Supercritical CO2 (sCO2) has attracted considerable attention in multiple thermal applications, such as power generation systems, aerospace, and electronics, due to its high energy density. Its unique properties facilitate the development of high-performance, cost-effective, and compact metal heat exchangers. In addition, innovative designs enabled by additive manufacturing can further enhance thermal performance by increasing surface area density while regulating the pressure drop. In this study, we developed a small-scale, multi-pass microchannel heat exchanger (MPMHX) with a volume of 115,679 mm3 and a surface area density of 989 m2/m3, which experimentally achieved a high power density of 45.4 MW/m3. The additive manufacturing process used to fabricate the HX introduced a channel relative roughness of 9.6 %, which increased the pressure drop by an average of 172 % compared to the smooth channel. Meanwhile, the roughness improved thermal performance by 31 % on average. In a comparison with other compact HX concepts in the literature, the MPMHX performance experimentally demonstrated the highest compactness (Q/V = 45.4 MW/m3, Q/V/dT = 0.34 MW/m3/degrees C) with a low pumping power of 11.75 W. This is the first study to provide experimental results for the additively manufactured multi-pass microchannel heat exchanger, demonstrating enhanced performance for high efficiency, extreme environment, and power generation applications.

K-12 schools are critical pillars of American communities and provide an invaluable service to future generations. Thesefacilities are part of the buildings sector, which accounted for 38.1% of American energy consumption and 35.1% of greenhouse gas (GHG) emissions in 2023 (EIA 2024). To avert the worst effects of the climate crisis, the United States National Climate Task Force aims to reduce national greenhouse gas emissions by 50-52% before 2030. Reducing climate change contributions from K-12 schools is critical to achieving these important goals. Many schools could benefit from infrastructure improvements that would reduce costly utility bills and harmful environmental impacts while optimizing learning spaces for students' academic achievement. This paper presents opportunities for energy efficiency measures in two large public high schools in different counties in the state of Maryland. Energy audits were performed at both facilities to identify specific areas for improvement. First, utility data was analyzed, and each facility was benchmarked using Energy Star Portfolio Manager. Then, a walkthrough inspection of each school was conducted to diagnose inefficiencies. Both schools demonstrated opportunities to improve energy efficiency and reduce GHG emissions. The EUI at both schools was significantly above the ASHRAE recommended level for a high school in the climate zone. Control strategies and equipment upgrades were recommended to facility management to improve each school's performance. The successful results of these audits should encourage future analysis of K-12 schools throughout the national public education system to reduce the environmental impact and energy costs in this building sector.

Buildings are responsible for 30-40% of global energy consumption, making them prime candidates for improvements in energy efficiency. To this end, states across the U.S. are actively implementing building performance standards to regulate the operations of the existing building stock. However, when multiple buildings are being considered for energy performance upgrades, it can be difficult for decision-makers to determine which facility to prioritize. This study presents the development of a multi-criteria ranking tool capable of facilitating rapid, reliable, and efficient virtual energy audits. The ranking generated by this tool can be used to conduct performance analyses and identify facilities with suboptimal performance. The proposed ranking of the cluster of buildings can be used to make decisions based on key metrics such as energy use intensity (EUI), net total CO2e emissions per square foot, and dollar-saving potential per square foot. By assigning different weights based on the performance of facilities relative to energy and greenhouse gas emission benchmarks from the Commercial Building Energy Consumption Survey (CBECS) and the local/regional mandates as applicable, a cumulative score is developed for a portfolio of buildings. For the case study analyzed here, the building portfolio's end-use energy data resulted in a minimum annual average energy savings potential of 1,522 terajoules (TJ) (1,442,951 MMBtu), representing a possible 45% reduction in energy consumption. This translates to approximately 567 megajoules per square meter (MJ/sq. m.) (50 kBtu/sq. ft.) over a six-year analysis period (2018-2023). As a result, this average annual energy reduction would lead to a yearly decrease in greenhouse gas (GHG) emissions of 115,249 metric tons of CO2e, also a 45% reduction, equivalent to roughly 43 kilograms per square meter (kg/sq. m.) (4 kg/sq. ft.) over the same six-year analysis period (2018-2023). Furthermore, the potential annual average dollar savings is estimated at USD 26 million [similar to USD 10/sq. m. (similar to USD 1/sq. ft.)], reflecting a 40% cost reduction. Multi-criteria ranking models, such as the one presented here, are essential for identifying and prioritizing subpar building performance, facilitating targeted energy improvements, and allocating resources toward sustainability goals.

Phase change materials (PCMs) can be utilized in buildings for peak load shifting in air conditioning systems, and the use of salt hydrate-based PCMs can reduce the cost of thermal energy storage devices. Glauber’s salt is an economical salt hydrate PCM with a melting point of around 32 °C. However, the desired melting range typically falls between 18 and 22 °C for building air conditioning applications. Although many researchers have characterized Glauber’s salt and its composites with modified melting points, enthalpy–temperature curves for composites of Glauber’s salt and NaCl are unavailable. In this study, we report the melting and solidification enthalpy–temperature curves for two different composites of Glauber’s salt and NaCl with a melting point of 21 °C obtained by the T-history method. Both composites contain NaCl to suppress the melting point, borax to reduce supercooling, and sodium polyacrylate as a thickener to enhance cyclic stability. The first composite with 12 wt.% NaCl demonstrated 139 kJ·kg−1 of latent heat of fusion, and the second composite with 9 wt.% NaCl demonstrated 171 kJ·kg−1. Both the composites have high volumetric energy densities compared to their organic counterparts with similar melting points.

Frost is an undesirable problem in energy conversion and engineering applications because it negatively affects the operating system performance by reducing the heat transfer for energy conversion systems and the coefficient of performance (COP) for refrigeration and air conditioning (HVAC) equipment. Among the various frost prevention or removal techniques, electrohydrodynamics (EHD) is an active frost prevention and removal technique that has been studied since the 1970s. This review paper clarifies the fundamentals of EHD, while offering a comprehensive review of the works published in the literature regarding both the influence of EHD on frost growth control and its effectiveness on frost removal. It is observed that while individual research works have drawn conclusions on the specifics of EHD for frost control and removal, there is no consensus in the literature on the specific effects of some of the critical parameters associated with EHD phenomena, such as the influence of electric field intensity and the use of AC and DC voltage, which can both affect frost growth. In addition, no baseline for comparison has been established, making it difficult to compare the results of various investigators. Finally, prospects and conclusions are discussed.This article is part of the theme issue 'Heat and mass transfer in frost and ice'.

Owing to the need for continuous improvement in building energy performance standards (BEPSs), facilities must adhere to benchmark performances in their quest to achieve net-zero performance. This research explores machine learning models that leverage historical energy data from a cluster of buildings, along with relevant ambient weather data and building characteristics, with the objective of predicting the buildings’ energy performance through the year 2040. Using the forecasted emission results, the portfolio of buildings is analyzed for the incurred carbon non-compliance fees based on their on-site fossil fuel CO2e emissions to assess and pinpoint facilities with poor energy performance that need to be prioritized for decarbonization. The forecasts from the machine learning algorithms predicted that the portfolio of buildings would incur an annual average penalty of $31.7 million ($1.09/sq. ft.) and ~$348.7 million ($12.03/sq. ft.) over 11 years. To comply with these regulations, the building portfolio would need to reduce on-site fossil fuel CO2e emissions by an average of 58,246 metric tons (22.10 kg/sq. ft.) annually, totaling 640,708 metric tons (22.10 kg/sq. ft.) over a period of 11 years. This study demonstrates the potential for robust machine learning models to generate accurate forecasts to evaluate carbon compliance and guide prompt action in decarbonizing the built environment.

This study designs and fabricates a small-scale, counter-flow Multi-Pass Microchannel Heat Exchanger (MPMHX) with a high surface area density of 989 m2/m3 using selective laser melting printing process, achieving a power density of 45.4 MW/m3. The MPMHX features fin and microchannel widths of 0.180 mm and 0.762 mm, respectively, to enhance power densities with sCO2. The ribbed manifolds ensure uniform flow, while the 173 mm multi-pass microchannel array was designed with the manifolds to fit within the printer volume. The high compactness of the HX poses printing challenges. So, the printing orientation, support structures, and printing parameters were developed to successfully fabricate the HX with a measured porosity of 3.8 % using the buoyancy method. Additionally, the long, narrow channels complicate powder removal. Therefore, a two-step process was developed, consisting of ultrasonic cleaning and pressurized air cleaning. Afterwards, the fabricated microchannels exhibited a relative roughness of 9.6 %, increasing pressure drop by 165 % compared to smooth channels. This study demonstrates that additive manufacturing can successfully fabricate MPMHX without substantial channel clogging, defects, or inaccuracies that hinder performance. It also provides clear guidance for future research on HX fabrication using additive manufacturing, paving the way for advanced highpower-density heat exchangers.

Heat exchangers operating in extreme environments need to meet strict size, weight, and power consumption (SWaP) requirements to enable an efficient thermal system. The development of high-performance, low-cost, and highly compact metal heat exchangers can be attractive and beneficial to multiple sectors, like aerospace, power generation, and electronics. This study investigates the implementation of the microchannel approach in the heat exchanger design to optimize heat transfer and endure high-pressure scenarios in such challenging environments. However, the design's complexity and compact nature pose significant challenges to traditional fabrication methods. To overcome this issue, direct metal laser sintering, as one of the additive manufacturing (AM) methods, was employed to customize and fabricate such type of heat exchanger with lower cost and achieve small fins (0.18 mm width) and channels (0.3 mm width). Upon creating a demonstration unit, experimental characterization was conducted, and the results were compared with model predictions to evaluate the heat exchanger’s hydraulic performance in laminar and turbulent flow regimes while accounting for roughness factors. This study demonstrates the feasibility of fabricating a compact microchannel heat exchanger using AM, while offering insights into its hydraulic performance through both empirical and computational techniques.

Frost accretion is a common problem in HVAC and refrigeration systems. Frost accretion strongly impacts the operating efficiency of HVAC equipment and leads to a considerable increase in energy consumption. Therefore, energy-effective frost control techniques can significantly enhance the coefficient of performance of HVAC systems while also enhancing the lifecycle durability of the equipment. More recently, passive and active frost control and defrosting techniques have been proposed to reduce/control frost growth on heat transfer surfaces and ultimately enhance the thermal/hydraulic performance of HVAC systems. This review paper is focused on active prevention and frost removal techniques in HVAC, heat pumps, and refrigeration systems. This work was conducted by categorizing the defrosting methods into frost prevention strategies and frost removal strategies. Frost prevention strategies include treating and conditioning the upcoming inlet airflow and vapor injection techniques. On the other hand, frost removal strategies include reverse cycle defrosting (RDC), oscillation and ultrasonic vibration, hot gas bypass, and the use of applied electric and magnetic fields, among other techniques. Despite extensive theoretical and experimental work on active frost removal and control techniques, no baseline for comparison has been established, making it challenging to compare different techniques. Finally, future prospects and conclusions are discussed in detail, and future research and development directions are proposed.

Mitigating CO2 emissions is essential to reduce climate change and its adverse effects on ecosystems. Photovoltaic electricity is 30 times less carbon-intensive than coal-based electricity, making solar PV an attractive option in reducing electricity demand from fossil-fuel-based sources. This study looks into utilizing solar PV electricity production on a large university campus in an effort to reduce CO2 emissions. The study involved investigating 153 buildings on the campus, spanning nine years of data, from 2015 to 2023. The study comprised four key phases. In the first phase, PVWatts gathered data to predict PV-generated energy. This was the foundation for Phase II, where a novel tree-based ensemble learning model was developed to predict monthly PV-generated electricity. The SHAP (SHapley Additive exPlanations) technique was incorporated into the proposed framework to enhance model explainability. Phase III involved calculating historical CO2 emissions based on past energy consumption data, providing a baseline for comparison. A meta-learning algorithm was implemented in Phase IV to project future CO2 emissions post-solar PV installation. This comparison estimated a potential emissions reduction and assessed the university’s progress toward its net-zero emissions goals. The study’s findings suggest that solar PV implementation could reduce the campus’s CO2 footprint by approximately 18% for the studied cluster of buildings, supporting sustainability and cleaner energy use on the campus.

A comprehensive energy audit of a light rail maintenance facility was performed to assess its energy performance and identify potential scope for improvements. The facility’s energy use intensity (EUI) for 2022 was 404 kWh/m2—more than double the benchmark EUI for maintenance facilities (151 kWh/m2) recommended by EnergyStar. Furthermore, the load factor was 0.22—significantly lower than the recommended minimum of 0.75 for an efficient building. The energy audit encompassed an in-depth evaluation of the facility’s structural and operational characteristics, comprising HVAC systems, lighting, the building envelope, and energy-intensive machinery. An energy model of the facility was developed to emulate the facility’s energy performance in 2022. Following the energy model’s validation, an analysis was conducted to identify opportunities for improving energy efficiency. Post-implementation of energy efficiency measures for the facility, the projected annual reductions are 1086 MWh of electricity, 5034 GJ of natural gas, utility savings of USD 162,402, and net GHG emissions reductions of 584 metric tons of CO2e. A subsequent 30% reduction in EUI to 283.6 kWh/m2 could be achieved with an 86% improvement in load factor, that is, increasing it from 0.22 to 0.41. This study emphasizes the need for energy audits and modeling for maintenance facilities to reduce Scope 1 and 2 emissions.

Compared to state-of-the-art heat exchangers, manifold-microchannel heat exchangers have shown superior heat removal density (kW/kg) at moderate pressure drops. However, manifold-microchannel heat exchangers made of Ni-based superalloys or other tough-to-machine materials can be a challenge to fabricate using conventional fabrication methods. This is mainly because of the inherently complex manifold microchannel geometry, as well as the required small feature sizes (e.g., fin thickness) that should be comparable, or smaller than state-of-the-art high-performance metallic-based heat exchangers (similar to 150 mu m or smaller). In this study, a direct metal laser sintering (DMLS) additive manufacturing technique was used to fabricate the compact high-temperature manifold-microchannel heat exchanger reported here. The additively manufactured manifold-microchannel heat exchanger was fabricated as a single object, which significantly simplifies the fabrication process. In this work, three different additive manufacturing machines were used to study the effect of laser power, powder size, and layer thickness on the fin and channel sizes of the fabricated microchannel heat exchangers. To evaluate the minimum wall thickness for holding the required design pressures, pressure containment tests were performed. As a result, a wall thickness of 0.3 mm was shown to withstand 340 kPa and be leakage-free. A detailed analysis of different printing orientations and their effect on the manifold-microchannel heat exchanger's design was also performed. Finally, a 76 x 76 x 76 mm(3) manifold microchannel heat exchanger was successfully fabricated with a fin thickness of 0.13 mm out of maraging steel. A second unit with dimensions of 94 x 87.6 x 94.4 mm(3) was successfully fabricated with a fin thickness of 0.22 mm out of Inconel 718. Details of the fabrication process and key take-away results are discussed in this paper.

This work presents the experimental results of a novel, air-to-air, additively manufactured manifold-microchannel heat exchanger with straight fins on both sides. The heat exchanger was made of Inconel 718 using a direct metal laser sintering technique. The overall core size of the heat exchanger was 94 mm × 87.6 mm × 94.4 mm, with a fin thickness of 0.220 mm on both the hot and cold sides. The heat exchanger was tested with pressurized nitrogen gas at 300 °C and 340 kPa for the hot side, while air at an ambient condition was used for the cold side. An overall heat transfer of 276 W/m2K was obtained for Reynolds number values of 132 and 79 for the cold and hot sides, respectively. A gravimetric heat transfer density (Q/m∆T) of 4.7–6.7 W/kgK and a volumetric heat transfer density (Q/V∆T) of 6.9–9.8 kW/m3K were recorded for this heat exchanger with a coefficient of performance value that varied from 42 to 52 over the operating conditions studied here. The experimental pressure drop results were within 10% of the numerical values, while the corresponding heat transfer results were within 17% of the numerical results, mainly due to imperfections in the fabrication process. Despite this penalty, the performance of the tested heat exchanger was superior to the conventional plate-fin heat exchangers: more than 60% of improvements in both gravimetric and volumetric heat transfer densities were recorded for the entire range of experimental data.

Implementing a Thermal Energy Storage (TES) system in a data center has several advantages. They reduce energy consumption, improve resiliency in emergency conditions and reduce the carbon footprint. Organic phase change materials (PCMs) are widely used in TES devices for data centers due to their ease of implementation, high cyclic stability, and low supercooling. However, organic PCMs have a low thermal conductivity of $\sim 0.1$ W/m-K, limiting the thermal performance of the TES. On the other hand, salt hydrates, a subclass of inorganic PCMs, generally have high energy storage density and thermal conductivity and are inexpensive compared to their organic counterparts. Therefore, they are a lucrative choice of PCM materials for low-cost, compact latent heat thermal energy storage (LHTES) devices for both electronics and data center cooling applications. However, salt hydrates suffer from issues such as supercooling and phase segregation. Sodium Sulphate Decahydrate (SSD), also known as Glauber's salt, is one of the most inexpensive salt hydrates PCMs, melting at around 32°C. For applications such as peak load shifting for data center cooling, the PCM melting point requirement could be between 20–30 °C. The SSD's melting point could be tuned by adding a melting point suppression agent/s. NaCl and KCl are the most popular melting point suppression agents used with SSD in the literature. While supercooling could be solved by adding Borax (a nucleating agent), phase segregation of SSD remains a critical issue leading to degradation in latent heat with its usage, limiting its useful life. Adding thickener increases the viscosity of the PCM and is the most popular way to minimize latent heat degradation during thermal cycling. Several thickeners were explored in the literature with SSD, but often without melting point suppression agents. This study investigated the thermophysical properties and short-term cyclic stability of two different melting point-suppressed SSD PCMs with different thickeners in each PCM. The target melting point range of the PCM is 20–25 °C, which is suitable for application in TES at data centers.

The increasing power densities of electronic devices due to more compact package requirements make their thermal management a major challenge. Adequate cooling of these devices is required to increase their lifespan and maintain operational capabilities. Over the past couple of decades, microchannel heat sinks, among other solutions, have been implemented to dissipate high heat fluxes under operational parameters of practical sig-nificance. Furthermore, convective heat transfer coefficients have been increased by using a two-phase flow and by reducing the hydraulic diameters of the channels. However, this incurs an increase in pressure drop and pumping power. A novel cooling technique, film evaporation with an enhanced fluid delivery system (FEEDS), has been demonstrated to simultaneously enhance the heat transfer coefficient while minimizing the increased pressure drop and pumping power requirements. The FEEDS cooler is a manifold-microchannel system in which an array of manifolds is positioned perpendicularly on a system of parallel microchannels. In the present work, two FEEDS coolers were designed and developed to manage high heat fluxes of electronic components. The geometrical parameters of the manifolds were varied to investigate the effect of manifold parameters on thermal and hydrodynamic performances. Both single-phase and two-phase tests were performed with R-245fa refrig-erant as the working fluid at different mass fluxes, and heat transfer and pressure drop performances were measured. Heat fluxes in excess of 1 kW/cm2 with a surface superheat of 38 degrees C and with a low-pressure drop penalty were achieved. Additionally, this FEEDS cooler showed a relatively constant heat transfer coefficient even after the optimal vapor quality was exceeded, indicating a stable liquid film.

Phase change material (PCM)-based thermal energy storage (TES) systems are widely used for repeated intermittent heating and cooling applications. However, such systems typically face some challenges due to the low thermal conductivity and expensive encapsulation process of PCMs. The present study overcomes these challenges by proposing a lightweight, low-cost, and low thermal resistance TES system that realizes a fluid-to-PCM additively manufactured metal-polymer composite heat exchanger (HX), based on our previously developed cross-media approach. A robust and simplified, analytical-based, 1D reduced-order model (ROM) was developed to compute the TES system performance, saving computational time compared to modeling the entire TES system using PCM-related transient CFD modeling. The TES model was reduced to a segment-level model comprising a single PCM-wire cylindrical domain based on the tube-bank geometry formed by the metal fin-wires. A detailed study on the geometric behavior of the cylindrical domain and the effect of overlapped areas, where the overlapped areas represent a deviation from 1D assumption on the TES performance, was conducted. An optimum geometric range of wire-spacings and size was identified. The 1D ROM assumes 1D radial conduction inside the PCM and analytically computes latent energy stored in the single PCM-wire cylindrical domain using thermal resistance and energy conservation principles. The latent energy is then time-integrated for the entire TES, making the 1D ROM computationally efficient. The 1D ROM neglects sensible thermal capacity and is thus applicable for the low Stefan number applications in the present study. The performance parameters of the 1D ROM were then validated with a 2D axisymmetric model, typically used in the literature, using commercially available CFD tools. For validation, a parametric study of a wide range of non-dimensionalized parameters, depending on applications ranging from pulsed-power cooling to peak-load shifting for building cooling application, is included in this paper. The 1D ROM appears to correlate well with the 2D axisymmetric model to within 10%, except at some extreme ranges of a few of the non-dimensional parameters, which lead to the condition of axial conduction inside the PCM, deviating from the 1D ROM.

The development of low-cost, high-performance, and compact heat exchangers for extreme environmental conditions will benefit multiple sectors, especially applications in power electronics and aerospace. These heat exchangers enable efficient thermal exchange systems with strict size, weight, and power consumption (SWaP) requirements. In this study, a counter-flow heat exchanger was designed using microchannels to achieve a high-power density (31.1 kW/kg and 195 kW/L) and is capable of operating in high-temperature (800 °C) and high-pressure (80 bar) environments. Two critical aspects of this microchannel heat exchanger for extreme environments (MHXEE) are included: (1) creep resistance associated with operating at high temperatures and (2) pressure drop associated with the microchannels. CFD and stress analyses were conducted to characterize the heat exchanger's thermohydraulic and mechanical performances. By adding internal ribs inside the manifold area, the heat exchange strength was improved at its design condition while the pressure drops remained at acceptable levels. In order to improve this design further, a topology optimization approach was developed using a 2D model, where different optimal material distributions could be determined that focused on optimizing either: (1) the uniformity of the velocity field or (2) the uniformity of the heat flux. This study demonstrated the potential for further using a robust and reliable yet simple topology optimization approach to improve the thermal and mechanical performance of the MHXEE.