This facility is composed of the following subsystems:

  • A 14-stage multi-effect distillation (MED) plant
  • A field of stationary large-size flat plate solar collectors
  • A water-based solar thermal storage system
  • A double effect (LiBr-H2O) absorption heat pump
  • A fire-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 (forward feed configuration). 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 kWth, with a performance ratio (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 one is 40°C with a maximum operating temperature of 70°C in the first cell. The system heat transfer fluid is water, which is heated as it flows through the solar collectors and energy collected is then transferred 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 (AQUASOL-II) is composed of 60 stationary flat plate solar collectors (Wagner LBM 10HTF) with a total aperture area of 606 m2 and is connected with a thermal storage system (40 m3) through a heat exchanger (More details about the solar field are supplied within its specific subsection).

The double effect (LiBr-H2O) absorption heat pump is connected to the last effect of the MED plant. The low-pressure saturated steam (35°C, 56 mbar abs) generated in this last effect supplies the heat pump evaporator with the thermal energy required at low temperature, which would otherwise be discharged to the environment, 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 that ensures the heat pump operating conditions (saturated steam at 180°C, 10 bar abs), as well as operating the MED plant in the absence of solar radiation.

Figure 29. The PSA SOL-14 MED Plant (a), double-effect LiBr-H2O absorption heat pump (b) and 606-m2 flat plate solar collector field (c).

Test-Bed for Solar Thermal Desalination Applications

The purpose of this facility is the study of the efficiency of large-aperture static solar collectors and its behavior in the coupling with thermal desalination systems minisat 60-90°C temperature levels.

The collector model installed is an LBM 10HTF with an aperture area of 10.1 m2, manufactured by Wagner & Co. The static solar field is composed of 60 collectors with a total aperture area of 606 m2 and a total thermal power output of 323 kWth under nominal conditions (efficiency of 59% for 900 W/m2 global irradiance and 75°C as average collector temperature). It consists of 4 loops with 14 large-aperture flat plate collectors each (two rows connected in series per loop with 7 collectors in parallel per row), and one additional smaller loop with 4 collectors connected in parallel, all of them titled 35° south orientation. Each row has its own filling/emptying system consisting in two water deposits, from which the heat transfer fluid is pumped to the collectors at the beginning of the operation and where all the water volume in the collectors is spilt either at the end of the operation or when a temperature limit is reached (above 100°C). The solar field has flow control valves that allow to have an equal distributed flow rate without further regulation. In addition, the facility has an air cooler that allows the entire energy dissipation from the solar field, which is useful for efficiency tests at different temperature levels. The five loops of collectors are connected with a thermal storage system through a heat exchanger. The thermal storage system consists of two water tanks connected to each other for a total storage capacity of 40 m3. This volume allows the sufficient operational autonomy for the fossil backup system to reach nominal operating conditions in the desalination plant.

The flexibility of the solar field allows the operation of each loop independently, through their own valves and pumping system. Each loop is connected to an individual heat exchanger that offers the possibility of coupling it with any low-temperature thermal desalination system for testing purposes.

Figure 30. The 606-m2 large-aperture flat plate solar collector field (AQUASOL-II).