*3.4. System Comparison*

The investigated system is compared to di fferent reference systems in order to further evaluate its energetic performance. These reference systems are designed as mathematical simulation models relying on measured data of the investigated system (DW-GEO). Thus, the thermodynamic processes of the considered reference systems were modeled relying on simplified thermodynamic relations and required assumptions. Measurement data of the investigated system were used whenever similar air states are expected for the reference systems in comparison to DW-GEO. In order to ensure a fair system comparison, oda and sup conditions as well as sup mass flow rate and room air conditions were assumed to be equal with one exception that is explained later on. Air dehumidification as well as air humidification within the reference systems are considered according to the equivalent operation modes of the investigated system. In each operation mode, summer and winter, two di fferent reference systems were designed. During summer operation, reference systems relying on an electrical powered vapor compression chiller are considered. Humidifying processes with adiabatic or isothermal air humidification are considered for the reference systems regarding winter operation. The individual characteristics of each reference system are summarized in Table 4.



Supply air temperature of the conventional reference system (DP-VC) was set to 18 ◦C in order to ensure a fair system comparison. Changes in thermal load discharge by air and cooling ceilings was considered. The second reference system for the cooling period is a hybrid air conditioning system (DW-VC); the BHE is replaced by a vapor compression chiller, while the rest of the investigated system remains unchanged. The specific layout of corresponding air handling units for the reference systems DP-VC and AH-GEO/IH-GEO is shown in Figure 11. Highlighted components are specific for the designated reference systems, whereas the rest of the air handling unit remains the same for these systems.

Figure 12 shows the results of the system comparison for the considered periods. Legend entry "supply" includes electricity demands of the compression chiller (DP-VC, DW-VC) or rather the BHE circulation pump (DW-GEO) during summer operation, see Figure 12a. During winter operation, shown in Figure 12b, it includes the heat pump and corresponding auxiliary energies, respectively. Electricity demand of the AHU and circulation pumps of the hydraulic circuits are summarized to category "air treatment and distribution" for both periods. Electrical and thermal energy demands are

related to the entire periods under consideration. All values of electrical and thermal energy demands shown in Figure 12 are also listed in Table 5 for more transparency.

**Figure 11.** Specific layout of air handling units for different reference systems.

**Figure 12.** Total electrical and thermal energy demands of the investigated system (DW-GEO) and reference systems (summer: DP-VC, DW-VC; winter: AH-GEO, IH-GEO) for the investigated periods: (**a**) cooling period; (**b**) heating period.


**Table 5.** Electrical and thermal energy demands for system comparison.

As shown in Figure 12a, the investigated system boasts significant reductions in electrical energy demand compared to the reference systems during summer operation. The savings sum up to 50% compared to the conventional system (DP-VC) and 34% compared to the hybrid system (DW-VC). These savings were primarily caused by the chiller unit (black column) that required 61% of the total electrical energy demand for the system DP-VC, whereas just 1% of the system DW-GEO was required for the BHE circulation pump. Due to the fact that only sensible cooling is required for the chiller unit of the hybrid system, its electricity demand was reduced by more than 50% compared to the conventional system. The overall AHU electricity demand of the system DP-VC was lower compared to the desiccant assisted systems (DW-GEO, DW-VC), because pressure drop across the desiccant wheel was saved for this system. Considering thermal energy demands, the conventional system shows the lowest thermal energy demand for reheating supply air. The required regeneration process within the desiccant assisted systems caused an increase of thermal energy demand by a factor of 1.2 compared to the system DP-VC. In total, the differences in thermal energy demand were not significantly high caused by the moderate summer period with a certain amount of oda conditions that required reheating but no or only moderate dehumidification. Nevertheless, the increased thermal energy demand of desiccant assisted air conditioning requires a convenient and low-cost heat source regarding primary energy demand of heat supply.

During winter operation, as shown in Figure 12b, the benefits of the investigated system (DW-GEO) are not as obvious as in summer mode. Thus, electrical and thermal energy demands have to be analyzed carefully. For the reference system with electric isothermal air humidification (IH-GEO) electricity demand for air treatment and distribution was increased by a factor of 1.8 compared to DW-GEO. This was significantly induced by the electrical steam humidifier that accounts for nearly 50% of the corresponding gray column. The corresponding electricity demand for AH-GEO was reduced by 16% compared to DW-GEO for the following reasons. First, the additional electricity demand for the impeller humidifier is low and second, the pressure drop associated with the enthalpy wheel is saved. This applies for IH-GEO, respectively. Due to the reasons mentioned above, the electricity demand to operate the GCHP system is almost equal for the systems DW-GEO and AH-GEO. For the reason of high temperature steam used to humidify supply air within the system IH-GEO, required GCHP power was lower in terms of adjusting sup stream to the desired sup temperature. In total, the electricity demand of DW-GEO was reduced by 18% compared to IH-GEO and it was increased by 7% compared to AH-GEO.

Thermal energy demand required for underfloor heating is assumed to be equal for the three systems. Thermal energy required for reheating sup to the desired sup temperature was about 44% higher for the DW-GEO and AH-GEO systems compared to the system with isothermal air humidification due to substitution of thermal energy by electricity as described above. Even though the differences are quite small, considering both, electrical and thermal energy demands, system

comparison shows lowest total energy demand for AH-GEO. This results from the slight advantage in electricity demand of AH-GEO against the investigated system. Thus, energetic benefits of the investigated system were limited for the considered heating period. It is expected that increased benefits will be achieved for winter terms with lower oda temperature and water content. Nevertheless, moisture recovery using an enthalpy wheel is beneficial against the reference technologies from a hygienic point of view, especially for the use of LiCl desiccant material.

Summarizing, taking summer and winter operation into account, the investigated system boasts significant reductions in electricity demand, resulting from the electricity savings during summer operation. Additional thermal energy demand required for regeneration of the desiccant wheel is not significantly higher at the same time. The required temperature level up to 70 ◦C can commonly be easily provided by solar thermal energy. Nevertheless, providing thermal energy as favorable as possible is crucial considering the economic e fficiency of the system. Another advantage of the investigated system is the reduced amount of mode specific equipment. Subsystems like the desiccant wheel and the geothermal system of the investigated system are used throughout the year, whereas this holds not true for the chiller and the air humidifier of the reference systems.
