Effects of Plug-in Hybrid Electric Vehicles on Vermont Electric Transmission System

Plug-in hybrid electric vehicles (PHEVs) have emerged as a near-term technology to reduce the nation’s dependence on imported petroleum, address rising gasoline prices, and reduce carbon emissions from the transportation sector. This paper presents the results of a PHEV grid impact study for the State of Vermont—a small power system with peak demand of approximately 1 GW. The study looked at three different PHEV penetration rates and three different charging scenarios. Uncontrolled charging regimes even at a low PHEV penetration rate of 50,000 vehicles (~ 9% of the Vermont light duty vehicle fleet) could lead to an increase in the peak demand for electric power in Vermont. However, a delayed nighttime charging regime could accommodate 100,000 PHEVs without adding to system peaks. Furthermore, next generation smart grid technology using optimal charging algorithms could accommodate 200,000 PHEVs, or approximately one-third of Vermont LDVs. As previous studies have found, displacing conventional ICE vehicles with PHEVs can reduce GHG emissions and decrease consumer fuel costs. INTRODUCTION Increasing the number of plug-in hybrid electric vehicles (PHEVs) on the nation’s roads has been identified as a critical strategy for reducing U.S. dependence on imported petroleum (1). Individual states have identified PHEVs as an important element in their transportation GHG reduction strategies (2). Vermont policy-makers have identified PHEVs as a critical greenhouse gas (GHG) reduction strategy for the transportation sector for several reasons. Vermonters are extremely dependent on personal automobiles for most trips with a per capita vehicle miles traveled (VMT) of 12,379 – the fifth highest in the U.S. (3). Yet the state has the lowest carbon electric generation mix in the nation because of the reliance on hydro-power and nuclear power (0.014 kg/kWh compared to a U.S. average of 0.61 kg/kWh) (4). Vermont state government, private utilities and other entities have funded a series of demonstrations and experiments with electric and PHEV vehicles starting in 1991. As the possibility of the wide-scale availability of PHEVs increases, electric utilities and Vermont policy-makers are interested in understanding the effect of large numbers of PHEVs on the electric grid, GHG emissions and end-user costs (5). This paper focuses on the effect on the electric grid of the deployment of 50,000, 100,000 or 200,000 PHEVs. A number of studies have modeled the impact of differing numbers of PHEVs on large state and regional electric grids, GHG emissions and end-user costs (6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16). We reviewed those studies for methodologies to examine the effect of the large-scale deployment of PHEVs in Vermont. The PHEV technical specifications that emerged from the Hybrid Electric Vehicle Working Group (WG) convened by the Electric Power Research Institute (EPRI) have served as essential groundwork on the effects of PHEV deployments. In the absence of manufactured PHEVs, the EPRI studies provide specifications for a mid-sized sedan PHEV (2001) and technical specifications for a compact sedan, and mid-sized and full-sized SUVs (2002) (6, 7). Table 1 lists the PHEV technical specifications described in the reports. TABLE 1 Technical Specifications for PHEV20 in Compact Sedan, Mid-Size Sedan, Mid-Size SUV, and Full-Size SUV Vehicle Platforms. Units PHEV20 compact sedan PHEV20 midsize sedan PHEV20 midsize SUV PHEV20 fullsize SUV Motor Rated Power kW 37 51 84 98 Nominal Battery Pack Size kWh 5.1 5.9 7.9 9.3 Battery Rated Capacity usable kWh + 4.1 4.7 6.3 7.4 Miles per Gallon Fuel Economy mpg (PHEV/ICE § ) 52.7/37.7 43.5/28.9 34.7/22.2 29.5/18.2 Electric Only Economy mpeg* 134 117 90.5 77 All Electric Efficiency (miles/kWh) 4.0 3.49 2.7 2.3 Mileage Weighted Probability Fuel Economy (MWP) @ 71.7 58 46.6 39.8 Vehicle Mass kg 1,292 1,664 2,402 2,824 Charging time (hours, 120 V 15 amp, 1 kW)^ 4 4.7 6.3 7.4 Charging time (hours, 120 V 20 amp, 1.3kW)^ 3 3.5 4.7 5.6 Charging time (hours, 240 V 40 amp, 5.7 kW)^ 0.7 0.8 1.1 1.3 + The battery rated size is assumed to be 80% of the nominal pack size. § This row gives the miles per gallon (mpg) for PHEVs as compared to internal combustion engines (ICE) – a standard gasoline vehicle. *The report expresses the all electric range as miles per energy equivalent gasoline gallon (mpeg). This calculation assumes 33.44 kWh per gallon of gasoline. @ The mileage weighted probability (MWP) fuel economy provides an estimate of a blended electric/gasoline operation efficiency. The MWP gives an estimation of what portion of PHEV’s daily annual mileage will be in all electric mode based on national driving statistics. The values presented in the table assume nightly charging of the vehicle. ^The charging rate per hour assumes an 80% required safety factor for continuous charging and assumes an 82% efficiency for 120 V chargers and 87% for 240 V chargers and 85% battery efficiency. The first measure of fuel economy in Table 1 is the gasoline miles per gallon, which indicates the lower bound mileage number based on the vehicle operating in charge sustaining mode similar to conventional hybrid electric vehicles sold today. The second fuel economy measure is based on operation of the vehicle in electric-only mode and is expressed as the miles per energy equivalent of a gallon of gasoline (mpeg). The energy content of a gallon of gasoline is expressed in terms of electrical energy at 33.44 kWh per gallon to derive this value. The MPEG serves as the upper bound efficiency potential of the vehicle. The ―Mileage Weighted Probability Fuel Economy‖ presented in Table 1 is an attempt to present a likely ―real world‖ fuel economy estimate based on a statistical approximation of the number of miles driven each year in all-electric mode and with the vehicle being recharged nightly. The technical parameters of PHEVs developed by the EPRI WG may not necessarily conform to those of PHEVs that ultimately reach the market but serve as an approximation in terms of what to expect regarding PHEV characteristics and performance. Building on the EPRI WG reports, five recent studies have analyzed the electric grid impacts from an emerging fleet of PHEVs. While there are some similarities across the studies, each one takes a different approach in terms of the electric system, PHEV configurations, and charging scenarios analyzed. The studies find generally that the existing electric power infrastructure is capable of charging large fleets of PHEVs without the need to build additional generation, transmission, or distribution infrastructure. Table 2 lists the grid impact studies reviewed here, along with some key features of each (8, 9, 10, 11, 12). TABLE 2 PHEV Grid Impact Studies. Title & Date Authors’ Name and Affiliation Geographic Focus Vehicle Configuration Charging Scenario(s) Emissions Assessment Impacts Assessment of Plug-In Hybrid Vehicles on Electric Utilities and Regional U.S. Power Grids (2007) Kintner-Meyer, Schneider and Pratt, Pacific Northwest National Laboratory Entire U.S., based on 12 modified North American Electric Reliability Council regions PHEV33, this vehicle configuration is used to estimate the electricity consumption that would satisfy the average daily commute as determined by travel survey data. The study assumes all excess capacity is used. Produces estimates based on 24-hour charging and 12-hour charging scenarios. Yes An Evaluation of Utility System Impacts and Benefits of Optimally Dispatched PlugIn Hybrid Electric Vehicles (2006) Denholm and Short, National Renewable Energy Laboratory Six different geographic regions, using hourly load data from electric utility control areas. This study simulated the energy requirements of a PHEV fleet that meets on average 40% of its daily miles traveled with electricity. This translates into a PHEV with an allelectric range between 20 and 40 Charging is based on an optimized 24hour cycle assuming direct utility control of when the vehicles are charged. No miles. Costs and Emissions Associated with Plug-In Hybrid Electric Vehicle Charging in the Xcel Energy Colorado Service Territory (2007) Parks, Denholm and Markel, National Renewable Energy Laboratory This study was focused specifically on Xcel Energy’s Colorado service territory. A mid-size PHEV20 vehicle with 37 mpg gasoline and 2.78 miles/kWh and 7.2 kWh of battery storage capacity. Four charging scenarios were evaluated: uncontrolled charging; delayed charging; off-peak charging; and continuous charging. Yes Effects of Plug-In Hybrid Electric Vehicles in California Energy Markets (2007) Lemoine, Kammen and Farrell, Energy and Resources Group at the University of California Berkeley This study used load data from the California Independent System Operator and thus was focused exclusively on CA. A compact PHEV20 vehicle with 50 mpg gasoline, 130 mpeg, and 5 kWh of usable stored energy. Also conducted sensitivity analysis using a fullsize SUV. Three charging scenarios were modeled: optimal charging, evening charging, and twice a day charging. No Potential Impacts of Plug-in Hybrid Vehicles on Regional Power Generation (2008) Hadley and Tsvetkova, U.S. Department of Energy Oak Ridge National Laboratory This study focuses on the entire U.S. but divides its scenarios into the 13 Energy Information Agency regions. A mixture of sedans and sport utility vehicles. 182 scenarios: 7 scenarios for each region for 2020 and 2030, with a base scenario, different electricity mixes, and 2 charging scenarios (5:00 pm and 10:00 pm) Yes Some of the studies detailed above and several others have looked at the emissions reductions from replacing ICEs with PHEVs (13, 14, 16). The EPRI WG studies, for example, calculated the net greenhouse gas emissions and smog precursor emissions on a per vehicle basis to allow for comparisons (6, 7). Three of the grid impact studies also assessed the net emission impacts from an emerging f