Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 142

4.6 SOLAR ENERGY COLLECTOR AND STORAGE
FOR THERMAL POWER GENERATION
Parabolic troughs, the most deployed widely solar energy concentrator, consist usually
of long curved mirror-surfaced troughs, which concentrate direct insolution on a tube
at the focal axis of a parabolic mirror (Fernandez-Garcia et al., 2010). Parabolic
trough concentrators are made usually of back-silvered glass for high reflectance and
durability. A stainless steel tube receiver is usually coated with a highly solar selective
absorbing ceramic and metal blend that is durable at high temperatures material. The
absorber is surrounded usually covered by a geometrically concentric and evacuated
borosilicate glass tube envelope.
In a central receiver system a large array of heliostats mirrors individually tracks
the sun to reflect insolation onto a fixed receiver mounted on a tower, that absorbs the
heat. The heated fluid (usually molten salt) convectively removes the receiver's heat
energy and is then transported from the receiver to drive a turbine generator or stored.
Parabolic dish concentrators reflect solar energy onto a receiver mounted at the
focal point. Parabolic dishes typically use multiple curved reflective panel segments
made of glass or laminated films. These concentrators are mounted on a two-axis solar
tracking system. The concentrated sunlight at the receiver may be utilized directly by
a cycle heat engine mounted on the receiver, or simply heats a fluid that is transported
for storage.
Using multiple smaller engines means: (i) smaller engines can be replaced readily
(ii) a plant can deliver close to rated power while engines are being repaired, and
(iii) the system can be easily expanded by adding modules to accommodate growth.
The thermal efficiency of an engine is proportional to the difference between
the maximum collector and heat rejection temperatures. Most real engines operate
with efficiencies of just over half of the ideal Carnot efficiency. Whilst engine effi-
ciency is higher at increased operating temperature, the efficiency of a solar collector
decreases as its operating temperature increases. The optimal operating temperature
thus depends on the particular efficiency trends of the specific engine and collector
employed. Typically organic Rankine cycle or Sterling engines are used with evacuated
tubes in parabolic troughs or two axis tracking parabolic dishes respectively.
Rankine and Brayton cycles both have constant-pressure heat-addition processes
readily applicable to solar heating. Stirling engines use a reciprocating piston design
can be solar heated directly.
A working fluid can pass through the absorber directly, or there can be an inter-
mediate heat-transfer fluid flowing in a closed loop between the absorber and a heat
exchanger to heat another fluid via a heat exchanger. Incorporating an intermedi-
ate heat-transfer fluid requires another pump, heat exchanger as well as two fluids.
Utilizing an intermediate heat-transfer fluid a lower vapor pressure reduces the
receiver's mass and obviates the need for high-pressure piping.
4.7 OVERALL SYSTEM OPTIMIZATION
Solar water heating systems can be designed to meet hot water heating (DGS, 2005),
industrial process heating (Gordon and Rabl, 1982); Kalogirou, 2003; Kulkarni et al.,