Optical Design and Optimization of Parabolic Dish Solar Concentrator with a Cavity Hybrid Receiver

R. Blázquez1, J. Carballo1, M. Silva2

1 CTAER Solar Department, Paraje los Retamares S/N. 04200, Tabernas -Almería (Spain)

E-mail address: rosa.blazquez@ctaer.com

2 Department of Energy Engineering, University of Seville, (Spain)

SIMULATIONS FOR THE OPTIMIZATION PROCESS

RESULTS

One of the main goals of the BIOSTIRLING-4SKA project, is the development of a hybrid Dish-Stirling system based on a hybrid solar-gas receiver, which has been designed by the Swedish company Cleanergy. A ray tracing study, which is part of the design of this parabolic dish system, is presented in this investigation. The study pursues the optimization of the concentrator and receiver cavity geometry according to the requirements of flux distribution on the receiver walls set by the designer of the hybrid receiver.

ABSTRACT

OPTICAL SYSTEM MODELThe main requirements assumed for the optical design are:  

Low spillage; Relatively homogeneous flux on the receiver walls with flux peaks below 1300 kW/m2, with

higher flux areas placed closer to the cavity aperture and negligible or very low flux on the receiver bottom;

The heat impinging the receiver walls is expected to be 40 kW at least; Concentrator optical parameters:

rim angle less than 45º at all points of the mirror aperture; 95.5% of reflectivity; concentrator slope error due to non-ideal orientation of the reflective surface of 1.7 mrad

and 0.5 mrad of specularity error.

The final concentrator geometry will be the result of an iterative process of simulations in order to reach a flux distribution on the receiver walls which is compatible with the restrictions.

Maximum flux on the receiver surfaces for different cavity lengths (4.8 m focal length;

0.2032 m diameter cavity; 0.150 m diameter aperture; total optical error, TOE=2.24 mrad )

Comparison between the maximum flux on the cylinder walls and on the bottom of the cavity for two cases: (a) a single focal length of 4.8 m (yellow and green lines) and (b) 2 focal lengths of 4.8 and 4.7 m (red and blue lines) and different aperture diameters (0.12, 0.15 and 0.19 m). TOE=3.54

The final study focused on the fulfillment of the requirements in terms of rim angle and heat absorbed by the walls, but in this case the heat losses were taken into account in the analysis. The preliminary results are used as starting point.

GEOMETRICAL PARAMETERS TO BE OPTIMIZED

HYBRID RECEIVER MODEL IN TONATIUH SOFTWARE

Concentrator parameters:A: inner radius; B: intermediate radius; C: outer

radius; D: cheese cake angle. Receiver cavity parameters: E: aperture radius; F: cavity radius; G: cavity length.

Parametrical study for the ‘non active’ section length (for 0.15 m in cavity aperture diameter)

Cavity aperture diameter (m)

Spillage (%)

0.12 16.33

0.15 6.64

0.19 1.84

The pedestal-based concentrator must have:a section of 30º removed to permit the dish rotationan effective collection area of 54.44 m2 (oversized in order to take into account the radiative heat losses in the receiver, estimated in 10 kW approximately)a 44º rim anglea double focal length, 5.5 outer focal length and 5.4 m inner focal length an inner diameter of 2.2 m and an outer diameter of 8.97 m

Surface Power (kW) for 48 pipesFirst row pipes 31.90

Second row pipes 12.27

Bottom wall 1.99

Cylinder walls 2.34

‘Non active’ section 3.22

TOTAL RECEIVER CAVITY 51.72

SPILLAGE (%) 5.02

The receiver cavity is 0.2513 m depth and 0.308 m in diameter. These values have been optimized along the different stages of the optical design process and set by the receiver designer in the last step of the process.

Spillage for different cavity aperture diameters (insulation

length of 0.1 m)

Bottom wall

Final flux distributions on the receiver walls

Cavity wall

BIOSTIRLING-4SKA Project, funded by the European Commission

Grant Agreement number: 309028