*3.3. Evaluation of Thermal Comfort*

The quality of indoor air conditions is a result of the performance and operation strategy of the overall system. To provide an overview of comfort conditions for the investigated periods, outside and room air conditions during system operation are shown in Figures 9a and 10a with simplified comfort areas according to DIN EN 15251 [38]. Comfort areas of category I and II are defined to ensure less than 6% (cat. I) and 10% (cat. II) of occupants being dissatisfied with the present indoor air conditions. In order to exclude start-up effects, the first hour of system operation is not considered.

**Figure 9.** (**a**) Outside and room air conditions during system operation for the investigated cooling period; (**b**) dependence of room air temperature on BHE outlet temperature.

For the investigated cooling period, indoor air conditions according to cat. I were maintained for 55% and cat. II was maintained for over 96% of operation time, respectively. The reasons for the remaining violations were different for cat. I and cat. II. Cat. I was violated primarily due to too high indoor air humidity, whereas too high indoor air temperatures caused violations of cat. II.

In order to further analyze the reasons for overheating, Figure 9b shows the dependence of room air temperature ϑroom on water outlet temperature ϑout,BHE of the BHE. Outlet temperature of the BHE is similar to the cooling ceilings' inlet temperature. With good approximation, a linear increase of indoor air temperature with increasing BHE outlet temperature above 18 ◦C is visible. Maximum room

air temperatures were right above 26 ◦C at maximum BHE outlet temperature of ϑout,BHE,max = 20.5 ◦C. This is an effect of peak loads that could not be covered by the geothermal heat sink due to its limited capacity and little controllable thermal power output.

**Figure 10.** (**a**) Outside and room air conditions during system operation for the investigated heating period; (**b**) dependence of room air temperature on underfloor heating inlet temperature.

According to the German Meteorological Service, the investigated heating period can be classified as moderate. The average oda temperature in January was 1.67 ◦C at an average water content of 3.3 gw ·kg−<sup>1</sup> air. The following months were characterized by higher oda temperature and water content. As shown in Figure 10a, nearly 100% of oda conditions were outside comfort area according to cat. I and II. Days with oda conditions within the desired comfort area just occurred during springtime period at the end of March.

Around 67% of indoor air conditions satisfied the requirements according to cat. I. Cat. II was maintained for 75% of system operation time, respectively. The remaining violations were primarily caused by too low indoor air temperature. As a result of the control strategy for the underfloor heating system (UHS) and insufficient internal loads, 85% of indoor air temperatures below the desired temperature level occurred during the first four hours of daily system operation. A similar characteristic applied to indoor air water content. The level of sup and indoor air water content is mainly depending on oda water content if constant internal latent loads are provided. Thus, 75% of indoor air conditions characterized by insufficient level of water content occurred at oda water contents below 2.5 gw· kg−<sup>1</sup> air. Without humidification of supply air, over 60% of room air conditions would have been outside of cat. I. Over-humidification of room air did not occur during the entire heating period.

Figure 10b shows the dependence of indoor air temperature ϑroom on UHS inlet temperature ϑin,UHS. A trend of increasing fluctuation of indoor air temperature with increasing UHS inlet temperature is noticable, especially for ϑin,UHS > 32 ◦C. This is an effect of insufficient thermal power supply during the first hours after system start-up at low oda temperatures. Taking this initial situation, improvement of the control strategy for the UHS is required in terms of limiting periods at insufficient thermal comfort. The GCHP system was not operated between 10 pm and 7 am, while underfloor heating was provided 24 hours a day. Providing and storing sufficient amounts of thermal energy can be achieved by operating the GCHP system additionally during night. Resulting shorter regeneration periods of the soil have to be considered for the proposed operation strategy focusing on increase in thermal comfort.

Table 3 lists the relative shares of operation modes for both periods for the given outside air conditions and the implemented control strategies. The significant amount of DW or EW mode represents the importance of desiccant assisted air conditioning to provide a high level of indoor air conditions throughout the year.


**Table 3.** Relative shares of operation modes.
