Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 288

These tests are achieved through simulations of the whole system in dynamic
regime.

D. Numerical simulation of the electrical subsystem

The analysis of the dynamic regimes is achieved on the time scale corresponding
to the evolution of the physical variables of the Stirling engine. In the following, a
time constant of 60 s was taken into consideration for the Stirling engine, to which
corresponds a transitory time of about 300 s. Obviously, on this time scale, the fast
dynamic regimes of the electrical processes are neglected.
The control law of the battery voltage loop, according to the principle scheme
given by Fig. 21 , may be of 0/1 or continuous control type. Further on, it is
considered the case in which the Stirling engine allows an on/off control, that
corresponds to a power in the range of 0

- 0.45P n , and a continuous control in the

range of 0.45P n - P n (P n denotes the nominal power). For smaller powers, under

0.45 P n , the control of the Stirling engine is achieved through 0/1 pulse width
modulation (PWM), the minimum period of these impulses being 30 min (the
minimum duration between two successive starts of the Stirling engine being
imposed by the engine characteristics). The controller used in the battery voltage
control loop is of PI type. Taking into account the global ef ciency of using the

η

electrical energy delivered by the Stirling engine, = 0.7, the controller will pro-

vide controls of PWM type in the range of 0 - 1.6 kW and continuous controls in the
range of 1.6

3.5 kW.
In these conditions, the numerical simulation scheme of the electrical subsystem
is given by Fig. 25 where the block

-

“ Stirling engine ” is detailed in Fig. 26 .

Figures 25 and 26 include also the Simulink blocks “ Electrical process controller ”

and respectively. These blocks are further presented in Figs. 89

and 91 b in Sect. 4.1 . The variation of the power consumed in 24 h, with the form
given in Fig. 22 , is generated by the block of the from File type, named

“ PWM Subsystem, ”

The
simulation scheme allows the recording of the main variables of the subsystem:
battery voltage, useful power delivered by the Stirling engine, difference between

“ var2. ”

var_ele1.mat
To File1

Surplus/deficit

Applied command

Stirling engine
power

Out1

Command var_ele.mat

To File

1
s
Integrator1

In1

Vm+(VM-Vm)/(WM-Wm)*(u(1)-Wm)

Out2
Striling engine

Battery

Battery model

voltage graph

PW M block
command

Out1
Out2
Out3
Electrical
process

kW W 48.5

In1

Accumulated energy graph (J)

var2
24h power profile

1000

1
s
Integrator2

Setpoint

In2

battery
voltage
Produced and consumed
energy graph (J)

Control loop
error

kW -W

Controller

1
s
Integrator3

command

controller

Necessary power
graph

Fig. 25 Power subsystem simulation scheme