hit counter
 

PS | How clean are EVs?

There’s no doubt that EVs are much cleaner and friendlier to be around than anything powered by an internal combustion engine, as they produce no local emissions (writes Peter Donaldson).


They also use energy more efficiently in stop-go traffic, particularly when fitted with regenerative braking systems. Even petrol-diesel electric hybrids allow the engine to be operated at constant speeds in sweet spots for efficiency – in series form (where the engine drives only a generator) at least.


However, their global impact depends on clean electricity production. Without that, even complete conversion to pure batterydriven EVs would simply amount to shifting pollution away from where the vehicles are operated to wherever the power stations are.


The same applies to hydrogen fuel cell vehicles. Hydrogen is so reactive that there are no natural reservoirs of the gas that can be readily tapped, so it has to be produced in industrial processes, most of which are far from clean or environmentally friendly. These days, most of it comes from steam reforming of natural gas, with some from naphtha or reforming techniques applied to off-gases from oil refineries or other industrial plants, and some from partial oxidation of coal and other hydrocarbons.


Steam reforming of methane, for example, happens at 700-1000 C and at 3-25 bar of pressure in the presence of a catalyst. This produces a mixture of hydrogen, carbon monoxide and a small amount of carbon dioxide, to which more heat is added in a separate vessel.


The carbon monoxide and steam are reacted again using a catalyst to bring the carbon and oxygen together and liberate more hydrogen. Finally, a pressure-swing adsorption process is used to separate the carbon dioxide and any other impurities from the gas stream, leaving almost pure hydrogen.


Steam-reforming processes can also separate hydrogen from other fuels, including coal and oil. A relatively small amount of hydrogen is also produced by the electrolysis of water, a process that itself obviously needs electrical power.


So whatever forms EVs take, there is no escaping the fact that they need power from the grid, and therefore their environmental credentials ultimately depend on how cleanly that power can be fed into the grid. In turn, that depends on the result of the political, economic, commercial and technological processes that will determine the mix of renewable and even nuclear electricity generation systems that the world migrates to if it is to be weaned off fossil fuels.


A large-scale switch to EVs will also add a great deal of load on grid systems, so the overall national capacity for generating power will have to increase and be able to cope with spikes in demand that coincide with commuting patterns. There is an opportunity here for the EV industry to make a contribution to smoothing the load while reusing old batteries at the same time, one that Honda and American Electric Power (AEP) for example have decided to explore.


In mid-May, the companies announced that they are to cooperate on a research project centred on integrating used batteries from Honda’s Fit EV into AEP’s electricity supply system. “Together with AEP, we are exploring opportunities to use the ‘second life’ battery to improve energy security, reduce CO2 and prepare for broad-scale electrification of the transportation ecosystem,” says Ryan Harty, manager of connected and environmental business for Honda in America. “Neither automakers nor utilities can address these complex technical, policy and business issues alone.”


The EV industry can do a great deal to make transport and industry friendlier to the environment, particularly when it comes to local air quality, but ultimately energy policy at government level and internationally holds the key.


Recent Posts

See All

PS | Superconducting aircraft motors

Power density and power-toweight ratio have always been very near the top of the list of essential attributes for aircraft propulsion systems, particularly in those elements that work at the sharp end