Archive for May, 2009

Energy Facts

May 3, 2009

I’ve been looking on the Internet for facts on energy usage including alternative energies and alternative fuels for cars. Most of what I’ve found is propaganda, and I don’t believe it. The solar people try to sell you that your electric meter will run backwards and you’ll make money. The electric car people call their cars “zero emission” and claim it will cost you pennies a mile to drive them. I analyzed some real data provided by the DOE, DOT, and other sources and computed some answers using simple math.

Total U.S. yearly electric power generation/consumption 5,000,000,000,000 kWh
Total U.S. Households 120,000,000
Total electrical power per household 4,756 W
Yearly electrical consumption  by households 1,300,000,000,000 kWh
Consumption per household by households 1,237 W
% of total electrical consumption by households 26.0%
U.S. Gallons of gasoline consumed per year 142,000,000,000 gallons
Energy in a gallon of gasoline 34.50 kWh/gal
Vehicle miles travelled/year 3,000,000,000,000 miles
cars/light trucks 2,775,000,000,000 miles
big rigs 225,000,000,000 miles
Average miles/gallon for cars/light trucks 19.54 mpg
Total energy in gasoline consumed per year 4,899,000,000,000 kWh
Hybrid (i.e. Toyota Prius)
efficiency of hybrid gasoline engine 35.0%
hybrid miles/kWh 4 miles/kWh
mpg of a hybrid 48 mpg
efficiency of average car 20.0%
efficiency of gasoline generator (8kw) 35.0%
efficiency of a coal burning power plant 40.0%
efficiency of a coal burning power plant after distribution network 37.1%
Registered Vehicles in U.S. (2007) 247,264,605 vehicles
Registered Cars in U.S. (2007) 135,932,930 vehicles
Registered Trucks in U.S. (2007) 110,497,239 vehicles
Electric car – typical miles/kWh 3.00 miles/KWh
Equivalent mpg for typical electric car @ 100% efficient engine 103.50 miles/gallon
Equivalent mpg for typical electric car @ coal efficiency power production 38.42 miles/gallon
Losses in Power grid 7.2%
Additional electrical power generation required for all electric cars 996,767,241,379 kWh
% increase in total electrical generation needed for electric cars 19.9%
% increase in household electricity usage for electric cars 76.7%

That’s a lot of numbers.

The first interesting number is that 26% of electrical energy use is in households, the rest is for commercial and industrial purposes. I started this simple analysis when I read a story on Wikipedia about solar energy plants in the Mojave desert. In the same article it said that one solar power plant that delivers 350MW could power 500K households while another 550MW plant could only power 400K households. They obviously assumed a different value for household usage, so I asked myself: which is correct?  I looked up on the DOE web site and found that on average a U.S. household consumes about 1200W (10,500kWh/year).  But 1200W is only 1/4 of the overall power consumed on a per household basis.  So a 550MW plant, which would be the biggest solar plant ever built at a huge cost, would supply power for a community of about 90K households and businesses, much fewer than either the 400K or 500K quoted in Wikipedia.  In fact that 550MW plant couldn’t even power the smallest of cities because it provides no power when the sun doesn’t shine, for example at night or on cloudy days. So in fact saying that the solar plant can provide power for even a single household is misleading. Except for remote cabins with huge batteries, solar does not provide primary power. Solar can only be used to reduce peak power consumption during the daytime, which is certainly a need, but not a primary need.

The second question that I have always wondered about is much energy is used in the form of electricity versus how much is used for cars. After doing some simple math I found that the amount of energy used by cars over one year is roughly the same as the amount of electrical energy produced over one year. That was unexpected. Unfortunately, the useful energy that we derive from gasoline is only 20% of the total energy because of the efficiency of the average car. If we move to hybrid cars this efficiency goes up to about 35% which would reduce the energy needed by cars, that is the amount of gasoline consumed, to 57% (a little more than half) of the current level. Some newer non-hybrid engines can also achieve this higher efficiency. A 50% reduction in gasoline usage would be a huge help both for global warming and for energy security, but what if we want to go further. Enter the electric car.

There is no free lunch with the electric car, it still requires electricity to run and where do we get that electricity?  From coal and gas burning power plants for the most part. So how to we compare the performance of electric cars with gasoline powered cars?  That is a very difficult question because while there are 250 million registered gasoline power cars in the U.S. today, there are less than 100 thousand electric vehicle and those are all basically golf carts of one sort or another (the term of art is “neighborhood electric vehicles”). The equivalent to mpg for electric vehilces is miles/kWh. While small vehicles can have miles/kWh of 5 or even higher, the unanswered question is what would the equivalent miles/kWh be for the entire fleet of 250 million gasoline power cars if they all converted to electric cars. For my analysis I chose a very generous 3 miles/kWh, which I’m sure the electric car advocates would say is low. But if you consider that a Prius is a 4, chosing a 3 means that the average car would be a little bit bigger than a Prius, which is obviously not the case today.

So that what is the bottom line about how much fossil fuel can be saved moving to electric vehicles? The answer can be seen by looking at the efficiency of those coal and gas burning plants. Turns out coal and gas plants have about the same efficiency (35% or maybe up to 40% for the newest designs) as a well designed hybrid gasoline power engine. So that means that electric powered vehicles are really coal power vehicles generating pretty much the same amount of greenhouse gasses as hybrid vehicles. But the amount of other crap, like radioactive isotopes, that come from burning coal is much worse than burning gasoline. The bottom line is that it looks like electric cars pollute much more than hybrids. On the outside, electric cars are promoted as the ultimate in clean, but on the inside they are very dirty things. The only hope for electric cars is nuclear power, which would truely provide a “zero-emission” vehicle, but would also pollute the ideology of the electric car.

A final note on the electric car. My analysis shows that you can expect your electricty usage to almost double if you use one, so make sure you get the cheapest electricity rate you can.