WO2009009873A1

WO2009009873A1 - Solar water desalination unit with superheater and heat exchangers

Info

Publication number: WO2009009873A1
Application number: PCT/CA2008/001270
Authority: WO
Inventors: David Holroyd
Original assignee: David Holroyd
Priority date: 2007-07-18
Filing date: 2008-06-30
Publication date: 2009-01-22
Also published as: WO2009009873A8

Abstract

A solar water desalination unit using a superheater to increase efficiency. Water vapor which comes out of the main evaporation chamber travels through a superheater, increasing its temperature by a few degrees. Then it goes into a condensing heat exchanger where it gives up its latent heat of vaporization to the water in the hot feedwater tank. The main evaporation chamber is fed from this hot feedwater tank. As the solar energy influx increases the temperatures throughout the system increase until the water at the top of the hot feedwater tank begins to boil. At that point the system becomes self-sustaining and continues with only nominal additional solar heat input until the water in the hot feedwater tank becomes saturated with salt and has to be dumped into a secondary evaporator. Use of the superheater allows steam at a few degrees above the boiling point to give up all of its latent heat to water that is near or at the boiling point 100 degrees C.

Description

TITLE: SOLAR WATER DESALINATION UNIT WITH SUPERHEATER AND

HEAT EXCHANGERS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 60/959774, filed July 18, 2007 by the present inventor. FEDERALLY SPONSORED RESEARCH Not Applicable SEQUENCE LISTING OR PROGRAM Not Applicable

BACKGROUND— -FIELD OF INVENTION

Many parts of the earth have become deserts because of human action. Ellsworth Huntington in his book "Mainsprings of Civilization" tells us that the earth has undergone many long-term cycles of wet and dry climate. During periods of wet climate the population of people and animals expanded too rapidly. When the climate changed the herds of animals overgrazed the land, especially during the first years of the dry periods, and left very little, if any, vegetation to protect the land from rains. The next rains then carried away a large fraction of the topsoil. Because of the decrease in vegetation and the accompanying decrease in transpiration from vegetation the climate became drier and the desert expanded again. We are frequently told that the desert expanded because the climate became drier but we are not often told that one reason for this was human action. Deserts are expanding now in many areas because some people have to walk as far as 20 miles to obtain firewood. They strip everything out from the nearest group of trees. Nothing is left to protect the land and the next rain washes away the topsoil, further expanding the desert. The Sahara Desert is rumored to be expanding a few miles every year because of this effect.

In a temperate climate forest only a few percent of rainfall runs off immediately. The rest is absorbed by the ground, the undergrowth, and the trees. Most of this is given off later by the leaves or needles of the trees, in effect recycling it back into the atmosphere. In a desert area nearly all of the rain runs off immediately, and it takes a large fraction of whatever topsoil exists with it. This makes the area even more arid and unproductive. The present invention is intended to make it possible to produce fresh water more economically. This fresh water is an essential part of any arid land reclamation project, although it must be properly used to do this.

Most agricultural crops require large amounts of water, typically 50-200 times the weight of the crop produced. The vast majority of the irrigation water is given back to the atmosphere by transpiration. Traditional methods of solar desalination require a collector area about the same size as the irrigated area. The present embodiment allows much higher production for a given collector size, making it practical to irrigate large areas. The transpiration mentioned above means that the areas downwind from these irrigated areas will obtain large increases in rainfall, increasing their agricultural productivity. If the irrigated area is large enough it could increase rainfall for hundreds of miles, depending on the geography and existing wind patterns.

BACKGROUND— -PRIOR ART

Traditional solar distillation units were very similar to each other. They used a cell with a glass surface inclined at an angle that approximated the latitude of the area where they were built. In equatorial regions the glass was set at any convenient angle but the slope had to be enough so that the water that condensed on the glass ran down to the collector instead of evaporating again. Saline water was poured into the bottom of the cell. Sunshine coming into the cell evaporated some of the water. It condensed on the glass top and ran down the glass. It was collected at the bottom of the sloped surface. The heat was given off by convection from the glass surface. One large installation of this type was built to supply a mining town in Chile about 1873. Capacity was typically about 4 liters per day per square meter of glass surface. The theoretical limit of this type is about 5.7 liters per day per square meter. This can be determined by taking average sunshine of about 350 Rayleighs, or 350 calories/square centimeter/day and dividing it by 620 calories per gram, representing 80 calories to raise the temperature from room temperature to boiling and another 540 calories to evaporate it. It is this latent heat of vaporization that is the limiting factor to these designs.

This theoretical limit means that to irrigate one hectare of land for agricultural purposes requires about one hectare of collector. This is only a rule of thumb and varies with different types of collectors, crops, and soil types but is a useful starting point for design and estimating.

The big problem was that the latent heat of vaporization was not recaptured. Instead a large condensation unit had to be built to supply the energy needed condensation. This is typical of all traditional units. PRIOR ART - MORE RECENT INVENTIONS

Patent searches in the US, Australian, and International patent bases show that most of the more recent inventions use some sort of pumping mechanism to create a partial vacuum and thereby decrease the amount of energy required to evaporate the feed water. Some use reflectors to concentrate sunlight on a tube of water. Compound distillation units have been produced that can extend the limit up to about 19 liters/day/square meter by condensing steam on heat exchangers that heat up the feedwater.

Some previous inventions, such as US patent 6,391 ,192 tried to limit the temperature in the warm water supply tank so that steam could condense there. Patent 4,135,985 uses a portion of the latent heat of vaporization to warm up the feedwater toward the boiling point but uses only a small fraction of its latent heat.

Patent 4,270,981 suffers from the same problem.

Patent 5,645,693 uses parabolic reflectors. It uses only a small fraction of the latent heat of vaporization. Patent 6,494,995 also uses only a small part of the latent heat.

SUMMARY

The present embodiment reverses traditional design by superheating a portion of the steam so that it will give up its latent heat to water that is already near or at the boiling point and raise its temperature above boiling, his allows effectively complete recapture of the latent heat of vaporization so that the process can continue with very little input heat energy. DRAWINGS

Fig 1 is a top view of one embodiment of the design. Fig 2 is a longitudinal section taken from Fig 1. Fig 3 is a transverse section taken from Fig 1. It shows the heat exchanger in the hot feedwater tank. Fig 4 is a longitudinal section of a different embodiment of the design.

Fig 5 is a section taken from Fig 4 showing the condensing heat exchanger.

Fig 6 is a section taken from Fig 4 showing the superheater and the tubes leading from the hot feedwater tank to the main chamber. Description of drawings

Figure 1 is a top view of one embodiment of the basic design. The portion of walls 102 which is below the condensate trough 206 (shown on figure 2) can be made of foam plastic coated with any suitable waterproofing material. The portion of walls 102 which are above the condensate trough 206 should be made of a transparent material, normally the same material as plastic cover 114. Supports 112 for the said plastic cover can be made of any suitable material. Care must be taken during construction to ensure that condensate water that runs down the plastic cover continues down to the drainage troughs and does not fall back into the main chamber to be evaporated again. This might include making the supports larger than they would otherwise be and making them concave on the lower surfaces. The cover 114 is referred to herein as plastic but could be any similar material that has good transparency and good thermal insulation properties. This has been done in the past by providing two skins separated by spacers but the current design would work with materials which provide the same qualities by other means. If no similar material is available glass can be used.

The superheater is composed of a flat sheet 108 and partitions 110, acting together with the plastic cover 114. These sheets can be made of sheet metal or some similar material and should be painted or lacquered to provide a black or very dark surface. As the steam passes through this superheater it absorbs the solar energy that comes through the plastic and this raises its temperature. The hot feedwater tank 104 is built of foam insulation or some similar material and covered with polyester or epoxy reinforced with fiberglass. The cold feedwater tank 106 is of similar construction. The feedwater goes through these two heating chambers so that it will be as hot as possible before it enters the main chamber. Figure 2 is a sectional view taken from Figure 1. It shows the mechanism in more detail. Condensate troughs 206 are attached to the sides of the main chamber 222. They carry the condensate that has run down from the plastic surface 214 along the walls of the main chamber 222 to the heat exchanger 318. This is made of a material such as copper tubing and consists of coils that carry the condensate back and forth in the feedwater tanks. The hot condensate runs through the tanks and gives up part of its heat to the feedwater. The condensate goes through the hot feedwater tank and then the cold feedwater tank. Superheater fans 208 should be sized in proportion to the rest of the unit. The dimensions of the superheater should be adjusted to fit the size of fans 208. Power for these fans can be provided by ordinary electric power as provided by normal utility companies where this is available. In isolated areas photovoltaic power can be used. The steam that is moved by these fans is forced into funnels that are formed by plastic cover 114 on the top, sheet 108 on the bottom, and separator sheets 110 on each side. These funnels absorb solar energy through the plastic cover 114. Solar energy that impinges on sheet 108 heats it up, and a large fraction of this heat is transmitted to the stream of steam through the separator sheets 110 and through the portions of sheet 108 that the stream of steam touches directly. Fans 210 blow across the surface of the water in the hot feedwater tank 104 to speed up evaporation. They should be about the same size as superheater fans 208. Figure 3 is a sectional view taken from Figure 1. It shows the portions of the unit where the steam from the superheater condenses. After each stream of steam is funneled to a small area it continues to a downcomer 302 and then is carried down to a main collection header 304 which is completely immersed in the hot feedwater tank 104. The said main collection header is drawn as horizontal for clarity but in fact should have a slight downward slope toward heat exchanger 314. The water that condenses in the said main collection header is carried by gravity to said heat exchanger 314, which is constructed of metal tubing or some similar material. Each element in the said heat exchanger slopes slightly so that the condensate will run down by gravity. After a few turns through the hot feedwater tank 104 the condensate flows into another similar heat exchanger in the cold water feedwater tank and then out through outlet 316. A similar heat exchanger 318 allows recapture of a portion of the heat in the condensate that comes from troughs 206. It flows out at outlet 320 into a suitable container, which is not part of this invention.

The drawings show two feedwater tanks but the licensees may change this to a larger number or combine the two feedwater tanks into one. The drawings show one unit. Normally the licensees would be expected to build and test one unit before starting commercial production but for commercial production a multiplicity of units can be built side by side without any partitions separating them. The heat that was lost through the side walls in a single unit will no longer be lost on the new interior partitions. This will increase the efficiency of operation.

The mechanisms to regulate the depth of water in the main chamber 222 and to regulate the flow from the supply to the cold feedwater tank and from there to the hot feedwater tank are not shown on the drawings. A float valve in the main chamber can be used to regulate the flow to the cold feedwater tank and then a simple overflow can be used to regulate the flow to the hot feedwater tank. Figure 4 shows a section through an alternative embodiment with a different design of superheater and condensing heat exchanger. The change in the superheater consists of changing the separation so that the cover is connected to the base by a series of Z-shaped partitions or a sheet of corrugated metal and all of these partitions run parallel to the flow. The condenser is made in a similar manner. The steam flows along the partitions and the full width of superheater is effective. This decreases the pressure loss during flow and therefore reduces or eliminates the need for the fans shown on the previous set of drawings. The transparent cover 404 covers the top surface of the apparatus. At the bottom there is a shallow tray 406 that holds a thin layer of water. This should heat to boiling point after the sun has been shining on it for a while in the morning, the time depending on the depth of water, the exact configuration of the apparatus, and the intensity of the sunlight. It will begin to steam a few minutes before this happens. Some of the steam will rise through the throat 408 into the superheater 410. This acts like a chimney with one side heated by sunlight shining on the lower surface 412. The superheated steam goes through a bend 414 into a condenser, where it gives up a part of its heat energy. The heat given off during condensation is conducted through the metal wall 418 of the condenser to the hot feedwater. Some of the heat energy goes through the /spacers 416 into the metal wall 418. The condensate goes down through a drain 420 to a serpentine tube 422 in the hot feedwater tank 421 where it warms up the incoming feedwater After passing through the hot feedwater tank it continues on and goes through the cold feedwater tank 424. It flows to an external tank through outlet 426.

Supply water comes in through the pipe connection 428 at the bottom of the cold feedwater heater. There are two floats to regulate the water supply. They are not shown in the diagram. One of them is regulated by a float valve (not shown) in a well in the bottom of the tank 430 so enough water is added to the feedwater heater to keep the water level almost constant in the tank 430. This can be done by connecting it to a valve on the line leading from the feedwater tank down to the main chamber. Because some water is evaporated directly from the top of the hot feedwater tank an additional float valve is needed at the top of the cold feedwater heater to replace this water. A tube from near the top of the cold feedwater tank to the bottom of the warm feedwater tank (not shown on the diagram) allows this water to be replaced by water from the cold feedwater tank. As the water in the top of the hot feedwater tank 421 heats up incoming water pushes hot water down tube 432 to the main chamber where liquid water will go down to the bottom of the tank 430. When the hot feedwater tank gets warm enough to vaporize some of the water the vapor will go down this same tube and then rise up and go directly through the superheater. A sheet of insulation 434 separates the hot feedwater tank 421 from the cold feedwater tank 424. All external surfaces must be insulated, and also interior surfaces where temperatures are different. The back surface 436 of the main chamber must also be insulated.

A drain for steam that condenses on the cover, similar to drain 206 on Figure 2, is not shown on this figure but its function would be exactly the same as in Figure 2.

Fig 5 shows a part section through the condensing heat exchanger. There are several different arrangements of stiffeners possible and this shows hat shaped stiffeners 416. There are many different types of corrugated metal that could be used here in place of the hat shaped stiffeners. These stiffeners also act as spacers to hold sheet metal piece 418 in place. Prime considerations are that the stiffeners are connected to the sheet metal piece 418 so that heat will be conducted very well between them, and that the material used for both of these should not cause local corrosion at the point where it meets the drain 420 that leads to the serpentine heat exchangers below. A connection is shown where the apparatus may be taken apart for cleaning if the licensee decides to do this. Other separation points might be used if desired. Using this connection point would require heat-resisting gaskets all around. Tubes 206 are not shown on this drawing but they would also need gasketing of some type unless they are below the feedwater tanks. This could be done by putting the joint on the drain tube outside of the hot feedwater tank. These points would then be used for alignment when replacing the back part of the apparatus. There are many existing methods of gasketing connections for low pressure water tanks and any one of them may be used at the licensee's discretion.

Fig 6 shows a section through the superheater and the tubes that lead down from the hot feedwater heater to the main evaporation chamber. These tubes were collectively shown as item 432 on Fig 4 but here we can see that one is much smaller than the others and is a little lower. It provides water to cover the bottom 406 of the tank. It does not need to be curved upward at the bottom as shown in Fig 4. Alternatively a float valve can be used in a well in the bottom of tank 406 and a valve placed in this line to regulate the flow to maintain the desired water level in the tank. The larger tubes are to permit the flow of boiling water and steam to the superheater when the apparatus is operating at maximum capacity. The superheater can be made of corrugated metal sheets or hat shaped sections. These metal partitions decrease turbulence in the flow. There are no problems of compatibility with other materials here because there are no connections to other metallic parts of the apparatus. The metal sheets should be painted black with heat resisting enamel. Insulation of all hot surfaces is required but is not shown explicitly here. DETAILED DESCRIPTION OF THE APPARATUS

The device has a base 406 which must be insulated and as level as possible. Input water should cover the entire base with the minimum depth being on the order of 1 or 2 mm. If the water is too shallow salt buildup can prevent water from covering the whole base and the solid salt can reflect incoming solar heat. The base has side walls 430 high enough so that the contents of the hot feedwater heater 421 can be dumped into it when necessary. The cover 404 can be made of glass, preferably non-reflecting glass, transparent double-wall polycarbonate, or any other material which is nearly transparent to light and a poor conductor of heat. Other materials can be used but some materials that seem to be suitable, such as heavy vinyl, may fog over when exposed to steam and stop acting as transparent covers. Adequate supports for this cover should be provided depending on the material and the size of unit. It should be readily apparent that the larger the area of the base 406 in comparison to the volume of the hot feedwater tank 421 the faster the system will get up to full operating temperature in the morning. The back wall 436 of the main chamber should be insulation with a black or nearly black finish. At the top of this wall it connects with the throat 408 of the superheater. The throat 408 is shown in the figure as having a rounded edge where the two meet. If the cover 404 is polycarbonate or similar material spacers will be required in the superheater. These can be Z shapes made of sheet metal and need to be stiff enough to support the cover during strong winds and heavy rains. They will be aligned along the flow. Their spacing should be determined by the support requirements of the cover. In Fig 7 one possible arrangement of these is shown.

When the early morning sun begins to shine on the water on the base 406 it will heat it fairly rapidly because the depth is only a few mm. Full direct sunlight could bring it up to boiling point in about 40 minutes but diffused light coming in from an angle will of course take more time. There will be very little vapor formed until the temperature is above 54 C. Above this temperature the amount of vapor produced will increase slowly until the boiling point is reached. This vapor will go through the superheater and condensing heat exchanger. The water that evaporates is replaced with water from the hot feedwater heater. Initially this replacement water will be cool but it will soon become warmer. Once hot water is used to replace water that is taken off as vapor the whole system will operate more efficiently.

While the vapor is going through the superheater it will absorb the solar energy that is incident on surface 412. At the top of the superheater it will go through an insulated bend 414 into the condensing heat exchanger 418. This could be formed from a piece of sheet metal 418 that has the edges bent over as indicated in the figure and connected to the backing piece 434. This backing piece must be insulated and must be sufficiently rigid to resist the expected water pressure. The sheet metal piece 418 must be reinforced to resist this pressure. The suggested method of doing this is by sheet metal stiffeners 416 that are bent into the shape of a Z or hat sections. They must be connected to sheet metal piece 418 so that heat can be conducted through them. They have to end above the bottom surface of the condensing heat exchanger to permit the flow of condensate. They therefore can act as fins, stiffeners, and spacers all at the same time. Of course other methods of supporting sheet metal piece 418 can be used but the stiffeners as shown make it very easy to brush salt off of the outside of the condensing heat exchanger.

It is assumed that the fins will be spaced about twice their width. Since vapor can condense on both sides, they double the effective area of the condenser at this spacing. A piece of shaped insulation 442 directs any bubbling in the feedwater heater 421 into the tubes 432, which are spaced along the length of the apparatus. The tubes 432 carry the hottest water from the hot feedwater heater into the pool of water in chamber 430 until boiling begins, then carry steam. When water flows down these tubes it drops into the main chamber. When steam comes down it rises directly into the superheater.

Condensate is drained from the condensing heat exchanger through drain 420 into the serpentine heat exchanger 422 where it loses heat while warming up the incoming feedwater, first in the hot feedwater tank and then in the cold feedwater tank. It eventually drains through drain 426. Makeup water is added through connection 428. The incoming solar radiation varies with the location and the time of year but a typical figure might be 350 Rayleighs, meaning 350 calories per square centimeter per day.

Some heat is lost at both ends of the apparatus. If the end walls are opaque they prevent early morning sunlight from coming into the main pool 430. If they are transparent they let some of the morning sunlight escape through the west end wall. The analogous situation occurs in the afternoon. One possible solution might be to provide end walls of one-way mirrors with transparent insulation, such as polycarbonate double-wall sheeting. There is also some heat lost since the end walls cannot be perfectly insulated. For large-scale desalination units, as opposed to experimental units, normally several units would be placed side by side so that the losses at each end are proportionally much smaller. No partitions are needed between units and this makes it easier to sweep out any salt that accumulates on base 406.

One way to increase output would be to use a water heater of some kind to preheat the water before it goes into the apparatus. This warmer water could be used to fill the unit in the morning or all day, depending on what the licensee decides. The outer insulation shown on the bend 440 could be hinged and come down at night to cover the top of the feedwater heater. Alternatively the water from the feedwater heater could be drained into an insulated container overnight and pumped back in the morning, so that the apparatus would begin the day with hot water everywhere.

The figure shows the superheater occupying half the dimension of the cover 404.

This is purely arbitrary but the lower section must be large enough to get the whole system into full boiling mode. If the supply water is warmer this could be smaller than it would be with cold supply water. When the system is in full operation the superheater is calculated as adding about 10 C to the temperature of the steam. The lower serpentine condenser 422 should be adequate to heat the supply water and cool the output.

The slope of the cover 404 should theoretically be equal to the latitude at the location. Some writers recommend making it up to 10 degrees steeper than this to get more energy early and late in the day. If more water is needed in the summer the slope should be flatter and if more water is needed in the winter the slope should be steeper. A difficulty of using a steeper slope is that if you are assembling a large number of units increasing the height of one row makes it necessary to increase the spacing between rows. If the installation is in the tropics the design should be changed slightly to make the serpentine heat exchanger 422 wider and lower and more fins should be added to the condensing heat exchanger 418 to decrease its required height.

The volume of the feedwater heater should be as small as practical while containing the heat exchangers. This will allow the apparatus to heat up as early as possible in the day.

In this manner the latent heat of vaporization is reused many times during the day and does not enter into the overall heat balance. Once it is warmed up we have energy coming in equal to the solar energy coming in through the cover 404, and energy losses of the heat given off through the cover 404 and any gaps in the insulation, as well as the increased temperature of outlet water from drain 426 compared to the lower temperature of the supply water coming in through pipe connection 428. There will be additional losses at the end walls of the apparatus. Because of this efficiency will improve when several units are built side by side with the end walls separated by several units, not just by one unit. Ocean water can typically be evaporated until it has lost about 90% of its volume before it becomes saturated with salt. When this happens the hot salty water must be removed and pumped to an evaporator or some similar apparatus to avoid having thick salt deposits foul the apparatus. This evaporator is not covered by the present invention, but would reduce the risk of environmental objections. It would be beneficial to assemble the apparatus so that the heat exchangers and the cold feedwater heater can be easily removed for cleaning. This could be done by making the condensing heat exchanger 416, the insulated wall 434, the serpentine 422, and the cold feedwater heater as one piece and attaching it to the rest of the apparatus by a series of wingnuts or clamps, and providing gaskets to maintain watertight connections. Once it is opened the condensed salt can be brushed out easily. This superheater is in the form of a multiplicity of channels where solar heat is collected and transferred to the steam that is passing through it. The superheated steam then goes through a bend in the channel to a condensing heat exchanger which is immersed in the hot feedwater. It gives off its latent heat of vaporization in this condensing heat exchanger and so heats up the water in the hot feedwater tank. This makes it possible to feed steam and boiling water into the main chamber to speed up the evaporation process. Once the steam is condensed the hot condensate goes through two heat exchangers to cool it off and warm up the water in the feedwater supply tanks. If flow through the superheater is too fast the steam will condense more slowly and decrease the partial vacuum at the condensing heat exchanger, slowing down the flow through the superheater and increasing the degree of superheat imparted to the steam.

Although it is only a small fraction of the steam generated by the current invention, the hot water that is condensed on the sloping surface runs down to a gutter. It then goes into a tube that is coiled inside the hot water supply tank and then inside the cold water supply tank. This cools off the condensate and warms up the supply water. This makes the main process work more efficiently than it otherwise would. The level of water in the main chamber is as low as possible without leaving dry areas on the floor. This means that it will heat up fairly rapidly in the morning. Both tanks are insulated so it should be clear that performance will improve after the first day, since the supply water will come into the main unit at a higher temperature and so begin to boil earlier in the day. Flow of water into the main chamber can be regulated so that it will be essentially dry at sundown to allow the sea salt or other dissolved substances to be swept out at night. This should be done often enough so that the salt that is deposited on the dark surface does not get so thick that it begins to reflect sunlight that would otherwise be absorbed by the dark surface.

Because of these improvements average production rates are much larger than in the traditional method of construction. Cost is more than the conventional construction but not by as much as the production so cost per unit of water produced is much smaller.

Current U.S.CIass: 203/DIG.1

Fields of search: 210/642; 203/10; 210/664; 210/416.3; 202/152;

202/159; 202/163; 202/167; 203/DIG.1 ;210/767; 203/22;

References Cited [Referenced By]

4,135,985 January 1979 La Rocca

4,270,981 June 1981 Stark

5,645,693 July 1997 Gode

6,494,995 December 2002 Battah

Lienhard & Lienhard, A Heat Transfer Textbook

Pitts & Sissom, Heat Transfer

Ellsworth Huntington, Mainsprings of Civilization

Claims

1. I claim a solar powered water desalination apparatus in which steam which is generated by solar heat is passed through a solar-powered superheater unit and then condensed in such a manner that its latent heat of vaporization is transmitted to the incoming feedwater to raise its temperature up to or above the boiling point to make it easier to evaporate.

2. I claim a solar powered water desalination apparatus of claim 1 in which the condensate is conducted through a heat exchanger to cool off the condensate and heat up the incoming feedwater.

3. I claim a solar powered water desalination apparatus of claim 1 in which the steam that condenses on the cover of the unit flows down by gravity to a heat exchanger in which it is cooled down while the feedwater is heated.