NORME EUROPÉENNEICS 91.140.10 English VersionHeating systems in buildings - Design of embedded water based surface heating and cooling systems - Part 3: Optimizing for use of renewable e
Data referred to the circuit
sp m & H , specific water flow, calculated on the area covered by the circuit kg/ (m 2 s) ; c w specific heat capacity of the water J/ (kg K) ;
T pipe spacing m ; d a external diameter of the pipe m ; s r thickness of the pipe wall m ; λ r thermal conductivity of the material of the pipe wall W/ (m K) ;
A Floor area cooled/heated by the circuit m 2 ;
P W maximum cooling (0) power for a conditioning plant W ;
0 θ w supply water temperature at the beginning of the simulation °C ; lim θ w minimum (in the cooling case) or maximum (in the heating case) supply water temperature obtainable by the machine °C
Data referred to the room geometry and the boundary conditions
A Walls overall area of vertical walls, external facade excluded m 2 ;
F v Floor-Ext Wall view factor floor-external wall;
F v Floor-Ceiling view factor floor-ceiling;
F v Floor-Walls view factor floor-walls;
R add Floor additional resistance covering the upper side of the slab (m 2 K)/W ;
R add Ceiling additional resistance covering the lower side of the slab (m 2 K)/W ;
The resistance of the surface layer of internal walls is measured in square meters Kelvin per Watt (m² K/W) The convective heat transfer coefficients are defined as follows: the air-floor coefficient is expressed in Watts per square meter Kelvin (W/(m² K)), the air-ceiling coefficient is also in W/(m² K), and the air-walls coefficient represents the heat transfer between the air and internal walls in W/(m² K) Additionally, the radiant heat transfer coefficients include the floor-walls coefficient, the floor-ceiling coefficient, and the ceiling-walls coefficient, all measured in W/(m² K).
C Walls average specific thermal inertia of the internal walls J/(m 2 K)
Tcomfort maximum operative temperature allowed for comfort conditions °C ; n
Q & Sun solar gain in the room in the present calculation time step W ; n
Q & incoming heat flux to the room from the external wall in the present calculation time step W ; n
Q & Air convective heat flux extracted by the air circuit W ; n
Q & internal radiant heat gain due to people or electrical equipment in the present calculation time step W ; n
The internal convective heat gain, denoted as Q, accounts for heat generated by people or electrical equipment during the current time step, measured in watts (W) The variable \( n \) represents the system's running mode, taking a value of 1 when the system is operational and 0 when it is turned off.
Data referred to the slab and its partitions
s 1 thickness of the upper part of the slab m ; s 2 thickness of the lower part of the slab m ;
J 1 number of material layers constituting the upper part of the slab dimensionless ;
J 2 number of material layers constituting the lower part of the slab dimensionless ;
The total number of material layers in the slab is represented by the equation \( J = J_1 + J_2 \) Each layer is characterized by specific physical properties, including density (\( \rho_j \)) in kg/m³, specific heat capacity (\( c_j \)) in J/(kg K), thermal conductivity (\( \lambda_j \)) in W/(m K), and thickness (\( \delta_j \)) in meters, where \( \delta_j = 0 \) if the layer serves only as thermal resistance Additionally, the number of partitions for each layer is denoted as \( m_j \), which is dimensionless.
R j thermal resistance summarizing the j-th layer m 2 K/W ,
Rj > 0 if the layer is a mere thermal resistance
However, if the simulation covers more than one running cycle, the choice of the initial values is not decisive
In fact, it will influence only the very first time steps of the simulation
3.5 Calculation of the temperature profile and the heat fluxes in the generic time-step n
At the conclusion of the previous time step, the temperature at a specific interface is utilized to calculate the heat fluxes impacting building structures and to determine the resulting temperatures for the current time step The key parameters involved in these calculations include the global specific convective heat gains (W/m²), the global specific radiant heat gains (W/m²), and the air temperature in the room (°C) for the present calculation time step.
Walls θ mean temperature of the walls in the present calculation time step °C ; n θ Op operative temperature in the room in the present calculation time step °C ;
− 1 n θ w supply water temperature at the end of the previous time step °C ;
− 1 n exit θ w outlet water temperature at the end of the previous time step °C ;
The temperature of the i-th interface at the end of the previous time step is denoted as \( n_i \theta \), measured in degrees Celsius, where \( 0 \leq i \leq i_L \) The results obtained at each time step include the supply water temperature, \( n \theta_w \), and the temperatures of the upper and lower sides of the slab, \( n \theta_F \) and \( n \theta_s \), respectively, all recorded at the end of the current time step in degrees Celsius.
I , θ temperature of the i-th interface, with 0 ≤ i ≤ i L , at the end of the time step in progress, °C
4 Relation to other EPBD standards
The present standard requires input from the following standards: prEN 15377-1, EN 15251, EN 15255 and
The present standard provides input data to the following standards: EN 15243 and EN ISO 13792
5 Optimisation of systems for facilitating the use of renewable energy sources
Transporting energy through water is more efficient than using air, as it requires less auxiliary energy for pumps and occupies less installation space Additionally, utilizing water at temperatures near room temperature can further enhance this efficiency.
Increasing pipe spacing and reducing the temperature difference between supply and return water in embedded radiant heating and cooling systems leads to water temperatures closer to room temperature However, this results in higher flow rates and longer pipe lengths, causing increased pressure losses Designers face the dilemma of either raising auxiliary energy use for pumps or opting for larger diameter pipes, both of which are not ideal A potential solution is to utilize more circuits with shorter pipe lengths Optimization of these factors should adhere to prEN 15377-2 standards.
Thermo-Active Building Systems enhance the utilization of renewable energy by minimizing peak loads, shifting energy consumption to off-peak times, downsizing energy generation systems, and improving efficiency through optimal water temperature levels This approach enables the effective use of various energy sources, including solar collectors, ground source heat pumps, free cooling systems, ground source heat exchangers, and aquifers.
6 The concept of Thermo-Active-Building-Systems (TABS)
A Thermo-Active-Building-System (TABS) utilizes a water-based heating and cooling mechanism, with pipes integrated into the central concrete core of a building This system facilitates heat transfer between the water in the pipes and the concrete, as well as between the concrete core and the room surfaces, including the ceiling and floor, ultimately regulating the temperature within the space.
Figure 1 – Thermo-active radiant system
A typical thermo-active system consists of a cooling system, such as a chiller or heat pump, that removes heat through pipes embedded in the slab This system can be categorized into several key elements, as illustrated in Figure 2.
3 = slab including core level with pipes
4 = possible additional resistances (floor covering or suspended ceiling)
5 = room below and room above
Figure 2 – Simple scheme of a thermo-active system
Peak-shaving allows for the heating and cooling of building structures during times when occupants are typically absent, such as at night, effectively lowering the peak power demand.
Reducing energy consumption can lead to the opportunity to take advantage of lower nighttime electricity rates, if available, and allows for the potential downsizing of the cooling system, including the chiller.
1 heat gain X-axis time of the day
2 power needed for conditioning the ventilation air Y-axis cooling power
3 power needed on the water side
4 peak of the required power reduction
Figure 3 – Example of peak-shaving effect
TABS can be utilized in conjunction with both natural and mechanical ventilation, depending on the weather conditions Mechanical ventilation with dehumidification may be necessary based on external climate and indoor humidity levels As illustrated in Figure 3, the cooling power needed for air dehumidification during the day is adequate for cooling the slab at night.
The designer must determine if the water temperature capacity is adequate to maintain the desired room temperature range Additionally, understanding the heat flow on the water side is essential for sizing the heat distribution system and the chiller or boiler This document outlines the methods necessary for these calculations.
Advanced building-systems calculation models have been created to analyze heat exchanges in single rooms under non-steady state conditions, assess thermal and hygrometric balance, predict comfort levels, evaluate surface condensation, explore control strategies, and calculate incoming solar radiation However, the complexity and time required for these simulations limit their practical use Therefore, there is a need for a more user-friendly tool, which is introduced here, enabling easy simulation of thermo-active systems.
Key θ mr mean radiant temperature X-axis time of the day θ air air temperature Y-axis, left temperature °C θ f floor temperature Y-axis, right PMV values θ c ceiling temperature θ w exit return water temperature
Figure 4 – Example of temperature profiles
Figure 5 illustrates the relationship between internal heat gains, water supply temperature, room-side heat transfer, operational hours, and water-side heat transfer The diagrams pertain to a concrete slab with a raised floor (R=0.45 m² K/W) and maintain a permissible room temperature range of 21 °C to 26 °C.
The upper diagram illustrates the relationship between the maximum allowable total heat gain in a space, measured in W/m², and the required water supply temperature in °C The x-axis represents the water supply temperature, while the y-axis indicates the total heat gain, which includes both internal and solar gains Various lines on the diagram correspond to different operational hours (8 h, 12 h, 16 h, and 24 h) and varying maximum energy supply levels in Wh/m² per day.
Calculation of the temperature profile and the heat fluxes in the generic time-step n
At the conclusion of the previous time step, the temperature at a specific interface is utilized to calculate the heat fluxes impacting building structures and to determine the resulting temperatures for the current time step The key parameters involved in these calculations include the global specific convective heat gains (W/m²), the global specific radiant heat gains (W/m²), and the air temperature in the room (°C) for the present calculation time step.
Walls θ mean temperature of the walls in the present calculation time step °C ; n θ Op operative temperature in the room in the present calculation time step °C ;
− 1 n θ w supply water temperature at the end of the previous time step °C ;
− 1 n exit θ w outlet water temperature at the end of the previous time step °C ;
The temperature of the i-th interface at the end of the previous time step is denoted as \$\theta_i\$, where \$0 \leq i \leq i_L\$, measured in degrees Celsius The results obtained at each time step include the supply water temperature, \$\theta_w\$, at the end of the current time step, as well as the temperatures of the upper and lower sides of the slab, \$\theta_F\$ and \$\theta_s\$, respectively, also at the end of the current time step, all expressed in degrees Celsius.
I , θ temperature of the i-th interface, with 0 ≤ i ≤ i L , at the end of the time step in progress, °C
4 Relation to other EPBD standards
The present standard requires input from the following standards: prEN 15377-1, EN 15251, EN 15255 and
The present standard provides input data to the following standards: EN 15243 and EN ISO 13792
5 Optimisation of systems for facilitating the use of renewable energy sources
Transporting energy through water is more efficient than using air, as it requires less auxiliary energy for pumps and occupies less installation space Additionally, utilizing water at temperatures near room temperature can further enhance this efficiency.
In normal embedded radiant heating and cooling systems, increasing pipe spacing and reducing the temperature difference between supply and return water leads to water temperatures that are closer to room temperature However, this adjustment results in higher flow rates and longer pipe lengths, which in turn cause increased pressure losses Consequently, designers face the dilemma of either raising auxiliary energy consumption for pumps or opting for larger diameter pipes, both of which are not ideal solutions To mitigate these issues, utilizing more circuits with shorter pipe lengths can be beneficial These considerations should be optimized in accordance with prEN 15377-2 standards.
Thermo-Active Building Systems optimize renewable energy use by minimizing peak loads, shifting energy consumption to off-peak times, downsizing energy generation systems, and enhancing efficiency through water temperature management This approach enables the effective utilization of various energy sources, including solar collectors, ground source heat pumps, free cooling systems, ground source heat exchangers, and aquifers.
6 The concept of Thermo-Active-Building-Systems (TABS)
A Thermo-Active-Building-System (TABS) utilizes a water-based heating and cooling mechanism, with pipes integrated into the central concrete core of a building This system facilitates heat transfer between the water in the pipes and the concrete, as well as between the concrete core and the room surfaces, such as ceilings and floors, ultimately regulating the temperature within the space.
Figure 1 – Thermo-active radiant system
A typical thermo-active system consists of a cooling system, such as a chiller or heat pump, that removes heat through pipes embedded in the slab This system can be categorized into several key elements, as illustrated in Figure 2.
3 = slab including core level with pipes
4 = possible additional resistances (floor covering or suspended ceiling)
5 = room below and room above
Figure 2 – Simple scheme of a thermo-active system
Peak-shaving refers to the ability to heat and cool buildings during times when occupants are typically absent, such as at night, which helps to lower the peak power demand.
Reducing energy consumption can lead to the opportunity to take advantage of lower nighttime electricity rates, if available, and allows for the potential downsizing of the cooling system, including the chiller.
1 heat gain X-axis time of the day
2 power needed for conditioning the ventilation air Y-axis cooling power
3 power needed on the water side
4 peak of the required power reduction
Figure 3 – Example of peak-shaving effect
TABS can be utilized in conjunction with both natural and mechanical ventilation, depending on the weather conditions Mechanical ventilation with dehumidification may be necessary based on external climate and indoor humidity levels As illustrated in Figure 3, the cooling power needed for dehumidifying the air during the day is adequate for cooling the slab at night.
Designers must assess whether the water temperature capacity can maintain the desired room temperature range Additionally, understanding the heat flow on the water side is crucial for sizing the heat distribution system and the chiller or boiler This document outlines the methods necessary for these evaluations.
Advanced building-systems calculation models have been created to analyze heat exchanges in single rooms under non-steady state conditions, assess thermal and hygrometric balance, predict comfort levels, evaluate surface condensation, explore control strategies, and calculate incoming solar radiation However, the complexity and time required for these simulations limit their practical use Therefore, there is a need for a more user-friendly tool, which is introduced here, enabling easy simulation of thermo-active systems.
Key θ mr mean radiant temperature X-axis time of the day θ air air temperature Y-axis, left temperature °C θ f floor temperature Y-axis, right PMV values θ c ceiling temperature θ w exit return water temperature
Figure 4 – Example of temperature profiles
Figure 5 illustrates the relationship between internal heat gains, water supply temperature, heat transfer on the room side, hours of operation, and heat transfer on the water side The diagrams represent a concrete slab with a raised floor (R=0.45 m² K/W) and maintain a permissible room temperature range of 21 °C to 26 °C.
The diagram illustrates the relationship between the maximum allowable total heat gain in a space, measured in W/m², and the required water supply temperature in °C It features lines representing various operational hours (8 h, 12 h, 16 h, 24 h) alongside different maximum energy supply levels in Wh/m² per day.
The lower diagram illustrates the cooling power in W/m² needed on the water side for sizing the chiller in thermo-active slabs, depending on the supply water temperature and operational duration Additionally, it presents the daily energy rejection measured in Wh/m² per day.
A =Maximum total heat gain in space [W/m 2 floor area]
O&E = occupants and equipment (acc to SWKI 95-3)
D =Mean cooling power tabs [W/m 2 floor area]
General
The following calculation methods can be applied:
rough sizing method based on a standard calculation of the cooling load (accuracy 20-30%) To be used based on the knowledge of the peak value for heat gains, see 7.2;
simplified sizing method using diagrams based on 24 one-hour values of the heat gains (accuracy 15- 20%), see 7.3;
A simplified model utilizing the finite difference method (FDM) achieves an accuracy of 10-15% This model conducts a detailed dynamic simulation of thermal conduction in a slab, incorporating 24 one-hour measurements of variable cooling loads and air temperatures, as referenced in section 7.4.
detailed simulation models (accuracy 6-10%) Overall dynamic simulation model for the radiant system and the room, see 7.5.
Rough sizing method
The cooling system must be designed to accommodate 70% of the peak cooling load, in accordance with standards EN 15255, prEN 15377-1, and prEN 15377-2 To achieve this, the cooling load calculation should utilize an operative temperature.
Simplified sizing method using diagrams
To calculate heat gains, a 24-hour analysis must be performed with an operative temperature set at 24 °C It is essential to add 10% of the solar gain each hour to account for external window contributions This approach assumes that the conductive slab maintains a constant temperature throughout the day, with the average slab temperature determined by the method and linked to the supply water temperature during the circuit's operation.
The following data and parameters are involved by this method:
The specific daily heat load on the room during the design day, denoted as Q, is measured in kWh/m² per day It represents the total of 24 one-hour heat gain values divided by the floor area Understanding the load profile pattern is essential for accurate calculations.
θ comfort : maximum operative temperature allowed for comfort conditions °C;
Exposure of the room, in order to determine when the peak load from heat gains occurs: East (morning), South (noon) or West (afternoon);
The number of active surfaces is crucial for determining whether the slab facilitates heat transfer from both the floor and ceiling sides or solely from the ceiling side, as illustrated in Figure 6.
θ s average slab temperature in °C, which depends on the number of active surfaces, the running mode
The average surface temperature of the slab is influenced by the duration of the internal load profile, whether it is 24 hours or 8 hours, and the presence of a lunch break This relationship is quantified using specific coefficients outlined in the method.
Q coeff comfort s =θ + ⋅ θ °C (1) where values of the coefficient are given in Table 1 and Table 2;
R t, total thermal resistance of the circuit in m 2 K/W, obtained by the Resistance Method This thermal resistance depends on the characteristics of pipe wall resistance, pipe diameter and pipe spacing (see
Figure 10), and is calculated according to B.1;
θ w, which is the required temperature of the supply water in °C obtained through the equation:
Example of slab acting through 2 surfaces Example of slab acting through 1 surface Example of slab acting through 1 surface
Figure 6 – Number of active surfaces where:
Figure 7 – Examples of conductive regions where:
UP = upper part of the conductive region
LP = lower part of the conductive region
ROS = rest of the slab
The coefficients for calculation of the average temperature of the slab are given in Table 1 and Table 2, depending on the shape of the internal heat gains profile
Table 1 - Constant internal heat gains from 8:00 to 18:00
Exposure of the room EAST SOUTH WEST Kind of floor
Coefficient for calculation of average slab temperature
Only ceiling C1 -6.3022 -7.2237 -7.7982 Floor and ceiling I2 -5.5273 -6.1701 -6.7323
Table 2 - Constant internal heat gains from 8:00 to 12:00 and from 14:00 to 18:00 (two peaks)
Exposure of the room EAST SOUTH WEST Kind of floor
Coefficient for calculation of average slab temperature
Only ceiling -7.9663 -8.7989 -8.7455 Floor and ceiling -8.1474 -8.758 -9.3264
Once θcomfort is defined, the tables can be summarized by diagrams For instance, if θcomfort = 26 °C, the diagram for constant internal heat gains from 8:00 to 18:00 is given in Figure 9
X-axis specific daily energy (kWh/m 2 per day)
Y-axis average slab temperature coding of lines: operation condition of the circuit (C = continuous, I = intermittent, 8 h) number of active surfaces (1 or 2) exposure of the room (E = East, S = South, W = West)
Figure 9 – Diagram for determining the average slab temperature in the case of constant internal heat gains during the day
• Q: 0,6 kWh/m 2 per day; shape of thermal loads: 2 peaks
• Exposure of the room: SOUTH
If λ of the conductive region
Simplified model based on finite difference method (FDM)
Cooling system
The cooling system's limited power must be considered, as it affects the ability to maintain a constant supply water temperature This temperature is influenced by the heat flux exchanged with the slab and the chiller's maximum power After each time step, the new inlet water temperature is calculated based on the heat fluxes from the previous time step.
Hydraulic circuit
The Resistance Method outlined in Annex B establishes a clear relationship between the inlet water temperature and the average temperature at the pipes plane, denoted as θ c To facilitate this analysis, the slab can be divided into two distinct sections: the upper slab, located above the pipes plane, and the lower slab, situated below the pipes plane, which are examined independently (refer to Figure 10).
R z thermal resistance between the inlet water temperature and the supply water tempera- ture along the pipe/circuit length
R w thermal resistance between the supply water temperature in the pipe and the internal surface temperature of the pipe wall
R r thermal resistance between the internal and external surface temperature of the pipe wall
R x thermal resistance between external surface temperature of the pipe wall and average temperature at the pipes plane
Slab
The Resistance Method enables the division of a slab into two sections, which are evaluated using an explicit finite difference approach This method incorporates resistance due to radiation exchange between the sections, with each slab surface linked to the wall-node via a resistance connection (refer to Figures 11, 12, and 13).
Figure 11 – General scheme of the Resistance Method where:
TES = transmission through the external surface
CIHL = convective internal heat loads
RIHL = radiant internal heat loads
Figure 13 – Heat loads involved acting on the room and how they take part in the calculations 7.4.5 Limits of the method
The following limitations shall be met:
Typical concrete slab structures should have a thermal conductivity (\(\lambda\)) ranging from 1.15 to 2.0 W/(mK) Additional materials, such as acoustic insulation or raised flooring, may be added above the slabs It is important to avoid using discontinuous light fillings in the construction of both lower and upper slabs.
If these conditions are not fulfilled, a detailed simulation program has to be applied for dimensioning the thermo-active system (see 7.5)
A cooling load calculation or simulation for a convective system can be performed over a 24-hour period, maintaining an internal temperature of 24 °C The key outputs of this calculation, which will serve as inputs for the simplified model, include solar gains and heat.
Limits of the method
The following limitations shall be met:
Typical concrete slab structures should have a thermal conductivity (\(\lambda\)) ranging from 1.15 to 2.0 W/(mK) Additional materials, such as acoustic insulation or raised flooring, may be added on top It is important to avoid using discontinuous light fillings in both the lower and upper slab structures.
If these conditions are not fulfilled, a detailed simulation program has to be applied for dimensioning the thermo-active system (see 7.5)
A cooling load calculation or simulation for a convective system can be performed over a 24-hour period, maintaining an internal temperature of 24 °C The outcomes of this calculation, which include solar gains and heat, will serve as essential input for the current simplified model.