What Temperature for Solar Panels

Aeneas

I am thinking about installing solar panels to provide domestic hot water and like many others I am confused by the conflicting advice and data available. I have been looking at threads on this site which give me a better understanding of the issues but there are a few basic questions the answers to which might help me understand the info better
- size of system. I have been advised to get three flat panels (approx 6.9 sq m) and a 300 litre tank for a two person house with occasional weekend guests. Roof faces due south. Pipe runs will not be long. Water will be used for shower, bathroom sink, kitchen sink, infrequent bath. Not for washing machine or dishwasher. My advisor says the system will be a little bigger than I need but it is better to have three panels to maximise solar gain. I note that oversizing can cause problems with something called stagnation. Is this a serious issue? Is my recommended system likely to be oversized? Or does stagnation refer only to those who put in very large systems?
- legionnaires. From the threads it looks like I need to bring the full tank to 60 degrees at least once a week to prevent the risk of disease. Does this mean that if the bottom thermostat (there will only be one on the tank) does not get to 60 at any point in a seven day period I should turn on the immersion or run the central heating? Would this not increase costs? Is regular temperature checking a part of solar panel management?
-temperature. I know that the temp in solar systems will vary considerably depending on time of day, month, weather. But what is a "normal" temperature for domestic hot water, generated say by an immersion?

quentingargan

Hi, Its hard to know - as others have said on this forum, panels can be variable in quality and output, but at almost 7 sq m for two people using mostly showers, this is a bit over-sized, but that just means that it will work further into the spring and autumn. However, personally if I wanted to extend my use into spring and autumn I would do it by using vacuum tubes as they are better insulated and work better at lower sunlight and air temperatures.

Regardless of cylinder size, if you only use 100L of hot water a day, the system will only have to heat 100L the following day. Once the cylinder is up to its temperature, the pump will knock off. The panel will continue to heat up to about 160 degrees, all the water in it will turn into steam and the water will be contained in the expansion vessel. The system will sit in this state until nightfall and this is called stagnation.

It is OK for this to happen occasionally, say during a power cut. However, on an ongoing basis the problems with this are;
1) Glycol used as anti-freeze becomes degraded at high temperatures.
2) Pipe insulation in close proximity to the panel becomes degraded
3) At high temperatures, the sealing rubber in the panel may age more quickly, shortening its life
4) Constant heating and cooling under pressure may gradually cause problems for some of the pipework in the system, causing it to leak (this ha been a common problem in N. IReland where analysis of old systems has been carried out.

You can get around that by running a heat dump. This is simply a diverter valve which pumps the panel into a radiator to dump the heat. In most cases the controller already has a relay for this function.

In terms of legionnaires, running the central heating only heats the top half of the cylinder. This is an issue in winter when the solar panel might regularly keep the bottom of a large cylinder at 30 to 40 degrees. More so in your case if you have a large cylinder and aren't using the water regularly enough. You need a system that heats the bottom of the cylinder to 60 degrees once a week. I prefer to use a stainless pump which circulates water in the cylinder (from secondary return to cold feed) once a week at a time when the central heating is on.

Normally water is brought to 60 degrees. With a solar water heating system, you can bring the temperature to 85 or 90 degrees to maximise heat storage, and then to prevent scalding, bring that water back down to a more acceptable 55 degrees by using a thermostatic mixing valve, which isn't expensive and should be standard on all systems anyhow. The only exception to this is if you have hard water where high temperatures would cause limescale to form on the coils of the cylinder. If your kettle is all white, you should stick to 60 degrees.

PM me if you want somone to call you for a consultation on all this. Q

Aeneas

Thanks for that very clear reply. I think I have a better grasp now. The third panel was recommended, as you say, to extend the solar heating season into spring and autumn. I have opted for panels over tubes because of the appearance in my particular situation and the risk of damage to tubes. I will look into the idea of a heat dump to reduce the risk of overheating/stagnation and damage to the system And also the idea of a circulating pump to mix the water in winter and bring the lower section to 60. The tank will have an element low down so I suppose turning this on once a week would have the same effect? I imagine the electrical usage cost would not be very high since I would only be bringing the bottom portion up from say 30 to 60. Finally I live in a hard water area. The system will have a thermostatic mixing valve but I am having a water softener installed also to protect the system.

quentingargan

If the electric element is low enough in the cylinder and you can use off peak electricity to run that once a week, that is a better and cheaper solution. Q

Aeneas

From the drawing of the tank (either a Joule or a Grant) the element will be about 20ins from the bottom, positioned just over the return to the pump. I don't have off-peak electricity but would that substantially affect the cost of a once a week operation?

quentingargan

Aeneas wrote: »

From the drawing of the tank (either a Joule or a Grant) the element will be about 20ins from the bottom, positioned just over the return to the pump. I don't have off-peak electricity but would that substantially affect the cost of a once a week operation?

Hmm. There is still water left unsterilised at the very bottom of the cylinder. There is a similar post to this on legionnaires going on here http://www.boards.ie/vbulletin/showthread.php?p=61431455#post61431455

Have a look at that and see what you think. Q

Aeneas

The thermometer for reading the water temp is a foot or more lower than the immersion - about nine inches from the bottom of the tank. So I guess if that reaches 60 using the immersion then I would be ok? The other question is cost. You mentioned that the immersion might be a cost effective way of dealing with the legionnaires issue. But I won't be using off-peak electricity. Do you think that once a week use in the cooler period of the year to raise the temp of the lower portion of the tank to 60 would be costly? I hardly think so, but would be glad of your opinion. (The other thread is interesting; this solar business is a bit more complicated than I thought.)

quentingargan

The immersion would be controlled by an internal thermostat, not by the one on the cylinder. Heat doesn't really travel downwards, so it would be an unreliable guide. Q

Aeneas

But wouldn't the tank thermostat tell me that the water had reached 60 degrees at the bottom of the tank? Even if I was using some other method to heat the water in the lower portion I would have to rely on the tank thermostat to tell me that the water had exceeded the critical 60 degrees. Isn't that the important thing? Or am I missing something here? Thanks for the quick replies. I will probably stick with the three panels proposal to ensure a longer solar heating season. And I will talk to my installer about the heat dump and the legionnaires issue.

quentingargan

Yes, the stat in the bottom of the cylinder would do OK, but you couldn't use the immersion 20" above this to do the heating in that case, unless you had some pump circulating the water.

I wouldn't be inclined to use full price electricity to heat the water by the way, but most houses could benefit from having a day/night meter and if you fund the change by switching clothes washing to night time, you would be helping the grid to absorb more renewable energy. I reckon virtually all houses will have smart meters in 10 years time with multiple price bands which will push us to use more power off-peak:)

macnab

I am using 2 panels (5m2) and a 300 litre water cylinder. The temperature has reached over 60 C at the bottom a few times and at the top quite often.

We use quite a lot of hot water (2 adults, 1 teen, 2 tots) and find this set up more than meets our requirements.

I would have thought that 2 panels and a 200 Litre tank would be more than enough for your 2 person household.

pool fan

hi,
im thinking of gettin this put in: http://www.atec.ie/index.php?id=specialoffer0

what do yous think?
Is there anything i need to look out for?

thanks

homer911

correct link

http://www.atec.ie/index.php?id=specialoffer

2sqm and your existing cylinder - sounds expensive

quentingargan

Very expensive. Half of the work of an installation is changing the cylinder to a properly insulated solar cylinder with twin coils.

2 sq m will only get you a grant of €600. On straightforward jobs, we have often installed 40 tubes (3.64sq m) with a replacement copper cylinder for €4,500, with a grant of €1092 instead of €600. So the nett price difference is €8.

Evergreen

Yes, I agree it is very expensive for what is on offer. If you get to keep your own cylinder then they must be using a Willis Solar Siphon, we have tried those and the efficiency is much lower than a dual coil cylinder (the return fluid temp is much higher than it should be).

The advert states that it is 2 meters of panels, but it is actually 2.85 m squared that sits on your roof, the output from this lot is only 1400 odd kWh/year (or 500 kWhr/m2). I don't think that they will have to worry about fitting an anti scald device

pool fan

The advert states that it is 2 meters of panels, but it is actually 2.85 m squared that sits on your roof, the output from this lot is only 1400 odd kWh/year (or 500 kWhr/m2). I don't think that they will have to worry about fitting an anti scald device



So your telling me theses tubes are poor quality?? What should i be looking for then?

quentingargan

So your telling me theses tubes are poor quality?? What should i be looking for then?

They aren't bad quality at all. There are issues with this particular type of tube because the replacement tubes are extremely expensive, whereas replacements for vacuum flask type systems are very cheap.

Tube and plate efficiency is measured on the aperture area - the area exposed to sunlight. The gross area (including frames) is a lot higher for tubes.

However, in terms of their characteristics overall, the main difference between tubes and flatplates is that flatplates work better in hot sunny conditions, tubes work better in cloudy or cold weather. If you want maximum heat gain, go for flatplate. However, often during the summer time tube or plate will provide more than you need anyhow.

If you want to extend the season into autumn and spring, go for some sort of tubes, but bear in mind replacement costs. They will lose their vacuum after about 20 years.

Of course if you think tubes look awful and would be an abomination on your roof, you may prefer flatplates for other reasons.

Aeneas

Well, I went ahead with the installation recommended by my engineer - three Grant Sahara flat panels, a 300l Joule tank and the other bits and pieces. No heat dump I'm afraid as I was assured this was not needed and the system was robust enough to withstand the maximum temperatures we are likely to get in Ireland - to date the maximum I have got on the plates is, I think, around 130. And that is in the sunny south east in August. Result: constant hot water. Even after two very overcast and very rainy days the tank produced enough warm water for a shower on the morning of the third day. On very sunny days the water in the bottom exceeds 60 degrees. So far so good.

bertie1

Aeneas wrote: »

From the drawing of the tank (either a Joule or a Grant) the element will be about 20ins from the bottom, positioned just over the return to the pump. I don't have off-peak electricity but would that substantially affect the cost of a once a week operation?

Wouldn't be worth it for that your standing charge for the newer meter would double and the savings would not be made up.

bertie1

Evergreen wrote: »

Yes, I agree it is very expensive for what is on offer. If you get to keep your own cylinder then they must be using a Willis Solar Siphon, we have tried those and the efficiency is much lower than a dual coil cylinder (the return fluid temp is much higher than it should be).

I have to agree with you about those solar syphons

We have 2 houses that are used as holiday homes & we installed solar panels in both. As there were already 300 liter cylinders in the properties we decided to leave them. In one property we used a solar syphon in the other a heat exchanger & bronze pump. The water in the property with the heat exchanger is always hotter than the other property, both are next door to each other. Even if the 2 houses were empty & you went in at 20.00 at night the cylinder with the heat exchanger will be a lot hotter than the other house. Both have the same amount of pipe & the same amount of panels the only variable is the heat exchanger & the solar syphon.

Both houses had the same occupany this year & the gas is used to top up the cylinder once a day for an hour. ( Thermostat on cylinder to shut off the gas if the cylinder is already hot form the sun. Gas bills came for both ( Solar Syphon house €48.00 for mid June - mid August ) (Heat exchanger house €16.00 mid June - mid August) .

It is a great improvement last year those bills would have been up over €200.00 each for that time

quentingargan

bertie1 wrote: »

We have 2 houses that are used as holiday homes & we installed solar panels in both. In one property we used a solar syphon in the other a heat exchanger & bronze pump. The water in the property with the heat exchanger is always hotter than the other property

I wonder are these flatplates or tubes? The solar syphon is a lot less efficient because it keeps the panel temperature higher. This would have a far greater effect on flatplates than it would on tubes, but it will affect both to some extent. Anyone using solar syphon with tubes out there?? Q

bertie1

Its a tube system we put in. Just not happy with the solar syphon

quentingargan

That's very interesting thanks Bertie1. If that is what happens with tubes, then the syphon system would be absolutely hammered with flatplates... Q

murfbrowne

just looking into getting the solasyphon with thermomax tubes, i have a 150 litre copper cylinder and a south facing roof with 2 adults and 2 tots.
Somebody said to me that i would need a dumping system but by the looks of the recent treads the water wont get that hot or will it?

is the willis solsyphon a good idea?
thanks

quentingargan

Hi Murphbrown. I am familiar with that package, and while it is tempting, the evidence suggests that there are better packages on offer which, for not much more money, will change your cylinder and give you a proper system.

However, one possible advantage of the particular tubes you are looking at is that there is a memory metal valve on the heat pipe which prevents the fluid in the system from overheating. So normally you don't use a heat dump with that system (I don't think you would need to anyhow, except possibly during the current hot spell if you were on holidays).

My reservation about this is that it means that the heat is held within the vacuum tube itself which can now get to extremely high temperatures internally.

Vacuum flasks are different from tubes because they are like a thermos - one piece of glass containing a vacuum, and no weld or seal. Tubes on the other hand rely on a weld between the copper heat pipe and the single layer of glass to hold in the vacuum. These materials expand at different coefficients as they heat and cool, and the more you do that, the shorter the life of the system will be. So by relying on holding the heat within the tube, you will save the pipework from coming under pressure, but I have seen situations where these panels lost their vacuum.. And the tubes are very expensive to replace. Thats why I prefer flasks.