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Rooftop solar and local battery storage has been widely adopted in many countries in recent years as the technology has become more affordable, and the cost of power from fossil fuels has skyrocketed.
In Japan, the total installed capacity of rooftop solar has leapt from fewer than 5 gigawatts to more than 20 gigawatts over the past 10 years. In fact, the amount of electricity generated by solar panels is more than 9% of the total power generation in Japan, and it is still increasing. With this trend set to continue — expedited by a local government mandate that all new dwellings in Tokyo be installed with rooftop solar from 2025 — there is an urgent need to overhaul how energy in the local and national grid is managed to maintain the stability of supply and limit outages and failures.
"Conventional power grid systems become unstable as the share of renewable energy increases," says Taiichi Otsuji, from the International Research Institute of Disaster Science (IRIDeS) at Tohoku University in Sendai, Japan. "This is projected to become a real issue within the next decade, so we must strive to establish a new standard for how this new energy mix is distributed and managed," he says.
Otsuji explains that through the nationally funded Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA) project under the financial support of the Japan Science and Technology Agency (JST), the university is bringing together industry and academic partners to rethink the architecture of the grid.
Otsuji is leading a six-year project called Power and Information and Communication Technology (ICT) Convergence. "Our goal is to establish an architecture for resilient electrical power and ICT, or R-EICT, that provides autonomous, decentralized and cooperative control of household and regional ''DC-microgrids''," he says (see Current ideas). "We can now achieve this by utilizing the network of 5G and/or Beyond-5G (B5G) telecommunications base stations combined with mobile edge computing to flexibly control and coordinate the rebalancing of power."
The Tohoku-led OPERA project is a collaboration between universities, research institutes and 13 ICT and electrical companies working on the elements of the R-EICT architecture. These include Nippon Telegraph and Telephone (NTT) Corp. for distributed data centre and their grid-networking, Panasonic Holdings Corp. for mobile high-speed communications infrastructure, and Furukawa Electric Co., Ltd. for intelligent on- and off-grid networking.
"Over the past six years, we have made a big effort to establish a non-profit research and development consortium with industry and academia under the financial support of the Japan Society for the Promotion of Science (JSPS) to study the issues, social and economic impacts and possible solution to problems that can come from new technologies, and such a new wave has led to the establishment of the OPERA project," says Otsuji.
Traditional centralized power distribution systems use a small number of large power plants to meet demand from households, business and infrastructure. This means the large power generators and the grid regulator control the flow and price of power.
With the advent of rooftop solar and battery storage, however, households, communities, and businesses now have the capacity to generate their own power, reducing their electricity needs from power plants, and exporting excess power to the grid for profit.
This reversal of such power flows has many benefits for consumers, but requires highly responsive and intelligent control of many systems to prevent surplus energy damaging the grid. Such controls are already in place — the charge controller units attached to rooftop solar systems sense the voltage in the network and can disconnect the systems in circumstances of oversupply — but Otsuji''s team aims to take this intelligence to another level.
Otsuji adds that "at Tohoku University, researchers had the very vivid experience of the 2011 Great Tohoku Earthquake to learn from, which caused widespread power outages that also brought down all communications. In the event of such an earthquake in the future, a system like R-EICT could ensure much greater resilience by allowing the autonomous control of clusters of DC microgrids with access to local sources of stored power."
The basic architecture of R-EICT is an interconnected network of DC microgrids that are connected at the cluster level to an AC distribution backbone (see Current ideas). A household-scale DC microgrid would operate autonomously and in coordination with other microgrids to maintain a stable DC power supply that is optimized for efficiency, storage and local consumption.
"Some of the microgrids would not be connected to the AC backbone at all, but through the power of edge computing we would be able to optimize many aspects of these power systems, and isolate subsystems that might have faults without impacting other microgrids in the cluster," says Otsuji.
As Hirohito Yamada — who leads the technological development of R-EICT at Tohoku University — points out, 5G and/or beyond 5G (B5G) combined with the ability to operate autonomously is key to a successful microgrid concept.
"DC microgrid distribution networks could resolve the problem of AC grid instability," he says. "However, most studies on controlling DC microgrid networks have been based on computing over best-effort, service-based ''Internet of Things'' technology, which can result in a loss of real-time power rebalancing control at times of high network congestion," he says.
"Only with 5G and/or B5G and mobile edge computing in local clusters is real-time, low latency power interchange and rebalancing feasible. R-EICT represents a revolutionary energy-distribution and communication network infrastructure."
Katsumi Iwatsuki, who leads the programme''s research strategy at Tohoku University, says the team has a designed a reference architecture model for R-EICT network systems, and carried out simulations and modelling that have demonstrated the scalability, resilience and energy-use efficiency that can come from R-EICT. "Sensor networking and micro-EV networking on DC microgrid systems will be key to advancing digital twins of compact city-based cyber-physical systems," he says.
To test the system practically, they are now constructing a test-bed DC microgrid network, as well as a corresponding digital twin — a complete virtual model of the test-bed and its operation.
The test-bed project is being implemented at the Tohoku University''s Aobayama campus, and involves installation of connected pilot-scale DC on- and off-microgrids, and distributed computing nodes on which the team are building a virtual data centre.
"In this way the computing and power consumption of the data centre will be distributed in space, which is a different computing architecture to the large centralized data centres such as those of Google and Amazon," says Otsuji.
With each DC grid connected by a single copper cable, the computing system models power generation, consumption and storage in real time. It will also intelligently control voltage levels in each grid so that the generation and consumption are balanced across the broader network.
"We are also planning to assist major shopping malls, and evacuation centres within the Tohoku region, with the installation of autonomous, intelligent, DC microgrid systems that would become active during disasters," says Otsuji.
Use in the wake of disasters could be some of the first ways these technologies are employed, he says. "But over the longer term we can see the systems becoming an integral part of future smart cities. They would be an example of a new standard for autonomous, decentralized and coordinated control of power rebalancing, using the capabilities of 5G/B5G and mobile edge computing."
Since the late 1800s, electricity has been generated and distributed in the form of an alternating current (AC) over two or three wires — a current that swings from positive to negative polarity 50 or 60 times per second. Not only is AC easier and cheaper to generate using traditional turbine-based power plants, it also allows the voltage to be readily stepped up and down using transformers, which makes it possible to use very high voltages for long-distance distribution with low losses while delivering safe lower voltages for household consumption.
The power derived from a battery, on the other hand, is direct current (DC) — a constant voltage that does not oscillate like AC. DC is the native form of current used by most small electric appliances and devices such as personal computers, mobile phones, TVs and LED lightings, all of which need an AC/DC converter to function if connected to a standard AC outlet. Even recent inverter-controlled devices such as air conditioners, refrigerators, and vacuum cleaners first convert AC to DC, and then reconvert the DC to appropriate frequencies of AC different from the energy grid''s native 50 Hz or 60 Hz to operate them.
Photovoltaic solar panels generate DC, which must be converted to AC to connect to the grid or wiring in the home, only to be converted back to DC for use. With the price falling for both rooftop solar and high-capacity lithium-ion batteries for energy storage, DC microgrids — with a second socket for DC devices — could become a feature of future smart energy grids.
Political and grassroots public support for a resilient, non-nuclear and fossil fuel-free future is gaining traction and spurring development of new microgrids in Japan.
Prime Minister Shinzo Abe''s governing Liberal Democratic Party (LDP) is advancing efforts to invest greater sums to develop smart villages, towns and cities with interconnected, low and zero-carbon microgrids and distributed energy.
"The national resilience program is deeply institutionalized," Rikkyo University professor of political economy Andrew DeWit told Microgrid Knowledge, adding that Prime Minister Shinzo Abe''s government seeks to spend $41 billion on new resiliency projects in 2018.
One such development comes comes in the form of a partnership between PanaHome, ENERES, IBJ Leasing and the government of Hyogo Prefecture. The project partners unveiled a plan on September 21 that calls for installing and integrating 117 home solar PV-battery energy storage systems to form a community microgrid in a new area of Ashiya City known as Smart City Shioashiya Solar-Shima. Panasonic, Tesla''s lithium-ion battery Gigafactory partner, is supplying the residential battery energy storage systems.
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