Archive for April, 2006

Staying focused

2006-04-29

A somewhat acrimonius debate on halfbakery mostly with jhomrighaus, with constructive work by Ling. Lots of questions and I wrote a lot of answers, usually too long winded and some respondents didn't get the point. It seems to have died down. I now have two entries in other/thermal energy, one with highest rating, one with lowest. They weren't all that different in principle. Replying there did keep me working on the problem.

I also started a new page on Renewable Energy Design, titled Hydrostatic Stirling cycle air compressor. It is aimed toward producing compressed air instead of raising water for a hydro plant to convert to electricity.
I do see that finding the largest delta T available is important, since equipment cost varies with amount of heat transferred, but output energy varies with delta T. I have been thinking about building a heat pipeline carrying waste heat up the mountain with cold refrigerant draining back down to create a cold reservoir at a hot location at base of mountain. It might include a gas compressor to increase the rate of heat transfer by pushing the gas upward faster. This also increases the rate of condensation on cold surfaces by increasing the concentration of gas in the neighborhood of the surface. The rate of evaporation depends only on the temperature, so the net rate of condenstation increases.

A compressed air pipeline from a location good for running the hydrostatic compressor (cold and hot nearby) to a very hot location might be useful. At the hot location, the compressed air would produce the maximum amount of work. Hmm. Does this do better than steam at a nuclear plant? Need to calculate that answer.

It still seems to me that the mechanical efficiency problem for an air compressor ought to be solvable, so that a net profit can be made off relatively low delta T. 

Pumped storage of energy from ambient temperature II

2006-04-17

While wordpress was down yesterday I lost a little work on the previous article because of a bad interaction between Firefox and the partly down wordpress server. The browser back button did not return to a page where I could save my text. Firefox had forgotten it after sending it to the server and receiving an updated page (showing an empty blog), and the server had no record of the text. So I started working on the article at renewableenergy.wikia.com. I hate re-doing things, but it all came out for the best because I finally got the design right.

Please refer to the image of the tanks, pipes, valves, and reservoirs. 

The article is still in a state of flux, but I believe I now have all of the elements of a workable design. I figured out how to make the water motion reversible: water is moved between many pairs of low-pressure tanks at different elevations by adjusting the air pressure difference so that water flows either up or down through a U tube (with valve) connecting each tank of a pair. The pressure difference is coupled to the hydrostatic pressure applied to pairs of high-pressure tanks so that expansion of many different levels of high-pressure gas exerts a low pressure force raising water from many lower tanks to many higher tanks at once, while the pressure generated by water flowing downward from many pairs of higher and lower tanks together creates an increase in the hydrostatic pressure of a column of water feeding the high-pressure tanks, which compresses the air.

This machine is so simple it is amazing. It isn't quite obvious, but it will seem so once built. During compression, the slight pressure created by water flowing downward in many pairs of tanks at once is combined and applied to the space above an enclosed water reservoir at about the same height as the upper lake. The level of the enclosed reservoir is restored to the level of the upper lake after every step. (I call it a step rather than a cycle, because this machine completes one trip around the thermodynamic cycle each day). Thus, water descending in many pairs of tanks at different elevations generates air pressure which combines to push down on the top of a column of water, which compresses air at many different pressures simultaneously.

During expansion, the air pressure inside the hydrostatic tanks causes air flow until the hydrostatic pressures equal the air pressures inside all tanks. This causes an increase in the height of the water in the enclosed top reservoir, a decrease in the available volume for the enclosed air, and an increase in the air pressure. This air pressure appears in the lower tank of a pair of low-pressure tanks, and forces water down through the U tube and up into the upper tank of the pair. Thus, expanding gas pumps water.

The enclosed reservoir drain can be connected to either the even or odd hydrostatic manifold, while the upper lake is connected to the other one. The even hydrostatic manifold connects to the drain ports on the even numbered hydrostatic tanks, while the odd manifold connects to the odd numbered tanks. The manifolds are completely filled with water from the surface of the upper lake or reservoir to the water surface inside those tanks which contain air, or else to the top of the tanks. So the hydrostatic pressure at the surface of the water equals the vertical distance from the reservoir surface to the water surface inside the tank. Water will flow and air will compress or expand until the air pressure equals the hydrostatic pressure. This can't take very long because the pressures are never very far apart.

Pumped storage of energy from ambient temperature changes

2006-04-15

In the latest iteration of my nearly-reversible no-solid-moving-parts heat engine design, I aim for a machine installed at a pumped storage site with lakes at two elevations and an existing hydroelectric pump / generator. The heat engine will pump water upward whenever the ambient is hot enough to provide excess energy, and will temporarily compress cold air when the weather forcast says the current temperature is at a minimum.

[still working on this.  A (hopefully small) second upper reservoir, slightly higher (or lower) than the upper lake is needed.  During compression, every other tank is completely full of water.  Air is moved from one tank to the next lower tank by connecting them at the top and letting water flow from the slightly higher reservoir into the higher tank, displacing the air into the lower tank and water from the lower tank up into the other upper reservoir. Not nearly as clear as the system with multiple reservoirs but multiple tanks has many advantages]

Because all compression and expansion must take place while connected to the same upper reservoir, a single working-gas tank will not suffice. The design specifies a series of tanks, each connected from its bottom port past a water pressure gauge and through a valve to the upper reservoir by a dedicated water pipe sufficiently large to avoid turbulent flow during expansion and compression of gas in the tank. The resulting system has a larger surface area devoted to heat exchange, and hence produces more power than the earlier design. Also, a tank used with a wide range of pressures repeatedly expands and contracts, and so might have a shorter useful life than many tanks each dedicated to a narrow range of pressures.

Heat sinks and sources are attached to the tanks by gravity-feed heat pipes. The heat sink, used during compression, is located above the tank. Heat pipes extend downward from the heat sink into but not through the tank. Liquid refrigerant in the bottom of a heat pipe evaporates as working gas warms due to work done on it during compression, and condenses at the relatively cool top of the heat pipe, delivering heat to the heat sink. Valves in the heat pipes are closed when the tank is not producing heat. A chimney above the heat sink carries lighter hot air upward, supplimented with a fan if necessary.

The heat source, used during expansion, is located below the tank. Heat pipes extend upward from the heat source into but not through the tank, delivering heat to relatively cool gas inside the tank as the working gas cools after doing work on the rising water. Valves in the heat pipes are closed when the tank is not consuming heat. A flue extending downward below the heat source carries heavier cool air downward. A fan is available if necessary.

A large high-pressure gas conduit runs from the top of each tank to the next, with one valve beside each tank, so that any pair of adjacent tanks can be connected by closing the flanking valves and opening the central valve. Thus, each tank is directly connected to a water pressure gauge and three valves, including the water valve at the bottom, and two gas valves at the top.

During a time of low temperature, … [still working this out. Having tanks at different levels makes it complicated…]

Discussion on halfbakery

2006-04-14

See halfbakery for a discussion of the previous post. The page ends up discussing the silo idea mostly. It got +12/-3 presently. There is some resistance to the idea that energy might be available just waiting for a simple machine to capture it.

There was also a comment and my reply on thinkcycle.

And I got two diggs on digg.com

I plan to make another halfbakery page discussing the tiny climbing ratchet machine because an "idea" page is supposed to be about one idea.

I posted to krazyletters to see if any of the students there might be interested. New text. First use of "Even among those who should know better, it is widely believed there is no free lunch. The sun is our free lunch."

I've started working some more on renewableenergy.wikia.com.

Compressed air is like the stock market: Buy low, sell high.

2006-04-10

Potential energy stored in a spring or in the position of a heavy object is like money in the bank — its value is not likely to change. Energy stored as gas pressure is like money invested in the stock market. The value varies with temperature. The inefficiency of the device used to convert pressure to potential energy is like the stock-broker's commission. Buy low, sell high, but be sure the commissions don't wipe out the gains.

Our whole energy problem arises from a general ignorance of the above facts.  Everyone knows we are just 20 years away from getting limitless energy from fusion.  That's been true for the past 50 years.  The energy available from fusion is like money in Uncle Bob's bank account — he might leave it to us, but then, he might not.  In the mean time, the sun is a working fusion reactor, and the neutrons it produces stay far away from us.  Air in a sealed tank gains and loses energy as it warms and cools.  We can play that market while waiting for Uncle Bob to die. 

One approach is day-trading. Convert your potential energy to compressed air just before dawn, when the most molecules of gas can be squeezed into a container by a given amount of potential energy. At the daily high temperature in the afternoon, sell some of the high-pressure air to customers. (They might use it to power their cars). Then convert the rest back to the same potential energy you started with, and wait for the next low temperature.

Another approach is to sell on the uptick and buy on the down-tick. Extremely low commissions and high volume are necessary, but think of the profits! The volume is there — every day much more energy arrives from the sun and is re-radiated back to space than humans use in a year. Just capture a tiny fraction of it using properly designed machines spread out over large areas of land, and there is no need to burn fossil fuel.

As you might have noticed, I have been thinking a long time about various ways of capturing energy from ambient temperatures. Very recently I thought of an approach that seems much more promising than anything I have encountered before. A machine using this approach does not operate between two heat reservoirs. Instead, it accepts heat from and rejects heat to its environment, either at daily lows and highs, or whenever the environment's temperature changes.

A day-trading setup might involve some concrete cylinders the size of grain silos for storing potential energy in the form of pumped water, and some high-pressure gas tanks. To convert stored water to high-pressure air, start with a large chamber of atmospheric pressure air connected to the water silos. Open a valve to the lowest silo, and let the water flow in until it stops. Then close that valve and open one to the next higher silo. Continue until the air in the chamber is all squeezed into the high-pressure tanks connected to the top of the chamber.

In the afternoon, the tanks are warmer and the pressure is higher. Transfer some of the air to storage tanks for later sale to customers, but you have to save some to "pay the broker". You need a little more pressure than you started with in the morning to get the water back to where it was. Reverse the morning process: open the valve to the highest silo, and let the water in the pressure chamber flow up into the silo until it stops. Then close that valve and move to the next. When you get to the last silo, you should have enough pressure left to empty the pressure chamber of water, leaving the water levels where they were and the chamber full of air at atmospheric pressure. It will help to have a lot of copper heat-transfer vanes inside and outside the compression chamber, and probably heat pipes too, so that the gas temperature stays at ambient. The process should be done slowly, over an hour or two, the water pipes have to be big so there is no friction losses, and you need enough different level silos so that even with big wide-open water pipes, the water moves without turbulence.

Obviously this scheme works. The thermodynamics is sound.  The question is, do you make a profit after paying for the equipment? I don't know. But if we don't find some way to replace fossil fuels, a lot of people are going to suffer.

The other scheme (frequent "buying and selling") avoids working with high-pressure air and massive equipment, but doesn't get a daily harvest. I imagine a cheap device mass produced and distributed far and wide over thousands of acres of open fields, or deserts. After a week or two, a harvester moves slowly over the fields, collecting stored potential energy from the devices but leaving them in place to collect more. Each device includes a pressure chamber with a good thermal connection to ambient temperature, and some means for storing potential energy.

To be vivid and a little cute, imagine that the fields are filled with posts several meters tall, and the devices store potential energy by climbing the posts. The harvester lowers each machine back to the ground while capturing the energy. So, how can a heavy machine lift itself up a pole using nothing but ambient temperature variations? When the price (temperature) is low, the machine buys gas pressure in exchange for potential energy, lowering itself a little down the pole. When the temperature rises enough, the machine sells the gas pressure for a boost up the pole, ending up a little higher than it started even after paying the broker.
There are endless possible variations on the theme, and local conditions would dictate adjustments. If there is a lot of sunshine, the machines might warm up their working gas with sunlight, store the energy, and then cool the gas by sending the heat into the ground. This is more like an ordinary heat engine, but during the night, the machine can still operate whenever a big enough temperature variation happens.

A vast quantity of energy comes and goes every day. We have been using fossil fuels because they were there, and we didn't see anything wrong with it, and we didn't have anything better immediately available. Now we understand that we can't keep using fossil fuels without big unintended effects. All we need to do is capture a tiny fraction of the solar energy the earth blocks, and delay its return to space by a day or two.

Interested parties are invited to join the Renewable Energy Design Wikia (formerly Wikicity) at renewableenergy.wikia.com and work out the details.

Countercurrent heat exchanger

2006-04-01

Recently I've been thinking about a cylindrical machine tilted at about 45 degrees which is all one piece, as described in the last post. It feeds hot water in at top center, and the water flows through and finally out at middle outside. There are two heat exchangers, both water to air, instead of one air to air. This avoids the need for high pressure piping because the high pressure air stays in deep water where its pressure equals the ambient.  Near the surface, hot low pressure air is cooled by cold water, and at the maximum depth, cold high pressure air is warmed by hot water.

Last post I asked what's the difference between the helix and a countercurrent heat exchanger? The purpose of the low pressure exchanger is to be sure the air is as cold as possible before any work is expended compressing it. If the exchanger has flexible walls, this means the air has to be at constant depth. The heat exchanger looks like a conical hat on the cylinder. With the cylinder axis at 45 degrees, the cone has a right angle at apex. The highest surface of the cone is horizontal. As the cylinder rotates about its axis, that surface will be vertical after 1/2 turn. The air stays under the horizontal part.

The cone has two parallel surfaces, top and bottom. (Really two cones). The space between is filled mostly with water, and an air pocket at minimum depth. The top surface has a spiral wall which leads the air toward the axis as the rotor turns. Since the air pocket is horizontal (45 degree cone tilted at 45 degrees), the air stays at constant pressure as it moves toward the axis.  The water flow needs to be in the opposite direction, from axis toward the outer edge.  There will be a second pair of surfaces in which the air moves away from the axis and the water moves toward the axis.