Comparison of Steady-State and Dynamic Load-Based Performance Evaluation Methodologies for a Residential Air Conditioner

Space cooling and heating equipment account for nearly 32% of the total residential electricity consumption in the U.S. In the residential space conditioning equipment market, air-conditioning and heat-pumping systems are prevalent, so even a slight improvement in these system efficiencies can have a significant impact on reducing the overall energy consumption. Over the years, the energy efficiency benchmarks established by the U.S. Department of Energy have been successful in encouraging manufacturers to develop higher efficiency equipment. These benchmarks are based on an energy efficiency standard, and these standards are based on a rating test procedure that forms the technical basis. Currently, in the U.S., AHRI 210/240 is the rating procedure for residential air-conditioning and heat-pumping equipment, which is based on a steady-state performance measurement method with a degradation coefficient to account for the cycling losses in part-load conditions. Although it provides a standard metric to compare different equipment performances, there has been a debate that this current methodology fails to appropriately characterize the performance of systems with variable-speed compressors and advanced control design. This is largely attributed to the steady-state nature of this current testing approach, which also involves overriding the equipment native control. In contrast to this, a load-based testing methodology has been developed in which the equipment responds to a simulated virtual building load, and the system dynamic performance is measured with its integrated controls. The load-based testing methodology is described in detail by Hjortland and Braun (2019), Patil et al. (2018), and Cheng et al. (2021), which forms the basis for CSA standard draft EXP07:2019 (CSA, 2019). In this paper, these two performance measurement methodologies, steady-state and dynamic load-based, are compared for application to a 5ton residential heat-pump system. The equipment performance was measured in cooling mode and the seasonal performance estimates based on the two testing approaches are compared. The differences in the two test methodologies\u27 performance evaluation results are discussed with a causal analysis of the observed differences

Demonstration of a Load-Based Testing Methodology for Rooftop Units with Integrated Economizers

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

Heat-Pump Control Design Performance Evaluation using Load-Based Testing

Space heating is one of the primary components of residential energy usage in the U.S., accounting for nearly 43% (EIA, 2015) of the total residential energy consumption. To reduce this energy usage, heat-pumps provide an energy-efficient alternative to currently prevalent systems such as electric heaters and gas furnaces. Advanced control strategies have the potential to further improve heat-pump system energy efficiency and comfort delivery. In recent years, advancements in the microprocessor field have made it possible to widely implement advanced energy-efficient controls within heat-pump systems. However, still only a very small fraction of residential air-conditioners and heat-pumps currently sold in the U.S. market utilize these next-generation controls (ACEEE, 2019). To facilitate an acceleration in the development and implementation of advanced control architectures within heat-pump equipment, a load-based testing methodology can be utilized. Load-based testing allows realistic dynamic behavior and performance evaluation of energy efficiency and comfort delivery for heat pumping and air conditioning equipment with embedded controls in a laboratory setting. In the load-based testing methodology, the sensible and latent loads of a representative residential building are emulated in the indoor psychrometric test room by dynamically varying the test room conditions utilizing a virtual building model. The test equipment responds dynamically to this virtual building with its embedded controls based on the thermostat sensing response. This enables engineers to evaluate the performance of a heat-pump in a controlled setting under dynamic conditions that are similar to a field application but with a significant reduction in testing time and cost. This paper demonstrates the application of load-based testing for evaluating the performance of a 5-ton split-type residential heat-pump with its integrated controls in a heating mode application. Furthermore, the effect of equipment oversizing and undersizing on the heat-pump energy consumption and comfort delivery are also presented

Validation of a Load-Based Testing Method for Characterizing Residential Air-Conditioner Performance

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

Performance Evaluation of Heat Pump Systems Based on a Load-based Testing Methodology

This paper presents results of testing variable-speed heat pumps using a new load-based testing methodology that is described in a companion paper. The testing methodology involves emulating the response of a building’s sensible and latent loads to equipment controls by dynamically adjusting the temperature and humidity setpoints of the psychrometric chamber reconditioning system using a simple building model. The advantage of this approach over existing testing approaches specified in ratings standards is that it considers the interaction of the integrated controls with the equipment. As a result, it better captures the full range of part-load operation and the benefits of improved controls. This paper presents performance results for application of the automated load-based testing methodology to different variable-speed residential heat pump systems. In order to assess the benefits of load-based testing versus existing standards, tests were also conducted based on AHRI 210/240 and seasonal performance estimates are compared using data obtained with the two testing approaches

Impact of Virtual Building Model and Thermostat Installation on Performance and Dynamics of Variable-Speed Equipment during Load-based Tests

To better characterize the performance of variable-speed DX (Direct Expansion) equipment in a laboratory environment, a load-based psychrometric chamber testing methodology has been developed as an alternative to existing steady-state testing approaches. The methodology allows equipment to respond dynamically to a virtual building model using its integrated controls. To mimic an actual building, a virtual building model incorporates sensible and latent loads along with simple lumped capacitance building dynamics that interact with the variable-speed equipment. The rated capacity of the test equipment is used along with a specified sizing factor and target sensible heat ratio (SHR) to specify the building sensible and latent load models. In addition, heuristic approaches are used to specify and scale sensible and latent capacitances of the virtual building model. Two companion papers present the overall methodology and results for different variable-speed heat pumps using default building parameters. This paper studies the impact of the virtual building load parameters on overall performance and dynamic behavior of the equipment for load-based testing. It is shown that equipment seasonal performance can increase significantly with increasing sizing factor. In addition, performance increases with decreasing building SHR results. In addition to simple lumped capacitance models, more detailed two-node models are investigated to evaluate more realistic dynamics and their impacts on seasonal efficiency ratings. In addition, the impact of the thermostat location on equipment dynamics and performance ratings is considered