Environmental Engineering Reference
In-Depth Information
spots. The separated liquid fraction is sent back at the inlet of the solar field by a
recirculation pump and is mixed with feedwater at 260 C. The main drawbacks of
the DSG configuration are the absence of available storage suitable for the steam, and
temperature control in the SH section.
14.5.2 Rankine cycle performance
When a Rankine cycle is adopted for power conversion, the power section efficiency
is defined as:
W gross , tur W pumps
W net
W aux
η PS =
h HTF , out ) =
(14.5.2)
· m HTF ·
· m HTF ·
( h HTF , in
( h HTF , in
h HTF , out )
where W gross , tur is the power output generated in the steam turbine [W], W pumps
accounts for pump (feedwater and condensate) consumptions, W aux considers aux-
iliaries consumption as condenser fan, · m HTF is the HTF mass flow rate [kg/s] and h HTF
are the enthalpies of the heat transfer fluid at the inlet and outlet of the boiler. In case of
direct steam generation, the denominator will be · m steam ·
h water , in ), where
m steam is the steam generated in the solar field [kg/s], h steam , TV is the enthalpy at the
outlet of the solar field (i.e. turbine inlet) and h water , in is the enthalpy of the water after
the feedwater section (i.e. solar field inlet).
Thermodynamic conditions and the performance of different power cycles at
design conditions are summarized in Table 14.5.3 and Table 14.5.4. The examples
given reproduce existing plant beside indirect molten salts; this is because there is no
stand-alone plant running with this HTF.
Molten salts can work at higher temperature, increasing the power cycle efficiency
by 12% compared to conventional synthetic oil configurations. Direct cycle config-
uration with superheating can achieve almost the same efficiency owing to higher
temperature at the turbine inlet, whilst it is penalized for the absence of RH. Lastly,
saturated steam cycle in direct configuration is penalized from an efficiency point of
view by about 3% points versus commercial synthetic oil.
A comparison of the yearly power block efficiency is more difficult since, besides
cycle configuration and HTF, this depends also on condensing technology and on
thermal storage size: the bigger the thermal storage, the closer power block efficiency
is to the nominal working conditions at constant thermal input. Condensing technology
strongly affects power block efficiency as, during the summer when solar radiation is
higher, ambient temperature also increases, together with the condensing temperature.
An evaporative tower is less penalized than an air-cooled condenser, since the wet bulb
temperature is more constant than that of the dry bulb, in particular in sites with low
relative humidity, such as deserts.
( h steam , TV
14.5.3 Stirling cycle
The Stirling engine is a closed cycle that has been designed for small solar power
applications (from a few kW up to 100 kW). It has a potential for high cycle efficiency,
the ideal Stirling cycle equalling the efficiency of the Carnot cycle. Efficiency is one
 
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