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Solar Thermal Desalination

Desalination of sea and brackish water to produce fresh water for domestic and industrial uses is becoming increasingly important in many places around the world. Presently, the major desalination processes in use are Reverse Osmosis (RO), Multi-Effect Distillation (MED), and Multi-Stage Flash (MSF).

Electrically-driven RO is typically used in small to medium-scale water production facilities.  Thermally-driven MED and MSF use low to moderate grade heat, and are used in most of the medium to large-scale systems installed today.

THERMOFLEX together with PEACE can model these desalination processes at the design-point and at off-design conditions.  Model results include detailed thermodynamic states, performance metrics, and installed system cost estimate.

The THERMOFLEX model below was supplied courtesy of Solar Power Group, GmbH.  It models a water and power producing plant in the coastal Surt region of Libya, owned by the Libyan Ministry of Energy.  Nominally, this plant produces 15,000 m3 per day (3 MIGD) of potable water and nominally produces 15 MWnet electric power.

Heat input is provided by the combination of a solar field and natural gas-fired boiler, installed in parallel.  This solar-fossil hybrid design leverages the site’s sunny climate and its proximity to natural gas fields.  The hybrid design can produce power and water 24 hours per day without a solar storage system.  The desalination system condenses steam turbine exhaust and produces potable water from seawater.  It takes the place of what would be a steam condenser in a pure power plant.

The solar field uses Linear Fresnel Collectors (LFC) with direct steam generation (DSG).  It consists of three sections; one to preheat water, one to evaporate water, and the final section to superheat steam.  The evaporator is designed to produce 30% quality steam.  A steam drum separates the phases; liquid recirculates to evaporator inlet, and dry steam flows to the superheater field.

A single-casing steam turbine operates at high speed with a reduction gear coupled to a synchronous generator.  Nominal turbine inlet steam conditions are 55 bar, 400 C.  Three steam extractions provide steam to a deaerator and two higher-pressure feedwater heaters.  Water is preheated to about 190 C before the economizer section of the solar field.  Feedwater to the gas-fired boiler comes directly from the feedpump.

The desalination system is comprised of two parallel units, each having a nominal capacity of 7,500 m3 per day (1.5 MIGD). The MED design has a nominal performance ratio of 10.4 (gain output ratio of 9.4).  This high level of efficiency is achieved by using twelve effects over a temperature range of 30 C.