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Old 09-13-13, 10:58 AM   #31
Acuario
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Well a slight delay whilst I waited for a few bits to arrive to complete the project (the keyboard, a MAX485 and DS18B20 temperature sensor to be precise).

Now all has arrived and I've built the main controller, finished writing and debugging the code and now all that's left is to fit the units - probably tomorrows job.

As the main board has a relay on it I've added a 4th machine (the oil boiler) that can be turned on if we get really cold weather. The main controller has a sensor for the water temperature in the tank and, depending on the temperature, turns on 1, 2 or 3 of the units and if that still isn't enough, as a last resort it turns on the boiler (although this can be disabled). The individual temperature thresholds for each machine are also set by the controller.

As the defrost temperature is difficult to know exactly the controller can re-program the individual slaves to change the temperature the defrost cycle kicks in at.

From the controller you can also set the operating mode (Heat/Cool) and turn on/off the units or force a defrost cycle.

The controller communicates with the slaves via 2 wires (RS485) and checks for existence of the slave and can request its current operative status and the condenser temperature. This is then displayed on the main screen.

So it's built, it should work (it does from my testing) so time to install it all and see if the reality coincides with the theory.

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Old 09-14-13, 02:07 PM   #32
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Man that thing looks cool. On the outside, it looks like a blast furnace control box. On the inside, it looks like robot guts. Best of both worlds.

So have you tested out your staged system? Or are you waiting to rig the controller? Does it pour on the BTU's on demand like it should?

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Old 09-15-13, 05:43 AM   #33
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I've installed the unit controllers and main controllers and it's all up and running.

I have done a few quick tests and (certainly with the current temperature here of 28C) it looks like I'm only going to be able to run 2 of the 3 compressors.

The problem is when the 3rd compressor kicks in the gas pressure goes up quite dramatically and I'm sure it's not safe. It also sounds like the compressor(s) start struggling to run. With just the main unit and one of the twin unit compressors running all seems fine.

When the second compressor kicks in there is definitely a good increase in heat output.

So it's now a waiting game until it cools down and I need to start heating the house - probably towards the end of October (I switched on the heating the 27th last year).
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Old 09-15-13, 08:43 AM   #34
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I am interested in designs where the COP (and EER) can be markedly increased by dual compressor technology. Differing gasses offer advantages at each differing temperatures (latent curve) and it might make sense to have the first compressor do an efficient job of maximizing heat throughput at low temperatures, whereas the second compressor boosts it up to a higher temperature.

Right now, the compressors are engineered so that they are just OK at boosting temperature through a wide range of temperatures.

This would have immediate applications for water to water heat pumps where it is very hard to get water output more than 120 F.

I do see technical (proprietary for now) designs that have COPS in the 10+ range with EERs in the low 100's. I know this sounds amazing, but only thirty years ago, it was stated that "for thermodynamic reasons, a COP of over 5 will be impossible".

Imagine going to a homeowner with an AC unit EER of 10 and showing then the 90% costs savings going with an EER of 100 . . . . Same with COP.

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Old 09-15-13, 11:51 AM   #35
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A COP of 5 was always possible, it just depended on the temp spread and the load. Those systems in the past needed higher discharge temps because the buildings were crap and a bit larger evap made the unit too expensive. Besides, American systems were just AC units with a reversing valve and a very crude timed defrost, never really designed for heating. The real SYSTEM COP includes the pumping or fan power which until recently was a PSC motor and poor bearings. We know better now.

There are thermodynamic limits to the refrig cycle and a COP of 6-7 is probably about as good as we will see for a long while. The COPs of 8-10+ have usually been debunked. That said, the pace of improvements has never been greater, most of which I put down to measurement and controls.
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Old 09-15-13, 10:18 PM   #36
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Quote:
Originally Posted by stevehull View Post
..."for thermodynamic reasons, a COP of over 5 will be impossible"...
I searched the Internet, but I couldn't find that quote.

Do you have a source for that quote?

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Old 09-15-13, 10:27 PM   #37
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AC, you have the quote, it was told to me. I was in graduate school getting an MS in engineering.

A similar quote was heard by my class by an engineer for GM where he stated that "for efficiency purposes, an internal combustion, reciprocating heat engine could not be made that could get more than 50 mpg in an automotive application".

Your point?

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Old 09-15-13, 11:39 PM   #38
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Seasonal energy efficiency ratio - Wikipedia, the free encyclopedia
The interesting part is that the article states that for a desert climate, the maximum EER is about 46, while Brittany Benzaia's prototype gets on the order of 40 SEER (equivalent) or about 35 EER. But that's a hybrid that makes use of evaporative cooling for a large part of the cooling, and the SEER equivalent probably means that it is equivalent to a 40 SEER rated unit in terms of energy use (including bypass mode, plus the fact that regular air conditioners are heavily disadvantaged by high ambient whereas a hybrid cares mostly about wet bulb temperature), not that it would actually achieve 40 SEER under realistic conditions.

Something else to consider is that increasing SEER tends to degrade dehumidification due to higher evaporator temperatures. The better built units have variable speed motors to mitigate that by running longer (at reduced power) in order to extract more moisture, but the cheaper units that are built simply to get a high SEER at a low price tend to dehumidify poorly. (I have that problem with the 14 SEER central units that were preinstalled on my house, hence why I'm building a small supplementary unit that is optimized for dehumidification.) As a result, the temperature has to be set lower to get the same comfort index (unless it's for machine cooling), undermining the very efficiency the unit was designed for.

Ducting causes very significant losses in both thermal leakage and fan power needed to overcome the pressure drop. Hence the most efficient units are ductless. Induction fan motors tend to be inefficient, especially in the smaller sizes. That's especially true of the multi speed versions where they achieve the reduced speed with more slip, plus the multiple speed taps increase the winding resistance per turn. In contrast, permanent magnet synchronous motors are very efficient, especially those using Cindy Wu sensorless FOC (Field Oriented Control) or some other FOC algorithm. In FOC, the controller tries to hold the stator flux at exactly 90 electrical degrees ahead of the rotor flux, where the most torque is produced for a given current. In practice, a synchronous motor can easily be twice as efficient as an induction motor for small fans, sometimes as much as 4 times.

Compressor motors tend to be very efficient, so there's little remaining improvement there. The compressor itself is also similarly efficient, so little improvement left there as well. Where variable speed is used, the highly dynamic load the compressor poses can actually make induction motors more efficient than synchronous motors under some conditions.

The expansion valve is actually a point where a lot of improvement is possible. In almost all current designs, the pressure imparted to the refrigerant actually adds unwanted load to the low side, greatly dropping efficiency. That's especially a problem with high pressure refrigerants like R410a and R744. One solution is to use an ejector to recover the energy. An added benefit is that it allows two stages of cooling, which dehumidifies more efficiently.
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Old 09-16-13, 12:06 AM   #39
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Quote:
Originally Posted by stevehull View Post
AC, you have the quote, it was told to me... Your point?
I don't think I have that quote anywhere. I never heard that quote before.

Since you were quoting it in your post, I thought you might be able to cite the source that your quote came from.

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Old 09-16-13, 11:26 AM   #40
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I found an article by Tom Murphy of his series "Do the Math" entitled "100 MPG on Gasoline; Could We Really?"

http://physics.ucsd.edu/do-the-math/...g-on-gasoline/

I know that when a number of engineers come together there are generally a number of opinions.

Personally I enjoy the posts by stevehull. I think he presents information from a different perspective and is a valuable asset to Ecorenovator, as are many other contributors.

I hope we can agree to disagree on some points and not take it personally.

Sincerely, charlesfl

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