ENERGETIC AND ECOLOGICAL ASPECTS OF WOOD DRYING WITH HEAT PUMPS

Abstract

About 45% of the Canadian territory is woodland, and the wood industry accounts for 3% of the country’s Gross Domestic Product, 10% of total foreign exchange, and 57% of the annual trade surplus. Canadian sawmills usually deliver 72 million m of softwood. About 85% of this production (white and black spruce, jack pine and balsam fir) is dried with bark boilers, 10% with fossil fuels (natural gas, propane or oil), and only 3% with low temperature heat pumps. Despite numerous attempts, the low temperature heat pump technology has not succeeded in securing a significant market share, the maximum operating temperature (50 – 55oC) being one of the reasons. However, in order to provide shorter drying times, softwood producers opt for drying at high temperatures (80 – 120oC). Advanced hybrid (electricity and fossil fuel) wood drying systems with high temperature heat pumps have been developed and field tested in the United States and Canada over the last six years. Several developments were achieved at the refrigerant flow control, dehumidifying rate and optimum drying schedule levels. This paper briefly presents energy calculations and overall operating parameters and performance, as well as the ecological aspects of high-temperature drying heat pumps. The study demonstrates that the thermodynamic efficiency and energetic performance of improved drying heat pumps reached designed targets, and that ecological impacts have to be carefully considered in the future.