ASHRAE Journal - February 2012 - 46

Mean Cooling Power TABS (W/m2 Floor Area)

the room temperature within a given comfort range. Moreover, the planner needs to know the heat ﬂow on the water side to be able to dimension the heat distribution system and the chiller/ boiler. There is an international need for a design manual for TABS. ISO11855-44 and Olesen, et al.,5 will give some help and guidance regarding the design of TABS. When using TABS, the indoor temperature changes moderately during the day, and the aim of a good design of TABS is to maintain daytime occupied space comfort conditions within the ranges (i.e., –0.5 < PMV < 0.5) speciﬁed by ISO EN 77306 or ASHRAE Standard 55-2010. The systems allow separation of ventilation and thermal conditioning. Ventilation systems are designed to provide the required amount of fresh outdoor air, remove latent heat loads and supplement the space sensible cooling in peak hours. In many cases, moisture should be controlled because of biological concerns (bacteria, viruses and molds), which will reduce the dew point and risk of condensation on cooled surfaces in a TABS. TABS are often designed with a pipe spacing of 150 to 200 mm (0.5 to 0.7 ft). A water ﬂow rate in a TABS system is often based on a temperature increase in the water temperature (difference supply-return of 3 to 5 K [5.4°F to 9°F]). This is to increase the cooling capacity, avoid too low supply temperatures (condensation risk) and keep a relatively small temperature difference between the water and the room.

Maximum Total Heat Gain In Space (W/m2 Floor Area)

60 Maximum Temperature Increase (21°C to 26°C) 50 358 40

Self-Regulating Effect of Slab

369

350 Wh/m2 d 300

30 Maximum 20 Minimum SWKI 95-3: Occupants and Equipment

250

SWKI 95-3: Lighting 0 16 17 18 19 20 21

24 h 16 h 12 h 8h 22

Inlet Temperature TABS (°C) 60 16 17 18 19 20 21 22

50 320 Wh/m2 d 40 240 Wh/m2 d 30

Indoor Environment and TABS

Due to the high thermal mass of the system, 0 the indoor temperatures will drift within a thermal comfort zone. This means that the heating or roomconditioning system does not keep a constant tem- Figure 4: Working principles of thermo active building systems. perature, but a comfort range. Kolarik, et al.,7 and Toftum, et al.,8 have shown that people ﬁnd temperature drifts ceiling panels. Other surfaces may be used for sound absorpwithin the comfort zone acceptable up to a rate of 4 K/h (7.2°F/h). tion; or partly covered ceilings or vertical positioned absorption In typical TABS’ buildings, the rate of change is 0.5 to 1.0 K/h (0.9 devices can be installed without impairing the heat transfer. to 1.8°F/h). It is important to notice that it is not acceptable for the occupants to drift outside the comfort range, which means a total Cooling Capacity of TABS drift of 3 K to 4 K (5.4°F to 7.2°F) during time of occupancy. Some detailed building system calculation models have been TABS do not have a direct effect on the indoor air quality. developed for the determination of heat exchanges under unHowever, an advantage is that in buildings with TABS, the steady state conditions in a single room, determination of thermal ventilation system is designed to meet the criteria for indoor and hygrometric balance of the room air, prediction of comfort air quality and/or the requirement for dehumidiﬁcation. As conditions, a check of condensation on surfaces, availability of the required water temperatures in most cases are higher than control strategies and calculation of the incoming solar radiation.9 19°C (66.2°F), it will also be possible in many applications to The use of such detailed calculation models is limited due to the have operable windows and rely on natural ventilation. high amount of time needed for the simulations. The developThe optimal capacity of TABS is obtained when there is free ment of a more user-friendly tool is provided in ISO 11855-3.10 heat exchange between the space and the concrete slabs. This The diagrams in Figure 4 (ISO 11855-3) show an exammay require solving acoustical issues without using suspended ple of the relation between internal heat gains, water supply
46 ASHRAE Journal ashrae.org February 2012

ASHRAE Journal - February 2012

Table of Contents for the Digital Edition of ASHRAE Journal - February 2012