Solar Energy Materials 24 (1991)204-209 North-Holland

Solar Energy Materials

Development and testing of a volumetric gas receiver for high-temperature applications

Mafio Posnansky and Thomas Pylkkiinen * Atlantis Energie AG, Thunstrasse 43a, CH-3005 Bern, Switzerland

Feasibility te~ts with a closed-loop volumetric gas receiver developed by Atlantis Energy Ltd, were conducted on the solar tower of the Sandia Natioiial I~aboratories, Albuquerque, New Mexico (USA), in Apr i l /May, 1990. The tests demonstrated that it i~ possible to build receivers which are based on a novel ceramic grid absorber concept for working temperatures in excess of 850 ° C. Theoretical and experimental data allow further to conclude that reasonable efficiencies (84-92~) can be achieved also at radiation levels above 900 k W / m 2 for both open-loop and closed-loop receiver configurations, Although the development of this technology was primarily aimed at providing receivers for solar driven chemistry, the near-term applications may be in the area of CRS power generation (or CRS cogeneration of power and heat - e.g. for seawater desalination).

Atlantis started working on high solar radiation flux trans|er problems back in 1978. A first volumetric receiver concept based on cascaded absorption of solar radiation with partially absorbing quartz glass elements was tested on a laboratory scale. It demonstrated als~ the feasibility of quartz glass as a transparent construction material - important e.g. for thermophoto- chemical processes. In 1988, a new volumetric ceramic grid absorber was conceived. Laboratory tests were conducted using a dish for high solar concentrations. Mean gas temperatures of 840 o C were achieved at a solar flux of 1500 k W / m 2.

In order to test the feasibility of the concept, a larger scale 500 kW test receiver was subsequently built for testing with a heliostat field. In order to gain exper;,ence for chemical applications, a closed-cycle receiver was chosen for the tests in Albuquerque. Crucial for the success of the testing at Sandia was the necessity to develop a radiation shield capable to protect receiver structural elements from the high-intensity incident solar radiation.

The project was supported by the Swiss Energy Research Foundation, the Swiss Federal Office for Education and Science, and the U.S. Department of Energy for the tests at Sandia National Laboratories.

I. Absorber concept

The ceramic grid absorber consists of straight heat-transfer channels with a very small area (fig. 1). The walls between the holes are thin, so that most of the solar radiation is absorbed on the inside surface of the channel.

This absorber design can be used with high incident solar radiation fluxes (1 MW/m 2) to achieve high gas outlet temperatures (800-I;~00°C) without absorber overheating, because - the heat transfer coefficient is large due to the small hydrodynamic diameter of

the channels,

0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

M. Posnansky, 7". Pylkkiinen / Volumetric gas receiver for high-temperature oppfications 205

I ° I' L ,

S o l a r ~ radiatio.~n ~ Air flow .

T I

,,T :arge

~..__~ :'"::::::::::~ i i i i i i i i i i i i I I I I i i i i i i i i

i i i i i i i ! 1 1

i i i i i i i i i i

I I I I I l l l l l l l l

T. / large Ceramic

gdd

Relative solar radiation flux along the channel

r I

i -

1000 r

Temperature along the channel (%)

. . . . L Distance along the absorber channel

Fig. 1. Ceramic grid absorber. Solar flux and temperature distribution in the absorber.

- the temperature difference between the gas and channel wall is high where the highest solar radiation flux exists,

- part of the solar radiation is absorbed witl,jn the channel (volumetrJ.'c effect), - the maximum absorber temperature is only slightly higher tha~ the gas outlet

temperature. A typical solar flux and temperature distribution in the absorber is presented in

fig. 1. Although the test program presented below used absorbers enclosed in quartz

glass tubes, this absorber concept can be used in an open air configurat:on as well (fig. 2).

R ( , -AP

a

~ I N J E G T I O N

S L O T

T O W E R b

Fig. 2. Central receiver tower with an (a) ope . and (b) closed cycle ceramic grid receiver.

206 M. Posnansky, T. Pylkkiinen / Volumetric gas receiver for high-temperature appfications

2. Prototype test receiver

Prior to the design and fabrication of the test receiver, laboratory tests were performed on a small (1 cm z) sample of the ceramic absorber at high solar concentrations using a parabolic dish. Mean gas temperatures of 840°C were achie,:ed at a solar flux of 1500 k W / m 2.

After many theoretical studies and experimental tests of components, a larger scale 500 kW test receiver was designed and fabricated for testing at the Sandia National Laboratories Solar Thermal Test Facility in Albuquerque, New Mexico (fig. 3). In order to obtain experience for chemical applications, an enclosed receiver configuration was chosen. The air is blown into the receiver from the top, passes through a two-stage absorber configuration placed in four quartz glass tubes, and is heated up to high temperature levels (fig. 4).

For protection against high-intensity solar radiation impinging outside the active absorber area (approximately 0.8 m × 0.7 m), water cooled shields with highly reflecting insulation were specially developed and installed.

Cut A-A

Absorber ~ ~ Quartz " ~ g.ass ~---~

/ - / / .... ./ /I_

A..o.. i Quartz ~,/~i£i~/ ~"~--~ glass tube ,k.~_

/

L/~---~ Test

~ ' l ~t " , ( - Pl atform

! : %,;:!.!",,i~;h>~:!'<~:~:?,i.i ';2;S~,2..',~;.2.~,2,~I/1~!

Radiation protection shield

Solar radiation

Fig. 3. Receiver test prototype, schematic side view.

M. Posnansky, T. Pylkkiinen / Volumetric gas receiver for high-temperature applications 207

Fig. 4. (a) One of the four tubes with the absorbers inside. (b) Receiver front view.

The test receiver was designed to withstand all differential thermal expansions expected during the tests and also the severe temperature-time transients.

The test receiver was developed and built by Atlantis Energy Ltd in Switzerland and sent to Albuquerque for testing.

3. Tests in Albuquerque, results and analysis

The receiver was tested in April/May, 1990, at the Sandia National Laboratories Solar Thermal Test Facility in Albuquerque. Advanced Thermal Systems, Inc. (of the USA) designed the test support systems and part of the instrumentation, assisted in test preparations and provided support for data evaluation.

Du~ag the tests, the maximum absorber exit air temperature achieved was 970 °C (fig. 5). The mean outlet temperature was lower, partly due to a non-uniform flow distribution (not yet optimized) and partly to cold air bypassing the absorber (leakage). These problems can be corrected by design changes in absorber sealing and support techniques. The receiver average outlet temperature can therefore approach the maximum air temperature measured behind the absorber.

The solar flux distribution on the receiver was measured by Sandia before or after each receiver test.

208 M. Posnansky, T. Pylkkiinen / Volumetric gas receiver for high-temperature appfications

~ )

bJ n . D h -

n . ILl G_

i , i

0 2 4 6 8 10 12 14 16 18 20 22 24

TIME ( M I N U T E S )

Fig. 5. Receiver tube 2 temperatures.

25 Z8 30 3z ,J4 o6

v

o z h i

o_ h i

r~ hi __> h i

w

86

8 5 -

8 ¢ -

8 3 -

8 2 -

81 -

80

79

73

77

0,6

I0= 900 kW/n~-,o(= 0.95, E= 0.8, 2 stages

I I 'i I I I

0,8 1 1,2

HYDRODYNAMIC CHANNEL DIAMETER (ram)

Fig. 6. Calculated closed cycle receiver efficiencies,

I

1,#

M. Posnansky, T. Pylkk~nen / Volumetric gas receiver for high-temperature applications 209

The receiver efficiency (delivered thermal power divided by the incident solar power received by the effective absorbing area of the receiver) was 70%. A higher efficiency can be obtained using a darker absorber coating. In fig. 6, calculated efficiencies are shown as a function of the hydraulic diameter of the air passages. For this purpose the temperature distributions along the absorber channels were calculated. The absorber absorptivity was assumed to be 0.95, u~e emissivity 0.8.

For a commercially available ceramic grid (channel size 1.1 ram) the calculated closed-cycle receiver efficiency is 84% at an outlet t~emperature of 800°C. The corresponding efficiency of an open receiver is 92%.

4. Conclusion

From the theoretical and experimental data it is concluded that receivers based on the ceramic grid absorber can be built to achieve 800-1000 °C gas temperatures at an incident solar flux density above 900 k W / m 2 on the receiver. Calculations show that realistic closed-cycle receiver efficiencies are 84~ for an outlet tempera- ture of 800 °C and 82~ for 1000 ° C. The corresponding calculated efficiency of an open receiver system is 92% (800 ° C) using a similar ceramic grid absorber.

The few problems which were experienced during the receiver feasibility tests at the Sandia National Labo,latories, particularly the non-uniform air flow distribution can be overcome by simple receiver design changes.

The good results achi,eved so far are encouraging for the further development of this gas receiver tyFe for both power generation and solar driven chemistry applica- tions.

Acknowledgements

We would express our appreciation to all the staff members of the Sandia National Laboratories solar tower test facility fer their excellent technical assistance which led to the successful execution of the receiver feasibility test program. In particular we thank J. Holmes and C. Cameron from SNL as well as D. Gorman from Advanced Thermal Systems, Inc., for their cooperation.