Energy 3

Earlier posts under this title have pointed out that energy consumption must increase despite advances in the efficiency with which energy is used. They have pointed out that fossil fuels and climate change are existential problems for mankind. Electricity is unique in its flexibility as an energy medium of transmission but the big problem with electricity is that it is difficult and expensive to store.

At present the transportation business is the second largest user of energy in society after space heating/cooling which is the largest. Various parts of transportation present their own unique features but the development focus has been on passenger cars. The technology most discussed with cars is electric vehicles.

Fundamentally there are two types of electric car, those purely driven by batteries and those which are hybrid having both a petrol and electric engines. While early on hybrids captured most of the market there has been a resurgence in pure electric cars. This surge has particularly been driven by Tesla a US manufacturer who have been very successful.

The issue with battery only cars has been range. Tesla is now selling cars with a 300 mile range and batteries which can be quickly charged. This recharging can be in tens of minutes to big fractions of the total capacity. However “range anxiety” is still a potent factor limiting all electric cars. Quick charging is not nearly as fast as filling up with petrol giving a typical petrol car a range of 400 miles plus. The anxiety over running out of charge has been compounded by the relative lack of charging facilities outside the home.

It is time to firmly lay to rest one myth. Electric cars may be pollution free at the tailpipe. However the electricity they use is supplied from the grid from multiple sources which include some renewables but often a lot of coal, oil and gas.. The overall energy equation needs to take account of the large amount of energy used in producing the car and battery.

Battery efficiency is key. The battery is certainly a lithium ion type which actually describes a range of broadly similar battery technologies. The battery is an expensive component which does not last the life of the car. Recycling of battery parts is claimed but not actually done as yet. To drive down battery production costs Tesla with Panasonic is building a big factory called the Gigafactory in the US. Batteries are getting more efficient but there is still a big need for improvement.

One important feature of most electric vehicles is regenerative braking. This means that when slowing the car turns the motor/generator putting electricity into the battery.

Hybrid electric is much more common than pure electric. There are fundamentally two types of hybrid,: parallel and serial. In the parallel both petrol engine and electric can drive the car  while in serial the wheels are always driven by the electric motor while the petrol engine charges the battery. Parallel is the most common but the best arrangement is far from agreed with many variations.

While originally hybrids only used a petrol engine to recharge the battery it is increasingly common for mains electricity to be also used for battery charging. This is the so called “plug in” hybrid. An abbreviation has come into use as PHEV which is Plug in Hybrid Electric Vehicle.

Incidentally the Toyota Prius, which was the first hybrid on the mass market, uses earlier battery technology earlier lithium battery technology..

Other transportation systems are being contemplated. The diesel electric locomotive is well established. Somewhat similar systems are possible for buses. The economy advantage is that the diesel engine can run in its most efficient range. In principle trucks could use a similar system although the tractive effort required is much larger.

There is interest in electric aircraft. The problem is twofold in that regenerative braking can only occur on landing but more importantly that weight is a big consideration. Hypothetical hybrids with many electric motors driven from batteries charged by a gas turbine have been suggested but not built as yet.

There are other possibilities for storing energy. One is to use a flywheel turning very fast which is spun up as the vehicle ( usually a bus ) brakes with the flywheel energy then used as the bus accelerates away from a stop

There are also possible alternative fuels. Hydrogen which is less dense than petrol and difficult to store could either power an internal combustion engine or be used in a fuel cell.  A fuel cell converts hydrogen and oxygen from the air into electricity. The big attraction is the possible generation of hydrogen on the car from petrol thus making use of the existing distribution infrastructure. No economically practicable method is known at present.

It is fair to say there is a ferment of research and development at present. Overall the electric vehicle looks to be firmly in the lead but we can expect lots of possibilities in the near future. To inject a note of caution, for all the glib talk of banning petrol and diesel fuels they are at present the economic choice and alternatives have a way to go before matching them.