Biomass drying increases the heating value of the fuel and can lead to increased use of this renewable energy resource. An efficient approach for such fuel processing is being advanced by researchers from VTT Technical Research Centre of Finland in the form of a hybrid biomass drying system combining a heat pump and solar thermal collectors.

This integrated system offers advantages especially in Nordic regions where solar radiation availability is relatively high only during the summer. By incorporating a heat pump, the feasible annual operating time can be substantially increased while the investment payback time is reduced.

An experimental setup consisting of two freight containers features a 24 kW heat pump, a condenser, an evaporator, a heat recovery unit, a heat exchanger and 12 flat-plate solar heat collector modules in the first container. The second functions as a drying chamber for fresh wood chips. Under hybrid mode operation, closed-loop drying conditions are maintained under which the same air is circulated through the system. If solar radiation is also available, the thermal collectors provide an additional heat input, and in its solar mode, the system only uses collectors to dry the biomass.

Tests conducted in Finland confirmed that hybrid drying results in higher and faster drying rates. The average drying rates were 32.96 kg/h with solar drying. However, as the heat pump was powered from the grid, the decreased drying duration was enabled by increased electricity consumption.

The heat pump required 8.24 kW to dry the wood, while the solar mode required 1.45 kW for the fan used for airflow. Due to the improved drying rate delivered by the system described in Energy Reports, the specific electricity consumption in hybrid operation mode is only 45% higher at 0.26 kWh/kg compared to 0.18 kWh/kg.