SOLAR DESALINATION

The facilities included are:


SOL-14 SOLAR THERMAL SEAWATER DESALINATION PLANT

This facility is composed of the following subsystems:

  • A 14 stage multi-effect distillation plant
  • A field of stationary CPC (compound parabolic concentrator) solar collectors
  • A solar thermal water storage system
  • A double effect (LiBr-H2O) absorption heat pump
  • A water-tube gas boiler

The multi-effect distillation unit is made up of 14 stages or effects, arranged vertically with direct seawater supply to the first effect. At a nominal 8 m3/h feedwater flow rate, the distillate production is 3 m3/h, and the thermal consumption of the plant is 190 kWt, with a performance factor (number of kg of distillate produced per 2326 kJ of thermal energy consumed) over 9. The saline concentration of the distillate is around 5 ppm. The nominal temperature gradient between the first cell and the last is 40ºC with a maximum operating temperature of 70ºC in the first cell.

The system heat transfer fluid is water, which is heated as it flows through the solar collectors to the storage system. The hot water from this storage system provides the MED plant with the thermal energy required for its operation. The solar field is composed of 252 stationary solar collectors (CPC Ao Sol 1.12x) with a total surface area of 500 m2, arranged in four rows of 63 collectors. The maximum working temperature is 100ºC since the collectors are connected to atmospheric pressure storage tanks in an open loop. The thermal storage system consists of two water tanks connected to each other for a total storage capacity of 24 m3. This volume allows the operation sufficient autonomy for the backup system to reach nominal operating conditions.

The double effect (LiBr-H2O) absorption heat pump is connected to the last effect of the MED plant. The low-pressure saturated steam (35ºC, 56 mbar) produced in this cell supplies the heat pump evaporator with thermal energy, which would otherwise be discharged to the atmosphere, cutting in half the thermal energy consumption required by a conventional multi-effect distillation process. The fossil backup system is a propane water-tube boiler with a 10‑bar saturated steam production capacity of 200 kg/h. This boiler ensures heat pump operating conditions (180ºC, 10 bar), as well as operating the MED plant in the absence of solar radiation.

STATIONARY SOLAR COLLECTOR TEST PLATFORM

This facility was built in 2002 for the purpose of offering additional services to the scientific research community, among which was the possibility of carrying out energy characterization of the stationary solar collectors, with special emphasis on its possible application to solar desalination processes.

2
General diagram of the stationary solar collector test platform
1
SOL-14 plant stationary solar collector field (500 m²)

The facility has three independent hydraulic circuits. In the primary fluid circuit (water or water mixed with antifreeze) is heated as it flows through the solar collector, delivering the energy acquired to the water deposited in a storage tank. In the secondary circuit, the water from the tank is pumped to the heat exchanger where its energy is transferred to the tertiary circuit. This heat exchanger simulates the hot water inlet in the first cell of a multi-effect distillation plant. Finally, the water that circulates through the tertiary circuit goes to a cooling tower where the energy from the secondary circuit is discharged into the atmosphere.