Current directives and national regulations do not always explicitly address all aspects of innovative renewable energy plant design. Integrated approval management is therefore indispensable whenever new power station projects are to be realized. In Spain, TÜV SÜD helped a solar thermal demonstration power plant based on the Fresnel collector technology progress from design to service. 

The 1.4 MW PE 1 near Murcia became the first Fresnel solar thermal plant to enter commercial operation in March 2009 Source: Novatec BioSol
Hans C. Schröder, TÜV SÜD Industrie Service, Germany

As renewable energies become increasingly important, innovative plant designs are pushing into the market. A study carried out by the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt eV, DLR) demonstrated that solar thermal power stations will account for a long-term market share of over 60 per cent in electricity generation in southern Europe, North Africa and the Middle East.

However, the existing directives and regulations do not always cover all aspects of these innovative plant designs. And as these power stations depend on locations in southern Europe or North Africa offering many hours of sunshine, specific national regulations may also have to be complied with.

Integrated project support and far-sighted approval management must cater to many requirements which may vary. Given this, disclosure of how licensing and permit requirements impact on the design, manufacturing, equipment, approval and operation of a plant is a must. TÜV SÜD Industrie Service accompanied the implementation of Novatec BioSol’s PE 1, a solar thermal demonstration power plant based on Fresnel collector technology from planning through construction to final approval.

In this project, TÜV SÜD was not only required to assess process and engineering aspects to ensure their compliance with the applicable statutory and regulatory requirements. The power plant’s innovative design also called for special classification of its individual components and alternative technical solutions for component design. Further services included integrated quality management and the safety assessment of the completed power plant.

Pilot project in Spain

The key components of this solar thermal power plant, which is located near the Spanish city of Murcia, are its solar steam generators. A mirror surface of around 18 000 m2 concentrates the solar irradiation onto two 800-metre coated absorber tubes and causes the water in these tubes to vaporize. In a conventional turbine, the energy of the hot steam, at a temperature of approximately 280 °C, is then converted into 1.4 MW of electricity. Downstream of the turbine, the steam passes through a water-cooled condenser before being pumped back as feedwater into the absorber tubes.

In addition to electricity generation, this type of power plant with a solar field may also be used for process steam and cooling applications, or the desalination of seawater. The linear Fresnel collectors are parallel rows of flat mirrors with a single-axis solar tracking system. The simplified design minimizes costs while improving environmental compatibility, i.e. more efficient land use and lower water consumption.

The solar thermal power plant essentially consists of four sections:

1. Steam production – the water in the absorber tubes is heated by solar energy

2. Conventional power station technology in conjunction with efficient heat storage systems

3. Electricity generation with a conventional turbine

4. Grid feed-in

Finding the optimum solution to compensate for changes in the solar field proved to be one of the biggest challenges at the demonstration power plant. The operating temperature of 280 °C causes the absorber tubes, originally 800 metres long, to expand by a further 4 metres in length during operation.

To compensate for this elongation, the plant designers used a corrugated hose made of special steel. This hose not only flexibly adjusts to the changes in clearance between the feedwater pipe and the connection to the solar field, but also compensates for operating stress caused by the temperature fluctuations produced by the power station’s daily start-up and shutdown.

I&C systems for high control performance

The designers had to first interlink the systems of the individual plant sections in a way that ensured safe operation while maintaining high availability and efficiency. As the position of the sun keeps changing, highly precise control performance of the flat mirrors was one of the priorities in the pilot project to make the most of the sun’s energy.

Given this, the TÜV SÜD experts also subjected the process control systems and the electrical systems to in-depth design examination. The electrical systems were assessed in accordance with the Low Voltage Directive 2006/95/EC, the EMC Directive 2004/108/EC and the Machinery Directive 98/37/EC. In this context, the experts also reviewed manufacturers’ declarations submitted by suppliers, facilities, equipment and technical installations. During plant construction, the safety specialists also inspected the quality of the welds.

In view of the wide array of design-related and regulatory requirements, a segmented approach to plant assessment based on the relevant regulations proved impractical. Instead, the plant as a whole had to be regarded as a functional unit and approved in an integrated approach. This impacted on the design, manufacturing, equipment, final approval and operation.

The simplified design of the linear Fresnel collectors minimizes cost and provides more efficient land use

For steam generators, for example, the regulations – e.g. the EN 12952 standard – officially require a device for preventing overheating. The solar power station, however, does not have a steam boiler with a traditional furnace and therefore requires a different approach. For the two absorber tubes to overheat, the supply of feedwater would have to be inadequate. In this case, the control system would automatically cause the flat mirrors to move out of the sun.

Even if the control system of the mirrors failed, the collector would only heat up over a short period, as the position of the sun keeps changing. This fact is taken into account in plant design. The possibility of a short-term temperature rise in the absorber tubes was taken into consideration by selecting suitable materials and coating for the collector. The experts also agreed that practice-oriented measures which simultaneously fulfilled safety functions, such as volume control of the feedwater supply, should be given priority over conflicting protective tripping systems.

The fact that national features also plays a role became evident in the earthing system for the low-voltage switchgear. While in Germany every neutral point of a transformer is earthed and a direct connection of the downstream system complexes is ensured, this neutral connection is not always required in southern Europe, where the connection to earth is mandatory for every downstream system. The electrical components must comply with harmonized European standards and regulations but also, for instance, local regulations (Real Decreto 842/2002).

To ensure the solar thermal power station operates efficiently throughout its entire life-cycle, reliable determination and optimization of life-cycle costs, which make up a significant part of the total cost of ownership, were indispensable. To be successful, a cost scheme had to include both warranty and guarantee costs plus expenditure on service contracts and potential replacement investments. In addition, a large number of technical specifications and standards had to be taken into account.

Classifying components and assessing implementation

Systems are classified into module categories on the basis of their operating parameters and in accordance with Directive 97/23/EC. With this approach, the entire plant can be divided into ‘assemblies’, which, in turn, can be classified into different module categories.

The respective components must be specifically designed and manufactured to meet the requirements of the respective category (ranging from I for low requirements to IV for high requirements). This particularly applies to key parameters such as pressure strength. A proof test, a declaration of conformity and a final assessment before the component is placed into service mark the end of the process.

Although the solar field is independent from, and not subject to, the Pressure Equipment Directive (PED), the collector needed for steam generation falls into module G, category IV, of the PED. This category requires testing by a notified body, i.e. an accredited third-party organization. On the basis of the EN 12952 standard, the collector was officially classified as a steam boiler.

But uncontrolled overheating, as in a fired steam generator, is unlikely in the collector tube. In the solar thermal power plant, the collector could also be regarded as a line pipe and grouped into the lower category III. After all, overheating over a lengthy period is impossible even if the supply of feedwater should happen to fail, as in this case the system would be protected by the implemented protective tripping systems.

The corrugated hose is classified as module A1, category II. In this case, responsibility for the design and the testing of the component rests with the manufacturer, although the formally required test pressure is not suitable for this component. The test pressure is relatively high and would cause preliminary damage during the proof test, significantly shortening service life. Modified component design associated with lower test pressure offered a solution that did not compromise on safety.

Project support and co-ordination

The numerous interfaces between complex systems and component solutions may also cause problems that impact on the expected quality of plant processes. Support and quality assurance during construction help systematically to prevent these weaknesses from design to construction and final approval before the plant is placed into service. This approach is based on the principle that economic and technical risks must be identified and controlled right from the outset and problems solved in a targeted approach. This also applies to the case under discussion.

When checking the welds in the Spanish solar plant, the experts not only inspected the welds but also examined the root sides using an endoscope and documented all welding parameters. Some of the assembly welds were inspected by means of non-destructive radiographic examination.

The examination results permitted a comprehensive and qualitative assessment of the integrity of the welded joints to be made. Engineering and business requirements were merged and set in relation to the projected service life. Owing to the impartial assessment of the most varied types of solutions, the experts achieved an excellent overall result.

Background information: welding technology

In a component, welds are considered an unwanted ‘disruption’. As far as quality, no weld is ever as homogeneous as the surrounding material. In addition, the welding process reduces the strength of the welded component around the weld, making the component more prone to defects.

The key damage mechanisms in this context include ‘notch geometry’ or ‘notch effects’ on the one hand and residual tensile stresses on the other. The latter are inevitable during welding when the briefly melted weld material cools down and changes its volume compared to that of the base material.

These high residual stresses in the heat-affected zone (HAZ) at the transition from the welding material to the surrounding base material result from welding and can generally reach values approximating the yield point. Experts have therefore long been striving to improve the fatigue and vibration strength of welded joints through post-weld treatment procedures.

Given this, improvement of the fatigue strength of welds is becoming increasingly significant in many industrial sectors. Welded structures are particularly prone to stresses caused by high temperature, cyclic load, vibration and the material fatigue associated therewith.

Experts counter the mechanisms of notch effects by ensuring high-quality welding processes and welds. In many instances, welded surfaces are also painstakingly ground after welding to remove any poor transitions between the welds and the surrounding base material. Post-treatment of welds by means of high-frequency hammer peening also increases the service life and fatigue strength of steel structures and components by combining improved weld geometry with an improved stress profile.

Integrated profitability analysis

The efficient operation of a solar thermal power station must be ensured throughout its entire life cycle. In this context, life-cycle costs are one of the key elements of the plant’s total cost of ownership. Reliable determination and optimization of these costs are crucial to successful cost planning – from plant design, construction, installation and commissioning to actual operation.

This also covers guarantee and warranty costs and the costing of subsequent service contracts or replacement investments which may have to be taken into account. At the same time, further technical specifications and standards must be complied with.

Even though engineering and business requirements imposed on the conceptual design of a plant may differ to some extent in terms of the entire service life, they still need to be harmonized effectively and efficiently.

Decision-makers in business and engineering must co-operate from an early stage to realize their joint goal of optimum life-cycle costs. In the PE 1 project, all stakeholders worked together right from the assessment process to reach their common goal. Although compromises were inevitable, the comprehensive and professional consulting services provided by the impartial and unbiased TÜV SÜD experts significantly improved the outcome.

Recommendation

Playing a major role in the future power generation mix, solar thermal plants are becoming increasingly important. Their innovative designs call for practical solutions that simultaneously maintain high safety levels. Key priorities should include premium control performance of the flat mirrors and their operation within the context of total plant design.

The approach follows the principle that a well-integrated control system is preferable to numerous single protective tripping mechanisms. When new plant designs are approved, the requirements – in particular the requirements resulting from the implementation of codes and standards – must be addressed in an integrated manner.

Plant owners and/or operators will reap the benefits of working with service providers that are familiar with the effects of process and engineering requirements and see them within the context of legal licensing and permit requirements. By opting for this approach they ensure planning certainty and a high level of availability and cost-effectiveness extending over the long term.

 

Hans Christian Schröder is head of Power Plant and Energy Services and Power Station Sector manager at TÜV SÜD Industrie Service GmbH, in Mannheim, Germany. For further information visit www.tuev-sued.de/is

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