Solar Energy Lec. 3 PDF

Solar Energy Lec. 3  Content Sun Solar thermal energy Nature Solar thermal types EM (electromagnetic) Spectrum Non-concentrating solar Visible...

Solar Energy Lec. 3  Content Sun Solar thermal energy Nature Solar thermal types EM (electromagnetic) Spectrum Non-concentrating solar Visible collectors Components of radiations Flat plate collector Green house Effect Evacuated Glass Tube Heat transfer Collectors   SUN The sun is a big ball of plasma composed primarily of hydrogen (92%), helium (8%), and small amounts of other atoms or elements. A plasma is where the electrons are separated from the nuclei because the temperature is so high (kinetic energy of nuclei and electrons is large). The sun is a stable main sequence star with an estimated age of 4.5 * 109 years and will continue for another 4 to 5 * 109 years before starting the next phase of evolution, the burning of helium. At that point, the sun will expand and be larger than the orbit of the Earth.  The sun’s surface, the photosphere, has a temperature of about 6000 K. There are “cold” sunspots, that is, regions with a diameter of up to 50,000 km, having a temperature of about 3800 K. The sun’s corona with a temperature over 1 million kelvin extends millions of kilometres into the space. The sun also emanates solar wind, that is, a low-density stream of charged particles propagating through the solar system at a speed of 450 km/s.  Nuclei are composed of nucleons, which come in two forms: protons (which have a positive charge) and neutrons (no charge). The mass of one helium nucleus is less than the mass of four protons. This mass difference Δm, called mass defect, is converted to energy according to the Einstein’s formula E = Δm c2, where c is the speed of light equal to 3 × 108 m/s. That energy is transferred to the surface of the sun, where EM (electromagnetic) radiation and some particles (solar wind) go off into space.  This tremendous amount of energy is radiated into space from the surface of the sun with a power of 3.8 * 1023 kW. The Earth only intercepts a small portion of the sun’s power; however, that is still a large amount. At the top of the atmosphere, the power intercepted by the Earth is 1.73 * 1014 kW, equivalent to 1.35 kW/m2. Remember that this surface is perpendicular (90°) to the sun. If a surface is at an angle to the sun, the same amount of energy is spread over a larger area. At the surface of the Earth on a clear day, this solar insolation is around 1.0 to 1.2 kW/m2 on a surface perpendicular to the sun from 9 to 15 h, depending on the amount of haze in the atmosphere and on elevation.   Nature Particles Waves They have mass, They have no mass They are localized in space They are spread out over space; They can have charge and other They obey the principle of properties superposition, which means that no two particles can occupy the two or more waves can occupy same space. the same space at the same time.   EM (electromagnetic) Spectrum EM waves travel at the speed of light and are described by their wavelength and frequency, which are related by where c is the speed of light, 3 * 108 m/s; λ is the wavelength, m, and f is the frequency in hertz, which is the number of cycle/second (as a wave moves by a point, the number of peaks or crests per second).  EM radiation consists of oscillating electric and magnetic fields, perpendicular to each other and perpendicular to the direction of motion of the wave. The EM spectrum is the range of EM radiation from very short wavelengths (large frequency) to very long wavelengths (small frequency). Sometimes, there is confusion between light, which refers to all EM waves, and the visible range.  At the atomic level, the two ways of describing nature are combined, so EM waves come in units called photons; their energy is given by where h is Planck’s constant, 6.6 * 10−34 kg m2/s. Large frequency corresponds to high-energy photons , such as x-rays and gamma (γ) rays, which can go through materials that absorb visible light and can cause damage to the materials. Low-frequency EM radiation can also go through materials that absorb light (e.g., radio waves go through the walls of houses).  Visible The range of the spectrum that we can see, visible light, is small, with red light (7 * 10−7 m) having a longer wavelength than blue light (4 * 10−7 m). A rainbow is a familiar example of the colours that we can see. White light is just a superposition (combination) of all the colours. All the different colours we can see and generate are just absorption and reflections of different parts of the visible spectrum.  Components of Radiation  The global solar radiation incident on the earth’s surface comprises beam (direct) and diffuse (sky) solar radiation.  Beam radiation is used to describe solar radiation traveling on a straight line from the sun down to the surface of the earth.  Diffuse radiation describes the sunlight that has been scattered by molecules and particles in the atmosphere but that has still made it down to the surface of the earth.  Insolation is the solar radiation at the surface of the Earth and is given in units of energy/square meter (for some time period, which is really power) or power/square meter, which is generally for an average by day, month, or year. It depends on the geographical latitude φ, local climatic conditions, season, time of the day, and the cloudiness of the sky. On clear days, that insolation is around 1.0 kW/m2.   Greenhouse Effect The greenhouse gases are water vapour, carbon dioxide, methane, and other trace gases. A large atmosphere of carbon dioxide can drastically change temperature at which the energy balance occurs, with Venus a drastic example(Venus is a hellscape with atmospheric pressure 90 times that of Earth and surface temperatures of more than 400 degrees Celsius). There is an increase in carbon dioxide in the atmosphere due to the increased use of fossil fuels, and most scientists say this results in global warming.   Heat Transfer The electromagnetic radiation is absorbed and becomes thermal energy (heat); therefore, we need to know how heat is transferred. Heat is just internal kinetic energy of a material and is the random motion of atoms and electrons. Heat only flows in one direction, from hot objects to cold objects, high temperature to low temperature. To move heat from a cold place to a hot place requires energy. For example, the air conditioner for your home uses a lot of energy. Heat can be transferred by conduction, convection, and radiation.  1. Conduction Conduction is the transfer of heat in a solid. If one side or end is at a high temperature and the other is at a low temperature, heat flows from the hot to the cold side. The amount of heat loss or gain by conduction is given by where A is the area, ΔT is the temperature difference, U is the thermal conductance and h is the number of hours.  Resistance (R) is a property of a material to retard the flow of heat and is the inverse of conductance. Good insulators have a high R value, which is low conductance. The total R value for a composite material is just the sum of the R values of the component parts. In the winter, you want to reduce the heat loss, and in the summer, you want to reduce the heat gain of the house. EXAMPLE (1)  2- Convection Convection is the transfer of heat by the movement of fluids—gases or liquids. Heated fluids can move by natural convection or by forced convection by pumps and fans. In natural convection or thermosiphoning, as a fluid is heated it expands and becomes less dense, thereby the hot air and the hot part of a liquid rise while the cooler part descends. In forced convection, the quantity of heat moved depends on the amount of fluid moved (rate) and the heat capacity of the fluid. It takes a lot more air than water to move the same amount of heat.  Calculation of infiltration, convection heat loss or gain through open doors and cracks, is just an educated estimate. The reduction of infiltration (in or out through the exterior) is more important and will save energy. where c is the heat capacity of the fluid, Q is the volume of air leakage per length of crack per hour, L is length of crack, ΔT is the temperature difference, and h is the number of hours. Example (2) solution  3-Radiation Radiation is energy that moves from one place to another in a form that can be described as waves or particles. Some of the most familiar sources of radiation include the sun, microwave ovens in our kitchens and the radios we listen to in our cars. Most of this radiation carries no risk to our health. But some does. In general, radiation has lower risk at lower doses but can be associated with higher risks at higher doses. All objects emit electromagnetic radiation, and the amount and wavelengths depend on the temperature.   Solar thermal energy Solar thermal energy technologies capture the heat energy directly from the solar radiations, to be used for heating purposes and to produce electrical energy. Solar thermal energy is quite different from the photovoltaic (PV) solar panels (capable of direct conversion of solar radiations into electricity).  The solar thermal Types Active solar thermal system Passive solar thermal system It requires continuously moving It have no such mechanical parts, such as pumps and fans, for structures, and these mainly rely on the circulation of fluids carrying the features of design, to capture the heat energy. the heat energy. EX.: The active solar thermal EX.: the design of a building to systems are usually equipped with regulate the amount of solar energy the rood mounted flat plate it receives in order to regulate it's collectors for the circulation of temperature. liquids or fluids.  Low temperature less than 100C temperature are commonly used for space heating and water heating. Medium temperature ranging from 100 C to The solar temperature thermal Types 250 C used as solar ovens temperature greater High than 250 C, use groups of reflecting temperature mirrors for the concentration of solar radiations on the central collector.  Active Solar Thermal Energy Solar Collectors An active heating system requires power for pumps or fans for moving the fluid (air, water, or even silicone) and power for the controller for turning on pumps, valves, and so on. A solar collector is a device that uses solar energy to heat a heat transfer fluid (HTF), such as water, air, thermo-oil, or molten salt. The collector absorbs a large portion of the incident solar radiation and converts it to useful heat. A certain part of incident solar radiation is lost due to reflection and heat transfer to the ambient environment.  Solar Collectors Non- Concentrating concentrating (focusing) solar solar collectors collectors  Non-concentrating solar collectors Non-concentrating collectors absorb both beam and diffuse radiation, and therefore still function when beam radiation is cut off by cloud. This advantage, together with their ease of operation and favourable cost, means that non-focusing collectors are generally preferred for heating fluids to temperatures less than about 80°C. Non-concentrating Collectors are classed either as flat plate or as evacuated collectors   Flat Plate Solar Collector 11 A flat plate solar collector has an absorber plate made of a material with high thermal conductivity, for example, copper or aluminium, a transparent cover made of solar glass, and tubes for collector HTF flow. The whole structure is assembled in a sealed housing with thermal insulation at the sides and bottom. The absorber can absorb up to 95% of incident solar radiation. The absorbed energy is partially (up to 50–70%) used for heating of the collector fluid HTF; the rest is lost into the environment.  A transparent cover reduces the collector energy losses caused by reflection, radiation, and convection, while thermal insulation reduces the heat losses by conduction. An FPC has a simple design and is suitable for heating water (and other liquids) or air up to 100°C. Typically the tube diameter is ~2 cm, the tube spacing ~20 cm and the plate thickness ~0.3 cm. In cold climates, two or three glazings are needed, and more insulation of the box is needed to reduce heat loss. The efficiency changes with number of glazings and the temperature differential between the outside and the absorber plate  13 https://ars.els-cdn.com/content/image/1-s2.0-S0957417407003685-gr1.jpg  14 https://cdn-ffejk.nitrocdn.com/iXdMqITbhAcxlhSYvKLmYwuGezdKjtzs/assets/images/optimized/rev-7ad4ad7/www.solartubs.com/images/2023/10/12/flate-plate-image-with-labels.png   Evacuated Glass Tube Collectors 15 ETC collectors are theoretically capable of heating the collector HTF up to 250°C. They are well suited to commercial and industrial heating applications and also for cooling applications by regenerating refrigeration cycles. They can also be used for domestic space heating, especially in regions where it is often cloudy.  EGT Construction An ETC consists of several glass tubes with a diameter of 50–100 mm and length of 2 m connected in parallel to a hot fluid header. An absorber copper plate is placed inside the glass tube and is connected to a copper U-tube or a heat pipe. The heat pipe is an efficient heat transfer device consisting of a sealed tube that is partially filled with an appropriate working fluid, which undergoes phase change, liquid to vapour in the evaporation zone where the solar radiation is absorbed. The working fluid vapour gives the heat away at a higher temperature in the condensation zone. This heat is used to heat water from Tcold to a required temperature Thot. 16  17 https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcT_JGlrryjElpAqwq5kY91ZyN38-c6NfwVWA8_5ebqWrqNWwBdp  18 https://5.imimg.com/data5/RH/BX/MY-5919580/evacuated-tube-solar-collector-500x500.jpg  ………...is the energy stored in an object or system of objects. a) Potential energy b) Kinetic energy A c) Electrical energy d) Bio energy "An incandescent light bulb that is turned off" is an example of …… a) Chemical energy b) Potential energy B c) Electrical energy d) Both B and C  ………….. From the advantages of non-renewable energy. a) Low energy output b) No pollution out C c) Abundance and affordability d) All of the previous ……..energy are exhaustible and cannot be replaced once they have been used up. a) Coal b) Nuclear c) Both A and B C d) Hydro  What is the rate of solar energy reaching the earth surface? a) 1016W b) 865W c) 2854W A d) 1912W Flat plate collector absorbs ______ a) Direct radiation only b) Diffuse radiation only C c) Direct and diffuse both d) None of the above  Which of the following is used to measure extraterrestrial radiation? a) Pressure b) Watts/square meter c) Joules/square meter B d) Torque Which of the following energy has the greatest potential among all the sources of renewable energy? a) Solar energy b) Wind Energy A c) Thermal energy d) Hydro-electrical energy