From 2006 to 2016, Luca worked as a system and hardware developer in the biomedical sector. In 2016, Luca joined the Energy and Industrial System Group at Analog Devices, in Edinburgh, UK. He can be reached at luca.martini@analog.com.

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