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OCEAN ENER GY SYSTEM
Ocean are large water bodies covering 70.8 percent of the Earths’ total surface areas and hold about 1445 cubic km of saline water. A part from being large reservoir of water they are huge reservoir of energy also. Following are the most common ways of obtaining energy from ocean;
- Tidal Energy
- Wave Energy
- Ocean Thermal Energy
TIDAL ENEGRY
Water level near the coasts rises up and fall twice a day( the time interval between the rise and fall is 12h 24min). The everyday movement of water level along the coasts is known as a tide.
Tidal Energy or Tidal Power as it is also called, is another form of hydro power that utilises large amounts of energy within the oceans tides to generate electricity. Tidal Energy is an “alternative energy” that can also be classed as a “renewable energy source”, as the Earth uses the gravitational forces of both the moon and the sun everyday to move vast quantities of water around the oceans and seas producing tides.
As the Earth, its Moon and the Sun rotate around each other in space, the gravitational movement of the moon and the sun with respect to the earth, causes millions of gallons of water to flow around the Earth’s oceans creating periodic shifts in these moving bodies of water. These vertical shifts of water are called “tides”. When the earth and the moons gravity lines up with each other, the influences of these two gravitational forces becomes very strong and causes millions of gallons of water to move or flow towards the shore creating a “high tide” condition. Likewise when the earth and the moons gravity are at 90 to each other, the influences of these two gravitational forces is weaker and the water flows away from the shore as the mass of water moves to another location on the earth, creating a “low tide” condition. This ebbing and flowing of the tides happens twice during each period of rotation of the earth with stronger weekly and annual lunar cycles superimposed onto these tides. When the moon is in perfect alignment with the earth and the sun, the gravitational pull of the moon and sun together becomes much stronger than normal with the high tides becoming very high and the low tides becoming very low during each tidal cycle. Such tides are known as spring tides (maximum). These spring tides occur during the full or new moon phase. The other tidal situation arises during neap tides (minimum) when the gravitational pull of the moon and the sun are against each other, thus cancelling their effects. The net result is a
smaller pulling action on the sea water creating much smaller differences between the high and low tides thereby producing very weak tides. Neap tides occur during the quarter moon phase. Then spring tides and neap tides produce different amounts of potential energy in the movement of the sea water as their effects differ from the regular high and low sea levels and we can use these tidal changes to produce renewable energy. So we can say that the tides are turning for alternative energy. So we now know that the constant rotational movement of the earth and the moon with regards to each other causes huge amounts of water to move around the earth as the tides go in and out. These tides are predictable and regular resulting in two high tides and two low tides each day with the level of the oceans constantly moving between a high tide and a low tide, and then back to a high tide again. The time taken for a tidal cycle to happen is about 12 hours and 24 minutes (called the “diurnal cycle”) between two consecutive high tides allowing Oceanographers and Meteorologist to accurately predict the ebb and flow of the tides around the oceans many years in advance. The main big advantage of this is that the tides are therefore perfectly predictable and regular unlike wind energy or solar energy, allowing miles of coastline to be used for tidal energy exploitation and the larger the tidal influence, the greater the movement of the tidal water and therefore the more potential energy that can be harvested for power generation. Therefore Tidal Energy can be considered as a renewable energy source as the oceans energy is replenished by the sun as well as through tidal influences of the moon and suns gravitational forces.
Tidal Energy Generation
Since the position of the earth and the moon with respect to the sun changes throughout the year, we can utilise the potential energy of the water contained in the daily movement of the rising and falling sea levels to generate electricity. The generation of electricity from tides is similar in many ways to hydro-electric generation we looked at in the hydro energy tutorials. The difference this time is that the water flows in and out of the turbines in both directions instead of in just one forward direction. Tidal energy, just like hydro energy transforms water in motion into a clean energy. The motion of the tidal water, driven by the pull of gravity, contains large amounts of kinetic energy in the form of strong tidal currents called tidal streams. The daily ebbing and flowing, back and forth of the oceans tides along a coastline and into and out of small inlets, bays or coastal basins, is little different to the water flowing down a river or stream. The movement of the sea water is harnessed in a similar way using waterwheels and turbines to that used to generate hydro electricity. But because the sea water can flow in both directions in a tidal energy system, it can generate power when the water is flowing in and also when it is ebbing out. Therefore, tidal generators are designed to produce power when the rotor blades are turning in either direction. However, the cost of reversible electrical generators are more expensive than single direction generators. Different Types of Tidal Energy Systems Tidal Barrage – A Tidal Barrage is a type of tidal power generation that involves the construction of a fairly low dam wall, known as a “barrage”and hence its name, across the entrance of a tidal inlet or basin creating a tidal
reservoir.
This dam has a number of underwater tunnels cut into its width allowing sea water to flow through them in a controllable way using “sluice gates”. Fixed within the tunnels are huge water turbine generators that spin as the water rushes past them generating tidal electricity.
- High predictability as high and low tides can be predicted years in advance, unlike wind.
- Tidal barrages provide protection against flooding and land damage.
- Large tidal reservoirs have multiple uses and can create recreational lakes and areas where before there were none. Disadvantages of Tidal Energy
- Tidal energy is not always a constant energy source as it depends on the strength and flow of the tides which themselves are effected by the gravitational effects of the moon and the sun.
- Tidal Energy requires a suitable site, where the tides and tidal streams are consistently strong. Must be able to withstand forces of nature resulting in high capital, construction and maintenance costs.
- High power distribution costs to send the generated power from the submerged devices to the land using long underwater cables.
- Intermittent power generation, only generates power ten hours a day during the ebb and flow of the tides.
- Build up of silt, sediments and pollutants within the tidal barrage from rivers and streams flowing into basin as it is unable to flow out into the sea.
- Danger to fish and other sea-life as they get stuck in the barrage or sucked through the tidal turbine blades.
Tide Characteristics
The tide is a complete cycle of tidal oscillations of sea level, including the flood-tide (rise of level) and the ebb-tide (drop of level). The cause of such tidal reversals, even in sites where the tides are strictly regular, remained unknown for many years. The ancient Greeks, even in their time, suspected that the tides are associated with the lunar phases, but the complexity and variety of tidal oscillations led to the ancient proverb: “Tides are the grave of human curiosity”, remaining plausible for many centuries. At present navigators have at their disposal tide tables where the height and the time of low and high tides for any day of any year are indicated for the main points of the coast. The tables are compiled with the help of harmonic analysis of data of observations. The tidal wave floods vast areas. Depending upon the position of the site on the globe, the coastal configuration, and bathymetry, the tidal range varies from a few centimeters in the land-locked seas (Black Sea, Baltic Sea, Mediterranean Sea, and others) to many meters at the funnel-shaped bays open towards the ocean. So, at the head of such a bay named Fundy, Canada, the highest Earth’s tide with a range equal to 17.3 m is observed. In most sea areas, two tidal rises and two falls occur during each lunar day, i.e. with the period of approximately 12 hours 24 minutes (semidiurnal tides) or only one high and one low tide occurring in each 24 hours 48 minutes (diurnal tides). In most cases, the actual tidal oscillations are a combination of both above-mentioned types and are named after that substantially prevailing over the other. If both types of tidal oscillations play an essential role, the resultant tide is called a mixed tide. The highest point reached by the sea in any one tidal period is called the high water, HW, and the lowest point reached by it is termed the low water, LW. The difference in the height of the water at low and high tides is known as the tidal range, A; the intensity of tidal oscillations may be characterized by the height of the
high or low water with respect to the mean sea level and is called tidal amplitude, A /2.
In the course of semidiurnal tides, the maximum amplitudes take place at new Moon or full Moon (spring tides) while the minimum amplitudes are observed about the time of the first and third quarters of the Moon (neap tides). In the course of diurnal tides, the maximum amplitudes are observed at the extreme declinations of the Moon (tropic
tides), and the minimum at the zero declination (equatorial tides). When the tropic tides are in phase with the spring ones the amplitude of the resultant tide reaches its maximum magnitude. The chart (see Figure 1) shows the values of maximum range, mean spring (tropical) range, and mean range in the sites that permit utilization of tidal
energy.
WAVE ENERGY
Wave Energy is a non-polluting and renewable source of energy, created by natural transfer of wind energy above the oceans, which itself is created by the effects of the suns solar energy. As the wind blows across the oceans surface, moving air particles transfer their energy to the water molecules that they touch. As the wind continues to blow more and more of its kinetic energy is transferred to the oceans surface and the waves grow bigger. These larger waves are called gravity waves because their potential energy is due to the gravitational force of the Earth. There is a lot of potential energy in the waves generated by the wind, to the point were large storm waves can lift ships high out of the water. As an ocean wave passes a stationary position the surface of the sea changes in height, water near the surface moves as it losses its kinetic and potential energy, which affects the pressure under the surface. The periodic or oscillatory nature of ocean waves means that we can use a variety of different Wave Energy Devices to harness the energy produced by the oceans waves. The problem lies in that the oscillatory frequency of an ocean wave is relatively slow and is much less than the hundreds of revolutions per minute required for electric power generation. Then a great variety of wave energy devices and designs are available to convert these slow- acting, reversingwave forces into the high speed, unidirectional rotation of a generator shaft. There are three fundamental but very different wave energy devices used in converting wave power into electric power, and these are:
- Wave Profile Devices These are wave energy devices which turn the oscillating height of the oceans surface into mechanical energy.
- Oscillating Water Columns These are wave energy devices which convert the energy of the waves into air pressure.
- Wave Capture Devices These are wave energy devices which convert the energy of the waves into potential energy.
Wave Profile Devices Wave profile devices are a class of wave energy device which floats on or near to the sea surface and moves in response to the shape of the incident wave or, for submersible devices, it moves up and down under the influence of the variations in underwater pressure as a wave moves by. Most types of wave profile devices float on the surface absorbing the wave energy in all directions by following the movements of waves at or near the sea surface, just like a float. The only wave energy devices that use wave profiling have been in practical use for some time, although on a fairly small-scale, are those used for powering navigation buoys. If the physical size of the wave profile device is very small compared to the periodic length of the wave, this type of wave energy device is called a “point absorber”. If the size of the device is larger or longer than the typical periodic wavelength, it is called a “linear absorber”, but more commonly they are collectively known as “wave attenuators”. The main difference between the two wave energy devices is how the oscillating system converts the wave energy between the absorber and a reaction point. This energy absorption can be achieved either by a floating body, an oscillating solid member or oscillating water within a buoys structure itself. The waves energy is absorbed using vertical motion (heave), horizontal motion in the direction of wave travel (surge), angular motion about a central axis parallel to the wave
compressed and decompressed by this movement every cycle. The air is channelled through a wind turbine generator to produce electricity as shown.
The type of wind turbine generator used in an oscillating water column design is the key element to its conversion efficiency. The air inside the chamber is constantly reversing direction with every up-and-down movement of the sea water producing a sucking and blowing effect through the turbine. If a conventional turbine was used to drive the attached generator, this too would be constantly changing direction in unison with the air flow. To overcome this problem the type of wind turbine used in oscillating water column schemes is called a Wells Turbine. The Wells turbine has the remarkable property of rotating in the same direction regardless of the direction of air flow in the column. The kinetic energy is extracted from the reversing air flow by the Wells turbine and is used to drive an electrical induction generator. The speed of the air flow through the wells turbine can be enhanced by making the cross-sectional area of the wave turbines duct much less than that of the sea column. As with other wave energy converters, oscillating wave column technology produces no greenhouse gas emissions making it a non-polluting and renewable source of energy, created by natural transfer of wind energy through a wells turbine. The advantage of this shoreline scheme is that the main moving part, the turbine can be easily removed for repair or maintenance because it is on land. The disadvantage though is that, as with the previous wave energy devices, the oscillating wave columns output is dependent on the level of wave energy, which varies day by day according to the season. Wave Capture Device A Wave Capture Device also known as a Overtopping Wave Power Device, is a shoreline to near shore wave energy device that captures the movements of the tides and waves and converts it into potential energy. Wave energy is converted into potential energy by lifting the water up onto a higher level. The wave capture device, or more commonly an overtopping device, elevates ocean waves to a holding reservoir above sea level. The overtopping waveenergy converter works in much the same way as an impoundment type hydroelectric dam works. Sea water is captured and impounded at a height above sea level creating a low head situation which is then drained out through a reaction turbine, usually a Kaplan Turbine generating electricity as shown.
Wave Capture Device The basic impoundment structure can be either fixed or a floating structure tethered to the sea bed. The wave overtopping device uses a ramp design on the device to elevate part of the incoming waves above their natural height. As the waves hit the structure they flow up a ramp and over the top (hence the name “overtopping”), into a raised water impoundment reservoir on the device in order to fill it. Once captured, the potential energy of the trapped water in the reservoir is extracted using gravity as the water returns to the sea via a low-head Kaplan turbine generator located at the bottom of the wave capture device. Other such wave capture devices are located at the shoreline were the waves are channelled along a horizontal man made channel. This channel is funnel shaped which is wide towards the sea where the waves enter and gradually narrows towards an impoundment reservoir at the other end. As the waves propagate along the narrowing channel, the wave height is lifted due to the funnelling effect to a level exceeding the horizontal upper edge of the channel wall, excess water from the wave is allowed to spill into a confined basin above the normal sea level. As the water is now at a height above the sea level, the potential energy of the water trapped in the basin is then extracted by draining the water back to the sea through a low-head Kaplan turbine as before.
Ocean Thermal Energy
Large amount of solar energy is stored in the oceans and the seas. Solar energy stored in the form of eat is called Ocean Thermal Energy. Te sun warms the ocean water at the surface and the wave motion mixed the warmed water downwards to the depth of about 100m. Te mixed warm layer is separated from the deep cold water layer and the temp. Difference between these layers ranges from 10˚c to 30˚c. It is this temperature between the surface of the ocean and the depth of about 100m which is used to produce electric power.
OCEAN THERMAL ENERGY CONVERSION TECHNOLOGIES
Among ocean energy sources, OTEC is one of the continuously available renewable energy resources that could contribute to base-load power supply. The resource potential for OTEC is considered to be much larger than for other ocean energy forms. Up to 88,000 TWh/yr of power could be generated from OTEC without affecting the ocean’s thermal structure Systems may be either closed-cycle or open-cycle. Closed-cycle OTEC uses working fluids that are typically thought of as refrigerants such as ammonia or R-134a. These fluids have low boiling points, and are therefore suitable for powering the system’s generator to generate electricity. The most commonly used heat cycle for OTEC to date is the Rankine cycle, using
Open-cycle OTEC uses warm surface water directly to make electricity. The warm seawater is first pumped into a low-pressure container, which causes it to boil. In some schemes, the expanding vapour drives a low-pressure turbine attached to an electrical generator. The vapour, which has left its salt and other contaminants in the low-pressure container, is pure fresh water. It is condensed into a liquid by exposure to cold temperatures from deepocean water. This method produces desalinized fresh water, suitable for drinking water, irrigation or aquaculture. In other schemes, the rising vapour is used in a gas lift technique of lifting water to significant heights. Depending on the embodiment, such vapour lift pump techniques generate power from a hydroelectric turbine either before or after the pump is used. Hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid, warm seawater enters a vacuum chamber and is flash-evaporated, similar to the open-cycle evaporation process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to produce electricity. The steam condenses within the heat exchanger and provides desalinated water.
Working fluids
A popular choice of working fluid is ammonia, which has superior transport properties, easy availability, and low cost. Ammonia, however, is toxic and flammable. Fluorinated carbons such as CFCs and HCFCs are not toxic or flammable, but they contribute to ozone layer depletion. Hydrocarbons too are good candidates, but they are highly flammable; in addition, this would create competition for use of them directly as fuels. The power plant size is dependent upon the vapor pressure of the working fluid. With increasing vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers increase to endure high pressure especially on the evaporator side.
HYDRO ENERGY
In many ways, Hydro Energy is very similar to “Wind Energy” in that a renewable energy source, in this case “water”, is used to rotate a turbine generator to produce electricity. Hydro Energy is the energy derived from the power of moving water. Today large hydro electric power plants generate about 15 percent of the world’s electricity by extracting the potential energy which comes from the vertical distance that water drops, called the “head”. This stored potential energy is released as work because the water is in motion, and the best way to put large amounts of water in motion is to let gravity do the work. Then the most important element for the production of “Hydro Energy” is not the water itself, as within reason any liquid could be used, but “gravity” as it is gravity that makes the water move. Then we can correctly say that hydro energy is gravity powered energy as we are generating electricity from gravity. Rivers and streams generate currents of water because the water in them is moving downhill, even if only slightly, flowing downwards by the pull of gravity. The water which is flowing downhill, being pulled by gravity, contains large amounts of kinetic energy that can be extracted and used by a water turbine or water wheel and even a small stream can produce enough kinetic energy to turn a wheel. The kinetic energy produced by the moving water is converted into either mechanical energy to perform some work or directly into electrical energy by means of an electrical generator, and this then is the basic science behind Hydro Energy production. Hydrokinetic Energy is a highly developed form of “alternative energy” that can also be classed as a “renewable energy source”, with the English word Hydro originating from the Greek word meaning “water”. The energy from the Sun heats large masses of water such as the sea, oceans and lakes, turning it into water vapour which rises forming clouds high in the sky. The cold air above the clouds condenses this water vapour which then falls back to Earth as rain or snow in the hills and mountains. This well known cycle keeps the mountain rivers and streams supplied with plenty of water. This cycle of evaporation and precipitation
(rainfall, snow, drizzle, etc), is called the “water cycle”, and will keep on going forever as long as the sun shines, and all for free, that’s why hydro energy is a renewable energy source. But to use this renewable energy source to generate electricity we must first use its gravitational potential energy to spin a water turbine generator. How does Hydro Energy work? We now know that hydro energy is the process of generating electricity from water which harness the power of the rivers and currents. But we can also use the power of stationary or slow-moving bodies of water to generate electricity by storing it in large dams. The gravitational potential energy that is stored in the water above the dams wall is released as it flows downwards through large pipes to the bottom of the dam. As the water flows down these large pipes it passes through turbine blades at the base of the dam causing the blades of the turbine to rotate which produces electricity. Because of the great height of the water above the turbine, called the “head”, it will arrive at the turbines at a high speed and pressure which means that we can extract a great deal of energy from it generating more electricity. Then there are different ways in which we can use “Hydro Energy” to produce mechanical or electrical power with the four common types of hydro energy sources being:
- Run of River – this is the cheapest and simplest form of hydro energy were an undershot waterwheel (or turbine) is placed directly into the natural flow of a fast-flowing river or stream. The water wheel is then rotated by the movement of the water flowing beneath it. Run of river hydro systems are commonly used to elevate water for the irrigation of surrounding farmland or to produce a mechanical power for grinding corn, etc. Minidams or weirs can also be built to increase or regulate the flow of water underneath the water wheel.
- Water Diversion – this is a low-head micro-hydro scheme which channels a portion of the rivers water down a man made canal, trough or aqueduct (called a penstock) to turn a waterwheel at the bottom. It may or may not need the use of a dam or weir to divert water away from its natural course and through the penstock which travels downhill using the force of gravity to feed an overshot waterwheel from above. The flow of the water and therefore its potential energy through the penstock design may be regulated by means of a sluice gate or water valve.
- Water Impoundment – this is a large man made system that uses a dam or weir to store the potential energy of large volumes of water in a lake or reservoir which can then be used when needed to generate electricity. Water impoundment systems are the typical hydro-electric power plants we see in mountainous areas and countries were high dams can be built and deep reservoirs can be maintained.
- Pumped Storage – pumped hydro storage) facilities store energy by pumping water from a lower reservoir to an upper reservoir when the need for electricity is low such as during the night time. During periods of high electrical demand, the water is then released back to the lower reservoir via turbine generators to generate electricity. Hydro Energy is a clean, green technology which produces no pollution. Unfortunately, most of the lakes and rivers that could easily be exploited have already been dammed, so the future of hydro energy lies in the development of less appealing sites. But hydro energy is more than just a clean source of electricity. When dams are built, they create large bodies of water for fish and wildlife to survive. These reservoirs and lakes have several other advantages and uses. For example, they provide opportunities for outdoor activities such as sailing boats, swimming, fishing, or just enjoying the surrounding outdoors as well as using the stored water from the reservoirs for drinking and to grow crops. The major benefit that all forms of hydro energy have is that they provide power without burning fossil fuels, but water power is not without its drawbacks and here are some of the advantages and disadvantages associated with hydro power. Advantages of Hydro Energy Hydro energy is a clean and reliable renewable energy resource.
Turbine Power Output In general, the turbine converts the kinetic energy of the working fluid, in this case water, into rotational motion of the turbine shaft.
Swiss mathematician Leonhard Euler showed in 1754 that the torque on the shaft is equal to the change in angular momentum of the water flow as it is deflected by the turbine blades and the power generated is equal to the torque on the shaft multiplied by the rotational speed of the shaft. See following diagram.
Note that this result does not depend on the turbine configuration or what happens inside the turbine. All that matters is the change in angular momentum of the fluid between the turbine's input and output.
Hydroelectric Power Generation Efficiency Hydroelectric power generation is by far the most efficient method of large scale electric power generation.. Energy flows are concentrated and can be controlled. The conversion process captures kinetic energy and converts it directly into electric energy. There are no inefficient intermediate thermodynamic or chemical processes and no heat losses. The overall efficiency can never be 100% however since extracting 100% of the flowing water's kinetic energy means the flow would have to stop. The conversion efficiency of a hydroelectric power plant depends mainly on the type of water turbine employed and can be as high as 95% for large installations. Smaller plants with output powers less than 5 MW may have efficiencies between 80 and 85 %. It is however difficult to extract power from low flow rates. Power from Dams (Potential Energy)
A hydroelectric dam installation uses the potential energy of the water retained in the dam to drive a water turbine which in turn drives an electric generator. The available energy therefore depends on the head of the water above the turbine and the volume of water flowing through it. Turbines are usually reaction types whose blades are fully submerged in the water flow.
The diagram opposite shows a typical turbine and generator configuration as used in a dam.
Source U.S. Army Corps of Engineers
Source: TVA
The civil works involved in providing hydro-power from a dam will usually be many times the cost of the turbines and the associated electricity generating equipment. Dams however provide a large water reservoir from which the flow of water, and hence the power output of the generator, can be controlled. The reservoir also serves as a supply buffer storing excess water during rainy periods and releasing it during dry spells. The build up of silt behind the dam can cause maintenance problems.
- Available Power
- Potential energy per unit volume = ρgh
- Where ρ is he density of the water (10^3 Kg/m^3 ), h is the head of water and g is the gravitational constant (10 m/sec 2 )
- The power P from a dam is given by
- P = ηρghQ
- Where Q is the volume of water flowing per second (the flow rate in m^3 /second) and η is the efficiency of the turbine.
- For water flowing at one cubic metre per second from a head of one metre, the power generated is equivalent to 10 kW assuming an energy conversion efficiency of 100% or just over 9 kW with a turbine efficiency of between 90% and 95%.
- Geothermal Power Plants – these harnesses the extremely hot water or steam using vertical
bore holes drilled deep underground and then uses the available geothermal steam to generate
electricity.
Then we can see that there are basically two main types of “Geothermal Energy”. One where
the energy can be used directly, as heat or hot water, and the other where it can be used
indirectly as a means of generating electricity. Geothermal is very well suited for home
heating and cooling by installing either a geothermal heat pump or a geothermal heat
exchanger for direct use. Since this energy resource is constantly being re-supplied by the sun
and the surrounding Earth, “Geothermal Energy” is a renewable energy resource.
Geothermal Energy Technologies:
- Geothermal Electricity Production
- Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found a couple of miles or more below the Earth's surface. There are three types of geothermal power plants: dry steam, flash steam, and binary cycle.
- Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.
- Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.
- Binary cycle power plants operate on water at lower temperatures of about 225°-360° F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.
- Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas, possibly even as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.
- Geothermal Direct Use
- When a person takes a hot bath, the heat from the water will usually warm up the entire bathroom. Geothermal reservoirs of hot water, which are found a couple of miles or more beneath the Earth's surface, can also be used to provide heat directly. This is called the direct use of geothermal energy.
- Geothermal direct use dates back thousands of years, when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Today,
hot springs are still used as spas. But there are now more sophisticated ways of using this geothermal resource.
- In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system - piping, a heat exchanger, and controls - delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface.
- Geothermal hot water can be used for many applications that require heat. Its current uses include heating buildings (either individually or whole towns), raising plants in greenhouses, drying crops, heating water at fish farms, and several industrial processes, such as pasteurizing milk. With some applications, researchers are exploring ways to effectively use the geothermal fluid for generating electricity as well. Geothermal Heat Pumps
The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature
between 50° and 60°F (10°-16°C). Like a cave, this ground temperature is warmer than the
air above it in the winter and cooler than the air in the summer. Geothermal heat pumps take
advantage of this resource to heat and cool buildings.
Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water. Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.
Geothermal Energy Advantages
- Geothermal energy is a well-proven energy resource that can be used to provide both heat and electricity.
- Geothermal energy is a clean energy source as no fossil fuels are burned, so there are no air pollutants.
- In most cases the geothermal fuel, the hot water is “free” for extraction.
- Geothermal energy produces little or no emissions.
- Once built, geothermal power station operating costs are small making geothermal generated electricity much cheaper.
- The steam used for electricity production is turned into water and recycled back into the Earth.
- Ground based geothermal heat pumps for heating and cooling can be used almost anywhere.
- Using geothermal energy directly for heating applications can be up to 70% more efficient.
- Geothermal energy is renewable only if the rate of fluid extraction is less than the recharge rate.
- Geothermal energy resources exist in many areas of the world for both high and low temperature applications.
Apex predators, such as orcas, which can consume seals, and short fin mako sharks, which can consume swordfish, make up the fifth tropic level. Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels.
Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers grow and reproduce slowly.
There is an exception with cyanobacteria. Marine cyanobacteria are the smallest known photosynthetic organisms; the smallest of all, Prochlorococcus , is just 0.5 to 0.8 micrometres across. Prochlorococcus is possibly the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion (~10^27 ) individuals. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans. The bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, and forms part of the base of the ocean food chain.
Osmotic Power
Osmotic power , salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electro dialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes. The key waste product is brackish water. This by-product is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.
Osmotic Power or Pressure Retarded Osmosis (PRO) is a burgeoning renewable energy source (RES) that converts the pressure differential between water with high salinity and water with lower or no salinity into hydraulic pressure. This hydraulic pressure can be used to drive a turbine that produces electrical energy. Osmotic power can occur naturally where a river meets the sea or by bringing together two manmade sources from processing plants (like a wastewater treatment or desalination facility. There are enormous benefits with osmotic power, namely that it’s the cleanest, most reliable source of renewable energy on the planet, it is always available and more cost-effective than solar or wind power, has a small ecological footprint and it recycles natures resources (seawater or wastewater).
Basics of salinity gradient power
Salinity gradient power is a specific renewable energy alternative that creates renewable and sustainable power by using naturally occurring processes. This practice does not contaminate or release carbon dioxide (CO2) emissions (vapor pressure methods will release dissolved air containing CO 2 at low pressures—these non-condensable gases can be re-dissolved of course, but with an energy penalty). Also as stated by Jones and Finley within their article “Recent Development in Salinity Gradient Power”, there is basically no fuel cost.
Salinity gradient energy is based on using the resources of “osmotic pressure difference between fresh water and sea water.” All energy that is proposed to use salinity gradient technology relies on the evaporation to separate water from salt. Osmotic pressure is the "chemical potential of concentrated and dilute solutions of salt". When looking at relations between high osmotic pressure and low, solutions with higher concentrations of salt have higher pressure.
Differing salinity gradient power generations exist but one of the most commonly discussed is pressure-retarded osmosis (PRO). Within PRO seawater is pumped into a pressure chamber
where the pressure is lower than the difference between fresh and salt water pressure. Fresh water moves in a semipermeable membrane and increases its volume in the chamber. As the pressure in the chamber is compensated a turbine spins to generate electricity. In Braun's article he states that this process is easy to understand in a more broken down manner. Two solutions, A being salt water and B being fresh water are separated by a membrane. He states "only water molecules can pass the semipermeable membrane. As a result of the osmotic pressure difference between both solutions, the water from solution B thus will diffuse through the membrane in order to dilute solution A".The pressure drives the turbines and power the generator that produces the electrical energy. Osmosis might be used directly to "pump" fresh water out of The Netherlands into the sea. This is currently done using electric pumps.
Methods
While the mechanics and concepts of salinity gradient power are still being studied, the power source has been implemented in several different locations. Most of these are experimental, but thus far they have been predominantly successful. The various companies that have utilized this power have also done so in many different ways as there are several concepts and processes that harness the power from salinity gradient.
Pressure-retarded osmosis
One method to utilize salinity gradient energy is called pressure-retarded osmosis. In this method, seawater is pumped into a pressure chamber that is at a pressure lower than the difference between the pressures of saline water and fresh water. Freshwater is also pumped into the pressure chamber through a membrane, which increase both the volume and pressure of the chamber. As the pressure differences are compensated, a turbine is spun, providing kinetic energy. This method is being specifically studied by the Norwegian utility Statkraft, which has calculated that up to 2.85 GW would be available from this process in Norway.
Reversed electrodialysis
A second method being developed and studied is reversed electrodialysis or reverse dialysis, which is essentially the creation of a salt battery. This method was described by Weinstein and Leitz as “an array of alternating anion and cation exchange membranes can be used to generate electric power from the free energy of river and sea water.”
The technology related to this type of power is still in its infant stages, even though the principle was discovered in the 1950s. Standards and a complete understanding of all the ways salinity gradients can be utilized are important goals to strive for in order make this clean energy source more viable in the future.
Capacitive method
A third method is Doriano Brogioli's capacitive method, which is relatively new and has so far only been tested on lab scale. With this method energy can be extracted out of the mixing of saline water and freshwater by cyclically charging up electrodes in contact with saline
