Distillation: apparatus – Apparatus – Types
Reexamination Certificate
1999-03-03
2002-03-12
Manoharan, Virginia (Department: 1764)
Distillation: apparatus
Apparatus
Types
C159SDIG008, C159SDIG001, C159SDIG002, C159SDIG002, C159S903000, C202S174000, C202S267100
Reexamination Certificate
active
06355144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to solar distillation systems. More particularly, the present invention relates to a high output solar distillation system that is practical to manufacture and efficient to operate.
2. Discussion of the Related Art
Most fresh water is obtained from rivers or lakes fed by rain or melting ice. When sufficient fresh water is not available; processes using thermal powered water distillation or pressure driven reverse osmosis are often used to convert brackish or salty water to pure water. These processes are expensive to set up and operate. They are less economical and often impractical for very small-scale versions.
Use of the sun to evaporate water, and use of glass or plastic covers to contain and condense the vapor has been long known. Nevertheless, only limited versions of such systems are currently in use. One major problem of previous versions of such systems was the limited amount of water that could be produced in a given area. This resulted in very extensive areas being required for reasonable production levels, and this contributed to the cost and maintenance requirements.
The solar flux is about 1.4 kW/m
2
at the earth's location. Just under 1.0 kW/m
2
reaches the ground at midday on a clear day and with a high sun angle, due to atmospheric losses. The effective daily total solar flux to a collector depends on the inclination of the collector, amount of clouds, seas, etc. The following discussion relates to a solar collector design, which uses stationary collectors inclined at an angle close to the average latitude angle, and facing the average noon sun. For this case, the maximum total effective daily solar flux is about 8 kW hours/m
2
/day. For most of the United States, the year round average is approximately 5 kW hours/m
2
/day. This energy can be used to evaporate water for purification.
Conventional single stage solar evaporator systems are constructed as enclosed structures that pass sunlight in through a sloping glass or plastic wall, have a dark floor covered with a thin layer of source water, and use radiative, convective, and conductive heat transfer to and through the glass to the outside air. The tilt of the wall is required to drain the condensate to collectors. About 92% of sunlight energy pass through the glass sheet, and the dark floor absorbs about 90% of the remainder. The energy raises the water temperature above the outside temperature. Since the glass is cooler than the water, the water vapor condenses on the underside of the glass surface and drains down to the collector. The absorption and heating effectively converts short wavelength sunlight to long wavelength radiation, much of which is trapped in the enclosure due to the spectral response of the glass. This phenomenon is commonly known as the greenhouse effect. Radiation from the heated water to the condenser surface, and the energy used to raise the water temperature above ambient temperature decreases the available energy, so that the effective energy to evaporate the water is typically only about 60% of the total. Glass is normally preferred to plastic for these types of systems due to the better wetting characteristics of glass. If plastic is used, the tilt angle and average drain distances are limited to a smaller range to avoid drops falling from the sheet. Glass is also more scratch resistant and lasts longer in sunlight than plastic. Plastic is sometimes the better choice, however, if its limitations are not overly restrictive, due to its lower weight and greater break resistance, and also due to its ability to be fabricated in complex shapes.
When the water is heated, some of it evaporates, and this removes about 540 cal/g of energy from the remaining water. It therefore requires 2.38 kW hours to evaporate 1 gallon of water at constant temperature. This means that the maximum of approximately 4.8 kW hours/m
2
/day that is available could evaporate up to 2.02 gallons per day, with a maximum year-round United States average of about 1.26 gallons per day. Due to other system losses such as heat conduction (including through the edge and backside), convective heat losses, and thermal capacity of excess supply water, the best single effect systems actually produce a peak of about 1.2 gallons/m
2
/day and an average of about 0.8 gallons/m
2
/day. Even these levels require a fairly high solar flux region.
The main limitation with the above single effect system is that all of the sunlight energy available goes to heat the water one time, overcome the heat of vaporization, and then the system dumps all of this energy into the air in order to cool and condense the vapor out. This process has very poor thermal efficiency.
Use of multiple effect systems can improve the production over single effect systems. The heat of vaporization used for one stage is recovered during condensation of the distillate and passed on to the next stage closer to the external surface. This multi-stage regeneration process, driven by a continual temperature drop stage to stage, can multiply the pure water production. Reflection, refraction, and scattering of the incoming light from each partition drops the energy reaching the darkened absorbing surface beneath the last partition more than for a single effect version, so the maximum distillate production per effect is decreased. The maximum total production in such a process is also limited by the higher temperatures obtained with such systems. The total production for practical multiple effect systems, however, can greatly exceed single effect systems. Unfortunately, previous versions of multiple effect solar distillation systems are either too inefficient or too complex or costly to be of practical use.
SUMMARY OF THE INVENTION
The solar distillation system of the present invention is capable of significantly greater production per area, with an easily installed and maintained structure. The new design can be moved easily, and is practical in small or large-scale systems. The new solar distillation system results in economical purification of water at all scales of operation. In addition, the system can be used to distill other liquids such as ethyl alcohol.
The present approach uses inclined parallel partition surfaces that are spaced a very short vertical distance apart, so that a very compact and strong panel structure results. The small distance between partitions results in diffusion being a major mode of water vapor transport within the chambers. This also results in a minimum temperature drop per stage to achieve the desired level of water vapor diffusion. It should be noted that most previous versions in the literature used fairly large spacing, and this was a major cause of low efficiency. The input sunlight has to pass through all of the partitions to the heat absorbing bottom layer. An insulation sheet is used behind the dark absorbing layer to maximize the energy used to heat the water. As in the single effect system, the usable solar flux energy available is considerably less than 1 kW/m
2
.
If a 3 effect is used as an example, the maximum available absorbed energy is estimated to be about 450 W/m
2
due to additional wall reflection, refraction, and scattering. The maximum temperature of this lower layer depends on the outside temperature, but is typically below 160 degrees F. This energy first evaporates supply water from the bottom of the lowest effect chamber. Since the temperature drops continually from the heat absorber to the exterior, some water condenses on the upper surface of the lowest effect chamber. This condensation gives the 540 cal/g due to the heat of vaporization back to the partition. Water is also run down the top of the second partition, and this water absorbs this energy of condensation. Since the temperature of the second partition is still reasonably high (but lower than the lowest partition), water evaporates from the second partition, and is condensed at the third partition up. The same process continues to the final partition
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