Solar Panels - How many

fitzie79 · 02-08-2009 03:27PM #1

I am putting solar panels into my new build. I have a 300l triple coil tank in the house. Until now I had been planning to place 3 solar panels (3m x 2m) on the roof. However, someone recently said that this might be a waste of money as I should have sufficient hot water from 2 panels and there would be no benefit from putting on a 3rd.

How do you decide how many panels should be used? Is there a figure for the expected amount of hot water that each panel can supply? There will only be 2 of us in the house initially but hopefully there will be a few additions to that in time

homer911 · 02-08-2009 04:20PM

Is that 6sqm or 18sqm?

What angle will they face?

How much pipe work between the panels and the cylinder?

Any obstructions in front of the panels?

Will you have a heat dump?

Are you in a soft or hard water area?

On the face of it this sounds like too much, but there are lots of factors which can influence panel size

I'm using 40 tubes facing due south/300l tank for a family of five - water can get up to 70C in summer, or even higher. I think my sizing is about right for year round use. I have a heat dump which has only been used a few times..

fitzie79 · 02-08-2009 04:59PM

homer911 wrote: »

Is that 6sqm or 18sqm?

18sqm

What angle will they face?

Close to direct south - probably slightly south east

How much pipe work between the panels and the cylinder?

It's a long run - probably about 20m from the garage to hot press

Any obstructions in front of the panels?

Not really - only late in the day will the shadow from the house impact the roof

Will you have a heat dump?

No

Are you in a soft or hard water area?

Not sure

On the face of it this sounds like too much, but there are lots of factors which can influence panel size

I'm using 40 tubes facing due south/300l tank for a family of five - water can get up to 70C in summer, or even higher. I think my sizing is about right for year round use. I have a heat dump which has only been used a few times..

What size panels does 40 tubes equate to? That size system sounds about right for what i would need based on your family size.

Do you need to boost the water in the immersion due to legionnaire's? Is this automatically taken care of by the system?

quentingargan · 03-08-2009 08:26AM

fitzie79 wrote: »

What size panels does 40 tubes equate to? That size system sounds about right for what i would need based on your family size.

Do you need to boost the water in the immersion due to legionnaire's? Is this automatically taken care of by the system?

18 Sq M would be a mad amount of panels! If you were using tubes, you would use somewhere between 4 sq m and 6 sq m of aperture area in most cases. I would go to the upper end of that, and maybe a bit more because you wil have a substantial loss on that 20m pipe run, but 18 sq m without a heat dump would spend a lot of time switched off, and this will damage glycol, damage pipe insulation, accelerate the ageing of the system and possibly cause leaks to develop over time. A heat dump just consists of a radiator and a high temperature 3 port valve. Most controllers include a relay for this already.

There are other posts on pipe runs, but generally they are best avoided if possible by putting the panels closer to the cylinder.

Interesting that you raise legionnaires. All training suggests that systems should raise the temperature of the entire cylinder to 60 degrees once a week. I haven't seen a single installation (apart from my own) that does this. It isn't needed in summer because the solar panels generally achieve this, but in the winter, the solar panel will bring the water in the bottom of the cylinder to 30 or 40 degrees. Ideal breeding temperature for legionella.

I would either

1) Put an immersion in the bottom of the cylinder and set this to come on once a week at night (or you can use a willis immersion heater to do this)
2) Run a stainless pump between the secondary return take off in the center of the cylinder to the cold feed, and run this pump at the same time as the central heating once a week.
3) You could put an immersion permanently operating at the top of the cylinder. This won't use much power because it is seldom needed. If you set your central heating to bring the cylinder to a higher temperature than the immersion, then once the immersion keeps water at the very top of the cylinder above about 55 degrees, legionnaires population will be reduced.

Some people argue that legionnaires is only a problem if you have a very large cylinder which isn't getting its water used regularly enough. That isn't the opinion of SEI or other authorities. Anyone got info on this? Without solid info it shouldn't be relied on....

I'd like to see a discusson on this though. Anyone out there got other ideas?

Q

holdfast · 03-08-2009 09:18AM

Would go for around the 6sq m alright if it meets the criteria in southfacing etc. Did a Ber for a guy with 27sq m did not improve his rating at all. mad he was told it would provide a some space heatine even though he had a heat pump. Very expensive way and not very effective way. I use 9 sqm got to 95 degs in june mad. I use with a heat pump and immersion ( but 90% of the time in winter just them)

CJhaughey · 03-08-2009 11:02AM

I have 6m2 of flat panels and a 400l tank.
No problem with heat, even the last few dull days the top of the tank is 48 deg C If the sky clears we get 55-60 no problem and the first year we had 80+
I don't see the need for more than that in my setup, saying that my pipe runs are small and direct.

Re legionella, how many cases have been reported in the last few years? Does it follow that the increase in solar DHW has led to an increase in Leg?
What are the German guidelines on DHW and legionella?

Why don't we follow a countries lead that has had solar installs for a lot longer than here?

quentingargan · 03-08-2009 11:18AM

CJhaughey wrote: »

Why don't we follow a countries lead that has had solar installs for a lot longer than here?

For the same reason that almost all systems have no heat dump. Everyone is competing on price.

OK - there is a difference of opinion on when and where a heat dump is needed, but there are sites that definitely should have a heat dump and don't.

In relation to Legionnaires though, I haven't heard of any fatalities from a solar water heating system. The problem is that one fatal case is one too many and the reality is that there is a theoretical risk which should be eliminated. If every single house had solar water heating, there would almost certainly be fatalities - just that the current numbers are too small.

Point is that all the guides and manuals state that you should have a legionnaires strategy. Some people believe this isn't necessary with a copper cylinder (copper is toxic to bacteria) and others say it isn't necessary unless you have a very large cylinder with a small number of users in the house.

However, that doesn't mean that someone coming back from a two week ski-ing break woudn't have the bottom of their cylinder laced with lethal doses of legionella bacteria at temperatures that would be acceptable for showering.

So you come home, switch on the central heating, heat the top half of the cylinder, switch off the heating, have a bath and use the portion of water in the cylinder that is above 60 degrees. Someone later has a shower using the water that has been at the bottom of the cylinder stewing legionnaires bacteria at 40 degrees for the last two weeks. BANG!:eek:

I know this is a lot of IFs, but it is a real risk. What's more, it isn't covered by the strategies imposed by SEI which only require that the top of the cylinder be heated to 60 degrees once a day.

03-08-2009 12:20PM

Re: legionella, I have 12m2 of flat plate panel and a 1000 litre tank, oversized as it will double as a buffer tank for a wood burning stove.

The tank frequently hovers between 40-55C but it is in the garage and there is a coil in it that extracts heat for the main thermal store in the house.

Most of the time the water for the thermal store is at 60C, when the system calls for heat this water is first passed via the buffer tank that brings it up to 40C and then into the oil boiler that takes it up to 70C before returning to the thermal store.

When I have the wood stove up and running, I should be able to bring the buffer tank up to 60C + thus resolving any legionella issues that I (probably) have in the garage.

The water in the buffer tank is in a closed loop with just the solar panels and stove.

quentingargan · 03-08-2009 01:49PM

One popular solution to Legionnaires on the continent is to use the cylinder as a thermal store, with a coil running through that cylinder which the used water runs through. This means that the cylinder itself can be open vented while the water supply is unvented.

It is also popular in the UK because plumbers there are have to have additional certification for unvented cylinders. Very few people selling such a system here. There is a slight problem in that there is a temperature drop across the coil...

03-08-2009 02:49PM

quentingargan wrote: »

One popular solution to Legionnaires on the continent is to use the cylinder as a thermal store, with a coil running through that cylinder which the used water runs through. This means that the cylinder itself can be open vented while the water supply is unvented.

It is also popular in the UK because plumbers there are have to have additional certification for unvented cylinders. Very few people selling such a system here. There is a slight problem in that there is a temperature drop across the coil...

That's how my system is set up, but with two tanks.
One for the solar water loop and one for the DHW and UFH water, the first tank feeding the second via an oil boiler.

holdfast · 07-08-2009 10:30PM

found this web page that might add to the already stated case for meeting

http://www.solartwin.com/solartwin-features/legionella-solar-water-systems-with-conventional-twin-coil-solar-cylinders/[/URL]

I have this problem and I have been looking for a way to remedy it. Baisc setup Heat pump with solar. immersion used to top of DHW from HP but solar thermostat says water in btm of tank hits around 40-50 degs. Showers are taken from the top of the tank. But the immersion is side mounted anyone no if you can get an immersion that is side mounting and is angled to reach btm of cyclinder.

quentingargan · 08-08-2009 12:01AM

No such immersion I'm afraid. The only options are as already outlined - a "Willis heater" which is an inline immersion that goes into pipework between the middle and bottom of the cylinder, or a pump that does likewise.

The Solartwin solution along with the Willis Sola Syphon both address legionnaires quite well, but at the cost of a system that keeps the water in the solar panel at the temperature of the top of the cylinder which makes the panels operate very inefficiently.

That is why the universally adopted system in other countries has been the twin coil cylinder - it produces enough additional energy that it is worth spending extra on a work-around if you are concerned about legionnella.

holdfast · 08-08-2009 09:38AM

cheers for the reply quentingargan I though as much as I have been looking at the interweb for a while. the rating for my immersion is 2.5kw would it be worth while upgrading. by the way the immersion is more than half way down in the tank.

Solar Panels - How many

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