The Efficiency of Wireless Charging

25 June 2019
Source: Viribright

Efficiency in wireless charging compares the power that is available to the charged device (e.g. the device connected to the wireless receiver) with the power used in the wireless transmitter. The higher this number is, the better.

For instance, if to deliver 1 Watt of power to a device you need to use 100 Watt on the transmitter, the efficiency is 1%.

Why is efficiency important?

Three main reasons:

1. Cost

According to the United States Energy Information Agency, the average cost of delivering 1-kilowatt hours of energy was 12.95 cents. 1-kilowatt hour means deliver 1000 watts for an hour. This is the cost of energy going into the transmitter. But, if only a tiny fraction of the energy going into the transmitter ends up as usable energy on the receiver side, the cost of wireless energy can be high. For instance, let’s assume you want to charge a phone and you are using fairly slow charging at 1 Watt and let’s round up the average energy cost to 13 cents per kWh:

  • If the efficiency of wireless charging is 10%, then for a 2-hour charge of the phone, your cost is about 0.26 cents. Here’s why: 13 cents / kWh means 0.013 cents per Wh, and thus if you need to deliver 1 Watt of power with 10% efficiency, you are consuming 10 Watts every hour. Thus, for 2 hours, this is 0.26 cents
  • If the efficiency of wireless charging is 1% then for the same 2-hour charge, you’re paying 2.6 cents
  • If the efficiency of wireless charging is 0.1% then you are paying 26 cents to charge your phone for 2 hours. Over time, wireless charging with such low efficiency can start adding up to a noticeable amount on your energy bill

2. Where does the excess energy go?

Have you ever noticed how hot a light bulb can become? An incandescent light bulb is about 5% efficient (and sometimes less). In the case of a light bulb, the other 95% turns into heat.

In the case of wireless power, some of the lost energy turns into heat, but depending on the technology (see below), some energy may be stray energy that baths the environment with unwanted radiation.

3. Safety

The device being charged does not care about the transfer efficiency – it just wants to be charged. So, if you need to deliver 1 Watt of power and you have a low-efficiency system, you’ll need to crank up the power to deliver the desired amount. Very quickly, for low-efficiency technologies, you’ll bump up against government safety limits. Thus, higher efficiency not only saves energy costs and reduces heat, but also allows you to deliver higher levels of energy while staying within safety limits.

What determines efficiency?

Wireless power works by converting electricity into some physical phenomena (RF, sound, light) on the transmitting side, sending it over the air, and then converting it back to electricity on the receiver side. Thus, wireless power efficiency is a combination of these three stages.

The choice of technology is critically important to efficiency. On the receiver side, for instance, this would be the antenna efficiency for RF or the photovoltaic cell efficiency for light.

The second stage – the efficiency of sending power over the air – is sometimes overlooked and is a critical difference between various technologies. In RF, for instance, the diameter of the power beam increases when leaving the transmitter due to diffraction and thus with any reasonable-sized receiver, only a small portion of the energy can be captured. Wi-Charge, on the other hand, uses a thin and focused beam of light and this allows the small Wi-Charge receivers to capture 100% of the energy sent from the Wi-Charge transmitter.

What is a good benchmark?

Wireless power transfer is never going to be as efficient as a wired connection, though people trade efficiency for convenience all the time. Wireless power is also a new technology so efficiency will surely improve over time.

But we need to start from someplace. I suggest we use the efficiency of an incandescent light bulb as the benchmark: if the wireless power efficiency is the same or better than a light bulb, this would be considered a workable solution for a wide range of devices. If efficiency is much lower than a light bulb, then such charging solutions should be relegated to only delivering tiny amounts of power.


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