engineering

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The Sun Solar System

The sun is the centre of our solar system. The energy it releases warms our planet and powers all life on earth. Through photosynthesis, solar energy is transformed in to organic matter - the food that makes our lives possible (Ramlow & Nusz, 2007).it is by far the most significant source of renewable energy available on earth (Quaschning, 2005).

The sun is the largest object (star) in the solar system taking up more than 99.8% of the solar systems total mass. (www.Nineplanets.org, 2010). The remaining 0.2% of the solar system mainly consists of the planets (including earth), asteroids, meteoroids, comets and dust. The sun is classified as a G2V star based on it Stellar Classification. The G2 designates that the surface temperature of the sun is approximately 5800K (www.Nineplanets.org, 2010) and the V indicates that the sun like most stars is a main sequence star therefore generates its energy by nuclear fusion. Though the true colour of the sun is white from earth it appears yellow due to the atmospheric scattering. Therefore the sun is formally designated a yellow star because a majority of it radiation is in the yellow green portion of the visible spectrum. Earth is approximately 149.6 million kilometres from the sun though this varies throughout the year. Light emitted from the sun takes a total of 8 minutes and 20 seconds to reach the surface of the earth.

The sun is made up of a number of different elements though the two main elements are hydrogen with 73.46% of the suns total mass and helium with 24.85%. The other elements such as Oxygen, Carbon, Iron and Neon make up the remaining 1.69% of the mass. These percentages are constantly changing slowly over time as the sun converts Hydrogen to Helium within the core. Each second approximaly 700,000,000 tonnes of hydrogen is converted to 695,000,000 tonnes of helium and 5,000,000 tonnes of energy in the form of gamma rays.

This process of Nuclear Fusion involves 4 hydrogen nuclei (protons 1p) fusing to form one helium nucleus (alpha particle 4a). This one helium nucleus/ alpha particle consists of two neutrons 1n and two positively charged protons 1p. The reaction between the hydrogen to helium produces two positrons e+ and two neutrinos ve as well as generating energy (Quaschning, 2005).

As the energy from the sun travels towards the earth the energy is continuously absorbed and remitted at lower and lower temperatures therefore by the time it reaches the earth's surface it is mainly visible light (www.Nineplanets.org, 2010). From this energy produced approximately 31% of this radiation is directly reflected back in to space at the surface of the atmospheric rim. The remaining 69% is in principle available for use on earth though approximately 4.2% of the radiation that reaches the earth is reflected back into the atmosphere (Kaltschmitt et al., 2007)

Without the sun no life would be possible on earth likewise no energy would be available. The earth is heated by sun and from this wind it created. The wind energy then creates waves energy which in turn can be used for wave power. Also because the sun is heating the earth up it creates an evapotranspiration cycle which is the movement of water. From the movement of water energy can be created in the form hydro plants (currently the largest source of renewable energy today) (European Renewable Energy Council, 2004). The suns potential to make plants photosynthesize allows them to grow and store energy which is basically a chemical storage of solar energy. This chemical storage in plants can be used in a wide range of biomass products which vary from wood to rapeseed. Tidal flows are produced by the interactions between the sun and the moon. These tidal movements can be capture and converted into electricity.(European Renewable Energy Council, 2004)

Energy from the sun is by far the greatest resource available on earth. Each year a total of 3.9 x 106J = 1.08 x 1018kW of solar energy reaches the earths surface. This solar energy equates to be around 10,000 times greater than the earths yearly primary energy demand and is much greater than all reserves available for energy. Figure illustrates by way of energy cubes that the annual solar irradiation greatly exceeds the total global energy demand and all total energy reserves.

Solar energy

The solar energy is received in earth and by heating the planet the sun generates wind. This wind then creates waves that can be used for wave power. The radiation from the sun also controls the evapotranspiration cycle which allows power to be generated by hydro plants (currently the largest source f renewable energy today) (European Renewable Energy Council, 2004). The suns potential to make plants photosynthesize which is basically a form of chemical storage of solar energy. This chemical storage in plants can be utilised in a wide range of bio-mass products which vary from wood fuel to rapeseed. Interactions between the sun and the moon produce tidal flows that can be intercepted and used to produce electricity (tidal movement mainly caused by the moons gravational pull with a small contribution from the suns gravity (European Renewable Energy Council, 2004).

Renewable energy sources have the capability to provide everything that fossil fuels currently have to offer in terms of energy though improvements in the efficiency of renewable is needed.

Wind Energy

Civilisations have been using wind as a source of energy for more than 100 years. Wind has been used from the very early stages to sail boats right up till today where it is used to provide a source of electricity. Wind energy was first used in the 1880 by P. La'cour to produce electricity.

Wind turbines can be either categorised in to horizontal axis or vertical axis turbines based on the positioning of their rotors. In a horizontal axis turbine the rotor axis are kept horizontal and aligned parallel in the direction of the wind. Where as in the vertical axis turbine the rotor axis is vertical and fixed, and remains perpendicular to the wind stream (Kothari). Wind turbines are usually fitted with blades, sails or buckets which are connected to a central shaft which rotates. The rotating of this shaft can be used to drive a pump or generate electricity.

Vertical rotor axis turbines

Wind turbines that incorporate rotor concepts with vertical axis are:

  • The Savonius Rotor
  • The Darrieus Rotor
  • The H-Rotor

The Savonius rotor was first developed in Finland in 1920 by an engineer called Savonius. This was one of the first vertical rotors which work on the drag principle. It is made up of two half-cylindrical blades placed facing each other. The blades slightly overlap to allow the redirected wind to flow from one blade to the other causing the driving force. The efficiency of the Savonius rotor is slightly better than one of a simple drag device as it also utilises the lift principle. However, the efficiency is much worse than that of a good lift device, reaching maximum power coefficients of the order of 0.25 (Hau, 2000). As with all rotors there is advantages and disadvantages the main advantage of this type of rotor is that they can operate at very low wind speeds though the disadvantage of them is that they have a high material.

The Darrieus rotor was first developed in 1929 by a French engineer called Georges Darrieus. The rotor is made up of two or three thin parabola blades made usually from flexible metal strips. The shape of the blade relates to the lift principle which the Darrieus rotor works. The Darrieus rotor is much more efficient than the Savonius rotor though it efficiency is only about 75% of a modern rotor with horizontal axis. The major disadvantage of a Darrieus rotor is that it is unable to start on its own but needs an auxiliary device such as a drive motor or even a coupled Savonius rotor to start.

The H-Rotor also known as the H-Darrieus rotor is another type of vertical axis turbine which can be used in wind energy. The H-Rotor has a permanent magnet generator directly built in to the structure. The H-Rotor works on the same principle as the Darrieus rotor though the three rotor blades are connected vertically. The H-Rotor was designed for extreme winds such as those found in the Antarctica or high mountains.

Horizontal Axis Systems can be made up of anything from 1 (low solidity) to 20+ (high solidity) blades from a single blade with contour weight to a multi-blade type. The most commonly produced wind turbine is a low solidity HAWT came from the first windmills. The rotors of a low solidity resemble an aircrafts propellers and their design is derived from the development in aircraft wing and propeller design.

The majority of wind farms today use rotors with horizontal axis. The main reason for the use of horizontal system over a vertical is that they have a greater efficiency and less of a material demand though the vertical has advantages like the generator and gearbox can be situated on the ground making the maintenance easier, a vertical axis system does not need to rotated to face the wind and that they are perfect for areas with high wind speeds. To date these advantages have not changed the decision to install horizontal systems.

The amount of wind energy available in the UK could produce well in the region of 1000TWh/year. This is more than the UK's total electricity demand. This also applies for the USA as they could also produce enough electricity per year through wind power to supply their total electricity demand (Quaschning).Germany is the leading country in the wind power market and in 2002 and had a turnover of nearly 4 million and created 45,000 new jobs (Quaschning).Altogether, 15,797 wind generators with a total capacity of 15,327 MW and an electricity generation potential of 30 TWh/year were installed in Germany by mid-2004 (Ender, 2004). This equates to nearly 6% of Germanys electricity demand.

Solar Kilns

A solar kiln is used to dry different products on a large scale such tea, corn and timber. The products are heated and dried by the use of solar energy; the kiln is made up of a transparent sheet of glass or plastic which allows solar radiation in but stops long wavelength radiation emitted from products such as timber. As with all drying process there are a number of factors which affect it such as the relative humidity and temperature of air, the rate at which the air is flowing, how saturated the product being dried is and to what degree the product is to be dried to.

As the air is circulated around it carries the heat from the solar absorbing plate through the fan assembly which aids the movement and circulation of the air. This heated air then passes through the product to be dried and evaporates any moisture within. There are two different types of solar kilns a Integrated Solar Dryer or a Distribution Solar Dryer.

Solar Distillation

A solar distillation is a device that produces potable water from the use of solar heat energy also known as a solar water still shown in Figure 1 (A Typical Solar Still) The solar still is used to distilled saline water either from underground or the ocean which can then be consumed in areas where safe drinking water is scarce. A solar still is a very basic concept, it is made up of a blacked bottomed basin, transparent air tight glass/plastic and a drain trough. Saline water is placed in the black bottomed basins at a shallow depth then covered with either the glass or plastic which is slanted and sealed tight. The solar radiation passes through the transparent cover and heats up both the saline water and black basin. The water is heated this way till the point when vapour is formed and condensates against the cold surface of the cover. This condensate water then runs down cover to be collected in troughs which are connected to a storage tank ready for consumption. A solar still is capable of producing in the region of 3 litres/m2/Day if situated in a good position. The performance of the solar still depends on a number of factors such as the intensity of solar radiation, the ambient air temperature, wind speed and cloud cover. The desalination process increases it output if there is a rise in ambient air temperature and is independent of the salt content int the raw water feed.

Solar Crop Drying

Agriculture products such as crops (dates, grapes, apricots, cashew nuts and chillies) can be dried by the means of solar energy to avoid any losses between harvesting and consumption. Crops that high in moisture content are more prone to fungus contamination and attack by pests and insects. Solar crop driers are able to dry crops without any ingression of dust, therefore meaning the product can be preserved for a greater length of time. A solar dryer is very simple in design and is only made up of an area covered with a transparent cover as shown in Figure 1 (A Typical Solar Crop Drier).

A Typical Solar Crop Drier

The crop dryer works on the basis of natural ventilation and the stratification of air. There are openings in both the bottom and top allow the air to circulate around. The crops that are to be dried are spread across perforated trays within the dryer. As the solar radiation enters the dryer it is absorbed by the crops and the internal surfaces of the dryer. This means that the inside of the dryer is getting warmer and evaporating any moisture in the drier. Temperatures inside the dryer can usually range from 50C to 75C which is ideal for drying most crops such as those listed above. The drying of the crops depends on a number of different factors but usually range from 2 to 4 days. Where larger scale drying is required such as timber, corn, tea and tobacco curing a solar dryer would not be practical and a solar kiln should be used.

Solar Crop Drying

Agriculture products such as crops (dates, grapes, apricots, cashew nuts and chillies) can be dried by the means of solar energy to avoid any losses between harvesting and consumption. Crops that high in moisture content are more prone to fungus contamination and attack by pests and insects. Solar crop driers are able to dry crops without any ingression of dust, therefore meaning the product can be preserved for a greater length of time. A solar dryer is very simple in design and is only made up of an area covered with a transparent cover as shown in Figure 1 (A Typical Solar Crop Drier).

The crop dryer works on the basis of natural ventilation and the stratification of air. There are openings in both the bottom and top allow the air to circulate around. The crops that are to be dried are spread across perforated trays within the dryer. As the solar radiation enters the dryer it is absorbed by the crops and the internal surfaces of the dryer. This means that the inside of the dryer is getting warmer and evaporating any moisture in the drier. Temperatures inside the dryer can usually range from 50C to 75C which is ideal for drying most crops such as those listed above. The drying of the crops depends on a number of different factors but usually range from 2 to 4 days. Where larger scale drying is required such as timber, corn, tea and tobacco curing a solar dryer would not be practical and a solar kiln should be used.

Passive Solar

There are two types of passive solar heating systems that can be integrated to most buildings. A passive building system carries out all functions such as the collection, storage and distribution by the building materials and does not incorporate the use of electrical or mechanical controls.

  • Direct System Gain

A direct system is designed to heat a room using the direct radiation from the sun in the winter but blocking it the summer. One way of doing this is to have a overhanging roof which blocks the summer sun as it is higher in the sky but allows the lower winter sun rays enter the room. Figure 1 (Direct Passive Heat Gain) shows the angles at which the sun's rays fall during summer and winter. A direct system is normally used in a colder climate to try and maximise the use of solar energy. There are a number of different ways in which to improve a direct heat gain building such as having south facing double glazed windows which would maximise the amount of sunlight available during the winter months, incorporating a shading device into the building either by a roof overhang or a brise soleil system which will provide shading in the summer. Also by having a large area of floor and walls this acts as a thermal store heating up during the day and gradually releasing the heat in the evening and night.

  • Indirect Gain Systems (Trombe Wall)

An indirect gain system such as a Trombe wall can be used to control the temperature of a room more effectively compared to a direct gain system. As with the direct system gain there is variation in air temperature trough out the day. A way in which to reduce the variations and maintain a constant temperature is by incorporating a Trombe Wall in to the design a thermal store between the glazing and the living space.

A Trombe wall is a wall is usually 400mm or thicker which is facing south and painted black to absorb the heat radiation from the sun. The wall is normally made of materials which are good absorbers and stores of heat such as concrete. A type of glazing either glass or plastic (double glazed with air gap) is sited a few inches in front of the wall which helps trap any heat in and minimises losses. During the day the sun heats up the Trombe wall and at night this heat is gradually given off by the wall to heat the building.

As the air between the glazing and the Trombe wall is heated the hot air flows from the bottom to the top of the Trombe wall and out through opening A due to natural ventilation. As the hot air in the room cools it sinks and is drawn in through opening B creating a natural circulation of air.

In the summer months vent A at the top of the Trombe wall is kept closed and vents B,C and D are opened. Air trapped between the glazing and the Trombe wall is exhausted through vent C therefore air flows in to the void from the room pulling fresh air into the room through vent D that is located in a cool shaded area. As in like the direct gain system a roof overhang can be incorporated in the design to minimise heat gains in the summer.

Hydro power

Hydroelectricity is a well established technology which has been producing relaible power at very competitive prices for about 100 years. In total the electricty produced covers about 18% of the worlds annual electrical output. It also accounts for about 90% of the electricty from renewables. (BP Boyle 192).

Each year the sun evapourates on average 980 litres of water per every square meter of the earths surface (500,000km3). Out of the suns total solar radiation reaching the earth 22% is required to maintain this water cycle.

For A hydro electric power plant to operate there must be a driving force created. This can be done by damming the source to create a weir in which head can accumulate. It is this potential difference in height that can be then used to produce power. The head of water forces through the turbine, which is able to turn the potential energy into mechanical energy. For the production of electricity this mechanical energy would be turning an electrical generator. There are three common turbines that are used: Pelton, Francis or Kapian turbines which can be used depending on the head height and flow rate. Finally a transformer converts the generator voltage to the grid voltage.

Some areas in the world use excess power generated to pump water up to a high level for storage in a basin of dam. This method is called pumped-storage hydroelectric power plant and is basically a means of electricity storage. When this water is allowed to flow back down through the turbine it will be again converted the potential energy back in to electricity.

Solar air-conditioning and refrigeration

Solar air-conditioning and refrigeration use the solar energy from the sun to either cool buildings or refrigeration of food. This type of cooling is most beneficial in countries with good solar gains. This is mainly because when the sun is at its strongest normally the cooling demand will be at its highest. There are three different types of solar cooling evaporate cooling, absorption cooling and passive desiccant cooling.

Evaporate cooling is best suited to hot dry climates and works on the basis that when air is used to evaporate water the air itself will become cooler and therefore can be used to cool any living space. There are two common ways that are used for cooling either the vapour absorption or vapour compression out of these two vapour absorption is seen the more practical as there is a link between the amount of cooling needed and the amount of solar radiation needed.

A typical schematic of a solar absorption cooling system is shown below. The system works on the basis of five major components: the flat plate collectors, generator, condenser, evaporator and absorber. Water is circulated by the use of a pump around the flat plate collectors and the heat exchanger in the generator. The generator is filled with a chemical solution suited best for absorption cooling such as NH3-H2O or LiBr-H2O. This chemical it therefore heated up by the water from the solar collectors. The refrigerant vapour is boiled off at a high pressure and pumped to the condenser. At the condenser the refrigerant is condensed rejecting the heat therefore turning to the state of a high pressure liquid. The refrigerant then passes through the expansion valve and evaporates in the evaporator. The refrigerant vapour is then absorbed into a solution mixture taken from the generator in which the refrigerant concentration is quite low. A heat exchanger is provided to transfer heat between solutions flowing between the absorber and the generator (Kothari)

The passive desiccant cooling method is best suited for warm humid climates. The dehumidification of room air can be done by either the absorbent or the adsorbent followed by the evaporative cooling of the air. This is the most workable air-conditioning method for use in a hot humid climate. Materials normally used for the desiccant wheel are silica gel, molecular sieve and triethylene glycol as all these materials have a high affinity for water vapour.

In the desiccant cooling process the air from the room which is hot and humid is first dehumidified either by a solid or liquid desiccant, then cooled by the exchange of sensible heat and finally it is evaporatively cooled.

A typical schematic of a solar dehumidification and evaporative cooling. The absorbent the system is using is an organic liquid absorbent triethylene glycol (TEG). To ensure rapid absorption of water vapour in the absorption chamber the TEG is atomized. The TEG is then pumped through the heat exchanger to the stripping chamber (regenerator), sprayed counter-currently to heat the air from the solar collectors. The hot air removes part of the moisture from the TEG solution and exhausts it to the atmosphere. The hot TEG is them pumped back a heat exchanger and to the dehumidifier/absorption chamber. The desiccated air from the dehumidifier passes through the evaporator cooler them to the room to be supplied.