modelAHU

Information

Overview

This is an ideal model of an air handling unit (AHU), primarily to calculate the thermal energy consumption of an air handling unit (AHU) but also the electric power. The model is mainly based on thermodynamic equations.

It is based on incoming and outgoing enthalpy flows of moist air (thermodynamic principle).

The model aims to need little computational effort. Therefore state machines represent the basis of the model and no Modelica Fluid is used.

If simulation runs instable, reduce clockPeriodGeneric and/or use one of the alternative equations for the calculation of p_sat_surface in both dehumidification state machines (see source code of these state machines).

Assumptions

For further explanation for each parameter see noted sources and [1]! Please note that the assumptions are made regarding AHUs which are implemented in laboratories.

• BPF_DeHu: by-pass factor of cooling coil during dehumidification; Expression for the amount of air that by-passes unaffectedly over a coil while the remaining fluid comes in direct contact with the coil. [2, p. 500]; BPF_DeHu between 10 % and 35 % acc. to [3]
• efficiencyHRS_enabled: temperature differential; efficiency of heat recovery system (HRS) when it is enabled (for HRSs without sorptive technology efficiencyHRS_enabled = 0.6 ... 0.8 [4])
• efficiencyHRS_disabled: temperature differential; efficiency of heat recovery system (HRS) when it is disabled (proposal for parameter efficiencyHRS_disabled = 0.2 ... 0.4 [1])
• dp_sup, dp_eta: pressure drop over ventilator; recommendation dp = 800 Pa [7]
• eta_sup, eta_eta: efficiency of supply/extract air fan. Assumed as constant in this model.

Known Limitations

• static model, not dynamic
• no technical restrictions (demanded goal/value will always be reached)
• no implementation of merging two or more mass flows
• only adiabatic humidification
• no moisture transfer in HRS
• the pinch temperature HRS component is 0 K
• if absolute humidity of outside air (input connector "X_outdoorAir) exceeds X_saturated(T_oda) calculated with phi=1, X_oda is set to X_saturated

Concept

• Producing output data (thermal and electric power and outgoing air flow rate) by using some input data (black-box-principle). The inner components of an AHU are considered within this model.
• Based on sketch/schema of AHU shown in figure 1 [5, appendix D].
• This model of an AHU is able to represent 5 cases: only heating, only cooling, dehumidification, humidification plus heating, humidification plus cooling.

Figure 1 [5, appendix D]

References

• [1] Mehrfeld, P. (2014): Experimentelle Untersuchung von Lüftungstechnik in Laboren (master thesis). RWTH Aachen University, Aachen. E.ON Energy Research Center, Institute for Energy Efficient Buildings and Indoor Climate; supervised by: Lauster, M.; Müller, D.
• [2] Khurmi, R. S.; Gupta, J. K. (2009): Textbook of Refrigeration and Air Conditioning. 4th ed. New Delhi: Eurasia. (682 pages). ISBN 9788121927819.
• [3] Lindeburg, M. R. (2013): Mechanical Engineering Reference Manual for the PE Exam. 13th ed. Belmont: Professional Publications, Inc. (1488 pages). ISBN 9781591264149.
• [4] Verein Deutscher Ingenieure e. V.: VDI 2071:1997-12: Wärmerückgewinnung in Raumlufttechnischen Anlagen. Richtlinie. Berlin: Beuth Verlag GmbH.
• [5] Deutsches Institut für Normung e. V.: DIN V 18599-3:2011-12: Energetische Bewertung von Gebäuden – Berechnung des Nutz-, End- und Primärenergiebedarfs für Heizung, Kühlung, Lüftung, Trinkwarmwasser und Beleuchtung – Teil 3: Nutzenergiebedarf für die energetische Luftaufbereitung. Vornorm. Berlin: Beuth Verlag GmbH.
• [6] Schweizerischer Ingenieur- und Architektenverein (2006): SIA 2024: Standard- Nutzungsbedingungen für die Energie- und Gebäudetechnik. Merkblatt.
• [7] Gilroy, E.: Designing - Building Services: fan power, it will blow you away! PM Group. Online available at http://www.pmgroup-global.com/pmgroup/media/News-Attachments/ Fan-Power,-it-will-blow-you-away!pdf, last accessed on 23 September 2014.

Example Results

20 validation experiments are documented in [1, chapter 4 and 5].

• February, 2016  by Philipp Mehrfeld:
  Added previous() functions in source code as decisions for state machines and calculations in state machines did not use values of the same time step.
• June 17, 2015  by Philipp Mehrfeld:
  Changes in Inputs, transition conditions, BPF_DeHu, etc. Added functionality for mode selection.
• Septmeber, 2014  by Philipp Mehrfeld:
  Model implemented

Parameters

Components

Contents