DC Power, Green Storage and Communication: Basis for Interconnected Micro Grids

By Dan Bishop

DC power distribution in a microgrid is significantly more efficient than AC power in communities with inputs from renewable power-generating sources (i.e. solar, wind power). DC power alleviates losses associated with design concerns on AC systems. When combined with recyclable and environmentally-sustainable DC energy storage, such as sodium-ion batteries and super capacitors, a reliable basis for long term energy independence and transition to renewable energy is possible. Through a hierarchy of immediate source storage to localized grid storage, surplus energy would become available to the primary (AC) grid in a controlled and productive manner. Conversely, microgrid energy storage would provide a controlled load when receiving power from the primary grid. By managing multiple energy inputs and outputs through an effective communication backbone, we can significantly reduce efficiency loss in the primary grid and have more options for supply and balance

In September 2017, Hurricane Maria delivered a tragic blow to Puerto Rico and its power infrastructure. Although news reports from March 2018 suggest that power has been restored to 90 percent of clients, a system using renewable power sources could create long-term energy independence, and serve as a proving-ground for the world. This is particularly an opportunity for the remaining 10 percent of customers who may otherwise be without power for some period of time. This approach also allows for conversion one region at a time, facilitating expansion and integration with existing grid infrastructure.

The proposal comprises three main objectives:

1. DC: Use of direct-current (DC) power within a microgrid as part of a distributed energy resource (DER), including use of DC power in households and industrial venues.
2. Multi-level storage/communication: Use of environmentally sustainable energy storage at multiple levels (combined with reliable communication). These designated storage levels are defined as follows:
   • Directly at the sources of “light” renewable power sources such as photo-voltaic (PV) or wind power, as “household-level” storage (HLS).
   • With renewables at “neighborhood-level” storage (NLS) or utility-level renewable (ULR) storage.
   • As a consolidated region (CR), for mass energy storage (high-level DER management).
3. Efficient interaction with main grid: Storage at the CR level would interface with the main power grid (using alternating-current [AC]) and with collective micro grids that use DC power. The AC component of the CR would utilize smart inverters to optimize delivery from storage to the main grid.

DC:
Distributed power within a microgrid is currently inefficient when integrating renewable energy sources as AC. The need to integrate multiple renewable sources into a DER filled with AC taps (supply and load) adds unnecessary burden. Running a distributed system on DC, however, eliminates challenges associated with lead/lag of current to voltage (and cycle synchronization), requiring only the maintenance of voltage level. This solution is complemented by the use of battery storage (a DC operation). As with any transmission system, wire losses are incurred. For longer distance exchanges (between HLS and NLS) the DC voltage can be stepped-up higher to reduce current flow. The loss from conversion efficiency is much smaller compared to wire losses over distance at lower voltages. For exchanges between a neighboring HLS the drop in voltage from wire loss will naturally favor the closest storage exchange (higher voltage contributes first) which ultimately reduces losses.

Addressing Transition to DC in the Household:
LED lighting, cell-phone chargers, laptops, and other commercial devices run on DC, and can easily use DC-DC converters instead of AC/DC converters. DC brush-less motors are already used in industrial applications, and could be manufactured cheaply if mass-produced. It would be necessary to create a DC standard for household supply, but transitioning to DC does not represent an unreasonable burden technologically or financially, especially in the case of a new installation. Additionally, a recent comprehensive DC conversion study showed that a DC household (with energy storage) can save between 14-25 percent energy using DC instead of AC. Losses from the diode rectification of AC was listed as a large contributing factor.

Multi-Level Storage with Communication:
The benefit of storage is that power can be exchanged in a predictable and timely manner. In this case, storage acts as a two-way load buffer that can provide notice to the next storage level (NLS or CR). On a CR level, providing information on available power sources and expected load allows the utility to adapt supply or to redistribute power.

By providing battery storage that combines communication between storage levels, power loads can become more predictable at all levels of transmission and distribution. For Puerto Rico, a devastated substation would be a good candidate for replacement with a CR storage facility.

Providing Greater Insight into Renewable Energy Supply Using DC:
Renewable sources exceeding load demand at the household level can be fed simultaneously into storage at the HLS bank and NLS, with the HLS providing a seamless transition when household-level renewable sources drop out. Storage output can be maintained in the household within a voltage range appropriate for batteries, but still in compliance with device requirements. As HLS capacity fills to a predetermined buffer threshold, the output DC voltage from HLS can be stepped up to be compatible with external community supply. The stepped-up output voltage can be controlled by the HLS to provide an acceptable current delivery for the size/delivery of household storage. The amount of deliverable current from the HLS along with the kWh capacity can be reported to the NLS for planning. A similar approach can be employed between the NLS and CR.

Current Challenge for Battery Energy Storage:
A challenge for battery storage is to make it recyclable (environmentally friendly) and sustainable (accessibility of raw materials). Lithium-Ion batteries are not currently sustainable for long-term mass storage. Standard Li-ion cells have a short operating life (~5 to 7 years), requiring a consistent supply for replacement. Additionally, a limited amount of lithium is available globally, and is supplied by a few companies. Lithium requires great amounts of energy to be recycled from used cells. Newer alternatives (still in development), such as solid-state sodium-ion batteries, are less energy-dense, but are manufactured from inexpensive, accessible materials, and are easily recycled. Sodium-ion batteries also can be shipped and maintained with zero charge. While Li-Ion batteries may fill the gap in short-term testing, a more sustainable approach soon will be required.

Efficient Interaction with Main Grid (Transmission Side):
For distribution, the CR acts as an intermediary to the main AC grid. Following the net load curve for renewables (“duck curve”), in the absence of load requirements during the daytime (when there is adequate solar or wind input), CR storage can provide a predictable load to the main grid. Controlled load can be applied during the day in anticipation of requirements at night, preventing a roller-coaster load schedule. Additionally, with the use of smart inverters, CR storage can fill power supply gaps in the main grid. While communication between CR and the main grid may not be possible initially in Puerto Rico, the CR can self-regulate load based on daily schedule while receiving feedback from HLS.

In summary, areas of grid infrastructure loss from Hurricane Maria can be transformed into a golden opportunity to advance the supply and distribution of power in Puerto Rico with a vision that reflects the need for modern, sustainable power.

This article was edited by Mehrdad Rostam.

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