This post is mainly to address the lack of understanding about solar powered (PV-to-Load) solid-state heating elements – also called a diode string or chain.
The Part 2 video presentation and this blog post should help fill in the blanks. The video itself is also posted here on this page.
Watching Part 1 is also recommended – link here
Why Diodes Are Better
Because diode string heating elements can “practically” track the Max Power Point (vMP) of the solar panel array – with no supporting electronics. It’s almost like “MPPT” or maximum power point tracking. Try that with a standard resistance heating element.
Simple fact: a string of diodes, properly sized to a solar panel array, will practically hold the maximum power point (or voltage of max power, vMP) of a solar panel array. It isn’t perfect, the voltage can certainly change some, but it’s not bad at all. Compared to a resistance element, it’s very good. A string of diodes in series – it’s so simple yet so effective.
It is a fact that diode strings can extract more power from the same solar panels versus a standard resistance heating element under changing solar conditions.
Understanding Ohmic Resistance Elements vs. Diode Strings
A standard resistance heating element could be called an “Ohmic” element. It has a Linear relationship between Voltage, Current and Resistance (Ohms, or Impedance).
But solar panels are not linear at all in their behavior. They want to hover at a certain efficient voltage range (vMP) while the current (amperage or quantity of power flowing) remains dynamic based on solar conditions. More sun gives more current flow. But the voltage needs to stay more or less constant for maximum power and efficiency.
A direct solar driven (PV-to-Load) resistance heating element cannot efficiently accommodate changing solar conditions; the voltage swings up or down with current and conditions. Simply put, this means a resistance heating element cannot maintain vMP by itself, so some of the solar electric power is not being extracted from the solar panel throughout the day. That’s why we have MPPT charge controllers and DC converters in the first place.
Note that Ohms are a measurement unit of resistance, or “impedance” to electrical current flow in a given electrical circuit.
Diodes on the other hand, are not Ohmic. They drop voltage. And they drop a set amount of voltage, which remains nearly constant. The result is, a string of diodes can lift the solar panel array voltage up into the efficient region and keep it there, while the current varies – extremely similar behavior to a maximum power point tracking circuit. Except it’s just a string of diodes! It’s so simple – no DC conversion, no microcontrollers.
Now there are voltage variations during operation, it’s not perfect, and it is tricky to get it all working properly. But silicon semiconductor diode strings are the best thing to happen to PV-to-Load heating and cooking. Holding vMP of the solar panel without any supporting electronics? It seems to good to be true – but it’s both good and true.
Why This Matters
Because diode strings (aka diode chains) place the ability to tap maximum heat energy directly from the solar panel array into the hands of the average DIY enthusiast – with minimal cost and complexity. And just a tiny bit of DIY electronics.
Furthermore diode strings open the door to small, portable ultra-efficient solar PV-driven cooking appliances – prototypes of which I am already building and testing in my solar workshop. Check out “boiling diodes” for a preview.
Right now, it is not common to find direct solar electric heating systems and cooking appliances. I pretty much have to build them myself as they don’t exist off the shelf. Some of the solar electric cookers and heaters I have built as prototypes used standard resistance heating elements, but they don’t tap the full potential of a solar panel array without supporting electronics.
I don’t want to add a lot of electronics, because they are too complicated for many DIY solar power enthusiasts to build and replicate. They just add cost and complexity. As a high-tech survivalist who strives for preparedness, I like simple, DIY and robust technological systems that I can maintain and work on myself.
Diode strings open the door to ultra efficient Solar PV-driven heating and cooking, straight from the solar panels using just a string of semiconductor diodes. It is rather exotic territory. And ironically, diodes are electronic components. But they are so simple in their operation.
As solar panels get cheaper and more prevalent, direct solar PV powered appliances may become reality. Why not just plug straight into the solar power, and bypass batteries, inverters and charge controllers? As the world continues to change, we may find those expensive discrete components, with their limited operating time before failure, becoming harder to get and more expensive.
Therefore, saving wear and tear on them would make sense. In fact, it already makes sense. I don’t see the point in wearing out a charge controller, battery bank and inverter just to simply extract heat.
Summary
Currently in my solar workshop, I’m researching and developing the use of solid state semiconductors (silicon diodes) to extract heat directly from solar panels. My goal is to find ways to make small efficient portable solar electric cookers and also space heaters. I have 2 cooking prototypes in testing and a solar electric hot water heater design.
Diodes are a bit tricky to understand and use, but after initial investigation I found the idea holds a lot of promise for the reasons stated. If you are interested in this technology, please consider subscribing to my YouTube channel. I will probably upload a few more videos about the topic.
I hope this blog post was informative and helped you gain more understanding into the topic. Thanks for reading and see you later – DD
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