POINTOFVIEW Future Vehicles Will Be Driven by Electricity, But Not as You Think By SEBASTIAN VERHELST Department of Flow, Heat and Combustion Mechanics, Ghent University, Gent, Belgium features of the gasoline and diesel engine cycles. In recent years, the pace seems to have picked up: around 2007, biofuels were in the picture (mainly ethanol); in 2009, EVs again came on stage, this time joined by plug-in hybrids and range extended EVs. Most recently, with increased shale gas development, natural gas vehicles (NGVs) have been on the rise. Looking at the situation today, it seems very difficult to pick the most likely candidate to emerge from this multitude of options. The very fact there are so many alternatives is easily explained given the complexity of the problem. We are looking for transportation to decrease energy use, greenhouse gas emissions, and pollutant emissions, and increase energy security, while remaining affordable and not compromising on driving range, interior space, and comfort. Furthermore, what is fre- quently forgotten is that any alterna- tive has to be scalable. Not only have we passed the 1 billion vehicle mark he debate on what will be the car of the future has been ongoing in2010,butalso,withpresent for many years now and only seems to intensify. Throughout the growth rates, this number is expected years, there have been many views, some would say hypes, on what to double by the early 2020s, with would be the most likely candidate technology enabling sustain- the BRICS1 countries taking the Table transportation. The early 1990s saw the end of the large-scale fleet trials of lion’s share of this growth. As we methanol-fueled vehicles, followed by the introduction of electric vehicles will discuss below, scalability has a (EVs) some years later. The new millennium started off with high hopes for the number of implications. hydrogen fuel cell vehicle (FCV). Homogeneous charge compression ignition (HCCI) engines were going to save the day a few years later, combining the best 1The acronym for five major emerging national economies: Brazil, Russia, India, China, Digital Object Identifier: 10.1109/JPROC.2014.2351191 and South Africa. 0018-9219 Ó 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. Vol. 102, No. 10, October 2014 | Proceedings of the IEEE 1399 Point of View So, given the above, what we set out to answer here is whether we can find out what would be the energy carrier for transportation in the future, and which type of powertrain will be used. We will test every solution we come up with at least by two criteria: sustainability and scalability. I. WHICH ENERGY SOURCE? The first question is how we will power our future society in general. This has to be answered before we Fig. 1. An integrated power, heat, and transport system combining renewable sources with narrow this down to transportation. synthesized fuels [7]. Luckily, Abbott has done an excellent job in addressing this question [1]. Comparing the potential of the energy sources at our disposal to the world- III. WHICH ENERGY to rely on one of these two energy wide energy needs, only solar energy BUFFER? carriers to power vehicles. Going to is shown to have the potential to As reflected by a special issue of the battery electric vehicles or hydrogen deliver these energy demands sustain- Proceedings of the IEEE [2], the fuel cell vehicles thus seems like the way ably, with enough excess capacity to intermittency of renewable energy to go. Again, the scale of the problem has cope with the increasing energy sources such as solar, hydro, and to be taken into account, in this case the demands. Other sources can contrib- wind is a major hurdle to their large- total number of vehicles worldwide. If ute(e.g.,windandhydro),butas scale use. The potential of these we want to equip such a number of these are in fact just very diluted sources can only fully be used when vehicles with batteries or fuel cells, to forms of solar energy, their potential there is a solution for buffering energy power electric drives, once more we is at least two orders of magnitude on a massive scale. One of the come up against the limits formed by the lowerthansolar.Inasustainable possibilities is to use hydrogen as an reserves of certain raw materials need- society, the base energy demand thus energy buffer.2 The most sustainable ed. Ramping up the production of hastobemetbysolarenergy. and scalable way for hydrogen pro- electric vehicles, for instance, has been duction is to split water using alkaline reported to be constrained, if not by electrolysis, when there is an excess of lithium resources [1] then by the II. WHICH TECHNOLOGY renewable energy. Converting hydro- availability of certain rare earth ele- TO CAPTURE THE gen back to electricity can then cover ments [3]. Hydrogen fuel cells need ENERGY? theperiodwhenenergydemandout- platinum as catalyst material, again Different possibilities exist to capture weighs renewable energy production preventing them from being a truly solar energy and convert it into an or the hydrogen can serve directly or scalable option. energy carrier. To select the most indirectly (see below) as an energy From the viewpoint of scalability, appropriate technology, we have to carrier for transportation. the tried and tested internal combus- look at the scale of the task. Again, tion engine (ICE) is a much more following Abbott’s reasoning [1], solar sustainable alternative. ICEs are made thermal plants emerge as the method IV. WHICH ENERGY from abundantly available and more- that is most easily scaled up. With CARRIER TO POWER over recyclable materials. Conse- TRANSPORTATION, AND quently, they are also cheap to solar thermal, solar energy is concen- 3 trated to generate heat, which is used WHICH POWERTRAIN? produce. Of course, in a sustainable to produce steam, which in turn As our sustainable society drafted above drives a steam turbine coupled to a relies on solar energy to produce 3 generator to produce electricity. The electricity, and buffers the energy using In fact, as they rely on a cyclic process in which peak temperatures only occur during a scalability stems from the relative hydrogen, the most logical way of brief part of the cycle, with cooling periods in simplicity of the technology and providing transportation means seems between, they can use cheap materials and still from the fact that it does not rely on get up to high peak temperatures, allowing relatively high peak efficiencies. Thus, they are raw materials of which reserves are 2Other options are detailed in the special probably the technology with the best ratio of limited (in contrast to photovoltaic). issue dedicated to energy storage [2]. energy efficiency to cost. 1400 Proceedings of the IEEE |Vol.102,No.10,October2014 Point of View society they can no longer rely on oil V. SO, AGAIN: WHICH well as oxygenates are the most inter- derivatives as fuel. However, ICEs ENERGY CARRIER? esting options. have the added advantage of being Ahlgren [6] concurs that a sus- fuel flexible and can also operate on, We have now determined the three tainable society needs renewable, for example, hydrogen. main criteria that should be met by liquid fuels. He advances two fuels, Hydrogen turns out to be an any energy carrier to be used in one based on nitrogen, a ‘‘nitrofuel,’’ attractive engine fuel, as its proper- transportation, before it can be eval- with ammonia serving as example and ties allow hydrogen engines to be uated against other criteria. one based on carbon, a ‘‘carbofuel,’’ operated at peak efficiencies not too 1) It should be sustainable, i.e., with methanol as illustration. The far off of practical fuel cell systems rely on an infinite energy nitrogen-based fuel is proposed be- [4], with ultralow emissions, for a supply and make use of a cause of the relative abundance of fraction of the cost of fuel cell closed cycle of resources, but nitrogen in the atmosphere and the systems and with proven reliability. that in itself is not enough, fact it is carbon free. This latter Naturally, upon combustion, water because of 2). argument is somewhat surprising, as vaporisformedsothatasustainable 2) It should be scalable, i.e., it wearenowtalkingaboutsynthesizing cycle results. As vehicles powered by should use abundantly avail- fuels from abundantly available build- hydrogen ICEs seem to meet the able and thus cheap resources. ing blocks, which are liberated again sustainability and scalability criteria, 3) It should be compact, i.e., when the fuels are used. Thus, a Abbott advanced them as the solu- offer high energy and power closed carbon cycle can be formed and tion for meeting our transportation density the fact that the fuel contains carbon is demands [1]. We believe these criteria are met no longer an issue. As Wallington et al. However, keep in mind that we by renewable, liquid fuels. These can pointed out [5], it is no surprise that are talking about transportation here, be synthesized using renewable elec- we are now primarily relying on i.e., vehicles, having to carry the tricity. Liquid fuels are a very efficient hydrocarbons to fuel our society; this energy onboard. Thus, next to sus- way of storing energy and can thus be can be concluded from first (chemi- tainability and scalability, energy handled, distributed, and stored in a cal) principles: these compounds density is crucial and, unfortunately, practical and affordable way. offer the best features desired from a this is where hydrogen fails. Even when Thequestionnowis:whichliquid fuel. Ammonia may offer an accept- highly pressurized (with 700 barV fuel? We believe simple molecules are able energy density and thus appears 10.000 psiVtanks currently most to be preferred, as their production is to be an attractive energy carrier, but favored) or liquefied (requiring tem- more efficient, requiring less proces- that does not make it a good fuel.