A Simplified Design Method for PEX Plastic Downhole Heat Exchangers

Abstract

In previous work, Chiasson et al. (2005) reported on the installation and monitoring of the first known downhole heat exchanger (DHE) constructed of cross-linked polyethylene plastic (PEX), a material known for its relatively high temperature and pressure rating, durability, and chemical resistance. The objective of this study is to develop a simple design method for determining DHE length based on field test data. Current design procedures rely on parameters that are difficult or impossible to quantify, and as a result, design lengths rely on empirical “rules of thumb”. The design method developed in this paper is based on the classical Line Source Solution for conduction heat transfer, which is used successfully to design of other types of borehole heat exchangers. The concept of effective thermal conductivity is used to combine the overall effects of conductive and convective heat transfer processes. The DHE thermal resistance and the effective rock thermal conductivity are readily determined from field test data. Use and verification of the design procedure are demonstrated using monitored data from an actual installation for the 2004- 2005 heating season.