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The goal of this study is to model the effects of wind on Cylindrical Trough Collectors (CTCs). Two major areas are discussed in this paper: 1) heat losses due to wind flow over receiver pipe and 2) average forces applied on the collector’s body. To accomplish these goals a 2D modeling of CTC was carried out using commercial codes with various wind velocities and collector orientations. Ambient temperature was assumed to be constant at 300 K and for specific geometries different meshing methods and boundary conditions were used in various runs. Validation was done by comparing the simulation results for a horizontal collector with empirical data. It was observed that maximum force of 509.1 Newton per Meter occurs at +60 degrees. Nusselt number is almost the constant for positive angles while at negative angles it varies considerably with the collector’s orientation.

Fossil fuels as a major source of energy have many advantageous such as low price, high availability and can produce a significant amount of energy per unit of weight. However, the environmental consequences of extensive use of fossil fuels, especially coal, are manifesting themselves through global warming and similar phenomena. According to [_{2}-eq/yr which has caused an increase in atmospheric carbon concentration from a pre-anthropogenic level of 280 ppm to 379 ppm in 2005. These global changes are pushing industries and politicians to find sustainable solutions to humanity’s ever growing energy demands.

Solar energy as an alternative is a clean, renewable and inexhaustible source of energy. All other forms of energy that we use are solar in origin. Oil, coal, natural gas and woods were originally produced by photosyn- thetic processes, followed by complex chemical reactions in which decaying vegetation was subjected to very high temperatures and pressures over a long period of time [

The two primary types of solar collectors that are using are distributed receivers and central receivers. This study is concerned with a specific type of distributed receivers called as trough collectors. Trough collectors are line-focus tracking reflectors that concentrate sunlight onto receiver tubes along their focal lines. Other types of distributed receivers include parabolic dishes, Fresnel lenses, and special bowls [

Sadly after 70’s, due to poor policies, progress in solar thermal power slowed down. However, the last five years have seen a resurgence of interest in this area [

According to Reference [^{◦}C, this makes cylindrical trough collectors a feasible option for most of such industries.

CTC’s are made by bending a sheet of a reflective material into a cylindrical shape, then a tube which is covered with another glass tube (to reduce heat loss) is placed at the focal line. This called “receiver tube”. When the collector is facing the sun, some rays that hit the reflector surface are reflected on the receiver tube. It is these reflected rays that heat up the working fluid which is flowing in the receiver tube. For efficient performance, CTC’s have to face the sun at tall time hence they are usually equipped with a single axis tracking system. This means that at different times of day reflector surface will have different orientation. Reflectors orientation can have a significant impact on structure and thermal efficiency of the collector.

Several studies have been carried out on solar collectors such as, wind load on residential and large scale solar collector models [

Collector Type | Temperature Range (˚C) |
---|---|

Flat-Plate Collector (FPC) | 30 - 80 |

Evacuated Tube Collector (ETC) | 50 - 200 |

Compound Parabolic Collector (CPC) | 60 - 240 |

Linear Fresnel Reflector (LFR) | 60 - 250 |

Cylindrical Trough Collector (CTC) | 60 - 300 |

Parabolic Trough Collector (PTC) | 60 - 400 |

Parabolic Dish Reflector (PDR) | 100 - 1500 |

Heliostat Field Reflector (HFR) | 150 - 2000 |

Geometry of this study consists of a CTC with variable collector angles. Collector is a 60 degree section of a 140 cm diameter cylinder as illustrated in

Grid structure is of critical importance in any CFD problem due to its contribution to accuracy and convergence efficiency. For this reason a structured c-h type grid is applied in the domain by Gambit. Boundary’s which were defined in the control volume are shown in

As it can be seen from

Simple pressure-velocity coupling, energy equation and RNG k-ɛ model (derived by renormalization group theory) was used in Fluent to solve the problem. This model is derived from the instantaneous Navier-Stokes equation using a mathematical technique known as Re-Normalization Group method hence the abbreviation RNG. RNG model which is similar in form to other k-ɛ models was proposed by [

In order to validate the computational analysis, two different methods were used, one was to verify Nusselt number and the other was to verify drag coefficient.

At 0 degree, the flow over the receiver pipe is perpendicular to receiver pipe (as can be seen in

In order to verify drag coefficient, a 180 degree sector of an empty cylinder (10 cm diameter) was modeled in Gambit and analyzed in Fluent with similar methods. Afterwards, pressures on both sides of the half-cylinder were plotted against the vertical axis (y) and through curve fitting, the pressure was estimated as a function of height. By integrating the pressure function between y = −0.05 and y = 0.05 and using Equation (2), the drag coefficient was calculated.

Comparison between the results from the computational analysis and drag coefficient given in [

Steady state flow in the control volume was analyzed using computational methods at 0, ±30, ±45 and ±60 collector angles and various wind velocities (2.5 m/s, 5 m/s, 10 m/s and 15 m/s) as inlet boundary condition. Con-

vergence of the numerical solution was obtained when the residual of each of the governing equations was less than 1e−5.

where θ is the collector angle in degrees. Values of ς, φ, ψ and ϑ which are required for calculating the net force are given in Equation (4).

where V is wind velocity. Equation (5) calculates the values of ς, φ, ψ and ϑ that are required for calculating the drag force:

In order to achieve acceptable precision, a higher degree polynomial equation (Equation (6)) is derived for calculating the lift force.

In Equation (6), f_{l} is the lift force and θ is the collector angle. All the other variables are defined in Equation (7) to Equation (13).

Forces acting on the collector (N/m) | Velocity | Direction | Collector angle | ||||||
---|---|---|---|---|---|---|---|---|---|

−60 | −45 | −30 | 0 | 30 | 45 | 60 | |||

2.5 m/s | Drag | −9.6 | −6.4 | −2.8 | 0.4 | 3.6 | 8.2 | 12.4 | |

Lift | 4.6 | 4.9 | 2.8 | −3.4 | −7.6 | −9.2 | −7.6 | ||

Total | 10.7 | 8.0 | 3.9 | 3.5 | 8.4 | 12.3 | 14.6 | ||

5 m/s | Drag | −38.3 | −24.2 | −9.9 | 1.2 | 13.2 | 30.5 | 48.8 | |

Lift | 18.3 | 18.4 | 9.6 | −13.8 | −26.6 | −34.0 | −29.8 | ||

Total | 42.4 | 30.4 | 13.8 | 13.8 | 29.7 | 45.7 | 57.2 | ||

10 m/s | Drag | −148.9 | −93.5 | −37.8 | 3.6 | 50.4 | 117.6 | 193.4 | |

Lift | 71.3 | 71.2 | 36.2 | −52.4 | −99.0 | −129.8 | −117.8 | ||

Total | 165.1 | 117.5 | 52.4 | 52.5 | 111.1 | 175.2 | 226.5 | ||

15 m/s | Drag | −331.6 | −207.5 | −83.6 | 7.1 | 111.6 | 261.3 | 434.8 | |

Lift | 158.9 | 158.0 | 79.9 | −115.6 | −217.8 | −287.6 | −264.7 | ||

Total | 367.7 | 260.8 | 115.6 | 115.8 | 244.8 | 388.6 | 509.1 |

have the same value. Another interesting fact about the lift force is that it reaches its peak at about +45 degrees. At 0 degree, as is obvious, the drag force is minimal for all speeds. Based on

As can be seen in

Based on

It is desirable to reduce the Nusselt number and consequently the heat losses caused by wind in solar collectors. At positive angles, the flow over the receiver pipe is almost cross flow therefore, regardless of collector angle the average Nusselt number almost remains constant for a fixed speed. But as for the negative angles, while collector is at smaller than 15 degrees angles, the flow over the receiver pipe is cross flow. At medium angles (15 to 45 degrees) the flow crosses the receiver pipe with a higher velocity which increases the heat loss. For large angles (larger than 45 degrees) the collector covers the receiver pipe therefore the Nusselt number is reduced.

Based on the results of this study, it is suggested that industries should consider structural forces and heat losses caused by wind before considering the use of CTC technology.