Report in reference to Trends in Sustainable Energy

lable at ScienceDirect

Vacuum 122 (2015) 369e375

Contents lists avai

Vacuum

journal homepage: www.elsevier.com/locate/vacuum

New frontiers in sustainable energy production and storage

J.M. Martinez-Duart a, *, J. Hernandez-Moro a, S. Serrano-Calle b, R. Gomez-Calvet c, M. Casanova-Molina d

a Applied Physics Department (C-XII), Autonoma University of Madrid and N. Cabrera Materials Institute, Madrid, Spain b School of Telecommunications, Polytechnic University, Madrid, Spain c Economics School, European University, Valencia, Spain d Physics Department, University of Valencia, Valencia, Spain

a r t i c l e i n f o

Article history: Received 30 October 2014 Received in revised form 6 April 2015 Accepted 23 May 2015 Available online 10 June 2015

Keywords: Sustainable energy Energy storage Climate change

* Corresponding author. Tel.: þ34 660446909. E-mail address: [email protected] (J.M. Mart

http://dx.doi.org/10.1016/j.vacuum.2015.05.027 0042-207X/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Evidently, one of the most effective ways to reduce CO2 emissions consists in the deployment of renewable energies, with the advantage of securing and expanding the energy supplies of a given country. The main problem that arises is due to the intermittent temporary character of many renewable resources like solar and wind. For this reason, it has been recently concluded that in case of high inte- gration of renewables into the distribution grid (more than about 30% of the electricity mix), the implementation of energy storage systems together with smart grids is necessary. Therefore, renewable energies for large scale power production in country would usually require the availability of a suffi- ciently large energy storage capacity. Unfortunately, current technologies can only provide energy storage to a very limited extent, and large R&D efforts will be needed to find adequate solutions. Due to these reasons, the European 2050 Energy Roadmap, should contemplate parallel development pro- grammes for energy storage technologies (batteries, power-to-gas, hydrogen, etc.), and the upgrading of distribution networks, including smart grid technologies. With respect to renewable energies, we would like to point out that, in addition to their sustainability with respect to the environment, we will also consider in this Conference their economic sustainability, or price of the generated electricity, since we are immersed in a globally competitive economy. Another main topic of this Conference deals with advanced materials for key enabling technologies for a sustainable development, as contemplated in the Horizon 2020 Program. Among the multiple examples of materials that are essential for the improve- ment of energy generation and efficiency, we will centre our interest in topics including solar cells, fuel cells, energy storage and electric vehicles.

© 2015 Elsevier Ltd. All rights reserved.

1. Energy utilization and climate change

During the last few decades the world has been involved in a transition from fossil fuels (carbon, oil, gas, etc.) towards renewable resources based on sustainable low-carbon technologies like solar, wind. The main reason for this transition is to avoid the large amounts of CO2 being emitted to the atmosphere which amounted globally to 34.6 gigatonnes in 2012 [1] and 35.3 gigatonnes in 2013 [2]. If the emissions continue to grow at the same rate as in the last decades, the increase in temperature by the end of this century could be as much as 5 �C or 6 �C. In addition, as recently pointed out by the Intergovernmental Panel on Climate Change (IPCC) [3], in

inez-Duart).

order to avoid a global warming greater than 2 �C by the year 2050, it would be necessary not to surpass the mark of 450 ppm in CO2 atmospheric concentration; however, at present we have already reached the 400 ppm level (May 2013). The IPCC has also docu- mented that in order not to reach a future global 2 �C increment, it would be required that the peak in annual emissions should occur not later than in the next 10e15 years.

In effect, according to the IEA Scenarios, from the representation in Fig. 1 of the annual CO2 emissions future evolution, it can be observed that, if the present trends in emissions continue, in the year 2050 they would almost exceed the actual values by about 50%, corresponding to a temperature increase close to 6 �C (upper curve 6D in the figure) [4]. However, if emissions are drastically reduced (lower curve 2D) as proposed by the Blue Map Scenario, the temperature increase would only be 2 �C. Between both curves of Fig. 1, the partial contributions to the reduction of emissions by

Fig. 1. Evolution (2011e2050) of CO2 emissions for the current situation (upper curve 6D), and for the Blue Map Scenario (lower curve 2D). The vertical axis indicates the contributions of the different sectors to the CO2 emissions abatement [4].

J.M. Martinez-Duart et al. / Vacuum 122 (2015) 369e375370

several technological sectors are indicated by several colored bands. Observe also in this figure that renewable energies can play a very important role (41%) in emissions abatement within the power sector. But also strong efforts to the decarbonization of the energy system are needed across many other sectors like transport (19%) and buildings (13%), which can contribute significantly [4]. As we have previously noted, even if we start now the strong limita- tions in emissions just mentioned, it will take still some years to turn down the slope of the lower curve (2D) of Fig. 1 and conse- quently, begin reduction of emissions.

The reduction of carbon emissions at the global scale can only be achieved if all major players in the world assume their responsibility for mitigating CO2 emissions. At present, international negotiations towards a global treaty on the reduction of CO2 emissions are taking place which should culminate in a new World Climate Change Agreement in Paris (December 2015) to be applied from 2020 on- wards [5]. The Agreement will contain a set of commitments which will be applicable to all countries and implemented as national laws. In addition, the Agreement should contemplate the recent recom- mendations from the 5th Assessment Report of the IPCC [3]. Among all major countries in the world, the role that will played by the United States and China will be crucial. At present the top contam- inating country in the world is China with emissions in 2013 almost

Fig. 2. Evolution (2000e2013) of the global cumulative i

doubling those of the United States which comes second. However, if one looks at CO2 emissions per capita in 2013, the United States (21 tonnes/cap), Australia (18 tonnes/cap), Canada (17 tonnes/cap) are more than double those of China (7.4 tonnes/cap) [2]. However, on the positive side we would like to remark that the emissions per unit of gross domestic product, or carbon intensity, are diminishing at a fairly high rate in most places. As a consequence, in the case of China for instance, during the last years the rate of growth of emissions has been continuously decreasing and it is planned that in the near future this tendency will continue at a higher pace [2].

2. Past and present status of renewable resources for power generation

First, we would like to point out the tremendous growth of the main renewable resources for power generation during the last decade as shown in Fig. 2. Thus, in 2013 alone, 35 GW of wind power [6] and 40 GW of solar photovoltaics (PV) [7] were installed throughout the world, reaching the cumulative installed power the values of 319 GW and 140 GW, respectively, at the end of this year. To put these numbers into context, let us observe that in the case of PV, for instance, in order to reach the values predicted by the IEA Scenarios for the decade 2030e2040, the corresponding cumula- tive power increase will have to reach more than several thousand GWs, and therefore enter the terawatt scale (1 TW ¼ 1000 GW) [4].

To better appreciate the past growth of PV, we have plotted in the left part of Fig. 3 the values of the PV cumulative installed po- wer Q(t) since the mid-1970s, when the technique was already well established, up to the end of 2012 [8]. Evidently, the plot has to be represented in a log scale since the values of Q(t) have increased during this period by more than four orders of magnitude until it has recently (2013) reached a global value of about 140 GW [7]. One interesting observation from Fig. 3 is that, after a further future increase of one additional order of magnitude, Q(t) will enter the terawatt (TW) scale, as predicted by the IEA Blue Map Scenario for 2050 [9,10]. It should also be remarked that for a technique to play a significant role in the electricity mix, at the global scale, the cor- responding power capacity should be of the order of a few TW. Finally, we would like to observe with the help of Fig. 3 that to reach a value of this magnitude, the annual growth rate of Q(t) from now to 2050 would only need to be on the average of about 7%, in contrast with previous growths of about 30e40% in the past. Note also that the proposed curve plotted in Fig. 3 after 2012 approaches

nstalled capacity for wind power and solar PV [6,7].

Fig. 3. PV Cumulative installed capacity for the period 1976e2012, left side, and predicted values to 2050, right side. (See text for references).

Fig. 4. Roadmap (2050) European Union CO2 emissions towards an 80% reduction with respect to the year 1990 [11,12].

J.M. Martinez-Duart et al. / Vacuum 122 (2015) 369e375 371

the year 2050 with a rather small slope since, otherwise, Q(t) would reach completely unrealistic values during the decades after 2050.

3. Energy scenarios and the European Union 2050 Energy Roadmap

As we have seen, there are several scenarios elaborated by the IEA and the IPCC in order to plan the future energetic needs that take into account the environmental impacts as a consequence of the energy consumption. From our point of view, the most frequently studied of all these scenarios is the so-called the 2DS Blue Map Scenario [9,10]. The main objective of this Scenario is to limit the global temperature increase to 2 �C until the year 2050, for which it would be necessary that the atmospheric CO2 concentra- tion does not surpass the 450 ppm limit. For this objective, emis- sions will have to be reduced by about 50% (see Fig. 1) of their value in the reference year 2005 and one way to accomplish it could be by a decided substitution of traditional power technologies by renewable ones. Therefore, according to this scenario, in 2050 the proposed contributions of solar and wind renewables to the power mix should be around 6% (PV), 5% (CSP) and 12% (wind) [9].

Recently, the European Commission has published its Energy Roadmap 2050 [11,12], in order to develop a long-term framework for the implementation of a set of objectives leading to a compet- itive low-carbon economy in 2050. One of the main targets of the Energy Roadmap 2050 is the reduction of greenhouse gas (GHG) emissions of 80e95% by 2050 according to the recommendations of the IPCC for developed countries, while the world's global emission reductions would only be about 50%. Central to the European En- ergy Roadmap 2050 are the emission reduction milestones which are represented in Fig. 4, together with the evolution of the emis- sions by sectors: power, transport, industry, etc. Notice also in Fig. 4 that the upper curve also shows the evolution of emissions under the current policies, i. e. without an extra effort in reducing them.

It can also be observed from Fig. 4 that the European power (electricity) sector is the one that can contribute the most to CO2 abatement since currently it is the one with highest emissions, but it will be the one with the least emissions in 2050. This implies a reduction of 90e95% of its present value, a very ambitious target

needing strong and rapid additional measures to accelerate the emissions abatement [11,12].

As renewable energies are continuously being incorporated into the power systems, the question of centralized versus distributed energy systems is currently under strong debate [13]. In this respect, most authoritative opinions convey that we will continue to move towards a distributed energy future, but this cannot meet all needs. In California, for instance, energy planners admit that in about one or two decades most residential and commercial spaces will get their power from distributed systems in a significant percentage [13] and for this the contribution of smart grids and energy storage (not yet economically profitable) will be necessary. However, there will still be the necessity of large and fast amounts of power to be supplied to large cities. For instance, it will be practically impossible for cities with populations of several million inhabitants, and little free communal space, to satisfy their power needs with only distributed energy, and, therefore, centralized generation will be also needed for the proper balance of demand and supply.

4. The levelized cost of electricity and its evolution. The case of photovoltaic electricity

The calculation of the Levelized Cost Of Electricity, (LCOE) is the method most frequently used when comparing power generation technologies and for evaluating the economic feasibility of an electric generation project. The calculation of the LCOE is based on the equivalence of the so-called present value of the sum of the discounted revenues and the present value of the sum of dis- counted costs (for details see Refs. [14e16]). In order to determine the future evolution of the LCOE for PV systems in the period 2012e2050 we use a model previously proposed by us [14]. This model is based on the discounted cash flow (DCF) economic tech- niques and the experience curves approach, and estimates the cost during the whole lifetime of the system. In the following equations for the calculation of LCOEs, all costs are given in 2012 US$, so that they are not distorted by inflation rates. According to this model, the LCOE in a given future year t is given by Ref. [14]:

LCOEðtÞ ¼ CðtÞ þ L þ PN

n¼1 � ðO&M þ IÞ·CðtÞ

� ð1 þ rÞn

�

PN n¼1

� S·TF·h·ð1 � dÞn

� ð1 þ rÞn

� (1)

In this equation, the cost of the system for a year t, C(t), is given by:

CðtÞ ¼ Cð0Þ·ðQðtÞ=Qð0ÞÞLogð1�LRÞ=Logð2Þ (2)

where C(0) is the initial cost of the system, i.e. in 2012, Q(t) is the cumulative installed capacity, and Q(0) is the value of Q(t) in 2012.

Fig. 5. LCOE evolution for PV systems for different solar resources (kWh/m2/yr) for the IEA Blue Map (left) and the IEA Roadmap Scenarios (right) [14].

J.M. Martinez-Duart et al. / Vacuum 122 (2015) 369e375372

LR is the learning rate, which according to the learning curve approach indicates the cost reduction per cumulative doubling of installed capacity, and taken as 18% for PV systems; L is the land costs; O&M and I are the operation and management costs, and the insurance costs, respectively, expressed as a percentage of the cost of the system; N is the expected lifetime of the system, (considered normally as 30 years for PV systems); S represents the solar resource, or the irradiation on a fixed optimally tilted module surface (units of kWh/m2/yr.); TF is the tracking factor; d the annual output degradation rate of the systems; and, finally, h is the so-called performance factor of the PV plant. Finally, r is the discount rate used in LCOE calculations, which reflects the return on the capital for an investor in the absence of specific market or technology risks (IEA 2010b). The specific values of the parame- ters in equations (1)e(2) are clearly specified in previous work [14e16].

In order to proceed with the calculation of the PV electricity cost evolution for the period (2012e2050) we assume in equations (1) and (2) the values provided by the IEA for the cumulative installed capacity, Q in MW, for some targeted years (2020, 2030, etc.), and for the cases of the IEA Blue Map 2DS Scenario, and the IEA Roadmap Scenario (for more details see Refs. [14e16]). Following this procedure, we represent in Fig. 5, for the period 2013e2050, the LCOE cost evolution for the two IEA scenarios and for several solar resources [14]. The global irradiations of 1300, 1800, and 2300 kWh/m2/yr correspond, respectively, to locations in Germany, central Spain, and California. For these representations, we have used a representative value of the discount rate of 5%. We would like also to remark that the progressive and sustained (until 2050) cost reductions in PV electricity shown in Fig. 5 are really a consequence of the high learning rate (18%) corresponding to PV modules. It can also be observed that the cost reductions are somewhat larger for the Roadmap Scenario in relation to the Blue

Fig. 6. World electricity capacity storage in 2012 represents 141 GW. Observe than less than

Map, the reason being the more ambitious targets in PV deploy- ment until 2050, which amount to 10% and 6% of PV in the 2050 electricity mix, respectively.

5. Energy storage

As the percentage of electricity from renewable intermittent sources gets above some 30%, the problem of integrating this var- iable electricity supply into the distribution grid rises very signifi- cantly [17]. Therefore, the large integration of renewable sources demands some combination of efficient and flexible backup plants, large energy storage systems, smart grids etc. and consequently the cost of electricity can further increase [18]. On top of this, base load power plants (for instance, coal fired or nuclear) will usually be needed to satisfy the required demand. In this context, the display of storage systems can reduce the curtailment of intermittent sources (wind, solar, etc.), provide grid stability, and enable to hourly shift the consumption of renewables for demand manage- ment. This is especially the case in countries like Germany which has plans to increase the contribution of renewables in the elec- tricity mix to 50% in 2030 and 80% in 2050 [4].

Lately, it has been shown that small-medium PV solar and wind systems combined with electricity storage find numerous applica- tions in off-grid uses like access to electricity in remote areas. When demand exceeds supply, storage can be used as backup; on the contrary, when the output of variable renewable energies genera- tion exceeds demand, storage can reduce curtailment. Storage is especially useful for solar applications in countries like Germany where capacity factors of power PV plants in 2012 were only about 10% [4].

As shown in Fig. 6, electricity storage capacity only represents a very small portion of about 3% of all the total capacity generation (5250 GW). Of this amount (141 GW), most of the electricity storage

1% of this amount corresponds to technologies different from hydro-pumped storage.

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(99.3%) is based in hydro-pumped storage (HPS), electrochemical storage representing less than 1% [4]. HPS has been traditionally used so far for large electricity storage, especially in countries with high reliefs, because of its efficiency and simplicity. For instance this is the preferred technique in the case of nuclear plants when the supply exceeds the demand.

As we have seen above, solar and wind renewables grew very quickly in the last few years, and according to the IEA 2DS Blue Map Scenario it is expected that in 2050 they would represent almost one fourth of the total electricity generation [4]. It should be observed that power operators have a notable experience in demand-side variability but not in supply-side variability, as it is necessary in the case of non-dispatchable renewable energies. For this reason, storage can make dispatchability much more effective by reducing curtailment periods of low demand, and therefore increasing the capacity factors of renewable systems. As a consequence, the CO2 emissions can be substantially reduced.

For energy storage applications in the case of electrical devices, there are different types of batteries, capacitors, fuel cells, etc. that can be used [4]. One of the main parameters to characterize these devices are the energy stored per unit weight, and also the power output that they can supply, as indicated in Fig. 7. In the case of the batteries, the type lithium-ion constitutes the dominant power source for most rechargeable devices. The electrochemical ca- pacitors, although they are very limited for energy storage, they can supply large amounts of power for short periods of time. In ultracapacitors, the energy is stored in a double layer at the sur- face of the electrode material. These devices are also called supercapacitors for the large amount of energy they can store in comparison with regular capacitors. Lastly, we would like to remark, that hydrogen can also be used as an energy source, for instance in automoviles by feeding it to fuel cells as we will see again in Sec. 8.

Currently, some of the most common thermal storage technol- ogies are based in the thermodynamic s ensible heat of molten nitrate salts. However these systems show several problems like freezing of the salts, and, at the other extreme, they cannot operate at temperatures higher than about 560 �C [19]. Therefore, at pre- sent thermal storage in solids is being investigated: concrete, graphite, phase-change materials (PCM), etc. Usually a combination of these materials is sought in order to get materials that combine high thermal conductivity together with high specific heat.

Fig. 7. Typical power density and energy density values for batteries, capacitors, and fuel cells.

6. Power-to-Gas

During the last few years, the technique known as “Power-To- Gas (P2G)” is emerging as one of the most interesting for the storage and distribution of large amounts of energy generated from intermittent and highly variable sources like solar and wind [20]. The P2G technology is based on the transformation of surplus intermittent electricity (solar, wind, etc.) into combustible gases (hydrogen, methane, etc.), which can be stored and transported through the existing natural gas distribution infrastructures (Fig. 8). The two main proposed P2G transformation techniques are: hydrogen production by electrolysis, and methane production from CO2 and hydrogen. It will probably be needed at least around a decade for P2G to become a reality [20] and a good push could come from both, a high penetration of renewable resources into the traditional energy system, and the development of efficient elec- trolyser (fuel cells) technologies.

7. Carbon dioxide recovery for sustainable development

Due to the increase of the world's population during the last decades, and the rising energy demands in countries like China and India, the primary energy consumption in the world has already reached the extraordinary value of approximately 500 EJ, where 1 exajoule equals 1018 J. Since most of the energy consumed has its origin in fossil fuels, the CO2 yearly emissions amounted to about 35.3 billion tons in 2013 [2]. The bad news is that there has been a growth in CO2 emissions of about 50% in the last 20 years, and, as a consequence, the C-atmospheric concentration has experimented a growth of about 10% during the same period already reaching the 400 ppm record recently. Evidently, it would have not been for the strong growth of renewables, especially solar and wind, the current CO2 atmospheric concentration would be still higher.

Since it is practically impossible to put all countries in agree- ment to drastically reduce future emissions during the next de- cades, it has been recently proposed to copy nature, and instead of considering the CO2 as the end-point of the fuel cycle, let us utilize it as a surplus energy, in the same way as plants do. But, how it can be done? An interesting answer that is now being considered is the following: let us capture the CO2 and add hydrogen, which would react according to the Sabatier Process, (CO2þ4H2 ¼ CH4þ2H2O), and produce methane (or natural gas) [20]. Evidently, other similar processes to the Sabatier reaction can be used, like the electrolysis of water for the synthesis of methane, in whose case the electricity could be obtained from renewable energies like solar or wind. Subsequently, the obtained natural gas can be used directly as fuel in transportation vehicles, or, alternatively, could be applied to the production of syngas by means of the steam-reforming reaction: CH4þH2O ¼ COþ3H2.

8. Green fuels for future cars

Hydrogen fuel cells vehicles (HFCV) are currently in an advanced stage of development with joint plans by BMW and Toyota to enter the market as early as in 2015; the type of fuel cells are usually of hydrogen fed proton-exchange membrane (PEMFC). Probably spurred by the huge car industry, PEMFCs have shown an extraordinary technology development and a large cost reduction during the last decade decreasing from a cost of 275 $/kW in 2002 to about 42 $/kW in 2013 and with a target before the end of this decade of 30 $/kW [21]. However, it is not probable that these cars will represent a big share of the market in the short-medium term because of the high fueling costs (hydrogen) and a lack of fuel- delivery infrastructures [22]. Recently, the improvements in the reliability, performance and lifetime of the PEMFCs have been

Fig. 8. Production of methane from renewable solar energy and further integrated into the natural gas network [20].

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extraordinary, showing currently target lives around 6000 h. Evidently, the production of HFCV vehicles will strengthen the implementation of renewable energies, especially wind and solar, for the synthesis of hydrogen by electrolysis [4].

CO2 can also be used as a raw material by making it to react with hydrogen to produce methane as fuel for vehicles. The hydrogen can be obtained from excess intermittent renewable energies (wind and solar). Then, the methane can be stored and distributed through the existing natural gas infrastructure. Audi has already built a 6 MW plant in Werite (Germany) to produce first hydrogen by the electrolysis of water, and then making the hydrogen to react with CO2 to produce the methane and water as output. As Audi has proven, cars using this type of fuel show very low C-emissions.

9. Advanced materials for low-carbon energy technologies

Another main topic of this Conference deals with advanced materials for key enabling technologies for a sustainable develop- ment and for strengthening innovation in Europe, as contemplated in the Horizon 2020 Program. Among the multiple examples of materials that are essential for the improvement of energy gener- ation and efficiency, we will centre our interest in solar cells, solar thermoelectricity, wind energy, fuel cells, materials for fusion re- actors, etc. In all these applications, we will emphasize the roles of surface engineering technologies, thin-films, hard coatings, and nanotechnologies.

The industry of photovoltaic solar cells is still predominantly based on crystalline silicon but there is a share of about 10% of thin- film second generation (2G) cells, like those based in CdTe, with lower prices since they can make use of large-area deposition techniques and low-cost substrates. In addition and in order to overcome the Shockley-Queisser limit for single-junction devices, there is a lot of research on third generation (3G) solar cells [8]. At present the concept of 3G cells is not clearly defined and it even includes non-semiconductor (for instance, polymers); therefore we

will not try to mention all those 3G solar cells considered by some authors as such. In our opinion, some illustrative 3G cells are rep- resented by the following types: quantum-dot, intermediate band gap, multi-junctions, hot-carriers, organic, dye-sensitized, perov- skite cells, etc. The quantum-dot cells often comprise semi- conductor nanostructures of different sizes, and consequently bandgaps, enabling in principle the absorption of practically the whole solar spectrum. Another interesting type of 3G solar cells is based on the intermediate semiconductor bandgap concept, ac- cording to which the absorption of photons with energies lower than the bandgap is allowed by means of a two-step photon process [23]. More recently, perovskite solar cells have drawn much attention since their efficiency has enormously increased in a few years from 4% in 2009 to more than 15% in 2013 [24] and besides they can be manufactured using simple wet chemistry techniques.

In the case of materials for solar thermoelectric direct energy conversion, the characteristic parameter is the so-called figure of merit ZT, which is proportional to the Seebeck coefficient and the electrical conductivity and inversely proportional to the phonon thermal conductivity [25]. It is also interesting to remark that in the case of nanomaterials, the values of ZT are in some cases twice as large as for bulk materials, the reason being the quantum- confinement enhancement of the Seebeck effect, together with the reduced thermal conductivity due to interface scattering.

As for materials for wind energy, some of the most important topics being investigated are: development of polymer-matrix composite materials reinforced with fiberglass or graphite fibers, powerful magnets made from rare earth materials for the turbine electrical generators, special alloys for the gearboxes in order to accommodate a wide range of wind speeds, the development of new coatings for improved corrosion resistance, and the progress in high temperature superconductors (HTS) generators, etc. [10].

In materials for smart grids, one future technology trend is to integrate sensors in the electricity distribution grids, to provide information and control capabilities to optimize grid operation and

J.M. Martinez-Duart et al. / Vacuum 122 (2015) 369e375 375

management of power flows [26]. There is also activity on the integration of micro-energy-storage, which can be used as distributed generation sources to help managing peak demand and supply. For transmission cables, the trend is the use of composite materials based on aluminium and, therefore, to offer better per- formance in terms of amperage capacity and temperature. In this field, also work is being conducted on developing advanced power conductors, as high temperature superconductors, which can ensure that electrical systems can respond more quickly to changes in operation, thus benefiting control systems [26].

The research in improved materials for fuel cells is nowadays very active in the following areas [27]: (i) In Proton Exchange Membrane Fuel Cells (PEMFC), the technology efforts are mainly devoted to the development of new catalysts for protons, electrons and oxygen to form water, which would replace the current high- cost platinum-based electrodes by cobalt and chromium Pt alloys. (ii) As for Solid Oxide Fuel Cells (SOFC), the research activities are mainly focused on trying to develop alternative materials to zir- conia oxide membranes showing higher electrical conductivities, so that they can operate at lower temperatures (at around 500 �C), instead of the current 1000 �C. (iii) The technology efforts in Molten Carbonate Fuel Cells (MCFC) focus primarily on the development of corrosion-resistant materials to increase the lifetimes of the fuel cells. The materials must withstand a minimum of 40,000 h at 650 �C in the presence of a molten salt in oxidizing or reducing environments.

Large-scale electricity storage is expected to play a significant role in balancing supply-demand in variable renewable energy systems. At present, the most mature battery technologies are the lead-acid (LA), sodium-sulphur (NaS), vanadium redox flow (VR), and the lithium-ion (Li-ion). Among them, during the last decade, the Li-ion batteries have been the most investigated, due also to their potentiality to improve their storage capabilities, allowing to increase their actual energy density of 300 Wh/kg to doubling this amount [28]. At present, the Li-ion batteries use a liquid electrolyte, but the research to use an all-solid-state electrolyte is well advanced and Toyota is already announcing that their electric ve- hicles for the 2020s will incorporate this kind of batteries [29]. One virtue of these batteries will be the elimination of the fire risk associated to the flammable liquid electrolyte.

In magnetic confinement nuclear fusion, the major techno- logical efforts are focused on the construction of ITER, which should be completed in 2018 [27,30]. The ITER roadmap considers that in the decade 2030e2040 the first commercial nuclear reactor should be in operation. The Tokamak concept includes the development of a spherical torus to try to improve the plasma confinement stability. Specifically, research is needed to design and produce new mate- rials to support the intense irradiation generated during the reactor operation, especially new types of stainless steel. Also more expe- rience is needed to control the handling of the very corrosive flow of liquid lithium within the reactor, as well as the synthesis process of tritium from neutron capture.

We would like to end this section on advanced materials with a reference to the very interesting rare earth elements or lantha- nides (La, Nd, Er, Y, etc), which are finding today multiple appli- cations in electrical devices for energy applications [31]: high- strength permanent magnets, advanced electric motors for wind turbine generators, high-temperature superconductors (LaeBaeCueO). The undersupply is increasing for the heavy rare earths like neodymium and dysprosium at least until the year 2025. In addition, lanthanides have very important applications as

catalysts, hydrogen storage media (LaNi5), light elements for avia- tion and aerospace, etc.

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