By Georges MOUGN -W.P.I
Tabular iceberg transfer is now becoming feasible, thanks to new satellites data and means developed by the oil offshore industry.
Control of iceberg melting is performed by an OTEC open cycle plant but without cold water pipe, named ITEC.
Lower investment cost and a better efficiency allows a kwh cost of one third of the equivalent OTEC kwh.
Tabular icebergs from the Antarctic are potential sources of fresh water and power.
Transfer of these large bodies of ice, up to 300 million tons, contemplated twenty years ago, was then postponed due to the lack of information on the bathymetry and on transient currents in real time.
These informations are now available, brought by satellites like TOPEX POSEIDON !V JASON !V ERS II and soon ENVISAT.
Most obvious uses for melted iceberg water is to augment supplies for both urban and agricultural uses.
In fact should iceberg's delivery be established, entirely new areas may be in time developed at formerly inhospitable locations.
Fresh water from iceberg also appears as a major possibility in combating rising saline water tables as experienced in many areas.
In all instances iceberg's exploitation requires the conversion of ice into water.
Solar radiation !V 5 to 8 109 J m-2 yr-1 can only melt in one year 6 to 12 m of ice thickness (with albedo attenuation) and even less in foggy areas like the PERU and North CHILE shores.
A larger yield is needed, and melting should be controlled.
An iceberg of 1011 kg represents through the ice latent heat quite a large cooling power !V 3.35 x 1016 J.
One use of it would be its conversion to electrical energy through a process similar to the OTEC open cycle.
By analogy it has been named ITEC for Iceberg Thermal Energy Conversion.
ITEC process is a new concept which has to be valided by experiments of condensation directly on ice to optimize ice chips size and the compressor for the gas released by the ice.
ITEC should present some advantages in comparison with OTEC:
|Investment cost is reduced !V Cold Water Pipe and pump are avoided|
|Cold water pump energy saved|
|Efficiency is improved by greater temperatures' difference between warm and cold water|
|Steam condensation taking place directly on a slurry of ice at 0¢FX.|
|Fresh water produced by steam condensation is directly added to the flow from the condenser without heat exchanger.|
The plant is similar to an open cycle OTEC plant but for the cold source.
A slurry of ice is produced in a centrally formed pond on top of the iceberg.
Iceberg top above sea level allows the condenser to be feeded by gravity. The plant being on a barge moored along the berg.
Ice volume is 10% air, to be released in the condenser, but the mass of ice to be introduced in it is 6.25% of the mass of cold sea water, warmed of 5¢FX, for the same cooling power.
Quantity of gas from the ice should be of the same order that the gas released from the sea water under 13 mbar.
If not, the compressor size has to be adjusted but the power requirement should not be increased significantly in regard of the energy gross output.
|No cold sea water pipe|
|No cold water pump|
|Simple condenser without heat exchanger|
These reduce the cost of the plant by 30 to 40%.
|Energy used by the cold water pump is about 15% of the gross energy output. Without this pump ITEC ratio net to gross power is increased by 1.23 !V (0.80 instead of 0.65).|
|Condensation is on ice at temperature near 0¢FX instead of 9¢FX in OTEC condenser. The temperatures' difference and Carnot efficiency are increased by 1.69 (ITEC difference of 22¢FX instead of OTEC 13¢FX) ; on the basis of cold water at 4¢FX warmed up of 5¢FX for condensation and warm water at 26¢FX cooled down of 4¢FX for evaporation.|
|Together for the same quantity of evaporated warm water, net energy output is more than double (1.23 x 1.69).|
Kwh Unit Cost
-Lack of maintenance for cold water pipe and its pump
-Lack of cleaning of the sea water biofilm in the condenser since only soft water is in it.
|The ITEC plant investment is between 0.6 and 0.7 of the OTEC one.|
|The energy output being double, the unit cost ot kwh ends up at one third of the OTEC one.|
|Some savings on unit cost are also provided by:|
ITEC PLANT CAPACITY
A medium sized iceberg, whose transfer is economically feasible by the water value, represents a mass of ice of 108 metric tons.
For its utilization in 2 or 3 years 5,000 or 3,400 tons of ice have to be melted every hour.
If entirely melted through an ITEC plant its capacity would be 25 or 17 MW.
Added water production from sea water condensation would be 750 or 500 m3 per hour.
Icebergs are important resources of fresh water and power - ITEC being the process to combine both productions.