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.

Completion of Civil Works at Perugia

Civil works began on December 3rd, 2020 at the Perugia Pilot. At that time, partner IDSGEORADAR performed a ground-penetrating radar measurements survey. Meanwhile, partner R2M operated UAV flights with and without a thermal camera. And all throughout the drilling phase, partner UNIPG completed the Structural Health Monitoring (SHM).

After one week, the works completed the shallow excavations of up to 2.5 meters deep. This meant the area was ready for the 50 centimeters of the sand bed that increases the conductivity on the ground.

Later, on January 11th, the company in charge of the fieldwork started the installation of the slinky GHEXs that go 2 meters deep. It considers 5 parallel trenches, with 1.35 m of mutual distance, 24 meters of average trench length, and 53 rings for each trench.

After the GHEXs installation, on January 18th, GROENHOLLAND came to the Perugia Pilot to install the fiber optic cable (FOC) monitoring. This technology has a particular approach that allows remote checking of the new system’s performance while matting underground temperature.

To increase the innovative aspects and the scientific relevance, UNIPG installed on the first two trenches, two parallel tubes of a drip plant that allows experimental evaluations and comparisons that consider different boundary conditions and configurations, such as with varying ground hydration content.

Drip plant in the first two trenches.

Finally, civil works were completed on January 28th as soon as all the excavated area was backfilled with an additional 50 cm of sand. Also, the ground excavated material was reused on-site.

Backfilling the area with other 50 cm of sand.

Furthermore, all the heating/cooling system components: heat pumps, boilers, tanks, are available on-site. And today, FAHRENHEIT’s chiller also arrived at Perugia!

Presently, UNIPG is working to complete the whole installation by the end of March and to move forward with the commissioning of the system.

D3.2 – Ground Source Heat Exchanger design framework

The goal of WP3 is to develop a design framework for novel ground (slinky/earth basket) type shallow heat exchangers. This design framework, based on developing theoretical models of heat transfer and on experimental data, will be implemented in a design- and engineering calculation tool to support the implementation of these new technologies in the market.

The design framework defines the goals of the (thermal and hydraulic) design (especially sizing) of the ground source heat exchanger, as a function of different boundary conditions (building energy demand, soil thermal parameters, required system performance etc.). Moreover, an engineering tool it is aimed at the overall system design and will support the engineer in the choices of heat exchanger technology (vertical, horizontal or earth basket/slinky) and other design parameterizations.

This deliverable describes the overall design process and provides information and procedures for data collection and evaluation. The detailed description of the design process for different types of Ground Heat Exchangers is based on the design of the actual GHEX systems implemented in the demo sites of the Geofit project and includes vertical borehole heat exchangers, shallow slinky heat exchangers and earth basket type heat exchangers.

This deliverable is suited to be implemented in a design handbook or procedure that can be part of an integrated quality control system.

D3.1 – Design methodologies strengths and weaknesses

In this deliverable the main fundamental processes of heat transport relevant to the ground heat exchangers have been described. For the purpose of the project, only heat transport due to conduction will be considered in detail.

There is abundant literature concerning methods to calculate the thermal response of ground heat exchangers and design. Using a few selected references an introduction to analytical and numerical methods is presented. A description of the different implementations of analytical and numerical modeling and design software codes is presented as well.

The different methods are classified for strengths and weaknesses based on criteria including methodology, complexity, number of input parameters required, processes considered, flexibility with regard to geology and parameters relating to the actual ground heat exchanger. The aim has not been to arrive at a ranking, but only at an overview of the capabilities of the codes and the ease or complexity of use. Cost is of course also an important aspect but is currently not included.

Based on the overview of different methodologies and implementations a roadmap for the development of the GHEX engineering tool within the work package has been defined. This roadmap is based on the expertise of the different partners and works from a detailed level, using the results to derive simplified models that can be integrated into a final engineering design tool.

European Heat Pump Summit Presentation (2019)

Presentation of GEOFIT at the European Heat Pump Summit (2019)

Title: GEOFIT: Ground source heat pump systems for energy efficient building retrofitting.

Language: English

Author: Michael Lauermann (*1)

*1: AIT Austrian Institute of Technology GmbH, Center for Energy,

Summary: The integration of geothermal systems for heating / cooling solutions in conjunction with heat pump technology is a major challenge, particularly in the case of renovation. In order to find the best solution for the increased flow temperatures compared to a new building for the renovation, various heat pump configurations are evaluated according to energy and economic criteria. A refrigerant with low greenhouse gas potential (GWP) is used as the working medium, e.g. R1234ze (E)) less than 10 used. A favorable refrigeration circuit configuration for high flow temperatures is a twin-circuit system, which essentially consists of two heat pumps with different condensing temperatures and the same evaporation temperatures. An alternative to this is a single-stage configuration with a significantly larger condenser for improved supercooling. Both systems should enable efficient operation when renovating buildings.

Deutscher Kaelte und Klimatechischer Verein (DKV) Presentation (2019)

Presentation of GEOFIT at the 45th Annual Meeting of the Deutscher Kaelte und Klimatechnischer Verein (2019)

Title: Erdwärmepumpen für die energieeffiziente Gebäudesanierung

Language: German

Authors: Michael Lauermann (*1), Johann Emhofer (*1), Edith Haslinger (*1), Karl Ochsner (*2)

*1: AIT Austrian Institute of Technology GmbH, Center for Energy,
*2: Ochsner Wärmepumpen GmbH

Summary: The integration of geothermal systems for heating / cooling solutions in conjunction with heat pump technology is a major challenge, particularly in the case of renovation. In order to find the best solution for the increased flow temperatures compared to a new building for the renovation, various heat pump configurations are evaluated according to energy and economic criteria. A refrigerant with low greenhouse gas potential (GWP) is used as the working medium, e.g. R1234ze (E)) less than 10 used. A favorable refrigeration circuit configuration for high flow temperatures is a twin-circuit system, which essentially consists of two heat pumps with different condensing temperatures and the same evaporation temperatures. An alternative to this is a single-stage configuration with a significantly larger condenser for improved supercooling. Both systems should enable efficient operation when renovating buildings.

Deutscher Kaelte und Klimatechischer Verein (DKV) Paper (2019)

Presentation of GEOFIT at the 45th Annual Meeting of the Deutscher Kaelte und Klimatechnischer Verein (2019)

Title: Erdwärmepumpen für die energieeffiziente Gebäudesanierung

Language: German

Authors: Michael Lauermann (*1), Johann Emhofer (*1), Edith Haslinger (*1), Karl Ochsner (*2)

*1: AIT Austrian Institute of Technology GmbH, Center for Energy,
*2: Ochsner Wärmepumpen GmbH

Summary: The integration of geothermal systems for heating / cooling solutions in conjunction with heat pump technology is a major challenge, particularly in the case of renovation. In order to find the best solution for the increased flow temperatures compared to a new building for the renovation, various heat pump configurations are evaluated according to energy and economic criteria. A refrigerant with low greenhouse gas potential (GWP) is used as the working medium, e.g. R1234ze (E)) less than 10 used. A favorable refrigeration circuit configuration for high flow temperatures is a twin-circuit system, which essentially consists of two heat pumps with different condensing temperatures and the same evaporation temperatures. An alternative to this is a single-stage configuration with a significantly larger condenser for improved supercooling. Both systems should enable efficient operation when renovating buildings.

GEOFIT: Poster for the Experiment on GHEX characterization (WP3)

GEOFIT is optimizing compact geothermal heat exchangers systems (GHEX) with a more compact design based on a helix. To help establish heat transfer behavior, experiments are being conducted by consortium partner AIT. This poster shows the objective, approach and set-up for this experiment, which belongs to WP3. The subsequent CFD-simulations based on this experimental data will help establish a reliable engineering design tool for the helically based GHEX systems.