Re: Some questions I got from a listener by email

Hello there,

I think, I have some answers or at least some hints, where to look for answers for some of the questions asked by Larry.

1. What is the relative energy efficiency and emissions levels of an all-electric plug-in vehicle, relative to a traditionally-powered car?

This is an very interesting question, but unfortunately it is incomplete. If you want to compare these types of energy conversion, then you have to keep in mind, that they are more or less chains of single energy conversions. That means, to compare them you have to have a equivalent starting point and ending. So, there is not much sense in analysing and comparing the energy efficiency of an all-electric plug-in vehicle with a traditionally-powered car, because they have different starting points. One starts with the calorific value of energy of the fuel and the other with the voltage and electric charge needed. Both of them have to be produced in the first place and for this production you also have to calculate the energy efficiency, unless they aren't provided by nature for free. Like solar radiation, wind power, water power or usable geothermal heat.

Nevertheless, if you do compare them, the result is, that the all-electric plug-in vehicle has an energy efficiency somewhere between 0.9 and 0.8 for converting electricity into motion, while the car powered by a turbocharged diesel engine and a mechanical transmission would have an energy efficiency of about 0.45 to 0.4, for converting the calorific value of energy of the fuel into motion.
For a petrol engine the energy efficiency is a little less, about 0.4 to 0.3, depending on the technical level of the engine used. (At this point I did not go further into detail. If you want to know more about it, that would lead us into the priciples of thermodynamics and electrical engineering, which takes too much space here.)

So, to take a common starting point, you can start with the same fuel. No matter, if it's fossil fuel or bio fuel, or if it's natural gas, coal, wood, petrol or what ever. Then you can say the energy contained in this fuel is converted into motion in two different ways. The first one is the direct way, where the fuel is used directly in the engine of the car with the energy efficiency mentioned before and the product is mechanical energy, which is turned into motion. The second option is to burn that fuel in large power stations and convert it into electricity. This electricity is transformed several times, stored in the battery of the car and finally converted into motion. These transformations and the storage are not free of losses. So, to get an equivalent total energy efficiency compared to the direct use, the power station has to be exactly that more efficient, what the electrical system is more unefficient, compared
to the direct use. (Now things get more complicated if you say that the all-electric plug-in vehicle comes with a build in start-stop capability and regenerative braking, because they lead to an increase of energy efficiency. But their influence is hard to calculate because they are depending on the driving habits.)

So that was also, albeit indirect, the answer to your 2'nd question. Which asked, if it would be more efficient, to fill tanks in cars with natural gas, or to power large generators with natural gas and use this to power the cars?

As far as I know, today the best available solution for a large power station, regarding the energy efficiency, is a combination of two thermodynamic cycles. Namely a Joule and a Rankine cycle. Practically this is achieved by a combination of a gas- and steam turbine system, where some heat from the gas turbine exhaust gases is recovered and used for the steam production for the steam turbine. This leads to an energy efficiency of about 0.55 for the conversion of the calorific value of energy of the fuel into electricity.

Instead of starting the calculation with the same fuel, it is also possible to start the calculation beginning with a natural source and calculating the energy efficiency for its extraction, mining or growth & harvest. Like a well-to-wheel calculation for the extraction, transport, refinement & distribution of oil as a fuel.

Regarding the emission levels, one can say, that they are depending on the fuel and the technical level and combustion process of the engine. (I once did measure the emission levels of my car, at a certain operating point, because I wanted to figure out, if there's a difference when it's running on canola oil compared to diesel fuel.) So, for the all-electric plug-in vehicle, as long as the source used to produce the electricity is producing almost no emissions, you will have almost no emissions. But unfortunately in most countries it's a mixture of different sources, e.g in Germany in 2006 the electricity was produced out of 13% renewable sources, 29% nuclear fission and 58% fossil carbohydrates. It is stated that this mixture produces 530 g of CO2 per kWh. For the all-electric
plug-in vehicle this end's up somewhere between 589 and 663 g CO2 per kWh of mechanical energy. To have a comparison I calculated the average CO2 emission of my car and that's 702 g of CO2 per kWh when running on diesel fuel only and 438 g when running on a mixture of canola oil and diesel fuel (This allready includes an input of 37% of primary energy to grow the canola.) So, here it turns out, that in my situation there is no advantage in changing to an all-electric plug-in vehicle. But that's also depending on the car I drive and my driving habits. (About 70% of the distance I travel by car in one year, is on highways and has an average distance of 500 km (310 miles) per journey. The average speed is 130 km/h (80 mph). The other 30% I travel through cities and on minor roads. So you can say my car's engine is mostly driven close to its designed operating point regarding the rpm and power output.)

3. question was: Would it make sense to power the generating plants with petroleum instead of natural gas?

That does not make too much difference regarding the relative energy efficiency. The difference is less depending on the fuel you use, but more on the thermodynamic cycle, the technical level and complexity of the power plant. That means if it's an gas turbine, steam turbine or a combination of both. Or if it is a diesel engine. (By the way also diesel engines can run on natural gas, only the injection system is a little more complicated and thus a little more expensive.) Then the question is if your plant has some kind of waste heat recovery or if it provides a district heating system.

4. If the nation were to switch to a fleet of vehicles powered by one of these natural-gas options, would there be enough fuel available?

Well, this question is a really philosophical one. Without notice it implies, that the word "enough" is correlating with a certain period of time and amount of consumption, unless this resource is not endless. Even if you think in a period of another 100 years and the same consumption as today, there will surely not be "enough" natural gas.

So I think there will not be a big one-fits-all solution regarding everyones energy demand, especially when it's about transportation. I think in the futur there will be a greater variety of fuels and therefore the future engines have to be able to run on "multi-fuel". And not at last people have to reduce their energy consumption, by changing their habits and way of live, to solve the energy shortage and climate change problem.

So long

Ernesto