. Discussion and Conclusions
Figure 2 and 3 show that the amount of emitted CO varies significantly for the different systems. There are several reasons for these differences. Each pellet heater has its own CO-characteristic for
the operation with a particular combustion power and during start and stop (Figure 1). The pellet heaters vary also in their nominal power from 6 kW to 20 kW which of course together with the size of the buffer volume, the boiler control mode and the way the heat is transferred to the building influence the number of starts and stops.
The boiler systems have large start/stop emissions whereas the start/stop emissions for the stove systems are very low. This is due to the much lower total number of starts and stops of the stoves using the complete building as heat storage. This is a simplification which provides that the heat can be freely distributed to the building. A more advanced multi zone building model for simulations of stove systems has been used by Persson. Perssons studies showed that stove systems have similar number of starts and stops as boiler systems if the convective and radiative part of the heat from the stoves can not be freely distributed to the complete building (see table 3.2 in [9]).
In Figure 3 the specific CO-emissions for each system are compared with the CO-limit values for two Eco-labels. All systems are below the rather high limit value of the Standard EN 303-5 of 1314 mg/MJ which is not indicated in this figure. However, it can be seen that only system 2, if on/off controlled, would fulfil the recently proposed limit values for the Svan-mark if the start and stop emissions and realistic conditions are taken into account. None of the stoves and boilers would fulfil the requirements for the Blauer Engel-mark. The dashed area shows the emissions of the stoves and boilers at nominal combustion power. Testing institutes usually use a mixture of the measured CO-emissions at nominal and minimal load to specify CO-emissions of the tested pellet heater. The comparison shows that this leads to a drastic underestimation of the real annual COemissions due to the fact that the start/stop emissions are not included. It is therefore suggested to revise the methods to determine the CO-emissions for pellet stoves and boilers in the current norms
nd eco-labels and to include an estimation of total annual emissions based on the operation of the boiler and the average load.
Combining solar and pellet heating systems can reduce significant CO-emissions compared to operating a single pellet heating system. This combination prevents the summer operation of the pellet heater with low efficiency and high emissions. Simulations for one system have shown that the CO-emissions can be reduced by almost the half compared to a single pellet heating system using the same boiler.
We are grateful to the Nordic Energy Research and the Dalarna University College for their financial support for this work within the REBUS project.
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