Master's Thesis Tobias Gülker
Modeling and implementation of moisture recovery systems in a planning tool for the energetic and economic evaluation of air handling unitsCopyright: EBC
The aim of the German climate protection plan is to save energy of buildings and to promote corresponding technologies. 60 % of the energy demand of non-residential buildings is required to condition room temperature and humidity. Thermal comfort is often achieved by using air handling units. Enthalpy recovery systems reduce the energy requirement of air handling units. Two common technologies for enthalpy exchangers are membrane-based enthalpy exchangers and rotary energy wheels.
An estimation of the resulting energy demand of air handling units over the course of seasonal weather changes is especially advantageous at the state of designing and planning. The use of a planning tool facilitates the extensive calculations of the components. A planning tool developed by the Institute for Energy Efficient Buildings and Indoor Climate does not take into account enthalpy recovery systems. Thus, enthalpy recovery systems were modeled and integrated into the planning tool. The modelling is based on a method using a constant efficiency, the Effectiveness-NTU method and performance charts.
Using the planning tool, a parameter study is then carried out comparing the utilization of membrane-based enthalpy exchangers and rotary energy wheels with heat exchangers. In the parameter study, internal loads, outside air temperature and humidity range are varied while keeping constant conditions of thermal comfort. In addition, seasonal climate data of the cities Aachen, Chicago and Bangkok are considered.
The results of the parameter study prove that enthalpy recovery systems are particularly advantageous in cold and dry as well as hot and humid outdoor air conditions. In Bangkok, this leads to the largest primary energy and economic savings. In consequence, enthalpy exchangers are superior to competing plate heat exchangers especially when dehumidifying and recovering cold.
The parameter study shows that all modeled methods reflect trends as expected. However, the constant efficiency method greatly simplifies transport processes. The results of the NTU method deviate up to 10 % from the performance charts method. Nonetheless, the NTU method has a 40 to 60 times shorter computation time. This results in a trade-off between accuracy and computation time.
With regard to subsequent work, negative internal loads have not yet been taken into account. In addition, the control logic of the enthalpy exchangers and the switch-on temperature of the preheater in combination with an enthalpy exchanger can be optimized. The validity range of the NTU method should also be extended. Furthermore, the implementation of this planning tool could be extended to non-full air handling units.