Parabola construction

To achieve cost-effectiveness in mass production, the collector structure must feature not only a high stiffness-to-weight ratio, to keep the material content to a minimum, but also be amenable to low-labor manufacturing processes. A number of structural concepts have been proposed, such as steel framework structures with central torque tubes or double V trusses and fiberglass (Kalogirou et al., 1994b). A recent development in this type of collectors is the design and manufacture of the EuroTrough, a new parabolic trough collector, in which an advanced lightweight structure is used to achieve cost-efficient solar power generation (Lupfert et al., 2000; Geyer et al., 2002). Based on environmental test data to date, mirrored glass appears to be the preferred mirror material, although self-adhesive reflective materials with lifetimes of 5-7 years exist in the market.

For the EuroTrough collector, a so-called torque-box design has been selected, with less weight and fewer deformations of the collector structure due to dead weight and wind loading than the reference designs (LS-2 torque tube or the LS-3 V truss design, both commercial in the Californian plants). This reduces torsion and bending of the structure during operation and results in increased optical performance and wind resistance. The weight of the steel structure has been reduced by about 14% as compared to the available design of the LS-3 collector. The central element of the box design is a 12 m long steel space-frame structure having a squared cross-section that holds the support arms for the parabolic mirror facets. The torque box is built out of only four steel parts. This leads to easy manufacturing and decreases the required effort and thus the cost for site assembling. The structural deformation of the new design is considerably less than in the previous design (LS-3), which results in better performance of the collector.

Another method for producing lightweight troughs, developed by the author, is with fiberglass (Kalogirou et al., 1994b). For the production of the trough, a mold is required. The trough is in fact a negative copy of the mold. Initially, a layer of fiberglass is laid. Cavities produced with plastic conduits, covered with a second layer of fiberglass at the back of the collector surface, provide reinforcement in the longitudinal and transverse directions to increase rigidity, as shown in Figure 3.15.

TRACKING MECHANisMs

A tracking mechanism must be reliable and able to follow the sun with a certain degree of accuracy, return the collector to its original position at the end of the day or during the night, and track during periods of intermittent cloud cover. Additionally, tracking mechanisms are used for the protection of collectors, i.e., they turn the collector out of focus to protect it from hazardous environmental and working conditions, such as wind gusts, overheating, and failure of the thermal fluid flow mechanism. The required accuracy of the tracking mechanism depends on the collector acceptance angle. This is described in Section 3.6.3, and the method to determine it experimentally is given in Section 4.3.

Various forms of tracking mechanisms, varying from complex to very simple, have been proposed. They can be divided into two broad categories: mechanical and electrical-electronic systems. The electronic systems generally exhibit improved reliability and tracking accuracy. These can be further subdivided into:

1. Mechanisms employing motors controlled electronically through sensors, which detect the magnitude of the solar illumination (Kalogirou, 1996)

2. Mechanisms using computer-controlled motors, with feedback control provided from sensors measuring the solar flux on the receiver (Briggs, 1980; Boultinghouse, 1982)

Parabole Solare

FIGURE 3.15 Fiberglass parabola details.

30 X 30mm RHS

2mm first fiberglass layer

2mm final fiberglass layer Glue

20mm dia. plastic conduit

2mm first fiberglass layer

30 X 30mm RHS

2mm final fiberglass layer Glue

20mm dia. plastic conduit

Receiver position

Parabola

FIGURE 3.15 Fiberglass parabola details.

A tracking mechanism developed by the author (Kalogirou, 1996) uses three light-dependent resistors, which detect the focus, sun-cloud, and day-night conditions and give instruction to a DC motor through a control system to focus the collector, follow approximately the sun path when cloudy conditions exist, and return the collector to the east during night.

The system, which was designed to operate with the required tracking accuracy, consists of a small direct current motor that rotates the collector via a speed reduction gearbox. A diagram of the system, together with a table showing the functions of the control system, is presented in Figure 3.16. The system employs three sensors, of which A is installed on the east side of the collector shaded by the frame, whereas the other two (B and C) are installed on the collector frame. Sensor A acts as the "focus" sensor, i.e., it receives direct sunlight only when the collector is focused. As the sun moves, sensor A becomes shaded and the motor turns "on." Sensor B is the "cloud" sensor, and cloud cover is assumed when illumination falls below a certain level. Sensor C is the "daylight" sensor. The condition when all three sensors receive sunlight is translated by the control system as daytime with no cloud passing over the sun and the collector in a focused position. The functions shown in the table of Figure 3.16 are followed provided that sensor C is "on," i.e., it is daytime.

The sensors used are light-dependent resistors (LDRs).The main disadvantage of LDRs is that they cannot distinguish between direct and diffuse sunlight. However, this can be overcome by adding an adjustable resistor to the system, which can be set for direct sunlight (i.e., a threshold value). This is achieved by

Sprocket wheel

Sensor-A

Sprocket wheel

Sensor-A

Eurotrough Drive System

Control system functions

Sensor-A

Sensor-B

Function

On

On

Motor off

Off

On

Motor on

Off

Off

Timer on

Collector frame

Collector frame

Receiver

Receiver

Compound Parabolic Collectors

FIGURE 3.16 Tracking mechanism, system diagram.

setting the adjustable resistor so that for direct sunlight, the appropriate input logic level (i.e., 0) is set.

As mentioned previously, the motor of the system is switched on when any of the three LDRs is shaded. Which sensor is activated depends on the amount of shading determined by the value set on the adjustable resistor, i.e., threshold value of radiation required to trigger the relays. Sensor A is always partially shaded. As the shading increases due to the movement of the sun, a value is reached that triggers the forward relay, which switches the motor on to turn the collector and therefore re-exposes sensor A.

The system also accommodates cloud cover, i.e., when sensor B is not receiving direct sunlight, determined by the value of another adjustable resistor, a timer is automatically connected to the system and this powers the motor every 2 min for about 7 s. As a result, the collector follows approximately the sun's path and when the sun reappears the collector is re-focused by the function of sensor A.

The system also incorporates two limit switches, the function of which is to stop the motor from going beyond the rotational limits. These are installed on two stops, which restrict the overall rotation of the collector in both directions, east and west. The collector tracks to the west as long as it is daytime. When the sun goes down and sensor C determines that it is night, power is connected to a reverse relay, which changes the motor's polarity and rotates the collector until its motion is restricted by the east limit switch. If there is no sun during the following morning, the timer is used to follow the sun's path as under normal cloudy conditions. The tracking system just described, comprising an electric motor and a gearbox, is for small collectors. For large collectors, powerful hydraulic units are required.

The tracking system developed for the EuroTrough collector is based on "virtual" tracking. The traditional sun-tracking unit with sensors that detect the position of the sun has been replaced by a system based on calculation of the sun position using a mathematical algorithm. The unit is implemented in the EuroTrough with a 13-bit optical angular encoder (resolution of 0.8 mrad) mechanically coupled to the rotation axis of the collector. By comparing both sun and collector axes positions by an electronic device, an order is sent to the drive system to induce tracking.

3.2.2 Fresnel Collectors

Fresnel collectors have two variations: the Fresnel lens collector (FLC), shown in Figure 3.17a, and the linear Fresnel reflector (LFR), shown in Figure 3.17b. The former is made from a plastic material and shaped in the way shown to focus the solar rays to a point receiver, whereas the latter relies on an array of linear mirror strips that concentrate light onto a linear receiver. The LFR collector can be imagined as a broken-up parabolic trough reflector (see Figure 3.17b), but unlike parabolic troughs, the individual strips need not be of parabolic shape. The strips can also be mounted on flat ground (field) and concentrate light on a

Sun rays

Sun rays

Downward Facing ParabolaParabolic Trough Cross Section
parabolic trough collector.

linear fixed receiver mounted on a tower. A representation of an element of an LFR collector field is shown in Figure 3.18. In this case, large absorbers can be constructed and the absorber does not have to move. The greatest advantage of this type of system is that it uses flat or elastically curved reflectors, which are cheaper than parabolic glass reflectors. Additionally, these are mounted close to the ground, thus minimizing structural requirements.

Receiver

Receiver

FIGURE 3.18 schematic diagram of a downward-facing receiver illuminated from an LFR field.

The first to apply this principle was the great solar pioneer Giorgio Francia (1968), who developed both linear and two-axis tracking Fresnel reflector systems at Genoa, Italy, in the 1960s. These systems showed that elevated temperatures could be reached using such systems, but he moved on to two-axis tracking, possibly because advanced selective coatings and secondary optics were not available (Mills, 2001).

In 1979, the FMC Corporation produced a detailed project design study for 10 MWe and 100 MWe LFR power plants for the U.S. Department of Energy (DOE). The larger plant would have used a 1.68 km linear cavity absorber mounted on 61m towers. The project, however, was never put into practice, because it ran out of DOE funding (Mills, 2001).

A later effort to produce a tracking LFR was made by the Israeli Paz company in the early 1990s by Feuermann and Gordon (1991). This used efficient secondary CPC-like optics and an evacuated tube absorber.

One difficulty with the LFR technology is that avoidance of shading and blocking between adjacent reflectors leads to increased spacing between reflectors. Blocking can be reduced by increasing the height of the absorber towers, but this increases cost. Compact linear Fresnel reflector (CLFR) technology has been recently developed at Sydney University in Australia. This is, in effect, a second type of solution for the Fresnel reflector field problem that has been overlooked until recently. In this design adjacent linear elements can be interleaved to avoid shading. The classical LFR system has only one receiver and there is no choice about the direction and orientation of a given reflector. However, if it is assumed that the size of the field will be large, as it must be in technology supplying electricity in the megawatt class, it is reasonable to assume that there will be many towers in the system. If they are close enough, then individual reflectors have the option of directing reflected solar radiation to at least two towers. This additional variable in the reflector orientation provides the means for much more densely packed arrays because patterns of alternating reflector orientation can be such that closely packed reflectors can be positioned without shading and blocking. The interleaving of mirrors between two receiving towers is shown in Figure 3.19. The arrangement minimizes beam blocking

Receiver

Receiver

Receiver e o

Heliostat Field Schematic

FiGURE 3.19 schematic diagram showing interleaving of mirrors in a CLFR with reduced shading between mirrors.

FiGURE 3.19 schematic diagram showing interleaving of mirrors in a CLFR with reduced shading between mirrors.

by adjacent reflectors and allows high reflector densities and low tower heights to be used. Close spacing of reflectors reduces land usage, but in many cases, as in deserts, this is not a serious issue. The avoidance of large reflector spacing and tower heights is also an important cost issue when the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost are considered. If the technology is to be located in an area with limited land availability, such as in urban areas or next to existing power plants, high array ground coverage can lead to maximum system output for a given ground area (Mills, 2001).

3.2.3 Parabolic Dish Reflectors (PDRs)

A parabolic dish reflector (PDR), shown schematically in Figure 3.20a, is a point-focus collector that tracks the sun in two axes, concentrating solar energy onto a receiver located at the focal point of the dish. The dish structure must fully track the sun to reflect the beam into the thermal receiver. For this purpose, tracking mechanisms similar to the ones described in the previous section are employed in double, so the collector is tracked in two axes. A photograph of a Eurodish collector is shown in Figure 3.20b.

The receiver absorbs the radiant solar energy, converting it into thermal energy in a circulating fluid. The thermal energy can then be either converted into electricity using an engine-generator coupled directly to the receiver or transported through pipes to a central power conversion system. Parabolic dish systems can achieve temperatures in excess of 1500°C. Because the receivers are distributed throughout a collector field, like parabolic troughs, parabolic dishes are often called distributed receiver systems. Parabolic dishes have several important advantages (De Laquil et al., 1993):

1. Because they are always pointing at the sun, they are the most efficient of all collector systems.

2. They typically have concentration ratios in the range of 600 to 2000 and thus are highly efficient at thermal-energy absorption and power conversion systems.

148 Solar Energy Collectors Sun rays

Receiver

Parabola

Two-axis tracking mechanism

Dish Collector Structurewww.psa.es/webeng/instalaciones/discos.html)."/>
FIGURE 3.20 Parabolic dish collector. (a) schematic diagram. (b) Photo of a Eurodish collector (from www.psa.es/webeng/instalaciones/discos.html).

3. They are modular collector and receiver units that can function either independently or as part of a larger system of dishes.

The main use of this type of concentrator is for parabolic dish engines. A parabolic dish engine system is an electric generator that uses sunlight instead of crude oil or coal to produce electricity. The major parts of a system are the solar dish concentrator and the power conversion unit. More details on this system are given in Chapter 10.

Parabolic dish systems that generate electricity from a central power converter collect the absorbed sunlight from individual receivers and deliver it via a heat transfer fluid to the power conversion systems. The need to circulate heat

Parabola Construction
FIGURE 3.21 schematic of central receiver system.

transfer fluid throughout the collector field raises design issues such as piping layout, pumping requirements, and thermal losses.

3.2.4 Heliostat Field Collectors (HFCs)

For extremely high inputs of radiant energy, a multiplicity of flat mirrors, or helio-stats, using altazimuth mounts can be used to reflect their incident direct solar radiation onto a common target, as shown in Figure 3.21. This is called the heliostat field or central receiver collector. By using slightly concave mirror segments on the heliostats, large amounts of thermal energy can be directed into the cavity of a steam generator to produce steam at high temperature and pressure.

The concentrated heat energy absorbed by the receiver is transferred to a circulating fluid that can be stored and later used to produce power. Central receivers have several advantages (De Laquil et al., 1993):

1. They collect solar energy optically and transfer it to a single receiver, thus minimizing thermal energy transport requirements.

2. They typically achieve concentration ratios of 300 to 1500 and so are highly efficient, both in collecting energy and in converting it to electricity.

3. They can conveniently store thermal energy.

4. They are quite large (generally more than 10 MW) and thus benefit from economies of scale.

Each heliostat at a central receiver facility has from 50 to 150 m2 of reflective surface, with four mirrors installed on a common pillar for economy, as shown in Figure 3.22. The heliostats collect and concentrate sunlight onto the receiver, which absorbs the concentrated sunlight, transferring its energy to a heat transfer fluid. The heat transport system, which consists primarily of pipes, pumps, and valves, directs the transfer fluid in a closed loop among the receiver, storage, and power conversion systems. A thermal storage system typically stores the collected energy as sensible heat for later delivery to the power conversion system. The storage system also decouples the collection of solar energy from its conversion to electricity. The power conversion system consists of a steam generator,

Heliostat Facets
FIGURE 3.22 Detail of a heliostat.

turbine generator, and support equipment, which convert the thermal energy into electricity and supply it to the utility grid.

In this case incident sunrays are reflected by large tracking mirrored collectors, which concentrate the energy flux towards radiative-convective heat exchangers, where energy is transferred to a working thermal fluid. After energy collection by the solar system, the conversion of thermal energy to electricity has many similarities with the conventional fossil-fueled thermal power plants (Romero et al., 2002).

The collector and receiver systems come in three general configurations. In the first, heliostats completely surround the receiver tower, and the receiver, which is cylindrical, has an exterior heat transfer surface. In the second, the heliostats are located north of the receiver tower (in the Northern Hemisphere), and the receiver has an enclosed heat transfer surface. In the third, the heliostats are located north of the receiver tower, and the receiver, which is a vertical plane, has a north-facing heat transfer surface. More details of these plants are given in Chapter 10.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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Responses

  • Procopio
    How to fibreglass parabolic trough?
    8 years ago
  • brigitte
    What structures are considered parabolas?
    8 years ago

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