Posted By: renewablejohnI dont know where I am supposed to have poured scorn on using low grade solar

Posted By: renewablejohnNow Fostertom, you are getting carried away. Solar powered Walls! I think I am borderline as it is but that would be OTT (both price and work)

It's OK anyway - just wanted to nudge you again, as you seem worth it - quick to learn.

A general strategy/guiding principle towards massive reductions in energy demand without suffering reductions in standard of living/progress of useful technology etc (Holgrem's Green Tech Scenario - the only one of his four Scenarios that has a long-term future), is taking care to use precious energy, for all purposes, in lowest-feasible-grade form. Going to higher-grade energy, on the whole is wasteful in one way or another, looking at in wide energy- and resource-accounting terms.

Space-heating is a very large and fruitful opportunity to do just that. It is huge - space-heating of buildings accounts for fully 47% of UK CO2 emissions. And no other use that I can think of can make full, uncompromised use of heat of such extremely low grade.

For a start, only by collecting at v low temp (collector flow temp about 21C) is it possible to do it thro the coldest months Dec and Jan. If expecting to collect at higher temp, you'll need to rely on heat collected up to October and after March, so you'll either need swimming-pool size super-insulated heat storage tanks, or you'll have to burn fuel thro Dec/Jan.

Yes, that is my quote. Not scorn though, tongue in cheek. You have been very helpful, Tom, but various nudges on this forum have probably quadrupled the cost and work I need to do (with obvious benefits). I know you have a penchant for 'solar walls' and they sound a good idea but a step too far for my situation. The bank manager and I are going to get to know one another well as it is! I suppose jocular comments don't translate so well when not spoken.
Thanks again
Paddy

Posted By: StuartBBank managers still exist?

Aren't they 'personal financial assistants', because your an individual just like everyone else

My personal 'business manager' is in a call centre 60 miles away and she is about 12 years old. I tried to discuss a creative business proposal with her and she said that wasn't her department.

Posted By: fostertom

Posted By: renewablejohnIf you design your thermal solar tubes with tracking reflectors you can multiply the solar energy collected per tube by a factor of 3 which then makes high grade solar available all year round

Yes, not bad, at a price, but we're looking at v high whole-system collection-and-use efficiency using large lo-tech collection and input means that double as part of the building fabric. In fundamental principle, it's wasteful to collect at temp higher than necessary and then dilute it down at point-of-use - tho there's reasons like compact storage density and small input radiators that seem to promote high temp collection.

Our philosophy has been to design the system to the maximum temperature which is achievable and with thermal oil and evacuated tubes that would be approx 230C. We then exploit the temperature gradient between 230C and 100C to produce electrical energy then any waste heat below 100C can be used for domestic hot water and central heating.

Posted By: renewablejohnthe maximum temperature which is achievable and with thermal oil and evacuated tubes that would be approx 230C

For which months of the year? and if 230C is the max, what temps do you get other times?

Posted By: evanThat's very interesting John!
I guess you store up a batch of high temperature oil and then fire up a steam engine generator at a peak time for selling, with the exhaust gas being condensed by the central heating / DHW?

How's the electrical efficiency, and what about the cost compared to a combination of PV and ST?

It could be as simple as that but we have it linked into a biomass system as well so that we have an option on how the thermal oil is heated. Yes we can fire up the steam engine on demand with approx 15% electrical efficiency (but a bristle turbine I am looking at might improve that figure) and overall efficiency of approx 75%. If running purely on solar. If using biomass then the figures rise to 45% electrical and 80% overall. Electrical made up of gas and steam turbines combined. PV was a non starter due to inefficiency of charging batteries. ST with reflectors approx 1/3 of normal price ST arrangement for the same heat output.

Posted By: fostertom

Posted By: renewablejohnthe maximum temperature which is achievable and with thermal oil and evacuated tubes that would be approx 230C

For which months of the year? and if 230C is the max, what temps do you get other times?

You can get 230C on a sunny day all year round as the tubes are in series and flow rate is adjusted to control temperature

Posted By: evanSounds good. I like the aspect of combining all sorts of high-ish grade heat, you could also use biogas from kitchen waste / sewage etc on a very small scale which is difficult to make use of otherwise.

But "PV was a non starter due to inefficiency of charging batteries. " - Huh?

We wanted a system that operates 24/7 not just daylight hours so by the time you have a 15% efficient PV panel charging a battery bank with 25% efficiency your overall efficiency could be as low as 4% electrical and no thermal heat captured (maybe the batteries heated up slightly but I would not consider it usable heat)

Posted By: SteamyTeaHow do you compare efficiencies between fuel/energy input/ourputs?

I use the methods as set out in the CHPQA guidelines which is the basis of any ROC payments that we receive.

Posted By: evan

Posted By: renewablejohn

Posted By: evan

But "PV was a non starter due to inefficiency of charging batteries. " - Huh?

We wanted a system that operates 24/7 not just daylight hours so by the time you have a 15% efficient PV panel charging a battery bank with 25% efficiency your overall efficiency could be as low as 4% electrical

Woah there, this is getting very hand-wavey. Battery banks and inverters can be far more efficient than that for round trip storage, better than 90% even if you use the best available. You also seem to be ignoring grid connection.

I think you'll be hard pressed to get anywhere near that with heat storage and steam engines.

By all means publish and prove me wrong, but please don't (erroneously) put down other technologies just to make your approach sound better.

Accepted if you have a large battery bank and can afford to top slice the discharge to less than 50% you will get better efficiencies. But in the real world batteries are the expensive item of a PV system and the temptation will be to reduce cost by having a small battery bank and to discharge to 20% reducing the number of cycles and reducing efficiency. Even at 90% efficiency your still down to 13% efficient compared to a solar thermal of 15% and overall of 75%.

In essence you're using stored hot liquid and making electricity from it on demand, instead of trying to store electricity as such, in batteries? And as a bonus, getting lots of useful heat. Sounds good.

I understand that by modulating flow thro the solar collectors you can get all your output @230C (which the system depends upon, otherwise no steam). But what kind of flow rate, therefore quantity of heat can you get in deep winter? I'd have thought that collecting at that temp, re-radiant losses to the cold winter outside scenery wd far outweigh any gain, even during (very intermittent) clear sky/sun spells? Unless you can collect @ 230C even during winter overcast periods, you'll have to store quantities of heat to bridge between the rare clear-sky/sun spells, to generate steam on demand through overcast spells - and your heat store will be useless for steam if it drops below 210C.

If in winter periods you're relying on biomass, then which time of year is your 'waste' heat useful?

If you can find a way to convert low grade heat into electricity, that's really the holy grail! It wd function like a heat pump, because the lo grade heat, once converted into electricity, cd be reconverted into heat at any temp you like! Wd depend on the efficiency of conversion - if v lossy then that's not so good, tho as in your present plan the 'waste' heat cd be used.

Posted By: renewablejohnOur philosophy has been to design the system to the maximum temperature which is achievable and with thermal oil and evacuated tubes that would be approx 230C. We then exploit the temperature gradient between 230C and 100C to produce electrical energy then any waste heat below 100C can be used for domestic hot water and central heating.

So I was asking about the waste heat that you get during periods when your hi-temp solar is the heat source.

You're saying that hi-temp solar generates electricity plus as a bonus (dramatically pushing up overall efficiency) the waste heat is used for space heating. I'm checking whether there's any such need for space heating during periods when your hi-temp solar is the heat source. Because if not, your solar collection isn't being used at all for space heating.

I was talking about lo temp solar collection for space heating but you preferred an alternative; hi temp collection cd do space heating plus electricity generation as well.