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.

Design Requirements for Condensation-Free Operation of High-Temperature Cooling Systems in Mediterranean Climate

Article pulished in Building and Environment (2020)

Title: Design Requirements for Condensation-Free Operation of High-Temperature Cooling Systems in Mediterranean Climate

Language: English

Authors: Henrikki Pieskä (*1) , Adnan Ploskić (*1,2), Qian Wang (*1,3).

*1: Division of Sustainable Buildings, School of Architecture and the Built Environment, KTH Royal Institute of Technology, Brinellvägen 23, SE, 10044, Stockholm, Sweden
*2: Bravida Holding AB, Mikrofonvägen 28, SE, 12637, Hägersten, Sweden
*3: Uponor AB, Hackstavägen 1, SE, 72132, Västerås, Sweden

Abstract: Radiant cooling systems are a subject of increasing scientific interest due to their efficiency and ability to use high-temperature cooling sources. In hot and humid conditions, they have generally been studied in combination with dehumidification systems. For retrofit projects, a control system that would eliminate the need for dehumidification would be beneficial. In the present study, a passive geothermal-based radiant high-temperature cooling system is studied in a Mediterranean climate. The system is operated with supply water temperature control using dew point temperature as a controlling variable. The system’s performance is compared with that of an all-air cooling system. The systems are evaluated using IDA-ICE building energy simulations, validated with on-site measurement data. The results show that the radiant cooling system produces the same level of thermal comfort with 40% lower energy use and 85% lower exergy consumption than the all-air system. The risk of condensation limits the cooling capacity of the radiant cooling system. Consequently, insufficient cooling capacity causes thermal discomfort for the occupants due to the operative temperature exceeding 26 ◦C.

BS Rome – IBPSA Paper (2019)

Presentation of GEOFIT at BS ROME – 16th IBPSA International COnference and Exhibition (2019)

Title: Environmental sustainability and Energy Efficiency in Historical Buildings: GeoFit Project Implementation in the Case Study of a medieval fortress in Perugia

Language: English

Authors: Jessica Romanelli (*1), Matteo Di Grazia(*1), Cristina Piselli (*1,2), Anna Laura Pisello (*1,2), Franco Cotana (*1,2)

*1: CIRIAF – Interuniversity Research Centre, University of Perugia, Italy
*2: Department of Engineering, University of Perugia, Italy

Abstract: Italian cities are mainly constituted by buildings constructed until the mid-20th century by pre-industrial construction techniques. A HVAC system for the energy retrofit of historical buildings is evaluated when applied in the case study of Sant’Apollinare. It consists of a ground source heat pump a water tank for thermal energy storage connected to a low-temperature radiant system and air handling unit. The building thermal-energy behavior, typically influenced by thermal inertia in historical buildings, and the novel HVAC system performance interactions are comparatively assessed together with more traditional scenarios. Energy demand decreases by about one third compared to the pre-retrofit situation.