EnOB: Dyn-GSGK: Using dynamic geothermal systems for sustainable desiccant assisted air conditioning

Project description

Object of the research project is the development of an air conditioning system for basic and peak loads based on renewable energies. Therefore, at the Institute of Engineering Thermodynamics the combination of an open cycle desiccant-assisted air conditioning system and a geothermal system is investigated. A key aspect is a reduced cooling demand and lower peak loads due to the decoupling of sensible cooling and dehumidification within the air conditioning process. Additionally, cooling power can be increased by using a dynamically controllable borehole heat exchanger (BHX) for the first time. The single components as well as the interaction of the components as a system are investigated. In case of uncomfortable room air conditions despite the additional cooling power, a backup system including phase change material to locally enhance comfort perception is analyzed.



In summer, air conditioning systems need to dehumidify and cool outdoor air. Conventionally, air is cooled below dew point temperature in order to dehumidify the air. Afterwards the air is reheated to reach supply air temperature level. In contrary, dehumidifying and cooling is decoupled within a desiccant assisted system. Hygroscopic materials like Lithiumchlorid or Silica gel are used for dehumidification. As a result of the decoupled process of latent and sensible cooling, the cooling demand is reduced and heat sinks with higher temperature levels (approx. 16 to 19 °C) can be used. Therefore, heat sinks like shallow geothermal energy can be implemented efficiently. Despite the benefits of the desiccant system, peak load demand still represents a problem. In the consequence of high ambient temperatures and a constantly high cooling demand, the cooling power of the soil can be exceeded. As a result, oversizing the geothermal system is needed to accomplish comfortable room temperatures during peak loads due to internal or external effects.



The objective of the research project Dyn-GSGK is to investigate covering of peak loads during summer operation by combining the patent-registered methods of desiccant dehumidification with subsequent geothermal cooling (applicant Prof. Dr.-Ing. Schmitz, EU-Patent 1368596) and the air injection borehole heat exchanger (applicant Prof. Dr.-Ing. Grabe, EU-Patent 09401001.4-2301). Initially, the desiccant assisted air conditioning systems reduces peak demands and the air injection borehole heat exchanger covers the remaining peak load due to the dynamic response characteristic.

By inducing an artificial ground water flow due to air injection inside the well the cooling power can be adjusted according to the demand within seconds. In contrast, conventional geothermal systems can not be adjusted in power or according to the demand. As a result of the dynamic behavior, oversizing of the geothermal system is not necessary. Besides the cooling demand, desiccant assisted systems need thermal energy in order to enable a continuous process. To further improve the coverage of the energy demand by renewable energies, pre-cooling of the process air is investigated. If, however, uncomfortable room air conditions occur, a backup system consisting of phase change materials (PCM) is investigated. The PCM is encapsulated in plates and placed in a rack. By means of computer case fans an airflow is induced throughout the rack. By bypassing the PCM the hot air is cooled due to the occurring phase change from solid to liquid of the PCM. As a result, the cooled air can be used to locally reduce the temperature and improve comfort perception.


Besides summer, winter operation is investigated for the first time. In order to achieve an equalized energy balance throughout the year, using the geothermal system as a heat sink in summer and as a heat source to power a ground-coupled heat pump in winter is meaningful. Additionally, the ceiling surface heat exchanger is investigated in regard to heating application. In terms of humidification, the hygroscopic material is used to humidify the supply air and improve indoor humidity during dry winter days. Compared to conventional systems, additional components to humidify the air in winter are not needed.

EnOB: Dyn-GSGK: Nutzung dynamischer geothermischer Systeme zur sorptionsgestützten nachhaltigen Klimatisierung

Project leader: Prof. Dr.-Ing. Gerhard Schmitz, Institute of Engineering Thermodynamics
Project partner: Prof. Dr.-Ing. Jürgen Grabe, Institute of Geotechnical Engineering and Construction Management
Project coordinator: Finn Richter, M. Sc., finn.richter@tuhh.de
Funding code: 03ET1421A
Project duration: 01.08.2016 – 31.10.2020
Project executing organization: PJT, Jülich
Funding agency: Federal Ministry for Economic Affairs and Energy

Supported by the Federal Ministry for Economic Affairs and Energy