Latent Power, Solar Desalination Plant

Patent application Nos. 0511946.6, 0608208.5



This desalination plant is not in competition with relatively compact reverse osmosis desalination systems.

It has the broader aim of creating a temperate indoor "climate" in hot sunny countries while producing
sufficient potable water and electricity for the indoor community to thrive.



1                    Basic solar desalination system theory


Basic, traditional solar powered desalination plants are ideal for small scale domestic use
in sunny climates because solar power free and the distilled water produced has a
high degree of purity.



Figure 1. The simplest form of solar desalination unit.


The principle weaknesses of the design are:

(i)                 The system is inherently inefficient because a large amount of heat (latent heat of vaporisation)
   is required to evaporate the water.

(ii)               It is difficult to dissipate this low temperature waste heat into the environment.



Figure 2. The heat dissipation problem can be reduced by allowing the condensation to take place in a chamber that is shaded from the direct heat of the sun.



Figure 3. The heat dissipation problem can be further reduced by using an optical system to concentrate the solar energy and raise the temperature of the system. The position of the optical system (lenses or mirrors) has to change throughout the day as the position of the sun changes.


These improvements can reduce the heat dissipation issues, but traditional solar desalination plants are
commercially uncompetitive because of the latent heat problem.



Figure 4. Even if the water is raised to boiling point using concentrated solar energy, the latent heat problem remains.



2                    Latent Power Solar Desalination


Key design considerations:


       Simplify the solar tracking system by keeping the optical system stationary and moving the location of the water being heated.

       Use lenses instead of mirrors, but keep the weight of glass down by using Fresnel lenses, similar to those used in flat credit card sized magnifying glasses.

      Recycle the released latent heat to evaporate additional brine at a slightly lower temperature.

      When the temperature falls too low for cost effective recycling of the latent heat, use it to generate electricity using a Latent Power Turbine.






Figure 5. By using a canopy made up of cylindrical Fresnel thin lenses (i.e., micro prisms) the sun can be tracked throughout the day without the complexity of the moving mirrors commonly
associated with large solar power units.

The troughs take the form of long (100 metes plus) channels running from North to South. This simplifies the Fresnel lens design because the lens only needs to focus solar radiation in the
 East to West plain.

At any given time, most of the troughs are relatively cool because they are in the optical shade.

The design does not require the optical sharpness of a reading glass, so relatively coarse Fresnel lenses can be used. Manufacturing costs per square metre should be similar to bathroom glazing glass.




Figure 6. A combination of conduits and valves delivers water vapour saturated air from the troughs to an underlying array of condensation chambers. Condensation is only possible in each chamber because the release of latent heat is used to evaporate brine in an overlying chamber.
The air gradually cools as it is drawn through the alternating condensation and evaporation chambers by a fan. In order to maintain pressure, the cross sectional are of successive chambers has to fall.
The coolest moist air emerging from the final evaporation chamber is used to generate electricity using a Latent Power Turbine.



Figure 7. This is a vertical cross section through the first pair of condensation and evaporation chambers.



Q. What happens when the sun goes down?

A. We can continue to generate power and potable water using the freeze desalination process. Dry air LP Turbines can operate below 0oC. This allows them to extract latent heat from brine as
pure water freezes out to form ice.




3      Using the the desalination plant as a horticultural glasshouse


The spare space under the roof at ground level can be used as a working area or for growing crops. The cropping area is in the optical shade and remains relatively cool.





Figure 8. Pleasantly humid living and cropping zones can be created by adding external glass walls.

Larger cropping zones and more intense focusing of solar radiation can be achieved by employing a higher Fresnel lens canopy.

The cropping zones are only illuminated by scattered sunlight from the sky. In effect, the inner glazed zone, occupied by the troughs, acts like a giant heat
pump, shunting heat away from the cropping zones.

Plants can also provide natural air cooling:

(i) Photosynthesis will convert about 10% of the solar energy falling on to the leaves in to chemical energy inside the plants.
(ii) Evaporation of water from the leaves of the plants provides further cooling.
This system will allow people to work comfortably in hot regions, with minimal need for air conditioning.



Figure. 9. Parallel solar desalination units may be added as required
There will be an upper limit for this type of colony if the presence of glass houses creates a cloudy micro-climate.


re. 10. If land space is scarce, pontoon systems can be used.


4    A simplified system for small scale units


Key by-product features
Using a heat exchanger for the condensation process, this system could provide heat for cooking food and the  sterilization of bacterially polluted water or sewage.


Figure. 11. The hot steam plus air output is passed through alternating condensation and evaporation chambers, as in figures 6 and 7.


5    A closer look at the canopy

Figure. 12.  Flat backed Fresnel lenses are easier to cast and pack for shipping to the building site.

As a bought in product, polycarbonate is currently cheaper than glass. But, if the raw glass making materials can be sourced locally and solar powered Latent Power Turbines are used to generate electricity for the manufacturing process, glass will work out cheaper for very large scale plants.


6    Ventilating the living and cropping areas


Figure. 13.  Evaporative cooling is used to cool and moisten the air entering into the enclosed area under the canopy. This process also takes the chill off the raw brine before it enters the solar evaporation troughs.


7   Taking the design concept forward - Student projects?

Before building a prototype system, proof of principle experiments will need to be carried out in the laboratory.

7.1 The primary solar evaporation system
In order to minimise construction costs and the size of the rig, the water could be heated directly, instead of using Fresnel lenses.

Figure. 14.  The first round of experiments can be done indoors using heating elements to model the input of solar energy. 

7.2 The array of secondary condensation and evaporation chambers.
It is only necessary to build a single pair of secondary condensation and evaporation units as shown in Figure 7 above.
In the first round of experiments pure steam at (stagnation) atmospheric pressure would be drawn into the system using an exit fan.
The exit temperature and dynamic pressure would be noted and used as the inlet temperature and fractional steam pressure for the second experiment.
In the second round of experiments, the steam would be bulked out by adding sufficient warm air that the mixture was entering at (stagnation) atmospheric pressure and its dew point


8 An alternative to the Fresnel lens canopy

On our Latent Power Turbine page we suggest a glasshouse system that uses ordinary sheet glass instead of Fresnel lenses.
This simplified system is cheaper to build, but is not very efficient for distilling water.

Figure. 15.  This appears as Figure 1 on our Latent Power Turbine page .


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