# Passive and Active Solar Systems in Eco-Architecture and Eco-Urban Planning
## 3 Research: Passive and Active Solar Systems in Eco-Architecture and Eco-Cities
### Heliostats and Their Application
#### 3.3.2 Heliostats: Selected Case Studies of Urban Design  
The sun plays a key role in urban space, has aesthetic and health-promoting functions, and affects the quality of life and comfort of residents. Sunlight allows well-known places to be looked at from a different perspective, bringing out their potential which is often extraordinary. One example of the use of heliostats on an urban scale is the Norwegian town of Rjukan, located in the Vestfjord valley adjacent to the Norsk Hydro hydroelectric power plant. Rjukan is surrounded by the Gaustatoppen Mountains where there is little sun from October to March. This location influences the inhabitants, their health and the economy. Indirectly, access to the sun or lack of it led to a situation in which the material and social status of the inhabitants were reflected in the insolation of their houses, depending on their height on the mountain slope [38]. For this reason, a group of architects proposed futuristic solutions based on an idea from over half a century ago to place heliostats on the slopes of the mountains, which would reflect and direct the sun's rays to the shaded town center. The scale and importance of the problem of insolation were proven by the fact that in 1928 a cable car route was opened, which allowed residents, for a small fee, to travel from the valley to the summit at a height of 886 m above sea level, from where they could rest in the sun's rays for at least a few hours during the day. In 2013, as part of the Rjukan Solar Mirror project, the Solveil heliostat system was installed on the hillside to reflect and direct the sun's rays to the main square of Rjukan. Installed 450 m above the market square, three huge mirrors, with a total area of 51 m\({}^{2}\), move approx. every 10 s such that the market square remains illuminated for most of the day. The entire system is computer controlled and powered by solar batteries. This old idea and the innovative technical solution of the present time have had a real impact on the quality of life of the inhabitants in terms of physical and mental health as well as social contacts.  
Another place that actively uses heliostats is the Austrian town of Rattenberg in the Tyrol. Here, a nearby hill to its south completely blocks sunlight from November to mid-February. Heliostats can solve this problem, and at Rattenberg, they are to be located approximately 400 m north of the town where their task will be to redirect sunlight to auxiliary floodlights located at the top of Castle Hill. Heliostats can move to different angles as they rest on a ball-joint, and their movement is controlled by a computer that automatically follows the movement of the sun.  
Currently, the use of heliostats in the urban fabric is associated primarily with the problem of lighting in areas characterized by dense and high buildings. One such installation is the heliostat in Teardrop Park, a small city park located in downtown Manhattan near the site of the World Trade Center. Teardrop Park is surrounded by tall buildings that shade its space. Currently, the park is illuminated by three 240 m diameter mirrors placed on top of a residential building in Battery Park City overlooking the park [39]. Another park illuminated with a mirror system is One Central Park in Sydney where the Architectural Heliostat and Lighting System was conceptualized by architect [PERSON] and designed by Kennovations. The One Central Park lighting system is made up of 40 motorized heliostats located on the West Tower roof and 320 reflective mirror panels located on the East Tower cantilever. The light is redirected onto retail and landscape terraces [40].  
A further interesting example of the use of heliostats to illuminate public spaces in Sydney is a 39-story tower at 14-22 Walker Street in Inner West Sydney. Heliostats placed on the roof of the building are 20 m high and reflect sunlight, directing it to the previously shaded Union Square [41].