| Title | Experimental Investigation of Innovative and Conventional Shallow Borehole Heat Exchangers |
|---|---|
| Authors | Matthias SCHUCK, Peter NIEMANN, Finn RICHTER, Jürgen GRABE |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | borehole heat exchanger, peak load, gas injection, field test |
| Abstract | Conventional closed geothermal heat exchangers possess the drawback of a weak dynamic behaviour. Considering a geothermal assisted air conditioning system, the load on the geothermal system is continuously varying. Designing a system that can cover high demands will probably result either in oversizing for conventional operation or in the requirement of backup systems. Both will decrease the efficiency of the overall system and increase the costs. To counteract this drawback a patented method called “gas injecting borehole heat exchanger†(GiBHE) will be presented that creates an artificial ground water flow around the borehole heat exchanger. By injecting gas, most commonly air, at the bottom end of the well the density of the groundwater changes along the vertical heat exchanger thus causing a circulating flow. Field tests with variable gas injecting rates have shown that the system’s heat transfer rate has a very fast response time. Therefore, the heat transfer performance can be optimized for the current load which increases the efficiency of the geothermal system. The control of the gas injection rate adds a further control-ability for geothermal systems which extends the scope of application. Another characteristic of the artificial groundwater flow is the increase of the overall transfer rate by a factor in the range of3 to 5. Due to the improved heat transfer between groundwater, soil and heat exchanger caused by convection the heat transfer rate per meter of drilling is additionally increased. The efficiency of such systems can be further improved by combining ordinary geothermal borehole heat exchangers (BHE) with gas injection ones. Within an appropriate distance around a GiBHE one or multiple ordinary BHE are installed. The artificial groundwater flow has positive effects on the ordinary BHEs. Therefore the overall performance of geothermal systems is enhanced and the installation costs and operating costs are reduced. According to the purpose of increasing the acceptance and applicability of geothermal system in many sectors the investigated technology is capable of offering significant advantages for high loads and fluctuating power demands. Results of a field test with a cluster of BHE and GiBHE will be presented which show the dynamic and increased performance of such systems. |