Chapter II Summary of literature review in the fields of solar energy and related
2.1 Solar energy and its availability
2.1.3 Solar energy collection and tracking configuration
2.1.3.3 Solar aiming and tracking
each one with its own characteristics which may be appropriate for some particular application. Just to mention some: cylindrical trough concentrator, spherical concentrator, evacuated tube collector, etc., as addressed in various literature. See Kalogirou (Kalogirou, 2004) for a detailed discussion of these.
Active tracking techniques (normally electrical/electronic based) involve the use of an active actuating device to provide the desired movement, commonly, motors of different types (stepper, DC, etc.). A monitoring and control apparatus is also required to properly control the actuator.
2.1.3.3.1 Types of collector positioning / sun tracking
Collector positioning is the task of aiming the collecting surface at the sun so as to null the light incident angle and thereby maximize the energy collecting efficiency.
It should be pointed out that the choice of a tracking strategy is influenced by collector type, along with a trade-off between performance and cost. Comments will be added later about the suitability or fitness of a particular collector for specific types of control strategies.
There are three major types of collector positioning / sun tracking: Fixed positioning (non-tracking), one axis tracking and two axes tracking. We describe each of these methods briefly:
2.1.3.3.1.1 Fixed (non-tracking)
In this configuration the collector assembly is fixed. The collector surface is tilted by the latitude angle of the location and the normal to the surface is made coplanar with the local meridian. In most cases the collector assembly is strictly fixed, with no adjustment whatsoever. However, in some cases, there may be seasonal adjustments of the collector’s tilt angle so as to account for seasonal variations of the solar light incident angle, where the gain in efficiency is considerable: For mid- latitudes, tilt angle adjustments at every three months, only increases the annual energy production by less than 5%; (Wenham, et al., 2007).
Fixed (strictly or seasonally adjusted) tracking is only suitable for collector shapes and configurations whose efficiency is not greatly affected by the cosine of the light incident angle.
2.1.3.3.1.2 One axis tracking:
In one of the possible configurations, the axis is made parallel to the earth’s rotation axis (i.e., it is a polar axis) and the corresponding tracking angle is made equal to the hour angle (ω). To make the axis parallel to the earth’s polar axis, the axis should be coplanar with the local meridian and its tilt angle (slope) should be made equal to the latitude of the location (see Figure 14). As in the case of fixed tracking, in some cases, there may be the need for seasonal adjustments of the collectors tilt angle so as to account for seasonal variations of the solar light incident angle. This is done in case where the gain in efficiency is considerable
compared against tilt adjustment implementation costs.
An azimuthal one axis tracking is also possible (Prapas, et al., 1986), where the axis is made vertical, i.e., collinear to the zenith line; the corresponding tracking angle is than the azimuth angle (γ).
It should be pointed out however, that for some types/shapes of solar collectors (like a PTC or a ), a one axis tracking, at the least, is compulsory.
Figure 14 - One axis sun tracking systems with tilt angle equal to the latitude angle (Alata, et al., 2005)
2.1.3.3.1.3 Dual axis tracking
There are two major types of two axes tracking: the polar (equatorial) (or hour- declination) tracking (Figure 15) and the azimuth-elevation (azimuth-altitude) tracking (Figure 16). Dual axis tracking may increase the annual energy production around 30% (Wenham, et al., 2007). It should be pointed out that for some types/shapes of solar collectors, the point focus type collectors (like the paraboloidal type concentrator), dual axis tracking is mandatory.
2.1.3.3.1.4 Polar (equatorial) tracking
In this type of tracking (see also Figure 15), the tracking angles are the hour (ω) and declination (δ) angles. The main axis (hour) is made parallel to the earth’s polar axis and perpendicular to the equator’s plane. This is equivalent to orienting this axis north-south and tilting it by the latitude angle of the location. When this axis rotates (eastwards or westwards) the tracking angle is the hour angle (null at solar noon when the sun is at the local meridian; positive westward and negative eastward), while the other axis is perpendicular to the main one and when it rotates (northwards or southwards), the tracking angle is the declination angle (null at equinoxes; negative southwards from September to March equinox and positive northwards from March to September).
Figure 15 - Dual axis equatorial tracking (north hemisphere) (adapted from Aiuchi, et al., 2006).
2.1.3.3.1.5 Azimuth elevation tracking
In this type of dual axis tracking (see Figure 16), the tracking angles are the azimuth (γ) and altitude (α) angles. In one of the possible approaches, the one axis is made vertical (i.e., collinear to the zenith line) and the corresponding tracking angle is then equal to the solar azimuth angle (γs), while the other axis is perpendicular to the first one and the corresponding tracking angle is equal to the elevation angle (Alata, et al., 2005).
Figure 16 - Dual axis azimuth/elevation tracking (adapted from Aiuchi, et al., 2006)
2.1.3.3.2 Polar axis versus azimuth-elevation axis tracking.
The major advantage of polar axis tracking (over the azimuth-altitude approach) is the slow variation of declination tracking angle. This allows for very long (time) step adjustments and thus saving power and allowing for longer motor and tracking assembly life expectancy. This same characteristic (of declination tracking angle low variation) is what favours the existence of a one-axis (polar axis) tracking needing only seasonal tilt angle adjustments.
Conversely, in the azimuth-altitude approach, both tracking angles (azimuth and altitude) have moderate to fast variations during one single day. That makes this tracking approach somehow power hungry and not very appropriate for collectors that are cosine-effect sensitive. In general that approach should be discouraged where low cost, sustainable and energy-centric solutions are desired.
2.1.3.3.3 Manual versus automatic tracking
Solar tracking may be performed manually or automatically.
In manual tracking, a human is in charge of periodically or seasonally changing the position of the tracking assembly aiming it at the sun, guided by a schedule. In automated tracking, a machine (a controller and actuator) is used to perform the tracking tasks. The major drawbacks of manual/human executed tracking are that the periodic step adjustments cannot be as short as can be with automated tracking and on the other hand they are subject to possible human timing and positioning errors. Manual tracking is mostly likely to be used with collectors whose efficiency is not greatly affected by the cosine of the sun light incident angle (θ).
2.1.3.3.4 Open loop versus closed loop tracking
Open loop tracking: the relative position of the sun is mathematically determined using the solar time and the suitable tracking angle equations. This does not account for possible disturbances, and other types of perturbations, widely addressed in next sections. This type of tracking is suitable for simple systems where a desired positioning accuracy is guaranteed and the effect of disturbances is negligible. Abdallah and Nijmeh (2004) built a PLC based open loop dual axis tracking system that was found to be efficient.
Closed loop tracking: the actual alignment of the controlled surface to the sun position is measured and compared to the desired position (the set point), and a corrective action taken to minimize this error. A number of closed loop systems for providing one axis and dual axis tracking were mentioned above including:
(Aiuchi, et al., 2006), (Kalogirou, 1996), (Khalifa, et al., 1998), (Prapas, et al., 1986) and (Roth, et al., 2005).
2.1.3.3.5 Continuous versus step tracking
When the variation of the tracking angles is relatively slow and/or the degradation of collector efficiency relative to the variation of the tracking angle is considerably low, a step tracking is advisable, since, most likely it saves power. Only for very accurate tracking needs (like in automatic pyrheliometer applications) a continuous tracking is compulsory. For parabolic or paraboloidal concentrators, the collector’s acceptance angle determines the maximum size of the tracking step. The tracking step may be defined in degrees or time (minutes), deriving it from the equations that relates solar time with the tracking angle.