BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates generally to a dust sensor for photovoltaic (PV) cells, in particular a sensor-based radio-frequency identification (RFID) tag having a passive RFID tag that can retain and transmit information wirelessly in conjunction with the dust sensor. Together with other sensors, health conditions of the PV cell can be detected in addition to the level of dust; the detected health information can be stored in and transmitted by the RFID tag.

Description of the Related Art

In moving toward greater use of clean energy sources, large fields of hundreds of PV panels (made up of solar modules), often referred to as solar farms, are being installed. A PV module may consist of a plane of an integrated group of multiple PV cell devices. Solar farms need to last about a decade in order to justify return on investment. On a lesser scale, it is becoming more common for arrays of tens or a few hundred PV panels or modules to be installed on or near homes and buildings. Because the PV modules are mounted in open fields, they are subject to various weather and environmental conditions that can affect their efficiency, and may even suffer various faults. Over time, PV modules exposed to the environment accumulate dust and organic matter. Dust and organic matter accumulation can lead to reduced power output. Other conditions such as temperature, humidity, and cloudiness may have an impact on efficiency. Faults and other malfunctions may be detected as hot spots in PV cells.

An emerging technology for radio-frequency identification (RFID) is passive RFID tags. Ultra-high frequency (UHF) radio-frequency identification (RFID) tags have become more reliable as integrated circuit techniques have evolved. Passive RFID tags offer features including battery-less operation, wireless communication, high flexibility & low cost. RFID is typically used in supply chain & warehousing. Radio frequency identification (RFID) technology in the ultra-high frequency (UHF) band is currently being used for identification and tracking of various products.

Recently researchers have been exploring the possibility of obtaining physical attributes of objects in the environment from low level processing of electromagnetic signals backscattered from RFID tags. UHF RFID technology has substantial growth potential in the area of computing and sensing applications. This potential could be implemented by integrating various sensors, such as temperature, humidity and acceleration sensors, to passive RFID tags (Juha Virtanen, Leena Ukkonen, Toni Björninen and Lauri Sydänheimo, “Temperature Sensor Tag for Passive UHF RFID Systems”, in IEEE sensors application Symposium, Feb. 22-24, 2011, San Antonio, Tex., USA—incorporated herein by reference).

Temperature sensing may be used to indicate that a PV cell has one or more hot spots. Hot spots may indicate internal failures such as shorts in electrical conductors of the PV cells or may be caused by cracks in the PV cells. Temperature sensing in the previous applications of RFID has led to many realizations of passive UHF RFID temperature sensors by various researchers. These past realizations can be grouped into two sub-categories. The first category consists of passive temperature sensors integrated into an RFID integrated circuit (IC) (P. Sunghyun, M. Changwook, C. Seong-Hwan, “A 95 nW ring oscillator-based temperature sensor for RFID tags in 0.13 μm CMOS,” IEEE International Symposium on Circuits and Systems, 2009. ISCAS 200, pp. 1153-1156, 24-27 May 2009; M. K Law, A. Bermak, H. C. Luong, “A Sub-μW Embedded CMOS Temperature Sensor for RFID Food Monitoring Application,” IEEE Journal of Solid-State Circuits, vol. 45, no. 6, pp. 1246-11255, June 2010—incorporated herein by reference). This approach removes the need for any external sensor component, eases the assembly of a sensor tag and provides savings in the manufacturing costs if mass-produced. Most importantly, these integrated sensors are able to output their data digitally, thus maximizing the accuracy of the sensing. However, the commercialization of such RFID ICs has not begun and most IC designs are still in the prototyping stage. The second category is formed by passive UHF RFID temperature sensor tags, which are equipped with an external sensor element. In this approach, researchers have integrated external sensor elements such as mechanical structures (R. Bhattacharyya, C. Floerkemeier, S. Sarma, “RFID tag antenna based temperature sensing,” 2010 IEEE International Conference on RFID, pp. 8-15, 14-16 Apr. 2010—incorporated herein by reference) or shape-memory alloys. These kinds of sensor tags are able to utilize commercial RFID ICs and because they have no additional temperature sensing electronics they usually possess long read ranges. The drawback is the high cost of sensing materials and the complexity of the tag structure. The interpretation of information from such external sensors is slightly more complicated than the use of fully integrated sensor devices. The sensor readout has to be made indirectly by monitoring changes in the tag's backscattered power, electronic product code (EPC) or threshold power i.e. the transmit power needed to activate the tag.

Regarding dust and organic matter, several sensors for detecting dust have been developed. Sensors for detecting dust may be classified as two types of sensing technologies. One technology is based on optical principles, where a light is illuminated onto the surface and the reflected light intensity is measured as an indicator for particle accumulation on the surface. A second technology is based on measuring a vibrator's frequency to decide if dust particles are sticking on the vibrator mounted on the surface.

Examples of these two types of sensing technologies may be found in patent literature. With regard to dust detection based on optical principals, U.S. Pat. No. 8,804,119—incorporated herein by reference—relates to a particle detection system that uses at least one light source adapted to illuminate a volume being monitored at at least two wavelengths. U.S. Pat. No. 8,638,391—incorporated herein by reference—relates to an image sensor assembly capable of effectively removing dust sticking onto a surface of an image sensor. U.S. Pat. No. 8,514,090—incorporated herein by reference—relates to a dust level sensor arrangement for dust collection system. The sensor has an adjustment to control the distance at which it picks up light off the collected material. U.S. Pat. No. 8,451,460—incorporated herein by reference—relates to a monitoring system for the acquisition of the layer thickness of dust in ventilation ducts. The monitoring system includes a light source and a sensor. The light source is arranged to transmit light across a detection surface and the sensor is on the other side of the detection surface. When a substance, such as dirt or grease, is deposited on the surface it obstructs the light and the amount of light reaching the sensor decreases. U.S. Pat. No. 8,342,710—incorporated herein by reference—relates to a loupe lens and lighting assembly for enhancing detection of dust particles on the optical sensor of a digital camera. U.S. Pat. No. 7,702,236 relates to a digital image acquisition device with built in dust and sensor mapping capability. The device derives a map of defects on the surface of the sensor from a calibration image acquired by the sensor when illuminated by a light source. U.S. Pat. No. 7,292,338 incorporated herein by reference—relates to a particle detection apparatus and particle detection method. T particle detection apparatus has a light source and a detection device for detecting light from the light source, and detects particles floating in the air at a position through which the light passes. U.S. Pat. No. 5,412,221—incorporated herein by reference—relates to a particle fallout/activity sensor. The particle fallout/activity sensor measures relative amounts of dust or other particles which collect on a mirror in an area to be monitored. The sensor includes a sensor module and a data acquisition module, both of which can be operated independently of one another or in combination with one another. The sensor module includes a housing containing the mirror, an LED assembly for illuminating the mirror and an optical detector assembly for detecting light scattered off of the mirror by dust or other particles collected thereon.

With regard to dust detection based on measuring a vibrator's frequency, U.S. Pat. No. 5,910,700—incorporated herein by reference—relates to a dust sensing apparatus that uses a transducer element oscillating at a resonant frequency to detect changes in dust concentration. U.S. Pat. No. 5,698,931—incorporated herein by reference—relates to a sensor element for detecting solid particles in a fluid. At least one of the detecting unit and the vibrating portion contacts with a solid particle in the fluid so that the piezoelectric film converts the vibration into an electric signal.

Accordingly it is one object of the disclosure to provide a dust sensor that does not sense other effects or lack of polishing when sensing dust and that does not cause wear on the object being sensed. It is a further object of the disclosure to perform health monitoring of PV cells using a dust sensor, and sensors for other conditions that can influence performance, such as temperature, humidity, and wind.

SUMMARY OF THE INVENTION

According to an embodiment of the present disclosure, there is provided a photovoltaic (PV) cell health monitoring apparatus. The apparatus includes a Radio Frequency Identification (RFID) tag mounted to the PV cell and having identifying information of the PV cell, and a sensor in communication with the RFID tag for measuring health information of the PV cell. The RFID tag stores the measured health information together with time and locality information of the PV cell and responds to an interrogation signal by transmitting the stored information together with the identifying information.

