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This is a pretty simple idea that could be simulated to determine efficiency gains, without much cost. Perhaps someone reading this could do the calculations.
I propose building an inexpensive series of pipes, designed to dissipate heat from ambient air being drawn into the combustion chamber of
a power station. Apparently, the temperature at about 3m below ground is fairly constant, no matter how hot it is above ground.
Instead of inhaling scorching 40C+ air directly, it would be piped underground where the air would cool to a much lower temperature. This cooler air would increase the efficiency of the power station’s thermal cycle.
It has no moving parts, except perhaps a diverter valve for when the night air is cooler than the underground temperature, and could be installed very cheaply.
Even if the efficiency gain is on the order of 1-5%, it’d pay for itself in no time at all.
Geo-thermal heat pumps
http://www.eere.ene.../geo_heatpumps.html
Nice and concise. Good statistics. Not completely passive but the savings more than makes up for this. [Tiger Lily, Oct 17 2004]
The Carnot cycle
http://hyperphysics.../thermo/carnot.html
Read this first before digging trenches [pluterday, Oct 17 2004]
[link]
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//It has no moving parts, except perhaps a diverter valve for when the night air is cooler than the underground temperature// |
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I don't know much about phisics (sp?) but sounds like a cheap way to do it. |
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How long would the airflow have to dwell below ground to cool appreciably? |
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It may be cheap, but it will be slow. |
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Fast heat transfer is far easier if you can involve atmospheric moisture levels in the deal. Water has a pretty high energy coefficient (It holds a lot of heat, and is practically the best way to add or remove heat, in a closed system). This is because it is easy to move air over a heat exchanger. |
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Dirt and rock are considerably less mobile, so heat load builds up quickly, around the exchanger buried in the ground. |
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These are good points. Let me address them: |
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"It requires a fan." - whatever the station uses to draw air in will be just plumbed into the heat exchanger, much like the intake of your home furnace is plumbed into the outside world. There may be a slight amount of pressure loss, but if designed correctly, this could be minimized. |
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"How long would the airflow have to dwell below ground to cool appreciably?" - That's a simple physics problem. Much like asking how long the coolant in your car engine must dwell in the radiator before it cools. It all depends on how much cooling you want, the surface area of the pipes, the temperature differential, the humidity etc. It'd be calculated to suit the particular power plant. |
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"Dirt and rock are considerably less mobile, so heat load builds up quickly, around the exchanger buried in the ground." - I had the same concern, but apparently this isn't a problem. Do a search under 'ground-source heat pumps'. This is a new application of proven technology. I want to keep water out of the system to keep it simple.
If you had an ample supply of water, you could build an evaporation pond to cool the pipes, or inject water into the intake stream directly.
Still, it seems weird to me to use the earth as a way of gaining or losing heat, but it does work. |
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"It may be cheap, but it will be slow." - oxygen diffusion into our lungs is slow too, but if you have enough surface area, it compensates for the slow movement of heat. The design would be such as to maximize cooling, whilst minimizing structure. |
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When your Unca Bubba was a lad, he and his Dad dug a defile into a hillside, lined it with concrete, filled it with about 30 cu. m. of basalt rocks (heavily pitted volcanic bombs) and put a roof and a sprinkler system in, to spray water over the rocks. A fan drew hot, dry air out of the house and into the bottom of the machine, and another drew the cool, moist air out of the device through straw pads and into the house. Excess water, ponding beneath the rocks, was recycled back to the sprinklers. |
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It made a very efficient air conditioner, in a dry part of the world. We worked on the principle it was usually going to be cooler underground than above. |
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I'd be surprised if this idea was as afficient as an evaporative type air conditioner, [TIB]. Can you point us at some of the research you have seen? |
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Nice system [Unabubba]! When I lived in outback Australia, we used small evaporative coolers to keep us cool . It was way cheaper than running an AC unit, and did a good job as the air was dry. Nothing like the scale that you and your dad did though. |
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How much water did your system use? Was the water you used cooler than the surrounding air? I only ask because I once measured the water temperature where I now live (in Canada) and was shocked to find it was only 5C (forget the season…). Since it does get warm here in summer, despite what many people think, I decided to use this water as a way to cool down my apartment. I got a box fan, some cardboard, a car transmission cooler, and a lot of tape, and put together my water-wasting room-cooler. The fan drew air through the transmission cooler, and exhausted it into my room. Water, from my bathroom tap, provided the cooling as it flowed through the transmission cooler, then exited down the drain. Simple and effective, but it sure used a lot of water L |
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As far as proving efficiency goes, I’ve only the reading I’ve done on ground-source heat pumps to go by, and my general knowledge to make a point. Punch in “ground source heat pump” into Google and you’ll see what I did. An evaporative cooler, like the ones we used in our childhood, will work far more efficiently than my idea of just running air through an underground maze of heat-exchanger pipes, but for the type of climate where this idea may come in useful, water will no doubt be a precious commodity. |
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Have you ever stood at the entrance to a very long railway tunnel in the heat of summer? I did this once, when it was about 40C, and the breeze coming out was like air conditioning, it was so cool. |
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It was in northern Australia. (Where necessity *is* the mother of invention). The water was bore water, at around 20deg C from the ground. I'm not sure how well it would have worked in, say, Blackall, southwest of where we were, where the water exits the bore at 62deg C. |
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I guess we used about 2000 litres a week. The house was quite large, and not designed to hold cool air in, very well. The pump and fans ran from the 32V
diesel generator we used for other power needs |
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Hey [Unabubba], I lived in a small town outside of Jabiru in the NT.
We should form a team, in combination with some other members here, and go on Junkyard wars. The 'halfbakers'. We'd win for sure!! |
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I was north of Pentland, near Charters Towers. The family still has cattle stations west of Greenvale and up in the Gulf country. An uncle paid $7.2M for one, last month. |
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I'm in Brisbane, these days. |
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//Even if the efficiency gain is on the order of 1-5%, it’d pay for itself in no time at all.// |
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I think [TIB] is referring to intake cooling of CCGTs. |
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I can't remember the details clearly, but this was always a difficult economic decision linked to the expected load cycle of the turbine. I guess precooling of intake flow would involve vast quantities of air and this would require a substantial heat exchanger, if the temperature differential and heat sink potential is as he describes. |
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Another technique I heard of that works quite well without doubling the capital costs of the plant is simple irrigation of the intake air. |
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//We should form a team, in combination with some other members here, and go on Junkyard wars. The 'halfbakers'. We'd win for sure!!// |
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We can never agree on anything. Before long someone will be sent to search the junkyard for custard, or Twiglets, or a genetic engineering kit with which to build the A/C. |
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Geo-thermal heat pumps are quite baked. They are designed around the very principles TIB describes though these systems are not entirely passive. However, the savings alone appear to make the passive issue moot, at least at this point in their evolution.
[link] |
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Tiger, I believe the idea at hand was to use the cooled ground air to increase the density into a power station. Not bad, although I shudder to think of the ground area a reasonably-sized power station's ground sink would cover. |
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No no no no no. (Is that about enough?) You WANT to have hot air, the hotter the better. Cooling the air REDUCES the efficiency. |
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[RayfordSteele], it's obvious to me now that I was in no
condition to 'bake' proficiently when I wrote my
annotation. However, I am confident of this: Tha' girl
[pluterday]--she be correct. |
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Sounds like a lot of hot air, to me. |
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I think cold air would work better. If you wanted to heat the intake air, then you could use the exhaust and a counter flow heat exchanger. |
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Think about the motor on a turbocharged, intercooled car. Air is compressed using a turbocharger which causes it to heat up. It’s then piped through a small radiator (intercooler) to cool off before entering the engine. Cooler air is more dense, and that’s what the engine wants. |
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Hey [RayfordSteele], I’m wondering how big of an area it would need? I agree it’d be pretty huge, but would love to have an estimate done by someone familiar with the physics involved. |
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All conservative estimates: say, a 50MW station, drawing 40C ambient air, converted to draw air through an underground heat exchanger that can bring the air temperature down to 20C. Also assume we’re using thin steel pipes as heat exchanger material. |
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1) Would there be any increase in efficiency with a temperature difference of 20C?
2) What kind of surface area would be needed? (lots of fudge here please)
3) What would the material cost on average? (again, fudge please) |
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I endorsed pluterday's comment
based on casual familiarization with fire ignition
theory. |
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As to area needed for embedding pipes for cooling
turbines <g>, it's probably
not as much as you think. Tubing is the standard
material and is often laid vertically, looping up and down,
in trenches that can be anywhere from 15ft to 200ft
deep. I think your cost would be mostly in labor and
design. |
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To give you a better contact area, for heat transfer, it might be best to run the tubes in trenches, as suggested, but fill those trenches with water, or concrete. Water particularly, has very good heat retention and absorption capabilities. |
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NB. An Australian engineering firm is putting a geothermal power station together in the Cooper basin, which is one of the world's hottest non-eruptive geothermal spots. |
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//An uncle paid $7.2M for one, last month//
That's a mighty expensive cow. |
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I am shocked, shocked, that my previous no’s have been ignored. Where is Worldgineer to back me up?
I guess I’ll have to do it myself. <Pluter puts up a big sheet of butcher paper and starts to draw, it gets really complicated, she erases a few things, tearing the paper, then, cursing, she pulls it down> Dang, it’s been too long since... But I’ll describe it in words.
Don’t get cars confused with power generation. That means you [TIB].
The most efficient power generation cycle is the Carnot cycle (you think I’m making a pun, but I’m not.) The efficiency is:
E = ((the hot temp.) – (the cold temp.) / (the hot temp)) x 100%
The entry air goes to the hot side, so you want that to be as hot as possible.
If you want to argue, do it with Worldgineer. But first read the link. |
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[TW], it's what your mob call a ranch. It's about 700,000 acres. Stockholding at the moment is about 32,000 head @ $1,000 apiece, so the land and improvements represent about $4M. |
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It's a pretty big place, though Ben has three other stations, as well. He's converting part of one of the smaller ones to intensive agriculture, growing grains and high protein grasses under irrigation, to allow him to fatten stock from the other three before turning them off. There will be roughly 26,000 acres planted. |
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He tipped in 20% equity, so he only owes $5.7M on the deal (If he's forced then he can destock, and reduce his debt to just over $2.5M, which should be manageable). I consulted to him on the finance package for the deal. |
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Yes, it's an expensive cow, but a cash cow for all that. |
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How big would it need to be? Well, for this go-round that would probably just be a straight calculation of the ratio of heat capacities as compared to an existing air/water system, plus perhaps a fudge factor to deal with poor heat dissipation and some packaging estimates. Find the R-value and heat capacity for geothermal, compare it with the heat capacity for a water system, and use that ratio to size up the volume of the current system. Me, I don't really have the time to run the numbers. |
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