Solar thermal power tested successfully in Spain

The next generation of solar thermal power plants will have reduced capital and power generation costs

A. Finker

FDE

M. Schmitz-Goeb, Steinmuller, and Dr. Streuber

Didie

Since 1986, companies and research institutes from Europe and the United States have been working on solar thermal (central receiver systems) power generation technologies. The basic technological concept is the central receiver power plant, in which solar radiation is focused by means of an array of mirrors to a central receiver mounted on top of a tower.

In March 1990 the “phoebus consortium” conducted a wide-ranging feasibility study to demonstrate the technical feasibility, economic viability and the financing of a 30-MWe plant in Jordan.

On the basis of extensive investigations this study came to the conclusion that the volumetric air receiver principle would be the best applicable for the central receiver. The reason for this is the technical advantages and very high degree of environmental compatibility of the technology.

One advantage of the concept is the heat carrier medium in the primary loop that uses air which is non-toxic, non-corrosive, easily handled and universally available. This is crucial for applications in developing countries and industrialized countries with high solar insulation.

A 200-kWt receiver operating on this principle was successfully tested in 1987/88 at the Plataforma Solar de Almeria in Spain, the European test facility of CIEMAT and DLR. For scale-up to the size required for a commercial 30-MWe plant, with a thermal capacity of the receiver of 115 MWt, an intermediate step was indispensable. To accomplish this, a technology solar air (TSA) receiver program was initiated, an important step towards the commercialization of the phoebus receiver principle. The TSA consortium was formed to build and test a 2.5-MWt air receiver including steam generator and thermal storage.

This consortium consisted of the following German companies:

– Fichtner Development Engineering, the managing partner with responsibility for

system engineering, the control system, and the power station contractors

– L & C Steinmuller, responsible for the receiver and system delivery

Didier M and P Energietechnik-thermal store

Other major contributors were Plataforma Solar de Almeria (PSA), where the tests were performed, H.W. Fricker, as expert, and the German Aerospace Research Establishment who was responsible for the test program and evaluation. The commercial aspects of the TSA program were finalized at the end of 1991 thus enabling the start of project activities in January 1992.

Priority was given to developing and designing the receiver and thermal storage system. To allow for the construction of larger units in the future a modular receiver was developed. In addition, the receiver has been designed for high availability and reliability, and for optimized air flow, recirculation and heat flow distribution. During this phase tests were carried out to demonstrate the steaming rate, response to fluctuations in solar radiation and startup/shutdown behavior.

The costs of the project was (US)$3.75 million. However, this does not include costs for performance tests and acquisition of property rights. The project is financed by the industrial partners, the German Federal Ministry of Research and Development, the Ministry of Economics of the State of Baden-Wurttemberg and Pacific Gas & Electric Co., San Ramon, Calif., USA.

System configuration

The basic configuration of the TSA plant, consisting of receiver, steam generator, solid heat storage module, two blowers and associated air ducts and dampers, is shown in Figure 1.

The air loop was built on top of the existing CESA 1 tower of the test facility (Figure 2). The heliostat field and the water/steam cycle of CESA 1 were also used for the tests.

The TSA plant is controlled by a digital control system (DCS). System design as well as hardware and software selection allow for easy operation, modification and reliability. A data acquisition system records transients. The user interface is based on dynamic graphic displays and the system is operated by keyboard and pointing devices. Special consideration was given to an easily-comprehensible display of process data with the system having approximately 160 analog values.

A number of open- and closed-loop control algorithms, to regulate air and steam parameters, are part of the TSA control system. During automatic operation, the system has been able to maintain constant air and steam parameters even during cloud transients.

The DCS is connected for online control of existing PSA systems including the heliostat array and feed water system. It is also hooked up to the PSA data acquisition computer for purposes of off-line data processing such as trending, performance evaluation and data archiving. Protective devices allow for safe operation of the receiver and heliostat array defocusing and protection of the solar air receiver. This system is also linked to the DCS.

Test program

The TSA test program, prepared by DLR Cologne in close cooperation with the members of the TSA consortium, closely simulated the typical operation modes of a phoebus power plant as defined in the Phoebus Feasibility Study.

These tests were carried out through a sequence of trial test runs to determine operating characteristics under conditions of warm start-up, cool-down, normal steady-state, during transients, thermal storage, emergencies, routine actions and overloading. All tests were scheduled and carried out over a period of six months. Between April and July 1993 the plant was successfully operated for a total of nearly 400 hours and major parts of the test program were completed.

The first test results obtained were encouraging with receiver outlet temperature of 700 C being easily achieved within 20 minutes of plant start-up. In addition, the temperature distribution behind the receiver absorber corresponded reasonably well to the flux density. Receiver efficiency, as predicted in the phoebus study, was confirmed and the nominal air receiver outlet temperature of 700 C was achieved up to a 3-MWt receiver load (design load 2.5 MWt).

Because of the blower`s control and thermal inertia, fluctuations in solar irradiation showed no effect on receiver outlet temperature. Likewise, charging and discharging characteristics of the thermal store worked as predicted. Operation of the plant is very straightforward and is capable of rapidly tracking changing meteorological conditions while keeping steam parameters constant. Similarly, there was no danger or damage during various emergency and upset situations. Figure 3 shows a typical graph of the results of short-term field test runs. The test program continued through the end of 1993 and was used to gain more operating experience of the solar tower and plant component reliability under realistic transient solar operating conditions.

Future project phases

After successful completion of the TSA program, the technology now exists for constructing the first phoebus plant with a rating of 30 MWe. In the following phase of the project, the TSA consortium had incorporated the findings and experience from this program into the phase 1B phoebus feasibility study.

As a means of providing an up-to-date picture of the technical and economic status of an actual phoebus project, referred to 1993, and so that this concept can be offered on a global basis, a post feasibility study was completed in March 1994. It is clear from the TSA results that it will be possible to substantially reduce capital costs and power generation costs on future solar thermal power plants.

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