Page 164 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

152 Fundamentals of Ocean Renewable Energy

6.3.5 Additional Benefits of OTEC Power Plants
A particular advantage of open cycle OTEC power plants is that they produce
desalinated water, which could be particularly beneficial in many island states
in the Caribbean and Pacific where there is a good OTEC resource and a paucity
of fresh water. In addition, the cold deep water used in both closed cycle and
open cycle OTEC plants can be introduced into air conditioning systems, after
the water has been used to facilitate condensation. Finally, deep ocean water is
rich in nutrients, and after desalination can be a useful product for agriculture
and aquaculture.

6.3.6 Environmental Impacts

A large OTEC power plant would require the transport of significant quantities
of warm surface water and cold water from around 1000 m depth. In the deep
water environment, the cold water tends to be rich in nutrients, and so there
could be direct consequences for deep water marine organisms due to the de-
pletion of nutrient-rich water from this environment. The plant operation could
alter the stratification of the water column, and this could influence the natural
process of upwelling of nutrients, affecting a range of marine species. Further,
the discharge of this colder water into warm surface waters could influence
marine life in the vicinity of the power plant, for example, it is suggested that
◦
‘exhaust’ water at 3 C below ambient could trigger algal blooms [14]. The warm
and cold water inlets of an OTEC power plant will include screens to prevent
debris and larger species from entering the power plant, and this could directly
influence marine life that become entrained in the screening system.

6.4 SALINITY GRADIENTS
When there is large difference in salinity between two fluids, for example, where
a river flows into the sea, it is possible to harness the energy of the salinity
gradient to generate electricity. The global potential for salinity gradient power
is surprisingly large—the global technical resource has been estimated by the
IRENA as 647 GW, which would represent almost 25% of global electricity
consumption [17]. The resource is largely concentrated in South America and
Asia, but there is also a considerable resource in North America (Fig. 6.6). The
Amazon River alone contains up to 20.8 GW of potential, and the Yangtze up to
2.29 GW (Table 6.2). The resource could be higher still if salinity gradient power
were extended to include ‘hybrid’ applications, for example, by exploiting very
large salinity gradients that exist between the brine ‘waste’ of desalination plants
and the ambient water.

6.4.1 Technology Types
There are two salinity gradient technologies: pressure retarded osmosis (PRO)
and reversed electro dialysis (RED).