LIFE CYCLE ASSESSMENT OF FUEL CELLS ELECTRIC VEHICLES

In recent years, regarding the influence of the production processes and vehicles on the environment, new technical solutions for reducing air pollutions have been studied and developed. One of the new constructions is fuel cell electric vehicle (FCEV). The production and running conditions of the vehicles are specific in different countries. Hence, a study of these conditions and fuel production process is needed. In this paper, a study of the FCEV efficiency, at different producing technologies of hydrogen (H2), is carried out. Life cycle assessment (LCA) method is used. A comparison, concerning fuel consumption and emissions as CO2 equivalent for the whole life cycle, is done for FCEV and conventional gasoline vehicle (GV). The influence of the energy mix and technology of production of hydrogen on spent energy and air pollution is analyzed. As the results show, in countries with CO2 emissions over 447 g per 1 kWh electricity, the technology of hydrogen production from natural gas is most effective. Now and in the near future, the ecological and financial advantages, connected to renovation of existing vehicle fleet with FCEV, are not absolutely verified.


INTRODUCTION
During the past decade, life cycle assessment (LCA) has become a dominant methodology in research studies concerning sustainable development of a product [14]. LCA is applicable also for studying the influence of a production process on the environment. Existing research studies [2,18,26,29,21] about the effectiveness of fuel production and use in vehicles stimulate environment protection and support development in this area.
Production and use of FCEV as an alternative to conventional vehicles require an assessment of their advantages and disadvantages for the life cycle. FCEVs, like battery electric vehicles (BEV), do not generate air pollutions during the motion process. The main difference between these two types of vehicles is supply sources for the electric motor with electric energy. In FCEV, the electricity is produced in motion, from fuel cells (FC), by continuous supply of hydrogen (H2) and oxygen (O2). Produced electricity is used not only for motion but also for charging the electric battery at some regimes.
The fuel cell is an energy convertor, with a theoretical efficiency up to » 83% [7]. If all losses in auxiliary systems of the cell are taken into account, the real efficiency of electric vehicle fuel cells is approximately 40-50 %. This value is nearly as efficient as the diesel ICE [7].
The main properties of the gasoline, natural gas, and hydrogen are presented in tab. 1. The gasoline is produced at normal atmosphere conditions through distillation of crudе oil at temperature from 30 to 200 о С. The main stages of the process are shown in Fig. 2.
The maximal and minimal values of the specific burning heat of coal are accepted respectively as 25.86 and 27.16 MJ/kg. There are three basic methods for hydrogen production [1,2]: reforming of natural gas, gasification of coal, and electrolysis of water (Fig. 1). In the last decade, production of H2 from biomass has increased. It is generated by the industry and farms. Electrolysis through solid oxides electrodes (SOEC) is one possibility to produce hydrogen using renewable energy sources. The properties of the basic technologies are summarized in Tab. 2. *LHV, HHV -respectively low and high limit of the value

RESEARCH METHODOLOGY
The used LCA takes into account all processes, connected with the product (in our case fuel)from extraction of raw material, production process, use in vehicles, and it recycling (eventually) [14]. Schematic, the LCA for hydrogen and gasoline is presented in Fig. 1 and Fig. 2. The recycling includes all facilities used in the process.
In the conduction of energy analysis, the maximal value of specific burning heat (HHV) is used (Tab. 1). It corresponds better with real energy content of the fuel, based on the principal of energy saving.
Life cycle assessment of fuel cells electric vehicles 155.  The needed primary energy is analyzed only concerning production of Н2 and its compression up to 700 bar or its condensation. The same also concerns the environmental estimation.
A comparison between structure of FCEV ( Fig. 3) and conventional GV shows that they have one similar part of construction -chassis, which includes steering system, brake system, suspension, and body. Nevertheless, propulsion system and its components are very different, and for its production, the spent energy and generated emissions will be different values. Usually, the FCEV has approximately 20% bigger mass. In this study, it is assumed that energy spent for production of chassis of FCEV and GV is equal and consists of 11900 kWh [19].
For production of the FC and its management systems, the spent energy is approximately 15% more than for chassis of vehicles [24]. For this reason, it can be accepted that production of the FCEV uses 80% more energy and generates 80% more emissions than production of a GV. However, 100% of GV parts can be recycled, but for FC, this percentage is only 75% [24,33].
When the needed primary electric energy for vehicle production is determined, the structure of country energy mix is considered (Tab. 3). The efficiency of the used technologies for electricity production is also taken into consideration [11].
Life cycle assessment of fuel cells electric vehicles 157. Table 3 Structure of the electric energy production (mix) of the EU-28 countries and Norway [11,27]  The fuel consumption is determined on the basis of HHV of Н2 and gasoline, as well as efficiency of the ICE and FC. That way, equal energy is used for motion of the two type of vehicles with equal mass. Determination of the energy spent during exploitation of the GV and the losses concerning life cycle of the fuel are calculated, and this way the efficiency of gasoline production is evaluated as 79.6% [11,23,28]. Considering the expected trends in development of FC production technologies, a value of FC efficiency of 50% [4] is used in calculations.
Life cycle assessment of fuel cells electric vehicles 159. Table 5 Main technical data of some modern FCEV

ANALYSIS OF THE EXISTING TECHNOLOGIES FOR PRODUCTION, STORAGE, AND TRANSPORTATION OF HYDROGEN AND GASOLINE
The global annual production of Н2 is more than 50 million tones. The main part of whole production is from natural gas -48%, from refinery waste gasses -30%, from coal -18%, and other 4% from biomass and through electrolysis [2].
Efficiency of hydrogen production from natural gas is between 65% and 80% [13,17,21,23,30]. The СО2 emissions per 1 kg Н2 are 9,066-10,728 g. On the basis of HHV values of natural gas and hydrogen (Tab. 1), and taking into account technological losses, it is evaluated that for production of 1 kg Н2 the needed natural gas is 3,17 kg. For the life cycle of FCEV, production of the hydrogen will use 9 840 kg natural gas and 3 450 kWh electric energy. Production of 1 kg Н2 , using EU -28 mix, generates 12,13 kg СО2 emissions (Tab. 2), or for life cycle of the FCEV, the mass of the emissions will consist approximately of 37 640 kg.
Effectiveness of hydrogen production from coal varies between 50% and 80% [9,30], depending on technology and quality of used coal. The losses in production of coal are 5-20%, depending on exploitation conditions and place of the mine [3,5], and losses for transportation can reach up to 15% [20]. Hence, for life cycle of FCEV, production of hydrogen from coal, a value of effectiveness of 50% can be used [23]. The mass of the СО2 emissions consists of 24,2 kg per 1 kg Н2 [2].
Production of Н2 from biomass will have important place in the future, because it is a renewable source. Effectiveness of hydrogen production through gasification of dry biomass (like wood, straw etc.) is into the limits of 65,7-79,1%. Generated СО2 emissions are up to 13,5 g per 1 kg Н2. The wet waste from biomass (like sediments, organic waste etc.) can be put to gasification (effectiveness of 35.8-40.3%) or bio-chemical treatment (effectiveness of 29.1-36.3%) [22]. Generally, the effectiveness of hydrogen production from biomass is accepted as 65.7% [22].
Alkali electrolysis is known and used since the 18th century. It is in the basis of technology and more of commercial electrolyzers. The produced hydrogen is very pure, but the price is higher because of low price of petrol (used in SMR) in comparison with electricity. Low-temperature polymer electrolyte membrane (PEM) and high-temperature electrolyzers of solid oxides (SOE) are two more effective future technologies. РЕМ is appropriate for production of small volumes of hydrogen. SOE electrolyzers can reduce consumption of electricity using thermal cracking process [5,25].
The use of RES for supply industrial electrolysis [15] is very small -about 3%. The main cause is low efficiency. With electricity from photovoltaic power-plant in technology with efficiency of 10-25% will give a total efficiency of electrolysis from 7.8 to 18% [32]. With the same technology using electricity from solar PS and Sterling motor and generator, the total efficiency can be increased to 28% [32].
Solar PS using cycle of Rankine and technology of solar tower can achieve annual efficiency of 15% and total efficiency of electrolysis of 14% [32]. The solar PS with parabolic reflectors has annual efficiency of 12%, and total efficiency of transformation process of solar energy into hydrogen using electrolysis is 11% [32].
Transportation of the hydrogen is realized by pipes or in special tanks (as gas or as liquid) using vehicles and railway or marine transport. The cheapest method for large volumes of H2 is transportation as gas in pipes. The losses during transportation of hydrogen are significantly higher than analog losses for natural gas, because of short distance between the compression stations [35,36].
Charging of the hydrogen on FCEV is made in special hydrogen stations at pressure 700 bar (70 MPa). Usually one charge is enough for a range of 400-500 km.
Compression of the hydrogen needs about 3.5 times more energy [35,36] in comparison with natural gas at same pressure (Fig. 4). Fig. 5 shows a possibility for reducing the transport losses -if the hydrogen is liquid [36]. The transformation process of Н2 in liquid phase generates losses up to 40% [12,36]. In analysis done below are used values for losses equal to 15.5% (from HHV) for compression up to 700 bar and 33.33% for transformation process of Н2 in liquid phase. Transportation of Н2 in pipe generates less energy losses. Transportation of natural gas at a distance of 5 000 кm generates losses of 10% (Fig. 6). For Н2 transport losses are 35%, because of energy spent for supply of the compressors, placed at each 150 кm (generated losses of about 1.4%) [36]. Storage of liquid Н2 generates the highest losses -5.5 kg per day for a reservoir of 725 kg capacity, which is 0.76% per day [10]. There is a tendency in future to decrease storage losses to 5% per 10 days.

LCA FOR FCEV AND GV. RESULTS AND ANALYSIS
Using the aforementioned given information, an assessment of constant energy losses and generated emissions for FCEV and GV was done. Following diagrams from Fig. 1 and 2, the two models were described -for FCEV and GV. The primary energy spent for the life cycle of the FCEV was evaluated by the following model , where ai is the part of electricity produced in different types of power-plants (as part of whole produced energy); -efficiency of electricity transfer; -efficiency of different power-plants, including production technology and fuel transportation; -energy spent for production and recycling of the vehicle, kWh; -energy spent for production of H2 for life cycle of FCEV, ; -energy spent for compression or transformation in liquid phase of H2, kWh.
The generated emissions were evaluated by the expression (2) where is emission factor for production of electricity, (see last column in tab.4). For GV, used equations are as follows: where is energy spent for production of gasoline for life cycle of GV, ;  (1) and (2). The calculations were repeated 4 times -for conditions in Bulgaria, Poland, Norway, and corresponding to energy mix of EU-28. The results are presented on Fig. 7 and 8 and also in Tab. 6 and 7.
The technology for production of hydrogen from natural gas is most effective by criterion of spent primary energy. Using it, at some conditions, FCEV can be a competitor of GV. The technology of production of Н2 from coal and by electrolysis, at the current stage of development, is less effective concerning primary energy for life cycle of GV vehicle. Significant use of the RES in energy mix of the country can give advantage of the FCEV -for example Norway (Fig. 7).
By criterion emissions, the technology using natural gas for production of hydrogen has advantage once again. At the moment, other technologies are less ecological and their use less ecological in comparison with GV. Only in Norway, thanks to the large use of the RES in energy mix, the FCEV is more ecological. Energy mix, including basically thermal PS on coal, is a factor for bigger losses of energy during life cycle of the FCEV and more СО2 emissions -for example Poland and Bulgaria ( Fig. 7 and 8).
One better assessment of the three used technologies for production of hydrogen can be done on the basis of needed primary energy (Fig. 9) and generated emissions in СО2 equivalent per 1 km (Fig. 10).
For the life cycle of the GV are spent 309 750 kWh or 1,068 kWh/km at accepted range of 290 000 km. By this criterion, FCEV is a competitor to GV only in case of using compressed hydrogen. In Norway, FCEV is more effective as it consumes less primary energy -15.5 and 25% for liquid and compressed Н2, respectively. The electrolysis is the worst of the three technologies. The energy spent for life cycle of the FCEV, depending on energy mix of the country, can be over 2.5 times higher than respective for GV. For the life cycle of GV are generated 59 750 kg СО2 emissions or 0,206 kg/km.
Results (Tab. 7) show that the best ecological technology is electrolysis for countries using a large part of RES in its energy mix. For example, in Norway, FCEV will have 16 times less emissions (Fig. 8). The structure of energy mix has the most significant influence on production of hydrogen through electrolysis in countries with high level of emissions per 1 kWh electricity. Most ecological for these countries is technology for production of Н2 from natural gas. Produced and used in these countries, FCEV will have ecological disadvantages in comparison with GV, independent of used technology for production of hydrogen (Fig. 8).
-for production and recycling of the vehicles; -for production of Н2 and gasoline; -for compression of Н2; -for transformation to liquid Н2 -for production and recycling of the vehicles; -for production of fuels, and from its burning; -for compression of Н2; -for transformation to liquid Н2

CONCLUSIONS
In this paper, a study of the FCEV effectiveness, at different producing technologies of hydrogen (H2) was carried out. Using life cycle assessment, a comparison, concerning energy consumption and air pollutions for fuel cell electric vehicle and conventional gasoline vehicle, was done. The influence of the energy mix and technology of production of hydrogen on spent energy and air pollution was analyzed on the basis of statistical data.
The obtained results show that the technology of hydrogen production from natural gas is most effective in countries with CO2 emissions over 447 g per 1 kWh electricity.
The energy spent for life cycle of the FCEV, depending on energy mix of the country, can be over 2.5 times higher than the respective for GV. The most ecological technology for production of hydrogen is electrolysis for countries using a large part of renewable energy sources in its energy mix. For example, in Norway, FCEV will have 16 times less emissions than GV.
Positive ecological and financial positive effects from replacing the vehicle fleet with FCEV, at the moment and in near future, are not strongly proven.
The results of the present study have to be understood as one indicative simulation, which highlights positive and negative features of FCEV.