Efficient Energy Recovery with Wood Drying Heat Pumps

Abstract

This article presents a 13-m³ 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 completed in order to determine the system's energy performance. Supplementary heating to compensate for the dryer heat losses was supplied using electrical coils or steam exchangers. The heat pump running profiles and dehumidification performance in terms of volumes removed and water extraction rates, coefficients of performance, and specific moisture extraction rates were determined for two all-electrical and two hybrid drying tests. The hardwood drying curves, share of the final moisture content, and final quality of the dried wood stacks, as well as total drying energy consumption and costs, were determined for each drying run. Finally, the total energy consumption of the drying cycles using a heat pump was compared with that of a conventional drying cycle using natural gas as a single energy source.

Keywords:

• Energy analysis
• Heat pump drying
• Wood drying

ACKNOWLEDGMENTS

The author gratefully acknowledges Hydro-Quebec's Energy End-Use Service for its unfailing support in this project. The author also acknowledges Marc Savard and Danny Normand, scientific researchers at FP Innovation, for their outstanding contribution to this study. Finally, the availability of the drying heat pump prototype, its technical improvements, and comprehensive running schedules were made possible with the help of a Canadian heat pump manufacturer.

Notes

All-EL, all-electrical; CONV, conventional; DB, dry bulb temperature; SP, set point temperature; WB, wet bulb temperature.

^a From the beginning of the drying cycle.

^a From the beginning of the preheating step.

MC_fin, final moisture content; MC_max, maximum moisture content.

N/A, not applicable.

B, heat pump blower; E, electricity; NG, natural gas; N/A, not applicable.