Daily Archives: June 28, 2017

Connect with a New Type of Li-Fi

Many of us are stuck with slow Wi-Fi, and eagerly waiting for light-based communications to be commercialized, as Li-Fi promises to be more than 100 times faster than the Wi-Fi connections we use today.

As advertised so far, most Li-Fi systems depend on the LED bulb to transmit data using visible light. However, this implies limitations on the technology being applied to systems working outside the lab. Therefore, researchers are now using a different type of Li-Fi using infrared light instead. In early testing, this new technology has already crossed speeds of 40 gigabits per second.

According to the Li-Fi technology, a communication system first invented in 2011, data is transmitted via high-speed flickering of the LED light. The flickering is fast enough to be imperceptible to the human eye. Although lab-based speeds of Li-Fi have reached 224 gbps, real-world testing reached only 1 gbps. As this is still higher than the Wi-Fi speeds achievable today, people were excited about getting Li-Fi in their homes and offices—after all, you need only an LED bulb. However, there are certain limitations with this scheme.

LED based Li-Fi presumes the bulb is always turned on for the technology to work—it will not work in the dark. Therefore, you cannot browse while in bed in the dark. Moreover, as in regular Wi-Fi, there is only one LED bulb to distribute the signal to different devices, implying the system will slow down as more devices connect to the LED bulb.

Joanne Oh, a PhD student from the Eindhoven University of Technology in the Netherlands, wants to fix these issues with the Li-Fi concept. The researcher proposes to use infrared light instead of the visible light from an LED bulb.

Using infrared light for communication is not new, but has not been very popular or commercialized because of the need for energy-intensive movable mirrors required to beam the infrared light. On the other hand, Oh proposes a simple passive antenna that uses no moving parts to send and receive data.
Rob Lefebvre, from Engadget, explains the new concept as requiring very little power, since there are no moving parts. According to Rob, the new concept may not be only marginally speedier than the current Wi-Fi setups, while providing interference-free connections, as envisaged.

For instance, experiments using the system in the Eindhoven University have already reached download speeds of over 42 gbps over distances of 2.5 meters. Compare this with the average connection speed most people see from their Wi-Fi, approximately 17.5 mbps, and the maximum the best Wi-Fi systems can deliver, around 300 mbps. These figures are around 2000 times and 100 times slower respectively.

The new Li-Fi system feeds rays of infrared light through an optical fiber to several light antennae mounted on the ceiling, which beam the wireless data downwards through gratings. This radiates the light rays in different direction depending on their wavelengths and angles. Therefore, no power or maintenance is necessary.

As each device connecting to the system gets its own ray of light to transfer data at a slightly different wavelength, the connection does not slow down, no matter how many computers or smartphones are connected to it simultaneously.

How Do Wind Turbines Work?

Wind turbines generate electricity from moving winds. You can see them in large numbers on wind farms both onshore and offshore. The blowing wind turns their blades, and rotates the shaft on which the blades are mounted. The shaft in turn, operates an electric generator and the resulting electric output is sometimes stored in a battery. A swarm of wind turbines can generate a substantial amount of energy. Wind turbines are often called a renewable source of energy, as they generate power from natural renewable sources, and do not consume fossil fuel, a source that cannot be replenished.

Inside the wind turbine, there are several control systems at work. These include its ability to turn the face of the turbine into the wind, called yawing, and its ability to control the angle of its blades, called pitch. Yawing and pitch extract the maximum amount of power from the blades rotating in the wind and require motors and controls. However, the yawing of a wind tower may actually twist the cables inside. The wind turbine usually has electronic intelligence built inside to untwist the cables.

The wind actually propels the blades, which are designed using the laws of physics and vectors to extract the maximum from the wind driving them. Speed analysis shows the maximum efficiency the wind turbine can achieve is about 59%. The laws of physics, especially Betz’s limit prevents the wind turbine from achieving efficiencies any higher.

As mentioned earlier, the main parts of a wind turbine are the blades mounted on a shaft. As the blades turn with the wind, the shaft rotates and spins a generator to make electricity. Most designs of wind turbines use electronic controls to generate the 60 Hz AC sine wave, although there are wind turbines that generate DC as well.

Typically, a doubly-fed induction generator is used to generate three-phase power. As this requires capacitors and a DC link, workers need to monitor the systems periodically to prevent failure of capacitors. Most of the control is similar to that used for controlling bidirectional motors, using IGBTs, and rectifier diodes in a full bridge arrangement. These components are rather large, considering the voltage generated is nearly 690 volts, and the power is in megawatts. Transformers step up the voltage from the generator to the grid power line, and there is built-in protection to limit the spikes as the speed of the wind increases.

The rotation speeds for wind turbine blades are 5-20 rpm, while a generator needs to rotate at speeds between 750 and 3600 rpm to generate power. Therefore, a gearbox in between translates the speeds. When maintenance time comes around, a combination of yawing and proper pitch is used to stop the rotation. Workers then insert pins into the shaft, locking the blades to prevent them from spinning.

Workers servicing and maintaining the blades have to dangle from ropes hundreds of feet above the ground in the air. Other parts, being within the tower, can be maintained more easily. In general, the maintenance and servicing for a wind turbine is similar to that required by any other turbine in a power generating station.