Model development and performance analysis of Novel Shallow Ground Heat Exchangers

by Henk Witte – Groenholland

One of the key aspects of the EU GEOFIT project is the development of integrated engineering design tools for different types of ground heat exchangers. This toolkit provides design methodologies for vertical borehole heat exchangers, shallow horizontal and slinky type heat exchangers, and earth basket (spiral) heat exchangers.

Ground heat exchangers (GHEX) are used to provide a heat source or heat sink used for heating or cooling a building. They are typically constructed of plastic pipes installed in different configurations in the ground. A fluid is circulated in the pipes and the GHEX extracts heat from the ground (heating operation) or rejects heat to the ground (cooling operation).

For the validation of the analytical solutions used in the integrated engineering design toolkit, especially the new finite line source solutions developed for earth basket (spiral) heat exchangers laboratory experiments (figure 1) and detailed numerical simulations (figure 2) have been performed.

Figure 1. Experimental sandbox setup for earth basket (spiral) heat exchanger characterisation (foto: AIT)

The performance of a ground heat exchanger can be summarized to two key parameters:

  1. Pressure drop: A measure of the pump energy needed to move the fluid through the heat exchanger.
  2. Thermal resistance between fluid and ground: A measure of the thermal performance of the GHEX.
Figure 2. CFD simulation of earth basket (spiral) heat exchanger (foto: AIT).

The goal of the performance analysis is to identify key-design parameters affecting the overall system performance. Parameters investigated include:

  • Diameter of the earth basket (spiral) heat exchanger
  • Pipe diameter in relation to flow rate and pressure drop
  • Distance between adjacent rings in relation to total length and buried depth
  • Soil thermal parameters

Evaluation of the results of the performance analysis should take into account the actual effect on system performance. As an example, it can be attempted to reduce the thermal resistance in all cases as much as possible. However, the effect on performance is related to the actual heat rate of the system (figure 3). It is clear that with a low heat rate (5 W/m) the thermal resistance can be allowed to be high without affecting performance. These results will have implications for operating these systems during part-load conditions, which is important in view of the application of frequency-controlled compressors in the heat pumps. In this way, the results of the GEOFIT project not only provide designers with the tools to evaluate different types of ground heat exchangers in one integrated tool but also allows optimization of actual system operational control.

Figure 3. Relation between thermal resistance (fluid to ground) and energy performance degradation for different specific heat rates.

REFERENCES

Meeng, C.L (2020). Development of an engineering tool for the design of novel shallow ground heat exchangers – GEOFIT. MSc Thesis TU Eindhoven.

Dörr, C.J. (2020). CFD Analysis of Ground Heat Exchangers. MSc Thesis Montan Universität Leoben, Austrian Institute of Technology.

Kling, S. (2020). Experimental characterization of Helix-Type Ground Source Heat Exchangers Configurations for Developing a Standardized Design Tool. MSc Thesis FH Burgenland University of Applied Sciences, Austrian Institute of Technology.

We end May with our 7th General Assembly!

The GEOFIT consortium successfully held its 7th Virtual General Assembly on May 20 and 21 2021, our first GA for 2021! The meeting was organised online in two intense half-day sessions during which work package leaders where able to present the progress made with the core technologies, demonstration activities and coordination work packages.

Kicking-off our 7th meeting!

The first-day session started with a focus on the state of the art of technical work packages (WP). Lead by our colleagues from IDS Georadar, Groenholland, AIT, and Nuig Galway, we got to review WP2 which focused on-ground research, worksite inspection, and improved drilling technologies, and wrapped up its work last April 2021. The presentations for WP 3, 4, and 5 presented the different advances made with our Shallow Ground Heat Exchangers, Heat Pumps, system integration and efficiency management. After a short coffee break, we had the presentation of the progress of pilot implementation. In this sense, we were celebrating the commissioning of our first pilot site in Perugia, an important milestone for the project! Our Bordeaux pilot has also taken important steps in the past months when it comes to drilling.

The GEOFIT Team

On the second day, we were able to review the advances made on our GeoBIM platform and the standardisation, exploitation, and communication activities and R2M started an open discussion on possible business cases, UNE presented different approaches to the standardisation language including the CWA and COMET updated on the most recent events and the new website design and materials.

The meeting wrapped up with high hopes that next time we might meet again in person and with a clear view of the next steps that will be taken by all partners during the next 6 months.