Modern test methods for air conditioners and heat pumps must reconcile two competing demands: accurately reflecting real operating behavior and ensuring practicality, repeatability, and interlaboratory comparability. Conventional fixed-speed, steady-state rating procedures fall short of this objective, as they exclude control dynamics and interactions with buildings and distribution systems, thereby limiting their representativeness of in-use performance.This review critically examines the limitations of current testing standards and synthesizes recent research and technical advances aimed at improving performance characterization and seasonal efficiency assessment. Emphasis is placed on load-based testing methodologies, emulator-based approaches, and hardware-in-the-loop (“field test in the lab”) concepts, which enable active-control operation under reproducible yet realistic conditions. Evidence from laboratory demonstrations, interlaboratory comparisons, and emerging standardization efforts is consolidated to assess the technical maturity, robustness, and scalability of these methods. The discussion also reflects ongoing international initiatives, including International Energy Agency Annex 88, “Evaluation and Demonstration of Actual Energy Efficiency of Heat Pump Systems in Buildings,” and the International Organization for Standardization Informal Meeting on “Load-based Test Methods.”Based on the reviewed methods, the paper identifies pathways toward next-generation testing frameworks that better recognize advanced system architectures and control strategies, support evidence-based policy and standardization, and provide consumers with performance metrics that more closely align with real-world energy outcomes.

This paper presents a field study aiming to understand and improve the energy and thermal comfort performance of air-source heat pumps (ASHPs) in cold climates. The study was conducted in a residential community of 22 single-family homes in Indianapolis, Indiana, equipped with smart thermostats, circuit-level power meters, and indoor temperature sensors. Field measurements and resident interviews were first used to characterize thermostat adjustments and their implications for heating-related electricity use. Baseline data revealed that heating-related electricity use varied significantly, by up to a factor of two, across physically similar homes, indicating that ASHP field performance was shaped not only by weather, equipment, and envelope characteristics, but also by how residents operated their thermostats and supplemental heating devices. A Smart Energy Assistant was then developed for the first time, to detect energy-inefficient thermostat adjustments and implement targeted control actions, while communicating each action and allowing residents to cancel or override it. These control actions were designed to reduce unnecessary auxiliary heat use, avoid inefficient recovery loads, improve temperature distribution, and encourage more efficient use of the central heat pump system. During the two-month deployment, the Smart Energy Assistant generated 1,643 actions across 22 homes, with 88% overall acceptance rate, but with uneven distribution across homes. Overall, the findings indicate that improving ASHP performance in occupied homes requires both better understanding of resident thermostat adjustments and equipment control strategies as well as transparent communication and consideration of resident thermal comfort and agency.

Hyeongseok Lee, Feng Wu, Hemanth Devarapalli, James E. Braun, and Panagiota Karava

Traditional dehumidification and drying processes are energy-intensive as they involve cooling the air below the dew point to condense and remove water vapor. Vacuum membrane dehumidification offers energy-saving opportunities, but its full potential remains undeveloped due to the presence of water vapor within the vacuum pump and excessively high pressure ratios. Recent studies proposed a dual-module humidity pump (DMHP) to address this, but the increased air permeation into the system requires strategies to prevent air pressure buildup and diffusion barrier formation. This study investigates a DMHP, specifically designed to address these issues by incorporating hollow fiber membranes for isothermal dehumidification in industrial heat pump dryers (HPDs). Membrane geometry and properties are coupled with a partial pressure-driven epsilon-NTU method, and a discretized model is used to identify water vapor transport under sub-ambient conditions. Thermodynamic models of the components are developed, and membrane-integrated HPDs are compared to a conventional HPD. The proposed system's specific moisture extraction rate (SMER) exceeds conventional HPDs by 69%. Global sensitivity analysis reveals that SMER is 7.2 times more responsive to ambient conditions than dryer inlet conditions, with membrane geometry and properties' interactions exerting greater influence than their individual effects. The optimum pressure ratio for the water vapor compressor, ranging from 1.2 to 3.8 and adjustable via synchronized control of rotational speeds with the vacuum pump, enhances SMER by up to 33.7% with a vapor balance ratio of 0.84-0.89. The results suggest that future work should investigate further optimization of membrane modules and variable built-in volume ratio compressors to unlock the full potential of DMHP technology.

Molecular Rayleigh scattering (RS) is sensitive to the pressure, temperature, velocity, and number density of the gaseous flow. Filtered Rayleigh scattering (FRS) utilizes a narrowband molecular filter to remove stray scattering and deconvolve the effect of flow conditions on the signal intensity. As single-frequency, intensity-based FRS techniques can typically only deconvolve a single parameter per detector angle, frequency-scanning (FS) FRS has been used to semi-spectrally resolve the signal and quantify multiple parameters using a single detector. In this work, the FS-FRS data rate was increased by a factor of 105 using a rapid wavelength-tunable burst-mode laser operated at 20 kHz. The technique is demonstrated for simultaneous, spatially resolved temperature, pressure, and radial velocity measurements time-averaged across 1 ms in an underexpanded jet, yielding a measurement rate of 1 kHz for potential use in high-speed flow test facilities.

This study presents an adaptive Gaussian Mixture Regression (GMR) framework for the fault detection and diagnosis (FDD) of radiant slab (RS) heating and cooling systems. This domain has not been extensively explored in FDD research. Our study addresses the challenge of identifying and adapting to previously unknown or unexplored operational states within HVAC systems, exacerbated by the influence of seasonal changes on fault characteristics. An evolving GMR model is developed that learns from the dynamic performance of building systems and their components across different seasons. Utilizing a comprehensive dataset collected from a real office environment during winter, spring, and summer, the model is rigorously tested against various faults. A feature selection process is applied to determine the most indicative features for fault diagnosis. The adaptive mechanism allows for updating the existing or integrating new Gaussian components upon detecting unknown states, thereby preserving its diagnostic precision over time. The GMR FDD model was trained with winter data and tested with winter, spring, and summer data. Our findings reveal that fault diagnosis accuracy is extremely high for winter, both before and after evolving the model, but is compromised without the model evolution for the data from the spring and summer seasons. However, once evolved, the model demonstrates a remarkable ability to assimilate new fault patterns, achieving high diagnostic accuracy rates-99.6% in spring and 97.3% in summer-thereby confirming its robustness and adaptability to seasonal variations in fault diagnosis.

This paper assesses the numerical modeling efforts of an experimentally built non-premixed 6 cm radius hydrogen-air rotating detonation combustor with an integrated short diverging wall as well as a post-processing routine to compare two computational solvers. The geometry and boundary conditions are detailed, and unsteady RANS simulations have been performed with CFD++ with a wall resolved mesh with multi-step chemistry. First, a one-dimensional detonation tube is modeled using both the NASA OpenNCC and Metacomp CFD++ solvers with coarse and fine grids. Second, a two-dimensional detonation tube is characterized to capture the detonation cells. A quasi-two-dimensional RDC is modeled with CFD++ and OpenNCC to understand the solver-to-solver differences. Finally, the 3D non-premixed combustor is analyzed for its cold flow (steady state) performance and hot flow performance and compared to the experiment. Overall, the three-dimensional simulations are in close agreement with experimental global parameters such as wave speed, chamber pressure, and coefficient of discharge as well as predicting low frequency oscillations.

This manuscript evaluates the use of an actively cooled aerodynamic probe body with integrated optics to perform short focal length laser diagnostics within high-temperature, high-supersonic flows. A novel probe with a “ship’s bow” shape and an open cycle cooling scheme ending in a matrix of effusion holes is presented. The flow around the probe is studied numerically using 3D Steady Reynolds-Averaged Navier-Stokes (RANS) simulations at Mach 6, with free stream total conditions of 1700 K and 44.2 bar. The laser trajectory is calculated using the ray propagation equation to ensure that it deviates minimally while going through regions of gradients in refractive index. The location and shape of the detached shock formed ahead of the probe is extracted, and the minimum focal length achievable for measurement of the undisturbed flow-field is determined. The sensitivity of the bow shock shape to changes in cooling pressure between 1.33 and 1.835 bar, and to blocked holes has been computed. Finally, the flow through the effusion system of the probe is tested experimentally, ensuring its proper operation, and checking the consistency of the effusion boundary conditions used in the numerical study.

The testing and rating procedure for central air conditioners and heat pumps ("CAC/HPs") detailed in AHRI 210/240-2024 employs a steady-state test procedure where test room conditions are maintained at constant values, and the CAC/HP system is controlled to operate at specified fixed compressor speeds and indoor airflow rates. For variable-speed and multi-stage systems, it includes some specifications on selecting compressor speeds and indoor fan speeds for various ambient conditions. However, this standard does not include a process to verify whether the performance under the selected component speeds in the testing agrees with the operations of the system at the same conditions under native controls. To address this problem, a Control Verification Procedure (CVP) has recently been developed as an additional test that is described in AHRI 210/240-2024. The CVP is based on the load-based testing approach developed through CSA SPE-07:2023 that measures performance of a test unit with its native controls that interacts dynamically with a building load. In the CVP, the indoor environment is controlled to dynamically respond to the test unit behavior using a representative virtual building model that has outdoor test conditions and equipment cooling/heating rates as inputs. For the testing sequence, the outdoor condition was varied between three steady-state conditions through two transitional periods. Then, the system dynamic performance at the intermediate load condition was used to validate the operation of the native control, including verifying the system operates with variable capacity. In this paper, results and assessments associated with applying the CVP to two variable-speed heat pumps are presented. One of the primary goals is to evaluate whether CVP can serve as an indicator of whether the existing steady-state testing for AHRI 210/240 accurately reflects the unit's performance under native control and dynamic load conditions.

The chemical looping heat pump (CLHP) is a promising electrochemical heat pump technology due to high system efficiency, scalability, and use of low-to-zero Global Warming Potential (GWP) fluids. However, similar to other emerging HVAC&R technologies, there is a lack of direct comparison and discussion of economics between CLHPs and conventional vapor compression (VC) heat pumps. In this work, a generalized modeling framework to estimate the levelized cost of energy (LCOE) for space conditioning applications is used to assess the early-stage economic feasibility of CLHP. The LCOE consists of two components: levelized operating expenditures and levelized capital expenditures. These clarify the influence of key factors such as annual cooling and heating delivered and price of electricity. The simulations show that the LCOE of CLHP could be less than that of VC in the case of unit utilization of >30,000 kWht yr(-1), operating current density of >0.4 A cm(-2), and 30% performance improvements. This is despite the projected capital cost of CLHP is nearly 1.6 times higher than that of VC system.

In the present work, a first-of-its-kind 3D large-eddy simulation (LES) study is conducted to numerically investigate the combustion dynamics as well as aero-thermal phenomena in a full-scale non-premixed hydrogen-air rotating detonation engine (RDE) (with a diverging-shaped lower-end wall), when integrated with nozzle guide vanes (NGV) acting as the turbine stator. The wall-modeled LES framework incorporates hydrogen-air detailed chemical kinetics and adaptive mesh refinement (AMR). A comparative analysis is carried out for two operating conditions with different fuel/air mass flow rates but global equivalence ratio of unity, and considering RDE configurations without and with stator. The LES model is validated against available experimental data for the low mass flux condition with respect to detonation wave speed/height, wave dynamics, and axial static pressure distribution. Numerical results indicate significant deflagrative combustion occurring in the fill region near the inner wall due to the formation of recirculation zones in the injection near-field driven by the backward facing step. The leading detonation wave is found to be trailed by an azimuthal reflected-shock combustion (ARSC) wave, consistent with experimental observations, which consumes unburned vitiated reactants that leak through the main detonation wave. The main detonation wave characteristics, such as detonation wave speed/height, and combustion efficiency do not change appreciably with the presence of NGV. A novel combustion diagnostic technique based on chemical explosive mode analysis (CEMA) is employed to quantify the fraction of heat release occurring in the detonative mode versus deflagrative mode for the simulated conditions. The exit flow is found to be nearly fully subsonic and supersonic for the low and high mass flux conditions, respectively. Further analysis of the exit flow profiles shows that the presence of the NGV renders the flow more axial and significantly impacts the exit Mach number and total pressure, while the total temperature shows negligible change. In addition, the low mass flux operating point, despite exhibiting more deflagrative losses within the combustor, yields overall lower pressure drop from plenum to exhaust, which is mainly attributed to lower pressure drop across the injectors. Lastly, the RDE-NGV configuration exhibits higher total pressure loss compared to RDE without stator across both the mass flux conditions. This study extends the state-of-the-art in numerical modeling of pressure gain combustion systems by demonstrating high-fidelity 3D reacting LES of a full-scale RDE-NGV systems for practical insights pertaining to RDE-turbine integration for power generation.

A large body of simulation research suggests that model predictive control (MPC) and reinforcement learning (RL) for heating, ventilation, and air-conditioning (HVAC) in residential and commercial buildings could reduce energy costs, pollutant emissions, and strain on power grids. Despite this potential, neither MPC nor RL has seen widespread industry adoption. Field demonstrations could accelerate MPC and RL adoption by providing real-world data that support the business case for deployment. Here we review 24 papers that document field demonstrations of MPC and RL in residential buildings and 80 in commercial buildings. After presenting demographic information – such as experiment scopes, locations, and durations – this paper analyzes experiment protocols and their influence on performance estimates. We find that 71

This paper investigates, for the first-time in the research literature, how the operational characteristics of air-source heat pumps in cold climates influence occupants’ thermostat adjustment behaviors. It presents a field study that monitors disaggregated energy use and occupant thermostat interactions under different heating system operational scenarios in 21 nearly identical single-family homes within a newly constructed residential community. Despite typical heterogeneous occupant behavior that exists in residential buildings, the results from our field study showed that in 62 % of the homes, the occupants selected lower temperature setpoints when the auxiliary heater was the primary source compared to the baseline heat pump priority mode that had higher supply air temperatures. Subsequently, to further investigate differences in setpoint preferences and the motivations behind setpoint adjustments, controlled laboratory experiments were conducted with 32 participants. The experiments emulated the operational characteristics of a single-stage heat pump with auxiliary heating observed in the field, and included a variable-speed heat pump test case with enhanced comfort as a baseline for comparison. According to the results, 19 out of 32 participants increased their setpoints even though the emulated single-stage heat pump had sufficient capacity to warm the indoor space. Cold air movement and indoor temperature fluctuations due to the heat pump cycling on/off were the main reasons participants reported increasing their setpoints. The laboratory study documented that these triggers can be mitigated by using variable-speed equipment, which provides better indoor temperature control in cold conditions.

The objective of the work described in this paper is the development of a novel wall-embedded micro heat pump (WEMHP) that is packaged and easy to install for retrofit and new building applications. A successful product could be a more efficient approach to providing local comfort and could contribute to reducing heating and cooling energy consumption and electrification of residential buildings. This new distributed comfort delivery approach has several distinct advantages compared to alternatives: (1) A WEMHP eliminates the need for a secondary water loop and circulation pump that could employed for some other comfort delivery approaches. It does not require separate indoor and outdoor units. Instead, a WEMHP unit operating in heating mode directly absorbs heat through an embedded heat exchanger (evaporator) at the outside wall surface and then conditions the indoor space using an embedded heat exchanger (condenser) at the indoor surface. (2) This packaged solution eliminates the need for extensive HVAC installation and on-site refrigerant charging, leading to reduced installation costs and minimized risk of refrigerant leaks. (3) The interior surface temperature of the exterior wall-section empowered by the micro heat pump is independently controlled, allowing for distributed space conditioning and delivery of radiant heating to meet diverse occupant needs in different zones. In this study, a prototype WEMHP was designed, assembled, and tested in a laboratory environment as a proof-of-concept demonstration. The developed prototype consists of a variable-speed miniature rotary compressor and aluminum compact roll-bond heat exchangers acting as evaporator and condenser, respectively, depending on the operating mode. To control the prototype, a miniature electronic expansion valve was developed and calibrated in-house. The system performance in heating mode was evaluated using a pair of psychrometric chambers under varying operating conditions. The test results demonstrated that the heating capacity under condition H1 reached around 164 W at a compressor speed of 3500 RPM with a COP of 1.67. Additionally, the system exhibited a fast thermal response, with a time constant t63 (the time it takes for the surface temperature to reach 63% of the difference between its final and initial values) of less than 0.5 hours and a t95 of approximately 1.5 hours.

The Tesla turbine, also known as a bladeless turbine, operates based on the principles of viscosity. These turbines are a promising alternative to expansion valves in refrigeration cycles as they recover a part of expansion work and improve the COP of the cycle. This paper presents a comprehensive numerical analysis of a bladeless turbine prototype developed by Tree Associates, utilizing Computational Fluid Dynamics (CFD) to evaluate its performance under various conditions with air as the working fluid. An unstructured meshing approach with refinement around the nozzle region was employed to capture the flow characteristics and predict turbine performance accurately. Mesh sensitivity analysis was conducted to ensure grid independence and optimize computational efficiency. Velocity triangles were analyzed to understandflow behavior and energy transfer mechanisms. The study explores the impact of nozzle shape (flat to convergent-divergent), nozzle inclination (15 to 75 degrees), and the number of nozzles (2, 4, 6 & 8) on overall turbine performance. Additionally, the effect of Mach number and shear stress are investigated. An experimental counterpart was set up and tested to validate the computational model and the results of the numerical simulations are compared with the experimental data.

Flow instabilities in high-Reynolds flows are difficult to characterize because the increasingly chaotic interactions of fluid particles allow for larger variety and superpositions of flow patterns. These are especially underexplored in liquid jets in crossflows (LJIC) that are a popular mixing strategy for high-Weber gaseous flows. This study utilizes proper orthogonal decomposition (POD) to identify periodic flow patterns in experimental spray images. A liquid ethanol jet is injected into both a linear subsonic (Mach 0.25-Mach 0.7) and an expanding Mach 2 gaseous crossflow with plain-orifice injectors of 0.5, 1, and 2 mm diameter. Across both regimes, crossflow mass flows vary from 0.5 to 12 kg/s with injectant momentum flux ratios between 0.3 and 180. Reynolds numbers scaled by the injector diameter varied between 4000 and 52,000. A high-speed camera is used with extinction imaging systems to collect transmittance contours of the spray at 50 kHz. Unique "dagger" and "hump" shaped spray features are identified and correlated with a newly ascribed wave-crest and the well-known catastrophic LJIC breakup modes, respectively. The same features are identified as "advection modes" with the POD, and the mode images provide an accurate representation of the phase-averaged size and shape of these periodic spray structures. A characteristic length is used to normalize the peak POD frequencies and produces a mapping of the wave-crest and catastrophic breakup modes into regimes of Weber number and momentum flux ratio.

Social energy games present strong potential to promote household energy awareness and motivate community-level energy-saving behaviors, especially when paired with incentive mechanisms that align with resident preferences and goal achievement capabilities. This study proposes a mathematical framework for designing social energy game mechanisms that promote heating and cooling energy savings in residential communities. The framework enables modeling household decisions and predicting counterfactual outcomes under diverse game scenarios in MySmartE, a social gaming platform that provides a goal-oriented collaborative game and a competitive lottery game to encourage energy-aware smart thermostat adjustments. First, utility models were developed to capture the unique decision-making characteristics of households participating in the games. Parameters, estimated through Hierarchical Bayesian model calibration using field data, captured large variations in residents’ preferences and responsiveness to indoor temperatures, game performance, and reward levels. The inferred model parameter space was then used to generate synthetic communities under four behavioral scenarios, covering all combinations of decision characteristics observed during the game. Finally, counterfactual simulations were performed to examine how community decisions would vary under different levels of goals and incentives for each synthetic community. The simulations employed a game-theoretic model where household decisions followed the dynamics of Fictitious Play. The results reveal that communities participating in the same game design can select substantially different room air temperatures, depending on the decision characteristics of the residents. The proposed framework offers a foundation for designing sustainable and impactful energy-saving programs tailored to diverse resident characteristics in affordable housing communities.

Heating, ventilation, and air conditioning (HVAC) systems account for more than 40% of the U.S. electricity usage, primarily utilizing vapor compression system (VCS) heat pumps, raising environmental concerns. The phase down of high GWP refrigerants, targeting an 85% reduction by 2035, prompts the exploration of novel heat pump technologies. Electrochemical looping heat pumps (ELHP) show promise by employing redox reactions for fluid compression to replace conventional compression technologies. However, energy-intensive redox interconversion necessitates catalysts to lower activation energy. Inspired by electrosynthesis principles, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) emerged as a successful homogeneous electrocatalyst for selective isopropanol oxidation to acetone for use in ELHP. HPLC and 13C NMR confirmed remarkable selectivity (~100%) at practical isopropanol concentrations. To enhance TEMPO's catalytic activity, various electron-withdrawing and donating groups were explored. TEMPO-OCH3, among seven derivatives, exhibited superior efficiency with a notable rate constant (6 M-1s-1) and turnover frequency (3.1 s-1). This study innovatively applies concepts from molecular catalysis for the development of efficient catalyst materials.