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John O | August 2018

Could vacuum-insulated tubing improve efficiency of borehole heat exchangers

By Josh Perry, Editor


Extracting geothermal heat is a growing field, particularly in European countries, and one of the methods that has been developed to get at the Earth’s crust is by using deep borehole heat exchangers (DBHE).


Research continues into efficiently extracting geothermal energy. (Wikimedia Commons)


Scientists from the AGH University of Science and Technology (Poland) and the University of Stavanger (Norway) have recently published research into DBHE technology that indicates vacuum insulated tubing (VIT), which is widely used in offshore drilling, could enhance the efficiency of DBHE installations. The research was recently explained in an article from Science Trends.


According to the article, “VIT constitutes of two coaxial steel pipes, where air between them is evacuated using a vacuum pump during the manufacturing process. After achieving the desired level of vacuum, pipes are sealed and welded. This process creates very high thermal insulation and minimizes heat exchange between the vacuum pipe and annulus.”


This decreases the heat transfer between the inner coaxial column and the existing well construction, removing heat exchange from convection, radiation, or conduction. VIT thermal conductivity ranged from 0.006-0.0008 W/mK, while conventional steel pipes have a coefficient of 40 W/mK.


VIT has considerable disadvantages in transport costs for the much heavier tubing, as well as high prices for materials and limited availability.


The research was recently published in Geothermics. The abstract read in part:


“One of the ways to extract geothermal heat from the Earth’s crust are Deep Borehole Heat Exchangers, which can be either drilled especially for such purposes or, what is more cost effective, reconstructed from already existing, negative or abandoned oil or natural gas wells.


“In many depleted boreholes, exploiting underground geothermal waters is impossible or technically complex, therefore potential reconstruction for heat exchanger might be the only possible solution that allows utilizing geothermal heat.


“Following paper investigates potential reconstruction of abandoned or negative wells into deep coaxial borehole heat exchangers. Main focus was put on selection of coaxial inner column which enables for heat carrier circulation. To maximize the heat uptake from geothermal systems based on borehole heat exchangers, it is advised to use inner column made from material with lowest possible thermal conductivity coefficient.


“Vacuum Insulated Tubing, as one of the options of insulated inner column, can significantly improve heat production and increase efficient energy use. This technology found plethora of applications in offshore and onshore petroleum industry and is yet to gain more popularity within geothermal sector.”

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