Collector Technology

The basic component of the solar field is the solar collector assembly (SCA). Each SCA is an independently trackin g parabolic trough solar collector made up of parabolic reflectors (mirrors), the metal support structure, the receiver tubes, and the tracking system that includes the drive, sensors, and controls. Table 2 shows the design characteristics of th e Acurex, single axis tracking M.A.N., and three generations of Luz SCAs. The general trend was to build large r collectors with higher concentration ratios (collector aperture divided by receiver diameter) to maintain collecto r thermal efficiency at higher fluid outlet temperatures.

Table 2. Solar collector characteristics [4,6].

Acurex

M.A.N.

Luz

Luz

Luz

Collector

3001

M480

LS-1

LS-2

LS-3

Year

1981

1984

1984

1985

1988

1989

Area (m2)

34

80

128

235

545

Aperture (m)

1.8

2.4

2.5

5

5.7

Length (m)

20

38

50

48

99

Receiver Diameter (m)

0.051

0.058

0.042

0.07

0.07

Concentration Ratio

36:1

41:1

61:1

71:1

82:1

Optical Efficiency

0.77

0.77

0.734

0.737

0.764

0.8

Receiver Absorptivity

0.96

0.96

0.94

0.94

0.99

0.96

Mirror Reflectivity

0.93

0.93

0.94

0.94

0.94

0.94

Receiver Emittance

0.27

0.17

0.3

0.24

0.19

0.19

@ Temperature (°C/°F)

300/572

300/572

350/662

350/662

Operating Temp. (°C/°F)

295/563

307/585

307/585

349/660

390/734

390/734

Luz System Three (LS-3) SCA: The LS-3 collector was the last collector design produced by Luz and was used primarily at the larger 80 MW plants. The LS-3 collector represents the current state-of-the-art in parabolic trough collector design and is the collector that would likely be used in the next parabolic trough plant built. A more detailed description of the LS-3 collector and its components follows.

One Axis Parabolic Trough Collector
Figure 3. Luz System Three Solar Collector Assembly (LS-3 SCA) [1].

Figure 3 shows a diagram of the LS-3 collector. The LS-3 reflectors are made from hot-formed mirrored glass panels, supported by the truss system that gives the SCA its structural integrity. The aperture or width of the paraboli c reflectors is 5.76 m and the overall SCA length is 95.2 m (net glass). The mirrors are made from a low iron float glass with a transmissivity of 98% that is silvered on the back and then covered with several protective coatings. The mirrors are heated on accurate parabolic molds in special ovens to obtain the parabolic shape. Ceramic pads used for mounting the mirrors to the collector structure are attached with a special adhesive. The high mirror quality allows 97% of th e reflected rays to be incident on the linear receiver.

The linear receiver, also referred to as a heat collection element (HCE), is one of the primary reasons for the hig h efficiency of the Luz pa rabolic trough collector design. The HCE consists of a 70 mm steel tube with a cermet selective surface, surrounded by an evacuated glass tube. The HCE incorporates glass-to-metal seals and metal bellows t o achieve the vacuum-tight enclosure. The vacuum enclosure serves primarily to protect the selective surface and t o reduce heat losses at the high operating temperatures. The vacuum in the HCE is maintained at about 0.0001 mm Hg (0.013 Pa). The cermet coating is sputtered onto the steel tube to give it excellent selective heat transfer properties with an absorptivity of 0.96 for direct beam solar radiation, and a design emissivity of 0.19 at 350°C (662 °F). The outer glass cylinder has anti-re flective coating on both surfaces to reduce reflective losses off the glass tube. Getters, metallic substances that are designed to absorb gas molecules, are installed in the vacuum space to absorb hydrogen and othe r gases that permeate into the vacuum annulus over time.

The SCAs rotate around the horizontal north/south axis to track the sun as it moves through the sky during the day. The axis of rotation is located at the collector center of mass to minimize the required tracking power. The drive syste m uses hydraulic rams to position the collector. A closed loop tracking system relies on a sun sensor for the precis e alignment required to focus the sun on the HCE during operation to within +/- 0.1 degrees. The tracking is controlled by a local controller on each SCA. The local controller also monitors the HTF temperature and reports operational status, alarms, and diagnostics to the main solar field control computer in the control room. The SCA is designed fo r normal operation in winds up to 25 mph (40 km/h) and somewhat reduced accuracy in winds up to 35 mph (56 km/h). The SCAs are designed to withstand a maximum of 70 mph (113 km/h) winds in their stowed position (the collecto r aimed 30° below eastern horizon).

The SCA structure on earlier generations of Luz collectors was designed to high tolerances and erected in place in order to obtain the required optical performance. The LS-3 structure is a central truss that is built up in a jig and aligne d precisely before being lifted into place for final assembly. The result is a structure that is both stronger and lighter . The truss is a pair of V-trusses connected by an endplate. Mirror support arms are attached to the V-trusses.

Availability of Luz Collector Technology: Although no new parabolic trough plants have been built since 1991, spare parts for the existing plants are being supplied by the original suppliers or new vendors. The two most critical an d unique parts are the parabolic mirrors and the HCEs. The mirrors are being provided by Pilkington Solar International (PilkSolar) and are manufactured on the original SEGS mirror production line. The Luz HCE receiver tube manufacturing facility and technology rights were sold to SOLEL Solar Systems Ltd. of Jerusalem, Israel. SOLE L currently supplies HCEs as spare parts for the existing SEGS plants. Should a commercial opportunity arise, it is likely that a consortium of participants would form to supply Luz parabolic trough collector technology.

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