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Research Papers

Analysis of Heliostats' Rotation Around the Normal Axis for Solar Tower Field Optimization

[+] Author and Article Information
Erminia Leonardi

CRS4, Center for Advanced Studies,
Research and Development in Sardinia,
Parco Scientifico e Tecnologico,
POLARIS, Edificio 1,
Pula 09010, Cagliari, Italy
e-mail: ermy@crs4.it

Lorenzo Pisani

CRS4, Center for Advanced Studies,
Research and Development in Sardinia,
Parco Scientifico e Tecnologico,
POLARIS, Edificio 1,
Pula 09010, Cagliari, Italy
e-mail: pisani@crs4.it

1Corresponding authors.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received August 4, 2015; final manuscript received February 2, 2016; published online March 9, 2016. Assoc. Editor: Mary Jane Hale.

J. Sol. Energy Eng 138(3), 031007 (Mar 09, 2016) (9 pages) Paper No: SOL-15-1247; doi: 10.1115/1.4032758 History: Received August 04, 2015; Revised February 02, 2016

The design of a solar field is one of the crucial aspects when a solar tower system is realized. In general, shading and blocking effects, which are the main causes of solar power losses, are minimized displacing the heliostats each other quite distant, with typical land coverage less than 20%, and thus, strongly limiting the construction of these plants to low value lands. A new method is proposed here to improve the collected energy for solar tower systems with high land coverage (greater than 30%), based on the chance for each heliostat to rotate about the normal passing through the center of its surface. Then, shading and blocking are minimized by optimization of the relative orientations. To this aim, a small solar field composed of 150 rectangular flat heliostats has been considered, and its performances with and without the proposed optimization have been computed and compared for a wide variety of cases. In particular, a systematic analysis is presented to study the effect of the shape of the heliostats on the solar field performance: in a series of simulations, maintaining constant the area of each heliostat, the ratio between its two sides has been varied in a range between 1 (squared heliostats) and 3 (very stretched heliostats), and optimized and nonoptimized systems have been compared. Also, the total energy collected by the solar field has been calculated for optimized and nonoptimized heliostats' orientations, considering towers of different heights. Finally, the real PS10 solar plant has been considered, demonstrating that also for an optimized, very low coverage plant (about 14%), heliostats rotation can still improve the energy collection efficiency by a non-negligible amount.

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References

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Figures

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Fig. 1

Scheme of the heliostat rotation about the normal passing through the center to its surface

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Fig. 2

Solar field configuration

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Fig. 3

The system of coordinates

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Fig. 4

Solar coordinates at Cagliari on 21st March

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Fig. 5

Effective area as function of the time corresponding to 21st March at Cagliari and ht = 40 m. Circular heliostats are compared with squared and rectangular ones with ratio Lx/Ly  = 3 and 1/3, respectively. The theoretical limit is also indicated.

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Fig. 6

Top view of the solar field (light grey points). Shading and blocking (dark gray points) for the case with Lx/Ly=3 (a) and Lx/Ly=1/3 (b). hh = 40 m, Ah=28.83 m2. Solar coordinates correspond to 21st March at 12:00 in Cagliari.

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Fig. 7

Dependence of the effective area on the ratio Lx/Ly of the heliostats, for the case with ht = 25 m. Comparison with the theoretical limit is also shown.

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Fig. 8

Dependence of the effective area on the ratio Lx/Ly of the heliostats, for the case with ht = 40 m. Comparison with the theoretical limit is also shown.

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Fig. 9

Dependence of the effective area on the ratio Lx/Ly of the heliostats, for the case with ht = 25 m. Comparison of the total effective area between the nonoptimized (a) and optimized (b) solar field.

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Fig. 10

Dependence of the effective area on the ratio Lx/Ly of the heliostats, for the case with ht = 40 m. Comparison of the total effective area between the nonoptimized (a) and optimized (b) solar field.

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Fig. 11

Shading and blocking with optimized and nonoptimized heliostats. Data correspond to 21st March at 8:00 and ht = 25 m. (a) Lx/Ly = 1, nonoptimized; (b) Lx/Ly = 1, optimized; (c) Lx/Ly = 3, nonoptimized; and (d) Lx/Ly = 3, optimized.

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Fig. 12

Shading and blocking with optimized and nonoptimized heliostats. Data corresponding to 21st March at 12:00 and ht = 25 m. (a) Lx/Ly = 1, nonoptimized; (b) Lx/Ly = 1, optimized; (c) Lx/Ly = 3, nonoptimized; and (d) Lx/Ly = 3, optimized.

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Fig. 13

Solar coordinates at Cagliari on 21st June (a) and 21st December (b), respectively

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Fig. 14

Dependence of ηrel on time, at Cagliari on 21st March (a), 21st June (b), and 21st December (c), respectively

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Fig. 15

Average DNI at Cagliari

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Fig. 16

Cumulated energy (MWh) as function of time, corresponding to (a) 21st March, (b) 21st June, and (c) 21st December, respectively, and ht = 25 m

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