Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 292

variable P tv is calculated on the basis of the relations in the thermal balance of the
residence, that is done using a distinct MATLAB program. This MATLAB program
considers the following values for the main temperatures, corresponding to the
winter regime: outside temperature t e =

17 ° C, temperature of the air in the resi-

−

dence, T a = 293 K (t a =20 ° C), temperature of the domestic cold water, and

t ar =10 ° C etc. In the summer regime, the following values were considered:

t e =35 ° Candt a =26 ° C.

A. Model of the thermal energy sources

In winter regime the subsystem includes two sources of the thermal energy: the
Stirling engine and the pellet boiler. The thermal power of the Stirling engine, P St,t ,
is in a ratio of 3:1 with respect to its electrical power. Since the electrical power is
controlled by the battery voltage controller in the electrical subsystem, so as to
maintain the battery voltage at a constant value, it results that the thermal power of
the Stirling engine has a random evolution. The pellet boiler is controlled by the
temperature controller, which maintains constant the temperature of the thermal
agent in the accumulation tank. This control has two objectives: bringing the
temperature to the setpoint value, T re c , and compensating the power variations of
the Stirling engine. In the simulation scheme of the thermal subsystem, the pellet
boiler was modeled as a simple dynamic system, with a time constant of 100 s.
The two sources transfer the thermal energy to the accumulation tank. Finally the
energy is transferred to the thermal load. The temperature in the accumulation tank
has a value sensitive to the difference between the produced power and the con-
sumed one. Through the control of this temperature, the balance between the two
powers is achieved.
In summer regime a third energy source is available: the solar thermal collector.
It was considered that this source is active in the thermal subsystem circuit between
6 am and 6 pm. Using the mathematical model of the solar thermal collector and
considering a day when the maximum air temperature reaches 38

C, the graph of
the diurnal power given by Fig. 32 is obtained. Integrating this power over a 24 h

°

Fig. 32 One day pro le of
the solar collector power

Solar collector power

15000

10000

5000

0

0 2 4 6 8

x 10 4

Time [s]