4.3.4 Energy carriers
Energy carriers include electricity and heat as well as solid, liquid and gaseous fuels. They occupy intermediate steps in the energy-supply chain between primary sources and end-use applications. An energy carrier is thus a transmitter of energy. For reasons of both convenience and economy, energy carriers have shown a continual shift from solids to liquids and more recently from liquids to gases (WEC, 2004b), a trend that is expected to continue. At present, about one third of final energy carriers reach consumers in solid form (as coal and biomass, which are the primary cause of many local, regional and indoor air-pollution problems associated with traditional domestic uses); one third in liquid form (consisting primarily of oil products used in transportation); and one third through distribution grids in the form of electricity and gas. The share of all grid-oriented energy carriers could increase to about one half of all consumer energy by 2100.
New energy carriers such as hydrogen (Section 18.104.22.168) will only begin to make an impact around 2050, whereas the development of smaller scale decentralized energy systems and micro-grids (Section 4.3.8) could occur much sooner (Datta et al., 2002; IEA, 2004d). Technology issues surrounding energy carriers involve the conversion of primary to secondary energy, transporting the secondary energy, in some cases storing it prior to use, and converting it to useful end-use applications (Figure 4.17).
Where a conversion process transforms primary energy near the source of production (e.g. passive solar heating) a carrier is not involved. In other cases, such as natural gas or woody biomass, the primary-energy source also becomes the carrier and also stores the energy. Over long distances, the primary transportation technologies for gaseous and liquid materials are pipelines, shipping tankers and road tankers; for solids they are rail wagons, boats and trucks, and for electricity wire conductors. Heat can also be stored but is normally transmitted over only short distances of 1–2 km.
Each energy-conversion step in the supply chain invokes additional costs for capital investment in equipment, energy losses and carbon emissions. These directly affect the ability of an energy path to compete in the marketplace. The final benefit/cost calculus ultimately determines market penetration of an energy carrier and hence the associated energy source and end-use technology.
Hydrocarbon substances produced from fossil fuels and biomass are utilized widely as energy carriers in solid, slurry, liquid or gaseous forms (Table 4.3). Coal, oil, natural gas and biomass can be used to produce a variety of synthetic liquids and gases for transport fuels, industrial processes and domestic heating and cooking, including petroleum products refined from crude oil. Liquid hydrocarbons have relatively high energy densities that are superior for transport and storage properties.
Table 4.3: Energy carriers of hydrocarbon substances.
|Primary energy ||Energy carriers of secondary energy |
|Solid ||Slurry ||Liquid ||Gas |
|Coal ||Pulverized coal Coke ||Coal/water mix Coal/oil mix ||Coal to liquid (CTL) Synthetic fuel ||Coal gas Producer gas Blast furnace gas Water gas Gasified fuel Hydrogen |
|Oil || || ||Oil refinery products ||Oil gas Synthetic gas Hydrogen |
|Natural gas || || ||LNG, LPG Gas to liquid (GTL) GTL alcoholics Di-methyl ethers ||Methane Hydrogen |
|Biomass ||Wood residues Energy crops Refuse derived fuel (RDF) || ||Methanol Ethanol Biodiesel esters Di-methyl ethers ||Methane Producer gas Hydrogen |