Electric propulsion, its been talked about for years what are its strengths and weaknessess and could it actually be used in aircraft propulsion. As you already know the ability to turn electricity into motion through a motor is very efficient, even approaching close to 95%. But, where does the electricity come from. In most electric propulsion, it has come from batteries that store the electricity in chemical bonds inside the battery, hence the different names of batteries, lithium ion, nickel cadmium, lead acid. All chemical combinations that can store and then easily release this energy into electricity. As an interesting side note, notice that almost each propulsion technology is named after its method of “storing” energy.
So with Electric storage we need to look at two flows to really get the big picture. First lets look at the flow from production to use.
We will assume the electricity is coming from a source of power that turns fuel into electricity at a rate of about 60%. That is the efficiency of some large powerplant turbine generators. So,
Fuel 46.4 MJ -->Industrial Turbine 60% efficient (46.4*.6)-->27.8 MJ rotational energy--> Industrial Generator 80% efficient (27.8*.8)-->22.24 MJ Electricity--> batteries 80% efficient (22.24*.8)--> 17.8MJ stored energy-->electric motor 95% efficient (17.8*.95)--> Work output 16.9 MJ
This process is about 36% efficient. The equivalent 300 mile tank in a normal vehicle would take this vehicle an additional 246 miles. That is an additional 82% in effective mileage. The problem as most know is battery storage, its both very expensive and very heavy. So only very low energy consuming vehicles can use this efficient flow, and unfortunately aircraft are high energy users, making this flow unpractical except for the lightest and slowest of aircraft.
Just for fun lets look at this from a different angle for those light and slow aircraft that might be able to use this power flow. Lets pretend that the energy from the wall was very cheap compared to the electricity we would have to produce ourselves in a hyrbid setup. (And in relation it is very cheap) How efficient will your flow be now. At 15 cents a KWH, a gallon equivalent of gasoline (132 MJ) costs about 5.50 $ this might seem expensive but not when you consider that were able to use such a high percent of it, compared to a gallon of gas.
Wall outlet 46.4 MJ--> Vehicle Batt 80% efficient (46.4*.8)--> 37.12 MJ stored energy -->electric motor 95% efficient (37.12*.95)--> Work output 35.26 MJ
That is an efficiency of 76%. Which means now a 300 gallon tank equivalent of “wall juice” is going to carry you 840 more miles. That's a 280% increase over current engines. The one drawback engineers are finding is it happens to be very difficult to put one gallon of fuel equivalent (132 MJ) of energy into a bank of batteries, let alone multiple gallons worth. So for now only very light energy applications will be able to use this flow. But, for those that can, it is an extremley efficient flow. At the current 20% consumption, (132 MJ) of energy is almost equal to 4 gallons worth. Making the equivalent wall energy about a $1.40 for the same usable energy.