EFFICIENT ENERGY RECOVERY WITH WOOD DRYING HEAT PUMPS

This paper presents a 13-m(3) wood dryer coupled with a 5.6 kW (compressor power input) heat pump. Drying tests with hardwood species such as yellow birch and hard maple were achieved in order to determine the system energy performances. Supplementary heating for compensating the dryer heat losses was supplied by electrical or natural gas heaters. The moisture extraction rates are provided as functions of wood initial moisture contents above and below the fibre saturation point (FSP). The electrical energy consumption of the heat pump (compressor and blower) ranged between 25% and 30% of the drying system energy consumption. The dryer central fan assumed around 8.5%, and the back-up energy about 61% of the system energy consumption. For initial moisture contents above 41%, the water quantities extracted above the fiber saturation point were 2.9 times higher than those removed below this point. Consequently, the dehumidification efficiency of the heat pump was up to 3 times higher above the FSP than it was below it. Finally, this study shows that hardwood drying with heat pump may reduce the total energy consumption by up to 56% compared to conventional drying cycles.