This PDF file contains the front matter associated with SPIE Proceedings Volume 8728, including the Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

In this work, efforts were conducted in order to mitigate the issue of polysulfides dissolution and hence to improve the capacity and efficiency of Li-sulfur cells. The first approach was achieved by optimizing the amount of sulfur that can be contained in the sulfur/carbon electrode. Five sulfur/carbon ratios were prepared- (1) 50/50, (2) 60/40, (3) 70/30, (4) 80/20, and (5) 90/10- to study the effect of carbon contents on electrochemical cycling. The second approach was by adding nano-sized TiO₂ particles having a large specific surface area as the polysulfide adsorbing agent in the electrodes. The impact of nano-sized TiO₂ particles in improving the electrochemical properties of sulfur electrodes was investigated using CV measurements and charge/discharge tests. To further enhance the efficiency and cycling stability of Li-S batteries, a novel polysulfide electrolyte was developed. This new electrolyte mainly consisted of pre-dissolved lithium polysulfides (Li₂S_x) as an alternative electrolyte salt to replace the lithium bis(trifluoromethanesulfone)imide (LiTFSI). We also used LiNO₃ to mitigate the shuttle mechanism that occurs in Li-S cells during the charge and discharge. By creating a dynamic equilibrium at the interface of the cathode and electrolyte, the dissolution of lithium polysulfides, and thus the loss of active materials from the cathode during the discharge and charge of the cell, was greatly prevented.

Secondary lithium-ion batteries have found multiple applications in portable electronics where high charge and discharge rates are not required to improve performance. However, lithium-ion batteries are currently being sought for high power applications that require long cycle life, such as those encountered in the transportation sector. To meet these performance requirements, the shortcomings that have relegated the use of conventional lithium-ion batteries to low-power applications need to be addressed. In an attempt to fabricate batteries with high power densities, current technology is moving toward electrode materials with irregular surfaces resulting in high interfacial surface areas and short characteristic lithium-ion diffusion lengths. The use of three-dimensional (3D) architectures with interdigitated electrodes with the above described electrode characteristics have been proposed to alleviate these shortcomings because it allows a significant decoupling of the inversely proportional relationship between energy and power density. This conference proceeding manuscript is focused on the idealized calculations of both nanowire and foam 3D architectures utilizing electrode and electrolyte components that are currently being developed. A brief discussion of the use of electrodeposition as the main synthetic technique towards realizing a truly 3D solid-state lithium-ion cell is also presented.

The conventional dialogues on the reasons for thermal runaway, venting and fire of Li-ion cells point to temperature increase and gas formation. There is, however, no consensus on which of these events occurs first, thereby enabling research to correctly target the root cause. The recent work conducted at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) demonstrates that neither the temperature increase nor gas formation may be the primary reason for the venting or fire. Instead, the primary reason could potentially be a thermodynamic parameter that is often associated with both anode and cathode, namely entropy. Typically, a decrease in entropy, and a concomitant increase in the electrode temperature have been observed in the anode during charging, and in the cathode during discharging. We find that under ambient operating conditions (0 to 40 °C), entropy-driven thermal energy accounts for more than 2/3rd of the heat. More importantly, sudden changes in the entropy increases the electrode temperature by an order-of-magnitude that if left unchecked could drive the electrode temperatures sufficiently high to disrupt the SEI layer, bringing the active materials in the electrodes in direct contact with the electrolyte, enabling exothermic reactions. Battery internal temperature (BIT) sensor, a technique that we recently developed at JHU/APL enables one to follow the anode and cathode temperatures in real time, while the cell is under charge and discharge. We will discuss the application of BIT sensor in estimating the entropy changes that define the limits of safety in Li-ion cells.

The voltage and current characteristic of a photovoltaic (PV) cell is highly nonlinear and operating a PV cell for maximum power transfer has been a challenge for a long time. Several techniques have been proposed to estimate and track the maximum power point (MPP) in order to improve the overall efficiency of a PV panel. A strategic use of the mean value theorem permits obtaining an analytical expression for a point that lies in a close neighborhood of the true MPP. But hitherto, an exact solution in closed form for the MPP is not published. This problem can be formulated analytically as a constrained optimization, which can be solved using the Lagrange method. This method results in a system of simultaneous nonlinear equations. Solving them directly is quite difficult. However, we can employ a recursive algorithm to yield a reasonably good solution. In graphical terms, suppose the voltage current characteristic and the constant power contours are plotted on the same voltage current plane, the point of tangency between the device characteristic and the constant power contours is the sought for MPP. It is subject to change with the incident irradiation and temperature and hence the algorithm that attempts to maintain the MPP should be adaptive in nature and is supposed to have fast convergence and the least misadjustment. There are two parts in its implementation. First, one needs to estimate the MPP. The second task is to have a DC-DC converter to match the given load to the MPP thus obtained. Availability of power electronics circuits made it possible to design efficient converters. In this paper although we do not show the results from a real circuit, we use MATLAB to obtain the MPP and a buck-boost converter to match the load. Under varying conditions of load resistance and irradiance we demonstrate MPP tracking in case of a commercially available solar panel MSX-60. The power electronics circuit is simulated by PSIM software.

Flexible photovoltaic cells with a specific power of more than 275 W/kg have been demonstrated by depositing copper indium gallium diselenide (CIGS) absorber layers on ultra lightweight and highly durable titanium foil. Advanced device designs employing nanostructured optical coatings and exploiting optical cavity effects provide a pathway to further increase power-generating capability. Single-junction CIGS devices can potentially outperform multi-junction III-V structures in some environments, including under high air mass terrestrial spectrums.

Most of investigations of quantum dot photovoltaic devices are aimed at the development of the intermediate band solar cell. To form the intermediate band by quantum dot electron levels, the dots should be placed close to one to another. This leads to strain accumulation and defects, which increase the photocarrier recombination, and recombination losses. To avoid the nanostructuring-induced recombination, we proposed and studied an alternative approach, which is based on the separation of quantum dots (QDs) or QD clusters from the conducting channels by potential barriers created by quantum dots with built-in charge (Q-BIC). Charging of QDs improves the performance of QD solar cells due to the following factors: Negative dot charging increases electron coupling to sub-bandgap photons and provides effective harvesting of IR energy. Because of the strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs act as deep traps for electrons, but they are shallow traps for holes. Thus, the holes trapped in QDs may be excited by thermal phonons, while excitation of localized QDs electrons requires IR radiation or the interaction with hot electrons. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Charging of QDs is also an effective tool for managing the potential profile at micro- and nanoscales. Filling QDs predominantly from dopants in the QD medium allows one to maintain the macroscale profile analogous to that in the best conventional single-junction solar cells.

A simple technique with low thermal losses is used to characterize the temperature-dependent properties of some well-known thermoelectric (TE) materials. With this measurement technique, a radiant heat source is used to impose a small temperature gradient (ΔT) on a TE sample in an otherwise isothermal chamber. The Seebeck coefficient is determined directly from the linear relationship of the sample voltage with varying ΔT. To determine thermal conductivity, a small electrical current (I) is passed through the sample which causes Peltier cooling at the heated surface and can be used to force the sample toward an isothermal condition with its surroundings. At steady-state, the thermal conductivity is determined from the ΔT vs. I relationship. Because the sample is nearly isothermal with its surroundings, thermal parasitic losses are negligibly small and thermal conductivity can be determined with high accuracy.

Concentrated photovoltaic (CPV) systems achieve the highest level of solar conversion efficiency of all photovoltaic (PV) technologies by combining solar concentration, sun tracking, and high-efficiency multi-junction PV cells. Although these design features increase the overall efficiency of the device, they also dramatically increase the cost and physical volume of the system and make the system fragile and unwieldy. In this paper, we present recent progress towards the development of a robust, reduced form-factor CPV system. The CPV system is designed for portable applications and utilizes a series of low profile optical and optomechanical components to concentrate the solar spectrum, enhance energy absorption, and track the sun throughout the diurnal cycle. Based on commercial off-the-shelf (COTS) single-junction PV cells, the system exploits the efficiency gains associated with tuning the wavelength of the incoming light to the band-gap of a PV material. This is accomplished by spectrally splitting the concentrated incident beam into multiple wavelength bands via a series of custom optical elements. Additional energy is harvested by the system through the use of scavenger PV cells, thermoelectric generators, and biologically inspired anti-reflective materials. The system’s compact, low-profile active solar tracking module minimizes the effects of wind-induced loads and reduces the overall size of the system, thus enabling future ruggedization of the system for defense applications. Designed from a systems engineering approach, the CPV system has been optimized to maximize efficiency while reducing system size and cost per kilowatt-hour. Results from system tests will be presented and design trade-offs will be discussed.

Energy (or power) harvesting can be defined as the gathering and either storing or immediately using energy “freely” available in a local environment. Examples include harvesting energy from obvious sources such as photon-fluxes (e.g., solar), or wind or water waves, or from unusual sources such as naturally occurring pH differences. Energy scavenging can be defined as gathering and storing or immediately re-using energy that has been discarded, for instance, waste heat from air conditioning units, from in-door lights or from everyday actions such as walking or from body-heat. Although the power levels that can be harvested or scavenged are typically low (e.g., from nWatt/cm² to mWatt/cm²), the key motivation is to harvest or to scavenge energy for a wide variety of applications. Example applications include powering devices in remote weather stations, or wireless Bluetooth headsets, or wearable computing devices or for sensor networks for health and bio-medical applications. Beyond sensors and sensor networks, there is a need to power compete systems, such as portable and energy-autonomous chemical analysis microinstruments for use on-site. A portable microinstrument is one that offers the same functionality as a large one but one that has at least one critical component in the micrometer regime. This paper surveys continuous or discontinuous energy harvesting and energy scavenging approaches (with particular emphasis on sensor and microinstrument networks) and it discusses current trends. It also briefly explores potential future directions, for example, for nature-inspired (e.g., photosynthesis), for human-power driven (e.g., for biomedical applications, or for wearable sensor networks) or for nanotechnology-enabled energy harvesting and energy scavenging approaches.

With increased focus on intermittent renewable energy sources such as wind turbines and photovoltaics, there comes a rising need for large-scale energy storage. The vehicle to grid (V2G) project seeks to meet this need using electric vehicles, whose high power capacity and existing power electronics make them a promising energy storage solution. This paper will describe a charging system designed by the V2G team that facilitates selective charging and backfeeding by electric vehicles. The system consists of a custom circuit board attached to an embedded linux computer that is installed both in the EVSE (electric vehicle supply equipment) and in the power electronics unit of the vehicle. The boards establish an in-band communication link between the EVSE and the vehicle, giving the vehicle internet connectivity and the ability to make intelligent decisions about when to charge and discharge. This is done while maintaining compliance with existing charging protocols (SAEJ1772, IEC62196) and compatibility with standard “nonintelligent” cars and chargers. Through this system, the vehicles in a test fleet have been able to successfully serve as portable temporary grid storage, which has implications for regulating the electrical grid, providing emergency power, or supplying power to forward military bases.

An energy harvester is proposed consisting of a unimorph where the active layer is a magnetostrictive Iron-Gallium alloy. A theory for predicting the open voltage of the energy harvester in a bending-mode configuration is derived. A constrained optimization is performed using the open voltage as the objective function to identify key geometric and physical parameters that produce the highest root-mean-square voltage. Conclusions about the sensitivity of the voltage to variations in the parameters are also presented. The useful power is calculated, and the effect of changing the pickup-coil configuration is explored.

Some long-term, remote applications do not have access to conventional harvestable energy in the form of solar radiation (or other ambient light), wind, environmental vibration, or wave motion. Radiation Monitoring Devices, Inc. (RMD) is carrying out research to address the most challenging applications that need power for many months or years and which have undependable or no access to environmental energy. Radioisotopes are an attractive candidate for this energy source, as they can offer a very high energy density combined with a long lifetime. Both large scale nuclear power plants and radiothermal generators are based on converting nuclear energy to heat, but do not scale well to small sizes. Furthermore, thermo-mechanical power plants depend on moving parts, and RTG’s suffer from low efficiency. To address the need for compact nuclear power devices, RMD is developing a novel beta battery, in which the beta emissions from a radioisotope are converted to visible light in a scintillator and then the visible light is converted to electrical power in a photodiode. By incorporating 90Sr into the scintillator SrI2 and coupling the material to a wavelength-matched solar cell, we will create a scalable, compact power source capable of supplying milliwatts to several watts of power over a period of up to 30 years. We will present the latest results of radiation damage studies and materials processing development efforts, and discuss how these factors interact to set the operating life and energy density of the device.

Electrets-based electrostatic energy harvester for harvesting electrical energy from the ambient vibration is introduced and described in this paper. A new design of electrode structure called the trapezoidal electrodes and its electrets counterpart are designed, modeled and analyzed thoroughly to evaluate its performance. First, the theory is explained and the mathematical analysis is performed using Matlab/Simulink tool. Results of the analysis shows that the average output power harvested from the trapezoidal electrodes is ~1 mW from 20 Hz at 1 g inputs. Then, the 3D model of the electrodes and electrets structures are constructed, simulated and analyzed with Finite Element Modeling/Analysis (FEM/FEA) tool. Further, mechanical analysis carried out on the trapezoidal electrodes model indicates that it displaces laterally at 94 μm and resonates at 113 Hz whereas the electrostatic analysis unveils 1895 pC of charge density induced on the trapezoidal electrodes from 450 VDC electrets potential. The optimized parameters derive from the analyses are used as a reference for fabrication of MEMS (Micro Electro-Mechanical System) physical device on a standard CMOS process technology.

Commercially available double layer capacitors store energy in an electrostatic field. This forms in the form of a double layer by charged particles arranged on two electrodes consisting mostly of active carbon. Such double layer capacitors exhibit a low energy density, so that components with large capacity according to large electrode areas are required. Our research focuses on the development of new electrode materials to realize the production of electrical energy storage systems with high energy density and high power density. Metal oxide based electrodes increase the energy density and the capacitance by addition of pseudo capacitance to the static capacitance present by the double layer super-capacitor electrodes. The so-called hybrid asymmetric cell capacitors combine both types of energy storage in a single component. In this work, the production routes followed in our laboratories for synthesis of nano-porous and aligned metal oxide electrodes using the electrochemical and sputter deposition as well as anodization methods will be described. Our characterisation studies concentrate on electrodes having redox metal-oxides (e.g. MnO_x and WO_x) and hierarchically aligned nano-porous Li-doped TiO₂-NTs. The material specific and electrochemical properties achieved with these electrodes will be presented.