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Wooden Mounting System Brings Down Cost of Solar Power Plant (page 2)

Costs less than steel systems, easier to handle

2014/03/01 18:59
Kenji Kaneko, Nikkei BP CleanTech Institute
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First, these logs are formed into a square with a length of about 4m. After completion, eight solar panels are mounted on this wooden frame. The frame is tilted at 20° and installed on concrete foundations. On the higher side, other logs are inserted vertically and obliquely (Fig. 3). The lower side is supported with iron poles that can adjust the space between the foundation and the log (Fig. 4). One unit (base) of the mounting system is formed this way, and 15 units are aligned sideways in a row.

Three types of chemicals prevent decay, termites

What is a serious concern when using wooden mounting systems is how to prevent them from decaying and termite attacks for 20 years while continuing the power generation business. Wooden buildings face the same risk, but wooden pillars are protected, for example, by exterior walls. In the case of wooden mounting systems that are continually directly exposed to wind and rain for 20 years, stronger measures against decay and termites are needed compared with general construction materials.

After consulting businesses engaged in decay- and termite-proof treatments, Sumitomo Forestry found out that sufficient measures against decay and termites can be achieved by pressure-injecting chemicals.

Sumitomo Forestry Kashima Solar Power Plant used three different kinds of decay/termite-preventive chemicals partly because this was Sumitomo Forestry's first wooden mounting system. They were boron compound-based borate salt (Fig. 5), copper compound-based Mitrex ACQ (Fig. 6) and CUAZ (Fig. 7). These chemicals are all widely used in parks, etc. In addition, Sumitomo Forestry used Hiba cedar, which is said to be highly resistant to pest damage and decay, and set up mounting systems that use non-treated square logs for empirical reasons (Fig. 8).

Ease of construction is an important measure to cut construction costs. The square logs used to build the mounting system are all pre-cut and treated in advance with decay/termite-preventive agents at an affiliated factory before being delivered to the construction site of a power plant and assembled by workers with connecting clamps and screws. No saws are used on-site. Basically, six screws are used to fix the joints (Fig. 9).

A special clamp, which uses two screws, was designed to hold the solar panels and square timber (Fig. 10 & 11). The length of the screw is about 5cm. They are screwed consecutively using electric screwdrivers. The workers who assembled the wooden mounting systems were not experts like carpenters. Most of them had never even done such connecting work before. Even so, as they got used to the work, they were able to assemble 15 units per day in a team of four.

Installation per 1kW costs less than with steel systems

The weight of the square timber is 140kg per unit. Given the larger size (volume) in comparison with steel mounting systems, the wooden system seems heavy, but it is actually lighter than steel systems. Of the panels used to realize Sumitomo Forestry Kashima Solar Power Plant's 876kW total output, the wooden mounting system was only used for about 30%, with the remaining panels set up on steel mounting systems.

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