Analyzing the Use of CO₂ as a Refrigerant in Vertical Direct Expansion Geothermal Evaporators

From eSociety, October 2019

To increase the use of ground source heat pumps, researchers are proposing using natural refrigerants such as carbon dioxide in direct expansion.

The article, “Theoretical and Experimental Analysis of a Vertical Direct Expansion Geothermal Evaporator Using CO₂ as Refrigerant,” from Science and Technology for the Built Environment explores theoretical and experimental investigations on a single U-tube vertical direct expansion borehole heat exchanger using CO₂ as the refrigerant.

One of the researchers, Messaoud Badache, Ph.D., who works at the CanmetENERGY Varennes Research Centre in Varennes, Quebec, discussed the research further.

1.     What is the significance of this research?

Ground source heat pump (GSHP) systems provide many possibilities for energy savings for cooling and heating in various applications ranging from space and water heating in buildings to industrial processes. Although well recognized as among the most efficient end-use technologies currently available, they are not widely used due to their cost, the relatively suboptimal efficiencies of the ground heat exchanger (GHE) and, to some extent, environmental challenges to the soil and the environment.  

The approach proposed to overcome these issues is to adapt natural refrigerants such as carbon dioxide (CO₂) in direct expansion ground source heat pumps (DX-GSHP). Carbon dioxide circulates directly in a metallic GHE, which acts as a condenser or evaporator, depending on the operation mode. A CO₂ in DX-GSHP system reduces the overall size and cost of the system, making it a promising environmentally friendly and energy-efficient alternative compared to existing equipment.  

2. Why is it important to explore this topic now?

One of the main components in transcritical CO₂ DX-GSHP systems is the GHE because it is the component where heat transfer between the GSHP system and the soil occurs. GHE heat transfer performance is the key factor influencing and limiting the operation and the performance of the entire system. At present, relatively few studies are reported on the DX-GHE. 

Analysis of the system performance and phase-change process of CO₂ inside the GHE are topics of great importance for a scientific appreciation of the technology to facilitate its widespread use. Research on this topic will also help to:

• Phase out of synthetic refrigerants and phase in using natural refrigerant 

• Reduce the cost of the geothermal loop using the DX concept

• Increase awareness of environmental benefits of using natural refrigerants compared to synthetic refrigerants

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

The cost and the effectiveness of the technology depend significantly on the GHE installation cost (drilling, grouting, etc.) and design. For the reduction of the cost of the GSHP, several strategies would be investigated/adopted.

• Reducing the GHE required length. This can be achieved only by precise design, using more elaborate and validated design tools, taking into consideration the complete heat transfer process in GHE. This strategy requires a precise soil temperature and thermal property determination, since the precise determination of borehole length is not possible without good estimation of these properties. 

• Thermophysical properties of the refrigerant in DX-GSHPs have a critical impact on the overall system performance. Thus, a second strategy would be to use CO₂, which is a natural refrigerant with very favorable properties for the application.

• Investigation of further new borehole configurations with CO₂ to increase heat extraction from the soil at reasonable costs. 

4. How can this research further the industry's knowledge on this topic?

This research familiarizes the industry with the concept of the direct expansion (DX) ground source heat pump and particularly the operation of the ground heat exchanger as the evaporator of the system. Furthermore, it demonstrates in detail the characteristics of CO₂, a natural refrigerant, as the working fluid of the geothermal boreholes. It also quantifies the performance of a CO₂ DX borehole under different operating conditions.    

5. Were there any surprises or unforeseen challenges for you when preparing this research?

Challenges and issues of this research are two-fold: experimental and modeling. Due to the fact that the refrigerant enters the boreholes and evaporates in DX systems, modeling is significantly more complex compared to a conventional system with water. Coupled pressure-temperature physics of the problem and abrupt changes of CO₂ properties in the transcritical region created some unforeseen convergence challenges in the modeling process. Using different techniques such as under/over relaxation helped us to overcome these challenges.    

To carry out experiments, researchers are always dealing with challenges such as uncertainty of measurements and stable operation of the tests. In addition to these challenges, when preparing the experiments of this research, we also confronted several unforeseen issues. Since there are no commercially available CO₂ heat pumps on the market, an air-source CO₂ refrigeration system has been installed and transformed completely to a ground source heat pump system. Control of a transcritical CO₂ heat pump brought some surprises during the operation such as unstable operations. Furthermore, finding relatively small components for CO₂ with high working pressure was always a challenge.