Recycling of wood waste as sustainable industrial resources—Design of energy saving wood-based board for floor heating systems

doi:10.1016/j.energy.2006.02.001

Energy

Volume 31, Issue 13, October 2006, Pages 2341-2349

https://doi.org/10.1016/j.energy.2006.02.001 Get rights and content

Abstract

Recycled wood has two important functions for sustainable development, as an industrial resource and as a means of longer storage of fixed carbon from carbon dioxide by photosynthesis. Recycled wood-based board, such as medium density fiberboard (MDF), is treated to produce a base material of floor heating system. The concept of functionally graded material (FGM) is introduced for the design of energy saving wood-based board. The heat transfer phenomenon of an FGM base plate used in a floor heating system is modeled and analyzed. The basic equation of the unsteady two-dimensional heat transfer problem and the analytical method for an FGM plate with wood material properties are proposed. The analytical method consists of the Laplace transformation and the perturbation method for arbitrary grading patterns. The results of the numerical calculation suggest that the optimum FGM wood-based board can save more energy than the homogeneous material.

Introduction

Forests have two important roles from the viewpoint of sustainable development; the first is fixing carbon from carbon dioxide in the air through photosynthesis, combating global warming; the second is providing a sustained supply of wood as a renewable industrial and energy resource, different from finite resources such as metals, coal and oil.

Although wood does not have ability of photosynthesis any more different from living trees, it also has another function as the storage of the fixed carbon, which had fixed during its living days through photosynthesis, in addition to the function as a renewable industrial and energy resource. This means that long-term use of wood continues the storage of the fixed carbon. The five-story pagoda at the Horyu-temple in Japan is a good example; the wooden main pole felled in 594 A.D. has stored fixed carbon for over 1400 yr until now [1]. The term of storing is about four times the 354 yr spent as a living tree. Wooden coffins of the mummy in ancient Egypt are another good example. Some of them have stored fixed carbon for about 5000 yr [2].

In spite of such advantages of wood for sustainable development, problems with wood resources, such as the increase in non-thinned forest and the amount of wood building waste, have been observed in Japan. Non-thinned forest does not produce good quality wood and its capacity for carbon fixing is decreasing. Few application of thinned wood is a reason of it. We have evaluated goodness of wood to increase its use [3].

On the other hand, the amount of wood-building waste generated in 2000 in Japan was approximately 5 million tons [4]. The wood waste has been disposed off in landfills or by incineration. Currently, the recycling of wood waste is strongly encouraged because of a lack of landfill space and a prohibition on waste incineration to eliminate dioxin emissions.

The recycling of wood-based materials effectively addresses problems of waste treatment and lack of resources. The recycling of wood waste provides longer storage of fixed carbon, preventing the atmospheric releases that would contribute to global warming. Wood-based boards such as particleboard (PB) and medium density fiberboard (MDF) are prospective recycled wood-based products. These boards have a wide range of density, controlled by compressing pressure. The grading density inside of the boards provides grading thermal conductivity because the thermal conductivity of natural wood is in proportion to its density. The wood-based boards provide better insulation or higher energy efficiency for floor heating systems.

In the current paper, we address the heat transfer problem of a functionally graded material (FGM) with wood material properties to design an optimized energy saving wood-based board for use in a floor heating system. This reuse would be effective against deforestation, would provide a solution to the wood waste treatment problem, dioxin problem, and would create energy and resource savings, encouraging sustainable development.

Section snippets

Floor heating system

The floor heating system shown in Fig. 1 consists of hot-water pipes, a wooden base material, an aluminum plate and a wooden facing plate. The bottom of the base material is heated partially by the hot-water pipe. The heat flows into the top of the base material, which is in contact with the aluminum plate. Although the aluminum plate is also heated partially, the high thermal conductivity of aluminum makes the temperature uniform. The facing plate is also heated uniformly.

When the heating

Material properties and calculation conditions

Fig. 3 shows the thermal conductivity for wood-based board, natural wood and plywood, measured using the hot wire method. λ_v is the thermal conductivity in the vertical direction of wood fiber. λ_vf contains both components, with the fabric direction and perpendicular to it. Although natural wood and plywood show anisotropic thermal conductivity, the wood-based board shows isotropic thermal conductivity. Thermal conductivity of wood and wood-based board is in proportion to their density. The

Concluding remarks

Lengthening the lifespan of wooden materials by recycling of wood waste is an important approach to combating global warming, because the longer use of wooden materials means longer storage of carbon fixed from carbon dioxide through photosynthesis. The application of recycled mean density fiberboard (MDF) from the wood waste to the base material for the floor heating system is evaluated and the concept of functionally graded material (FGM) is considered. The basic equation and the analysis

References (6)

Y. Obata et al.
Research on woods as sustainable industrial resources: evaluation of tactile warmth for woods and other materials
Energy
(2005)
Article of Mainichi Newspaper. Main pole was cut down in A.D. 594. Look over the argument on rebuilding of Horyu...
Wooden coffin yields ancient mummy,...

There are more references available in the full text version of this article.

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