thermodymic panel for water heating

zeupater

1echidna wrote: »

Hi Z, I've done calculations professionally on industrial refrigeration and cryogenic systems with compressors of up to 30,000 HP (yes 30,000 Horse Power). I can assure you that your analysis of what goes on tubeside in these evaporators is wrong. Effectively we have the refrigerant in nucleate boiling at -26C (one of the commercial sources quotes this as the temperature of evaporation of the refrigerant at the appropriate compressor suction pressure) Nucleate boiling gives the most efficient heat transfer regime there is and the whole tubeside temperature will be very close to -26C up and until superheating occurs which may be a relatively minor part of the exchanger if heavily loaded.

As for the 25C, it is an unusual but not unknown temperature on which to base a peak performance, particular in full sunshine.

Hi

The real issue is what would the standard operating COP be over a full year of operation in the UK as this is the only relevant point deserving of discussion as it compares directly with manufacturer claims (their marketing comparison suggests an average annual COP of 8.4 based on running costs) ... again standard theoretical or lab-condition calculations are irrelevant .... this is similar to establishing that a 4kWp pv system is capable of generating over 17000kWh/year by multiplying 4x12x365 ... in the 'real world' it's impossible, just as an average annual daily temperature of 25C is in the UK ....

HTH
Z

Kernel_Sanders

zeupater wrote: »

At a temperature of around 20C at sea level air weighs about 1.2kg/cubic metre ....

Does air weigh anything? I guess it must, otherwise a hot air balloon wouldn't work!
I only ask because in O-Level Physics (which I failed) I learned that something can have mass (measured only in grams, kilos or tonnes) but not weight.
If so, how is the weight of air measured?

1echidna

Martyn1981 wrote: »

May I ask some stupid questions?

Assuming (for arguments sake) that such a system runs at 390W, would that remain a constant energy consumption, regardless of the temperature differential outside (or in the tubes)?

In other words, would it make more sense to run such a panel(s) when the temp outside is high, and ideally when the panel(s) are in the sun, or would the 390W demand vary accordingly?

I'm wondering if such a system, whilst able to work year round and day or night, might still profit from intelligent interference, eg the MkI eyeball, or some sort of temperature activated switch?

It also occurs to me that such panels could be used to cool an office if placed internally, but I can't work out what to do with all that hot water, plus I'm probably just re-inventing some sort of A/C to water heating system that already exists and is far cheaper/efficient?

Mart.

I imagine the control system will work on the compressor suction conditions controlling the pressure let down valve.




In all probability the compressor will be a constant speed device, only absorbing maximum power at the highest operating suction pressure (when the evaporator is operating at peak load). At lower suction pressures the power consumption may well be less, though how much less will depend on the operating characteristics of the compressor. Of course under these conditions the mass flow and temperature of condensation will also be less.

zeupater

Martyn1981 wrote: »

May I ask some stupid questions?

Assuming (for arguments sake) that such a system runs at 390W, would that remain a constant energy consumption, regardless of the temperature differential outside (or in the tubes)?

In other words, would it make more sense to run such a panel(s) when the temp outside is high, and ideally when the panel(s) are in the sun, or would the 390W demand vary accordingly?

I'm wondering if such a system, whilst able to work year round and day or night, might still profit from intelligent interference, eg the MkI eyeball, or some sort of temperature activated switch?

It also occurs to me that such panels could be used to cool an office if placed internally, but I can't work out what to do with all that hot water, plus I'm probably just re-inventing some sort of A/C to water heating system that already exists and is far cheaper/efficient?

Mart.

Hi Mart

The plate condenser on the back of a (hot climate variant) large freezer can typically be ~1.2mx0.5m .... would two of these connected through an outside wall linked with a freezer compressor (evaporator outside/condenser inside) provide half of the heat requirement for a standard size house in the UK .... ?? .... no ??, then what's different with this system as it's effectively the same ..... most freezers will work with a static speed compressor, but in answer to your query, inverter compressors can vary the duty and therefore power consumed ...

A single panel system which has a power input of 390W and a annual average COP of 8.4 (implied marketing claim) would output 3.276kW on average whilst running for an average 5 hour day, so extending the run to 24 hrs that's 78.6kWh/day .... which is enough to heat a house ... as already mentioned, of the 3.276kW average, the direct solar heating would likely only provide 500W on a really bright day, so overall it's pretty irrelevant ....

If this was the case then all of the world's refrigeration industry, including a company I once worked for, would have been making these systems years ago, and as they're not that should simply make people think why ....

HTH
Z

zeupater

Kernel_Sanders wrote: »

Does air weigh anything? I guess it must, otherwise a hot air balloon wouldn't work!
I only ask because in O-Level Physics (which I failed) I learned that something can have mass (measured only in grams, kilos or tonnes) but not weight.
If so, how is the weight of air measured?

Hi

I think I simply used 'weight' in answer to a question ....

A simple test .... weigh an open container containing air at 1atm, close the container and pump pressurised air into it to let's say 10atm, let it cool to ambient temperature and then weigh the container again .... if the container weighs more then air has weight because the force on the scales due to gravity has changed, and the only change is the mass of the air within it, therefore the air must have weight ...

Another way of thinking about it would be that if gasses didn't have a 'weight' then whatever their mass might be gravity would have no effect and they would simply drift away into space ....

Well, that's my take on it ....

HTH
Z

1echidna

I've been waffling on about kW absorbed in an evaporator but to really check this out I would need to examine the thermodynamic properties of the refrigerant and the characteristics of the compressor. It may just not be possible to compress the required quantity of refrigerant using 390W of power. Perhaps they have a very efficient compressor here but the claimed COP seems high not just because of questions about the evaporator.

grahamc2003

Kernel_Sanders wrote: »

Does air weigh anything? I guess it must, otherwise a hot air balloon wouldn't work!
I only ask because in O-Level Physics (which I failed) I learned that something can have mass (measured only in grams, kilos or tonnes) but not weight.
If so, how is the weight of air measured?

Air (and everything else) has weight when acted on by gravity, so the weight of everything varies depending where it is (i.e. things weigh less on the moon than they do on earth). When far away from a massive object, then air (and everything else) weighs very little. The constant property is mass - which is the same throughout the Universe. Strangely enough, the unit of mass is kilogram, and that of weight is Newtons.

A cubic meter of air (at standard temperature and pressure) has a mass of about 1.3kg, and that applies everywhere in the Universe. The weight, on earth, of the same volume at stp is about 12.7 Newtons, and on the moon, about 2.1N.

1echidna

My reading of the appropriate graphs for R134a is that the energy required to evaporate the liquid at -26C is approx 220kJ/kg. To compress the refrigerant from evaporating conditions to produce a fluid condensing at 70C takes about 70kJ/kg This would give a COP of approx 4. To get higher COPs to my mind there must be a clever control system which utilises a lower compressor discharge pressure when the tank is cold?

grahamc2003

zeupater wrote: »

Hi



Looking at your calculation In have some observations ... firstly, the average annual daily temperature in the UK is more like 10C than 25C, and although the temperature of evaporation could be -26C (depending on gas, it's probably more like -45C), this will not be the temperature of the entire plate, just the point where the compression pressure drops and evaporation occurs, after that the liquid is simply gathering heat until the next compression phase where it releases it again .... it's no use considering ideal conditions, what is needed is a 'real' world scenario, if not the 'ideal' could simply be used to mis-sell product, and no-one wants that ...

Z

The temperature of the plate will not vary to the extent which you imply, and will be, if well designed, reasonably constant and cold over most of the surface. This is because the heat is picked up mainly as latent heat of evaporation at the boiling point (i.e. constant) temperature of the refrigerant. It isn't substantially picked up as a temperature rise, which will only happen when all the refrigerant has boiled. As you say, the heat absorbtion will drop with a rising plate temperature - hence why all refrigeration cycles use fluids with high latent heat of evaporation. the fluid state within the plate will be both liquid and gas at the boiling point, with more liquid at the start and no liquid near the exit (and only from this point the temperature will rise). (This is why the refrigerant volume in heatpumps is very important, and I suspect why some heatpump systems work well and others dont).

zeupater wrote: »

Hi

Take a number of these plates, fold them up , put them in a box and add a fan and you have an ASHP and we are all aware of the claims made by many manufacturers in that sector, however, I'm pretty sure that I've not seen many claims of a COP of 6,7 or 8 being possible when transferring to water at 55C ....
Z

Don't know why you are still saying this. It is completely incorrect. The plate works by completely different physics to the coils in an ashp as previously explained. Your apparent reluctance to accept that is leading you to make many mistakes in your analysis.

zeupater wrote: »

Hi

I remain a total sceptic on this product .... if the manufacturers want to have one tested by a sceptic, then I'm their man .... I've got pv and thermal, but these only cover about 20% of our roofspace and we have plenty of walls to cover too .... :cool: .... chances of a freebie to test, absolutely none ...

HTH
Z

I'm naturally sceptical too, but this isn't on a par with finned rare-metal filled heaters (as you perversely seem to be treating them).
They may work well or they may not, but certainly the system is within the bounds of thermodynamics. I think one main problem colouring your analysis is the lack of recognition of the plate temperature which is crucial to the physics under which it operate. For example, the heat pickup on a sunny day will not come solely from the sun (which is mainly the case with other solar panels at ambient temperature). A cold plate will pick up all the direct radiation from the sun, plus radiation from everything else which is warmer than the plate (which is everything, including roof tiles the other side of the panel points at).

Also I think there's a misconception in your understanding of the details of the refrigeration cycle itself at the compression stage. The compression is designed to be isentropic (i.e. performed at constant entropy) and all analysis assumes that, hence why a 390W compressor delivers 390W (almost) into the working fluid as a temperature and pressure increase - it's not about reclaiming lost heat as you implied, this is a critical energy input into the system.

1echidna

grahamc2003 wrote: »

..........

Also I think there's a misconception in your understanding of the details of the refrigeration cycle itself at the compression stage. The compression is designed to be isentropic (i.e. performed at constant entropy) and all analysis assumes that, hence why a 390W compressor delivers 390W (almost) into the working fluid as a temperature and pressure increase - it's not about reclaiming lost heat as you implied, this is a critical energy input into the system.

This is very interesting and makes a lot of difference, and is outwith my experience. The best compressors I have heard of have an isentropic efficiency in the high seventies (although expanders may achieve 95%). In all honesty I can't see how 100% isentropic efficiency is possible.