We need to reduce the atmospheric CO2 level – use quantitative methods …
Everyone wants to reduce the CO2 concentration in our atmosphere. In 1750 it was 280 ppm and in 2018 it is now 410 ppm, leading to temperature rises worldwide. So what can we do to counter this effect? As engineers we believe that the best way is to use numbers to quantify what is happening now, and to calculate the probable effect of different new schemes that the world could adopt. Many of the numbers used in this article are for the UK, but the analysis and principles apply to most developed countries in the world. It is interesting to note that the quantitative engineering methods used in this article to analyse world and national climate and energy issues, apply equally to new product design and development.
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Man-made CO2 emissions are about 5% of the world’s natural cycle. Each year the natural world generates about 771 Gtonne CO2e, and pulls out about 788 Gtonne CO2e. Man generates an extra 35 Gtonne CO2e, leading to an annual increase in the atmosphere of about 18 Gtonne CO2e. If man’s annual emissions reduced to 17 Gtonne CO2e now, the CO2 concentration would stabilise at about 410 ppm. This would mean that our population of 7.5 billion would have an annual emissions budget of 2200 kg CO2e each. In 2014 the annual CO2 generation per person was about 6740 kg in the EU, 6760 kg in China and 16600 kg in the USA. Our net CO2 emissions are way above sustainable levels, so we need to generate less, and help the world to pull out more CO2 from the atmosphere (e.g by planting more trees).
Energy consumption is our biggest CO2 generator
Most of our CO2 emissions come from our energy usage. In 2017 the UK used 2327 TWh of primary energy, generated 367 Mtonne of CO2 and 89 Mtonne of CO2e from other greenhouse gases (giving a total of 456 Mtonne of CO2e). Per person this is an annual average of 35 MWh and 6910 kg CO2e, which is a daily average of 96 kWh and 19 kg CO2e.
Our future energy consumption must generate much less CO2
To reduce our CO2e emissions, we need to either reduce our energy consumption, or use energy that emits much less carbon (e.g lower g CO2e/km for cars), or both. It is important to consider both operating emissions and embedded emissions. New equipment may be more efficient, but it will include a lot of embedded energy and carbon for its manufacture and transportation. Total CO2e emissions will sometimes be lower if you continue running old equipment, instead of replacing it with new equipment (e.g for a car, if you don’t drive many km each year).
When analysing energy we must be careful to differentiate between primary energy (e.g calorific value of a hydrocarbon fuel) and the useful work done (e.g pushing a car through the air). Heat losses are an unavoidable effect in all heat engines, so when comparing alternative schemes it is important to compare like with like for the whole energy chain (e.g well to wheel for cars). In some applications the lost heat can be usefully used locally, and in other applications it cannot!
So where do we use most of our energy? In his excellent 2008 book ‘Sustainable Energy – without the hot air’ (free download from www.withouthotair.com) Sir David MacKay suggested that a useful simplification to understand the UK’s primary energy consumption is that it spends about a third on heating, a third on transport and a third on generating electricity. This applies quite well for many developed countries and is still roughly true today. Note that the media and politicians often say ‘energy’ when they mean ‘electricity’ (a much smaller number).
How can electricity help us use energy in a low-carbon manner?
One of the strategies for reducing carbon emissions is to electrify applications (e.g transport and heating). This decouples the application from the primary fuel. An electric car doesn’t mind whether it is charged with electricity generated from gas, wind or nuclear. It enables a smooth transfer of energy infrastructure from high-carbon to medium-carbon to low-carbon to zero-carbon solutions. It also means that any CO2 emissions occur at a centralised electricity power station (where they are easier to capture), instead of at the end point of consumption (where they are harder to capture).
Like hydrogen, electricity is not an energy source, it is just an intermediate form for carrying energy. It can be generated from many sources, and it can be used in many applications. If we want to make more applications electric, then we need to generate more electricity. It is useful to understand the energy sources for our current electricity generation, and our options for generating more in a low-carbon manner, in future.
In 2017 the UK used 2327 TWh of primary energy, with 724 TWh of this being used to generate 352 TWh of electricity (40 GW average). The electricity output was 137 TWh from gas, 70 TWh from nuclear, 44 TWh from wind, 32 TWh from biomass, 23 TWh from coal, 18 TWh from solar, 6 TWh from hydro, 2 TWh from oil, 5 TWh from other sources and 15 TWh from imports (from other countries).
A rough estimate indicates that local energy efficiency can increase by about 2:1, when an application changes from hydrocarbon to electricity. Using this assumption means that if the UK changed all its applications to be electric (transport, heating, cooling, infotainment, industry, agriculture, etc), it would need to generate about 1160 TWh of electricity in a year (half of 2327 TWh). 1160 TWh is 3.3 times higher than the 352 TWh currently generated. i.e. the average electric power output would increase from 40 GW to 133 GW.
How can we generate more electricity sustainably?
If the UK wants to generate all its electricity from local renewable sources, it will be constrained by the available land and sea area. The area of the UK is:
- Northern Ireland 14,130 sqkm
- Wales 20,735 sqkm
- Scotland 80,077 sqkm
- England 130,279 sqkm
- Total 245,221 sqkm
To produce all of the 133 GW from a single UK renewable would require:
- 66,500 sqkm of onshore wind turbines (annual average = 2W/sqm), or
- 44,300 sqkm of offshore wind turbines (annual average = 3W/sqm), or
- 13,300 sqkm of solar PV panels (annual average = 10W/sqm), or
- 8,900 sqkm of high efficiency solar PV panels (annual average = 15W/sqm)
Another way of looking at this is to say that if the UK wanted to generate 1160 TWh of annual electricity from wind only, it would have to increase its wind output by a factor of 26 (from the 44 TWh it generated in 2017).
Renewables are diffuse energy sources, so you need country-sized installations to produce enough electricity to satisfy our current energy appetite. In addition, a huge amount of embedded energy and carbon is needed to manufacture and install so many renewable devices (which have finite lifetimes - about 20 years).
In practice the UK will continue to generate electricity from a variety of energy sources (Nuclear, Wind, Solar, Hydro, Gas, Biomass). Coal and oil will stop. Wind, solar and nuclear will increase, but are unlikely to match the increased demand for electricity. It is likely that there will be more gas power stations to fill this gap (as they can be installed and commissioned quickly). This will still reduce the UK’s carbon emissions as electricity generated from gas is 400g CO2e/kWh, compared to 900g CO2e/kWh from coal. Biomass (mainly wood pellets) is treated as a zero carbon fuel, but this is only true if new wood mass is grown as fast as it is chopped down. Is this the case for trees felled in the USA, then shipped across the Atlantic to be burnt in UK electricity power stations? To get more zero-carbon electricity, the UK needs to consider installing more nuclear power stations or importing clean electricity (maybe from solar farms in the Sahara one day).
The energy transition to electricity needs to match Demand with Supply
It is difficult to store electric energy, so in practice it helps if the growth in electric supply can be matched by the growth in electric demand (and vice-versa). It is not attractive to generate lots more electricity if there is insufficient demand to consume it at the same time.
Electric demand and renewable supply vary significantly with time. In practice the gap is currently met with gas-powered electricity generating stations (which can be turned on and off relatively quickly). i.e. at present, gas is the dominant energy storage used for UK electricity. This strategy will not be able to continue at times when renewable electric supply exceeds the total electric demand. This is when alternative energy storage schemes will be needed.
So how will electricity demand increase? It will probably come from transport before it comes from heating. Local heat losses are a waste for transport but can be used usefully for heating.
We can already see the success and quick uptake of eBikes, but electric vehicles (EVs) are not being adopted so fast yet. Users have a number of concerns about buying an EV, including:
- Recharge time
- Recharge availability (charging stations)
These are big challenges and opportunities. The installations of national scale charging networks will be huge projects.
Users need to at least be convinced that running an EV will be more environmentally friendly than running an efficient petrol or diesel car. To achieve this an EV really needs to offer:
- Lower CO2e/kWh than the best turbo diesel and petrol cars, even if it is charged with electricity from a gas power station.
- Good performance in winter. An EV uses more of its battery energy to heat the cabin in winter. In addition, the batteries operate less efficiently at low temperatures. Some EVs use electrical energy to heat up the batteries in winter before they can be used efficiently for driving.
- Good safety. Thermal runaway can be a real issue in Lithium Ion batteries, which is why they always need a battery management system (BMS). Furthermore, the BMS needs to keep operating at all times, even when the car isn’t being driven. If the EV is parked in a hot place, the BMS must continue to use energy to protect the batteries.
- Long life – years and km. The key concern is the battery.
- Good re-cycling at the end of life. The key concern is again the battery.
EVs with large batteries will cost more and take longer to charge but will have longer range. People are used to petrol and diesel vehicles where a low cost car does not have a low range. This is not the case with EVs. The battery is a significant proportion of the total cost, so a lower cost EV has a shorter range.
For years the automotive industry has had small evolutionary changes from one vehicle generation to another. This has all changed now. With the change to eMobility (EVs and Hybrids) there are revolutionary changes from one car generation to another. The industry is full of change, innovation and opportunity. In addition to the drive chain, there are significant developments in autonomous vehicle and driver assist functions. Artificial intelligence and machine learning have now come of age.
Opportunities and challenges in power electronics
We can see that the world is electrifying many of our energy functions. Many applications are moving from petrol/diesel/gas/hydraulic/pneumatic actuators to electric actuators, especially electric motors.
This means that products will need far more power electronics design, e.g:
- Motor drives
- Converters (AC to DC, DC to AC, DC to DC)
- Battery management
Thermal management is a key issue of power electronics. Increasing the efficiency of power electronics will reduce the heat losses. An increase from 97% to 99% will reduce heat losses by a factor of 3. In some cases this could mean that air cooling can be used instead of water cooling, which can reduce the cost and mass of a system considerably.
The power semiconductor industry is releasing many new power devices enabling designs to run at higher voltages. This reduces the current and cable size needed for a given power output. There are interesting new devices in silicon and silicon-carbide that are enabling lower total cost and mass in such new products.
These are just two examples of how innovative design is needed to achieve the quality and performance of these new power products.
This article has considered climate, then CO2, then energy, then electricity supply, then electricity demand, then electric transport, then power electronics, by numbers. These issues are huge and cannot be solved by qualitative methods alone, quantitative methods are needed.
It shows the joined-up and quantified thinking that is needed from end to end, to decarbonise our world with clean energy. This raises many new opportunities and as engineers, we are looking forward to designing the new products and systems of the future.