Environmental Engineering Reference
In-Depth Information
Table 2.1
Performance of parabolic trough and power tower systems. (Source: http://www.eere.
energy.gov/consumerinfo/pdfs/solar_overview.pdf)
Parabolic trough
Power tower
Size
30 - 320 MW
10 - 200 MW
Operating temperature ( ° C/ ° F)
390/734
565/1049
23 - 50%
a
20 - 77%
a
Annual capacity factor
Peak effi ciency
20%(demonstrated)
23%(predicted)
Net annual effi ciency
11(demonstrated) - 16%
7(demonstrated) - 20%
Status
Prototype
Demonstration
Storage available
Limited
Yes
Hybrid designs
Yes
Yes
a
The top end capacity factors are for systems including substantial thermal storage.
The primary advantage of these adaptations is to the capability to supply power when the
sun does not shine, in other words to be dispatchable.
A smaller scale approach, suitable for units in the range 10-50 kW, is possible using para-
bolic concentrators. These also work at high temperatures of 600-1000 °C and might be
suitable for distributed electricity generation.
Thermal - to - electric effi ciency is in the range of 20-40% depending on the design, with the
resulting overall solar - to - electricity conversion effi ciency in the range 13-25%.
Table 2.1 (adapted from material available from the website given in Reference [14] when
it was accessed in January 2006) compares the performance of the parabolic trough and power
tower systems.
2.6 Tidal Power
2.6.1 The Resource
The moon and sun's gravitational fi elds cause the natural rise and fall of coastal tidal waters.
Since the moon is closer to the earth, albeit much less massive, it has a dominant effect upon
tides. As the moon is 2.2 times more infl uential than the sun, it could be considered that tidal
energy is mostly a form of lunar energy!
The earth rotates on its axis once every 24 hours. In the earth's frame of reference the sun
orbits the earth once every 24 hours. The moon orbits the earth once every 29 days approxi-
mately. In the earth's frame of reference, the moon appears to orbit the earth once every 24
hours and 50 minutes. This difference in periods between the apparent orbits of the sun and
moon leads to phase changes with larger
spring tides
during in-phase behaviour and smaller
neap tides
when the sun and moon are out of phase. Spring tides can be twice as large as
neap tides.
A 1991 study commissioned by the EU estimated that the technically feasible energy
resources from tidal barrages across the EU could be as much as 105 TW h/year (from 64 GW
of installed capacity). This resource is unevenly distributed across Europe with the UK
