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The emphasis of this report is on the actual technology of small-scale pellet combustion units and important regulations concerning emissions and pellets. Wood as a heating source has a long tradition in Sweden, but the use of compressed wood pellets in domestic stoves and boilers is rather new. Based on a literature survey, information from manufacturers and test institutes, this report gives an overview about existing technology and investigates how mature it is already. Some comparisons were made to similar heating units in Austria, where this technique is also widely used. It could be seen that the Austrian boilers are more sophisticated including a high level of comfort for the user. On the other hand the simpler Swedish boilers are significantly less expensive, and it is questionable if Swedish costumers are willing to pay for the higher comfort. Relevant regulations concerning emissions and fuel quality are also reported and compared between Sweden, Germany and Austria. In addition some interesting non-official certificates and environmental labels are presented. These give an outlook to future, probably more stringent, regulations.

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 often lower efficiency of pellet heaters compared to oil or gas heaters increases the final energy demand. Consequently, heat losses of the various systems have been studied. The systems have been modeled 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 I 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 placed inside or outside of the heated area. Unlike the expectations and results from other studies, the operation of the pellet heaters with modulating combustion power is not necessarily improving the performance. A large potential for system optimisation exists for all studied systems, which when applied could alter the relative merits of the different system types.

Emission and electricity consumption are important aspects of a pellet heating system. Low noxious emissions, particularly carbon monoxide, are a measure of a well-performing system. High carbon monoxide emissions are often caused by unnecessary cycling of the burner, poor adjustment of the combustion air and insufficient maintenance. The carbon monoxide output, the thermal performance and the electricityconsumption for modulating and non-modulating operation mode have been investigated by simulations of four stoves/boilers as part of combined solar and pellet heating systems. The systems have been modelled with the simulation programme TRNSYS and simulated with the boundary conditions for space heating demand, hot water load and climate data as used in earlier research projects. The results from the simulations show that operating the pellet units with modulating combustion power reduces the number of starts and stops but does not necessarily reduce the carbonmonoxide output. Whether the carbon monoxide output can be reduced or not depends very strongly on the reduction of starts and stops and how much the carbon monoxide emissions increase with decreased combustion power, which are in turn dependent on the particular settings of eachpellet burner and how the heat is transferred to the building. However, for most systems the modulating operation mode has a positive impact oncarbon monoxide emissions. Considering the total auxiliary energy demand, including the electricity demand of the pellet units, the modulatingcombustion control is advantageous for systems 1 and 4 for the used boundary conditions. The study also shows that an appropriate sizing of the stove or boiler has a huge potential for energy saving and carbon monoxide emission reduction.

n this study, an optimisation method for the design of combined solar and pellet heating systems is presented and evaluated. The paper describes the steps of the method by applying it to an example system. The objective of the optimisation was to find the design parameters that give the lowest auxiliary energy (pellet fuel + auxiliary electricity) and carbon monoxide (CO) emissions for a system with a typical load: a single family house in Sweden. Weighting factors have been used for the auxiliary energy use and CO emissions to give a combined objective function. Different weighting factors were tested. The results show that extreme weighting factors lead to their own minima. However, it was possible to find factors that ensure low values for both auxiliary energy and CO emissions, and suitable weighting factors are suggested.

At the beginning of 2003 the four year long research project REBUS on education, research, development and demonstration of competitive solar combisystems was launched. Research groups in Norway, Denmark, Sweden and Latvia are working together with partners from industry on innovative solutions for solar heating in the Nordic countries. Existing system concepts have been analyzed and based on the results new system designs have been developed. The proposed solutions have to fulfill country specific technical, sociological and cost requirements. Due to the similar demands on the systems in Denmark and Sweden it has been decided to develop a common system concept for both countries, which increases the market potential for the manufacturer. The focus of the development is on systems for the large number of rather well insulated existing single family houses. In close collaboration with the industrial partners a system concept has been developed that is characterized by its high compactness and flexibility. It allows the use of different types of boilers, heating distribution systems and a variable store and collector size. Two prototypes have been built, one for the Danish market with a gas boiler, and one for the Swedish market with a pellet boiler as auxiliary heater. After intensive testing and eventual further improvements at least two systems will be installed and monitored in demonstration houses. The systems have been modeled in TRNSYS and the simulation results will be used to further improve the system and evaluate the system performance.