I've noticed an increasing number of these are switching from the older through-flow water design to a single-ended heatpipe design, where the water flows through a manifold linking the array of tubes and the tubes themselves contain a separate liquid/vapour working fluid in a heatpipe.
Would anyone care to suggest good reading material on the relative pros and cons? Preferably something with decent numbers in it.
Thanks
Best folk to ask are alt.solar-thermal, or is it alt.solar.thermal
NT
As far as I can see, a wasteland of trolls, spam and crazy 'merkins.
8-((or is that just Usenet?)
Try also the forums on the Navitron website.
Might be more to do with convenience than performance.
cheers, Pete.
We had these installed at the last house, back in Suffolk. Advantages were :-
- Self-regulating, in the event of mains or pump failure while the sun was shining the tubes 'somehow' self-regulate their temperature...
- Easy to replace individual tubes - as their connection into the circulating circuit is via the manifold - which encloses the circulating water
- Easy to add extra tubes - our original system had (I think) 12 tubes with some 'positions' in the manifold empty - and we added an extra 6 tubes to up the output, without needing to drain down the water at all
- Circulating water doesn't go through the collectors themselves - so no problems with corrosion / limescale / whatever
- Collectors were claimed to be more efficient than conventional flat plate collectors...
These advantages were claimed by the manufacturer / installer -
In all we were very pleased with the system - it produced most of the summer dhw for the two of us - and we simply had an electric immersion heater on a timeswitch that came on as necessary in the evening to bring the water up to the required temperature.
HTH Adrian
On Tue, 08 Jul 2008 20:49:08 +0100 someone who may be Adrian wrote this:-
Only some tubes have the first feature, but it does have advantages.
An additional advantage is that just having water in the header reduces the volume of water in the solar circuit quite a lot. That means the system starts producing useful heat more quickly.
AIUI, they don't - they regulate the _manifold_ (i.e. water) temperature. This is done by a simple bimetallic widget that 'uggers the heatpipe behaviour once hot. In the UK we don't get hot enough that we hit the collector limits themselves, so we don't need reflector detuners etc.
This looks attractive, as a feature of a long-term high-capital-cost system. However there are no free lunches. Are tubes with this nice feature still reliable? Still well-insulated? My obvious concern is a heat leak added around the manifold plug-in connector.
That's the trouble. I keep being told self-evident bollocks as to how they're better than a painted radiator (one should certainly hope so!), but nothing comparing heat-pipe vacuum tubes to through-flow vacuum tubes, which is the real comparison of interest. There's a lot of extra stuff added to a het-pipe, when the killer problem for UK solar is getting the capital cost to repay itself faster than the MTBF. Complexity thus scares me, unless it's giving better efficiency too.
Thats the trouble with most of this greenwash technology. It isn't actually cost effective when overall cost of ownership is taken into account. The only two things that look like being well worth the cost, at least in my situation are:-
-insulation.
-heatpumps.
Frankly my hot water requirements in summer are almost zero, being limited to the odd bath/shave/wash or three every week, and coffee. Its winter when I want it, and in winter these things dont produce much of it.
{{fact}} {{unreferenced}}
Global irradiance varies throughout the course of the day because the path length of the solar radiation through the atmosphere changes. For the same reason, there are variations with season and latitude. The total solar energy received in a day (known as the insolation or solar irradiation) can vary from 0.5kWh m²/day in the UK winter to 5kWh/m ² /day in the UK summer and can be as high as 7kWh/m²/day in desert regions of the world. Many tropical regions do not have large seasonal variations and receive an average 6kWh/ m²/day throughout the year.
I NEED around 240 KWh/day to heat this house. so thats 480 square meters at 100% efficiency.
lets be kind, and say this lot does 30% efficiency. So I need no more than 1200 sq meters - 1.2 hecatres, or about what - 4 acres, to heat my house.
What do 4 acres of solar panels cost?
Ok.lets be kind and say that we will only heat 250 liters of bathwater with it. its winter, so thats coming in at 5 degrees C. I need it as 65C so thats a 60c rise, on 250 liters. thats 15mega calories, or about 17Kwh.
Thats better, only 34 square meters of panel at 100% efficiency.
So lets say that I cover my whole roof with solar panels, pointing vaguely southwards. and get a shade more than that and do indeed heat all my bathwater with solar energy?
my 34 sq meters of panels are saving me..17 units a day ?
Gosh. Thats over 100 pounds a year. or almost £2 a week! I can save that much by not mowing the grass in summer, or not drinking a pint of beer once a week.
If you can get me 34 square meters of solar panels for near £1000 (installed) at 100% efficiency, that have ZERO maintenance, I reckon you have a deal.
If not, f*ck off and take your greenwash somewhere else.
Or compare that with a heatpump. which in conjunction with the actual land area I have..about an acre CAN turn te average insolation of that into low grade heat that actually *will* save me something like £1000 quid a year in heating at todays oil prices, at a capital cost of about
10,000..and CAN deliver me a FULL house heating, and if using Sizewell B. which I probably am, at NO carbon cost whatsoever.
Order of magnitude error there! 10^4 m^2 in a hectare, not 1000. I make it just under 0.3 acre, so stop exaggerating :~)
Your consumption does seem quite high for a recently built - presumably well insulated - place. What's the floor area?
about 3500 sq ft I think. Upstairs and downstairs. Mmm. that seems rather huge. I thought it was nearer 2000..
Takes worst case input of 10KW to keep warm at -5C exterior. According to SAP calcs dne by a heating consulatnt.
About 30W/sq meter. The UFH is rated at 50W/sq meter and its marginal in ultra cold northerly winds. Two bloody great open chimneys and underfloor fire vents don't help either.
Anyway the point is that at even 30W/sq meter, a 2 storey house needs something like 60W/a sq meter footprint of insolation to stay warm in winter, or around 1.4kwh/day at 100% efficiency. The chart I posted shows that it wont get that in November December January and February, and if it did, actually bloody great triple glazed south facing windows would be the best way to collect it.
Well we don't have those, but we have a lot of glass facing south in the living room, and that DOES help a LOT when the sun comes out..2-3C rise pretty quickly.
Now I don't know what efficiency these so called vacuum tube collectors achieve, but it ain't over 100%.
To quote David Mackay 'a lot of littles makes..a little'
I don't need greenwash solutions. I want a way to knock a grand off my heating bills.
if I covered the whole of the south facing thatched roof with solar panels (100 sq m), it would knock about 300 grand off the value of the house, cost me gawd knows how much in installation, and at the best I can estimate, would give me an annual return of about £2000 IF I could use the heat it produces when I dont actually need it, in summer.
The reality is, I cant. I can at best store a tank of hot water a day.
In winter I need a huge area to do even that. In summer I have hot water that I cannot use, and the output of the panels gets thrown away.
Its the old windmill bollocks all over again. Without the ability to store energy when times are good, you cant do without massive capacity at huge expense when times are bad. but here it's the worst of all possible worlds. Ok at least it gets light once a day, but the REAL cycle is annual. How much hot water do I have to store in summer to take me thorough a winter? something the size of a small olympic swimming pool, and even that will be distinctly tepid come March.
HOWEVER I CAN store all of that average insolation from summer in the winter in terms of an acre of land with a load of pipes buried in it. And PUMP it into my house at higher temps. And by the time the subsoil temps have bottomed, its March and I don't need to heat quite so much..anyway.
Lets take current heatpump figures as a basis.
no one is quoting less that a 3:1 uplift in energy by pumping, so at a sort of 12p a unit, each Kwh of heat out costs 1/3rd a unit. 4p/Kwh say.
units are guaranteed a minimum of 25 years. So if I take my installation at say £10k over 25 years, the amortized cost is £400 a year.
Now solar energy is free, and if we take Apricus' figures of 60% efficiency and an average insolation (which we cant use anyway as its all when we don't need it, but let's assume we could) that's around
2Kwh/day per sq meter, we are effectively getting 24p a day per square meter return on our investment.The lifetime I cant get, because the warranty page on their site just hangs...haha.lets assume 25 years..so in 25 years this one sq meter of panel will generate £2190 of heat energy. Compared with an electric immersion heater. Compared with a heat pump, its about £700 quid.
Now going to something that has even remotely the output of a heatpump..lets say we need 1kw in winter. 24Kwh per diem. And compare it with our 10Kw peak heatpump running at an avarege of 24Kwh per diem... Costing 10 grand. And costing in electricity about 4p a Kwh output So over its lifetime it costs the installation plus £8760 of electricity..so £18,760 in all.
Thats about 12 sq meters of solar collectors. So assumeing we can alwasy use the summer heat 0- which we cant, and the panles are as effeicient as claimed, and they need zero maintenance, or break even costs with respect to our heatpump are £730 a square meter INSTALLED and FULLY COMMISSIONED. Including all the other stuff. l Note that this is completely biased in favour of the solar panels: I am assuming that :
- I can fully utilize the output of 12 sq meters of panel at any time and..
- there are no maintenance costs associated with them, like scraping algae and birdshit off them.
- I can fully run the house off them (I cant) at an average figure of just 1Kw. When although that IS my AVERAGE heating requirement,in reality is zero in summer (when the sun shines and they work) and about
20% of that in winter (when I need 10Kw).In order to satisfy that winter requirements I need to store really about 4 months of summer energy for 4 months of winter. Spring an autumn are energy neutral.
So lets look at THAT.
4 months at 24Kwh/day is about 120 days. 2.88 Mwh So lets see how bihg a tank opf hot summer water at say 60C, it takes to store 2.88MWh of energy before it drops to say 40C. we can storte with a 20C difference 20 calories per cc..thats .023 watt hours per cc or 0.23 kwh per liter.So to do 2.88 Mwh will be around..er..125 thousand liters give or take a bath or three. Thats 125 tonnes of water..or a 5 meter cube of hot water, insulated well enough to stay that way with no appreciable drop for 6 months. Only about the size of a small flat full of stinking hot water. A mere nothing. Or think of it as a 5 meter wide 20 meter long swimming pool 1.25 meters deep.
Peanuts cost wise, I mean even the insulation of that cube is only 125 sq meters..I would say the while thing could be dug insulated and cast in concrete for way less than 20,000 ... Lets ay a measly 12000 quid..oh dear..
...that has made our entire installation instead of generating money, actually cost so much that it never ever pays for itself at any price..
Which is pretty much the way every single eco bollocks solution I have actually costed out, has ever been.
I think its important to distinguish between solutions for people who can do sums, and solutions for people who can't, dont you?
In terms of saving money, being feasible and generally working, and being carbon free, I have been saying for some time that nuclear energy and electric cars will actually WORK. The only other element to add to that is heat pumps. They are actually the most cost effective way to utilise solar energy. 125 tonnes of wet soil is available pretty much to every household that has a back garden. Go deep enough ands its its own insulator too. There is the winter storage.
Couple up to a cheap nuclear power station, and there's your carbon free house heating.
.
You, the Queen and Nicholas van Hoogstraten maybe.
Is that really the best you could come up with?
You don't need 240kWh/day to heat a house. Palace maybe. That's a constant 10kW average! How big is this place, how hot do you need it, and why haven't you insulated it yet?
10 kW sounds quite low for such a big place - certainly consistent with "well insulated". But your consumption is high. Something isn't quite adding up here in my mind. Let's see: 10 kW with (say) a 25 deg. temp difference means it needs 400 W per degree (steady state). An average heating season in East Anglia is something like 2200 degree-days on average, so that's 880 kW-days per season, i.e. about 21 MWh. But you said you use 6000 litres of oil a year, and that's about 60 MWh. So either:
- you're burning the oil very inefficiently (unlikely), or
- there's serious heat leakage somewhere, or
- you've phenomenally high hot water usage, or
- you like it hot, or most likely
- some combination of the above.
Serious heat leakage?
True in context, but...
... That's about 30% by the sound of it. In that context 5% saved here and 10% there, etc., can soon add up to a significant amount.
[Big snip]
Interseasonal storage - there are people doing it now:
Except that what used to be considered superinsulation is now pretty mainstream. I wouldn't be quite so quick to dismiss these things as bollocks. The rules of the game could change (although the massive increases in energy prices that some people were predicting after the
70s oil crisis never materialised) and exploring all options seems a good idea. Groups like CAT are, I think, are much to be admired for being prepared to try out ideas and sometimes fail.Yes. Science and sound engineering will always prevail in the end "for nature cannot be fooled" [Feynman]. But don't dismiss experimenters just because what they do isn't economic today.
It's not the only valid option though - see Mackay's five plans. Time will tell.
getting _seriously_ into this for heting glasshouses. Impressve numbers too.
Don't you have a shower or bath every day in summer? :)
cheers, Pete.
Guess what. They use heat pumps.
You won't get any help by telling people to f*ck off, will you?
cheers, Pete.
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