Water Desalination Process through Solar Evacuated Tube Collectors and Vacuum Membrane Distillation

Stephen Yaghmour Author

08/04/2020 Added

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This presentation gives a numerical analysis for the utilization of Solar Evacuated Tube Collectors and Vacuum Membrane Distillation with respect to a Water Desalination process.

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[00:00:02.642]Hi my name is Stephen Yaghmour and I am a
[00:00:05.332]member of Dr. Nejati's Research Team, and
[00:00:07.972]today I will be discussing water desalination process through solar
[00:00:11.212]evacuated tube collectors and vacuum membrane
[00:00:14.152]distillation. So first as in introduction, the whole
[00:00:19.822]topic surrounding this project that we have
[00:00:21.782]is water scarcity. Water scarcity's prevalent
[00:00:24.272]around the world today. With less than 0.01%
[00:00:27.367]of the Earth's water being readily
[00:00:29.737]available and readily consumable for humans,
[00:00:31.907]many either continue to thirst or will turn to
[00:00:34.467]unsanitary resources to find water,
[00:00:36.487]to be able to drink water. Current methods
[00:00:38.702]for water desalination, in addition, are not
[00:00:41.162]energy-efficient, and they can contribute
[00:00:43.472]to global warming. In the United States
[00:00:45.562]alone, over 44 million tons of Greenhouse
[00:00:47.932]Gases are emitted each year due to water treatment
[00:00:50.882]processes. Solar energy, as a solution, can be
[00:00:54.395]used for water desalination while lowering
[00:00:56.705]energy expenses. This is a solution that
[00:00:59.025]we potentially have. The Evacuated Tube
[00:01:01.255]Collectors, or ETCs, can be used to
[00:01:03.455]efficiently harness the sun's energy and thus
[00:01:06.235]efficiently allow for energy to be used
[00:01:08.465]in this process. This energy can be used to perform
[00:01:12.135]Vacuum Membrane Distillation, or VMD,
[00:01:15.105]over seawater to produce clean water. This
[00:01:18.205]study that I have here analyzes the overall
[00:01:20.605]numerical models for both solar ETCs and
[00:01:23.335]VMD for this desalination process.
[00:01:26.885]This overall design of the water desalination
[00:01:29.475]system was designed by the entire research
[00:01:31.755]team of Dr. Nejati while adhering to two basic principles.
[00:01:35.185]The conservation of mass, which states that the sum of the
[00:01:38.455]mass of the inlet streams equals the sum of the mass
[00:01:41.245]of any outlet streams. And it also adheres to the
[00:01:44.075]conservation of energy. The conservation of energy
[00:01:47.575]is represented in most intersections that we
[00:01:50.205]see here as the following:
[00:01:52.255]The sum of the change in enthalpies of the
[00:01:54.735]inlet streams equals the sum of the change
[00:01:57.305]in enthalpies of the outlet streams.
[00:01:59.635]Other intersections, however, may have some heat
[00:02:02.505]input or output represented by Q. And we will
[00:02:05.537]see an example of this shortly.
[00:02:08.507]To model the overall system that was created,
[00:02:11.967]MATLAB was utilized to analyze the mass flow rate,
[00:02:14.847]the salt water concentration, the temperature,
[00:02:17.047]and the enthalpy of each stream within the system.
[00:02:19.736]When enthalpy was known, the corresponding
[00:02:22.146]temperature of a stream was calculated using
[00:02:24.746]the bisection method, which is
[00:02:26.256]essentially a method in which you
[00:02:28.206]guess a couple of approximate values
[00:02:30.456]for what the solution could potentially be
[00:02:32.646]and you essentially narrow your options -
[00:02:35.256]you pinpoint your options until you have
[00:02:37.506]an exact solution of what the value may be.
[00:02:41.136]And so, in this study here, not every single
[00:02:44.033]component of this water desalination
[00:02:46.723]system is analyzed, only the solar ETCs and
[00:02:50.653]the VMD is analyzed here. So first, we're
[00:02:53.273]starting off with the tube collectors.
[00:02:55.643]The Solar ETCs were analyzed, I mean were
[00:02:58.483]utilized to allow for the transformation
[00:03:00.873]of solar energy to thermal energy
[00:03:02.953]such that the inlet seawater could be
[00:03:04.813]raised to a high enough temperature for
[00:03:06.573]the VMD to be effective. And here we have the
[00:03:09.033]overall schematic of the process. Here
[00:03:11.573]in Stream 1 we have the inlet seawater
[00:03:15.123]and so Stream 1 isn't just directly
[00:03:18.134]the seawater. This doesn't come directly
[00:03:19.964]from the sea. First it does pass through
[00:03:22.004]a series of heat exchangers to thus
[00:03:23.744]raise the temperature a little bit before
[00:03:25.544]getting to the solar collectors. This was
[00:03:27.604]not represented here, however, for simplicity.
[00:03:30.494]Stream 1 is met with Stream 5, a recycle stream,
[00:03:33.494]and is joined to combine to Stream 2. Stream 2
[00:03:36.454]is sent to the solar collectors. And here
[00:03:39.084]is where we have this heat input Q. Heat is added.
[00:03:43.084]The solar collectors absorb heat and that heat is
[00:03:45.729]transferred to Stream 2 to create
[00:03:47.309]Stream 3. This heat in Stream 3 is sent to a
[00:03:50.089]Buffer Tank, which is then split off
[00:03:52.319]into two separate streams: the water fed
[00:03:57.299]to the VMD Stream 4 and the recycle stream Stream 5.
[00:04:01.389]And so here is what the overall system
[00:04:04.164]would look like. To model the energy that's
[00:04:07.824]captured, a single equation is able to be
[00:04:10.644]used. This equation here. And the variables:
[00:04:13.994]Q-SA is the solar energy that is absorbed
[00:04:16.974]through the solar ETCs, tau right
[00:04:20.244]here is the transmissivity of the ETCs,
[00:04:23.264]alpha is the absorptivity of the ETCs, and
[00:04:26.914]capital A is the surface area of the ETCs.
[00:04:30.564]I, which is a function of t, represents
[00:04:32.985]the solar irradiance at a certain time t.
[00:04:35.905]This function I(t) can be approximated as
[00:04:39.145]this function here. A few of the variables
[00:04:42.165]here: t0 is what we refer to as solar noon.
[00:04:45.605]Solar noon is the time at which the maximum
[00:04:48.075]solar irradiance would occur. In most of
[00:04:50.415]the United States, the average solar irradiance
[00:04:52.895]would be approximately about noon.
[00:04:56.225]I0 is the solar irradiance at time t0,
[00:04:59.975]or essentially the maximum solar
[00:05:01.385]irradiance throughout the day, and d sub e
[00:05:03.755]is the number of hours of active irradiance
[00:05:06.175]in a day. In the United States, this number
[00:05:09.075]is approximately 20 hours. We have about 20
[00:05:11.505]hours in which the solar irradiance
[00:05:13.275]will be active and would be meaningful.
[00:05:16.685]And so here is a quick plot of what this
[00:05:20.775]function would look like. As we can see,
[00:05:22.505]as we approach hour 12 we're at our
[00:05:24.625]maximum solar irradiance with an approximate
[00:05:27.005]cosine graph decreasing as we go from 2
[00:05:30.255]as we get to 22. And here the
[00:05:32.095]dependent variable is the hour of the day while
[00:05:34.645]measuring the solar irradiance represented
[00:05:37.365]in watts per meter squared. And so as
[00:05:41.675]mentioned earlier a Buffer Tank is used,
[00:05:44.725]represented right here, to accumulate
[00:05:46.714]the heat that is absorbed by the solar
[00:05:48.594]ETCs. This accumulation is important as it
[00:05:50.794]allows for the heat to absorb in that
[00:05:52.954]buffer tank to allow for the temperature
[00:05:55.544]to essentially increase or decrease
[00:05:57.654]throughout time, and we want this temperature
[00:06:00.064]to increase so that temperature 4, or the
[00:06:02.424]temperature of Stream 4, can increase.
[00:06:05.474]That is essentially our overall goal here
[00:06:07.154]we want the stream, we want the
[00:06:08.874]temperature of Stream 4 to increase so that
[00:06:11.174]we have an effective VMD process so we can
[00:06:14.604]get the clean water. And so here we can see
[00:06:17.714]a model once again through MATLAB that
[00:06:19.864]as we start at time t = 12 or noon, we
[00:06:24.152]will start at 0 and the temperature of the
[00:06:27.902]Buffer Tank will increase to about 70
[00:06:29.952]before it just slightly decreases, and
[00:06:32.072]this slight decrease is due to the solar irradiance
[00:06:35.002]slowly decreasing from noon to 2:30 since we
[00:06:39.002]passed our maximum time for solar
[00:06:41.172]irradiance. And so the Buffer Tank here is able to
[00:06:50.142]allow for Stream 4 to have its increase in
[00:06:52.632]temperature so an effective VMD process
[00:06:55.062]can take place. The properties of the overall
[00:06:57.752]solar collector system determine the amount
[00:06:59.772]of clean water that can be produced
[00:07:01.482]per amount of seawater fed. For example
[00:07:03.882]the number of ETCs used can increase the
[00:07:06.202]clean water production as represented
[00:07:08.302]by this figure here. As we can see, as we
[00:07:11.832]increase the number of ETCs, or the
[00:07:15.572]surface area of the ETC, we will have
[00:07:17.812]an increase in the mass flow rate of water
[00:07:20.262]recovered in kg/s, so we can see
[00:07:23.122]this bottom plot here represents N = 1.
[00:07:27.122]N = 1 is a basis, some arbitrary number of
[00:07:30.052]ETCs or some arbitrary surface area and
[00:07:32.762]N = 2 is double the number of ETCs here
[00:07:36.016]N = 3 is triple the number of ETCs here.
[00:07:39.902]And so this directly correlates to the amount
[00:07:41.572]of seawater produced and to optimize we
[00:07:43.585]want to increase our surface area or number
[00:07:47.705]of ETCs that we use for our solar collector
[00:07:50.115]process. And then this process is, in the
[00:07:53.685]end, utilized to allow for an effective
[00:07:56.205]Vacuum Membrane Distillation process
[00:07:58.385]or VMD process. The VMD system utilizes
[00:08:00.755]a vacuum to create a water flux through
[00:08:02.845]the semipermeable membrane. The overall
[00:08:05.045]schematic is presented here, in which we
[00:08:07.135]Stream 4 which is the seawater from the
[00:08:09.285]Buffer Tank, represented right here
[00:08:11.537]passes right through this Vacuum Membrane
[00:08:14.017]Distillation system to create both Stream 6
[00:08:16.867]which is some remaining seawater which
[00:08:18.977]will be recycled through the process
[00:08:20.807]again and Stream 7 with is a fresh water
[00:08:23.327]permeate. Stream 7 is created due to there
[00:08:27.637]being a significant difference in pressure here.
[00:08:30.621]Creating a vacuum creates a different
[00:08:32.382]pressure and having this pressure be
[00:08:34.812]different from the vapor pressure at which
[00:08:37.762]the water is flowing through allows for this
[00:08:41.182]process to occur allows for this permeate
[00:08:43.802]to occur here. And so this permeate will
[00:08:45.602]actually be a water vapor. It does require a
[00:08:48.647]significant difference in the vapor pressure
[00:08:50.386]though because the water production in Stream 7
[00:08:53.807]is represented by this equation here,
[00:08:55.976]C1 plus C2, here we have our difference term
[00:08:58.836]in which we have the difference between
[00:09:01.146]the vapor pressure of the liquid and the
[00:09:03.506]pressure of the vacuum and we multiply by
[00:09:05.796]the molecular weight of water and the
[00:09:07.586]surface area of the membrane. As we can
[00:09:10.336]see here, we do have two constants C1 and
[00:09:12.796]C2. These constants are dependent on
[00:09:15.126]various membrane properties such as the
[00:09:16.876]membrane's tortuosity, porosity, and
[00:09:19.326]radius. And so, as indicated here by these graphs here,
[00:09:24.005]Stream 6 from noon to 2:30 starts at, if
[00:09:27.075]we start at a basis of 8 kg/s - if
[00:09:29.785]that is the basis we start at - we do see
[00:09:32.145]a slight drop - not too much in the grand
[00:09:34.805]scheme of things, this is a per second
[00:09:36.905]basis it is a pretty significant drop here
[00:09:39.625]but so we drop by about 0.15, 0.15 kg/s
[00:09:46.135]until we slowly increase as time gets to
[00:09:48.445]150 min. Here, in Stream 7, Stream 7 being
[00:09:52.775]the actual permeate that we are gaining,
[00:09:55.005]the clean fresh water. We can see we start at
[00:09:57.715]zero we start right at noon, and we
[00:09:59.975]will increase to about this 0.15, 0.12
[00:10:03.975]value here and we will see a steady, slow
[00:10:07.975]but steady decrease, which once again corresponds
[00:10:11.432]to the decrease in the solar irardiance as
[00:10:14.052]represented earlier. So you can see the
[00:10:16.552]numerical model shows that when we have this
[00:10:19.332]difference in vapor pressure with
[00:10:20.942]the vacuum pressure and we do have
[00:10:22.912]correct temperature at which we can start at
[00:10:25.012]we can find that there will be a
[00:10:27.152]significant pressure difference to allow
[00:10:29.252]for the production of clean water through
[00:10:31.592]the Vacuum Membrane Distillation system.
[00:10:34.042]And the research team is working to
[00:10:35.712]utilize these numerical models in which we
[00:10:37.842]can eventually create this idea - this
[00:10:39.846]concept - into a reality. Thank you.

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Water Desalination Process through Solar Evacuated Tube Collectors and Vacuum Membrane Distillation

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