Wooden Mounting System Brings Down Cost of Solar Power Plant

Costs less than steel systems, easier to handle

2014/03/01 18:59
Kenji Kaneko, Nikkei BP CleanTech Institute

The Kashima Plant of Sumitomo Forestry Crest Co Ltd, a Sumitomo Forestry Group company, in Kashima City, Ibaraki Prefecture, is located on the Kashimanada coast, where a steady wind blows throughout the year and large windmills operate in the neighboring areas.

In November 2013, "Sumitomo Forestry Kashima Solar Power Plant" with 876kW output started operation in the site adjoining the plant (Fig. 1). Canadian Solar Inc's solar panel and Toshiba Mitsubishi-Electric Industrial Systems Corp's (TMEIC) PV inverter were adopted (Fig. 2). These products boast large market shares and are often seen at other solar power plants in Japan.

When you look behind the solar panels, however, you will recognize this power plant's unique feature: the mounting systems are assembled with wood. Compared with inorganic steel mounting systems, the natural warmth and presence that the wood surface brings about is impressive.

Sumitomo Forestry Co Ltd is known for its timber houses that primarily use domestic materials. Its "Mocca (Wood Use Integration) Department" is developing wood applications other than houses. "Can you build a wooden mounting system for a solar power generation system?" The Mocca Department was asked by a private entrepreneur during the sharp rise in the number of construction plans for solar power generation facilities that resulted from the implementation of the feed-in tariff (FIT) program.

Simple assembly of square cedar logs

Sumitomo Forestry established the Environmental Energy Group in the Forestry & Environment Division to deal with biomass and solar power generation businesses. At that time, the plan was devised to construct Sumitomo Forestry Kashima Solar Power Plant with 876kW output in the idle site adjacent to the Kashima Plant of Sumitomo Forestry Crest, a group company that manufactures and sells wood housing materials and equipment. In light of this plan, the company hurriedly started considering building part of the power plant using a wooden mounting system.

The mounting system had to be strong enough to withstand typhoons and other strong winds and durable enough to support the solar panels for 20 years, even under such severe weather conditions. In addition, cost becomes a key factor in the effort to get the wooden mounting system construction on track as a business. Business sustainability cannot be secured unless the wood mounting system is manufactured for an equivalent or lower cost compared with iron, aluminum and steel models.

After formulating a design plan and estimating the strength, durability and cost, the Mocca Department reached the conclusion that it could commercialize the wooden system and so made a full-fledged start.

Considering future volume production and cost, Sumitomo Forestry deployed 105 x 105mm kiln-dried (KD) square cedar logs that were generally used as a building material. In addition, the company aimed at a structural design assembled with these standard logs to be as simple as possible.

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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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The West Holdings group, which provided engineering, procurement and construction (EPC) service for the overall solar power generation system, built the steel mounting systems. But the installation cost per 1kW was lower with the wooden mounting system that Sumitomo Forestry assembled.

Moreover, many workers claimed "the installation work was easier with the wooden system because its materials were lighter than expected." As a result, it turned out that the workers feel less burdened when setting up the wooden mounting systems compared with the steel ones.

As for other benefits of the wooden mounting system, Sumitomo Forestry cited that it is, unlike steel systems, free of decay caused by salt even along the coast, and it can be effectively used as a fuel for biomass power generation when removing it upon the completion of solar power generation business 20 years later. From now, Sumitomo Forestry plans to propose the use of the wooden mounting system across Japan, providing structural calculations in accordance with the location where it will be installed as well as proposals in view of effective use of local materials.

As the job creation effect is negligible at mega (large-scale) solar power plants, it is always a challenge how to contribute to local communities. The wooden mounting system that can utilize local timber is also likely to be focused on from that viewpoint.