Amount Of Stored Energy Research Articles

Experimental evaluation of solar collectors

The purpose of the present work was to investigate the association of two important engineering devices, the two-phase thermosyphon and the solar collector. A cycle to obtain high vacuum was set up to manufacture thermosyphons, which were inserted into evacuated glass tubes to compose the structure of a solar collector prototype. The prototype was operated over a period of eight months, during which two other solar collectors were tested simultaneously for comparison purposes. The other two collectors were purchased on the market, one of which already included the thermosyphons, while the other operated without them. Temperature readings were taken at the manifold inlet and outlet, as well as in the main water supply, and in the storage tank. The temperature data were analyzed and could be condensed into just two 24-hour periods, as they showed reproducible behavior. A calorimeter allowed estimating the amount of stored energy, which in this case used twenty 24 hour periods for analysis. It was concluded that the solar collector that uses the thermosyphon can reach higher temperatures at the manifold outlet, but the storage tank can interfere, causing a decrease in the temperature of the water that reaches the consumer.

Influence of ultrafine-grained structure of Ti-42Nb-7Zr alloy on energy dissipation and storage under quasi-static tension

The ultrafine-grained (UFG) structure and plastic deformation of the Ti-42Nb-7Zr alloy have been investigated using infrared thermography. The initial stage occurs at a constant temperature when the total energy of plastic strain absorbed by the alloy is 1.75 times higher for the UFG alloy, than for the alloy with a coarse-grained structure. This fact is due to substructural strengthening caused by severe plastic deformation and dispersion hardening with ω-phase nanoparticles. The amount of heat released before the UFG alloy fracture is 2 times larger than the amount of stored energy, and this results in localized softening of the UFG alloy prior to its fracture.

Experimental dynamic thermal properties determination of EPDM/NBR panels with a shape stabilized Phase Change Material based on summer daily temperatures of four cities in Italy

In this work, the evaluation of E-PCM panels based on an elastomeric matrix containing a shape-stabilized paraffinic phase change material with a melting point of 28 °C was carried in a climatic chamber, simulating the summer daily temperature profile of four geographical locations in Italy at different latitudes. The characterization, carried out on testing boxes insulated using the E-PCM panels, allowed the determination of the delay (time lag) and of the dampening (decrement factor) in the propagation of the heat wave when passing from the outer to the inner surface of the walls; the results were correlated to two important parameters, the characteristic admittance and the heat capacity. From the difference between the heat fluxes on the outer and inner surface of the panels it was also possible to quantify the amount of stored energy and to do a comparison with a reference insulating panel (aerogel). Aerogel panels, despite the low thermal conductivity but due to the limited heat capacity and characteristic admittance, were unable to provide an effective insulation during summer conditions while E-PCM panels led to excellent outcomes. Results showed that with a peak of 33 °C in the external temperature, the phase change material limited the internal one at 27 °C with a maximum delay in the peak temperature of 6 h, an overall decrement factor of 0.4, time lag of 3 h and stored energy of 740 kJ/m2 per day. Finally, also the influence of the testing speed on the response of the E-PCM was evaluated through repetition of heating/cooling cycles in the temperature range 19–35 °C, highlighting the necessity of improving the thermal conductivity of the system in case of fast-response applications (cycle length < 3.5 h) and/or to optimize the PCM amount in the panel depending on the energy to be absorbed.

A Symmetric Sixth-Order Step-Up Converter with Asymmetric PWM Achieved with Small Energy Storage Components

This research explores an improved operation of a recently studied converter, the so-called two-phase sixth-order boost converter (2P6OBC). The converter consists of a symmetric design of power stations followed by an LC filter; its improved operation incorporates an asymmetric pulse width modulation (PWM) scheme for transistor switching, sometimes known as an interleaved PWM approach. The new operation leads to improved performance for the 2P6OBC. Along with studying the 2P6OBC, one of the contributions of this research is providing design equations for the converter and comparing it versus the interleaved (or multiphase) boost converter, known for its competitiveness and advantages; the single-phase boost topology was also included in the comparison. The comparison consisted of a design scenario where all converters must achieve the same power conversion with an established maximum switching ripple, and then the stored energy in passive components is compared. Although the 2P6OBC requires a greater number of components, the total amount of stored energy is smaller. It is known that the stored energy is related to the size of the passive components. Still, the article includes a discussion of this topic. The new operation of the converter offers more streamlined, cost-effective, and efficient alternatives for a range of applications within power electronics. The final design of the 2P6OBC required only 68% of the stored energy in inductors compared to the multiphase boost converter, and 60% of the stored energy in capacitors. This result is outstanding, considering that the multiphase boost converter is a very competitive topology. Experimental results are provided to validate the proposed concept.

Different energy storage techniques: recent advancements, applications, limitations, and efficient utilization of sustainable energy

In order to fulfill consumer demand, energy storage may provide flexible electricity generation and delivery. By 2030, the amount of energy storage needed will quadruple what it is today, necessitating the use of very specialized equipment and systems. Energy storage is a technology that stores energy for use in power generation, heating, and cooling applications at a later time using various methods and storage mediums. Through the storage of excess energy and subsequent usage when needed, energy storage technologies can assist in maintaining a balance between generation and demand. Energy storage technologies are anticipated to play a significant role in electricity generation in future grids, working in conjunction with distributed generation resources. The use of renewable energy sources, including solar, wind, marine, geothermal, and biomass, is expanding quickly across the globe. The primary methods of storing energy include hydro, mechanical, electrochemical, and magnetic systems. Thermal energy storage, electric energy storage, pumped hydroelectric storage, biological energy storage, compressed air system, super electrical magnetic energy storage, and photonic energy conversion systems are the main topics of this study, which also examines various energy storage materials and their methodologies. In the present work, the concepts of various energy storage techniques and the computation of storage capacities are discussed. Energy storage materials are essential for the utilization of renewable energy sources and play a major part in the economical, clean, and adaptable usage of energy. As a result, a broad variety of materials are used in energy storage, and they have been the focus of intense research and development as well as industrialization. This review article discusses the recent developments in energy storage techniques such as thermal, mechanical, electrical, biological, and chemical energy storage in terms of their utilization. The focus of the study has an emphasis on the solar-energy storage system, which is future of the energy technology. It has been found that with the current storage technology, the efficiency of the various solar collectors was found to be increased by 37% compared with conventional solar thermal collectors. This work will guide the researchers in making their decisions while considering the qualities, benefits, restrictions, costs, and environmental factors. As a result, the findings of this review study may be very beneficial to many different energy sector stakeholders.

Self-healing in dielectric capacitors: a universal method to computationally rate newly introduced energy storage designs.

Metalized-film dielectric capacitors provide lump portions of energy on demand. While the capacities of various capacitor designs are comparable in magnitude, their stabilities make a difference. Dielectric breakdowns - micro-discharges - routinely occur in capacitors due to the inevitable presence of localized structure defects. The application of polymeric dielectric materials featuring flexible structures helps obtain more uniform insulating layers. At the modern technological level, it is impossible to completely avoid micro-discharges upon device exploitation. Every micro-discharge results in the formation of a soot channel, which is empirically known to exhibit a semiconductor behavior. Because of its capability to conduct electricity, the emerged soot channels harm the subsequent capacitor performance and decrease the amount of stored energy. The accumulation of the soot throughout a dielectric capacitor ultimately results in irreversible overall failure. We have developed a universal method for predicting the composition and evaluating the properties of the decomposition products obtained after the dielectric breakdown of a metalized film capacitor. This method applies to both existing and newly developed designs of capacitors. In our work, we compared samples based on polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC), and Kapton. We found that the decomposition of the PP-based composition yields the greatest number of gaseous products. The corresponding soot has the lowest electrical conductivity compared to other samples. The smallest fraction of gaseous products and the highest conductivity corresponded to the Kapton-based system. According to the electrical conductivity, the obtained soot samples have been ranked in the following order: PP < PET < PC < Kapton. The resulting gas phase content is as follows: PP (12.3 wt%) > PC (6.4 wt%) > PET (6.2 wt%) > Kapton (5.1 wt%). The obtained results are in agreement with the experimental data on the self-healing efficiency of metalized-film capacitors. The novel method qualitatively correctly rates the performances of the known capacitors. The method relies on various electronic-structure simulations and potential landscape explorations. The reported advances open an impressive avenue to computationally probe thousands of hypothetical capacitor designs and boost engineering practices.

Effect of inner channel geometry on solidification performance of phase change material (PCM) in double-pipe thermal energy storage

Advantages such as high energy storage density and low heat losses have made phase change materials (PCMs) preferable in energy storage. However, the very low thermal conductivity of PCMs is the most important obstacle to improving melting and solidification performance of PCMs. To improve the thermal conductivity of PCM, the combination of PCM with nanoparticles, fins, and metal foam is among the methods widely researched in the literature. The innovation of this study was to examine the effect of inner and outer channel configurations on the solidification rate in double-pipe energy storage with PCM. Thus, it was aimed to benefit from the same amount of stored energy more and at a lower cost. In this regard, solidification analyses were carried out for three different basic inner channel geometries. With the triangle-in-circle configuration, the solidification rate was increased by 5.5% to 13.2% compared to other configurations. It is anticipated that the data obtained in this study will be a guide for novel inner channel geometry designs that will increase solidification performance and heat transfer in double-pipe energy storage with PCM.

Potential Electrical Power Generation and Installation Cost Analysis of a Biogas Plant Chicken Farm

Poultry waste is a promising renewable energy source for electricity generation. A poultry farm comprising 20000 chickens can generate a maximum of 2000 kg of waste daily, and this waste contains a substantial amount of stored energy. Typically, this waste is disposed of resulting it doesn't become a daily burden. The main objective of this research was to focus on the extraction of electrical energy from waste through the utilization of a biogas plant. For this study, a digester with a diameter of 8.24 m and a height of 3.30 m was designed for the biogas plant, which can produce up to 148 m3 of gas per day. Consequently, a total of 208.45 KWh of energy can be generated by employing a 13-kW generator. Regarding biogas plants, the estimated cost for installation amounted to approximately BDT 1145500. Furthermore, the payback period for this plant was calculated to be 2 years.

A Multilevel Boost Converter with Reduced Inductor Current

DC–DC converters are gaining attention due to their importance in key applications like renewable energy generation. A desirable feature in new DC–DC converters is a reduction in the size, which can be achieved with a reduction in the energy stored in the inductors. This article introduces a new step-up DC–DC converter topology with the following advantages: (i) a larger relation of duty cycle vs. voltage gain compared with the classical boost topology and (ii) an inductor with smaller current and smaller inductance (for the same power conversion characteristics) compared to the traditional boost converter. The smaller inductor current is an advantage against many step-up topologies with the inductor in series with the input (and then the input and the inductor currents are equal). The necessary inductance to achieve a certain current ripple is also reduced compared to the classical boost topology. This results in an inductor with a smaller amount of stored energy, lower inductance, and lower current. The proposed topology can be scaled to have a full family of large-voltage-gain converters. This paper presents the mathematical analysis, simulations, and experiments to assess the benefits of the proposition.

From computer systems to power systems: using stochastic network calculus for flexibility analysis in power systems

As power systems transition from controllable fossil fuel plants to variable renewable sources, managing power supply and demand fluctuations becomes increasingly important. Novel approaches are required to balance these fluctuations. The problem of determining the optimal deployment of flexibility options, considering factors such as timing and location, shares similarities with scheduling problems encountered in computer networks. In both cases, the objective is to coordinate various distributed units and manage the flow of either data or power. Among the methods for scheduling and resource allocation in computer networks, stochastic network calculus (SNC) is a promising approach that estimates worst-case guarantees for Quality of Service (QoS) indicators of computer networks, such as delay and backlog. Promising QoS indicators in the power system are given by the amount of stored energy, the serviced demand, and the demand elasticity. In this work, we investigate SNC for its capabilities and limitations to quantify flexibility service guarantees in power systems. We generate and aggregate stochastic envelopes for random processes, which was found useful for modeling flexibility in power systems at multiple time scales. In a case study on the reliability of a solar-powered car charging station, we obtain similar results as from a mixed-integer linear programming problem, which provides confidence that the chosen SNC approach is suitable for modeling power system flexibility.

Composited phase change material with hierarchical metal foam for efficient thermal energy management

The composited phase change material (CPCM) with highly-conductive supporting foam material is promising in thermal energy management, which overcomes the disadvantages of low heat conduction capacity and possible liquid-phase leakage for organic PCMs, i.e. paraffin wax (PW). However, its energy storage efficiency (ESE) is not fully represented by the composite thermal conductivity. Practically, the phase change efficiency is mainly dominated by the coupling effect of heat conduction and natural convection, which is strongly related to the structural configuration of composite. In the present study, a simple and feasible hierarchical metal foam (HMF) structured by adding interior fins inside each pore is produced by shrinking Voronoi tessellations to achieve precisely tunable microstructure and porosity. Then the PW is integrated with the HMF to form an enhanced PW/HMF composite system. The hierarchical CPCM’s melting mechanism and energy storage behavior is studied under the condition of bottom heating through a numerical model, which is validated by comparing to the melting experiment. Subsequently, the influence of hierarchical topology representing by various fin sizes is quantitatively investigated on heat transfer behavior governed by the nature convection and heat conduction and the composite’s ESE denoting the amount of stored energy per unit melting time is assessed. To reasonably describe the composite’s ESE, a performance map related to different hierarchical topologies is provided to guide the design and application of the PW/HMF composite under the conditions that the foam porosity is different or same.

Enhancing solar thermal collector systems for hot water production through machine learning-driven multi-objective optimization with phase change material (PCM)

Energy storage and supply in solar thermal collector systems rely heavily on phase change materials (PCM). It is vital to establish the appropriate values for energy discharge time (tPCM) and net amount of stored energy (Qnet) within the PCMs in order to increase the efficiency of solar thermal collectors. For this study, we implemented a multi-objective evolutionary algorithm based on decomposition (MOEA/D) for the optimization process using the MATLAB simulator software. The simulation findings show that, under ideal circumstances where tPCM is large and Qnet is low, these two goal functions are inversely connected to one another. According to an analysis of how input parameters affect the objective functions, the mass of PCM is minimized and the mass flow rate of the input water is increased under ideal circumstances. The relationship between the tube's inner diameter and the tPCM objective function shows that an increase in the tube's diameter corresponds to an increase in the energy discharge time. The amount of stored energy in the PCM increases when the system parameters are held constant while the tube diameter is increased. Additionally, we investigated the effects of several solid PCM materials on the discharge time and the energy stored.

Virtual Inertia Control for CHB-STATCOM in a Power Electronics Dominated Grid

Integration of renewable energy sources (RESs) leads the power system to be a low inertia system, resulting in frequency deviation in the grid. This paper presents a cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) providing virtual synchronous inertial support. This topology achieves two functionalities. 1. It reduces the system frequency deviation in a power electronics dominated grid (PEDG) following the updated grid codes. 2. Without disturbing the normal operation of CHB-STATCOM, it provides reactive power support. This topology uses the cumulative stored energy in the distributed DC-bus capacitors of CHB-STATCOM to improve the frequency nadir/zenith and the rate of change of frequency (RoCoF), preventing frequency instability. A closed- loop control algorithm is developed, which injects an adequate amount of stored energy to prevent frequency variation. It also facilitates re-energizing the capacitors in a smooth fashion, so that subsequent frequency events can be taken care of. For this, a high-pass-filter (HPF) is designed for autonomous DC- link voltage restoration for CHB converter. Relationship between frequency variation and inertial energy support by CHB is analyzed in depth. Finally, the viability of the proposed theory is verified in OPAL-RT test bench for step-up and step-down load change.

Molecular dynamics study of the formation of the nanocrystalline structure in nickel nanoparticles during rapid cooling from the melt

The formation of a nanocrystalline structure in nickel nanoparticles during their rapid cooling from the molten state was studied using the molecular dynamics method. The influence of the cooling rate in the range from 5∙1010 to 5∙1012 K/s and particle diameter from 1.5 to 11 nm on the features of the resulting structure, the amount of stored energy, and the temperature of the onset of crystallization were considered. At a rate above 1013 K/s, crystallization did not have time to occur in the model used; at a rate from 1012 K/s to 5∙1012 K/s, a nickel particle crystallized with the formation of a nanocrystalline structure. With a decrease in the cooling rate, a decrease in the density of defects and the number of grains in the final structure of particles was observed. It is also shown that the crystallization temperature during rapid cooling is inversely proportional to the particle diameter and increases with decreasing cooling rate. The melting of a nanocrystalline particle, due to the high content of grain boundaries and other defects, occurs at lower temperatures compared to a single-crystal particle.

Optimization Method of Energy Storage Configuration for Distribution Network with High Proportion of Photovoltaic Based on Source–Load Imbalance

After a high proportion of photovoltaic is connected to the distribution network, it will bring some problems, such as an unbalanced source and load and voltage exceeding the limit. In order to solve them, this paper proposes an optimization method of energy storage configuration for a high-proportion photovoltaic distribution network considering source–load imbalance clustering. Taking the minimum total voltage deviation, the minimum total power loss and the minimum total operating cost as the objective function, and considering various constraints such as power balance constraints and energy storage operation constraints, a mathematical model for energy storage configuration optimization is established. Firstly, the source–load imbalance of the distribution network with a high proportion of photovoltaic is defined. Therefore, according to the 24 h photovoltaic and load data, the 24 h source–load imbalance can be obtained, and the optimal k value can be determined by the elbow rule, so that 24 h a day can be clustered into k periods by the k-means algorithm. Then, the fuzzy comprehensive evaluation method is used to determine the weight factors of each objective function in each period, and three scenes are determined according to the different amount of energy storage. Then, the hybrid particle swarm optimization algorithm proposed in this paper is used to solve the model, and the minimum objective function value, optimal position and optimal capacity of each energy storage grid in each scene are obtained. Finally, it is applied to an example of IEEE33. In the results, the total voltage deviation is increased by more than 10%, the total power loss is increased by more than 8% and the total operating cost is increased by more than 12%, which verifies the effectiveness of the proposed model.

Comparison of the consequences of state of charge and state of health on the thermal runaway behavior of lithium ion batteries

In this work the influence of the SoC and the SoH on the thermal runaway was analyzed in depth (SoC 30–100 % /SoH 75–100 %). Generally, the results show a decrease in the strength of the TR reactions with decreasing SoC/SoH. The determined surface temperatures decrease by >100 °C when SoC and SOH are reduced to 50 % SoC or 75 % SoH because there are fewer stored and active lithium ions as well as less overall stored energy and more inactive components. Released particles and gas caused a mass loss reduction of 12.5 % (SoC) and 7 % (SoH), respectively, with measurable gas concentrations of decomposition products and linear hydrocarbons decreasing. At the same time the concentrations of gaseous electrolyte components increase. A comparative analysis at SoC 80 % or SoH 80 % shows that the amount of stored energy or Li-ions incorporated at the time of penetration into the anode is responsible for the main reaction characteristics, which include the course of the voltage drop, the temperature evolution and the composition of the reaction gases. However, despite aging effects occurring with SoH reduction, the surface temperatures and determined gas concentrations reach the same values. At microscopic scale, changes in the reactions, e.g. SEI-decomposition, are expected, but no effect on the macroscopic TR is measurable. In general, it has been shown that both lower SoC and SoH minimize the severity of an occurring TR. In summary, it could be determined that cell surface temperatures drop by the same average values, up to 100 °C, for the same reduction in SoC and SoH. Gas concentrations decrease, as do cell mass loss parameters. With lower energy, the amount of carbonate compounds in the gas phase increases, with decomposition products increasing with increased energy. In general, it has been shown that both lower SoC and SoH minimize the severity of an occurring TR. In a comparative analysis of decreased SoC and SoH, it was shown that the type of decrease in stored Li-ion amounts, setting SoC 80 % or SoH 80 %, does almost not affect the macroscopically detectable responses of TR. The surface temperatures reached on the cell, as well as the gas concentrations determined and parameters such as the penetration depth to the TR and the mass loss show identical values for SoC 80 % and SoH 80 %. In the final analysis, it is the quantity of stored Li-ions that determines the effects and strength of the TR, and effects such as cyclic aging or state of charge are negligible on a macroscopic level. The results help to better allocate the effects of aging and charging to the utilization phase and existing hazards, and form the basis for further predictions and simulations.

Development of a Scalable MMC Pulsed Power Supply through HIL Methodology

Nuclear fusion experiments are becoming one of the most interesting focuses of research, given the hope of generating programmable, safe, and green energy. Among them, ASDEX (axially symmetric divertor experiment) upgrade has been operating at the Max Planck Institute for Plasma Physics (IPP) research center since 1991. To ignite and confine the plasma, several coils must be supplied through controllable high-current pulsed power supplies. The toroidal field magnets are here considered and a modular multilevel converter (MMC)-like system was designed and tested thanks to a small-scale prototype in previous works. The MMC-like topology, consisting of full-bridge submodules (SMs) with IGBTs and supercapacitor and exploitable also for other industrial applications, was chosen because of its modularity, redundancy, fault tolerance, and large amount of stored energy. The prototype, made of four SMs, was necessary to highlight critical key points in the design process. However, its scalability must be further tested before building a full-scale power supply, meant to reach almost 2400 SMs to guarantee the energy required by the load. This paper aims at validating hardware-in-the-loop (a powerful, safe, and relatively inexpensive real-time simulation environment that enables testing with real control boards) as a useful technology for power supply scalability studies and not only for control strategy tests. The results obtained previously from the prototype will allow us to finally increase the number of SMs and test the MMC-like scalability.

Numerical investigation of melting process with natural convection inside multi-tube horizontal cylinder by SPH method

The application of Phase Change Materials on Latent Heat Thermal Energy Storage has attracted wide focus on its thermal flow analysis. This study applied Smoothed Particle Hydrodynamics to the melting process inside a multi-tube horizontal container. The Lagrangian nature of the present method simplifies the moving boundary treatment and phase change calculation process, the melting and natural convection are solved by the enthalpy method and Boussinesq assumption, respectively. After the validation of the present method in a square cavity, the melting performance inside a multi-tube container is conducted. Two main factors, Rayleigh number and arrangement of the inner tubes, are calculated and analyzed. The results reveal that natural convection basically enhances the overall thermal performance. With a large Ra number, the melting speed is accelerated and the amount of stored energy is also increased. The melting rate is faster in the upper-half region than that in the lower-half region due to the Buoyancy force. Besides, under the multitube configuration, the thermal performance is more sensitive to the change of the distance than the angle between inner tubes. With a larger distance, the thermal stratification effect in the bottom of the container is weakened, and the melting time is shorter.

Selection of appropriate configuration for optimal cascaded thermal energy storage system under the effect of outlet threshold temperature constraint

For analyzing the performance of the studied configuration, consideration of its application is highly crucial. Therefore, analyzing the amount of discharged energy against the temperature margin of the outlet is vital. A 2-D Numerical analysis was performed on various TES cases (under dynamic solar energy conditions). The type of storage media for each column was subject to choice and optimized using a genetic algorithm. The optimal distribution of the HTF rows altitude showed a variety of 12.3 mm (Center) to 29.1 mm (Sidewall) within the structure. The study objective is the selection of the best configuration to maximize the system discharge energy following two scenarios, first when there is no applied outlet threshold temperature constraint for the TES and second, when a specific outlet threshold temperature constraint is applied on the system. Generally, the cascaded structure showed a 14.7 % higher amount of stored energy in comparison with the average of cases with only one PCM, whereas the hybrid-cascaded configuration showed a 7.8 % higher stored energy compared to the same cases. At the first scenario (under no outlet temperature constraint) outputs, a higher figure of energy was discharged by the cascaded structure against the hybrid-cascaded one, however, the difference in the energy discharged amounts has decreased, describing the better thermal performance of hybrid cascaded setup while discharging. In the second scenario (with applied outlet temperature constraint), it was found that the useful discharged energy of the hybrid-cascaded structure is higher, meaning that more amount of energy is discharged in the required temperature range and thermal control is enhanced, and this is the most important result of this research and shows that the use of the sensible storage material as well as latent storage material will cause a very favorable change in the performance of the system.

Driving the quadricyclane-to-norbornadiene isomerization by charge separation with perylenediimide as electron acceptor.

Through comprehensive photo-assays, this study investigates the reaction coordinate governing the interconversion between quadricyclane (QC) and norbornadiene (NBD) upon photo-irradiation up to a wavelength of 550 nm. To harness this spectroscopic range for energy release, we link the NBD-core with a highly electron-accepting perylenediimide (PDI) with broad absorption, achieving strong electronic coupling between them. We detail the successful synthesis and present extensive DFT calculations to determine the amount of stored energy. By means of transient absorption spectroscopy, an oxidative electron transfer is observed during the QC-to-NBD isomerization following the initial PDI photoexcitation. This charge-separated state is key to triggering the back-isomerization with visible light excitation.