No abstract

Electrical energy is a high quality form of energy that can be easily converted to other forms of energy with high efficiency and, even more importantly, it can be used to control lower grades of energy quality with ease. However, building a cost-effective electrical energy storage (EES) system is a challenging task despite steady advances in the design and manufacturing of EES elements including various battery and supercapacitor technologies. As of today, no single type of EES element fulfills high energy density, high power delivery capacity, low cost per unit of storage, long cycle life, low leakage, and so on at the same time.Unlike conventional EES systems, we introduce a HEES (hybrid EES) system comprising heterogeneous EES elements. Our proposed HEES system builds on the concepts of computer memory system architecture and management in order to achieve the attributes of an ideal EES system through appropriate allocation and organization of various types of EES elements. We also introduce a HEES design considerations which should be taken into account to optimize the amortized cost for the system, including the initial cost (cost per capacity), the operating cost (efficiency), the maintenance cost (cycle life and disposal cost), and so forth.

An LCD (Liquid Crystal Display) is a standard display device for hand-held embedded systems. Today, color TFT (Thin-Film Transistor) LCDs are common even in cost-effective equipments. An LCD display system is composed of an LCD panel, a frame buffer memory, an LCD and frame buffer controller, and a backlight inverter and lamp. All of them are heavy power consumers, and their portion becomes much more dominant when running interactive applications. This is because interactive applications are often triggered by human inputs and thus result in a lot of slack time in the CPU and memory system, which can be effectively used for dynamic power management.In this paper, we introduce low-power LCD display schemes as a system-level approach. We accurately characterize the energy consumption at the component level and minimize energy consumption of each component without appreciable display quality degradation. We develop several techniques such as variable-duty-ratio refresh, dynamic-color-depth control and backlight luminance dimming with brightness compensation or contrast enhancement. Each method exhibits power reduction of 260mW, 250mW and 480mW, respectively. The aggregate energy reduction ratio is 28% out of total energy consumption including the CPU and the main memory system when we execute a document viewer. We also demonstrate that we can extend the battery life about 38% and 20% for a text editor and an MPEG4 player, respectively.

It is important to maintain high efficiency when charging electrical energy storage elements so as to achieve holistic optimization from an energy generation source (e.g., a solar cell array) to an energy storage element (e.g., a supercapacitor bank). Previous maximum power point tracking (MPPT) methods do not consider the fact that efficiency of the charger varies depending on the power output level of the energy generation source and the state of charge of the storage element. This paper is the first paper to optimize the efficiency of a supercapacitor charging process by utilizing the MPPT technique and simultaneously considering the variable charger efficiency. More precisely, previous MPPT methods only maximize the power output of the energy generation source, but they do not guarantee the maximum energy is stored in the energy storage element. Note that the load device takes its energy from the storage element so it is important to maximize energy transfer from the source into the storage element. We present a rigorous framework to determine the optimal capacitance of a supercapacitor and optimal configuration of a solar cell array so as to maximize the efficiency of energy transfer from the solar cells into a bank of supercapacitors. Experimental results show the efficacy of the proposed technique and design optimization framework.

Unlike liquid crystal display (LCD) panels that require highintensity backlight, organic LED (OLED) display panels naturally consume low power and provide high image quality thanks to their self-illuminating characteristic. In spite of this fact, the OLED display panel is still the dominant power consumer in battery-operated devices. As a result, there have been many attempts to reduce the OLED power consumption. Since power consumption of any pixel of the OLED display depends on the color that it displays, previous power saving methods change the pixel color subject to a tolerance level on the color distortion specified by the users. In practice, the OLED power saving techniques cannot be used on common user applications such as photo viewers and movie players.This paper introduces the first OLED power saving technique that does not result in a significant degradation in the color and luminance values of the displayed image. The proposed technique is based on dynamic (driving) voltage scaling (DVS) of the OLED panel. Although the proposed DVS technique may degrade luminance of the panel, the panel luminance can be restored with appropriate image compensation. Consequently, power is saved on the OLED display panel with only minor changes in the color and luminance of the image. This technique is similar to dynamic backlight scaling of LCDs, but is based on the unique characteristics of the OLED drivers. The proposed method saves wasted power in the driver transistor and the internal resistance with an amplitude modulation driver, and in the internal resistance with a pulse width modulation driver, respectively. Experimental results show that the proposed OLED DVS with image compensation technique saves up to 52.5% of the OLED power while keeping the same human-perceived image quality for the Lena image.