Evacuated tubes for pool heating - how many? « Alternative Technology Assn Forums

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Evacuated tubes for pool heating - how many?

(24 posts) (13 voices)

Started 2 years ago by aburnett
Latest reply from SgtThursday

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aburnett
Member

We've completed a major renovation, and in the process removed the 25 year old solar heating system which was a John Sharpe design.

We kept the piping from the pool filter to the roof with the intention of installing a bank of evacuated tubes. The water from the roof goes through a heat exchange to heat the water going into the pool. That is, we don't pump pool water through the system at all.

The pool is about 65,000 litres, and indoors which is great for keeping leaves out but not so great for direct solar heating.

My question, given our Sydney location, is how many tubes will be enough to maintain a temperature of about 25-28 degrees? Or do we simply add 12, see how it goes, add another 12, and so on?

Commercial responses welcome.

Posted Monday 30 Nov 2009 @ 1:20:02 am from IP #
dymonite69
Member

I am not sure if this is the best purpose for an evacuated tube system. It certainly is not the most cost-effective way to heat a low performance system like a pool. Hot water heaters are designed to bring up water to a high temperature whilst reducing heat losses. The higher you heat anything the faster it will cool down.

With a pool heater you don't need hot water, just warm water.

You say that you aren't recirculating the pool water through the heater. So how will you prevent the inevitable heat loss (apart from a pool cover)?

Posted Monday 30 Nov 2009 @ 2:19:18 am from IP #
dymonite69
Member

Here are some designs for pool heaters:

http://www.builditsolar.com/Projects/PoolHeating/pool_heating.htm

Posted Monday 30 Nov 2009 @ 2:38:30 am from IP #
Sunshine
Member

aburnett,

I believe it would be more cost effective to use conventional solar pool heating collectors on the roof.
A solar pool blanket also helps help heat the pool during the day, and keep it warm overnight.
I have both solar pool heating and a solar pool blanket and they both work well.

Posted Monday 30 Nov 2009 @ 2:45:39 am from IP #
aburnett
Member

Thanks dymonite69,

Re heat loss - The pool is indoors so heat loss to the atmosphere is not that affected by wind or evaporative losses. On the coldest days we can close all the windows to keep the air temperature in the room relatively high.

The project is an upgrade of a 25 year old solar pool heating solution. We already have in place the plumbing, heat exchanger, gas pool heater, pump and solar controller.

The issue we have is the gas heater we have to heat the pool presents us with huge gas bills. It is a Raypak RP2100 similar to http://raypak.com.au/default.asp?V_DOC_ID=790 and PDF instructions http://raypak.com.au/files/operation/RP2100%20Pool%20and%20Spa%20Heater%20Instructions.pdf

The idea is to have the tubes heating the pool water whenever the sun is shining, and then only need to use the gas on rare occasions. The only cost from here should be the tubes and a small amount of labour.

It is better to have a small amount of very hot water trickling into the pool whenever the sun shines compared to putting the gas heater on for a solid 24 hours to raise the whole pool in a short time.

Posted Monday 30 Nov 2009 @ 3:07:54 am from IP #
dymonite69
Member

So you don't want to recirculate the pool water back into the solar collector to reheat it?

Posted Monday 30 Nov 2009 @ 6:16:36 am from IP #
Sunshine
Member

I read somewhere that the salt/chlorine in pools is unsuitable for evaucated tubes.
However if you have a heat exchanger system, where water from pool doesn't circulate through evaucated tube system should work.
I still reckon that a black pool solar collector using 35m2, which is what I run in my pool, has a better ability to heat pool than the evacuated tube system.

aburnett, why not go for the tradional pool solar collector?

Posted Monday 30 Nov 2009 @ 6:55:01 am from IP #
JohnB
Member

Many years ago I had a small involvement in the pool solar heating industry.

The 65kl indoor pool sounds like a perfect candidate for retaining a normal low temperature pool specific rooftop collector. The fact that the pool is indoors and quite large in volume helps to keep the temperature stable, so a few cloudy days here and there shouldn't make the pool cool down too quickly - as long as you don't mind the pool enclosure being quite warm.

One issue that is quite important is that any water entering the pool shouldn't be hot. Rooftop systems typically drain back to the pool when the pump stops - so that there isn't a chance of overheated water entering the pool when the pump restarts. The heat exchanger system may well have the capacity to 'mix' hotter water with cooler pool water, but it's certainly something to watch if you're planning to use a high temperature system to make tepid pool water.

There's also the efficiency issues pointed out by the others too.

Posted Monday 30 Nov 2009 @ 8:53:41 am from IP #
Benny
Member

No one is answering your main question aburnett in terms of how many tubes will be needed - and I'm sure thats because its not an easy question to answer. Its critically dependent on how quickly heat will be lost from the water, i.e. ambient temperature ?, insulated pool ?, room insulation ? double glazing ? All quite tricky without a sophisticated model.
To get a ball park estimate though, you can do some simple calculations.
1 sqm of collector area (and remember ET's have only a small fraction of the array area actually as an absorber - I think about 1.8m x 25mm = 0.5sqm per tube) - will at the absolute best collect about 600W of heat - esp in winter.
Over say 5 hours this could be .6 x 5 * .5 = 1.5kWh of energy.
1.5kWh is 1.5 x 5hr x 3600s/hr = 27 MJ
It take about 4000J to heat 1kg (say 1 litre) of water by 1degC.
So 27MJ would heat 65kl by 27 M / (65k x 4k) = 0.1C.
So thats 0.1C increase per tube per day. I stress this is a rough guide so don't be too critical if my estimates are out but I'm happy for others to correct mistakes. (-;

I disagree a bit with what others have said about efficiency. I agree that operating a water heater at a higher temp than is needed can be inefficient due to higher losses, but remember that an ET system is much better insulated than a normal black pipe pool heater and so would be much more efficient at collecting the heat. If the circulating pump is run faster than normal, the transfer water could be kept cooler too and reduce losses and of course insulate the transfer pipes. With the pool being so cool there would be a good transfer of heat in to the pool water.
All up it could be much more "efficient" than a black pipe heater, but being of so much smaller area, might still collect less heat.

Posted Tuesday 1 Dec 2009 @ 3:04:35 am from IP #
dymonite69
Member

Benny,

I did a similar calculation on the steam cooker thread.

Your estimate is on the generous side. Aperture area for one tube is about 1/5th that you quoted and Sydney insolation varies from 2.47kWh/m2 (winter) to 6.63kWh/m2 (summer). Efficiency is about 80%.

I estimate an output varying from 0.72 MJ to 1.8 MJ/tube/day.

ET system is more efficient but at the low temperatures required, not much less than an uninsulated pool collector. What you want is large ABSORBER area to capture as much solar energy as possible. This is doable with ET but very expensive. Black pipe heater is cheap and you can literally cover your roof with the stuff for the same cost.

Posted Tuesday 1 Dec 2009 @ 4:30:03 am from IP #
Sunshine
Member

dymonite69,

I agree.. I asked Aburnett about using a tradional pool solar collector, but he didn't reply.
It would be the way to go as they are off the shelf products specifically designed for pools, and probably cheaper than evacuated tube.
Maybe he doesn't have the roof area needed and needs a compact system.

Posted Tuesday 1 Dec 2009 @ 4:43:54 am from IP #
aburnett
Member

Thank you all for the contributions - very helpful indeed. The reasons I'm keen on ET and not black polymer tubing are several:

1) Black tubing is ugly, prone to cockatoo attack, and interferes with the rain harvesting ability of the new Zincalume roof. We get a lot of leaves landing on the pool roof (one of the reasons for enclosing it in the first place), so a carpet of black tubing will make cleaning the leaves that much harder.

2) We're heading towards a huge array of ETs to perform a variety of applications around the house, including solar hot water, heating the concrete slab which has tubing laid ready, heating the pool, and last but but no means least power generation. Yep, power generation. Pool heating was considered the easiest application to knock over, and would give us insight into the performance of ETs.

By using a tried and tested brand of evacuated tubes with the pool heating, we can hopefully expand the array when we add in the other applications above.

Our architect had suggested a product that was in development that used ETs to collect and store very hot water/steam in two 3000 litres tanks. Attached to the tanks was a steam turbine, similar to some other discussions elsewhere in this forum. The idea was that power could be generated from the stored hot water 24 hours a day - especially appealing given the 60c gross feed-in tariff we might be getting in NSW. The same tanks supplied the house with hot water, heated the slab, and heated the pool.

Alas, the product's inventor didn't have the knowledge to take it commercial, and has been distracted into other areas of research. So we're back at the ideas stage, with a renovated house that is wired and plumbed ready for conversion to becoming 100% water and energy sustainable house.

We're considering another avenue, hydrogen electrolysis, but this technology is more geared to new developments of about 60 houses: http://www.azureenergy.com.au and is also in the prototype stage.

I have uploaded some photos to http://picasaweb.google.com/findnearest/PoolHeating# so people can understand what we're dealing with.

Thanks again for the contributions - and the opportunity to share our sustainable renovation with the wider ATA community.

Antony

Posted Tuesday 1 Dec 2009 @ 10:05:09 pm from IP #
dymonite69
Member

ABurnett,

Sounds like expense is not a barrier and you are willing to experiment with a few cutting edge technologies. I would imagine that converting heat to steam then into electricity would occur significant energy losses. How does it compare just to photovoltaics or wind? Perhaps you should then consider how to use pure heat energy to drive as many appliances as you can. Stirling engines come to mind (I am not sure if there are any commercially available ones for domestic use) and also solar powered cooling and refrigeration.

Posted Tuesday 1 Dec 2009 @ 10:41:01 pm from IP #
ghostgum
Member

aburnett mentions an interesting idea - use solar hydronic to heat the house in winter, and in summer use the excess heat to generate electricity. This has the added advantage of dealing neatly with the problem of excess solar hot water in summer.

Posted Tuesday 1 Dec 2009 @ 11:52:01 pm from IP #
sun2steam
Member

Creating electricity from low temperature heat energy is just not worth it as the maximum efficiency as can be calculated by the Carnot process is just to low. In addition it is not possible to convert heat into mechanical or electrical energy. It is always necessary to have a cooling medium which can be heated up in the process. And if you don't want to waste water in a cooling tower there is only warm air available which limits the driving temperature difference even further.

It does no matter which kind of engine like Stirling engine or others are being used. The all are limited in their efficiency by the rules of thermodynamics.

Use solar heat for thermal processes and possibly for cooling.

Posted Wednesday 2 Dec 2009 @ 12:09:16 am from IP #
dymonite69
Member

I wasn't thinking of using a heat engine to make electricity but to drive low powered motors like fans.

The Solar Cooling interest group reckons you can get around 0.66 kW of cooling per m2 of solar collector.

http://www.ausscig.org/heat_collection.html

Which works out that for every 6m2 of floor space you would need 1m2 of collector (rule of thumb being 0.15kW/m2)

Posted Wednesday 2 Dec 2009 @ 12:47:54 am from IP #
aemtei
Member

My question is not as specific as the number of tubes - has anyone used their ET system to do complimentary heating of a pool/floor etc? How did it go.

We're in Perth and getting a 30T/315L Apricus system, gas boosted. We have a 30kl pool and underfloor hydronic heating (~200m2). The pool heating from ETs is an interesting idea. We won't have the room on the roof for ETs, PVs and Pool heating. And i'm not keen on having a gas boiler for the pool, another for the hydronic underfloor and yet another for the Solar DHW.

I'm thinking heat exchangers (aj bowman in the UK do a solar pool heat exchanger) and a lot of control monitoring.

aburnett - did you get anywhere with, what sounds like, an awesome setup?

Posted Wednesday 18 Aug 2010 @ 5:55:36 am from IP #
aburnett
Member

Hi aemtei,
Didn't get to where we wanted, and stuck using gas heater. Until the other day that is - the gas heater blew up and will cost $6000 to replace. Helluva cost when that doesn't include the gas bills we get.
Looks like we'll go for plain old black polymer, and perhaps change the pool pump to a more efficient model. The tubes are going to collect leaves like anything, but we currently have a pool too cold to swim in and its summer.
Antony

Posted Tuesday 21 Dec 2010 @ 10:42:02 am from IP #
termite
Member

Would it be possible to put hinged glass panels over the polymer tubing, to create a smooth surface for easy cleaning and water collection? This would presumably also reduce heat loss from the tubing.

Posted Tuesday 21 Dec 2010 @ 1:07:36 pm from IP #
aburnett
Member

Currently investigating a product mentioned elsewhere in these forums at http://www.ata.org.au/forums/topic/no-mains-power-solar-pool-heating which looks promising if it can be roof-mounted.

Posted Tuesday 21 Dec 2010 @ 8:15:43 pm from IP #
Anonymous
Unregistered

Hi there i install and sell the only successful evacuated tube solar pool heating system in Perth Wa, can send you information and testimonials if you like from clients

Posted Friday 7 Jan 2011 @ 5:24:51 am from IP #
Infinitybatman
Member

G'day aburnett, wish i found this conversation earlier.
I have an indoor swimming pool on the coast in Victoria, which is about 65,000 litres. Me and my wife run a swim school from here which we opened last April. We have the temperature at 32-33 degrees year round ( classed as a thermal pool, for babies and re-hab.) I bought a heat pump from U.S.A which cost me 6,000 deliverd to my door (taxes customs,etc. included). This is currently our only source of heating, but i'm in the process of having professionals do me a computer model test for running evacuated tubes.
Our heat pump costs about 5-$6000 a year to run, i reckon you could do it somewhere between 1-$3000.
Heat pumps are much more energy efficient than gas and they last longer too. The only down fall is that they can be a little bit noisy ( newer models are getting quieter ) and it takes longer to heat up.
I've tried to keep this brief, let me know if you want any more info.

Posted Friday 25 Feb 2011 @ 11:10:30 am from IP #
Anonymous
Unregistered

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Posted Friday 19 Aug 2011 @ 8:20:08 am from IP #
SgtThursday
Member

aburnett said:
2) We're heading towards a huge array of ETs to perform a variety of applications around the house, including ... power generation. Yep, power generation.

...

Our architect had suggested a product that was in development that used ETs to collect and store very hot water/steam in two 3000 litres tanks. Attached to the tanks was a steam turbine, similar to some other discussions elsewhere in this forum. The idea was that power could be generated from the stored hot water 24 hours a day - especially appealing given the 60c gross feed-in tariff we might be getting in NSW.

...

Alas, the product's inventor didn't have the knowledge to take it commercial, and has been distracted into other areas of research.

...

Antony

Howdy... I realise I've picked this up well after it's gone cold, but I think the message is still valid...

BEWARE of this idea! I became aware of someone trying to hawk a similar system (quite likely the same) back in 2008, promising 24-hr power generation at 5 kW. Short answer is - the maths and physics just don't add up to anywhere remotely near that amount.

Always be cautious when someone offers a fantastic-sounding product that is "still under testing" or "just waiting on certification". Some products maintain that status for a very very long time.

Be *particularly* wary of anyone offering "free electricity". Or websites that proudly display a patent as proof that their system works. (Patents prove that nobody else has patented that device - NOT that a system works).

Posted Thursday 8 Sep 2011 @ 8:53:37 am from IP #

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