Solid State Solar Electric Heating Elements

Background

Update: further research has been conducted into this topic, and it has been very fruitful. Videos showing some of the latest results are posted on this page.

I’ve spent a lot of time working with resistance style heating elements to extract heat from solar panel arrays. Uses include space heating, cooking food and heating water.

Resistance heating elements are simple and generally robust, but they do have downsides. Recently I read some research papers about using solid state semiconductors to generate heat from solar panels. In short, diodes can fulfill this purpose. After reading the research papers I decided to give this idea a try in my solar workshop. The tests were documented on my YouTube channel, the video is posted here for reference.

Diodes for Solar Electric Heating

Diodes are in fact quite fascinating. They are actually installed in most larger solar panels as “by pass diodes” but are not intended to generate heat, although they often do fail because they get too hot.

Ironically, this very fact is quite useful. A diode drops about a half of a volt (DC) when in use. If you drop a half of a volt at 10 amps, that is 5 watts of heat dissipated.

Oddly enough, if you put a whole string of diodes in series, you can connect a solar panel to the string and dissipate pretty much the entire available power as heat!

The Main Benefit of Diodes

What makes this even more interesting, is that the diodes themselves behave differently compared to resistive heating elements. The diodes drop approximately the same amount of voltage all the time – but the current varies the most. Voltage does vary some, but it takes only a little change in the voltage to make a huge change in the amount of current flowing across the diode – and thereby the amount of power (watts, heat, BTU) that is dissipated.

Why is this so interesting? Well it turns out a solar panel wants to be at a certain voltage to maximize efficiency. This is called the Maximum Power Point voltage, or Vmpp for short. A resistive heating element does not change much as the solar conditions change, meaning it cannot alone extract the maximum amount of power from the solar panel reliably without some kind of extra help.

Diodes on the other hand, theoretically more or less keep their voltage drop constant, while the current varies.

This means with the right number of diodes in the string, they will tend to stay near or at Vmpp without any supporting electronics. The current will vary as the solar conditions change, but the voltage will tend to be somewhat stable.

The end result is that diodes used as solid state heating elements can potentially extract more power from the same solar panel versus a standard resistance heating element.

Initial Testing in my Solar Workshop

I wanted to see this for myself, so I decided to start small. Some of the first tests were conducted with a single 100 watt solar panel mounted on the outside of my solar workshop (a metal building which I oufitted with a lot of solar equipment and workbenches).

The early tests were very promising. In fact I verified this idea works 100% and is totally a viable way to extract heat from a solar panel array. I literally hooked up a number of diodes in series to the test 100 watt solar panel, and they got hot. I could feel the heat coming off the diodes. The voltage stays rather stable as is generally expected. By switching diodes in and out of circuit, the characteristics of the heating element can be changed in real time.

Primary Challenges

The main challenge for me seems to be cooling the diodes, as they will most certainly fail and burn out if they get too hot. But that is a problem that can be solved with quite a few different means. Probably in the future there will be better diode packages available for this purpose. Right now I only have access to standard axial diodes and the common TO-220 packaged diodes. I don’t think those are ideal for solar applications, and hope there will be something better in the future.

If you want to watch these tests performed on my workbench, please check out the video on this page.

PS If you want to try making a heating element with diodes, here are the types of diodes I used in my tests. Please note these are affiliate links and I may get a small commission if you use them to shop. Thanks for your support 🙂
20A Silicon Rectifier Diode https://amzn.to/4b3niOb
20A Diode variety kit https://amzn.to/3O5h1I3

I hope this blog post was helpful to you. Thanks for reading and see you later! -DD, SPE