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The extension of time-of-use (TOU) rates to residential buildings has incentivized the use of thermal energy storage (TES) to shift heating and cooling loads away from on-peak hours. At the same time, the growing use of solar photovoltaics (PV) for on-site energy generation with variable buy back rates also encourages the use of TES to store energy generated at hours of peak solar radiation. Metal hydrides are reversible chemical compounds that form when certain metals react with hydrogen. Since these reactions depend on both temperature and pressure, metal hydrides can be used for thermochemical energy storage across a range of temperatures. Because of this, metal hydrides can be used as storage for both heating and cooling, enabling year-round utilization. In this paper, we present and evaluate a system that combines metal hydride energy storage with heat pumps and on-site PV. The system is designed to reduce operating costs by discharging the energy storage system during on-peak hours when electricity is expensive and charging the system during off-peak periods when electricity is less expensive and there is less incentive to sell excess PV electricity generation back to the grid. The TOU rates considered are designed for a future scenario with high solar penetration, where greater solar generation in the afternoon leads to on-peak periods in the mornings and evenings rather than the afternoon. The payback period of the energy storage system is estimated using energy savings relative to a baseline of a solar home without an energy storage system along with estimates of additional installed costs for the storage system. Payback period calculations for a range of rates show that rates with on-peak demand charges result in larger cost savings than rates with high on-peak rates, but even for the most favorable rates, the payback period is too long for the system to be economically viable.

Most air conditioning and refrigeration systems that employ the vapor compression cycle rely on oil circulating with refrigerant to lubricate the bearings and other contact surfaces in the compressor. The lubricant acts as a sealant to reduce leakage losses during the compression process and it also helps to absorb some of the excess heat that is generated in the compression chamber. However, this oil circulation results in oil retention in various other components outside the compressor depending on the physical interaction between lubricant and refrigerant and their transport properties. Other factors, such as geometry and orientation of connecting lines and system operating conditions (e.g., refrigerant flow rate and oil circulation ratio), also impact the oil retention. As a result of oil retention, the oil level in the compressor reduces, which may ultimately affect its efficiency and life span. In addition, the pressure drop across the system increases and the efficiency of heat exchangers (evaporators and condensers) decreases with oil retention. The current line sizing rules reported in the ASHRAE Handbook Refrigeration have only limited consideration of the effects of oil in the system. With the increasing development of variable speed systems as well as future use of newer refrigerants, there is a need in the industry for upgrading the line sizing recommendations to consider the effects of oil retention, especially the connecting gas lines of unitary split systems. To address this issue, a physics based model has been developed to predict oil retention in horizontal lines. The model is validated using experimental data collected for R410A-POE32. The developed model will be a backbone of a design tool, which will provide more information on oil retention in refrigerant gas lines of the commonly used refrigerant-lubricant combinations in the HVAC&R industry.

There is a significant opportunity to improve building energy efficiency and indoor environmental quality by accurately monitoring CO2 levels. However, current CO2 sensors tend to be expensive or require regular recalibration. This work presents research related to the initial development and evaluation of two novel CO2 sensors based on chemiresistive and resonant mass sensing techniques. Prototype sensors were assessed in a bench-top test chamber at temperatures, humidity levels, and CO2 concentrations, typical of indoor environments. Under these conditions, prototype sensors required only 60 mW of power, or less. Further, each sensor was developed to have a footprint of less than 25 mm2 and a cost of less than $50. Given the relative low cost, small size, and potential for low power consumption, these sensors may serve as an attractive alternative to the commercial CO2 sensors that are currently available.

A future is envisioned where buildings are assembled on-site from factory manufactured modular elements that integrate the smart technology needed to enable scalable, cost-effective solutions with autonomous, occupant-responsive, healthy, and sustainable features. The use of modular elements would mean that buildings are assembled rather than constructed on-site with better quality control, less material waste, and more predictable schedules. The use of manufactured building elements can enable more cost-effective integration of new sensors, embedded intelligence, networking, adaptive interfaces, renewable energy, energy recovery, comfort delivery, and resiliency technologies, making high-performance buildings more affordable. To explore and evaluate these modular and intelligent comfort delivery concepts and advanced approaches for interaction with occupants, a new human-building interaction laboratory (HBIL) has been designed and is under development. The facility has a modular construction layout with thermally active panels. The interior surface temperature of each panel can be individually controlled using a hydronic system. Such configuration allows us to emulate different climate zones and building type conditions and perform studies such as the effect of different active building surfaces on thermal comfort, localized comfort delivery, and occupant comfort control, among others. Moreover, each panel is reconfigurable to allow investigating different interior surface treatments for different visual and acoustic comfort conditions. In this paper, the overall design approach of the facility is presented. Furthermore, a prototype panel has been constructed to validate the design and assess the dynamic and steady-state thermal performance. Test results for the prototype panel are also presented here with a discussion on their agreement with design phase modeling results.

Seasonal performance assessments of air-conditioning and heat-pump systems are typically carried out based on performance measurement of equipment in a test laboratory. The performance ratings that arise from these assessments are important in providing information to consumers, and in influencing policymakers to determine appropriate incentives for high-efficiency equipment in the marketplace. The current testing and rating approach for performance evaluation of residential air-conditioning and heat-pump systems is based on steady-state performance measurements, with a degradation coefficient to account for the cycling losses that occur during part-load operating conditions. However, this current methodology fails to appropriately characterize the true performance characteristics of these systems in the field, and as a consequence, SEER (seasonal energy efficiency ratio) improvements have not resulted in proportional savings in energy. As an alternative, a load-based testing methodology has been developed with the motivation of capturing realistic equipment performance in a laboratory setting while operating similar to field application conditions. In this approach, the equipment responds to a simulated virtual building load, and the system dynamic performance is measured with its integrated controls and thermostat. However, there is a lack of field-testing data to characterize how well the load-based testing approach captures equipment performance and dynamic behavior compared to a typical field application. To fill this gap, a 3-ton heat pump system was tested within the Residential Home Ecosystem at the Helix Innovation Center where a 2-story house is located within an environmental chamber that can vary external ambient temperature and humidity conditions. During tests, the house was subjected to cooling loads resulting from different outdoor temperature conditions, and its air conditioning system responded accordingly. Similar cooling equipment was also tested within psychrometric chambers at the Ray W. Herrick Laboratories using the load-based testing methodology. A comparison of the test equipment performance and its dynamic behavior in cooling mode between testing performed at the Helix Center and at the Herrick Laboratories is presented in this paper.

Implementation of an ejector for expansion work recovery in transcritical carbon dioxide (CO2) cycles provides an opportunity to improve the efficiency of these environmentally-friendly refrigeration systems. However, literature outlining an approach to ejector design for a given application is lacking. This paper presents a tool to design a complete ejector applied in a vapor compression cycle. In this work, the developed design tool was validated using experimentally-derived polynomials for air-conditioning conditions. Then, constant values for nozzle and mixing section efficiencies were used as inputs into design tool to broaden the analysis outside of the application boundaries of the experimentally-derived polynomials to study a transcritical CO2 system with an ejector operating in the evaporating temperature and gas cooler pressure in the range of -15 °C to 20 °C and 80 bar to 110 bar, respectively. The design tool allows for the calculation of the motive and suction nozzle throat diameters, the mixing section diameter, and the diffuser outlet diameter, as well as the lengths of each section, to output a full internal geometry of the ejector based on performance requirements. Individual component sub-models are presented within the proposed model structure. The model which forms the basis of the design tool was experimentally validated with a mean absolute error (MAE) between 3% to 4%. Additionally, the sensitivity of the ejector geometry and performance to component efficiencies, operating conditions, and component versus system optimization was investigated. The optimization and parametric studies provided novel insights into the impact of desired efficiency and operating conditions on ejector geometry, thus allowing a designer to make decisions based on the tradeoff between ejector size and performance. For example, as the diffuser length increased by 5.1 mm to obtain an efficiency increase, to obtain a further efficiency increase of the same amount would require a 17.1 mm length increase in diffuser length. Potential model improvements and other future work are also discussed.

Compressor testing is an essential task to characterize compressor performance, but often requires significant time to be executed. This study suggests a method that could greatly reduce time needed for compressor testing by inferring steady state performance from transient data rather than waiting for true steady state conditions to be measured. The key finding is that the overall isentropic efficiency in transient operation is almost identical to its true steady state performance value after applying very simple data processing. The paper describes a simple data processing method that extracts steady-state performance from transient data. The proposed processing should not be understood as a general rule to all compressors, but as a positive result for this particular compressor and a first glimpse into the value of transient data for performance estimation.

HVAC, refrigeration, and water heating accounted for approximately 22 quads of primary energy consumed in the United States in 2018, according to US EIA. Most of HVAC&R industry still relies on vapor compression and heat-driven technologies. The development of highly efficient technologies that would significantly improve both COP and annual energy savings is an open challenge for the new decade. Among the novel technologies, the Chemical Looping Heat Pump (CLHP) combined with a reverse fuel cell has been modeled with estimates of a COP increase of over 20% relative to a conventional vapor compression (VC) cycle. However, limitations of simplified modeling efforts necessitate the development of a comprehensive mechanistic model to predict several physical phenomena for varying operating conditions and more accurately estimate performance. In this work, a charge-sensitive mechanistic modeling approach is utilized to predict the performance of the CLHP system. A thermodynamic model is coupled with a discretized fuel cell model to estimate the energy savings potential. A moving boundary model is adopted to assess the steady-state heat transfer rate in the heat exchanger. Sensitivity analyses are used to identify the system behaviors and performance.

This paper presents a reduced order modeling approach for dynamic vapor compression cycles (VCC) based on a trajectory piecewiselinear (TPWL) approximation and Proper Orthogonal Decomposition (POD). Unlike other model order reduction techniques that apply to nonlinear dynamic heat exchanger (HX) models, the TPWL approach first represents a finite volume HX model with a weighted combination of linearized models along some state trajectories. Linearization points are selected by the kmeans clustering algorithm, which enables control of the number of linearized pieces. Then, each of the full order linear pieces is reduced by projection onto a POD basis to form a reduced order TPWL model. Reduced order HX models were generated and integrated with other quasistatic component models to complete a reduced order VCC model. Simulation results were compared with experimental data for cycle loadchange transients over a wide range of operating conditions. The proposed method is an alternative to a nonlinear model order reduction framework that is presented in a companion paper (Ma et al., 2020) and may be particularly useful for cases where it is challenging to stabilize nonlinear reduced order models. Comparisons were drawn between these two approaches in terms of generalization, stabilization and computational efficiency.

Current performance evaluation approaches for commercial packaged air conditioning and heat pump equipment (e.g. AHRI 340/360) utilize full-load steady-state performance tests to estimate system EER (energy efficiency ratio) at different ambient conditions and part-load steady-state tests to estimate an IEER (integrated energy efficiency ratio), a figure of merit for system part-load performance. There are some limitations of the current testing approaches and performance metric estimations, including that they do not consider the effects of: 1) test unit embedded controls and their realistic interactions with the building load; 2) different climate zones and building types; and 3) economizer operation. As a result, the overall performance measurement procedure does not appropriately incentivize the development of better performing controls and economizers. In this paper, an improved testing procedure applied to packaged air conditioning equipment, such as rooftop units (RTUs), that include the effects of embedded controls, economizers, climate, and building type is presented. The testing approach is based on allowing the integrated equipment system and controls to respond naturally to a “virtual building load”. This is termed load-based testing and involves dynamically adjusting the indoor room temperature and humidity setpoints for the psychrometric chamber reconditioning system in a manner that emulates the response of a building’s sensible and latent loads to the test equipment controls. The developed test methodology is demonstrated to evaluate the dynamic performance of a 5-ton variable-speed RTU with an integrated economizer in a psychrometric test facility.