When ηopt < 1, Equation (2) becomes
ηrec = 1 - σ(TR,H^4 - TL^4 )/ICηopt. (6)
This is the receiver efficiency when conduction and convection losses are negligible. The engine
efficiency must account for actual losses in its various components and the generator efficiency.
Equation (3) therefore becomes
ηeng = (1 - TL/TE,H)(1 - Leng)ηgen , (7)
where
Leng = combined fraction of all the component and parasitic losses
ηgen = generator efficiency.
Note that in a real system, the effective receiver reradiation temperature TR,H and the upper
engine temperature TE,H are usually not the same.
An expression for real system efficiency is obtained by introducing Equations 5, 6, and 7 into
Equation 4.
Typically, as the engine size increases, ηeng also increases, and its specific cost (dollars per
kilowatt) is reduced. On the other hand, the high concentration required to maintain high power
conversion efficiency during operation at high temperatures (Figure 73 and Equations 1, 2, and
3) is more difficult to attain in large systems, and it is associated with optical efficiency
reduction. This difficulty is compounded by the continuous change of the sun’s location, which
causes the annual-average components and system efficiencies to be significantly lower than the
respective design point efficiencies. Innovations that could alleviate or resolve the apparent
mismatch between the optimum size and configuration of the optical and power conversion
components in a solar thermal system are vital in achieving the cost reduction required for
competitiveness.
The traditional approach of the solar community has been that large-volume production is the
key to cost reduction. Although this might be true, the development and utilization of lower-cost
materials, implementation of more efficient methods, and improvement of key components can
also lead to significant cost reduction.
Three optical configurations developed for concentrated solar thermal systems are as follows:
line focus systems, central receiver systems, and on-axis tracking systems.
Line Focus Systems. In line focus systems, sunlight is “folded” in one direction — from a
plane to a line. In most cases, the optical configuration is that of a trough tracking the sun from
east to west and a target that rotates accordingly (see Figure 74); recently, linear Fresnel
reflectors with stationary target have also been used (Mills et al. 2004). The heat transfer fluid is
heated in the tubular receiver. It then provides thermal energy to drive a Rankine cycle and