thermodymic panel for water heating

Kernel_Sanders

zeupater wrote: »

literally tonnes of air

The last time I weighed some air it didn't come to anywhere near that

1echidna

(Source)

My reading of this is that for low ambient temperatures (say refrigerant at -20C and surroundings at 0C) the contribution of radiant heat gain is likely to be of the order of 100W/m**2 to the evaporator plates. Convection is likely to be considerably more significant in these conditions assuming absence of sun.

In the event that there is insignificant heat transfer to the evaporator I would expect the refrigerant compressor to trip out and the circuit would accomplish nothing.

grahamc2003

zeupater wrote: »

Hi

Just read the thread ....

I've seen a system which is similar, probably on a much larger scale - see recent post ...(http://forums.moneysavingexpert.com/showpost.php?p=55976915&postcount=2510) ....

My take on it .... if you consider that the selling point of the unit over standard solar thermal is the ability of the plate to absorb heat from the surrounding air even at night then the maths is quite simple - how much heat can you exchange between air and a square metre of passive surface at a given temperature differential ?, then multiply up to the scale of the installation ....

With an ASHP you are using the fan to force literally tonnes of air through a heat exchanger with a surface area which will be measured in the tens or even hundreds of square meters ... and this is what you'll be attempting to do with a few square meters of painted metal and a negative convective flow ... it will definately work - but I'd guess that the COP would be much closer to 1 than any other integer ....

Okay, so it works better than solar thermal during the day ? ... well, I suppose that this depends on how you sell the word 'better' ... would it provide more heat, or would it provide a better COP .... our thermal utilises ~20W of (pv self generated) electricity to provide a return of ~1000W.t when running, that's a COP of 50, so my guess is that a salesman or literature wouldn't compare on this basis ....

A different approach ... it's cloudy outside at the moment .... a pv system rated at ~4kWp (ours) is generating 450W, that's just about describing the total irradiation falling on the plate surface as being 100W/sqm (~450W/~25sqm/~17.5%efficiency), so a ~200W (guess - basis, large freezer) compressor could only be collecting 200W from a 2sqm plate which would equate to a COP of 1.0 ... IF THE SYSTEM WAS 100% EFFICIENT !! ...

For less than £9k, I'd buy a pv system, or for less than half of that I'd buy a solar thermal system and simply keep an eye on this technology, and it's pricing, as it develops, or fades away ...

HTH
Z

hI z, I think your take on the physics of this system isn't quite right. The first two paragraphs assume that the heat gathering is by conduction from the air (which, as you state, how ashps gather heat). The heat gathering is by radiation, not conduction, which is a different kettle of fish altogether, and often counterintuitive (as this thread illustrates in one or two instances). Also the discussion about night operation was to specifically answer the op's question, and isn't a selling point (or at least not a rational sp).

Pra 3 - yeah, I agree with that. There are pros and cons to conventional panels vs this radiative panel. he advantage of this system is the cliam to heat water higher to normal domestic temperatures, saving an immersion top-up. Also they deliver heat at a higher rate than a passive system (under sunny conditions). They claim 2.9kW max - seems reasonable to me. A passive system (with less area and at ambient temperature) will perhaps deliver 400W (?, I'm asking). But for those cliaimed advantages (and the claims seem reasonable to me and within the bounds of physics and thermodynamics), as you point out, you've got to use a 390W compressor, as opposed to a 20W pump in a passive system.
My view is probably like yours - I'd rather pay less for a conventional thermal system and have lower running costs, at the cost of slightly lower temp water and a longer heating time. These panels can be linked into systems with 2 or 3 panels (when they claim cops of 7 measured at 7C, probably in bright sunshine too) to provide whole house heating, where they may (and I don't know) compete with Ecodans etc (again pros and cons for each system).

Para 4 is a bit confused. In the refrigeration cycle thermodynamics, if you use a 200W compressor and extract 200W from a heat source, then the heat delivered is 400W, a COP of 2, not 1. (The compressor input is an input of heat into the system too, with little heat loss). Related to this, the outside fan on an ashp is simply an energy loss from the system (so a source of inefficiency) - the radiative panel doesn't suffer this loss, one of its pros).

zeupater

Kernel_Sanders wrote: »

The last time I weighed some air it didn't come to anywhere near that

Hi

I understand your point, but consider the following ....

At a temperature of around 20C at sea level air weighs about 1.2kg/cubic metre .... typical ASHP/Air conditioning evaporators when running at maximum rate pass air over the fins at around 50m3/minute .... thats 3.6Tonnes/hour (1.2kg*50m3*60mins) .... (literally tonnes) .... heat is transferred by using a flowrate to increase the mass of the air which comes into contact with higher density materials within a given timeperiod .... factor in relative specific heat capacities for the materials, the contact surface area and the conductivity of the material to be cooled and you can calculate the air/liquid flowrate to provide the required cooling/warming duty ....

HTH
Z

zeupater

grahamc2003 wrote: »

hI z, I think your take on the physics of this system isn't quite right. The first two paragraphs assume that the heat gathering is by conduction from the air (which, as you state, how ashps gather heat). The heat gathering is by radiation, not conduction, which is a different kettle of fish altogether, and often counterintuitive (as this thread illustrates in one or two instances). Also the discussion about night operation was to specifically answer the op's question, and isn't a selling point (or at least not a rational sp).

Pra 3 - yeah, I agree with that. There are pros and cons to conventional panels vs this radiative panel. he advantage of this system is the cliam to heat water higher to normal domestic temperatures, saving an immersion top-up. Also they deliver heat at a higher rate than a passive system (under sunny conditions). They claim 2.9kW max - seems reasonable to me. A passive system (with less area and at ambient temperature) will perhaps deliver 400W (?, I'm asking). But for those cliaimed advantages (and the claims seem reasonable to me and within the bounds of physics and thermodynamics), as you point out, you've got to use a 390W compressor, as opposed to a 20W pump in a passive system.
My view is probably like yours - I'd rather pay less for a conventional thermal system and have lower running costs, at the cost of slightly lower temp water and a longer heating time. These panels can be linked into systems with 2 or 3 panels (when they claim cops of 7 measured at 7C, probably in bright sunshine too) to provide whole house heating, where they may (and I don't know) compete with Ecodans etc (again pros and cons for each system).

Para 4 is a bit confused. In the refrigeration cycle thermodynamics, if you use a 200W compressor and extract 200W from a heat source, then the heat delivered is 400W, a COP of 2, not 1. (The compressor input is an input of heat into the system too, with little heat loss). Related to this, the outside fan on an ashp is simply an energy loss from the system (so a source of inefficiency) - the radiative panel doesn't suffer this loss, one of it's pros).

Hi

I think that the logic is likely correct. The system actually combines two forms of heat transfer .... conductive and radiative collection .... during the day the majority of the collection will be solar radiation, at night, differential temperature conductive transfer from the ambient air temperature .... this is why I have specifically split the two (day/night) in order to remove the complexity of combining the two ... the important point to be considered is that at a given ambient temperature, there will be no(/little) difference between the conductive transfer during the day or night ....

Leaving the conductive transfer aside, the only advantage that I can see of the system in question over standard solar thermal is the relatively low temperature which the panels can operate at .... standard solar thermal starts to transfer heat into the how water system when the differential between the panels and the bottom coil in the cylinder is above a preset level and stops again when the differential is small .... ours is set to start at a differential of 9C and stop pumping at 4C - after cutting out the panels warm to +9C again and the pump cuts back in .... this is how the system works in lower light conditions. As the temperature within the HWC increases the panels need to operate at higher temperatures (ours currently tank bottom 41C / panels 48C and rising, not pumping) ... this is where the technology of the solar thermal panels comes into play ....

Panels will radiate and convect heat to the atmosphere at a given rate related to differential temperature to ambient. When the differential temperature results in a rate of heatloss which matches the gain from solar radiation then equilibrium is reached and the panel will not heat any further. This rate of heatloss from the panel can be reduced by insulating. glazing, double glazing or introducing a vacuum gap, thus increasing the equilibruim point ... the important point in all of this is that with a negative heat differential this approach becomes irrelevant, and that's what is being sold ... the benefit of an uninsulated flat plate with a minimal heat gain, not loss - against an insulated panel system with adequate insulation to provide minimal heat loss in order to conserve solar gain ...

At this point we should now consider the marketing based system benefit claims. Our system will heat the water in the cylinder to a level which will match anything claimed by the marketing literature ... we run our system to a maximum cylinder temperature in the mid 70'sC, the reasoning for this purely being to extend the lifespan of the mechanical elements (pump) as the heating circuit operating temperature would be mid 80'sC, but I have no doubt that the system would be capable of boiling our cylinder and providing many mugs of tea directly from the tap, after removing the Thermostatic Mixing Valve first ... :D ... all of this with a 20W pump .... now, at 7C but bright sunshine outside what would the effect of this be on our COP ... absolutely nothing, our panel equilibrium temperature is that high that if it was -30C outside we would still be able to maintain a COP of 150+ ....

Regarding the maximum system provision question of 400W .... our's will be pumping more than four or five times that into the cylinder on a clear day .... today's mostly cloudy and what sunshine there has been is hazy, but we'd still expect over 3x that figure for extended periods (10 minutes+) ... this being from a roof area equivalent to four pv panels .... when it's cloudy it's different ... off, then on at ~2kW.t and within minutes falling below 1kW.t, then off for 5 - 10 minutes, then on again .....

To put this into perspective ... the last time we heated our water with gas was in April. In that month gas was used three times to boost the water temperature, the total gas burn was below 1.2cubic metres and equated to just over 13kWh of fuel .... since then, nothing, although we came close to using gas again last week before a couple of better days raised the temperature at the top of the tank again .....

Regarding para4 ... I was working on the principle that the compressor would be mounted somewhere in the loft space in order to reduce the refrigerant pipe runs to the panels and minimise noise, therefore the heat from this and the delivery pipe runs would be outside the normal heated volume of the house .... I assume that the heat is then transferred to a pumped water circuit for transfer to the HWC .... if this is not the case and a good proportion of the heat from the compressor can be captured and delivered to the hot water cylinder,and not the surrounding air, then the COP would be higher, possibly 2.0, but again ... IF THE SYSTEM WAS 100% EFFICIENT !! ...

HTH
Z

zeupater

1echidna wrote: »

(Source)

My reading of this is that for low ambient temperatures (say refrigerant at -20C and surroundings at 0C) the contribution of radiant heat gain is likely to be of the order of 100W/m**2 to the evaporator plates. Convection is likely to be considerably more significant in these conditions assuming absence of sun.

In the event that there is insignificant heat transfer to the evaporator I would expect the refrigerant compressor to trip out and the circuit would accomplish nothing.

Hi

I tend to agree .... in the absence of light as the energy source 'conductive heat transfer' would be more appropriate .... heat in the air molecules would transfer their higher state heat energy to the lower energy state molecules in the cooled 'absorption plate' through standard conduction, exactly the same principle as is used in forced flow heat exchangers, the natural direction of energy transfer is highstate to lowstate ... I would be pretty amazed if any surface would actually absorb radiated heat at 100W/m2 at night considering the temperatures involved ... radiate, yes ... absorb? ... the vast majority of low level radiated energy available at night will have warmed the air and been moved away by convection long before it reaches the absorption plates ....

HTH
Z

1echidna

http://www.greenserveuk.com/

Looking at the above link, which seems to be the appropriate panels, I found the admission:

In areas that suffer from prolonged cold spells below freezing, it is possible to put the panels underground where the temperature is above freezing at all times and therefore continue to operate effectively

See under 'how do the work in detail' http://www.greenserveuk.com/faq/

The suggestion leaves aside the question of whether the heat flux in an underground location is OK and won't freeze the soil.

To my mind these panels may well be OK in the South and South West but the possibility of prolonged cold spell is greater elsewhere.

1echidna

zeupater wrote: »

Hi

I tend to agree .... in the absence of light as the energy source 'conductive heat transfer' would be more appropriate .... heat in the air molecules would transfer their higher state heat energy to the lower energy state molecules in the cooled 'absorption plate' through standard conduction, exactly the same principle as is used in forced flow heat exchangers, the natural direction of energy transfer is highstate to lowstate ... I would be pretty amazed if any surface would actually absorb radiated heat at 100W/m2 at night considering the temperatures involved ... radiate, yes ... absorb? ... the vast majority of low level radiated energy available at night will have warmed the air and been moved away by convection long before it reaches the absorption plates ....

HTH
Z

The graphical figure I posted gives the radiation from one plate and the absorption on the other when one is (for the sake of argument) 20C above the other. The amount radiated is equal to the amount absorbed if an equilibrium exists. For the temperatures quoted (surrounds 0C and plate -20C) I believe I have interpreted the physics correctly in saying that a heat flux of around 100WperM2 from radiation might be expected.

Actually the full heat transfer calculation for this situation are complex and the contribution of convection needs to be taken into account. I believe it will govern, as you say under cold night time conditions.

I would be surprised if it was not normal for at least part of these 'thermodynamic' plates to be frost covered in almost all conditions.

suffolkpumpkin

It does look like a right thing for sure. But the chap said that you have to expose them to influx of air, strange to read that you can put them underground.

OK: what I've learned in a very simple language
1) the price quoted is too much (£8.9K, 1 panel just to heat water, new water tank, labour and everything else)
2) you have to accept that below -10C they don't work (or almost don't work) and you have to put the booster on.
3) it's not really advisable to paint them in other colours apart from black, so just as well you can leave them as they are.

So many thanks!!!!:T

zeupater

1echidna wrote: »

The graphical figure I posted gives the radiation from one plate and the absorption on the other when one is (for the sake of argument) 20C above the other. The amount radiated is equal to the amount absorbed if an equilibrium exists. For the temperatures quoted (surrounds 0C and plate -20C) I believe I have interpreted the physics correctly in saying that a heat flux of around 100WperM2 from radiation might be expected.

Actually the full heat transfer calculation for this situation are complex and the contribution of convection needs to be taken into account. I believe it will govern, as you say under cold night time conditions.

I would be surprised if it was not normal for at least part of these 'thermodynamic' plates to be frost covered in almost all conditions.

Hi

I simply read the chart as pure heatloss from a perfect black body (emissivity=1) heated surface into the surroundings .... this is very similar to the 'delta T' sizing for central heating radiators (heated surface vs air temperature) except for the temperature ranges and the emissivity of a painted radiator likely being closer to 0.8 .... for a body to efficiently absorb heat radiated from another body the two surfaces would need to be in very close proximity with nothing to absorb & convect the heat away .... so we're really describing a vacuum, not really the condition which exists on most roofs ...

Regarding 'frost covered' ... I have seen similar passive heat transfer technology used with a heatpump first hand and it is frost coverered most of the time, the unit I know of also had an almost permanent area of 'snow' below it on the occasions which I saw it working .... (see earlier referenced post)

HTH
Z