May 7, 2018

What would it take to power all of Tacoma’s cars with electricity?

 

 

 

 

 

 

We aim at estimating the total annual energy consumption per household for light-duty vehicles in Tacoma, in the context of the City of Tacoma’s Livable City Year partnership with the UW. The most straightforward approach to estimate the consumption was found in taking the energy contained in the respective fuel needed, and then extrapolating the annual fuel consumption based on the expected consumption of this fuel per household based on average parameters for vehicle miles traveled, etc.

Annual Energy Consumption – Current

To pursue this, we collected data on the different propulsion systems in use, the vehicle miles traveled and the efficiency of the respective vehicles. The Washington State Transportation Commission released a very detailed private household vehicle inventory report in 2014, which provides more information than other sources that can be traced back to what the circumstances are in Tacoma. Even though the report dates back a few years, we decided to use it as a baseline for our calculations, as the trade-off for the assumptions that are necessary to get the same information out of newer, larger scale data appears to be too high. However, whenever better data is available, we will make use of this. For urban settings in Washington, the report provides an estimate of 1.7 vehicles per household. More recent data on car ownership for the City of Tacoma confirms this. We then want to look at the vehicle miles traveled per year per household. Here, our baseline report also provides helpful data with 19,418 VMT per household in Pierce County. Given the fact that Tacoma is the biggest city in Pierce County, contributing significantly to this value, and assuming that the travel demand did not change significantly since 2014, we could assume that this is representative. However, we need to take into account that Tacoma is an urban area, while the rest of the county is mostly suburban and rural. Thus, we need to consider Tacoma as an urban area, for which the same report provides a number of 13,206 VMT per household per year. This is much lower than the Pierce county average and appears to be more realistic, given the fact that Tacoma is still within the larger metropolitan area of Seattle and King County, which shows a more comparable value at a VMT of 14,036.

Now that we know our values for vehicle miles driven per household, we need to find a proper estimate for the fuel consumption of the vehicles used. As mentioned earlier, we know that the average household in Tacoma owns 1.7 vehicles. Thus, it would be ideal to take the MPG of these different vehicles into account, as we can assume that the VMT is not equally shared between primary and secondary vehicles per household. Moreover, a detailed estimate should account for different propulsion systems that require different fuels and as a consequence, different energy consumption. However, the data on that detail level is not publicly available for Tacoma. However, the share between propulsion systems for urban settings in Washington shows a strong dominance of gasoline powered vehicles with over 97% of households owning a vehicle have a gasoline powered vehicle. Considering that the total number of vehicles in Washington is 2,863,653 cars, of which only 24,624 were Battery Electric Vehicles as of June 2017, which is less then 1% (even less during earlier years), and that urban settings show very low numbers of diesel powered vehicles (4% of households own a diesel powered vehicle), we can neglect the effect of the differences in fuel consumption and thus, base our calculations on gasoline powered vehicles. Looking at the average MPG of vehicles in urban areas, we find a value of 24.8 for all registered vehicles. For primary vehicles, this value is slightly higher at 25.3 MPG. Since these values have changed with the introduction of new vehicles over the last years, we can assume slightly higher values and hence, slightly lower energy consumption in the end of our calculation. Since the primary used vehicles are proportionally used the most on urban roads, and considering the efficiency improvements mentioned above, we will use the corresponding value for our calculations. Our formula ends up to be:

This value gives us a rough estimate on how much energy is consumed per household just for private vehicle travel. Considering a 12 months rolling average to the current date in gasoline prices for mid-grade fuel in the greater Seattle area, the price per gallon is $3.15. As a result, this would lead to average annual expenses of

per household for the energy consumed for transportation. Given the fact that we estimated based on Washington state urban data for VMT (which includes Seattle), the demand could actually be slightly higher in Tacoma than this estimate shows. At the same time, the potential improvement in fuel efficiency could have led to a decrease in energy need in Tacoma.

Annual Energy Consumption – Conditional on BEVs

To create a scenario on what would happen if we would do an estimate based on the same parameters for travel demand, fleet mix in terms of vehicle sizes and but just for a purely electric fleet of cars, we can do our calculations through converting the energy need through the efficiency of internal combustion versus electric engines. We use this approach by using the data from electric vehicles currently available was not seen as representative, given the fact that the current portfolio is mostly focused on sub-compact cars, which is not representative of the vehicle type split we see today. As the U.S. Department of Energy provides a range for an estimated Power-to-wheel-ratio of 16-25% for internal combustion engine vehicles, and 77-82% for all-electric vehicles. If we take the average of these values, we can calculate with 20% and 80% correspondingly to trace back our energy need.

We start with the inverse of our average MPG rate, which results in a need of 0.0395 gal/mile needed to move a representative internal combustion engine powered vehicle.

Applying the loss of 80% of this energy consumed we end up with:

If we apply the efficiency of an electric engine powered car, we get a gross need of energy of:

Assuming equal travel demand per household, this would result in an annual consumption of:

As we can see, the gross energy need for the same transportation effort, would decrease by roughly 75% per household. Considering the low energy prices per kWh for electric energy in Tacoma with $0.085 per kWh, we can calculate the total cost to:

Which is only 22.5% of the expenses for the current expenses.

However, the question remains whether this comparison is reasonable. Given the fact that a massive drop in mobility cost which could be achieved through such a change the transition dynamics could actually cause people to drive more, as the marginal cost decrease drastically, similar to effects that could be the results of large fleets of autonomous vehicles. Moreover, it is very unlikely that the travel needs and behavior will remain the same over a development of the next 20-40 years (realistic time estimate for full implementation). In addition, the energy that is now taken from gasoline would have to be substituted by electric energy and it is questionable, whether this will not have a negative effect on the price benefit. Lower demand for gasoline will make the prices decrease, while a higher average need of electric energy will possibly require enhancements on the power generation structure and hence force the electricity prices to rise. Finally, this calculation assumes that all kinds of private vehicles can be replaced by electric vehicles and still fulfill the needs of today’s society. Thinking of light trucks, which are optimized for offering large cargo space while ensuring off-road capabilities, large battery storages could become an issue in the utility of these vehicles.

Hence, we can use this calculation to illustrate the savings potential, but a transition would probably include trade-offs that might decrease the efficiency advantage and the cost benefit as mentioned above.

Power Requirements

Assuming a level 2 charger at 6.6 kW, which is the common threshold for every car manufacturer but Tesla, each household would have to charge for 660 hours annually (Assuming 100% efficiency of electric car charging). Considering that in reality the efficiencies of the charging cycles will be lower, we need to take additional energy need into account when it comes to estimating peak hour charging capacities. We find the population of Tacoma as 211,277, with 2.49 people per household. The total number of households in Tacoma is 84,850. With 1.7 cars per household as referenced in the previous section, this results in 144,245 cars in Tacoma in total. If we now assume that all these vehicles are now electric, we can make use of the annual energy consumption per household as calculated in the previous section, we find the total annual energy usage for light-duty electric vehicles as:

To put this into perspective, the current yearly electricity demand per household in Tacoma is 11,315 kWh and the total energy demand can be calculated as:

Thus, a fully electric car fleet would increase the energy need of Tacoma by roughly 40%. The additional energy power needed to produce this power, assuming smoothed out operations, was calculated according to the formula E = Pdt to:

Thus, Tacoma would need to supply an additional 42.2 MW of power to supply power to an entirely electric fleet of light-duty vehicles in Tacoma, given the current numbers and the current travel behavior. This is slightly more than what is produced by one of the four generators at the Mayfield Dam today. This smoothed out operation would be the best-case scenario to cause as little disruption to the grid as possible. However, this illustrates that if creating smooth demand, this appears to be attainable with an extension of the existing power supply network. If we now look at the scenario of having all these vehicles charged at the same time, we assume a power input of 6.6 kW, assuming that most people would charge with home chargers (up to level 2), we would receive an energy demand of:

A power demand of 952,000 kW would be equivalent to 6 Mayfield Dams. Considering that we calculated based on up to level 2 charging, this energy demand could be even higher if faster charging solutions would be widely distributed, which is very likely by the time this scenario appears to be realistic. This would be highly disruptive to the grid and perfectly illustrates why peak charging times can be such a problem and need special considerations to distribute the charging times across the day. Thus, encouraging different charging options is a key enabler of a better distribution of charging times. Charging at places other than home, and at times other than after coming home for the night should be available such as work, school, shopping malls, grocery stores, movie theaters, and other places people go daily. Having charging options away from home not only reduces range anxiety but evens out the power demand on the grid due to charging and is integral to the success of electric vehicles in Tacoma. This could be extended by integrating energy storing solutions for alternative energy sources, such as Community Solar.