PHYS 471 Solar Energy 2004 1 Concentrating Collectors

PHYS 471 Solar Energy 2004 -1 Concentrating Collectors Instructor : Prof. Dr Ahmet Ecevit Prepared by: Serkan Kapucu

Table of Contents 1. Introduction. . . . 4 2. Concentrating collectors. . . . . 5 3. Types of concentrating collectors. . . . . 6 3. 1. Parabolic trough system. . . . 7 3. 2. Parabolic dish system. . . . 11 3. 3. Power tower system. . . . . 14 3. 4. Stationary concentrating solar collectors. . 16

4. Working principles of concentrating collectors. . 17 4. 1. Trough Systems. . . . . 18 4. 2. Dish Systems. . . 21 4. 3. Central Receiver Systems. . . . 23 5. Technology Comparison. . . . . 25 6. Calculations. . . . 28 7. Economic and Environmental Considerations. . 37 8. Conclusions. . . . 39 References. . . . 41

1. Introduction For 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 [1].

2. 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 [2].

3. Types of concentrating collectors There are four basic types of concentrating collectors: Parabolic trough system Parabolic dish Power tower Stationary concentrating collectors

3. 1. 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 [3]. Their shapes are like letter “u” as shown figure 3. 1. 1 below. Figure 3. 1. 1 Crossection of parabolic trough [4]. The parabolic trough sytem is shown in the figure 3. 1. 2 below. Figure 3. 1. 2 Parabolic trough system [3].

Parabolic troughs often use single-axis or dual-axis tracking. The below figure 3. 1. 3 shows one axis tracking parabolic trough with axis oriented E-W. Figure 3. 1. 3 One Axis Tracking Parabolic Trough with Axis Oriented E-W [8]. The below figure 3. 1. 4 shows two axis tracking concentrator. Figure 3. 1. 4 Two Axis Tracking Concentrator [8].

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 [3]. Parabolic trough combined with gas turbines is shown figure 3. 1. 5 below. Figure 3. 1. 5 Parabolic trough combined with gas turbines [4].

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 [4].

3. 2. 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 [3]. The below figure 3. 2. 1 shows crossection of parabolic dish. The Parabolic dish collector is shown in the below figure 3. 2. 2. Figure 3. 2. 1 Crossection of parabolic dish [4]. Figure 3. 2. 2 Parabolic dish collector with a mirrorlike reflectors and an absorber at the focal point [Courtesy of Sun. Labs - Department of Energy] [3].

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 [3]. The right figure 3. 2. 3 shows the solar dish stirling engine. Figure 3. 2. 3 Solar dish stirling engine [9].

Engines currently under consideration include Stirling and Brayton cycle engines. Several prototype dish/engine systems, ranging in size from 7 to 25 k. W 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 [4].

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 [3]. The Power tower system is shown in the figure 3. 3. 1 below. Figure 3. 3. 1 Power tower system [4]. Heliostats are shown in the figure 3. 3. 2 below. Figure 3. 3. 2 Heliostats [4].

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. Highpressure 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 collectors Stationary 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 collectors Unlike 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 (k. W) 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 [5].

4. 1. Trough Systems These 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 storagesetting 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 Systems Dish 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 k. W of electricity and can be used independently or linked together to increase generating capacity. A 250 -k. W plant composed of ten 25 -k. W 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 k. W 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]. The right figure 4. 2. 1 shows the combination of parabolic dish system. Figure 4. 2. 1 Combination of parabolic dish system [4].

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 heattransfer 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 shows the process of molten salt storage. Figure 4. 3. 1 The process of molten salt storage [11].

5. Technology Comparison Towers and troughs are best suited for large, gridconnected 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].

Table 5. 1 highlights the key features of the three solar technologies. Parabolic Trough Dish/Engine Power Tower Size 30 -320 MW 5 -25 k. W 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 Status Commercially Scale-up Prototype Demonstration Available. Demonstration Technology Development Risk Low High Medium Storage Available Limited Battery Yes Hybrid Designs 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].

6. Calculations Heat from a solar collector may be used to drive a heat engine operating in a cycle to produce work. A heat engine may be used for such applications as water pumping and generating electricity. The thermal output Qout of a concentrating collector operating at temperature T is given by Qout = F'[gamma. Ainqin - U. Arec(T - Ta)], Ain : the area of the incident solar radiation (m 2).

Arec : the area of the receiver (m 2) gamma: optical efficiency qin : the incident solar irradiation (W/m 2) Ta : the ambient temperature (°C) U : the heat loss coefficient (W/m 2 K) F’ : collector efficiency factor The quantity Ain/Arec is called the concentration ratio.

High concentration ratios are obtained by making Ain the area of a system of mirrors designed to concentrate the solar radiation received onto a small receiver of area Arec. Heat losses from the receiver are reduced by the smaller size of the receiver. Consequently, high concentration ratios give high collector temperatures. The stagnation temperature Tmax is given by: gamma. Ainqin = U. Arec(Tmax - Ta).

For example, if the optical efficiency is gamma = 0. 8, the incident solar irradiation is qin = 800 W/m 2, the ambient temperature is Ta = 30°C, and the heat loss coefficient is U = 10 W/m 2 K, then a concentration ratio Ain/Arec = 1 (no concentration) gives Tmax = 94°C, and a concentration ratio Ain/Arec = 10 gives Tmax = 670°C.

The collector efficiency etac at operating temperature T is etac=Qout/Ainqin = F'[gamma-U. Arec(T -Ta)/Ainqin] = F'gamma(Tmax - T)/(Tmax - Ta). The available mechanical power from thermal power output of the collector that would be obtained using a Carnot cycle is Qout(1 - Ta/T), where the temperatures are absolute temperatures.

The second law efficiency eta 2 of a heat engine is defined by eta 2=(mechanical power delivered) /(available mechanical power). Suppose a heat engine with second law efficiency eta 2 uses as input thermal power Qout from the solar collector. The first law efficiency of the engine is eta 1 = (mechanical power delivered)/Qout = eta 2(1 - Ta/T),

where Tmax depends on the design of the collector and on the solar radiation input qin. Now, given F', gamma, eta 2, Ta, and Tmax, we can find the maximum efficiency obtainable, and the optimum operating temperature Topt from the condition d(eta)/d. T = 0. This occurs at the optimum temperature Topt = [Tmax. Ta], ½ and the maximum efficiency is obtained by putting T = Topt in the equation eta = etac. eta 1.

For example, putting F' = 0. 9, gamma = 0. 8, eta 2 = 0. 6, Ta = 30°C = 303 K, we get the efficiencies etamax for different degrees of concentration shown in Table 6. 1. Very low overall efficiencies are obtained unless operating temperatures greater than 500°C are used. Expensive concentrating systems are needed to reach these high temperatures, so commercial viability is difficult [12].

Efficiencies for Converting Solar Radiation to Work Tmax Topt etamax 100°C 63°C 2. 2% 200°C 106°C 4. 8% 400°C 179°C 8. 5% 800°C 297°C 13. 2% 1600°C 480°C 18. 4% Table 6. 1. Different degrees of concentration [12].

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 1980 s. 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¢/k. Wh, and the potential for cost reduction are expected to ultimately lead to costs as low as 5¢/k. Wh [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 1980 s. 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 solaronly 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].

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