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Warning: Tritium is radioactive but safe to handle as long as it is contained within its vials.
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Work carefully to avoid breaking the vials.
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If they do break, leave the area and ventilate it for a few hours to disperse the gas.
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Greetings fellow nerds.
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In this video we're going to make a very simple nuclear battery.
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But as usual i need to crush your expectations.
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Due to the limited amount of radioactivity that can be safely handled in a home lab,
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our nuclear devices must necessarily be weaker than the professional devices.
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Do not expect free energy or vast amounts of power.
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Our objective here is to explore the science.
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As you may know nuclear batteries, or radioisotope thermoelectric generators,
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are famously used on space probes due to their high power density, reliability and longevity.
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They are also occasionally used in remote locations like unmanned lighthouses in the former soviet union.
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The military also uses such batteries for remote radar stations.
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Going to change the batteries on a regular basis was extremely costly so a nuclear battery was thought to be a better option.
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Unfortunately these types of nuclear batteries are strictly regulated and totally impossible for the average person to obtain.
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Now there exists smaller nuclear batteries you can buy commercially.
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This one from City Labs is well known and can be purchased without any special nuclear device permits.
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It works by converting the beta radiation from radioactive tritium into electricity using what is essentially a solar cell.
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But instead of photons it absorbs beta particles.
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It is thus called a betavoltaic cell rather than a photovoltaic cell.
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My inquiries on the exact pricing were ignored but you can find them online from resellers for three thousand dollars.
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In terms of power their website lists their top unit has a voltage of 2.4 volts with a short circuit current of 350 nanoamps.
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This is quite small but it's meant more for lower power devices like the memory or clock circuits in highly critical machines where chemical batteries are not as reliable.
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Other uses include medical implants like pacemakers,
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or remote sensors where changing the battery is not physically allowed or even possible.
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In hardware security modules changing the battery isn't allowed for security reasons.
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So the battery must last on its own for the lifetime of the device.
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Let's try and make our own nuclear battery and see how it compares.
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It'll likely be more expensive and less powerful than the device from City Labs but we're here to explore the science.
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Now the City Labs' device is categorized as a direct conversion radioisotopic battery
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in that the radiation directly interacts with betavoltaic cell to produce power.
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There isn't a commercial source of betavoltaic cells so we're going to have to use a photovoltaic cell.
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But photovoltaic cells only convert light into electricity, not beta radiation.
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We need to convert the beta radiation into light first.
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This technique is already well known and is categorized as indirect conversion.
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The simplest way of doing this is to put a phosphor in front of the radiation source which converts it to light.
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Fortunately for us, there is an easy to get commercial supply of safe radiation sources that have light conversion already built in.
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It's these tritium key ring lights.
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You can also find them as fishing lures, gun sights, emergency signs, military lights and so on.
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They work by having a sealed glass vial of tritium gas and an internal phosphor coating.
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The radioactive tritium gas emits beta radiation that strikes the phosphor and emits light.
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So the hard part of generating light from a radioisotope has been solved for us.
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This lets us avoid the safety issue of handling a radioactive gas.
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Now i bought mine online and they come encased in plastic.
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The actual tritium vial is much smaller.
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I could have bought the vials directly but the seller i got these lights from must have had a pricing error
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because they were selling for less than the price of the bare vials.
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So I took advantage and I bought 14 before they pulled their listing, presumably they realized their mistake.
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Score one for me.
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But i still have to remove the plastic casing to get at the vials.
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Now the casings dissolve in dichloromethane solvent but only sparingly.
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To help along the process I use this Soxhlet extractor i showed in a previous video.
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Briefly, the extractor constantly exposes the tritium lights to freshly distill dichloromethane
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and then drains it away where it is reboiled and distilled again.
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Thus i can use very little dichloromethane but thoroughly dissolve the plastic from the lights.
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Another reason why i used the extractor rather than just stirring with lots of dichloromethane is that the extractor is very gentle.
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There isn't a fast spinning stir bar to smash the delicate glass vials of tritium once they are exposed.
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While the radioactivity is small it's not a good idea to take any chances.
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Now i can do tritium vial decapsulation cheaply because i already have a soxhlet extractor.
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If you're making your own nuclear battery then just buy the vials directly.
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Unless you take advantage of pricing errors like I did the naked vials are usually cheaper than the encased lights.
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Anyway, there we go, tritium vials.
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Before you get too excited, they're not actually as bright as my camera suggests, I'm just maxing out the exposure settings.
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Anyway, I have more to go so i'm going put a fresh batch in and keep going.
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Now I noticed my vials had some insoluble rubber at the ends.
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Presumably it's silicone rubber and likely used to protect the vials from damage by cushioning from hitting the ends.
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If you want you can remove them by placing the vials in concentrated sulfuric acid.
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Silicone rubber is easily destroyed in sulfuric acid and after an hour they can be removed and washed with water.
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Nonetheless this is strictly optional and not actually necessary.
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Now we have cleaned tritium vials.
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And here we have it, 14 vials of tritium.
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Turn off the lights and make sure they still work and haven't leaked.
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Now to generate our power we put them on a solar cell.
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I'm going to use this amorphous silicon type solar panel you might find in calculators and other devices meant for indoors.
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Don't worry i'll test the more efficient monocrystalline solar panels later.
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Now i intend to build a box but for now a quick and easy way of mounting the tritium is to first align them so they match the shape of the panel.
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And then taking some tape and picking them up.
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Make sure they match.
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Now just place the panel on them and tape over.
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And now we have our simple nuclear battery.
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This is also called a radioisotope photovoltaic generator.
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It's not a thermoelectric generator because it doesn't use heat but just light.
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And we're using a photovoltaic cell and not a thermoelectric one, so it's photovoltaic.
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Now i'm going to put a second panel on top in the final version but for now this is good enough for performance testing.
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To ensure we get accurate results we encase the unit in aluminum foil to block outside light.
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Make sure the lead wires stick out.
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Now outside light can only help in terms of power generation.
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But we want to focus on just the nuclear energy part for our analysis.
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So let's get some performance numbers.
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We can measure the voltage and the panel seems be generating around 1.9 volts.
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So we're off to a good start.
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Let me switch over to measure current.
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Let's see what we get.
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And it looks like we have extremely low current.
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Barely one microamp.
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Now this reading is likely completely inaccurate.
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For all we know our device is generating half that current and my meter is rounding up.
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My meter could even have measurement errors and the true value could even be lower.
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Before you get too disappointed, keep in mind the professional device built by City Labs only produces 350 nanoamps,
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or about 0.35 microamps, and it's about half the size of our device.
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We shouldn't expect to be able to beat actual nuclear scientists and engineers.
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Now i still want accurate performance numbers to compare to the professional device.
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We're going to need a way of measuring nanoamps of current at various voltages.
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My multimeter can't go that low.
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I could buy a better ammeter but i'll save some cash and build a circuit.
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And this is my circuit.
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Here is my circuit diagram showing how it works.
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It's more complicated than a simple ammeter circuit
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because this circuit will also let us examine other performance characteristics like the current versus voltage curve
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and find the maximum power point.
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First, this potentiometer here, these resistors over here and this battery
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form a simple voltage source and divider that will allow us to impose a voltage on the nuclear battery.
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We'll be able to control the magnitude and even the direction of that voltage with this potentiometer.
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The voltage will be measured by this voltmeter i've inserted here.
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Now to measure the current we'll be using this voltmeter and this one megohm shunt resistor.
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By using such a large resistor the voltage difference across it caused by nanoamps of current reaches millivolt levels.
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A voltmeter can easily measure millivolt voltages and thus we can calculate the current
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by simply dividing the observed shunt voltage by the shunt resistance.
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Now a special point i need to make.
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The shunt resistance is not just this one megohm resistor.
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There is actually a ten megohm equivalent resistance across the voltmeter.
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Ideally a voltmeter will have infinite resistance but practically a real voltmeter has some very high but finite resistance.
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So the actual shunt resistance is actually a combination of the voltmeter internal resistance and the external shunt resistance.
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Now we don't know exactly what the internal resistance is but we don't have to.
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We can simply measured the combined resistance directly by connecting the other voltmeter to it
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switching to ohmmeter mode and measuring it on the megohm scale.
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That value there is the combined shunt resistance of the voltmeter and the external shunt and we can use that in our calculations.
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It's not often that the non-idealities of your multimeter will affect your circuits, but you need to be aware when they do.
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Now at this point you might ask, if the voltmeter has significant error at the nanoamp current levels.
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Wouldn't my measurement with this ohmmeter also have errors in it as well?
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You're absolutely right but in that case the makers of the multimeter know this
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and have calibrated the internal circuitry to compensate for that.
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So measurements, even in the megaohm range, are still accurate to within 1% or 2%.
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Okay now that we know the total shunt resistance and have recorded it in our lab notebook let me reassemble my circuit.
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Now if you look at my circuit again knowing how big the shunt resistance is you'll notice something.
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I've put the voltage measuring voltmeter across both our current measuring shunt and our solar panel.
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If i want to measure just the solar panel, why am i putting it across both? The voltage difference across them will be the shunt and the solar panel combined.
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Since the shunt voltage will be on the millivolt range, that would directly and significantly affect my results.
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So why am I doing that? Well you can put the voltmeter directly across the solar panel.
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But this actually makes the measurement harder.
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Remember when i said the voltmeter itself has a large but finite resistance?
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Well at nanoamp current levels this affects the results of the voltmeter and the ammeter circuit.
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I can prove it here.
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I've connected the voltmeter directly to the solar cell.
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As you can see, when i connect or disconnect the voltmeter the ammeter shows a change.
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The current that is being drawn off by the voltmeter and its internal circuitry is significantly affecting results.
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While it is possible to measure the internal resistance of the voltmeter and mathematically correct for it,
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it is mathematically simpler to measure the shunt voltage and the solar cell voltage together
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and just subtract the shunt voltage which we can read off directly on the ammeter circuit.
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So i put the voltmeter on these points.
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In my case though since my current measuring voltmeter is connected more or less backwards.
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I just add the voltages rather than subtract but the mathematical principle is the same.
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I put it backwards so i can read current on the positive axis.
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Now before we start taking measurements be absolutely certain no extra light is getting in.
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Just shade the aluminum envelope containing the nuclear battery to check.
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If the readings change then there is some light leaking in and you need to encase it better.
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This was a problem in my earlier attempts before i finally wrapped it properly.
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Anyway, we can start taking measurements.
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It's just a matter of reading off the voltages and rotating the potentiometer to read the next voltage.
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We can sweep across the voltage range and get current readings each voltage.
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Granted you have to sit down and take a bunch of data points but it works.
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Now at this point, i'm pretty sure you actual electrical engineers are reacting to my circuit like this.
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Bender: Ahahahah! Oh wait, you're serious.
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Bender: Let me laugh even harder. AHAHAHAHHAH!
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Yes i know, a completely automated circuit would have some sort of digital voltage regulator
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and a couple of ADCs and could sweep all the voltages in a few seconds and record them into a computer.
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I'm sure there are arduino or raspberry pi devices and shields that handle everything in one go.
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But then again that's why i'm the chemist and you guys are the engineers.
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You may continued laughing.
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Bender: HAHAHAHAHAHA
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Okay once we collect all our data we can now start processing it.
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We add together the shunt voltage and total voltage to find solar cell voltage.
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I must emphasize again that i can add them because i wired the currenting measuring voltmeter in the opposite direction of the total voltage measuring voltmeter.
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If it were the same direction you'd subtract the voltages.
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Now to find the current we divide the shunt voltage by the total shunt resistance
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that we found to be around 0.89 megohms for the combined external and internal resistors.
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Finally we find the power by multiplying the solar panel voltage by the current.
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Now we plot the current and the power versus voltage.
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And that is beautiful.
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There is nothing quite like plotting scattered data points and watching the underlying science emerge through the curves.
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One of the tiny little joys of research.
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So let me explain what we're seeing.
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This blue line here is the current versus voltage.
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The current at zero volts represents the short circuit current is the maximum current the solar cell can produce.
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Now to the right where the current is zero this is the open circuit voltage.
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This is the voltage if the solar cell is completely unloaded.
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As you can see we can draw more and more current very easily
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until we reach a kink or knee where the solar cell can no longer generate much more current to meet demand.
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At that point the voltage decreases rapidly with only modest increases in current draw.
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This is the maximum power point and can be found easily by simply plotting the power output versus voltage.
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We can see it as this orange plot.
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The very top of the curve is the maximum power point and as the name suggests
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you get the most power from your solar cell if you can design your circuitry to load your solar cell to this voltage.
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In this case around 1.6v.
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Now you might be wondering how i can have numbers into the negative region of my chart.
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My measuring circuit can apply voltages beyond the limits of the solar cell.
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There is nothing particularly special about those limits so the trends in solar cell behaviour continue.
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We're just now draining power rather than generating it.
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At the negative voltage region we have a very sensitive light detector and this is the basis of some photodiode circuits.
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Beyond the voltage maximum is where light emitting diodes work and instead generate light rather than absorb it.
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A solar cell however won't actually emit light since it's not designed that way and will just heat up.
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Anyway on the whole i'm very satisfied with these results.
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We were able to measure the performance characteristics like short circuit currents and maximum power.
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We're getting almost exactly 1 microwatt of power.
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Let's see if we can do better.
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Now amorphous solar cells are not very efficient, about several percent and that's it.
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If you've looked into solar cells you know there are monocrystalline solar cells that you can buy at around 20% efficiency or higher.
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So let's try that and see if we can do better.
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Now i know what you're thinking, this monocrystalline solar panel i'm using is much smaller than my amorphous cell.
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It won't be a fair comparison because it will generate less power than a solar cell of equivalent size.
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You're absolutely right but we can scale our results by measuring for area.
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I measured the monocrystalline panel to be 450 square millimeters in size
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and i measure the amorphous panel to be 1092 square millimeters in size.
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So the scaling factor is around 2.4267.
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I just need to multiply the current by that number and they'll be comparable.
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Voltage is constant regardless of surface area so it doesn't need to be scaled.
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So i've assembled the monocrystalline solar panel into our measuring circuit.
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Let me gather the data and start plotting.
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And here are our results.
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As said before i'm multiplying the current by the scale factor so we can compare them to our amorphous silicon panel.
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I had to put the power and current in separate charts since the numbers were so different.
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Now the monocrystalline cell produced a lot more current compared to the amorphous cell but the voltage max was much less.
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We can attribute some of this to the monocrystalline panel having fewer individual solar cells inside of it.
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But the most interesting difference is this fairly constant slope for the current trace and maximum power point is much lower.
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At best we're only extracting 71 nanowatts.
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And that's already been scaled to match the size of the amorphous panel.
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This monocrystalline panel is only giving us seven percent the performance.
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At first i thought i was doing something terribly wrong
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but i looked up the research by other groups and found this is a fairly well known observation.
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High efficiency monocrystalline solar cells are only efficient in very bright light.
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In very dim light like the weak glow of these tritium vials they actually perform very badly.
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Amorphous silicon solar cells while being very inefficient in bright light give much better performance in low light conditions.
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In thinking about this, I've actually seen this before but never really noticed it.
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Devices meant to work indoors with low light like this solar keyboard or calculators almost exclusively use amorphous solar cells.
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I've never seen one use monocrystalline solar cells.
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I even found this advertisement page extolling the virtues of using amorphous solar cells for low power.
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So apparently everyone knew that amorphous solar cells are better for indoor light except me.
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Bender: AHAHAHAHAHAHA
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Now to understand why this is we can look at this solar cell model circuit.
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A solar cell behaves like a current source in parallel with a diode and shunt resistor.
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There is also an equivalent series resistance.
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But for this analysis we can ignore this.
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Now the shunt resistance isn't an actual device you can find,
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it's just a representation of the combined properties of the solar cell like crystal structure, defects, purity of silicon and so on.
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In a monocrystalline cell the current source is very powerful and produces a lot of current but the shunt resistance is rather low
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So at small power levels like low light it dominates the circuit and shorts out what meager power is available.
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In an amorphous solar cell the current generator is weak but the shunt resistance is huge so it has very little effect at low light.
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So for low light conditions the amorphous solar cell produces more usable power than the monocrystalline one.
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For bright sunlight though the massively powerful current source dominates the circuit in a monocrystalline cell
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and thus they are used in bright sunlight where they give the most benefit.
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Anyway, I was going to try other solar panel types like polycrystalline
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but it seems the consensus among all the other researchers is that amorphous solar panels are the best for low light nuclear batteries.
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So i won't bother.
00:19:54
Okay so since we're going to use amorphous solar cells exclusively let me rebuild it.
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You might be wondering if different colors or shapes would be better.
00:20:03
I was only able to get discounted tritium vials for the color green, so that was all i was able to test.
00:20:09
The articles i read from other researchers mostly used green and some tested blue
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but i didn't find any broad rigorous testing to determine the relative qualities of all the available colors.
00:20:20
So that's something you could try looking into if you want.
00:20:23
As for shapes, I have seen rectangular tritium lights but those seemed to be more expensive for the same given area.
00:20:30
Now we should be able to almost double our output doing the obvious, sandwiching a second solar panel on the device.
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And there we go, a double sided nuclear battery.
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Once again i'm going to put the unit in foil to protect it from external light so we get accurate measurements.
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I've inserted the device back into my measuring circuit.
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Now i've connected the solar cells in parallel to get increased current.
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You can connect them in series for increased voltage if you want, the total power will still be the same.
00:20:58
Running the voltage sweep again we then plot the data and see how we did.
00:21:02
Interestingly enough rather than getting twice the maximum power we're only getting about 23 percent more.
00:21:08
The maximum power point is 1.23 microwatts rather than the 1 microwatt of the single panel device.
00:21:14
At first i thought i did something horribly wrong and rechecked my connections and work.
00:21:19
Turns out this data is correct.
00:21:21
The reason why the single panel nuclear battery attempt had such a good power rating
00:21:26
was because the aluminum foil i used to block the light actually reflected the other side of the tritium vials
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and returned the light back to solar panel.
00:21:34
Now It had to go through the tritium tubes again so it lost some power and thus didn't give the same power as a double panel device.
00:21:42
But it still gave more than half a double panel device, a good 60% more power.
00:21:48
In my double panel device there is no reflection so we're not getting that boost and thus not getting double power.
00:21:54
Still, a 20% increase is not negligible and worth keeping the extra panel since that's still cheaper than 20% more tritium vials.
00:22:03
Now that we have our relatively optimized device, let's compare it to the professional one.
00:22:08
Now betavoltaic technology is different from photovoltaic technology but it should have a maximum power point as well.
00:22:16
Unfortunately City Labs don't give us a current voltage graph like mine so we don't know what that maximum power point is.
00:22:23
But let's give them the benefit of the doubt and assume they have perfect technology and their device gives perfect power.
00:22:29
According to their specifications they can provide 350 nanoamps at 2.4 volts or 0.84 microwatts.
00:22:38
Our device at 1.23 microwatts is actually better than the professional device.
00:22:43
And we haven't even factored in cost yet.
00:22:46
While i got my tritium vials for a steep discount,
00:22:49
if you bought the components and the tritium tubes at market price then cost comes out to about $300 canadian.
00:22:56
Meanwhile the reseller was selling the City Labs device for $3000 canadian.
00:23:03
We could build ten of our devices.
00:23:06
That being said the City Labs device does beat mine in terms of size and weight.
00:23:11
It is both smaller and lighter and that could be an important consideration in devices where that is a premium
00:23:17
like in spacecraft or in medical devices like pacemakers.
00:23:20
Nonetheless i'm totally blown away.
00:23:22
I'm not even an electrical engineer let alone a nuclear engineer
00:23:26
and i've got something that is both cheaper and more powerful than the professional version.
00:23:30
Now why is their stuff more expensive? They got two things working against them.
00:23:35
First, they need nuclear regulatory approval to build and sell their devices.
00:23:40
I'd imagine getting the legal paperwork done and meeting nuclear safety standards is not exactly cheap.
00:23:45
Second, at just 0.84 microwatts the number of applications isn't that high and it's unlikely they're selling in massive volumes.
00:23:54
So they need to charge more per device to stay in business.
00:23:57
We kind of cheat in that tritium vials are mass produced in huge quantities for all sorts of uses
00:24:03
so economies of scale and the free market have made them affordable and well within the reach of the average person.
00:24:09
We also don't need nuclear regulatory approval.
00:24:12
Anyway, so at this point you're probably wondering, how long does this last? Well tritium has a half-life of 12.3 years.
00:24:20
So ideally this should gradually halve its power output every 12.3 years.
00:24:25
But there is another decay mode.
00:24:27
The phosphor in the tubes themselves are constantly being bombarded with beta radiation and they will decay as well.
00:24:33
So the total power output will decay faster than what the tritium half life suggests.
00:24:38
I don't know what that rate is but it's something to keep in mind if you want to use this device for the very long term.
00:24:44
I also recommend using a better case than plastic tape.
00:24:48
A custom metal box is best for the long term.
00:24:51
So what can we power this?
00:24:53
As I said before, this is meant for extremely low power devices where longevity and reliability are absolutely paramount.
00:25:00
For comparison, if we wanted half a watt, which is enough to run a low end cell phone,
00:25:06
we'd need 400000 of these, or about 120 million canadian dollars worth.
00:25:12
If your expectations weren't crushed already they should be now.
00:25:16
Now i did want to use this for something but I'm not an electrical engineer
00:25:20
so i can't build the nanowatt level circuits that would take advantage of this battery.
00:25:25
But i'm sure guys have better ideas and we should discuss them in the comments.
00:25:29
So, that is how you make a very simple nuclear battery or radioisotope photovoltaic generator,
00:25:35
for a fraction of the cost of the commercial units.
00:25:37
Thanks for watching.
00:25:39
Special thank you to all of my supporters on patreon for making these science videos possible
00:25:44
with their donations and their direction.
00:25:46
If you are not currently a patron, but like to support the continued production of science videos like this one,
00:25:50
then check out my patreon page here or in the video description.
00:25:53
I really appreciate any and all support.
