Mathematical models for single components to be simulated

Most of the components to be simulated are already included in the TRNSYS standard library except for the cogeneration unit and the biomass boiler. Hence, new mathematical models have been appropriately written. These models include on one hand operating characteristic curves, on the other hand internal control logics. The characteristic curves have been derived by the elaboration of technical specifications acquired by various manufacturers and they refer to the following ranges of sizes: [70; 500] kWe for gas engine based cogeneration units and [15; 1000] kWth for biomass boilers.

Both the created models implement the following steps:

• Once the nominal heat power has been determined according to the subchapter 3.5, the model calculate left nominal features, e. g. the efficiency and the primary energy consumption at nominal condition;

• Once the mass flow and the temperature of the fluid entering each machine are given as input and a desired outlet temperature is selected, the models calculate the heat required for the stream to reach the set temperature. According to the selected control logic (discrete or continuous modulation), the model identifies the current load rate, the corresponding efficiency and primary energy consumption and the fluid outlet state.

For instance, the algorithm of the cogeneration unit is shortly described.

The first input required by the algorithm is the nominal heat power (Qreq, nom). The corresponding electrical power (Pnom) is determined by:

Pnom = 0.6148Q„ + 0.9282 [kW] (1)

The nominal electrical, thermal and first law efficiencies (ne, nom, nth, nom and respectively) are

calculated according to:

nenom = 0.0061Pnom + 31.284



Vhnom =-0.0133Pnom + 58.266



ninom =-0.0071Pnom + 89.51



The inlet mass flow and its temperature being known, the model calculates the heat required to heat up the stream to a previously selected temperature. Then, the transferred heat Qcog in [kW] is identified according to the internal control logic (heat load control and continuous modulation). The electrical power Ppart which is related to Qcog can be calculated through the following equation:


^cog = 0.6685-^ + 0.3315 [%] (5)


£^req, nom nom

Afterwards, the electrical efficiency ne, part in [%] corresponding to Ppart is determined according to:

Подпись: = -0.40
Подпись: e, part
Подпись: Пе image078 image079 Подпись: [%] Подпись: (6)


For the absorption chiller simulation, the model presented in [4] is used. The characterization of the chiller, in terms of distribution of the heat exchange area between the components and the typical internal flows, is made referring to a commercial chiller. The maximum relative error calculated comparing the model output to the manufacturer performance data does not exceed 10% and it is found for off design conditions (cooling water inlet at 35 °C).

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