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I'm in the early phases of designing a new underfloor heating controller for retrofitting solar panels to locations with existing underfloor heating cables, which now take their power from the grid.

The question is: can any type of underfloor heating wire be driven with DC voltage? E.g. use 230 volts DC instead of 230 volts AC?

Can there be any adverse effects of sustained DC voltage, for example in the carbon polymer material of self regulating heating cables? Is there something bad that can happen, perhaps something that resembles connecting an electrolytic capacitor with wrong polarity? Can anyone think of anything else that should be considered for this kind of system?

An old college buddy of mine was concerned that the plastic polymer resistor found in two wire heating cables will ionize over time when DC is applied, eventually converting the plastic resistor into a short circuit. That I would obviously like to avoid, but is there any truth to his concern?

The basic idea, obviously, is to generate some amount of heat in the building from DC voltage without the need of a DC/AC inverter. (e.g. I'm trying to eliminate the inverter cost from the system or use a smaller inverter than what would be otherwise required). A portion of panels in the roof would be configured as a separate string that has a suitable DC voltage at some conditions. And this DC voltage would be fed into some segments of the underfloor heating using this new kind of underfloor heating controller.

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    \$\begingroup\$ As long as it is a purely resistive material, it will just heat the same. If you are afraid about chemical reactions, you'd have to ask the manufacturer, we can not possibly know all products out there. If you want to heat with sun power though, solar heat collectors directly heating some water used for interior heating is far more efficient. \$\endgroup\$ – PlasmaHH Aug 21 '18 at 9:01
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    \$\begingroup\$ just out of interest: which country is this? Germany's underfloor heating (where it exists; rather a luxury thing you'd have in your middle- to upperclass bathroom) would always be hot-water based with a central heating that's typically not heated by electricity \$\endgroup\$ – Marcus Müller Aug 21 '18 at 9:01
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    \$\begingroup\$ Oh btw. if you worry about the inverter being too inefficient, well it dissipates that energy as heat. \$\endgroup\$ – PlasmaHH Aug 21 '18 at 9:07
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    \$\begingroup\$ @MarcusMüller: There are a lot of surprises out there. We were living for years in an apartment that had district heating since the 80s and additionally electric floor heating in the bathroom freshly installed (we made sure to never touch it). Recently been in an 80s building that has floor electric heating everywhere, is being remodeled, but owners are afraid of gas, thus want to keep electricity. \$\endgroup\$ – PlasmaHH Aug 21 '18 at 9:14
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    \$\begingroup\$ You might want to work out how much of a magnetic field it will create, in case you want to use a magnetic compass near the room. It could be miniscule, or it could be significant. \$\endgroup\$ – Andrew Morton Aug 21 '18 at 18:42
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Can there be any adverse effects of sustained DC voltage, for example in the carbon polymer material of self regulating heating cables? Is there something bad that can happen, perhaps something that resembles connecting an electrolytic capacitor with wrong polarity? Can anyone think of anything else that should be considered for this kind of system?

An old college buddy of mine was concerned that the plastic polymer resistor found in two wire heating cables will ionize over time when DC is applied, eventually converting the plastic resistor into a short circuit. That I would obviously like to avoid, but is there any truth to his concern?

Probably not, if your heating cable looks like the one's below it probably has insulation material made with PVC (maybe an older cable), or a polyethylene variant, or teflon or teflon variant. All of these materials are widely used in both DC and AC applications, I am also unaware of any plastic insulation material that is not suitable for both DC and AC applications.

If the heating cable is just one conductor (which is probably older) and has no shield, then you probably wouldn't have to worry about the insulation either as insulation doesn't care about if the field is varying.

One problem that may result from a wire that is unshielded and double insulated is corrosion, but this will happen no matter the polarity. If the insulation is compromised and galvanic corrosion occurs, it's not going matter if you use DC or AC, it will occur either way. (That is why the newer cables are double insulated)

These wires usually have two conductors with a shield and can be connected to on one end of the wire, the other end is shorted to provide a path for the return current through both wires.

Usually identifying information is printed on the cable, like the model and manufacturer. The manufacturer could be contacted to find out exactly what it is, but it's very probable that it is one of the materials listed above.

The biggest worry would be overheating the cable with the application of overvoltage as some of these materials are only rated for 120C, again any information of that you have on the cable will be beneficial.

enter image description here Source: https://www.thermosoft.com/en-US/radiant-under-floor-heating/for-tile-ceramic-stone/installation

enter image description here Source:https://www.heatingelementsplus.com/heat-trace-cable/pvc-pex-pipe-heat-trace-cable.html

(cross linked means a polyethylene variant)

The cables also usually have a shield which should be connected to ground in the event of an internal cable short.

enter image description here

It would also be wise to not use an inverter, but you still need to use an MPPT tracker to make sure you are getting optimal efficiency from you solar panels and to match the load to the source. MPPT trackers are available to convert DC to DC. Otherwise you'll be hanging out at the low power end of the spectrum (red circle) because heating element is a low resistance load. (the power and voltage scales will vary with your system so the axis of the graph is different, but the shape of the curve is the same).

It may be beneficial to measure the load with a meter in ohms mode to find the resistance. Then take the voltage that you'd like to run the heater at and find the current (V/R = I). The current could help you size your MPPT tracker.

enter image description here Source: https://www.homepower.com/maximum-power-point-tracking-mppt

EDIT sizing the solar cells

First off, these calculations (or any other info in this post) are just a guide and not to be used as design information, you should run your own calculations and understand the ramification of your own design or get someone that is qualified to do so.

If the load is 1000W at 230V then that would be ~53ohms. Not exceeding the voltage rating of the wire is important, so 8 modules of TSM-PD05.08D would be 229V. The load line is shown below, so as far as the load is concerned you might be able to get away without having an MPPT tracker. But I would get one anyway to make sure there aren't any problems with overvoltage. enter image description here

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  • \$\begingroup\$ This is all good info, especially the MPPT, in addition, I would add that MPPT is usually about 72~82% of Voc and then end-result uses a load that matches the PV negative source impedance over the range of current. Voc/Isc=Zpv (open cct/short cct= Zpv, impedance which is a function of solar power where lowest |Zpv| is at max rated power. thus Zpv=Vmp²/Pmax e.g. 230Vdc² /10kW= 5.3 Ω or~ 250m of AWG 16 pair. ALthough an MPPT regulator is a DC-DC converter so it stores energy such that the voltage is as above and incremental source reactive impedance matches the load resistance. \$\endgroup\$ – Sunnyskyguy EE75 Aug 27 '18 at 17:55
  • \$\begingroup\$ Yeah, I guess the OP would need to know what the load is to size the MPPT \$\endgroup\$ – laptop2d Aug 27 '18 at 17:57
  • \$\begingroup\$ Yes both load R and source -Rs range and PV power, Voc \$\endgroup\$ – Sunnyskyguy EE75 Aug 27 '18 at 17:58
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    \$\begingroup\$ This is a great and thoughtful answer, thank you. Although I'd think I'd be more on the "Open Circuit" side of the graph with the red circle. I mean if I'd connect something like 8 to 10 of normal 24V panels, like the one your graphs is from, and try to sink the 8 amps they generate into a heating wire that normally takes only 4 amps. So I think you're correct about the need for the MPPT equipment, as much as I'd like to avoid as much electronics as I can.. \$\endgroup\$ – PkP Aug 27 '18 at 19:40
  • \$\begingroup\$ Can you post the model number of your panels? You might be able to find the datasheet and figure out which side of the curve your on. What is the loading of the heater? Can you measure it? \$\endgroup\$ – laptop2d Aug 27 '18 at 19:44
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Some older installations around my way use steel cable (often stuff used for lifting, but now worn out) set directly into poured concrete slabs. The concrete is insulated below, and has a large heat capacity. The heating runs overnight when electricity is cheap, and the concrete stays warm through the day.

These systems are designed to be AC. Applying DC for long periods may cause galvanic corrosion at the steel-concrete join. How bad depends on the condition of the concrete when you start. Fortunately, it's only a problem if the steel cable is positive with respect to ground, if it's negative, then it is protected instead.

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Electrochemical mechanisms such as corrosion at the junction of dissimilar materials are a possible concern, as mentioned in some other answers.

However these mechanisms will be slow to take effect, so you could mitigate any consequences of using DC by reversing the DC polarity periodically - for example once per day or per several hours of energisation. You might be able to achieve this using some kind of time switch or timer-driven contactor.

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    \$\begingroup\$ Electrochemical reactions are not necessarily reversible. Namely, you cannot restore iron from rust by reversing current. \$\endgroup\$ – Dmitry Grigoryev Aug 23 '18 at 13:25
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230V AC is the effective voltage value for sinewave waveform found in mains network. An effective value means exactly that is a voltage that would produce the exact amount of heat compared to DC voltage when connected to a heater.

Therefore, 230VDC would behave as 230VAC when connected to a pure resistive load - for example a heating wire.

There are drawbacks: disconecting AC is better, since the electric arc is self estinguished when voltage crosses zero volts. In case of DC, the relay has to be bigger or some DC solid state relay (transistor, not a triac) has to be used.

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DC itself is fine. Although the value is not an easy one. How will you get 230VDC? Also you need some way to regulate power, so so think about variable power supply.

By the way, there is a cool method i floor heating: by lighting high power LEDs and passing their light through semi transparent material.

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Most of the issues mentioned by others are worth checking but will usually not be "show stoppers". However, one aspect is vitally important.

  • Heating in a resistive load is the same for DC or RMS AC of the same voltage.

    If the supplied DC can be consistently converted to a voltage in the say 220-240 VDC range then the heating will be the same as for AC. I say 230V +/- 10V as that is the order of range specified for 230 VAC equipment. The actual heating varies with the square of the voltage, assuming that element resistance change is relatively minimal with temperature.
    Heating at 220 V is 91.5% of the 230 V value (220/230)^2,
    and at 240 V is 109% of the 230V value.

    • The following is not complex but may require a small amount of "extra thinking" to follow. It's probably requied reading for PkP, but downvoters should just move along, nothing to see here, these are not the heating comments you want.

    • If the DC is supplied by PWM from a higher voltage DC source or by rectification of AC boosted from a lower voltage DC source then it needs to be filtered to the extent that the applied DC voltage does not vary greatly from the target DC voltage.

    • If this is not done then excessive heating (proportional to V^2) and current (proportional to V) may occur on voltage peaks and element failure may occur.

    At voltages above 230 VDC, heating surges will be "slugged" by thermal time constant but may still cause significant stresses eg at poorly made joints.
    As an unlikely example, with a voltage high enough to make the affects clear,
    if DC at say 345 VDC (230 VDC + 50%) was applied using 2/3 duty cycle PWM then

    • the mean voltage when smoothed to DC would be 230 VDC (2/3 x 345 = 230),

    • BUT if the PWM'd and unfiltered DC was applied to the element directly the heating would be (345/230)^2 = 225% as high as 230 VDC when on.

    • The mean heating would be 2/3 x (345/230)^2 or 150% of the 230V value!

    To PWM-regulate unfiltered 345 VDC to an equivalent 230 V heating level would require 44.44% on PWM so heating = 0.4444 x (345/230)^2 = 100%, but the heating peaks would still be 225% of the mean 230 VDC value.

    Filtering 0.666 on PWM of 345 VDC to smoothed 230 VDC would produce the desired result without heating or current peaks.

  • galvanic corrosion issues need to be checked,

  • magnetic field will be different but unlikely to be important in most cases,

  • .... .

However

Marko mentioned the difference in switching requirements for DC & AC.
This is vitally important.

At a given voltage DC will draw and hold an arc far more easily, of far greater length and with much greater difficulty to extinguish. A switch rated at 230 VAC at the current levels liable to be used for heating would be destroyed by DC very rapidly (if not at the first use) and the risk of fire is extreme.

Switch equipment MUST be rated and certified for DC use and meet appropriate wiring codes. This is well covered under the specified practices for solar power applications and must not be overlooked if converting an installation from AC to DC.

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