Wanted: 750W solid-state water heater for PV

Discussion in 'The Projects Forum' started by calypso_rae, Apr 16, 2012.

  1. calypso_rae

    Thread Starter New Member

    Apr 15, 2012
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    On a good day, our PV panels generate around 10kWh of electrical energy, enough for several hot baths. Sadly, most of that free energy disappears away into the grid and our DHW remains cold. I need a way of routing 'my' precious electricity for my own purposes rather than someone else’s.

    Switching on the immersion heater during the day is no use because PV-generation is so variable. Power can drop ten-fold in just a few seconds as dense clouds go by. Instead, the load needs to be adjusted dynamically to match the surplus power. The monitoring aspect is well covered at http://openenergymonitor.org

    Solid-state controllers are frequently used to reduce the power that is consumed by an appliance by rapidly interrupting the supply. That’s fine when power can only flow in one direction. But when the sun comes out, our mains current can flow in either direction. When power is being drawn from the grid, even momentarily, our energy use is precisely metered and we are charged for it. But for any periods when we generate more that we consume, the surplus energy just disappears off into the grid without even a Thank You. As Baloo might well have observed, " It's gone man, Solid Gone!"

    We therefore need a continuously variable 'electronic load' which has similar VI characteristics to a resistive load of the appropriate value. When there is 3kW to spare, it needs to emulate a 20 Ohm resistor; and at 1 kW, 60 Ohms. By putting the electronic load in series with a standard 3kW immersion heater, any rating from 0 – 3kW can be accommodated. Peak power at the electronic load will be at the half-power point when each load is dissipating 750W.

    When operated in its linear mode, a solid-state device dissipates the same amount of energy as if it were a resistor with a similar voltage drop across its terminals. To carry heat energy away from an electronic component, you need a heat-sink, and water-cooled ones work best. An array of solid-state devices operating in linear mode with a large DHW-cooled heat-sink should do the trick.

    Is anyone else thinking along these lines? There must be thousands of PV owners out there who would just love to have a system like this …
     
  2. wayneh

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    Sep 9, 2010
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    So you don't want to EVER send power to the grid if you can avoid it? I thought the whole point of the expensive grid tie-in equipment was to sell your excess energy. Pay for natural gas to heat your water and pocket the difference in energy cost.

    Anyway, I think what you want is a traditional immersion (resistive) heater under PWM control via a MOSFET, or several in parallel. A resistive heater isn't too picky about voltage and current as long as you don't exceed its ratings. The control system needs some thought but shouldn't be too hard, and will be as fast as you need. It'll be at the logic level, meaning low power until you hit the MOSFETs.
     
  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I'm building a small scale hot water pre-heater using excess solar power. Using a simple uC pwm module and a FET driver for a 300W 12vdc load.

    Just testing it now with a few old projector bulbs to simulate the DC load.
    http://flic.kr/p/bNaCvT

    FET load driver:
    http://farm8.staticflickr.com/7205/7086005545_cebd99299c_z_d.jpg
    ILQ2 opto-isolator for the PWM signal from the controller:
    http://farm6.staticflickr.com/5316/6939933018_36fe6c5bf7_z_d.jpg

    DC heater elements for tanks:
    http://www.mwands.com/index.php?main_page=index&cPath=40
     
  4. calypso_rae

    Thread Starter New Member

    Apr 15, 2012
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    @wayneh I thought the whole point of the expensive grid tie-in equipment was to sell your excess energy.

    Yes, in part, but the term "Feed-in Tarrif" is very misleading. PV users generally get paid the same rate regardless of whether their generated power is used on-site or exported. So when you have both (free) surplus power available and cold water that needs heating, it makes good sense to combine the two.

    @wayneh A resistive heater isn't too picky about voltage and current as long as you don't exceed its ratings.

    If a PWM-fed immersion heater is used, a modern supply meter will correctly ignore any PV power that is exported (lost) during the 'off' phases. But with a properly controlled electronic load that continuously emulates a resistor of the appropriate value, no export would occur, so the local use of PV-power would be optimised.

    Heat that is dissipated by power controllers is generally wasted. But in this case, as cooling could be provided by the very water that we want to heat, the process should be highly efficient.

    MOSFETs are great for switching applications that require minimum power loss. But for linear applications, I think that bipolar components may be best. A bank of 2N3055s maybe? How to control such a mains-powered load from a microcontroller is a problem that I've yet to solve ...
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Curious. Did you apply for and receive a government grant for this system?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    OK, well the concept (on/off) is the same except the modulation frequency needs to be in hours or days. As long as your heater bank can take the full capacity for hours on end, you just need to determine when to dump into the heater bank and throw a big MOSFET switch.

    I recommend against the BJTs, because they require a base current of 1/10th the load current, are more difficult to parallel, and I think will cause more heat problems. With the MOSFETs, you can probably mount them all to one big heat sink and be done.
     
  7. calypso_rae

    Thread Starter New Member

    Apr 15, 2012
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    Unfortunately, if there's only 1kW of spare PV-power available and I turn on a 3kW immersion heater, we get billed for 2kW. With a modern electricity meter, I believe this will happen on all timescales, whether PWM or macro. Hence the load needs to be both continuous and variable.

    Yes, I agree entirely with your comments about BJTs, but I doubt whether MOSFETs would give the linearity and power dissipation that this novel application appears to require.

    @CDRIVE
    No, we didn't get a grant for our system, so I'm keen to maximise its return. Quotes for PV installations generally include a reduction for future electical bills because of the power that the panels will provide. But if the generated power is not used at the instant that's it becomes available, then it's gone. In this regard, the sales hype seems quite misleading.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I don't see the need to use the transistors as a load in this application. The immersion heater will actually heat more water for the same amount of power , digital PWM control at a high (1khz+) freqency will appear just like a resistive load to the power meter and "this is very important" move the heat from the electronics. Using an electronic device as a heater is the wrong way due to issues with thermal stress and reliability. Using BJT transistors in a high power application like this would require 10x the component count of a simple MOSFET switched load design. With a cheap 8 bit uC and a 1000W resistive heater load you could easily get 1W to 1000W control in 1W steps.

    If your power tap is on the AC line just use a diode bridge to convert the voltage to DC first.
     
  9. anotheruser1

    Member

    Dec 6, 2011
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    I have a battery bank for surplus energy from my panels. I don't let a single electron go to waste if I can help it. Dude where is your batteries?
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    I don't want to muddy the water with trivial minutia but I don't want a student or noob to read this without clarification. As I see it, calypso_rae wants the BJts to operate 'none saturated' to drop voltage to the load. Because of this the 1/10th the load current rule isn't applicable, as it's an accepted rule when saturating a transistor delivering power. Note that I said power because this rule is relatively meaningless when collector load current is low.

    This doesn't mean that I think BJTs (operated like this) is a good idea. I don't. If PWM and FETs can get the job done it would be far more efficient, lower component count and be far less prone to failure. Using a BJT to make heat will shorten its life and probably result in thermal runaway.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Can you not tap off the excess power before it hits the grid system? I mean, many folks don't do grid tie-in and it sounds to me like you want to remove that function from your system. Not sure of the lingo, but to me "grid tie-in" means you can inject power into the grid. Merely being able to use grid power when the sun goes down is not enough.

    A schematic of your system might save a lot of confusion.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Readers, please ignore the "Bravo" title here, as it was sadly misplaced. You will soon discover this by further reading.
     
    Last edited: Apr 22, 2012
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    I see that wayneh caught this but I missed it. Pity too, because it's stuff like this that sets my good nature into a nose dive. Do I understand this correctly? ... If you've been feeding 1KWH into the grid and then you turn on a 3KW load they bill you for 2KW without any regard to the fact that you back fed a KWH? Please tell me it ain't so! :eek:
     
  14. nsaspook

    AAC Fanatic!

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    I don't think that's what's happening with his utility meter. If he has a proper grid-tie system with a net meter it reads both ways with a balance of power. Some places require two meters with a incoming and outgoing meter to be logged by the utility every month. If this is a guerrilla install then the original meter sometimes shows all power as incoming, you then can get billed for power you send to the grid. :mad:

    Reconditioned meters are cheap to install for back-feed power drain system.
    http://www.hialeahmeter.com/
     
  15. wayneh

    Expert

    Sep 9, 2010
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    I think that's the solution here - proper metering.

    Making hot water is a pitiful waste of precious PV power, because the efficiency is so much lower than direct solar heating and the energy quality is lost. "Better than nothing" is a weak argument, IMHO.

    I like to think of energy "quality" as the difference between a computer, a light bulb, and a heater. All might draw the identical power from an outlet, but the computer gives information, light and heat. The bulb gives only light and ultimately heat. The heater gives nothing but heat. The same thermodynamic endpoint for all 3, but clearly the computer is the higher quality load.
     
  16. calypso_rae

    Thread Starter New Member

    Apr 15, 2012
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    Wow, this thread has certainly covered some ground!

    On this side of the pond, the UK government pays out a "Feed-In Tarrif" of 43.3p for every kWh that you generate with PV panels in a grid-tied system. You also get 3.1p for every kWh that you export to the grid. Well, that's the theory. In practice, very few installations have an export meter so an assumption is made that 50% of all generated power is used locally and 50% is exported. So, having been paid "up front" for all the power that my panels have generated, it makes sense to then use that power myself rather than let it drift off into the grid for someone else to use. I can't realistic intercept the power before it goes through the inverter because then I wouldn't get paid for it. Power taken from the grid here costs around 15p/kWh.

    My idea for an electronic load was based on the assumption that our supply meter would accurately follow the duty cycle of a PWM-fed load. However, having done some checks with a kettle today, it seems that there is sufficient slack in the system to allow a PWM-based approach to be used. Within a window of 1/1000kWh, our meter allows energy to be lost during the 'off' phase and then freely reclaimed during the 'on' phase. This is quite neat because the PV system in standalone mode would doubtless complain at this treatment.

    Using PV power to heat water may not seem sensible at first sight, but that's the way that the system is set up over here. Now, I'm really looking forward to my first solar-heated bath ...
     
  17. #12

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    Nov 30, 2010
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    So...you've been paid for the power you generate and this is a scam to avoid letting that power get used by the people that paid for it?
     
  18. CDRIVE

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    Jul 1, 2008
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    I'm giving this another day, waiting for an answer, before I go back and edit my "Bravo" post. Prior to that, I asked if there was a govt grant involved. The answer was unexpectedly "no" but I now suspect that I didn't get the whole story. I hope I wasn't too quick to give accolades. I'm pleased that you were on the ball though.
     
  19. calypso_rae

    Thread Starter New Member

    Apr 15, 2012
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    @Cdrive:
    Your question to me via the forum a few days ago was:
    "Curious. Did you apply for and receive a government grant for this system?"

    My answer to your question, quite correctly, was "No". As a house owner, it cost me around £10K to have this "retrofit" system installed, and no government grant was available to reduce this outlay.

    But if, instead, you had asked me whether PV-generated energy in the UK is supported by a government-backed tarrif scheme, I would have said "Yes".

    Whether you feel that the UK's tarrif-based scheme, or my small degree of involvement in same, is worthy of "Bravo!" status is another matter. Free use of PV-generated power is a feature of the scheme that is explained to potential investors from the outset. The rationale for the UK scheme appears to be linked with its obligation to reduce CO2 levels under EU mandate.
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    I keeping with civil forum decorum I respectfully decline further comment.
     
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