EuroSun2008-2

Modelling in TRNSYS

The modelling of the systems in TRNSYS is based on the system models and boundary conditions used in IEA-SHC Task 26 Solar Combisystems [2], which includes both the building (single node in type 56) and heat distribution using a radiator and PID controller modelling the thermostatic valve. For the system with air cooled pellet stove, no radiator was used. The boundary conditions for the systems are defined by the climate, in this study Stockholm, the domestic hot water (DHW)

load and the space heating demand. The DHW load has been modelled with a load profile developed by Jordan et al. [4] assuming a daily hot water demand of about 200 litre (~3100 kWh/year). The space heating demand is modelled by an one zone building model developed for IEA-SHC task 26 giving a yearly heat demand of approximately 12200 kWh (87 kWh/m2) for Stockholm.

Modelling of pellet stoves, burner and boiler were implemented with TRNSYS-component type 210 [8]. This dynamic model can be used to simulate pellet stoves, pellet burners and 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 model also calculates the CO-content in the flue gas, including the emissions during the start and stop phases. The parameter values used in this study were derived from parameter identification using measured data from the stoves/boilers, and have been verified against measured data [3; 10]. The parameter values for each of the pellet heaters used to simulate the CO-emissions of the pellet heaters can be seen in Figure 1. The model calculates the CO-emissions as the sum of a power dependent part during normal operation and a lumped constant amount per start and stop.

2 4 6 8 10 12 14 16 18 20

Combustion power [kW]

Fig. 1. CO-emissions during operation (graph) and start/stop (table) of the six pellet heating units.

Two variants of operating strategy were chosen for simulations of each system. On/off control using the full power of the heaters and modulation control was used with the measured modulation range for the specific heaters simulated in the systems. For comparison, system 5 has also been simulated with only the boiler or stove as main heat source and without solar heating system (solar collector loop and combistore).

3. Results

Figure 2 shows the CO-emissions for the six systems in kg divided in start/stop emissions, emissions during operation and standby emissions. The latter occur only for the boiler in system 4 which has an option to operate in a standby mode when there is no heat demand. Keeping the

boiler in this standby mode (by constantly combusting a little amount of pellet) increases the CO­emissions dramatically. The assumption here is that the start emissions are the same as if the boiler would not kept in standby. This has not been investigated in detail and the standby operation has not been included in the system simulations. Instead, the standby emissions in Figure 2 have been determined by separate calculations based on measurement of the boiler during standby operation.

From Figure 2 it can bee seen that the amount of emitted CO varies significantly for the different systems. The boiler systems have large start/stop emissions whereas the start/stop emissions for the stove systems are very low. The pellet stove in system 2 emits with 7 kg in on/off mode the lowest amount of CO per year whereas the boiler in system 5 emits 37 kg CO per year if on/off operated. The stove systems (system 1 and 2) emit most CO during operation whereas the combisystems (system 3-6) emit most CO during start and stop when on/off operated. For system 3, 4, 5 and 6 the start/stop emissions decreases drastically if controlled with modulating power. The CO-emissions of system 2 are much higher when operated with modulating power. The CO-emissions for system 1 are almost the same regardless if the stove is operated with on/off or modulating combustion power.

The pellet consumption is not the same for all systems. For a qualitative CO-emission comparison of the different systems it is therefore necessary to express the CO-emissions in a specific form, in kg per MJ pellet (Figure 3).

Together with the specific emissions of each system the limit values for CO from two eco-labels are indicated. The relative high limit value of the Standard EN 303-5 of 1314 mg/MJ is not indicated. 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 from lab measurements at constant nominal combustion power. These are much lower than the average annual emissions except for the stove in system 1 that has very little start and stop emissions. Note that for system 4 only the emissions for start/stop and normal operation are included but not the emissions for standby. These emissions have been excluded because no measurement data for the pellet consumption during standby were available.

In Figure 4 the annual CO-emissions of the pellet boiler used in system 5, with and without solar heating system, and the CO-emission of system 6 (with a solar heating system) are compared. It can be seen that the CO-emissions of system 5 can be reduced by almost the half by adding a solar system. This is mainly due to the reduction of the number of starts and stops from 3352 (on/off controlled) and 1601 (modulating power) to 1758 (on/off controlled) and 675 (modulating power). For system 6, that uses an Austrian pellet boiler with relatively low start/stop emissions, the annual CO-emissions would be only a third of the boiler used in system 5.