*2.1. Air Conditioning System*

As shown in Figure 2, system layout of the installation can be divided basically into three subsystems in form of the reference room, the air handling unit and the hot and cold water circuit. The air handling unit is designed as hybrid system combining an open desiccant assisted air handling process with closed-loop heating and cooling circuits similar to the system presented by in [33,36].

**Figure 2.** System layout of the test facility as used during summer and winter operation.

A brief description of system operation in summer and winter mode is given for the sake of completeness. Summer and winter operation are considered separately according to [36]. Considering dehumidification mode during summer, outside air (oda) is dehumidified within a desiccant wheel (1 →2) and precooled by a sensible rotating heat exchanger (2 →3). Water vapor is accumulated at the hygroscopic coating of the desiccant wheel (DW); lithium chloride (LiCl) is used as desiccant. Afterwards, process air is finally cooled or heated to the desired supply air (sup) temperature within a sensible water to air heat exchanger (3 →4). Extract air (eta) from the reference room is preheated (5 →6) by the heat recovery wheel (HRW) and further heated to the required regeneration air temperature within another sensible water to air heat exchanger (6 →7). Finally, eta is used to regenerate the desiccant material (7 →8), before it is emitted to the environment in form of exhaust air (exa). To achieve efficient operation, di fferent components of the air handling unit can be bypassed as shown in Figure 2. Thus, electricity demand of the fans is reduced for demand-oriented air conditioning.

Regarding winter operation, the desiccant wheel is operated as enthalpy wheel at higher rotational speed for coupled heat and mass transfer (1 →2) relying on passive air humidification. Oda is remoistened and reheated within this process using the eta stream. If oda humidity is within comfort limits regarding humidity ratio, it is preheated using the regenerative heat exchanger (2 →3); the desiccant wheel is bypassed in this case. Otherwise, the heat recovery wheel is not utilized. The reheater (3 →4) is used to adjust process air to the desired sup temperature. Eta is either used for sensible heat recovery (5 →6) or coupled heat and moisture recovery (7 →8). The heater (6 →7) is not operated in winter operation mode.

Both wheels have a diameter of 0.6 m. The reference room is connected to the air handling unit on the supply and extract air side for air exchange. Furthermore, to cover sensible heat and cooling loads directly, it is equipped with underfloor heating and cooling ceilings.

Desiccant assisted air conditioning enables the integration of shallow geothermal energy for cooling in summer due to the fact that the required temperature level for cooling applications is above dew point temperature at any time. Due to the capacity of the soil, a cold water storage is not integrated into the cold water circuit with respect to summer operation. Utilizing the geothermal system during full year operation as heat sink and heat source is essential for the reason of improving the annual energy balance of the soil as well as for the reason of maximizing the use of renewable energies around the year. Thus, heat supply during winter is primarily relying on a ground-coupled heat pump system ( . *Q*GCHP,nom = 5.1 kWth at BW5/W30). Solar thermal energy is utilized as primary heat source during summer mode ( *A*STU = 20 m2). A small-scale gas driven cogeneration (CHP) unit ( . *Q*CHP,nom = 12.5 kWth, *<sup>P</sup>*CHP,nom = 5 kWel) is used as backup system and to cover peak loads throughout the year. Integrating a stratified thermal storage system ( *V* = 1 m3) into the hot water circuit enables heat supply and heat demand to be decoupled temporally.
