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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]

Despite the increasing interest in organic Rankine cycle (ORC) systems and the large number of cycle models proposed in the literature, charge-based ORC models are still almost absent. In this paper, a detailed overall ORC simulation model is presented based on two solution strategies: condenser subcooling and total working fluid charge of the system. The latter allows the subcooling level to be predicted rather than specified as an input. The overall cycle model is composed of independent models for pump, expander, line sets, liquid receiver and heat exchangers. Empirical and semi-empirical models are adopted for the pump and expander, respectively. A generalized steady-state moving boundary method is used to model the heat exchangers. The line sets and liquid receiver are used to better estimate the total charge of the system and pressure drops. Finally, the individual components are connected to form a cycle model in an object-oriented fashion. The solution algorithm includes a preconditioner to guess reasonable values for the evaporating and condensing temperatures and a main cycle solver loop which drives to zero a set of residuals to ensure the convergence of the solution. The model has been developed in the Python programming language. A thorough validation is then carried out against experimental data obtained from two test setups having different nominal size, working fluids and individual components: (i) a regenerative ORC with a 5 kW scroll expander and an oil flooding loop; (ii) a regenerative ORC with a 11 kW single-screw expander. The computer code is made available through open-source dissemination. [ABSTRACT FROM AUTHOR]

Passive ceiling cooling systems can lead to reduced cooling requirements, less fan energy and downsized ductwork, compared to conventional all-air systems. Additionally, radiant cooling of occupants allows for improved comfort while allowing for higher operating temperature, improving chiller efficiency. This paper presents a comprehensive review of current modelling approaches for passive ceiling cooling systems in order to document the state of the art and identify current research gaps and modelling development needs. Modelling methods are separated in three main categories, based on the domain of interest:componentor “passive ceiling cooler” models, “indoor environment” models and “integrated” models. Simplified, detailed and empirical models are presented for each category. Different modelling approaches may be appropriate for different purposes (design vs. control analysis, and system simulation vs. whole building performance). The study summarizes useful findings, modelling limitations and applications, and presents needs for further modelling and simulation research, including passive chilled beams. [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: The primary goal was to evaluate and enhance a virtual refrigerant charge (VRC) sensor developed by Li and Braun (2006, 2009). Based on the evaluation, the VRC sensor was found to work well in estimating refrigerant charge for systems that do not utilize accumulators when using the default parameters. For systems with accumulators, however, the parameters needed to be improved for better accuracy. To overcome those limitations, a simulation method for estimating parameters was developed that depends on test data at rated conditions and system information. The simulation parameters decreased the errors between the actual and predicted charge. Even better performance was achieved for the VRC sensor when the parameters were tuned by minimizing charge prediction errors using test data and linear regression. The overall RMS error for the VRC sensors using tuned parameters was 3.8% over a wide range of operating conditions for different types of systems. [Copyright &y& Elsevier]

Abstract: Models for unitary air conditioners and heat pumps typically have difficulties in providing accurate predictions for the following situations: 1) charge modeling over a large range of charge levels, 2) two-phase refrigerant entering fixed-orifice expansion devices, 3) two-phase refrigerant entering compressors, and 4) non-uniform refrigerant mass flow allocation among evaporator circuits at low indoor air flow rates. This paper gives descriptions and assessments of improved modeling methodologies for all four issues. The study utilizes data obtained from extensive testing at a wide range of operating conditions for two R-410A units and one R-407C unit. [Copyright &y& Elsevier]

Abstract: A companion paper [Kim, J.-H., Braun, J.E., Groll, E.A., 2009. A hybrid method for refrigerant flow balancing in multi-circuit evaporators: upstream versus downstream control. International Journal of Refrigeration doi:10.1016/j.ijrefrig.2009.01.013 presented a hybrid approach for providing control of refrigerant flow distribution in evaporators that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. Furthermore, the companion paper demonstrated that the flow balancing valves should be located upstream rather than downstream of the evaporator in order to realize significant benefits. The current paper utilizes the model presented in the companion paper to more fully evaluate the effects of uneven air and refrigerant flow distributions and the benefits of upstream hybrid control in response to these effects. [Copyright &y& Elsevier]

Abstract: A hybrid method for optimizing refrigerant distribution in evaporators is presented that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. The flow balancing valves could be located upstream or downstream of the evaporator. This paper presents the results of a study to investigate the benefits of this hybrid scheme for both upstream and downstream flow balancing for the case of air flow mal-distribution. In order to perform this investigation, a simulation model was developed to consider evaporator flow mal-distributions for a 10.55kW residential R410A heat pump and then validated through comparisons of predicted results with measurements. Simulation results show that there are significant benefits in controlling the superheat of each circuit of evaporators through the hybrid–individual superheat control method. Furthermore, the upstream refrigerant flow control consistently outperforms the downstream refrigerant flow control, and recovers most of the loss in cooling capacity and COP due to non-uniform air flow distribution. [Copyright &y& Elsevier]

Most of the previous diagnostics research for vapor-compression equipment has been performed for packaged air conditioners and chillers. This paper describes application of a decoupling-based diagnostic technique that was originally developed for air conditioners to equipment used in walk-in coolers and freezers. Testing was performed on a small, restaurant- style walk-in cooler and a small walk-in freezer for faulted and unfaulted conditions. The data was then used to characterize how the fault features respond to faults and to evaluate the performance of the decoupling-based diagnostic technique. [ABSTRACT FROM AUTHOR]

Abstract: Evaluation of a building for application of demand-controlled ventilation (DCV) typically involves the use of computer simulations to predict energy use/costs for both fixed ventilation and ventilation adjusted to maintain fixed CO2 levels within the space. The simulation tools incorporate models for predicting CO2 concentrations in response to internal sources (people), infiltration/exfiltration, and ventilation. This paper presents a detailed evaluation of different modeling approaches for predicting levels of CO2 in occupied spaces for small, single-zone commercial buildings employing packaged air-conditioning equipment. Two-zone and three-zone transient models were compared with a quasi-static equilibrium model applied to three distinctly different building types. Baseline data were derived from computational fluid dynamic models that were developed for field sites. A complete building system simulation model was then used to compare the impact of the different modeling approaches on the predicted energy cost savings associated with application of DCV in each building type. The use of a transient CO2 model did not have a significant impact on model prediction accuracy and energy cost savings predictions as compared with the quasi-static model. The difference in predicted annual energy costs between the various CO2 modeling types were small and less than might result from errors introduced by factors such as CO2 sensor uncertainty. Therefore, the use of an equilibrium model is sufficient for use in evaluating DCV for small commercial buildings. [Copyright &y& Elsevier]

A detailed model for hermetic scroll compressors was presented in a companion paper. The current paper presents validation results for this model based upon detailed experimental measurements and application of the model to study the impact of design changes. The experimental study included a range of operating conditions that were controlled using a hot gas bypass compressor load stand. The model was validated in terms of overall performance predictions (refrigerant mass flow rate, discharge temperature, and compressor power consumption) and predictions of internal measurements (refrigerant pressures within the scrolls as a function of orbiting angle, temperatures along the scroll wraps, and temperatures at various points within the shell). The model performed very well in terms of predicting overall performance and was adequate in predicting the internal measurements. The parametric modeling study identifies some areas for potential improvement. Most significantly, a relative improvement of about 5% in overall compressor efficiency can be achieved through small changes in the scroll geometry. [ABSTRACT FROM AUTHOR]

In comparison with other types of conventional positive displacement compressors for domestic refrigerators, oil-free linear compressors have a number of advantages due to their compactness, low friction, limited number of moving parts, and excellent performance. Limited work was found in the open literature related to comprehensive compressor modeling and model validation of commercially available linear compressors. A comprehensive and generalized simulation model of linear compressors has been recently presented by the same authors. The current paper expands on this work by using experimental data for the simulated compressors to characterize their performance over the operating envelopes and to validate the linear compressor model. The validated model was then used to quantify the major sources of losses and analyze system vibration as well as piston eccentricity, which ultimately could be used to design the next generation of linear compressors. [ABSTRACT FROM AUTHOR]

Abstract The energy savings potential of passive chilled beams in various climatic zones is analyzed in this study. A passive chilled beam model, developed based on full-scale experiments, is used as a system module in a whole building simulation tool to account for the convective and radiative effects of passive chilled beams. The model was validated with measurements from a field study in an open plan office equipped with multiple passive chilled beams. In addition, a parallel field study in an adjacent identical office space equipped with an air (VAV) system was conducted to compare the resulting energy consumption with the two systems. To further study the energy savings potential and indoor thermal conditions with passive chilled beams, four different chilled beam system configurations, combined with a parallel variable air volume system, were studied using the whole building model in different US climatic zones. The results showed that using a dedicated outdoor air system, or a separate chiller for the passive chilled beams, or a desiccant wheel after the cooling coil may result in significant relative energy savings depending on the location. The last option proved to be the most efficient, providing up to 12% for hot and humid climates and up to 20% for hot and dry climates –compared to a conventional variable air volume system. Finally, the radiative-to-total cooling effect of passive chilled beams varied between 7% and 8% and has only a small impact on energy savings and indoor thermal conditions. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]