Assessment of the pointing error of heliostats with a single not polar rotation axis for urban applications

•The behavior of Fahrenheit heliostat is studied when it is not orientated along Earth’s polar axis.•The heliostat proposed by Torres-Roldán et al. (2015) is an improvement of the Fahrenheit one.•The angle between the heliostat axis and the normal to the mirror depends on the declination.•The closer is the heliostat to the location of polar behavior, the better it works.•The pointing errors of the heliostat proposed are assumable for urban applications. Electric lighting is the most important source of power consumption in buildings. For that reason, one of the most common measures adopted when constructing sustainable buildings is the use of elements of daylighting to reduce the use of artificial lighting. However, high density of buildings in cities complicates the introduction of natural lighting in buildings, being necessary to resort to concentration and tracking devices, such as heliostats. One of the most important limitations of heliostats for urban applications is that, when using a polar heliostat, like the Fahrenheit one, its location is determined by the direction of the Earth’s rotation axis and the point toward radiation needs to be redirected. However, this location may not be available due to architectural restrictions. In this paper, the behavior of this kind of heliostat as a generic one, whose axis does not need to be oriented along the direction of the polar axis, has been studied. It has been found out that it is possible to define a direction around the heliostat must rotate in order to reflect sunbeams toward the desired point with high precision. Additionally, the pointing errors of the device proposed have been analyzed. Considering that the lengths traveled by reflected sunbeams in urban applications are of about tens of meters, the errors estimated (0.01rad average pointing error) are assumable since they would entail absolute errors of the order of centimeters.