Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 439

temperatures. In particular, the yearly optical efficiency of two-axis tracking is more
constant than single-axis collectors. On the other hand, the very same aspects penalize
this configuration from a cost point of view and land occupation of the solar field.
The concept of point focus is to transfer the collected solar radiation to a fluid; the
thermal power produced is then converted into electricity by a thermodynamic cycle.
Considering the very high concentration ratio, the thermal losses of central receiver
systems are usually less important than optical losses when compared to linear focus
systems. On the contrary, since heat fluxes are significant (500-1000 kW/m 2 ), heat
transfer phenomena on the receiver, together with transient conditions, become more
important and must be carefully investigated.
Point focus technologies can be divided into two main categories featuring different
power outputs: (i) the solar tower (also named power tower) concept for net power
output above 10 MW; and (ii) the solar dish or dish Stirling system which has a power
output of up to 200 kW.
Since all of the solar radiation is collected at the same point, there is no necessity
for a piping system as in the trough configuration. In particular, in the solar dish
configuration the conversion from heat to electricity is performed directly at the focus
of the collecting systems, while with power towers, thermal power is transferred to a
fluid and then converted to electricity at the bottom of the tower.
By the end of 2012, very few solar plants based on point focus technology were
operating worldwide, producing a total installed capacity close to 40 MW, with another
17 MW under construction in Spain and 110 MW in Nevada, US (NREL). Over the
next few years the construction of 12 new heavy-duty plants in the United States and
Spain will yield a total installed capacity of more than 1.5 GW. The most important
power tower plants are the PS-10 and PS-20 as well as Gemasolar plants in Siviglia
(E), though there are several plants under construction, in particular in the United
States. With regard to dish Stirling technology, there is just one plant running in the
US, producing a total output of 1.5 MW. It must be noted that dish Stirling holds the
record for solar-to-electricity efficiency, with a value of 31.25%.
From these numbers, it can be noted how this technology is still under development
and far behind linear focus technology.

14.3.13 Central receiver systems
Central receiver systems, sometimes named power tower systems, are based on several
sun-tracking mirrors (usually called heliostats) which reflect incidence radiation onto
a receiver. The concentration ratio of this type of technology, defined as receiver area
divided by mirrors area, is usually in the range of 500-1000 suns.
The CR in this particular case is defined as:

total mirror aperture area
receiver area

n · A mirr

A rec

CR = = (14.3.15)

where n is the number of heliostats and A the area of each heliostat [m 2 ].
Since there are few operating plants based on central receiver technology, most of
the example described will refer to the PS-10 plant in Spain.