Frank Fiedler, Svante Nordlander, Tomas Persson, Chris Bales
Solar Energy Research Center SERC, Dept. of Mathematics, Natural Sciences
and Technology, Dalarna University College, S-7188 Borlange,

Phone: +46 (0) 23 77 87 11, Fax +46 (0) 23 77 87 01, ffi@du. se

Abstract — Various pellet heating systems are marketed in Sweden, some of them in combination with a solar heating system. Several types of pellet heating units are available and can be used for a combined system. This article compares four typical combined solar and pellet heating systems: System 1 and 2 with a pellet stove, system 3 with a store integrated pellet burner and system 4 with a pellet boiler. The lower efficiency of pellet heaters compared to oil or gas heaters increases the primary energy demand. Consequently heat losses of the various systems have been studied. The systems have been modelled in TRNSYS and simulated with parameters identified from measurements. For almost all systems the flue gas losses are the main heat losses except for system 3 where store heat losses prevail. Relevant are also the heat losses of the burner and the boiler to the ambient. Significant leakage losses are noticed for system 3 and 4. For buildings with an open internal design system 1 is the most efficient solution. Other buildings should preferably apply system 2 or 3. The right choice of the system depends also on whether the heater is place inside or outside of the heated area. A large potential for system optimisation exist for all studied systems, which when applied could alter the relative merits of the different system types.

KEYWORDS: Pellet heating systems, heat losses, flue gas losses, leakage losses

1. Introduction

A variety of system concepts for solar heating systems for new one — and two-family houses can be found on the European market. In Sweden electrical heaters are often used as the auxiliary heat source but the use of wood pellets in pellet stoves and pellet boilers is becoming more and more popular. Sweden is already today the most developed European market for pellet heating systems for one-and two-family houses. Approximately 30000 pellet heating units have been installed by the end of 2001 (Hadders, 2002). There are several Swedish manufacturers active on the market but also a number of manufacturers from other countries. For this study two Swedish, one Finnish and one German product, all marketed in Sweden, have been investigated.

The efficiency of pellet heaters varies significantly between different manufactures and designs and is generally lower than for comparable gas or oil boilers (Fiedler, 2004). For this reason the heat losses from these units are important parameters to evaluate the complete heating system.

2. Method

The systems investigated in this work are considered to represent typical solutions within a wide range of design variants. The systems are for the most part taken as they can be found on the market whereas system 4 is somewhat an exception. This system is
not available as a complete system but at least the pellet boiler and the store are standard products.

This study uses the results of comprehensive measurements on two pellet stoves, one pellet boiler and a storage integrated pellet burner at the Solar Energy Research Center SERC. Several tests have been performed to evaluate the performance of these units and to acquire data enabling an identification of characteristic parameters (Nordlander and Persson, 2003). The investigated system designs have been modeled in the simulation environment IISiBat/TRNSYS (Klein et al., 2002) based on a simulation model created for IEA task 26 Solar Combisystems (Bales, 2003). The same house model (Streicher and Heimrath, 2003) and other standard boundary conditions have been applied.

For the modeling of the pellet heating units TRNSYS type 210 that has been recently developed by Nordlander (2003) has been used. This dynamic model can be used to simulate pellet stoves, pellet burners and to a certain extent also pellet boilers and gives flue gas losses during operation and in standby mode (leakage losses), as well as heat supplied to water in a mantle and to the surroundings. The applied parameters are obtained from the parameter identification and have been validated by simulations comparing simulated and measured data. The models for the pellet heating units have been integrated into the system models and yearly simulations have been performed for a building with an annual heat demand for the Stockholm climate of approximately 12200 kWh (87 kWh/m2) and a hot water demand of 3100 kWh. The results from these simulations have been analyzed and used to compare the four systems.