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
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.
(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.
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.
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.