A REPORT ON

SOLUTIONS FOR TRAPPING SOLAR ENERGY

Contributed by:

MAYURESH DESAIDIPALI DONDEVIJAYA DONGEANUJ DOUND

PRAFUL GAIKWADVIDYASAGAR GOGATE

AKSHAY GOKHALEADITYA GOREHITESH HIRANANISH JOSHI

LITERATURE REVIEW

Solar energy is the most abundant source of energy available on earth. Though fossil fuels are available in concentrated form, are extremely polluting and are depleting fast. Also ever escalating costs are a major concern. Contrastingly, the solar power would be a clean source of energy with practically zero fuel costs. But a major problem is the diffused state of the available solar energy. So, a considerable amount of research is being done throughout the world for efficiently trapping the solar energy and harnessing it for different purposes. Some of the technologies have been well established through continuous research and rigorous experimentation. Solar liquid flat plate collectors (LFPCs) and photovoltaic cells are such technologies that are presently being applied on a large scale throughout the world. Currently, research is being made for developing innovative waves of trapping and utilizing solar energy. Solar ponds, solar nano-cells, solar power paints, solar screens, nano-antennas, etc. are the recent areas of research in the solar energy field. This report on “Solutions for trapping solar energy” discusses various traditional and innovative ways of trapping solar energy.

TABLE OF CONTENTS

CHAPTER NO. NAME1. Introduction2. Thermal Collectors3. Power Generation4. Solar Ponds5. Photovoltaic Utilization: Conventional Techniques6. Photovoltaic Utilization: Future Techniques7. Storage Of The Trapped Solar Energy8. Application: A Solar Airplane9. Conclusion

CHAPTER 1:

INTRODUCTION

1.1Man and Energy:

Man has needed and used energy at an increasing rate for his sustenance and well-being ever since he came on earth a few million years ago. Industrial revolution that began with the discovery of steam engine (AD 1700) brought about great changes. For the first time man began to use a new source of energy i.e. coal, in large quantities. Further developments led to use of newer fuels namely petrol, diesel, natural gas, etc. These are better known as fossil fuels. But, the main disadvantage with such kind of fuels was environmental pollution. So man began searching for newer and cleaner sources of energy, and sun was an obvious contender. From then various attempts have been made to trap and utilize the most abundant form of energy available on earth: the solar energy.

1.1Solar Energy Utilization:

Solar energy is the basic source of energy on earth. It can be utilized in the following ways:

Direct Methods:

Thermal:This includes the use of solar radiation for water heating as in a liquid flat plate collector.Photovoltaic:This includes the use of solar energy for electricity generation. For example: street lighting, etc.

Indirect Methods:

Water power: Water is heated by solar radiation and is used for power generation.

Wind: The phenomenon of wind occurs due to differential heating of land and sea due to the sun.

Biomass: Its degradation is performed by microbes by the organic processes using sunlight.

Wave energy and Ocean temperature differences: These are again caused by the differential heating of land and sea causing winds which lead to formation of waves that can be harnessed for energy production.

Diect

Figure (1.1) Schematic Representation Of Utilization of Solar Energy

SOLAR ENERGY UTILIZATION

Direct Methods Indirect Methods

1.Thermal

2. Photovoltaic

1.Water Power

2.Wind

3.Biomass

4.Wave Energy & Ocean Temperature Differences

CHAPTER 2:

THERMAL COLLECTORS

2.1 Flat Plate Solar Collectors:

2.1.1 Introduction:

A typical flat-plate collector is a metal box with a glass or plastic cover (called glazing) on top and a dark-colored absorber plate on the bottom. The sides and bottom of the collector are usually insulated to minimize heat loss.

Figure (2.1) LPFC

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use.

2.1.2 Applications:

The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive due to frequent laundry washing.

Commercial applications include Laundromats, car washes, military laundry facilities and eating establishments. The technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water.

Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather or can be drained easily during cold weather.

While solar collectors are most cost-effective in sunny, temperate areas, they can be cost effective virtually anywhere in the country so should be considered.

2.1.3 Performance/Costs:  

To compare performance ratings, look for a Solar Rating & Certification Corporation (SRCC) or Florida Solar Energy Center (FSEC) sticker on the equipment you are considering. Paybacks - (The amount of time required - usually in years - for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.) - vary widely, but for a well-designed and properly installed solar water heater, you can expect a simple payback of 4 to 10 years, depending on climate and utility costs. FSEC found that solar water heaters offer potential savings, compared to electric water heating, of as much as 50% to 85% in the water heating portion of the utility bill.

Flat plate water heating systems range in price from about $2,000 to $4,000 installed for residential systems (for 40 to 80 gallons per day usage), and $2,000 to $50,000 for commercial systems (for 40 to 1700 gallons per day usage). The following chart compares the percent of water heating energy that solar can provide in various cities for a 48-square-foot flat plate solar hot water system based on average water usage for four persons.

2.1.4 Availability:

Flat plate solar water heating systems are available in most areas of the United States and many other countries. FSEC currently lists 192 solar collector panel models and 280 solar systems that they certify. They also provide a list of manufacturers of flat plate collectors and systems. In addition, six mail-order catalogs sell solar water heating systems.

2.2 CONCENTRATORS:

The optical principle of a reflecting parabola (as discussed in Chapter 8) is that all rays of light parallel to its axis are reflected to a point.  A parabolic trough is simply a linear translation of a two-dimensional parabolic reflector where, as a result of the linear translation, the focal point becomes a line.  These are often called line-focus concentrators.  A parabolic dish (parabolic), on the other hand, is formed by rotating the parabola about its axis; the focus remains a point and is often called point-focus concentrators. 

If a receiver is mounted at the focus of a parabolic reflector, the reflected light will be absorbed and converted into heat (or directly into electricity as with a concentrating photovoltaic collector).  These two principal functions, reflection to a point or a line, and subsequent absorption by a receiver, constitute the basic functions of a parabolic concentrating collector.  The engineering task is to construct hardware that efficiently exploits these characteristics for the useful production of thermal or electrical energy.  The resulting hardware is termed the collector subsystem.  This chapter examines the basic optical and thermal considerations that influence receiver design and will emphasize thermal receivers rather than photovoltaic receivers.

 Also discussed here is an interesting type of concentrator called a compound parabolic concentrator (CPC).  This is a non-imaging concentrator that concentrates light rays that are not necessarily parallel nor aligned with the axis of the concentrator. 

Some of the concentrators are shown below:

 Parabolic Concentrator

 

Parabolic dish prototype use in the solar thermal cogeneration project at Shenandoah, GA.

CHAPTER 3:

POWER GENERATION

3.1 POWER GENERATION

The generation of electrical power is one of the most important applications of energy source.

Solar thermal power cycles can be classified as low, medium and high temperature power cycles. Low temperature cycles work at maximum temperatures of about 100 c while medium and high temperature cycles work upto and above 400 c respectively.

Low temperature systems:

The energy of the sun is collected by water flowing through array of flat plate collectors. In order to get maximum possible temperatures booster mirrors which reflect radiation on flat plate collectors are used. The hot water at temperature close to 100 c is stored in well insulated thermal storage tank. Then it flows it flows through a vapour generator through which working fluid passing through rankine cycle is also passed. The working fluid has low boiling point. Thus vapour at about 90 c and pressure of few atmospheres leaves the generator. This vapour then executes a regular rankine cycle by flowing through a prime mover, a liquid condenser and a liquid pump. Working fluid normally used are organic fluids like methyl chloride and toluene and refrigerants like R-11, R-113 and R-114.Overall efficiency of such system is 2 %.

Solar chimney power plant.

In this a tall central chimney is surrounded by a circular green house consisting of a transparent cover supported a few metres above the ground by a metal frame.Sunlight passing through the transparent cover causes the air trapped in the greenhouse to heat up.A convection system is setup in which this air is drwn up through the central chimney,turning a turbine located at base of the chimney.The hot air is continuously replenished by fresh air drawn at periphery of the greenhouse.

Medium temperature systems.

They use line focusing parabolic collectors technology.In this the cylindrical parabolic collectors used have their axis oriented north-south.The absorber tube is made of steel and has specially designed selective surface. Its surrounde by glass cover with vacuum.The collectors heat a synthetic oil to a temperature of about 400 c with a collector efficiency of abot 0.7 for beam radiation. The oil is then used for generating superheated high pressure steam which executes rankine cycle with efficiency of 38 %.

High temperature systems.

In paraboloidal dish concept, the concentrator tracks the sun by rotating about two axes and the sun’s rays are brought to a point focus.A fluid flowing through a receiver at focus is heated and used to drive a prime mover.

Central receiver power plants.

In this solar radiation is reflected from arrays of large mirrors called heliostats and is concentrated on a receiver situated on top of a supporting tower.A fluid flowing through the receiver absorbs the concentrated radiation and transports it to the ground where it is used to operate a rankine power cycle.

A schematic representation of of main components of a central receiver power plant is shown in which water is converted into steam in the receiver itself.Alternatively the receiver is used to heat

a molten metal or a molten salt and this molten fluid is passed through a heat exchanger in which steam for power cycle is generated.

CHAPTER 4:

SOLAR POND

4.1 Introduction:

Figure (4.1) Solar Pond

One way to trap solar energy is through the use of solar ponds. Solar ponds are large-scale energy collectors with integral heat storage for supplying thermal energy.

4.2 Principle:

The solar pond works on a very simple principle. It is well-known that water or air is heated they become lighter and rise upward e.g. a hot air balloon. Similarly, in an ordinary pond, the sun’s rays heat the water and the heated water from within the pond rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise.

A solar pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration.

When solar radiation (sunlight) is absorbed, the density gradient prevents heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C. The trapped (solar) energy is then withdrawn from the pond in the form of hot brine from the storage zone and can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive a turbine for generating electricity.

There are 3 distinct layers of water in the pond

The top layer which has a low salt content and called as UCZ(Upper Convective Zone)

An intermediate insulating layer called as NCZ (Non-Convective Zone) that prevents heat exchange by natural convection. Here the salt content increases as depth increases, thereby creating a salinity or density gradient.

The bottom layer, known as the storage zone or LCZ (Lower Convective Zone).It has a high salt content. It is this zone that collects and stores solar energy in the form of heat. This gradient zone acts as a transparent insulator permitting sunlight to reach the bottom zone.

It is economical to construct them at places where there is low cost salt and bittern, good supply of sea water or water for filling and flushing, high solar radiation, and availability of land at low cost.Though solar ponds can be constructed anywhere,. Coastal areas in Tamil Nadu, Gujarat, Andhra Pradesh, and Orissa are ideally suited for such solar ponds.

4.3 Applications:

It can be use for various applications, such as process heating, water desalination, refrigeration, drying and power generation.

The heat from a solar pond is usually extracted in one of two ways. The first method is to pump the hot brine from the storage zone of the pond to a heat exchanger located near the pond. The second method is to pump a heat exchanger fluid, usually fresh water, through a heat exchanger located within the LCZ of the pond. Both have advantages, but pumping the hot brine to an out-of-pond heat exchanger tends to be the most cost-effective and trouble-free system.

1. Energy to drive desalting units

fresh water production for municipal water systems

energy producing receptacle for waste brines

brine concentration

2. Supplemental energy source

peaking electrical power production base load power for remote locations

3. Process heat for production of chemicals, foods, textiles, and other industrial products

4. Heat for separation of crude oil from brine in oil recovery operations

5. Receptacle for brine disposal using waste brines from crude oil production

6. Heat for:

greenhouses livestock buildings

other low-temperature agricultural applications

7. Space heating and absorption cooling systems

8. Low-temperature aquaculture applications

9. Surface water clean-up

irrigation return flows saline waste waters

river desalination

10. Thermal energy storage systems in areas where brine is available to create the ponds and waste thermal energy is available

power plant cooling tower blow down systems cogeneration systems

11. Control of crystallization in certain mining operations

4.4 Advantages

Solar ponds address three environmental issues arising from the use of conventional fuels. First, heat energy is provided without burning fuel, thus reducing pollution. Second, conventional energy resources are conserved. Third, solar ponds coupled with desalting units can be used to purify contaminated or minerals-impaired water, and the pond itself can become the receptacle for the waste products.

They have a low cost per unit area of collection and an inherent storage capacity. Also, they can be easily constructed over large areas, enabling the diffuse solar resource to be concentrated on a grand scale.

The approach is particularly attractive for rural areas in developing countries. Very large area collectors can be set up for just the cost of the clay or plastic pond liner.

Advantage of desalination by solar ponds is that they can utilize what is often considered a waste product, namely reject brine, as a basis to build the solar pond. This is an important advantage when considering solar ponds for inland desalting for fresh water production or brine concentration for use in salinity control and environmental cleanup applications.

4.5 Disadvantages: The evaporated surface water needs to be constantly replenished. The accumulating salt crystals have to be removed and can be both a valuable by-product

and a problem.

4.6 Efficiency:

The energy obtained is in the form of low grade heat of 70 to 80 °C compared to a 20 °C ambient temperature, which has an upper Carnot-cycle extractable efficiency of 1-(273.15+20)/(273.15+80)=15%. By comparison a solar concentrator system with molten salt delivering high grade heat at 800 °C would be able to convert 73% of absorbed solar heat into useful work, and be forced to divest only 27% as waste heat to the cold temperature reservoir (ambient air.)

4.7 Development:

Further research is aimed at addressing the problems, such as the development of membrane ponds. These use a thin permeable membrane to separate the layers without allowing salt to pass through.

CHAPTER 5:

PHOTOVOLTAIC UTILIZATION: CONVENTIONAL TECHNIQUES

5.1 Photovoltaic Cells:

The basic physics:

What do we mean by photovoltaic? First used in about 1890, the word has two parts: photo, derived from the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta. So, photovoltaics could literally be translated as light-electricity. And that's what photovoltaic (PV) materials and devices do — they convert light energy into electrical energy (Photoelectric Effect), as French physicist Edmond Becquerel discovered as early as 1839.

Commonly known as solar cells, individual PV cells are electricity-producing devices made of semiconductor materials. PV cells come in many sizes and shapes — from smaller than a postage stamp to several inches across. They are often connected together to form PV modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output.

The size of an array depends on several factors, such as the amount of sunlight available in a particular location and the needs of the consumer. The modules of the array make up the major part of a PV system, which can also include electrical connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use when the sun isn't shining.

The Photoelectric Effect:

The photoelectric effect is the basic physical process by which a PV cell converts sunlight into electricity. When light shines on a PV cell, it may be reflected, absorbed, or pass right through. But only the absorbed light generates electricity. The energy of the absorbed light is transferred to electrons in the atoms of the PV cell. With their newfound energy, these electrons escape from their normal positions in the atoms of the semiconductor PV material and become part of the electrical flow, or current, in an electrical circuit. A special electrical property of the PV cell—what we call a "built-in electric field"—provides the force, or voltage, needed to drive the current through an external "load," such as a light bulb.

To induce the built-in electric field within a PV cell, two layers of somewhat differing semiconductor materials are placed in contact with one another. One layer is an "n-type" semiconductor with an abundance of electrons, which have a negative electrical charge. The other layer is a "p-type" semiconductor with an abundance of "holes," which have a positive electrical charge. Although both materials are electrically neutral, n-type silicon has excess electrons and p-type silicon has excess holes. Sandwiching these together creates a p/n junction at their interface, thereby creating an electric field. When n- and p-type silicon comes into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.

Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet—what we call the p/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, where they become available to the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

How do we make the p-type ("positive") and n-type ("negative") silicon materials that will eventually become the photovoltaic (PV) cells that produce solar electricity? Most commonly, we add an element to the silicon that either has an extra electron or lacks an electron. This process of adding another element is called doping.

5.2 PV Devices:

Photovoltaic devices can be made from various types of semiconductor materials, deposited or arranged in various structures, to produce solar cells that have optimal performance.

In this section, we first review the three main types of materials used for solar cells. The first type is silicon, which can be used in various forms, including single-crystalline, multicrystalline, and amorphous. The second type is polycrystalline thin films, with specific discussion of copper indium diselenide (CIS) cadmium telluride (CdTe), and thin-film silicon. Finally, the third type of material is single-crystalline thin film, focusing especially on cells made with gallium arsenide.

5.3 PV Systems:

A photovoltaic (PV) or solar cell is the basic building block of a PV (or solar electric) system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of power. To boost the power output of PV cells, we connect them together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, we can build PV systems able to meet almost any electric power need, whether small or large.

PV systems can be classified into two general categories: flat-plate systems or concentrator systems. By themselves, modules or arrays do not represent an entire PV system. We also need structures to put them on that point them toward the sun, and components that take the direct-current electricity produced by modules and "condition" that electricity, usually by converting it to alternate-current electricity. We might also want to store some electricity, usually in batteries, for later use. All these items are referred to as the "balance of system" (BOS) components.

Combining modules with the BOS components creates an entire PV system. This system is usually everything we need to meet a particular energy demand, such as powering a water pump, or the appliances and lights in a home, or, if the PV system is large enough, all the electrical requirements of a whole community.

A system consists of a photovoltaic module, charge controller and battery.

During the day, when there is daylight, the photovoltaic module produces enough energy to power the load and to charge the battery.

During the night, the load continues to be powered, drawing on energy stored in the battery.

PV in Use: Getting the Job Done with Solar Electricity:

DOE and one of its partners, the West Bengal Renewable Energy Development Agency, are working to improve socioeconomic conditions in the Sunderbans region of West Bengal, India. These rooftop PV modules on a village health center in West Bengal provide power for refrigerators containing medicines and vaccines, for lights, and for other important needs.

After decades of use on Earth and in space, solar electricity made its debut on another planet in 1997 when "Sojourner" began exploring Mars. High-efficiency photovoltaic (PV) cells located on top of the Sojourner vehicle generated 16 watts of power at noon on Mars, which was enough to carry out a day's mission.

Future photovoltaic cell technology:

One way to achieve further cost reductions in photovoltaic cells is to use considerably less semiconductor material per cell. There are limits to how much reduction can be made to the amount of crystalline silicon in a conventional cell because it is not that strong an absorber of light.

There are other possible semiconductors that can be used in very thin layers (thin film photovoltaic cell technology). These do away with the conventional wafer approach to making photovoltaic cells and instead use deposited layers less than 1 micron (one thousandth of a millimeter) on a material such as glass, steel or plastic.

At present thin film photovoltaic cells do not have as high efficiency as crystalline silicon cells. The cost today per peak Watt is about the same. Their cost per square metre of photovoltaic module is about one third the cost of crystalline silicon technology. If the efficiency of thin film photovoltaic cells can be raised to the same level as that of today's crystalline silicon cells, then the economics will be about right for large scale use as ac power producers. At present, most of the larger size photovoltaic systems are based on crystalline silicon photovoltaic cell technology, and this will continue to be so for some years.

5.4 Affordability:

Generally, PV energy costs are higher than those of energy bought from your local utility. However, if you need power in an area not served by a utility, PV may be the most cost-effective option. All over the world, PV system installations are increasing. As more people learn about this versatile, clean power option, this trend will continue, bringing costs down and affordability up.

Most of us must consider our goals in light of our wants and needs when determining affordability. Availability is an important determinant, and it has a unique meaning for a PV system. This is because it depends not only on reliable equipment but on the level and consistency of sunshine, and the capabilities of the energy storage system, at your site.

Because the weather is unpredictable, designing a PV system to be available at all times and conditions is expensive and often unnecessary. PV systems with long-term availabilities greater than 95% are routinely achieved at half the cost or less of systems designed to be available 99.99% of the time. Designing for lower availabilities decreases the size of the PV array and batteries and saves many dollars.

Another way to resolve the cost and availability issue is to specify a hybrid system, which includes another energy source (usually one that runs on a fossil fuel such as propane). Although saving money is important, you'll want to acquire a safe system that will last 25 years or more. Quality may cost more initially, but it will save money in the long run.

5.5 Printing Energy – Solar cells in the screen printing:

Members of the Fraunhofer Institute for solar Energy Systems ISE, Germany, have developed a new kind of the dye solar module, the dimensions of a door – two meters high and 60 centimeters wide, and are presented at Tokyo in Nanotech 2008, world’s largest nanotechnology trade fair, in Feb 2008.

The core component of the new modules is an organic dye, in combination with nanoparticles sunlight into electricity formats. The nanoparticles provide due to their small size, that the solar modules semitransparent. It therefore qualifies for integration into facades. The solar module prototype manufactured by the researchers at Fraunhofer ISE is amber in colour, but the modules in other colour or with printed images are also possible so that the modules look like decorative slices.

This results in completely new application possibilities: Instead of the electricity generators mounted on the roof, you can make it in glass facades. The new technology protects this building before disturbing direct sunlight and produces both electricity. The wafer-thin electricity-generating film, which lies between two glass panes, is produced from nanoparticles and applied using screen printing technique.

The dye solar module is still a prototype. One particular challenge posed by the new technology is that the narrow gap between the two glass panes must be hermetically sealed so that no air can get in and destroy the reactive substances inside.

The fraunhofer experts came up with a special solution to this problem – istead of using polymeric glue, they have instead decided to work with glass frit, i.e. glass powder is screen-printed onto the panes, and fuses with them at a temperature of around 600 degree Celcius.

DID YOU KNOW?

In 1954, US-based Bell laboratories found that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical solar cells with

a sunlight-energy conversion efficiency of around 6 per cent. This innovation led to Russia launching the first artificial satellite using solar cells in 1957

5.6 Solar – power paint and other solar surprises:

Solar paint - just paint it on your wall, car, boat - and you can start generating electricity.

Researchers Chemical engineer Cyrus Wadia and others at UC Berkeley's interdisciplinary Energy and Resources Group working long hours in the lab "synthesizing super-small nanoparticles" in a three-necked flask.

The scientists are developing a way to paint solar cells onto the steel sheets commonly used to clad large buildings.

Steel sheets are painted rapidly in steel mills by passing them through rollers. A consortium led by Swansea University, UK, hopes to use that process to cover steel sheets with a photovoltaic paint at up to 40 sqmts per minutes.

The paint will be based on dye-sensitised solar cells. Instead of absorbing sunlight using silicon like conventional solar panels, they use dye molecules attached to particles of the titanium dioxide pigment used in paints. While less efficient than conventional cells, dye-based cells do not require expensive silicon, and can be applied as a liquid paste.

Nanotech solar cells are only a few years old. At the University of Toronto in 2005, electrical and computer engineering professor Ted Sargent announced that he had developed a new plastic nanotech material containing solar cells. The Berkeley research takes the technology a step farther.

Traditional silicon based photovoltaic, PV, cells have been around for decades, but they are fragile, heavy, and costly.

Now solar is hot as Berkeley researchers attempt to bring solar technology to the next level, some by improving "first-generation" silicon-based PV, others by developing entirely new light-converting technologies.

"In the past few years, student interest has risen dramatically in all energy-related matters, especially photovoltaics," observes Eugene Haller, professor of materials science and engineering. "Many of the best applicants to our graduate program want to work in this field."

Materials science and engineering PhD student Becca Jones is researching exotic materials such as indium gallium nitride, a semiconductor material that shows promise

for use in long-lived and highly efficient solar cells. Indium gallium nitride is the light-emitting layer in blue and green light emitting diodes, or L-E-Ds.

"Coupled with inexpensive mirrors or lenses," says Jones, these devices could focus "a lot of light on a very small solar cell."

Jones co-founded Students for Greener Berkeley and helped win student approval last spring of a $5 per semester fee increase to fund projects to "green" the campus.

Postdoctoral researcher Lucas Wagner, a quantum physicist who does theoretical studies of nanostructure systems for PV applications, says, "It's interesting science, and you feel you can face yourself when you go home at night."

Solar-cell researchers from across campus meet twice a month at midday to discuss their research problems, experiments, findings, and frustrations. Launched in spring 2006, the grad student-run PV Idea Lab has grown from about 10 initially to a regular cast of up to 30. Free lunch helps. But the main draw is highly technical and stimulating conversation in which issues like ohmic contact and semiconductor band-bending figure large.

"It's very directed and very driven, because we all want to solve the same problem, and we can help each other," Wadia says. With representatives from eight different teams now in the room, "it's really starting to feel like a multi-lab collaboration."

5.7 Electricity from the slide:

World-wide research teams are working on the development of organic solar cells. Fraunhofer Institute for Solar Energy Systems ISE presented on 13 – 15 February on the nano tech 2008 in Tokyo, the world's largest nanotechnology trade fair, ways to industrial mass production.

Organic solar cells have good future prospects: they are inexpensive to produce, because they can apply to thin foils. This requires both a special adaptation of solar cells superstructures and the coating materials and substrates. "Since the procedure is a high-throughput allowed to fall mainly in material costs," says Michael Niggemann, ISE

Nevertheless, the organic solar cell is not against the traditional silicon cell compete - that is their efficiency still far too low. Since it is flexible but can new application areas open up that plastic solar cells could, for example energy for small mobile devices such as MP3 player or electronic ski passes. It would also, on a small strip of plastic solar cells, sensors and switching to a micro energy system to unite.

In Tokyo, the Fraunhofer experts a flexible solar module of the size of a book page. It was produced with a process that is readily on the reel-to-reel technology can be transferred - a key step on the road to mass production.

Also a new design principle helps to save money for the front, the sun-facing electrode used so far mostly expensive indium-tin oxide because it is transparent. But it is also different: The Fraunhofer crew, the connection of the cell to the rear moves the numerous holes in the other side is connected. Dieses This construction has an enormous advantage: It is inexpensive transparent polymer electrodes. The idea has already been patented.

On the nano tech 2008 show Fraunhofer researchers together two companies with their developments. The consortium, in addition to seven other initiatives by the Federal Ministry for Education and Research BMBF selected to participate in the campaign "Nanotech Germany" the state of German research in the future.

5.7 e-sensitized solar cells:

Dye-sensitized solar cells (DSC) are a relatively new class of low-cost solar cells. They are based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. These cells were invented by Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne in 1991 [1] and are also known as Grätzel cells.

Principle:

Dye-sensitized solar cells effectively separate the two functions provided by silicon in a traditional cell design. Normally the silicon acts as both the source of photoelectrons, as well as providing the potential barrier to separate the charges and create a current. In the dye-sensitised solar cell, the semiconductor is used solely for charge separation, the photoelectrons are provided from a separate photosensitive dye. Additionally the charge

separation is not provided solely by the semiconductor, but works in concert with a third element of the cell, an electrolyte in contact with both.

In the case of the original Grätzel design, the cell has three primary parts. On the top is a transparent anode made of fluorine-doped tin oxide (SnO2:F) deposited on the back of a (typically glass) plate. On the back of the conductive plate is a thin layer of titanium dioxide (TiO2), which forms into a highly porous structure with an extremely high surface area. The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO2. A separate backing is made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The front and back parts are then joined and sealed together to prevent the electrolyte from leaking. Although they use a number of "advanced" materials, these are inexpensive compared to the silicon needed for normal cells because they require no expensive manufacturing steps. TiO2, for instance, is already widely used as a paint base.

In operation, sunlight enters the cell through the transparent SnO2:F top contact, striking the dye on the surface of the TiO2. Photons striking the dye with enough energy to be absorbed will create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO2, and from there it moves by a chemical diffusion gradient to the clear anode on top. Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not provided. The dye strips one from iodide in electrolyte below the TiO2, oxidizing it into triiodide. This reaction occurs quite quickly compared to the time that it takes for a the injected electron to recombine with the oxidized dye molecule, preventing this recombination reaction that would effectively short-circuit the solar cell. The triiodide then recovers its missing electron by mechanically diffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit.

Features:

High Quantum efficiency (the chance that one photon (of a particular energy) will create one electron.) is very high about 90% compared to conventional PV cells. The 10% losses are mainly from optical loses from top surface(due to reflection) and conduction loses from TiO2

Bandgap is lower in case of Tio2 than Silicon and also the electrolyte limits the speed at which the dye molecules can regain their electrons and become available

for photoexcitation again. These factors limit the current generated by a DSSc, for comparison, a traditional silicon-based solar cell offers about 35 mA/cm², whereas current DSSc's offer about 20 mA/cm².

The potential difference generated is 0.7 V in DSSc compared 0.6 V in silicon. Combined with a fill factor of about 70%, overall peak power production for

current DSSc's represents a conversion efficiency of about 11%, whereas (as noted earlier) common low-cost commercial panels operate between 12% and 15%.

As a result of both of these features —low losses and lack of recombination— DSSc's work even in low-light conditions. DSSc's are therefore able to work under cloudy skies,

major disadvantage to the design is the use of the liquid electrolyte, which has temperature stability problems. At low temperatures the electrolyte can freeze, ending power production and potentially leading to physical damage. Higher temperatures cause the liquid to expand, making sealing the panels a serious problem.

5.8 Nano antennas:

Researchers in the US at Idaho national Laboratory(INL) – along with partners at Microcontinuum Inc, and Patrick Pinhero of the University of Missouri – have imprinted micro anennas on flexible materials to create a solar cell that will harness energy even after the sun has set.

The new approach uses a special and economical manufacturing process to stamp tiny square spirals of conducting metals onto a sheet of plastic. Each interlocking “nanoantenna” is as wide as 1/25th the diameter of a human hair. Such size enables the nanoantennas to absorb energy in the infrared part of the spectrum, just outside the range of what is visible to the eye, thus gives higher efficiency han conventional solar cells.

The team estimates that the new individual nanoantennas can absorb close to 80 per cent of the available energy. The circuits can be made of a number of different conducting metals, and the nanoantennas can be printed on thin, flexible materials like polethylene – a plastic that’s commonly used in bags and plastic wrap.

5.9 MISCELLANEOUS TECHNOLOGIES:

1) Sunshine to Petrol

Sunshine to Petrol (S2P) technology seeks to use heat from concentrated sunlight to drive chemical reactions that allow carbon dioxide gas in the atmosphere to be broken down into oxygen and carbon monoxide. The CO may then be used to artificially synthesize gasoline. This could be then used in conventional internal combustion engines (ICE) or as an energy storage medium.

2) A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is rather high.

3) Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is small and decreases by the square of the distance from the sun, but unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the sun shines and the sail is deployed and in the frictionless vacuum of space significant speeds can eventually be achieved.

4) Solar chemical

Solar chemical processes use light (photonic) and heat from the sun to drive chemical reactions. These processes offset energy that would otherwise be required from an alternate source, produce no pollution, and can serve as reversible method of storing solar energy. Pioneering work in photochemistry by Schenck et al. in 1943 successfully produced the anti-helminthic drug ascaridole. Solar chemical technologies are currently at the experimental stage with the primary focus on concentrating solar thermal technologies.

Zinc Oxide (ZnO) can be decomposed at high temperatures (1200-1750 °C). The resulting pure zinc can be marketed directly or the zinc can be reacted with water at 350 °C to produce ZnO and hydrogen.[79]

A solar furnace can be used to produce high purity lime and can reduce CO2 emissions associated with cement production by 20-40%. A prototype 10 kWth solar furnace at the Paul Scherrer Institute produced lime at 64.2 grams per minute with a solar energy to chemical energy efficiency of 34.8%.[80]

Water can be directly dissociated at high temperatures (2300-2600 °C). These process have so far been limited due to their high level of complexity and low solar-to-hydrogen efficiency (1-2%).[81]

5) Solar mechanical

Crookes radiometer

Solar mechanical technologies use sunlight to produce a mechanical effect. They were widely investigated by solar pioneers such as Auguste Mouchout, John Ericsson, Charles Tellier and Frank Shuman. These devices generally concentrated sunlight on a boiler to produce steam which was then used by a steam engine to perform useful work. Most of these technologies were displaced early in the 20th century as increasingly cheap fossil fuels made them economically noncompetitive but several solar mechanical technologies have since been developed.

A light mill or Crookes radiometer is a simple solar mechanical device consisting of a glass bulb containing a set of vanes mounted on a spindle. Each vane has a dark side (which absorbs light energy and changes it to heat energy) and a reflective side (which stays relatively cool). Due to the motion of gases around the hot and cool sides of each vane, the vanes rotate with the dark side retracting, and the reflective side advancing towards the light. The rotation is proportional to the intensity of light. The power levels are low however and no practical application has been found for this device

Passive solar tracking devices use imbalances caused by the movement of a low boiling point fluid to track the movement of the sun. These systems can improve performance by 25% over fixed tilt PV systems.

Passive solar shading systems also reposition with the sun according to the movement of balancing fluids. They are used in buildings to maximize natural lighting during winter, and reduce summer glare and cooling loads.

5.10 Artificial antenna system – artificial photosynthesis:

We must first understand the basic principles that govern the transport of the excitation energy. Fortunately, this understanding is very advanced. It goes back to the pioneering

work of Theodor Förster. A chlorophyll molecule consist mainly of a positively charged backbone and some delocalized electrons. By absorbing a photon, its energy is transformed into kinetic energy of one of these delocalized electrons. This fast moving electron causes an alternant oscillating electromagnetic field. An neighboring acceptor molecule A bearing states that are in resonance with the excited state of the donor D* can take over the excitation energy. The rate constant for this energy transfer process is inversely proportional to the 6th power of the D*-A distance R, the matching of the resonance condition J, the relative orientation of the transition dipole moments of the donor and the acceptor and inversely proportional to 4th power of the refractive index of the environment. This principle works not exclusively with chlorophyll molecules but with any exaction, either in molecules, clusters, quantum dots or semiconductors. Based on this ….

we made a model that mimics the key functionality of the antenna system of green plants, JPC B 1997. The model consists of cylinders containing green molecules and at one end red acceptors. The latter correspond to the “entrance of the reaction center RC”. Light absorbed by one of the green molecules is hopping among them until it is caught by an acceptor, which can be seen by monitoring the luminescence of the red dye. This is a system worthwhile to be tried in the laboratory.

CHAPTER 6:

SOLAR ENERGY STORAGE

6.1 Introduction:

Collecting solar energy is the preliminary stage for harvesting it. The major challenge lies in storing the solar energy. Solar energy can be stored in form of thermal, electrical, mechanical or chemical.

Thermal energy can be stored as sensible heat or as latent heat

6.2Sensible heat storage:

It is done in an insulated container containing a liquid like water or a porous solid in the form of pebbles or rock. The first type is preferred with liquid collectors and the second one with air collectors. It operates over certain range of temperatures. In the case of latent heat storage, heat is stored in substance when it melts and extracted when it freezes. This system operates at temperature at which the phase change takes place.

Solar energy can be stored in form of mechanical energy in compressed air and or in flywheel.

Compressed air:

A Solar Compressor

The tanks are operated in pairs disposed above the surface of the earth. Each tank is adapted with sun shading deflectors selectively operated so as to permit the air contained within one such tank to be heated and thus elevated in pressure, by the rays of the sun. A receiving tank, disposed below the surface of the earth, collects the pressurized air and enables the cooling thereof. Condensation occurs while the pressure level within the receiving tank is elevated. A plurality of such units comprising above ground pairs of tanks and below ground cooled receiving tanks are arranged in a series circuit so as to increase the available air pressure at the last receiving tank. A centralized condensate tank collects all the water condensed within each receiving tank and utilizes the elevated air pressure there within to discharge the water as required. The air pressure at the last receiving tank may be utilized to drive a motor which in turn can operate an electric generator.

6.3 Fly wheel:

A satellite power regulation and pointing system is disclosed that comprises a power bus (104) and first and second flywheels (114-116) capable of storing rotational energy. Each flywheel (114-118) comprises a flywheel motor/generator (202) for increasing the rotational energy in its associated flywheel when storing power in its associated flywheel and for reducing the associated flywheel rotational energy when drawing power from its associated flywheel. The system also includes individual flywheel regulators (108-112) connected to the power bus (104) and to the flywheel motor/generators (202). Each flywheel regulator (108-112) includes a power control circuit (204) that allows power to flow to a flywheel motor/generator (202) from the power bus (104) during an energy storage period and that allows power to flow to the power bus (104) from the flywheel motor/generator (202) during an energy drawing period. The flywheel regulators (108-112) also include feedback control loops.

6.4 Solar batteries (Electrical Storage):

A method of producing an array of photovoltaic cells responsive to incident radiation by forming heterojunction-forming material layers over a transparent substrate panel having a transparent electrically conductive coating and thereafter removing selected portions of the materials to form a plurality of cells on a common substrate. The cells are then electrically interconnected by depositing electrically conducting materials over substantially the entire panel and removing only those portions of the deposited materials required to form series electrical connections.

Solar battery and its array are shown below:

CHAPTER 7:

APPLICATION: A SOLAR AIRPLANE

“The pioneer is not always the one who succeeds, but the one who is not scared to fail . ”

- Bertrand Piccard

7.1 Solar Impulse, The zero fuel air-plane

7.1 History:

The Solar Impulse is not the first solar airplane imagined by man, but it is certainly the most ambitious. None of its predecessors has ever managed to make an entire night flight with a pilot on board...

Solar aviation began with reduced models in the 1970s, when affordable solar cells appeared on the market. But it was not until 1980 that the first human flights were realised.

7.2 United States:

Paul MacCready's team developed the Gossamer Penguin, which opened up the way for the Solar Challenger. This aircraft, with a maximum power of 2.5 kW, succeeded in crossing the Channel in 1981 and in quick succession covered distances of several hundred kilometres with an endurance of several hours.

In 1990, the American Eric Raymond crossed the United States with Sunseeker in 21 stages over almost two months. The longest lap was 400 kilometres. The Sunseeker was a solar motor bike-sail plane with a smoothness of 30 for a tare weight of 89 kg and was equipped with solar cells of amorphous silicon.

7.3 Europe:

During the 1980’s, Günter Rochelt was making his first flights with the Solar 1 fitted with 2500 photovoltaic cells, allowing the generation of a maximum power of 2.2kW.

In the middle of the 1990s, several airplanes were built to participate in the "Berblinger" competition. The aim was to be able to go up to an altitude of 450 m with the aid of batteries and to maintain a horizontal flight with the power of at least 500 W/sq.m of solar energy, which corresponds to about half of the power emitted by the sun at midday on the equator.

The prize was won in 1996 by Professeur Voit-Nitschmann's team of Stuttgart University, with Icare 2 (25 m wingspan with a surface of 26 sq.m of solar cells.)

Even if it did not allow a pilot on board, one could not forget Helios, developed by the American AeroVironment Society on behalf of NASA. This remote controlled aircraft, with a wingspan of more than 70 m, established a record altitude of nearly 30'000 m in 2001. It was destroyed during a flight two years later, probably because of turbulence, and crashed in the Pacific Ocean.

In 2005, Alan Cocconi (picture), founder of AC Propulsion, succeeded in flying an unmanned airplane with a 5 m wingspan for 48 hours non-stop, propelled entirely by solar energy. This was the first time an airplane of this type was able to fly through a whole night, thanks to the energy collected by, and stored in, the solar batteries mounted on the plane.

7.4 The technology:

The plane uses solar panels along its 80 m wingspan, about the same size as the wingspan of an Airbus A380, to harness energy from the sun. It is then stored in batteries overnight, giving the plane enough power to glide at a lower altitude through the night.

The heavy batteries mean the cockpit can only accommodate one pilot, with an advanced autopilot system designed to tell when the pilot is asleep and awake. The plane will weigh about 2 T and fly at an altitude of 12,000 metres by day and glide at 3000 m at night.

7.5 The Project:

The Solar Impulse is a revolutionary concept that will push back the limits of our knowledge in the field of materials, energy management and the man-machine interface. It is an aircraft with an inordinate wingspan for its weight and of an aerodynamic quality that to this day has not been equaled, capable of tremendous resistance, despite its light weight.

From the solar captors to the propellers, it is all about optimizing the different links in the propulsion chain and integrating an environment that is as hostile to the materials as it is to the pilot and of course to respect the weight and resistance constraints.

The construction calls on the most advanced technologies and stimulates scientific

research in the field of composite structures, the so-called intelligent light materials, and the means of producing and storing energy. It will be possible to use these results as much in the construction of the aircraft as, subsequently, in numerous other applications useful to society.

The design of the aircraft, pure and futuristic, will itself be the symbol of the spirit of the project in the sky.

The question of energy determines the whole project, from the structure’s dimensions to the extreme weight constraints.

At midday, each m2 of land surface receives the equivalent of 1000 Watts, or 1.3 horsepower of light power.

Over 24 hours, this averages out at just 250W/m2. With 200m2 of photovoltaic cells and a 12 % total efficiency of the propulsion chain,

the plane’s motors achieve no more than 8 HP or 6kW – roughly the amount of power the Wright brothers had a available to them in 1903 when they made their first powered flight.

And it is with that energy, optimized from the solar panel to the propeller by the work of a whole team, that Solar Impulse is striving to fly day and night without fuel.

7.6 Energy Resources:

Multiple forms of energy have to be managed and their conversion phenomena understood and optimized:

Photic – the mechanics of solar radiation Electrical – in the photovoltaic cells, the batteries and the motors Chemical - inside the batteries Potential - when the plane gains altitude Mechanical - through the propulsion system Kinetic - when the plane increases speed Thermal – the various losses (friction, heating…) to be minimized at all costs

7.7 Efficiency and Storage Capacity:

The 12,000 photovoltaic cells are in 130 micron monocrystalline silicon, selected for its capacity to combine lightness and efficiency. Their efficiency could have been higher, following the example of the panels used in space, but their weight would then have penalized the plane during night flight. This phase being the most critical, the main constraint of the project today lies with the batteries. Still heavy, they require a drastic reduction of the weight of the rest of the plane, so as to optimize the whole energy chain and to maximize the aerodynamic performance provided by a large wing span and a wing profile designed for low speeds. With an energy density of 200W/kg, the accumulators needed for night flight weigh 400kg, or more than ¼ of

the total mass of the plane. Improving battery capacity would eventually allow a second pilot, a smaller wingspan or a higher flight speed.

7.8 Central Intelligence:

The on-board computing system gathers and analyses hundreds of flight management parameters, giving the pilot information to interpret for making decisions, transmitting key data to the ground team and, above all, providing the motors with optimal power for the particular flight configuration and battery charge/discharge status. In this way the plane can self-correct and minimize its energy consumption.

7.9 Propulsion system:

Under the wings are four pods, each containing a motor, a polymer lithium battery consisting of 70 accumulators, and a management system controlling charge / discharge and temperature. The thermal insulation has been designed to conserve the heat radiated by the batteries and keep them functioning despite the -40 °C encountered at 8,500 meters. Each motor has a maximum power of 10 HP. A gear box limits the rotation of each 3.5 metre diameter, twin-bladed propeller to 200-400 revolutions/minute.

7.10 Structure and materials:

To attain a 61m wingspan with the necessary rigidity, lightness and flight controllability, and with just 1500kg take-off weight is a challenge which has never been achieved until now. Solar Impulse is constructed around a sort of skeleton in a carbon fibre-honeycomb composite using a sandwich structure. The undersides of the wings are covered with flexible film and the upper surface with a skin of encapsulated solar cells. One hundred and twenty carbon fibre ribs placed at 50cm intervals profile these two layers and give the body its aerodynamic shape.

7.11 Aerodynamics:

Maximum altitude …………………………………8500 m

Outside temperatures ……………………………...+ 80°C to -60°C

Maximum weight ………………………………….1500 kgAverage speed ……………………………………..70 km/h

Wingspan: 61 metres

Comparable to the Airbus A340, in order to minimize induced drag and to provide a

maximum surface area for the solar cells.

Wing-load …………………………………………Less than 10 kg/sq.m

7.12 Propulsion:

Power of the engines ………………………………Max. 30 kW

The average engine power provided by the sun over a 24h period is 12 CV comparable

to that used during the first flight by the Wright brothers in 1903

7.13 Materials & structure:

Essentially constructed from carbon fiber, sandwich structure Using very thin materials with the lowest possible densities.

7.14 Energy management:

Batteries Lithium, weight of 400 kg, from 200 Whr/kg battery capacities Solar cells Monocrystalline silicon, 150-micron thickness, about 200 sq.m surface, min 20 % photovoltaic efficiency Ultra-thin and integrated in the wings

CHAPTER 8:

CONCLUSION