Cycles: The Mysterious Maisotsenko Cycle

 The M-cycle is touted as a new thermodynamic cycle that will solve the world's air-conditioning problems (natch, by the companies selling them).  But is it really that good, and just how does it work?  When I look at drawings of air conditioners that use the M cycle it seems pretty confusing with all the different pieces and "wet channel" and "dry channel" stuff.  Not to easy to figure out, perhaps deliberatly so.  But by looking more closely at our trusty Psychometric chart things start to become much clearer.

If you have looked at my previous blog posts on DIY A/C you have already seen this:

It shows the different "paths" taken by evaporative coolers (solid line) and the more common compressor-based A/C systems, shown by the dotted line.

Suppose we sort of combine them.  Let's add a special type of heat exchanger, very similar to what's called an HRV, a Heat Recovery Ventilator.  It is an air-to-air heat exchanger used to replace stale air inside a house with fresh exterior air, while recovering the heat contained in the exhaust air.  They typically are cross-flow devices that use stacked corrugated plastic sheets -- the interior air flows across the outside surfaces of the sheets and the exterior air flows at right angles through the channels formed by the corrugations.  Or vise-versa, makes no difference.  This is a simplification because the air paths have to be kept separated so they only exchange heat -- they can't mix.  I have seen a number of DIY versions so making your own HRV is definitely feasible.  The biggest problem is that the corrugated plastic sheets are somewhat expensive, but I think I can make a similar kind of device using corrugated metal roofing with insulated panels on each side to force the air to flow down the corrugations.  It's much less expensive but (probably) more bulky.  Since it would be a type of counterflow system rather than the conventional cross-flow of other HRV's it might be pretty efficient.  The corrugated-roofing approach will likely be the subject of another blog.  For now, I just need to point out that making your own air-to-air HRV is not much of a stretch for an intrepid DIYer.

So, let's place our home-made HRV inline with our home-made evaporative chiller.  The chiller's input air comes from the output of one of the HRV channels, and the chiller's output air is routed to the other HRV's channel.  In this way the input air to the chiller is cooled before it enters it.

This might seem like a waste of a perfectly good HRV because we know that the RH of the cooled air increases, which decreases the effectiveness of our chiller.  And so it does, but that is more than offset by the attendent decrease in the resultant wet-bulb temperature.  I can show that by modelling our new system in a stepwise manner, like this:

Step 1:  We turn our chiller on.  The air entering it is at ambient temperature.  The air passing through the chiller follows the solid-line path on the psychrometric chart, and exits at a temperature close to the wet-bulb temperature.  It won't be equal to the wet bulb temperature because chillers aren't 100% efficient at transferring the full temperature drop of the water to the air.  Let's say that the chiller is 90% effective at that, so the air exits at 22.3C.  From there, it passes through the HRV, cooling the air entering the chiller.  Let's say that the HRV also is 90% efficient.  That translates to the chiller getting air that's been cooled to 23.3C.

Step2:  The chiller further cools the 23.3C air.  Looking at our psychrometric chart, we follow the dotted line over to where it intersects the 23.3C point on our temperature axis and see that the wet bulb temperature now is 18.5C.  This is almost 5 degrees Fahrenheit lower than the exit air we got in step 1.

Let's do one more step, just to see what happens.

Step 3:  Given the same efficiencies of our chiller and HRV, the air entering the chiller now is at 20.3C, giving us a wet-bulb temperature of 17.5C.  This is a further temperature reduction of 1 degree Centigrade, for an overall improvement of 6.7F.  Assuming the same efficiencies as before, the ambient air at 90F has been cooled to 64.4F.  For comparison, a single-pass chiller would output air at about 74F.

If we model our system in a continuous rather than stepwise manner we will find that the chiller's exit air asymtotically approaches the dew point, which is about 15C.  It will never get there because we have to evaporate SOME water to get any kind of cooling at all.  And in a real-world A/C system using this approach there will be significant heat input from the house we are trying to keep cool.

I think this is the basis of M-cycle air conditioning.  One additional wrinkle is that the M-cycle messes around with the relative volumes of air (via the Wet and Dry channels) so the cooled air delivered to living space isn't as humid as it would be in my example above.  However, since I'm going to run the chilled water through a water-air heat exchanger placed inside the house, I don't need to worry about the RH of the air exiting my DIY M-like  A/C system.  Just water leaks, perhaps from condensation on the heat exchanger (HX for short).

A system like this, unlike a compressor-based system, does little to nothing to address the increased RH due to the temperature drop.  However, there are ways to address this, also in a DIY manner that I will describe in yet another blog post.  It uses calcium chloride, but not as a one-shot "dry-z-air" type of system.  That's all I will say for now on that subject.  It gets complicated when we throw in dehumidification.

To summarize, we can noticeably improve the effectiveness of an evaporative cooler by adding a relatively simple air-to-air heat exchanger to the air flows entering and exiting the chiller.  

A do-able DIY system would likely be an indirect-cooled one, where the cold water in the chiller would be pumped through a water-air HX inside the house.  The HRV could be made from either a stack of metal sheets ($$$), corrugated plastic sheets ($$) or -- perhaps -- corrugated metal roofing panels ($).  In addition to cost, those options are approximately ranked in order of their physical size.  I'm guessing about the use of the corrugated metal but I think it's likely to take the most room.  However, it will be outside the house so that will be less of an issue.  If need be I think it's possible to stack the metal panels so we still get decent HRV efficiency in a smaller space.  The HRV design will be more complicated but, again, feasible for a good DIYer to make.

The chiller design also will be more complicated because the supply air has to come from our HRV and its exit air has to be routed back into the HRV.  My original open-sided design would have to be put in a sealed box that (1) provides for relatively unrestricted air flow and (2) keeps the input and output air flows well separated.  The four-sided tower scheme might have to go away.  A chiller using a single evaporation pad would be very easy to make (just a box with the chiller in the center), but would have to be pretty big to have the same surface area as the tower.  Maybe a set of pads placed in a "W" pattern?  How do I get water to them without introducing air leaks?  And just how much surface area do I need for the pad(s), anyway?  Does the enclosure need to be insulated? Time to do some thinking and sketching..

Evaporative Cooling Vs. Compressor-Driven A/C

In this post I'm going to explain more concerning what I've learned these two types of air conditioning.  In case new readers are wondering why I'm interested in evaporative cooling, it's because the technology is pretty easy to build yourself -- but with that, there are definite limitations that come along with it.

The psychrometric chart shown below has been marked to illustrate the two different kinds of cooling.  I'll then discuss some interesting differences between them.

I've drawn a solid line and a dotted line.  They both start at the same point, 32.2C (approximately 90F) and 36% relative humidity (RH).  That was the outside afternoon temperature at our house a week or two ago.   The solid line is drawn along a constant-enthalpy line, which just means that the total energy of the system remains constant.  Note that the relative humidity increases and so does the humidity ratio (basically, the amount of water in the air air, shown on the right side of the chart).  This shows what's going on when evaporative cooling is taking place.  You might think that this mechanism can't occur without a change in energy because the air is being cooled:  but that is balanced by the energy carried away by the water as it changes from a liquid to a gas.  To distinguish the two "forms" of heat, the energy contained by the air (oxygen, nitrogen and a small amount of carbon dioxide) is called "sensible heat".  Possibly because it can be "sensed" by a thermometer?  I haven't investigated the origins of the name so that is just a guess.  And the energy contained by the water vapor is called "latent heat", because it only plays a role when the water either evaporates or condenses.  Latent heat is a big deal in the A/C world because in humid climates it can be a substantial contributor to the energy (as in, coming out of a wall socket) needed to cool and condition air.

The humidity ratio for evaporative cooling increases because evaporating water is being used to cool the air, so the amount of water in the air increases.  So the latent heat increases, balancing the sensible heat drawn out of the air:  so the overal energy (enthalpy) remains constant.

The problem with so-called "swamp coolers" is that they are not very effective in humid climates, for two reasons.  As the humidity increases, the wet-bulb temperature increases so the chiller can't deliver air that's much colder than what entered it.  And the second problem is that the chiller increases the relative humidity of the air that exits it.  This reduces our body's ability to cool itself via evaporative cooling, so our perception of comfort is reduced

Now lets move on to the solid horizontal line.  That is what is going on when conventional compressor-driven cooling occurs.  The line follows a constant-humidity line because the amount of water in the air doesn't change.  Since there is no phase change, at least down to the dew point, the total energy in the air decreases:  the enthalpy decreases.  However, closer examination of the line shows that the relative humidity increases.  This is because cooler air has a reduced capacity to hold water vapor.  When the temperature reaches the dew point (at 15C/59F), the relative humidity reaches 100% and water starts to condense.   It takes a LOT of energy to condense water so once that happens it suddenly takes a bigger A/C unit to get the temperature to decrease.  The other factor that comes into play is our perception of comfort when humid air is cooled.  59F is pretty chilly, so let's say we just cool the air down to 68F (20C).  Our chart indicates that the air's relative humidity now is about 75%.  This is pretty humid so we don't feel all that comfortable -- our body's ability to cool itself is reduced because we can't cool ourselves as effectively due to _our_ evaporative cooling.  At the dew point our body can't cool itself at all via sweating so 59F would actually feel very uncomfortable.  The other downside to high humidity is that it promotes the growth of mold and mildew, steel parts rust and so on.

To improve the comfort level, most A/C systems deliberately cool the air to the dew point in order to force it to condense.  The cool air exiting the A/C unit has a lower relative humidity due to the condensation.  But now we have the reverse problem -- the air feels TOO cold for comfort.  How many of us have had the misfortune to be seated at a restaurant directly below an air conditioner vent?  Feels pretty cold, huh.  Well, it actually could be worse because commercial systems actually use extra energy to deliberately WARM that cold air back up some.  More sophisitcated A/C systems can recycle the heat they extract from the incoming air via a heat exchanger so the energy cost is lower:  but the cost of such an A/C system is higher.  

Here's a factoid.  An A/C system that returns all the heat energy back to the interior space it's in might seem ridiculous because it doesn't cool the room -- but it DOES reduce the relative humidity.  This type of system is called a dehumidifier.

So on the one hand we have evaporative cooling systems that work well in very dry climates but become less and less effective as humidity increases.  Unfortunately, in many parts of the world high temperatures are accompanied by high humidity so they aren't nearly as prevalent as compressor-driven A/C systems.  

In contrast, compressor based A/C can dry the air too much if it's used in dry areas of the world; and in humid areas a large percentage of the energy they consume is just used to pull water out of the air.  In hot humid locations, the energy consumed by A/C can be a large percentage of the total energy consumption of a household.

Both systems have their advantages and disadvantages, so it's no surprise that there still is considerable research and development going on to mitigate the disadvantages.  I'll go over some of those efforts in furture blog posts on the subject.

My Evaporative Cooling Test Bed, A Review

While working on my previous post regarding the usefulness of the Psychrometric Chart, I had a thought regarding the test bed I built last season.  I noticed large differences in the exit air temperature between my setup and a similar one built by Desertsun02 -- his system was outputting colder air than mine.

In retrospect, this probably is due to different cooling pads.  Doing some online searching revealed that different pads have higher efficiency compared to some of the synthetic ones, like the ones I'm using.  The old-fashioned shredded Aspen pads apparently are pretty good, as well as paper ones with a honeycomb pattern.  One big difference may be that the synthetic pad type I'm using isn't very thick so the air doesn't have as long a "dwell time" in the pad compared to thicker ones.

I'd like to get the exit air temperature lower because I think I can use it to pre-cool the air flowing _into_ the evaporative cooler.  That could get me closer to performance like the Maisotsenko Cycle, which theoretically can output air that is very close to the dew point. So it looks like I need to experiment with a different pad, along with everything else.

Air Conditioning With A "Swamp Cooler": the Psychrometric Chart

 As the climate heats up and energy resources become more and more stressed, interest in alternative approaches to compressor-based A/C has increased.  A lot.  The basis of many alternatives circulates around evaporative cooling technology, most commonly referred to as the swamp cooler.

As a kid I remember swamp coolers in two different situations.  For some time we lived in the Four Corners area, where Colorado, Utah, Arizona and New Mexico meet at one point.  The area is high desert, hot and dry in the summer; and that's perfect for the old-style swamp cooler.  The version we had in our house took hot and dry air from the outside and blew it through a water-soaked membrane.  Evaporation occurred, which cooled the air and also added some welcome humidity to the air, which usually had a very low relative humidity.  The cooled and wetter air was blown into the house using the same heater ducts that were used to heat the house in the wintertime.

The other situation was during summertime visits to relatives in Oklahoma.  At that time compressor-type A/C was expensive, so they couldn't afford to get that.  So they used swamp coolers there, too.  But in that case, while the air temperature was comparable to what we got in the Four Corners area, the humidity was much higher.  In that case, the swamp cooler was less effective for two reasons.  First, the high humidity reduced the amount of evaporation that could occur in the swamp cooler.  The second has to do with our perception of comfort.  When in a high-humidity environment WE also are less able to cool ourselves, because our sweat is less able to evaporate.  The end result was that the swamp cooler in Oklahoma really didn't make me feel any cooler than just staying outside in the shade, hoping for a bit of wind to come by.

This is where the Psychrometric Chart comes in, to help us understand what's happening.  I'm working on an upcoming post where I (hopefully) explain how a relatively new evaporative cooling technology based on something called the "Maisotsenko Cycle" works, and how a version of it can be relatively-easily added to your basic evaporative cooler, to significantly improve its performance; and the explanation heavily depends on use of the Psychrometric Chart.

Anyway, back to our simple swamp cooler.  Today I measured the exterior peak temperature and ambient relative humidity and got 32.2C (just shy of 90F) and about 37% relative humidity.  I plotted that point on a copy of my Psychrometric Chart and it looks like this:

The vertical axis is the temperature, but the relative humidity curves are the upward-trending ones as you look from left to right.  The dark point shows the conditions at our house.  Another set of curves are straight lines that go down from left to right, not quite at a 45 degree slope.  Those are lines of constant wet-bulb temperature, and give the temperature of the water in the swamp cooler membrane.  In this case we get about 70 degrees Fahrenheit.  That sounds pretty good, dropping the exterior air temperature down to 70F:  but that's the temperature that the WATER gets to.  My previous experiments with a home-brew swamp cooler show that the exit air temperature can be ten degrees higher than that, perhaps more if the air flow is excessive.  This means that the air temperature coming of out my swamp cooler might be about 80F.  Better than 90, but that wet-bulb temperature sure sounds better.

Some may wonder why the air and water temperatures aren't the same.  I think that's because the air carries off the heat extracted from the evaporating water.  Also based on my experiments, excess air flow also can be a factor.

The difference between the wet-bulb temperature of 70F and the exit air temperature makes the use of a slightly-more complicated system attractive.  That's an indirect evaporative cooler, where the chilled water is piped into the interior space and passed through an air-water heat exchanger.  In this case, the water also is continuously circulated through the evaporative cooler because we want to use the chilled water to cool the house.  A level sensor detects when the water level in the chiller falls to the point where it needs to be topped-up.

In either case, once the exterior relative humidity rises above about 50% they become pretty ineffective as an A/C system.  But there is a way to get the water in the chiller colder, approaching the dew point temperature (rather than the higher wet-bulb temperature).  In the case of my example, the difference is about 5 degrees C,  which would produce water at about 60F, ten degrees colder yet.  More on that in another post, which includes a more in-depth exploration of the Psychometric Chart.

Some may wonder why the wet-bulb temperature is higher than the dew point.  I think that is because there are two effects that are in equilibrium at the wet bulb temperature.  The first is the heat extracted from water by evaporation.  The second is the heat contained in the air being transferred to the wet bulb.

Check Valve Redux

 I had an opportunity to test my bearing-ball check valve idea, at least in terms of how it works in a pneumatic sense.  I discovered that my idea was flawed by the need to incorporate two incompatible requirements.  The first:  the ball has to fit easily into the end of the tube to seal.  The second:  the force pulling the ball into the tube has to be small so the check valve works with a very small pressure differential.

The practical result of these requirements was that the hole in the piston where the ball goes has to be very nearly the same diameter as the ball.  But this means that the air has to flow through a pretty small constriction around the ball. The end result was that there wasn't much of a difference in the flow rates.  I thought of some ways to get around this problem but they all had their own complications -- including noticeably more machining work.

So instead of that approach I went with a much simpler flap valve arrangement, made with a square piece of acrylic film.  The film is placed over a hole drilled in the base, which serves as the air inlet.  To promote free flow of the air, I also milled a shallow slot from the end of the base up to the hole.  I cut a square piece of the plastic film and then made three cuts in the form of a U, to free that portion of the plastic, enabling it to move up (away) from the hole, or down toward it to form a seal when the piston is being pushed into the cylinder.  That worked OK, but it worked even better when I made more cuts to widen the gaps between the flap and the rest of the plastic sheet.  I think the narrow slits didn't allow the flap to completely seal.

Putting a little lubricating oil in between the plastic and base would probably work even better -- for awhile, but the oil could attract dust and then the seal would likely fail.  In this case a little less (of a seal) is more, in terms of longevity of the damper.  That said, I also made the damper so it can be taken apart and cleaned if that becomes necessary.

A Check Valve For My Soft-Close Drawer Project

 I had originally intended to use an off-the-shelf check valve to use in my pneumatic soft-close mechanism, but the more I played around with the overall idea, the less suitable it appeared to be.  It would have to hang off the side of the cylinder.  The piston needed to be "special" to accommodate the need to put a hole in the cylinder for the air to pass through.  And so on.

So I was thinking about integrating my own check valve into the piston itself.  The original design used the aluminum stem for the air flow (it is a tube), with a ball bearing to act as the seal.  The bearing would be pushed against the end of the tube with a small spring.  The tube, bearing and spring would be installed in the piston.

But my piston is only .75" long, and it was hard to find the right itty-bitty spring so I decided to replace the spring with yet another magnet that would pull the bearing onto the end of the tube.  The magnet would be a ring magnet that the aluminum tube slides through, and would be glued to the top (or outer) side of the piston.  I liked this approach because it's based on a simple physical mechanism (magnetism) and should work OK for a very long time.  The only thing that might mess it up is dust and dirt between the ball and end of the tube; and if that becomes a problem I could glue a small air filter to the bottom of the piston to junk out.  Maybe I'll be pro-active and just do that up front :).

But the question arose:  would there be enough force acting on the bearing ball to pull it into the end of the tube?   Or would the force be too great so the check valve wouldn't permit the damper to easily open (and therefore cause the drawer to not easily open)?

This looked like another simulation to perform using FEMM, Finite Element Modelling, so I could answer these questions.  Long story short:  it looks good.  The attractive force is small, on the order of 3.9 grams; but since the check valve will be on the horizontal plane it won't take much force to pull the ball toward the end of the tube.  And 3.9 grams over an area of .049 square inches (the cross-sectional area of the tube) indicates that the check valve should open with a pressure differential of just .17 pounds per square inch.

Here's a screen shot of the simulation:

I included a steel plate that will be used to mount my damper on the back of the base unit where the drawer goes, just to make sure it wouldn't cause problems in the operation of the check valve.  And it doesn't alter the results to any great extent.  It does increase the attractive force between the magnet and baseplate, but it still is pretty low, about 14 grams.  That's less than one ounce.  At that point in the operation of the system -- damper plus long-distance magnetic latch -- the attractive force of the mag-latch is MUCH higher so it won't materially alter how the overall system behaves.

Forever Products -- Why Not?

 A significant part of the waste we all send to the landfill are products that fail due to some proprietary component,  or because they weren't designed to be repairable.  Often the bad component can't be replaced because the manufacturer either (A) doesn't offer it  (B) they did but only for a short while; or (C) it isn't possible to replace the failed part because the product wasn't designed to permit that.

All these issues are things that can be addressed in a variety of ways.  While the "right to repair" movement has gained some traction, my examples in the previous paragraph show that it can only go so far -- unless the design process used to make our "stuff" includes the requirement that the item can be repaired for a very long time, even long after the original manufacturer has gone out of business.

Our military has some similar requirements, considering how large their inventory of materiel can be, and concerns regarding the availability of replacements in a wartime situation.  But, considering the concurrent issues of waste reduction and the reduction of resource depletion -- both mineral and energy resources -- and the rather large multiplier of a consumer economy at work -  that also has large implications, given current trends.

So, what's in the way of making products that use off-the-shelf components as much as possible, so they can be replaced long after the manufacturer has declared the product obsolete?  What's in the way of requiring manufacturers to provide design data on their proprietary components for those same products so they can be made with 3D printers?  Many companies these days employ designed-in obsolescence as a part of their business model, so they can sell new stuff.  But that tactic has become a larger and larger problem, given the issues of waste and all the resource-consuming aspects of making new items that, in many cases, aren't any better (often less) than what they replaced.

This is where government has a role to play, basically drawing a line and saying that youse-guys have to clean up your act.  Of course, manufacturers should be able to offer new products with new features:  but they need to both design their products so they can be repaired; or if some unique parts are in there, once they have come out with a new model they have to make the design information available for the older item so replacement parts can be fabricated by a third party, or, if possible, with a 3D printer.

The overall impact of this would be multifold.  For starters, manufacturers that simply "churn" their products so older, but equivalent, products become obsolete, will have a greatly reduced incentive to do so.  To appeal to consumers, new products would only succeed if they offered better functionality, or added functions.  This would promote innovation rather than just putting a different color of lipstick on the same pig.  Of course, the new products would have to be Forever Products too, so the pattern of innovation would continue.  Or maybe some vendors would just offer Forever Products and tap into the demand for something that can be repaired until long into the foreseeable future.

Some manufacturers are very good about providing replacement components for the products they sell, but they seem to be in the minority.  That has to change.