Energy in motion

Electric motor, biofuel and hydrogen economy - can they be an alternative to refined oil in driving vehicles? About the materials that will drive vehicles in the near future

By Judy and Aryeh Melamed-Katz | Galileo Magazine

The global oil market has been in turmoil over the past five years. The general trend during this period was characterized by sharp price increases, so for example the price of a barrel of crude oil was almost five times higher in July 2008 than its price five years earlier.

The main disadvantage of electric cars lies in the limitations of the battery. The maximum range of a single trip is usually less than 100 kilometers.

The reasons for this instability, which sometimes (including in recent months) also included sudden declines, are not quite clear. They may be related to concerns about the depletion of oil reserves or they are an expression of changes in demand, and there may be more complex political reasons. One way or another, the current crisis, the results of which are difficult to predict, increases the economic viability of investing in alternative energy resources - an investment that can help oil-poor countries, such as Israel, to achieve "energy independence" already during the first half of the 21st century.

As of today, the propulsion of transportation vehicles is almost entirely dependent on refined petroleum products, referring mainly to gasoline and diesel fuel for land and sea vehicles and kerosene for aircraft jet engines. In recent years, there has been a shift in relation to the use of hydrocarbon gas (natural gas) as well as in regards to the implementation of alternative energy solutions for transportation vehicles, among other things with a trend to reduce the emission of greenhouse gases into the atmosphere. It is difficult to predict the end of the process, but it is already clear that it will have an extensive impact on the global economy, due to the large share of motorized energy consumers in the global energy market.

Glossary of important terms:

• Biogas - gas produced from biomass. This usually means methane.
• Biodiesel - Biofuel intended for combustion in a diesel engine. It is usually made from vegetable oils or fats.
• Biofuel - fuel derived from biomass. For example, ethanol (wine spirit, alcohol in everyday language) and butanol (butyl alcohol), which are produced from plants in a process of fermentation and distillation, can be used as fuel for internal combustion engines of vehicles.
• Biomass - the total mass of living things. The same term is also used to describe fresh biological raw material used to produce fuel.
• Fossil fuel - fuel created by long-term geological processes. This usually refers to coal, natural gas (a mixture of methane with other gases), oil shale and crude oil. In the refining process, a variety of fuels can be produced from the oil, such as gasoline, diesel and kerosene.
• Hydrogen economy - an approach according to which hydrogen gas will replace the types of fuel commonly used today to drive vehicles. Hydrogen can be used in an internal combustion engine or as a raw material for a fuel cell.
• Electric car - a car based on propulsion using an electric motor connected to a battery (electric battery). The battery is basically a chain of electrochemical cells.
• Hybrid car - a car that combines several types of drive, or that can be used on different fuels. Usually it means a car that combines an electric motor with an internal combustion engine.
• Solar car - an electric car whose energy supply is made through photovoltaic cells that convert the sun's energy into electricity.
• Internal combustion engine - an engine based on burning fuel in a small chamber. The drive is obtained with the help of the gas pressure created in the combustion process. In cars, two types of internal combustion engines are common: gasoline engine and diesel engine.
• Fuel cell - a device similar to an electrochemical cell that produces electricity through a chemical reaction. Unlike an electrochemical cell, a fuel cell requires a constant supply of materials (the reactants). One of the types of fuel cells intended for cars is based on a reaction between hydrogen and oxygen. The only product of such a reaction is water.


The electrical solution

The desire to build a vehicle that moves on its own is ancient. The first cars, developed at the end of the 18th century, mark the beginning of a period rich in inventions that changed the face of transportation. The early version of the motorized vehicle was based on a steam engine, a method that over the years became the main means of propulsion for trains and ships.

During the 19th century, internal combustion engines also began to be used, whose modern versions, a gasoline engine and a diesel engine, took over the car market during the 20th century. However, towards the end of the 19th century, the electric car was much more common. The development of the electric car, about 170 years ago, was made possible thanks to the invention of the electric motor. Such a motor, which is connected to a battery, can turn electrical energy into mechanical energy and drive the vehicle's wheels.

As mentioned, in the long-standing battle between the electric engine and the internal combustion engine, the latter ultimately had the upper hand, at least as far as vehicles are concerned. Starting from the second decade of the twentieth century, the production of cars using an internal combustion engine became cheaper, and their use became more convenient. One of the main reasons for this was the invention of the starter, which is used to start the internal combustion engine, and is basically a small electric motor connected to a battery.

Additional small electric motors, which are also connected to the battery, are used for a number of secondary purposes such as opening and closing the windows, but during normal driving, the vehicle is driven and the battery is charged using the internal combustion engine only. Burning gasoline or diesel fuel in such an engine is one of the causes of the increase in the concentration of carbon dioxide in the atmosphere during the last decades, as well as the severe air pollution in the big cities. Electric cars are "green" in this respect.

Limits on the use of electricity

The main disadvantage of electric cars lies in the limitations of the battery. The maximum range of a single trip is usually less than 100 kilometers, although this distance is expected to increase in the coming years. Another problem is the short lifespan of the accumulators, which currently stands at less than 5 years, but in this respect too a considerable improvement is expected.

The main limitation, which prevents a more vigorous penetration of electric cars into the vehicle market, is the long charging time of the battery, which must be done in parking mode. In fact, a full charge usually takes a few hours. The long time is due to the need to lower the charging current in its final stages, when the battery is 80% full or higher, because a significant part of the charging energy turns into heat at this stage. These problems limit the electric car market, although electric drive can be an excellent solution already today for certain purposes, such as using the car for city trips only.

From a broader perspective, one should dwell on the way in which the electricity used to charge the battery is produced. If, for example, this electricity is produced by power plants that burn fossil fuels, the use of an electric car does not make much sense in terms of reducing the emission of carbon dioxide into the air (for "fossil fuels" and other terms in the article, see below). Therefore, a wide transition to electric cars should be done in an overall framework of developing alternative energy for electricity production.

In a similar context, we can also mention the solar car, which is actually an electric car whose upper surface is coated with photovoltaic cells that convert the sun's energy into electricity. Its main disadvantage lies in the low power that the cells can produce.

An area of ​​several square meters will not be able to contribute more than a few horsepower, even on a bright sunny day and with high efficiency of the cells, and this is much less than the power needed for an average modern car. A serious problem exists, of course, at night, and on cloudy days. However, it is likely that the car of the future will receive some of its propulsion through solar energy, even if this electricity will mainly be used to slowly charge the battery when the car is parked.

Dual driveThe

The old idea of ​​a hybrid car, based on a combination of a large electric motor and an internal combustion engine, has recently been revived, due to the desire to reduce the vehicles' fuel consumption. Such a car does not suffer from the disadvantages of the electric car because the battery is charged while driving. Ferdinand Porsche designed and produced the first successful model of a hybrid car as early as 1902. He used a serial drive system in which an internal combustion engine rotates a generator used to charge the battery. The battery itself supplies electricity to an electric motor that is connected to the wheels.

Today, manufacturers prefer a different design - a parallel drive system. In such a design, the two engines are connected to the wheels. When driving slowly, or when stopped, the car's computer usually only activates the electric motor. The internal combustion engine is started by the electric motor at speeds where the electric motor has difficulty providing the full energy requirements of the car.
When burning ethanol, carbon dioxide is emitted, and when ethanol and gasoline are used to drive vehicles, they are also responsible for emitting greenhouse gases into the atmosphere

From that moment, the internal combustion engine is also used to charge the battery through a generator (alternator) that converts mechanical energy into electricity. A combination of electric drive with an internal combustion engine currently places hybrid cars at the top of the list of economical cars in terms of fuel consumption.

A typical structure of a parallel drive system in a hybrid vehicle. Both engines can drive the vehicle's wheels together or separately. Charging the battery (marked in green) is done while driving using a generator connected to the drive system. During braking, the electric motor works as a generator and provides additional current to charge the battery.

Charging while braking

The battery in the hybrid car, as well as the electric car, provides direct voltage that can directly run a DC (direct current) motor. However, today there is a tendency to prefer using an AC (alternating current) motor that is connected to a DC to AC (direct voltage to alternating voltage) converter. The voltage required for AC motors is usually higher (around 300 volts), and the motors are more expensive, but these motors have an important advantage - they can be used to charge the battery while braking the vehicle.

This method is called regenerative braking: pressing the brake pedal cuts off the flow of electric current from the battery to the electric motor, and the motor starts working in the opposite direction, as a generator, that is, the wheels drive its rotor and it produces electricity that charges the battery. According to Lenz's law, which is an expression of the law of conservation of energy, the current that develops in the generator creates a magnetic field in the opposite direction to the magnetic field that created it, thus slowing down the rotor, i.e. braking the vehicle.

In addition to the regenerative braking system, normal brakes are also installed in these vehicles, in order to help bring the vehicle to a complete stop, but their operation is limited and they wear less than typical brakes in a normal vehicle

drive on alcohol?

The term hybrid car usually refers to a car that combines an electric drive with an internal combustion engine, but in a general sense any car in which two types of drive are used in combination can be considered a hybrid car. Such is, for example, a car that is powered by a mixture of gasoline and ethanol.

These cars have a "flexible" propulsion system (which is why they are flexible-fuel machines or for short flex-fuel) that allows them to detect the concentration of ethanol in the fuel mixture (fuel that contains ethanol is marked with the letter E. For example, E85 contains 15% gasoline and 85% ethanol mixed with a few other substances such as methanol), and adjust the fuel injection mechanism and spark ignition timing for optimal combustion.

Another requirement for such a car is the need for the parts of the fuel system to be resistant to ethanol, which is considered a more corrosive substance than gasoline. Henry Ford was one of the first to realize the idea of ​​consuming ethanol in an internal combustion engine. The evaporator (carburetor) of the "Ford Model T", which was produced between 1908 and 1927, was adjustable and could work with different fuel mixtures.

Ethanol - advantages and disadvantages

Due to the oil crisis that occurred due to the Arab oil boycott, a global revival began in the 70s regarding the production of ethanol for driving cars. Brazil was a pioneer in allocating large areas to crops intended for ethanol production. Today, Brazil, which produces ethanol from sugar cane, and the United States, which produces ethanol from corn, provide about 90% of the world's consumption of ethanol as a fuel.

Brazil is also a pioneer when it comes to the transition to a car powered by ethanol only (E100), and the major car manufacturers have begun to produce models that can run on clean ethanol - models that are currently intended mainly for Brazil and Argentina, which share a similar policy. Brazil took another step towards the introduction of biofuel, a fuel of biological origin, in the vehicle market, and today it is no longer possible to find fuel that contains less than 26% ethanol. The popularity of ethanol in Brazil is partly due to its low price, and this without a government subsidy.

Today there is a tendency to try to produce biofuel from plants that are not consumed as food or to use plant parts that are not used by the food industry.

The question of extensive land allocation for biofuel production has made headlines over the past year due to the steep increase in food prices in the world. This issue, which has been called "food vs. fuel", raises the question of whether it would be better to use these areas for crops intended to be used as food. This is a global question that has implications when it comes to the issue of fighting hunger in particularly poor countries that hardly consume fuel, unsurprisingly.

Even from an ecological point of view, ethanol is not an optimal solution. During the combustion of ethanol, as in the combustion of other organic compounds, carbon dioxide is emitted, and when they are used to drive vehicles, ethanol and gasoline are responsible for a similar amount of greenhouse gases that are emitted into the atmosphere for each kilometer traveled. The main argument of the supporters of ethanol focuses on the way in which it is produced - after all, the crops from which the ethanol is produced consume carbon dioxide in the process of photosynthesis, thus obtaining a cycle of the gas.

Fuel from the field

Biomass is a general name for fresh biological raw material used to produce biofuel, for example ethanol. Ethanol is produced through fermentation followed by distillation. In such a process it is possible to reach a mixture containing 96% ethanol and 4% water (weight ratios), because in such ratios the mixture boils together and its components cannot be separated by normal distillation (azeotropic solution). You can get clean ethanol by adding benzene during distillation. Such ethanol must not be used for food because the benzene, whose remains may remain in the ethanol, is a carcinogen.

The alcoholic fermentation of the raw material, carried out by yeast, can only occur under non-aerial conditions. In the first step, the yeast turns the sugars from the raw material into pyruvic acid, and in the process they gain available chemical energy. Then, they break down the pyruvic acid into carbon dioxide and acetaldehyde, and it reverts to ethanol.

The process is well known in the food industry: the carbon dioxide produced by the yeast in alcoholic fermentation is what makes the dough rise, while the ethanol, which evaporates during the baking of the bread, is actually a desirable product in the fermentation of grape juice to wine.

Collecting methane to create biogas

Biogas is another example of biofuel. It is mainly methane gas created from organic matter in several stages. The final stage of methane production, called methanogenesis, is carried out exclusively by archaeons ("primitive bacteria"), under non-aerobic conditions, i.e. in the absence of oxygen. They consume simple molecules, such as molecular hydrogen (H2), carbon dioxide and acetic acid (acetic acid), and turn them into methane (CH4).

The initial raw material for those simple molecules can be extremely diverse, for example polysaccharides, proteins and fats, so it is not surprising that methane is spontaneously formed in municipal landfills. In the first stage, various microorganisms secrete enzymes that break down the long chains of the organic molecules into their basic units - glucose, amino acids and fatty acids - which themselves become smaller molecules with the help of those microorganisms.

The methane generated in landfills can be collected relatively easily and this is currently implemented in Israel. This way you can get biogas at a low price, and at the same time prevent methane, which is one of the greenhouse gases, from being released into the atmosphere.

Ethanol production from cellulose

Alternatively, methane and carbon dioxide can be produced proactively in a municipal facility designed to separate types of garbage and treat garbage. Similar to natural gas, which consists mostly of methane, biogas has many uses. Although its burning is accompanied by the emission of carbon dioxide, using methane in power plants is considered much more environmentally friendly than coal, for example. The amount of carbon dioxide emitted in the combustion of methane is low compared to any other organic material, and likewise no polluting particles are emitted in the process and there are no dangerous by-products.

Studies dealing with biofuel production have become popular in recent years. Today there is a tendency to try to produce biofuel from plants that are not consumed as food or to use plant parts that are not used by the food industry. Biofuel of this type, which does not pose a problem in everything related to the "food for fuel" issue, is called "second generation biofuel".

An example of such a development is the idea of ​​producing ethanol from cellulose (cellulose), which is not digestible by humans. Cellulose is a polysaccharide that is built as a long chain of glucose molecules. One of the difficulties in preparing cellulose for the fermentation process stems from the need to separate the cellulose from non-sugar components of the tree, such as lignin.

Another difficulty lies in the need to break down the cellulose into glucose molecules. The decomposition can be done with an acid or with an enzyme called cellulase. In nature, enzymes belonging to the cellulase group are secreted by fungi and microorganisms, such as for example microorganisms living in the digestive system of green leaves, thanks to which these animals can digest cellulose. Due to the support of the American government in the production of cellulosic ethanol, the idea has gained considerable progress in the last two years, especially with regard to the production of improved enzymes and the establishment of plants for the production of cellulosic ethanol.

The challenge: a diesel engine

The diesel engine presents a different kind of challenge to the biofuel industry. This engine requires a relatively heavy fuel that ignites quickly when injected into the engine where there is high pressure air. Today, it is customary to fuel vehicles equipped with such an engine with diesel fuel, one of the products of oil refining, but there are other options.
Hydrogen was not as successful as gasoline among land vehicles, but in the space race liquid hydrogen played an important role as fuel for the rocket engines of the launchers

Amazingly, Rudolph Diesel (Diesel) himself believed that vegetable oil was the preferred fuel for the engine he invented. Using vegetable oil, fresh or used, is indeed possible for this purpose, but its high viscosity may prevent the liquid from separating into tiny droplets when injected into the engine, thus damaging the injection mechanism and causing incomplete combustion of the fuel.

This problem has a fairly simple solution - a chemical process called transesterification. In this process, alcohol (ethanol or methanol) is added to oil or fat. The triglycerides react with the alcohol in the presence of a base that acts as a catalyst. In the process, glycerol and mono-ester are formed. The ester created in the process, which has a relatively low viscosity, is suitable for use as biodiesel, i.e. biofuel designed for a diesel engine.

Algae or not to be

One of the interesting directions in the field of biodiesel is the use of algae as a raw material. In 1978, the American government announced a research program designed to promote this direction in many ways - starting with finding algae strains that contain a large amount of fatty acids and ending with improving the methods of extracting oil from algae. For example, it was discovered that it is possible to increase the production of fatty acids produced by algae not only through appropriate nutrition, but also through genetic methods.

The researchers found that overexpression of the gene that codes for the production of the enzyme acetyl-CoA carboxylase, an enzyme that is also of interest in the study of obesity in humans, can significantly increase the percentage of fatty acids in the algae cells. The program was stopped in 1996 due to the assessment that growing algae for biodiesel production would only pay off if the price of oil rose significantly, which indeed happened ten years later.

Algae have some significant advantages compared to other crops for the production of biofuel - they grow quickly and hardly consume any space. You can grow them in a bag or a transparent container and thus increase the amount of light that reaches them. The tendency is to establish algae farms near factories that emit large amounts of carbon dioxide, and to flow the gas, necessary for the photosynthesis process, directly to the farm. This increases the growth rate of the algae and at the same time enables the recycling of the greenhouse gas.

An algae farm can be used not only for biodiesel production. The sugars remaining after the extraction of the oil can be fermented and produce ethanol, and the proteins can be used in the food industry. The main difficulty in growing algae is their sensitivity to infections and environmental conditions, so the growth should be done under controlled conditions.

Hydrogen economy

The only fuel that does not emit carbon dioxide when burned is hydrogen. When burning hydrogen, the hydrogen combines with the oxygen and the only product is water. The transition to the use of hydrogen gas as the main fuel source for vehicles will require the preparation of extensive infrastructures, which will include not only the development of vehicles with appropriate propulsion systems, the development of a system for transporting large quantities of hydrogen and the development of a convenient refueling system, but also a system for producing hydrogen - a gas that is found in minute amounts in the atmosphere. The approach that supports replacing the fuels used to drive vehicles with hydrogen gas is called a hydrogen economy.

The energy density of hydrogen, meaning the amount of energy that can be produced from each kilogram of the substance, is the highest among all existing combustion materials, and it is no wonder that it has been used in internal combustion engines since their invention in the early 19th century. Admittedly, hydrogen was not as successful as gasoline among land vehicles, but in the space race liquid hydrogen played an important role as fuel for the rocket engines of the launchers.

For efficient use in cars, the gas must be compressed up to 700 atmospheres. It is not an easy task that requires a lot of energy investment, but once the gas is compressed it will be relatively easy to transport it to the gas station and refuel the cars using a sealed fueling pipe.

It should be noted that as long as the hydrogen does not come into contact with oxygen, its use is less dangerous than the use of gasoline, due to its high autoignition temperature. above 500 degrees Celsius). On the other hand, leaking hydrogen that comes into contact with oxygen may ignite at a low temperature.

A multi-fuel car

Already today it is possible to switch to using hydrogen as a fuel in gasoline engines, and the consumer can choose between buying a new model or adapting an old vehicle to drive with hydrogen. The essential changes are the replacement of the fuel injection mechanism and the adjustment of the materials from which the combustion chamber is built, but most of the cost of the adjustment stems precisely from the need to replace the fuel tank with one made of carbon fiber, a light and strong tank that will hold the compressed hydrogen gas.

The main limitation today stems from the need for a huge fuel tank - a typical tank of fuel for a family car will be enough to travel less than a hundred kilometers, even if liquid hydrogen is used.
An option that is suitable for the near term is a "multi-fuel car", which will have several tanks designed for different types of fuels, such as gasoline, ethanol, natural gas and hydrogen gas

New materials for fuel tanks are under development. These materials will be able to increase the amount of stored hydrogen through the combination of hydrogen with the material itself. An example of such a substance is a compound of sodium and aluminum which in the presence of hydrogen can become a hydride. The challenge is to reduce the temperature at which the hydrogen is released from the material, which currently stands at around 300 degrees Celsius.

An option that is more suitable for the near term is a "multi-fuel car", which will have several tanks designed for different types of fuels, such as gasoline, ethanol, natural gas and hydrogen gas. The car's computer will automatically adjust the engine's injection and ignition systems according to the type of fuel selected by the driver.

Fuel cells to drive an electric motor

In the longer term, the hope of the hydrogen economy rests on another propulsion method - an electric vehicle that receives its electricity from fuel cells. A fuel cell is a device similar to an electrochemical cell, except that unlike an electrochemical cell, a fuel cell needs a constant supply of reactants. On the other hand, a fuel cell does not need charging like a normal car battery. The reactants in a hydrogen fuel cell are hydrogen and oxygen, so it needs a constant supply of molecular hydrogen and air. The hydrogen is flowed at high pressure towards the anode, where it undergoes an oxidation reaction in the presence of a catalyst, such as platinum (see figure).

As a result of the oxidation reaction, each hydrogen molecule (H2) is separated into two protons and two electrons. The electrons flow through an external electric circuit to the consumer of the current, the electric motor, while the protons cross the electrolyte located in the center of the cell. At the cathode located on the other side of the electrolyte, which prevents the passage of electrons through it, a redox reaction of oxygen (O2) occurs in the presence of a catalyst. The electrons, which flowed in the electric circuit, are absorbed by the oxygen molecules, which originate from the air flowed to the cathode, and then the negative oxygen ions react with the positive hydrogen ions (protons) and water is formed.

A hydrogen fuel cell produces an electrical voltage of 0.7 volts, so a large collection of fuel cells is needed to drive an electric motor. The production cost of such an electric vehicle is high, due to the price of the fuel cells, and it must be assumed that such cars will not be produced in large quantities until progress is made in the development of fuel cells. A hydrogen fuel cell uses the energy that is released when hydrogen combines with oxygen. The reverse process, in which hydrogen gas is produced from water, is a process that consumes a lot of energy.

Methods for producing hydrogen

Among the methods for hydrogen production, the most interesting are those that can be performed using solar energy and without the use of fossil fuels. The simplest process, called thermolysis, is based on heating water to a temperature of about 2,500 degrees Celsius, which can be achieved by concentrating sunlight. At such a temperature the water molecules split into molecular hydrogen and molecular oxygen.

The tools used for the process that requires such high temperatures are made of ceramic materials, such as zirconium (Zr) compounds.

Another method, which requires a lower temperature, is called a thermochemical cycle. In the first step, a metallic oxide, for example zinc oxide, is separated into metal and oxygen. This step is carried out at a high temperature of about one thousand to two thousand degrees Celsius (depending on the type of oxide). In the second stage, which occurs at a lower temperature, the metal is allowed to react with water; In this reaction the original oxide is formed and hydrogen is released.

The hydrogen economy is an interesting option for the State of Israel. There is indeed a risk in such an investment, because the hydrogen market depends on the major car manufacturers and the development of the fuel cell sector, as well as on the pressure exerted by the major oil suppliers. However, if there is indeed a global shift towards a hydrogen economy, Israel has a chance to become a major player in this market by investing in hydrogen production plants using the most available resource in our country - solar energy.

* Jodi Melamed-Katz is a chemist, currently completing a doctorate in the Department of Plant Protection at the Hebrew University.

* Aryeh Melamed-Katz is an electronics engineer and a doctor of physics, a lecturer on science topics and engages in scientific consulting and writing.