*1.1. Desiccant Assisted Air Conditioning*

Valkiloroaya et al. [6] presented an overview of di fferent strategies and technologies to reduce energy demands related to air conditioning in general. Desiccant-assisted air conditioning has been found to be a promising alternative to conventional air conditioning processes relying on a vapor compression chiller in terms of reducing the electricity demand for air conditioning. From a global perspective, air conditioning systems are primarily used during summer operation, providing cooled and dehumidified air. Thus, a lot of di fferent studies dealing with the improvement and evaluation of desiccant materials and di fferent system configurations for dehumidification mode. Several studies provide overviews of di fferent concepts for desiccant assisted air conditioning systems with both solid and liquid desiccant material [7–11]. Within the field of systems relying on a solid desiccant material, a considerable amount of studies investigate design and performance of desiccant wheels [12–14]. Desiccant assisted hybrid systems are known as system configuration relying on an open sorption process and closed-loop cooling circuit. Several studies have been undertaken to investigate energetic advantages of hybrid systems for di fferent locations [15–21]. To further reduce the electrical energy demand related to air conditioning, shallow geothermal energy is shown as promising alternative and renewable heat sink [22–25].

With respect to full year operation, final energy demand for space heating is currently still higher than final energy demand for space cooling applications from a global perspective [4]. But even though winter mode is obviously an essential part of full year operation, especially for heating dominated regions, winter mode as well as full year operation of desiccant assisted air conditioning systems are addressed only in few studies. Beccali et al. [26] investigated a hybrid system during summer and winter operation experimentally for the climate conditions of southern Italy. The presented system is relying on solar thermal heat supply with additional gas boiler backup system; a compression chiller is utilized for cooling. During summer operation, a reduction in primary energy demand of nearly 50% was achieved compared to a conventional reference system. Required information about system performance regarding moisture control in winter mode are not provided. De Antonellis et al. [27] investigated experimentally and numerically humidification of outside air using a desiccant wheel with silica gel coating for Mediterranean winter conditions. The authors investigated energy demand and occupants' discomfort for the considered system configuration and highlight the dependence of air humidification performance and required regeneration air temperature. To further evaluate the system's performance, a comparison with conventional humidification technologies is presented from an energetic point of view. Simulation results show reduced primary energy demand for air humidification using the proposed system compared to reference systems with adiabatic and electrical steam humidifiers for di fferent working conditions. Compared to reference systems with steam to steam humidifier primary energy demand of the proposed system was increased for the considered boundary conditions. Kawamoto et al. [28] investigated the combination of a desiccant-assisted system and a heat pump that is used for heat supply on the regeneration air side experimentally in Japan. La et al. [29] examined a system configuration with solar thermal heat supply and one-rotor two-stage desiccant wheel for winter in Shanghai experimentally and numerically. The proposed system uses extract air from the conditioned space to humidify supply air preheated with solar thermal energy. The study shows significant increase in thermal comfort due to air humidification. Furthermore, the authors draw attention to the space requirements for solar collectors to improve thermal comfort. Full year operation of a desiccant- assisted evaporative system in Austria was investigated experimentally by Preisler and Brychta [30]. The investigated system achieved a reduction in primary energy demand of 60% in comparison to a reference system relying on a vapor compression chiller, considering full year operation. The authors outline high energy saving potentials of the investigated system, whereas details about the humidification process and boundary conditions of system comparison are not provided.

#### *1.2. Air Dehumidification and Moisture Recovery*

Regarding desiccant wheel performance in dehumidification and enthalpy recovery mode, Zhang and Niu [31] investigated different desiccant wheels numerically by means of a two-dimensional heat and mass transfer model. From their simulation results the authors conclude that the optimal rotational speed of a wheel used for dehumidification is much lower than optimal rotational speed of a wheel utilized for enthalpy recovery.

Increasing the moisture level of supply air is a sensitive but often little noticed comfort aspect during winter. Dry indoor air conditions can adversely affect occupants' comfort, especially in modern buildings relying on mechanical ventilation without additional humidification systems during winter. Conventional air conditioning systems require additional components to achieve sufficient supply air humidity ratios. This is an advantage of desiccant assisted systems, because moisture recovery by means of the existing hygroscopic material is possible. A further hygienic advantage of desiccant assisted moisture recovery against conventional air conditioning relying on adiabatic or isothermal air humidification is the fact that no liquid or vaporous water is sprayed into the process air stream. Thus, emission of bacteria caused by air humidifiers as for example described by Strindehag and Josefsson [32] is avoided.

#### *1.3. Previous and Ongoing Investigations of the Considered System*

To the best of the authors' knowledge there is no study investigating summer and winter operation of an air conditioning system relying on desiccant assisted dehumidification, enthalpy recovery and a ground-coupled heat pump for heating dominated climate conditions. In [33,34] the considered system is evaluated during summer operation mode, using a borehole heat exchanger (BHE) for cooling. The system is verified to be promising against conventional air conditioning systems in temperate climate regions. Furthermore, Speerforck et al. [35] proved applicability of the proposed system at different investigated locations by means of a dynamic system model using modeling language Modelica®. The authors investigated summer operation, whereas winter mode is not observed.

Within this study the geothermal and desiccant assisted system is investigated experimentally during summer and winter operation to show system performance throughout the year. Moisture control is achieved by a desiccant wheel (summer) or enthalpy wheel (winter) to improve indoor air conditions; sensible cooling loads are primarily covered by cooling ceilings, whereas heating loads are primarily covered by underfloor heating, respectively. In combination with a geothermal system, temperature levels of cooling ceilings and underfloor heating enable efficient operation of shallow geothermal energy in combination with a ground-coupled heat pump (GCHP) during winter. Regarding an equalized energy balance, utilizing the soil for cooling and heating is essential.

This study is structured into three parts. First, a short description of the investigated system, operation modes and data acquisition is provided. Afterwards, the performance and limitations of the investigated system are analyzed. Especially, performance of the air handling unit and the geothermal system are considered. The effects on indoor air conditions in terms of thermal comfort are investigated in detail. Additionally, the system is compared to different reference systems regarding electrical and thermal energy demands. Finally, the main findings are summarized and future research work is addressed. This study is an extension of Niemann et al. [36], providing a previous experimental analysis on summer and winter operation of the investigated system.
