151: Lateral thoughts #7 - the problems with wind power

Fig. 151.1: Wind turbines.

There are many claims that are made about wind power, not least that it is cheap. It isn't. In fact it costs almost the same as nuclear.

A nuclear power plant costs about £10bn and delivers 1GW of power almost constantly over a lifetime of up to fifty years. So that is £10 of capital cost per watt of output.

A 1MW wind turbine costs about £1.25m (offshore turbines cost even more). So that is only £1.25 of capital cost per watt of nominal output, much less than nuclear. But wind turbines rarely deliver their maximum or nominal output because they cannot operate in high winds for safety reasons, and at normal wind speeds (v) the output varies as v³. So a drop in wind speed of 50% results in the output power dropping to an eighth of its previous value (see Fig. 151.2 below). 

 

Fig. 151.2: The observed power output of a 1.5MW wind turbine.

 

But there is another problem, and that is that wind speeds are weighted in their frequency of occurrence towards lower values (see Fig. 151.3 below). The result is the power output is both highly variable and weighted towards low values, and so most turbines struggle to deliver more than 33% on average over time of their nominal output. So the true capital cost of a wind turbine is about £3.75 per watt of output. But if we also factor in the 25 year lifetime of wind turbines (i.e. half that of nuclear), then the true capital cost relative to nuclear is going to be about £7.50 per watt. So wind is only marginally cheaper, and remember, offshore wind is even more expensive.

 

Fig. 151.3: The observed frequency of wind speed.

But this is not the biggest problem with wind power. That is the energy storage or backup dilemma. What do we do when the wind doesn't blow?

This was the problem in December of last year. The UK experienced a cold snap with temperatures dropping below -10°C. This is not unusual: it happens every year and is caused by an area of high pressure sitting over the UK. So, just as the UK needed more power for extra heating in the cold weather in mid-December, there was no power coming from the UK's main renewable source: wind power. But this is not just a winter problem. A similar phenomenon is seen during heatwaves in summer. In both cases the wind across most of the UK drops to almost zero for days, or sometimes even weeks on end. So how do we compensate for this?

Well, there are two options. We can either build extra wind turbines to generate surplus electricity in times of plenty and store the excess energy, or we can build backup generators using different and more reliable energy sources.

The problem with the energy storage route is the sheer amount of storage required. The cold snap described above lasted about a week but could have lasted up to twenty days. According to Worldometers, the UK generated 318,157 GWh of electricity in 2016, or about 870 GWh per day. That is 3132 TJ per day, or the energy equivalent of exploding fifty Hiroshima-sized atomic bombs every day. So twenty days of storage would require the equivalent energy storage of over one thousand atomic bombs. And if we want to completely de-carbonize our energy and transport systems that number could easily double. That would require an awful lot of batteries and so is totally unrealistic. It cannot be done.

So what about backup alternatives? Well the issues here are cost and reliability. Because wind is unreliable the backup source needs to be very reliable and immediately accessible. But it also needs to be green. So the obvious candidate is nuclear. But nuclear is more expensive than wind power, so using it as a backup means adding its capital cost to that of wind power when it is rarely going to be used. That makes no sense economically. If we are going to build enough nuclear power stations to satisfy all our electricity needs when the wind isn't blowing, then we may as well use them continuously all the time rather than keeping them idle as backups. If a backup is only going to be used sporadically then its capital cost needs to be much smaller than that of the primary generator it is backing up otherwise it is just an unnecessary additional cost. That leaves only two viable options for backup energy sources: coal and natural gas.

Coal and gas powered generators are up to ten times cheaper than the equivalent-sized nuclear station or wind farm, so their capital costs are negligible in comparison. They are also reliable, but they are not green. That said, they would only be used intermittently so their carbon emissions would be low.

So here is the dilemma. If we stick with wind power then we will need to compromise and allow some fossil fuels to be used as backup supplies in times of need. This will still massively reduce our CO₂ emissions but it will not make us carbon neutral. The only alternative is to abandon wind power and go nuclear.

90. Lateral thought #5: Without fossil fuels there would be no Champagne

 

Yes, amazing as it seems, without coal we might not have Champagne. Well, possibly.

This tale arises out of the English energy crisis in the early 17th century (see here). After over one hundred years of shipbuilding for the new Royal Navy the country ran out of wood. There were of course other reasons such as population growth and increased urbanization that were equally to blame, just as they are today. But the net result was there were just not enough trees. So much for renewable energy.

In response a worried King James I banned the use of wood for non-essential purposes. And one of the areas to feel the heat was glass-making. In response they switched to using coal, which had previously been considered a dirty and ungodly fuel even though it was abundant. Yet needs must when the Devil drives. And there was an unexpected bonus, as there often is when the Devil is in play. By using coal English glass-makers were able to achieve higher kiln temperatures. This meant they could make glass bottles that were thicker and stronger - just what you need to make Champagne. Otherwise the bottles have a tendency to explode.

So, three cheers for fossil fuels. And yet another reason why the French love the English.

Of course this does raise an important philosophical question. Can you be a Champagne socialist and still fight for climate justice? I don't know. And until I do I'll probably stick to the Château Haut-Brion.

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.