You cannae change the laws of physics

Materials World magazine
4 Dec 2011

As you may recall from classroom physics, the First Law of Thermodynamics states that energy can neither be created nor destroyed – it can only be changed from one form to another. The two most important kinds of energy for transport applications are the change from potential (stored) energy to kinetic (motion) energy. As far as road transport goes, the debate rages over the most efficient, least polluting way to achieve this transformation, and the relative efficiency of electricpowered vehicles compared to the internal combustion engine.

The efficiency of the internal combustion engine is surprisingly low. Energy is wasted as exhaust and radiator heat, and in turning the pumps, fans and alternators that keep the engine running. Some estimate the overall efficiency is about 20% – that is, only a fifth of the thermal energy content of the petrol is converted into mechanical work.

The electric motor also loses energy in several ways, including the rotor and stator winding resistance, magnetic energy dissipated when the magnetic field is applied to the stator core, stray losses dissipated as currents in the copper windings and harmonic flux components in the iron parts. There are also mechanical losses including friction in the motor bearings and cooling fan. Nevertheless, the electric motor achieves a remarkable 80-90% efficiency in converting electricity into work, but that electricity has to be generated and stored in some way.

In the UK, more than 90% of our electricity is from coal, gas or nuclear and the efficiency of a coal- or gas-fired power station is 30-60% at best. Meanwhile, car manufacturers are looking to lithium-ion batteries to store electricity in electric and hybrid cars. Researchers at the Massachusetts Institute of Technology, USA, recently reported a lithium battery efficiency of 77% using a catalyst consisting of nanoparticles of a gold and platinum alloy. In other words, 77% of the electricity used to charge the battery was recovered, up from the previous record of about 70%. Putting aside the environmental questions of mining, extracting, manufacturing and disposing of the lithium and the cost of such catalysts, the overall combined efficiency of the electric car is only 18-36%, with the not unreasonable assumption that a fossil or nuclear fuel generated the electricity in the first place.

Charles Snow, the English physicist and novelist, neatly summarised the three principal Laws of Thermodynamics as follows –

  1. You can’t win – you cannot get more energy out of the system than you put into it.
  2. You can’t break even – any transfer of energy will result in some waste of energy unless a temperature of absolute zero can be achieved.
  3. You can’t get out of the game – you cannot achieve absolute zero.

Or as Scotty, Chief Engineer aboard Star Trek’s USS Enterprise, put it, ‘You cannae change the laws of physics’. All those advocating the electric car as the answer to emissions’ reduction should take note.