Building integrated concentrating solar systems - Solar Energy Sciences and Engineering Applications - page 556

17.2 BUILDING INTEGRATED CONCENTRATING SYSTEMS

In the field of Building Integrated Concentrating Solar systems (BICS), solar thermal
as well as Photovoltaics (PVs) are included. First, some previous concepts regarding
solar concentration are introduced ahead of the presentation of BICPV systems. Some
of the configurations presented (e.g. Fresnel concentrators) can also refer to a Building
Integrated Concentrating Solar Thermal (BICST) system, since the receiver CPV (or
CPVT) can be replaced by a Concentrating Thermal (CT) unit. Later, some represen-
tative BICST technologies are included. Lastly, building integration requirements for
concentrating systems are summarized.

17.2.1 Physics of concentrating solar system
17.2.1.1 Why solar concentration?

The general concept of the PV solar concentrator is to reduce the amount of expensive
solar cell by low-cost optical material. The sunlight either focused to a point or to a
line is reflected by or refracted through an optical element to increase the solar flux at
the solar cell, thus the electrical power of the system. The solar flux at the solar cell can
be increased by light trapping using the total internal reflection using polymer material
which has properties like glass. A PV solar concentrator increases insolation intensity at
the PV surface, reducing the area of photovoltaic material required per unit of power
output. A cost reduction can be achieved for the overall photovoltaic/concentrator
system when the concentrator cost is lower than the displaced PV material cost. In
the case of thermal receivers, the use of solar concentration enables the attainment
of higher working fluid temperatures. This leads to the more effective use of thermal
concentrating systems for some applications, such as solar cooling with double-effect
absorption chillers or concentrating solar power with higher-efficiency cycles. Optical
concentrators can be reflective, refractive, diffractive or a combination of these.

17.2.1.2 The concentration ratio of a CPV system

The concentration ratio determines the increase in relative radiation at the surface of
the exit aperture/absorber. The concentration ratio can be defined in several ways,
as described below.

17.2.1.2.1 Geometric concentration ratio
The geometric concentration ratio is defined as the ratio of the area of aperture to the
area of the receiver (Duffie and Beckmann, 1991), i.e. C

A a /A r . This ratio has an
upper limit that depends on whether the concentrator is three-dimensional, such as a
paraboloid, or two-dimensional, such as a compound parabolic concentrator. In terms
of the half acceptance angle, the concentration ratio is defined as (Rabl, 1976b):

=

⎫
⎬

1
sin θ s

C = for a two-dimensional system

1
sin θ s

2 (17.2.1)

⎭

C = for a three-dimensional system