Assessment of downhole heat exchangers in Puna district, Hawaii

Abstract

"Downhole heat exchangers (DHEs) have the potential for use in direct (non-electric) applications of geothermal energy. Their benefit is that no groundwater is extracted from wells. Some targeted uses of DHEs in Hawaii are low temperature agricultural applications such as greenhouse bottom heating and soil pasteurization. The main objective of this study was to conduct a so-called thermal response test on a DHE in an existing well in the Puna District of Hawaii County, Hawaii. Thermal response tests are conducted to determine the thermal properties of an aquifer, namely the effective thermal conductivity, and also the thermal resistance of the DHE assembly. The effective thermal conductivity describes heat transport in an aquifer by combined conduction and groundwater flow, and the thermal resistance combines the effects of the conductive heat transfer through the pipe wall and convective heat transfer from groundwater within the well bore. The general test method involved injecting 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 well selected for the thermal response test was an existing well located on the Malama Ki Agricultural Experiment Station in the Pohoiki area of Puna District, Hawaii County, Hawaii. The DHE was fabricated of cross-linked polyethylene (PEX) tubing that was transported and assembled on site. The overall DHE assembly consisted of two closed-loop U-tubes constructed entirely of ¾-inch (19 mm) nominal diameter PEX plastic. The resulting effective thermal conductivity (including the effects of groundwater flow) is 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 given aquifer and DHE assembly, an equation is presented to determine DHE thermal output under any given set of heating design conditions. This project has verified the Geo-Heat Center’s experience that PEX DHEs are lower in cost and easier to install than other geothermal energy extraction methods for direct heating applications."