9.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Ocean thermal energy conversion | 9/10 | https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion | reference | science, encyclopedia | 2026-05-05T07:35:54.854158+00:00 | kb-cron |
Warm water mass flow rate,
M
w
˙
=
M
T
m
w
˙
˙
{\displaystyle {\dot {M_{w}}}={\dot {M_{T}{\dot {m_{w}}}}}\,}
Cold water mass flow rate
M
c
˙
=
M
T
m
C
˙
˙
{\displaystyle {\dot {{\dot {M_{c}}}={\dot {M_{T}m_{C}}}}}\,}
=== Closed Anderson cycle === As developed starting in the 1960s by J. Hilbert Anderson of Sea Solar Power, Inc., in this cycle, QH is the heat transferred in the evaporator from the warm sea water to the working fluid. The working fluid exits the evaporator as a gas near its dew point. The high-pressure, high-temperature gas then is expanded in the turbine to yield turbine work, WT. The working fluid is slightly superheated at the turbine exit and the turbine typically has an efficiency of 90% based on reversible, adiabatic expansion. From the turbine exit, the working fluid enters the condenser where it rejects heat, -QC, to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, WC. Thus, the Anderson closed cycle is a Rankine-type cycle similar to the conventional power plant steam cycle except that in the Anderson cycle the working fluid is never superheated more than a few degrees Fahrenheit. Owing to viscosity effects, working fluid pressure drops in both the evaporator and the condenser. This pressure drop, which depends on the types of heat exchangers used, must be considered in final design calculations but is ignored here to simplify the analysis. Thus, the parasitic condensate pump work, WC, computed here will be lower than if the heat exchanger pressure drop was included. The major additional parasitic energy requirements in the OTEC plant are the cold water pump work, WCT, and the warm water pump work, WHT. Denoting all other parasitic energy requirements by WA, the net work from the OTEC plant, WNP is
W
N
P
=
W
T
−
W
C
−
W
C
T
−
W
H
T
−
W
A
{\displaystyle W_{NP}=W_{T}-W_{C}-W_{CT}-W_{HT}-W_{A}\,}
The thermodynamic cycle undergone by the working fluid can be analyzed without detailed consideration of the parasitic energy requirements. From the first law of thermodynamics, the energy balance for the working fluid as the system is
W
N
=
Q
H
−
Q
C
{\displaystyle W_{N}=Q_{H}-Q_{C}\,}
where WN = WT + WC is the net work for the thermodynamic cycle. For the idealized case in which there is no working fluid pressure drop in the heat exchangers,
Q
H
=
∫
H
T
H
d
s
{\displaystyle Q_{H}=\int _{H}T_{H}ds\,}
and
Q
C
=
∫
C
T
C
d
s
{\displaystyle Q_{C}=\int _{C}T_{C}ds\,}
so that the net thermodynamic cycle work becomes
W
N
=
∫
H
T
H
d
s
−
∫
C
T
C
d
s
{\displaystyle W_{N}=\int _{H}T_{H}ds-\int _{C}T_{C}ds\,}
Subcooled liquid enters the evaporator. Due to the heat exchange with warm sea water, evaporation takes place and usually superheated vapor leaves the evaporator. This vapor drives the turbine and the 2-phase mixture enters the condenser. Usually, the subcooled liquid leaves the condenser and finally, this liquid is pumped to the evaporator completing a cycle.
== Environmental impact == Carbon dioxide dissolved in deep cold and high pressure layers is brought up to the surface and released as the water warms. Mixing of deep ocean water with shallower water brings up nutrients and makes them available to shallow water life. This may be an advantage for aquaculture of commercially important species, but may also unbalance the ecological system around the power plant. OTEC plants use very large flows of warm surface seawater and cold deep seawater to generate constant renewable power. The deep seawater is oxygen deficient and generally 20–40 times more nutrient rich (in nitrate and nitrite) than shallow seawater. When these plumes are mixed, they are slightly denser than the ambient seawater. Though no large scale physical environmental testing of OTEC has been done, computer models have been developed to simulate the effect of OTEC plants.
=== Hydrodynamic modeling === In 2010, a computer model was developed to simulate the physical oceanographic effects of one or several 100 megawatt OTEC plant(s). The model suggests that OTEC plants can be configured such that the plant can conduct continuous operations, with resulting temperature and nutrient variations that are within naturally occurring levels. Studies to date suggest that by discharging the OTEC flows downwards at a depth below 70 meters, the dilution is adequate and nutrient enrichment is small enough so that 100-megawatt OTEC plants could be operated in a sustainable manner on a continuous basis.
=== Biological modeling === The nutrients from an OTEC discharge could potentially cause increased biological activity if they accumulate in large quantities in the photic zone. In 2011 a biological component was added to the hydrodynamic computer model to simulate the biological response to plumes from 100 megawatt OTEC plants. In all cases modeled (discharge at 70 meters depth or more), no unnatural variations occurs in the upper 40 meters of the ocean's surface. The picoplankton response in the 110 - 70 meter depth layer is approximately a 10–25% increase, which is well within naturally occurring variability. The nanoplankton response is negligible. The enhanced productivity of diatoms (microplankton) is small. The subtle phytoplankton increase of the baseline OTEC plant suggests that higher-order biochemical effects will be very small.
=== Studies === A previous Final Environmental Impact Statement (EIS) for the United States' NOAA from 1981 is available, but needs to be brought up to current oceanographic and engineering standards. Studies have been done to propose the best environmental baseline monitoring practices, focusing on a set of ten chemical oceanographic parameters relevant to OTEC. Most recently, NOAA held an OTEC Workshop in 2010 and 2012 seeking to assess the physical, chemical, and biological impacts and risks, and identify information gaps or needs. The Tethys database provides access to scientific literature and general information on the potential environmental effects of OTEC.
== Technical difficulties ==