Solar Energy Exercises

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Exercises I. Essay Questions 1. What does each of the angles of altitude, azimuth, latitude, and longitude represent? What angles are needed to locate the position of the sun from a point in space? From a point on the surface of the earth? What are factors that affect solar insolation? 2. How do solar chimneys work, and how can they be used for generating electricity? 3. What are advantages and disadvantages of passive and active solar energy systems? 4. What are the principles of operations of OTEC and solar ponds? 5. How do photovoltaics work? What are their advantages over solar thermal for power applications? 6. What are the advantages of electrically conductive plastics over solar cells? II. Multiple Choice Questions 1. The sun is a sphere consisting mainly of a. A solid core covered by a layer of molten liquid and hot gases b. A liquid core covered by a mixture of hydrogen and helium gases c. A mixture of hot helium and hydrogen gases d. A mixture of noxious ammonia and sulfur gases e. The same material that makes the planet earth 2. Solar thermal energy has its origin in the heat released by the a. Various chemical reactions at the center of the sun b. Various chemical reactions at the surface of the sun c. Fission reactions occurring at the center of the sun d. Fusion reactions occurring at the center of the sun e. Latent heat as liquid hydrogen converts into gaseous form 3. Solar energy is ultimately responsible for which kind of energy? a. Fossil fuel b. Photovoltaic c. Wind d. Wave e. All of the above 4. The sun’s interior and exterior temperature are roughly a. 5 million and one million degrees celsius b. 10 million and 10,000 degrees celsius c. 20 million and 5,500 degrees celsius d. 50 million and 10 million degrees celsius e. 5,000 and 300 degrees celsius 5. Seasons are primarily a result of a. Rotation of the earth b. Rotation of the sun c. Varying distance of earth from the sun d. Tilt of the earth e. Orbiting of the earth around the sun 6. Which of the following is not a form of solar energy? a. Biomass b. Wind c. Wave d. Hydropower e. Geothermal 7. The fraction of solar energy that falls in the visible range is a. Less than 1% of the sun’s total radiant energy b. About 10% of the sun’s total radiant energy c. About 50% of the sun’s total radiant energy d. About 90% of the sun’s total radiant energy e. More than 99% of the sun’s total radiant energy 8. In what frequency range does the earth radiate the most? a. Cosmic ray b. Ultraviolet c. Visible 239 Chapter 10 - Solar Energy d. Infrared e. Radio wave 9. The infrared portion of solar radiation consists of roughly a. 1% of total radiation b. 10% of total radiation c. 25% of total radiation d. 46% of total radiation e. 90% of total radiation 10. The angular location of a point on earth relative to the equator is called the a. Latitude b. Altitude c. Zenith d. Meridian e. Azimuth 11. The angle between a north-south line on the earth’s surface and the horizontal projection of the sun’s rays is a. Latitude b. Altitude c. Zenith d. Meridian e. Azimuth 12. The sun’s location relative to an observer on the ground can be determined by a. Azimuth and altitude b. Latitude and altitude c. Azimuth and latitude d. Latitude and longitude e. All four angles 13. Albedo is a. The fraction of incident light that is absorbed by earth b. The fraction of incident light that is reflected by earth c. The fraction of incident light that is transmitted through the atmosphere and reaches earth. d. An organism exhibiting deficient pigmentation e. Unnaturally strong sexual drive in humans 14. Solar insolation at a point on the earth depends strongly on a. The day of the month, the time of the day, and the weather b. The topography of the area and the amount of shade c. The albedo of the earth d. The type of clothing we wear e. All of the above 15. The best orientation for installing fixed flat plate collectors is a. Parallel to the ground surface, because maximum radiation occurs when the sun is directly overhead b. Tilted at an angle roughly equal to the latitude of the location where they are going to be installed c. Tilted at an angle roughly equal to the complement of the latitude of the location where they are going to be installed d. Perpendicular to the ground surface to occupy the least space e. It does not make any difference, because the sun is traveling across the sky 16. Flat plate collectors a. Are large arrays of photovoltaic cells b. Work by using sunlight to electrolyze water to its components hydrogen and oxygen c. Are faced toward the wind to catch the most wind d. Are useful for heating domestic water heating systems e. None of the above 17. The majority of the solar radiation reaching the earth’s surface is a. Ultraviolet and infrared b. Infrared and radio wave c. Visible and ultraviolet d. X-ray and gamma ray e. Visible and infrared 18. An overhang on a southern-facing window is useful because a. It blocks the summer sun while enhancing 240 heating in the winter b. It prevents rain from hitting the side of the house c. It prevents squirrels from looking in d. It makes for more space in the ceiling for insulation e. It allows rain to drain efficiently 19. For maximum efficiency, flat plate solar panels a. Must be horizontal b. Must face south and vertically c. Must be painted white d. Must face north e. Must be perpendicular to the sun’s rays at all times 20. The best solar system for producing process steam at temperatures of 300-400oC is a. Flat plate collector b. Parabolic trough c. Parabolic dishes d. Heliostat with flat mirrors e. Photovoltaics 21. The primary drawback of electricity produced by solar energy is that it is a. Intermittent b. In the wrong frequency c. Unpredictable d. Dispersed e. All of the above 22. Some of the byproducts of an OTEC plant include a. Desalination b. Space air conditioning c. Chilled soil agriculture d. Industrial cooling e. All of the above 23. OTEC systems a. Can be used anywhere in the northern hemisphere b. Can be used only with areas with hot summers c. Is best in latitudes of between 10o N and 10o S d. Only in Hawaii e. Oceans with a minimum of 50oC difference between surface and deep water temperatures 24. OTEC technology can be used to a. Generate a large amount of electricity without the environmental impacts associated with fossil fuels b. Produce a large amount of drinking water c. Produce some fruits, vegetables, and marine organisms otherwise impossible to produce in a tropical environment d. Act as a large heat sink for many industrial processes e. All of the above 25. The main advantage of solar cells over other methods of electricity generation is that a. They are cheaper than the power the utilities can produce by burning coal b. Most utility plants are located in sunny areas c. They are clean sources of energy with no known adverse environmental impact d. Their power correlates with the utilities’ daily load patterns, because the power is available when it is needed most – during daylight hours e. Both c and d 26. Which one of the following ways of harnessing solar energy does not involve thermal energy? a. Active heating b. Passive heating c. Indirect generation of electricity d. Direct generation of electricity e. Passive cooling 27. PV cells are most frequently used in a. Consumer products, such as calculators, wristwatches, and solar-power radios b. Remote power applications of 100 W or less c. Areas close to or far from utility grids d. Satellites e. All of the above 28. The most common material used in PV modules today is a. Single crystalline silicon 241 Chapter 10 - Solar Energy b. Polycrystalline silicon c. Amorphous silicon d. Cadmium telluride e. Gallium arsenide 29. Which of the following statements is correct in regard to solar ponds? a. For a solar pond to be effective, surface water temperature must be about 50oC warmer than the water at the bottom of the pond. b. Temperature as high as 95oC can be reached at the bottom. c. Works best in areas of high winds. d. Works best in the same geographic areas that OTEC technologies are applicable. e. All of the above. 30. Thermophotovoltaics a. Is another name for solar cells b. Works best with visible lights c. Is most sensitive to ultraviolet lights d. Used mainly when waste heat is available e. Operates only when temperatures in excess of 2000oC are available III. True or False? 1. The Sun is the ultimate source of all our energy. 2. Albedo is another name for reflectivity of the surfaces on the earth. 3. Factors affecting light intensity are cloud cover, humidity, and atmospheric conditions. 4. The total energy incident on a southward-facing window is greater during summer than winter. 5. OTEC technology is an economical way of producing power from a few kW to hundreds of MW. 6. The fraction of solar energy in the visible range is called solar insolation. 7. Since dishes have a smaller aperture than trough reflectors, these systems are best suited for small-scale power production or as a stand-alone unit in remote areas away from power grids. 8. Both solar ponds and OTEC plants exploit the difference between higher surface temperatures and colder temperatures in the bottom layers. 9. The higher the frequency of photons, the more energy and the higher efficiency solar cells will have. 10. Since the output of solar cells is in the form of direct current, they cannot be directly fed into electrical grids. IV. Fill-in the Blanks 1. The angular distance measured east or west from the Prime (International) Meridian is called the ___________. 2. The amount of solar energy that reaches a surface on earth depends on the position of the surface and __________. 3. The first OTEC plant was constructed off the coast of ____________. 4. For a solar pond to work, there must be a higher concentration of _________ at the bottom than at the top. 5. A typical 10 cm by 10 cm commercial cell produces as much as _________ watts of electrical power. 6. ___________ cells are advantageous over flat panel cells, because flat panel cells only collect maximum sunlight if the sun is directly overhead. 7. Current PV technologies have typical commercial module conversion efficiencies between ____________ percents. 8. OTEC systems using water as the working fluid are of the ___________ type. 9. _______________ plastics are special kinds of plastics that collect light and convert it to electricity. 242 10. The higher the ____________, the higher will be the collection efficiency of the incident solar radiation. V. Project I - Solar Power Generation The objective of this project is to design a power generation plant capable of producing 10 MW of electricity. The power plant is located in a tropical region along a coast with plenty of sunshine. The ocean water is 27°C at the surface but drops to 12°C at a depth of about 500 meters. On the average, 1,200 W/m2 of solar insolation is available. Three options are being considered: a. Photovoltaic solar cells using poly-crystalline silicon cells with an efficiency of around 15%. b. A closed OTEC system using ammonia as the working fluid. Ammonia has a maximum temperature of 25 degrees and a minimum temperature of 15 degrees centigrade. 30% of the power generated is wasted through various frictional losses. c. A solar heliostat composed of plane and parabolic mirrors. The heliostat can heat water to steam at 500°C. Calculate: 1. For option a, the total area of photovoltaic cells required. 2. For option b, the theoretical maximum and actual efficiency of the OTEC plant. 3. For option c, calculate: a. The ideal Carnot efficiency (Eq. 5-1), b. The total amount of heat that must be disposed into the atmosphere. Hint: Recall that efficiency is the ratio of work delivered per heat input, and that conservation of energy requires that the part of energy not converted to work must be disposed into the atmospheric sink. Project II - Solar Photovoltaics for Homes You are asked to evaluate the economics and the environmental merits of building a solar electric system for your home instead of buying electricity from your local utility. Obtain a copy of your electric bill (desirably for a peak month) and note the following: a. The monthly electric bill ($) b. The total electric consumption for the month (kWh) Calculate: 1. Average cost of unit electricity ($/kWh) 2. Average power (kW) 3. Average solar insolation for your area. Insolation maps and tables are generated from historical data and are readily available for many cities. US data can be found from the National Renewable Energy Laboratory web site at (http://rredc.nrel.gov/solar/pubs/redbook). 4. Pick a commercial manufacturer and determine cost, peak and average power delivered, and efficiency. How much solar cell do you need? What is the cost? 5. Calculate peak power available from your solar system. 6. Estimate the total cost by adding the cost of the cell, the inverter, and other control devices you may need. Add 50% for the cost of installation. 7. What is the payback period? 8. Assuming your utility company uses natural gas to produce electricity, how much less carbon dioxide are you producing annually (tons/year) when you choose the solar option?