Nº 1, april

Energy area. Renewable energy: environmental applications of solar energy and characterization of the solar radiation

Scientific Investigator: Diego-César Alarcón Padilla

Plataforma Solar de Almería

The Unit of Environmental Applications of solar energy of CIEMAT has the following objective: generation of knowledge associated to a technological development that allows the effective use of the solar radiation in processes of environmental character, contemplating the treatment and purification of water and air, like main nuclei of activity.

Between the activities emphasizes the processes of solar seawater desalination by thermal treatments.

At the moment they are working actively in the development and improves of the multieffect distillation technology (MED), incorporating a solar energy power supply with the main objective of approach this technology to the thresholds of the conventional processes, and at the same time, to reduce it´s impacts on the environment. Several ideas and technological innovations will be incorporated. These ideas are developing into the AQUASOL European Program.

Among the results of their research can be highlighted:

• Development ofa hybrid technology of advanced solar-gas based on staticsolar collectors with zero waste.
• Design, construction and test of experimental pilot plant based in the distillation plant multieffect SOL-14, previously existing in the PSA.
• Development of a test bank of static solar collectors (CIEMAT-PSA).

At the moment there is a destilation multieffect plant connected to solar collectors, SOL-14 (CIEMAT-PSA) and to a solar static collectors (CIEMAT-PSA).

The SSPS-DCS plant, with solar desalination system, is a installation with a nominal potence of 1,2 MWt, and it have four subsystems:

• Asolar field consists of 40 parabolic-trough model Acurex 3001, grouped into 10 parallel rows, with 4 collectors connected in series within each row. The total area of these solar collectors is 2672 m2, and the rotation axis is oriented in the east-west direction.The fluid used by these collectors is Santotherm 55, wich owns a maximum temperature of work of 300ºC.The absorbent tubes of the solar collectors are without emptiness and own a selective covering with black chromium.This solar fieldhas a global yield of 50%, with power of 1.3 MWt for a direct solar radiation of 950 W/m2. The average of daily thermal energy is 6.5 MWt.
• Athermal storage system consisting of a thermocline oil tank with an internal volume of 140 mand a storage capacity of 5 MWht for temperatures loading / unloading of 295/25 ° C.It is equipped with an automatic fire system, automatic vent valves and a system for condensed volatiles.An oil cooler, cooled by water, allows the quenching of the oil to be able to realise tests of transitory.
• A system of electricity generation of 500 kWe, using a cycle of water Rankine/steam.This systemconsists: steam generator fed by hot oil that supply the solar field and / or the storage tank, turbine of steam, generator and circuit of refrigeration by evaporative towers.
• The MED desalination plant consists in a multieffect  distillation plant with 14 stages, denominated SOL-14, which is connected to the system of  thermal storage previously described. For a nominal production of 3 ms ³ /h of distillate, the consumption of the plant is of 190 kWt, with an efficiency factor (number of kg of distillate produced by each 2,300 kJ of energy consumed) greater of 9.Thesaline concentration of the distillate is about 50 ppm.The gradient of nominal temperature between the first and last stages is 40 ° C, with an operating temperature of 70 ° C in the first stage.The vacuum system consists in two hydroyectors of seawater at 3 bar.This vacuum system is used to evacuate air from the unit at the beginning of the operation and to compensate the small amounts of air and gases released with the water supply, and the small losses that may occur in the various connections.

The following arethe technical specifications of the solar desalination plant SOL-14:

• Capacity: 3 m3/hour
• Number of effects: 14
• Water of entrance: Seawater (35.000 ppm)
• Product: Distilled(£50 ppm)
• Recovery factor: 38%
• Feedwaterflow: 8 m3/hora
• Thermal consumption (FR): 63 kWh/m3 (>9)
• Electric consumption: 3 kWh/m3
• Solarpower:1,2 MWp
• Yield: 50%
• Collectorarea: 276 m2
• Type of collectors: ACUREX 3001 (CCP)
• Storage: 5 MWh (Therminol 55)
• Maximumoil temperature: 300°C

Más información

The projects in which the unit of Environmental Applications of Solar Energy has been involved are:

1. Hybrid Technology of Solar-Gas Desalination based in Static Solar Collectors(SOLARDESAL). Ministry of Science and Technology; Reference project: REN2000-0176-P4-04Ministerio de Ciencia y Tecnología; Referencia proyecto: REN2000-0176-P4-04.

   Participant Organizations:CIEMAT (Coordinator), Universidad de la Laguna, INABENSA and ECOSYSTEM.

2. Enhanced Zero Dischage Seawater Desalination using Hybrid Solar Technology (AQUASOL). EU. DGXII. Programa: Energy, Environment and Sustainable Development. Contrato EVK1-CT-2001-00102.

   Participant Organizations:CIEMAT (Coordinator), INABENSA (Spain), Ao Sol Energias Renováveis (Portugal), National Technical University of Athens (Greece), INETI (Portugal), Cajamar (Spain), Hellenic Saltworks (Greece), Comunidad de Regantes de las Cuatro Vegas de Almería (Spain) y Weir-Entropie (France).

The common objective of both projects is the development of a hybrid technology solar/gas to seawater desalination based on the multi-effect distillation process (MED) that fulfills the efficiency principles at the same time, energetic efficiency, low cost and null spill.

Particularly,the AQUASOL Project is focused in the technological development of three fundamental aspects:

1. Incorporation of a power source of hybrid solar/gas energetic supply based on parabolic static composed solar receivers (CPC) of high efficiency and low cost.

2. Development of anabsorption heat pump, of double effect (LiBr/H20) optimized for it´s connection to the MED process that allows to reduce the power consumption required by a conventional MED plant

3. Reduction to zero of any type of spill of the distillation process by the recovery of the salt from brine.

In 2004 the phase of project investigation has concluded and the result is the final specification of the subsystems that constitute the desalination plant AQUASOL (heat pump by absorption of double effect, field of solar receivers of type CPC, thermal storage, gas boiler and solar dryer).

The first effect ofthe MED plant will be fed directly with hot water at 66.5 ° C from the storage tank, which in turn is fed by the solar collector field, or by the absorption heat pump dual effect.The operation at these levels of temperature presents two advantages: firstly, the consumption of chemical additives is reduced because the formation of incrustations is low; secondly, the energy efficiency of static solar collectors is greater whichever minor is the difference between the temperature of the fluid and the room temperature.The series connection of two storage tanks has to objective improve the controllability of the system and maintain the temperature difference necessary for efficient operation of the heat pump.

Due to the limitation in the maximum temperature that can be obtained with standard solar receivers CPC, the only possibility of double effect heat pump operation is using steam to high pressure (180°C, 10 bar) from the gas boiler.Therefore, the yield factor of the plant (kg distillate produced / 2300 kJ energy input to the process) will oscillate from a value of 10 (only-solar mode) to a value of 20 (only-gas mode).  Is plannedwithin the project, the research of hybrid operation modes in which the heat pump can operate at partial load (30% -100%) in combination with a contribution [Hublitz et al., 2004]