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Concentrating Solar Power Plants: Clean And Renewable Energy For India

Mike Schlaich

Schlaich Bergermann und Partner and Technische Universität, Berlin, Germany 

Abstract:

Early Dish Stirling (3*50 KW) in Saudi Arabia

Current energy provision systems based on exhaustible fuels like coal, hydrocarbons and uranium are damaging the environment and are non-sustainable. Increasing prices due to limited resources and the rising demand pose serious problems even to rich countries. Poverty, population explosion and migration are, amongst other reasons, also a consequence of insufficient energy supply and high energy costs. Which source can supply this energy without an environmental breakdown, without safety hazards and without rapidly exhausting their natural resources at the expense of future generations? The source is the sun, and its energy can be harvested all over the world either by high-tech methods or by indigenously built power plants. This paper gives a short introduction to the work of Schlaich Bergermann und Partner and its approach to solar power plants as well as an overview on Concentrating Solar Power (CSP) systems in general. Finally, one of the first power plants of this kind in India, the Godavari project, is described.

Keywords: solar power plants, concentrating solar power (CSP) systems, environmental-friendly source of energy, Godavari project

Concentrating Solar Power Plants: Our Experience

For more than thirty years, Schlaich Bergermann und Partner has been an independent engineering office specialised in the subject of renewable energies. Already after the first oil crisis in 1973, we started to develop concepts and ideas for solar power generation and gained experience in the planning, construction and operation of solar thermal power plants in the framework of first projects in Germany and abroad. Some of our other building projects realised in those days, such as the second Hooghly River Bridge (Fig. 1) in Kolkata, India, showed us that there can be no prosperity and human existence without sufficient energy supply. 

For us as a civil engineering practice, the focus of our first projects was naturally on the development of new and innovative structures for renewable energy systems. Examples of this include the development and construction of high towers for the first solar updraft tower in Manzanares, Spain (Fig. 2) or the introduction of pre-stressed metal membranes for the design of large parabolic mirrors used for purposes of solar energy supply close to the city of Riyadh in Saudi Arabia (Fig. 3). Over the course of the years, however, it turned out that solar energy would develop into a completely new field of activity for engineers that cannot be covered effectively and successfully by one of the traditional engineering disciplines alone. Only with structural engineering knowledge, but without an understanding of the thermodynamic features, without control and drive

Over the course of the years, however, it turned out that solar energy would develop into a completely new field of activity for engineers that cannot be covered effectively and successfully by one of the traditional engineering disciplines alone. technology, without knowledge of the reflecting and refracting optics in concentrating solar structures, etc., efficient and cost-effective systems cannot be developed. 

The classical way of implementing complex projects, i.e. the cooperation of several offices with different areas of expertise, leads to high expenditures in terms of interface management, a consequential loss of innovation, systemic ‘blindness’ and the inability to optimise the overall system with the objective to create trend-setting integrated system solutions. Based on this insight, a highly productive team has emerged in the course of the projects and years, initially consisting only of structural engineers and then extended by mechanical engineers, electrical engineers, aerospace engineers, physicists, mathematicians and specialists for optical systems and thermodynamics, with which the complex tasks can be performed on a comprehensive basis.

Why The Sun?

Prototype for the Solar Updraft Tower in Manzanares, Spain

The greatest source of energy mankind has at its disposal around the world is the radiation energy of the sun. Year after year, the earth’s outer atmosphere is hit by radiation energy of around 1.5 x 1018 kWh, or 5,461,000 ExaJoule, corresponding to around ten thousand times the level of today’s primary energy consumption throughout the world. In 2010, the latter amounted to 600 ExaJoule approximately.

Only if mankind succeeds in using this free energy supply in a cost-effective way through the development of suitable new technologies, a resource-saving and environmental-friendly source of energy will be available to the world for a period of time indefinite by human standards (Fig. 4). 

The greatest source of energy mankind has at its disposal around the world is the radiation energy of the sun. Year after year, the earth’s outer atmosphere is hit by radiation energy of around 1.5 x 1018 kWh, or 5,461,000 ExaJoule, corresponding to around ten thousand times the level of today’s primary energy consumption throughout the world.

As a result of the anticipated further increase in the world population, exhaustion of the hitherto used fossil sources of energy, like crude oil and natural gas, and the desire to provide the entire world population with a truly human existence including access to sufficient and affordable energy, it will be the task of the present and next generation of engineers to promote solar technology with new ideas and technologies on a large scale. For more than 30 years, we have now committed ourselves to this task, learned, gained experience, coped with setbacks, handled the most various systems and continue taking pleasure in shaping this part of the future.

What Are The Objectives?

Fig. 4: Global fossil energy reserves and respective reserves to production ration compared to selected energy flows

Global energy supply, which will have to rely more and more on renewable energies in the future, requires a wide range of technologies from hot water collectors on the roof of a residential building, wind power plants, tidal power plants, small and large photovoltaic power stations and process heat provision systems down to large solar thermal power plants with two- and three-digit Megawatt outputs. Especially in the field of solar thermal power plants, Schlaich Bergermann und Partner has gained vast knowledge and experience, starting from the preparation of new concepts down to engineering design services for large power stations.

In addition to economic efficiency and environment-friendliness, the security of supply is one of the three main objectives of energy policy and future renewable power supply. Solar thermal power plants provide a possibility to achieve this goal: by means of a large collector field, a heat transfer medium (be it oil, water, salt or air) is heated to temperatures of 200°C up to partly more than 1,000°C through concentration of the solar radiation by point or line focusing collectors. The thus created heat can be held available either directly or by means of heat exchangers at large storage facilities and then transferred to a conventional steam or gas turbine if required, which converts the heat into mechanical energy and then into electricity by means of a connected generator. The development, optimisation and use of these so-called CSP (Concentrating Solar Power) technologies in power plants—and hence at relevant levels in energy economy terms—is our most important objective.

How To Achieve The Objectives?

At the beginning of a new project or a new development, there is almost always a techno-economic consideration: besides the technical function and elaboration of a new idea, another focus always is on the anticipated energy costs. Renewable energy systems are capital-intensive. As a rule, they do not cause fuel costs, which represent the dominant and ever-increasing share of energy costs in the case of conventional fossil power plants. On the other hand, the initial investment costs for the construction of a renewable energy plant are high. The objective with respect to all new developments is to keep these construction costs as low as possible. In this context, not only material costs are relevant, but also the cost of constructing and operating the plant. At the same time, it is a matter of course that as much useful energy as possible needs to be provided by the system.

The resulting optimisation tasks and solar energy generation in general provide us engineers with an exciting and multifaceted area of activity, which we cover from early concept development down to the last screw of the serial production process. Especially the field of solar energy generation, whether concentrating or non-concentrating as in case of the solar updraft tower except perhaps for the technology of parabolic trough power plants is still very young. Therefore, it is continuously required to break new ground and develop solutions that are adjusted to the respective task a challenge, yet one that makes sense and that we are pleased to face.

The resulting optimisation tasks and solar energy generation in general provide us engineers with an exciting and multifaceted area of activity, which we cover from early concept development down to the last screw of the serial production process.

What Are The Trends?

Almost all energy forecasts for the coming years and decades raise a similar prospect. It is expected that, by the middle of the century, approximately 50 to 75% of the global energy requirements will be covered from renewable sources. This requires an enormous structural change of the power plant park. Since the renewable energies are often fluctuating sources, the development and proliferation of energy storage facilities are of major importance. Only by this means the generation and utilisation of renewable energy can be uncoupled in terms of time and reliable energy provision be guaranteed. Moreover, the electricity transportation networks need to be adjusted to the upcoming change of energy distribution/availability in terms of space and time.

On the one hand, all this is certainly a great challenge for the provision of the necessary political framework conditions and financial resources. On the other hand, it is a still greater challenge for engineers, technicians and developers. In all areas, new technologies have to be conceived, developed, tested and introduced to the market. This ranges from comprehensive energy-saving measures and the development of new renewable energy generation technologies to the construction and integration of facilities and power plants into existing or newly designed infrastructures. Certainly, it would be presumptuous to say today which mix of technologies will have eventually established itself in the market at the end of the century.

However, the present and next generation of engineers will have to set the crucial trends in this respect. It is foreseeable that, besides wind, wave and tidal power plants, biomass utilisation and geothermal energy, the use of sun radiation in its different variations will play the most important role in the context of future energy supply.

Concentrating Solar Power Plants: Technology

View of power plant, Andasol, Spain

Solar thermal power stations transform solar radiation first into heat, then into mechanical energy by means of a thermal engine such as a turbine or a Stirling engine and, finally, into electrical energy by means of a generator. Today, steam turbines are almost exclusively used as thermal engines in solar thermal power stations; the steam produced on the basis of solar radiation and expansion within the turbine needs to be condensed again in a cooling device and then pressurized by means of a feed-water pump (Fig. 5). 

In times of an excess supply of solar energy, storage can be charged with heat from the solar field. This energy is then used to operate the turbine in times of insufficient solar radiation or during the night; alternatively, heat can also be provided through the combustion of a fossil or biogenic fuel. In this case, one refers to hybrid power plants as they use two different sources of energy.

The most important types of solar thermal power plants are classified in Fig. 6. In this context, one distinguishes between line-focus plants (parabolic trough and linear Fresnel power stations) and point-focus systems (central receiver systems and Dish/Stirling). A special case is the solar updraft tower, which works without the concentration of solar radiation.

In times of an excess supply of solar energy, storage can be charged with heat from the solar field. This energy is then used to operate the turbine in times of insufficient solar radiation or during the night; alternatively, heat can also be provided through the combustion of a fossil or biogenic fuel. 

Parabolic Trough Stations

Andasol 1 in Southern Spain is a typical modern 50 MW parabolic trough station with storage (Figs 5 and 7) and is to serve as an example in this context. The power stations Andasol 2 and 3, built in the meantime in immediate proximity of Andasol 1, are almost identical in construction with the first power plant. For all three power stations, our office planned the collector field and supervised the process of manufacturing and assembly.

For Andasol 1, a total of 7,488 individual collector elements, each with 12 m in length and 5.8 m in width, were installed on an area of 1.3 x 1.5 km. 12 collector elements each are combined into a solar collector assembly (SCA) of approximately 150 m in length, which follows the sun thanks to a hydraulic drive mechanism installed at the centre of the SCA. The collectors are arranged in a north-south direction.

Four collector units each form one collector loop. As a consequence, a collector field results in a total of 156 loops and an overall reflecting surface of approx. 510,000 m². The heat transfer medium a synthetic oil is pumped into the individual loops at a temperature of around 300 °C, heated up to just under 400 °C as it passes through the loops and then led to the power block by means of collecting pipes. Heat exchangers then transfer the heat to the steam cycle.

The Andasol power stations are equipped with heat storage (steel storage tanks filled with molten salt), which store the heat generated by the solar field on an intermediate basis.

Like all thermal power stations, whether operated on a fossil or solar basis, parabolic trough plants also require a cooling system. In the case of Andasol, these cooling systems are wet coolers working with water that partially evaporates. In regions with a water shortage, dry coolers, basically working like motorcar cooling devices, are the most suitable solution. Whereas the power plant efficiency slightly decreases in this event, the advantage is that no cooling water is consumed.

The Andasol power stations are equipped with heat storage (steel storage tanks filled with molten salt), which store the heat generated by the solar field on an intermediate basis. The collector field is dimensioned in such a way that, in the course of day, it does not only make sufficient thermal energy available for the operation of the turbines, but also charges the storage so that the turbine can be operated at full load for up to seven hours after sunset. As a result, the power stations make reliable power plant capacity available to the public supply network.

Linear Fresnel Power Stations

Linear Fresnel power stations likewise number among the line-focus solar thermal power plants, but differ primarily in terms of the collector field (Fig. 8). The reflectors consist of relatively narrow, slightly curved strips, arranged at a horizontal level. The individual reflector strips follow the course of the sun and reflect the radiation onto the linear receiver, which is installed on a stationary basis several meters above the reflectors in the focal line. Today, water is used as the heat transfer medium and evaporated directly in the absorption pipes. Currently, only saturated steam is generated in the commercial facilities. In the future, however, overheated steam will also be generated there. 

Classification of solar thermal power plants

Layout of a parabolic trough power plant and overview photo Andasol 1

From a technical as well as from an economic point of view, heliostats are the most important components of central receiver systems. Therefore, we have already furthered the development of precise, small or large, durable and at the same time favourably priced heliostats since the 1990s (Fig. 10). Moreover, the design and dimensioning of the towers as well as their influence on the layout of the heliostats field and its optimisation are an interesting area of activity.

Dish/Stirling Systems

Linear Fresnel plant (Ausra)

Central receiver system

It consists of the parabolically curved, 'dish-shaped' concentrator, a solar receiver and a Stirling engine as the thermal engine with a directly coupled generator (Fig. 11). The parabolic concentrator is made to follow the sun by means of a two-axis tracking system, so that it reflects the direct solar radiation onto the receiver, which is placed at the centre. From the receiver, the heat is transferred to the Stirling engine, which converts it into mechanical energy (usually the rotation of a cranked shaft). Finally, a coupled generator transforms the mechanical into electrical energy.

Stretched metal membrane heliostat

Dish Stirling

Solar Updraft Tower

Solar updraft tower

In the Solar Updraft Tower, the three familiar construction elements greenhouse, chimney and turbine—are combined in a new way (Fig. 12). The principle is depicted in Figure 12. Air is heated by solar radiation under a low transparent circular roof open at the periphery; this and the natural ground below it form a hot air collector. In the middle of the roof is a vertical chimney with large air inlets at its base. The joint between the roof and the chimney base is airtight. As hot air is lighter than cold air it rises up the chimney. Suction from the chimney then draws in more hot air from the collector, and cold air comes in from the outer perimeter. By means of this mechanism, solar radiation creates a continuous updraft in the chimney. The energy contained in the air current is used with the help of turbines, installed at the base of the chimney. 

The Godavari Project

The Godavari 50 MW power plant

Team chart for the Godavari project

Starting with the Andasol projects mentioned above, the so called EuroTrough was first in the long list of collector designs built in Spanish CSP plants. It was also first on the African continent with the project of Kuraymat, Egypt. Now the pioneer design in India is the 50 MW CSP project Godavari in Rajasthan. The collector field consists of 120 EuroTrough loops with an accumulated collector aperture area of 392,000 m². The plant was opened in early 2013 and is successfully running.

In India, the Rajasthan solar policy, National Solar Mission, was launched with the target of installing 20 GW of solar by 2022. One project developed under the National Solar Mission is the 50 MW CSP Godavari Project.

Robot Welding (Spain) and Manual Welding (India) of the Top Frame

The fast growth of India’s industrial market and population is directly linked to a fast increase of power demand. Fossil fuels are a finite resource and occurrences are limited in India, as well as the associated green house effect through CO2 emissions have been confirmed as facts. Further, nuclear power experiences more and more criticism within the Indian population. In order to face the challenging increase of national power demand and promote ecological sustainable growth the Jawaharlal Nehru National Solar Mission was launched by the Indian government with the target of installing 20 GW of solar energy by 2022. The mission is set up in three phases. Phase 1 has the ambitious target of realizing projects with a total of 1000 MW of solar energy. By 2012, 150MW of photovoltaic electricity and 450MW of CSP power plants were allotted.

One reason for the high partition of solar thermal technologies is the high local content of manufacturing of these systems and the ambition to establish India as a global leader in solar energy. In most CSP solar fields materials like structural steel, glass and concrete are used which are available on the Indian market. Further, thermal energy has the capability to be stored at affordable costs and be delivered on demand.

One of the first seven projects of phase 1 is the 50MW CSP Godavari project in Rajasthan. Figure 14 demonstrates the general arrangement and loop layout of the solar field for the Godavari project 

The owner of the solar power plant is Godavari Green Energy Limited (GGEL). The EPC contractor is Lauren-Jyoti (Mumbai, India), which is a joint venture of Lauren Engineers and Constructors (Abilene, USA), CCL Optoelectronics (Mumbai, India) and the steel fabricator Jyoti Structures Limited (Mumbai, India). Our office Schlaich Bergermann und Partner joined the Godavari project as the licence holder of the EuroTrough collector and the main solar field designer in collaboration with the solar division of Flabeg (Cologne, Germany) who was responsible for the piping design, control systems and the conceptual layout of the assembly line. Figure 15 presents

The fast growth of India’s industrial market and population is directly linked to a fast increase of power demand. Fossil fuels are a finite resource and occurrences are limited in India, as well as the associated green house effect through CO2 emissions have been confirmed as facts.

the chart for the solar field design and construction of the Godavari project. It was connected to the grid by March 2013. Therefore, the design period for the solar field was very short. After awarding the contract in the beginning of October 2011 until the issue of the complete design package for fabrication and assembly, a period of four weeks was given and successfully achieved.

In addition to our role as solar field designer, our team was also involved in the fabrication jig conceptual and final designs, supervision of assembly and fabrication jig manufacturing and the quality control of the component fabrication during the manufacturing phase. On site, the team was also present to support and supervise the collector assembly and erection in the solar field.

The site and the final assembly of the collectors are located 34 km north of Phalodi in Rajasthan. However, the fabrication of the individual sub-components and assembly jigs was allocated to four different contractors in the whole of India. Cantilever arms, HCE arms, middle and end pylons are fabricated at Jyoti Structures Ltd in Nashik. The drive pylon is fabricated at Tiwari Engineering Works in Baroda, Vadodara, the box frames at Mastech Technologies in Alwar and the assembly jigs at Ashoka in New Delhi. These four main contractors themselves partly distributed the work to further subcontractors in their surroundings.

As mentioned above, the collector structure is that of the Eurotrough. As the final accuracy of the collector assemblies is achieved during the on-site assembly, the fabrication tolerances required from the subcomponents

are moderate. Therefore, the fabrication process of the sub-components can be adapted accordingly depending on the expertise and traditional fabrication procedures of chosen fabricators, the overall time schedule of the project and the local costs of labour. The wide range of fabrication possibilities of the components is shown in Figure 16 for the torque box frames of the EuroTrough. Fully automated robotic welding was chosen by the manufacturers for the Andasol 3 project in Spain whereas more labour intensive manual welding of the frames was undertaken for the Godavari project in India.

Its structural system qualifies this design for fabrication all around the world, including less developed industrial environments, due to:

• availability of primary material worldwide;
• adaptability of primary structure to local standards (material grade, cross sections); and
• adaptability to most diverse fabrication possibilities like full automated robot welding or labour intensive
• manual welding. 

It is our experience that all complex structures from long-span bridges to solar power plants can be successfully built all over the world as long as we bear in mind the local boundary conditions. If we consider this context, quality can be achieved be it by all-robot fabrication or indigenous building.

Acknowledgement

This text is an edited and much reduced version of the two documents listed in the literature below. The permission of the authors, all members of Schlaich Bergermann und Partner, to do so is gratefully acknowledged. 

References

1. Balz M., Keck T., Schiel W. and Weinrebe G. (2012), "SonneSun", Publication by Schlaich Bergermann und Partner, Stuttgart, Germany (free .pdf copy available via solarinfo@sbp.de ).
2. Schweitzer A., Meese L., Birkle M., Schiel W. and Balz M. (2012), "Pioneer again - Eurotrough goes India, 50MW CSP plant Godavari in Rajasthan", Conference Proceedings, SolarPaces.