Development of a Dynamic Modelling Technique for Sizing Wood Chip Boilers

Abstract

Scandinavian countries in particular have low winter temperatures and rural areas with low building densities. It is quite common in these situations to build small and medium size district heating networks to provide buildings with heat. These networks incorporate typically one boiler house that takes care of the heat generation. This boiler house is equipped with one or more suitable boilers that may be oil or gas fired or they may consume some solid fuel which is often wood based, mainly wood chips or pellets. This kind of district heating (DH) system is owned by the municipality, energy service company/co-operative or both, In many cases the municipality owns the network but the heat supplier owns the boiler house and other equipment that relate to the heat production.

The purpose of this study was to develop three models that allow more accurate dimensioning of wood chip boilers.

- The first model developed was a steady state one. It relied on average temperature data and the supposition that the need for heat in buildings is almost linearly dependent on the outside temperature.
- The second model was divided in to two sub-models; a dynamic model for an annual heat consumption calculation in a single building and a dynamic model that enables the connection of the consumptions of several buildings of a district heating system to be studied on an annual basis.
- The third model was a short term simulation model that makes it possible to study interactions in the district heating system including the boiler plant, the distribution network and the consumer sub-stations.

According to the study, the steady state model gives satisfactory results, notably if the domestic hot water consumption is estimated to be somewhat more than it evidently is. The steady state model gave smaller percentages of annual heat production for the wood chip boiler if its capacity was less than approximately half of the peak, compared with the dynamic model. As the capacity of the wood chip boiler was more than approximately half of the peak, the steady state model gave bigger percentages for the wood chip boiler than the dynamic model. The turning point seemed to relate to the minimum sustainable output of the wood chip boiler so that as the minimum output came down the turning point went up.

The study gives some general outlines about the dynamics of the system. One of the most useful is the understanding that a peak in the network immediately causes a certain load in the boiler plant, and another load takes place when the cooled water returns from the consumer to the boiler plant. If the consumer is located close to the boiler plant and the peak lasts for a while, these two loads may overlap. By proper sizing of crucial pipelines, the designer may impede the overlapping and size the boiler somewhat smaller.

Date of Award	Nov 2005
Original language	English
Supervisor	John Ward (Supervisor) & Steven Wilcox (Supervisor)