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A strong interest in combining solar generation with electric vehicle charging

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As additional models of plug-in electric vehicles enter the marketplace, the challenges of electric vehicle charging infrastructure and its impacts on the utility grid are a growing concern. Integrating the following elements: renewable energy sources, the electric grid, battery storage, and management and monitoring systems, have already been put together in one zero-net design. In 2011, EPRI (Electric Power Research Institute) worked with the Tennessee Valley Authority (TVA) to create solar charging for electric vehicles, using a prototype of a six-port charging station with solar energy generation for plug-in electric vehicle (PEV), which were built at the EPRI laboratory in Knoxville, Tennessee.

The construction model included these elements: solar arrays installed in a canopy atop the parking bay, networked EVSE charging stations under the canopy, battery storage, networking capabilities, software, and more. This prototype has been replicated since and is a doable model of plug-in electric vehicle charging infrastructure at the workplace, parking lots and garages of various venues and commercial buildings. In addition, the prototype had implications for public charging infrastructure in community facilities, malls, hospitals, art and entertainment buildings, etc.

Using energy storage is a crucial part of the solution. The batteries store energy at night and offset the impact of the vehicles on the local electricity distribution during the day.

The Tennessee Valley Authority Smart Modal Area Recharge Terminal (SMART) Station Project report by EPRI is available at no charge and in PDF: Click here to download the SMART report.

This report documents the base design for a Smart Modal Area Recharge Terminal (SMART) station. The base design included a 10-space public parking facility that was designated for electric vehicle charging, a solar photo-voltaic (PV) canopy-top arrays and battery storage. Battery storage systems require a climate-controlled area. The project investigated the opportunities and challenges in this comprehensive system. The design also explored feasibility and cost of modularity, as well as to develop a base design that can be used by other entities, that might have a similar infrastructure.

Scaling considerations

Per each site application, every element in the designed system can be scaled independently. The cost of the system is primarily determined by the size of the solar array and the amount of battery storage. The size of the solar array, the number of charging stations are also scalable.

In order to maintain a reliable, public accessible and cost effective electric service, which are fundamental in the efforts to electrify fleet and private transportation, understanding the impact on the electric grid of PEV charging systems, managing this load growth, and planning the resources to meet future demands are essential.

The report concludes that the development of public charging infrastructure that incorporates multiple technologies has many challenges. Further, consumer behavior in regards to the use of plug-in electric vehicles is not well understood. Still, decisions are made in regards to potential use and location of EV infrastructure must be made. In scaling of the various system elements are made on consumers’ use assumptions for the system. Based on these decisions, there are possible oversights which may have cost implications. According to EPRI, the objective of undertaking this design was to identify the opportunities and challenges that would need to be addressed to deploy more complex vehicle-charging systems in the public space.

How would plug-in hybrid electric vehicles impact the grid?

Utilities have been supporting the early Plug-in EV and Hybrid PEV adopters and have started to prepare for the installation of charging infrastructure in their service areas. However, with mass adoption and commercialization of PHEVs and BEVs (battery electric vehicles) and the growing charging infrastructure, managing the new loads on the electric distribution system is a new challenge. Charging can happen at different types of locations: at the workplace, at event-based venues, community facilities, shopping sites, etc. But, generally, most of the charging is expected to occur at the residencies.

According to EPRI’s research, The short-term impacts for most utilities should be minimal and localized. While residential charging could reach a power draw of up to 19.2 kW (80 amps at 240 volts), most vehicles are expected to charge at requirements below 7kW. Charging levels of 120V/1.2kW, 240V/3.3kW, and 240V/6.6kW are expected to be the most widespread.

Still, there is a possibility of isolated impacts on some distribution transformers and secondary drops, especially in neighborhoods with older distribution systems, where residential PEV charging may cause issues. Older distribution systems initially were designed for much lower per-customer load than their current operation, and PEV charging might impact these systems more than in the newly designed systems.

EPRI concluded “that potential stresses on power delivery systems can be mitigated through asset management, system design practices, controlled charging of PEVs, or some combination of the three.”

ADDITIONAL RESOURCES

1. SMART Report: http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?productId=00000...

2. EPRI analysis "Potential Impacts of Electric Vehicles on the Grid": http://et.epri.com/Potential_Impacts_of_Vehicles_on_the_Grid_2011.html

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