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A study of the performances of low-temperature heating systems

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Abstract

In Europe, high energy consumption in the building environment has raised the need for developing low-temperature heating systems both in new buildings and in retrofitting buildings. This paper addressed many different topics related to energy saving in central heating systems with reduced supply temperature and radiant panel heating including floor heating, ceiling heating and wall heating. The paper investigated the performance of these different types of low-temperature heating system using numerical modelling, simulation tools and also site measurements. Thus, energy performance of radiator and radiant floor heating systems connected to a ground-coupled heat pump (GCHP) is compared, as obtained with experimental measurements in an office room. Furthermore, the thermal comfort of these systems is compared and a mathematical model for numerical modelling of thermal emission from radiant floors is developed and experimentally validated. Additionally, a comparative analysis of the energy, environmental and economic performances of floor, wall, ceiling and floor-ceiling heating using numerical simulation is performed. Finally, the energy efficiency of a heat pump in conjunction with a radiator or radiant floor heating system is calculated for different supply, return, and air design temperatures. This study showed that floor-ceiling heating works better than other low-temperature heating systems regarding providing better thermal comfort, lower energy consumption, lower CO2 emission and lower operating cost.

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Abbreviations

A R :

Radiator surface (m2)

a :

Air thermal diffusivity (m2/s)

c el :

Specific cost of electricity (€/kWh)

c g :

Specific cost of natural gas (€/m3)

c p :

Hot water specific heat (J/(kgK))

CO2 :

Carbon dioxide emission (kg)

COP:

Heat pump coefficient of performance

COP a :

Annual performance coefficient

COP syst :

System coefficient of performance

c v :

Coefficient of variation

D e :

External pipe diameter (m)

E el :

Consumed electrical energy (kWh)

E g :

Consumption of natural gas (kWh)

E t :

Heating usable energy (kWh)

EU:

European Union

g :

Gravitational acceleration (m/s2)

GCHP:

Ground-coupled heat pump

g el :

CO2 emission factor for electricity (kg CO2/kWh)

g g :

CO2 emission factor for natural gas (kg CO2/kWh)

GHE:

Ground heat exchanger

Gr :

Grashoff number

h :

Room height (m)

i M :

Metabolic rate (met)

k :

Correction coefficient of the natural gas consumption

L :

Characteristic dimension of the element surface (m)

m :

Mass flow rate of water (kg/s)

N :

Number of measured data

Nu :

Nusselt number

PMV:

Predicted mean vote

PPD:

Predicted percent dissatisfied (%)

Pr :

Prandtl number

q :

Heat flux (W/m2)

q r :

Radiant flux (W/m2)

R :

Thermal diffusion resistance of compound floor layers above the tube (m2 K/W)

R 2 :

Coefficient of multiple determinations

R cl :

Clothing thermal resistance (clo)

R i :

Heat transfer resistance at the internal surface (m2 K/W)

RMS :

Root mean square

r :

Distance from point P to the centre of the tube cross section (m)

r’ :

Distance from point P to the centre of the virtual tube cross section (m)

s :

Arrangement step of the radiant floor tubes (m)

T 1 :

Absolute temperature of radiant floor (K)

T 2 :

Weighted average absolute temperature of all room walls (K)

t ag :

Mean heat carrier temperature (K or °C)

t c :

Operative (comfort) temperature (K or °C)

t d :

Supply hot water temperature (K or °C)

t e :

Outdoor air temperature (K or °C)

t f :

Mean floor surface temperature (K or °C)

t hs :

Heat source temperature (K or °C)

t i :

Indoor air temperature (K or °C)

t mr :

Mean radiant temperature (K or °C)

t P :

Temperature at floor surface point P (K or °C)

t wi :

Hot water temperature in the inlet section of pipe (K or °C)

t wo :

Hot water temperature in the outlet section of pipe (K or °C)

TRNSYS:

Transient Systems Simulation

U :

Coefficient of heat transfer (W/(m2 K))

w Z :

Uncertainty in the result Z

y mea,m :

Measured value of one data point m

y com,m :

Computed value

\( {\overline{y}}_{mea,m} \) :

Mean value of all measured data points

α:

Radiator exponent

α c :

Convective heat transfer coefficient (W/(m2 K))

α i :

Superficial heat transfer coefficient of the floor surface (W/(m2⋅K))

α r :

Radiative heat transfer coefficient (W/(m2 K))

β:

Volumetric expansion coefficient of air (/K)

δ j :

Thickness of layer j (m)

Δt :

Logarithmic mean temperature difference (K)

Δt w :

Hot water temperature drop (K or °C)

Δt f-a :

Temperature difference between the floor surface and air (K or °C)

ε1 :

Floor surface emittance

ε2 :

Room wall emittance

η iz :

Thermal insulation efficiency

λ:

Air thermal conductivity (W/(m K))

λ j :

Thermal conductivity of layer j (W/(m K)

ν:

Air kinematic viscosity (m2/s)

ρ:

Tube radius (m)

σ:

Stefan-Boltzmann constant (W/(m2 K4))

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Authors and Affiliations

  1. Department of Building Services Engineering, Polytechnic University Timisoara, Piata Bisericii 4A, 300233, Timisoara, Romania

    Ioan Sarbu & Calin Sebarchievici

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  1. Ioan Sarbu
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  2. Calin Sebarchievici
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Correspondence to Ioan Sarbu.

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Sarbu, I., Sebarchievici, C. A study of the performances of low-temperature heating systems. Energy Efficiency 8, 609–627 (2015). https://doi.org/10.1007/s12053-014-9312-4

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