Optical Energy – Storing the Sun without Solar Cells
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Presented by: Elizabeth Cloos
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Optical Energy – Storing the Sun without Solar Cells
- 1. Optical Energy – Storing the Sun without Solar Cells Elizabeth Cloos University of Michigan 2013-10-24 1
- 2. Outline Background Solar Cell Technology Electric Capacitors Parametric resonance Our Idea: Optical Capacitors The Future 2 W.M. Fisher and S.C. Rand, J. Appl. Phys. 109, 129901(2011)
- 3. Current Tech: Solar Cells 3 Image: http://www.redarc.com.au/images/uploads/content/solar_cell.png
- 4. Current Tech: Solar Cells - Problems Absorb incident light and generate heat Only work for a few wavelengths of light Only <30% efficient Semiconductors are expensive 4 SOLUTION: Optical Capacitor
- 5. Review: Electric Capacitor 5 Image: http://teachingphysics.files.wordpress.com/2010/04/capacitor_schematic_with_dielectric-svg.png 𝑉 = 𝐸𝑑 = 𝑄 𝐴 𝜀 𝑑 Charge +Q Electric field E Plate area A Plate separation d 𝑈 = 1 2 𝑄𝑉
- 6. Parametric Resonance 6 Parametric resonance effects in a gantry crane, YouTube User: luizomar
- 7. What is light? 7 E B k Image: http://mysite.du.edu/~lconyers/SERDP/em_electric_magnetic_propagating_waves.jpg
- 8. Our Idea: Optical charge separation 8 z x e- E B k Charge displacement!
- 9. How does this make an Optical Capacitor? 9 +Q -Q V Sin(Ωt)
- 11. Schematic 11 W.M. Fisher, "Transverse Optical Magnetism." PhD diss., The University of Michigan, (2012)
- 12. 12 Solar Concentrator +Q -Q V Sin(Ωt) Fiber Optical Capacitor
- 13. Example: Ω = 25 MHz switching rate L = 10 m of Sapphire Fiber ω0 = 50 μm fiber radius I = 1 kW/cm2 PMAX = 0.299 kW Efficiency = 30% 13
- 14. Example (cont.): 50/1 step-down transformer Iavail / Isat = 0.1 V = 3.56*105 Volts Extractable Power: 29.7 W 14
- 15. Benefits The optical storage is 100% efficient! Magnetic fields do NO work! We are limited by the switching rates of the electronics Effect occurs in all dielectric materials Water, CCl4, Crystals Little heat generated Broadband light collection 15
- 16. Potential Problems Energy extraction is limited by switching rate of light direction High saturation intensities (so far) 16 𝐼𝑆𝑈𝑁 = 10−1 𝑊 𝑐𝑚2 𝐼𝑆𝐴𝑇 = 108 𝑊 𝑐𝑚2<<
- 17. So where are we now? Investigating new materials GOALS: Find materials with low intensities Develop a model for what materials are best 17
- 18. Acknowledgements MURI Center for Dynamic Magneto-Optics Air Force Office of Scientific Research University of Michigan National Science Foundation GRFP 18 This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. F031543. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
- 19. 19
Notas del editor
- Good afternoon. My name is Elizabeth Cloos and I am from the University of Michigan where I am pursuing a PhD in Optics. Today I will explain the theory behind how we might store the sun’s energy without solar cells, that is, with an optical capacitor. For all of you who are expecting a ready-to-market piece of technology, I just want to disclaim that this idea is still in fundamental research. Also, I misspoke in the abstract. This optical capacitor is still a photovoltaic technology since photovoltaic simply means getting an electric voltage from an optical source. It does, however, differ from conventional solar cells.
- Before I get started how many people are familiar with how solar cells work? How many know how electric capacitors operate? How many vaguely remember electrons? Good. I’ll do my best at bringing all of you along today to understand this potential technology. I’ll start with a background on solar cells and electric capacitors to try to bring you all up to speed. Then I’ll get into the details of what makes an optical capacitor possible. I also want to note that this work is based on the paper by my colleagues, Will Fisher and Steve Rand.
- I’d like to start with a reminder on how solar cells work. Solar cells operate off of the photovoltaic effect, which describes how light can transfer its energy to electrons causing them to move and generate an electric current. For a given material, there are two main layers: n-doped and p-doped semiconductors which form a “p-n junction”. N-doped semiconductors have an excess of electrons so it has a negative net-charge while p-doped semiconductors have an excess of holes giving it a positive net-charge. The light transfers enough energy to the excess electrons in the n-doped region that they overcome the barrier between the regions, combine with the holes in the p-doped region, and generate an electric current.
- However, there are a few main problems with solar cells. Sun light enters a cell with a broad range of energies or wavelengths, but only specific energies can excite the electrons. It depends on the specific electron “band gap” or energy difference of a material. The rest of the energy is wasted and turned into heat. Because of this most of the current commercial solar cells are <30% efficient. Engineers have worked on the efficiency problem by stacking multiple p-n junctions together so that more wavelengths are turned into current, but there is still no complete broadband solution. Another problem with solar cells is they are made of semiconductor materials which are expensive to manufacture and can be toxic to the environment. For a sustainable energy source, we want something better for the environment. Thus, I propose a new solution, an optical capacitor.
- Let’s next review the fundamentals of an electric capacitor. A simple electric capacitor is two parallel plates with opposite charge separated by a distance d. This “charge separation” creates a static electric field between the two plates. The voltage of a capacitor is just the magnitude of the electric field times the separation distance or the charge times the distance divided by the area of the plates and the electric permittivity of the filler material. Thus, Potential energy is stored in a capacitor by the potential difference and charge of the two plates. The key thing to take away from this slide is that a capacitor voltage is formed by an electric field from charges separated by a distance d.
- I want to digress into one more topic to review, parametric resonance. To explain parametric resonance in detail, we look at this example of Parametric resonance effects in a gantry crane. This is only a 15 second clip from a longer video. The pendulum starts stationary, but as the crane moves up and down vertically, the pendulum begins to swing side to side. This parametric resonance occurs when the vertical frequency of the crane is a 2/n integer fraction of the natural frequency of the pendulum. Thus parametric resonance is simple coupling between vibrational and rotational modes of a system. (http://www.youtube.com/watch?v=ekaeS8AmerY)
- Now into the optics! Most of you probably remember this, but as a quick refresher, light is an electromagnetic wave. This means than it has both an electric field component and a magnetic field component. The electric and magnetic fields are perpendicular to each other and travel in the same direction. In most cases we only consider the effects of the electric field on matter, but in a optical capacitor we must consider the magnetic field as well.
- The main idea behind an optical capacitor is static charge displacement. In all materials there are electrons bound to atoms. These electrons interact with light. The electric field can cause these electrons to start to oscillate in the same direction as the electric field. In optics this is referred to as an electric dipole polarization. Once the electrons start moving, the magnetic field acts upon them. The magnetic field couples the vertical oscillation of the electron into a torsional oscillation. This is a parametric resonance effect just like in the gantry crane. Oscillatory motion becomes rotational motion. Mathematically, this can be described using Mathieu equations. Note, the electron is now displaced from where it started. This is a static charge displacement and is driven by the magnetic field.
- Now that we have seen a charge displacement, how can we make an optical capacitor? Well, a static charge displacement corresponds to a voltage generation. The voltage generated by one electron cloud polarization is minimal, but if many are put together it can become something large. Just like stringing together batteries in series. If a potential difference or voltage is created in a material, it can be extracted using electrodes at the end of the material. However, In order to maximally extract voltage from an optical capacitor, the incoming direction of light must change to change the polarization of the electron clouds and generate an AC voltage. This is done effectively by switching the electrodes rapidly.
- Now, for efficient optical power generation, the charge separation must be repeated as rapidly and as often as possible. Power depends directly on the switching rate. This is because current is released when the electron moves back to where it started when the light is turned off. This is exactly how a electric capacitor only generates a current when whatever charged it is removed. Also, the power depends on the length of the material as well. The more charges stacked together in a row, the higher the voltage. Thus, to increase power extraction, we need a fast switching rate, a long length of material, and a high intensity of light.
- To make this capacitor a reality, here is a basic schematic for a capacitive energy harvesting circuit. It is based on an ac-dc rectifier with an output capacitor load, and an adaptive control dc-dc converter that maintains optimal power transfer. Basically, the optical capacitor generates AC voltage due to the switching of polarizations which then has to be rectified into a DC voltage in order to be stored long tem in a battery.
- Now, let’s look at a theoretical example. Sunlight enters a solar concentrator to increase the intensity of the light and focus it into an optical fiber. This focused light causes a charge separation. By switching the electrodes rapidly back and forth, an AC voltage is generated.
- Let’s look at some numbers. Assuming a 25 MHz electrode switching rate, in a 10 m length of sapphire fiber with a core radius of 50 microns, the maximum power that could be drawn from electrodes on the fiber ends is 0.3 kW if the incident light is 1kW/cm^2. This means a 30% efficiency. This efficiency is driven down by the low electronic switching rate.
- Now, including the conversion electronics the efficiency is much lower. Even lower than solar cell efficiency. This low efficiency is due to the high voltage and the low input intensity in this example. The charge separation which powers an optic capacitor is an effect whose strength increases quadratically with the input intensity of light, but at a certain intensity, a “saturation intensity”, it stop increasing. In order to maximally exploit this effect, we want to operate as close to the “saturation intensity” as possible. This saturation intensity is different in each material. Since the extracted power is directly related to the ratio between the input intensity and the saturation intensity, light must be focused to an intense enough spot for maximum efficiency.
- To conclude, the main benefits of this optical capacitor schema is that the optical storage part is 100% efficient. Magnetic fields do no work so no energy is lost in the charge separation. We are only limited by the electronic extraction speeds and high-voltage circuitry. This effect can occur in all dielectric materials. We have tested it in water, CCl4, cubic YAG, and a few other materials. Since light is minimally absorbed in the process, little heat is generated as the entirely broadband light is collected.
- However, there are also a few potential problems with an optical capacitor schema. These need to be worked out before it could be realized. Because the charge separation happens on the order of 100 fs, it is virtually instantaneous compared to the switching rate. Therefore, the power extraction is limited chiefly by the switching rate of the electrodes. Also, this charge separation in current materials saturates at around 10^8 W/cm^2 which is a billion times more intense than the sunlight incident on the surface of the earth. Thus, to bring this to fruition we need to both concentrate the solar energy and investigate materials with lower saturation intensities.
- So, where are we now? My research studies a related phenomena of transverse optical magnetization. I am studying different materials to find where this effect and the magnetization saturate. So far we have studied water, benzene, carbon tetrachloride, ethylene glycol and the cubic crystals YAG and GGG. Each material saturates at a different intensity with the solid saturating at intensities of at least an order of magnitude lower than the liquids. My thesis work is trying to develop a model for why different materials have different saturation intensities. My goal is to find materials that saturate at lower intensities so that we can make this optical capacitor a reality.
- Thank you all for your time!