DIRECT UTILIZATION OF SOLAR ENERGY full report
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DIRECT UTILIZATION OF SOLAR ENERGY
The main modes of direct collection of solar energy like passive solar system, solar heat collectors and thermal concentrating system are discussed in detail. Common designs adopted in these modes are also explained with suitable photographs. The main designs in thermal concentrating systems like parabolic trough, parabolic dish and central receiver are discussed in detail. The advantages and the future of solar energy are also discussed in brief. A general idea about the various methods of direct utilization of solar energy is conveyed by this seminar and presentation.
In passive solar system the techniques and the design features for the direct gain of solar energy are described in detail. Under solar heat collectors , solar heating and solar cooling are described in detail. Also the three methods under solar thermal concentrating system are also discussed in detail.
THE SOLAR RESOURCE
Solar energyâ€power from the sunâ€is free and inexhaustible. This vast, clean energy resource represents a viable alternative to the fossil fuels that currently pollute our air and water, threaten our public health, and contribute to global warming. Failing to take advantage of such a widely available and low-impact resource would be a grave injustice to our children and all future generations.
The aim of this work is to create an awareness among the students about the abundant renewable energy(mainly solar energy) available and about the methods by which this energy can be effectively utilized.
In the broadest sense, solar energy supports all life on Earth and is
the basis for almost every form of energy we use. The sun makes plants grow, which can be burned as biomass fuel or, if left to rot in swamps and compressed underground for millions of years, in the form of coal and oil. Heat from the sun causes temperature differences between areas, producing wind that can power turbines. Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning hydroelectric turbines as it passes. But solar energy usually refers to ways the sunâ„¢s energy can be used to directly generate heat and lighting.
The amount of energy from the sun that falls on Earthâ„¢s surface is enormous. All the energy stored in Earth's reserves of coal, oil, and natural gas is matched by the energy from just 20 days of sunshine. Outside Earth's atmosphere, the sun's energy contains about 1,300 watts per square meter. About one-third of this light is reflected back into space, and some is absorbed by the atmosphere (in part causing winds to blow). By the time it reaches Earth's surface, the energy in sunlight has fallen to about 1,000 watts per square meter at noon on a cloudless day. Averaged over the entire surface of the planet, 24 hours per day for a year, each square meter collects the approximate energy equivalent of almost a barrel of oil each year, or 4.2 kilowatt-hours of energy. every day
Our campus is 30 acres(121405.68 square meter) of land. So the sunlight falling on our campus per day is approximately equivalent to 509.903 mega-watts of energy.
The present situation of power crisis can be overcome by depending on renewable energies like solar energy, which finds a range of applications on various fields. India must take serious steps for adopting technologies in the renewable energy field so as to meet the increasing power demands in future.
NATURAL TRANSFORMATION OF SOLAR ENERGY
Natural collection of solar energy occurs in the Earthâ„¢s atmosphere, oceans, and plant life. Interactions between the Sunâ„¢s energy, the oceans, and the atmosphere, for example, produce the winds, which have been used for centuries to turn windmills. Modern applications of wind energy use strong, light, weather-resistant, aerodynamically designed wind turbines that, when attached to generators, produce electricity for local, specialized use or as part of a community or regional network of electric power distribution.
Approximately 30 per cent of the solar energy reaching the outer edge of the atmosphere is consumed in the hydrological cycle, which produces rainfall and the potential energy of water in mountain streams and rivers. The power produced by these flowing waters as they pass through modern turbines is called hydroelectric power.
Through the process of photosynthesis, solar energy contributes to the growth of plant life (biomass) that can be used as fuel, including wood and the fossil fuels that are derived from geologically ancient plant life. Fuels such as alcohol or methane can also be extracted from biomass.
The oceans also represent a form of natural collection of solar energy. As a result of the absorption of solar energy in the ocean and ocean currents, temperature gradients occur in the ocean. In some locations, these vertical variations approach 20Ã‚Â° C (36Ã‚Â° F) over a distance of a few hundred metres. When large masses exist at different temperatures, thermodynamic principles predict that a power-generating cycle can be created to remove energy from the high-temperature mass and transfer a lesser amount of energy to a low-temperature mass. The difference in these two heat energies manifests itself as mechanical energy (for example, output from a turbine), which can be linked with a generator to produce electricity. Such systems, called ocean thermal energy conversion (OTEC) systems, require enormous heat exchangers and other hardware in the ocean to produce electricity in the MW range.
2.1 Direct collection of solar energy
2.1.1 Passive Solar Systems
2.1.2 Solar Heat Collectors
2.1.3 Thermal Concentrating Systems
These methods are discussed in detail in the following chapters.
PASSIVE SOLAR SYSTEMS
Passive solar energy systems involve designing the structures themselves in ways that use solar energy for heating and cooling. Passive solar systems capture and use solar energy without the aid of mechanical or electrical devices.
One simple, obvious use of sunlight is to light our buildings. If properly designed, buildings can capture the sun's heat in the winter and minimize it in the summer, while using daylight year-round. Buildings designed in such a way are utilizing passive solar energyâ€a resource that can be tapped without mechanical means to help heat, cool, or light a building. South-facing windows, skylights, awnings, and shade trees are all techniques for exploiting passive solar energy. Buildings constructed with the sun in mind can be comfortable and beautiful places to live and work.
3.1 Direct Gain Passive Solar Design Techniques
Passive solar design strategies vary by building location and regional climate, but the basic techniques remain the same-maximize solar heat gain in winter and minimize it in summer.
Specific techniques include:
Â¢ Start by using energy-efficient design strategies.
Â¢ Orient the house with the long axis running east/west.
Â¢ Select, orient, and size glass to optimize winter heat gain and minimize summer heat gain for the specific climate.
Â¢ Consider selecting different glazings for different sides of the house (exposures).
Â¢ Size south-facing overhangs to shade windows in summer and allow solar gain in
Â¢ Add thermal mass in walls or floors for heat storage.
Â¢ Use natural ventilation to reduce or eliminate cooling needs.
Â¢ Use daylight to provide natural lighting.
These techniques are described in more detail :
3.1.1 Cutting Losses: A passive solar home should start out well sealed and well insulated. By reducing heat loss and gain, remaining energy loads can be effectively met with passive solar techniques. Approaches that contribute to minimizing heating and cooling loads include using advanced framing guidelines , properly installing insulation, using recommended insulation levels (International Code Councilâ„¢s International Energy Conservation Code, (703) 931-4533, or the U.S. Department of Energyâ„¢s Insulation Fact Sheet,DOE/CE-0180, (800) DOE-EREC),reducing duct losses, and tightening the building envelope.
3.1.2 Site Orientation: The buildingâ„¢s southern exposure must be clear of large obstacles (e.g., tall buildings, tall trees) that block the sunlight. Although a true southern exposure is optimal to maximize solar contribution, it is neither mandatory nor always possible. Provided the building faces within 30Ã‚Â° of due south, south-facing glazing will receive about 90 percent of the optimal winter solar heat gain.
3.1.3 Window Selection: Heating with solar energy is easy: just let the sun shine in through the windows. The natural properties of glass lets sunlight through but trap long-wave heat radiation, keeping the house warm (the greenhouse effect). The challenge often is to properly size the south-facing glass to balance heat gain and heat loss properties without overheating. Increasing the glass area can increase building energy loss. New window technologies, including selective coatings, have lessened such concerns by increasing window insulation properties to help keep heat where it is needed.
In heating climates, reduce the window area on north-, east-, and west-facing walls, while still allowing for adequate daylight.
3.1.4 Shading: The summer sun rises higher overhead than the winter sun. Properly sized window overhangs or awnings are an effective option to optimize southerly solar heat gain and shading. They shade windows from the summer sun and, in the winter when the sun is lower in the sky, permit sunlight top as through the window to warm the interior. Landscaping helps shade south-, east-, or west-facing windows from summer heat gain. Mature deciduous trees permit most winter sunlight (60 percent or more) to pass through while providing dappled shade throughout summer.
3.1.5 Heat Storage: Thermal mass, or materials used to store heat, is an integral part of most passive solar design. Materials such as concrete, masonry, wallboard, and even water absorb heat during sunlit days and slowly release it as temperatures drop. This dampens the effects of outside air temperature changes and moderates indoor temperatures. Although even overcast skies provide solar heating, long periods of little sunshine often require a back-up heat source. Optimum mass-to-glass ratios, depending on climate, may be used to prevent overheating and minimize energy consumption (The Sunâ„¢s Joules,solstice.crestrenewables/SJ/passive-solar/136.html). Avoid coverings such as carpet that inhibit thermal mass absorption and transfer.
3.1.6 Natural Cooling: Apt use of outdoor air often can cool a home without need for mechanical cooling, especially when effective shading, insulation, window selection, and other means already reduce the cooling load. In many climates, opening windows at night to flush the house with cooler outdoor air and then closing windows and shades by day can greatly reduce the need for supplemental cooling. Cross-ventilation techniques capture cooling, flow-through breezes. Exhausting naturally rising warmer air through upper-level openings (stack effect; e.g., clerestory windows) or fans (e.g., whole-house fan) encourages lower-level openings to admit cooler, refreshing, replacement air.
3.1.7 Natural Lighting: Sometimes called day lighting, natural lighting refers to reliance on sunlight for daytime interior lighting. Glazing characteristics include high-VT glazing on the east, west, and north facades combined with large, south-facing window areas. A day lit room requires, as a general rule, at least5 percent of the room floor area in glazing. Low-emissivity (low-E) coatings can help minimize glare while offering appropriate improved climatic heat gain or loss characteristics. Sloped or horizontal glass (e.g., skylights) admit light but are often problematic because of unwanted seasonal overheating, radiant heat loss, and assorted other problems.
Solar design, better insulation, and more efficient appliances could reduce this demand by 60 to 80 percent. Simple design features such as properly orienting a house toward the south, putting most windows on the south side of the building, and taking advantage of cooling breezes in the summer are inexpensive yet improve the comfort and efficiency of a home. In India some people are not yet aware of these simple techniques that helps them to save power.
3.2 Five elements of passive solar design
Here are the five elements that constitute a complete passive solar design, using a direct gain design as an example. Each performs a separate function, but all five must work together for the system to be successful.
Aperture (collector) -- the large glass window.
Absorber -- masonry wall, floor.
Thermal mass -- wall, floor
Distribution -- conduction, convection, radiation
Control -- roof overhangs
So it is clear that by making small modifications in the building design, as discussed above, the energy consumption (mainly electrical energy) can be reduced to large extend. The other two methods for direct collection of solar energy are discussed in the following chapters.
SOLAR HEAT COLLECTORS
Besides using design features to maximize their use of the sun, some buildings have systems that actively gather and store solar energy. Solar collectors, for example, sit on the rooftops of buildings to collect solar energy for space heating, water heating (Fig 4.1), and space cooling. Most are large, flat boxes painted black on the inside and covered with glass. In the most common design, pipes in the box carry liquids that transfer the heat from the box into the building. This heated liquidâ€usually a water-alcohol mixture to prevent freezingâ€is used to heat water in a tank or is passed through radiators that heat the air.
Oddly enough, solar heat can also power a cooling system. In desiccant evaporators, heat from a solar collector is used to pull moisture out of the air. When the air becomes drier, it also becomes cooler. The hot moist air is separated from the cooler air and vented to the outside. Another approach is an absorption chiller. Solar energy is used to heat a refrigerant under pressure; when the pressure is released, it expands, cooling the air around it. This is how conventional refrigerators and air conditioners work, and itâ„¢s a particularly efficient approach for home or office cooling since buildings need cooling during the hottest part of the day. These systems are currently at work in humid southeastern climates such as Florida.
Solar collectors were quite popular in the early 1980s, in the aftermath of the energy crisis. Federal tax credits for residential solar collectors also helped. In 1984, for example, 16 million square feet of collectors were sold in the United States, but when fossil fuel prices dropped and tax credits expired in the mid-1980s, demand for solar collectors plummeted. By 1987, sales were down to only four million square feet. Most of the more than one million solar collectors sold in the 1980s were used for heating hot tubs and swimming pools.
The number of Indians choosing solar hot water could rise dramatically in the next few years. With Liquified Petroleum Gas (LPG) prices at historically high levels, solar water and space heaters have become much more economic. According to the Indian Renewable Energy Development Agency (IREDA), water heating accounts for about 15 percent of the average householdâ„¢s energy use. As LPG and electricity prices continue to rise, the costs of maintaining a constant hot water supply will increase as well. Homes and businesses that heat their water through solar collectors could end up saving as much as Rs.5000 to Rs.10000 per year depending on the type of system being replaced.
Fig4.1 (Solar Water Heater)
SOLAR THERMAL CONCENTRATING SYSTEMS
By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems can produce very high temperaturesâ€as high as 3,000 degrees Celsius. This intense heat can be used in industrial applications or to produce electricity. Solar concentrators come in three main designs: parabolic troughs, parabolic dishes, and central receivers.
5.1 Parabolic Troughs
Long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. Parabolic troughs are the only commercially available solar concentrator that can be used to deliver high temperature thermal energy. Industrial Solar Technology is one of the few companies in the world that manufactures parabolic trough concentrators.
Parabolic troughs are the most utilitarian of solar collectors in terms of the markets they can serve. IST troughs can deliver heat at temperatures ranging from 40 C-300 C for applications such as hot water, space heating, air-conditioning, steam generation, industrial process heating, desalination and power generation.
It is a principle of geometry that a parabolic reflector pointed at the sun will reflect parallel rays of light to the focal point of the parabola. A parabolic trough is a one-dimensional parabola that focuses solar energy onto a line. Physically, this line is a pipe with a flowing liquid inside that absorbs the
heat transmitted through the pipe wall and delivers it to the thermal load. A trough captures sunlight over a large aperture area and concentrates this energy onto a much small receiver area, multiplying the intensity of the sun by a concentration ratio in the range of 30-80. It is the process of concentration that allows troughs to delivery high temperature thermal energy. However, to achieve such concentration, a trough tracks the sun in one axis continually throughout the day. The required tracking accuracy is within a fraction of a degree.
Establishing the concentration ratio is the major tradeoff in designing a trough concentrator. The goal is to balance the interception of solar energy at the receiver against heat losses from the receiver. The larger the absorber diameter the greater the heat loss from the absorber area. However, the absorber must be large enough to intercept most of the sunlight reflected from the mirror. This intercept is affected by factors such as the accuracy of the parabola, the size of the solar disk (the sun is not a point source), the quality of the reflector, the accuracy of collector tracking and location of the receiver with respect to the true focal point. IST's contribution to the development of parabolic trough concentrators is a patented design concept by which a concentrator that is accurate, lightweight and strong enough to survive in the outdoor environment can be built at a reasonable cost. To maximize the sunlight incident on the absorber, the reflectance of the parabolic reflector must be as high as possible. Aluminum or silver reflectors are used. Silver has the higher reflectance, but is harder to protect against the corrosive effects of the outdoor environment. It is also important to keep the reflectors clean since dirt will degrade the reflectance of light from the parabola.
The receiver of a trough concentrator is typically a metal absorber surrounded by a glass tube. The absorber is coated with a selective surface. This is a surface that has a high absorptance for incoming light in the visible range, and a low emittance (or radiative loss) in the infrared wavelength. The surrounding glass insulates the pipe from the effects of the wind and greatly reduces convective and conductive heat loss. The gap between the absorber tube and the inside of the glass is sized to minimize heat loss across the air gap. Glass is also a radiation barrier to infrared light so it reduces heat loss due to radiation. Since the light from the parabola must first pass through the glass before it hits the absorber, the glass is a source of optical inefficiency since some light is reflected from the inside and outside glass/air surfaces, and absorbed in the glass itself. IST reduces the negative effect of the glass tube by coating it with an anti-reflective surface to minimize optical losses due to reflectance. Taking all these factors into account, the peak optical efficiency of a parabolic trough is in the range of 70-80%. Since thermal losses from the receiver are relatively small and increase only moderately as operating temperatures increase, at peak conditions, a trough can be expected to deliver 60+% of the radiation incident on the collector even when taking into account heat losses in the solar field piping.
Parabolic troughs are highly modular. IST troughs are aimed at commercial and industrial markets, but they can be configured in any reasonable collector area to meet the desired load. Though east-west or north-south orientation of the collector axis is typically specified for year-round or summer-peaking loads, respectively, troughs can actually be oriented in any direction. The arrangement of troughs in parallel rows simplifies system design and field layout, and minimizes interconnecting piping. IST has trough models that can be mounted on the ground or on a roof.
Tracking of a parabolic trough involves fixed costs associated with the drive and control system. In large systems for commercial and industrial applications, costs for the drive and control system are relatively less pronounced and the cost of the collectors dominates the overall system cost. Materials are a major component of collector costs. IST's contribution to the progression of parabolic trough technology includes the development of a lightweight solar concentrator. Compared to a flat plate collector, an IST parabolic trough module is 3 to 4 times less weight, and consequently large trough systems are less costly that equivalent flat plate or evacuated tube collector installations. Though the tracking of troughs involves more maintenance compared to flat plate and evacuated tube collectors, the cost of electricity to power trough systems is less because pumping power to circulate the collector fluid is reduced several times. Importantly, troughs can meet temperature demands for energy far beyond the capabilities of none tracking collectors.
5.2 Parabolic Dish
These concentrators are similar to trough concentrators, but focus the sunlight on a single point. Dishes can produce much higher temperatures, and so, in principle, should produce electricity more efficiently. But because they are more complicated, they have not succeeded outside of demonstration project and implimentations.
Fig 5.2(Parabolic Dish)
The fluid in the receiver is heated to very high temperatures of about 750oC. This fluid is then used to generate electricity in a small Stirling Engine, or Brayton cycle engine, which is attached to the receiver. Parabolic dish systems are the most efficient of all solar technologies, at approximately 25% efficient, compared to around 20% for other solar thermal technologies. The Australian National University and Wizard Information Systems have negotiated the terms of a licence to commercialise the Big Dish solar concentrator technology and is working towards construction of a demonstration plant in Whyalla, South Australia(Fig 5.3) .
Fig 5.3 (Demonstration Plant in Whyalla, South Australia)
A more promising variation uses a stirling engine to produce power. Unlike a carâ„¢s internal combustion engine, in which gasoline exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. These dish-stirling generators produce about 30 kilowatts of power, and can be used to replace diesel generators in remote locations.
5.3 Central Receiver
The third type of concentrator system is a central receiver. Central receivers (or power towers) use thousands of individual sun-tracking mirrors called "heliostats" to reflect solar energy onto a receiver located on top of a tall tower.
Fig: 5.4(Solar Two Power Tower System)
The receiver collects the sun's heat in a heat-transfer fluid (molten salt) that flows through the receiver. The salt's heat energy is then used to make steam to generate electricity in a conventional steam generator, located at the foot of the tower. The molten salt storage system retains heat efficiently, so it can be stored for hours or even days before being used to generate electricity. Therefore, a central receiver system is composed of five main components: heliostats, receiver, heat transport and exchange, thermal storage, and controls.
One such plant in California features a "power tower" design in which a 17-acre field of mirrors concentrates sunlight on the top of an 80-meter tower. The intense heat boils water, producing steam that drives a 10-megawatt generator at the base of the tower. The first version of this facility, Solar One, operated from 1982 to 1988 but had a number of problems. Reconfigured as Solar Two(Fig-5.5) during the early to mid-1990s, the facility is successfully demonstrating the ability to collect and store solar energy efficiently. Solar Twoâ„¢s success has opened the door for further development of this technology. The parabolic trough has had the greatest commercial success of the three solar concentrator designs, in large part due to the nine Solar Electric Generating Stations (SEGS) built in Californiaâ„¢s Mojave Desert from 1985 to 1991. Ranging from 14 to 80 megawatts and with a total capacity of 354 megawatts, each of these plants is still operating effectively.
Solar energy premiums and other incentives under review in Spain create an attractive market opportunity, providing the economic incentives needed to reduce the initial high cost and risk of commercializing a new technology. The Spanish project and implimentation, called "Solar Tres" or Solar Three, will use all the proven molten-salt technology of Solar Two, scaled up by a factor of three. Although Solar Two was a demonstration project and implimentation, Solar Tres will be operated by industry as a long-term power production project and implimentation. This utility-scale solar power could be a major source of clean energy world wide, offsetting as much as 4 million metric tons of carbon equivalent through2010.
Fig 5.5(Solar Two near Barstow, California.)
THE FUTURE OF SOLAR ENERGY
Developing countries in Asia, Africa, and Latin America-where half the population is currently without electricity and sunlight is usually abundant-represent the biggest and fastest growing market for power producing technologies. A number of project and implimentations are being developed in India, Egypt, Morocco, and Mexico. In addition, independent power producers are in the early stages of design and development for potential parabolic trough power project and implimentations in Greece (Crete) and Spain. If successful, these project and implimentations could open the door for additional project and implimentation opportunities in these and other developing countries.
Solar energy technologies are poised for significant growth in the 21st century. More and more architects and contractors are recognizing the value of passive solar and learning how to effectively incorporate it into building designs. Solar hot water systems can compete economically with conventional systems in some areas. And as the cost of solar PV continues to decline, these systems will penetrate increasingly larger markets. In fact, the solar PV industry aims to provide half of all new U.S. electricity generation by 2025.Aggressive financial incentives in Germany and Japan have made these countries global leaders in solar deployment for years. But the United States is catching up thanks particularly to strong state-level policy support. The rolling blackouts and soaring energy prices experienced by California in 2000 and 2001 have motivated its leaders to create new incentives for solar and other renewable energy technologies. In January 2006, the California Public Utility Commission approved the California Solar Initiative, which dedicates $3.2 billion over 11 years to develop 3,000 megawatts of new solar electricity, equal to placing PV systems on a million rooftops.
As the solar industry continues to expand, there will be occasional bumps in the road. For example, demand for manufacturing-quality silicon from the solar energy and semiconductor industries has led to shortages that have temporarily driven up PV costs. In addition, some utilities continue to put up roadblocks for grid-connected PV systems. But these problems will be overcome, and solar energy will play an increasingly integral role in ending our national dependence on fossil fuels, combating the threat of global warming, and securing a future based on clean and sustainable energy.
The methods by which the solar energy can be utilized directly were discussed in the above chapters. At present India is far behind in the field of utilizing renewable energy resources compared to countries like Germany and America.
If India adopts these methods discussed above and other improved technologies, the country can overcome the present situation of power crisis.
Books and Journals:
1. J.D. Balcomb, (1992), Passive Solar Buildings , MIT Press, 528 pp
2. M. Freeman, (1994), The Solar Home: How to Design and Build a House
You Heat with the Sun, Stackpole Books , 240 pp.
3. Power Towers: Proving the Technical Feasibility and Cost Potential of Generating
Large-Scale Electric Power from the Sun When It Is Needed, produced by NREL for
DOE, August 2000.
4. Solar Trough Power Plants: Concentrating Power Plants Have Provided Continuous
Generation Since 1984, produced by NREL for DOE, August 2000.
5. Distributed Power Technologiesâ€Concentrating Solar Power
6. DOE Office of Building Technology, State and Community Programs
7. National Renewable Energy Laboratory
8. Parabolic Troughs: Solar Power Today
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