Sources of electricity – wind

Wind energy tapes the Earth’s winds to create electricity through wind turbines. Wind turbines can be HAWTs (Horizontal-Axis Wind Turbines), widely used, or VAWTs (Vertical-Axis Wind Turbines). VAWTs are designed to be used mainly within urban areas. The turbines can be deployed onshore (on land), producing cheaper electricity or offshore (near cost), producing more reliable electricity.

A wind turbine uses the inflow of wind to activate the blades and the rotor. They spin the main shaft and gearbox, which spin the generator, producing electricity. Blades work basically like a reversed airplane wing.

The design of a wing facilitates lift, while the wind turbine blade facilitates push, with the most important part (and most expensive materials) at the top of the blade, because most of the aerodynamic loading is created there. The blade is made of fibreglass (cheaper than aluminium, with similar structural resistance).

The rotor assembly is the main focus now for improving the turbine (Bazilevs, Wind Turbines: Future of Energy, 2014).

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.