Dedicated vs Generic Generation Schemes/Ideas

[SimonHobson]

Mention thermal store and some will state with 110% certainty that the only way to deal with it is to rip it out. I wind some of them up by using the term "plumber" as a derogatory term

Troll like behaviour in any Forum rule book. Life's too short to pick through your capacious posts and work out who you're trying to bait/wind up so be warned 

Not on this forum, and in the context I do it it's not unreasonable. Eg :

"A heating engineer would <blah blah>, a plumber would have a loong suck through his teeth, then rip it out and fit a combi"

When someone claims to be a plumber (in a positive way), but cannot grasp/accept there being any valid use case for something as complicated as a thermal store, then they epitomise all that's wrong with a certain section of those calling themselves plumbers - I'm sure we've all met at least one of them. They may well be capable of bunging in pipework, but they don't understand their business.

[Simes123]

On domestic scale pumped storage, I doubt that many will have the scope for doing anything big enough to cover the cost - as you point out, economies of scale mean that a very small pumped storage system will be proportionately much more expensive than a larger system.
Lets look at some figures. Say we can get 10m head (generous unless you have a 3 storey house), and can lift 1000l of water (probably the upper limit without major structural work). The kinetic energy available from that setup would be 98,000J. That's enough to run a kettle for 32s without taking into account any losses - so more realistically probably only 20s or less. To do that, you'd need to shift the water at 50l/s which is a fair old bit of plumbing.
So again, could be done but realistically you'd struggle to justify the cost - and space needed. I think you'd find batteries/inverter would be easier.

I think that it would be better to  leave the water in place  and use it  to pressurize  air bags under the water.   The pond level would of course change but not as radically as a flow system.  I am constructing a pond at present that will have variable depth using sloping sand covered sides.  It is  intended to  act  as a reservoir for the one beside the house  but it might  be  possible to test the idea for electricity storage on a small scale.   It would need some way of keeping the air bags submerged  and an efficient way to inflate and generate  from them.  Perhaps a Tesla turbine for the  generation part.   

Interesting -- I thought at first you wouldn't be gaining your Potential Energy, and then realised you are actually compressing Air.  Which made me think - ok, modify the proposal to run a small compressor via PV, into a large resevoir tank or tanks (eg cheap empty 75kg Propane cylinders on Ebay).  PV would run during the day, charging the tanks, ready to meet demand loads at night. Apart from the risk of explosion (!), compressed air seems to be quite an efficient energy resevoir system, and would be much lighter than a thermal store.  I realise the advantage you have is more constant pressure with the bag idea, but a large enough resevoir would mitigate that problem I'd think.  How efficient could a Tesla turbine generator be I wonder?

[Heinz]

Why use PV to run the compressor ? Too much energy lost in the system. Better to use a wind turbine direct driving a compressor pump ?? I have been scribbling such a devise to supply my workshop with air, I'll probably never get around to making it though...

Heinz

[derekmt]

I don't think there is anything on that list that isn't already possible. I think practicality and convenience are big factors, plus an element of "it's not what we do" and the "WTF" moments when dealing with traditional trades.

So, taking your LED lighting, it's perfectly doable but brings a load of problems - nothing insurmountable, just a new pile of issues to consider. The first is that once you go to a low voltage circuit for the lights, you are then committed to only using (for example) 12V DC lighting - with a corresponding restriction in the fittings available to you (many luminaires using LEDs still expect 240V AC). Volt drops become a serious issue as well - losing 3V at 240V is insignificant, losing 3V at 12V is very significant and not hard to do if you don't use thick cabling.
So on one hand, yes it can be done. On the other, the savings vs running an inverter and a more traditional 240V AC system aren't likely to outweigh the benefits - especially considering all the other stuff you'll probably end up running an inverter for.

...

0.5 mm2  (the smallest reasonable size cable ) feeding a 9W led light,  what voltage drop are you expectng over 10m?
 the IEE tables say 0.5v
I dont think you would wire up a ring main for 12V

[SimonHobson]

0.5 mm2  (the smallest reasonable size cable ) feeding a 9W led light,  what voltage drop are you expectng over 10m?
 the IEE tables say 0.5v
I dont think you would wire up a ring main for 12V

1/2 a volt - doesn't sound a lot does it ?

I'll put that in perspective. I put in some LED strip lights at work (the peel and stick flexible strip type), which has multiple sections, each with 3xLED chips plus a resistor. While playing, I determined that the voltage required to turn the LEDs on "at all" was something like 7V. So that means (neglecting the increase of forward voltage with current) something like 5V across the resistor.

1/2 a volt is 10% of 5V. In practice, the forward voltage of the 3 LEDs is more than 7V when fully lit (so less than 5V across the resistor).
So your 1/2 a volt drop represents more than a 10% drop in LED brightness.

And then someone switches on a light in the next room, so double the volt drop in at least part of your run ...

It can be dealt with. You'd want to use regulated LEDs and start with a higher voltage, or use "proper" drivers.

If the complete assembly (LED + driver) is well designed, it will drive the LED in current control mode and unless you drop below it's minimum supply voltage then the LED brightness will be constant. However, "proper" drivers soon push the cost up - and further restrict your choice of fittings.

At the cheaper end of options, many LEDs (whether packages or "sticky strips") include (I assume) a low drop out voltage regulator. Above a set voltage, the regulator takes over and limits voltage to the LED/resistor string. Below that, the output will be voltage dependent.

Of course, while you are charging, your batteries will be up at 13.something volts. Depending on capacity, weather, amount of use, etc, then it will drop somewhat lower during use.


Again, nothing that can't be dealt with - it just needs careful consideration up front and selection of the right kit. Whether the cost is justifiable is another matter - only you can decide that.
Unless you really expect to be living there for "a long time", then I'd suggest at least wiring the 12V circuit to the same standards as you would if using 240V AC. That way, it would be much easier to swap out all the lights and convert to something saleable.

[dhaslam]

Interesting -- I thought at first you wouldn't be gaining your Potential Energy, and then realised you are actually compressing Air.  Which made me think - ok, modify the proposal to run a small compressor via PV, into a large resevoir tank or tanks (eg cheap empty 75kg Propane cylinders on Ebay).  PV would run during the day, charging the tanks, ready to meet demand loads at night. Apart from the risk of explosion (!), compressed air seems to be quite an efficient energy resevoir system, and would be much lighter than a thermal store.  I realise the advantage you have is more constant pressure with the bag idea, but a large enough resevoir would mitigate that problem I'd think.  How efficient could a Tesla turbine generator be I wonder?

[/color]

I think they are very efficient under ideal conditions,  they have quite low torque so need to be matched to a high speed generator with low starting resistence.

This is a small one that reaches  25000 rpm and produces 60 watts.  They were sold for a while as a complete unit.
http://www.youtube.com/watch?v=fYhADZHXvnQ

I am not sure whether a compressor can be driven directly off a wind turbine.   It possible  would make an interesting combination with a couple of air bags and a pond.  There might be a miss match with the pressure required for the generator.  No batteries, just a voltage regulator and a valve on the air supply for control.

[Simes123]

I couldn't do a wind turbine unfortunately - within a small town, and although the property is large, the plot doesn't offer much garden.  I think there is more likelihood of wind than Sun given it is in Scotland   It might be better to store in batteries from PV to minimise conversion losses, but I like the idea of unlimited cycling with air storage.

[derekmt]

Unless you really expect to be living there for "a long time", then I'd suggest at least wiring the 12V circuit to the same standards as you would if using 240V AC. That way, it would be much easier to swap out all the lights and convert to something saleable.

if by insulation yes
if by conductors size no! you wire it up better than the usually cheapskate single  ring 1.0 or 1.5 mm sq to do an entire floor.  but more than 0.75mm2 per luminare at 12v is ott and at 240v is north of 1kw

[SimonHobson]

Actually I do mean use 1mm² cable of the proper design (typically PVC T&E). Cable CSA is about more than just current carrying capacity - as I've been trying to explain here. Volt drop is as important, and in many installation will dictate a larger CSA than the current carrying capacity requires. Also, lighting circuits don't use rings. The only situation where rings are used is a power circuit where 2.5mm² cable can be used in a ring configuration when connecting only BS<something> accessories (ie 13A sockets, fused connection units, and the like).

That way, if the idea doesn't work out or you need to move, you can swap out the fittings, stick the supply end into the CU, and easily convert to mains lighting without a rewire. The cost difference in cable will be negligible when you consider what you'll be paying for the lights and far less than the future cost of having to rewire.