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Steam Engine
  • Vote Up0Vote Down
    MarkG
     
    February 2013
    As I understand it, OSE is still working on a piston steam engine design. I tend to agree that a piston expander is the way to go for its relative ease in fabrication. Unfortunately (at least from my perspective) the OSE guys seem to be emphasizing high thermal efficiency, and things start to get really complicated when one tries to get high thermal efficiency in a small steam system. So, I'm just throwing out my concerns for anyone who might find interest.

    If biomass is the fuel source, and the system is used on a residential scale or any setting where the waste heat can be put to use, then it seems preferable to sacrifice high thermal efficiency for higher condenser temperature in order to make the most of the heat available from the system. There are so many ways to use this heat that can displace electricity otherwise used. A few examples beyond the obvious like space heating and water heating include water pasteurization, space cooling, biomass fuel drying, air circulation in the home by convection, clothes drying, and food drying. In particular, I think there is promise in a desiccant evaporative cooling system. Here the heat from a high temperature steam condenser can be used to boil the water out of the desiccant for regeneration, and the steam generated from this process can be used again in other heating applications. The distilled water that results is used for evaporative cooling. There are many other possibilities. Basically, I think an emphasis on high thermal efficiency could be counterproductive for some settings. Actually, I prefer a configuration where electricity use is minimized to the point where a micro heat engine is really unnecessary. Let photovoltaics and wind turbines do this. A micro heat engine might be useful as a backup only. However, a biomass furnace can provide the bulk of energy required for a home as a source of heat for the aforementioned applications.

    With respect to a remote solar heat engine where putting waste heat to full use might be impractical, then I can understand a focus on higher thermal efficiency. However, even here it can be impractical. After all, since the solar "fuel" is free, then the efficiency of a solar power system is a four letter word: cost. Sacrificing some thermal efficiency is justified if it can get the overall costs down. I have yet to see a mention of the following configuration on the OSE forums, so I will mention it here. I wonder if using conventional double acting, two cylinder compound, piston steam engines (with slide valves) will yield good results by integrating a heat transfer fluid (such as a thermal oil) and a bottoming cycling using an organic working fluid. In this configuration steam could be generated with thermal oil at a temperature that allows for circulating lubricating oil with the steam/condensate. The exhaust from the first compound engine can be kept at a high saturation temperature to transfer its heat to an organic working fluid selected to keep the same pressures as the steam engine at these lower temperatures (NOTE: see saturation conditions for butane - it might be a good candidate). Sure, there are some disadvantages here, but the advantages include not having to maintain a high vacuum in a steam condenser, and being able to use conventional steam engine systems with little modification. There would be additional pumping losses required for an ORC, but perhaps the higher pressure and smaller cylinders can reduce friction and thermal losses for this to be a wash. There would also be losses involved in the transfer of heat from the steam to the organic fluid, but my research suggests that surprisingly small copper heat exchangers can see very high heat transfer rates when saturated vapor/condensate compounds with high density are used, and with little temperature difference. Anyway, if the OSE guys are having difficulties in developing a piston steam engine with high thermal efficiency, then just toss this idea around.

    NOTE: This configuration would also show minimal superheat on the steam side, and the pressures could be limited to 500 psig or so. This would be a lot safer than many other approaches... especially in a DIY setting.
     
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    ckurtz
     
    February 2013
    MarkG:  Am not sure I follow your thoughts here regarding the claim that "the OSE guys seem to be emphasizing high thermal efficiency". The 'bump' valve piston steam engine design that is currently the only design on the OSE Wiki, is any thing but efficient thermally or mechanically. The same thing holds true for the single-axis electronic tracking strip solar collector design. Higher temperatures may translate to higher operational efficiencies for the specific engine they feed, but they are also always associated with the potential for much higher systemic thermal losses as that is just the nature of the heat energy beast.

    Translation of mechanical reciprocating motion into rotary via wrist pins and cranks is inherently fraught with friction and parasitic power losses with their attendant increased maintenance demands, not to mention all the resources required to manufacture the needless mess to begin with. Why cranks were even included in the design when the OSE emphasis is on hydraulic interfaces is an interesting insight into group mentality. On the solar side, while cheap electronic tracking controllers may now be available the problems associated with maintenance of an electro-mechanical tracking system which now requires an electrical power sub-system still exist, especially ones exposed to all weather conditions such as extreme wind gusts, not to mention tornadoes, which can simply rip such mechanisms apart or just toss trees on them. And when things are going well and the system is producing temps up to 500 degree C, what are the thermal losses going to be from the transport system and/or the called for 3 day storage system? Large delta-Ts create very large problems, which like the fabled Sword of Damocles, are a threat constantly dangling over your head.

    In regards to your concept of putting waste heat to use, this seems to be part of holistic or integrated farming or whatever the term is. Although this term doesn't seem to be specifically used by OSE it is definitely implied in statements such as: "The most important feature of the Global Village Construction Set is its nature as an integrated tool-set or ecology of products that fit together like a Lego set or jigsaw puzzle for building real infrastructures of communities.", "Imagine that now we could transcend this game – by using modern technology to convert sunlight to sustainable energy (solar, wind, biomass, water, others) to process the abundant “dirt and twigs” under our feet into the substance of modern civilization."

    Your concept is certainly valid but the specific means to accomplish it of course depends on what avenues OSE finally chooses to follow in regards to energy collection/generation/distribution. For instance instead of using a dual system of oil and an OCR which is somewhat complicated, there is a 'proven' option to just use air which provides for exactly the low temperature thermal system you seem to be after.

    The fullest expression of this concept is the liquid air system referenced, although not discussed, in this Energy section by Beluga. This is not new technology, but rather can be traced back to the 1890s when air was first liquefied by Linde, I believe, and in a few years Germany had a huge industrial stockpile of liquefied air that supposedly no one understood the reason for. Today of course large liquid air based electrical generation facilities have been the focus of much construction activity supposedly as grid backup installations. The minimal press coverage this activity has received is related to the fact that they are efficient durable scalable environmental energy extraction plants. If the public ever understood exactly what that means, then the old tired argument against compressed air power systems being nonviable because the heat of compression is always lost and that the air freezes when released from the pressure vessels, would die the idiotic death it deserves.

    For instance, lets say that in your scenario instead of using an OCR for heat recovery from a steam engine a system is implemented using a compressed air power system. To generate power the expansion engine of your choice is set between the great out doors and the compressed air tanks. Up-stream of the engine is a heat exchanger through which the hot air produced by the waste heat from the steam engine gives up its excess heat to the compressed air which allows it to recoup its pre-compression energy. The re-energized compressed air then powers through the expansion engine without freezing the poor thing allowing it to run a multitude of applications.

    Simple reliable scalable expansion air engines of the basic piston design everyone seems to love, can be manufactured at a much lower cost than steam engines and can be used for almost any application. Air conditioning and cold storage needs are directly addressed, and work shop tools can be compressed air based which is pretty much the norm anyway. Passive hot water flat panel systems can be backed up by heat pump air-to-water heat exchangers. And speaking of water, the most economical, low maintenance, dependable DIY water pumping system known is the air lift system, which in conjunction with water aeration makes a very desirable package.

    This is all simple proven standard compressed air / refrigeration / heat pump design minus proprietary exotic refrigeration fluids.

    The sweetener with a low temperature energy collection heat pump system that no other system can match is that the storage vessels themselves, not to mention any return lines, are un-insulated energy collection devices as well. Instead of a high temperature energy system subject to continual heat losses, a low temperature system collects environmental energy continuously 24/7. Paint it black, put it in a green house enclosure, and step back. If you wish to augment it with flat plate solar collector panels or under-ground piping, have at it. And to top it all off, industrial systems don't even use mechanical compressors but rather venturi systems with no moving parts and almost zero maintenance.

    Air, like sunlight, costs nothing, it can't be broken, it isn't toxic, and our atmosphere is the largest most accessible solar energy storage medium on the planet.

    The question then is what to use for producing the initial energy to power the air compressors to begin with. I probably shouldn't give in to humorous impulses but what the hay - if it isn't fun why bother. Here is another OSE quote: "All wealth comes from sunlight, rocks, soil, plants, and water - the abundant feedstocks of modern civilization. We are creating an open source toolkit that allows people to convert these resources directly to a modern standard of living - at a cost of no more than 2 hours of work per day. This is an ultimate standard of human potential.... (OSE: Strategic_Plan_2_Ambitious.htm)"

    In the spirit of that vision of excess leisure time and zealous dedicated community members, one option could be summed up by a cartoon of two lines of OSE staffers facing each other over extended handles similar to the old fashion see-saw like fire department water pumpers. The compressor handles are blurred with the flying effort, and the caption reads: "Marcin, get your butt down here, the induction furnace is coming on line and we need another erg!!"

    Well, if that doesn't get me kicked off the forum nothing will.
     
  • ckurtz, forgive me for not replying to all your comments, but that is a rather daunting task. My only purpose in suggesting the aforementioned solar heat engine configuration is that it can achieve a thermal efficiency in the range that OSE has discussed (17-26% net thermal efficiency as I recall, engine cycle only) with relatively low peak temperatures (500-600F), and using existing expander designs that are relatively easy to fabricate. Lower peak temperatures will lessen the thermal losses that you heralded before (and yours is a good point). Old compound steam engines are simple and durable, so it's a possibility that should not be casually dismissed. Also, I emphasize (again) that thermal efficiency on the engine cycle is not so important as overall costs (especially over the long term). Again, in the stationary setting where heat can be put to productive use, then high thermal efficiency may not be desired. Furthermore, in the stationary setting where direct solar is the heat source, then seeking a thermal efficiency beyond a certain point may also be counterproductive due to the high peak temperatures required which tends to increase the engineering costs on many fronts. I fear that a myopic focus on high thermal efficiency might be symptomatic of an emotional need to seek extremes (i.e. mechanics often sacrifice cost and common sense for high power, and engineers and physicists often do the same for high efficiency). What we need here is just something that actually works reliably and can be fixed easily should a problem arise. Beyond this, then let the overall costs set the thermal efficiency.

    As far as the compressed air systems go, well, you mentioned it yourself... how to compress the air in the first place. Energy storage is a problem secondary to the acquisition of this energy. Compressed air could be a good energy storage medium if mechanical energy suitable for compressing the air were in abundance, but there are significant losses there not unlike those seen in heat storage... if not more. This might be overcome with, well, heat storage to raise the temperature of the compressed air beyond ambient, but then that's getting ridiculous. Making the most of limited peak temperatures, and using a fairly simple, reliable, and cost effective heat engine design, seems the most practical way to go.

    Also, I'm actually more optimistic about the use of biomass gasification for fueling internal combustion engines, and with extensive waste heat recovery. However, I would favor a biomass-fueled piston steam engine if a good system were available.
     
  • MarkG:


    My apologies for seemingly going somewhat off topic in my previous comment. It seemed, as I mentioned, that you were attempting to look at steam engine design from a more holistic perspective, which I agree with. However my perspective was apparently a step or two even further back, which perhaps made my comments rather irrelevant to your concerns.
     

  • I've been aware of this for a while. I like the idea. In my opinion, it's not suitable for micro scale CHP, but I think it has promise for medium scale installations - and this seems to be the target market. 

    The way I understand this system based on what I read, and extrapolating from my knowledge of steam power, is that a two cylinder compounded engine is used. These systems provide steam at high pressure to a small cylinder where the steam expands, then exhausts the lower pressure steam to a larger cylinder for additional expansion before finally exhausting to the condenser. The system here uses Skinner unaflow engines that were manufactured as late as the 1940's. These engines had a reputation for high efficiency, and extreme reliability. During the day when the solar concentrators are generating high pressure steam, then the steam is sent to the high pressure cylinder. The larger low pressure cylinder is not used when additional energy storage is desired. Rather, the steam is exhausted to a large insulated steel pressure vessel filled with water. The steam exhaust raises the temperature and pressure of the water. When sufficient energy storage is achieved, or whenever higher power is desired, then the low pressure cylinder may be used in addition to the high pressure cylinder. When solar in not available (such as at night), then high pressure steam is not available, and the system draws low pressure steam from the pressure vessel by flashing water to steam to drive the low pressure cylinder. Turns out that there is negligible loss of efficiency by going this route. That is, while the engine is most efficient when full expansion is achieved with both cylinders, the energy otherwise used to drive the low pressure cylinder is stored in the pressure vessel for later use. Since the efficiency of these engines approach 20%, the overall efficiency of this system can be quite high with good concentrators. More important, the system achieves 24 hour power generation from solar without batteries, and the storage system is the most cost effective I've yet seen. I've argued before several times that when the "fuel" is free (as in solar), then "efficiency" is a four letter word: COST. For medium scale distributed solar power, this solution looks great to me.

    The discussions on the web site about the benefits of piston steam engines in lower power ranges are spot on. Turbines are great for high power and constant output. Below a certain power, the piston steam engine is more efficient, and the efficiency of piston steam engines vary little as the power varies. So, anything below about a megawatt favors the piston steam engine.
     

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