Vegetable oils are extracted from biomass on a considerable scale to be used in soap-making, other chemical processes and, in additional refined form, for cooking.
Concentrated vegetable oils may be used directly as fuel in diesel engines, but difficulties arise from the high viscosity and from the combustion deposits, as compared with conventional (fossil) petroleum-based diesel oil, especially at low ambient temperature (≤5_C).
Both difficulties are overcome by reacting the extracted oil with ethanol or methanol to make the equivalent ester. Such esters, called biodiesel, have technical characteristics as fuels that are better suited to diesel-engines than petroleum based diesel oil.
5. Wave Power
Very large energy fluxes can occur in trouble sea waves. The power in the wave is proportional to the square of the amplitude and to the period of the motion.
Therefore, the long period of _̴10 sec and large amplitude of _̴2 meter waves have considerable interest for the power generation, with energy fluxes commonly averaging between 50 and 70kWm−1 having width of oncoming wave.
The possibility of generating electric power from these trouble waves has been recognised for several years, and there are countless ideas for machines to extract the facility. For example, a wave power grid was utilized in California in 1909 for harbour lighting.
Modern interest has revived, particularly in Japan, UK, Scandinavia and India, so research and development has progressed to commercial construction for meaningful power extraction.
Very small scale autonomous systems are used for marine warning lights on buoys and much larger devices for grid power generation. The provision of power for marine desalination is an obvious attraction.
With all of the renewable energy supplies, the dimensions of operation possesses to be determined, and present trends to support moderate power generation at about 100 kW–1MW from modular devices each capturing energy from about 5 meter to 25 meter of wavefront.
Initial designs are for operation at shore-line or near to shore to give access and to lessen, hopefully, storm damage. It is important to understand the various difficulties facing wave power developments.
Wave patterns are irregular in an amplitude, phase and the direction. It is difficult to style devices to extract power efficiently over the wide selection of variables. There is always some probability of utmost gales or hurricanes producing waves of freak intensity. The structure of the power devices must be able to withstand this.
Commonly the 50 year peak wave is 10 times the peak of the typical wave. Thus the structures have to withstand ∼100 times the power intensity to which they are normally matched. Allowing for this is often expensive and can probably reduce normal efficiency of power extraction.
Peak power is usually available in trouble waves from open-sea swells produced from long fetches of wind, e.g. beyond the Western Islands of Scotland (in one of the most tempestuous areas of the North Atlantic) and in regions of the Pacific Ocean.
The difficulties of constructing power devices for these types of wave regimes, of maintaining and fixing or mooring them in position, and of transmitting power to land, are fearsome.
Therefore more protected and accessible areas near to shore are most commonly used. Wave periods are commonly ∼5–10 s (frequency ∼0_1Hz). It is extremely difficult to couple this irregular movie to electrical generators requiring ∼500 timesgreater frequency.
So many sorts of device could also be suggested for wave power extraction that the task of choosing a specific method is formed complicated and somewhat arbitrary.
The large power requirement of commercial areas makes it tempting to hunt for equivalent wave energy supplies. Consequently plans may be scaled up so only large schemes are contemplated in the most demanding wave regimes.
Smaller sites of far less power potential, but more reasonable economics and security, could also be ignored. The development and application of wave power has occurred with spasmodic and changing government interest, largely without the advantage of market incentives.
Wave power needs an equivalent learning curve of steadily enlarging application from small beginnings that has occurred with wind generation. The distinctive advantages of wave power are the massive energy fluxes available and therefore the predictability of wave conditions over periods of days.
Waves are created by the wind, and effectively store the energy for transmission over the great distances. For instance, large waves appearing off Europe will are initiated in stormy weather within the mid-Atlantic or as far because the Caribbean.
6. Tidal Power
The level of water within the large oceans of the world rises and falls consistent with predictable patterns. The power of tidal currents may be harnessed in a manner similar to wind power; this is also called ‘tidal stream power’.
In practice, tidal flow is probably going to be attractive for power generation only where it's enhanced in speed by water movement in straights between islands and mainland, or between relatively large islands.
Therefore the opportunities for viable commercial sites are unusual. Other sites with large tidal range, such as the Severn estuary in England and the Bay of Fundy on the eastern boundary between Canada and the United States, have been the subject of numerous feasibility studies over the past hundred years.
Sites for tidal range power are chosen for their large tidal range; a characteristic that is associated with estuaries having large areas of mud flats exposed at lower tides. Tidal range power depends on the placing of a barrier for a height difference in water level across the turbines.
In operation, (i) the level of water in the basin is always above the unperturbed low tide and always below the unperturbed high tide, (ii) the rates of flow of both the incoming and the outgoing tides are reduced in the basin, and (iii) sea waves are stopped at the barrier.
For optimum electric power generation from tides, the turbines should be operated during a regular and repeatable manner. The mode of operation will depend on the scale of the power plant, the demand and the availability of other sources.
The analysis of tidal behaviour has been developed by many notable mathematicians and applied physicists, including Newton, Airy, Laplace, George Darwin (son of Charles Darwin) and Kelvin. We shall use Newton’s physical theory to explain the phenomena of tides.
However, present day analysis and prediction depends on the mathematical method of Fourier analysis developed by Lord Kelvin in Glasgow. A complete physical understanding of tidal dynamics has not yet been attained due to the topological complexity of the ocean basins.
The seas are liquids held on the solid surface of the rotating Earth by gravity. The gravitational attraction of the Earth with the Moon and the Sun perturbs these forces and motions so that tides are produced.
Tidal power springs from turbines set during this liquid, so harnessing the K.E. of the rotating Earth. Even if all the world’s major tidal power sites were utilised, this is able to cause an additional slowing of the Earth’s rotation by no quite at some point in 2000 years; this is often not a big extra effect.
Near coastlines and between islands, tides may produce strong water currents that can be considered for generating power. This may be called tidal-current, tidal-stream or tidal-flow power.
The total power produced might not be large, but generation at competitive prices for export to a utility grid or for local consumption could also be possible. The theory of tidal river power is analogous to wind generation.
The advantages are (a) predictable velocities of the fluid and hence predictable power generation, and (b) water density 1000 times greater than air and hence smaller scale turbines.
The main disadvantages of tidal river power are (a) small fluid velocity and (b) the intrinsically difficult marine environment.
7. Ocean Thermal Energy Conversion (OTEC)
The ocean is the world’s largest solar collector. In tropical seas, temperature differences of about 20−25 °C may occur between the nice and cozy, solar-absorbing near-surface water and therefore the cooler 500–1000 m depth ‘deep’ water at and below the thermocline.
Subject to the laws and practicalities of thermodynamics and heat engines can operate from this temperature difference across this huge heat store. The term ocean thermal energy conversion (OTEC)refers to the conversion of a number of this thermal energy into useful work for electricity generation.
Given sufficient scale of efficient equipments, generation of electricity power might be sustained day and night at 200kWe from access to about 1km2 of tropical sea, like 0.07% of the solar input.
Pumping rates are about 6m3 s−1 of water per MWe electricity production. The technology for energy extraction is analogous thereto used for energy efficiency improvement in industry with large flows of heated discharge, but on a way larger scale.
The attractiveness of OTEC is that the seemingly limitless energy of the warmer surface water in reference to the colder trouble and its potential for constant, base load, extraction.
However, the temperature difference obtained is extremely small then the efficiency of any device for transforming the thermal energy to the mechanical power also will be comes very small.
Even for heating, warm seawater can't be spilt ashore thanks to its high salt content. Moreover, large volumes of seawater need to be pumped, so reducing the net energy generated and requiring large pipes and heat exchangers.
A device transfers heat from one fluid to a different, while keeping the fluids apart. Water flows by a method through the tubes while the working fluid flows through the shell round the tubes. The most fundamental of those arises from the relatively small thermal conductivity of water.
The inside of the pipe is vulnerable to encrustation by marine organisms, which will increase the resistance to heat flow, and thereby reduce the performance.
Such biofouling is one of the major problems in OTEC design, since increasing the surface area available for heat transfer also increases the opportunity for organisms to attach themselves.
Among the methods tried to keep this fouling under control are mechanical cleaning by continual circulation of close fitting balls and chemical cleaning by additives to the water.
The effect of all these complications is that the need for cost saving encourages the use of components working at less than optimal performance, e.g. undersized heat exchangers.
8. Geothermal Energy
The inner core of the earth reaches a maximum temperature of about 4000 °C. Heat passes out through the solid submarine and land surface mostly by conduction – geothermal heat – and occasionally by active convective currents of molten magma or heated water.
Geothermal heat is generally of low quality, and is best used directly for building or process heat at about 50–70 °C, or for preheating of conventional high temperature energy supplies. Such supplies are established in several parts of the world and many more projects are planned.
Occasionally geothermal heat is available at temperatures above about 150 °C, so electrical power production from turbines can be contemplated. Several important geothermal electric power complexes are fully established, especially in Italy, New Zealand and the USA.
In other geothermal sites, however, the current of heat is increased artificially, e.g. by fracturing and actively cooling hot rocks, or by drilling into hot aquifers, and so the supply is not renewable at the extraction rate on a long time scale. Such finite supplies are included in this text only because they are usually included with other ‘alternative’ supplies.
Heat passes from the crust by (1) natural cooling and friction from the core, (2) radioactive decay of elements such as uranium and thorium, and (3) chemical reactions.
The time constants of such processes over the whole Earth are so long that it is not possible to know whether the Earth’s temperature is presently increasing or decreasing. The radioactive elements are concentrated in the crust by fractional recrystallisation from molten material, and are particularly pronounced in granite.
However, the production of heat by radioactivity or chemical action is only significant over many millions of years, consequently geothermal heat extraction relies on removing stored heat in the thermal capacity of solid material and water in the crust, rather than on replenishment.
If conduction through uniform material was the only geothermal heat transfer mechanism, the temperature gradient in the crust would be constant. However, if convection occurs ‘locally’, as from water movement, or if local radioactive or exothermic chemical heat sources occur, there are anomalous temperature gradients.
Moderate increases in temperature gradient to ∼50°C km−1 occur in localized regions away from plate boundaries, owing to anomalies in crust composition and structure. Heat could also be released from such regions naturally by deep penetration of water in aquifers and subsequent convective water flow.
The resulting hot springs, with increased concentrations of dissolved chemicals, are often famous as health spas. Deep aquifers can be tapped by drilling, to become sources of heat at temperatures from ∼50 to ∼200°C. If the anomaly is associated with material of small thermal conductivity, i.e. dry rock, then a ‘larger than usual’ gradient occurs with a related increase in stored heat.
The principal components of a heat plant are the boreholes then heat extraction from the depths up to 15 km are often contemplated.
There are three classes of geothermal region:
1. Hyperthermal. Temperature gradient ≥80°Ckm−1. These regions are usually on tectonic plate boundaries. Such type of the first region to be tapped for the electricity generation was at Larderello in Tuscany, Italy in 1904. Nearly all geothermal power stations are in such areas.
2. Semithermal. Temperature gradient ∼40–80°Ckm-1. Such regions are associated generally with anomalies faraway from plate boundaries. Heat extraction is from harnessing the natural aquifers or fracturing the dry rock. A well-known example is the geothermal district heating system for houses in Paris.
3. Normal. Temperature gradient <40°Ckm-1. These remaining regions are associated with average geothermal conductive heat flow at ∼0_06Wm−2. It is unlikely that these areas can ever supply geothermal heat at prices competitive to present (finite) or future (renewable) energy supplies.
