Array Design

An array usually includes many individual groups of collectors, called modules, to provide the necessary flow characteristics. To maintain balanced flow, an array or field of collectors should be built from identical modules. Basically, two types of systems can be used: direct return and reverse return. In direct return, shown in Figure 5.22, balancing valves are needed to ensure uniform flow through

Supply manifold

FIGURE 5.23 Reverse-return array piping.

Return manifold the modules. The balancing valves must be connected at the module outlet to provide the flow resistance necessary to ensure filling of all modules on pump start up. Whenever possible, modules must be connected in a reverse-return mode, as shown in Figure 5.23. The reverse return ensures that the array is self-balanced, as all collectors operate with the same pressure drop: i.e., the first collector in the supply manifold is the last in the return manifold, the second on the supply side is the second before last in the return, and so on. With proper design, an array can drain, which is an essential requirement for drain-back and drain-down freeze protection. For this to be possible, piping to and from the collectors must be sloped properly. Typically, piping and collectors must slope to drain with an inclination of 20 mm per linear meter (ASHRAE, 2004).

External and internal manifold collectors have different mounting and plumbing considerations. A module with externally manifolded collectors can be mounted horizontally, as shown in Figure 5.24a. In this case, the lower header must be pitched as shown. The slope of the upper header can be either horizontal or pitched toward the collectors, so it can drain through the collectors.

Arrays with internal manifolds are a little more difficult to design and install. For these collectors to drain, the entire bank must be tilted, as shown in Figure 5.24b. Reverse return always implies an extra pipe run, which is more difficult to drain, so sometimes in this case it is more convenient to use direct return.

Solar collectors should be oriented and sloped properly to maximize their performance. A collector in the Northern Hemisphere should be located to face due south and a collector in the Southern Hemisphere should face due north. The collectors should face as south or as north, depending on the case, as possible, although a deviation of up to 10° is acceptable. For this purpose, the use of a compass is highly recommended.

Return

'-'

_ii 3-5°

k Supply

Î 35

(a) External manifold

Return

(a) External manifold

Supply

Return

(b) Internal manifold FiGuRE 5.24 Mounting for drain-back collector modules.

Return

FIGuRE 5.25 Row-to-row collector shading geometry.

FIGuRE 5.25 Row-to-row collector shading geometry.

The optimum tilt angle for solar collectors depends on the longitude of the site. For maximum performance, the collector surface should be as perpendicular to the sun rays as possible. The optimum tilt can be calculated for each month of the year, but since a fixed inclination is used, an optimum slope throughout the year must be used. Some guidelines are given in Chapter 3, Section 3.1.1.

SHADING

When large collector arrays are mounted on flat roofs or level ground, multiple rows of collectors are usually installed. These multiple rows should be spaced so they do not shade each other at low sun angles. For this purpose, the method presented in Chapter 2, Section 2.2.3, could be used. Figure 5.25 shows the

0

Collector slope angle, p (degrees)

FiGuRE 5.26 Graphic solution of collector row shading.

Collector slope angle, p (degrees)

FiGuRE 5.26 Graphic solution of collector row shading.

shading situation. It can be shown that the ratio of the row spacing to collector height, bla, is given by

A graphical solution of Eq. (5.47) is shown in Figure 5.26, from which the bla ratio can be read directly if the shading angle, 9s, and collector inclination, (3, are known.

Equation (5.47) neglects the thickness of the collector, which is small compared to dimensions a and b. If the piping, however, projects above the collector panels, it must be counted in the collector dimension a. The only unknown in Eq. (5.47) is the shade angle, 9s. To avoid shading completely, this can be found to be the minimum annual noon elevation, which occurs at noon on December 21. However, depending on the site latitude, this angle could produce very large row gaps (distance b), which might be not very practical. In this case, a compromise is usually made to allow some shading during winter months.

THERMAL ExpANSioN

Another important parameter that needs to be considered is thermal expansion, which affects the modules of multi-collector array installations. Thermal expansion considerations deserve special attention in solar systems because of the temperature range within which the systems work. Thermal expansion (or contraction) of a module of collectors in parallel may be estimated by the following (ASHRAE, 2004):

where

A = expansion or contraction of the collector array (mm). n = number of collectors in the array.

tmax = collector stagnation temperature (°C), see Chapter 4, Eq. (4.7). ti = temperature of the collector when installed (°C).

Table 5.5 Galvanic Series of Common Metals and Alloys

Corroded end (anodic)

Magnesium Zinc

Aluminum Carbon steel Brass Tin

Copper Bronze Stainless steel Protected end (cathodic)

Expansion considerations are very important, especially in the case of internal collector manifolds. These collectors should have a floating absorber plate, i.e., the absorber manifold should not be fastened to the collector casing, so it can move freely by a few millimeters within the case.

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Solar Panel Basics

Solar Panel Basics

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