EuroSun2008-2

REDUCTION OF CO-EMISSIONS BY COMBINING PELLET. AND SOLAR HEATING SYSTEMS

F. Fiedler1*, T. Persson1 and C. Bales1

1 Solar Energy Research Center SERC, Hogskolan Dalama, S-78188 Borlange, Sweden
* Corresponding Author, ffi@du. se

Abstract

Emissions are an important aspect of a pellet heating system. High carbon monoxide emissions are often caused by unnecessary cycling of the burner when the burner is operated below the lowest combustion power. Combining pellet heating systems with a solar heating system can significantly reduce cycling of the pellet heater and avoid the inefficient summer operation of the pellet heater. The aim of this paper was to study CO-emissions of the different types of systems and to compare the yearly CO-emissions obtained from simulations with the yearly CO-emissions calculated based on the values that are obtained by the standard test methods. The results showed that the yearly CO-emissions obtained from the simulations are significant higher than the yearly CO-emissions calculated based on the standard test methods. It is also shown that for the studied systems the average emissions under these realistic annual conditions were greater than the limit values of two Eco-labels. Furthermore it could be seen that is possible to almost halve the CO-emission if the pellet heater is combined with a solar heating system.

Keywords: Carbon monoxide emissions, Pellet and solar heating systems

1. Introduction

The dramatically increased prices for oil and electricity over the last few years encourage many house owners with electric heating or and oil heating systems to convert their heating systems. Today in Sweden mainly heat pumps are installed but also pellet heating systems become more and more popular. Studies have shown that the combination of conventional boiler heating systems with solar heating is beneficial in terms of fuel savings and lower emissions since the boiler usually in the summer can be turned off when it’s efficiency is low [11; 14].

Emissions of harmful gases are important parameters in addition to the efficiency and the thermal performance of pellet heating units. The national building codes and emission regulations include limits of allowable emissions of noxious gases for wood pellet boilers. More stringent limit values are applied by the Swedish Testing Institute (SP) and eco-labels such as the Svanmark [7; 13]. The limit values can be expected to further sharpened when comparing the limit values from other European regulations and eco-labels [1; 12]. More stringent limit values have also been proposed by the Nordic eco-label Svanmark [6]. In Table 1 the official limit values for emissions and efficiencies for pellet boilers and pellet stoves are compared with the current limit

In this study the emphasis has been on CO-emissions released from different pellet heating systems with different operating strategies combined or not combined with a solar heating system. CO­emissions from pellet stoves/boilers are highest during the start and stop phase. By operating the burner with modulating combustion power the number of starts and stops and consequently the start/stop CO-emissions can be reduced. On the other hand, the longer operation time leads to higher total CO-emissions during normal combustion. Both these effects are simulated in this study, and results are given for complete annual simulations with sub-hourly time step.

2. Method

This work compares and analyses the simulation results of six combined solar and pellet heating systems that have been chosen from a variety of design variants. Four of them were chosen to represent the range of commercially available solutions found in Sweden. The systems contain: a water mantled stove; an air cooled pellet stove; a store integrated pellet burner; and a standalone pellet boiler. The fifth system is similar to the system with the standalone boiler but uses a boiler with an adequate size of 12 kW. The sixth system is based on a completely new system concept using a very efficient Austrian pellet boiler. The pellet heating units in these systems had been previously tested at the Solar Energy Research Center, Borlange [10]. A detailed description of the systems can be found in [3] and [11].

The systems were modelled in the simulation environment IISiBat/TRNSYS [5]. The systems have been simulated for one year for the same boundary conditions. Particularly design parameters such as the boiler combustion control have been varied to study the effect on the CO-emissions of the systems. For comparison one system has also been simulated with only the boiler as main heat source and without solar heating system.