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Trying to figure something out PV Solar and Hot Water

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  • 19-07-2019 5:40pm
    #1
    dingding
    Registered Users Posts: 4,931 ✭✭✭


    Hi

    It has been over 30 years since I studied electrical theory and something does not make sense so I am not sure what I am missing.

    I was looking at PV cells with the diverter which heats the hot water.

    https://www.amazon.co.uk/Solar-iBoost-water-Immersion-controller/dp/B00HZ4GZBW

    This appears to have a CT that you install between the meter and the consumer unit to measure whether electricity is being imported or exported. If the electricity is being exported then it heats the hot water.

    A couple of things.

    1. As the current is AC, then it will need to reference with the AC supply so that it can decide the direction of current flow. How does this work.

    2. AS it just switches on the hot water, it will need to have a certain amount of electricity exporting to make sense. If it is a 3KW element, anything less than 3KW generated will mean that the shortfall is made up by the mains. In this case unless you have a large PV array then you are still paying a percentage of the cost to heat the water and it might be more efficient to heat on the night rate without the solar.

    Thinking of putting in a PV system any help on the above questions appriciated.


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    #2
    pale rider
    Registered Users Posts: 264 ✭✭pale rider


    Technical answer required which I cannot provide but I do have one of these, there appears to be a slight draw from the grid since my install for reasons that are unclear to me, these past few days do not deliver hot water in my tank, warm yes but a boost necessary to have a shower, the system does not delver full power to your immersion just surplus, I have seen as little as 200 watts going into the unit which cycles on and off and does not do much t get heat into the cylinder, a hot sunny day will be different.


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    #3
    dingding
    Registered Users Posts: 4,931 ✭✭✭dingding


    Hi Pale Rider, do you find the system good, how many panels do you have, Cost etc.... Any info appriciated,

    Cheers


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    #4
    pale rider
    Registered Users Posts: 264 ✭✭pale rider


    Too soon to say and weather is not great but today is first day we went to grid in almost a week so that is a positive, 14 panels, 4.5 kW, 6kw battery and diverter which thus far is not impressive but will be on better days Im sure,


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    #5
    TheChizler
    Registered Users Posts: 9,426 ✭✭✭TheChizler


    On the first question, current clamps are directional, in that the current flowing in the wire will induce a voltage across the clamp terminals that is proportional to the current, with the polarity positive when current is flowing in one direction and negative in the other. The flow direction will change 50 times a second as you know. To calculate the power you need to measure the voltage as well which also inverts polarity 50 times a second.

    Since power is voltage across times current through a load and if the voltage and current are in phase, i.e. positive at the same time your instantaneous value for power will be positive so you could say power was being consumed by the load. If voltage and current are 180 degrees out of phase then your power value will be negative and you could say power is being generated by the 'load'.

    There are some nice pictures here demonstrating the concept. https://www.digital-toolbelt.com/arrow-on-current-clamp/

    On the second question, typically diverters limit the average current in the water heating element in a way that only excess power is used, its not just 3kW on or off.


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    #6
    dingding
    Registered Users Posts: 4,931 ✭✭✭dingding


    Thanks for this. The current clamp had a directional arrow and while it is wireless, it must reference the voltage cycle at the diverter.

    Also that the diverter limits the current is interesting also.


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    #7
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    dingding wrote: »
    Hi

    It has been over 30 years since I studied electrical theory and something does not make sense so I am not sure what I am missing.

    I was looking at PV cells with the diverter which heats the hot water.

    https://www.amazon.co.uk/Solar-iBoost-water-Immersion-controller/dp/B00HZ4GZBW

    This appears to have a CT that you install between the meter and the consumer unit to measure whether electricity is being imported or exported. If the electricity is being exported then it heats the hot water.

    A couple of things.

    1. As the current is AC, then it will need to reference with the AC supply so that it can decide the direction of current flow. How does this work.

    2. AS it just switches on the hot water, it will need to have a certain amount of electricity exporting to make sense. If it is a 3KW element, anything less than 3KW generated will mean that the shortfall is made up by the mains. In this case unless you have a large PV array then you are still paying a percentage of the cost to heat the water and it might be more efficient to heat on the night rate without the solar.

    Thinking of putting in a PV system any help on the above questions appriciated.

    If you have a home generation setup, and your current load is 1.5kw, and you generate 2kw, it is not simply that the 2kw you are generating is going to your appliances. After all, you might have a 100w lamp in a shed, and a neightbours loads might be nearer. It is a solid copper wire to their loads as well, at least on the same phase.

    But what does happen is, you generate 2kw, so if your 2kw just happens to go straight to your connected loads, 500w goes out back through the meter.

    Now if your 2kw only supplies 1kw of your loads, then 1kw is out through the meter, but 500w is in from network to supply the 500w your generator is not "seeing". So no matter how your generated power divides up, the surplus at the meter will be 500 watts.

    It is a bit simplistic looking at it like that. But the reality is, your home generation supplies the grid. It doesnt just "see" the loads on your side of the meter.

    Your meter measures the out v the in, in whatever way it divides up at that point. It doesnt just supply your loads, then start flowing out if its a surplus.

    So if you generate 1kw, you are 500w short, whether your generator divides up in all directions, your meter will see the in v out, again, in a simplistic view of it.


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    #8
    dingding
    Registered Users Posts: 4,931 ✭✭✭dingding


    Thanks Bruthal,

    I had not considered that scanario.


    Just remainds me of some of the complicated kirchofs laws scanarios from senior trades. The meeter used to have a ratchet and pawl in it so it would not rotate backeards if the current flow was reversed.



    i suppose with the PV system any power that leaves your system wont be captured by your meter, how ever when your power goes into a neighbour the supply company get paid for it.

    I suppose there is no gurantee that your generated power will be used in your premesis. I suppose the trick is your generated voltage v's the grid voltage.


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    #9
    TheChizler
    Registered Users Posts: 9,426 ✭✭✭TheChizler


    dingding wrote: »
    I suppose there is no gurantee that your generated power will be used in your premesis. I suppose the trick is your generated voltage v's the grid voltage.
    At the end of the day it doesn't matter where the actual electrons end up or came from, what matters is what's measured at the meter and if you're generating more than what you're using your bill will be 0 for that period.


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