Solar Multi-Effect Distillation Facility

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 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 kW_th, 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 ºC with a maximum operating temperature of 70 ºC 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 ºC, 56 mbar abs) produced 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 ºC, 10 bar abs), as well as operating the MED plant in the absence of solar radiation.

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 at 60-90 ºC temperature levels.

The collector model installed is a 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 kW_th under nominal conditions (efficiency of 59% for 900 W/m² global irradiance and 75 ºC 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 35º 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 ºC). The solar field has flow control valves that permit to have an equal distributed flow rate without further regulation. Also, 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.

Test-Bed for Solar Membrane Distillation Applications at Pilot-Scale

The installation is designed for evaluating solar thermal desalination applications. There are two solar fields of flat-plate collectors available: one of 20 m² with two parallel rows of five collectors in series (Solaris CP1 Nova, by Solaris, Spain), and another one of 40 m² with four large-aperture collectors in parallel (LBM 10HTF, by Wagner Solar, Spain). Both fields are connected to water storages of 1500 litres acting as heat buffers for thermal regulation and storage, and have a distribution system which enables simultaneous connection of several units. The test-beds allow for a stationary heat supply using the thermal heat storage or for direct supply of solar energy without buffering. The installation is fully automated and monitored (temperatures and flows), and allows for heat flow regulation. The maximum thermal power is 7 kW_th in one case and 14 kW_th in the other, and hot water can be supplied with temperature up to about 90 ºC.

The installation has a separate water circuit that can be used for cooling (about 3.5 kW_th) in the desalination units and as a device for supplying simulated seawater, with the possibility of working in open or closed loop. In the latter case, both the distillate and brine flows are collected and mixed together to be fed again into the desalination units after passing through a heat dissipation system. The installation currently operates with Membrane Distillation modules, and has a wide range of different commercial and pre-commercial units from all manufacturers. The list of MD modules that have been evaluated or are under evaluation is:

1. Plate and frame air-gap (AG) MD commercial modules from Scarab (total membrane area 2.8 m²).
2. Two plate and frame permeate-gap (PG) MD prototypes from Keppel Seghers (both with total membrane area 9 m²), a compact one (M33) and another which is split in three separate modules connected in series for higher energy recovery (PT5).
3. Spiral-wound PGMD commercial modules Oryx 150 from Solar Spring (10 m²).
4. Two spiral-wound AGMD modules from Aquastill with membranes area of 7 m² and 24 m² each.
5. WTS-40A and WTS-40B units from Aquaver, based on multi-effect vacuum membrane distillation technology using modules fabricated by Memsys (5.76 m² and 6.4 m² total membrane area respectively).