46. The problem with electric vehicles

 

If there is one thing that is synonymous with carbon-free green energy it is probably the electric vehicle (EV). But if there is one thing that highlights the gap between idealism and the reality of green energy it is probably the electric vehicle. That is not to say electric vehicles are bad, or totally impractical. But they do have their limitations, and more importantly, they probably always will. The problem is, much of the media and the green movement have so far failed to acknowledge this, and probably never will.

Traditionally, electric cars have suffered from two major drawbacks: cost and vehicle range. However, over the last ten years we have seen significant improvements in both to the point where, for many people, electric vehicles are now both affordable and practical. This has prompted the UK government to announce recently its plan to ban the sale of new petrol and diesel cars by 2030, and to effectively force people to buy electric vehicles instead. This is part of its plan for a green industrial revolution; a policy that is intended to boost growth and save the planet. Whether those two aims are mutually compatible has yet to be demonstrated, but the plan itself is not wholly without merit. For one thing, it would certainly help to improve air quality in our major cities and thus help prevent many unnecessary and premature deaths such as that of Ella Kissi-Debrah which hit the national news headlines recently. 

Unfortunately, there is a major problem with this policy: the recharging time for electric batteries, and therefore for EVs as well. Except that even here there is now some really good news. This week it has been reported that new battery technologies are being developed that can be recharged in under five minutes. So to the casual observer this may look like another triumph of technology, but unfortunately it is not quite that simple. This is because there are some things in physics that cannot be circumvented, like the law of conservation of energy. 

Electric vehicles use electrical energy. Batteries store that energy, and when it is used up it has to be replaced. And the faster you try to replace it, the more electrical power you need to do so. The root of the problem here is the vast amount of energy that needs to be replaced. So to replace that amount of energy in a very short time (like five minutes) requires very high rates of energy transfer: i.e. very high power for your power source. And it is the consequence of using very high power to recharge EVs that is the problem, particularly with regard to customer safety. The only way to eliminate this problem is to reduce the amount of energy EVs use, but as I will explain that is virtually impossible to do.

i) Energy comsumption

In a petrol or diesel car the energy is stored in the form of chemical energy in the fuel (petrol). This is highly concentrated (13 kWh/kg) and easy to replenish. Combustion releases this energy and allows the car to do work overcoming external forces such as air resistance and gravity (if travelling uphill) in order to first accelerate to its cruising speed, and then to maintain that speed against the forces of air resistance and friction. In order to do this effectively engines in most family cars need to be able to generate over 120 brake horsepower (bhp), which is the equivalent of about 90 kW. Even when cruising at 70 mph they still usually require over 50 kW of power to maintain a constant speed. This is because of the the power needed to overcome air resistance, friction and gravity.

For the case of air resistance, the power required to overcome it increases as the cube of the vehicle speed, v, while also being proportional to the density of air (ρ = 1.3 kg.m^-3), the cross-sectional area of the vehicle (A ~ 2 m²) and the drag coefficient (C_d ~ 0.4). So if v = 31.3 m/s (i.e. 70 mph), the power needed just to overcome air resistance is 15 kW ( = ½C_dAv³). This means every mile of travel at 70 mph requires almost 0.2 kWh of energy (in one hour the car will travel 70 miles and use 15 kWh of energy). The only way this can be reduced is by reducing the speed, the size of the vehicle (A), or its drag coefficient. The first of these would increase journey times, while the last two are more or less fixed and already optimized in the car's design (unless you want to drive around in a torpedo).

The second source of energy loss comes from friction with the road. This is proportional to the car's mass (m ~ 2000 kg) and speed (v), the acceleration due to gravity (g = 9.81 ms^-2), and the rolling resistance coefficient of friction of the car tyres (C_rr ~ 0.01). This adds about 6 kW to the required power at 70 mph. 

Then there is the energy needed to overcome gravity when travelling uphill. Even a modest incline with a gradient of only 5% would require a power of 30 kW to overcome the effects of gravity when travelling at 70 mph. The net result is that most engines operate at between 20 kW and 50 kW when travelling at 70 mph. If we take the midpoint of these two values, this amounts to 0.5 kWh of energy per mile (i.e. in one hour the car would travel 70 miles and use 35 kWh of energy). The key point here is that none of the numbers listed above can be significantly improved upon. They are all set by the physical properties of the world we live in, such as gravity, air resistance, friction, and the the size of a typical human.

ii) Recharging power

Now suppose you want the range of your electric vehicle to exceed 300 miles. This will require an energy storage capacity for your battery of 150 kWh. Currently most EVs have a capacity of less than half this (the Nissan Leaf is 40-62 kWh at 350 V).  

In the UK the standard mains voltage is 230 V, and the maximum current of most domestic circuits is 13 amps. That equates to a charging power of about 3 kW. So it would take 50 hours (or about two days) to fully recharge your EV.

You could of course use higher voltages (e.g. a 3-phase supply of 400 V) and currents of up to 30 A. The recharging power is now 12 kW, and the time required to recharge your EV battery is only 12.5 hours. But this is still about 2.5 hours of charging for every one hour of driving at 70 mph.

So what about using this new battery technology that can recharge in five minutes? Well if you want to recharge a 150 kWh battery in five minutes you would need a 1.8 MW power supply. That is almost the equivalent of the output of a small power station. And then you need to consider the currents and voltages that would be required.

A 350 V supply would require a current of over 5000 A to provide an energy transfer rate of 1.8 MW. Now assuming the power cable used to carry this current from the generator to the car was about five metres long and had a cross-sectional area of about 5 cm² (which is a fairly chunky cable), the power dissipation in the cable would exceed 4 kW. That would require some pretty heavy-duty insulation and cooling.

Alternatively, you could reduce the current to a few hundred amperes and operate at voltages of over 10 kV, but I doubt the HSE would look that favourably on such an outcome. However, irrespective of which current-voltage option is chosen, 1.8 MW high power charging points would place an enormous strain on the national grid.

Finally, consider this. There are currently about 40 million cars in the UK and the average driver drives 10,000 miles per year. That is 400 billion miles in total. If this is to be achieved using only electric vehicles it will require an extra 200 billion kWh of electrical energy to be generated, or 23 GW of generating capacity. That represents an increase of about 30% in the current UK generating capacity, or the equivalent of over twenty new power stations, and a 60% increase in total electricity usage. And all this is to be achieved in ten years.

Summary

For those who only use their cars for short journeys recharging times are not a significant issue. This is because the amount of energy such journeys need is small, and so the recharging time is much less than the time that the vehicles remain idle for. The problem only really becomes acute where the journeys are long, undertaken at high speed (i.e. over 60 mph) and are repeated daily. This is not just a problem with the distance that electric cars can travel on a single charge, although that can still be an issue. Nor is it a problem with a lack of available charging points, which also needs to improve as well. Even if both these problems are overcome the underlying problem remains: the recharging time and the rate of energy transfer.

Those who don't understand the physics, or have maybe just failed to fully consider the implications of the physics, may believe that this is just another technical issue that technology will fix in time. It is not. The key point is this: if you want to increase the range of EVs, then you need to increase the energy storage capacity of their batteries. But that energy will need to be replaced on a regular basis and the rate you can do this is not set by the battery; it is set by the safety regulations around the charging point.