To estimate the required storage capacity of wood chips or wood pellets for a ´domestic´ sized boiler, follow this simple rule of thumb. It assumes a 8 week supply§, equivalent to 4 weeks at full operation and 4 weeks at 50% load.
- 8 hours operation x 5 days a week x 4 weeks x peak load (of boiler) x boiler efficiency = Energy used (kWh)
- -- plus --
- 8 hours operation x 5 days a week x 4 weeks x half load (of boiler) x boiler efficiency = Energy used (kWh)
- -- adjust the hours and days according to use --
- next divide the energy used by the heat content (CV) of the wood per kg taking account of the boiler efficiency
- so, energy used in period kWh /CV (kWh/kg) x boiler efficiency (say 0.9) = wood mass volume (kg)
- divide this figure by the bulk volume to get storage volume
- wood mass volume (kg)/ bulk volume (kg/m3) = minimum storage volume (m3)
§Note a 8 week supply with 3 winter fills and 1 summer fill should cover the whole heating season - 32 weeks supply which is approximately equivalent to the annual heating period
Example Wood Store Size
The storage for a 60kW boiler using pellets would be as follows:
- (8 x 5 x 4 x 60 x 0.9)+ (8 x 5 x 4 x 30 x 0.9) = 8,640 + 4,320 = 12,960 kWh
- so, 12,960 / 4.4 = 2,945 kg of pellets
- therefore the minimum volume for pellet storage is 2,945 / 650 = 4.5 m3
- volume for wood chip storage is(12,960 / 3.1) / 230 = 18.1 m3
Where wood has the following properties:
| CV | Bulk Density | |
| Wood Chip | 3.1 kWh/kg | 230 kg/m3 |
| Pellets | 4.4 kWh/kg | 650 kg/m3 |
The storage volume should be at least twice the delivery volume.
