Irradiance gain for surface tilt or tracking

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If a solar energy system tracks the sun so that the angle of incidence is virtually zero, the energy yield increases significantly. The higher direct irradiance on a surface perpendicular to the solar radiation causes the increased energy yield. During days with high direct irradiation, tracking can achieve energy gains over horizontal orientation in the order of 50 per cent in summer and up to 300 per cent in winter, depending on the latitude of the location (see Figure 2.15). However, tracking can cause a reduction in energy yield in overcast conditions because the contribution of diffuse irradiation from behind the surface is lost. Tracking achieves the main energy gain in summer. On the one hand, the absolute energy gain in summer is higher than in winter; on the other hand the number of overcast days is usually lower in summer.

There are two principle options for solar energy system tracking: one-axis and two-axis tracking. Two-axis tracking systems move a surface always into an ideal position; however, two-axis tracking systems are relatively complicated and thus one-axis tracking systems are preferred in some instances. One-axis tracking systems can follow the daily or annual path of the sun. Tracking to the annual path of the sun is relatively simple: the surface tilt angle need only be changed once a week or even once a month.

A two-axis tracked solar energy system installed in middle-European latitudes can obtain an energy gain in the order of 30 per cent compared to inclined non-tracking systems. The energy gain of a one-axis tracking system is slightly lower; it is closer to 20 per cent. Regions with higher annual irradiation also have a higher absolute energy gain because typically the direct beam contribution is much higher. However, tracking systems are more complicated, more expensive and have higher operating and maintenance costs. The tracking system must also resist strong winds. There are two

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- - - - 2-axis tracked -horizontal

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Time 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00

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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.

Table 2.12 Ratio of the Global Irradiation on a Tilted Surface to a Horizontal Surface in Berlin and Cairo Calculated Using the Perez Diffuse Irradiance Model

Berlin Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec Average

Table 2.12 Ratio of the Global Irradiation on a Tilted Surface to a Horizontal Surface in Berlin and Cairo Calculated Using the Perez Diffuse Irradiance Model

Berlin Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec Average

Horizontal

1.00 1.00 1

.00 1.00 1.00 1.00 1.00 1.00 1.00 1

.00 1.00 1.00

1

.00

10°

South

1.33 1.23 1

.13 1.08 1.04 1.03 1.03 1.07 1.12 1

.17 1.30 1.36

1

.09

30°

South

1.90 1.61 1

.32 1.17 1.06 1.03 1.04 1.13 1.28 1

.43 1.81 1.99

1

.19

60°

South

2.39 1.88 1

.38 1.11 0.93 0.87 0.90 1.03 1.29 1

.57 2.23 2.54

1

.15

90°

South

2.34 1.73 1

.15 0.83 0.63 0.58 0.60 0.74 1.04 1

.38 2.14 2.51

0

.89

45°

E/W

0.98 0.99 0

.98 0.92 0.91 0.91 0.90 0.90 0.94 0.95 1.02 1.03

0.93

Cairo Jan Feb Mar Apr MayJune July Aug Sep Oct Nov Dec Average

Horizontal

1

.00 1

.00 1

.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.00

10° South

1

.19 1

.14 1

.08 1.04 1.00 0.98 0.99 1.02 1.06 1.12 1.16 1.20

1.06

30° South

1

.47 1

.33 1

.17 1.04 0.93 0.88 0.90 0.98 1.11 1.26 1.40 1.51

1.10

60° South

1

.61 1

.36 1

.08 0.85 0.67 0.59 0.62 0.74 0.96 1.23 1.49 1.68

0.96

90° South

1

.37 1

.08 0.75 0.49 0.31 0.25 0.26 0.37 0.61 0.93 1.24 1.45

0.63

45° E/W

0

.89 0

.89 0

.88 0.86 0.86 0.85 0.84 0.85 0.86 0.88 0.88 0.90

0.87

However, near the equator an almost horizontal plane receives the highest annual irradiation. Table 2.12 shows the energy production over the year for some surface orientations in Berlin and Cairo.

Solar energy systems are often installed on pitched roofs, which are usually not at the optimum tilt angle. A badly oriented roof can reduce the energy production significantly; however, the acceptable orientations for the roof have a rather large tolerance, as shown in Figure 2.16 and Figure 2.17 for Berlin and Cairo.

North Easl

600 700 800 900 >950

Figure 2.16 Annual Irradiation on Various Inclined Surfaces in Berlin

Figure 2.16 Annual Irradiation on Various Inclined Surfaces in Berlin

Irradiance Berlin

Figure 2.17 Annual Irradiation on Various Inclined Surfaces in Cairo

Figure 2.17 Annual Irradiation on Various Inclined Surfaces in Cairo

In the southern hemisphere, the optimum tilt angles are similar, with the optimum surface azimuth facing the equator, i.e. north. Close to the equator, very flat tilt angles are ideal because the sun is in the zenith for long periods of time. Therefore, the irradiation losses for vertical surfaces are much higher at low latitudes.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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