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Modelling and simulation of radiant heating and cooling systems
REHVA Journal – December 2011
KarelKabele
Professor
Czech Technical University in Prague
Department of Microenvironmental and Building Services Engineering
kabele@fsv.cvut.cz
Pavla Dvořáková
Assistant Professor
Czech Technical University in Prague
Department of Microenvironmental
and Building Services Engineering
pavla.dvorakova@fsv.cvut.cz
Introduction
There are several
aspects that emphasises the application of radiant heatig/cooling systems in
form of embeded capillary mats (Figure 1) to be an up-to-date topic in Central
Europe. Desired reduction of energy consumption including primary energy, a
recently formed demand for active cooling systems in this region and general
expectations of high indoor environmental quality open a wide area for
integrated radiant systems. Hand in hand with more and more common applications
of these systems various questions arise. This paper has ambitions to answer
often asked questions about radiant heating/cooling applications.
Figure 1 Integrated heating/cooling ceiling system with
capillary mats before covering with the plaster.
Buildings in Central Europe
The typical need
for climate conditions of Central Europe (Czech Republic) is space heating during
heating period (approx 230 days a year). Up till recent days there were no active
cooling systems considered for a traditional building. Due to low internal
gains, high thermal inertia and optimised glazing ratio such a building could
be operated without any active cooling. For up-to-date buildings (especially
office buildings) with high internal gains, high galzing ratio, low mass and
well-insulated walls (U value less than 0.3 W/m²K ) and windows (U value
less than 1.8 W/m²K ) an active cooling system seems to be essential
whenever we want to follow the comfort requirements during summer period.
Numerous of these buildings demonstrate the cooling load to be higher or much
more higher than the heating load.
Traditional
approach to the technical solution of a heating /cooling system is to design
two independent systems - radiator heating and split units cooling. One of the
latest technical solutions, which could be considered as a sustainable one, is
an integrated radiant heating/cooling system.
Radiant heating/cooling systems
Heat transmition
to a room equipped with this system is mainly by radiation and convection.
Final assesment of the room thermal comfort should not be based just on dry
bulb temperature but it is adequate to asses it according to the room resultant
temperature – numerical values of these two paramenters might differ
significantely during both the winter and the summer period. This fact means
that by using this system it is possible to reach lower air temperature in
winter and higher air temperature in summer to get the same perception of
indoor climate.
Radiant systems
are used as ceiling, wall or floor systems.
Comparing to a
traditional system, this technical solution integrates two systems into one and
it may reduce air change rate to hygienic minimum. The strong point of this
solution is a significant reduction of used material or better to say reduction
of embodied energy in a heating, cooling and ventilating system (Roulet et al.
1999).
There are several
limits for application of these systems. During the heating period the ceiling
system output is limited by a hygienic limit reducing highest intensity of
radiation on the skull-cap up to 200 W/m². For underfloor heating there is
a limit for floor surface temparature – it should not exceed 29°C. In case of
wall heating system the surface temperature should be within 35–50°C, based on
the room operation. During the cooling period the surface temperature should
not drop below the dew point temperature to avoid condensation.
The whole system
solution must not cause any local discomfort like asymmetric thermal radiation,
large vertical temperature gradient or draught.
Capillary mats
Furthermore paper
deals with the radiant heating/cooling system in form of capillary mats.
Capillary mats, embedded
in the gypsum plaster layer of the ceiling structure (Figure 2), are supplied by both the heating and the cooling water (four-pipe
system) (Figure 3). Ceiling surface transmits energy into
the heated/cooled room via radiation and convection heat transfer modes. As the
output of radiant system is limited by acceptable surface temperature, often
asked question is whenever is this system suitable to use.
Let´s list the elements that might infuense the
applicability of integrated radiant systems. These factors come from the limits
mentioned above taking into account the type of building and its operation.
Figure 2 Integrated heating/cooling ceiling system with
capillary mats.
Figure 3 Four pipe system – connection circuits.
Factors with crucial impact to
the system applicability (Figure 4)
·
Indoor
sensitive heat load
·
Indoor
latent heat load
· Fresh air volume for ventilation
· Total air volume for ventilation
· Indoor humidity control
· Glazing ratio
· Glazing quality – U-value, g-value
· Active shading
· Room geometry
· Building orientation
· Heating setpoint
· Cooling setpoint
· Activity and clothing level of building inhabitants
· Climate
Figure 4
Factors influencing indoor
environment in the rooms with radiant heating /cooling.
All of these listed parameters affect the design and operation of HVAC systems. Forgetting these parameters may result in undesirable state of indoor environment. The system does not cover all the demands (heating/cooling) or surface condensation appears.
Modelling and energy performance simulation
One of the ways how to investigate the performance of a radiant heating/cooling system is the use of computer modelling and simulation. By a virtual model with various cases of application it is possible to reach the knowledge about the system operation under different boundary conditions. Several methods and approaches to the building energy performance modelling and simulation are available. However, the issue is very complex, full of bonds between different factors with the basic impact to indoor environment and final applicability of the system. Therefore it is necessary to use simulation tools that can cover all of these effects. Regarding energy performance, control algorithm and whole year IEQ assessment it seems that tools based on method of dynamic heat balance are the best to fulfil our expectation. We can find program ESP-r in this category. It provides time dependent energy flow patterns, temperature and humidity levels in processed parts of the building and some other important values for overall analysis.
If we are looking for a detailed description of temperature and velocity airflow patterns in defined closed space, tools based on the CFD (Computational Fluid Dynamics) method seems to be the best choice e.g. Fluent or Flovent.
Capillary mats modelling
It is possible to find several studies aimed to the applicability of the system with capillary mats in Czech climate conditions. Modelling and simulation of energy performance is effective for complex systems, but it is very sensitive to the way of the task and boundary conditions definition.
To build a model of a room fitted with a heating/cooling ceiling system it is necessary to specify dimensions, building envelope characteristics, environment behind the envelope and orientation of the building. Furthermore it is necessary to set ventilation, lighting, inner loads, activity and presence of users with relevant timing and loads. For the heating/cooling part it is necessary to define maximum output and control loops. Thus defined, the model is loaded with effects of climate conditions of the site, which are again defined by the time dependence of air temperature, relative humidity, wind direction and speed and intensity of solar radiation. Length of the simulated period is chosen depending on which parameters we are interested in. For either only heating or only cooling focused models a characteristic winter or summer week usually provides enough information. A whole year period is preferable for integrated systems. This will also affect the transition period and can alert the otherwise hardly identifiable marginal stages.
Following case study is an example of using the ESP-r simulation tool to analyse energy performance of a room equipped with integrated heating/cooling ceiling system.
Problem descriptionThe main purpose
of the study is to investigate integrated heating/cooling system performance
during typical Central Europe climate conditions with office operation load
profile. This task arose from common practice, when several problems with
system application occured despite following all the common recommendations
(Kabele et al. 2002). The issue
appeared with following questions. Is the integrated ceiling heating/cooling
system able to secure compliance with comfort requirements during the whole
year operation in a modelled case? Are the current design recommendations in
terms of maximum heating/cooling output of the ceiling applicable
particularly in climate conditions of Central Europe? (Figure 5)
Figure 5 Annual ambient air temperatures in the Czech Republic. Research methodProblem analysis followed by
computer simulation of an annual building energy performance was used on a
case study to analyse selected parameters, that may have any influence on the
possibility of system application. ESP-r, an energy system performance
simulation tool, was used for this purpose (ESRU 2004). Modelling and simulationModel DescriptionA five - zone modelwas created (Figure 6). The model contains four equal zones with following dimensions 5 m x 9 m x 3 m, each facing different cardinal point, and a corridor in the centre with following dimensions 4 m x 4 m x 3 m. Each of the zones has a window 5 m x 1.6 m in a longer exterior wall. Medium-heavy constructions were considered with the value of overall coefficient of heat transmission according to Czech building regulations (ČSN 730540 2005). For an external wall U = 0.239 W/m²K, for an internal wall U = 1.561 W/m²K and for a window it is 1.198 W/m²K. No heat flux through ceilings and floors was assumed.
Figure 6 ESP-r model of the building. Heating and cooling system is radiant low temperature heating/high temperature cooling system with capillary mats placed inside the layer of gypsum plaster on ceiling construction and defined by heating capacity controlled according to established practice in a range of 0-130 W/m², cooling capacity 0–80 W/m² in each of the rooms (Jeong et al. 2004). This technical solution is carried out in the model by placing a cooling and heating capacity to the axis of gypsum plaster layer. (R.K. Strand and K.T. Baumgartner 2005) The active ceiling construction contains layers according to Figure 7.
Figure 7. Active Ceiling/Floor construction: 1 Flooring, 2 Polyurethane foam board, 3 Heavy mix concrete, 4 Gypsum plaster with capillary mats. The
control of the system is running as basic heat
and cool controller according to sensors, located in each of the rooms sensing
dry bulb temperature. Set point for heating is Ventilation. Mechanical ventilation 0.7 h-1 and infiltration 0.3 h-1 is considered. Supply air temperature is 20°C. Occupation and casual gains. Sensitive and latent heat load by persons is 7.8 W/m² during working hours (weekdays 7:00-18:00), sensitive heat load by equipment is 15 W/m²and by lights 25 W/m²during the whole day. SimulationThe whole year period was studied using Prague (Czech Republic) ASHRAE IWEC climate files. An integrated building simulation was used, with time step of 1 hour and initial period of 11 days. The discussion of the results was focused on heating/cooling energy consumption. PMV and PPD parameters were used to evaluate thermal comfort (Yang 1997, ČSN EN ISO 7730 2005). The third thing to follow was the possibility of condensation on the ceiling surface during the cooling period. Simulation results and discussionIn following tables and diagrams there are selected simulation results from ESP-r. EnergyTable 1. Annual energy h/c consumption.
Thermal comfort – resultant temperatureThe temperature curve confirmed the ability of the heating/cooling system capacity to guarantee set temperatures inside all of the zones almost during the whole year. Set point for cooling was exceeded only in several hot days during summer with maximum value 31.5°C (Table 2) Table 2. Resultant temperature extremes.
Thermal comfort – PMV, PPDThermal comfort evaluation is based on PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied) classification of heated/cooled spaces. PMV is defined by six thermal variables for indoor-air and human condition. It is air temperature, air humidity, air velocity, mean radiant temperature, clothing insulation and human activity. The value of PMV index has range from −3 to +3 which corresponds to human sensation from cold to hot. The null value of PMV index means neutral. It is desirable to maintain the PMV at level 0 with a tolerance of ±0.5 to ensure a comfortable indoor climate. Comfort evaluation is based on activity level 1.2 met with clothing level equal to 0.7 clo (ASHRAE 2005, CSN EN ISO 7730) in this case study. The results show PMV index to be from 0.0 to 2.0. Index PPD in 48% of the time is up to 10% which means that during this time the number of dissatisfied occupants will not exceed 10%. Index PPD during 99% of the time is up to 50%. From the point of heating there is no problem with thermal comfort in all of the examined cases; minimum value for PMV during heating period is 0.0, which means neutral. On the other hand several problems with thermal comfort during cooling period were detected. In this case the maximum PMV index reached 2.0, which means warm (Figure 8).
Figure 8 Annual distribution of PMV and PPD index (ESP-r). Active ceiling surface temperatures and surface condensationThe active ceiling surface temperatures coming out from the simulation are in figures. Figure 9shows temperature difference between active ceiling surface temperature and the dew point temperature during the year. Critical period (the value is below zero) is marked with circle. Detailed analysis of critical time is on the Figure 10. The possibility of surface condensation occurred in a range of one or two hours during one critical day of the year in summer when the exterior relative humidity was very high.
Figure 9 Temperature difference between active ceiling surface temperature and the dew point temperature.
Figure 10 Integrated look at energy and environmental performance during critical days. OccCasG - internal heat gains, CoolInj-cooling system injection, AmbientRH-ambient air relative humidity, dbT- zone dry bulb temperature, Ambient dbTmp- ambient dry bulb temperature, room1RH- zone relative humidity, Surf-5:room1Dsur-dpT difference between cooling surface temperature and dew point. Critical hours marked with circle. ConclusionIs it possible to use an integrated heating/cooling ceiling system in a modern building, built according to valid Czech standards with respect to energy efficiency in Czech climate conditions? The question has been analysed. The purpose of created case study was to predict thermal behaviour of the room heated/cooled with this system and to describe thermal comfort behaviour in time during a whole year operation. ESP-r, a modelling tool was applied. Based on the simulation results no problems with the heating were detected anywhere. The system can reliably guarantee the required temperature during the whole year. At the same time the simulation shows that common designed heating/cooling capacities (130 and 80 W/m²) of the ceiling surface are appropriate. Several problems appeared with the cooling. The designed capacity was not able to cover the temperature requirements and occasionally a short-term condensation appeared. This means that the application of this integrated system is limited by its capacity. Especially in the buildings with higher internal gains this application is arguable. Following the effect of building orientation individual control of the zones is recommended. The results from above and the conclusions made from them are valid for the conditions of the simulation. |
Acknowledgments
This paper was supported by Research Plan CEZ MSM 6840770003.
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