Analysis of Round-Trip Efficiency of an HVAC-Based Virtual Battery

The flexibility in power consumption of heating, ventilation, and air-conditioning (HVAC) systems in buildings can be utilized to provide a battery-like service to the power grid. Recent work has reported that using HVAC systems in such a manner may lead to a net increase in energy consumption compared to normal operation, similar to a low round-trip efficiency (RTE) of a battery. In our previous work we showed that the low RTEs reported were due to the way the experiments/simulations were performed, and that using an HVAC system as a virtual battery repeatedly leads to an asymptotic RTE of one. In this work we show that when an additional constraint is imposed---that the mean temperature of the building must remain at its baseline value---the asymptotic RTE can be lower than one. We numerically investigate dependence on parameters such as building size and time period of charging/discharging

Thromboembolism in Children with Multisystem Inflammatory Syndrome: A Literature Review

Multisystem inflammatory syndrome in children (MIS-C) is a hyperinflammatory response observed in children several weeks to months after acute infection with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). On review of all published cases of thromboembolism (TE) as a complication of MIS-C, 33 cases of TE were found with incidence ranging from 1.4 to 6.5%. TE occurred mostly in children aged 12 years and above. One-third of the cases were cerebral infarcts and the remaining cases included intracardiac and radial arterial thromboses, upper and lower extremity deep vein thrombosis, pulmonary embolism, and splenic infarcts. Five were asymptomatic cases and 3/33 (9%) patients (all three with cerebral infarcts) died. To conclude, TE appears to be a significant complication of MIS-C caused by SARS-CoV-2 infection, associated with morbidity and/or mortality. Patients ≥12 years are affected more often, and TE occurs despite thromboprophylaxis in some patients. Thromboprophylaxis should be considered in all cases after reviewing the concomitant bleeding risk. Prospective studies are needed to confirm the role of standard-dose thromboprophylaxis and to explore whether higher-dose thromboprophylaxis is required in certain high-risk patients with MIS-C

A Unified MPC Formulation for Control of Commercial HVAC Systems in Multiple Climate Zones

Model predictive control (MPC) has been widely investigated for climate control of commercial buildings for both energy efficiency and demand flexibility. However, most MPC formulations ignore humidity and latent heat. The inclusion of moisture makes the problem considerably more challenging, primarily since a cooling and dehumidifying coil model which accounts for both sensible and latent heat transfers is needed. In our recent work, we proposed an MPC controller in which humidity and latent heat were incorporated in a principled manner, by using a reduced-order model of the cooling coil. Because of the highly nonlinear nature of the process in a cooling coil, the model needs to be modified based on certain weather/climatic conditions to have sufficient prediction accuracy. Doing so, however, leads to a mixed-integer nonlinear program (MINLP) that is challenging to solve. In this work, we propose an MPC formulation that retains the NLP (nonlinear programming problem) structure in all climate zones/weather conditions. This feature makes the control system capable of autonomous operation. Simulations in multiple climate zones and weather conditions verify the energy savings performance, and autonomy of the proposed controller. We also compare the performance of the proposed MPC controller with an MPC formulation that does not explicitly consider humidity. Under certain conditions, it is found that the MPC controller that excludes humidity leads to poor humidity control, or higher energy usage as it is unaware of the latent load on the cooling coil

Study of Adsorption of Methanol in an Activated Carbon and Carbon Nanotube Matrix for Use in a Solar Based Refrigeration Cycle

This thesis seeks to investigate the adsorption capabilities of activated carbon and carbon nanotubes. The adsorption of methanol on both of these substances was tested for their application in a solar based refrigeration cycle. Research on carbon nanotubes and their growth has been carried out for applications in the semiconductor industry. Enough focus has not been given to the use of nanotubes for refrigeration purposes. Adsorption refrigerators have been designed with the energy source being solar energy. Various adsorbent/adsorbate pairs have been tested in literature. The present work focuses on carbon nanotubes because theoretically, nanotubes should be able to adsorb better than activated carbon due to their high surface to volume ratios and hence a higher number of adsorption sites available for methanol to adsorb. The amount of adsorption of methanol on nanotubes depends on whether the end caps of the nanotubes are open or closed and also on the hydrophilic nature of the nanotubes. Nanotubes with ends closed are supposed to adsorb less than the nanotubes with their ends opened. The ends of carbon nanotubes can be blocked because of iron and other impurities. In this project, nanotubes are annealed under high vacuum to open the end caps. The hydrophobic nature of the nanotubes is corrected by treating them with concentrated nitric acid. The hydrophobic nature of the nanotubes is corrected by treating them with concentrated nitric acid. The acid treated nanotubes are used to obtain adsorption data at different temperatures. The adsorption of methanol on activated carbon, pristine and treated carbon nanotubes is measured at different temperatures. Electron microscopy is used to validate that annealing the nanotubes at high temperature under vacuum opens the end caps of the nanotubes. Finally, a matrix of nanotubes and carbon powder is prepared with different concentrations. The mixture is tested for adsorption of methanol. It is observed that the carbon nanotubes, pristine or treated, do not perform better than activated carbon. However, performance seems to increase when mixtures of activated carbon and carbon nanotubes are used as adsorbent. Also, it is found that mixtures containing annealed nanotubes perform better than mixtures with pristine nanotubes. Kinetics of the adsorption process is calculated for the different adsorbents used, which is used to explain the increase in the amount of methanol adsorbed for the activated carbon-carbon nanotube mixture

A Control-Oriented Dynamic Model of Air Flow in a Single Duct HVAC System

A model of a variable air volume (VAV) system is developed that can predict air flow rates, fan pressure rise, and fan power consumption in response to changes in fan speed and damper positions. The system consists of a fan, ductwork, and a number of dampers, one in each VAV box. The model can be used for conducting simulation studies of how advanced control algorithms that seek to provide various services (energy efficiency, personalized comfort, and demand-side flexibility to the grid) may behave when deployed in a building with an existing climate control system, or to do model-based control computations for such services. Comparison of the model\u27s predictions with experimental data from a small commercial building is presented for the single-zone version of the model. The multi-zone model structure is described, but its validation is left for future work. Due to the strong non-linearities in the steady state relation between inputs and outputs, and due to the fast transient response observed in experiments, the dynamic model is constructed to be of Hammerstein type, with a linear dynamic system in series with a static nonlinear model

Low-Power Phase Frequency Detector Using Hybrid AVLS and LECTOR Techniques for Low-Power PLL

Wireless communication is a fast-growing industry and recent developments focus on improving certain aspects of the area and reducing the power con- sumption while maintaining the frequency of operation. Phase Locked Loop (PLL) is an integral part of commu- nication circuits which operate at very high frequencies. Phase Frequency Detector (PFD) is the first block of PLL and is key in determining the computational ca- pacity of the PLL. The power consumption of the PFD has to be reduced to minimize the overall power con- sumption of PLL. The PFD architecture used is based on Double Edged Triggered D Flip-Flop (DET-DFF), which is free of dead zone. Stack, LECTOR, AVLS and hybrid low-power approaches are implemented to reduce the power consumption of DET-DFF based PFD architectures. The PFDs power, delay and power delay product analysis is performed using Cadence Virtuoso and Spectre in CMOS 180 nm and 90 nm technology. A power reduction of upto 32 % has been observed while keeping the transistor count to a minimum