Tower of Power

A recently completed solar power project near Sevilla (Seville), Spain, illustrates this point. Known as PS-10, the project consists of several hundred large movable mirrors, known as heliostats, that reflect direct solar radiation to a central receiver located near the top of a 115 m high concrete tower. Components within the receiver convert the solar radiation into electricity. Because the tower is unique in many ways, its design proved challenging. In particular, the tall structure had to be sufficiently robust to support large loads yet maintain an aesthetically pleasing appearance that would be compatible with the surrounding environment.

With regard to the potential development of solar energy, Spain is one of the most attractive countries, as it enjoys the greatest amount of sunshine of any European country. In fact, the Plataforma Solar de Almería is Europe's largest center for research, development, and testing of technologies used to concentrate solar energy. Located in Almería, Spain, the facility is part of the Centro de Investigaciones Enérgeticas, Medioambientales y Technológicas. Moreover, Spain is the world's fourth largest manufacturer of solar power technology.

Given these factors, the Spanish government is committed to producing 12 percent of its primary energy that is, the total amount of energy supplied by the nation's power grid from renewable sources by 2010. This plan includes an installed solar generating capacity of 400 Mw. In March 2004 it removed economic barriers that prevented the connection of renewable energy sources to the power grid, making it easier and safer for investors to develop new projects in this field.

Facilities of the PS-10 type employ a field of heliostats with a control system to reflect direct solar radiation to a central receiver located on a mast or post. The concentrated solar radiation heats up a fluid located in the receiver to temperatures between 500°C and 1,000°C. This thermal energy can then be used to generate electricity.

In fact, PS-10 is Europe’s first commercial generating facility to be powered by the sun, having begun operations earlier this year in southern Spain in Sanlúcar la Mayor, near Sevilla. It is the first of a set of solar power plants to be structed in the same area that will have a total generating capacity of more than 300 MW by 2013. Power will be generated using a variety of technologies.

PS-10 produces electricity with 624 heliostats, each having an area of 120 m². All told, the power plant occupies 60 ha of land. Directed by a central computerized system, the heliostats concentrate the sun's rays at the top of the tower, where a solar receiver and a steam turbine are located. The turbine drives a generator, producing electricity.

With a nominal capacity of 11 MW, the plant produces 24.2 GWh of electricity annually. Construction of the plant cost €35 million (U.S.$47.8 million). of that amount, €5 million (U.S.$6.8 million) came from the European Union's Fifth Framework Programme, a fund that supports research and development work on innovative technologies. Although power produced by PS-10 is three times more expensive than that from conventional sources, the cost in similar plants is expected to fall as the technologies develop.

The PS-10 plant is owned by Solúcar Energía, S.A. The firm, commonly referred to as Solúcar, designs, promotes, finances, constructs, and operates power plants that use the sun as their primary energy source. Solúcar is a subsidiary of Abengoa, of Sevilla, a technology company whose various business units concern themselves with solar energy, bioenergy, environmental services, information technology, and industrial engineering and construction.

Solúcar subcontracted the construction of the PS-10 plant to Abener Energía, S.A., a firm that is one of Abengoa's subsidiaries and focuses on construction. In turn, Abener subcontracted the design and construction of the solar tower to Alternativas Actuales de Construcción S.L. (ALTAC), of Madrid, Spain. ALTAC specializes in designing and constructing industrial chimneys and tall concrete structures.

At the beginning of the project, the tower design was not established. Therefore, it was necessary to assess all requirements to define a suitable, feasible, and attractive solution. For example, the tower had to support various types of large, heavy equipment:

• The solar receiver comprises four interchanger panels that are 5 m wide and 12 m high. Covering an arc of 180 degrees and located on the tower at an elevation of 100 m, the receiver has a total weight of 240.5 metric tons. 
• A 15 m long steam drum is used to collect the steam, which drives the turbine. with a total weight of 74.5 metric tons, the drum is situated atop the tower 113.5 m aboveground. 
• Four pumps below the receiver are needed to pump up water. weighing a total of 12.5 metric tons, the pumps are located at an elevation of 85.8 m. 
• A 5 m long reheater with a total weight of 6.4 metric tons is located 34.1 m aboveground. 
• A 3 m long degasifier weighing 3.6 metric tons is located at the same elevation as the reheater. 
• Ductwork and pipes for water and steam, distributed among several platforms, have a total weight of 176 metric tons when full.

The architectural design also had to be carefully considered during the design stage. recognizing that the solar tower would be a remarkable project in the renewable energy sector, the client wanted to have a unique, memorable structure. Furthermore, the city council of Sanlúcar la Mayor was concerned about the power plant's visual obtrusiveness and opposed a massive concrete tower.

Project participants realized that school groups and other members of the public would want to visit the site as part of guided tours. Because it affords a superb view of the heliostat field, the tower was seen as an ideal location for installing viewing platforms for visitors. However, allowing visitors inside the tower made it necessary to install an elevator and a system for double independent access—that is, a ladder and a stairway—for safety reasons in the event of a fire, a plant failure, or some other emergency.

Taken together, these requirements constituted a new engineering and architectural challenge, as no similar structure had ever been designed. Therefore, the tower's design had to be executed from scratch.

Abener Energía decided to split the tower into two main packages: equipment and civil works. The equipment package was awarded to the energy efficiency and renewables division of Madrid-based Técnicas Reunidas, commonly known as Tecnical.

Because it does not specialize in designing concrete structures, Abener Energía asked several companies in the concrete chimneys market to bid on the civil works package. ALTAC was awarded the civil works package even though no architect had been involved in developing the proposed tower design.

Since the specifications were quite loose with regard to architectural design, the bidders had to propose certain solutions. ALTAC presented several preliminary designs to the client with different levels of service, aesthetic sophistication, and construction difficulty. The different designs depended on, for example, whether a straight or a tapered geometry was required and whether external or internal platforms were desired. each design was named according to its shape. For example, the various tower shapes suggested a cylinder, a wine glass, a telecommunications tower, and a torch.

The design ultimately chosen by the client was a double-shaft straight tower. In plan view, the tower is a trapezoid with rounded corners, the longer side, measuring 22.3 m in length, located to the north, and the shorter side, 15.6 m in length, located to the south.

Two internal walls perpendicular to the tower's parallel sides divide the structure into three main areas, referred to as the east, west, and central shafts. This trapezoid shape is adapted to the geometry of the solar receiver, whose panels cover an arc of 180 degrees. with this nonaxisymmetric shape, the tower seems to be "looking" north to the heliostat field.

The east shaft houses an elevator that extends to the pump platform, along with a stairway leading to the solar receiver level. The west shaft houses the preheater and the degasifier. In the event of fire, the west shaft also provides an alternative means of egress by way of steel service platforms and ladders situated at 10 m intervals.

From the front, the solar tower resembles a twin-edged blade distinguished by three well-defined areas. Completely closed because of structural requirements, the lowest part of the tower supports the visitor platform at an elevation of 31.0 m. Beginning at the visitor platform and continuing upward to an elevation of 81.7 m, the internal walls form two independent shafts, the space between them being open. The shafts are used for such purposes as access, service, equipment placement, and maintenance.

From the outside, the top of the tower appears similar in form to the base. However, the top is fundamentally different in that it has no internal walls. These walls were removed to allow enough space for the solar receiver to be installed.

Obviously, an opening was required in the north wall to enable the sun's reflected rays to reach the receiver. Beginning at an elevation of 100 m, the opening extends 14.1 m in width and 15.3 m in height.

The four pumps that convey water to the solar receiver are located below the receiver in an independent internal platform located in the central shaft. This platform also supports some ducting and minor equipment.

The top platform supports the steam drum and several steam ducts that connect the drum to the solar receiver. None of the water and steam ducts are visible from the outside, in keeping with the initial architectural criteria. The tower's exterior was painted so that it would fit in better with its surroundings. Finally, the tower is equipped with a lightning protection system and three high-intensity aircraft warning beacons.

By adapting the tower's shape to the special requirements of the equipment, the design minimized the quantities of materials needed for construction. Moreover, the opening between the two shafts helps to reduce the wind load in the tower's weakest direction, that is, north-south, which has the lowest inertia.

The design stage was exciting, as all involved knew that they were creating something original rather than following established paths. However, the design process was not without its share of challenges, particularly as ALTAC was developing the structural design at the same time that Tecnical was developing the process and equipment design. occasionally, modifications were made to the platform levels, equipment loads, or access requirements. In fact, the basic architectural design was probably the only design that was defined at the bidding stage.

Located at an elevation of 100 m, the solar receiver is formed by four saturated steam interchanger panels supported in a tubular steel structure. Measuring 5 m wide by 12 m high, each of the panels is formed by a set of pipes that are bent from side to side and from bottom to top. Pipes from one panel are connected to the pipes from the adjoining panel.

The set of panels covers an arc of 180 degrees and acts as the target for the heliostat field. To provide support and ensure that the cavity is properly insulated, all of the panels are mounted in the tubular steel structure, which incorporates an insulated floor and ceiling. Because the tubular steel structure is completely closed, all solar rays entering the cavity are directed to the panels. water coming from the pumps flows through the entire solar receiver, and the resulting steam then enters the turbine, which is located on top of the tower.

The framed steel structure has five column supports to transmit the vertical load. Because the cavity is closed, creating additional wind load, two horizontal supports are included in the top of the steel structure to connect to the tower's north concrete wall and transmit the wind load.

Initially, the tubular steel structure was to be supported in a composite platform. However, other factors precipitated a change in plans. First, this level of the tower required many supports for ducts associated with the pumps located below and the steam drum located above, as well as electrical equipment. All told, these features impose a total load of 80.5 metric tons. Second, a service platform would need to encompass the tower's entire width at this elevation. These facts prompted the designers to divide the loads among different structures to properly transmit them to the tower's concrete walls.

The composite platform encompassing the tower's width serves as a service platform and a support for the ducts and equipment. Made of I-shaped beams situated in a north-south direction, the platform is located below the duct supports, decking, and a 14 cm thick concrete compression slab connected by means of studs.

To bear the solar receiver's load of 240.5 metric tons, another steel structure is located 0.2 m below the composite platform. Because the structure must cover a width of 11.4 m, its two main beams were designed as I-shaped trusses about 1.2 m high. The steam drum located in the top of the tower and the pumps located below the solar receiver also were supported by means of composite platforms.

Because of the large loads and the need to span lengths of as much as 11.4 m, most inner platforms were designed compositely in accordance with the european Commission's Eurocode 4: Design of Composite Steel and Concrete Structures. Composite platforms also were the easiest to erect. The steel beams supported in the concrete shell varied in height from 240 to 500 mm. All plates and anchors in the connections installed in the concrete shell were designed using Profis Anchor software, developed by Hilti, the manufacturer of the anchors, which has its headquarters in Schaan, Liechtenstein. Load values for those connections varied from 0.5 to 20.0 metric tons.

The elevator can lift four people and is equipped with such standard safety devices as a bidirectional communication set. The staircase has been designed to be directly supported in the tower walls rather than by pillars. This decision was made so that visitors would see that aesthetic details were also taken into account in the tower's interior.

Made of galvanized steel, the visitor platform, located, as mentioned above, 31 m aboveground, affords visitors an excellent view of the heliostat field while they listen to an explanation of the overall plant's design.

Because the ground at the site had limited bearing capacity, the preliminary design of the tower foresaw a foundation that would rely on deep piles. However, once the tower's geometry was defined, the foundation had to be redesigned. Since the tower's shape was not cylindrical, the wind load was not independent of the direction. Because the tower's wind load was quite high and its north-south orientation bore the main wind load yet had the lowest inertia, it was not economical to design a deep foundation. Such an approach would also have required a thick slab to connect the tower's concrete wall and the piling. Therefore, the foundation eventually was redesigned to be round and shallow with a diameter of 35.0 m and a thickness varying from 1.0 to 3.0 m.

Preliminary Tower Solutions

ATLAC

Fortunately, the water table was below the level of the foundation. Stability and ground capacity were verified to ensure that they afforded the necessary serviceability limit states. reinforced concrete was designed for serviceability limit states and ultimate limit states in accordance with the requirements of the Spanish standard Instrucción Española de Hormigón Estructural.

The tower was designed with a constant wall thickness of 250 mm, as this would make it easier to erect by means of a straight slip form. For the same reason, all openings were designed with flat edges. The structure was also designed in accordance with Spanish codes governing serviceability limit states and ultimate limit states.

The design was conducted using a finite-element model that incorporated both the foundation and the tower to account for the interaction between the foundation and the structure. The model used shell elements for the concrete and beam elements for the steel beams.

The complete analysis included six basic loads and 20 load combinations. Both the foundation and the tower were designed to withstand dead, service, wind, and earthquake loads, all of which were defined according to Spanish codes. The service load was defined as 5 kN/m² and applied to all platforms. For the wind load definition, a basic wind speed of 27 m/s was used. The earthquake load was determined using a basic acceleration of 0.08g.

The project did not require the use of special materials. The concrete used for the tower and foundation has a compressive strength of 30 MPa, and the reinforcement steel has a yield strength of 500 MPa. The staircases and steel platforms are made of galvanized steel, and the steel beams used in the composite platforms consist of regular carbon steel with a yield strength of 275 MPa.

Construction was scheduled to begin in June 2005 and conclude in February 2006, the most critical stages involving the slip form work and lifting and assembling the solar receiver and the steam drum. The schedules for civil and mechanical works therefore overlapped and required extensive on-site coordination.

The unique nature of the installation caused some delays. For example, platform loads were redefined frequently (some being recalculated 12 times), and the client decided to include extra steel service platforms in the tower's west shaft. Moreover, parts of the tower had to be insulated externally to avoid high temperatures in the concrete shell.

The foundation, consisting of nearly 2,000 m³ of concrete, was built during the summer with temperatures above 35ºC. with shifts working around the clock, the foundation was completed in one weekend. During this process, the temperature of the concrete had to be checked every two hours to avoid cracking.

Construction of the tower was accomplished by means of a straight wooden slip form, the openings and corners of which required extreme care to ensure that their geometry precisely matched that of the design. Temporary columns were used to form the tower's opening, which is located between the elevations of 100 and 115.3 m.

Once the slip form was dismantled, some temporary platforms were installed around and inside the tower to facilitate the remaining work. From then on, two separate crews worked simultaneously, one in the central area and the other in the lateral shafts.

One crew installed the steel beams of the composite platforms using a crane and trained personnel. Then the decking and concrete slabs were constructed, and the solar receiver and the steam drum were installed. Special care was taken to verify that the beam supports were level, as that was crucial to ensuring the correct distribution of the loads in the platforms. Meanwhile, one of the two crews installed the elevator, the staircase, the ladder, the visitor platform, and the steel service platforms.

To fulfill the client's requirements, the completed tower was painted a soft clay color to help it blend in with its surroundings. However, a minor problem arose near the project's conclusion. Because of an engineering redesign in the last stage that required the steam drum to be installed 3 m higher than originally planned, the drum was located above the tower's external walls and its aluminum cladding appeared too bright when viewed from a distance.

Two main options were evaluated: increase the tower's height by constructing a 3m high external concrete wall or paint the aluminum cladding the same color as the tower. From an architectural point of view, the first option was preferable because it would have concealed the steam drum. However, the project was almost finished, and adding an extra wall would have required a delay of at least two months. Such a delay was not possible, as the power plant's inauguration had already been postponed. Therefore, the steam drum was painted, and the result was deemed acceptable.

Construction was completed at the site in February 2007, and the PS-10 plant was inaugurated in June. The facility seems to be working properly in terms of power generation. Moreover, the owner is pleased with the tower's aesthetic appeal because people identify the PS-10 project with the tower. In fact, Sevilla residents have already assigned nicknames to the plant, two of which translate as the wind nail and the bright tower.

Gonzalo García-Sobrinos is the construction manager, Ignasi Salvador-Villà, ICCP, the technical manager, and Jesús Serradilla-Echarri the managing director of Alternativas Actuales de Construcción S.L. (ALTAC), of Madrid, Spain.

Project Credits
Owner: Solúcar Energía, S.A., Sevilla, Spain
Architectural design, structural engineering, and construction: Alternativas Actuales de Construcció S.L. (ALTAC), Madrid, Spain
Process design: Energy efficiency and renewables division of Ténicas
Reunidas (Tecnical), Madrid, Spain
Project management: Abener Energía, S.A., Sevilla, Spain
Geotechnical consultant: GEOCIS, S.A., Sevilla, Spain
Civil engineer of record: Ignasi Salvador-Villà ICCP, for ALTAC