Solar Energy Materials 24 (1991) 683-695 North-Holland

Solar Energy Materials

Review of PSA activities in the area of solar thermal energy conversion

Manuel Sanchez and Alfonso Sevilla Portili0 Plataforma Solar de Almeria, Apartado 22, E-04200 Tabernas (Almeria), Spain

The Plataforma Solar de Almeria (PSA) has recently been awarded a three year contract within the European Community's "Access to Large Scale Scientific Installations Program". This contract enables PSA to diversify and engage in new activities, while at the same time, continuing the program of work initiated by the Spanish German Cooperation, Agreement of 1987. In this paper, we will present an overview of all activities in the area of solar thermal energy conversion in which the PSA is currently engaged. New activities started this year under the EC program will be commented in more detail, whereas recent results from existing lines of solar thermal R&D will be presented in separate papers.

Introduction

Following the guidelines approved at the beginning of the Spanish German Coopera t ion A g r e e m e n t (1987), the PSA activities are grouped into the following categories: I. Solar chemistry II. Electrici ty III. Componen t development IV. Process heat V. Mater ia l testing

As a result of the three-year contract (1990-1992) granted under the European Communi ty ' s "Access to Large Scale Scientific Installations Program", new activi-

t i e s such as low-temperature solar detoxification (solar chemistry), Zirconia (Z rO 2) ceramic h igh- tempera ture melt ing experiments and metallurgical surface treat- ments (materials testing) have been added to the PSA's traditional activities in the area of solar tkermai energy conversion.

1. Solar chemis t ry

1.!. Solar steam reformino~ experiment (ASTERIX)

The objective of this first demonstrat ion-scale exper iment in solar chemistry at the Pla taforma Solar de Almer ia is to demons t ra te the feasibility of running a well-known industrial chemical process with solar energy. The h igh- tempera ture

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

684 M. Sanchez et al. / Review of PSA activities

(970 ° C) process heat required for the endothermic steam reforming of methane in the ASTERIX experiment (Advanced Steam Reforming in Heat Exchange) is provided by the GAST ceramic receiver on top of the CESA-1 central receiver system tower at the Plataforma Solar. This ceramic receiver was developed within the joint German Spanish Gas Cooled Solar Tower (GAST) Technology Program and is able to produce air at temperatures up to 1000 °C at 9 bar.

The particular experimental goals to be achieved with the ASTERIX expert- ment are: - collection, conversion and storage of a maximum amo,?.nt of solar energy - maximum C H 4 conversion i~ - production of a consistently high synthesis gas quality

Further details are presented at this Conference in a separate paper by M. B~ihmer, U. Langnickel, and M. Sanchez.

1.2. Solar detoxificatior.

The constant discharge of chemicals, pesticides and herbicides, still common practice in industry and increasingly applied in intensive agriculture, pose a serious threat to our supplies of drinking water. Many of these hazardous substances, such as chlorinated aromatics and aliphatics strongly resist natural decomposition. Some others are formed, for example during halogenation of water in water treatment plants.

Within the framework of the EC program, a cooperative effort involving photochemical research groups, defined the outline of testing to be done at the PSA and cooperated on the design and setup of a detoxification loop based on the SSPS (Small Solar Fower Systems) Project HELIOMAN two-axes-tracking parabolic troughs (fig° 1).

The following will be investigated within this program: 1. Typical contaminants and pollutants such as:

• Organic compounds Haloaliphatics (e.g., trichloroethylene) Haloaromatics (e.g., 4-chlorophenol) Water-soluble miscibles (e.g., 2-ethoxyethanol) Herbicides (e.g., atrazine) Surfactants

• Heavy metals Cromium (VI) Mercury (II) Cyanide

The photo-oxidation process will be examined in detail, emphasizing identifica- tion of even trace intermediates, and quantifying final product formation.

2. Improvement of the overall process efficiency (quantum yield, rate), essentially by oxygen enrichment.

3. Preparation of supported catalysts

M. Sanchez et al. / Review o f PSA activities 685

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In order for large-scale photocatalytic detoxification processes to become eco- nomically more competitive and scalable, costly processes commonly used in the laboratory such as filtration and recirculation of the catalyst must be avoided. This may be achieved by fixation of the catalyst on a stationary support. Anether important area of study is the kinetic description of the photocatalytic

reactions. In fact, kinetic models describing their evolution will be essential to future l~,rocess design.

Tests containing more than one pollutant will be also be studied in order to reflect real world conditions.

The iLv:stigation will further be extended to other classes of contaminants such as ionic surfactants, fluorcompounds, and other pesticides and herbicides, in order to check the general applicability of the process.

2. Electricity production

2.1. CESA.1 hybrid operation

The steam superheating stage of the CESA-1 central receiver was replaced by a fossil superheater in order to maintain the electricity production capacity of the plant. Presently, the CESA-1 saturated steam receiver is being operated with the fossil superheater and the possibilities of a water/steam hybrid solar tower concept is being investigated (fig. 2).

686 M. Sanchez et al. / Review of PSA activities

3. Component development

3.1. Novel heliostats

PSA is currently performing experiments aimed at the enhancement of conven- tional back surface mirror facets while developing new kinds of reflector units. Back surface glass and front surface metal mirrors are considered near term options for central receiver plants like Phoebus, where uncertainties and risks inherent to any novel heliostat design should be avoided. More innovative materi- als like metallic membranes, glass fiber reinforced polyester sandwiches and holographic concentrators are being tested for forthcoming plants and future applications.

Three different kinds of heliostats are being developed and tested at the Plataforma Solar during 1990. One of them consists of a light stretched-membrane structure heliostat with a double facet and 39 m E of reflective surface. The facets each have 12, 1.5 m 2 mirrors manufactured by Flachglas. No bonding or fixing of the mirrors is necessary. Stretching and focusing is performed on the metallic structure by means of a metallic wire mesh, and complete assembly is carried out in the field.

A number of small stretched membrane modules showing beam dispersion (two sigma) of between 3.5 and 3.8 mrad have been tested. A small (10 m 2) rectangular module is currently being tested at the PSA to see how well the non-circular shape works. A similar 60 m 2 prototype, now under construction, will soon also be installed in the CESA-1 field.

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M. Sanchez et al. / Review of PSA activities 687

Glass fiber reinforced polyester sandwich facets showed good stiffness and significant reduction of weight for experimental 1.4 × 2.3 m e facets. Even though flat sheets showing no grainy texture (i.e., no "print through" effect), could be manufactured, significant problems were encountered when manufacturing curved facets. Focal lengths 2 and 3 times shorter than the expected nominal focal length were observed after resin curing. Efforts are being made to improve the curing process before delivery of the first 12 experimental facets.

3.2. Hermes H measuring system

In 1982, the German Aerospace Research Establishment (DLR) developed the HERMES (Heliostat and Receiver Measuring System) camera system for remote measurement of surface temperature and solar radiation distribution. An im- proved version of this system, the so-called HERMES II camera system, is now installed at PSA. This system will be tested and available in the beginning of 1991.

3.3. Reconcentrators

A prototype hexagonal secondary concentrator, designed by the University of Sevilla, and built by DLR Stuttgart, is an essential coml:,o~ent of the modular, pressure-loaded volumetric receiver concept for large-scale applications. A sec- ondary concentrator and volumetric receiver test campaign started in May is to be continued in September of this year. The experimental results will verify the optical design program used for the prototype reconcentrator. Such a reconcentra- tor used in connection with a volumetric receiver would: - Concentrate solar flux in the focus of the heliostat field, thus allowing a

reduction ill receiver size and - Redistribute the Gaussian flux into a more desirable rectangular shape.

3.4. Volumetric receivers

Volumetric Receivers have shown important advantages over tube receivers: simple design, low weight, simple mounting, quick and cheap absorber repair, no receiver leakage problems, highly effective absorption, low thermal inertia, etc. Therefore, many different volumetric absorber designs using different geometries (wire, foil, foam) and materials (metal and ceramic), have been proposed. Some of these were built and tested at the PSA during recent years.

The first so-called Wire-pack Volumetric Receiver Experiments, designed by the Swiss company, SULZER, tested coiled, knitted wire and wire mesh ring absorbers at temperatures of up to 800 °C at the Plataforma Solar de Almeria from Summer 1987 to Spring 1988. Testing of a metal foil absorber designed and built by Interatom/Emitec followed in November, 1988 to March, 1989. Sandia Ceramic Foam Absorber tests from May to July, 1989, allowed a detailed charac- terization of its behaviour. The receiver was operated under a large range of operating conditions, from low incident power and low air temperatures up to 750 °C average air outlet temperature.

688 M. Sanchez et al. / Review of PSA activities

During the first half of 1990, two new ceramic absorbers, one designed by DLR, Cologne, and the other by the Italian company, Conphoebus, were tested and characterized. Results will be presented in separate papers.

Between these two latest absorber tests, the PSA carried out the preliminary testing of the pressurized volumetric receiver, designed and constructed at DLR Stuttgart (see also 3.2, Reconcentrators). Tests with this receiver will continue in September, 1990. This receiver is capable of producing hot air at up to 1000 ° C at 10 bar.

3.5. Heat transfer loop for storage experiments

The PSA has installed a new test facility for lab scale thermal storage module tests at up to 400 ° C, using Monsanto VP-1 thermal oil as the heat transfer fluid. Two small concrete storage modules with different thermal storage capacities are being tested in predefined charge and discharge modes.

Preliminary tests designed in the TESCA 200 study will verify whether concrete, the storage medium chosen, is the most suitable for the medium temperature range in LUZ SEGS plants.

The main test loop components are" the heater, oil cooler, expansion tank and oil pump. The oil is heated in an electric heater, where the outlet temperature is controlled by the heater power control. The mass flow to the test modules is measured by a flow meter and manually throttled with a three-way valve in a by-pass. An additional water-cooled oil cooler, permits simulation of storage module discharge conditions.

Siempe!kamp Reaktortechnik, Germany, designed and built two lab-scale con- crete test modules, (fig. 3) based on thermodynamic computer simulations carried out by the University of Essen, Germany~ for the TESCA 200 study (Thermal Energy Storage Capacity 200 MWh). One module has an admixture of steel needles (75 kg /m 3) in order to increase the thermal conductivity and specific heat. These modules will be studied to: - Determine properties of concrete such as:

- Specific heat Cp - Thermal conductivity

- Determine the effective heat transfer coefficient aef f between the heat transfer medium, oil, and the storage material, concrete.

- Test long term structural stability of the concrete and the tube/concrete contact under periodical thermal stress.

4. P r o c e s s h e a t

4.1. Desalination

The CIEMAT-IER (Spa;n) and DLR (Germany) launched in 1988 a research project to determine the technical and economic feasibility of using solar power for thermal desalination processes.

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For this purpose, a multi-effect (ME) type desalination plant was selected for the following reasons: • Of the evaporation desalination processes, this had the fewest thermal and

electrical energy requirements, and thus, the highest potential for solar energy use .

• This process has proved highly flexible. The plant (fig. 4) connected to the Acurex field had the following general

characteristics:

Type of plant:

Production Capacity:

Performance Ratio:

Number of stages:

Thermal consumption (70 ° C):

Salinity of output:

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Phase i goals were to: - Determine problems and solutions for solar desalination plants. - Determine operation and maintenance requirements.

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- Determine the large scale behaviour of this concept. - Find out the ways to make this concept competitive.

Based on the optimistic results of this first phase, (presented in a separate paper), Phase I1 has been outlined and launched, with the following objectives: - Determine ways to improve the plant. - Define and test an appropriate solar collector system. - Define and implement a heat pump system able to reduce the thermal consump-

tion to economical levels. Phase II activities have consumed most of the 1990 effort. The heat pump

should be ready by March, 1991. Evaluation will continue through December, 1991.

5 . M a t e r i a l t e s t i n g

5. !. Hermes material tests

Within the framework of the C-1 phase already approved by the European Space Agency (ESA), Avions Marcel Dassault-Breguet Aviation (AMD-BA) repre- sentatives have contacted the PSA requesting continuation of the thermomechani- cal reentry cycles performed here on C-C and C-Sic Wing Leading Edge (WLE) samples: - Testing at the automatic conditions set up by the remodelling tasks carried out

during 1989 under AMD-BA contract. - Testing one shingle unit at nominal reentry conditions - Up to 16 eentry cycles on new W.L.E. samples made of C-C and C-Sic to be set

at 1550 ° C and one at 1700 o C. - Preliminary nosecap test definition for the 1991 0.6-scale model tests which

would establish future full scale test conditions. The aim of these tests is to evaluate hot structure material behaviour under

controlled conditions: - Thermal cycle and temperature profiles - Mechanical traction and compression loads under stationery conditions.

The repeatibility of nominal conditions has proved to be excellent (fig. 5), opening the way to evaluate the behaviour of C-C and C-Sic composites.

In addition to the reentry test activities, the testing of a 0.6-scale nosecap is foreseen. Several tasks associated with these tests can be summarized as: - Numerical simulation of the test facility - Installation of a new test room - Manufacture of a ceramic model, to be used for debugging tests - Closed loop heliostat field control system

This ample program of activities demonst ra t~ how solar technology is providing a unique tool to evaluate ceramic materials at high temperatures and is helping to expand knowledge of the young field of materials technology within the strict requirements imposed by aerospace standards.

692 M. Sanchez et aL / Reciew of PSA activities

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• 7 5.2. Meta lurgicat surface treatment

As part of the CEC Access to Large Scientific Installations program, a coopera- tive project involving several European Community university research groups is directed at the superficial treatment of metals by inducing structural transforma- tions which would improve mechanical properties such as hardneg~ and wear resistance.

The program will also study the possibility of achieving a surface alloy by melting of the base material and solid-to-gas reactions developed in the controlled atmosphere of a solar furnace with quartz window.

Base matcrials with s~Jitable properties and applications and widely used engi- neering alloys (steels, cast iron-aluminum alloys) as well as the more recently developed advanced materials based on nickel and titanium will be studied.

For the treatment of such materials, the PSA provides a focussed beam of energy over 1 m 2 area with peak flux of 2.5-3 W/em 2.

In 1990, preliminary activities were carried out at the 25 m level of the CRS tower using its 93 heliostats. Future activites will take place either at the tower facility or at the soon to be built solar furnace.

5.3. The PSA solar furnace

Activities have been centered on the development of a Solar Furnace which by concentrated radiation would obtain very, high temperatures. The project goal

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consists of offering tile scientific and engineering community a powerful device for Physics, Chemistry and Materials Technology research experiments as well as developing new furnace techniques. The Solar Furnace project is part of the European Community's "Access to Large Scientific Installations Program".

A four-heliostat field, selected from the existing MBB he!iostat field, tracks the .~un's movement and reflects the sunlight toward a McDonnell Douglas parabolic dish concentrator with 94 mirror modules, gross ~,perture of 11 m, aad an estimated concentration factor of 10,000. An attenuator mounted between the the heliostat field and the dish, similar to Sandia's new furnace, controls the flux incident on the target, which is mounted on a testbed movable in three directions and controled with the same control and data acquisition system which moves the attenuator.

The first experiments with the PSA Solar Furnace will be in the area of high temperature ZrO 2 research. The final goals of these experiments which will be prepared and carried Out in collaboration with material scientists of the universi- ties of Sevilla and Paris, are ZrO 2 fiber pulling and high temperature behavioural testing.

To verify the performance of the PSA Solar Furnace for these experiments a study on the temperature distribution of a ZrO 2 probe in the focus of the parabolic dish concentrator has been carried out and a special sample chamber for high temperature testing of ceramic materials has been investigated.

6. Conclusions

After a first stage in which the various power plants were operated as electrical generators, a new stage c~f widely diversified use has been initiated.

The results of the f~rst phase paved the way to the design of co~:mercial power plants in the multimegawatt range, as well as the optimization of components such as heliostats and recievers. Recent joint activities by its supporting instkutes, the Centro de Investigaciones Energ6ticas Medioambientales y Tecnol6gicas (CIEMAT) and the Deutsche Forschungsanstalt fiir Luft- und Raumfahrt e.V. (DLR), now utilize the Plataforma Solar de Almeria for diverse industrial solar thermal technology R & D programs in:

Solar chemistry Electricity Component development Process heat Material testing In addition to this significant program, the outstanding and even unique

Plataforma Solar facilities are available upon request to academic and ~ndustrial research centers for sophisticated test requirements in the fields of space, materi- als, optics, etc~

M. Sanchez et al. / Review of PSA activities 695

We at the Plataforma Solar de Alrneria, believe that this very ambitious program will demonstrate that solar thermal technology has numerous applications which even the most optimistic would not have expected a short time ago and we are convinced that exciting results will be forthcoming for the international energy and scientific communities.