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Nick Raaum
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Not certain exactly where this idea belongs in this forum as it is an idea that integrates many subsystems like aquaponics, solar thermal ect...Nevertheless I wanted to try to explain a concept I've been dreaming up over the last year and this forum seemed like a good place to get feedback on it. So what follows is my outline of a synergestic integration of several solar technologies into an electricity, heat, food and fuel prodicing system. I believe that this could be realized at relatively simple, and small scales, though have yet to bootstrap an expirement together to do verify. In the meantime looking any feedback on the system outline below. Thanks!







The conceptual plant above consists of two closed loop systems that are integrated to promote symbiosis. These two main systems are the steam cycle and the water cycle.

Steam Cycle:
The steam cycle has two primary functions, to produce electricity and heat. This particular cycle is designed to operate as a baseload micro plant. Its hybrid fuel source accommodates variations in seasonal solar inputs through the ability to efficiently generate from biomass or fossil fuel sources.


Caveat: As illustrated above this design operates around low temperature solar energy. The premise and for this is at this point theoretical, though there exists promising historical data points to suggest it to be a very worthwhile approach to investigate (see Shumar vacuum solar engine). Should this concept however not prove effective, the integrated plant concept could still be realized economically through either integration around an organic rankine cycle or a conventional concentrated solar power steam cycle. I have choosen though to illustrate the integration concept around the novel low pressure steam cycle because I believe it could lead to the most dramatic cost breakthroughs as well as the widest area of impact.

Condenser: The condenser is an industry standard heat exchanger designed to operate at 1.5psia of pressure. Cooling water is provided from the aquaculture pond, temperatures from this pond will be moderated to an average 75 deg in most climates. After initial evacuation of noncondensibles nearly all vacuum is provided by the condensing of steam into water in this closed heat exchanger. From here the condensate is pumped to the low pressure solar boiler.

Condensate Pump:
The condensate pump will sit in a deep well to provide needed net positive suction heads, its discharge pressure will be marginally above the cycles maximum pressure which will be around 13-14psia.

Solar Boiler:
This boiler will be a cross flow closed heat exchanger where solar heated water at a temperature of 210 deg f will transfer energy to the low pressure condensate. The condensate will flash to steam and pressures of around 10-12 psia will be generated. By operating at these low tempratures and pressure the solar energy will provide the nergy needed for phase change of the condensate to steam. Subsequent superheating will optimize efficiencies while still allowing for 75% of the cycles energy to be derived from low cost solar energy.

Biomass Superheater:
This can consist of several styles of combustors, probably a simple grate style boiler will accommodate the widest range of fuels at smaller energy scales. Unlike conevtnional boilers which require water waters and drums this boiler will simply be a couple sections of boiler tube heating elements arranged in the gas path in a crossflow pattern (inlet steam paired with outlet flue gas). Exiting flue gas is ducted into the algae scrubber for cleanup.

LP Steam Turbine:
To optimize efficiencies blading would need to be redesigned for this cycles low pressures. However even without doing this a used LP from a retired conventional power plant would run on superheated low pressure steam with a total differential pressure of 10 psig.



Water Cycle
The water cycle consists of three main subsystems, the aquaculture pond, the algae scrubber, and the solar water heater collector.

Aquaculture Pond:
The aquaculture pond serves two main purposes, it provides cool water for the steam condenser while also producing warm or even potentially cold water fish. A continual circulation of thermally disinfected solar discharge water aerates and helps control the biology of the system. Additionally low cost algae protein provides a near zero cost symbiotic food input.

Algae Scrubber:
Condenser discharge at 110 deg F is at an optimal temperature for growing alage, this is promted through a managed algae raceway where circulation and water chemistry is optimized for low cost open algae production. Algae growth is further enhanced by scrubbing the plants exiting warm 200 degree flue gas of CO2. This acts as a dual energy recovery technique. Thermal energy is captured in the raceway, while the CO2 enhances algae growth for greater bioenergy yields. The algae can be pressed for liquid fuels, used as a feed source or simply composted as a CO2 sequestering and soil amendment strategy.

Open Solar Thermal Collection System:
To date concentrated solar power (CSP) has only been viable in the sunniest of regions, while organic rankine cycles (ORC) the other low temperature solar thermal option has proven to have too low of efficiencies to be commercially viable. In an attempt at a solar breakthrough this cycle utilizes the best of both low temp systems and high temp systems. This is done through fuel source hybridization. Solar energy is collected at low temperatures with water, under the premise that if this low cost solar energy can provide the bulk of the cycles energy (75%) by providing the heat of vaporization that the remainder (25%)can come from a high temp combustible source and raise overall efficiencies (400% not a typo). This hybridization may provide cost breakthroughs.
In order for that to happen though the collector must take advantage of the lower temperature requirements through a very simple low cost design. As crudely illustrated in the sketchup picture an open trough feedwater system may be able to achieve this breakthrough. The system could consist of little more than a shallow trough pitched and dug in the earth then blacktopped over and covered with three layers of low profile greenhouse film. Water would flow through these shallow troughs and become heated to 120-150 deg on most days. A variation of this design would provide the final heating of water, here a deeper parabolic earth trough is constructed, the walls are lined with mylar (or other low cost comparable reflective) . A mirror reflects the energy gathered in the walls onto the flowing stream of water below and raises temperatures to just below boiling point. System controls optimize flow through these heaters to maximize energy capture. Excess production is stored in an insulated tank for night time and cloudy day reserves.

After water runs through the solar boiler it is pumped back to the aquaculture pond at a temperature of 90 deg F. This water serves as continual supply of thermally disinfected water for the aquaculture system.

Summary:
Though too complex to quickly calculate the proposed system promises greater efficiencies than operation of any of the subsystems separately. Waste heat and flue gas from the electrical thermal cycle becomes a productive input into the other subsystems. What needs to be determined is if this approach will enhance efficiencies and lower economic barriers significantly enough to begin to displace conventional fossil fuels.. Thoughts suggestions??


Solar-Thermal-Plant.jpg
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Abe Connally
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It looks like you would need a full time staff to keep it running. Steam is not a leave it and forget it technology.

But the overall concept does have merit, though some things might need to be tweaked for performance and hands-off operation. I think you are going to have to work with much higher temps to get water to flash to steam, as well
 
Marcos Buenijo
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I appreciate this kind of thinking at it encourages one toward thinking and learning. However, on casual examination it seems that the system you have contemplated here has too high complexity combined with too low efficiency to be practical (particularly for small scale applications). I agree that the efficiency of a low pressure Rankine cycle system can be dramatically increased with sufficient superheating and with extensive heat regeneration (in principle), but the cost of the prime mover alone would be prohibitive for anything but a very large system. More important, superheating saturated steam at low pressure will require an enormous heat exchanger that would likely not prove economically viable (the heat exchanger would be too large, and the radiative heat losses from such a system would be too high).

If your interest is small or medium scale systems that are independent of fossil fuels, then there are many practical solutions that you could consider. What you are considering here is not practical.



 
Nick Raaum
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Thanks for the input Abe. You are correct steam plants are not hands off, and the concept as sketched out here would probably be more applicable to a town size energy system producing between.5-3MW.

Your comment about water not flashing to steam at those temps shows I did a poor job of explaining one of the key features of this proposed cycle. You see in a steam cycle with a condenser, steam is brought to sub-atmospheric pressures to enhance overall cycle efficiencies (by allowing steam to be expanded to lower temperatures). The vacuum needed to drive this cycle is created by condensing steam. In a conventional cycle this condensate is then pressurized to extremely high pressures so that it will be able to overcome boiler pressure. What I am proposing is to not pressurize condensate but instead utilize the fact that low temperature solar energy could be used to boil water under a slight vacuum. This would be inefficient though hence the biomass hybridization...Anyways I will think about how to clarify this idea, also FYI this was done succesfully pre WWI by a famous American engineer who among other things pioneered solar power and patented this "vacuum steam engine" concept which I am describing http://books.google.com/books?id=ygQiAQAAMAAJ&pg=PA28&lpg=PA28&dq=frank+shuman+steam+vacuum&source=bl&ots=eJY7p4hT9K&sig=9880F3eidUM9NoHKGlNrU2zSseg&hl=en&sa=X&ei=hIWpUMjXDcSVqgGxn4CACg&sqi=2&ved=0CEMQ6AEwBQ#v=onepage&q=frank%20shuman%20steam%20vacuum&f=false

The cycle could be designed to work in a very hands off scenario at much lower efficiencies without the biomass heat input. At those efficiencies (~3-5%)I would not bother trying to produce electricity, instead I'd simplify it into an integrated self energy supplying aquaculture system with water heat output for house heating. The steam would then drive a simple free piston water pump, which could be used both in the aquaculture system and also in the transport of hot water to the house heating system.
 
Nick Raaum
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Marcus, Thanks for the input. I do agree that the low pressure steam system is quite an unknown. However if either an organic rankine cycle or a high temperature solar steam powered cycle were utilized I would argue that this system would only be the combination of several existing and functional systems in a way that closes the cycle and makes them more efficient than they would be on there own. This sort of mutualism seems to be a big part of natural systems, and a big part of the permaculture philosophy. What I am exploring in this idea is whether the concept of synergestic integration can be applied to industrial systems.
 
Nick Raaum
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Marcus, also I am not certain if the costs to a low pressure heat exchanger would prohibitive, the reason is that though a greater surface area would be required, the pressures are negative, so the high safety factors and very conservative boiler code that governs dangerous high pressure steam would go away as the system would be relatively failsafe (hole in a line results in air in leakage and system shutdown not explosion). This would also mean that though the volume would increase the thickness would decrease, and reduce the amount of mass used on a sq surface area basis relative to a higher pressure heat exchanger. Also radiative losses could be a non issue if "boiler" was essentially a large pipe inside of a pipe were the flame was completely contained within and steam flowed inbetween the walls, though yes higher volume would equal higher convective losses. I need to do the math to quantify those.

I am also not certain about too low of efficienices, when I do the math for a rankine cycle like this without any regeneration operating at 1000 deg F I get over 30% mechanical output efficiencies, that iseems pretty decent for a cycle operating on 75% solar energy.

The prime mover is indeed the sticking point, so what I'd propose is to run the system at scales large enough that you could buy industrial scale Low Pressure Turbine cycles and condensers.
 
Marcos Buenijo
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Nick Raaum wrote:Marcus, also I am not certain if the costs to a low pressure heat exchanger would prohibitive, the reason is that though a greater surface area would be required, the pressures are negative, so the high safety factors and very conservative boiler code that governs dangerous high pressure steam would go away as the system would be relatively failsafe (hole in a line results in air in leakage and system shutdown not explosion). This would also mean that though the volume would increase the thickness would decrease, and reduce the amount of mass used on a sq surface area basis relative to a higher pressure heat exchanger. Also radiative losses could be a non issue if "boiler" was essentially a large pipe inside of a pipe were the flame was completely contained within and steam flowed inbetween the walls, though yes higher volume would equal higher convective losses. I need to do the math to quantify those.

I am also not certain about too low of efficienices, when I do the math for a rankine cycle like this without any regeneration operating at 1000 deg F I get over 30% mechanical output efficiencies, that iseems pretty decent for a cycle operating on 75% solar energy.

The prime mover is indeed the sticking point, so what I'd propose is to run the system at scales large enough that you could buy industrial scale Low Pressure Turbine cycles and condensers.


In principle, I think the concept is viable. My main grievance was that I don't see it being competitive with small scale alternatives. Now that it's clear the system cannot be small scale, the question is whether or not it can be competitive with large scale alternatives. There's no way to answer this without knowing the actual costs involved.
 
Nick Raaum
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Marcos, Well coal plants are being retired and being pushed into retirement quite frequently, you could land a semi suitable LP turbine for less than $100/kW of course that still is a fraction of overall system cost. I still think there is a way to build an efficient steam driven thermal engine at a small scale, perhaps open sourcing, and CNC machining will drive down barriers to producing turbines on a small scale. If not then there is always modernizing the steam engine. The biggest barrier to an efficient steam engine is figuring out how to lubricate at high temps as that puts a limit on cycle efficiency.

My interest in this idea has nothing to do with me wanting to go off grid, I know how to do that. What I am seeking is a system that is non dependent on fossil fuel, simple enough to be manufactured and maintained regionally, but still produces enough surplus to have something beyond a subsistence economy. PV and wind do not do this, they are still nursing off of the industrial teat so to speak, and it is unclear (EROEI) if they will ever get off of them. If you do the math on biomass you know that won't work, so it really suggests to me that integrated solar thermal systems really might be an important approach to evaluate.
 
Marcos Buenijo
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Nick Raaum wrote:Marcos, Well coal plants are being retired and being pushed into retirement quite frequently, you could land a semi suitable LP turbine for less than $100/kW of course that still is a fraction of overall system cost.


There are just too many unknowns here to make any rational calculations with respect to long term costs.

Nick Raaum wrote:I still think there is a way to build an efficient steam driven thermal engine at a small scale, perhaps open sourcing, and CNC machining will drive down barriers to producing turbines on a small scale. If not then there is always modernizing the steam engine. The biggest barrier to an efficient steam engine is figuring out how to lubricate at high temps as that puts a limit on cycle efficiency.


There is a way. Unfortunately, it will take a lot of time and money for development. For small scale (less than 1000 hp) the piston engine is the way to go. Cyclone Power Technologies is making a lot of advancements, but their high efficiency engines are rather complex. Their engines are water lubricated to allow for much higher steam temperatures (1200F boiler temps and 1400F+ reheat). Piston rings are made of a high temperature plastic and are actively water cooled - there are other means to protect the rings, and the bearings are all water lubricated as well.

Nick Raaum wrote:My interest in this idea has nothing to do with me wanting to go off grid, I know how to do that. What I am seeking is a system that is non dependent on fossil fuel, simple enough to be manufactured and maintained regionally, but still produces enough surplus to have something beyond a subsistence economy. PV and wind do not do this, they are still nursing off of the industrial teat so to speak, and it is unclear (EROEI) if they will ever get off of them. If you do the math on biomass you know that won't work, so it really suggests to me that integrated solar thermal systems really might be an important approach to evaluate.


Let the prices for fossil fuels continue to rise, then we'll see changing incentives. With respect to biomass - I say biomass to liquid fuels is nonsense. However, certain regions can make use of biomass efficiently to displace the use of fossil fuels. These can be used for cogeneration and some transportation applications using gasification. If you haven't yet done a serious study of biomass gasification for fueling internal combustion engines, then check it out. You might be surprised at what's possible. As it stands now, there is little incentive to pursue these alternatives, and I don't foresee a major move there without much higher sustained energy costs. In any case, I am convinced there is no magic bullet. With respect to integrated solar systems, I rather like the prospect of solar concentrators focused on a central receiver to heat compressed air to drive a gas turbine in a Brayton cycle or similar configuration.

 
Nick Raaum
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Marcus,
I think we are in agreement on most points you made, and my statement about biomass was a broad blanket statement meant only to apply the fact that we can't power our current centralized industrial society with it. It certainly will play a role in future energy, probably most significantly in transport fuels, yes I am aware of gasification and many other effective ways various cellulosic materials could be used for this purpose. Also aware how woody ag and trees can provide this feedstock input without jeopardizing the soil.

You are right, now silver bullet, if anything intelligent regional adaption is the silver bullet. Your CSP Brayton engine is a great example of this, should be a killer cycle in the clear southwest, but not a winner in the northwest. You somehow tripped a memory I have from some late night googling of a guy who was building a solar brayton cycle from piston engines (one acted as the compressor the other the exander) can't find it, but anyways maybe some DIY potential on that one...Also while I'm recalling google anamolies have you seen this solar evaporation cycle ? http://en.wikipedia.org/wiki/Barton_evaporation_engine
 
Nick Raaum
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Here is good conceptual snapshot for a piston brayton cycle....http://digilander.libero.it/digitalrino/warm_air_engine_with_brayton_cyc.htm
 
Marcos Buenijo
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Nick Raaum wrote:You are right, now silver bullet, if anything intelligent regional adaption is the silver bullet. Your CSP Brayton engine is a great example of this, should be a killer cycle in the clear southwest, but not a winner in the northwest. You somehow tripped a memory I have from some late night googling of a guy who was building a solar brayton cycle from piston engines (one acted as the compressor the other the exander) can't find it, but anyways maybe some DIY potential on that one...Also while I'm recalling google anamolies have you seen this solar evaporation cycle ? http://en.wikipedia.org/wiki/Barton_evaporation_engine


You're bringing back my own memories. I considered a piston Brayton cycle to drive directly the blower fan for a desiccant evaporative cooling system for cooling a home in a hot and very humid region. The heat source would be a small updraft biomass furnace. The hot air exhaust from the engine is used to periodically dry one of two desiccant beds in an alternating fashion. I still think that idea has promise, and I'll eventually pursue it when I have the opportunity. Advantages in that application are that low thermal efficiency will still yield good performance, a wide variety of unrefined biomass fuels can be used, and the system could operate for extended periods at low output and low speeds for longevity.

I haven't seen the solar evaporation engine cycle. I'll check it out.

 
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