Imagine you are standing in the streets of the city you live in, and you are trying to look up at the clear sky but instead what do you see first? sky with beautiful clouds, no!, Birds chirping around, not at all nowadays, It’s actually a bunch of stringing wires. What if we told you that we can remove those wires smoothly without any consequences and still get the power that we are getting today!?
This could be the future of wireless energy. Wireless electricity sounds like a science-fiction film, but the technology is already realized and primed for a utility-scale case study. In a first-of-its-kind pilot program, “Powerco” — New Zealand’s second-largest electricity distributor — has combined with a startup called “Emrod” to test this technology.
Wireless Power Transfer (WPT) is the technology in which electrical energy is transmitted from a power source to an electrical load without any electrical or physical connections. Compared to traditional power transfer with cord, WPT introduces many benefits such as better operational flexibility, user friendliness, and product durability.
Hitherto, wireless technology has been mainly studied based on magnetic induction, magnetic resonance, and electromagnetic wave methods. The magnetic induction and resonance based ones do work stupendously for short distances but their efficiency drops drastically as we go to meters. The electromagnetic wave method enables long distance transmission of several kilometers compared with the other two methods; however, it is harmful to the human body.
With the near-field wireless power technology such as Capacitive power transfer (CPT) and Inductive Power Transfer (IPT) reaching a mature stage for domestic and industrial applications , far-field wireless power research has been gathering momentum in the last decade. Both the microwave power transfer (MPT) and laser power transfers (LPT) have the ability to transfer several kilowatts of power over long distances up to several kilometers*, which are more flexible than those near-field wireless power technologies.Laser power transmission (LPT) is one of the most promising technologies in the long-range wireless power transfer field.
WHY WPT?
With that being said, many of us might wonder why we need WPT? When we have the conventional wired power transfer which is efficient, safe and while we are constantly discovering many new methods of wired power transmission, then why WPT Technology?
We will accept this as a valid question, without any objections.
We can all agree on the fact that, “Imagination is the key to innovation & new discoveries”. This technology that we are talking about today is not a new technology which emerged in this decade. The seed for this technology was sowed in 1886 by Hertz performing a successful experiment with pulsed wireless energy transfer. But the seed grew into a marvellous tree and is still standing strongly just because the mastermind Nikola Tesla entered this field and made Wireless Power Transmission as one of his dreams to achieve. From 1891 to 1898 he experimented with the transmission of electrical energy using a radio frequency resonant transformer of the Tesla coil, which produced high frequency AC.
I will illustrate with one simple example for why we need WPT? Many of you might have travelled to hill stations during your summer vacations. There, you will get power constantly in your place of stay. Think for a couple of seconds, is it easy to bring the electrical energy, which is generated somewhere in the terrain, all the way up to the hilly regions. Is it easy for the electricity board people or whoever it may be to set up transmission towers, to lay the transmission lines and those require constant surveillance & maintenance. Trust me the process is very difficult but once we optimize the WPT technology the process will become easy. It is just one of many difficult situations where WPT can save us all.
So with this information in mind, I hope we all can agree on the fact that the groundwork for the WPT was laid back in the 80s’ by great minds of that time and it is necessary. So now we will discuss how this technology works, its efficiency and how we can possibly optimize it.
* LPB — Laser Power Beam, LPT — Laser Power Transmission
How do we get Power out of a LASER?
The diagram above gives a rough idea on the working of LPT(Laser Power Transmission) systems. There are essentially 2 parts in the system — Transmitter and Receiver.
The transmitter of the system converts power from the source it may be battery, generator, or grid into a monochromatic beam of light via a laser.This laser beam is then shaped with a set of optics and directed via a beam director to the PV receiver.
The Receiver has a specialized PV array matched to the laser wavelength and beam intensity to convert the photons back into electricity to supply the load or charge a battery.
As we can see from Fig.3 , One can easily draw an analogy between wired and laser transmission. As said in the introduction we can comfortably remove the stringing wires in LPT technology.
The working of LPT depends on Laser diodes and solar cells .To Understand the LPT technology and its efficiency , we have to look at the working and efficiency of each component individually.
LASER TECHNOLOGY:
The selection of laser for the LPT system needs to comply with some fundamental constraints related to the:
1) the ability to transfer the energy through the atmosphere
2) the ability to transfer the energy as long as possible
As the atmosphere is composed of various gases that fluctuate in composition due to environmental conditions, it will absorb certain energies at particular wavelengths. Thus, the lasers must operate in the wavelength range centered around the spectrum in which the atmosphere is nearly transparent in order to maximize energy transfer.
From the graph it’s clear that the bin between 780 and 1100nm is only possible for laser technologies to produce sufficient power for wireless power transmission.
The other factor that needs to be considered is that the beam of the laser should be bright enough to ensure that energy can be transferred over a long enough range.
The flux Φ delivered to the receiver at slant range L where
- Rsource is the radiance (power per unit area per steradian) of the laser source, a constant that indicates the beam quality
- Asource is the total area of the beam source, possibly spread across more than one telescope aperture
- ηtrans is the transmission efficiency through the atmosphere
From the equation, if flux Φ is given, the transmission range L is limited by the source irradiance Rsource and source aperture size Asource. This limits the types of laser that could supply enough radiance Rsource
PHOTOVOLTAIC CELLS:
When we look into the usual Photovoltaic systems, to maximise the power produced by the Solar cells we use a lot of techniques and we also have to consider so many parameters. When it comes to WPT, we may need to add a few more parameters to consider for effective transmission.
Therefore at the receiver end an appropriate PV cell should be carefully designed so that power can be effectively converted into electricity. In order to do so, the factors of the laser power, wavelength, temperature, and the material of the PV cells should be considered.
Also the photons must have energy greater than or equal to the band-gap in the material in order for the cell to generate electricity. The energy of a photon is proportional to its frequency so PV cells respond to particular frequencies of light corresponding to the cell’s band-gap energies.
The ideal light source for us to use should be monochromatic and should be at an ideal frequency for the PV material, but the sunlight is spread across a broad spectrum of frequencies.
From the Fig4 we can see the graph of Wavelength vs. Conversion efficiency, the most widely used PV cells are Si and GaAs,they reach the highest conversion efficiencies when illuminated with monochromatic beams of wavelength 900 nm and 850 nm, respectively.
The GaAs PV cell has greater than 50% efficiency under the light of the right wavelength,thus it is the most preferred one to be used in this WPT technology, which can be observed from both the figures Fig4 & Fig5.
Efficiency of LPB System
After considering the constraints laid by this technology on each of laser and PV systems, one can say that Laser Diode and GaAs PV cells tend to be the best option for the LPB system.
The commercial off-the-shelf LD can offer high efficiency of 40–60% with laser power around 1 kW.Specialized GaAS PV cells can achieve more than 50% efficiency.However, the efficiency of the PV array is roughly 30–40% or even lower.
So by putting the math we get a system efficiency of about 23%.Scientists around the world are exploring new techniques and methods to push this number higher for practical applications. Some of them are MPPT based feedback models and Pulse mode operation of LD.
SOME OF THE POSSIBLE LPB SYSTEM COMBINATIONS
LPT is still a work in progress,most of the LPT based systems can transmit power only with low efficiency and are far from practical implementations. In order for the LPT system to become a viable option in near future, each component of the system must ensure sufficient high efficiency to provide for a high end-to-end system efficiency.
SOME LPT BASED PROJECTS IN THE PAST
- Researchers at the university of Korea Advanced Institute of Science and Technology (KAIST) have developed an electric transport system that is called Online Electric Vehicle, OLEV. The vehicles get power wirelessly from cables via non-contact magnetic charging that are kept under the surface of the road. To manage traffic congestion and to improve efficiency by reducing energy consumption, this technique is established.
- Laser powered mini-rover by EADS Space Transportation facility in 2002.The demonstration was based on Nd:YAG laser at 532 nm with an output power of 5W. A tracking system was developed to maintain an orthogonal angle between the PV cell panel and the laser beam over a distance up to 280 m.
- Laser powered Aircraft flight by NASA in 2003.The aircraft was fitted with a custom thin-film PV array, and an adjustable 1.5-kW diode array at 940 nm with 50% efficiency was chosen.The 500-W laser power beam manually tracked the aircraft’s flight path at 15 m range, resulting in a laser power of 40 W to the PV array and 7W of power to the motor to sustain flight for 15 min.
- In 2016, Russia’s Rocket and Space Corporation had successfully charged a cell phone across 1.5 km using a laser and a 60% efficient photoelectric converter.
The Important factor — SAFETY!
What is the use of a new technology if it doesn’t consider the safety use? Especially power transmission technology should consider the safety issues to its core, because it will be implemented among the general public.
The word LASER itself is giving us the denotative meaning of “Danger”, “Caution” and so on and so forth. We wouldn’t blame you, we must use high power density laser like Class 4 laser in the LPT systems to achieve the maximum efficiency. The lower grade (Class) laser like class 1, class 2, Class 3 will have very low power density if we are using the Class 3R lasers, which are limited to 5 mW of power output for continuous lasers. If we are planning on using class 3R lasers, we might need 740,000 3R lasers focused on our target to collect the desired output without taking into account any losses in diffusion,scattering, or conversion to power up the 370W load.
Even if we go for class 3B lasers, which have 0.5W of power output, we would require 740 lasers to be directed towards the target at a time without taking into account those same losses. Even the class 4 lasers seem like good enough but we might want to discard it if we are considering the safety issues. If safety is not a requirement we can get the lasers of 370W pretty easily. There are companies who manufactures these kind of lasers such as “Coherent | DILAS”. But we can develop the safety system, which can shut the whole system down when there’s a living or nonliving thing approaches the beam and quickly reactivate the whole system once the target is clear.
THE FUTURE SCOPE
Current laser technology and reasonable apertures can produce useful beam intensity at the receiver within a range of 10 km with efficiency of 20–25%.
Though LPT may seem as a futuristic tech,it can’t be seen as a practical solution as of now with very low system efficiency and high initial costs though. But further research in the areas of Photovoltaics, Control, Laser, Material science, and optical will one day transform this WPT technology as the one we will use in our day to day life. With more and more scientists and professionals working on innovative methods , we can safely say that LPT is definitely coming very soon.