Low cost stirling engine and water purifier

I found this project by Dean Kamen to be interesting, The filter will require a low cost Stirling engine using local biomass,

http://www.plasticsnews.com/article/20130904/NEWS/130909978/plastics-play-a-role-in-dean-kamens-water-purification-system

I checked out this system just a moment ago. At first I didn't understand why electricity should be required, but I understood after a few minutes of consideration. It's very clever. A compressor is used to increase the steam pressure allowing it to condense at a higher temperature. The higher pressure steam can now be used to heat more water and make more steam that is supplied to the compressor, which allows it to make more steam at a higher pressure and distill the water faster, which supplies more steam to the compressor, etc. Basically, the heat in steam gets leveraged by the compressor to a higher temperature. Since the steam temperature doesn't have to be very much higher than the water to boil it (assuming very good heat exchangers are used), then the work required from the compressor is relatively low for the mass flow rate of steam sent through the compressor. In other words, the compressor has to supply a lot of steam at a higher pressure, but the pressure increase is very slight... I don't know the specs, but just a few psi would be enough. That seems the toughest part (getting a good compressor design to work well here). The efficiency of the compressor and the heat exchangers are what limits the output.

Note that the latent heat in the steam is not allowed to be dumped to the air... it keeps getting reused to heat water and make more steam, and this is possible by the leverage provided by the compressor. According to one source the system generates about 250 gallons of distilled water in 24 hours of continuous operation at 1 KWe. This is 24 KWh of electricity, or 81,888 btu. If this electricity was used to heat water directly in a still, then it would yield only 9 gallons. Of course, one could use the fuel otherwise burned in the Stirling engine to distill directly, and this would provide about 60 gallons of distilled water.

I think this is brilliant. Although, I do think it can be simplified. For example, it's possible to compress the steam with heat. In this case the water to be distilled is supplied to the heaters with a pressure pump. The first heater is heated by a small furnace. This generates steam at a fairly high pressure, and this high pressure steam heats another stage, which generates more steam to heat another stage, which generates more steam to heat another stage, etc. Only about 4 or 5 stages would be enough for the same amount of fuel used to power the stirling engine to provide the same amount of distilled water, and the pump would consume electricity at a rate of about 50 watts (instead of 1000 watts!). Note that it should be possible to use some of the heat from the system to power a small plunger pump with a linear organic rankine engine, so the system can be fully heat powered.

Bill Mcgee : Thanks for finding and posting this, this system was of course created to be used for Long periods of time with little or No maintenance in third
world counties, About the first thing that needs to be done is research whether this is covered by Creative Commons, or if it was 'protected' by the regular
patenting process, I do not think that Coca Cola is doing this totally out of the kindness of its corporate heart !

Marcos Buenijo : As you say, most of the performance margin must be carefully preserved/protected by the clever use of good heat exchangers, what I
want to hear is that with enough insulation we can 'just' increase dwell time for the heat streams as they pass through the heat exchangers.

I know a glass blower who works from his home, working his own hours, who can make/repair -recycle a distillation tube, and he supports
himself making high-end glass Christmas tree stuff, can improved insulation use and due diligence over come price here?

I personally will need a refresher course to understand how much of the Sterling cycle I will have to know to be able to operate such a unit. My Excuse for
my ignorance is due to the apparent lack of a practical use of the work cycle of a Sterling, that having been proven, I would like to play catch-up and at least
rise to the point of understanding my own ignorance !

Part of what I am saying is can you expand on the information shared here, hopefully helping us see how to take advantage of this cleaver re-alignment of
parts and technology ? For the good of the Craft ! Big AL

allen lumley wrote:Marcos Buenijo : As you say, most of the performance margin must be carefully preserved/protected by the clever use of good heat exchangers, what I want to hear is that with enough insulation we can 'just' increase dwell time for the heat streams as they pass through the heat exchangers.

I personally will need a refresher course to understand how much of the Sterling cycle I will have to know to be able to operate such a unit. My Excuse for
my ignorance is due to the apparent lack of a practical use of the work cycle of a Sterling, that having been proven, I would like to play catch-up and at least
rise to the point of understanding my own ignorance !

Part of what I am saying is can you expand on the information shared here, hopefully helping us see how to take advantage of this cleaver re-alignment of
parts and technology ? For the good of the Craft ! Big AL

A stirling engine is not required. It's mentioned here only as a means to generate the electricity required for the system. I suspect a stirling engine was selected for its wide fuel capacity since many in "third world" regions do not have ready access to refined fuels. However, I prefer a system that does not require electricity, or that requires a great deal less than this system. When I first read the description of the product, I was bewildered at the suggestion of providing a product for "third world" use that requires 1 KWe input. That is serious power! However, the system does leverage that energy well. Still, it IS possible to devise a system to require a great deal less electricity, and even no electricity. Consider the stirling engine at 10% overall efficiency (fuel to electricity). If providing 1 KWe, then the system requires 34120 btu per hour of fuel (0.3 gallon of gasoline, or 5 pounds of dry wood). A highly efficient (but conventional) water distillation system could use this fuel directly to make 3.8 gallons of distilled water. The system here uses the same fuel burned in a stirling engine to make 1 KWh of electricity, which in turn makes three times as much distilled water. However, it requires the stirling engine and the very sophisticated distillation device. Hmmm. Why not devise a system powered by a small biomass furnace, a small pressure pump, and using a series of small heat exchangers and small insulated pressure vessels to do the same? Furthermore, the pump may be heat powered since it's power is low, and this would make a fully heat powered system possible. What I'm saying is that a fully heat powered system can be had that uses the same fuel burned in the stirling engine used to power the system that requires 1 KWe, and yet provide the same amount of distilled water. Furthermore, a simpler system like this (no stirling engine and no 1 KW compressor) that generates less distilled water seems justified for the additional simplicity. I think three stages is sufficient to justify it.

Here's the basic idea (note that this is just to illustrate the basic principles, I am not suggesting a system actually be constructed this way): Imagine a pressure cooker filled with water to be distilled. Connect the steam line from the pressure cooker to a heat exchanger placed inside another vessel filled with water to be distilled. Heating the pressure cooker will generate steam at a pressure above atmospheric. This pressurized steam moves to the heat exchanger, and since this steam is at a temperature well above 212F, then it will boil the water inside the second vessel. The steam in the heat exchanger condenses, and it moves out a drain line that sets pressure in the pressure cooker using a relief valve (keeps pressure at a particular value). This condensate leaving the relief valve is collected as distilled water. Now, the steam from the second vessel is condensed and also collected as distilled water. So, this system nearly doubles the amount of distilled water generated. Already we've gone from 3.8 gallons of distilled water to about 7 gallons. With one more stage, then a good system can provide nearly the same amount of water as the more sophisticated design.

Note that there is water pumped into each vessel, and this water must be pressurized. This requires a pressure pump. The steam leaving the final stage (the steam at atmospheric pressure) can be condensed in a heat exchanger through which this water is pumped. This will increase the efficiency of the system by preheating the water in this way. Ideally, the system will provide enough stages so that the rate at which water flows to the vessels is sufficient to fully condense the steam from the final stage. However, I suggest not making so many stages so that enough steam is available from the final stage to power the pump. The steam has no pressure to work with, so I suggest using the heat in the steam to heat an organic working fluid that can produce pressure to power the pump. Alternatively, one can use an electric motor to power the water pump, and this is probably the most practical way to go about it. However, a fully heat powered system can absolutely be done. I have already devised such a pump for another project, so I know it can be done. The system would allow for starting a fire in the furnace, connecting a hose to the dirty water source to supply water to the pump, then the unit will generate hot distilled water as long as the furnace is operating - no electricity, and it can provide the same amount of distilled water as the 1 KWe system with the same amount of fuel used to power the stirling engine that generates electricity for that system.

NOTE: A steam engine can be used to power a compressor directly, and the steam engine exhaust can provide heating as well. Basically, the steam engine exhaust is condensed at increased pressure to heat the water to be purified. This steam distillate generated moves through a high volume but low pressure steam compressor which sends the high pressure steam through a heat exchanger also contained in the water to be purified. The compressor here must not use lube oil and must not contaminate the steam as this steam is used to provide the final product. Since the temperatures and pressure are so limited, then many plastic products can be used. Considering the losses in generating electricity and then the losses in the compressor motor, and being able to use the heat from the engine that is wasted in the stirling engine in heating air, then this seems a very good idea to me. HOWEVER, getting a good compressor would be difficult. A compressor has to deliver steam at a very high mass flow rate and do so with low friction, so this would be a sophisticated device. Another prospect can be to use the heat from a furnace to power a simple and small low efficiency piston steam engine which also provides its steam exhaust at high pressure to the first stage of the kind of system I suggested previously. The engine can be used to power the pumps required of the system.

HowStuffWorks covers the Slingshot Water Purifier:

http://science.howstuffworks.com/environmental/green-tech/remediation/slingshot-water-purifier.htm

Marcos,
Thank you for your brilliant analysis of this system. And also simpler alternatives. It looks like Kamen and his DEKA company has been working on small Stirlings for years. He rides a motorcycle powered with a Stirling and is working with European automakers to use Stirlings in hybrid cars.
It looks like his technology is being developed for off-grid and decentralized grid-tied fuel cells, as well.

I wonder if the Slingshot water distiller is a solution in search of a problem. It does seem awfully high tech for applications in the poorest parts of the world. On the other hand Coca Cola manages to deliver soda in these areas. I would guess they would be selling the water? The stirling generator is said to be the size of a washing machine. Coke has a network of mom and pop shops selling from houses in every village. I wonder if this will also be a co-generator providing electricity. I could see solar to preheat the water for the slingshot distiller

I'll offer another way to explain this system that might clarify it for some readers. Consider a heat pump. These use a refrigerant compressor to increase the pressure of a refrigerant vapor so that it can be condensed. This causes the refrigerant to release its latent heat at increased temperature. The now liquid refrigerant is passed through a restrictor to a low pressure region where the lower pressure allows the refrigerant to vaporize at a lower temperature. It does this by picking up heat from its surroundings.

This system is similar, but it's not a closed cycle, and this can confuse. What this system does is operate a heat pump using water as the refrigerant. However, instead of releasing its latent heat to outside air (which is what air conditioners do), or to inside air (which is what a space heating heat pump does), this system releases the latent heat into water for distillation purposes. Since the compressor here obtains its steam from the same water source, then this process is a positive feedback - that is, the more steam generated, the more steam is supplied to the compressor, and the more steam is used to heat the water, and this supplies more steam, etc. The process is limited by the capacity of the compressor. The steam that moves through the compressor condenses to provide the end product. There may be excess steam generated that does not move through the compressor, and this steam would be condensed in a heat exchanger through which water is pumped before entering the system thereby getting preheated. It's simple, but brilliant... I suspect the main hurdle was getting a compressor to work well in this application.