Daihong Yu

A review of virtual sensing technology and application in building systems

A virtual sensor uses low-cost measurements and mathematical models to estimate a difficult to measure or expensive quantity. Virtual sensors have been sucessfully developed and applied in other fields within the past two decades. This article reviews developments of virtual sensors in other fields and early applications for buildings. It is believed that widespread application of virtual sensors for buildings would enable a level of building optimization and improvement not previously considered to be economical. It is hoped that this article can provide a resource for these future developments and applications.

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.

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.

An improved virtual calibration of a supply air temperature sensor in rooftop air conditioning units

Accurate supply air temperature (SAT) measurements are vital for improving the energy management of packaged rooftop air-conditioning units (RTUs) through better sequencing control and real-time automated fault detection and diagnosis (FDD). However, the accuracy and reliability of single measured manufacturer-installed supply air temperature (MSAT) are greatly compromised in the heating mode due to severe temperature stratification and high thermal radiation. There exists unacceptable erratic measurement errors and the traditional calibration method can hardly overcome the defect. An easy-to-use and very cost-effective nontraditional calibration method was proposed previously to calibrate virtually an MSAT sensor in RTUs. In order to overcome the deficiencies of the virtual calibration (VCal) method in fault tolerance and fault diagnostics, experiments with wider combination and coverage are investigated in this study. It is found that an improved virtual calibration (IVCal) method can be obtained by correlating the offset errors with available system information (the outside air damper status[OADst]) and low-cost temperature measurements (the MSAT and outside air temperature [OAT]). Further experimental evaluations demonstrate that the IVCal method could predict the true value of SAT with greater accuracy (at a relative error of only ±1.4°F [0.8°C]) with excellent fault tolerance and significantly enhanced performance of a virtual supply airflow rate meter. Additionally, the IVCal method also inherits the good characteristics of the VCal method, such as its high cost-effectiveness, ease of use, and applicability to all RTUs with similar constructed gas furnaces.

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.