Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 279

temperature in the rooms (T a ) should be obtained, is determined in conditions of
extreme values of the external temperature in winter and summer regimes.
In the thermal balance, the delivered power has the following components:

the power delivered by the pellet boiler, P ph , established by the temperature
controller;

￿

the thermal power of the Stirling engine, P St . In the thermal subsystem, it is a
random variable, because in the operating regime of the Stirling engine the ratio
P St /P Se is constant and the power P Se modi

￿

es randomly, due to the fact that it is
a control variable in the voltage control loop;

the power of the solar thermal collector, P PT , during the interval in which this
source is active; and

￿

the power transferred from the thermal tank, P Tto , when the sum of the powers
delivered by the sources mentioned above do not cover the thermal load.

￿

The thermal load has the following components:

￿ the power necessary for heating the residence during the cold season, P hl ;

￿ the power consumed by the air conditioning equipment in the hot season, P acl ;

￿ the power consumed in the domestic water circuit, P dhw ; and

￿ the power consumed for completing the energy accumulated in the thermal tank,

P Tti , that is the power used to increase the temperature T up to the setpoint, T sp ,
when the total thermal power delivered by the sources is greater than the total
thermal power used by the consumers.

From the interconnected operating mode of the two subsystems, it results that the
electrical subsystem the thermal subsystem. The electrical energy

variations of the Stirling engine, which take place in the process of balancing the
produced power with the consumed one in the electrical subsystem (through the
voltage controller), determine important variations of the thermal power produced
by the Stirling engine. These variations must be compensated at the level of the
temperature control system, through the adjusting of the pellet boiler power. To
exemplify, let us consider the situation in which, at a certain moment, there is an
important consumption of electrical energy in the electrical subsystem and the load
is reduced in the thermal subsystem. In the above-mentioned situation, the control
system previously presented imposes that the Stirling engine should produce the
nominal electric power, through the temperature control loop. Simultaneously, it
also delivers the nominal thermal power so that, if the energy accumulation in the
thermal tank is already at the nominal level, the total produced thermal power is
greater than the current consumption (if the solar thermal collector also operates). It
is obviously that the excess of thermal energy must be dissipated, which reduces the
economical ef

“ subordinates ”

ciency of the mCCHP system. This situation occurs more frequently
when monovariable controllers are used for the system control. There is another
possibility of adjustment, which takes into account the interaction between the
electrical and the thermal subsystems. In the situation presented above, when the
electrical load is big and the thermal load is small, a multivariable controller