Objective
The main objective of this project is the utilization of low enthalpy geothermal energy as the heating medium in a 250 m3/day sea-water desalination plant on Kimolos Island, Cyclades. The innovative aspect of this project is the actual use of low enthalpy geothermal energy as the heating medium in a multi-effect distillation (MED) sea-water desalination plant with evaporation under vacuum at low evaporation temperature (approx. 70 deg. C) in vertical tubes (MED-VT). The unit will be designed to produce 68450 m3 of fresh desalinated water annually (Load Factor of 75%). The exploitation of low enthalpy geothermal energy in this application will result in the production of 16,43 billion Kcal/year which is equivalent to 2053 TOE/year.
The project aim is to ensure, for isolated area such as islands, at a cost effective and environmentally safe way tap water production.
The drilling of borehole Prassa-1 commenced on about the 20th of August 1995 and was drilled to a depth of 186m, where temperatures of 60 to 62% were measured. Results from pumping tests (24 hour-test) are very promising showing that at borehole Prassa-1 over 100 m3/h can be pumped and that the composition of the geothermal water is very similar to that of the sea-water. In the first week of September 1995, borehole Prassa-2 will be drilled to a depth of 235 m and the whole subcontract to IGME will be completed by the end of September 1995. In our design phase we are already considering and examining 3 different scenarios for the final utilization scheme, depending on the well head temperature of our geothermal fluids:
1) if the geothermal fluid has a temperature of over 62 deg. C then desalination of seawater will be achieved by direct heat exchange;
2) When the geothermal fluids have well-head temperatures of under 62 deg. C two scenarios will be investigated: either a) The temperature level will be raised by the use of an electrically driven absorption heat-pump from below 62 deg. C (for example 56 deg. C) to over 70 deg. C (COP of 2-2,5) and then the sea water will be desalinated; or b) if the geothermal fluid composition will allow it then the geothermal water will be desalinated itself. The scenario 2b is the most efficient and inexpensive.
The proposed sea-water desalination unit on Kimolos island will utilize low enthalpy geothermal energy as the heating medium. This Multi-effect Distillation Vertical Tube (MED-VT) unit is expected to produce 250 m3/day or 10.42 m3/h of freshwater (TDS less than 5 ppm). The virtual use of low enthalpy geothermal energy as the heating medium in a MED-VT desalination unit is entirely innovative, since until now this method has been performed only with other, mostly conventional energy sources.
The unit design provides for the drilling of two (2) production wells at Prassa site of Kimolos Isl. (NNE region of Kimolos Isl), drilled to a depth of 250-300 m, producing 150 m3/h of geothermal water with a well-head temperature of 70 to 75 deg. C, providing the necessary thermal load (approx. 2.500.000 Kcal/h).
The proposed MED-Vertical tube method is based on the multi-effect distillation rising film principle at low evaporation temperatures (less than 70 deg. C). The rising film principletakes advantage of the fact that the inner tube surfaces are always covered with a thin film of feed water which prevents scale formation.
The working principle is as follows : A controlled amount of sea feed water (ca. 225 m3/h) is led into the bottom of each effect and into the tubes of the heat exchanger, where the low enthalpy geothermal energy, in the form of hot water of 75 deg. C, as the heating medium, heats it up. Part of the sea water is evaporated under vacuum, which is created by means of water ejectors connected to each effect. The vacuum makes it possible to lower the boiling temperature and thereby minimizing the amount of energy necessary for evaporating the feed water, and futhermore the low boiling temperature prevents deposits of scale inside the tubes.
The vapour generated in the first effect passes a separation compartment where the remaining feed water droplets are separated from the vapour and are extracted with the brine. The separated vapour leaves the first effect and flows through the vapour connection pipe to the heat exchanger of the second effect, and the vapour is now used as the heating medium for the second effect. Brine extracted from the first effect is mixed sea feed water and is brought to evaporation by means of the heat from the vapour generated in the first effect. This process is repeated in the second and third effect. The heated vapour condenses on the outside of the heat exchanger tubes and flows into the flash tank, where it is extracted by the freshwater pump.
The use of low enthalpy geothermal energy in this unit reduces the annual operational costs of the unit to 1/4 of the cost for the unit operation with conventional energy sources.
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Coordinator
190 Pikermi
Greece
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