Australasian Science Magazine

A BEV’s size and range are determined by the weight of batteries it needs to carry. Credit: babimu/Adobe

The BEV Range Conundrum

BEVs generally have a small driving range: the Nissan Leaf’s range is approximately 150 km, depending on how you drive it. However, the top-of-the-range Tesla Model S has a claimed driving range of up to 500 km.

Among FCEVs the small Toyota Mirai and the medium-sized Hyundai ix35s both have a claimed range around 500 km on a single 5 kg tank of hydrogen.

What’s going on here? Why can’t all BEVs have a 500 km range. How does a small FCEV manage this trick? To understand what’s going on, we should first remember that power (measured in kilowatts) is the rate of production or use of energy (measured in kilowatt-hours).

The key point about the FCEV is that the energy-containing hydrogen fuel is retrieved from the fuel tank at the rate needed by the fuel cell to provide the required electric power to the traction motors. The maximum continuous power of the vehicle is set by the fuel cell and the motors. The range is set by the volume of the fuel tank, just as in an ICEV.

In other words, power production in a FCEV is decoupled from its energy storage capacity. As long as the volume of the fuel tank can be accommodated within the vehicle’s envelope, the range can be extended to match an ICEV with very little need for increased fuel-cell and motor power, because the hydrogen tank constitutes a small proportion of the vehicle’s weight.

Now let’s think about the BEV in the same way. Vehicle batteries are very heavy compared with a hydrogen tank for the same stored electrical or electric-equivalent energy. The BEV battery pack is modular, so to make a bigger battery that stores more electrical energy we just add more modules. This sounds simple and obvious, but the consequences are far-reaching for our BEV’s design.

The maximum power of the vehicle is set by the traction motors, so the battery pack should supply this power, but greater power cannot be used unless the traction motors are upgraded. That’s the point: the power rating and energy storage capacity of a modular battery pack are proportional, so adding more batteries to increase the driving range also adds more power rating, whether or not it is needed. It also adds more weight to the vehicle.

To get a big driving range requires a big, heavy battery pack, and this big, heavy battery pack can deliver awesome power. This big, heavy, awesomely powerful battery pack is also very expensive, so we end up with a big, heavy, awesomely powerful and expensive luxury vehicle as the logical outcome of wanting a 500 km driving range.

If we opt for a small battery pack with modest power output, we end up with a small, relatively cheap BEV with a restricted driving range. The driving range problem of current BEVs comes down to the fact that the power production and energy storage are naturally coupled in modular battery packs.

The top-level Tesla Model S is a BEV with a claimed range of up to 500 km.

By Evan Gray

How much can electric vehicles reduce Australia’s carbon emissions, and what are the factors limiting the transition from Australia’s fleet of conventional combustion engines?

In pursuit of the goals of the Paris Agreement on climate change, now in force, the 2°C scenario of the International Energy Agency attributes more than 300 Gt of avoided carbon dioxide emissions by 2050 to a revolution in electricity generation. Industry is supposed to contribute about 150 Gt, with transport just behind at about 140 Gt.

The challenge to meet the goals in all sectors is immense, especially in electricity generation, so is it a good idea to add to the demand for electricity by a global rollout of vehicles powered by electricity?

Battery-Electric Vehicles

When most people think of an electric vehicle, they imagine a battery-powered car that is plugged into the electricity mains to recharge the battery. In these battery-electric vehicles (BEV), an on-board battery provides electricity to run the electric power train. In its May 2016 summary of the global electric vehicle outlook, the International Energy Agency reported that the number of BEVs had passed 750,000 (http://tinyurl.com/IEA-2016-EV-outlook). More than 20 manufacturers now offer highway-capable BEVs.

Most people also know about range anxiety – that awful feeling that you will run out of fuel before reaching the next refuelling station – in relation to BEVs because most BEVs have much shorter ranges than the vehicles we are used to (typically 150–200 km). The Tesla Model S is a notable exception.

At first glance, the replacement of our current petroleum-burning vehicles by BEVs that don’t produce CO2 or other pollutants during their operation is a great idea, but only if we consider the vehicle in isolation. Following the energy back to its source, we should ask how the electricity is generated, and 67% of the world’s electricity is still generated from fossil fuels (http://tinyurl.com/IEA-2016-key-world-stats). Consequently we should ask whether replacing an internal combustion engine vehicle (ICEV) with a BEV actually reduces CO2 emissions.

A BEV doesn’t avoid the pollution from electricity production – it is released at the power station rather than on the highway. Even worse, more BEVs will increase the demand for electricity from a grid that already can’t cope, unless the demand is managed to be outside peak periods. If the electricity comes from burning coal, the reduction in CO2 emissions from taking an ICEV off the road is offset by the increase from burning extra coal. So thinking a little about the proposal to replace ICEVs with BEVs raises a number of very important issues to resolve.

This is a complex matter, but a back-of-the-envelope calculation helps to sort it out. Let’s take a small-ish BEV with a 50 kWh battery pack and a fairly generous 300 km range. If it travels 10,000 km/year, 33 complete recharges of the battery are required, corresponding to 1.65 MWh of electricity per car per year (neglecting various small inefficiencies). If we roll out a fleet of 10 million of these cars across Australia, the total electricity requirement is then 16.5 TWh/year.

Australia’s electricity consumption through the National Energy Market is about 200 TWh/year (http://tinyurl.com/Aus-electricity-consumption), so the effect of adding a very large number of small-ish BEVs is not huge. This much additional sustainably generated electricity could be provided by new generators amounting to about 2 GW (equivalent to a single very large power station) operating continuously or 10 GW operating at a typical 20% availability for solar or wind power. This is comparable to the total solar and wind generation capacity in Australia at present (~9 GW from photovoltaics and ~6 GW from wind), but is very achievable.

Next, what is the quantity of CO2 emissions avoided by taking ICEVs off the road? Let’s assume that the CO2 emissions associated with manufacturing a BEV or ICEV are about the same, and just compare the emissions arising from running the vehicles. A reasonable figure for the exhaust emission of a modern small-ish ICEV is 125 g/km, so 10 million cars driving 10,000 km/year amounts to 12.5 Mt/year of CO2 avoided by our hypothetical replacement BEV fleet.

But what if the electricity powering the BEV fleet was produced from coal instead of solar or wind power? The amount of CO2 emitted by a black-coal-fired power station is about 900 g/kWh of electricity, so the electricity demand of our hypothetical fleet of BEVs (16.5 TWh/year) would send an extra 15 Mt of CO2 per year into the atmosphere, exceeding the 12.5 Mt of CO2 saved by replacing ICEVs.

The take-home message is that the replacement of a very large number of ICEVs with BEVs is feasible in terms of the additional electricity required, but pretty much pointless if the electricity is generated by burning coal and emitting the CO2 produced into the atmosphere.

The flip side is that even a national fleet of 10 million small-ish BEVs corresponds to only a fairly small fraction of the present national electricity consumption, so much more must be done to reduce national CO2 emissions than decarbonising the national fleet.

Fuel-Cell Electric Vehicles

Another type of electric vehicle is now coming into consideration. This is the fuel-cell-powered version (FCEV) in which hydrogen fuel is stored on board and fed to a fuel cell that produces electricity to run the electric power train. The only by-product at the vehicle itself is water.

FCEVs have entered the market later than BEVs because the underlying technology is much newer, but they are now becoming commercially available. Toyota’s Mirai sedan and Hyundai’s ix35 SUV are being sold in small numbers in several countries, and are both on trial in Australia. Honda’s 2017 Clarity is also on sale outside Australia. More than 10 mainstream manufacturers have FCEVs in or near production.

This also sounds great, but the same problem as BEVs crops up: nearly all hydrogen still comes from fossil fuels (by reforming natural gas), and the associated pollution is released at the hydrogen production plant, not avoided.

Hydrogen production from electricity – which would need to be sourced from renewables – by electrolysis of water is a well-known industrial technology but, like renewable electricity production itself, is in its infancy compared with the scale needed to have a major impact on global CO2 emissions. If the electricity is generated without CO2 emissions, then a closed, sustainable cycle is formed in which hydrogen is extracted from water using electricity and used to run the FCEV, producing the same amount of water again. In this scheme, electricity and hydrogen are the carriers through which the source energy (solar, wind) is eventually transformed into motive power.

Generating the required electricity at the scale required to operate a national fleet is feasible. However, significantly more electricity generation is required than in the BEV case because the efficiencies of the electrolyser (producing hydrogen from water) and the fuel cell (producing electricity and water from hydrogen) are both about 50%; in contrast, the efficiency of the battery in a BEV is probably above 90% depending on how the vehicle is driven.

So why would we even consider hydrogen-powered vehicles, and why are major automotive manufacturers bothering to build and sell FCEVs? One answer is their range, which easily exceeds that of a BEV (see box: The BEV Range Conundrum). Another is the different demand for materials. Whether enough materials can (or should) be supplied to make the batteries necessary for a global BEV fleet is a very big question, since relatively scarce elements such as lithium are used.

In the FCEV case, pressurised hydrogen is stored in a very sophisticated tank made from common elements, principally carbon (in carbon fibre and polymers) and aluminium. The difference between this and a BEV battery is that the hydrogen is stored as itself, just compressed, so only a container is required, not a storage medium. The FCEV does have a battery to smooth the power demand on the fuel cell, but it’s much smaller than in a BEV.

Electric Trucks and Buses

The BEV–FCEV range comparison holds true for buses and trucks. Battery-powered buses and trucks, with ranges around 200 km, are excellent in urban situations. For long-haul or infrequent refuelling, commercially available hydrogen-powered buses have ranges around 500 km, and the Nikola One and Nikola Two fuel-cell semi-trailer trucks are claimed to have a range of 1300–1900 km.

The contrast between the achievable range for current batteries and fuel cells is starkly illuminated by the Nikola One and Nikola Two. Their relatively massive 320 kWh battery – about three times the capacity of a high-end Tesla Model S battery – can run the 745 kW drive motors at full power for only about 35 minutes (to 80% depth of discharge), so ~200 km of level terrain might be crossed on a single battery charge. The majority of the range comes from recharging the battery on the fly using the 300 kW hydrogen fuel cell and stored hydrogen.

Bumps in the Road

There are several big bumps in the road to electric vehicles. The lack of refuelling infrastructure is one: vehicle buyers are wary of the scarcity of refuelling/recharging stations, and investors won’t bankroll the needed infrastructure unless there are vehicles to use it. This affects FCEVs more than BEVs, since the roll-out of BEVs and charging points is further advanced. BEV charging points are simple and cheap compared with a high-pressure hydrogen refuelling station, but more are needed because of the BEV’s range problem.

Another bump is cost, but that is the case with all new technologies. Early adopters pay more, but high-volume production brings down the price dramatically – look at the example of photovoltaic panels – and this bump will vanish. In January 2017 there were 274 hydrogen refuelling stations around the world, and the number is growing at about 50% per year).

The biggest bump – of major concern in relation to lowering CO2 emissions to zero to minimise global warming, and fundamentally the reason to change to electric vehicles – is really the need for sustainably produced electricity to recharge BEVs and generate hydrogen for FCEVs. The associated lesson is that solutions to a complex problem that ignore the connected nature of the problem never work.

BEVs or FCEVs?

Which type of electric car to go for amounts to “horses for courses”. For short trips, modest performance and a daily distance travelled of 100 km or so, a small BEV recharged at home or work will be convenient for many drivers. For long trips in a modestly sized vehicle with decent performance and a quick refuel, FCEVs are the logical choice.

Either way, as long as the electricity or hydrogen is sourced from renewable resources rather than fossil fuels, we can buy electric cars in the knowledge that we really are doing something positive for the planet.