swanning_it
Member
To all you technical types who understand the principles of thermodynamics, may I ask how the COP is calculated.
I know S2S has said part of it, but I was wondering how they calculate (or measure) the COP. The electrical part I understand, the temperature part I've no idea!
swanning_it
Member
Coefficient of Performance....sorry, just wondering how the input energy is measured against the output energy.
I think the temperature side just involves taking the specific heat of air, the quantity of air and the temperature difference an multiplying them all together. This gives you the amount of heat energy output by the unit.
How exactly they measure the air quantity and temperature I don't know but I presume there are standard methods.
I understand the COP to be the amount of heat energy delivered divided by the amount of electrical energy taken to deliver that heat.
swanning_it
Member
Hmmm, I don't even know, what I don't know!
What I gather is that the amount of MJ of heat "moved" from say inside to outside (in the case of a fridge) has to be measured to work out the COP (the electrical energy is easy to measure). So what's the contributing factors in these calsulations?
Let's take a fridge....I was thinking the volume of the space would play a part, but then I thought no it doesn't, because the heat energy moved should be a constant, therefore independant of the volume. The desired temp doesn't factor in either because a thermostat will simply switch off the compressor when the desired temp is reached (assuming the gas used will allow the desired temp to be reached).
So what are the factors that determines the amount of heat which can be moved? I'm sort of leaning toward the available heat in the atmosphere, the type of gas used in the compression, which will then effect the condensation but as I said previously.....I've no idea.
More to the point, how is this energy migration measured? I agree munter that the formula is COP = energy moved/energy used, and I also have no problem with both calculating and measuring the energy used part of the formula, but how do they calculate (and/or measure) the heat energy moved?
swanning_it
Member
I guess the surface area of the coils as well plays a part, as well as some kind of measure of heat transference (thermal conductivity?) of the metal type, but once again I'm still guessing.
TonyT
Member
swanning_it asked:
... how the COP is calculated.
sun2steam provided an excellent definition of COP at
http://www.ata.org.au/forums/topic/heat-pump-hws-using-co2-with-coeff-of-performance-8
swanning_it
Member
Yes TonyT, in my first post I did mention that and I think S2S did a good job of explaining the formula (for me anyway), however he also stated that was a "theoretical maximum".
Hence my question. I'm not after theoretical maximum, unless that's achievable. I'm trying to establish what factors effect the COP and more importantly how the research teams work them out. Or is it just trial and error? Surely there's a bit more theory to it than simply measuring the heat inside and the heat outside? I haven't tried it, but I'm guessing if I measured the inside and outside temp and the compressor run requirements of my fridge and slotted them into the formula, it wouldn't work out. For a start, the temp. inside the fridge is changing. Which value do you use, the temp at start up or the temp at the end of the cycle.....or the temp at a saturation level (where the temp won't change no matter how much more you run the compressor)? I'm also guessing it would be the temp at saturation level which in a perfect world would equal the theoretical maximum but S2S indicated otherwise, so is there a factor that can be used to multiply the theoretical maximum to achieve the actual maximum (or close to it)?
Another thing that puzzles me, is whether that COP is linear through it's range of heating/cooling or exponential? If it's exponential, than the COP (if measured at maximum exchange) would be misleading if the fridge/air cond./HWS was controlled by a thermostat.
So many quaetions, so few brain cells......I knew I shouldn't start drinking!
swanning_it
Member
I think I'm just starting to understand what it is that I don't understand.
To start with, I'm trying to complicate an uncomplicated issue. The COP is just that. I doubt it can be calculated (certainly not easily) but it can be measured (fairly easily). So a manufacturer can state a correct COP simply by measurement. What makes more or less efficient COPs are the variables that are hard to identify (like thermal conductivity and the properties of freeze and boil of varying refrigerants etc), but in themselves they aren't part of the basic formula for determining an actual COP from the measurements.
Actually TonyT, it was from your thread that got me started on this. You made a comment about whether the Eco Cute was heading toward a combined HWS and fridge, so I started wondering what the problems were and why no one had done it so far. I think I see one problem. That being that to heat water you need heat in the atmosphere. If you tried to draw that heat from a fridge compartment, it would take longer to achieve a smaller change and it would be difficult to control your fridge temp.(typically you want a fridge to stay between 2 and 5 degrees C and the freezer between -14 and -18 degrees C). So the change in the fridge might equal 3 degrees, which is hardly worth the effort for HWS heating unless that change occurred frequently which brings into quaestion the higher electrical energy use caused by frequently starting and stopping an inductive motor. To compound the problem, every time someone takes a hot shower, the tank temp. drops considerably so some kind of "normal" atmospheric heat use would be required which must mean secondary coils, contol logic and associated diversion valves, which then starts impacting on the embedded energy of the equipment.
I think it could be done and the ability to have just one compressor and one lot of refrigerant is certainly something worth thinking about. The other issues I see, is similar to combined inverter and battery chargers.......one fails and you loose both!
This is a simple explanation: http://en.wikipedia.org/wiki/Coefficient_of_performance
and this is a detailed one: http://www.freestudy.co.uk/thermodynamics/t5201.pdf .
Generally speaking you started to complicated as internal heat transfer issues do not need to be addressed directly. They are disregarded in the calculation of the ideal case (Carnot cycle or similar) and they appear as cause for the deviation of the real system from the ideal system. However for measuring the real system we can use a simple approach:
Let's use the equation
COP = Q/W
where
* Q is the heat supplied to the hot reservoir
* W is the work consumed
Q cab be measured various data. Q=m*cp*delta T
with m = mass of the medium in kg
cp = the heat capacity at constant pressure (can be looked up in tables)
delta T = change in temperature
This is where the difficulties start in the real world:
as cp is changing a bit with temperature, usually the measurement will be done with a small temperature change only to keep errors small
However then the temperature difference is small and requires precise temperature measurement.
Also all unaccounted heat losses must be avoided or at least added as a correction term in the final equation.
So depending on the system the measurement could be done in various ways:
a) groundwater heat pump:
Just measure the electrical power input, the flow rate of water going through the system and the temperature change between input and output. Done!
b) fridge:
You might put a very big drum with water into the fridge with the fridge thermostat setting say at 10 degrees C . (the water must be continuously stirred to avoid temperature differences in the water) Leave it for a day to allow the system to reach a constant temperature.
Monitor electrical power input and temperature.
Change thermostat setting to another temperature say 5 degrees C.
The temperature recording should show a slow drop of temperature which eventually stabilizes at a lower temperature. Keep running and recording for some more time.
There are a few difficult bits.
We have mass and heat capacity of water, But we must allow also a heat capacity of the drum wall and the cold part of the fridge. There are ways to make at least good estimates, but I will not get too much into details.
We must also measure the electrical power consumption of the fridge while the temperature does not change anymore. This gives us an estimate of heat losses which we must address in our final equation as well.
Also the electrical power consumption is not yet the mechanical work. We must allow for motor efficiency and depending on our target also for compressor efficiency. However for real life applications this is mostly disregarded because motor efficiencies are close to 100%.
And a last word. This is about physics where terms (words) with defined meaning are being used. Otherwise it would be a Babylonic confusion. That's why I have been insisting that COP is just COP and not COP_cold PLUS COP_hot.
TonyT
Member
Great links, thanks sun2steam !
Hello swanning_it:
I think that 2 compressors are being used.
The waste heat from the cooling unit is used as part input to the heating unit.
Ecocute seems to be a generic name for Japanese heat pumps amd many companies are making Ecocutes: Mycom, Daiken, Chofu, Corona, Panasonic, Hitachi.
Having said all that, it is of course possible to make a more efficient use of a heat pump if you can use both the temperature increase on the cold side and the temperature reduction on the hot side.
so for instance cooling your house and warming up the pool could be an option. Or in a mild winter warming the house and cooling the pool until it nearly freezes.
However running the freezer or fridge in a combined system is difficult as it was already pointed out as the heat capacity and temperature range of these appliances is quite small.
But something that is easily achievable is to blow the air coming from a heat pump hot water system into the hose. This is a free air conditioner on hot days. The downside is that you have to move away from cheap night tariffs because that's when you don't need an air condition.
The very small ones (rather Eskys) have peltier elements and are very inefficient.
The multi-fuel fridges that can also be driven by gas use the absorption refrigeration concept (invented by Albert Einstein before he became relative). Also less efficient than compression heat pumps with COP a bit more than one. However this is related to the energy of the gas. If you use the heat energy of the fuel to calculate the COP of compression heat pumps they are not much better because we can get just about 40% of the fuel energy as electricity at our power points.
Buzzman
Member
Ahem....
<Quote> The "Einstein Refrigerator was jointly invented in 1926 by Albert Einstein and his former student Leó Szilárd and patented in the US on November 11, 1930 (U.S. Patent 1,781,541).
This is an alternative design from the original invention of 1922 by the Swedish inventors Baltzar von Platen and Carl Munters.<Unquote>
Okay, so the quote is from Wikipedia, but it is cited.
http://en.wikipedia.org/wiki/Refrigerator
The absorption cycle on which both were based was first demonstrated in the mid 18th Century, but the earliest known 'fridge' was invented by an Arab scholar in the 11th Century.
Enjoy. 
Jeffbloggs
Member
S2S
Thanks for the info. But on refrigeration compressors, even though the electric motor components are +90% efficient, the compressors are substantially less. Do you have any figures for AC compression efficiency? Typical air compressors are under 50% eff., most even run at 10%, so theoretically a doubling of COP is possible from what is considered the norm just by employing better compressors, even if they "must" conform to energy conservation principles. I would like to know what the max compressor eff. is in AC units, but I haven't found much info on them so far.
-
@All
Using the both sides of the heat pump is a logical step. Have a look at this link for a common AC with hot water heat exchanger, that has this function.
http://www.alibaba.com/product-gs/249698111/Hot_water_heat_pump_air_conditioner.html
I think the important thing to understand with all of these devices is that they do not make heat, they only forcefully transport it from one location to another. It's a bit like filling up a water tank with a bucket, the bucket you are using does not make any water, it only collects it from one place and puts it in another. A heat "pump", like a "water pump", does the same with heat.
In any event it will create a potential temperature difference to ambient temperature, it will make one side colder than ambient, by removing the heat there and moving it to a warmer than ambient one, making that one even warmer. For any AC condenser or evaporator to function effectively, they need to have a relatively high temperature difference to the medium they are absorbing heat from (ie cooling) or dissipating heat to (ie Heating). The smaller the temperature difference to the medium, the less effective they become. That is why it is so important to operate all forms of heat pumps from optimal ambient mediums. Earth tube and/or hydronic thermal storage systems can greatly improve their effectiveness by introducing a "constant" and reliable base temperature medium from which to "heat pump" from. Hydronic type systems also allow for "harvesting" at the most advantageous times. For example running the AC at night when ambient air is cooler, and storing it in a insulated thermal store for hot daytime use, like this one:
http://www.ice-energy.com/ice-bear-energy-storage-system
For heating the opposite type system can be used to harvest daytime max temperatures for use at night, via an insulated HWS type water store. Solar can also be used to further add heat if the weather permits.
I have grown to the understanding that in warm weather one can harvest energy from the environment to maintain interior climate, however, in cold weather there is less readably available energy, especially with cloud cover, and therefore one tends to require the addition of "artificial" fuel sources ie timber.
Initially this type of system might seem overally complex, but if all these components are streamlined into one overall unit, the cost can be significantly reduced, and it's functionality and efficiency increased.
One of the things I have been considering is using only the water thermal mass as a "heat/cold battery" for all household type devices, from fridges to ovens, AC cooling to HWS and climate control etc. By de-coupling the operating times and the operating temperatures of heat pumps from demand, the gains, especially for those on cheap night time tariffs can easily repay the capital investment costs. The reason I came about this was from some designs, which resulted in us finding that water, with an added coolant to restrict it from boiling/freezing, can store more heat/cold energy than a equivalent battery, at the cost of only a simple insulated water tank. Plus the heat/cold stored can easily be distributed for refrigeration/climate control/HWS with minimal losses and little equipment, as a simple closed circuit water pump and valves with temperature sensors was sufficient.
As for peltier elements they have yet to achieve a COP of 1.
There is another better device however for this called a thermoacoustic engine/pump.
See here: http://en.wikipedia.org/wiki/Thermoacoustic_hot_air_engine
or a third world version (named "SCORE")here:
http://www.score.uk.com/research/default.aspx
Some local development here:
http://www.mecheng.adelaide.edu.au/avc/thermoacoustics/
JB, sorry I do not have real life data for compressors. This is not my area of expertise.
What is AC compression? Any difference to DC compression? How should the compressor know what kind of electricity is driving the motor? Sorry, I just realized you most likely used AC for air conditioner. But I think you mean compressor heat pump? You see, by using unclear terms communications becomes complicated.
But I doubt that a better compressor can double COP. There are thermodynamic limits. In your contribution you mentioned the best way to ramp up COP: use the best source for hot or cold that you can get. This helps a lot because COP is highly dependent on the base temperature and the temperature difference as can be seen from the basic definition. In addition heat transfer resistances (the real world issues) will become less important if you have good driving temperature differences between the inlet respectively outlet of the heat pump and the media like 'cold' supply or 'hot' supply. (Sorry, here I am messing up terms myself as I cannot find the proper words.).
Jeffbloggs
Member
Hey S2S
I know you don't like my use of worms. I like using worms for all kinds of things...ahem I mean "words" of course! 
On AC compressors, you correctly assumed that I meant Air-Conditioning (AC) compressors. I will now use the definition "air-con" to reduce confusion.
However, like you I am still in the dark on air-con compressors efficiencies. I will try to research this some more to get a better idea of what is normally employed in air-con units and will post my findings.
On the temperature source for heat pumps: This is probably one of the biggest untapped resources for consumption reduction in households. Over 30% of energy consumption in Australian houses is from climate control, and with the increase usage, due to increasing affordability, of air-con, these percentages are likely to rise further. This is obviously not a good thing and some appropriate "temperature source" type installations would be a good addition. It also would counter peak electricity loading (a prob in WA) and generation derate for higher ambient temperatures, along with the ability to offset air-con consumption to cheaper off-peak rates (even if it is from coal!).
-
Tony T
Was that meant to be a compliment or a general observation?
As humans we are 100% dependent on tools for survival. Nature has not provided us with the necessary tools for survival. Bar one. Ingenuity. Our technological creations are the crutches of our life, with which we can use them as environmental levers to change our environment, to make it more hospitable. Animals adapt to their environment or go the way of the dinosaur, we humans on the other hand, adapt our environment through the use of tools. It makes us "unnaturally" powerful (ie nuclear) but also unnecessarily vulnerable to those tools (ie fossil, capatilism & CC).
(S2S look away, I know you don't like this statement!)
On cheap energy I always say: "we cannot afford free energy". The reason is not because it does not exist physically, for instance in the sun, biomass wind etc, it is because we are "economically" not capable of having such a system. The affordability is an economical problem (due to greed, unbalanced trade and capitalist system) and not a physical one. Besides all energy already exists, (according to 1st law TD) so how much money can it cost to make some more energy?!?
The problem is only with what tools we utilize it, energy cannot be "used" per se. To change our behavior and "consumption" we need to change the tools, and adopt those which are effective of providing results with the minimal of effort on the environmental lever, but without burdening other "life" that creates an "ecosystem" wide disruption to naturally occurring balance of processes. Our ingenuity is key to our survival as a species, likewise it could be cause for our demise. It is a battle of good vs evil.
TonyT
Member
Jeffbloggs asked
"Was that meant to be a compliment or a general observation?"
Gratitude - many thanks !
off-topic but following your last para ...
Dan Winter (electrical engineer, research physicist, scientist, lateral thinker, humble) was in Adelaide recently.
Unexpectedly, I attended what was for me a truly remarkable weekend.
Dan linked together most of the eclectic collection of info that has interested me and lodged in my memory.
Google if you are interested.
Jeffbloggs
Member
Tony T
Thanks for the info. Always good to get some "new" perspective. In the middle ages electricity was considered wizardry. Today nearly everyone excepts it as scientific fact, their belief has become fact. Star Trek "invented" teleportation because the show couldn't afford the special effects to land model spacecraft on model planets (!), today it is being successfully used to develop super-fast computing. Who knows the limit to our ingenuity?
BTW I was trying to gleam some of natures glory with my compliment statement...
Jeffbloggs
Member
Hey Diver
Yes I did. Thx a bunch. I will respond via email.
Jeffbloggs
Member
Andy
Historically nothing is new. It's only being re-found. Nature existed like it does today, back in the middle ages as well. I would dare say we have lost as much technology and understanding of the world, as we think we have gained in the last two century's or so.
In some instances we are a lot more out of tune with nature then we have ever been, so can one really consider modern scientific achievement as a good thing? The tools have corrupted us as well, and we need to step back and consider how we use them. You can use a hammer to build a house, and to knock it down again. You can also hit your neighbor on the head with it. What do you call a Gorilla with a machine gun? "Sir!" The force we can apply on nature with our unnatural levers earns us "respect", even if we don't deserve it.
Windraider
Member
There are some Free Pdf's to download on my Site http://www.sustainabilitymeasurement .com which may be helpful.
Although I used to teach ThermoDynamicsI have wriiten them in what I hope is a straight forward manner to communicate the difference between Temperature and Heat Energy Transfer ------- Andrew Reid
