HEAT PUMPS TAKE CENTER STAGE IN PARIS

International Congress of Refrigeration Disseminates Latest Heat Pump Technologies

The International Congress of Refrigeration ‎(ICR) is a quadrennial conference. It was most recently held at the Paris Congress Center (Palais des Congrѐs de Paris), a venue that is just over a kilometer from the Arc de Triomphe.

Figure 1 – Visitors to the MicroGroove booth at ICR 2023 were lucky recipients of a Flash Drive loaded with the latest version of the HXSim heat exchanger simulation software program courtesy of the International Copper Association.

MicroGroove is a Silver Sponsor under the auspices of the International Copper Association. The benefits of smaller diameter copper tubes were promoted on the exhibit floor under the MicroGroove banner (Figure 1). Recent white papers on MicroGroove technology were distributed at the booth along with microgroove-branded flash drives loaded with the HXSim simulation software. See the "In the Spotlight” column in this issue for more about the HXSim.

This issue is limited to a brief review of select papers from Commission E2 on "Heat Pumps and Energy Recovery." Due to the large number of papers, select papers from other Commissions (e.g., B1, B2 and E1) will be reviewed in the next issue of the MicroGroove Update. Table 1 lists several of the Technical Sessions of interest within the E2 Commission.

Table 1 — Select Technical Sessions within Commission E2 on Heat Pumps and Energy Recovery

Plenary from UNEP RTOC

A comprehensive plenary paper was delivered by Lambert J.M. Kuijpers from the United Nations Environmental Program, RACHP Technical Options Committee (UNEP RTOC). Kuijpers reviewed the parallel, closely linked histories of Refrigeration, Air-Conditioning & Heat Pump (RACHP) technology and climate change in a presentation titled “From past to present: a net-zero framework with various hurdles and opportunities for RACHP” [2].

Kuijpers provides unflinching insights into the challenges to be confronted and overcome. The emphasis in RACHP is on the "HP" with no less than 787 mentions of heat pumps in his paper. Point 12 in the Concluding Remarks section is worth sharing here:

Net-zero will not be possible to achieve without the RACHP sector. Net-zero and certainly the 1.5 °C emissions reduction timeline is unattainable without the application of efficient ACs and heat pumps since RACHP – particularly with heat pumps included – is the most efficient technology to replace fossil-based technologies for heat production, given adequate renewable capacities available.

Kuijpers speaks from a position of deep knowledge of available technologies. A 307-page “Assessment Report” in 2022 provides a detailed evaluation of refrigerants, appliances, food service, refrigeration equipment, AC, heat pumps and much more [3].

E2 Keynote on Heat Pumps

It’s no wonder that the IIF/IIR E2 Commission on Heat Pumps & Energy Recovery was a track followed closely by many at the Congress. Björn Palm kicked off the E1 track with a keynote on “The role of heat pumps in the future energy system” delivered in the first Technical Session (TS 1) [4].

Palm is from the Division of Applied Thermodynamics and Refrigeration at the KTH Royal Institute of Technology in Stockholm, Sweden. His E2 keynote was short and cogent. After highlighting recent climate and geopolitical events, he provided a detailed assessment of energy use in the EU and the role of heat pumps. He raised a cautionary voice about reliance on HFOs and the demand in Europe to phase out all types of PFAS, suggesting that there is a need to phase out F-gases and PFAS and transition to natural refrigerants.

E2 Technical Session on Heat Exchangers

The E2 track on Heat Pumps and Energy Recovery included one technical session (TS 36) dedicated to heat exchangers.

Yoram Shabtay from Heat Transfer Technologies presented a paper on “Principles of evaporator coil design for air source cold climate heat pumps using smaller diameter ‎copper tubes and low GWP refrigerants [5]. The first part of the paper reviewed the effects of tube diameter, internal enhancements (microfins), fin spacing and fin type on a simple heat exchanger with R290 refrigerant using HXSim software.

Next, Shabtay presented several examples of evaporator designs for cold climate monobloc heat pumps. Simulations were run on a 15 kW prototype with six rows of copper tubes (Fig. 2). Furthermore, a prototype with 54 copper tubes in each of four rows (216 tubes total) as built and tested by Lordan is described in the paper. The paper and slideshow are available for downloading at microgroove.net/HXSim.

Figure 2 – Yoram Shabtay presented a technical paper on the use of heat exchanger simulation software in the design of cold climate heat pumps (CCHPs) using R290 as the refrigerant.

Variable Geometry Heat Exchangers

A paper by O’Malley et al. titled “Review of variable geometry air-to-refrigerant heat exchangers” was presented by Vikrant Aute from the Center for Environmental Energy Engineering (CEEE) at the University of Maryland. This review paper includes a literature survey with a summary of major findings [6].

A summary of the research from 1993 to 2022 related to variable geometry heat exchangers (VGHX) is presented in tabular format. Variables include fin pitch, fin curvature, fin thickness, tube shape, tube placement and tube diameter among other geometrical features.

Thinking outside-the-box, these features can be varied within the same heat exchanger. For example, different tube spacings or tube diameters can be used in different zones of the heat exchanger allowing for additional degrees of freedom in heat exchanger design. Most of the referenced research was based on numerical simulations, although several papers on experimental methods were included in the survey. This review article includes 46 references of value to students of VGHX as well as seasoned designers of heat exchangers.

Advanced Heat Exchangers from the LU-VE Group

Among the many excellent papers in the E2 Commission on Heat Pumps & Energy Recovery was the paper titled “Geometry miniaturization in fin-and-tube heat exchangers for refrigerant charge reduction” presented by Stefano Filippini from the LU-VE Group [7].

The paper was presented on the last day of ICR 2023 in TS 96 on “Refrigerant Charge Assessment and Reduction.” It provides a close look at the new heat exchanger geometry using very small diameter tubes (i.e., 4 mm diameter copper tubes). Experiments set up to validate the theoretical analysis show good agreement between theory and experiment regarding heat transfer performance as well as air pressure loss. This paper will be featured in an “In the Spotlight” column on the LU-VE Group in a future issue of the MicroGroove Update newsletter.

CO₂ Heat Pumps: A Technology to be Reckoned With

One can hardly mention “heat pumps” without mentioning systems that use CO₂ (i.e., R744) as a refrigerant. In fact, there were three different technical sessions devoted to CO₂ (TS 08, TS 43 and TS 63) in the E2 Commission alone.

Noteworthy are three papers in Technical Session 63 coauthored by Professor Armin Hafner and researchers from SINTEF and the Norwegian University of Science and Technology (NUST). Both SINTEF and NUST are are headquartered in Trondheim, Norway, the hometown of Gustav Lorentzen.

The SINTEF/NUST papers are about system designs for cooling and heating applications for hotels [8-10]. These systems use CO₂ as a refrigerant for both heating and cooling in hotels. Thermal energy storage (TES) is also implemented in these applications. Singh et al. describe a state-of-the-art implementation [8]. Henke et al. describe numerical simulations for heating, cooling and thermal energy storage (TES) in tropical weather conditions [9]. Elarga et al. describe reheating of the TES system in a paper presented by Professor Hafner [10]. These papers provide a good overview of the state-of-the-art R744 systems that are integrated for heating, cooling and TES.

Of course, the use of CO₂ refrigerant figures prominently in the Commission B2 on Refrigeration, to be reviewed in the next issue of the MicroGroove Update.

Ongoing Research on HFO Blends

Two technical sessions (TS 20 and TS 48) within the Commission E2 were devoted to Refrigerants and Refrigerant Mixtures. Numerous papers on HFOs were presented in Commission E2 as well as other commissions.

A paper in TS 48 by Petersen et al. from Trane compared the properties of R410A with four different blends (including R454B, DR-4, R454A, and R454C), spotlighting the effects of increasing the amount of R32 compared to R1234yf in these blends [11]. The thermal glide increases from 1.0 to 7.8 as the percent of R1234yf increases from 31.1 to 78.5. Likewise, the system COP is reduced as the percentage of the HFO increases.

Similarly, Bo Shen et al. from Oak Ridge National Laboratory evaluated refrigerants having a GWP less than 150, including several HFO blends [12]. They then built a laboratory prototype to optimize the performance of a high glide HFO blend (R457A) with a check valve assembly to maintain a uniform cross counter flow configuration in both cooling and heat modes. The outdoor unit had a reversible cross counter flow configuration with three rows of 5 mm copper tubes. According to calculations from system level measurements, the R457A compressor isentropic efficiency at the high stage 47 °F (8.3 °C) heating condition reached 70%, which is comparable to a similar R410A scroll compressor.

A Wealth of Research

Other papers in Commission E2 on Heat Pumps & Energy Recovery addressed topics such as Thermal Energy Storage (TES) and Phase Change Materials (PCM); Large Thermal Lift Heat Pumps; High Temperature Heat Pumps; and Hot Water Production. Many riches of research could not be covered in this brief recap of a small number of select papers. Clearly, however, the topic of heat pumps was front and center at ICR 2023. More research is needed to increase the practicality of heat pumps as an alternative to fossil fuels in all kinds of heating applications.

This review of ICR 2023 will be continued in the next issue of the MicroGroove Update with emphasis on Commission B1: Thermodynamics and Transfer Properties. Several papers specifically on smaller diameter inner-grooved copper tubes are among the papers in the technical sessions of Commission B1.

References (Main Article)

1. ICR 2023 Final Program, https://www.icr2023.org/pdf-final-program. See also https://iifiir.org/en/commissions-and-sub-commissions-of-the-iir.
2.  Lambert Kuijpers, ““From past to present: a net-zero framework with various hurdles and opportunities for RACHP,” Plenary Lecture, ICR 2023, Paper 1160.
3.  UNEP, “Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee, 2022 Assessment Report” https://ozone.unep.org/system/files/documents/RTOC-assessment%20-report-2022.pdf
4. Björn Palm, “The role of heat pumps in the future energy system,” E2 Keynote Lecture, TS1 on Energy Management, ICR 2023, Paper 1158.
5. Yoram Shabtay, Peter Mostovoy, Ji Song, Yifeng Gao, “Principles of evaporator coil design for air source cold climate heat,” E2, TS36 on Heat exchangers, ICR 2023, Paper 0191. Available for free download at www.microgroove.net/HxSim.
6. Brian O'Malley, Vikrant Aute, Daniel Bacellar, Reinhard Radermacher, “Review of Variable Geometry Air-to-Refrigerant Heat Exchangers pumps using smaller diameter copper tubes and low GWP refrigerants,” E2 TS36 on Heat Exchangers, ICR 2023, Paper 0622.
7. Stefano Filippini, Umberto Merlo, Dario Demurtas, Federico Volonté, Luca Molinaroli, Ennio Macchi, “Geometry miniaturization in fin-and-tube heat exchangers for refrigerant charge reduction,” E2 TS96 on Refrigerant charge assessment or reduction, E2 TS63 on CO₂, ICR 2023, Paper 0340.
8. Simarpreet Singh, Armin Hafner, Hagar Elarga, Kumodak Sharma, “Integrated R744 Heat Pump/chiller for Hotel: State-of-the-Art Implementation,” ICR 2023, Paper 0645.
9. Leon Henke, Hagar Elarga, Armin Hafner, “Numerical simulation of an integrated CO2/chiller and thermal storage system for a hotel in tropical weather conditions,” E2 TS63 on CO₂, ICR 2023, Paper 0380.
10. Hagar Elarga, Ángel Á. Pardiñas, Silvia Minetto, Armin Hafner, “Innovative reheat approach of the TES system during the hotel’s standstill operation utilizing a CO2 heat pump/chiller unit,” E2 TS63 on CO₂, Paper 0457.
11. Evaluation of R-410A alternatives with lower Global Warming Potential in Air Conditioning and Heat Pump applications » Mr. Michael Petersen, Mr. Stephen Kujak, Dr. Gurudath Nayak, E2 TS20 on Refrigerants and Refrigerant Mixtures, ICR 2023, Paper 0102.
12. Bo Shen, Zhenning Li, Hanlong Wan, Samuel Yana Motta, Kyle Gluesenkamp, “Direct Expansion Heat Pump Using High Glide Low GWP Refrigerant” E2 TS48 Refrigerants and Refrigerant Mixtures, ICR 2023, Paper 0689.

IN THE SPOTLIGHT

HXSim Software Gives Powerful Insights for RACHP Design

Simulations Show Advantages of Copper in Heat Pumps

HXSim Software from the International Copper Association places a powerful simulation tool in the hands of anyone who aspires to build better heat exchangers for Refrigeration, Air Conditioning and Heat Pumps (RACHP). Each of these product categories is benefitting from the advantages of smaller diameter copper tubes.

HXSim Software Distribution at ICR 2023

At the recent International Congress of Refrigeration, full working versions of HXSim software were given to interested attendees at no charge [1]. The software was loaded on flash drives that were freely distributed at the ICA MicroGroove Booth.

In case you missed the Congress, you can still download a full working copy of HXSim from the MicroGroove.net website for no charge. When you run the software for the first time, a registration ID will pop up in a window. You will then need to email the registration ID to yyli@craheata.org and, within a few days, you will receive a passcode that will unlock the software for one year.

Is It Safe?

First time users occasionally ask, “Is it safe?” In these days of heightened online security and harmful cyberattacks, that is a legitimate question. According to Kerry Song, who manages the Heat Exchanger Simulation Software program at ICA, the HXSim software is completely safe to use. He emphasizes that “No user information is collected or shared by ICA or its Copper Alliance members. The user’s email address is used only to deliver the passcode needed to unlock the program on the user’s computer."

The HXSim Software Package is guaranteed safe by ICA. “There is no risk of viruses or spyware,” says Mr. Song

Where to Begin

The user can begin simulating the performance of heat exchangers immediately after unlocking the program. Practice on simple heat exchangers is recommended before tackling more complex heat exchanger designs.

A user manual written in English by Professor Guoliang Ding from Shanghai Jiao Tong University includes a guide to the commands and controls [2].

Figure 1 – When the user starts a "New" design the default heat exchanger has two rows of 20 tubes. The space bar can be used to toggle between the front and rear views (180 degree rotation). 

The user begins by selecting “New” under the “File” tab on the main navigation bar. At once a graphic of a heat exchanger “Block” appears on the screen, along with an Input manifold and Output manifold. There are no connections at first between the tubes within the block or between the block and the manifolds. Initially, the block is an I-Type Block (that is, an I‑Block) with two rows of twenty tubes.

The next lesson is how to manipulate the image of the heat exchanger block. The user should practice using these controls: The block can be translated, rotated, enlarged or flipped 180 degrees. Also, the image can be toggled between 2D and 3D. Once these controls are mastered, then the user can begin connecting tubes. The trick is to select the end of an unconnected tube in the circuit; then use the left mouse button to create a joint. This process is repeated until one complete, continuous circuit (or multiple circuits) from Input-to-Block-to-Output is created.

At this point, once the tubes are all connected into circuits, the user can input various operating conditions via a pop-up input menu. The user can specify the mass flow of the refrigerant, the airflow outside the heat exchanger, the ambient temperature and other variables.

Next is when the fun begins. The “Run” command under the Simulation Tab is selected (or one could type CTRL-R on keyboard). Voila! Within seconds the simulation is complete.

Figure 2 – Once the tubes have been connected into complete circuits, and the operating conditions selected, the simulation can be run. Here, in this simple example, results are displayed as a color-coded temperature map in three-dimensions.

Countless Variations

Things really start to get interesting when the user selects “Edit Block” under the Edit Tab in the main navigation bar. The pop-up window that appears is called the “Input” window.

Initially, the user will want to choose the geometry of the heat exchanger. By “geometry” is meant whether the block is a simple I‑Block or a more complicated L‑Block or C‑Block. These more complicated shapes may have the same number of tubes as an I‑Block, but they require specifying two or more straight or curved sections. The curved sections are defined by a radius of curvature and an angle.

Other “geometrical” inputs include the block dimensions (height and depth) and tube arrangement (staggered, inline or custom) and the number of holes in the block.

Figure 3 – The user has compete control over the operating conditions, the refrigerant and the geometry of the fins and tubes via a simple "input" window.

HXSim Design Study Presented at ICR 2023

A paper presented at the 2023 ICR in Paris includes a design study based on the use of HXSim software [3]. “Principles of evaporator coil design for air source cold climate heat ‎‎pumps ‎using smaller diameter ‎copper ‎tubes and low GWP ‎refrigerants” is the title of the paper 0191 by authors from the International Copper Association (ICA Shanghai Office) as well as Heat Transfer Technologies (HTT, Prospect Heights, Illinois) and Lordan (Kfar Szold, Israel).

The E2 Commission on “Heat Pumps and Thermal Storage” selected the paper for presentation in the technical session TS 36 on “Heat Exchangers.” Yoram Shabtay, who is also the corresponding author, presented the paper. The paper and the slideshow presentation are available for download at no charge from the microgroove.net website courtesy of the International Copper Association, MicroGroove program [3].

Among the most useful features of the HxSim software are the fin and tube correlations that are built into the program. Using the “Input” pop-up screen mentioned above the user can choose from scores of tube and fin geometries.

Besides tube diameter, users can experiment with various microfins geometries (tube enhancements) and see for themselves how the microfins increase the efficiency of the heat exchanger under various operating conditions. The HXSim software gives accurate simulations for different tube enhancements and fin types as well as different refrigerants and tube diameters. Simulations demonstrate the degree to which fin density influences airside pressure drop and capacity. Such studies can be important for cold climate heat pumps, since ice and snow in combination with high fin density can block airflow. In the design study, simulations were run on the same two-row, I‑Block coil with various fin types and densities outside the tubes and various geometries of microfins inside-the-tubes, including smooth tubes and surface enhancements.

The HXSim simulation software allows the designer to explore the effects of these and other design features. Once these effects are understood, the designer can advance to the design of larger and more complex heat exchangers, including L‑Blocks and C‑Blocks. The design example presented in the ICR paper was for a CCHP evaporator. Specific for CCHP, the design calls for six rows of tubes. Test criteria for the CCHP included the following operating conditions:

• 15 kW fixed capacity
• Heating water to 35 °C
• Ambient air temp 7 °C dry bulb / 6 °C wet bulb
• Evaporation temperature 2 °C
• Superheat 2 K
• Air speed 2 m/s

Laboratory tests were conducted on the prototype heat exchanger as built by Lordan. The test results compared well with the simulations. The ICR 2024 paper by Shabtay et al. gives details of this design study [3].

Origins of HXSim Simulation Software

The current version of HXSim Software represents the high point of more than a quarter century of continual improvements. Lui et al. outlined the core equations of tube-fin heat exchanger simulation software in 2004 and provided ‎‎references to earlier work [4].

In the ensuing decade, simulations were motivated by the desire to reduce materials usage in the high-volume ‎‎manufacture of air conditioners. Subsequently, meticulous laboratory work was undertaken to measure the heat ‎‎transfer coefficients (HTCs) inside smaller diameter copper tubes. For example, laboratory measurements ‎‎of condensing R410A refrigerant were made inside smooth copper tubes with small diameters and microfin tubes. Simulations were also developed for tube circuitry and fin efficiencies. See the ICR 2023 paper for references [3].

Soon after ICA launched its MicroGroove campaign in 2010, it began to promote the availability of the heat exchanger simulation software from SJTU. The software was further improved to allow for simulations of commercial refrigeration equipment and large heat exchangers. Over the years, more refrigerants and correlations were included in the software. Graphical output of 3D color-coded, heat-exchanger temperature maps and a user-friendly interface were added as well.

Building on the steady advances in laboratory research and scientific computing, the International Copper Association and Shanghai Jiao Tong University are proud to offer a ‎user-‎friendly simulation software program that can directly model many types of copper tube heat ‎‎exchangers.

Toward Better CCHPs

Simultaneously, there has been a steady advancement of copper tube technology throughout the past two decades. Five-millimeter diameter copper tubes ‎with ‎ internally enhanced surfaces are now routinely used for air-‎conditioning ‎and refrigeration applications. And now their advantages are also used in the design of cold climate heat pumps (CCHPs). Simulations demonstrate why ‎the technology is ‎advancing in the direction of smaller diameter tubes as well as the advantages of enhancing ‎the inside surfaces of ‎the tubes.‎ Most recently the simulation software also allows for the selection of copper tubes with outer diameters of four millimeters.

What began as a project to reduce materials costs in air-conditioning is now an invaluable tool for the design of all sorts of condensers and evaporators for countless RACHP products. The vapor compression cycle (VCC) is central to a major paradigm shift in heating, and round tube, plate fin (RTPF) heat ‎exchangers are at the heart of the VCC.

More and more, the advantages of air source heat pumps (ASHPs) for heating residential and commercial ‎buildings is a topic of great interest to policymakers, OEMs, environmentalists and the General Public. The ‎practicality of cold climate heat pumps (CCHPs) is garnering attention around the globe.

Improvements in CCHPs are taking place at an astonishing pace as ‎OEMs seek to become the first to market with superior products. There is much room for improvement ‎using creative design and attention to fundamentals.‎ HXSim software demonstrates the advantages of smaller diameter copper tubes and the performance improvements possible using various combinations of tube diameters, microfins, tube circuitry, tube spacing, fin design, fin ‎density and heat exchanger shape.

References (In the Spotlight)

1. International Copper Association, HXSim Heat Exchanger Simulation Software, Available for downloading from www.microgroove.net/HXSim.
2. Guoliang Ding, "User Guide for Heat Exchanger Simulation for Refrigeration," Published by ICA/SJTU, 63 pages. Available for download at www.microgroove.net/HXSim.
3. Yoram Shabtay, Peter Mostovoy, Ji Song, Yifeng Gao, “Principles of evaporator coil design for air source cold climate heat,” E2, TS36 on Heat exchangers, ICR 2023, Paper 0191. Available for free download at www.microgroove.net/HxSim.
4. Liu, J., Wei, W.J., Ding G.L., Zhang C.L., Fukaya, M, Wang, K.J., Inagaki, T., 2004. A general ‎steady state mathematical model for fin-and-tube heat exchanger based on graph theory, Int. J. ‎Refrigeration 24 (8): 823-833.

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White Papers Recently Published in Appliance & HVAC Report

MicroGroove White Papers

Frank Gao, Harry Schmitz, Yoram Shabtay, Kerry Song, Appliance & HVAC Report, “HVACR Engineers Focus on Cold-Climate Heat Pumps,” October 2023, Page 22.

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Frank Gao, Kerry Song, Yoram Shabtay, and Harry Schmitz, Appliance & HVAC Report, “Heat Pumps ‎Deliver High Efficiency Heating: Five Trends in Heat Pump Design for Residential Heating,” October ‎‎2022, Page 10.‎

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