Advice on proposed installation please

[Allen]

Please - anyone who can help,

I am a total beginner in the area of solar hot water installations, although I have installed several central heating systems (normal boiler-powered types) in my own homes.

I would appreciate any advice that can be given on the system proposed below:

Type of property:                      Bungalow
Pitch of roof:                            22 degrees
Direction of roof:                       245 degrees
Size of existing water cylinder:     Vented, 400 x 1050 (123 litre, one indirect coil to existing boiler)
Usage:                                     Husband and wife - very economical, only showers, no bath installed!

Proposed system:

Solar Panel:                              4730(SFB30-47) 30 tube 47mm
Heat Exchanger:                        Willis Solasyphon connected to existing H/W cylinder
Expansion Vessel:                       Presssys 12 litre with kit
Controller:                                 TDC3
Pump:                                       Pump1

and the general pipes and glue to hold it all together!

Specific questions:

Should I increase the inclination of the panel beyond the 22 degrees of the roof?
Is the Willis Solasyphon a good choice?
Is there anything else that I should be considering?   

Many thanks for any help that anyone can give,

Allen
 

[Justme]

Should I increase the inclination of the panel beyond the 22 degrees of the roof?

 

Unless you want to apply for planning permission you wont be able to increase the angle by much. You only have 200mm to play with & nearly half of that will be the frame & header. So you will have an increase of 100mm over about 1.2m.

Not worth the hassle & extra cost for the frame alterations.

[DonL]

I haven't come across the Willis Solasyphon before so I had a quick search and it looks really interesting. The main difference from conventional systems is that it heats the tank from the top down so rather than getting a tank full of lukewarm water in marginal conditions you could expect to get a smaller amount of usable hot water. Any practical experience from a forum member would be really helpfull.
An option to this, if you have an immersion heater would be to use a retrofit coil http://www.navitron.org.uk/product.php?proID=91
Don

[Allen]

Don,

I have looked at all the references to the Willis Solasyphon, and also read the research paper - it certainly looks to be a very interesting unit.  Yes, it does appear to be able to give very hot water at the top of the cylinder, rather than lukewarm throughout the cylinder - which I think must be a real advantage.
There is the possibility of converting the existing immersion heater into a duel purpose heat exchanger / immersion heater, but is that an advantage  over the Willis?  Possibly easier to fit than the Willis, but the plumbing isn't difficult....

Does anyone actually have experience of using the Willis Solasyphon?

Allen

[acresswell]

I haven't got a solasyphon.... and it certainly looks like an interesting way to re-use an existing cylinder but I think its current levels of insulation may be inadequate.

The solasyphon sheel itself is 75mm diameter.  Overall diameter including insulation jacket is 112mm. This means that the insulation jacket is only about 18mm thick.

I know that 18mm is about the same as normal pipe lagging, but normal pipework isn't designed to set up a convection current that could cool your whole tank quite quickly overnight. I think it'll be important to add extra lagging round the solasyphon and associated pipework.  I'm sure you could modify a standard hot water tank jacket.  Once it was working and leak-free I think I'd also box it in and then pour vermiculite round it, which you could remove fairly easily for inspection/maintenance. 

This isn't a reason to avoid the solasyphon, but it might involve you doing a bit of extra (easy) work.  My plans include a big thermal store with 100mm thickness of insulation, but I'm still planning on using up my offcuts of celotex between it and the external wall.

[Hugo]

I have the same panel as you are getting and a 186 litre twin coil cylinder.
Two people using it and also it is a bungalow.
The water gets very hot and I now know that the cylinder size should be 200 litres.
I have no knowledge of a Willis solasyphon so cant help with that.
I do live on the south coast that does help with the temperatures mind you.

[Allen]

HUgo,

Thank you for the information - it may appear that I should reduce the size of the panel if you are finding that the water is getting too hot with a larger cylinder than we have installed.
Does your panel face south?   We are in Epping, Essex, so the conditions are not too different to the south coast.....

Many thanks,

Allen

[djh]

Should I increase the inclination of the panel beyond the 22 degrees of the roof?

Unless you want to apply for planning permission you wont be able to increase the angle by much.

Unless you are able to mount the panel vertically on the south-ish facing wall.

That will also reduce the maximum output in summer while significantly increasing the output at other times of the year.

Wookey has a successful homebrew system using a PHE that works in a very similar way to the Willis.

[SimonHobson]

I haven't come across the Willis Solasyphon before so I had a quick search and it looks really interesting. The main difference from conventional systems is that it heats the tank from the top down so rather than getting a tank full of lukewarm water in marginal conditions you could expect to get a smaller amount of usable hot water. Any practical experience from a forum member would be really helpfull.

Indeed, it would be useful to hear from someone that has this. Personally I'd have thought it would give poor performance in marginal conditions.
Unless your panel is hotter than the DHW cylinder, then you won't get any heat transfer. So under marginal conditions you may get no output at all. With a single coil in the bottom of the cylinder, you'd get more output - but maybe end up with half-hot water.

Since you're going to need a backup heat source, my gut feeling says you're better off getting all you can and reducing the "top up" cost of getting the top of the store up to full heat via alternative means. Eg, if your panel can give you (say) 40˚ over the whole tank, then you are only adding 20˚ to get 60 for the upper section. The alternative may be getting no output if the panel doesn't reach 60˚ and thus having to supply all the heating via the backup.

Or don't panels work like that ? Is it a case of if you leave them without flow, you'll get the hotter temperatures anyway - but can only run the pump intermittently ?

[Allen]

Simon,

Thanks for your very peceptive thoughts.
I'm a complete 'newbie' to all this, but doesn't the 'swicth on' of the additional heat source (i.e. gas fired boiler) depend on the thermostat setting - and if this is postioned, AND set correctly, then won't it limit the activity of the boiler?

I wish I knew more about this....

Allen

[SimonHobson]

Yes, the thermostat does control the firing of the backup source (boiler), and it's position dictates how much of the store can be heated by it.

What I was getting at is this...

Suppose we want to run the store at about 60. Under one scenario, we wait until the solar panel reaches 60 before we start the pump - and then we load the store top down. That's great when there's lots of sun and the panel easily reaches 60+. If there isn't enough sun for that, then we get no solar input and all our heating will have to come from the boiler.

The other scenario is that we start the pump as soon as the panel is hotter than the lower section of the store. Now we'll heat the store bottom up and gradually increase the temperature and we collect energy in the panels. As suggested, under poor conditions, we'll end up with a store full of tepid water. But we've still added energy and the boiler now has less to do as it's starting from tepid instead of cold.

I suspect the answer if to have a dual mode control - if there's enough sun then hold off the pump until the panel is hot enough to top-load the store, if there isn't then just do what we can and bottom load the store. That's complicating things somewhat and is there a reliable way to work out if you'd get hot enough water out if you turn off the pump for a while ?

[Allen]

Simon,

I e-mailed Willis Solasyphon, and had a very ineresting reply - original e-mail and their response are below:

Original e-mail:

Dear Sir,

I am considering installing a solar hot water system, and have been attracted by the simplicity of the Willis Solasyphon and the ease of incorporation into our existing vented hot water tank.

Your on-line documentation clearly explains the method of installation and operation, however, I am trying understand its operation in various scenarios when compared to the conventional indirect coil inside the hot water cylinder.

I would be grateful if you could describe the effect on the temperature and amount of heated water in the following situation:

The solar panel reaches a temperature of 65 deg C for a short time (say 3 hours), and the Solasyphon heats the water in top of the tank so that the stratified layer starts descending, with the temperature being maintained at approx 65 deg C.  With a conventional indirect coil, the whole tank starts generally heating up, but is unlikely to reach 65 deg C even at the top, and this shows the clear advantage of the Willis Solasyphon.

The solar panel then receives less sunlight, the solar heated water temperature drops to (say) 40 deg C.
 
What happens to the hot water tank with the Solasyphon?   Does it reduce the temperature of the stratified layer which will be at 65 deg C?  How can it, through the hot water siphon effect, have any effect on the rest of the water in the tank, below the stratified layer, which will be at a temperature less that the 40 deg C that the solar panel is producing?  Will it heat this cooler water at all, or is all the heat being produced by the solar panel wasted?

The conventional indirect coil will be still capable of raising the general temperature of the water in the tank with an input at 40 deg C from the solar panel, providing the tank temperature is <40 deg C, and the temperature sensor were correctly set to operate the pump.

I would be very pleased if you can explain what will happen in the above situation – I have been unable to find any satisfactory answer in all the research I have carried out.

Thank you in anticipation of your help,

Yours faithfully,

Allen Cherry

the reply:

Dear Mr Cherry
 
Thank you for your interest in the Willis Solasyphon.

I hope I can help you further with your understanding of how the Solasyphon works in an already stratified tank. There has been an independent study carried out to examine this particular situation . This study is available to download from our web site. It was carried out by Prof M Smyth at the University of Ulster in their state of the art solar simulator.

In essence what happens in this situation is that with the same collector area the Solasyphon is only trying to heat less than 2 lit of water at any given moment as compared to a conventional coil in tank system that is trying to heat a much larger volume of water.
 
With the Solasyphon the thermosyphon flow through it will vary depending on both the solar radiation and the temperature of water in the tank.

In the case you describe the secondary water will not circulate out of the Solasyphon until the temperature has risen above that in the tank.

The 2 lit of secondary water will delay in the syphon and correspondingly the solar primary return will be pumped back up to the collector at virtually the same temperature as the solar primary flow. This results in a rise in the collector temperature even in marginal solar conditions. This process happens quite quickly due to the small volume of water that is being heated at any one time.

As soon as the temperature of the 2 lit of secondary water reaches a temperature greater than that in the tank it thermosyphons into the tank under its own buoyancy.


In good solar conditions this process produces a continuous flow, and in more marginal conditions the process will slow down to match the solar conditions. We like to think of the process as “natures pump” .

The big advantage of this process is that it is always extracting the maximum temperature possible out of the available solar radiation.

I hope this helps.
Regards,
John Willis

[DonL]

Hi Allen
Again a really good post. The technical paper makes really interesting reading and the conclusions seem credible. That is, you heat from the top down and under most draw off conditions one will get hotter water than with a conventional coil system but the overall efficiency is lower. With a conventional coil more heat is actually transferred.
So if you want to get hotter water for intermittent use it seems a good bet but if you want a large volume of water at an even but lower temperature you're better off with a conventional coil.
I think we'd all still like to hear from someone who's got one!
Or maybe you should install it and tell us the results 
Don

[djh]

I think we'd all still like to hear from someone who's got one!

I already mentioned that Wookey has a homebrew system that works on the same principle. I think I've read reports about the solasyphon but I don't remember where. google may help you.

[SimonHobson]

Hmm, now I've read the report, I think it raises as many questions as it answers !

The first thing I notice is that the test methodology only simulates a 6 hour day of bright sunshine AND they always start from a cold tank. So we get little idea what happens starting with a hot or warm tank, or what happens on a dull day.

It's clear that using the coil rather than the syphon, more heat is put into the cylinder - the result being that the next day the cylinder starts from a higher temperature. It would have been interesting if they'd done some multi-day runs to see how the two setups compared after several days of solar heating and hot water drawoff. As it stands, their results suggest that most people would be topping up the store with an alternative input (boiler) and so long term you should be using less alternative input if you can extract more input from the solar collector (as their figures say you would with a coil).

The figures given show a significant difference in energy gained, with the cylinder bottom coil coming out significantly more efficient than the syphon. This fits with my "gut feeling" that you'll extract more heat by heating all the cylinder at a lower temperature than you will by heating some of it to a higher temperature - especially on a dull day. This effect would reduce after several days since the tank would be starting from warm instead of cold. But, each time you draw off cold water, you put cold in at the bottom - and this will extract maximum heat from a marginal solar input.

What would be useful would be a set of curves showing heat transfer vs temperature vs "sunniness" - because I suspect at low input levels, you'll get less out at higher temperature. Does anyone know if these exist ?

One thing I think the unit would benefit from in order to gain the main function it's sold on would be a thermostatic valve in the cylinder side. Set to prevent flow until the exchanger reached the desired cylinder top temperature, it would give far better results (especially from cold) - at least in terms of maintaining usable water temperature. As it is, the syphon starts up quite early with a cold tank - meaning you need to wait several hours before their's properly hot water.

Another thing to bear in mind is what happens if you use a thermal store instead of DHW cylinder. With these, much of the DHW heating occurs lower in the cylinder - taking heat from the cooler water. I suspect the DHW output would be somewhat reduced if the bulk of the tank was cold and only the top section was hot. But that doesn't appear to be their target market - the sales blurb seems to be selling it as an alternative to swapping out the existing DHW cylinder.