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Microhydro-powered Heatpump Pages: 12

microHydro on Sat April 24, 2010 4:09 AM User is offline

I am designing and building a heat pump to be directly driven by a turgo runner (like a pelton wheel spun by a jet of water). I need help selecting an efficient compressor.

I was drawn to this forum reading detailed scroll compressor discussions by ice-n-tropics:
Small Compressor

A few initial questions:
1. Which open-drive or automotive compressor would be the best choice? Efficiency is a high priority.
2. How do I couple the turgo runner to an automotive compressor?


Shaft Mechanical Input Power: 0.7-2.8 kw
Shaft Power is limited.
It would be best to utilize a range of input power (.7kw/water jet).
Shaft RPM will be fixed.
I can select from the range: 1500-2500 rpm,
- depending on runner diameter.
1500-1900 is an optimal range.
The heat pump is a water-to-water design.
Input: My water source is a 40 degree F spring running at 5gpm.
Output: Hot water will be used for hydronic heating

Pictures of the turgo runners I use:

Hartvigsen Runners

NickD on Sat April 24, 2010 5:27 AM User is offline

Thought maybe you lived near a damn where you could drill a hole in the bottom of it can get some decent pressure. A vane type pump would be far more efficient than that open design you have. Turbine with progressive diameters are even more efficient, but this is old science.

I wouldn't try that at home, my city water bill is greater than my electrical bill. Son tried to talk me into going half and half on a cabin on a lake front up north, pictures on the net were pretty and the prices came way down. But we drove up there to learn these cabins were on mud. Using hydroelectric power and drained the lakes dry, just not getting enough rain. But if you have a stream you could dam up, technology already exists to make your own hydroelectric plant.

microHydro on Sat April 24, 2010 12:11 PM User is offline

In fact, I built a micro-hydro-electric unit which powers my home. Now I would like to utilize the 300% COP efficiency gain of a water-to-water heatpump to heat my home.

Regarding the vane pump suggestion: Turgo runners are very efficient. More importantly, they can accommodate a variety of flows by changing nozzle size and number of jets.

Take a look at the wiki:Turgo Runner
Pelton Runner

Edited: Sat April 24, 2010 at 12:14 PM by microHydro

mk378 on Sat April 24, 2010 1:00 PM User is offline

Scroll compressors don't work very well at such low rpm if you were to direct couple. At 2000 rpm you could use a rather large auto compressor as you're underspinning it. There are two other kinds of car compressors, rotary vanes and piston type. Overall though, car parts are designed to be cheap as a priority versus efficient.

You will get much better COP going directly to air on the hot side. Heating water to hydronic temperatures then incurring a large irreversible temperature drop to the living space is very inefficient for a heat pump. The difference between evaporator and condenser temperatures determines the theoretical efficiency. Ideally (from a thermodynamic standpoint) the air leaving the condenser would be only slightly warmer than the room.

microHydro on Sat April 24, 2010 2:09 PM User is offline

My hydronic application is optimized to minimize condenser temperature. I have a huge surface area of pex pipeing (2000 lineal feet) thermally coupled to concrete. The concrete is itself coupled to the room air with a huge surface area (5000 sf). The result of this heat exchanger oversizing is that I can realize significant heat transfer with as little as a 5 degree F difference between hydronic fluid and room temperature. If the target room temperature were 70 F, and the target hydronic fluid temperature were 80 F, how would that inform your choice of compressor?

ice-n-tropics on Sun April 25, 2010 5:40 PM User is offline

The performance curves (cooling and power vs. rpm) for Sanden scroll compressors are found at

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

NickD on Mon April 26, 2010 9:01 AM User is offline

Ever see a SEER rating for a MVAC system? If you are limiting yourself to a MVAC compressor, also limiting yourself to MVAC low pressure relatively inefficient refrigerants, pressures are of far more concern than efficiency. Would look more into the HVAC end of this business, especially if you are interested in a heat pump.

But its your time and money.

microHydro on Mon April 26, 2010 12:16 PM User is offline

Thanks for the info, ice-n-tropics.

I think I have a good choice for a scroll compressor: Sanden TRSA09

I have about 2.8 kw of shaft power to share between my hydroelectric unit and the heatpump. I will use auto sprinkler valves to shunt water between the heatpump and the hydroelectric unit, regulated based on my household electrical needs. I would expect to want at least one jet running the hydroelectric unit all the time (~700 watts shaft power). So I thought I'd design the heat pump to handle 700-2100 watts shaft power. This TRS09 looks about right.

Will I be able to run the compressor at various power levels?
1 Jet = 700 watts
2 Jets = 1400 watts
3 Jets = 2100 watts

nickD: I had not looked at a SEER rating. I'll need to learn about that. My application is not automotive, and is outdoors, so I could choose another refrigerant. Is there a better choice?

I looked for open drive scrolls in HVAC, but they are only made in large sizes. I can't use a hermetic unit. The Sanden TRSA12 has a Volumetric Efficiency of 91% in the 2000 RPM range I use. I assume the TRSA09 is similar. This seemed reasonably efficient - is it?

Edited: Mon April 26, 2010 at 12:18 PM by microHydro

NickD on Tue April 27, 2010 7:29 AM User is offline

This is a MVAC website, I can't even turn the hub on an automotive compressor with a 1500 psi pressure washer let alone a water stream. Suggest you find one of those green sites for your questions.

microHydro on Tue April 27, 2010 11:35 AM User is offline

Those green guys do not have a detailed understanding of compressors and refrigeration systems. The experts in this forum are most valuable to me.

NickD on Wed April 28, 2010 4:01 AM User is offline

Energy cannot be created nor destroyed, can only be changed from one form to another, can however bankrupt you. My governor is an idiot pushing ethanol from corn, like farmers can grow corn on the same field year after year without killing the fields. Encouraged many plants to make ethanol and the only way they can stay alive is by a nice subsidy and using practically as much as fossil fuels to make an equivalent amount of the energy in ethanol. A farmer could make more money by selling his corn rather than using it to feed a cow to get milk, cheese, or beef, so the prices of these human needed foods skyrocketed so our governor could feed his SUV while putting the state in a hole.

They love to use the term, renewable energy, no such thing as renewable energy. Science has already figured energy conversion units between heat, electrical, chemical, and mechanical energy, learn them. Any conversion from one form of energy to another is less than 100%, sometimes far less, our vehicles are a perfect example of that with efficiencies less than 15%. The real problems were never addressed and today we have more lies than pure facts.

You have to know the source of your energy and learn the most efficient way to convert it to a usable form. Refrigeration is of some interest as its more of a heat transformation concept than using fossil fuels to directly convert chemical energy into heat. But the current refrigerants have a very limited temperature range in doing this efficiently with far more emphasis on making them environmentally safe than effective. Ironically, the hotter it gets, the less efficient these system get or in reverse, the colder it gets in like a heat pump, efficiency drops to nothing.

Automotive carries any additional curse, all specifications are kept esoteric and even in engineering, efficiency does not play a role, make it near satisfactory to please both marketing and the stockholders, all in other words, make it cheap.

I agree with you, most green people are idiots, but if you have an interest, know your energy source, the quantity of it you have, and learn how to efficiently convert it, can't do this on this board. But can guarantee you, you will run into many governmental obstacles, that is the key problem when you have idiots running this country. I gave up a long time ago.

ice-n-tropics on Wed April 28, 2010 10:57 AM User is offline

Volumetric efficiency is the measure of the actual quantity of gas that is pumped compared to the ideal quantity and is just one of the factors that determines energy efficiency.
Volumetric efficiency of TRSA09 is probably a pinch better than the TRSA12. The gas bypass is similar between the scroll flank and tip, if the machining and assembly (clocking) accuracy is the same. Next, is the number of turns of scroll wrap. The TRSA12 is somewhat compromised to keep the OD small enough to fit into the underhood real estate. Therefore the TRSA09 volumetric efficiency is equal or better as compared to the TRSA12.
Older TRS models have better volumetric efficiency than the newer TRSA models because the scroll center shapes are not comprimised to increase fatigue strength and because they do not have an internal bypass to return oil (and some gas) from the discharge to the suction.
Mitsubishi makes a TRSA12 scroll equivalent which is on newer Chevy Trailblazers
Scrolls gain some mechanical efficiency over a design which wastes energy, e. g., reciprocating pistons which totally change direction.
Isentropic efficiency is very close to adiabatic efficiency and is a more accurate measure of energy efficiency than VE.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Wed April 28, 2010 at 11:05 AM by ice-n-tropics

microHydro on Wed April 28, 2010 11:33 PM User is offline

Thanks ice-n-tropics for the info!

Are Isentropic efficiency numbers available for the Sanden scrolls? You mentioned that the 012 model has fewer wraps than the 09. Does it then produce lower compression? My application has a narrower temperature range between evaporator and condenser than normal auto AC. Is there a way to gain efficiency from that smaller delta T? For example, could I use an Automatic Expansion Valve adjusted for higher evaporator pressure and gain efficiency? Would a different refrigerant be better suited to my temperature application?

Heat Flow Temperatures:

40 F Spring water cooled to 32 F
Evaporator 20 F
Refrigerant 10F

70 F Hydronic Fluid heated to 80 F
Condenser 90 F
Refrigerant 100 F

Do the evaporator, condenser, and refrigerant temperatures seem right?

Edited: Wed April 28, 2010 at 11:47 PM by microHydro

bohica2xo on Thu April 29, 2010 1:31 PM User is offline

An interesting plan, with a few issues. I think you may need more than 2.8kw worth of flow to really make this pay off, but let's cover the bigger issues.

Heat transfer on the low side is the first thing that comes to mind. You may need to meter the refrigerant flow to keep the 40f water from forming ice, unless you have a lot of area. The object is to make the suction gas from the evaporator as warm as you can.

Oil return is another issue. This type of system with very cold evaporators, and moderate refrigerant flow will not return oil easily. It may be necessary to use an oil separator on the compressor discharge to keep the oil out of the refrigerant loop. This does increase heat transfer as well.

Orientation of the unit is also an issue. MVAC compressors are designed to run with the shaft horizontal. All DIY turgo installations I have seen were vertical. Large power generation has been done both ways, so there is plenty of info to draw on for the horizontal shaft turgo setup. Take some time, and look through the hundreds of pages at
French River Land Co. Those folks specialize in the actual engineering & repair of huge & ancient hydroelectric stuff.

You should consider a jackshaft arrangement with a way to step up the drive speed if you plan to use a scroll compressor. I will not re-start the compressor selection pissing contest here. You can probably find several threads with the search function if you want more compressor info. There are some runner / nozzle combinations that will spin 3,000 rpm, but MVAC compressor shafts are not designed like alternators, and will not handle direct coupling anyway. You are sort of locked into a belt drive of some sort.

So, wheel vertical, shaft horizontal running in a pair of pillow blocks. PTO from the far end of the shaft - nice & dry there too.

Refrigerants. Get ready to hear some of the MVAC guys wail. This system operates more or less in the open air. The turbine house is most likely some distance from the dwelling, and the heat is secondary loop inside the concrete slab. The refrigerant of choice for me would be R290 in this case. It is well suited to liquid chillers not operated in enclosed, occupied spaces. R290 is also known as Propane. Cheap, works with any kind of compressor oil... and "green". I have run R290 in chillers for decades, and there is a small but measurable energy savings over 134a in the same unit.

Good luck with your project.

Edited to add:

Here is a link to a single project at FRL with a Pelton wheel on a horizontal shaft - for readers without the patience to look through the FRL site:

Lake May Pelton Wheel


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Thu April 29, 2010 at 1:48 PM by bohica2xo

microHydro on Thu April 29, 2010 3:27 PM User is offline

Wow, bohica2xo, what an informative post! I'll post follow-up questions to your ideas in separate posts.

1. Heat transfer on the low side is the first thing that comes to mind. You may need to meter the refrigerant flow to keep the 40f water from forming ice, unless you have a lot of area. The object is to make the suction gas from the evaporator as warm as you can.

I've been designing a system to control ice formation, and maybe even capture some latent heat of freezing. I plan to use coils of tubing inside a barrel of antifreeze solution. The 40 F spring water is gravity fed to my site, with a static pressure of almost 50 psi. It will sustain 3.3 gpm. When cooled to 32F, this is a heat flux of about 13,000 btu/hr.
Construction: Fill a 55 gallon barrel with antifreeze. Immerse a copper refrigeration evaporator coil from the heat pump. Arrange the evaporator coil so that refrigerant enters the barrel at the bottom and exits at the top. Surrounding that coil in the barrel, immerse 350 feet of 1/2" pex flowing the spring water. Arrainge the pex so that the spring water enters the top of the barrel and exits out the bottom. the 350 foot length will convert the 50 psi static pressure to heat as friction loss and flow about 3.3 gpm. Control the cooling action of the evaporator coil to limit the minimum temperature of the antifreeze in the barrel to a few degrees below freezing. My hope is that some ice crystals would form in the spring pex coil, but would be swept along in the 5.5 fps flow and not buildup inside the pex.

I think that the refrigerant could be significantly below freezing and still not freeze the spring pex line. However, If I could design my system to have higher refrigerant flow and higher evaporator temperatures, then I might have more efficiency.

What would you recommend for a design like this?

Edited: Thu April 29, 2010 at 4:05 PM by microHydro

bohica2xo on Fri April 30, 2010 1:55 AM User is offline

Well, I like the barrel. The pex tube & antifreeze? not so much.

Pex has a poor heat transfer compared to copper. So you are planning to use a LOT of it to make up for this. But you give up some power with the losses in the tube. I would rather see the power available to spin the compresor... 55 gallons of antifreeze? Costly, and another maint. item.

The barrel however will do nicely for the pit & part of the tailrace. Imagine a divider in the barrel, that runs from top to bottom, but stops short of the bottom by about a half barrel diameter. Near the top of the barrel bend the divider over to deflect the falling water from the turbines in to one side. On the opposite side, a few inches below the top of the barrel cut a vent or spillway for the water to leave. Now the water must travel down, under the divider - then up & out the vent on the other side. You can easily pipe it away, if that will make the install cleaner.

Place your evaporator coils in the barrel. Sure, you will have a little corrosion over time. Easy to service when the time comes. Hydro stuff wears out & corrodes. Just part of the job. Should be able to build it in a day. If you have the full 5gpm of flow to work with, that gives you about 17,400 btu/h with a 7 degree delta. Actually a fair bit of heat if you can grab it.

If you use a TXV to maintain evaporator flow, it may take a little tinkering to get the proper superheat setting. A dodge H block type TXV is externally adjustable.

What are you using for power generation right now?


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

microHydro on Fri April 30, 2010 2:21 AM User is offline

Interesting barrel idea.

I have two water sources. The water that spins the turgo runner is from the creek, and cold in the winter (32F with ice crystals). The spring is independent from the creek, and needed only as a heat source. The spring actually runs at 3.3 gpm.

The antifreeze is recycled from friends with big rigs, and free. In my barrel application, it needs only not freeze, purity and age are not an issue. I could use a short copper coil for the first upper part of the spring coil, and then transition to pex in the lower-colder part. The main advantaqe of pex for the spring water is that if I cool the barrel too much and freeze the spring line, it won't damage the pex.

Thanks for the tip on the TxV

My hydroelectric unit is a super efficient EcoTech alternator directly coupled to the green turgo runner shown.

What do you think about the possibility of harnessing some of the latent heat of freezing?

bohica2xo on Fri April 30, 2010 8:52 PM User is offline

Ok, I thought your only water came through the penstock. If you are not giving up horsepower, the the only downside to a mile of PEX is the cost.

I think the idea of recovering the latent heat of the water phase change is interesting, but might require a fairly complicated control system.

I presume the alternator is rectified at some point, and you are using batteries & inverters. If this is the case you should consider a bigger runner for the horizontal shaft system.

By sizing the new runner to get maximum power from 100% of the available penstock flow, you can drive multiple things from the output shaft. Lundell type alternators are more effective at higher speeds. Most refrigeration compressors are equipped with a clutch, and some are constant run with PWM displacement control.

If you were to use a second alternator on the shaft, you could control the field to hold the load on the runner in the sweet spot. When not running the refrigeration system, you could use the power for electricity. At night, when electrical loads are small, the heat pump could run full power... Good job for a small PLC.

With only 3gpm of water, a conservative figure would be 10k btu/h. That is in the lower range of most MVAC compressors. You might not need to spin a piston type up too fast to get what you need. Below is a couple of graphs for compressors:

You can see where the 2.8 kW line is...

Are you planning to use this system for cooling in the summer?


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

microHydro on Sat May 01, 2010 1:15 PM User is offline

Lots of great ideas, bohica2xo.

Next post I'll talk about compressors. Now we can close out the evaporator discussion.

I think the idea of recovering the latent heat of the water phase change is interesting, but might require a fairly complicated control system.

Hmm. The control needed is really dependent on just how much heat I can pull out of the spring water. If my 2.1 kw heatpump operates at a COP of 3:1, then it will need 14,280 btu/hr from the spring water. This 14,280 btu/hr plus the 7140 btu/hr from the work of compression yields a 21,420 btu/hr system. Awesome! In six months of heating that saves me 8.5 cords of firewood. Cooling 3.3 gpm spring water from 40 F to 32F gives 13,210 btu/hr. 1070 btu/hr would then need to be recovered from the latent heat of freezing. So about 0.4% of the spring water would need to be frozen. It seems possible. Perhaps I could use an Automatic Expansion Valve and adjust the evaporator pressure to hold the bottom part of the barrel a few degrees below freezing when it is in full-power steady-state operation. There would be no concern of freezing for less than about 1.8kw shaft input power.

Most refrigeration compressors are equipped with a clutch, and some are constant run with PWM displacement control.
Is this like a copeland digital scroll? Is there really an open drive digital scroll made?

I run a battery - inverter system. You are the second person to suggest putting both the compressor and the alternator on the same runner. I am hesitant to do that for several reasons. 1. The current alternator installation works very well - it is optimally efficient. Direct coupling of the alternator gains me 10% to 15% in efficiency. 2. The alternator needs bearing service every 14 months. This is a very disruptive process to my household. When something goes wrong with either the alternator or the compressor I don't want it taking the other system out of service too. 3. Two separate runners would allow a greater range of flows.

Edited: Sat May 01, 2010 at 2:21 PM by microHydro

ice-n-tropics on Sat May 01, 2010 5:51 PM User is offline

Haven't found any isentropic efficiency on any of the 4 Sanden web sites. But COPs are given.
I just thought, Heck, I could do this too.
Tells us about the alternator, battery and inverter system, e. g., quantity of batteries, alt specs, inverter spec., power output.
I have a 45 gpm spring fed stream with a 4 foot waterfall out of a 18 inch metal culvert on my pine tree/hunting farm. Also have a 3 gpm spring fed overflow on a 20 foot dam overflow.
Have been using a 110 volt Onan gen set for 1 ton summer A/C. Also have two twelve volt batteries for night lights in the mobil sleeper.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Sat May 01, 2010 at 5:54 PM by ice-n-tropics

bohica2xo on Sat May 01, 2010 8:56 PM User is offline


My concern with the phase change is the possibility of it "running away" if something goes wrong. If you manage to pull the barrel of antifreeze down to 25f, and lose flow on the PEX you will have one very long helical ice cube.

I would not get rid of the original alternator & runner. I was proposing a secondary alternator on the shaft system to take advantage of the redundancy as well as the additional output during some modes of operation.

I understand the reason you put up with the ongoing service requirement of the close coupled design. I do a lot of alternator work, and hate to see the shaft wet, but it does reduce the drive losses. A tradeoff to be sure. Adding a second alternator on the "heat pump" drive would ease the pain of servicing the primary alternator. "Two is one, one is none."

Depending on the grade near any of that flowing water, you may have a sizable output just drifting away. If you can put in a penstock from that 45gpm flow that is 40 (or more) feet lower than the origin that is a lot of power. The 3gpm from the dam is already 20 feet at the base - if you went downstream, how far would you have to go to be 100 feet below the top of the dam?

The OP posted the link to Hartvigsen in his first post. They make a good selection of runners, and provide an Excel spreadsheet on the site to calculate output vs flow. There is a fair bit of micro hydro scattered across this nation, most of it built by the owners like the OP.

Have a look at the French River links I posted, you should enjoy the dinosaurs they work on...


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

microHydro on Sat May 01, 2010 9:14 PM User is offline

For hodrodac (Tex?)

Power watts= Net Head (ft) * Flow (gpm) /5.32
For you this is only 34 potential watts
The best water to wire efficiencies are about 50%, so that leaves you 17 Watts. As bohica indicates, you really need more head. My household started to come into its own when I started making 500w.

Battery - inverter systems are very useful as they can start big peak motor loads, and operate small loads efficiently. I use an Outback inverter with 30 salvaged golf cart batteries. The Outback is bulletproof and field serviceable. A 24V dc alternator, battery and inverter system is an excellent choice. The outback has a built in automatic generator transfer switch and battery charger. An outback and a few deep cycle batteries will save a lot of generator gas until you get a hydro system going.

Edited: Sun May 02, 2010 at 2:28 AM by microHydro

microHydro on Sun May 02, 2010 2:19 AM User is offline

bohica2xo: The runaway freezing is an excellent design consideration. I might put a standard hose thread connection on each end of the pex coil so that if it freezes as I dial in the evaporator temperature I can just pull it out and bring it inside to thaw.

I see the utility of your suggestion for the new heatpump to have an alternator also, in addition to the main hydroelectric unit. Load sharing between heat pump and alternator would be much faster by varying the alternator wound field strength instead of switching nozzles on and off between separate units. As the field strength went down, the compressor RPM would increase until the compressor used up the extra power. The RPM efficiency curve for a turgo runner or pelton wheel is pretty flat in the middle, so I could match the center of the turgo sweet spot with 1500 watt RPM for the compressor. Then extra power would spin the compressor faster, and less power would spin it slower. In the graph below the runner sweetspot is 1800rpm. The runner would loose 5% efficiency running at 1500, or 2200 RPM, and could operate at two other output levels.

Hotrodac: In cooling applications, what is COP a measure of? Can we turn the Sanden COP data into useful information?

Edited: Sun May 02, 2010 at 2:24 AM by microHydro

bohica2xo on Sun May 02, 2010 10:32 AM User is offline


Looks like you got it. By adjusting the field current on ALT-2 you can adjust the turbine speed for optimum power. All you need is a speed sensor, and a very simple controller.

The freezing issue is a "murphy" thing. Running close to the edge of freezing all of the water is where the gain is, but any deviation in water flow at that point can lead to solid ice. If the stream flow drops to 2.7gpm because of some trash - you get ice. One degree colder on the inlet water? Ice.

The dynamics of the PEX in antifreeze mean that you will have a lot of stored energy in that barrel. The antifreeze will have to be well below freezing when things are working right. R290 boils @ -40 at atmospheric pressure. Your evaporator temperature is controlled by the pressure in the evaporator, so pulling the barrel down to 10f is certainly possible... Yeah I would make the PEX coil removable.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

ice-n-tropics on Tue May 11, 2010 2:55 PM User is offline

Sorry for late reply to your COP question, but I had launch activities for "How to Air Condition Your Hot Rod" which I co-authored and is now published (in China) and available from Amazon.
COP = Evap Heat-X in BTU/hr divided by Comp Power Input converted to BTU/hr
Comp power = refrigerant mass flow rate X enthalpy of compression.
E. G., 20,000 BTU evap/(4.95 hp X 2545.9 BTU/Hp) = 1.59 COP
COP is greater than one because the energy of vaporized refrigerant is greater than the power into the compressor for a properly designed A/C system.
COP can be optimized by lower discharge pressure & increased sub cooling leaving the condenser, lower evap superheat, less suction pressure drop to retain max. suction gas density (larger suction hose).

I don't have a simple conversion from COP to Isentropic efficiency except that they both are good indicators of how much power will result in the required BTU/hr

For your thermal ice storage tank freeze up preventer mechanism you could create a liquid/ice slush paddle torque sensor:
I did a layout for the ice sensor for partial freezing for the ICEE frozen carbonated drink machine we called the "Torque Sensor". It is a paddle driven by a shaft coupling with tapered teeth that slide on each other and open or close a gap between the tapered roots of the toothed coupling halves. A switch extended arm senses the amount of gap resulting from a thicker ice slush and turns off the compressor before freeze up. You can see this mechanism by looking through the back perforated screen of a blue ICEE machine. Back when I was just a kid fresh out of high school at John E. Mitchell Co/MARK IV and worked cheap, the Mitchell bro's hired me at $2.75/hr to make all the ICEE drawings and do various ICEE refrigeration research projects.
A thermal storage tank could include immersed banks of evaporator tubing in the shape of a spiral scroll separated by 4 to 6 inches so that ice slush could build up between them. A larger space on the edges to the containment tank would encourage liquid around the ice mass.
Hope this helps a little,

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

Edited: Tue May 11, 2010 at 4:08 PM by ice-n-tropics

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