Are hydrogen cars the panacea we need to get off fossil fuels? Perhaps not. 

Arguably the next 'big' thing in sustainable transportation, hydrogen vehicles have been getting a lot of press attention lately. But, what exactly are they, and how do they work? 

Are they a potential silver bullet for solving rising global temperatures? Or are they a potential environmental disaster waiting to happen?

How do hydrogen cars work?

Hydrogen cars, or hydrogen fuel cell electrified vehicles (FCEVs), are basically a variant form of an electric vehicle. Other types of hydrogen engines are called hydrogen internal combustion engine vehicles (HICEV), but here we'll focus on the former. 

Hydrogen fuel cells are set to change man industries including mining. Source: Anglo American

Unlike EVs, instead of using batteries to provide the power needed for the vehicle, special fuel cells consisting of a mixture of hydrogen and oxygen are used. 

The hydrogen is stored onboard the vehicle using special tanks, while the oxygen can be sourced from the atmosphere and fed into the fuel cell, much like aerated combustion engines. 

These two elements react inside the fuel cell to provide the needed 'juice' to run the vehicle. However, unlike battery-powered electric vehicles, hydrogen fuel cell ones do produce water as a byproduct that needs to be emitted from the vehicle.

While this might sound like a relatively modern invention, hydrogen-powered engines are nothing new. More than 200 years ago, one French inventor, Francois Isaac de Rivaz, managed to develop a primitive hydrogen-powered engine powered by hydrogen and oxygen and ignited by an electric spark.

Called the De Rivaz engine, it was, in fact, a form of early combustion engine that used hydrogen as a fuel source. The engine was later refined into a working model that could actually provide propulsion for a simple vehicle called the Charette of de Rivaz. 

Modern hydrogen fuel cell vehicles, on the other hand, provide power through a chemical reaction rather than combustion – propulsion systems built around the fuel cell lack any moving parts, much like other electric vehicles. 

Renault is another company working on hydrogen cars. Source: Renault

When in operation, the hydrogen fuel is obviously exhausted over time and so needs to be topped up from time to time. Since the hydrogen tends to be stored in tanks, it can be refuelled in a fashion remarkably similar to petrol, LPG, or diesel engines and takes about the same amount of time. 

If all these sounds of interest, you'll be pleased to know that hydrogen fuel cell vehicles are currently commercially available, like the Hyundai NEXO, but more on that later. 

The hydrogen fuel cell cars on sale today are highly credible and – in many ways – remarkable for how ordinary they are to look at, be in and drive.

However, it should be noted, that most manufacturers of hydrogen-powered cars admit that they are currently only really viable for a small niche of customers. This is for various reasons, but chief among them is whether tax incentives are on offer and whether that is the necessary refuelling infrastructure available.

As you can imagine, the technology is rapidly developing, with more and more companies turning their research and development budgets towards making them cheaper and more efficient. Much like the initial issues with electric vehicles, developing a network of refuelling stations is a primary objective for private enterprises and governments alike. 

Some experts even go as far as to predict that battery-electric cars will only provide a short-term solution to the needs of green transport and that, in fact, hydrogen fuel cell cars will deliver the longer-term solution. 

There are even plans to make hydrogen-powered flying cars. Source: Maca Flight

Hydrogen fuel cells, not just those currently in development to propel cars, are seen mainly as a potential solution for many of our power needs, from replacements for conventional gas boiler heating systems to potentially providing thrust for aircraft of the future. 

Are hydrogen cars better than electric?

Since the only real difference between the two is the source of the electricity to power the vehicle's systems, you'll not be surprised to hear that choosing either is very much a compromise or trade-off. 

We've touched on the main benefits of this type of vehicle above, but it is probably constructive to look at the relative downsides. 

As we've touched on above, the main issue with hydrogen fuel cell cars is the lack of infrastructure for refuelling. This is a severe handicap; if you cannot top up the hydrogen tanks on a long journey, you'll soon be stranded.

This is very much less of an issue for electric vehicles, which have seen an explosion in charging point infrastructure around the world. Thankfully, many experts in the field are confident that hydrogen refuelling points can be readily scaled, so it is only a matter of having enough customers to make it commercially viable to do so. 

Another issue with hydrogen cars at present is their relative cost. Since they are very much a new kid on the block, the price of currently available units is relatively high. This was also the case for early electrical vehicles, so the cost should be expected to fall significantly for consumers with enough time and development. 

BMW Hydrogen 7. Source: More Cars/Wikimedia Commons

Since refuelling stations are also relatively uncommon worldwide, the cost per top-up is also currently significantly higher than electric vehicles. To give you some idea, hydrogen fuel is roughly four times the cost of recharging an electric car in the United States. However, this looks set to change too, and the cost of hydrogen fuel cells has already dropped by more than 80% in recent years.

For example, in the United Kingdom, the latest costs we could find are between £10 and £15 a kg. So, to fill a Hyundai NEXO's 6.33 kg tank (with a range of about 414 miles or about 670km) should cost between £63 and £95. Some companies claim they can bring this cost down significantly, especially by using renewable power sources to isolate and refine the hydrogen. 

In electric cars, on the other hand, the relative charge-to-range is less favourable when compared with hydrogen cars. While this does, of course, vary depending on the make and model of the EV, generally speaking, you'll need to stop and top up your EVs battery more often than refuelling a hydrogen car of comparable power and size. 

The prevalence of high-power charging stations that allow for faster recharging is improving, but they can also be more expensive. To give you an example, in Germany (where charging stations bill per kilowatt-hour), charging using high-power stations tends to be €10 more expensive per kilowatt-hour than more 'conventional' charging stations. 

The actual cost of recharging can also vary between countries too. In France, costs are about €2.10 to travel 100km (based on an average of 15 kWh per 100km), while the same distance would cost €3 in the UK.

These issues, as well as range anxiety, can be mitigated by using a hybrid electric/combustion engine system, where the traditional engine system can act as a backup or support for the battery.

What are some key differences between EVs and hydrogen cars? 

It's all very interesting, but how do they stack up on some key issues?

Range and refuelling infrastructure differences aside, electrical and hydrogen fuel cell vehicles have some other significant differences. 

One of the key ones is safety. Since hydrogen is a highly flammable fuel (think the Hindenberg airship), the idea of driving around with a tank full of the stuff had better be safe.  

Thankfully, modern technological advances in the field have resolved many potential hazards of the technology. For example, Toyota and its 'Mirai' uses a patented design to prevent issues should the car be involved in an accident.

These innovations include a special valve that shuts off hydrogen fuel and vents the gas into the atmosphere in case of a leak. Since hydrogen is much lighter than air, it can quickly diffuse rapidly. So long as the vehicle is in open space (and not somewhere enclosed like an underground parking lot).

Obviously, EVs don't have this kind of problem, but they are not immune from their own inherent safety concerns. This is primarily due to their lithium-ion batteries, which can, and do, overheat or overcharge. 

The Toyota Mirai. Source: Toyota

This can cause fires to start, igniting the batteries that can burn very hot. Should this occur, the fires are also challenging to put out as the fuel for the fire is not vented away as with hydrogen. While many EV manufacturers have been working hard to find a reliable failsafe for this sort of thing, it will also be a potential issue with lithium-ion or other chemical batteries. 

Another comparison to make is their relative CO₂ emissions. While neither technology releases carbon dioxide during operation, each vehicle (and all of its components) needs to be manufactured from materials including metal and plastics. Large parts of the supply chains for those materials will use fossil fuels at some point. 

Lithium-ion batteries, for example, are incredibly energy-intensive to manufacture but also require the extraction, processing, etc of some extremely toxic materials. But, that is not the end of the story.

Those batteries also need to be charged regularly. This is provided by coal and gas power stations attached to the grid in many cases. Although, as more power is generated from alternative fuels, this will improve.

Hydrogen fuel cells are not that much better either. There are also energy costs to producing the hydrogen fuel too. 

Of course, the environmental impact of the supply chains of either technology can be greatly improved by integrating renewable energy sources (though even that is not without its environmental cost).  

Finally, and the most important for potential consumers, is the cost of the technology. EVs can be very expensive to buy, especially higher-end models like some Teslas, although the price for many EVs is now comparable to that of new ICEs. There may also be other running costs for battery maintenance or rental and, of course, the cost of charging them. Although, running costs for EVs are almost always much lower than for ICEs. 

Hydrogen vehicles, however, tend to be much more expensive than electric vehicles. Also, unlike EVs, there are no 'budget' options on the market. To give you some idea, the price tag for a new hydrogen vehicle is comparative to that of a top-end electric vehicle. But, prices are becoming more reasonable. The Toyota Miria, for example, starts at a little less than $50,000 (about €47,000). 

Generic anatomy of a hydrogen fuel cell car. Source: US Department of Energy

In addition, the cost of refuelling can be a lot more than a full EV charge too. 

What are the environmental impacts of hydrogen cars?

We've already touched on some of the major potential environmental impacts of hydrogen cars, but one of the most important is rarely, if ever discussed.

This is that their 'only' emission is water vapour. 

But, how can water be considered a potential environmental pollutant? Without it, after all, life would not be possible on this planet in the first place? 

Well, as it turns out, liquid water is a fantastic and necessary asset for any life-bearing planet, but, in its gaseous form, it can be a far more potent greenhouse gas than carbon dioxide. 

Since most discussions on 'green' technologies tend to revolve around their potential to reduce human greenhouse emissions, undoubtedly, the potential addition of a large amount of a more potent greenhouse into the atmosphere should probably be thought about first? 

Let's explain why. 

Is water vapour worse than CO₂ for global warming?

As it turns out, water vapour is not just very potent as a greenhouse gas but is also the world's most significant greenhouse gas. It is responsible for about half of the planet's greenhouse effect and is partly why Earth can retain heat and support life on this planet as we know it. 

It is, in essence, critical. 

Without any greenhouse gases at all, Earth's surface temperature would be somewhere in the order of 59 degrees Fahrenheit (33 degrees Celsius) colder. That would turn most of Earth's surface into a frozen wasteland in a brief period of time. 

Source: EPA

Regarding the source of water in the atmosphere, somewhere in the region of 90% comes from the evaporation of water from water bodies. The bulk of the remaining water vapour is released by mechanisms such as plant transpiration, etc. 

Water vapour is crucial to life on the planet. Its presence in the air is usually transitory, as it will inevitably return to the surface in the form of precipitation as part of the all-important water cycle. 

Since it is a greenhouse gas, there is a positive relationship between the amount of water vapour content in the atmosphere and the average global temperature. 

"Data from satellites, weather balloons, and ground measurements confirm the amount of atmospheric water vapour is increasing as the climate warms," explains Nasa. With global temperatures rising roughly 2 degrees Fahrenheit (1.1 degrees Celsius) since the late-1800s, water vapour has also increased by about 1-2% every decade.

But, correlation does not necessarily mean causation. 

As best we can ascertain, it appears that water vapour increase in the atmosphere tracks or follows rising temperatures, not the inverse. If true, that would mean that warmer air has a greater capacity to 'hold' more water vapour.

Like carbon dioxide, rising levels of water vapour trigger positive feedback, further raising temperatures and enabling more water vapour to be released. This is primarily due to the higher temperatures increasing the evaporation rate of water from water bodies worldwide. But, the higher average temperatures of the atmosphere, specifically at higher altitudes, reduce the likelihood of water precipitating out and falling back to the surface.

This sounds rather worrying, but remember that water vapour is very different from other more commonly cited greenhouse gases. This is because it is of a type referred to as condensable. In other words, unlike non-condensable (carbon dioxide, methane, etc), it can readily change state from gas to liquid and back again. 

In other words, to use an analogy, water vapour is like an amplifier to a sound system. It cannot increase the volume (temperature for the Earth's climate) on its own. That requires volume control, which in this analogy would be rising carbon dioxide, methane, etc. levels. 

So, you may ask, does it make sense to artificially add more water vapour to the atmosphere via hydrogen cars? Wouldn't we be simply exchanging one problem (CO₂) with another, potentially worse one (H₂0)? 

The answer to this is a complex one, but needless to say, that would be a case of putting the horse before the cart. You cannot 'pump' water vapour into the atmosphere unless temperatures also rise. 

The water in the air is in equilibrium until more energy is available to enable it to 'carry' more water. This is called its saturation point. 

To give you some idea of the 'saturation point' for water vapour, an air mass with a temperature of 30 degrees Fahrenheit (-1.11 degrees Celsius) has a water vapour capacity of 3.368 grams of water per kg of air (k/kg). Suppose you raise the temperature to 60 degrees Fahrenheit (15.56 degrees Celsius). In that case, the body of air has a capacity of 10.699 g/kg, and at 90 degrees Fahrenheit (32 degrees Celsius), the capacity is 30.052 g/kg.

So, unless hydrogen cars also release excess heat when in operation, it is unlikely that the water vapour they exhaust would be a potential environmental disaster waiting to happen. 

Hydrogen fuel cells and hydrogen-fuelled combustion engines are rapidly developing and are already making waves in various industries. As they become more efficient and acquire more interest, it is probably a safe bet that they will become as common as electric vehicles in the not too distant future.

There are quite a few technical obstacles to overcome (like the cost of making the fuel and providing refuelling infrastructure). Still, so long as there is a demand for this technology, these issues will quickly be ironed out. 

However, a transition towards this technology will require careful planning to reduce non-condensable greenhouse gas emissions before mass roll-out. Otherwise, we'll exacerbate the problem they are developed to solve.