Heat Exchanger Models for Ground Source Heat Pumps

Photo of Pedro Carneiro Soares, Pedro Ferreira, Mikko Ponkala Students: Pedro Carneiro Soares, Pedro Ferreira, Mikko Ponkala

Sponsor: The University of Texas at Austin

Date: Fall 2010

Requirements:
The primary requirement requested by the sponsor was that the heat exchanger models must be adaptable to a variety of manufactured heat exchangers. Other requirements included validation against published empirical heat exchanger data, and the model must be based on first principles and basic heat transfer and thermodynamic relationships. To interface the heat exchanger models with the rest of the cycle model, all the code was developed in MATLAB. In essence, the heat exchangers will be subroutines in the overall cycle models being developed by the sponsor. Time and budgetary constraints prohibited the purchase of concentric tube or fin-and-tube heat exchangers for validation purposes.

Problem:
This project focused on development of computer models of evaporators and condensers for both a air-source heat pumps (ASHP) and closed loop ground-source heat pumps (GSHP). Both the evaporator and the condenser in the ASHP are fin-and-tube refrigerant-to-air heat exchangers. The evaporator in the GSHP is a fin-and-tube exchanger similar to that in the ASHP, while the condenser is a concentric tube refrigerant-to-water heat exchanger. Additionally, the concentric tube heat exchanger model may be used to model a desuperheater, which is placed at the compressor outlet to provide domestic hot water in a combined cycle set up.

Solution:
For the fin-and-tube heat exchanger, a model developed by Oak Ridge National Laboratory (ORNL) was chosen. For the concentric tube heat exchanger, a desuperheater model developed previously at UT Austin was used. After the selection process, property calculators for R410A, air, and water were formulated. The fin-and-tube heat exchanger was modeled after the analysis performed in the ORNL, whichwas adapted to predict outlet air and refrigerant conditions given inlet temperatures, inlet flow rates, and parametric heat exchanger data. Even though the fin-and-tube model is not fully functional, the iterative procedure required has been prepared and can be used with more accurate heat transfer correlations. For the concentric tube heat exchanger, the chosen model was intended to be used for desuperheaters only; the subcooled region of the refrigerant was added by UTME-GHP. The subcooled refrigerant was modeled as an incompressible liquid similar to water. Even though the original provided pressure drop and convective heat transfer coefficients for both superheated and two-phase regions, only the convective heat transfer corresponding to two-phase was used and the pressure drop was not accounted for.. Even with these limitations, the concentric tube heat exchanger model predicted the water outlet temperature and capacity correctly while overestimating the refrigerant outlet subcooling. From the condenser, a desuperheater was also developed by eliminating the two-phase and subcooled regions.

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