How to vary power to a resistance heater?

lazzalazza Solar Expert Posts: 336 ✭✭✭
HI

Still on the idea of using excess solar energy in PV systems to avoid grid injection...

If I have, for example, a simple 1kW resistance heater to heat say a room. How can I vary the power that it consumes?(assuming it has no variable power knobs or switches)

Would I need a varying transformer to reduce the voltage? or would I need some kind current limiter? (bearing in mind the idea is to consume say 300W instead of 1kW)

Thanks
Larry

Comments

  • BB.BB. Super Moderators, Administrators Posts: 33,003 admin
    Re: How to vary power to a resistance heater?

    You can use "Dimmer" technology to control the energy flow to your electric heater.

    The simple lamp type dimmer (cheap to build, pretty reliable) simply delay when they turn on the power to the load.

    The dimmer turns off when the voltage hits zero volts (Triac), and a variable delay circuit turns on a part way into the next voltage sine wave. Then stays on until the next zero crossing.

    Would not truly prevent reverse current flow to the grid, but in terms of metering, this would control power into the grid (i.e., if you chop 1/2 the size wave, the first 1/2 cycle of current flows to the grid, the second 1/2 cycle of current is diverted to the load). I believe that this would make for "bad power factor" if you are charged for poor power factor loads--But I am not sure.

    So, the other 1/2 of the design is to design a control circuit that can vary the Off/On timing of the dimmer circuit.

    Nice technical block source of information/circuits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    Still on the idea of using excess solar energy in PV systems to avoid grid injection...

    I don't see how that avoids grid intertie. If you generate 1000 watts with a GT inverter and use 1000 watts of power you're still grid intertied. Are you thinking of this as a way to "hide" from the power company?
    Would I need a varying transformer to reduce the voltage?

    A variable transformer (variac) would be the most effective in reducing backfed power. A dimmer type system chops up the waveform so you'd still be backfeeding power for the first half of the AC cycle. If it works well you will have increased the risk of unintentional islanding.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How to vary power to a resistance heater?
    I don't see how that avoids grid intertie. If you generate 1000 watts with a GT inverter and use 1000 watts of power you're still grid intertied. Are you thinking of this as a way to "hide" from the power company?

    Grid "injection": he doesn't want to sell back to the grid because they don't pay for it. So the GT system can offset household use but not exceed it. This is explained in another thread about determining direction of AC current flow (not easy) so that he can determine when the GTI's output is exceeding usage - and then do something about it. The latest plan is to add load, which is fine if that load can do something useful.

    This is similar to the problem off-gridders face once the batteries are full but the panels can still produce. But it's a bit easier to deal with off grid as one can use systems that detect battery SOC (one way or another) to automatically connect/disconnect loads. A straight GT system does not have that ability, because there are no batteries to measure.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to vary power to a resistance heater?

    Hi Lazza,

    Have you done the sums on just making this a battery based project with grid backup? If the grid is already there, you could install a very small battery bank and just use the built in transfer switch to call on the grid whenever there's an energy or even a power shortfall.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    Grid "injection": he doesn't want to sell back to the grid because they don't pay for it.

    OK, that sounds like a much easier task. A bang-bang controller that just turns a load on and off if there is extra power could do it. You could probably just buy an old glass power meter on Ebay and instrument it to close a relay when power sent back exceeds some (small) limit.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    OK, that sounds like a much easier task. A bang-bang controller that just turns a load on and off if there is extra power could do it. You could probably just buy an old glass power meter on Ebay and instrument it to close a relay when power sent back exceeds some (small) limit.

    The problem there is that you do not want to turn on a load that is large enough to bite into your direct loads, causing you to buy power instead. I think that is the reason that the OP suggested a variable load. Pushing back a small amount of power (wasted) is probably a better compromise than buying a similar small amount of power.
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    inetdog wrote: »
    Pushing back a small amount of power (wasted) is probably a better compromise than buying a similar small amount of power.

    Maybe not. If I recall correctly from the OP's other thread, he is charged for power he feeds to the grid. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: How to vary power to a resistance heater?

    HI in reply to the various posts:

    1. Hybrid systems will be much more expensive due to the batteries needed and higher cost of the inverter (plus the problem of constantly cycling the batteries)... although is the easiest solution, the batteries put a spanner in the works, we want something simpler and cheaper. In my new house, monies permitting, i will be installing such a pilot system, hopefully next year.

    2. We wish to adjust the power depending on the excess production: firstly, to not draw more power than necessary from the grid (hence increasing electricity bills); and secondly because as vtmaps mentions.. the just and fair electricity companies here are programming the digital meters to count excess production as consumption and hence you're paying them to be able to sell your excess electricity to your neighbours.

    A dimmer switch is interesting, but could it take the load of say, a resistance heater?
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to vary power to a resistance heater?

    I think the Emma product will do what you want, but it's not cheap: http://www.coolpowerproducts.com/uk/index.html

    Showing the energy mafia over here the middle finger should also be valued into the cost/benefit equation ;)
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: How to vary power to a resistance heater?

    Yeah Emma product was way out of the reasonable price range unfortunately. We need something reliable & cheap that can stick it to the energy fascists :) ... if only i had more time:cry:
  • BB.BB. Super Moderators, Administrators Posts: 33,003 admin
    Re: How to vary power to a resistance heater?

    Cheap dimmers for homes take around to 1500 Watts in a standard wall switch size.

    -Bill

    That is 600-1500 watt... My phone won't edit correctly.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    BB. wrote: »
    Cheap dimmers for homes take around to 1500 Watts in a standard wall switch size.

    -Bill

    That is 600-1500 watt... My phone won't edit correctly.

    In my experience, cheap ones go to 300 or 600 watts, with 1000 being more than four times the price and 1500 being only available in the commercial specification very costly units.
    SMA SB 3000, old BP panels.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    lazza wrote: »
    A dimmer switch is interesting, but could it take the load of say, a resistance heater?

    That's actually what they work best with. Incandescent lights are mainly resistive but have very high turn on surges. Regular resistance heaters (like hot water heaters) have lower surge currents.
  • BB.BB. Super Moderators, Administrators Posts: 33,003 admin
    Re: How to vary power to a resistance heater?

    You are correct... Been too long since I last looked.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: How to vary power to a resistance heater?

    Sounds like dimmer switches will not be sufficient in power rating, and the problem that during some of the cycle power is going into the grid.

    Why cant Variable Frequency Drives be used--- or do these only work for motors?

    Also, how do AC resistance heater operate under DC.... do they behave identically?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How to vary power to a resistance heater?
    lazza wrote: »
    Sounds like dimmer switches will not be sufficient in power rating, and the problem that during some of the cycle power is going into the grid.

    Why cant Variable Frequency Drives be used--- or do these only work for motors?

    Also, how do AC resistance heater operate under DC.... do they behave identically?

    Changing the frequency to a resistance load doesn't make much difference to it. I see no advantage to this.
    An AC resistance heater will operate pretty much the same using DC - again it's just a big resistor. But the controls certainly won't; the DC can weld thermostat contacts shut or burn them to bits.

    You really still have that basic problem of detecting if current is flowing to the grid and using that info to add load to use up the extra available power. I wonder if something simple like differential magnetic fields between the incoming utility and outgoing inverter power could be used in a simple form to activate a relay. (Just musing out loud here.)
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    lazza wrote: »
    Sounds like dimmer switches will not be sufficient in power rating, and the problem that during some of the cycle power is going into the grid.

    Correct. Some of each phase period will get back into the grid. The meter may or may not detect that power. However, there are very few power controls with ratings as high as dimmer controls. Theatrical dimmers go to 10kW.
    Why cant Variable Frequency Drives be used--- or do these only work for motors?

    Resistive loads do not care about frequency much, so changing frequency won't change power.
    Also, how do AC resistance heater operate under DC.... do they behave identically?

    In general they work the same; they are just resistors. You could rectify and then PWM the power to the resistive load; it would give you pretty fine control over power used.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to vary power to a resistance heater?

    Just saw this product on the SMA website: http://files.sma.de/dl/13660/METERBOXSBU-IA-IEN120932.pdf

    Now they clearly state that it can only be used with their sunny backup system.... but maybe it's hackable to work without it :)

    Let me beat that grid-supported-off-grid horse one more time, because it's still twitching:
    Disregarding PV for both a grid-tie vs. grid-supported-off-grid (let's assume 2kW of pv for both) the cost differences would break down as:
    1. Grid Tie: GT inverter (1000 Euros) + magic device that will dump excess power (400 Euros?) = 1400 total
    2. Grid-supported-off-grid: (MPPT charge controller 500Euros) + 200Ah 24 battery (560 Euros) + 2kW battery inverter (1300 Euros) = 2800 euros total

    AND you have the option of using more of your generated power by shallow cycling the batteries, let's say you discharge them 30%, so that's 1.4kWh per day saving you 30cents per day or 107 Euros/year, which will pay for itself in 13 years. Of course you may need to replace those batts at around that time too... Still, giving the cost saving and the simplicity of the setup I think it's a viable option.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to vary power to a resistance heater?

    Don't know if you're following this thread on the spanish solar forums: http://www.solarweb.net/forosolar/fotovoltaica-sistemas-aislados-la-red/23300-aprovechamos-la-energia-que-nos-sobra-15.html but the last 2 posts included a link to a component that can be used to vary the output to a resistive load. So I think it would be possible to DIY the device you're looking for like so:
    - Wire a one-way electronic meter in reverse so that it only counts electricity exported.
    - Use an arduino to read the energy, as described here http://openenergymonitor.org/emon/buildingblocks
    - program the arduino to convert the pulses into a 1-5V output signal
    - Use this output to drive this device: http://www.kemo-electronic.de/en/Transformer-Dimmer/Converter/M150-DC-pulse-converter.php
    - Which then drives this device: http://www.kemo-electronic.de/en/Light-Sound/Effects/Modules/M028N-Power-control-110-240-V-AC-4000-VA.php
    - Which then controls the output of an immersion heater
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: How to vary power to a resistance heater?

    HI Sounds good. Concerned though, that the dimmer-type control will be injecting at certain points in the phase cycle. Anyone know how these gadgets work?

    If it isnt a dimmer-type system and controls power in a linear fashion then it would be perfect!

    in terms of the hybrid system:

    1. There is a bigger difference between the inverters in terms of costs (800 € grid-tie... 1400 € intelligent battery inverter with AC input (2kW systems)).
    2. More batteries would be surely needed and with so much cycling I think you'd be lucky if they lasted 5 years (12V singleblock type).. I reckon you will need at least 500Ah to have useful energy storage/usage at night).

    We aim at 5 to 8 years payback so the extra costs that all this imposes just makes the hybrid system too costly.. as things stand. Hence the quest for the magic device :). If you want, when I've some more time, I could go through the costs in more detail and say exactly the price difference for similar systems, grid-tie and hybrid.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: How to vary power to a resistance heater?
    lazza wrote: »
    If it isnt a dimmer-type system and controls power in a linear fashion then it would be perfect!

    It doesn't control it in a linear fashion. It's a phase control dimmer. You won't find anything that does truly linear regulation; at best you will find a PWM that switches fast enough that it isn't seen by the meter. (i.e. the ripple will be below some limit.)
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