Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

Buy individually or both books together. Delivery is free!


widget @ surfing-waves.com




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013
     
    Now that the cost of PV is coming down (~50p/watt) and the FIT is not as attractive as it was and I am not MCS approved, I have coonected 1kW of PV direct to my 3kW 240VAC immersion heater, that is approx 120V DC.
    Yes it works but its effectiveness I assume is governed by the I*I*t relationship (Joules Law?), bearing this in mind I am assuming either a MPPT or switchable 120V battery bank between the PV and immersion would help, to ensure max current is going to the immersion in bursts rather than low current trickle charge throughout the day?
    I am new to the forum, I have looked but cannot find this in any previous threads.
    Thanks
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013
     
    I agree, the drop in current causing a corresponding drop in voltage results in very inefficient use of available power in less-than-fully bright conditions. A post on my blog on the subject:

    http://edavies.me.uk/2012/11/pv-immersion-gotchas/

    Sorry if it's grandmothers and eggs but do note the issue of using a thermostat designed for AC with the DC output of a PV panel. Could result in a nasty accident.
    • CommentAuthorSprocket
    • CommentTimeFeb 23rd 2013
     
    I don't agree.
    PV cell is (mostly) just a current source with a maximum voltage.

    Joules law just IS the amount of power in that current. There is no way to maximise it.
    The voltage will depend mostly on the resistance of your heating element.
    Power out of panel = power into heating element. It has nowhere else to go.

    Whilst there is an optimal peak point to do with internal losses, as long as the heating element resistance is selected such that the voltage across it at your current does not exceed the nominal cell voltage you should get pretty close to maximum power transfer.

    Once that voltage drop reaches you PV max output voltage then you cannot transfer any more power.
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013
     
    Posted By: SprocketWhilst there is an optimal peak point to do with internal losses, as long as the heating element resistance is selected such that the voltage across it at your current does not exceed the nominal cell voltage you should get pretty close to maximum power transfer.

    Indeed. But if you pick the resistance to be good for one level of sunlight it will be a very bad choice for significantly different levels.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013
     
    I agree Ed, and I concor with your blog and as you state the logistics of multiple elements becomes difficult. Have you considered a small battery bank 15/20 Ahr but 100V or so with a simple voltage controller to switch a relay to enable the immersion to be run when the battery is full down to say 40% dod, then switch off and allow the PV to top it up again. This will crudely keep the PV operating at peak power even at low light levels and would be cheaper than a Power Point tracking inverter?
    • CommentAuthorSprocket
    • CommentTimeFeb 23rd 2013 edited
     
    > But if you pick the resistance to be good for one level of sunlight
    > it will be a very bad choice for significantly different levels

    OK, maybe, if the internal resistance of the PV cells is high enough, but can you explain a bit more what you mean? What are these losses for typical useful irradiance levels?

    I can see how if you choose for something like 400W and then generate 500W the extra power is lost because the panel reaches maximum voltage and the resistance of the heater will then limit the current to below the maximum.
    But I can't see how if you choose for maximum output you will lose much efficiency when the irradiance is lower. The mismatch will mean more a bit more heat lost in the panel's internal resistance but not all that much surely? Sure, if light levels are really low, but do we really care about that? I would quite like to see numbers for just how many watts in what circumstances.

    Charging and discharging batteries will be horribly inefficent. Even ignoring charger losses, if you put 100Wh into any battery, you certainly don't get 100Wh out. Not to mention the cost and deterioration of the batteries with use.

    If you really want to match the panels to the heater for that extra bit of efficiency at low light levels surely it's just a matter of a few relays and a simple controller to adjust number of elements in series or parallel. There are no significant losses in the relays so that would be close to peak possible performance. This is way simpler electronics than a typical PV inverter.
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013 edited
     
    Posted By: SprocketBut I can't see how if you choose for maximum output you will lose much efficiency when the irradiance is lower. The mismatch will mean more a bit more heat lost in the panel's internal resistance but not all that much surely?

    All that much, surely.

    Sure, if light levels are really low, but do we really care about that? I would quite like to see numbers for just how many watts in what circumstances.

    My blog post goes through the numbers for the case where the immersion is matched to the panels in bright sunlight but the actual sunlight gives about 50% available power resulting in 25% power into the water. Is there something wrong with that example?

    I agree, though, that batteries are not a good idea. The losses would probably be tolerable (i.e., you'd win overall) but the degradation of the cells would be a problem. On the other hand, it might not be a bad use for cells which are pretty much at the end of their life anyway. Even then, 40% DOD is probably unnecessarily punishing.

    Just using ordinary capacitors might work. The Centre for Alternative Technology has (or had, it's a while since I went) a nice demo of some PV panels running a pump. They had a couple of big electrolytics charged by a PV panel with a bit of electronics to turn a pump on when they were fully charged and off when the voltage got a bit low. Nice in that example as the high voltage and current capability of the fully charged capacitor can deal with the start current required while allowing the pump to run efficiently on the lower voltage as the cap discharges.

    Same sort of thing with a power FET circuit to switch the immersion reasonably quickly might be feasible. I haven't done any calculations on the capacitor sizes, etc, though.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013
     
    The resistance of a 3kW immersion is about 20 Ohms, so at say 120V it will draw 6A which the 1kW (4x 250W 30Vdc panels in series) can just about do from 70-100% sunlight reasonably efficiently.
    When the light drops, the current will drop so say at 3A (38% sun, assume max current approx 8A) the immersion will run the panels at only 60VDC (pulling only 180W from the PV array losing a massive 200W! (38% of 1kW = 380W)...lower levels are even worse.

    Yes I agree switching the panels from 4x1 in series to 2x2 and 1x4 is possible, but would requires quite sophisticated electronics to control and would happen frequently, so would prefer to use transistor rather than relay switching, requiring some serious design, heatsinking, weatherproofing, mounting etc..
    I accept that batteries wont be that efficient but its got to be better than doing nothing! When the light level rises the batteries wont be required so I guess a bypass could easily be designed for this.
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013
     
    Posted By: welshjim99Yes I agree switching the panels from 4x1 in series to 2x2 and 1x4 is possible
    Interesting idea. Both my blog post and Sprocket's comment assume switching the elements rather than the panels. I think the elements would be easier to deal with. You could do series/parallel switching with the elements but I think it'd be easier just to switch separate elements on and off. The elements don't have to all be the same resistance making different combinations more accessible.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013 edited
     
    Not sure what you mean by 'elements', do you mean the strings of cells within the panels themselves? these are not normally accessable, or do you mean the individual cells?
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013 edited
     
    No, the immersion elements. Assumes more than one which might not be practical for an existing tank though I think you can get multi-element heaters to go into a normal immersion boss. E.g., 1 kW and 2 kW instead of a single 3 kW element.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013
     
    The capacitor idea was my orginal thought but unfortunately the equation CV=It, so C=It/V, means a capacitor of thousands of Farads which i dont think exist just yet!

    Yup.. got it re Heating Elements, but again quite sophisticated switching electronics and only 2 different elemnts in one boss doesnt give much room for power matching. Probably better matching could be achieved switching the PV panels
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013
     
    You can get 60 µF, 250 V DC capacitors at sensible prices. £13.13.

    http://www.digikey.com/product-detail/en/C4ATDBW5600A30J/399-6223-ND/2783185

    With 10 A from the panels (1 kW at 100 V) that would give a voltage slew rate of 166×10³ V/s. Say you operate the capacitor over the range 90 V to 100 V (i.e., turn the immersion on when the voltage is 100 V and turn it off again when the voltage drops to 90 V) then you'd be switching 16'000 times per second. Definitely not a job for a relay but quite feasible electronically though you'd probably have to have some ferrites around to avoid radiating too much hash.

    Of course, at peak current you wouldn't be switching, it's when the sun's less bright so the panels are producing less current that you need to switch so the actual in-use frequencies would be lower.
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013 edited
     
    Posted By: welshjim99...and only 2 different elemnts in one boss doesnt give much room for power matching

    Yes, but you don't need much more. For your 1 kW example suppose you had a 600 W element and 2 x 200 W then you could have 200, 400, 600, 800 or 1000 W easily. That should match pretty well. Three relays driven via transistors from an Arduino or something which can measure the panel voltage. So long as the software doesn't keep clicking the relays all the time that should be fine, I'd think.

    [Edit: better 150, 300 and 600 W elements allowing 150, 300, 450, 600, 750, 900 or 1050 W. ]
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 23rd 2013
     
    Here is the power distribution of the solar resource down my way. If you can find out the cell resistance and voltages at the different input levels you can work out the optimal resistance for the element.
    Over half the time it is dark, and below 50 W/m^2 for 25% of the time that it is light.
    Amazing anything comes out of it at all really.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 23rd 2013
     
    Posted By: Ed DaviesSo long as the software doesn't keep clicking the relays all the time that should be fine, I'd think.
    Just put a cycling delay on it, say 5 minutes between samples regardless of what is happening.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 23rd 2013
     
    Ed I like the different elements idea, I can see that as a working solution, but I'm being a bit thick.. isnt "150, 300 and 600 W elements " 3 elements in one boss? even 2 would be handy though, are you aware of companies that do custom element mixes.

    My only other concern with playing with the immersion was that if I ever need the dreaded mains then I can always switch 240V AC on to it.
    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2013 edited
     
    Maybe these people:

    http://tpfay.co.uk/

    I think I got that reference from Paul Camilli (http://lifeattheendoftheroad.wordpress.com/) who I also think has had “funny” elements made up by them.

    If you had three elements designed to be 500, 1000 and 2000 W at 240 V you could connect them up fine (in parallel) for 240 V operation. At 120 V DC they'd be 125, 250 and 500 W giving the options of 125, 250, 375, 500, 625, 750 or 875 W with spare power capacity to run the PV at a bit more than Vmpp in bright sunshine.

    [Edit: should really only use the 1000 and 2000 W elements at 240 V.]
    • CommentAuthorSprocket
    • CommentTimeFeb 24th 2013
     
    (I'm lagging behind a bit here)
    Doh! Of course. Thanks Ed & Jim for the patient explanations.

    I notice the standard 3phase immersions at tpfay are in star connection at the lower power levels.
    Even one of those with three identical elements could still get you four power levels if one config puts two elements in series.

    2 in series, or 1,2,or 3 in parallel.
    Can't do 3 in series unfortunately because of the common connection of course.

    But the 500W+1000W+2000W setup for 7 power levels plus easily switched to mains use does sound really tidy.
    • CommentAuthorwelshjim99
    • CommentTimeFeb 24th 2013
     
    Thanks Ed & Sprocket, had some sun today at last and read a massive 6.37A through the immersion, rathar than the 0-1A whislt cloudy confirming my suspicions.
    I am endeavouring to log all data; current, voltage and sunlevels from a hand made reference cell and of course water temperature, and will happily present the findings, with of course an automated immersion switching circuit, probably using immersion voltage as the reference source.
    I will contact tpfay and see what they can make for me...its a shame I've got to go to work tomorrow!
    Thanks again...
    • CommentAuthorjohnuready
    • CommentTimeJul 30th 2015
     
    welshjim99

    How did you get on with the project? were are you with it?

    john
    • CommentAuthorEd Davies
    • CommentTimeJul 30th 2015
     
    Welshjim99 hasn't logged on to the forum since this conversation. He might still be watching but not logging on but I rather doubt it. Pity, as it would be nice to know the ending.

    Had a brief exchange of email the other week with somebody (British, I assume from his name) in Spain wanting to do something similar and having read my blog post referenced above. His problem was that the immersion in question had a very small boss (1.25", I think) so the option of multiple immersions actually in the tank wasn't there. I did suggest immersions in a side-arm heat exchanger. I'll hope the hear what he comes up with.
  1.  
    Posted By: Ed DaviesI did suggest immersions in a side-arm heat exchanger.


    Is that like the Willis or Geeza type?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 31st 2015
     
    Sounds more like Smith and Weston
    • CommentAuthorEd Davies
    • CommentTimeJul 31st 2015
     
    Not familiar with the Geeza type but, yes, like the Willis thermosiphon heat exchangers for solar thermal but getting the heat from electricity.

    The bit I'm not clear about is that if you plumb in to the feed and outlets of the tank how you make sure that there's enough resistance in the side-arm path to prevent a “short-circuit” when water is drawn off but keep the resistance low enough to ensure the thermosiphon works well enough. I guess the resistance of the tank is low enough that this really isn't a problem but I'm not sure. Worse case, put some restriction in the side-arm circuit and pump it when required but I don't think that's necessary. E.g., Wookey's system seems to work:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2563
    •  
      CommentAuthordjh
    • CommentTimeJul 31st 2015
     
    Posted By: Ed DaviesThe bit I'm not clear about is that if you plumb in to the feed and outlets of the tank how you make sure that there's enough resistance in the side-arm path to prevent a “short-circuit” when water is drawn off but keep the resistance low enough to ensure the thermosiphon works well enough.

    Good question. I suppose that (a) with a PHE, the heat exchanger itself has a significant resistance and (b) the side arm is probably plumbed with a smaller diameter pipe than the main feed and is certainly much longer than the feed length into the top of the tank. So there's much less resistance for water coming from the top of the tank and the vast majority will come from there. Hand wave, hand wave and no numbers, sorry!
    • CommentAuthoralistairm
    • CommentTimeJul 31st 2015
     
    I havent read over all the posts as I dont have time just now.

    there is a system called EMMA which transfers power to emmersion heater when there is no demand from power outlkets. Not sure if it would work on off grid system but nice piece of kit all the same

    http://coolpowerproducts.com/
    • CommentAuthorskyewright
    • CommentTimeJul 31st 2015
     
    Posted By: alistairmthere is a system called EMMA which transfers power to emmersion heater when there is no demand from power outlkets. Not sure if it would work on off grid system but nice piece of kit all the same

    Nice kit yes, but have you seen the price of the EMMA (then added in the price of an inverter to convert the DC from the panels to the AC that EMMA works with)?
    Even if possible it's unlikely to be an economic solution for a simple case.

    I have heard of people using much cheaper immersion diverters with off-grid systems that are AC coupled (e.g. Sunny Island), but that's a long way from "direct connection to an immersion"...
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 31st 2015
     
    Our old mate JSH has details of his power diverter over on the other place.
    • CommentAuthorEd Davies
    • CommentTimeJul 31st 2015
     
    But the point here is just using the PV to directly heat the water as a substitute for solar thermal (but with less maintenance). Cheaply.
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press