Chapter 10 CSULB

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Chapter 10
Solar Energy
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The Sun
Material
Size
Temperature
Radiation Intensity
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Radiation Spectrum
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Solar Energy 
The amount and intensity of solar energy available vary depending on:
Geographical location, 
Position of the sun in the sky, 
Weather conditions, and 
Orientation of the collector and surrounding objects.
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Solar Position
 The geometric position of the sun 
Altitude (q)
Zenith (p/2 – q)
Azimuth (f)
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Solar Position
The geographical location of the observer on Earth
Latitude ()
Longitude ()
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Earth-Sun Position
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Solar Intensity Map
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Solar Insolation 
(not the Insulation!)
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Sun Paths 
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In the Northern Hemisphere.
The opposite is true in the Southern Hemisphere. 
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Radiation Reaching the Surface
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1m2 of collector area receives 1 kilowatt of sunshine, and produces 100 W of electricity. 
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Heating and Cooling Loads
The load increases with temperature difference between inside and outside, and depends on… 
building design, 
building materials,
season and time of day, 
atmospheric parameters such as humidity and wind speed.
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Passive Solar Heating
Solar rooms
Solar chimneys 
Trombe Wall
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Passive Cooling
Overhang 
Natural ventilation
Cross ventilation 
Evaporative cooling 
Roof ponds
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	A simple overhang can block much of the summer heat. 
To keep a building cool,
reduce the losses by keeping heat out
insulate the walls, making walls thicker (bigger thermal masses)
reduce surface area of south-facing windows
plugging all the leaks
Solutions…
Overhang 
Natural ventilation
Window openings on opposite sides of the building to enhance cross ventilation driven by breezes. 
Evaporative cooling by means springs and fountains
Roof ponds
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Solar Chimney
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Active Solar Heating
Flat-plate collectors
Evacuated-tube collectors 
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Flat Plate Collector
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Active Heating
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Active Cooling
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Electrical Power Generation
Solar Thermal
Photovoltaic
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Solar Thermal Power Plants
Concentrators
	(1) Parabolic troughs 
	(2) Parabolic dishes 
	(3) Fresnel lens 
 
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Concentrated Solar Power (CSP) 
Source of energy comes from  concentrated sunlight. 
Mechanism: Concentration can be achieved through reflective surfaces in the forms of parabolic troughs, dishes, Fresnel lenses, or an array of mirrors forming a heliostat. 
CSP is advantageous in areas with high direct solar gain and access to large plots of land. 
Solar collectors fall into two general categories:  non-concentrating and concentrating. 
In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). 
In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area.  Where temperatures below about 200o F are sufficient, such as for space heating, flat-plate collectors of the non-concentrating type are generally used. 
Note: Byproduct of thermal plants -- waste heat -- can be used to desalinate seawater, used for thermal cooling, or used directly in industrial applications. 
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Solar Concentrators
Concentration Ratio  
 the degree to which solar energy is concentrated by a given collector. 
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Concentration ratio is defined as the average solar flux through the receiver aperture divided by the ambient direct normal solar insolation. Since parabolic dishes distribute the energy over a smaller collector surface (around a focal point), they have a higher concentrating power than parabolic troughs. Furthermore, it is a matter of common sense that the bigger and more curved the collector area is, the higher the degree of concentration and collection efficiency will be.
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Solar Concentrator Comparison
	Solar concentrators				
		CR		 	Status
	Flat Plate Collectors	1	100°C	20%	Commercial 
Stage
	Parabolic Trough	10-50	400°C	56%	Commercial 
Stage
	Fresnel Lens	50-100	1000°C	76%	Prototype
Stage
	Parabolic Dishes	30-1000	1200°C	80%	Prototype
Stage
	Power Tower	-	2000°C	86%	Demonstration
Stage
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Solar Electric Generating Stations (SEGS)
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SEGS solar power plant at Kramer Junction
Each plant consists of a collector field of many troughs aligned on a north-south axis, which reflect sunlight onto receiver pipes filled with oil. Each trough is mounted on a tracking device that can follow the sun from east to west, during the day, to ensure that the sunlight is continuously focused on the receiver pipes.
 
The collectors concentrate sunlight 30 to 60 times the normal intensity on the receiver, heating the oil as high as 390°C. Additional troughs allow some heated oil to be stored in large storage bins filled with salt and turning the salt into liquid. As evening falls, the stored energy is redirected to the oil, generating electricity for several hours after the sun has set. To assist solar collectors in meeting demands during nighttime, extreme cold, rainy or cloudy days, some gas-fired generators are added to supplement the energy requirement.
Mojave Desert (Kramer Junction), California
World’s largest parabolic trough facilities
Ranging in size from 14-80 megawatts for a combined electric generating capacity of 354 MW, enough to meet the needs of 500,000 people. 
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Concentration Dishes
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Stirling Energy Systems, Inc. proposes using a 12-meter-wide array of curved-glass mirror facets (called Sun Catchers) in the shape of parabolic dishes. Each module generates 25 kilowatts of electricity.
Concentric dishes 
Light is focused to a receiver placed at their focal points. 
Because light is concentrated to a much smaller area, much higher temperature is achieved -- enough to run an engine; the power generation is rather limited, however. 
To maximize efficiency, the assembly automatically tracks, collects, and focuses the sun. 
The concentrated light heats up a working fluid which drives a closed-cycle, four-cylinder Stirlingengine. 
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Solar Furnace
The largest solar furnace in the world is in Odeillo, France. 
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The heliostat consists of 8,570 mirrors (not shown here) automatically tracks the sun and reflect its rays into a fixed parabolic mirror reaching temperatures of 3,800°C for a maximum of 1 MW of power. 
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Tower of Power
SCE Solar Two, Dagget CA 
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Heliostat (NREL)
Solar Two—a demonstration power tower located in the Mojave Desert—can generate about 10 MW of electricity. In this central receiver system, thousands of sun-tracking mirrors called heliostats reflect sunlight onto the receiver. Molten salt at 554ºF (290ºC) is pumped from a cold storage tank
through the receiver where it is heated to about 1,050ºF (565ºC). The heated salt then moves on to the hot storage tank. When power is needed from the plant, the hot salt is pumped to a generator
that produces steam. The steam activates a turbine/generator system that creates electricity. From the steam generator, the salt is returned to the cold storage tank, where it stored is and can be eventually reheated in the receiver. By using thermal storage, power tower plants can potentially operate for 65
percent of the year without the need for a back-up fuel source. Without energy storage, solar technologies like this are limited to annual capacity factors near 25 percent. The power tower's ability to
operate for extended periods of time on stored solar energy separates it from other renewable energy technologies.
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Ocean Thermal Energy Conversion (OTEC)
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Ocean Thermal Energy Conversion (OTEC) takes advantage of the temperature difference between the warm surface waters and cold, deep waters of the oceans to drive steam turbines and produce electricity. OTEC plants can be constructed to operate in closed or open cycles. 
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Open and Closed OTEC
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Close-cycle OTEC
Open-cycle OTEC
Closed-cycle systems use the ocean’s warm surface water to vaporize a working fluid, such as ammonia, Freon, or another refrigerant. The vapor expands through a turbine, turning a generator to produce electricity. The vapor eventually is condensed by transferring its heat to the cold water of the deep ocean, before being pumped back to the evaporator and completing the cycle. In 1999, the Natural Energy Laboratory of Hawaii tested a 250-kW pilot OTEC closed-cycle plant, the largest such plant ever put into operation.
In open-cycle systems, water itself is the working fluid. Open-cycle systems work by boiling the surface water into a flash evaporator operating at a pressure so low that water boils at the temperature of the surface water. Salt is left behind, while pure steam drives a low-pressure turbine to generate electricity. The steam is condensed back to liquid water in a condenser, which is then cooled by deep seawater before being discharged. The output of the condenser is desalinated water, one of the desirable byproducts of the OTEC system. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment.
In addition to open and closed cycles, an OTEC plant also can be designed as a hybrid. In a hybrid cycle, steam is generated by flash evaporation of warm surface water similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.
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Advantages and Disadvantages of OTEC System
Merits
Clean source of energy with relatively little environmental impact
Deep-water Aquaculture
Air conditioning, Desalination, and Chilled soil agriculture
Convenient sink for waste heat
Demerits
High initial cost
Limited suitable sites
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Deep water is clean and has a significantly higher amount of nutrients that allow enhanced growth of algae and other marine organism such as salmon and lobster (Aquaculture)
After steam is expanded in turbine and condensed in the condenser, the condensate can be used in a number of applications such as A/C, desalination, etc.
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Solar Ponds
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Photovoltaics (PV)
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Facts: 
 Worldwide, 1 gigawatt of electricity is generated by photovoltaics. 
 Japan currently leads in solar cell manufacturing and controls nearly one-third of the global solar cell market; Germany is second and the United States is third, and China and India are catching up rapidly. 
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Photovoltaics
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The pv cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight.  Beginning in the late 1950s, pvs were used to power U.S. space satellites.  The success of PVs in space generated commercial applications for pv technology.  The simplest photovoltaic systems power many of the small calculators and wrist watches used everyday.  More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes. 
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Transportation Systems
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Operation of a photovoltaic cell
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PHOTOVOLTAIC ENERGY
Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (PVs) cell, commonly called a solar cell.  A photovoltaic cell is a nonmechanical device usually made from silicon alloys. 
When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed.  Only the absorbed photons provide energy to generate electricity.  When enough sunlight (energy) is absorbed by the material (a semiconductor), electrons are dislodged from the material's atoms.  Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface. 
 
When the electrons leave their position, so-called holes are formed.  When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery.  When the two surfaces are connected through an external load, electricity flows. 
The photovoltaic cell is the basic building block of a PV system.  Individual cells can vary in size from about 1 cm (1/2 inch) to about 10 cm (4 inches) across.  However, one cell only produces 1 or 2 watts, which isn't enough power for most applications.  To increase power output, cells are electrically connected into a packaged weather-tight module.  Modules can be further connected to form an array.  The term array refers to the entire generating plant, whether it is made up of one or several thousand modules.  As many modules as needed can be connected to form the array size (power output) needed.  
The performance of a photovoltaic array is dependent upon sunlight.  Climate conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a PV array and, in turn, its performance.  Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight with further research being conducted to raise this efficiency to 20 percent. 
Types of Solar Cells
Monocrystalline
Thin-film solar cells 
Microcells 
Concentrated cellsSpherical cells
Polycrystalline
Amorphous
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The efficiency is limited by the number of photons of appropriate frequency. Photons of lower frequency do not have sufficient energy, whereas higher frequency photons may cause overheating of the substrate and decline in efficiency. Because of its abundance, silicon remains the most common cell material used today. 
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Fresnel lens
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Multi-junction (tandem) Solar Cell
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It’s a latest promising technology.
Unlike conventional photovoltaic cells that absorb light only in the red part of the light spectrum, these cells consist of several very thin layers of light- absorbing materials sandwiched together. 
Each layer absorbs a different color of light, increasing the overall efficiency. 
Application:
The technology has been implemented on the Mars Rover, but remains too expensive to be used in commercial products. In one configuration, a triple-junction cell constructed of GaInP/GaInAs/Ge (gallium indium phosphide, gallium indium arsenide on a germanium substrate) was shown to achieve efficiencies as high as 41%. 
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Spherical Cells
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Concentrated Light
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Solar Power Satellite
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Joint Project by the US DoE and NASA
5,000 MW Plant
10 times bigger than conventional Plants
5 km x 10 km
50,000 tons
Power beamed as microwave (2.45 GHz)
While NASA has abandoned research in solar sail propulsion. Japan's JAXA successfully tested IKAROS in 2010. The goal was to deploy and control the sail and for the first time determining the minute orbit perturbations caused by light pressure. 
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Solar Sail (Ikaros)
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While NASA has abandoned research in solar sail propulsion,[1] Japan's JAXA successfully tested IKAROS in 2010. The goal was to deploy and control the sail and for the first time determining the minute orbit perturbations caused by light pressure. Orbit determination was done by the nearby AKATSUKI probe from which IKAROS detached after both had been brought into a transfer orbit to Venus. The total effect over the six month' flight was 100 m/s.
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Hybrid Systems
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High Gain Solar (HGS) system (developed by Skyline SolarTM)
Combination of traditional PV and long-reflecting trough surfaces
Operation: Arrays of low-cost reflective metal, silver-coated glass mirrors concentrate light onto narrow bands of conventional photovoltaic cells.
Single-axis tracking system 
following the arc of the sun from sunrise to sunset, concentrating sunlight by a factor of 10. 
Because it can be built in small modular building blocks, it can be used in plants ranging from less than 100 kilowatts to many megawatts.
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