Nihon Dengyo Kosaku Co Ltd (Dengyo) developed a new product in the "Microwave Regenerative Converter" series of 2.4GHz-band rectennas.
The new product is much smaller than the other products in the series. It can be used for harvesting microwave oven's or industrial microwave heating machine's electromagnetic energy that does not contribute to heating food or materials and is discarded.
A rectenna is a device combining an antenna and rectifier, and it converts electromagnetic waves into direct-current power. Its regeneration efficiency (ηt), which is the ratio of the power regenerated by a rectenna to the power input to a microwave oven, can be calculated by using the following formula.
ηt = ηm x (1 - ηh) x (1 - LC) x ηr x ηC
In this formula, ηm is the efficiency of converting alternating-current power into microwaves with a microwave oven, etc. ηh (heating efficiency) is the ratio of the energy used for heating food or materials to the energy of the generated microwaves. LC is the loss of the transmission circuit. ηr is the efficiency of the rectifier, and ηC is the efficiency of the boost circuit or superposed circuit.
Among those parameters, ηh (heating efficiency) fluctuates most because it depends on food or materials. It ranges from 0 to 90%.
"Food and materials that contain much water tend to have high ηh, and those that contain only a small amount of water tend to have low ηh," Dengyo said.
The lower the heating efficiency (ηh), the higher the regeneration efficiency ηt. It is because a low heating efficiency means a large electromagnetic energy received by the rectenna.
For example, when ηh is 50% and the values of the other parameters are typical (ηm = 70%, LC = 10%, ηr = 80%, ηC = 90%), the regeneration efficiency (ηt) is 22.7%.
On the other hands, when ηh is 10% and the values of the other parameters are the same as above, the regeneration efficiency (ηt) is 40.9%. In both of the cases, relatively high recycling efficiencies can be expected.
Though rectennas have never been used for such an application, Dengyo plans to promote it by using the new product.