thermodymic panel for water heating

grahamc2003

1echidna wrote: »

It is as well to note that in the event the outside temperature falls to -10C the system just won't work. It is an air source heat pump with a new name.

I'd disagree with that. Ashps blow air over the evaporator, so the source of the heat is air (as you'd expect). The source of heat for these 'thermodynamic' panels - (and what a stupid name!) - is anything radiating heat (i.e. clouds, your house wall, trees etc, in fact anything both sides of the panel point at). There'll be an element of conduction from the air in contact with the panel surface, but most of the heat comes in via radiation. This is why they work at night, as opposed to conventional solar thermal, which aren't cooled below ambient. They'll 'work' anytime the working fluid gets boiled, and I'd guess the boiling point is below -10C, it's just that the cop will probably be less than 1 at those temps, so pointless.

1echidna

grahamc2003 wrote: »

I'd disagree with that. Ashps blow air over the evaporator, so the source of the heat is air (as you'd expect). The source of heat for these 'thermodynamic' panels - (and what a stupid name!) - is anything radiating heat (i.e. clouds, your house wall, trees etc, in fact anything both sides of the panel point at). There'll be an element of conduction from the air in contact with the panel surface, but most of the heat comes in via radiation. This is why they work at night, as opposed to conventional solar thermal, which aren't cooled below ambient. They'll 'work' anytime the working fluid gets boiled, and I'd guess the boiling point is below -10C, it's just that the cop will probably be less than 1 at those temps, so pointless.

I think you are getting lost in thermodynamic mumbo jumbo. If the surroundings, clouds etc were warm, yes you would get a degree of black body radiation from one body to another but they aren't and you won't (or at least not of the order of kW or similar.)

In fact because the refrigerant is at -20C or there about there is a real risk of ice build up at any ambient temperature below 5C if conditions are at all humid. Ice is a bad conductor of heat. Some air source heat pumps have a defrost cycle, no mention of one here.

1echidna

Ideally I would like to see the heat transfer calculations showing the contribution of convection, conduction and radiation under various operating conditions. Perhaps I should delete my previous post?

Ben84

suffolkpumpkin wrote: »

Thank you, it was very useful indeed.
We were quizzing this chap about the winter months. I don't think the panels themselves will freeze over: the chemicals inside the panel freezing T is 70C (I think). But, what matters to us how much they'll slow down heating the water. And this info I could not get in "black and white" neither from him or the internet. Unless I'll start digging really deep (i.e. name of the chemicals and hit the PubMed or similar :-(
It wouldn't be nice to get up on the frosty morning and find out that there is not enough hot water having paid all these moneys :-(

I think you're concerned that the chemicals stop water changing temperature so quickly? But that's not now any antifreeze I know of works. I wouldn't at all expect antifreeze chemicals to change the heat carrying capacity of the water, they will just stop it forming ice crystals so easily. I don't think this needs to be a worry.

Anyway, I have other issues about the idea, mainly it looks expensive and the time until it breaks even looks likely to be pretty long? What other options do you have available for similar or less price range that this is competing against?

grahamc2003

Ben84 wrote: »

I think you're concerned that the chemicals stop water changing temperature so quickly? But that's not now any antifreeze I know of works. I wouldn't at all expect antifreeze chemicals to change the heat carrying capacity of the water, they will just stop it forming ice crystals so easily. I don't think this needs to be a worry.

Anyway, I have other issues about the idea, mainly it looks expensive and the time until it breaks even looks likely to be pretty long? What other options do you have available for similar or less price range that this is competing against?

Ben, it's not water or antifreeze circulating, it's a refrigerant (Tetrafluoroethane or r134a) which wont freeze. It's basically a heat pump with novel heat collection, unlike conventional thermal panels.

I agree with you that at 6 grandish, it's expensive for just hot water. However, it's classified as solar thermal, so will attract the rhi (and the rhpp) once it's decided.

suffolkpumpkin

Thank you for all your posts, lovely people. We didn't look into these green technologies previously, so it is our first attempt.
As I mentioned before the house is heated with oil (supplemented with log fires in winter. We wanted wood burners to heat hot water, but were advised against it, due to the exposed pipe work) and we would like to reduce its usage. We have some limitations to invest into green technologies in terms of the initial capital outlay and the fact of living in the grade II listed buiding.
I agree that the panel is very expensive and we'll have to consider all facts really carefully before committing ourselves (or not).
any other suggestions we can explore?

grahamc2003

suffolkpumpkin wrote: »

Thank you for all your posts, lovely people. We didn't look into these green technologies previously, so it is our first attempt.
As I mentioned before the house is heated with oil (supplemented with log fires in winter. We wanted wood burners to heat hot water, but were advised against it, due to the exposed pipe work) and we would like to reduce its usage. We have some limitations to invest into green technologies in terms of the initial capital outlay and the fact of living in the grade II listed buiding.
I agree that the panel is very expensive and we'll have to consider all facts really carefully before committing ourselves (or not).
any other suggestions we can explore?

Could I just check on thing? Was your quote of £9k for a single panel and just to heat the domestic hot water, or was it for a couple of panels and designed to heat your whole house through existing radiators? It seems way OTT for the former, and probably not too bad for the latter. My quote of £6k was just for hot water, and not whole house heating.

I assume you're not on the gas grid, and if not, you'd potentially qualify for the probable big grants looming for renewable heating systems. You could consider an air to water heatpump (Ecodan probably market leader, or Daikin systems) - they are designed for whole house heating. There are threads with users of these systems, so they are tried and tested (but not always without issues!). The annual rhi atm looks like ranging from generous to incredibly generous, and there's an £850 rhpp you get up front.

suffolkpumpkin

8.9K just for heating hot water, single panel. I've realised now, after reading your comments that it is too much. We have no main suppliers for anything at all.
Thank you for the other pointers too, we'll have to get our heads around different options.

suffolkpumpkin

sorry, of course, we have electrocity and the telephone wires. and that's it. Our own bore hole for water+septic tank+oil for the rest.

zeupater

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