The modeling of pellets stoves, burner and boiler has been realized with a new TRNSYS — component type 210 that, unlike other models, takes the dynamic behavior during the start and stop phase into account. Type 210 has been developed for pellet burners and pellet stoves and can be even used for boilers where no internal hot water preparation needs to be modeled. The model is calculating the fuel consumption, combustion air flow and the exhaust gas flow and provides data for the delivered energy to the ambient, the heating circuit connected to the water jacket and to the exhaust gas. The model calculates also the air leakage losses when the burner is not in operation, the number of starts and stops and the consumed electricity.

The model separates the pellet heater into two main thermal masses. Mi represents the part of the stove that transfers the heat to the ambient and m2 representing the water jacket of the stove/boiler. Fuel and combustion air entering the stove, combust and form a combustion gas that transfers heat first heat to m-i and then to m2 before leaving the stove. Heat transfer coefficients define the heat transfer between the hot air mass flow, the thermal masses, the ambient air and the fluid in the water jacket.

For the two stoves and the integrated burner a verification has confirmed the correctness of the identified parameters. Simulation tests with parameter for the pellet boiler showed good energetic accordance to the measured data, but no exact verification has been performed thus this boiler is being considered as a generic boiler.

System 3 uses type 210 as a pellet burner, where all heat except the heat losses from the burner itself is transferred to the flue gas before entering the air to liquid heat exchanger of the combistore. Consequently the total flue gas losses for this system need to be calculated separately using the exhaust gas temperature of the air to liquid heat exchanger outlet.

Qflto, = mfl ■ CPfl ■ (Tohxb — TrooJ (1 )

where m’fl is during the combustion phase calculated by type 210 and after the burner is out of operation by equation 4 using the temperature difference between the gas leaving the heat exchanger in the store and the outdoor temperature. The leakage losses have been determined by:

Qleak = | m fl ‘ Cp fl ‘ (Tihxb Tohxb ) I for Tohxb >Tihxb (2).

A modification has been accomplished for the model of system 3 where the factory settings for the placement of the temperature sensor of the pellet burner have been adapted. The simulations showed that the sensor was placed too far in the top of the store causing a delayed start of the burner and thus giving in the meanwhile the electrical heater the possibility to heat up the store. To prevent the electrical heater turning on during normal operation the sensor was placed lower and also the set temperature of the electrical heater was reduced to 55 °C, the same value as for system 4. Moreover the maximum heating power has been simulated with the lower summer adjustment (12 kW).

In the model of system 4 the control settings for the boiler pump have been changed so that almost all the heat of the boilers water volume is transferred to the combistore once it is heated.

For the simulation a one zone building model has been used, which implies that the results for system 1 are only relevant if good heat distribution can be achieved from the stove to the whole building. The losses from the store and, in the case of systems 3 and 4, also from the boiler, are not used as heat input to the building model. The domestic hot water load has been modeled with the load profile developed by Jordan and Vajen (2002) assuming a daily hot water demand of 200 liter. All four DHW-stores and combistores have been model with TRNSYS type 140 (Druck and Pauschinger, 1996).