Sunday, May 23, 2010

What if 25% of the Cars were plug in...How much power is needed?

Last week Toyota announced a partnership with Tesla motors backed by $50M in investments. Tesla is the manufacturer of the trendy $100K all electric plug in sports car and has a model for us all in the works, the Model S. Toyota wants the technology and I can just imagine a Tesla/Prius in every garage. Gov. Schwarzenegger hailed the joint venture as the future and asked us all to imagine CA with more plug ins.

“What we are witnessing today is an historic example of California’s transition to a cleaner, greener and more prosperous future. We challenged auto companies to innovate, and both Tesla and Toyota stepped up in a big way, not only creating vehicles that reduce emissions and appeal to consumers but also boosting economic growth,” said Governor Schwarzenegger.

How will all these plug ins be powered? Everyone seems to think that electricity comes from a plug in the wall. Power has to come from somewhere.  How will we make a green lifecycle from source to vehicle?  Wind turbines? Coal? Gas? Solar? Nuclear? 

Lets break it down.

136,000,000 registered passenger vehicles in 2007. Lets say 25% of the cars suddenly become plug ins. Therefore: 34,000,000 vehicles.

16.8 KW = 56miles charged per hour per the Tesla website.

Assume 12,000 miles per year driven we have 214 hrs of charge at 16.8 KW or 3600 KW-Hrs per car.

with 34M cars we have 1.2 x 10^11 KW-Hrs
A new nuclear power plant generates 13 billion kilowatt-hours (kWh) or 1.3 x 10^10 (assuming 1600MWe and 92% availability).

So the final answer is: 9.4 new nuclear plants would be required to keep all those vehicles charged.  One plant charges 3.6M vehicles. There were 16,153,952 new vehicles (cars trucks and SUVs) sold in 2007.

Conclusion: We need one new 1600MWe plant a year if 25% of the new cars are all electric using the numbers and 2007 sales rates above.

Electric vehicles are great, we just need to remember that the power source is part of the equation and that conservation and alternative energy will not be enough to account for future energy demands.

3 billion barrels of gasoline were refined in 2006 out of 5.5 billion barrels of crude oil.   1.6 x10^9 gallons or 3.8 x 10^7 barrels of gasoline would be removed per year if 25% of new cars were all electric.  Using the ratio of gas to oil equates to  7 x 10^7 barrels of crude oil saved per year (2006 refining and 2007 car sales and 30mpg).

Numbers and calculations are for illustrative purposes.  I am hoping for credit for error carried forward--ECF.

Good points raised from readers comments:
1. The number of cars calculation I used omits trucks and SUVs reducing the overall number of cars.  
2. What about reduced electricity demand off peak at night?  Good question.  I did not take that into account, however smartgrid technology and offpeak charging will mitigate the effects of EV.  There is also talk of VTG or vehicle to grid where the electric vehicle could actually supply power during peak or the most expensive time of day and then charge during off peak or cheaper times of day.


Monday, May 3, 2010

Safety is designed into US Reactors--The Power of a Negative Temp. Coefficient of Reactivity

Let's step back and explore one of the fundamental concepts of reactor theory and the FACTS that make reactors in the US inherently safe. I am talking about the temperature coefficient of reactivity. Oh sure everyone knows about that. Well I think if they did know it would help to alleviate some of the concern with reactors supposedly being able to "blow" up or melt down in some China Syndrome event.

The sad news for the nay Sayers is that reactors are safer than ever and US reactors are designed such that they shutdown when something goes wrong. Current reactor technology uses less equipment and less automation, focusing on passive systems. When something goes wrong in a nuclear reactor temperature is likely to rise in the reactor core. A negative temperature coefficient of reactivity means that as temperature goes up...reactivity goes down. When reactivity goes down the reactor is essentially turning itself off like pulling your foot off the gas of your car.

Reactivity is the engine of fission in a reactor. Reactivity equals more neutrons per unit time (neutron density) and therefore more fission, therefore more energy released, therefore an increase in temperature. That increase in temperature is harnessed as steam to drive a turbine and create 20% of the power in the US.

A negative temperature coefficient of reactivity makes a reactor inherently stable. Example: As power demand increases on the turbine, more steam is used, the coolant circulating through the steam generator and the reactor is cooled slightly. As the temperature goes down the reactivity....goes up! So we push on the gas pedal and get more neutrons and energy as we increase fission and compensate for the temperature drop by increasing reactivity and reactor power to match steam demand.

As you can see this stability allows for a mitigated emergency response for a major casualty leading to an increase in temperature. If I lose reactor coolant and cannot cool the core as effectively the reactor will shutdown (to a point see emergency cooling below).

Contrast this with Chernobyl. Russian designed reactors had essentially a net overall positive temperature coefficient of reactivity (graphite moderator with water coolant thus positive steam void reactivity and positive reactivity of initial control rod motion [Ref1]). See where we are going here?!? Temperature goes up and reactivity goes up. Therefore power goes up and therefore temperature goes up.... leading to disaster. Chernobyl also did not have sealed containment. It also had an enormous reactor core which lead to fluctuating reactivity and flux..essentially three or four different reactors all within the same core behaving independently yet as a whole. All of this lead to a difficult to control reactor that was not inherently stable.

When the casualty hit, the reactor essentially was unable to be controlled (There are multiple factors) and fission products and gases were released to atmosphere (no containment) NRC analysis When disaster struck three mile island, containment was in place and there was very little release to the environment (maximum offsite radiation dose 0.1 rad and total population dose was approximately 10 person-rems [ref1]and NRC analysis plus an overall mitigated reactor response due to the negative temp. coefficient of reactivity.

US reactors have containment and inherently stable reactors. Other safety systems such as the emergency core cooling system (ECCS) ensure that the reactor is cooled even with a loss of coolant. Without emergency cooling the temp. coefficient of reactivity will not help as the uncovered fuel rods melt due to fission product heating leading to various exothermic chemical reactions between the molten material and the water steam mixture.

Test to follow next Tuesday........

For extra credit: I would be remiss in not clarifying that we are talking about the moderator (water coolant) temp. coefficient of reactivity above. The prompt temp. coefficient of reactivity describes the affect of the change of temperature of the fuel itself and determines the first response of a reactor to changes in either fuel temp or reactor power. The NRC requires all reactors to have a negative prompt temp. coefficient of reactivity.

Ref 1 Intro to Nuclear Engineering, John Lamarsh and Anthony Baratta