Photovoltaic sample-and-hold circuit enabling MPPT indoors for low-power systems

Photovoltaic (PV) energy harvesting is commonly used to power autonomous devices, and maximum power point tracking (MPPT) is often used to optimize its efficiency. This paper describes an ultra low-power MPPT circuit with a novel sample-and-hold and cold-start arrangement, enabling MPPT across the range of light intensities found indoors, which has not been reported before. The circuit has been validated in practice and found to cold-start and operate from 100 lux (typical of dim indoor lighting) up to 5000 lux with a 55cm2 amorphous silicon PV module. It is more efficient than non-MPPT circuits, which are the state-of-the-art for indoor PV systems. The proposed circuit maximizes the active time of the PV module by carrying out samples only once per minute. The MPPT control arrangement draws a quiescent current draw of only 8uA, and does not require an additional light sensor as has been required by previously-reported low-power MPPT circuits

Ultra low-power photovoltaic MPPT technique for indoor and outdoor wireless sensor nodes

Photovoltaic (PV) energy harvesting is commonly used to power wireless sensor nodes. To optimise harvesting efficiency, maximum power point tracking (MPPT) techniques are often used. Recently-reported techniques focus solely on outdoor applications, being too power-hungry for use under indoor lighting. Additionally, some techniques have required light sensors (or pilot cells) to control their operating point. This paper describes an ultra low-power MPPT technique which is based on a novel system design and sample-and-hold arrangement, which enables MPPT across the range of light intensities found indoors and outdoors and is capable of cold-starting. The proposed sample-and-hold based technique has been validated through a prototype system. Its performance compares favourably against state-of-the-art systems, and does not require an additional pilot cell or photodiode. This represents an important contribution, in particular for sensors which may be exposed to different types of lighting (such as body-worn or mobile sensors)

High-performance flexible energy storage and harvesting system for wearable electronics.

This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm(2) and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices

Flexible and stretchable power sources for wearable electronics.

Flexible and stretchable power sources represent a key technology for the realization of wearable electronics. Developing flexible and stretchable batteries with mechanical endurance that is on par with commercial standards and offer compliance while retaining safety remains a significant challenge. We present a unique approach that demonstrates mechanically robust, intrinsically safe silver-zinc batteries. This approach uses current collectors with enhanced mechanical design, such as helical springs and serpentines, as a structural support and backbone for all battery components. We show wire-shaped batteries based on helical band springs that are resilient to fatigue and retain electrochemical performance over 17,000 flexure cycles at a 0.5-cm bending radius. Serpentine-shaped batteries can be stretched with tunable degree and directionality while maintaining their specific capacity. Finally, the batteries are integrated, as a wearable device, with a photovoltaic module that enables recharging of the batteries

Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination

This is the first report of an investigation on flexible perovskite solar cells for artificial light harvesting by using a white light-emitting diode (LED) lamp as a light source at 200 and 400 lx, values typically found in indoor environments. Flexible cells were developed using either low-temperature sol–gel or atomic-layer-deposited compact layers over conducting polyethylene terephthalate (PET) substrates, together with ultraviolet (UV)-irradiated nanoparticle TiO2 scaffolds, a CH3NH3PbI3–xClx perovskite semiconductor, and a spiro-MeOTAD hole transport layer. By guaranteeing high-quality carrier blocking (via the 10–40 nm-thick compact layer) and injection (via the nanocrystalline scaffold and perovskite layers) behavior, maximum power conversion efficiencies (PCE) and power densities of 10.8% and 7.2 μW·cm–2, respectively, at 200 lx, and 12.1% and 16.0 μW·cm–2, respectively, at 400 lx were achieved. These values are the state-of-the-art, comparable to and even exceeding those of flexible dye-sensitized solar cells under LED lighting, and significantly greater than those for flexible amorphous silicon, which are currently the main flexible photovoltaic technologies commercially considered for indoor applications. Furthermore, there are significant margins of improvement for reaching the best levels of efficiency for rigid glass-based counterparts, which we found was a high of PCE ~24% at 400 lx. With respect to rigid devices, flexibility brings the advantages of being low cost, lightweight, very thin, and conformal, which is especially important for seamless integration in indoor environments.</p

Which type of solar cell is best for low power indoor devices?

Low power devices such as sensors and wireless communication nodes, focused towards indoor applications, face serious challenges in terms of harvesting nearby natural sources of energy for power. Nowadays, these wireless systems use batteries as source of energy. These batteries need to be replaced in due time and this factor plays a major role in determining the life of the device. Often, the cost of replacing the battery outweighs the cost of the device itself. Also from an environmental perspective, reducing battery waste is laudable. In order to obtain an “infinite” lifetime of the system, the device should be able to harvest energy from renewable resources in the device’s environment. Photovoltaic (PV) energy is an efficient natural energy source for outdoor applications. However, for indoor applications, the efficiency of classical crystalline silicon PV cells is much lower. Typically, the light intensity under artificial lighting conditions found in offices and homes is less than 10 W/m² as compared to 100-1000 W/m² under outdoor conditions. Moreover, the spectrum is different from the outdoor solar spectrum. Although the crystalline Si cell is still dominating the PV market, second generation solar cells, i.e. thin film technologies, are rapidly entering the market. The different PV cells are rated by their power output under standard test conditions (AM1.5 global spectrum and light intensity of 1000 W/m²) but those conditions are not relevant for indoor applications. The question therefore arises: which type of solar cell is best for indoor devices? This paper contributes to answering that question by comparing the power output of different thin film solar cells (CdTe, CIGS, amorphous Si, GaAs and an organic cell with active layer P3HT:PCBM) with the classical crystalline silicon solar cell as reference. This comparison is made for typical artificial light sources, i.e. an LED lamp, a “warm” and a “cool” fluorescent tube and a common incandescent and halogen lamp, which are compared to the outdoor AM1.5 spectrum as reference. All light sources (including the outdoor spectrum) are scaled to an illumination of 500 lux to obtain a correct comparison. The best artificial light source for all cell types is the incandescent lamp which, for Si and CIGS, improves the performance of the cell with a factor of 3 compared with AM 1.5. The LED lamp is the worst light source for indoor PV with a decrease in performance of a quarter for amorphous silicon to two thirds for crystalline silicon cells. The best solar cells for indoor use depend heavily on the light source. For an incandescent lamp, crystalline silicon remains the best. However, for an LED lamp or “warm” fluorescent tube, amorphous silicon is significantly better. For “cold” fluorescent tubes as light sources, CdTe solar cells perform the best