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Energy positive, "all inclusive" engineering concept

  1. Jun 25, 2014 #1
    So this is my first post to PF, but I've been a long time reader. I graduated with a civil engineering degree a year ago and since just before graduation, I've had an engineering concept that has stayed in my mind very persistently for the entire time. I will warn you ahead of time; this will probably be a lengthy post as there is a lot of moving parts.

    I have a solid grasp of general engineering mechanics, but I have little advanced aptitude in practical mechanical/electrical systems. The system I'm going to lay out here is definitely something that I would not be able to successfully optimize myself, especially if scaled, but I would appreciate candid feedback anyways.

    The idea stems from three major efforts underway in renewable energy/sustainability:
    1) Access to potable water
    2) Renewable electricity
    3) Hydroponics (somewhat of a lesser extent; this idea came into play after the initial concept was thought through)

    All three of these technologies have massive amounts of resources currently being funneled into them, but the money is focused on exclusive development of the technology. The motivation in this case is to combine these three ideas into a single system that can produce the following (albeit in a manner that is less efficient compared to an independent manufacturing process for each product):
    - Mostly deionized water
    - Minerals to be used in high quality fertilizers
    - Electricity (enough to run on site processes; possibly enough to sell excess to the grid)
    - A highly productive producer of organic protein and plants.

    How it works:
    So the technology that inspired the parts of the system include the solar trough invented by some grad students at MIT and various hydroponics inventions of the last 30 years.

    The "plant" will be located ocean-side in a sunny, lower latitude location, positioned in such a way that it sits just below sea level. The first step in this process is to use a gravity fed filtering mechanism to take water from the ocean to a shallow basin located somewhat near the shore. After the water has been filtered of sediments, debris, large organic matter, etc., it will fill the basin until the design capacity has been reached. The basin itself will be thermally insulated with a composite cover that has a low emissivity, designed to act as a solar oven. The water will be optimized with respect to depth and area coverage (to maximize radiation exposure). If economical, mirrors can be used to increase radiation exposure.

    The flooded basin will then be sealed and subject to the sun's radiation. The shallow water will reach the boiling point in quick order, as would a solar oven, and it will begin to vaporize in the containment basin. When enough pressure has built up and the steam has become hot enough, it will work its way through a system of collection pipes located at the high point of the basin (spaced in the most efficient manner) to either a heat exchanger or turbine, whichever is determined to be most efficient. Vapor that condenses within the system of boiler feed pipes before reaching the turbine will work its way back to the basin through a return capillary mechanism attached to the bottom of the boiler feed pipes.

    This vapor will then pass through a turbine or heat exchanger that will include a carbon scrubber. The idea is to capture energy from this steam while trying to capture the carbon dioxide that will be leftover from dissolved carbon in seawater. After the vapor has passed through, the carbon dioxide will be vented off into the greenhouse which will be discussed later. The vapor that has performed work going through the turbine will condense on the other side and will then go off to a storage facility.

    After an amount of time, basin will be completely dehydrated (hopefully by the end of useful sunlight), leaving purified water, carbon gases to be sent off to the greenhouse, and minerals at the bottom of the basin which will be useful for high quality salt, potash production, etc.

    Now, the final stage of the process, to maximize the utility of the system, is to integrate a greenhouse and fishery at the end. The purified water can both be harvested and sold to industries that require highly pure water or it can be sent to a freshwater breeding pool. With water that is sent to the fishery, it will constantly be receiving new, fresh water, optimal for creating a healthy population of fish. The waste that's created, full of nitrates and other plant nutrients will be delivered to the greenhouse to grow plants in a very controllable, efficient way.

    The greenhouse will receive exhaust from the evaporation process, providing ample carbon dioxide to the plants that are growing. The plants will receive nitrate rich water from the fishery waste. This can then be combined with nutrients left over from ocean water evaporating to create a crop that grows with alarming efficiency. The plants themselves will also act as a secondary filtering mechanism for the waste water. With the plants on raised beds, water that seeps through the roots can be captured and recycled to other parts of the facility.

    Finally, the intended results:
    - Create high quality water from seawater as an alternative to other energy intensive methods.
    - Recover high quality products from ocean water which are very desirable and sought after.
    - Create enough energy from the collection process to at least power the other processes within the system; preferably enough to sell electricity to the grid, especially when demand is high (hot sunny days: the days when my system is most productive)
    - Grow fully organic protein and crops with no irrigation water or reliance on outside sources of water in a very efficient manner.

    Clearly the process that I have laid out here is not the most efficient ways to get each of the individual products that it creates. The intention is that the products are created in a manner that is not reliant on fossil fuels, is independent of mass energy, and to a lesser extent, is not climate dependent.

    I know this was very long winded, but the idea has been like a monkey on my back since I randomly conceived it last summer. What are your thoughts on the whole process? What is the appropriate scale?
  2. jcsd
  3. Jun 25, 2014 #2


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    Low emissivity also means low absorption. You'll want a material with a high emissivity (especially in the visible light).

    Good luck trying to boil a whole basin with sunlight. It is hard enough to construct suitable pipes in specialized solarthermal power plants. Even if you manage it: the temperature difference between water at 100°C and the environment is small, so your efficiency will be very low.
    Using it to heat the remaining water in the basin (to get it to boil) looks more interesting, but then you will need external power to run it.

    Anorganic carbon in water has a concentration of ~2.3mmol/kg (source), corresponding to ~90mg/kg CO2 (assuming all anorganic carbon is from CO2). This is below the concentration of CO2 in air (~400ppm or ~600mg/kg). If it would be worth the effort to filter it out of sea water, you could do the same with air. Or with exhausts of coal power plants (~300,000 mg/kg). Even there, it is discussed, but too expensive to be realized now. Forget the CO2 part.

    What remains in your basin is a nasty, highly corrosive mixture of salt, water, some other previously dissolved compounds and dirt. Yes it is possible to get clean salt out of it, but it is a lot of effort on its own. And putting that salty mixture directly on plants is not a good idea to grow them.

    Water distillation is an extremely expensive way to get fresh water for fishes. You can just use rivers.
  4. Jun 26, 2014 #3
    I understand the issues mentioned (and thanks for the emmisivity correction). I definitely don't intend the entire process to be the most efficient for each individual product. But here's my reasoning on some of them:
    1) The basin is very shallow (and wide compared to depth). As in 12-18" deepand 10' wide. It will also have an optimal geometry. What I had in mind was a snaking basin, perhaps several thousand feet long, that wraps in on itself to reduce the spacing needed for steam exhaust.
    2) I definitely understand that the efficiency of the power generated is not going to be that high, but the purpose is how its created. Also, if you insulate it in a similar manner to a solar oven, then it should boil relatively quickly, as internal temps easily reach 300+ in commercial solar ovens. But, as I said, scaling it would be an engineering challenge no doubt. And, if that turns out to be unachievable, then a form of heat exchanger might be more efficient?
    3) The only part I don't necessarily agree with is the perceived uselessness of the water. The optimal location for something like this might be a San Diego, or Dubai, in which there are no natural, permenant sources of fresh water close by. So the fact that your water is created without using any energy that you didn't create yourself is important.
    4) The CO2 scrubber is a generic term. I simply mean to separate the gaseous CO2 from the steam before it recondenses. This is probably doable by having a baffle system just after the exchanger. Vent off the CO2 before it reabsorbs into the feed water.

    Also, proof* that the products left over have mineral value in fertilizer (read the abstract):
    *I'm not denying that there might be an energy intensive secondary process to separate it, but the money from the minerals might outweight that.
    Last edited: Jun 26, 2014
  5. Jun 26, 2014 #4


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    To boil water ~50cm deep, you need about 1GJ/m^2. If your mirrors achieve a 30:1 concentration of sunlight, this would still need a whole day of full sunlight. Good luck with the insulation of such a shallow container for a full day. I don't say it is impossible, but it will be really expensive.
    Using reverse osmosis might be easier here.

    I think it is absolutely necessary to get relevant quantities of water to boil.
    I think those towns are not the ideal places to grow fishes that need a lot of fresh water.
    Why? The amount of CO2 you catch is negligible and some CO2 in the water should not harm for almost all applications.
    Most of it will leave the water before it reaches its boiling point, by the way.

    Sure, sea water is used both for fresh water and for salt. That's nothing new.
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