Environmental Engineering Reference
In-Depth Information
Figure 2.15
Irradiance on Horizontal and Two-axis Tracked Surfaces for
Cloudless Days at a Site at 50° Latitude
methods for achieving the motion required for tracking: electrical motors and
thermohydraulic systems. The electric motor driving the tracking unit needs
electrical energy and thus reduces the energy gain of the system. Prototypes
with thermohydraulic drives have shown some operational problems. If the
tracking system fails, more often than not it is stuck in an inefficient position
and hence the system output is very poor until the system is repaired.
The energy gain of tracked solar energy systems does not usually
compensate for the disadvantages. Hence, there are only few operational
tracking systems at present. Only large systems in regions with a very high
annual irradiation can achieve economical advantages from tracking
(Quaschning and Ortmanns, 2003).
The situation is totally different for concentrating solar energy systems in
which optical systems concentrate the energy to a much smaller area. Such
systems are in operation for solar thermal troughs, solar tower power plants
(see section headed 'Use of direct solar energy') or concentrating photovoltaic
systems. These systems have very narrow angles of acceptance for incoming
irradiance and thus do not work satisfactorily without tracking. However,
most concentrating systems can only use direct solar irradiation.
Using the optimal orientation for non-tracked solar energy systems can
also increase the energy gain significantly. The optimal orientation for solar
energy systems operating the whole year at latitudes higher than 30° is about
30° to the south in the northern hemisphere and 30° to the north in the
southern hemisphere. The optimum tilt angles for systems that only work in
summer are flatter, but are much steeper for those that only work in winter.
