The vapor-compression process uses mechanical energy rather than direct heat as a source of thermal energy. This method increases the productivity but also increases the cost. The corrosion damage and scale deposit formation became serious problem.
An improved version of the vapor-compression process is using negative pressure to decrease the boiling point. The operation temperature is lower than 70 C. It lowers the cost and extends the life span of the equipment (see United States Patent Atwell 4,285,776). The disadvantage of this method is relying on vacuum pump to maintain the negative pressure.
3. Summary 1. With the help of gravity, pressure of outgoing end of the siphon pipe (which is full of feed salt water) will be higher than the input end. This pressure forms the negative pressure at the top of the siphon pipe. Salt water will flow through the pipe. The bigger elevation difference of the two ends the higher speed the water will move through the pipe. High water flow speed will depress the scale deposit formation.
2. Pressure and boiling point has positive correlation, the lower pressure the lower boiling point. Low boiling point will lower the consumption of heat energy. Both low boiling point and low pressure will depress the scale deposit formation.
3. Air pressure and speed of evaporation has negative correlation. The lower pressure the higher evaporation speed. Water will be separated from salt solution by evaporation then steam condenses to fresh water. So the lower pressure the higher efficiency of desalination.
4. Using solar energy as heating source, heated by gravity and air pressure and with the help of the siphon structure the salt water can be automatically desalinated by this apparatus.
4. Description The invention solar powered automatic desalination system using simple design structure. It is a combination of multiple-effect distillation (MED) and siphon effect.
This invention has two forms by applying different solar energy heating system.
One is using hidden solar heating storage pool and the other one is using monobloc solar water heater.
Design 1: solar powered automatic desalination system using hidden solar storage pool Three parts construct the system (Figure 1).
First part is a heating system, which includes Solar heating storage pool, alternative heating source and exchanger.
The second part is distillation tower, which is a vertical mounted multi-effect distillation system. It includes distillatory, hot salt water pipe and steam pipe.
The third part is cooling system, which includes cold feeding water pipe, fresh water pipe and brine.
Heating system Solar heating storage pool (11) is a salt-water pool with stable grads of concentration. It is 1-3 meters deep. There are great amount of over saturated salinity at the bottom of the pool. From bottom to surface, the concentration of the solution in the pool decreases, solar energy can be converted into heat in this pool, then quickly goes down bottom and absorbed by the salt dissolving. A stable high temperature region will be formed at the bottom of the pool. This combination of heat collection and storage can provide heating source over seasons, it won't be affected by daytime or nighttime. Experiments show the high temperature region can reach 100-180 degree Celsius. The advantage is it can store large amount of heat energy, but the disadvantage is the big size and weight.
Heat exchanger (12), it will transfer heat from solar pool to distiller (10) and heat the cold feed water for distillation.
Alternative heat source (12) will provide energy when the solar energy is not enough.
Distillers Distillers (10) mounted vertically. There are two spaces in each distiller, one for hot salt water (4) and the other one for steam (5). Solar energy will boil the salt-water force the steam separate from salt water.
Hot salt-water pipe (8) connects the hot water (4) of near by distillers (10);
steam pipe (6) connects the steam (5) of near by distillers (10). Steam pipe (6) zigzag through hot salt-water release energy evaporate more water in the distillers.
The energy release procedure also cools down the steam. That will increase thermal efficiency dramatically. If there are right number of distillers (10) steam's temperature will be lowered enough to condensate to water. Set up separation layer in last few distillers close to the top to ensure the fresh water can be separate from the brine.
Cooling system The cooling container is the expansion of cold salt-water pipe (2). Its bottom must lower than the bottom of the distillers tower. The top of cooling container connects to hot salt-water pipe (8) of the top distiller (10). The bottom of the container is brine discharge pipe. The top of Fresh water pipe (3) connects the fresh water of the top distiller (10), the pipe zigzag through the cooling container release the heat energy.
The fresh water production discharged from its bottom.
Feeding salt-water pipe zigzag through the cooling container cools down the fresh water pipe and the brine, in the mean time it absorb the heat from them and warm up the feeding salt- water.
Design 2: solar powered automatic desalination system using monobloc solar water heater Compare to design 1, distiller tower and cooling system are identical. It uses monobloc solar water heater(s) as heating source. To increase the heating ability, monobloc solar water heaters can mount on top of each other. The advantage of this design is smaller in size and weight and also higher heat collection efficiency. Water temperature can reach 70-150 degree Celsius. But it was greatly affected by daytime/nighttime strength of the sun radiation. Additional heating sources must be added for continuously produce fresh water.
There are three layers of siphon pipe in the system. Cooling container and distillers forms one layer of siphon pipe. Water goes into distillers and come out from cooling container. Feeding salt-water pipe(2) and hot salt-water pipe(8) forms the second layer of siphon. Water goes into feeding salt-water pipe and goes out from hot salt-water pipe. Fresh water pipe and steam pipe forms the third layer of siphon. Water goes into steam pipe and goes out from fresh water pipe. All these three layers have a common feeding pipe salt-water pipe (1) and driven by air pressure. Their common lowest pressure area is in top distiller (10).
The invented apparatus use solar energy, gravity and air pressure as power and with the help of the three layers of siphon, automatically desalinate salt-water.
Compare to MED, this invented system doesn't need vacuum pump and cooling device and reduced the throughput of salt-water. In that way, it lowers the device cost and operation costs.
The invented apparatus can be installed on seashore, islands and even be able to mount on ships. It can also use other heating resources like engine exhaust heat. It can also use for contaminated water purification.
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Fig 1. Illustration of the automatic desalination system 6. Implementation steps The implementation of the invented apparatus should following steps:
Step 1, heating resource check Check the temperature change in the tank of solar heating system. If the high temperature stables, the heating resource is ready. Because the energy collection procedure of Solar heating storage pool is slow, to get it ready for operation need some time.
Step 2, pressure check The invented apparatus will work under negative pressure, so the pressure check turns to be crucial. The test should be done from part to whole system. Each part should be checked before the final construction. To check the whole system, we use water check.
I. Set outgoing valve There are two outgoing valve (7) in the top distiller close to the top (10) and the separator (9) (see Fig. 1) II. Pump in water Close valve (2) and (3), feed water from salt-water feeding pipe(1). When the water runs out from valve below the separation (7), close it. When water runs out from the valve close to the top, close it. Now the distillers and the cooling system are filled with water.
III. Pressure test Close feeding pipe (1), extend the fresh water pipe (2) under the surface of the fresh water tank and the brine pipe (Z) under the surface of salt-water. This will prevent the air goes into system. Open the valve of both pipes (2)(3). Check if water runs out. If the water doesnt run out after one to two days, the system is stable and the pressure test passed.
Step 3: Test operation Open the feeding pipe, fresh water pipe and the brine pipe. Once the heat transferred into distillers the fresh water and brine starts to run out. When the feeding pipe establishes a stable intake, the test operation is done. If the solar energy is not enough to boil the water in distillers, the alternative heating resource should start to work.
Step 4: operation status determination There are four major parameters need to be monitored during the operation.
Fresh water productivity change: Daily monthly and yearly productivity Fresh water quality change: salt remaining, bacteria and other materials Heat efficiency change: Highest temperature changes of the solar heating system, lowest temperature of the pipes changes and amount of energy input through the alternative heating resource.
Pressure change: The pressure changes in top of the topmost distiller.