Simulation of Legionella concentration in domestic hot water: comparison of pipe and boiler models

Abstract

The energy needed for the production of domestic hot water (DHW) represents an important share in the total energy demand of well-insulated and airtight buildings. DHW is produced, stored and distributed above 60°C to kill Legionella pneumophila. This elevated temperature is not necessary for DHW applications and has a negative effect on the efficiency of hot water production units.

In this paper, system component models are developed/updated with L. pneumophila growth equations. For that purpose, different existing Modelica pipe and boiler models are investigated to select useful models that could be extended with equations for simulation of bacterial growth. In future research, HVAC designers will be able to investigate the contamination risk for L. pneumophila in the design phase of a hot water system, by implementing the customized pipe and boiler model in a hot water system model. Additionally it will be possible, with simulations, to optimize temperature regimes and estimate the energy saving potential without increasing contamination risk.

Keywords:

• domestic hot water (DHW)
• Legionella pneumophila
• pipe model
• boiler model
• contamination risk
• energy use

Supplemental data

Supplemental data for this article can be accessed at https://doi.org/10.1080/19401493.2019.1583286.

Nomenclature

A(T) [–]	=	Growth function depending on water temperature, the species of the organism and the chemical nature of water
B(T) [–]	=	Death function depending on water temperature, the species of the organism and the chemical nature of water
C0 [cfu/m3]	=	Start concentration of L. pneumophila in water entering system
C(t) [cfu/m3]	=	Concentration of L. pneumophila in water at time t
Cprevious [cfu/m3]	=	Concentration of L. pneumophila in water on previous timestep. Cprevious = C0 on first timestep.
Cin(t) [cfu/m3]	=	Concentration of L. pneumophila in water entering system
Cout(t) [cfu/m3]	=	Concentration of L. pneumophila in water leaving tap
Cb,in(t) [cfu/m3]	=	Concentration of L. pneumophila. entering biofilm
Cb,out(t) [cfu/m3]	=	Concentration of L. pneumophila in water leaving biofilm
Cb(t) [cfu/m3]	=	Concentration of L. pneumophila. in biofilm at time t
Cb,previous [cfu/m3]	=	Concentration of L. pneumophila in water on previous timestep. Cprevious = C0 on first timestep.
$c_v$ [J/kg·K]	=	Heat capacity
dC(t)/dt [cfu/m3·s]	=	Changing concentration of L. pneumophila over time
dCb(t)/dt [cfu/m3·s]	=	Changing concentration of L. pneumophila in biofilm over time
D [m]	=	Tube diameter
g [m/s2]	=	Acceleration due to gravity
K [cfu/m3]	=	Carrying capacity
k [W/m·K]	=	Thermal conductivity
$\dot{m}$(t) [cfu/m3·s]	=	Change in concentration of L. pneumophila. due to growth or starvation
${\dot{m}}_b\left(t\right)$ [cfu/m3·s]	=	Change in concentration of L. pneumophila in biofilm due to growth or starvation
P [Pa]	=	Total pressure
Qin(t) [kg/s]	=	Mass flow rate of L. pneumophila in water entering system
Qout(t) [kg/s]	=	Mass flow rate of L. pneumophila in water leaving tap
Qb(t) [kg/s]	=	Mass flow rate of L. pneumophila entering/leaving biofilm
$\dot{q}$ [W/m3]	=	Volumetric energy generation rate
t [s]	=	Time
T [K]	=	Absolute temperature
Vp [m3]	=	Volume of water in pipe
Vb [m3]	=	Volume of biofilm in pipe
y [s]	=	Multiplication time of L. pneumophila in water dependent on temperature
yb [s]	=	Multiplication time of L. pneumophila in biofilm dependent on temperature
v [m/s]	=	Mass-average velocity for multicomponent mixture
μ [Pa·s]	=	Viscosity
ρ [kg/m3]	=	Mass density of mixture
Φ	=	Function of fluid viscosity and shear strain rates

ORCID

Elisa Van Kenhove http://orcid.org/0000-0002-4648-0551

Lien De Backer http://orcid.org/0000-0002-0476-3537

Arnold Janssens http://orcid.org/0000-0003-4950-4704

Jelle Laverge http://orcid.org/0000-0002-5334-1314

Additional information

Funding

This work was supported by the Agency for Innovation by Science and Technology-Belgium (IWT/VLAIO) under Grant 141608. The authors thank Filip Jorissen for Modelica modelling guidance. The authors thank the Belgian Building Research Institute (BBRI/WTCB/CSTC), the partners of the ‘Instal2020’ project and especially Karla Dinne and Bart Bleys for building a Legionella test rig and making their experimental data available, these data will be used in future research to validate the Legionella growth in DHW systems in laboratory conditions. The authors thank the partners of the Proeftuin R&D building projects to make real case validation measurements possible.