Sources of electricity – oil

Oil or petroleum was once a key player in the electricity sector, but now it is used only marginally, usually as back-up reserve in diesel generators for major consumers, such as factories, hospitals, airports or as an electricity source in islands (for example in Greece).

Oil lost its share because of price, it is far more expensive to burn oil than burning coal or gas.

Merits of oil include high energy density, easy to transport and very stable composition, remaining liquid in most climatic conditions. Drawbacks of oil are price are environmental concerns (Webber, 2014).

Sources of electricity – biofuels

Biofuels are mostly used to replace oil as fuel in internal combustion engines, such as corn and cellulosic ethanol, jatropha, cyanobacteria, diatoms and green algae. However, some are used to create electricity, mainly from biomass (Mayfield, 2015).

Biomass uses mainly waste biomass gasification to produce electricity. Waste biomass could be poplar trees or tall grasses, but also agricultural waste (almond shells, corn stover), forest clearings and municipal solid waste. All this is cellulosic biomass, which has strong molecular connections, therefore strong forces need to be used to extract energy, such as heat, steam or acid.

The most used technique uses heating. In large vessels (called fluidized bed gasifiers) steam is pumped (because the reaction is endothermic, needs energy input, in the form of heat in this case) below the biomass (technically known as bed material) to heat it. Heated to 600-800 degrees Celsius it produces, among others, synthetic natural gas. At 400 degrees Celsius, biomass heating results in solids known as bio-coal, the process being known as torrefaction. Bio-coal has better storage qualities than waste biomass. Bio-coal and synthetic natural gas are later burn in conventional power plants to produce electricity (Herz, Thermochemical Conversion of Biomass to Fuel: Future of Energy, 2014).

Another biomass source is algae. Algae are grown in an open or closed (closed bioreactors can be flat-plate, tubular or column, each option having its specific advantages.) to environment bioreactors (also known as photobioreactors).

The biggest problem of algae is crop protection from pests, the reactors, particularly the open ones, can be very easily infested and destroyed within 48 hours. After the algae has grown in the pond, it is harvested, by either centrifuge, filter, flocullation (letting the organism settle) and dissolved air flotation, and used to produced heat, through direct combustion.

The main effort of the technology developers now is to improve Energy Return on Investment (EROI), basically to make it commercially sustainable (McBride, Production Processes for Biofuels from Algae: Future of Energy, 2014).

Sources of electricity – nuclear

Nuclear energy is based on heat released by atomic (uranium) fission reactions which proceed via a chain reaction. Various technologies compete in the sector, mainly divided into light water reactors (LWRs), more popular, and heavy water reactors (HWRs). The main difference between them is that LWRs need enriched uranium, while HWRs can use natural uranium.

The development is now at the third plus generation, focusing mainly on safety measures, such as simplified core design and natural convection-driven cooling in case of loss-of-coolant (LOCA) incident.

Simplifying, core design measures include: natural convention air design (uses air cooling), gravity drain water tank (moved water on top of reactor, so no need for pumps), water film evaporation, outside cooling air intake (another measure to use external atmospheric temperature for cooling) and steel containment vessel (better protection). Simplified core design is aimed to reduced complexity and consequently increase reliability.

The main problem for nuclear resides mainly in the economics of a project, needing high capex and having long rate-of-investment; spent fuel handling and storage; and nuclear proliferation (atomic bombs). Fusion can be a player in the future, mainly due to better safety measure (there cannot be a core melt-down) and shorter (10s-100s) lived activated reactor components (Tynan, The future of Nuclear Energy: Future of Energy, 2014)

Sources of electricity – natural gas

Natural gas is primarily composed of methane, an odourless gas, made from carbon and hydrogen. Mercaptan is added as odour to identify leaks. Burning pure natural gas makes dioxide of carbon and water. Natural gas was formed similarly as coal.

It is usually exploited through drilling of gas streams, many co-located with oil or coal reserves. Modern techniques include horizontal drilling, boring the well along a horizontal stream and fracking, which means pumping at high pressure a liquid mixture into a drilled well, thus fracturing the earth to allow higher gas flows out from formations. Fracking was a technique known from the 1980s, but combining it with horizontal drilling made significant higher exploitation returns.

Gas is later transported through pipelines towards gas-fired power plants. There is also LNG (Liquified-Natural-Gas) as an option for transport, where gas is cooled at -162 degrees Celsius, changing its state into liquid and, consequently, it’s volume, then moved into specialized vessels called cryogenic sea vessels and then unloaded at LNG terminals, where gas is returned into the gaseous form and reintroduced into the gas network.

Gas-fired power plants are divided into two major types. Firstly, there are power plants using gas turbines, where water for cooling is not needed. A gas turbine is essentially a modification of a fighter jet engine. Secondly, an update of the power plant is the combined cycle gas turbine (or CCGT), where heat resulted from gas burning is used to create a second cycle, using steam, similar with nuclear or coal cycles, in order to create electricity, with efficiencies reaching 55-60% (GasNaturally, 2015; Sheldon, 2013).

In short, merits of gas are abundance, reliability of fuel supply and burning relatively cleaner than coal and oil. Drawbacks are environmental impacts from burning and flaring, safety concerns from leaks and volatile prices.

Sources of electricity – hydroelectric

Hydroelectric power uses height differences and large volumes of water to spin a turbine’s blades and create electricity. Dams can be massive, such as Three Gorges Dam, or smaller, such as those found in mountain rivers.

Dams are highly efficient in converting mechanical energy into electrical energy, efficiencies reaching almost 90%.

Merits of hydroelectric energy are high efficiency, low emissions, relative reliability and additional benefits (flood control, water storage, irrigation). Drawbacks are generation limits almost reached in developed countries, high environmental footprint, disturbance of water ecosystems, siltation (which might limit the life of a dam to 100 years) and unsolved questions on end-of-life dams (Webber 2014; Sheldon 2013)

Sources of electricity – coal

Coal-fired electricity is the most used source to create electricity and, until 2014, it had the fastest growth wordwide (IEA, Coal Information 2015).

Coal forms through the process of coalification, where peat undergoes several changes as a result of bacterial decay, compaction, heat and time. Peat is the plant remains from a water-saturated environment, such as a bog or a mire. This process happens in a water-saturated environment because only in this particular environment there is a lack of oxygen which favours a specific bacterial decay. The degree of alteration of the peat marks the rank of the coal, broadly divided into low-rank coals, such as lignite and sub-bituminous coals, which have lower calorific value and higher moisture levels, and high-rank coals, bituminous and anthracite coals (also known as hard coal), which have more carbon and higher energy content.

Coal mining methods can be either through underground mines: drift, slope and shaft mining or surface-mines: area, contour, mountain top removal and auger mining. Coal mining equipment makes the largest human-built machines on the planet.

Leaving aside other applications from coal, we will discuss how coal is used in electricity production. After coal is mined, coal is taken to power plants through trains and conveyor belts. Coal is then blown in a combustion chamber of a boiler and burned at around 1,400 degrees Celsius. Surrounding the walls of the boiler are pipes filled with water, which are heated to make superheated high-pressure steam. The steam passes through a turbine, causing it to rotate, that turns a generator, creating electricity. Efficiencies of these power plants can reach 46% (EURACOAL, 2013).

In short, merits of coal are abundance, affordability, reliability of fuel supply, easy to store and transport. Drawbacks are land disturbance during mining for surface mines and the release of harmful pollutants during burning.