Field Performance Assessment of Downhole Heat Exchangers in Puna District, Hawaii

Abstract

This paper reports on the field performance testing of a downhole heat exchanger (DHE) in a low-temperature geothermal well in the Puna District of Hawaii County, Hawaii. The performance assessment consisted of three components: field installation (and removal) of a test DHE, thermal response testing of the DHE, and analysis of the test data. The DHE was field-fabricated entirely of cross-linked polyethylene (PEX) plastic tubing and fittings, and consisted of two closed-loop U-tubes with a nominal diameter of ¾-inch (19 mm). The thermal response test involved applying a constant power pulse on a fluid flowing through the DHE and recording the inlet and outlet fluid temperatures. A mathematical solution to the so-called Kelvin Line Source model, which describes heat conduction from an imaginary line into a semi-infinite medium, was used to analyze the experimental test data. The resulting effective thermal conductivity (including the effects of groundwater flow) was found to be one order of magnitude greater than without groundwater flow, and the DHE thermal resistance (including the effects of convection within the well bore) is 40 times greater than for a DHE without well bore convection. With these parameters known for a similar aquifer and DHE assembly, an equation is presented to estimate DHE thermal output under a given set of heating design conditions. This project has verified recent experiences that PEX DHEs are lower in cost and easier to install than other geothermal energy extraction methods for direct heating applications.