29 thoughts on “Stirling Engines”

  1. “The Stirling is quieter than an internal combustion engine, and it’s more efficient because the heat is retained inside the engine by to do more work rather than allowed to escape to the environment.”

    All your base are belong to us?

  2. Lots of incorrect information in that article. It says: ”
    The Stirling is quieter than an internal combustion engine, and it’s more efficient because the heat is retained inside the engine by to do more work rather than allowed to escape to the environment. Though Stirlings are highly efficient, they haven’t caught on because it takes them a while to warm up and they can’t change power output quickly. That makes them unworkable for cars and trucks, but potentially ideal both for power generation and water heating.”

    First, quieter than an IC engine? New cars are quiet too; they have mufflers. Second, the maximum efficiency is Carnot (not strictly Carnot, but the numbers work out the same). Takes a while to warm up? Maybe 10 seconds for a well built example. MTI in Latham NY was running a Stirling car around in the 1980s and drivability was the same as an IC engine. The biggest reason you don’t see them in cars is that they cost more. The hot parts get hotter and take higher pressures than typical IC engines, hence stronger, higher temp rated materials. Finally, good for water heating? You can get 98% efficient at that with a far cheaper hot water heater.

    Don’t get me wrong. I like Stirling engines; they’re multi-fuel and easier to get good emissions control. I was really hoping Stirling Energy Systems would be successful with their solar tracking dish. 32% efficiency from solar input to 60 Hz AC out.

    1. Yes you can heat water very efficienly but the neat thing with the Stirling is that you can use your waste heat from the power generation to heat water. That is if you are making a cabin in the woods and you compare the difference between a gas powered Honda + gas water heater against a Stirling generator/water heater the Stirling will win. Not too many Hondas recover heat into a water heat exchanger. Now I wonder if there was a way to use solar to boost it even more? Or would that just add complexity without much gain?

      1. Comparing Stirlings in general to a single manufactured IC engine is not a fair comparison. Marine IC engines typically have engine coolant heat exchangers to make hot water. Some of them also have exhaust -to-water heat exchangers.

  3. Every martian habitat should have one. I’m thinking the temperature difference between outside ambient and life support inside should keep it humming. It’s not perpetual motion but it would seem like it.

    1. Nicely leaving aside the matter of what creates the high end of that temperature gradient needed to support life in the first place. It can’t be the stirling engine itself or it would be a perpetual motion machine. There needs to be some reliable, continuous external source of energy to draw from or that cozy and warm Martian hab with a stirling engine in one wall will quickly drop its inside temp to nearly exterior ambient as it slows to a stop and freezes its occupants. TANSTAAFL.

      1. Yes, the Stirling engine would cool the habitat and heating would have to come from some other source. The point is there will be a temperature differential that could be used to produce electricity because the temperature differential will have to exist. That electricity doesn’t have to be used to produce heat for the habitat. Instead it might be used to produce methane from the atmosphere. That methane is a store of energy that could heat the habitat. Bodies don’t produce heat without energy input either (food.) In both cases that energy come from the sun.

        The issue isn’t just energy production but keeping the flow of energy smooth from various sources. They should have nuclear, but they could get by with enough solar panels as long as they can get through dust storms, night time, etc.

        The Stirling would be most efficient when outside temp drop. How much energy could it produce for a ten degree loss of habitat temp? Certainly it would get you through the night and take up some of the battery slack? Then again, batteries will be very simple to produce ISRU in high quantities on mars.

        We should try many options to see what works best.

    2. Sorry. That’s a no-go. Energy still has to come from somewhere to heat the habitat in the first place.

      Let’s assume that your Mars habitat is perfectly insulated and has 4 people living in it. The average person puts out maybe 100 W of metabolic waste heat on average. If all of that waste heat could somehow be captured and turned into useful work (ignoring the 2nd law of thermodynamics), then you will have 400 W that you might be able to power a laptop with– assuming 100% conversion of waste heat into electrical energy.

      In the real world efficiency is never 100%. Assuming that a Carnot engine (the most efficient heat engine possible) was somehow rigged between a 70°F habitat and a -67°F martian environment, then you can get at most 26% efficiency in energy conversion. Therefore, the most waste heat you could hope to recapture and turn into useful power is 103 W which may be enough to run a couple lights.

      Remember, if it looks like perpetual motion, it probably is perpetual motion and, therefore, impossible no matter how much wishful thinking you put into it.

    1. A thermal kW is about 3400 BTU. A 60,000 BTU/hr gas-fired water heater would not be uncommon, suggesting that at least 15 kW thermal, 5 kW electrical at 33% efficiency is doable.

      The 10 kW version shouldn’t be more of a strain on your gas supply than a high capacity home gas furnace.

      1. At current gas and electricity prices in southern California, it might not take that long to pay for itself. I’d have to run the numbers, though.

        1. I was on an airplane a few months ago, talking to a petroleum engineer. He was saying that they are burning off a lot of the gas they recover in South Dakota just for lack of transportation capacity.

          1. I’ve seen plenty of gas flares while over the Dakotas, so I’m inclined to believe that engineer.

            There’s probably a good reason for doing this though – lack of pipelines. And, we can’t build those pipelines because then the gas would be burned doing useful work (instead of wasting it) and that would never do, because “environment!”.

          2. Seems like it would make more sense to use the gas locally (near the well) to do some crude process that requires a burner to melt something.

          3. the cost of a pipeline is high and the value of the gas is low
            and there is no carge for flaring the gas.

  5. The latest wonder gadget that will induce eye-rolling and snoozing to the crowd from the same guy who brought us the Segway. This one seems to want to cash in the same market of wannabe microgeneration tax credits that the Bloom Energy guys tried to get a couple years back using SOFC.

    I remember hearing about Stirling engines when I was a kid. The best description I have heard of them called them an external combustion engine that can run on anything with theoretically awesome efficiency. The only problem is that to get that theoretically awesome efficiency you need to build a heat exchanger out of materials that do not exist.

    1. For those of us that DO remember Themodynamics class a heat engine is limited in efficiency by the temperature of the hot reservoir and the cold reservoir. A natural gas power plant is large enough to make the installation of cooling towers economically viable unlike these microgenerators. Modern combined cycle natural gas power plants are 60% efficient. More if you use cogeneration with for e.g. district heating schemes when they can get up to 95% efficient.

      Microgeneration schemes using natural gas or other combustion schemes to generate electricity for stationary applications in places connected to the electric grid are a scam fueled by tax credits. Nothing more. Nothing less.

    2. I loved this quote:

      “I’d say yes. Ten years from today the probability that you are depending on wires hanging on tree branches is as likely as that you’ll still be installing land lines for telephones. Close to zero.”

      Yes. Because in his marvel world instead of mostly harmless electric wires people will have fat exploding natural gas pipelines going into their houses. A vast improvement indeed.

      1. We already have natural gas pipelines going into our houses, and they seem remarkably reluctant to explode.

        I would love to have one of these things to produce power, because, when the power goes out in the winter at 40 below zero due to a tree falling down or someone shooting the power lines, we can’t run the electronically-controlled, electrically-forced-air furnace even though we have lots of gas coming in to burn.

      2. “Yes. Because in his marvel world instead of mostly harmless electric wires people will have fat exploding natural gas pipelines going into their houses. ”

        What are you talking about? I’ve lived in homes that have natural gas piped in all my life.

        I’d love to have one of these machines powering my house instead of relying on the grid, if it turns out to be economical. And I’d love to sell any excess power back to the grid so long as I made a profit over the cost of the gas plus wear & tear.

        1. Which part of it is going to be less efficient than a combined cycle natural gas power plant did you not get? This thing is an economic disaster. The only reason why it is showing up is because of government tax credits for microgeneration.

          1. While a large powerplant is much more efficient than one of these small Stirling generators, a substancial percentage of the electricity is lost in transmission. America’s national electric grids are large, fragile, under developed, and somewhat inefficient.

          2. “Which part of it is going to be less efficient than a combined cycle natural gas power plant did you not get? ”

            Um, perfessor — are you quoting efficiency based on Upper Heating Value (UHV) or Lower Heating Value (LHV)?

          3. If you want to know more details about combined cycle power plants go to the General Electric website. e.g.
            http://efficiency.gepower.com/pdf/GEA31097%209HA_Gas_Turbine_FINAL.PDF

            I have other things to do. I am not a mechanical engineer. Not that I need to be one to see that this Stirling engine business is a crock to avoid paying taxes using microgeneration tax breaks just like the Bloom Boxes were. Except it is even less energy efficient than the Bloom Boxes and has less margin for technological improvement.

            As for electric grid transmission losses they are less than you seem to think they are. Most losses are in the electricity generation process proper. Natural gas pipelines do not require zero energy to move the gas either.

  6. “Which part of it is going to be less efficient than a combined cycle natural gas power plant did you not get? ”

    I didn’t get the part about how in the Great White North, where I consume almost 900 therms (100,000 BTU units — about 80% of a gallon of gasoline) per season anyway for space heating. If I had a Kaman Stirling engine generator to consume 1350 therms to heat the house with the 900 therms in waste heat to generate 10,000 kWHrs of electricity, I didn’t get the part that I would be “ahead of the game”, even without the tax credits.

    That 10,000 kWHrs retails for $1400 where I live and the extra 450 therms costs about $600. I could pay back a $10K generator in about 10 years.

    YMMV

    1. If you just want to have a heater that burns natural gas it is a lot cheaper than this Stirling engine. Depending where you live a heat pump, especially a ground source heat pump, is a lot more energetically efficient and cheaper way to provide house heating than burning natural gas. Of course it only provides low grade heat.

  7. It isn’t off the grid, it’s just substituting the natural gas grid for the electrical grid. If it were to be used off the grid, one would have to ship in or produce the natural gas on-site. For that application, a diesel generator is probably more economical. However, where the natural gas grid already exists, it’s a great idea.

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