2. LITERATURE REVIEW
2.2 Parabolic trough systems
2.2.1 Reflective parabolic collectors
In general the useful power ( ) in Watts extracted from a set of parabolic solar collectors can be characterized by the following equation reported by Hoyer et al. (2009) and utilized in FLAGSOL’s performance model “PCTrough”.
· · (2.1)
where is the beam insolation measured in W/m2, A is the aperture area of the parabolic trough collector in m2, is the optical efficiency multiplied by efficiency derating factors and is the sum of the thermal or heat losses obtained from a suitable model or measurements. In Equation 2.1 refers to instantaneous beam radiation. In general is used to refer to a summation or integration of beam radiation over an hour.
The aperture area (A) is dependent on the width and length of the collector assembly as determined by manufacturers. SAM has a library of collectors with aperture areas that vary from 235 m² for the Luz LS-2 collector to 817.5 m² for the EuroTrough ET150.
The derating efficiency factors used by SAM to determine the design optical efficiency ( ) are tracking error, geometry effects, mirror reflectance, dirt on mirror and general optical error. The product of these derating factors is the optical efficiency ( ).
The heat loss of a collector as a result of radiation, conduction through its structural support members and convection as the wind blows past the collector is difficult to model. Hence, SAM focuses on modelling the heat losses on the linear tubular receiver. These losses are discussed in section 2.2.2.
2.2.1.1 Quantifying reflective parabolic collector efficiency
In order to direct the beam component of solar radiation onto a receiver effectively, a reflective parabolic collector must have a surface with high specular reflectance for radiation in the solar spectrum. Hence, most parabolic trough reflectors are specialized mirrors.
The efficiency of a parabolic trough collector, η, can be defined as the ratio of the useful power given by the collector, Pcoll, and a solar power reference. Rojas et al. (2008) found two main approaches in the literature when defining the reference solar input:
Approach 1 uses the component of direct solar irradiance normal to collector aperture (Ebcosθ; where Eb is the beam irradiance and θ is the solar incidence angle). This approach is mentioned in Kreith and Kreider (1978). The efficiency is calculated as follows:
(2.2)
where A is the aperture of the collector as above in section 2.2.1.
Approach 2 uses the beam or direct solar irradiance (Eb). This approach is followed by many authors and most classical reference texts on solar parabolic troughs according to Rojas et al.
(2008). It results in an expression of the efficiency as
(2.3)
It is clear that with a zero solar incident angle both efficiencies are equal. Therefore, in principle, the choice between these two approaches is somewhat arbitrary.
2.2.1.2 Reflective surface technologies
Duffie and Beckman (2006) suggest that back-silvered glass has excellent specular reflectance and durability provided the reflective coating is adequately protected. The transparency of the glass in this application is very important as the radiation will pass through the equivalent of twice the thickness of the glass and twice through the front glass, surface-to-air interface.
The glass mirror range comprises thick, heavier more durable glass and thin, lightweight and more fragile glass. The mirrors may also be front or back surfaced which impacts on the reflectivity. Considering transport, implementation and maintenance of this technology in Upington the thicker glass may be preferable.
Flabeg manufacture low-iron 4 mm float glass mirrors with a solar-weighted transmittance and reflectance of 98% and 93.5% respectively when new. These are used in the SEGS plants on Luz LS-3 parabolic trough concentrators and operational experience with them are
reported to be very good. Despite this they are susceptible to wind-related breakage and are expensive to transport and install.
Glass mirror alternative development and implementation has been occurring for a number of years and Jorgensen reported on a number of these in 2001. ReflecTech, Inc. has co- developed a mirror film with NREL which is used by SkyFuel, Inc. on their SkyTrough parabolic trough concentrator. Before this development most researchers were in agreement that glass alternatives did not demonstrate low cost, good performance and durability characteristics required for commercial trough development, confer with Price et al. (2002).
However, ReflecTech’s most recent technical release (2009) quotes a loss of less than 0.5%
across the solar spectrum after 6.25 years of measurements.
ReflecTech mirror film is a 0.10 mm thick pressure sensitive adhesive film made of multiple polymer film layers with one silver layer to give mirror like reflectance while protecting against UV radiation and moisture. It is designed for application to smooth surfaces, has a peel-off back liner to cover the adhesive until application and a peel-off top liner to protect the mirror surface during processing, handling, transportation and installation.
ReflecTech quote a specular reflectance of 94%, a solar-weighted hemispherical reflectance of 94% and a maximum operating temperature of 60°C. The mirror film is laminated onto curved aluminium panels integrated with an aluminium space frame forming the parabolic mirror of SkyFuel’sSkyTrough parabolic trough concentrator.
Maintenance of high specular reflectance of parabolic collectors is a challenge as dirt or degradation of the reflective coating result in loss of reflectance. As parabolic collector reflectance directly affects solar field performance, cleaning of parabolic collectors is of vital importance. Cohen et al. (1999) experimented with two mechanized cleaning methods and found that a high-pressure, demineralised-water sprayer with rotating heads mounted on a rig that could be operated by one person worked most effectively. SAM has mirror washing input fields which contain the amount of water used to watch each square meter of collector mirror and the number of washes per annum. These inputs are used to calculate the annual wash water usage for the solar field which is presented in the modelling results in chapter 6.
The optical losses and solar absorption of reflective parabolic collector mirrors is difficult to model accurately.
Parabolic collector reflectance has a strong influence on system efficiency. Forristall (2003) reports that at a working fluid temperature of 400°C, a 15% decrease in reflectance results in efficiency decreasing by 24.5%.
2.2.1.3 Parabolic trough support structure technology
The structure supporting the parabolic reflector is an important factor when considering construction of a parabolic trough power plant. The metal support structure for parabolic trough collectors was pioneered by the Luz LS-2 and LS-3 designs implemented at the SEGS plants.
Abengoa Solar Industrial Solar Technology (IST) produced a parabolic trough collector mainly used in low temperature process heat applications called the PT-1. Solúcar, the solar technology business unit of Abengoa then developed the PT-2 based on the PT-1 design. The updated collector’s small aperture raises concerns that a larger solar field area may be required to produce the same output as a smaller solar field with larger aperture collectors.
The EuroTrough consortium formed by European research laboratories and companies (CIEMAT, DLR, FSI, CRES, Iberdrola, Inabensa, Fichtner Solar, SBP, Solel) have completed the development and testing of two parabolic trough collector models, namely the ET100 and ET150 (Geyer et al., 2002). These collectors have a torque box design which is light-weight and easy to assemble, which is a significant advantage for installation (Geyer et al., 2002).
FLAGSOL GmbH, a subsidiary of The Solar Millennium Group, has developed two collectors based on the EuroTrough design: the Skal-ET and the new HelioTrough. The differentiating factor between the two collectors is that the Skal-ET uses the torque box design developed by the EuroTrough consortium while the HelioTrough uses a torque tube design developed by Flagsol and Schlaich Bergermann und Partner (SBP) (Flagsol, 2010).
The Skal-ET parabolic collector has been used in the Andasol projects in Granada, Spain as well as the Kuraymat project in Egypt, and as such is a proven design (Herrmann and Nava, 2005). The HelioTrough has recently been integrated into the SEGS V plant (December, 2009) and is undergoing performance and operational tests.
SENER, the Engineering, Procurement and Construction (EPC) contractor for the Andasol projects, has developed its own parabolic collector called the Senertrough. SENER installed and tested a 600 m Senertrough loop at Andasol-1 (Vázquez and Castañeda, 2008) and then implemented the design in the 50 MW Extresol-1 solar thermal plant in Extremadura, Spain
(Vázquez et al., 2009). The collector’s distinctive elements are its torque tube and 28 stamped cantilever arms attached to the tube, to which the mirrors are directly attached.
Solargenix Energy developed, installed and commissioned their parabolic trough collectors at the 1 MW Arizona Public Service (APS) Saguaro Solar Trough Power Plant. The collector uses an all aluminium space frame and is called the DS1. The design was based on the LS-2 collector but Solargenix Energy claim “superior structural properties, weight, manufacturing simplicity, corrosion resistance, manufactured cost and installation ease.” (Solargenix Energy, 2004)
National Renewable Energy Laboratory reported that Solargenix Energy designed a new SGX-1 collector under the U.S. Department of Energy’s USA Trough Initiative. The SGX-1’s aluminium design is lighter than comparable steel designs and is assembled with fewer fasteners. This avoids the need for specialised manufacturing, welding and component alignment in the field.
In collaboration with NREL, a second generation SGX-2 collector was developed by Solargenix Energy. The SGX-2 collector has an improved space frame design that reduces fabrication time and cost. It is also extremely accurate, light and easy to assemble without a fabrication jig.
The SGX-2 collector has been used in the 64 MW Nevada Solar One project and therefore is considered as successfully proven.
SkyFuel have most recently developed the SkyTrough parabolic trough concentrator. In their own words:
“The SkyTrough consists of ReflecTech mirror film laminated onto curved aluminum panels; which are integrated into an aluminum space frame to form the parabolic [mirror]. The space frame [is made of] extruded aluminum struts and other components [that] are self-aligning when joined together with fasteners, requiring no welding. The entire assembly is mounted on pylons and attached to a self-locking rotary hydraulic drive enabling the [collector] to pivot and track the sun.”
(SkyFuel, 2009: 2)
SkyFuel have signed an agreement in 2009 to install SkyTrough collectors at SEGS I & II (now known as the Sunray 43 MW parabolic trough generating plant).
To summarise, reflective parabolic receivers consist of a highly reflective glass mirror or a mirror filmed parabolic reflector which is supported by an aluminium frame of varying complexity and weight. A number of manufacturers have developed different designs which can be likened. The latest available data from the most prominent manufacturers are compared in Table 2-1.
Table 2-1: Reflective parabolic receiver metrics (Grey areas indicate data not publicly specified by manufacturer)
Manufacturer IST Solúcar Flagsol Flagsol SENER Solargenix
Energy SkyFuel Collector PT-2 Skal-ET 150 HelioTrough Senertrough SGX-2 SkyTrough Module aperture
length [m] 12 12 19.1 12.27 12* 13.9
Module aperture
width [m] 4.4* 5.77 6.77 5.774 5.77* 6
Module aperture
area [m2] 52.8 69.24 129.3 70.8 69.24* 83.4
No. of modules per solar collector assembly
12 12 10 12* 8 Total solar
collector assembly length [m]
148.5* 148.5 191 100-150* 115
Net aperture area
[m2] 817.5 1263 667
Mirror type
Glass mirror Glass mirror Glass mirror Glass mirror Silver polymer film Solar-weighted
reflectance [%] 94
Total mirror area
[m2] 750
Mirror
area/aperture area ratio
1.12
Focal length [m] 1.7* 1.71 1.71 1.71
Gemoetric
concentration ~63* 82* 82* 75
Structure Parabolic sheet
with front lattice Torque Box Torque Tube Torque Tube Space-frame Space-frame Materials of
construction Sheet metal Galvanised steel Recycled aluminium
Pre-galvanized steel sheet
Recycled
aluminium Aluminium Drive type Linear
actuator/hydraul ic
Two hydraulic
cylinders Hydraulic Hydraulic Self-locking rotary hydraulic
Rim angle [°] 72* 80* 82.5
Module weight per
m2 [kg] ~17* ~28 ~22*
Erection method On site factory
assembly* Jig Concrete jig Jig No jig required Foundations Concrete
caissons* Pile Spread foot foundation Wind load design
basis [m/s2] 35.8* 31.5* ~33 40
Control system
Feedback Flagsol proprietary*
Acciona proprietary*
SkyFuel SkyTrakker Peak optical
efficiency 75* 80 77*
Overall optical
efficiency -- 78
Geometric concentration (aperture width)/(absorber tube diameter) [sun]
99 Varies with HCE used
Varies with HCE used
Varies with HCE used
Varies with HCE used
Varies with HCE used
*Kearney, DW. 2007. Parabolic Trough Collector Overview. Parabolic Trough Workshop 2007.
Golden, CO, USA.