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The primary goal of this work was to develop, demonstrate, and evaluate a fault detection and diagnostics (FDD) system that incorporates integrated virtual sensors and fault impact models for decision support. FDD systems have the potential for improving energy efficiency along with reducing service costs. To achieve this goal, virtual sensors and fault impact models that only require low cost measurements are implemented to detect the cause of the fault, diagnose the severity of a fault and assess the performance reduction. The integrated virtual sensors can handle multiple simultaneous faults to isolate a specific fault from other faults. The fault impact models can decouple performance impacts of individual faults under multiple fault conditions. A complete diagnostic FDD system was implemented and demonstrated for a rooftop air conditioner. The test conditions included a wide range of fault and operating conditions with multiple simultaneous fault situations. Laboratory tests were performed to define reasonable thresholds for the FDD system with multiple simultaneous faults to assess whether RTU service should be performed. The FDD system provided the following diagnostic outputs for and impacts of: 1) loss of compressor performance, 2) low or high refrigerant charge, 3) fouled condenser, 4) evaporator filter, and 5) faulty expansion device. [ABSTRACT FROM AUTHOR]

In recent years, variable electricity pricing has become available to residential consumers to incentivize demand reductions during midday peak hours. Thermal energy storage (TES) systems enable consumers to store cooling energy when demand is low and assist A/C operation during peak demand periods. However, the cost savings achievable using TES are highly dependent on how the system is operated for a given utility rate structure. This study investigates control strategies for a packaged chiller unit integrated with ice storage that leverage available residential utility rate structures in the U.S. to reduce consumer electricity cost. The present work describes the development and evaluation of a generalized rule-based control strategy inspired by the performance of an optimal controller that minimizes monthly electricity cost considering both time-of-use energy and demand charges. The generalized rule-based controller is compared against the optimal controller as well as to heuristic control strategies for TES that were originally developed for commercial buildings for a range of equipment cooling capacities, TES sizes, geographic locations, and residential utility rates. The total electricity cost is determined using a simulation model that includes models for the chiller unit, ice storage tank, and secondary loop components, along with a building load model. Results show that the generalized rule-based controller can approximate the performance of the optimal controller within 20% for all cases tested, and within 10% of the optimal cost in 53% of the cases tested. The controller also performs significantly better than the heuristic strategies for commercial buildings that were evaluated. [ABSTRACT FROM AUTHOR]

• Experiments were conducted to optimize the control of TES defrosting for CASHPs. • To regulate heat distribution, the LSC and HSC refrigerant flow rates were adjusted. • Optimization included changing the LSC compressor frequency and HSC EEV opening. • LSC compressor frequency of 90 Hz is recommended. • Reducing the HSC EEV opening is recommended. Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively. [ABSTRACT FROM AUTHOR]