Thermal Comfort Performance in Summer

5.2Thermal Comfort Performance in Summer

Room temperatures during summer

Figure 4 illustrates the range of operative room temperature during the time of occupancy in summer for the individual buildings. Monitoring results are presented as boxplot indicating 50 % of the temperature measurements as well as maximum and minimum occurrences. Obviously, temperature values scatter over a wide range considering both the individual building and all buildings within one category. This depends on the prevailing ambient air temperature, the building physics, the use of the buildings, the behavior of the occupant, and the cooling system.

Central results can be summarized as follows:

4. 
   The mean room temperature during the time of occupancy in summer is 24 °C for the buildings with passive and air-based mechanical cooling. Considering the water-based mechanical and air conditioned buildings, the mean room temperature is with 23.5 °C slightly lower.
   
5. 
   Maximum temperature occurrences are about one Kelvin higher in the buildings with passive and air-based mechanical cooling that the buildings with water-based and air-conditioned cooling. All buildings have exposed rooms where operative room temperatures reach a maximum of up to five Kelvin higher than the average.
   
6. 
   However, each category contains buildings that exceed the mean temperature level of the category significantly.
   
7. 
   Obviously, the occurring range of operative room temperature in the buildings with passive and air-based mechanical cooling is markedly wider than in the other buildings.
   
8. 
   Considering the three buildings in South and South-East Europe (Italy, Greece, and Romania), room temperature are elevated significantly in comparison to the North European and German buildings. (Cooling concepts for different European climates will be studied further in Chapter 6).
   
9. 
   The German building with full air-conditioning (cooling and dehumidification) has the narrowest temperature band during occupancy, that is 22.5 to 23.5 °C.
   
10. 
    Surprisingly, the room temperatures of the buildings with water-based cooling scatter noticeably around the mean temperature of 23,5 °C. Some buildings have relative elevated temperature ranges (e.g., the German buildings EBD, LHL, SCF, TMZ, and the Italian building). On the contrary, some buildings are mainly below the average temperature range of the building category (e.g., the German buildings GMS, BOB, EGU, ZUB). Low-energy buildings that reduce and cover the energy demand mainly by passive technologies and environmental heat sources and sinks are usually heavy-weight constructions. Due to the thermal mass, they behave indulgently towards exterior and interior changes, e.g. higher ambient air temperature or higher internal loads due open solar shading or open windows. This means that the heavy-weight building construction buffers the rise of the operative room temperature for a certain period avoiding overheated offices and rooms. However, the monitoring results indicate that the buildings with cooling by means of radiant cooling systems and environmental energy -systems with lower cooling capacity- are nevertheless sensitive towards the applied control and operation algorithms and towards the occupant behavior. Therefore, room temperature cannot be kept in a narrow range.
    
11. 
    Evidently, the comfort performance is not strongly affected by the type of environmental heat sink employed (i.e, using the ground, ground water, ambient air), provided that the heat sinks is adequately dimensioned and well operated.
    
12. 
    An unexpected result is the wide range of comfort ratings within a given building, e.g. some monitored rooms do not violate the comfort boundaries at all, whereas others reveal a significant exceedance. This discrepancy within a building is mainly affected by the orientation of the rooms (Figure 3 ), the presence of the occupants and the occupants’ behavior in terms of opening windows and using solar shading, which is not monitored in this investigation
    

Figure 4 Operative room temperatures during occupancy in summer [°C] illustrated as boxplot. Buildings are grouped according to the building categories introduced. The European buildings are indicated by an orange shading. Further given are the median (solid grey line), 75^th and 25^th percentile (dashed grey line) of all values considering on building category. Remark: Highlighted are the European buildings.

Since there are just two buildings for the category of mixed-mode cooling, this building category cannot characterize sufficiently.

Thermal Footprint during Summer

13. 
    An unexpected result is the different frequency of exceeding the upper and the lower comfort boundaries, respectively. Taking the European buildings as an example, fewer offices exceed the comfort boundary for class II, however, mostly during less than 5 % of the occupancy. On the contrary, in many office rooms the operative room temperatures fall significantly below the lower comfort boundary of class I and II, even at higher ambient air temperatures.
    
14. 
    Buildings with passive cooing: Thermal comfort in the buildings with passive cooling differs a lot, achieving the comfort classes I to II or even failing the classes. Most of the buildings show an exceedance of comfort class II during 2 to 10 % of the time of occupancy. Moreover, temperature measurements violate clearly the lower comfort boundaries, in particular during summer periods with moderate ambient air temperature between a running mean of the outdoor temperature between 15 and 20 °C. Most of the buildings employ free night ventilation by means of open windows and ventilation slats, which does not enable to influence the room temperature. However, room temperatures hardly fall below 21 and 20 °C.
    
15. 
    Buildings with air-based mechanical cooling: The buildings with air-based mechanical cooling comply during 80 to 95 % of the occupancy with comfort class II considering the adaptive approach. Buildings that employ mechanical night ventilation by means of an exhaust or supply and exhaust system allow for a fixed air change rate between 2 and 4 ACH. Therefore, cooling capacity is higher and less depend on prevailing wind situation and temperature difference between indoor and outdoor temperatures. Although the building ISE that uses nighttime ventilation only, performs surprisingly well in 2002 (class I), a persistent heat wave in 2003 resulted in an extensive violation of comfort class II. On the contrary, the building BOB using TABS in combination with the ground as heat sink can be assigned to comfort class II in summer 2003. Evidently, air-based systems are not as effective as water-based thermo-active building systems, in particular during prolonged periods with high ambient air temperatures. Taking the extreme summer conditions of 2003 in Europe as indicative for a global warming scenario, thermal comfort in compliance with EN 15251:2007-08 cannot be ensured in buildings that employ night ventilation only. Rising ambient air temperatures decrease the cooling potential of night ventilation and demand elevated air change rates for a mechanical ventilation system. Besides, longer hot periods exhaust the building’s thermal storage capacity. TABS are essentially unaffected by these disadvantages, and thus present an effective concept for conditioning buildings in Central European climates, even in very hot summers, given a building with a high-quality envelope, solar shading devices, and reduced internal loads. The naturally ventilated buildings LAM and POL provide good thermal comfort during the hot summer 2003. This reduced cooling demand attributes to the high quality of the building envelope and the stringent load-reduction strategy (day lighting concept, solar shading system, energy efficient office equipment).
    
16. 
    Buildings with water-based mechanical cooling: Except two buildings, water-based mechanical cooling by means of thermo-active building systems and environmental energy provides good thermal comfort during summer. Applying the adaptive comfort approach, most buildings meet the upper comfort requirements of class II. Obvious is the frequent violation of the lower comfort boundaries. This can be observed in all buildings studied. Reason for an insufficient thermal comfort in those buildings can often be attributed to an insufficiently sized cooling system (e.g., cooling capacity of heat sink not sufficient at buildings TMZ and EBD) or inadequate control and operation algorithms (e.g., building IHD during the first year of operation).
    
17. 
    Buildings with air-conditioning: The German building PFI as an example for a fully air-conditioning building provided excellent thermal comfort during summer considering the upper boundaries of the PMV comfort approach. An air-conditioning system allows for well-defined and narrow temperature ranges during the time of occupancy and the dehumidification of the supplied air. However, monitoring results show a tremendous violation of the lower comfort boundaries, as well, which might result in dissatisfied occupants feeling too cold.
    

• 
  Considering the total exceedance of the upper and lower comfort boundaries, very few of the buildings monitored comply with the comfort classes II and II of the adaptive approach. This is almost exclusively attributed to the violation of the lower comfort boundaries. Unexpectedly, the results do not vary much taken different building spaces into consideration, i.e., 95, 84, 75, or 68 % of the building space. Most of the buildings have to be assigned to the overall comfort class III. This conclusion cannot be supported by the authors’ experiences with the performance of low-energy buildings and the user satisfaction. Further research is necessary in order to verify and adjust, if required, the upper and lower comfort boundaries for low-energy buildings that employ cooling systems with a limited cooling power such as thermo-active building systems. Measurements of long-term monitoring of building and plant performance as well as thermal comfort have to be related to comprehensive post-occupancy investigations. As a result, current approaches and guidelines for the evaluation of thermal and overall interior comfort need to be verified, perhaps revised and redefined for particular building categories. It might be necessary to adjust the slope of the lower comfort limit allowing greater temperature amplitudes between the lower and the upper limits, as for example suggested by the Dutch Guideline NPR-CR 1752 [van der Linden et al. 2006].
  
• 
  All buildings investigated prove to be energy efficient. The total primary energy consumption for heating, cooling, ventilation and lighting ranges between 32 and 240 kWh_prim/(m²_neta). The night-time ventilation concept provides useful cooling energy in the range of 5 to 18 kWh_therm/(m²_neta). Is an earth-to-air heat exchanger employed, the cooling energy is supplied with an energy efficiency of SPF 20 kWh_therm/kWh_prim (related to primary energy). The mechanical ventilation systems provide cooling energy with an efficiency of SPF 0.5 to 6 kWh_therm/kWh_prim (again related to primary energy). The environmental cooling systems provide useful cooling energy in the range of 5 to 44 kWh_therm/(m²_neta), which is provided with an efficiency of SPF 1.3 to 8.0 kWh_therm/kWh_end of the entire cooling system (related to the end energy use).
  

Figure 4 Thermal comfort footprint: Exceedance of both (top), upper (middle) and lower (bottom) comfort boundaries during the time of occupancy in summer for the buildings allocated to the proposed building categories. The adaptive comfort approach according EN 15251:2007-08 is used for the buildings with passive, air-based mechanical, water-based mechanical and mixed-mode cooling. The PMV comfort approach is considered for the two buildings with air-conditioning.