A.3 Further opportunities
507
on the temperature difference between the warm surface water of a maximum of 22 to approximately 28 °C and the cold deep waters of approximately 4 to 7 °C (Fig. A.9). The efficiency of such a cycle depends on the useful temperature difference. Due to the small available temperature differences of a maximum of about 20 K, these power plants can only achieve very low efficiencies that vary between 1 and a maximum of 3 %. In addition, for this electricity generation method enormous volumes of water need to be circulated. Furthermore, the water needs to be transferred from deep sea layers to the ocean surface and vice-versa. The design of these plants is thus very expensive. To keep energy transfer to the customer to a minimum, ocean thermal energy conversion plants are mainly built near the coastline. In parallel, an easy access to cold deep water is required to run such a cycle. As the energy yield is relatively low when compared to the large mass or water flows to be transferred, this technology, generally referred to as OTEC (Ocean Thermal Energy Conversion) has, to date, not been applied due to economic aspects. Since there still remain unsolved technical problems with regard to the successful operation of such ocean thermal energy conversion plants, their application in the sunny parts of our planet cannot be expected within the years to come.
Warm water inflow
Warm water inflow
Electrical energy
High pressure NH3-steam Water discharge
Low pressure NH3-steam
25 °C
30 m diameter
20 °C
Turbogenerator
7 °C 10 °C
Evaporator
Condenser
700 – 1,000 m
10 °C 10 m diameter
10 °C
23 °C High pressure liquid NH3 Warm water discharge
Cold water inflow
Pump
Low pressure liquid NH3
5 °C Cold water inflow
7 °C
Cold water discharge
Fig. A.9 Example of an OTEC plant (left: layout; right: power plant cycle; according to /A-1/)
A variety of this technology suitable for geographical areas, where both cold ocean currents and geothermal energy are available, has only lately been introduced. This method benefits from the temperature differences between warm areas near volcanic structures and cold ocean water. However, it is unlikely that this method will be put into practice in the near future due to considerable technical problems.