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Multi-Mirror Concentrators

Multi-Mirror Concentrators

Multi-mirror concentrators is a form of non-imaging optics, where a maximum fraction of incident energy is cast onto the target area but without creating an image.

Why Concentrate?

The prime motive for concentrated photovoltaics (CPV) is cost reduction, as is indeed, the purpose of all improvements in business strategies. Concentrating solar irradiance from over a large area and focusing it onto a smaller area. This permits use of less semiconductor area for conversion of solar energy collected from a much larger area. Since solar cells still form a very significant part of the photovoltaic equipment cost, this scheme helps reduce the overall cost. Thus, concentrated photovoltaics make systems more cost effective.

Use of Multijunction Cells

Multijunction solar cells add to the conversion efficiency by using multiple junctions which simultaneously convert a greater part of the solar spectrum. But these high technology devices are still pretty costly. This becomes an added reason for using them with concentrators only. Thus, multi-junction cells will normally go together with use of solar photovoltaic concentrators.

System Efficiency

The total system efficiency, say, ηtot, consists mainly of the product of optical efficiency ηopt and receiver electrical efficiency ηelec (electrical efficiency of a PV array) where ηopt is the fraction of incident rays that reach the receiver after traveling through the optical system. We say: ηtot∝ηopt. ηelec. The optical efficiency depends on mirror reflectance ρ, receiver absorbance α and the intercept factor AF. which itself is defined as the ratio of the radiation power intercepted by a central, square receiver of area Ai and the total radiation power reaching the focal plane.

ηopt = ρ α ΑF.

Assuming an ideal optical system, ie, reflectance is 100 percent and receiver absorbance is 100 %, ηopt becomes equal to AF. Thus, AF remains a critical parameter.

The Intercept Factor

A perfect concentrator will throw an exact image of the sun on its focal plane. Practical concentrating surfaces do not have the perfect shape for point concentration, but the fact of concentration means that the target area will be illuminated in a circular pattern with maximum intensity at the center reducing, more or less, monotonically outwards. Target areas are of square or rectangle shape. Either the corners will be illuminated very little, or if we make the beam broader, a lot of energy will be lost around the sides. Thus, a good fraction of energy that has been collected does not fall on the target cell. This necessarily reduces the intercept factor AF, affecting the overall efficiency adversely.

Uniformity of Irradiance

Because of the same reason, some parts of the array in single mirror concentrators get intense radiation and others get relatively much less. Now the electrical efficiency, ηelec of a PV array is very closely linked to uniformity of irradiance on the receiver array.  As cells are series connected ηelec is dependent on the cell-to-cell uniformity. Cells in a string which are poorly illuminated will hamper the output of the other cells. A single poorly illuminated cell can badly affect the string output and even cause local hot spots.

Opposite Demands

Thus, with conventional concentrator reflectors, improving irradiance uniformity necessarily means loss of power and, hence, reduced conversion efficiency. That is where the multi-mirror concentrators show a promise.

The Effect of Shading

Recall that a single mirror concentrator will produce an image of the light profile available at its aperture. A shaded patch will be imaged at the receiver as a shaded patch and a bright region will be imaged as such. If a cell in an array gets shaded, it gives no output. Being in series it will impede the current of all cells in series with it. A hot spot may result. A shaded cell is like a dead soldier whose body must be carried boy others.

Multi-Mirror Concentrators Concept

multi-mirror concentrators avoid these shortcomings.
  • Uniformity-Individual mirrors are laid out and focused to spread the irradiance uniformly over the entire target array surface.
  • High Active Area Fraction-The irradiation pattern need not be circular or linear. Virtually any simple shape including a square target area can be illuminated with virtually uniform radiation. Theoretically, an infinite number of mirrors can permit absolutely uniform illumination of any target shape. Thus, a higher Active Area Fraction can be achieved with use of multiple mirrors.
  • High Ground Coverage Ratio-Because multiple mirrors are forming part of the reflector, the reflector need not have a circular or elliptical aperture. Circular and elliptical apertures will allow a smaller ground coverage ratio (GCR) than square or rectangular aperture. Tessellation of multiple mirrors permits creating reflector apertures of rectangular shapes and hence, permit better ground coverage ratio.
  • Mitigating Effect of shading- Multiple mirrors also reduce the ill effect of shading by distributing the shading effect uniformly over a large part of the array.
     
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