Dynamics Characteristics of an Indirect District Heating System and Operational Optimization

Abstract

Based on energy conservation principle and heat transfer mechanisms, a set of dynamic model was established for an indirect district heating system. Dynamic start-up process under ideal condition was simulated and the influence of heat loss from piping network, make-up water and intermediate heat exchange station, and radiator area surplus on the dynamic characteristics of the system was discussed. Dynamic process without control was simulated under the hourly outdoor temperature measured in the days from February 13 to 19, 2012. To overcome the obvious indoor temperature variation with outdoor temperature, four PI control strategies were proposed. The dynamic characteristic and energy consumption of the system operated under these four control strategies were discussed. It is found that the control strategy 3, controlling the fuel flow rate, and water flow rate in the first loop not only produce stable working performance but also obtain considerable energy saving potentials. These results are very helpful to realize the dynamic characteristics of the indirect district heating system and to select suitable control strategy and optimal performance.

Appendix

Symbols
c	Specific heat based on mass/${\text{kJ}} \cdot ({\text{kg}} \cdot^{ \circ } {\text{C}})^{ - 1}$	$u$	Valve opening, 0–1
C	Heat capacity/k${\text{J}} \cdot^{ \circ } {\text{C}}^{ - 1}$	$f_{{{\text{ex}},i}}$	Surplus heat transfer area coefficient of IHE
G	Mass flow rate/kg/s	a	Coefficient related to boiler efficiency
$H_{v}$	Combustion heat value per unit volume under standard state/${\text{kJ}} \cdot ({\text{Nm}}^{3} )^{ - 1}$	$G_{f}$	Volumetric flow rate/${\text{Nm}}^{3} \cdot {\text{s}}^{ - 1}$
$k_{p}$	Proportional coefficient in PI control algorithm	$\eta$	Boiler efficiency
$k_{i}$	Integral coefficient in PI control algorithm	${{\upalpha}}_{1}$, ${{\upalpha}}_{2}$	Fitting coefficients
$U$	Heat transfer rate driving by 1 °C per unit area of IHE/${\text{W}} \cdot {\text{K}}^{ - 1}$	$b_{i}$	The index related to heat transfer coefficient. $b_{1} =$ 1, accounting for the effect of radiation heat transfer, $b_{2} \,{\text{and}}$ $b_{3}$ = 0.28 with only convective heat transfer
T	Temperature/$^{ \circ } {\text{C}}$	e	Temperature deviation
${{\uptau}}$	Time/s
subscripts
z	Zone	b	Boiler
d	Design	rb	Return water into boiler
ex	Heat exchanger	w	Circulating water
f	Fuel of boiler	s	Supply water
htr	radiator	r	Return water
sh	user inlet	sp	Set point
1, 2, 3	1#, 2# or 3# zone	p	Parameters related to pipelines
o	Outdoor parameters	${\text{rex}}$	inlet of make-up water pump in the secondary system
max	Maximum	int	Internal heat gain
mk	Make-up water	sol	Solar gain
1rgg	Joints on the return water pipe in the primary system to 2#, 3# zones	1r11	Joint on the return water pipe in the primary system to 1#, zones 1#