Publication Detail

Evaluation of a PV Powered EV Charging Station and its Buffer Battery


Research Report

Sustainable Transportation Energy Pathways (STEPS)

Download PDF

Suggested Citation:
Zhao, Hengbing and Andrew Burke (2015) Evaluation of a PV Powered EV Charging Station and its Buffer Battery. European Electric Vehicle Congress

Plug-in electric vehicles (PEV) are expected to become much more common in upcoming decades. California’s Zero-Emission Vehicle (ZEV) Action Plan calls for 1.5 million ZEVs on the road by 2025. Most of ZEVs will be PEVs including electric-only PEVs and Plug-in hybrid electric vehicles (PHEV). Presumably electric vehicle (EV) charging will occur at night during off-peak demand hours, when electricity price are relatively low. However, at least some EV charging will be needed during the day and even peak demand time periods. This will require the same sort of ubiquitous electric vehicle charging infrastructure, also known as Electric Vehicle Supply Equipment (EVSE). On the utility side,the electricity sector is increasing the share of total electricity generated from renewable sources such as wind and solar. The efficient use of renewable energy resources relies on the ability to store energy when it is produced and disburse it when it is needed. Both EV charging and renewable power sources pose challenges to decades-old electrical grid systems. Growing demand for EV charging facilities may play a role in meeting the challenges of EV charging and the renewable electricity grid. Therefore, building and installing EVSEs requires long-term and smart infrastructure investment. Properly designed EV charging stations not only benefit EV owners, but also the businesses that provide charging services and the utility grid.

Various EV charging stations have been developed recently [1-5]. Most of EVSES are connected to the grid directly, which incurs the high cost of demand charges caused by spikes in power usage, often a barrier to installing EV changing stations. Demand charges are part of every commercial electricity bill and are determined by the highest 15 minutes of use during a billing cycle. In California, rates can skyrocket during midday periods of high demand in summer. Some solar PV-Powered EV charging stations (PV-EVSE) can reduce high power demand during peak times, but are subject to the change of the weather. Battery-integrated EVSEs can significantly smooth the charging spikes and mitigate charging from on-peak time to off-peak time. Combining EV charging equipment (EVSE) with an energy storage system (ESS) and solar PV eliminates the high cost of demand charges caused by spikes in power usage and maximizes the usage of renewables locally. Using storage batteries and solar PV panels with intelligent energy management system reduces the energy exchange with the grid, manages and smooths EV charging demand spikes, and avoids high demand charges during on-peak time periods. A solar PV powered EV charging station with energy storage (PV-ESS-EVSE) may be more efficient and economical than a solar PVintegrated EV charging stations (PV-EVSE) and a charging station with buffer battery only.

A Solar PV-integrated electric vehicle charging station with energy storage (Figure 1) has been developed and demonstrated at the University of California – Davis, West Village, the largest planned zero-energy consumption community in the U.S. The charging system has a 5 kW PV array, a 6.6 kW level 2 charging unit, a 35 kWh lithium iron phosphate battery pack, and a 10 kW load response bi-directional inverter. The bidirectional inverter controls power flow between the different units. It has two DC ports which are connected to the PV panel and battery storage and two AC ports tied to the utility grid and EV charger, respectively. The solar PV electrical energy forecasting and EV charging demand projection are introduced to the energy management system to optimize the state of charge (SOC) of the buffer battery, maximize the usage of PV electricity for EV charging, and avoid EV charging during peak hour time.

The charging station has been operated continuously and used by several EV users for a year. The EV charging load, PV power, grid power, and the cell voltage of each 220 cells have been recorded every minute. The research collects data from both the battery management systems (BMS) and the real-time controller and assesses the system operation and evaluates the battery performance. This paper is organized as follows. First, the power flow control, solar PV electricity forecasting, EV charging project, and the battery optimization are descripted. Then, the data from each unit are analysed for assessing the control strategy, and the cell voltage standard deviations are calculated for evaluating the cell consistency. Finally, the conclusions and future work are presented.
Presented at the European Electric Vehicle Congress 2015 in Brussels, Belgium, December 2-4, 2015.