Wind turbine and solar hot water

[Magpie]

Finally at the stage of organising our power, and for a wind turbine, we have been recommended a Lakota - anyone know anything good or bad about these?
Also, we had been intending getting a Solahart( ie with panels) hot water system, but our supplier is saying the evacuated tube type is better - again, any opinions? We had been concerned it would be a bit fragile.

[Martin]

starting backwards - another installer has a vacuum tubed system on his roof, and was worried what would happen if it were hit by a flying ball, so he clamped a tube in a frame, and bounced a leather football from 5 metres away onto it, several times - he no longer worries! - they're made to withstand large hailstones, and are made from "tough as old boots" borosilicate glass (pyrex).
Let noone pull the wool over your eyes about tubed systems - they are a Chinese invention (developed in one of their universities in the 70's/80's) - and most sold worldwide come off the same chinese production lines!
(much as many swiss and german companies try to rebrand them as "swiss" or "German")
With their superior performance in spring and autumn..........
As for the turbine mentioned, I honestly don't know - I gather it's one of many US manufacturers - I've heard no reports of their products either way - best advice, shop around, and make very sure you site it right (if you have any doubts at all, invest in a wireless weather station, and collect wind data for a few months!

[Magpie]

Thanks Martin. That's what our supplier told us about the evacuated tubes, too - that they are Chinese - so at least he's honest that way! He has also offered to lend us his wind-measuring thingo, so he does seem to be doing things in our interest. Good to also hear from you about their strength, supplier told us he's only had to replace 2 tubes out of 100's of installations, and they were both broken during installation. He also said he intaleed one on his own house, after his panel-style one froze and burst a panel!!

Here, not many people know anything about alternative power sources, so it can be hard to get accurate local knowlege.

I was a bit miffed when the parents-in-law said they had watched "it's not easy being green", and what a lot of good ideas those people had - it's only the same things we have rabbitted on about for the last 5 years!!

[Martin]

Looks to me that you've got someone offering good, sound advice - obviously compare prices, but I'd tend to trust them, they're going about it in an ethical way!

[contadino]

I believe that panels are as efficient and robust as tubes nowadays. For me the only deciding factors would be cost and whether you wanted to integrate it into a roof. Panels are cheaper and can be easier roof-integrated. One advantage of tubes is that individual tubes can be replaced, whereas I expect panels would need the whole thing replacing if there was a fault.

[Martin]

certainly as robust, but I would personally always prefer tubed systems because they tend to perform better in spring and autumn (they all work in summer!)
Better not get too sniffy about flat panels I suppose, we're going to be offering them as well as tubed systems soon (mostly to fit in with planners, who tend to view flat plates as "just like a Velux window" so can often allow them and not tubes in conservation areas, on listed buildings etc.)
Given similar systems, I believe that flat panel systems will be a smidgeon cheaper than tube systems, but not by a lot (often the thermal store costs more than the tubes/panels), so the panels are often a relatively small part of the costs of a full installation

[Magpie]

Well, another thing I remember the supplier say was these new tube models are true thermo-syphon - the have only 800ml fluid in them, as it is the hot air trapped in the tube that does the heating - the water doesn't run through the tubes at all. That's important for us, as we often get freezing weather of some sort in Winter, so when the pump goes on to move the fluid to prevent freezing, it only pumps that 800ml, not the whole tubes' or panels' worth, thus saving power that way too. That's how his panel burst - he turned the pump off, as it was running all night - not good on a solar/wind system!

[Martin]

we do it a bit differently - there's lots of different designs of tube "innards" - ours have a copper "heat tube" inside the vacuum tubes - it contains a tiny amount of water in a vacuum - the sun heats the tube, the tiny bit of water turns into steam, and rises to the top of the tube, where it condenses -
here's a picture -
- the top of the heat tube fits into an insulated manifold, where the heat is transferred into the water flowing through the manifold - so the amount of water going round the solar loop is quite small, and it never goes down into the tubes. Rather than run the pump (and lose heat) in freezing conditions, the solar loop contains 40% antifreeze!
Here's a typical simple layout

[Muddypause]

Why do you need an expansion vessel in an open vented system, Martin?

[Martin]

only the solar loop is pressurised - the main thermal store (tank) is conventionally vented. The solar loop contains quite a small amount of liquid (around 5 litres), and runs at around 1 bar
The choice is to use a couple of litres of antifreeze every 2-3 years, or you use energy to run the pump to stop freezing - but that also loses energy from the tank as you're using it to heat the atmosphere!
(there are other ways - "drain downs"- but they do present quite a lot of practical problems in implementation, so tend not to be much used, and there is one manufacturer that uses a lot of plastic, and reckon their pipes and panels can freeze unscathed.........).
I would say there are many different configurations - to that basic circuit we would expect to add a thermostatic mixing valve to limit the hot water output temperature, and also a heat dump circuit.
If the system is properly sized, it should give you most or all of your domestic hot water April - October - that means that if you go off on holiday for a couple of sunny weeks, it's still heating the water, and noone's using it.........The systems are "fail safe", and would eventually vent. Our systems are built to withstand it, but I prefer to have another valve, actuated by the controller when it reaches a hefty temperature, that runs the solar hot water round a long length of uninsulated pipe, skirting the loft where it gets the cooling air - or if space is limited, through a radiator in the loft, or in an adjacent room - that way, the system is never stressed
probably clear as mud!

[Martin]

ps the reason why a pressurised loop is used - nearly always, the solar panel is at the highest point of the system, and the increase in pressure raises the boiling point of the water in the loop to around 120C, giving an increased safety margin, and aiding efficiency!

[Muddypause]

only the solar loop is pressurised

What I should have asked was, why does a pressurised loop have an open vent? But in any case, I realised soon after posting that what is in you diagram is probably not an open air vent but an air bleed valve.

[Jack]

Gidday

I also have heard that the tube heaters are by far the best and have even heard of one joker in the Hawkes Bay, who gets enough heat from it even right through the winter, and frosts are not unheard of there.

But Martin, that diagram amazes me. It shows the hot water from the solar panels entering the heat exchanger in the hot water cylinder near the bottom. That means that the entire volume of water in the cylinder has to be heated before you get hot water for your taps. If it entered at the top and exitted at the bottom the draw off water would be the first to be heated and ready for use.

[Martin]

Muddy - it's a pressure release valve which lets off pressure above 3 bar!
The solar coil is at the bottom of the tank because it is far more efficient for heat transfer - being at the bottom of the tank, the temperature differential between the heated water from the solar loop and the water it's heating is far higher, hence heat transfer is more efficient (and hot fluids rise!):cooldude:
There are some systems that use some form of external heat exchanger, which then dump the resultant hot water into the top of the tank!