A. Clemente, T. Grassie, D. Henderson and J. Kubie

Napier University, 190 Colinton Road, Edinburgh, EH10 5DT Scotland, UK

Abstract

A novel solar dryer for drying woodchip has been developed in Scotland. In this paper, designs and performance of both solar collector and dryer have been presented separately. Woodchip drying performance has been analysed for a range of temperatures (10°C to 51°C) and flow rates (70m3/h to 280m3/h). Page model has been used for modelling the drying curves as a function of temperatures and drying velocities.

The thermal solar system considered consisted of a solar collector based on the transpired plate type and a small 10We PV panel unit employed to run a 5We fan. The performance of the system is presented in terms of air flow rate and temperature increments as a function of irradiance levels.

Keywords: Solar air heating, solar dryer, woodchip

1. Introduction

Woodfuel is a clean energy resource that reduces the dependency on imports of fossil fuels and contributes to the reduction of CO2 emissions that cause climate change. The main production of woodfuel in Scotland comes from forestry and timber industries. Changes in the energy policy and high production of forestry mass give a significant role to the wood fuels in the Scottish heat power market. [1]. Researchers have predicted that wood fuel production will be equivalent to 4.5 TWh or 11 % of the heat demand in Scotland [2].

Woodchip for burning is a bulky fuel characterized by the size and the shape of the chip and its heating value, highly dependant on the moisture content, MC. The percentage of water in a fresh cut Sitka Spruce wood sample can be up to 65 % MC on wet basis. Thus removing water from the woodchip is a necessary step in the wood fuel chain supply in order to improve the quality of the product: reduce storage and haulage costs and enhance the burning performance [3]. Drying wood requires time and energy. As an alternative to natural drying or fuel heated dryers, solar thermal systems can be used as a cheap and sustainable method to reduce the drying times, suitable for small scale producers [4].

Scotland is located at high latitudes (between 50°N and 60°N) and it has a moderate maritime climate. Despite the low average temperatures, there is a long period of daylight during the spring and summer time that makes solar energy an important power resource for preheating air applications. Previous works on solar ventilation have been accomplished in Scotland as a solar slate system by Odeh [5] and solar heater for pebble bed stores by Grassie [6].

A novel solar dryer has been designed in order to assess the capacity of drying woodchip using exclusively solar energy. The solar thermal system consists of a solar collector that increases the

temperature of air that has been delivered by a fan connected to a PV-panel. This warm air passes through the wet wood chip located on a tray.

The design and operation of the present system is considered in respect of drying woodchip in a small scale. Although woodchip is commonly dried in high volume rates in forestry factories, the decentralization of wood fuel production in Scotland leads to its use in medium and small size installations where users look for minimizing production costs [2] Woodchip usually is stored outdoors drying in natural air ventilation. So a solar thermal system can be used as a backup to reduce the drying times for a small woodchip production.

For effective woodchip drying it is necessary to supply the maximum flow rate at higher temperatures. The solar thermal system when run at high flow rates yields lower flow temperatures and vice-verse. The optimum system design and operation are a compromise between the performance of the woodchip dryer and the solar collector.

The solar dryer tests were taken in the workshops of Napier University in Edinburgh. The project consisted of two independents parts that were studied separately: the dryer and solar collector. The dryer was designed and built on basis of the outlet flow from a solar collector described in the paper. After the study of the woodchip drying performance, the dryer was connected to the solar collector for the study of the solar collector and solar dryer.