• Energy Research

The prospect of physical exertion commonly acts as a deterrent to the adoption of cycling for everyday transport. A battery powered assistance torque electric motor could alleviate such physical exertion by reducing the effort required by the cyclist. This study investigates the potential effectiveness, efficiency, and energy saving of electrically-assisted cycling when assistance torque of a switched reluctance motor is designed to vary in accord to the cyclist instantaneous torque at the pedal cranks. Specifically, the modulated motor assistance torque is delivered at the least efficient human input torque points on the cycle. For a representative short distance cycling schedule modulating the instantaneous torque of the on-board electric motor causes the electric energy expenditure to not exceed that of the assisted cycling mode of an identical constant-torque motor. Furthermore, for the same speed profile cycling journey with added road gradient and head wind resistance, the energy expenditure of the modulated torque motor is equal to the constant torque motor. These findings indicate significant improvements in the cycling experience.

Energy harvesters are being used to power autonomous systems, but their output power is variable and intermittent. To sustain computation, these systems integrate batteries or supercapacitors to smooth out rapid changes in harvester output. Energy storage devices require time for charging and increase the size, mass, and cost of systems. The field of transient computing moves away from this approach, by powering the system directly from the harvester output. To prevent an application from having to restart computation after a power outage, approaches such as Hibernus allow these systems to hibernate when supply failure is imminent. When the supply reaches the operating threshold, the last saved state is restored and the operation is continued from the point it was interrupted. This paper proposes Hibernus++ to intelligently adapt the hibernate and restore thresholds in response to source dynamics and system load properties. Specifically, capabilities are built into the system to autonomously characterize the hardware platform and its performance during hibernation in order to set the hibernation threshold at a point which minimizes wasted energy and maximizes computation time. Similarly, the system auto-calibrates the restore threshold depending on the balance of energy supply and consumption in order to maximize computation time. Hibernus++ is validated both theoretically and experimentally on microcontroller hardware using both synthesized and real energy harvesters. Results show that Hibernus++ provides an average 16% reduction in energy consumption and an improvement of 17% in application execution time over state-of-the-art approaches.

For decades, the design of untethered devices has been focused on delivering a fixed quality of service with minimum power consumption, to enable battery-powered devices with reasonably long deployment lifetime. However, to realize the promised tens of billions of connected devices in the Internet of Things, computers must operate autonomously and harvest ambient energy to avoid the cost and maintenance requirements imposed by mains- or battery-powered operation. But harvested power typically fluctuates, often unpredictably, and with large temporal and spatial variability. Energy-driven computers are designed to treat energy-availability as a first-class citizen, in order to gracefully adapt to the dynamics of energy harvesting. They may sleep through periods of no energy, endure periods of scarce energy, and capitalize on periods of ample energy. In this paper, we describe the promise and limitations of energy-driven computing, with an emphasis on intermittent operation.This article is part of the theme issue ‘Harmonizing energy-autonomous computing and intelligence’.

Dust accumulation on photovoltaic (PV) modules significantly reduces their performance, especially in desert environments. Cleaning can be costly or not feasible. This paper presents a comprehensive study of PV modules performance in a desert environment, focusing on the impact of dust on power output reduction at various tilt angles to determine the optimal angle in uncleaned conditions. Seven pairs of PV modules were installed on the roof of the Faculty of Engineering in Jeddah City at angles of 0°, 15°, 25°, 45°, 60°, 70°, and 90°. The output power of both the cleaned and dusty modules was recorded over a 12-month period. The results show that dust accumulation, tilt angle, and rain significantly reduce power. The optimal tilt for maximum average output power varies with the seasonal position of the sun and the amount of dust on the module’s surface. After 183 days of dust accumulation without rain, the power reduction for the dusty modules reached 80.4%, 75.6%, and 60.2% at tilt angles of 0°, 15°, and 25°, respectively. In the rainy period, the highest performance of the dusty modules was observed at a 45° tilt angle, with a power reduction of 5.9%. Conversely, during the dry period and throughout the year, the tilt angle that generated the highest power output was 25°, with power reduction of, respectively, 28.7% and 20.7%. These findings provide valuable insights into the impact of dust and tilt on PV module performance and contribute to the development of predictive models and optimization strategies for solar panel systems in harsh desert conditions. This research highlights the importance of strategic tilt selection to enhance the performance and longevity of PV installations in desert environments.

Display Omitted TALiSMaN detects road users and sets streetlight brightness appropriately.A utility model is detailed to quantify the usefulness of street lighting.Street lighting schemes are evaluated with StreetlightSim.TALiSMaN offers comparable usefulness as conventional lighting schemes.TALiSMaN consumes 2-55% of the energy of conventional/state-of-the-art schemes. Street lighting is a ubiquitous utility, but sustaining its operation presents a heavy financial and environmental burden. Many schemes have been proposed which selectively dim lights to improve energy efficiency, but little consideration has been given to the usefulness of the resultant street lighting system. This paper proposes a real-time adaptive lighting scheme, which detects the presence of vehicles and pedestrians and dynamically adjusts their brightness to the optimal level. This improves the energy efficiency of street lighting and its usefulness; a streetlight utility model is presented to evaluate this. The proposed scheme is simulated using an environment modelling a road network, its users, and a networked communication system - and considers a real streetlight topology from a residential area. The proposed scheme achieves similar or improved utility to existing schemes, while consuming as little as 1-2% of the energy required by conventional and state-of-the-art techniques.

Radio frequency energy harvesting (RFEH) and wireless power transfer (WPT) are increasingly seen as a method of enabling sustainable computing, as opposed to mechanical or solar\ud EH WPT does not require special materials or resonators and can be implemented using low-cost\ud conductors and standard semiconductor devices. This work revisits the simplest antenna design,\ud the wire monopole to demonstrate the lowest-footprint, lowest-cost rectifying antenna (rectenna)\ud based on a single Schottky diode. The antenna is fabricated using a single Litz-wire silk-coated\ud thread, embroidered into a standard textile substrate. The rectifier is fabricated on a compact lowcost flexible printed circuit board (PCB) using ultra-thin polyimide copper laminates to accommodate low-footprint surface mount components. The antenna maintains its bandwidth across the\ud 868/915 MHz license-free band on- and off-body with only −4.7 dB degradation in total efficiency in\ud human proximity. The rectenna achieves up to 55% RF to DC efficiency with 1.8 V DC output, at 1\ud mW of RF power, demonstrating its suitability as a power-supply unit for ultra-low power e-textile\ud nodes.

In Radio Frequency (RF)-powered networks, peak antenna gains and path-loss models are often used to predict the power that can be received by a rectenna. However, this leads to significant over-estimation of the harvested power when using rectennas in a dynamic setting. This work proposes more realistic parameters for evaluating RF-powered Body Area Networks (BANs), and utilizes them to analyze and compare the performance of an RF-powered BAN based on 915 MHz and 2.4 GHz rectennas. Two fully-textile antennas: a 915 MHz monopole and a 2.4 GHz patch, are designed and fabricated for numerical radiation pattern analysis and practical forward transmission measurements. The antennas' radiation properties are used to calculate the power delivered to a wireless-powered BAN formed of four antennas at different body positions. The mean angular gain is proposed as a more insightful metric for evaluating RFEH networks with unknown transmitter-receiver alignment. It is concluded that, when considering the mean gain, an RF-powered BAN using an omnidirectional 915 MHz antenna outperforms a 2.4 GHz BAN with higher-gain antenna, despite lack of shielding, by 15.4× higher DC power. Furthermore, a transmitter located above the user can result in 1× and 9× higher DC power at 915 MHz and 2.4 GHz, respectively, compared to a horizontal transmitter. Finally, it is suggested that the mean and angular gain should be considered instead of the peak gain. This accounts for the antennas' angular misalignment resulting from the receiver's mobility, which can vary the received power by an order of magnitude.