GB2484526A

GB2484526A - Rectenna array for solar energy conversion

Info

Publication number: GB2484526A
Application number: GB1017401.9A
Authority: GB
Inventors: Yi Huang; Steve Hall; Yaochun Shen
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-10-14
Filing date: 2010-10-14
Publication date: 2012-04-18
Also published as: GB201017401D0

Abstract

The rectenna array consists of inter-connected rectenna elements in a grid and each element is formed by a circular patch antenna and a rectifier operating from 215 to 750 THz, which covers the visible light and far-infrared frequencies of the solar spectrum. The rectenna array is of cross-polarisation and broad unidirectional radiation pattern which improves the energy conversion efficiency. The rectenna array is of planar structure and made of metals and dielectrics rather than expensive semiconductor based materials. The described photovoltaic technology is scalable and low-cost.

Description

A High-Efficiency Rectenna Array for Solar Energy Collection

DESCRI PTION

1. The Background

Global warming has become a real threat to the future of our planet. More than 85% of the world's power is at present generated by combustion of fossil fuels, which is a major contributor to the global warming. Furthermore, worldwide demand for energy has been increasing steadily at a rate of around 5% per year. There is therefore a most urgent need for green and clean renewable alternative energy sources for all countries. The renewable energy challenge has now become many countries' top priorities.

Solar energy: main features and spectral density Solar energy has been identified as one of the best alternatives. This is because of the following reasons: * It is abundant: the amount of the solar energy falling on the Earth within in one hour could meet our energy requirement for one year. It is the largest, more than any other forms of energy.

* It is free and renewable: there is no need for us to generate it. It is there for us to collect and use.

* It is green: no waste and no CO2 are generated.

Photovoltaic technology: current situation and limitations Photovoltaics (PVs) are arrays of cells containing a solar photovoltaic material (such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulphide) that converts solar energy into direct current (DC) electricity. Photovoltaic technology is the most mature of the solar-energy-harvesting techniques. The worldwide market for PV has increased at an annual rate of 20% over the last 10 years, making it the world's fastest-growing energy technology. At the end of 2009, the cumulative global PV installations surpassed 21,000 megawatts, and it is estimated that as much as 18 billion watts per year could ship by 2020. Whether this goal can be achieved is very much dependent on the successful development of the related technologies.

The PV concept is based on traditional p-n junction theory whereby an incoming photon above the energy bandgap of the PV material generates a single electron-hole pair, which delivers energy proportional to the bandgap to the load as DC energy. A highly energetic photon still delivers the same amount of energy as a photon exactly matched to the bandgap but the excess energy will be dissipated within the semiconductor as heat or may be reflected. This means that the efficiency of PV is fundamentally limited by the match of the bandgap to the solar energy.

Much of the solar radiation reaching the Earth is composed of photons with energy greater than the bandgap of suitable PV materials such as silicon. For single-junction solar cells, this sets an upper efficiency limit of near 30% [W. Shockley and H. Queisser, Journal of Applied Physics, Vol. 32, 510, 1961]. Even with complex (and expensive) multi-junction designs, the theoretical efficiency is limited to around 50% [C. H. Henry, Journal of Applied Physics, Vol. 51, 4494, 1980]. In fact, it was reported that current state-of-the-art solar cells can achieve 20% efficiency for single cells and about 30% efficiency for multi-junction systems in laboratory environments.

Although great improvements have been made in the last 20 years, in practice, the efficiency of most today's solar cells is still well below 20% and is typically about 10- 15%. Furthermore, the current multi-junction PV designs required to overcome the efficiency limitation, do not appear to be cost-effective solutions. Thus there is an urgent need for new high-efficiency technologies to improve solar-conversion technologies to meet the increasing demands of green and renewable energies.

Rectenna: concept and problems A rectenna is a rectifying antenna, a special type of antenna that was introduced in the 1960s to directly convert RF (radio frequency) and microwave energy into DC electricity; that is to allow wireless power transmission. It consists of two key elements: an antenna and a rectifier. The antenna is to receive electromagnetic wave/energy and then convert it to electricity; it acts as a transducer. The rectifier converts the high frequency electricity at the output of the antenna to DC power. The original motivation was to use rectennas to provide microwave power (around 2.4 GHz) to high altitude atmospheric platforms, like helicopter, aircraft or satellite, to Unlike conventional PVs, rectennas do not have to use solar power and can operate at other frequencies such as infrared and microwave. Thus they can be used in both day and night times. An even more important advantage is that, unlike PVs, there is no fundamental limit on the efficiency of rectennas -in principle, the efficiency of the rectenna can be as high as 100%. Although in practice we have to take things like thermodynamics, feasible design limitations and associated losses into account, the achievable efficiency is still very high, much more than 30%. For example, in laboratory environments, efficiencies of 85% have been observed at microwave frequencies [Y. H. Suh, and K. Chang, "A High Efficiency Dual Frequency Rectenna for 2.45 and 5.8 GHz Wireless Power Transmission", IEEE Transaction on Microwave Theory and Techniques, Vol. 50, No. 7, Dec. 2002.].

Due to their promise of high efficiency and relative economy, there have been some attempts to extend the concept of rectennas to infrared and optical frequencies for use as energy collection devices [J. C. Fletcher and R. L. Bally, "Electromagnetic waver energy converter", US Patent No. 3760257, 1973. A. M. Marks, "Device for conversion of light power to electric power," US Patent No. 4445050, 1984. 0. H. Lin, R. Abdu and J. Bockris, "Investigation of resonance light absorption and rectification by subnanostructures", J App. Phys. Vol 80(1), pp. 565 -568, July 1996. B. Berland, "Photovoltaic technologies beyond the horizon: optical rectenna solar cell", Final Report, NREUSR-520-33263, ITN Energy Systems, September 2002. M. Sarehraz, Novel Rectenna for Collection of Infrared and Visible Radiation, PhD thesis, University of South Florida, 2005. D. K. Kotter, S. Novack, W. Slafer and P. Pinhero, "Solar nantenna electromagnetic collectors", Proc of Energy Sustainability, 2008], but so far all these attempts have failed to demonstrate that the idea can be indeed realised in these higher frequency regimes largely due to technological problems, not least around the rectifier. For example, the very recent work by the group of [D. K. Kotter, S. Novack, W. Slafer and P. Pinhero, "Solar nantenna electromagnetic collectors", Proc of Energy Sustainability, 2008] has only managed to show that the frequency selected surfaces developed in [B. Monacelli, J. Pryor, B. Munk, D. Kotter, and 0. Boreman, "Infrared frequency selective surfaces based on circuit-analog square loop design". IEEE Transactions on Antennas and Propagation, Vol. 53, No.2, Feb 2005] could be manufactured using the latest nano-technology while the functionality for collecting energy was not demonstrated. The slow progress in this area is mainly due to the following reasons: i) Lack of good understanding and design of rectennas at IR and optical frequencies.

Since the material properties at these high frequencies are very different from those at RF and microwave frequencies, the simple frequency scale-up designs of conventional microwave rectennas do not work well. We need to re-design the rectenna using the correct material properties and associated theory and taking into account the special features of the high frequency solar signals. For example, substrate modes are normally not a problem at microwave frequencies but could be a serious problem at higher frequencies. Finally, the necessary impedance matching between antenna and rectifier was realised at microwave frequencies but not at IR and optical frequencies.

ii) Limited nano fabrication capability and reliability.

This was a major problem before -the fabrication was expensive and unreliable, but now it has been much improved.

iii) The published papers indicate that the teams working in this area did not have the requisite multi-and inter-disciplinary expertise to allow optimisation of each element of design: antennas and circuit design, opticallTHz engineering, device micro-fabrication and technology.

2. The Invention As shown in Fig. 1, the proposed rectenna array consists of many inter-connected rectenna elements in a grid and each element is formed by a circular patch antenna and a rectifier without bias operating from 215 to 750 THz, which covers the visible light and far-infrared frequencies and contributes over 80% of the solar energy falling on the Earth over this band of the spectrum. This device can convert this part of the solar energy to usable direct current (DC) electricity. Although each element has a very small size and the associated power/signal is very limited, these elements are connected in series in a 2D grid and the rectified DC current will be combined constructively to generate a large current hence power to achieve the aim of harvesting the solar energy.

A closer look of the rectenna element is given in Fig. 2 where "1" is for the circular patch antenna and made of Metal 1; "2" is the rectifier; "3" is the Si substrate with thermal oxide; "4" is the deposit Metal 1 (Au or other); "5" is the deposit dielectric layer; "6" is the ground plane and made of metal.

The fabrication process is illustrated by Fig. 3. Three masks and some of the state-of-the-art nano-fabrication facilities such as the Atomic Layer Deposition (ALD) are required. It demonstrates that this design can be fabricated using the facilities available.

The simulated reflection coefficient of the element antenna is shown in Fig. 4. It has an extremely broad bandwidth over the frequency range from 250 to 810 THz which is close to our desired frequency band.

The total radiation of the rectenna is also simulated and given in Fig. 5. A very wide unidirectional radiation pattern is obtained -this means that the rectenna can receive the solar energy over a wide incident angle. Furthermore, the rectenna array has cross-polarisation to meet the challenge of the incoming solar light with random polarization. As a result, this new rectenna design can provide high energy conversion efficiency. The rectenna array is of planar structure and made of metals and dielectrics rather than expensive semiconductor based materials. The technology is therefore intrinsically scalable and low-cost, and it is of great commercial values.

The novelty of this design consists of the following aspects: a) A planar-circular dipole/patch antenna array is integrated with rectifiers to form a rectenna array; b) The rectenna array is made in nano-scale to receive very broadband signals in visible and infrared frequencies; c) The solar energy can be received over a wide-incident angle (from about -70 degrees to +70 degrees) and converted to DC electricity at a high efficiency.

The major benefits over the existing designs are a) Planar structure -easy to be applied in practice; b) Metal and dielectric based -cost-effective; c) Realisable -it can be made and demonstrated.

Claims

CLAIMS1. A rectenna array consists of many rectenna elements on a conductive ground plane (6) which are inter-connected in a 2D grid.
2. Each rectenna element is made on a substrate with thermal oxide (3) whose thickness and material properties can be varied.
3. A layer of metal 1 (4) is deposited on the substrate using a mask. The thickness and pattern of the metal can be changed although rectangular is recommended.
4. A layer of dielectric material is then applied to the top of the structure with a thickness typically between 1 to 10 nm for metal-insulator-metal (MIM) design. For metal-insulator-insulator-metal (MIIM) design, two layers of different dielectric materials are applied. The thickness is also typically between I to 10 nm.
5. Mask 2 is employed to make desired pattern and etch the dielectric down to mental electrode and open up contact via for the final metal layer.
6. A layer of the second metal, which can be different from the first one, is deposited on top of the structure using Mask 3 and it is the antenna element. Its pattern can vary although the recommended one is circular in shape with a diameter about 300 nm. The separation between two adjacent antenna elements is a variable for the design. The recommend one is about 30 nm.
7. The complete structure is the invention, a rectenna array, which can be used to receive solar energy.