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Renewable Energy: Solar PV - water heating & monitoring questions for a new install

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  1.  
    We are slowly finalising the solar specification for our new build and I have a few questions the forum can help with:
    1. We will be at work during the day and therefore (hopefully) generating excess electricity. I'm assuming one of the best things to do is to use the excess electricity to heat our hot water? We are currently planning to have a 250l Megaflo Eco Indirect tank.

    2. Our installers are recommending www.powerdiverter.com as the kit to heat our hot water. Has anyone used one of these before or can recommend an alternative?

    3. We are going to have an east / west install split 2.25kwh and 1.75kwh. They are recommending either SMA or Solarworld Sunplug inverters. I'm not sure if the forum has any recommendations on these two. I'm assuming I'll need two inverters, one for each aspect based on comments I've previously read.

    4. Finally, I'd like to monitor my PV generation. For Code for Sustainable Homes points, I also need to monitor my consumption of mains electricity. I haven't been able to find a device which does both. Is anyone aware of one? I have read (on the Owl website) that the diverters used for water heating can affect the accuracy of the monitors as well. No real point getting one if they aren't going to be accurate.

    Thanks for any thoughts
    • CommentAuthorEd Davies
    • CommentTimeAug 1st 2015
     
    3. Not necessarily two inverters - often inverters can take two separate strings of panels and track each independently.

    By the way, it'll be 2.25 kW and 1.75 kW, not “kwh”. Sorry, I don't have anything useful to say on your other questions.
  3.  
    Thank you for the answer on the inverter and the correction to my units! ;-)
    • CommentAuthorgravelld
    • CommentTimeAug 1st 2015
     
    1) Yes, make sure immersion is at the bottom of the tank.

    2 and 4) Optimmersion and Wattson work well together. There are plenty of other solutions.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 1st 2015
     
    I would think that an SMA 4 kW inverter (not sure of the number now, used to be TL4000), it has built in monitoring via Bluetooth, two separate Maximum Power Point Trackers, is reliable.
    Just get the inverter put somewhere cool (though I think they have stopped putting them in lofts now).

    I don't know enough about power diverters to comments. Others have made there own with varying success.

    There are many systems to monitor your generation and usage. I have my favourite, but there is the Open Monitoring lot (our old mate Damon HD).
    •  
      CommentAuthordjh
    • CommentTimeAug 3rd 2015
     
    Posted By: richardelliotI have read (on the Owl website) that the diverters used for water heating can affect the accuracy of the monitors as well. No real point getting one if they aren't going to be accurate.

    There are various different ways in which the diverters work. Some ways might affect monitors and might cause dimmable LEDs to flicker, for example. I don't know anything about the Powerdiverter so I can't advise, but do make sure that your installers will help you sort out any such issues (or provide an effective guarantee that they will not happen). Perhaps they'll be able to supply the monitoring system that you need as well?
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2015
     
    The SMA has a relay built in that can be set to divert power I think.

    http://www.alternergy.co.uk/store/sma-sb-4000tl-21-transformerless-inverters.html
    •  
      CommentAuthordjh
    • CommentTimeAug 3rd 2015
     
    Posted By: SteamyTeaThe SMA has a relay built in that can be set to divert power I think.

    The relay seems to be an option. It doesn't seem to offer diversion of all excess power, only power above a configured threshold.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2015
     
    I am not too sure of all the details, seem to remember someone saying that it can be activated once the modules are delivering an amp, so that would be somewhere around 100W. Most houses have close to that as a baseload, so not really loosing anything if that is the case.
    •  
      CommentAuthordjh
    • CommentTimeAug 3rd 2015
     
    Posted By: SteamyTeaI am not too sure of all the details, seem to remember someone saying that it can be activated once the modules are delivering an amp, so that would be somewhere around 100W. Most houses have close to that as a baseload, so not really loosing anything if that is the case.

    What a diverter does is divert all available power over the baseload (or whatever is being used instantaneouly elsewhere in the house) to the diversion load. As long as the load is something like a resistive heater, it just soaks up the power until its nominal rating. So a typical 3 kW immersion absorbs anywhere from 1 W to 3000 W depending on how much is available.

    A relay that closes as soon as 100 W is available will simply switch 3 kW to the load.
  11.  
    We have now been in our house a month and have the power diverter (an iboost) connected to the bottom immersion of our 250l megaflow tank.

    One thing I'm not sure how to measure is whether the power diverter is sufficient to meet our hot water needs. We also have the gas boiler on and set to heat the hot water. I'm a bit too scared to turn of the boiler in case I get in trouble with the family for cold showers.

    Are any other forum members using power diverters to help heat their hot water? Is there a way to tell if the solar PV diversion is enough? Is there a recommended setting to have your boiler on as a 'top up'?
  12.  
    Hi Richard,

    probably not of great use, I'm 'off grid' and totally electric but it may help. I've a large thermal store and draw DHW from the top of the tank which has a regular 3kW immersion in wired through a stat. In the summer I turn the stat right down to 60 degrees (usually it's at it's highest setting) which is just hot enough for a shower. The tank has several temp sensors in it, all of which are visible from the living area. You soon get a 'feel' for what's going into the tank from PV or solar HW. My finding so far with 4.75 kW PV and 60 evacuated tubes is that the regular immersion does very little in the summer. Mind you, my store is 1500lt and my teenage son 'boards' during the week.

    The rest of the tank is heated by various coils from heat pump, solar tubes and SSR controlled immersion elements (like your power diverter), seems to work better than I expected :-)
    • CommentAuthorDandJ
    • CommentTimeAug 1st 2016 edited
     
    <blockquote><cite>Posted By: richardelliot</cite>We have now been in our house a month and have the power diverter (an iboost) connected to the bottom immersion of our 250l megaflow tank.

    One thing I'm not sure how to measure is whether the power diverter is sufficient to meet our hot water needs. We also have the gas boiler on and set to heat the hot water. I'm a bit too scared to turn of the boiler in case I get in trouble with the family for cold showers.

    Are any other forum members using power diverters to help heat their hot water? Is there a way to tell if the solar PV diversion is enough? Is there a recommended setting to have your boiler on as a 'top up'?</blockquote>


    We have an iBoost also but have turned off every other way of heating the water in order to check if the PV can satisfy our hot water demand. So far (since around April/May) it has been fine. Not sure at what point in the autumn we will change the setup.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 1st 2016 edited
     
    Posted By: richardelliotAre any other forum members using power diverters to help heat their hot water? Is there a way to tell if the solar PV diversion is enough? Is there a recommended setting to have your boiler on as a 'top up'?
    Can you set up your system on the timers i.e. gas only during hours of darkness, so 10PM to 4 AM and immersion on only during hours of peak solar, say 9AM to 4PM in summer.
    You may have to change the times you use the hot water as you need to start the day with a cold store, not a hot one.
  15.  
    Posted By: richardelliotI'm a bit too scared to turn of the boiler in case I get in trouble with the family for cold showers.

    Go for it ! In any event it will only be a couple of days of flak, soon forgotten, and justify it as teething issues understanding the system.:devil:
    •  
      CommentAuthordjh
    • CommentTimeAug 1st 2016
     
    Posted By: richardelliotOne thing I'm not sure how to measure is whether the power diverter is sufficient to meet our hot water needs.

    I measure simply by touching various points on my tank to see how hot it is and where. I have a bunch of 1-wire temperature sensors that I will install sometime. I also measure by reading the display on my diverter, which tells me how much energy it has put in the tank each day. That's usually enough to see whether the showers will be hot enough. It's a soft failure in any case though. The showers don't go cold, they just start not being as hot as they should be, at which point I turn on the immersion boost.

    The thermostat fed by my diverter is set to a higher temperature setting than the one used in winter on an E7 timeswitch. So if the water is hot enough from solar diversion, no mains electricity (or gas in your case) is used. If it's not hot enough, then mains electric or gas automatically tops it up. Note that the timeswitch ensures that the mains heating is only ever used overnight.
    • CommentAuthorgravelld
    • CommentTimeAug 1st 2016
     
    Posted By: Peter_in_Hungary
    Posted By: richardelliotI'm a bit too scared to turn of the boiler in case I get in trouble with the family for cold showers.

    Go for it ! In any event it will only be a couple of days of flak, soon forgotten, and justify it as teething issues understanding the system.http:///newforum/extensions/Vanillacons/smilies/standard/devil.gif" alt=":devil:" title=":devil:" >
    Yes, I've ridden this one out too, YMMV :D
  18.  
    Thanks for all the responses, it's reassuring to know that we should be able to rely on PV only for a good portion of the year. I might risk turning off the boiler at the weekend (I think I'd get in too much trouble on a work day).

    We shower in the morning and that is the main hot water use of the day, the only other real use if washing up dishes in the evening. As an initial step I will look to change the timer on the boiler so that it only comes on during the night for a couple of hours and will then experiment with turning off the boiler completely.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 1st 2016
     
    Set the gas thermostat to the lowest temperature you can get away with, and then only have it come on for an hour or two, just before dawn.
    That way you can reduce standing losses.
    • CommentAuthorjamesingram
    • CommentTimeAug 1st 2016 edited
     
    Not read all the replies , if new install
    Id get a solaredge dumb inverter and optimisers .
    Let's all panels function individually.
    HW dump I'd go iboost+ with 2 dump circuits it'll let you feed 2 immersions . Get a cylinder with 2 top/bottom
    Then wire top first for some winter HW at usable temperature.
    Iboost+ comes with add on a wireless buddy that shows available exported energy for use.
    • CommentAuthorringi
    • CommentTimeAug 1st 2016 edited
     
    Posted By: richardelliotWe shower in the morning and that is the main hot water use of the day


    That makes it very easy....

    (I am assuming you have the correct safety mixer to the output of the DHW tank)

    Set the electrical immersion thermostat to 80c, the tank thermostat that controls the gas boiler to 60c and the boiler timeswitch so that the gas boiler only heats the tank after the sun has set (say 4am).

    A simple hour’s run meter connected so that it is only powered when the boiler is heating the DHW will tell you how much the boiler is having to run.

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