ebin Yu

Virtual calibration of a supply air temperature sensor in rooftop air conditioning units

Supply air temperature (SAT) measurement is an important element in sequencing control and automated fault detection and diagnosis (AFDD) in HVAC systems to ensure the comfort of building occupants, decrease energy consumption, and lower maintenance cost. But in rooftop air conditioning units (RTUs) with gas-fired heating, the accuracy and reliability of manufacturer-installed supply air temperature (MSAT) sensors are notoriously difficult to attain. Experimental evaluations in this study, covering both the cooling and heating modes and using both direct measurements of a MSAT sensor and a multi-sensor measuring grid, demonstrate that direct measurements cannot obtain the true value of SAT in RTUs in the heating mode. Erratic measurement errors exist due to nonuniform temperature distribution and intensive thermal radiation in a compact chamber. An innovative indirect virtual calibration method for an MSAT sensor is proposed in this article to solve this issue. It demonstrates that a virtual calibrated MSAT sensor can provide accurate results when combined with a linear correlation for offset error that depends on heating stage and outside air damper signals. The linear correlation could be determined using the calculated temperature difference between the predicted theoretical true value of SAT and the direct MSAT measurement. This virtual calibration method is generic for all RTUs with similar construction of gas furnaces and can be implemented for long-term use. Further experimental evaluation and uncertainty analysis prove that the virtual calibration method can accurately predict the true value of SAT in RTUs within ±1.2°F (0.7°C) uncertainty. This economical technology will not only improve energy management of packaged units in sequencing control but also better facilitate real-time automated control and fault detection and diagnosis.

Optimal building energy management using intelligent optimization

The building thermal capacity can be used for shifting on-peak load and reducing peak cooling/heating in commercial and residential buildings. The optimization of the pre-cooling/pre-heating is a complicated problem of several major factors, including utility rates, load profiles, building storage characteristics and weather conditions. This paper introduces an intelligent search algorithm, Particle Swarm Optimization (PSO), to find the near optimal solution. A simulation model of a single floor with multi-zone based on a real lab building in Carnegie Mellon University is built with Energyplus to simulate the proposed algorithm. By using MLE+, the EnergyPlus model of the building becomes an S-function block in the Matlab/Simulink, and all available Matlab toolboxes can be used for control and optimization purposes. Simulation results demonstrate the feasibility and the effectiveness of the proposed method.

A Gray-Box Based Virtual SCFM Meter in Rooftop Air-Conditioning Units

Knowledge of supply airflow rate (SCFM) measurement in packaged rooftop air-conditioning units (RTUs) is vital for improving energy management and indoor air quality and facilitating real-time automated control and fault detection and diagnosis. Despite the importance of SCFM measurement in RTUs, the conventional SCFM metering devices are very vulnerable. The credibility of SCFM measurement would be compromised dramatically after a long-term use in adverse duct work surroundings. Moreover, application of conventional SCFM meters in RTUs is very costly in regard to procurement, installation, and periodic maintenance. A cost-effective and accurate nonconventional first principles based SCFM meter in RTUs was proposed previously to virtually monitor SCFM measurement. In order to overcome the deficiencies of the first principles based virtual SCFM meter in model implementation and fault diagnostics, experiments with a wider combination and coverage are investigated in this study. It is found that a gray-box based virtual SCFM meter can be obtained with available system information (outside air damper status) and low-cost temperature measurements (direct measurement of a manufacturer-installed supply air temperature sensor (SAT mfr,meas ) and outside air temperature). Further experiment evaluations demonstrate that the gray-box based virtual SCFM meter could predict the true value of SCFM very accurately (the uncertainty is ±5.9%) with significantly enhanced applicability in model implementation and capability in fault diagnostics. Additionally, the gray-box based virtual SCFM meter also inherits good characteristics of the first principles based virtual SCFM meter, such as high cost-effectiveness, good robustness against variations in multivariable operating conditions, and applicability to similar RTUs. This innovative virtual meter could serve as a permanent monitoring tool to indicate real-time SCFM measurement and/or to automatically detect and diagnose an improper quantity of SCFM for RTUs.

Supermarket system characteristics and operating faults (RP-1615)

Supermarkets undergo various types of operating faults that can require costly preventive maintenance, service, and repairs. Because supermarkets are growing in number and in the variety of system types and characteristics, information about common faults and equipment is essential prior to investing effort on the development of new fault detection and diagnostics (FDD) methods or other improvements to system design and operation. A study was conducted to investigate supermarket equipment characteristics, such as the prevalence of central vs. distributed systems, condenser types, control strategies, and common operating faults such as refrigerant leakage, failed evaporator or condenser fans, failed compressors, etc. The data come from four sources: expert surveys, facility management system messages, service calls, and service records. Some of the results of the study are that centralized direct expansion systems with on/off compressor capacity control strategy and air-cooled condensers are the most common system types and refrigerant charge problems are the most common source of equipment failure. Case-related problems, in aggregate, are the most frequently occurring faults in supermarkets.

Automated fault detection and diagnosis methods for supermarket equipment (RP-1615)

Many automated fault detection and diagnostics methods have been developed for application to building mechanical systems over the past 20 years because they have the potential to reduce operating costs and energy consumption by providing early warning of performance degradation faults. Supermarkets could be a very beneficial setting to deploy automated fault detection and diagnostics, particularly in the refrigeration systems, which are major energy users and are known to commonly suffer from significant refrigerant leakage problems. The current article provides an overview of the common mechanical systems deployed in supermarkets, and then describes a comprehensive review of the literature on automated fault detection and diagnostics methods from other systems that could potentially be applied in supermarket settings. A collection of supermarket field data is analyzed in the context of its potential use in automated fault detection and diagnostics methods from other systems. The review includes methods to categorize and assess the automated fault detection and diagnostics approaches, from the perspective of a potential adopter of automated fault detection and diagnostics technology for a supermarket setting. The article concludes that supermarket automated fault detection and diagnostics is still in the early stages of development and that there is a need to further develop automated fault detection and diagnostics methods for supermarket applications. To facilitate the development of supermarket-specific automated fault detection and diagnostics approaches, additional data sets from refrigeration equipment are needed.

Heat and mass transfer enhancement potential on falling film absorbers for water-LiBr mixtures via a literature review (RP-1462)

Significant process intensification (PI) of heat and mass transfer is indispensable in building compact and energy efficient absorption refrigeration systems. High potentials exist to achieve the required PI through (1) development of active heat and mass transfer enhancement techniques and (2) combining the active enhancement mechanism with proven and widely used passive enhancement techniques in transport processes. There is limited research on the effect of active mechanisms, such as vibration, on heat and mass transfer coefficients in absorption systems with falling film horizontal-tube absorbers. In this general survey, with the aim to enlighten the path for active mechanisms development, recorded heat and mass transfer enhancements via active mechanisms were extracted from pertinent research works, and were summarized in tables suitable for evaluation and comparison. The potential for future research on enhancing heat and mass transfer in absorption chillers was identified.

Experimental study on the active enhancement mechanisms of heat and mass transfer in an absorption chiller (RP-1462)

Based on the introduction of periodic vibration equipment in a sorption fluids system, the effect of an active mechanism on the heat and mass transfer enhancement in an absorber and the coefficient of performance (COP) enhancement in an absorption chiller was investigated. Different combinations of frequency (15–30 Hz), amplitude (0.1–0.4 mm), and flow rate (0.1–0.5 m3/h), with and without additive (2EH), were compared quantitatively. The results indicated the heat and mass transfer coefficients of the absorber and the COP of the absorption chiller significantly improved—by about 50% under certain conditions. There was a critical value for the amplitude (0.2 mm) and an optimal frequency (25 Hz) in the current study. The additive and solution flow rate also affected performance, but no exclusive conclusion was obtained from our experiment. A properly controlled and applied active mechanism would be of great value not only for the absorption chiller, but also for other chemical engineering applications. The results also begin to fill the knowledge gap between the analysis of mechanical motion and absorption chiller technology. The next phase of work is to find the active mechanism of heat and mass transfer enhancement from the microflow perspective.

Characteristics of flow and heat transfer of nanofluids under laminar states

ABSTRACT The current article used four different models (single-phase, Mixture, Eulerian, and discrete phase model) to investigate the flow and heat transfer characteristics of nanofluids under a laminar state. We explored the Al2O3-water nanofluid inside a microscale trapezoidal channel and the CuO-oil inside a circular channel with a regular size. The velocity and temperature fields of nanofluids were discussed by comparing the differences among each model. It is revealed that the change of flow characteristics of nanofluid plays a more decisive role in its heat transfer enhancement besides the improvement of its physical properties.

Oversizing Analysis of HVAC System in Prototypical Commercial Buildings

Oversizing of heating ventilation and air-conditioning equipment becomes an intractable fault once the building system is designed and constructed. It imposes control challenges, impacts the indoor air condition, increases energy consumption, and reduces the equipment life span. As a system-level fault, it is difficult to duplicate and quantify in an experimental environment. This study analyzes the oversizing issues in prototypical commercial buildings. Data of system operations from 12 retail stores in different climatic regions in the US are collected. We applied three parameters, including cycling number, run-time fraction, and maximum cycling number, to capture the oversizing signature of a RTU based on the annual design conditions. Two different effective dead-band temperatures are adopted to evaluate the potential uncertainty. The findings can be used to assess the oversizing level of RTUs, quantify the average energy penalty of sample buildings, and guide future design. More importantly, the methodology can be automated and applied in smart building management systems for soft-repairing of an oversizing issue.