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Innovative Applications of Solar Trough Concentration for Quality Fresh Water Production and Waste Water Treatment by Solar Distillation

Final Report Summary - AQUA SOLIS (Innovative Applications of Solar Trough Concentration for Quality Fresh Water Production and Waste Water Treatment by Solar Distillation)

The AQUASOLIS project was thought as a support action for of a larger project dedicated to the use of solar concentrating plants for the production of hot water and air conditioning for Southern Mediterranean countries. AQUASOLIS found that the technology used in these plants can be used to generate fresh water at no additional cost of equipment and at times in which there is an excess energy available that, otherwise, would be lost. The solution to the water problem, as well as that of power, lies in renewable energy. However, renewable sources remain costly and the combination of the existing desalination plants (usually large scale ones) with renewable plants (usually small scale) is problematic and economically inefficient.

The main objectives of the project were the following:
- Data on the climatology and the socio economical framework of the three Mediterranean partner countries identified as targets of the project: Morocco, Lebanon and Jordan.
- Review of the state of the art of water production processes.
- Examination and evaluation of the concept of water production by means of the solar cooling system consisting of a solar parabolic trough and a single effect lithium bromide-water chiller.
- Organisation of a conference for the diffusion of the results of this support action. The conference was held as a workshop within the 'Desalination and the Environment' Congress that took place in Halkidiki (Greece) at the end of April 2006.
- Other dissemination actions.

The production of the final feasibility study on the use of the solar cooling system used in the Sixth Framework Programme (FP6) REACT has started from the study of the state of the art. After this phase, two main decisions were taken about the work that should be actually performed in the project. The research team was concentrated on two different technologies for the production of water: extraction of air humidity from the atmosphere and desalination by humidification / dehumidification of air.

In order to properly simulate the proposed technologies, meteorological data were needed. Since the in depth analysis of the meteorological scenario that is being carried out in the frame of the REACT project was not ready at the time AQUASOLIS was in progress, a choice has been made to use meteorological data synthesised by historical databases by means of software called Meteonorm. The input data used for the simulation were the meteorological data collected on an hourly basis for the eight location selected in the three target countries. Namely, the data used for the simulation were: solar global radiation, ambient temperature and relative humidity.

In order to simulate such a complex system like the humidification dehumidification desalination system only the option of developing an ad hoc solution is feasible, since existing software packages (typically expansions of Matlab / Simulink or Ipsepro) are too specific and tailored on the system they were designed to simulate. In AQUASOLIS particular attention has been paid to the generality of the system simulated with less regard to in depth accuracy. The three main parts of the system: solar collector field, evaporator, condenser, have been simulated separately in order to calculate a set of steady operating points. The constraints on the boundary regions were set by the bordering subsystems. It should be noted that the results were not very site specific. In order to have a comparison for the HD system, a set of simulations has been run also on a solar still. The solar still simulated here had an aperture area of 100 m2 and presented value of yearly production that was smaller by a factor of 8 to one with respect to the HD figures. Accordingly the specific production was nearly an order of magnitude lower than that of HD.

The second system under study was based on the extraction of atmospheric water by cooling of moist air below the dew point. The system consisted of a fan that can be fed by electrical energy (ideally coming from PV). The task of the fan was to fix the volume flow of air on the surface of the heat exchanger. The value of the fixed air flow is calculated taking into account the minimum output of water in terms of litres per hour and the minimum value of the relative humidity for the selected location. In this simulation the value of the moist air flow was fixed to 8 000 m3/h. The efficiency of the fan is given by the sum of three contributions: intrinsic fan efficiency, belt efficiency, motor efficiency. It became apparent from the results that both the quantity of water produced and the specific energy consumption are more favourable in locations near the sea (Bayrouth, Tripoli, Casablanca) or in Irbid, an inland location but with high relative humidity throughout the year. In the three sea locations the specific energy figure is four time bigger than the theoretical figure of 680 kWh/m3, that is the latent heat of vaporisation and is stated as a theoretical minimum in some literature. The results of the simulation point out a strong dependence of water production on season. This explained by the higher solar radiation and the biggest amount of sunny hours during the day in that period. The specific energy is very high and so is the total energy consumption, while water can be extracted in important amount only during summer.

The concept that AQUASOLIS has extracted from 10 months of work is that fresh water obtained from atmospheric humidity or from desalination using solar concentrating plants can be seen as a way to store solar energy, transforming it into a useful product. The AQUASOLIS project has examined in detail two methods of freshwater production that can be used in conjunction with the linear parabolic troughs being designed and built in the REACT project. For a comparison, also the traditional solar still method was simulated.

Desalination by humidification dehumidification of air coupled to solar concentrators can be applied when:
- seawater or brackish water is available. Suitable areas are remote areas on the seaside or inland where brackish wells are available.
- solar concentrating HD can be performed using robust materials (such as plastics) and can be operated and maintained with a complexity that is equal or lower to the complexity in operating a solar cooling system.
- it presents a limited additional cost for the contextual installation with the solar cooling system, while providing very high hields in terms of specific production and energy compared to other thermal desalination processes.

Extraction of water from air can be applied when:
- it is a by-product of an existing solar cooling system;
- no sea water or underground water resource is present or exploitable.

Whereas so far the approach has been to minimise costs by increasing the size of desalination plants, the AQUASOLIS approach showed the possibility of small size plants usable for small communities where fresh water is a by-product of a multi-purpose approach. The AQUASOLIS approach located therefore a possibility for a further economic gain of cylindrical solar collectors that the REACT project had not - so far - considered. It is hoped that this first survey of the matter can be put to test in the future with real multi-generative plants.

Further work is needed for a better understanding of the economic and technical implications of fresh water production from solar concentrating plants. In particular, the safety of distilled water for human consumption should be carefully assessed. However, the diffusion of plants that can produce water as an additional economic output to heating and cooling for can be seen as a boost for renewable energy which will kick start the diffusion of solar energy in Mediterranean countries.

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