(94c) Wood Pellet Boiler Heating Systems Evaluation and Optimization

The use of wood pellet boilers for residential space heating has significantly increased over the past decade. Wood pellets are a biomass-based renewable energy made by pelletizing debarked wood fiber. Compared to log wood and wood chips, wood pellets have higher energy density, relatively uniform fuel quality, easier to automate their use, and therefore, are being more widely used.

A typical wood pellet heating system has three components: the boiler (energy generation unit), the thermal energy storage (TES) tank (energy storage unit), and the building (energy consumption unit). The three components form into two loops: the boiler to TES tank loop and the TES tank to building loop.

Three modern wood pellet boilers were installed and monitored in this research. Two 25 kW boilers (PB and WPB) were installed in the end of 2014 and a 50 kW boiler (LGB) was installed in March of 2016. PB is used solely for radiant floor heating and WPB is used for traditional baseboard/cast iron radiators space heating as well as providing domestic hot water (DHW) supply. LGB boiler is used for concrete slab heating. The three boiler installations provide different methods of heating and different levels of building heat demand.

This research first evaluated the performance of residential scale wood pellet boilers (25 – 50 kW) in terms of boiler operation characteristics, thermal efficiency, boiler emissions, TES tank stratification and discharge efficiency, etc. Comparisons were also made among different boilers with suggestions for improvements were made.

A process dynamic simulation using VMGSim was built up based on the field monitoring data. Good agreement between the simulation and field data was found. The model was then used to size the TES tank with respect to different boiler capacities and heating demands. The results will be used to compose a guidance document for wood pellet boiler heating systems.