posted on 28th Nov 2017 14:02
The discovery of ways of using fossil fuels has resulted in significant advances for humanity, while fossil fuels made available to mankind far more energy than the human body, through its muscles, is able to produce.
Each resident in the Czech Republic consumes a daily average of 102 kWh of energy generated by fossil fuels. Of this 13 kWh are derived from hard coal, 36 kWh from brown coal or lignite, 28 kWh from oil products, and 25 kWh from natural gas. The human body working on its own is only able to create physical work in the order of between 60 and 80 W, over a period of between eight and 12 hours per day. That amounts to a mere 0.5 to 1.0 kWh of mechanical energy per day - barely a hundredth of the amount generated by the use of fossil fuels in one day.
The use of fossil fuels during the 19th and 20th centuries has supported the development of industries, transport and housing, together with many other economic activities. It has also brought about an essential improvement in the quality of life. This improvement has been on a scale never before witnessed in the history of mankind. Thanks to the use of fossil fuels we are able to enjoy warmth in heated buildings in winter, and light during the hours of natural darkness. We can use machines to assist in the cultivation of land, in the construction of buildings and other forms of infrastructure, and in the running of factories.
In the field of transport, the availability of fossil fuel energy has changed our mobility patterns in a manner hitherto unthinkable. It has made possible the permanent settlement of areas which would in the past have never been inhabited, and also had a profound impact upon social geography. Mobility has made possible access to education, access to health care, and enriched our living standards. One positive end result of this has been that life expectancy has been prolonged.
These are all pluses which have been derived from the use of fossil fuels. But there are also four negative side effects which should merit our consideration:
Naturally, humanity is concerned over the need to maintain current living standards, even after we have used up all the Earth’s „geological gift“ of fossil fuels. The fear is that otherwise we would have to return to the frugal existence of life as it was before the 18th century, which was based on a great deal of hard manual work. However this lifestyle was completely and indefinitely sustainable, being in balance with nature. It sufficed from the start of civilisation until the late 1700s. Few people nowadays would be able to cope with such a lifestyle, since society has, over the past couple of centuries, drifted so far from it. We have lost the habit of hard physical work.
Our lives have become more complex, but easier, thanks to the use of coal, oil and natural gas. Our affluence makes it difficult for us to envisage a return to a lifestyle without these „essential“ hydrocarbon fuels. Over the past two centuries our dependence on the latter has steadily increased, rather than remained stable, so that all aspects of our present day lifestyles, such as technology, life expectancy, education and housing depend on them.
The challenge facing society nowadays is to move in the opposite direction - to wean ourselves off total dependency on fossil fuels. A major step forward was taken in spring 2016. This was an essential follow-up to the UNFCC’s Paris Agreement, when at the COP21 summit held in December 2015, 196 countries agreed to adopt the text of a document strictly limiting CO2 emissions. This text was not, however, legally binding. On 22 April 2016 at the UN headquarters in New York delegates from 175 countries signed an intent to ratify the Paris Agreement and thus make it legally binding. That will enable the putting in place of a worldwide programme setting humanity on course for a new era of sustainable energy policies.
Everyday events raise various concerns which have to be addressed at international and national levels. Among these are the questions of how to deal with fossil fuel consumption and its negative effects. This is necessary at a worldwide scale, since these effects occur at this level and affect all of us. That is why agreements such as the 2015 COP21 Summit are necessary. But finer details of how to approach anthropogenic climatic change also have to be thrashed out at more local levels, and that is why the EU has its own range of strategies for reducing the emissions of anthropogenic CO2.
Between 23 and 24 October 2014 Brussels was the venue for a significant EU Energy Summit, attended by the prime ministers of the member countries, the objective being to establish an energy strategy in line with earlier agreements, which had specified a reduction of greenhouse gas emissions by 20 % by 2020, a 20 % increase in the use of renewable/replaceable energy sources by 2020, and a 20 % increase in the efficiency of energy usage by the same date.
Negotiations were not easy, because two conflicting attitudes exist with regard to how to address problems of global fossil fuel consumption and its effects on climatic change:
The outcome of the EU Energy Summit resulted in the acceptance of a strategy for future energy policy. These follow on from the previous targets set for fulfilment by 2020, and cover the period up to 2030. This new strategy received legislative acknowledgement in a publication accepted during the Summit entitled „Conclusions on 2030 Climate and Energy Policy Framework, SN79/14, 23 October 2014“. This defines the latest targets as:
The results indicate that in the EU the majority of member countries are in favour of necessary policy changes in fossil fuel consumption and environmental protection. This is in spite of populist tendencies to favour the use of these fuels without any limitation. The EU has only a partial influence on global policies affecting fossil fuel consumption and climatic change policies. However as a major economic power, Europe also has an influence in determining trends in technological innovations. This brings benefits not only for Europe, but also for the rest of the world.
An explanation is begging of the three different percentages, 40 %, 27 % and 27 %, cited above. Why are not all three numbers identical? This is because a reduction of anthropogenic CO2 (a reduction in our „carbon footprint“) is reached by two compatible strategies:
Therefore, the 40 % reduction in CO2 production is greater than the 27 % increase in use of environmentally friendly sources of energy and the 27 % increase in energy efficiency.
Significantly, many countries are not waiting for EU-level imposed restrictions on fossil fuel usage, even though these are sure to come in the near future, and instead are independently seeking ways of using alternative sources of energy. Technologies are being developed to enable this, and emission-free transport systems are being established. This is happening in the face of general worldwide attitudes, which initially regarded such strategies as being rather eccentric. Over the years the sense underlying such strategies has been gradually accepted and understood.
Consider, for instance, the German Energiewende, which started after the Fukushima accident in 2011 as an anti-nuclear energy campaign, and endeavoured to demonstrate that a switch to alternative energy supplies (hence the name) were possible. Germany is now a model for all of how an industrialised, modern economy can reduce its dependency on fossil fuels. In fact the Energiewende principles have now been adopted by the global programme for technological change with respect to usage of energy sources.
Industry 4.0, the fourth Industrial Revolution, refers to the current trend of automation and data exchange in manufacturing technologies. Reducing our dependency on fossil fuels forms a natural part of Industry 4.0 and so does the behaviour of modern industry, which instinctively adjusts itself so that it is independent of expensive and uncertain inputs - these include employees and environmentally unfriendly energy sources. Innovations tend to eliminate these factors from the production procedure.
As far as transport is concerned, it is practically impossible to use any of the renewable/replaceable sources of energy (sun, wind, water, or biomass) as a direct „fuel“. However all four can be transformed into electricity and thus conveyed by power lines to the point of use. So, electricity is of fundamental importance for „clean“, environmentally friendly transport systems, the advantages being:
The main disadvantage with the use of electric energy is that it is difficult to store it. Storage facilities in general are large, heavy, expensive, and not very durable. Some significant advances are, however, now being made. During the 20th century the only type of battery available was a lead-acid or alkaline type, which had a specific energy of around 25 kWh/t, which was difficult to maintain. Nowadays the most common type used on board vehicles are maintenance-free lithium-ion batteries, whose specific energy is 100 kWh/t.
This is still much lower than the specific energy offered by liquid hydrocarbon fuels, which offer around 4,500 kWh/t delivered by mechanical work of an internal combustion engine. But the 100 kWh/t available from a lithium-ion battery is better than the calorific value obtained using coal and water for a steam locomotive, which is about 70 kWh/t.
We can thus conclude that sustainable mobility, free from the use of fossil fuels and energy-saving, is best achieved using electric energy. Especially when applied to rail-based transport, which in principle, is very energy-saving. Therefore it is profitable to construct supply systems for rail-based transport systems in areas where large numbers of people, or large quantities of freight have to be moved. There is still a lot of potential for development - in EU member countries at present only 4 % of all transport is realised using electric energy (see Fig.1).
If the direct use of oil fuels is to be replaced by the use of electricity, some sort of infrastructure is necessary:
Extension of transport systems with vehicles powered by electricity also requires certain other, relatively minor, investments. Moreover some costs can be reduced because such vehicles are fitted with recuperative braking, enabling the reuse of kinetic and potential energy.
To become transport-independent of oil fuels other possibilities have been advanced. One of these is Compressed Natural Gas (CNG). In some countries substantial state subsidisation is made available for CNG projects. This subsidisation can take the form of grants for the purchase of vehicles and the creation of a CNG distribution system (filling stations) or tax reductions on revenue obtained through operation. But just how effective is CNG?
As noted earlier, the EU’s CO2 policy has some clear targets. By 2030 CO2 production is to be reduced by 40 %, and the use of renewable/replaceable fuel types is to be increased to a 27 % share, with a 27 % increase in energy efficiency. It is questionable how these targets could be fulfilled by rebuilding existing vehicle types, such as urban buses, that are currently powered using blended diesel fuel, for powering using CNG.
Fossil fuels do not come in a standard format. They are natural substances, and their composition varies according to the geological conditions that exist where they are exploited. To enable standardised, precise calculations, the EN 16258 norm was created, defining fuel characteristics for energy consumption and the amount of greenhouse gases which the fuels generate. EN 16258 also defines two calculation methodologies:
Each of these methodologies has its pluses and minuses. Well-to-wheels methodology covers the whole processing and usage cycle. It uses, however, fixed values which do not reflect the actual operating conditions found in real situations. Here we use the methodology from an operator’s point of view, which approximates to the Tank-to-wheels methodology and separates clearly transport and energy activities (and responsibilities). Moreover the Tank-to-wheels methodology complies with existing practices for evaluation of vehicle’s fuel consumption and noxious exhaust emissions.
It is essential that the latter methodology is not based on searching in data tables, but on basic knowledge in the areas of mathematics, physics, chemistry and biology, even though the input from these subjects might only be at an elementary school level… In spite of this fact the result is clearly evident.
In a hydrocarbon fuel energy is stored in both the hydrogen and carbon. When oxidation occurs as the fuel is burnt, an exothermic reaction releases heat from both these elements. The difference between individual hydrocarbon fuels lies in the size of the share of carbon in the structure of the fuel. This determines the amount of CO2 released during the burning process. The structure of CO2 consists of carbon (atomic weight 12) and the rather heavier oxygen (atomic weight 16). So, for example, when 1 kg of carbon is burned, the amount of CO2 produced is calculated as follows:(12 + 2 . 16)/12 = 3.67 kg CO2
Natural gas (also known as hydrogen carbide) is a hydrocarbon fuel. Its composition includes a considerable share of methane (CH4 - one atom of carbon and four of hydrogen), the actual share of which varies across different gas fields. Compared with liquid hydrocarbon fuels, the structure of methane incorporates a rather greater share of hydrogen. This creates heat during combustion, but unlike carbon (C) it does not generate CO2.
So, used as a fuel, hydrogen carbide has a slightly smaller carbon footprint than do other types of hydrocarbon fuel. Compared with oil, the production of CO2 from natural gas is about 80 %. Compared with ga-soline it is about 79 %, for the same calorific value (see Table 1):
The carbon footprint of the fuel is only one of the contributory factors involved in evaluation of the usefulness of a fuel type. Also of importance is the quantity of fuel it is necessary to burn to achieve the same results. This is of key importance with regard to powered vehicles, where efficiency (the ability to haul a load of a given weight) is related to energy requirements.
Another factor that must be considered in analysing carbon footprints is that only fossil fuels are evaluated, and not renewable sources, which include biofuels. These are created by biological processes that take place within the present and, therefore, does not result in an continuous increase in atmospheric CO2 concentration. Burning biofuels can be seen as only a part of the natural CO2 cycle.
Living plants extract CO2 from the atmosphere through photosynthesis, then return it to the latter when they rot, or are burnt. Similarly, all living organisms (animals and people) generate CO2 by breathing and by digesting food (including plants) and expelling waste, and also when they die, through decomposition or burning. Animal organisms receive energy from carbohydrates, amyloids (protein fragments) and fats, which serve as natural „fuels“. But these are all natural increases in CO2 and do not result in a permanent increase in greenhouse gas concentration.
The natural abstraction from and release into the atmosphere of CO2 can be shown graphically. With most land masses on the globe being situated in the northern hemisphere, natural CO2 atmospheric concentration decreases during the northern summer and early autumn as plants grow, then increases during the late autumn, winter and early spring, as rotting and burning of dead organic matter takes place. This sinusoidal pattern, which has a maximum seasonal variation of a mere 2 ppm of CO2, is superimposed on the exponentially increasing curve of anthropogenic CO2 atmospheric concentration.
Diesel is a 100 % fossil fuel. Converted to 1 kWh of usable heat energy it represents:
At present blended diesel fuel is used by vehicles in the Czech Republic. This contains around 6 % of Fatty Acid Methyl Ester (FAME), produced from crops of rapeseed, which is not, of course, a fossil fuel in itself, and whose burning does not add to the anthropogenic CO2 atmospheric concentration, since rapeseed crops, like other organic matter, rely on photosynthesis for growth. Converted to 1 kWh of usable heat energy, the blended diesel with 6 % of FAME represents:
Natural gas is a 100 % fossil fuel. Converted to 1 kWh of usable heat energy, the natural gas represents:
Note that the burning of natural gas, which has the same usable heat energy as diesel fuel, produces 80 % of the CO2 generated by the burning of diesel fuel, even though both are fossil fuels.
Besides natural gas, biogas can also be used as a fuel. Biogas is a natural substance, generated by the spontaneous rotting of vegetation. That is why it is often called marsh gas. However marsh gas can also be produced as an industrialised process, using waste material from agricultural or industrialised activities, or occurring as a by-product of the latter. Examples of industrialised marsh gas production include sewage water treatment installations, waste dumps, compost pits and pig farms.
However, unlike natural gas, biogas is of a very varied composition, depending on what produced it. Its desirable methane content varies between only 40 and 75 %, and there are also other types of gas present, including CO2, and harmful ones such as hydrogen sulphide. As a result, it is necessary to remove these before biogas is used. Because of its varied and potentially noxious contents, biogas is not used nowadays as a fuel for road vehicle internal combustion engines.
When road vehicles are authorised for using gas as a fuel, this is standardised only for the use of natural gas, which consists almost entirely of pure methane. That ensures that exhaust emissions are standardised and guaranteed, something which it is not possible to achieve using biogas. In principle, an engine which can use natural gas can also run off biogas, but only once the methane content has been extracted for use. This is a costly technology, and the end result is no longer a biogas, but bio-methane, whose chemical composition, involving a high methane concentration, approximates to that of natural gas. Moreover bio-methane is, on cost grounds, not competitive with natural gas.
Nevertheless, in spite of the cost involved, bio-methane is still a renewable/replaceable energy source, and some countries, such as Sweden, are considering the use of this form of technology. The question remains of whether it is better to modify biogas for powering vehicles, or to use it in its raw, untreated form, to power static machinery, such as in cogeneration power/ heat complexes where waste heat could also be used to advantage.
Within the EU biogas forms a significant share of all types of fuels used. Its most common uses are to be found close to its place of production, for instance on livestock farms, or at sewage water treatment installations. Biogas is also used in cogeneration power stations, and this is a highly efficient use.
The basic component in the cogeneration power/heat station is a piston-type internal combustion power unit with an electric generator. The power unit can either be of ignition or dual-fuel type, while the generator can convert around a third of the heat energy released by the fuel to electric energy. The remaining two-thirds of the fuel heat energy warms cooling liquids in the combustion engine and the exhaust gases. The latter are used in heat exchangers, to provide heating systems for buildings, for industrial processes and to heat water for domestic and/or commercial use.
In this manner between 80 and 90 % of the energy generated by the fuel can be made use of. However there are certain limitations with cogeneration. There is a limited distance over which hot water, either for direct use or for heating, can be transported by pipeline, especially in cold climates. So, cogeneration complexes have to be located in areas close to the places where there is a considerable level of demand for use of the heat - such as housing estates, blocks of flats, factories and hotels. At present both natural gas and biogas are used in cogeneration complexes. However as far as road and rail vehicles in most European countries are concerned, if gas is used for their powering, natural gas is used, rather than biogas.
Progress in the technological development of coal-burning power stations has resulted in an increase in their efficiency from 30 % to the present 40 % level, resulting in a decrease in coal consumption and consequently in the amount of emissions. As for the remaining 60 % of fuel energy not used for the generation of electricity, this only heats the immediate surrounds of the power station, and most of it escapes through the cooling towers. In the vicinity of large power stations, with output in the range of several hundred megawatts, it is difficult to find sufficient demand for the consumption of heated water produced, which is also in the region of several hundred megawatts.
In general, large power stations of such dimensions are found in close proximity to the mines from which they receive their coal, this location policy intended to save transport costs. Most power stations nowadays incorporate technology to desulphurise their emissions, so at a local level they are not great polluters.
As far as utilisation of waste heat is concerned, a large network of densely distributed small cogeneration complexes is more effective than large power stations, their locations adjacent to areas of demand for both electricity and heat. This leads us to the study of decentralised energy production strategies, which ensure that as much as possible of the waste heat generated during the production of electricity is used. One possible strategy in this direction would be the replacement of gas-fired water heaters used not only domestically but also by the service sector of the economy and by industries, by small cogeneration stations within the latter complexes and in homes.
Compared with the amount of CO2 produced through burning blended diesel fuel with a 6 % content of FAME, the burning of fossil natural gas of the same usable heat energy produces 85 % of this quantity. This (a saving of only 15 %) is rather more than the 80 % (and a 20 % saving) produced when pure, not blended, diesel fuel is burnt. The biofuel component in blended diesel fuel does not, by itself, create an anthropogenically-induced (created by human activity) carbon footprint, while natural gas is a 100 % fossil fuel.
As an example of one country’s use of liquid biofuels, the Czech Republic’s State-level energy policy envisages that in the period up to 2020 there will be an increase in the amount of liquid biofuels used for transport. Over a five-year period from 2015 the increase is expected to be up to 7.8 billion kWh per annum, resulting in the energy produced by liquid biofuels comprising 12 % of all energy generated by liquid fuels used for transport purposes within the country. This means a saving of 13 % in the anthropogenic production of CO2 over this five-year period, as a result of using blended diesel fuel as opposed to natural gas.
The energy policy also envisages that after 2020 it will be impossible to increase the amount of energy from liquid biofuels used for transport, for simple agricultural reasons. To obtain the above-mentioned 7.8 billion kWh per annum, after consumption at source is taken into account, it will be necessary to sow an area of close on 1,000,000 hectares with oilseed rape crops. These yield on average 4 t of rapeseed per hectare, suitable for the production of 2 kWh/kg of methyl ester.
1,000,000 hectares amounts to roughly a third of all the arable land available within the Czech Republic, so it is essential to keep the remainder for crop growing for human and livestock consumption. It is thus understood that the primary purpose of agricultural land is to sustain humanity, and livestock, and not to feed the appetites of cars.
The low efficiency of rapeseed methyl ester for transforming solar energy into heat energy should also be borne in mind. During a year each square metre of land in the Czech Republic receives, on average, rather more than 1,000 kWh of solar energy. But out of this only 0.8 kWh of the energy of rapeseed oil methyl ester is gained (once the productions’ own consumption has been taken into account). The efficiency of this method is just a mere 0.08 %. Although they have been often subjected to criticism, solar power stations are over 200 times more efficient, at around 18 %. We can conclude that the cultivation of biofuels is a costly, and not very effective means of trying to replace conventional diesel-based fuels. Moreover, it consumes substantial State subsidies.
The fundamental disadvantage of natural gas is its low density, of only 0.0007 kg/dm3. This makes it very unwieldy, compared with diesel fuel, whose density is 0.83 kg/dm3. One kg of diesel fuel can be stored in a container with a volume of just 1.2 litres. For the same amount of natural gas, in uncompressed form, a 1,400-litre container is necessary. For natural gas to be used realistically as a vehicle fuel, it first has to be compressed.
CNG is natural gas compressed 200 times, to a compression of 20 MPa. Nevertheless, the resulting storage tank is still large, over five times that required for the same weight of diesel fuel. To have a container for CNG with the same volume as that for an equivalent amount of diesel fuel, it would be necessary to compress the gas overa thousand times, to 110 MPa. Although this is in practice feasible, the state of technological development of gas compression is such that it would be a commercially unreal proposition.
The compression of natural gas involves two additional energy factors. Energy is needed to compress the gas, and the energy needed to carry heavy storage tanks on board those vehicles using CNG. To compress one kg of natural gas, which has a calorific value of 13.61 kWh, to a pressure of 20 MPa, it is necessary to use around 0.02 kWh of electric energy to power a compressor. That means (given the present energetic mix available in the Czech Republic) the consumption of 0.03 kWh of fossil fuels to produce electric energy (1.74 kWh/kWh). The resultant carbon footprint is 0.60 kg of CO2/kWh, and this has a negative influence on CNG’s energy balance (see Table 2).
Converted to 1 kWh of usable heat energy, CNG compressed to 20 MPa represents:
In comparison with the use of 6 % blended diesel fuel in the Czech Republic (in 2015), the saving in CO2 generation decreases from 15 % (blended diesel fuel) to 11 % (CNG) on account of the energy consumption required for compression purposes. 0.226 kg is generated using 6 % blended diesel, compared with 0.254 kg using CNG. Total energy consumption is increased to 102 %, while fossil fuel consumption is increased to 112 %.
The density (or specific weight) of CNG at 20 MPa is calculated by 200 x 0.007 kg/dm3, which is 0.14 kg/dm3. The high pressure means that storage canisters must be robust, made of thick steel. For each litre of internal storage space these tanks weigh around 0.9 kg, and thus weigh 6.4 times greater than the weight of the CNG stored in them. In fact in this way the weight of the stored gas is around seven times greater. The consequence is that when mounted on a vehicle, the latter has to use more energy to move them. This situation is exacerbated when CNG tanks are mounted on an urban bus, where weight-dependent rolling resistance is significant, as is the increased fuel consumption on account of frequent stopping and starting. It can be estimated that the amount of energy used by such vehicles is about 6 % greater than in the case of urban buses powered by diesel fuel. The end result is increased energy consumption, increased fossil fuel consumption, and increased CO2 production. Converted to 1 kWh of usable heat energy the result of the greater weight of a CNG-fuelled vehicle in the energy balance is as follows:
Given the increase in the weight of a CNG-fuelled vehicle, in comparison with the 2015 use of 6 % blended diesel fuel, the saving in CO2 generation decreases from 15 % (blended diesel fuel) to 5 % (CNG). Total energy consumption is increased to 108 %, while fossil fuel consumption is increased to 119 %.
For purposes of objectivity we ought to add that the latest technology for storing CNG is the use of canisters made of carbon composites, either incorporating some metal content, or purely of carbon. These are rather lighter than steel gas canisters, but they still take up a large amount of space, compared with conventional liquid fuel tanks. They are also more expensive than steel gas canisters. However even the use of carbon composite canisters does not sway the balance in favour of using CNG as a vehicle fuel.
Most buses powered using liquid hydrocarbon fuels use diesel fuel, rather than petrol. Diesel fuel has a lower igniting temperature than does petrol. This enables it to be used in internal combustion engines with fuel injection and compression ignition. A diesel engine has a rather higher efficiency (ideally around 42 %) compared with a petrol engine. This results from the higher compression ratio and the more volatile nature of diesel fuel. Fuel injected and sprayed into hot air ignites and fills the combustion chamber immediately.
Natural gas has a higher ignition temperature than that achieved by compressed air in internal combustion engines. Because of this it is impossible to use natural gas as a fuel for ignition in an internal combustion engine, since it would not be possible to ignite it using the level of air compression. Natural gas must be ignited using a spark, so it is only possible to use it directly as a fuel in Otto Cycle (gasoline) engines which have spark firing.
It is technically feasible to rebuild a diesel ignition engine into a petrol-fuelled engine, and thus use CNG. The result is that the efficiency is reduced slightly, to about 39 % in the best of cases, and fuel consumption thus increases. In such a modified Otto Cycle engine fuel consumption is about 1.08 times higher than in a Diesel engine, because the Otto Cycle engine has a somewhat lower compression ratio and a lower burning rate. The fuel is ignited by the spark at one location, then the burning spreads gradually sideways. By way of contrast, compression ignition results in all the injected fuel being ignited practically simultaneously within the combustion chamber.
On account of the low combustion speed, spark ignition can only be used in internal combustion engines with small diameter cylinders. When large diameter cylinders are involved, the spark firing principle is impossible, since the fuel ignition would take too long. That is why, when large gas combustion engines are required (such as for power stations and marine), two-stroke engines are used. They suck in a mixture of air and gas, with ignition involving a small amount of diesel fuel which ignites by the heat of compression. But the latter type of engine is not common in ordinary urban buses.
With the efficiency of a petrol internal combustion engine being about 8 % lower than an ignition diesel engine (39 % versus 42 %), there is a fuel consumption increase of about 8 % in a petrol engine fuelled by CNG compared with an oil-fuelled diesel engine. The result is increased energy consumption, increased fossil fuel consumption, and increased CO2 production. Converted to 1 kWh of usable heat energy a CNG-fuelled vehicle with an internal combustion engine of lower efficiency has an energy and CO2 balance as follows:
In comparison with the use of 6 % blended diesel fuel (Czech Republic, 2015), the saving in CO2 generation decreases to -1 % by using a petrol engine fuelled by CNG. The total energy consumption is increased simultaneously to 116 %, while fossil fuel consumption is increased to 128 %. Figures 2 and 3 compare the energy balance, fuel requirements and environmental impacts of urban buses fuelled by different systems.
To complete the picture, Tables 3 and 4 show Well-to-wheels data using the EN 16258: 2012 standard. The latter increases the fuel consumption of the vehicle (based on the EN 16258: 2012 Tank-to-wheels methodology) and its footprint using adjustments to take into account the energy required for extracting the fuel and moving it to where it is consumed. The coefficients for natural gas (energy 1.12, carbon 1.15) are slightly lower than for diesel fuel (energy 1.19, carbon 1.21) and for diesel fuel which is blended with 6 % of FAME (energy 1.24, carbon 1.26).
It is questionable, though, how accurate these coefficients are when applied to individual circumstances, since they are very generalised. The high coefficients for 6 % blended diesel fuel are noteworthy, since these results from the considerable amount of energy used in the production of rapeseed oil methyl ester. Here the correction factor for energy is 2.09, and the carbon footprint is 0.163 kg of CO2/kWh. Using Well-to-wheels methodology slightly changes the results calculated by the Tank-to-wheels methodology, the one favoured by operators, making natural gas usage look more positive (see Table 4). However that still does not mean that the use of CNG is an optimum solution for vehicle powering.
The aims of the autumn 2014 EN Energy Summit mentioned in the document entitled SN/79/14 (Conclusions on 2030 Climate and Energy Policy Framework) are to reduce CO2 generation by 40 %, increase reliance on renewable/replaceable energy sources up to 27 %, and increase overall energy use efficiency by 27 %. By replacing diesel-fuelled buses by gas-fuelled vehicles these aims are far from being fulfilled: the use of CNG results in an increase in the consumption of both energy and fossil fuels and only in a minor reduction in carbon footprint.
To check the above mathematical argument, actual measurements of energy consumption under conditions of simultaneous operation of diesel buses and gas buses on an urban transport network were assessed. According to statistical data published by the urban transport operator DP města Pardubice (see Table 5) the following consumption values were recorded for 12 m buses:
As a comparison of the values (4.77/ 4.17 = 1.14) clearly demonstrates, the replacement of diesel fuel by CNG causes an increase in the consumption of energy directly by the vehicle by 14 % (this value does not include the energy required for the compression of the gas).
The foregoing indicates that the replacement of the blended diesel fuel currently sold in the Czech Republic with CNG is not a way forward in an endeavour to reduce the carbon footprint of motorised transport, while CNG does not result in any significant reduction in the production of CO2. But how does CNG measure against blended diesel fuel when we consider the local emissions - the noxious emissions generated by vehicles as exhaust when they are in use?
Table 6 shows what steps have been taken since shortly after the turn of the millennium to limit road vehicle noxious emissions. It should also be borne in mind that an internal combustion engine can be adjusted in various ways, each affecting the quantity of noxious emissions:
In each instance the adjustments required to the engine are different. To achieve one objective, such as to reduce exhaust emissions, the other performance parameters will deteriorate. Gradual steps to reduce air pollution, therefore, naturally lead to a decrease in the utility value, relative to other parameters, of the internal combustion engine, hence the usual technique is to try to achieve an acceptable compromise.
In the especial case of turbo-charged engines, there are both stabilised and transition regimes. Here an endeavour to reduce exhaust emissions results in a lowering of engine temperament (the rate of power increase, dP/dt). The turbocharger is in effect a time-delayed positive feedback system. As more strict exhaust emissions are imposed, more technical devices, such as filters, catalytic converters and fuel additives, have to be used. The result is an increase in both the price and the operating cost of both the internal combustion engine and the vehicle itself.
Users expect that manufacturers ensure that their car fulfils virtually all their stated car performance parameters, such as power rating, speed, starting acceleration, braking performance, weight, fuel consumption and stability. However, in contrast the vehicle’s carbon footprint is not usually of interest to the majority of car purchasers or users, so long as their vehicle fulfils the legislative standards. This makes it easy for manufacturers to practise fraud and deception when they publish the emission values of their vehicles. Neither buyers nor sellers want to make cars more complicated, less powerful and more costly than is necessary, by minimising the vehicle’s environmental impact.
Nevertheless, since the turn of the millennium, as shown in Table 6, new road vehicles have been subjected to progressively tougher noxious exhaust emission limits (Euro VI and Euro 6, valid from 2014/15), in order to protect the environment. Note the use of the adjective „new“. There is no legislation which obliges owners of older vehicles to meet current standards.
In general, natural gas used in an internal combustion engine produces fewer noxious emissions than either diesel fuel or petrol, even without the additional devices which engines using the latter two fuels require, and which need certain working conditions, which are not always fulfilled. These concern both the exhaust emissions from liquid hydrocarbon-fuelled engines, as specified in regulations, and also the harmful emissions which are not regulated, but endanger human health. The latter include polycyclic aromatic hydrocarbons, which bind to micro-particles and which are both mutagenic (changing the genetic properties such as the DNA of organisms) and, worse, carcinogenic.
With the development of catalytic converters and carbon filters for petrol- and oil-fuelled engines, the amount of noxious exhaust emissions has in recent years declined significantly. It is however necessary to ensure that vehicles are really equipped with such devices, and that they are in an operational state.
The most recent Euro VI noxious exhaust emission limits, as specified in the ECE 49 regulations, have to be fulfilled not only by diesel-fuelled engines, but also by those fuelled by CNG. Manufacturers at present offer operators a choice between diesel-fuelled and CNG-fuelled urban buses, both with similar emission levels.
As indicated in Table 6, the Euro VI emission limits are essentially similar for both petrol- and diesel-fuelled engines, though in the case of older engines, not built to Euro VI standards, this difference is substantially greater. And it must be recalled that most vehicles in circulation at present were built prior to the introduction of Euro VI and Euro 6. The former (with Roman numerals) is the standard used for buses and heavy commercial vehicles, while Euro 6 (Arabic numerals), defined by the emission Test Cycle ECE 83, is used as the current standard for private cars and light commercial vehicles, and was enforced between September 2014 and September 2015, according to the specific category of vehicle.
Since the turn of the millennium a number of urban areas throughout the world have introduced Low Emission Zones (LEZs), from which vehicles whose noxious exhaust emissions exceed a certain level are excluded. One of the first was Tokyo in October 2003, with certain urban areas in Germany and Britain following suit in 2008, and then Sweden, Denmark, the Netherlands and Italy.
Clearly the ranking of vehicles according to the level of noxious exhaust emissions they produce is a much more objective measure than only evaluating them on the type of fuel they consume. There has been much opposition to LEZs, both from the general public and from municipal authorities. Sadly, people would rather look after the welfare of their old cars than care about their own health, and that of their offspring. Unfortunately that is the way of the world.
To cover all possible forms of vehicle propulsion, it is necessary for us to consider the combustion turbine alternative. Compared with piston internal combustion engines, a pure rotary turbine is a simpler structure, but has a rather lower level of efficiency. A combustion turbine can be powered using natural gas, as is the case, for example, in gas/steam power stations. The main advantage is the ability of a quick transition from cold to power supply status.
In a gas/steam power station, the hot exhaust gases first pass through the combustion turbine, and then the heat is used in the boiler for the production of steam for a steam turbine. It is of no consequence here that the combustion turbine is of lower efficiency than a piston internal combustion engine, since any remaining heat can be used for steam production in the boiler. However the power station configuration, even on a small scale, would be too large to install in vehicles, since there would be insufficient space for a boiler and steam turbine. The efficiency of the combustion turbine itself is significantly lower than that of an internal combustion engine, and is hence a disadvantage.
The basic advantage of a combustion turbine compared with a piston internal combustion engine is its low weight, hence this technology is much appreciated by the aerospace industry. In the case of rail vehicles there are not such strict weight constraints as there are in aircraft design, so there are no reasons for using machines of lower efficiency.
On the contrary there are other constraints applying to the use of turbines in rail vehicles:
These, in addition to the high energy demands, were the reasons why attempts with combustion turbines used to power trains met with mixed success. These vehicles only ever formed a small percentage of the entire rolling stock/ motive power fleets of certain operators, such as SNCF’s fleet of turbine-powered multiple units, the same type being exported to the USA, Egypt and Iran, and DB’s Class VT 11.5 TEE multiple units, and modifications of the Class V 160 locomotive design. There were also others, but in these instances the combustion turbines were powered using liquid fuel, rather than gas.
What was the cause of the recent massive increase in the number of CNG-fuelled cars in the Czech Republic and certain other EU countries? The explanation is a prosaic one: the growth in popularity of building insulation. Prior to this the suppliers and distributors of natural gas for home heating were very successful as more and more households and heating companies switched from coal to natural gas, and gas distribution networks (pipelines) served more and more settlements. The consumption of gas rose in a satisfactory manner. And so eventually did the price of gas.
But sales slowed down when it became popular to save energy by improving the thermal insulation of buildings to ensure that unwanted cold air does not penetrate, and internal heat is conserved. Instead of standard masonry structures, new techniques were adopted for buildings, with new designs of windows, doors and wall cladding. Governments introduced a policy of realising thermal evaluations of buildings. Modern water heaters have evolved, with automatic temperature control, this often being dependent on the ambient exterior temperature.
The thermal insulation policies also affected older buildings, so that costs for heating could be reduced. Households and companies were offered generous subsidies by the State, a further motivation being that as a result of insulation, heating costs savings would be accrued. The outcome was successful - to the delight of the EU, individual countries, companies and households there was a perceptible decrease in natural gas consumption for heating purposes. In the Czech Republic, for example, natural gas consumption fell by 10 % between 2010 and 2015, at an average rate of 2 % per annum (see Table 7). As shown in Figure 4 and Table 7, a further decrease in gas consumption is expected over the coming years.
Now this is a very positive trend, when viewed from the economic and environmental point of view. Money and natural gas are being saved, and the manufacturers of modern construction materials are enjoying increased sales. The only losers are the suppliers and distributors of natural gas, who faced a gloomy long term future after 2010 or thereabouts. They were also hit by the fact that some of their clients also decided to switch from gas heating to rely solely electricity. This was because there is a large fixed element in both electricity and gas tariffs, and people in the Czech Republic were unwilling to pay two bills for the use of energy, when using one source of energy could suffice.
The gas companies were soon to realise that the largest and most profitable market for gas in the future, according to the EU’s energy balance statistics, would be transport. Especially when, as explained below, a CNG-fuelled private car is three times less efficient at converting gas to energy than is a modern water heater.
Evidently natural gas suppliers and distributors are not concerned about conserving this fossil fuel for future generations, and equally the effect of gas burning on anthropogenic atmospheric concentration of CO2 is of little worry to them. Their main objective is to maximise sales and maximise profits, which pay the wages of their workers. This is of course standard business practice.
The general public is lured to the new potential use of gas - CNG for cars - by massive advertising campaigns, which often filled several pages in the national newspapers.It is crucial that the general public must be made aware of the pitfalls in these persuasive pro-CNG arguments. The same applies, and even more so, to those state institutions which support programmes for the acquisition of CNG-fuelled vehicles, using public money, since these strategies do ultimately regulate the market. There is only one remedy against crafty advertising campaigns - a quality all-round education!
To complete our picture of heating and transport energy consumption, it should be noted here that the modern domestic gas-fuelled water heater has an energetic efficiency of around 94 %. In commercial situations condensing heaters are used, taking advantage not only of the calorific value of natural gas, but also of the combustion heat, which is about 11 % higher. In the heat exchangers of a condensing heater the waste gases (created by the burning of the hydrogen component of methane) are condensed from water vapour to liquid, thus recovering the latent heat of vaporisation.
The ratio of heater output to fuel thermal power (standardised fuel energy usage) is increased, as high as 108 %. The laws of Physics do not, of course, enable an efficiency of 108 %, which is here related to the combustion heat rather than to the calorific value of the fuel. Thence the efficiency of a condensing heater is stated as 97 %. Compared with the average modern water heater, the CNG-fuelled combustion engine in vehicles has an efficiency, related to the calorific value of the fuel, of only 35 - 39 %, almost three times less.
One day, in the near or distant future, depending on future energy policies, we will reflect on the fact that natural gas reserves, like those of coal and oil, are finite. Anthropogenic atmospheric concentration of CO2 is irreversible. We have a social responsibility to halt this. Our reserves of natural gas must be used sparingly and prudently. Natural gas should be used where it is most effective, either for water heaters or in cogeneration complexes with an efficiency of 94 - 108 %.
It should not be used as a fuel where its energetic efficiency is as low as between 35 and 39 %, where the losses through waste heat are as high as between 61 and 65 %, in other words, greater than the share of the fuel that is actually used for heating purposes. It is thus a foregone conclusion that the use of CNG for powering internal combustion engines in vehicles is highly inefficient compared with the use of natural gas (in uncompressed form) for water heater. Moreover in vehicles there are no opportunities to make full use of the every high heat energy of exhaust gases and cooling water.
Not only is CNG an inefficient energetic source for powering vehicles, all fossil fuels share the same handicap. With the current endeavour to minimise the production of anthropogenic CO2 it is surely wrong to waste fossil fuels in combustion engines, especially where no system exists for making use of the residual waste heat.
The Carnot Cycle states that there is an upper limit of around 40 % to the efficiency of any standard thermodynamic engine during the conversion of thermal energy into mechanical work. The heat loss is thus 1.5 times greater than the amount of energy used for mechanical work, and moreover, the resultant carbon footprint is 2.5 times greater than would be the case if all 100 % of the fuel energy were used.
Also to be excluded from future energy strategies should be large thermal power stations, unless they are situated near locations which consume much if not all of their waste heat. In a similar manner it is not possible to contemplate the use of internal combustion engines to power vehicles, because it is impossible to make a meaningful use of the thermal energy generated by the cooling water and exhaust emissions.
At present there are numerous examples of both these types of squandering of thermal energy, only serving to demonstrate to us all that there are potentially enormous energy savings to be made in the future. As far as transport is concerned, the future pathway lies in the direction of using electrically powered vehicles, the electricity coming from renewable/replaceable sources.
We talk of oil being of organic origins - the decay of vegetation which took place at least 250 million years ago. However there also have been encouraging (?) suggestions by scientists that oil is of mineral (abiogenic) origin and that at depth in the Earth’s crust there are enormous deposits of it. Their research is open to much speculation, but if indeed it is true, it is good news for the manufacturers of internal combustion engines, but very bad news for climatologists and the rest of us. For if such oil reserves do exist, and can be exploited economically, there will be no restraints or curbs on the production of anthropogenic CO2. The amount of CO2 in the atmosphere will simply increase, exponentially, and so will anthropogenic climatic change, with ever-accelerating global warming. The prospects are terrifying.
One major lever on economic behaviour is the amount of taxation, or other charges, superimposed on the cost of products or services. Consumers are influenced by price when selecting products and services, including transport. If all these products and services are taxed equally, then consumers choose what they want by comparing price and value, in a straightforward manner. But if the price of a product is distorted by tax - such as an exceptionally heavy percentage applied, or an exceptionally low one, in comparison with other products, consumer behaviour is distorted accordingly.
Natural gas is a case in point, since in many countries it enjoys reduced excise taxes in relation to its energy content, compared with other fossil fuels used for transport, notably diesel fuel and petrol. Although for consumers it is agreeable to take advantage of the lower prices resulting from tax allowances, it has to be borne in mind that to achieve a balanced economy other products have to be taxed more heavily.
Sustainable energetic policies and sustainable transport policies must be founded upon sustainable economic policies. The goal must always be to ensure that profitable operation is ensured, without the need to depend interminably on the provision of subsidies.
In spite of generous support through reduced excise tax, the Czech Republic has not managed to use natural gas for transport purposes in the manner specified in the State Energy Policy (SEK - Státní energetická koncepce). According to the latter in 2015 4.25 billion kWh of oil fuels used for transport purposes were to be replaced by natural gas. Instead, in 2015 barely a tenth of the substitution had occurred, with CNG accounting for just 0.414 billion kWh.
Clearly the State benefited through the lower amount of a CNG taxation support. However the SEK strategy failed: visions that in 2020 use of CNG in the Czech Republic will account for 7.444 billion kWh (see Figure 5) and in 2030 as much as 12.25 billion kWh now seem unrealistic indeed.
Transport is one area in which, over many years, the market conditions have been distorted by taxation. The following summarises how this occurs:
As far as taxation on diesel fuel, petrol, marine diesel fuel and aviation spirit are concerned, rail operators and road users are usually charged, whereas airline operators and shipping companies are not usually charged.
The costs in the economy are distorted by factors such as taxes, tax relief and subsidisation. A transport operator is prompted to realise activities which might from the business point of view appear profitable, but which in reality only survive because they receive Government subsidisation. Ideally, conditions should be equal for all modes of transport, but they are not. If they were so transport systems would perform according to the natural laws of economics. So differential taxation, or the absence of taxation on various types of hydrocarbon fuel, is a further distortion on the playing field of transport operation. Former US President Ronald Reagan described the situation succinctly: „If it moves, tax it. If it keeps moving, regulate it. And if it stops moving, subsidise it.“
It is not that surprising, therefore, that transport operators, other companies, and individuals make use of state subsidisation, if it is available, and if they can benefit from it. CNG enjoys a low level of taxation, compared with petrol and diesel fuel, so it is logical that transport operators find it interesting and opt for using it. Moreover there is State support available to reduce the cost of buying CNG-fuelled vehicles, and for extending the network of CNG compressor stations.
The logic involved is twisted, though. On the one hand the State provides grants for the insulation of buildings, and the use of gas-fuelled water heaters, which have a heating efficiency of between 95 and 105 %. Given their efficiency, this is a perfectly acceptable strategy. Simultaneously, grants are available for the purchase of CNG-fuelled cars, which with their combustion engines with spark ignition have an efficiency of between just 35 and 39 %.
At this juncture, to cover all fuel options, we must take a brief look at the possibilities of hydrogen for transport purposes. Hydrogen is a lightweight fuel and its calorific value is 33.2 kWh/kg. It can be used either as a substitute either for petrol or diesel in internal combustion engines, or in fuel cells, to be converted into electric energy to power traction motors.
The first option (a substitute fuel) results in the same problem as with other fuels (diesel, petrol and natural gas/methane). An internal combustion engine has an efficiency of conversion of energy to mechanical work of a mere 35 to 42 %. With hydrogen being such a costly fuel to produce (see below), this is not a practical option since there is a critically low conversion efficiency of energy to mechanical work.
The other option is to use fuel cells. Fuel cells enable hydrogen to be transformed by oxidation into water. This process releases electrical energy. Fuel cells are slightly more efficient than internal combustion engines at transforming energy into mechanical work, but not that much more so. Moreover, fuel cell technology is very costly: the fuel cells are expensive, and it is difficult to regulate them.
As in the case of a nuclear power station reactor, fuel cells must run continuously (until they are exhausted). It is thus necessary to provide some form of energy storage device. There are two possibilities: one is a double-layer supercapacitor, which charges rapidly, and the other is a lithium-ion electrochemical battery, which can be used for long term energy storage. The result is a hybrid vehicle, in which the fuel cell replaces the internal combustion engine. The fuel cell can also be used as an extension to the life of the battery between recharges, thus extending the operating range of the vehicle (a plug-in hybrid).
Hydrogen is thus an attractive option, but the cost of its use is beyond realistic levels for most public transport operators at present. For it to be used on a universal scale there would be questions of the technical, economic and security aspects of distribution, refuelling and on-board storage to be addressed. Moreover, fuel cells require a supply of ultra-pure hydrogen, without any additives or contamination.
The actual process of converting electrical energy to hydrogen energy involves electrolysis, and then from hydrogen energy to electrical energy in a fuel cell, is essentially that of an open cycle electrochemical battery. It is similar to the process of charging a battery, putting its charged electrolyte into another uncharged battery, and then discharging it. Compared with the use of standard batteries, fuel cells have a lower weight. But not only is hydrogen fuel cell technology costly, and can not be overloaded, it has a low efficiency of only about 40 %, so there is a 2.5-fold increase in energy consumption.
The really big obstacle involving the use of hydrogen for transport purposes is this: unlike coal, oil and natural gas hydrogen is not available „on tap“ in nature. It has to be manufactured. It is produced mostly using hydrocarbon fuels (coal, oil, natural gas), or it can be produced from electricity (via electrolysis). Therefore, at the present level of technological development it is not possible for hydrogen to be considered as a substitute for existing energy sources. Instead, it is a product of them.
In spite of the aforementioned obstacles there seems to be one real future for the use of hydrogen-based technologies in transport - the storage of surplus electric energy. One of the main possibilities will occur when excess electricity is being generated - when renewable/replaceable sources of electricity become the majority sources of power. Performance will then be unpredictable, and difficult to control. There will be periods when production exceeds demand and the price of electricity for consumers will then fall. Thus surplus electricity can be used in the production of hydrogen.
In similar, for industrial concerns it will be worthwhile to buy some electricity at the lowest or the night tariffs, and use it in the processes with a low efficiency. For instance, for the long distance transmission of electricity, storage of electricity in electrochemical accumulators, power management using smart electricity grids - and the production of hydrogen via electrolysis. This is best achieved close to where electricity is produced. The ideal locations would be large aeolic energy complexes - wind farms, either on land in windy locations, or on artificial islands, in shallow seas, close to the coast.
One of the main problems with hydrogen is that for safety reasons it is not easy to store or transport it. Laboratory tests have been carried out on a technique whereby CO2 is added to H2 (manufactured by electrolysis) resulting in CH4. This methane can then be distributed by conventional gas pipe networks, so that provided that these networks are comprehensive, even the smallest centres of population can be connected up to methane, for use in domestic appliances and to power other forms of machinery.
Using hydrogen generated by electrolysis at aeolic energy complexes can be perceived as a renewable source of natural gas. In the future, this could liberate us from exploiting natural gas fields, and would not result in any increase of anthropogenic atmospheric CO2 concentration. It would also result in significant changes in the market for natural gas.
Fracking technology can be seen as an analogous procedure, although regarded by many as a controversial activity. When the technology of extraction of natural gas from shale was developed, the price of gas extracted from natural gas fields fell. What happened was that shale gas obtained through fracking was originally consumed only in the USA. However the USA then started to export shale gas, and the producers of natural gas lost their monopoly and had to reduce their prices. The USA is now the world’s main producer of shale gas.
Whether the gas is natural gas from underground deposits, or produced artificially as methane, the same usage constraints apply. Its most efficient use is in water heaters, which use up practically all the thermal energy. Its use in internal combustion engines, with their low efficiency, results in two thirds of its fuel energy being transformed into waste heat.
It makes no sense to try to seek a way where there is in effect no way. The EU’s White Papers entitled „European Community COM (2011) 144 Roadmap to a Single European Transport Area“, and „Conclusions on 2030 Climate and Energy Policy Framework (SN 79/14)“ set out a concise strategy for the development of transport and energy. An emphasis is placed on electrified rail networks. It would be folly to ignore these policy statements. It would be a supreme folly to hope that a miracle might turn up, allowing people to maintain their present mobility-based lifestyles. Miracles do not happen. It would be wiser to opt for independence from fossil fuels in developing our future energy and transport strategies, rather than to prolong our stubborn reliance on personal motorised mobility and non-renewable energy sources.