The plug in hybrid and the all-electric car

The hybrid car

It does not sound particularly logical that in order to increase efficiency we equip a car with two engines: an ordinary internal combustion engine and electric motor. Yet this is indeed the case with a hybrid car. It derives its energy exclusively from petrol or diesel, so all it does is use energy more efficiently than an ordinary car. How does it achieve this?

The secret is that its electric motor – in combination with a relatively large battery – makes driving the city, with its frequent stop-and-go cycles, much more efficient than an ordinary car. At traffic lights, the motor simply turns off, it does not idle the engine of an ordinary car. Furthermore, it recovers its braking energy by converting it into electricity that recharges the battery. That energy is lost in the form of heat in the brakes in the ordinary car. So, all of sudden, driving in the city has become much more efficient. This may be not surprising if we recall that the air resistance (or drag) is small at low speeds. All that is left to conquer is the rolling distance. Thus, the total resistance is small, allowing for greater driving efficiency.

On the highway, this advantage disappears. Moreover, the battery only has a relatively short range and the car must revert to running on petrol or diesel, just like a ‘normal’ car. This means that, for long-distance travel, a ‘normal car with the same amount of streamlining is just efficient.

Since the electric motor may be used in conjunction with normal engine for a short time, to go up a hill or to overtake another car, it does not hurt to have a smaller internal combustion engine than you would normally find in such a car. This can further enhance engine efficiency, since the efficiency increases when the engine ‘has to work harder’.

In conclusion, the advantage of a hybrid car is mainly evident in the city. So, it may be a good idea for taxes to switch to hybrid cars. Not only would that driving more economical, it would also reduce air pollution in the city.

The plug in hybrid and the all-electric car

The hybrid car discussed above still gets all its energy from fossil fuel. We can go a step further and really drive electrically. For long-distance travel this is not yet a practical option, since current battery performances keep up us from having enough energy on board.

An intermediate option is the ‘plug-in hybrid’. This enables us to drive on electricity drawn, for example, from the power outlet in our homes. If the batteries are running low, we can still cover a reasonable distance by switching to petrol. The charging can be done at selected times when electricity consumption is low (and cheap), for example at night. Alternatively, if solar electricity is used, the batteries can be charged in the middle of the day during periods of maximum sunshine.

The all-electric car is currently an option for short-distance travel only. Its range (typically 150 km) may be sufficient for most commuter traffic. If and when battery development improves, cars may drive electrically for distances comparable to those commonly covered by current combustion-engine cars.

Obviously, the question arises whether the electric car is more energy-efficient than the traditional car running of fossil fuels. To compare the two, we must compare the efficiency of an internal combustion car engine (20% on average) with electricity generation. If we use a power plant fuelled by oil or natural gas, a net efficiency of 35% is a reasonable figure. So, if losses in the battery-charging process and in the electric motor are small, the electric car would prevail. The advantage is even larger when we consider the lighter weight of the electric motor compared to the internal combustion engine, plus the fact that an electric car is more efficient in city traffic with its many stop-and-go cycles, since it consumes zero energy when it stands still. However, energy losses in the charge/discharge cycles can largely offset this advantage.

Electric cars may also be powered by electricity from fuel cells.

Hydrogen may become useful as fuel for internal combustion engines, like we currently use petrol. But a better decision is to use it in the form of a fuel cell. Fuel cell works like electrolysis in reverse. It directly converts H2 and O2 into electricity. Since electric motor is more efficient than an internal combustion engine, this seems to have a future because we avoid the inefficient step of converting heat to mechanical energy. An efficiency of 60% in fuel cell operation has thus far been achieved, which is twice as efficient as the internal combustion engine.

Hydrogen is a clean fuel, which is especially useful for mobile applications like cars in the city. But it needs to be transported to where it’s needed, and must be conveniently stored. Transporting H2 between two fixed points is easy, for example H2 gas can be transported through pipelines. However storing H2 for mobile applications remains a challenge. This is because at ambient temperature, H2 is a gas, as opposed to petrol or diesel, which are liquids. This means that we either have to compress the gas or turn it into a liquid, if we want to store a large amount in a small volume.

There are currently three ways to store hydrogen in a car in a relatively compact volume.

1. As a gas at high pressureThisrequires strong (and thus heavy) containers. For city buses, for example, this is not really a problem. They have mere space, weight is less crucial and they return to the garage frequently.

2. As a liquidHydrogen only turns into a liquid at -253 C. This makes handling, loading and transport cumbersome. Moreover, the tank must be superbly insulated to prevent rapid evaporation. Even super-insulation means a ‘boil-off’ of 1-2% per day. We may compare this to a bottle of water in a 300 C environment. Even with excellent insulation (like thermos flask) it will inevitably boil down and eventually empty, like a pan of boiling water on a tiny gas burner.

3. Sorbed into solidSome metals are capable of sorbing large amounts of hydrogen, even to extent that the amount of hydrogen per unit volume is larger than in the liquid phase, which is very good. The disadvantage is, however, that the ‘tank’ is very heavy compared to the weight of the hydrogen inside. This may seem obvious because, after all, hydrogen is the lightest of the elements and metals tend to be heavy. An example of metal-hydrogen compound (‘metal hydride’) is LaNi5H6, where hydrogen is stored in an alloy of lanthanum and nickel, both heavy metals. If we look up the masses of La and Ni, we find that the mass ratio of hydrogen to metal is 6 to 432. But the good news is that 6kg of hydrogen enables us to travel about 600km.

Using metal hydrides requires a temperature-control system that controls sorption and desorption, since this process is very temperature dependent. It is comparable to a fluid’s evaporation and condensation process.

Given these complications, we can predict that hydrogen vehicles will, at least in the short term, fill their tanks with hydrogen gas at high pressure (350 to 700 bar). The 700 bar is already being used in Norway, on HyNor. So far, carrying liquid hydrogen on board is only practical in exceptional cases, like racing cars. This is still too complicated for ordinary cars. Storage of H2 as hydride in metals on a large scale will, alas, require more research and development.

Hydrogen on the ‘Hyway’

Obviously, re-filling with hydrogen along the highway requires a special infrastructure: ‘gas stations’ that deliver hydrogen, with highways that provide hydrogen refueling called ‘highways’ for obvious reasons. California began a hydrogen highway network in 2005, primarily to reduce air pollution, aiming to have 50 to 100 operable refueling stations along the West Coast by 2010. There were only about 30 such filling stations in 2010, with several more under construction. Future development depends on how many hydrogen cars are sold, and vice versa, which implies slow growth. Europe has also several of these projects. For example, Germany, Sweden and Denmark have plans for a system of interconnected hyways. Norway, meanwhile, is building a n580-km hyway between Oslo and Stavanger (HyNor, or Hydrogen road of Norway).

There is only one drawback: leaked hydrogen enters the atmosphere, where it may affect the ozone layer. The severity of this has yet to be determined, but there is reason enough (also in view of security) to insist on leak-proof refueling stations.

Date: 2016-04-22; view: 856