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Selecting appropriate working fluids is important for improving the technology readiness level of electrochemical heat pumps (EHP), which is a promising emerging technology for space conditioning and thermal management applications. The major contribution of this paper is to develop and evaluate key metrics to identify electrochemically active working fluids based on heat pump cycle requirements. The key metrics include the possibility of phase change after a chemical reaction, reversible cell voltage and power consumption, cooling and heating capacity, and environmental and safety aspects. Such metrics are applied to conduct fluid screening to assess their suitability. Isopropanol/acetone showed the highest potential to deliver efficient cooling and heating when applied to an EHP system; the use of ethanol/acetaldehyde and methanol/formaldehyde would easily enable electrochemical fluid condensation over a wide range of operating temperatures. Although the approach is demonstrated for selected organic fluids, the screening framework can be readily employed for other chemical compounds expanding the design space of electrochemical heat pumps. • Developed a tool for thermodynamic evaluation of electro-active working fluids. • Evaluation metrics include thermophysical, reactions, GWP, and safety properties. • In cooling mode, system efficiency is improved by 15%–25% using isopropanol/acetone. • Ethanol/acetaldehyde system can operate down to −27.4 °C in heating mode. • Flammability and toxicity of the fluids should be addressed using secondary loops. [ABSTRACT FROM AUTHOR]

A load-based testing methodology has recently been developed for laboratory testing of residential cooling equipment with its integrated controls that employs a model to emulate building dynamics and their interaction with equipment controls. This approach is part of a new performance rating standard that utilizes a bin method to estimate a seasonal energy efficiency (CSA EXP-07, 2019; Cheng et al., 2021). However, a "holy grail" for future equipment performance rating is to be able to map equipment performance using load-based testing results and then implement the map as a "model" in building energy simulations to generate seasonal performance ratings that are specific to various building and climate types. With this goal in mind, this paper presents a performance mapping methodology that incorporates a "gray-box" model structure that uses inputs that are consistent with building energy simulation programs (sensible cooling load and equipment inlet conditions) and outputs key performance metrics (total sensible and latent cooling rates, power consumption, and COP). A strategy and a test matrix for training the model with a relatively small number of testing points were established by developing a successive optimization approach to identify the best 12 test conditions to apply for training from a larger set of available data. In order to develop, demonstrate, and validate the modeling and training approach, load-based laboratory testing was set up within psychrometric chambers and testing was performed to generate 39 quasi-steady-state data points over a range of loads and boundary conditions with a test unit operating with its normal integrated controls. The best model trained using the optimal training subset of 12 data points was able to represent the equipment performance across the operating envelope within ± 10 %. [ABSTRACT FROM AUTHOR]

• A thermostat environment emulator was developed for use in load-based testing. • Typical flow conditions are provided, and appropriate building dynamics are emulated to the test thermostat. • Details of the design and operation of the emulator are described to allow future reproduction. • A 2-D dynamic model of the thermoelectric conditioning devices in the emulator is presented. • The emulator could improve load-based testing reproducibility across different test facilities. To test unitary residential cooling equipment with integrated controls and typical load responses, a load-based testing methodology has been developed where a virtual building model is used to emulate building-equipment dynamic interactions. With this approach, the temperature of an environmental chamber that simulates indoor conditions continuously tracks virtual room temperature setpoints that are adjusted based on outputs from the virtual building model. The test equipment thermostat senses the floating air temperature and communicates with the test equipment as if it were installed in the field. In order to achieve realistic dynamic behavior, it is important to properly emulate inlet flow conditions and inlet temperature dynamics provided to the thermostat that are representative of field conditions and that can be reproduced across different test facilities. The goal of the work described in this paper was to develop a thermostat environment emulator for use in load-based testing. The proposed emulator can dynamically control the thermostat inlet air temperature to the virtual room temperature setpoints using thermoelectric devices. Sample test results obtained using the emulator are presented in this paper to demonstrate its ability to track emulated building responses during load-based testing. A description of the design and control of the emulator is provided with sufficient detail to allow future researchers to reproduce it. Additionally, a two-dimensional dynamic model of the thermoelectric conditioning devices is presented that guided the overall design process. This emulator is a basis for a standardized solution for applying load-based testing that is reproducible across different facilities. [ABSTRACT FROM AUTHOR]

This paper presents a near-optimal control algorithm for minimizing energy costs in hybrid cooling plants in response to cooling loads. The approach is based upon an approximate solution to the partial differential equations involved in minimizing the cost of energy consumed by the plant with respect to two control variables: cooling tower airflow and condenser water flow. The parameters of the algorithm can be determined with design information of the chillers and cooling towers, along with some measurements of total condenser water flow and pump power consumption. In addition to reducing plant operating costs, the algorithm simplifies tower control and is more stable compared with conventional tower control strategies such as constant condenser water supply temperature or constant approach to wet-bulb. A large-scale cooling plant model was utilized to evaluate the performance of the approach compared with optimal control and two heuristic control strategies over six months. The difference between the energy costs associated with the near-optimal control approach and optimal control was 1.7%. Comparison with the heuristic control strategies shows that significant energy savings can be achieved with the proposed algorithm. [ABSTRACT FROM AUTHOR]

This paper presents the application of a multi-agent control methodology to large chiller plants. The approach, originally conceived to automate controller design and reduce engineering costs in building energy systems, consisted of a multi-agent simulation framework with distributed consensus-based optimization algorithms. To adapt the approach to this scenario, agents representing physical components of a cooling plant were developed and incorporated in the framework along with two alternative optimization methods: centralized, parallel optimization with a genetic algorithm (GA), and a combination of the GA with a quasi-newton method to handle non-linear equality constraints associated to physical component behavior. An existing cooling plant was utilized as case study to simulate the performance of the methods under different operating conditions. The results demonstrated the difficulty of the consensus-based algorithms to find optimal solutions. The GAs, on the other hand, showed that significant energy savings can be achieved through the implementation of multi-agent control with algorithms capable of handling non-convex objective functions and a combination of discrete and continuous variables, which are characteristic of central cooling systems. [ABSTRACT FROM AUTHOR]

Buildings are responsible for more than 73% of the total electricity usage in the United States and have a significant contribution to peak electrical demand of the power grid. Thus, proper building demand management could improve the robustness and efficiency of grid operation and could also result in economic benefits for building owners. Almost half of the building electricity usage occurs in the commercial building sector where 60% of the installed cooling capacity is attributed to small commercial buildings employing packaged air-conditioning systems (e.g., rooftop units). Thus, understanding the demand response potential and economic benefits for the small commercial sector is of great importance. In particular, the effect of climate, building type, and utility structure on demand response potential is valuable information for building managers in identifying the most beneficial demand response strategies and for utilities in developing better utility structure rate incentives to achieve demand reduction. This article presents a simulation-based assessment approach and highlights the key assessment results for three demand response strategies—thermostat, shade, and lighting controls—across a range of building types, vintages, locations, and utility rate structures. The control performance is evaluated in terms of utility cost savings, peak demand reductions, and indoor comfort impacts in comparison to a baseline night setup strategy. The tradeoff between cost savings and comfort is also explicitly investigated. The assessment results show that the demand response controls lead to slightly worse indoor comfort compared to the baseline control, with an increase in the median percentage of people dissatisfied from 5.7% to 8.1%. However, significant demand reductions and cost savings could be achieved across all tested cases with the magnitudes dependent on the weather condition and building type. Annual electricity cost savings of 5 to 25 cents/ft2 (54 to 269 cents/m2) were demonstrated for time-of-use energy rates with demand charges, whereas the savings were relatively small, below 5 cents/ft2 (54 cents/m2) for most cases, when buildings subscribed to flat rates without demand charges. [ABSTRACT FROM AUTHOR]

Abstract The power grid has seen record demand for frequency regulation capacity in recent years due to the increased employment of renewable energy resources worldwide. Building thermal loads are flexible and thus, can be used as regulation reserves with proper control strategies. Previous studies have shown that building HVAC equipment can provide high-quality power grid regulation service with PJM performance scores of up to 0.98 and buildings' participation in the regulation market could bring significant economic benefits for building owners. However, the power flexibility in buildings is not persistent and the available HVAC regulation capacity has significant hour-by-hour variation due to building load and other operating constraints. This paper presents a regulation capacity reset strategy for HVAC regulation control that identifies the available regulation capacity and baseline power on the fly with real-time load and operation data. The strategy relies on a steady-state HVAC performance model derived from manufacturer performance data and implements a pseudo-optimization that seeks the maximum regulation capacity while respecting all operating constraints. The proposed strategy was implemented on a variable-speed rooftop unit (RTU) and validated with laboratory tests in psychrometric chambers. The test results show that the proposed reset strategy is effective in providing consistent high-quality regulation service with negligible impact on the indoor temperature control; the zone temperature deviation from the setpoint was within 0.3°C for all the performed tests. The reset strategy was also simulated with a prototypical building diurnal load model to quantify the integrated regulation capacity for a typical summer day. Simulation results indicate the integrated HVAC regulation capacity throughout a summer day equals approximately 1/4 of the daily electrical energy use; and the estimated daily regulation credit can offset up to 26% of the daily HVAC electricity cost based on PJM historical prices. [ABSTRACT FROM AUTHOR]

Abstract Real-time power supply and demand balances are critical to ensure stable power frequency and quality power services. However, the growing integration of renewable energy increasingly challenges the power infrastructure because most of the renewable resources, e.g., solar and wind energy, are intermittent and difficult to predict. To meet stringent power frequency requirements, more fast reacting frequency regulation resources are being brought online among which grid-level batteries are the dominant ones. Buildings consume more than 73% of the electricity in the US, offering significant regulation reserve for the power grid. Variable-speed air-conditioning and heat pump systems are taking an increasing share of the market due to higher efficiency requirements imposed by federal agencies. In addition to efficiency benefits, variable-speed cooling/heating systems are also perfectly suited to provide frequency regulation service since these units can modulate their power continuously over a wide range. This paper presents a methodology and case study results for laboratory-based assessments of power frequency regulation service performance for variable-speed HVAC cooling equipment. The assessment methodology involves the use of both open- and closed-loop testing in psychrometric chambers. The open-loop testing quantifies the maximum possible performance of regulation services that is building independent, whereas the closed-loop testing provides representative performance for regulation services when the equipment must maintain comfort conditions for typical "virtual" buildings. Two air-conditioning units, a variable-speed packaged rooftop unit (RTU) and a split heat pump with an after-market variable-speed retrofit, were utilized for regulation tests performed in psychrometric chambers. Both open-loop and closed-loop regulation control strategies were developed and tested for this equipment. Open-loop test results showed that variable-speed HVAC equipment can provide high quality regulation service with performance scores of up to 0.98. With PJM historical wholesale energy and regulation prices, the estimated regulation credit can offset up to 48% of the electricity price. The closed-loop regulation control was shown to have negligible impact on indoor comfort with temperature fluctuations smaller than 0.1 °C. Power regulation also did not cause any efficiency degradation for the closed-loop tests. In fact, for more humid climate conditions, the electrical input for cooling can even be reduced with regulation control due to reduced moisture removal. [ABSTRACT FROM AUTHOR]

A load-based methodology for testing and rating the performance of unitary air conditioning equipment has been developed and demonstrated using a laboratory psychrometric chamber test facility. The methodology simulates representative sensible and latent loads of a commercial building as well as the thermal and latent capacitance of buildings using psychrometric chamber test facilities. The advantage of the proposed test methodology is that there is no requirement to disable the native control systems designed by equipment manufacturers. This results in more realistic representations of the installed performance of equipment that include interaction of their embedded controls with the building. The load-based control approach is described and applied to the control system of an existing psychrometric chamber test facility. Results using the developed test methodology collected from fixed-speed and variable-speed RTUs show significant differences between actual part-load performance and steady-state performance at equivalent ambient operating conditions. A test procedure that includes interaction of the equipment controls with a virtual building could enable future equipment ratings that are both climate and building-type specific. This would not be possible for the current testing approaches that override native controls with no building interaction. [ABSTRACT FROM AUTHOR]

Abstract The need for more energy efficient heating and cooling technologies will increase over the coming decades. Climatic concerns associated with greenhouse gas emissions as well as energy costs associated with powering heating and cooling systems necessitate more efficient systems. This work presents an alternative to traditional vapor compression cycles used for heating and cooling. The chemical looping heat pump system is an indirect chemical heat pump that utilizes the possibility of condensation after a chemical reaction to allow the working fluid's compression to take place in the liquid phase. A thermodynamic model of the cycle is developed using electrochemical cells as the reaction drivers of the cycle. Sensitivity analysis is used to identify the most critical components of the system. The chemical looping heat pump may provide a potential COP increase of over 20% when compared to traditional vapor compression systems for representative device efficiencies. [ABSTRACT FROM AUTHOR]

Highlights • A numerically efficient but accurate model for frosting evaporators is developed. • The enthalpy-based reformulation and linearization method are employed • Simulation results on flat plate and finned-tube heat exchangers are provided. • It is 8 to 20 times faster than reference models but withcomparable accuracy. Abstract Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy. [ABSTRACT FROM AUTHOR]

Highlights • A complete and low cost MPC solution for small/medium sized commercial buildings was developed. • Handling unmeasured building disturbances for modeling and control phases was focused. • Performance was evaluated using both laboratory and field experiments. • A long-term performance evaluation of the MPC at a field site was performed. • Over 10% energy savings and 18. Abstract Small and medium sized commercial buildings, such as retail stores, restaurants and factories, often utilize multiple packaged air conditioners, i.e. roof top units (RTUs), to provide cooling and heating for open spaces. A conventional control approach for these buildings relies on local feedback control, where each unit is cycled on and off using its own thermostat. The lack of coordination between RTUs represents a missed opportunity for operating more efficient units when there is strong coupling between the spaces they serve and can lead to unnecessarily high electrical demand due to the inherent randomness of unit cycling. This paper presents an overall model-based predictive control (MPC) approach for RTU coordination that includes a description of the control architecture, modeling approach, implementation, and assessment. We provide results of laboratory and field tests that demonstrate the short-term and long-term performance of the MPC solution in terms of energy and demand savings. [ABSTRACT FROM AUTHOR]

Packaged air-conditioning systems, such as rooftop units (RTUs), are widely used in small commercial buildings in the United States. Although new regulations and standards require new air-conditioning equipment to have multi-speed supply fan operation, most of the installed units employ fixed-speed supply fans. The electrical energy usage associated with constant volumetric air delivery fan systems is on the order of half the total RTU electrical energy usage because supply fans operate continuously during the occupied periods in commercial buildings for ventilation. In addition, compressors within RTUs typically feature single- or two-stage capacity controls leading to frequent unit cycling and thus, deteriorated performance in energy efficiency. This paper presents relatively comprehensive assessment results for three RTU variable-speed retrofit options: 1) retrofits of fixed-speed supply fans with variable-speed fans for single-compressor-speed units, 2) replacement of existing fixed-speed RTUs with RTUs having two-speed compressors with variable-speed supply fans and 3) retrofits of RTUs with new RTUs that incorporate variable-speed compressors and fans (indoor and outdoor). The assessment study involves parametric simulations across different building types and locations in the U.S. to understand the performance variation with respect to location and building use. Performance improvements in terms of energy efficiency, operation cost and indoor comfort delivery for the various retrofit options are demonstrated compared to conventional single-stage RTUs with constant speed supply fans. The assessment results show that all three retrofit options offer significant fan energy savings, but much smaller savings potential for compressor and condenser fan electricity consumption. The variable-speed-unit retrofit provides the highest cost savings potentials, ranging from 40 to 250 $ per rated cooling ton per year depending on the building type and location. In addition to efficiency and economic benefits, the two-stage-unit and variable-speed-unit retrofits also lead to improved indoor humidity regulation performance due to reduced unit cycling. [ABSTRACT FROM AUTHOR]

• Extensive experiments for R454C through an EEV were carried out and studied. • Two semi-empirical correlations were developed for mass flow rate prediction. • The proposed correlations can predict 95% of the experimental results within 10%. • A potential relation between molar mass and mass flow coefficient was observed. The use of electronic expansion valves (EEVs) in heat pump systems has seen a rapid increase in recent years due to their advantages such as fast response, accurate control, and ability to improve seasonal performance. Modeling of the refrigerant flow characteristics through EEVs is needed during design for optimizing system performance and operation. Previous studies have not led to any generalized correlations for EEVs that accurately predict performance for different refrigerants. When working with a new refrigerant and/or EEV, it is necessary to perform mass flow measurements for the EEV over a wide range of operating conditions that are then used to develop and validate a refrigerant and EEV specific correlation. R454C, a low-GWP refrigerant mixture, is gaining attention as a potential replacement for R404A and R410A systems. However, there is currently no available EEV experimental data or correlation specifically for R454C in the literature. The objective of this paper is to present experimental results and characterization of R454C flowing through an EEV in a heat pump split system across a wide range of operational conditions. The impacts of valve opening, subcooling degree and inlet pressure on mass flow rate though the EEV were investigated. Two semi-empirical correlations were developed that map the experimental data based on the Buckingham π theorem and a polynomial function. Both mapping methods predicted mass flow rate for test data within 10%. The Buckingham π correlation was slightly more accurate. Both correlations were developed to enable mass flow rate predictions for R454C passing through EEVs with different valve sizes. Finally, a relationship between refrigerant molar mass and mass flow coefficient was observed that could be useful in developing a general EEV correlation that can be applied for different refrigerants. However, further investigation is needed to validate this relationship. [ABSTRACT FROM AUTHOR]

This article introduces a simple empirical model that can be readily implemented in building simulation programs to model the effects of refrigerant charge on air conditioner performance. Under- or over-charged refrigerant faults are relatively common and impact performance of equipment. It is important to understand their overall impacts in the context of whole building energy consumption and comfort in order to prioritize their importance relative to other building issues. This article develops an empirical model using some existing data. The model applies corrections to the rated values of cooling capacity, energy input ratio, and sensible heat ratio of existing air-conditioning equipment modeling approaches. Model predictions are compared with experimental results obtained from three air conditioners that have fixed orifices as expansion devices. The accuracy of the model is compared with a more complex empirical model and is shown to provide better accuracy for predicting cooling capacity and sensible heat ratio. [ABSTRACT FROM PUBLISHER]

A figure of merit, V, to characterize the value of an AFDD maintenance tool is presented. The AFDD tool is fed data representing air-conditioners operating with and without faults across a range of conditions. The calculation of V considers the probability of each scenario occurring, the tool's response, and the resulting implications. These results are summed to give a typical average value for deploying the AFDD tool, as compared to maintenance being performed without AFDD. Case studies illustrate the calculation of V. Six AFDD tools and two idealized tools are evaluated. Five of the six real tools give negative V, meaning that their use causes more harm than good. The sixth shows approximately $10 benefit per nominal ton of air-conditioner capacity per typical maintenance visit. This represents about half of the maximum potential value. These results demonstrate the importance of measuring AFDD performance and the potential monetary benefits of AFDD. [ABSTRACT FROM AUTHOR]

The paper presents an algorithm to minimize the amount of data collection for calibration of steady-state virtual sensors when obtained during normal operation of HVAC equipment in the field. If virtual sensors were calibrated using laboratory data, the amount of the calibration data could be minimized using design of experiment techniques. However, these techniques are not applicable to data from field operating equipment since most of the operating conditions are not controllable (e.g., ambient temperature, loads). In this paper, an algorithm to minimize the data collection period for calibration of steady-state virtual sensors is developed based on sensor accuracy, reliability and applicability. Application of the method is demonstrated for calibration of hourly electricity consumption virtual sensors for 3 packaged air conditioners that are serving a small commercial building, and the method terminated the calibration process after about 3 weeks of data collection for all units with very accurate estimates of electricity consumption. [ABSTRACT FROM AUTHOR]

This article presents a general approach for determining maximum monthly energy cost savings associated with optimal supervisory control for cooling in commercial buildings in the presence of utility rates that include both demand and time-of-use energy charges. The resulting tool has a month-long time horizon because of the nature of demand changes and is only useful for benchmarking the performance of simpler and shorter-term demand response and optimal control approaches. Attempts to solve this optimization problem using a centralized formulation failed and, therefore, the benchmarking problem was formulated as a dynamic optimization problem within a multi-agent control framework so that the monthly optimization problem is segmented into several sub-problems where each sub-problem involves system optimization for a shorter period of time, for example, a 1-day period. The daily-scale optimization involves determination of trajectories of zone set-point temperatures that minimize an integral cost function with a demand cost constraint determined by a demand agent. In order to further simplify the daily optimization, dynamics associated with the cooling system are neglected and optimal control of the cooling system is assumed to be based on heuristics determined through upfront analysis. This approach leads to significantly reduced computational requirements and more importantly, it provides guaranteed convergence in the multi-agent optimization. Results for a single-zone building case study are presented to illustrate the potential cost savings. In addition, a simpler and more practical short-term optimization approach with a demand-limiting heuristic is proposed and evaluated in comparison to the benchmarking optimization results for this case study and achieves most of the potential savings. [ABSTRACT FROM AUTHOR]

In regions with sub-freezing winter temperatures, the performance of conventional air-source vapor compression heat pumps drop drastically during extended periods of low outdoor ambient temperatures. Modifying the basic cycle configuration with liquid flooded compression and regeneration reduces the thermodynamic inefficiencies associated with the heat pump equipment and increases energy efficiency with less degradation in heating capacity especially at low ambient temperatures. Flooding non-volatile liquid along with refrigerant vapor stream into the compressor leads to a near isothermal compression process and thereby reduces desuperheating losses in the condenser. Regeneration improves the system performance by reducing throttling losses in the expansion device. The performance of a heat pump operating with flooded compression and regeneration has been analyzed for three sub-freezing outdoor ambient climatic regions: Boston, Indianapolis and Minneapolis. The results indicate that at −10 °C ambient temperature, COP (heating) of the flooded compression with regeneration cycle was approximately 13% higher than the standard vapor compression cycle. The Heating Seasonal Performance Factor (HSPF) of a flooded compression with regeneration cycle for Boston, Indianapolis and Minneapolis was approximately estimated to be 9%, 10% and 13% higher than the standard vapor compression cycle respectively. [ABSTRACT FROM AUTHOR]

Metal hydrides have been investigated for use in environmentally friendly heat pumps. These systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator). Some challenges faced by compressor driven metal hydride heat pumps are poor heat transfer in the metal hydride beds and high compressor discharge temperatures. To overcome these challenges, this article investigates the use of a metal-hydride slurry in conjunction with various isothermal compression techniques. Liquid-flooded, electrochemical, and liquid piston compressors were modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system. [ABSTRACT FROM PUBLISHER]

Oil flooding is a technique that can be utilized in compression systems to achieve near-isothermal compression. This can lead to a boost in system efficiency and a reduction in compressor power consumption. In this paper a semi-empirical model for oil flooded compressors using liquid injection was developed. The model is validated with experimental data and integrated into a thermodynamic model of a vapor compression system with oil flooding and regeneration. The performance of the heat pump system is predicted and the semi-empirical model is used to identify and estimate the magnitude of the irreversibilities during the compression process. A method for generalizing the model for different working fluids is also presented. Using this model, design recommendations are made to improve the efficiency of the studied liquid flooded compressors for heat pump applications. [ABSTRACT FROM AUTHOR]

Virtual sensors use data from low-cost measurements and calibrated models to provide outputs that would either be too expensive or impossible to measure directly. Virtual sensor technology has the potential to enable cost-effective implementation of advanced monitoring, diagnostic, and/or control features for buildings. While it is commonly known that the reliability of virtual sensors depends on the amount and conditions of calibration data, no methods have been presented that quantify the effect of the conditions of calibration data on virtual sensor output uncertainty. In this paper, a general method is presented for estimating the virtual sensor output uncertainty in terms of the uncertainty, conditions and amount of calibration data. The method is demonstrated with a power consumption virtual sensor for packaged air conditioning systems. [ABSTRACT FROM AUTHOR]

Refrigerant mass flow rate is an important measurement for monitoring equipment performance and enabling fault detection and diagnostics. This paper presents and evaluates three different virtual refrigerant mass flow (VRMF) sensors that use mathematical models to estimate flow rate using low cost measurements. The first model uses a compressor map that relates refrigerant flow rate to measurements of condensing and evaporating saturation temperature, and to compressor inlet temperature measurements. The second model uses an energy-balance method on the compressor that uses the compressor power consumption. The third model is developed using an empirical correlation for an electronic expansion valve (EEV) based on an orifice equation. The three VRMFs are shown to work well in estimating refrigerant mass flow rate for various systems under fault-free conditions with less than 5% RMS error. The combination of the three VRMFs can be utilized to detect and diagnose when the compressor and/or expansion device is not providing the expected flow. [ABSTRACT FROM AUTHOR]

In this article, virtual sensors for air-side diagnostics are developed and validated for rooftop air conditioners. The virtual air-side sensors require indoor fan differential pressure and speed measurements. Experimental data from laboratory measurements are used to evaluate the accuracy of the virtual sensors. Methodologies for improving the accuracy of air-side temperature sensors often used for outdoor-air economizer diagnostics have also been developed and evaluated. Outdoor-air temperature correction correlations improve single-point temperature inaccuracy caused by exhaust-air recirculation. A model correcting a single-point mixed-air temperature measurement for inaccuracy caused by thermal stratification and non-uniform velocity distributions at the evaporator inlet has also been described and validated. Finally, methods for estimating outdoor ventilation air fraction are presented and compared to measurements. [ABSTRACT FROM PUBLISHER]

• Addressed a real-world challenge for load-shifting multiple ON/OFF staged units. • Presented a unique MPC that shifts load while eliminating undesired peak. • Presented comprehensive 2-month experimental MPC demonstration results. • Achieved significant load shifting with over 30% demand savings. • Discussed experimental charging and discharging processes of building thermal mass. Small and medium-sized commercial buildings (SMCB) are significant demand response resources, and it is important to develop grid-responsive control algorithms that exploit those resources and create financial benefits for building owners and HVAC service providers. Furthermore, unlike large-sized commercial buildings, there is an opportunity to have universally applicable control solutions for many SMCBs since those buildings have a consistent HVAC system configuration: SMCBs are commonly served by multiple-staged air conditioning units controlled by their own thermostats. Despite the demand response potential and scalability, however, very few control solutions are available for SMCBs. Typical model predictive control (MPC) and heuristic control approaches for cooling load shifting that lower thermostat setpoints before an electric price jump are suitable mainly for large-sized commercial buildings where a continuous capacity modulation is possible, e.g., via dampers in variable air volume terminal units. However, those approaches can cause undesired, high peaks for SMCBs due to the nature of ON/OFF unit staging and narrow thermostat deadbands. This could discourage the use of advanced grid-responsive controls for SMCBs due to the concern of high demand charges, and has to be resolved. This paper presents a MPC solution that overcomes this challenge. It has a hierarchical MPC structure where an upper level MPC is responsible for electrical load shifting in response to an electric price signal while a lower level MPC is responsible for coordinating compressor stages to eliminate unnecessary peaks and follows the setpoints determined by the upper level MPC. Two one-month, comprehensive laboratory tests have been carried out to demonstrate load shifting and cost savings for the algorithm. Interesting trade-offs between energy efficiency and load flexibility were observed and are discussed, and lessons learned for applying MPCs for SMCBs are also presented. [ABSTRACT FROM AUTHOR]

The primary goal of the work described in this paper was to evaluate and extend a virtual refrigerant charge sensor (VRC) for determining refrigerant charge for equipment having variable-speed compressors and fans. Based on the evaluations, the original VRC sensor (termed model I) was found to work well in estimating the refrigerant charge level for systems with a variable-speed compressor. However, for extreme test conditions such as low compressor speed, the model I needed to be improved. To overcome the limitations, the model was modified to include a term involving the inlet quality to the evaporator (termed model II). The model II gave better performance for systems with a variable-speed compressor. When the superheat of the compressor was zero, neither model II could accurately predict charge level. Therefore, a third approach (Model III) was developed that includes the discharge superheat of the compressor. [ABSTRACT FROM AUTHOR]

This article presents a generalized control heuristic to reset supply air temperature set-points for direct-expansion units with capacity modulation and variable airflow and integrates the heuristic into a simplified model predictive control framework that precools building thermal mass through zone temperature set-point adjustments. The resulting control methodology is appropriate for control of medium-sized commercial buildings that employ variable air volume air distribution with direct-expansion units for primary cooling. The heuristic is shown to work well for different direct-expansion systems with different compressor types and fan–duct combinations. To assess the integrated energy savings potential of the heuristic strategy and simplified model predictive control, a simulation platform was developed for a medium-sized commercial building. Simulations were carried out for a 100-day cooling season in different U.S. locations and under different control strategies. The heuristic control provided energy savings of between 5% and 10% relative to conventional control, depending on the system type and location. Approximately 4% additional energy savings was achieved when the heuristic was integrated into a simplified model predictive control having only moderate computational requirements. The cost savings could be significantly greater if time-of-use and demand charges were considered. [ABSTRACT FROM PUBLISHER]

Residential variable-speed ductless heat pump (DHP) systems are claimed to operate more efficiently than conventional single-speed ducted systems because of fewer on-off cycles and smaller fan power consumption. However, few methods exist for quantifying the energy-saving benefits of DHP technology because of complicated and proprietary control strategies employed in the DHP system. In this paper, a model in the literature is used to assess the energy savings relative to a baseline ducted system with the same heat exchangers and rated capacities. Process efficiencies are introduced to highlight the sources for differences in performance between systems. Analyses of process efficiencies at different building loads and ambient temperatures show that DHP systems perform better than conventional systems primarily because of lower fan power consumption associated with the use of variable-speed fans. Greater benefits exist at lower heating loads because of lower air flow requirements and the cubic dependence of fan power on flow. In contrast, conventional heat pumps typically operate with on/off fans. Heating seasonal performance factors (HSPF) were determined for different climatic regions and the DHP system was found to perform 22%–107% better than a conventional system, depending on the climate. [ABSTRACT FROM AUTHOR]

Modifications such as economization aim to improve the efficiency of vapor compression equipment by cooling the refrigerant during the compression process. A previous study ( Mathison et al., 2010 ) explored the theoretical limit to cycle performance with economizing, which was defined as the performance when a saturated vapor state was maintained in the compressor by continuously injecting a two-phase mixture. However, achieving continuous injection and controlling the quality of the injected refrigerant poses a substantial challenge. Therefore, the current paper investigates the ability of an economized cycle with saturated vapor injection through a finite number of ports to approach the limiting cycle performance. For an air-conditioner using R-410A with an evaporation temperature of 5 °C and a condensing temperature of 40 °C, the model predicts that injecting saturated vapor through three ports will improve the COP by 12%, which is approximately 67% of the maximum benefit provided by economizing in the limiting case. [ABSTRACT FROM AUTHOR]

The current testing and rating procedure for residential air conditioners and heat pumps is based on a steady-state performance measurement approach with a degradation coefficient to account for cycling losses at part-load conditions. Test equipment performance is measured under various ambient conditions with varying compressor and fan speeds, and the results are propagated through a temperature-bin method to estimate seasonal performance. Although the current rating approach offers a standardized performance metric for comparing the relative performances of different equipment, it involves disabling the native controls and, as a result, does not consider the impact of integrated controls for test units and their dynamic interactions with representative building loads. As an alternative, a load-based testing methodology (CSA EXP07) has been developed in which the dynamic performance of equipment is measured in a test facility by allowing it to respond to a simulated virtual building model. This study compares the steady-state and dynamic load-based performance measurement methodologies for use on a 5-ton residential heat pump system in order to understand the differences and their significance for the next-generation rating procedure. The differences in the two test methodologies' performance evaluation results are discussed with a causal analysis of the observed differences. [ABSTRACT FROM AUTHOR]

The market for variable-speed ductless heat pump (DHP) systems has grown in North America in recent years. However, the tools available for modeling their performance within building simulation programs have not kept pace. In general, simple empirical models are typically used for characterizing equipment performance for use in system simulation. However, unlike more conventional single-speed systems, DHPs have complicated control algorithms for managing electronic expansion valve opening, compressor speed, fan speed, and defrost operation. Very little work has been done in the development of empirical models that explain the impact of these characteristics on the performance of DHP systems. In this article, an empirical approach is introduced to model the heating and defrost performance of DHP systems under a wide range of conditions. The model incorporates separate relationships for performance associated with maximum, minimum, and intermediate (part-load) heating capacities. The defrost operation of the systems is also modeled empirically. The approach was tested on data from two DHP systems and the agreement is very good. The performance of the systems under different building loads was simulated and it is shown that the performance is highly dependent on the system control characteristics. [ABSTRACT FROM AUTHOR]

Abstract: Multi-split ductless heat pumps are touted as an energy efficient option for air conditioning and heating in residential and small commercial applications. However, their use of multiple indoor units with variable speed compressor and fans and complicated controls leads to difficulties in understanding their characteristics by experiments only. A method to model the system using gray-box component models and an empirical control algorithm is described in this paper. The model was constructed from experimental results of an 8 kW R410A dual-unit ductless heat pump system having an accumulator. Strategies for modeling systems with accumulators and saturated control inputs are presented. Parametric studies with varying indoor air temperature, individual indoor unit load ratio, indoor fan speed and total indoor load were conducted. The results show that the model predicts system performance accurately and can help to understand the effect of saturated control inputs and changes of accumulator status on system performance. [Copyright &y& Elsevier]