Antennas for Mobile Frequency Harvesting and Energy Harvesting
What Is Energy Harvesting?
Energy harvesting, also called power harvesting or energy scavenging, attempts to capture energy from natural sources and the waste energy from other processes. Solar panels capture sunlight via photovoltaic cells. Wind turbines capture energy from the wind. Energy harvesting technologies are in development to capture energy from footfalls, vibrations, thermal gradients and waste heat to turn into more power for the grid.
Many proposals for waste energy harvesting would collect electromagnetic energy in the environment in the RF spectrum. This is called mobile frequency harvesting.
An Introduction to Energy Harvesting Methods
Energy harvesting antennas in an indoor environment could be designed to use any frequency range that would not harm humans. In practicality, efficiency and safety take center stage.
For example, wireless power transfer or WPT, also called wireless energy transmission, over short distances use magnetic fields. This is safe for someone to be close to as they don’t have a pacemaker or other biomedical implant that could be effective. This type of wireless energy transfer is sent by one magnetic source and received by the destination device via inductive coupling. It isn't openly radiated out, which could cause havoc with unshielded devices.
Power beaming, also called power beaming, radiative power transfer or far-field energy transfers, would transmit power over long distances. The receiving antennas in power beaming systems would use microwaves, though lasers could be used as well. Microwave antennas would be the most likely because it can contain more energy in a narrow beam than lower frequencies. (Microwave radiation uses frequencies between 1 GHz to 30 GHz, and wavelengths from 30 centimeters to a tenth of a centimeter.)
To either send or absorb any signal, the antenna must be at least half of the wavelength of the transmission. This design constraint means that smaller wavelengths and higher frequencies are preferred for energy transmission systems. What types of antennas could be used with power beaming or radiative power transfer?
Energy Harvesting Antennas
Let us assume you want to harvest energy from the local RF radiation. Patch antennas collect more energy than other designs, but they are directional. You can use patch antennas to receive beamed energy if it is facing the beam, but it can’t collect energy emanating from all directions.
Dipole antennas are simpler. Dipole means that it has two poles or elements, usually equal in length, pointing in opposite directions and perpendicular to the connector. (Dipole antennas are also called doublet antennas.) A simple way to describe this is that dipole antennas look like an upper case T. The benefit of these antennas for energy harvesting is that they can absorb energy in almost all directions. However, the actual implementation of energy harvesting antennas is much more challenging.
Problems with the Actual Implementation of Energy Harvesting Antennas
If you're talking practicality and the sheer amount of power available, set up an energy harvesting antenna to collect FM frequencies. However, unless you're right next to the transmitting station, it won't generate much energy, even with the five foot long antennas you need to do receive energy from FM broadcasts.
Trying to collect energy from wifi would be even more difficult. The antennas are much smaller than those required to receive power from FM broadcasts. After all, they can be only a few inches tall. The downside is that wifi has a limited range, and thus there isn’t much energy to collect unless you’re standing right next to the wi-fi hot spot.
Why Isn’t Electromagnetic Energy Harvesting More Common?
In short, it is because energy is lost in the conversion, even in literally high energy environments. It is like trying to capture the waste heat produced by a combustion engine – all the work lets you harness more and more of the thermal output, but you never get to 100% because of the waste, and at some point, the diminishing returns of the effort to collect nearly 100% of the power becomes a waste.
Your energy harvesting antenna – no matter what frequency of the radio frequency spectrum it tries to collect – will never collect 100% of the radiated energy on that frequency range, and it loses some energy in the conversion process from RF to electrical. Nor can you try to collect energy on all frequencies using one antenna, while setting up multiple different antennas to collect energy from all practical energy sources starts to hit the limit based on the return on investment.
The only practical use of the energy harvesting antenna would be through long distance energy transmissions, such as if we had solar collectors in space sending down power to large microwave antennas that then sent that energy to the power grid. On a local scale, more wireless power transmission in enclosed spaces to reduce the need for wires will come into use.
Yet we cannot expect to capture much of the RF noise around us to turn into power except as a minimal source of additional energy unless the energy harvesting antennas are sitting right next to the transmitter – and in that case, they are interfering with the broadcast.
An Explanation of the Scales Involved
Your radio and TV transmitting towers can be thought to put out 30,000 Watts. Your local cell sites are putting out more like 300 Watts. Why can't you use these signal sources to power your cell phone?
It is like standing in a coal mine looking at a light bulb 1000 feet down the tunnel. You can hold up a solar panel. It will receive a tiny amount of light. But the battery discharge rate of the battery connected to the solar panel is greater than the small amount of power the solar panel could generate.