Introduction
A little over five years ago, coastal communities in New Jersey were devastated by Hurricane Sandy. According to the Department of Energy, over 2.6 million households in the state lost power, and more than 12000 households in the local Marlboro Township community were seriously impacted with several losing power for nearly two weeks. During this period, high winds downed trees and overhead power lines everywhere, leading to widespread power outages that plunged the entire town into darkness. Schools and businesses remained closed. Forced to manage without lights, communications, and heat during chilly fall nights, many residents relied on the generosity of neighbors owning generators or found shelter elsewhere. Fast forward to the present and little has changed. Marlboro’s residents and businesses are still heavily dependent on electric power from local utilities which in turn rely on centralized power generation and distribution through overhead transmission lines. Tree branches still dangle precariously close to power lines and can easily knock down electric poles or wires, causing widespread power outages or creating safety hazards for residents. To prevent disruptive power outages in the future, it is essential for the township council to consider embarking on a fundamental transformation of Marlboro’s current electric grid to a highly resilient electric grid that is 99.999% available to its residents. Two engineering alternatives are envisioned to satisfy these requirements: burying electric cables in the existing distribution network or building a resilient microgrid termed the M2-grid using clean renewable energy alternatives such as solar power.
Improving grid availability with underground cables
One approach to building a more reliable electric grid is to dig trenches in the ground and bury all the overhead power cables. Clearly, there is a significant safety and reliability benefit from burying these electric cables underground since they would become immune to high winds. Consequently, a major storm with gale-force winds is unlikely to cause any power outages. However, burying all the overhead cables requires a significant construction effort which will consume substantial time, incur excessive costs, and encounter many other obstacles such as access to private property, uneven roadways, or barriers. Underground cables require the installation of accessible concrete vaults for splicing cables every 1000 to 4000 feet depending on the type of cable used. Underground cables are also not cooled by air flow like overhead cables and may require backfilling with special types of soil that allow better cable cooling. Lightning arresters must be installed at transfer points between overhead and underground cables to prevent damage arising from large voltage fluctuations. Additionally, while faults in underground cables are less likely to occur than in overhead cables, the repair of underground cables is expensive and disruptive to residents. Overall, the time and cost incurred in transitioning to an electric grid with underground cables can prove to be excessive. According to a report by the Public Service Commission of Wisconsin, “The estimated cost for constructing underground transmission lines ranges from 4 to 14 times more expensive than overhead lines of the same voltage and same distance.” Moreover, these cables are owned and operated by private electric utilities that are unlikely to bear the costs.
Improving grid availability and resilience with the M2-grid
Another alternative is to build a resilient microgrid using a clean renewable energy alternative such as solar power. According to the National Renewable Energy Laboratory of the U.S. Department of Energy, the cost of utility scale and residential scale solar photovoltaic power have dropped to approximately $1 per Watt and $2.80 per Watt, respectively. Much of this cost reduction is driven by the large scale manufacturing and deployment of solar panels. Simultaneously, this rapid growth in solar power generation has spurred the development of residential energy storage solutions. Rechargeable lithium ion batteries with 10 kiloWatt hour capacity that can meet the needs of an average sized home for a day are now cost-effective and costs are expected to drop further. The combined availability of solar panels with small-scale distributed energy storage technology has created an opportunity for Marlboro to create an ultra-reliable and resilient electric grid called the Marlboro Micro-grid, or M2-grid, incorporating the following 5 essential elements: ubiquitous solar panel installation, distributed energy storage, microgrid integration into the utility’s electric grid, connected sensors, and smart meters.
Ubiquitous solar panel installation
In order to build in resilience and capacity to serve Marlboro’s electricity needs in the event of an outage, solar panels must be installed at as many locations as possible. Panels may be installed on rooftops of residences and businesses or in open spaces that are zoned as solar energy farms. Property tax incentives can be provided to spur solar panel installation. Often, private utilities may also be willing to install panels at residences as extensions of their own electric grid.
Distributed energy storage
To minimize the widespread impact of downed power lines on local households and businesses, it is essential to build in distributed energy storage in the form of rechargeable lithium ion batteries. These may be installed at residences, businesses, or at solar energy farms. When an outage occurs, batteries in close proximity will automatically serve as the source of electricity.
Microgrid integration into utility’s electric grid
The M2-grid’s solar photovoltaic cells and batteries must be interconnected with the existing utility owned electric grid to generate and distribute electricity locally in the event of an outage. This should be carried out in cooperation with the local utility to ensure that no instability is created in the grid by incorporating a multitude of power sources. During normal operation, any local power generation can serve local consumption or storage needs. A private utility may also draw on this power to serve its own demands while compensating residents for its consumption.
Connected sensors
Connected sensors must be installed to detect faults such as downed power lines so that the system can immediately isolate the fault with respect to the rest of the grid. Electricity can then be automatically restored within the microgrid(s) by drawing on local solar power generation and energy storage sources.
Smart meters
Traditionally, private utilities supply electricity to consumers and charge by metering consumption. In the case of the M2-grid, electricity may be locally generated, supplied by a neighbor, or by the utility. The use of smart meters to track local power generation and consumption through web and smartphone applications allows residents and businesses to be suitably credited or charged according to whether they are producing or consuming electricity.
The M2-grid provides clear benefits in ultra-reliability and resilience since dependence on centralized power generation is reduced. In addition to reducing reliance on fossil fuels, the cost is expected to be much lower than underground cables because existing utility owned overhead electric cables are still expected to be used. A potential limitation of this approach is that an outage in a single downed cable may impact residents in its immediate vicinity, but it is unlikely to propagate to a wider area; this can be tested by simulating a cable outage and measuring the fraction of sites for which electricity is restored with the M2-grid. The M2-grid also needs cooperation and action by residents, businesses, and utilities but these can be enabled by providing incentives in the form of zoning for solar farms or property tax rebates.
Conclusion
The M2-grid is a cost-efficient, clean energy alternative that can meet the future electricity needs of Marlboro’s residents and businesses and can serve as a model for all of suburban America. By leveraging advances in photovoltaic solar panels, lithium-ion batteries, connected sensors, and smart meters, it can provide the resilience and availability needed without incurring the excessive costs of underground cabling. Hurricane Sandy was a wake up call to the residents of Marlboro. By building the ultra-reliable and resilient M2-grid, Marlboro township can seize control of its energy future.
References:
U.S. Department of Energy Office of Electricity Delivery and Energy Reliability. (2013, April). Comparing the Impacts of Northeast Hurricanes on Energy Infrastructure. Retrieved from https://energy.gov/sites/prod/files/2013/04/f0/Northeast%20Storm%20Comparison_FINAL_04 1513b.pdf
Martin, R. (2015, September 17). Home energy storage enters a new era. MIT Technology Review. Retrieved from https://www.technologyreview.com/s/541336/home-energy-storage-enters-a-new-era/
McKenna, P. (2015, May 1). Why Tesla wants to sell a battery for your home. MIT Technology Review. Retrieved from https://www.technologyreview.com/s/537056/why-tesla-wants-to-sell-a-battery-for-your-home/
Public Service Commission of Wisconsin (2011, May). Underground Electric Transmission Lines. Retrieved from https://psc.wi.gov/Documents/Under%20Ground%20Transmission.pdf
Fu, R., Feldman, D., Margolis, R., Woodhouse, M., & Ardani, K. (2017, August). U.S. Solar Photovoltaic System Cost Benchmark: Q1 2017. National Renewable Energy Laboratory Technical Report NREL/TP-6A20-68925. Retrieved from https://www.nrel.gov/docs/fy17osti/68925.pdf
Ton, D. T. & Smith, M. A. (2012, October). The U.S. Department of Energy’s Microgrid Initiative. The Electricity Journal, Retrieved from https://www.energy.gov/sites/prod/files/2016/06/f32/The%20US%20Department%20of%20Energy%27s%20Microgrid%20Initiative.pdf