“On the tail of the dragon”: the solar tower in Badaling, China
The solar tower is located in Badaling, near Beijing, at approximately 5 km from the Great Wall. As it ican be seen in the picture, the area is flat and the sinusoidal lines of the landscape can be seen in the distance, with the Great Wall line that is the only human manufact on the Earth that can be seen from the space.
Therefore, the vicinity of the tower to the Great Wall and its morfological structure are the main design features.
The aspect of the tower
The vicinity to the Great Wall and the relationship with the location and the ancient Chinese architecture, have required the use of simple and compact forms to be made with economic materials that can be easily maintained (reinforced concrete and corten steel). The shape has historical and symbolic meanings (a big illuminated dragon is erected near Beijing) but it is also using a contemporary architecture style based on different architectonic elements and pure geometric lines.
The tower, characterized by having a square basement (12 m. x 12 m.), will appear on the horizon as a mainly closed structure, similar to the pre-medieval towers that characterize the Great Wall sky-line. The vertical development of the tower is reinforced by the long tail that is shaped as the surrounding landscape.
Particular care has been dedicated to the design of the visual effects in the night time.
The idea is to characterize and reinforce the relationship between the inner and outer spaces and its function to characterize the urban land-mark.
The objective is to transform the tower into a illuminated sculpture, on object with an aesthetic value with the objective to conjugate the architectonical, iconic and functional aspects.
During the night, the red coloured lights should highlight the architectonical lines and and the structures: for example, the north side, characterized by a corten steel (used because it does not need any maintenance), will appear as a large geometric pattern which is developing vertically; the east and west views will appear transparent and deep, due to the presence of the lifts and the stairs; the south view will be the most compact because of the small square windows and the size of the vertical body that is linked to the horizontal ground.
The program requirements
The competition requires the design of a 100 m high solar tower, containing three receivers located at three different levels: 70, 80 and 90 meters from the round level. The tower should have two lifts, one of which for workers, a stair, some services and technical spaces for the passage of pipes from the receivers to the ground level, working spaces at all levels, and two spaces for visitors at 50 m and on the top of the tower.
The attached proposal was strictly based on the above requirements.
The lay-out is compact and rational to avoid empty spaces and therefore higher costs.
As far as the space organization is concerned, some functions have been placed under the “dragon tail” with a line arrangement.
This location allows to save some space at the ground level, making it possible to organize the area to have more functions that are not considered at the moment.
We have considered two main points to stay within the budget foreseen for the construction: materials and construction techniques: smoothed and painted reinforced concrete and corten steel screens.
The structural configuration
The cantilevered structural configuration of the tower represents a very simple scheme and allows for some direct evaluations aimed at checking the safety of the designed scheme.
The vertical loads determine axial forces increasing from the top to the bottom sections. If lateral actions are present, the critical section is the bottom one, where the bending moments are maximum. Moreover, due to a concentration of masses toward the top, caused by the presence of the equipments, the sensitivity to the lateral action increases, especially if the dynamic behaviour is considered, and an amplification of the oscillations shall be expected.
The tower is made of reinforced concrete. Ordinary strength value can be hypothesized, i.e. concrete 30/37 or even lower. Due to the large vertical load at the tower base, a deep foundations' system should be provided, consisting of high diameter piles (1000-1200 mm) below a thick plate built at the base of the elevation.
Even is the structure is very high, the assumed configuration of the transverse section and the steel bracing system adopted, allow for a "tube - like" configuration with significant stiffness. A period of the fundamental oscillation mode equal around 3 seconds is estimated.
Accounting for the flexibility of the tower, the wind action can be considered as the most relevant lateral action. Preliminary analyses carried out referring the assumption provided by the Chinese code GB50009-2001 have been made. Prudential increased values of the wind pressure (varying from about 2.0 to 3.5 kN/m2) have been adopted in the verifications, for accounting for the rectangular shape of the tower transverse section. The adopted dimension of the resistant section allows for a large safety factor against the wind action, even with a minimum, constructive, rebar percentage. Due to the relevant height of the tower and to its shape, more accurate analyses based on numerical simulation and on tests in wind tunnels should be done at the stage of the detailed design, as specifically provided by the Chinese code.
The partial concentration of the masses toward the top of the tower, configuring an inverse pendulum system, makes the tower also sensitive to the seismic actions. Due to the relevance of the structure, a large return period of the reference earthquake should be adopted. A site hazard analysis should be done before the detailed design. In the absence of a specific site characterization, some preliminary evaluations have been carried out assuming a reference peak ground acceleration of 0.25-0.30 g and a standard response spectrum shape. The forces evaluated at the base section result of the same order of those induced by the wind, therefore the structural solution can survive also extreme seismic events without significant damage.
Plant’s lay-out and safety requirements
The three receivers will be placed with the center at 70, 80 and 90 m respectively, held in place by steel structures that are fixed on the tower concrete side walls. The receivers will be insulated on the internal side with 30 cm insulation with a stele containment structure that will limit the temperature of the internal side to an acceptable level for safety measures.
On the back of the receiver, the workers will be able to reach the receivers’ surface with platforms to execute measurements in a safe environment.
The pipes will be mounted internally along both tower side walls. On one side the pipes will contain the cold flows, on the other the hot flows. One of the pipes will be used for air and the other for saturated and superheated steam.
The stairs are conceived to safely allow workers and visitors to evacuate the tower in case of emergency and/or malfunctioning of the lifts. The size of the stairs has been designed according to the EU legislation. The roof of the solar tower will be accessible for visitors, as well as the platform at 50 m above ground level. The access to both viewing locations will be clearly delimited to avoid interference with the workers activities and to avoid contact with dangerous parts.
The dragon tail on the ground level is used to host a large part of the technical spaces and machines. The largeness of the tail has allowed to located most of the energy systems under the cover. All the requirements of the programme have been fulfilled in terms of spaces dedicated to each function.
The sequence of the functions located under the “tail” has been decided according to the energy flows and energy storage, whereas the workshop and maintenance spaces have been moved on the side. The organization and the shape of the tail have also required a rearrangement of the internal streets, to move people and parts on the ground.