Bachelor's thesis Maximilian Szymula


Simulative Investigation on the dimensioning of rotary enthalpy exchangers with regard to efficiency and profitability

recovered heat QERG in dependence of wheel diameter DR Copyright: EBC recovered heat QERG in dependence of wheel diameter DR

To decrease the primary energy consumption of air handling units, energy recovery systems are installed. A very efficient way to recover thermal energy from the exhaust air to condition the supply air is a rotary enthalpy exchanger. In comparison to other alternatives, rotary enthalpy exchangers are far more efficient regarding energy recovery. Rotary enthalpy exchangers are that efficient because they do not only recover thermal energy, but they can also be used to humidify or dehumidify the supply air due to their sorptive coating.

The energetic and cost specific potentials of an air handling unit with a rotary enthalpy exchanger were investigated. Simulations were used for the investigation, due to their time and cost efficiency compared to other methods. Therefore, a dynamic model of a rotary enthalpy exchanger was developed using the modeling language Modelica. All state conditions are passed on using Real-Connectors to avoid any discontinuities in the fluid-models of the Modelica-library. Furthermore, the model has a variable number of grid points in axial and tangential direction to calibrate the model quality in comparison to the CPU-time. To implement the number of revolutions of the rotary enthalpy exchanger the boundary conditions of each volume are switched every half revolution. To switch the boundary conditions a new approach was developed in this thesis, which is 30 % more time-efficient than an if-condition.

To conclude the model a control for the number of revolutions was implemented. With the control, temperature and absolute humidity are controlled by using the target values of a setpoint field. Therefore, the wheel is slowed down before the rotary enthalpy exchanger influences the supply air negatively.

Several variations of wheel diameter and wheel depth were investigated to optimize the dimensioning of the rotary enthalpy exchanger. To simulate the air handling unit in a moderate climate, a test reference year of the city of Cologne was chosen. Based on the usage profile of the room model the air stream is varied between 0,2 to 0,8 kg/s.

From the investigation, the optimal dimension of the rotary enthalpy exchanger with respect to cost and energy efficiency was found to be a 0,8 m in diameter and 0,25 m in depth. If the optimal dimensioning is compared to other dimensioning alternatives, it was discovered that due to increasing the investment based on a higher material need larger dimensioning than the optimum is not reasonable regarding profitability.

Especially heaters and humidifiers can be bought in a cheaper version due to the high potential of heat and humidity recovery by the rotary enthalpy exchanger. The rotary enthalpy exchanger recovers 86,5 % of the heat and 97,05 % of the water required by the air handling unit.