CONTACTS 

Keith Parks, Paul Denholm, and Tony Markel 

National Renewable Energy Laboratory (NREL) 

1617 Cole Blvd. 

Golden, CO 80401-3393 

keith_parks@nrel.gov, paul_denholm@nrel.gov, 

tony_markel@nrel.gov 

Strategic E nergy

SEAC Analysis C enter

PHEV Impacts on Regional Systems 

Results 

The study quantifies benefits to the utility and 

the consumer for plug-in hybrid electric vehicles. 

PHEVs have implications at the individual and 

societal level regarding reduced gasoline consumption, vehicle operating cost reductions, and 

lower emissions. Xcel Energy can control impacts 

while increasing sales through modest incentives 

or controls. 

Charging Profiles 

The timing of charging reflects real driving patterns 

with and without incentives and/or controls. Three 

of the four charging 

profiles represent charge at home scenarios (Do 

Nothing, Delay to 10pm, Optimized to Off-Peak); 

the last (Opportunity Charging) assumes vehicles 

have access to plugs throughout the day. Typical 

vehicle consumption for charge at home scenarios 

is 5.3 kWh/dayt (1.9 MWh/yr). For opportunity 

charging, consumption goes up 75% to 9.4 kWh/day 

(3.4 MWh/yr). 

Avoided Gasoline 

The largest single benefit is reduced 

gasoline consuption for the consumer. For 

charge at home regimes, 55% of fleet 

gasoline consumption is avoided. With 

opportunity charging, 73% of gasoline is 

avoided. At $3/gallon gasoline and 

$0.09/kWh electricity, that translates to 

annual consumer savings of $710 and $860 

respectively. At 50k vehicles, or 3% penetration of household vehicles, the annual 

household savings is $35.5M and $43M. Increased electricity consumption translates 

to 96.7 GWh and 171.5 GWh annually. 

Production Costs 

The additional load due to PHEVs will result 

in additional production costs.The timing of 

the charging matters. As charging is pushed 

to the off-peak, production costs decline. 

Fuel costs dominate the total costs. By 

implementing a program to encourage offpeak charging, Xcel Energy could save 

$53/car annually by optimizing dispatch of 

the vehicle, or $41/car annually by delaying charging to 10pm. At 50k vehicles, this is 

$2.65M and $2.05M annual savings. 

Emission Footprint – Utility Scale 

Since PHEVs substitute electric-drive for gasoline-drive, they will displace tailpipe 

emissions upsteam to Xcel Energy. The total amount of emissions depends on the 

number of vehicles and the timing of the charging. Emissions go up as charging is 

pushed further into the off-peak period. Opportunity charging has the largest 

total CO2 emissions, largely due to the larger total energy consumed. Total PHEV 

emissions is small compared to total electricity sector emissions. 

The emission rate associated with different charging regimes has the same pattern; 

as charging is pushed to the off-peak, the emission rate goes up. This is due to 

more coal being used to supply power for the PHEVs. 

Note the incremental emission rate is less than half of the system average emission 

rates. That is, incremental generation reduces the average emission rate. 

Figure 3. PHEV Electricity Average Cost (50k Vehicle) 

Figure 2. Comparative Fuel Consumption for PHEVs 

Figure 1. Charging Profiles 

Problem Definition 

Plug-in hybrid electric vehicles (PHEVs) offer an intriguing solution to displacing 

oil, off-setting emissions, and benefiting both the utility and customer with cost 

savings. While impacts can be generalized, specific costs and benefits are relatively 

unknown. Using the Xcel Energy Colorado service territory and customer base as a 

proxy to estimate these impacts will help further knowledge and benefits of PHEV 

technology. 

Goals 

As a result of additional PHEV loading, quantify the following items: 

• Avoided gasoline 

• Distribution impacts (feeder and bank loading) 

• Increased utilization of existing infrastructure 

• Vehicle emission footprint 

Solution Concept 

Develop PHEV charging profiles derived from actual GPStracked vehicles simulated to be PHEV with a 9.0kWh battery 

(ie PHEV20 ) using NREL’s ADVISOR vehicle simulation model. 

Simulate the Xcel Energy Colorado system assuming various 

penetrations and charging strategies of PHEVs using PROSYM, 

a chronological production cost model. Develop a PHEV load 

analysis tool to analyze capacity impacts at the system, bank, 

and feeder level. 

Four different charging regimes were analyzed: 

1. Do Nothing* – Vehicles leave the for day; return home and begin charging immediately 

2. Delay to 10pm* – As above, but a timer delays charging until 10pm 

3. Optimize to Off-Peak* – As above, but control vehicle charging to occur in the lowest load hours. 

4. Opportunity Charging – Vehicles have access to the grid throughout the day charging whenever they are parked. 

*Options one through three represent charge at home regimes 

The resulting loading profiles were scaled to simulate penetrations of 50k, 250k, and 500k vehicles corresponding 

to 3%, 15%, and 30% penetration of household vehicles in the Xcel Energy Colorado service territory. 

Emission Footprint – Vehicle Scale 

The vehicle emission footprint includes 

tailpipe emissions (aka vehicle), upsteam 

utility emissions, and upsteam gasoline 

refinery emissions. The PHEV total 

emission for NOx, SOx, and CO2 goes 

down relative to future combustion 

vehicles and hybrid electric vehicles for 

most charging regimes. The notable 

exception is the Optimized to Off-Peak 

charging scenario which has nominally more NOx and SOx emissions (but marked 

reductions in CO2). To contrast, the Opportunity Charging regime has the greatest 

reduction of emissions for all emissions. The tailpipe emissions are reduced in all 

charging cases. This is significant with regard to urban air-shed issues, or the 

amount of emissions released in or near urban areas. 

Benefit Analysis 

The total benefit of PHEVs to Xcel 

Energy is a mixed bag. In the Optimized 

to Off-Peak scenario, costs can be saved 

in exchange for nominal increase in 

total emissions. For Opportunity 

Charging, the most gasoline is avoided 

combined with the most emission 

reductions with more total electricity 

sales. The trade-off is this scenario has the 

highest production and capacity cost impacts. Delay to 10pm is a compromise of 

these two extreme cases providing cost savings while providing equal or less total 

emissions. Do Nothing has lower emissions, higher production costs, and 

additional capacity costs. 

Figure 4. Comparative CO2 Emissions for Regional System 

Figure 5. Upstream Emission Rates Impact of PHEVs (50k Vehicles) 

The information contained in this poster 

is subject to a government license. 

CU Energy Initiative/NREL Symposium 

University of Colorado, Boulder 

October 3, 2006 

NREL/PO-640-40694