Solar Power

SOLAR POWER

COMPLETE DESCRIPTIONBy

A.S.Krishna

Putting Solar Energy to Use: Heating Water by Flat Plate

Collectors Two methods of heating

water: passive (no moving parts) and active (pumps).

In both, a flat-plate collector is used to absorb the sun’s energy to heat the water.

The water circulates throughout the closed system due to convection currents.

Tanks of hot water are used as storage.

Heating Water: Active System

Concentrating collectors

IntroductionFor applications such as air conditioning, central power generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective.

They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used.

These are devices that optically reflect and focus incident solar energy onto a small receiving area. As a result of this concentration, the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several thousand degrees Celsius. The concentrators must move to track the sun if they are to perform effectively.

Concentrating collectors

Concentrating, or focusing, collectors intercept direct radiation over a large area and focus it onto a small absorber area. These collectors can provide high temperatures more efficiently than flat-plate collectors, since the absorption surface area is much smaller.

However, diffused sky radiation cannot be focused onto the absorber. Most concentrating collectors require mechanical equipment that constantly orients the collectors toward the sun and keeps the absorber at the point of focus. Therefore; there are many types of concentrating collectors

Types of concentrating collectors

Parabolic trough system Mirror strip (or)Parabolic dish  Power tower Stationary concentrating collectors 

There are four basic types of concentrating collectors:

Parabolic trough system 

Parabolic troughs are devices that are shaped like the letter “u”. The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the trough. Sometimes a transparent glass tube envelops the receiver tube to reduce heat loss

Figure 1.2 Parabolic trough system . Figure 1.1 Crossection of parabolic trough

The parabolic trough sytem is shown in the figure 1.2 below.

Their shapes are like letter “u” as shown figure 1.1 below.

Receiver

Sun rays

Tracking mechanism

Parabola

Parabolic troughs often use single-axis or dual-axis tracking.

Figure 1.3 One Axis Tracking Parabolic Trough with Axis Oriented E-W

Figure 1.4 Two Axis Tracking Concentrator

The below figure 1.3 shows one axis tracking parabolic trough with axis oriented E-W.

The below figure 1.4 shows two axis tracking concentrator.

Temperatures at the receiver can reach 400 °C and produce steam for generating electricity. In California, multi-megawatt power plants were built using parabolic troughs combined with gas turbines Parabolic trough combined with gas turbines is shown figure 1.5 below.

Figure 1.5 Parabolic trough combined with gas turbines [4].

ECONOMIC ASPECTS:

Cost projections for trough technology are higher than those for power towers and dish/engine systems due in large part to the lower solar concentration and hence lower temperatures and efficiency.

However with long operating experience, continued technology improvements, and operating and maintenance cost reductions, troughs are the least expensive, most reliable solar thermal power production technology for near-term

Mirror Strip (or)Parabolic dish

systems A parabolic dish collector is similar in appearance to a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. It uses a dual axis sun tracker .

Figure 2.2 Parabolic dish collector with a mirror-like reflectors and an absorber at the focal point [Courtesy of SunLabs - Department of Energy]

Figure 2.1 Crossection of parabolic dish .

The below figure 2.1 shows crossection of parabolic dish.

The Parabolic dish collector is shown in the below figure 2.2.

A parabolic dish system uses a computer to track the sun and concentrate the sun's rays onto a receiver located at the focal point in front of the dish.  In some systems, a heat engine, such as a Stirling engine, is linked to the receiver to generate electricity. Parabolic dish systems can reach 1000 °C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small-power capacity range.

Figure 2.3 Solar dish stirling engine.

Engines currently under consideration include Stirling and Brayton cycle engines. Several prototype dish/engine systems, ranging in size from 7 to 25 kW have been deployed in various locations in the USA. High optical efficiency and low start up losses make dish/engine systems the most efficient of all solar technologies. A Stirling engine/parabolic dish system holds the world’s record for converting sunlight into electricity. In 1984, a 29% net efficiency was measured at Rancho Mirage, California ].

3.3. Power tower system 

A heliostat uses a field of dual axis sun trackers that direct solar energy to a large absorber located on a tower. To date the only application for the heliostat collector is power generation in a system called the power tower .

Figure 3.3.2 Heliostats [4].Figure 3.3.1 Power tower system [4].

Heliostats are shown in the figure below.

A power tower has a field of large mirrors that follow the sun's path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected rays of the sun are always aimed at the receiver, where temperatures well above 1000°C can be reached. High-pressure steam is generated to produce electricity [3].The power tower system with heliostats is shown in the figure 3.3.3 below.

Figure 3.3.3 Power tower system with heliostats [4]. 

3.4. Stationary concentrating solar

collectorsStationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun tracker. This class of collector includes parabolic trough flat plate collectors, flat plate collectors with parabolic boosting reflectors, and solar cooker. Development of the first two collectors has been done in Sweden. Solar cookers are used throughout the world, especially in the developing countries [3].

4. Working principles of concentrating

collectorsUnlike solar (photovoltaic) cells, which use light to produce electricity, concentrating solar power systems generate electricity with heat. Concentrating solar collectors use mirrors and lenses to concentrate and focus sunlight onto a thermal receiver, similar to a boiler tube. The receiver absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is converted into electricity. There are three main types of concentrating solar power systems: parabolic troughs, dish/engine systems, and central receiver systems.These technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid connected applications of 200-350 megawatts (MW) or more. A concentrating solar power system that produces 350 MW of electricity displaces the energy equivalent of 2.3 million barrels of oil

4.1. Trough SystemsThese solar collectors use mirrored parabolic troughs to focus the sun's energy to a fluid-carrying receiver tube located at the focal point of a parabolically curved trough reflector [5].It is shown in the figure 4.1.1 below.

Figure 4.1.1 Parabolic trough with mirrored parabolic troughs [10].

The energy from the sun sent to the tube heats oil flowing through the tube, and the heat energy is then used to generate electricity in a conventional steam generator. Many troughs placed in parallel rows are called a "collector field." The troughs in the field are all aligned along a northsouth axis so they can track the sun from east to west during the day, ensuring that the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate about 80 MW of electricity.

Trough designs can incorporate thermal storage-setting aside the heat transfer fluid in its hot phase allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuels to supplement the solar output during periods of low solar radiation. Typically, a natural gas-fired heat or a gas steam boiler/reheater is used. Troughs also can be integrated with existing coal-fired plants [5].

4.2. Dish SystemsDish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish center. A dish/engine system is a stand alone unit composed primarily of a collector, a receiver, and an engine. It works by collecting and concentrating the sun's energy with a dishshaped surface onto a receiver that absorbs the energy and transfers it to the engine. The engine then converts that energy to heat. The heat is then converted to mechanical power, in a manner similar to conventional engines, by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a piston to produce mechanical power. An electric generator or alternator converts the mechanical power into electrical power.

Each dish produces 5 to 50 kW of electricity and can be used independently or linked together to increase generating capacity. A 250-kW plant composed of ten 25-kW dish/engine systems requires less than an acre of land. Dish/engine systems are not commercially available yet, although ongoing demonstrations indicate good potential. Individual dish/engine systems currently can generate about 25 kW of electricity. More capacity is possible by connecting dishes together. These systems can be combined with natural gas, and the resulting hybrid provides continuous power generation [5].

Figure 4.2.1 Combination of parabolic dish system [4].

The right figure 4.2.1 shows the combination of parabolic dish system.

4.3. Central Receiver Systems

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 tall tower. 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 [5]. In this system, molten-salt is pumped from a “cold” tank at 288 deg.C and cycled through the receiver where it is heated to 565 deg.C and returned to a “hot” tank. The hot salt can then be used to generate electricity when needed. Current designs allow storage ranging from 3 to 13 hours [4].

Figure 4.3.1 The process of molten salt storage [11].

Figure 4.3.1 shows the process of molten salt storage.

5. Technology Comparison

Towers and troughs are best suited for large, grid-connected power projects in the 30-200 MW size, whereas, dish/engine systems are modular and can be used in single dish applications or grouped in dish farms to create larger multi-megawatt projects. Parabolic trough plants are the most mature solar power technology available today and the technology most likely to be used for near-term deployments. Power towers, with low cost and efficient thermal storage, promise to offer dispatchable, high capacity factor, solar-only power plants in the near future.

The modular nature of dishes will allow them to be used in smaller, high-value applications. Towers and dishes offer the opportunity to achieve higher solar-to-electric efficiencies and lower cost than parabolic trough plants, but uncertainty remains as to whether these technologies can achieve the necessary capital cost reductions and availability improvements. Parabolic troughs are currently a proven technology primarily waiting for an opportunity to be developed. Power towers require the operability and maintainability of the molten-salt technology to be demonstrated and the development of low cost heliostats. Dish/engine systems require the development of at least one commercial engine and the development of a low cost concentrator [4].

  Parabolic Trough Dish/Engine Power Tower

Size 30-320 MW 5-25 kW 10-200 MW

Operating Temperature (ºC/ºF)

390/734  750/1382 565/1049

Annual Capacity Factor  23-50 % 25 % 20-77 %

Peak Efficiency  20%(d)  29.4%(d) 23%(p)

Net Annual Efficiency  11(d)-16% 12-25%(p) 7(d)-20%

Commercial StatusCommercially Scale-up

Prototype Demonstration  AvailableDemonstration

Technology Development Risk

Low High Medium

Storage Available Limited Battery Yes

Hybrid Designs Yes Yes Yes

Cost USD/W 2,7-4,0 1,3-12,6 2,5-4,4

(p) = predicted; (d) = demonstrated;

Table 5.1 Key features of the three solar technologies [4].

Highlights the key features of the three solar technologies.

7. Economic and Environmental Considerations

The most important factor driving the solar energy system design process is whether the energy it produces is economical. Although there are factors other than economics that enter into a decision of when to use solar energy; i.e. no pollution, no greenhouse gas generation, security of the energy resource etc., design decisions are almost exclusively dominated by the ‘levelized energy cost’. This or some similar economic parameter, gives the expected cost of the energy produced by the solar energy system, averaged over the lifetime of the system. 

Commercial applications from a few kilowatts to hundreds of megawatts are now feasible, and plants totaling 354 MW have been in operation in California since the 1980s. Plants can function in dispatchable, grid-connected markets or in distributed, stand-alone applications. They are suitable for fossil-hybrid operation or can include cost-effective storage to meet dispatchability requirements. They can operate worldwide in regions having high beam-normal insolation, including large areas of the southwestern United States, and Central and South America, Africa, Australia, China, India, the Mediterranean region, and the Middle East, . Commercial solar plants have achieved levelized energy costs of about 12-15¢/kWh, and the potential for cost reduction are expected to ultimately lead to costs as low as 5¢/kWh [6].

8. Conclusions

Concentrating solar power technology for electricity generation is ready for the market. Various types of single and dual-purpose plants have been analysed and tested in the field. In addition, experience has been gained from the first commercial installations in use worldwide since the beginning of the 1980s. Solar thermal power plants will, within the next decade, provide a significant contribution to an efficient, economical and environmentally benign energy supply both in large-scale gridconnected dispatchable markets and remote or modular distributed markets. Parabolic and Fresnel troughs, central receivers and parabolic dishes will be installed for solar/fossil hybrid and solar-only power plant operation. In parallel, decentralised process heat for industrial applications will be provided by low-cost concentrated collectors.

Following a subsidised introduction phase in green markets, electricity costs will decrease from 14 to 18 Euro cents per kilowatt hour presently in Southern Europe towards 5 to 6 Euro cents per kilowatt hour in the near future at good sites in the countries of the Earth’s sunbelt. After that, there will be no further additional cost in the emission reduction by CSP. This, and the vast potential for bulk electricity generation, moves the goal of longterm stabilisation of the global climate into a realistic range. Moreover, the problem of sustainable water resources and development in arid regions is addressed in an excellent way, making use of highly efficient, solar powered co-generation systems. However, during the introduction phase, strong political and financial support from the responsible authorities is still required, and many barriers must be overcome [7].

References

[1]http://aloisiuskolleg.www.de/schule/fachbereiche/comenius/charles/solar.html

[2]http://www.tpub.com/utilities/index.html

[3]http://www.canren.gc.ca/tech.appl/index.asp

Some Other Useful Data

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Types of solar collectorsTypes of solar collectors

Motion Collector type Absorber type

Concentration ratio

Indicative temperature range (°C)

Stationary

Flat plate collector (FPC) Flat 1 30-80

Evacuated tube collector (ETC) Flat 1 50-200

Compound parabolic collector (CPC) Tubular1-5 60-240

Single-axis tracking

5-15 60-300

Linear Fresnel reflector (LFR) Tubular 10-40 60-250

Parabolic trough collector (PTC) Tubular 15-45 60-300

Cylindrical trough collector (CTC) Tubular 10-50 60-300

Two-axes tracking

Parabolic dish reflector (PDR) Point 100-1000 100-500

Heliostat field collector (HFC) Point 100-1500 150-2000

Note: Concentration ratio is defined as the aperture area divided by the receiver/absorber area of the collector.

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Modes of TrackingModes of Tracking

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Comparison of energy absorbed for various modes of tracking

Comparison of energy absorbed for various modes of tracking

Tracking modeSolar energy (kWh/m2) Percent to full tracking

E SS WS E SS WS

Full tracking 8.43 10.60 5.70 100.0 100.0 100.0

E-W Polar 8.43 9.73 5.23 100.0 91.7 91.7

N-S Horizontal 6.22 7.85 4.91 73.8 74.0 86.2

E-W Horizontal 7.51 10.36 4.47 89.1 97.7 60.9

Note: E - Equinoxes, SS - Summer Solstice, WS - Winter Solstice

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Parabolic Trough SystemParabolic Trough System

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Parabolic trough collectorsParabolic trough collectors

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Parabola detailParabola detail

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Receiver detailReceiver detail

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Central receiver systemCentral receiver system

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Heliostat detailHeliostat detail

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Central receiver-1Central receiver-1

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Central receiver-2Central receiver-2

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Central receiver-3Central receiver-3

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Central receiver-4Central receiver-4

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Central receiver-5Central receiver-5

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Central receiver-6Central receiver-6

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Central receiver-7Central receiver-7

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Central receiver-8Central receiver-8

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Solar energy should be given a chance if we want to protect the environment. We own it to our children, our grandchildren and the generations to come.

Solar energy should be given a chance if we want to protect the environment. We own it to our children, our grandchildren and the generations to come.

Thank you for your attention,

any questions please….