Tag Archives: solar

Fun with Failure

Have you ever come up with a neat idea?  An idea for some really useful sort of device?  And then you find out that it is, at least for now, a physical impossibility?

I had just such a neat idea this week.  At least, I thought it was a neat idea.  You see, for the past few weeks, I’ve been traveling around the country interviewing for various different jobs.  One of these jobs is with John Deere in Waterloo, Iowa.  Now for those of you who don’t know, it gets cold up in Iowa.  Their average January temperature is a mere 16°F.  I was imagining myself leaving work after a long day, going out into the cold, and then shivering in a very cold Jeep Liberty (my car) for the drive home.  So I’ve been thinking, wouldn’t it be nice if I could somehow heat my car while it’s outside?

A Solar-Heated Jeep?

Well, I don’t want to run a heater using my car battery, for obvious reasons.  However, I have this nifty 90W solar panel that I’m not really using at the moment (although it is acting as a glorified cellphone charger, and has been put to good use in the past).  Perhaps I could mount this to the top of my Jeep and use it to run a small heater.  My only question – would 90W be enough power to effect a substantial (20-30°F) temperature rise?

Initially, my gut feeling said no.  For one thing, from experience I know that my 90W photovoltaic (PV) panel will never produce 90W if mounted horizontally on the roof of my car.  Even on a perfectly clear and sunny winter day, it won’t be getting strong, direct sunlight.  So perhaps I could expect 60W at best, but on a cloudy day I might see less than half of that.  But more importantly, on a sunny day, wouldn’t I already be trapping more than 90W via my windows and the greenhouse effect?  Because if so, this effect has never produced a particularly warm interior.

Of course there’s only one way to answer these questions.  Science!  Particularly, the study of thermodynamics.  I’ve taken a couple of courses on the subject, but I still had to refer to my book for this formula, derived from the first law of thermodynamics:

An equation for the rate of temperature change for an object at a given power input/output.

This says that the rate of change of an object’s temperature (dT/dt) is equal to the power (P) absorbed or released by that object divided by its mass and its specific heat.  So for my first experiment, I decided to heat the interior of my Jeep by idling the engine and turning the heat to full-blast.  I then hopped out and remotely measured the interior temperature during cooling.  How did I accomplish this?  I temporarily re-purposed my Doom Box, which now contains both a LM34 temperature sensor and an XBee wireless transceiver.  Here’s a snapshot of the latest PCB revision, powered by an ATMega644P MCU:

Doom Box PCB (Rev3)

So here’s what the temperatures looked like during my first test.  The yellow vertical line below indicates when the engine was shutdown and cooling began:

Test #1 Graph

I should note that the Jeep’s temperature measurement was filtered by a 40 second (eight sample) moving average filter implemented within LabVIEW.  Also, the internal temperature sensor was placed on the floor in front of the back seats.  Exterior temperature measurements were taken manually using a digital thermometer and are represented by a linear trend-line shown in red above.

So during this test, my jeep was outdoors, parked in the shade, from 3-5PM.  You’ll notice that during this time the exterior temperature dropped by about ten degrees as the sun started to set.  However, the temperature inside the Jeep dropped slightly faster, as you would expect.  There was also very little wind during this test.

Now to apply the formula!  The drop in temperature between t = 4000s and t = 6000s is easily determined from the graph: 4.7°F or 2.6°C.  The trick is in determining values for mass and specific heat.  For this first test I decided to simply approximate the mass of air and metal within the car.  I calculated that the Jeep contained about 4kg of air at a cp of 1005J/kg-K as well as about 20kg of steel (e.g. in the seats and wheel) at a cp of 490J/kg-K (it’s pretty interesting to note that air has a higher specific heat than most metals).  I decided to ignore small pieces like plastic and seat stuffing, as well as everything under the hood (an assumption which proved later to be somewhat stupid).

So plugging all of that into the equation above yields a power loss of just about 18W.

Well now, eighteen watts is pretty surprising.  I wasn’t expecting losses to be that low for an interior-exterior temperature difference of 25°F.  This called for a second round of testing.  This time, I moved my Jeep into an insulated, closed garage.  Doing this allowed me to get rid of temperature variations due to solar radiation and wind.  It also prevented rapid changes in exterior temperature.  I then inserted a controllable heat source (an old laptop) into the car, located a few feet away from my interior temperature sensor.  The laptop was configured in such a way as to draw about 46W via a 120VAC inverter.  In theory, this level of power should have raised the Jeep’s internal temperature by about 12°F per hour.  Did it?  Well, I think the graph below speaks for itself:

Test #2 Graph

Yep, that’s definitely a failure.  It turns out that an input of 46W could only raise the car’s interior temperature to about five degrees (F) above ambient.  So the bottom line is my 90W solar panel isn’t going to have much of an effect.

So what went wrong with that first test?  Why did I calculate the need for only 18W?  Two reasons.  First, I didn’t account for all the residual heat in the engine compartment.  This helped keep the Jeep’s interior temperature higher than it would have been otherwise.  I likely also underestimated both the mass and heat capacity of the car’s interior.  In my defense, it’s not exactly an easy thing to determine.  In thermodynamics classes, you get problems about isolated, well-defined blocks of aluminum and ideal, constant heaters.  In reality, you get an ill-defined mixture of materials and variable heat sources.

Just out of curiosity, I used the first few minutes of my second test to make a better guess at the Jeep’s mass and specific heat.  In five minutes (300s), the temperature rose by about 0.7°F or 0.4°C.  Assuming this rise came only from heat input by the laptop, the term mcp = 34,500J/K.  That’s actually not too far from my initial guess for the first test, which worked out to 13,800J/K.  What this likely means then is that in my initial outside test, the Jeep was losing about 45W instead of just 18W.  But I’m also guessing that a lot of heat was still coming in from the engine compartment.

Anyway, I hope these results aren’t too confusing.  I’m not making any guarantees about the correctness or validity of my methods or assumptions.  What I’m really trying to say here is that you’ll need a lot more than 90W of PV panels to heat your car’s interior.  That, I can say with confidence.  I’d love to hear comments on this, particularly if you’re an expert in thermodynamics!  Where have I gone wrong here? 🙂

Unique Energy Storage

What do you think poses the greatest challenge for technological advancement today?

Well if you were to ask me, I’d say energy – both its creation and storage.  This may be obvious, but without sufficient energy, manufacturing, research, and educational facilities won’t be accomplishing much.  But even if scientists were to find a source of limitless, cheap energy, if it’s not portable, there’s still a problem.  For instance, what’s holding back the advancement of electric cars?  The cost and weight of batteries.  Why aren’t solar and wind energy more widely accepted?  Admittedly, cost is a big factor, but renewable sources like these operate on their own timeline.  Clearly, we can’t rely on solar power at night, so we need a means of storing up that energy during the day.

Now sadly, I don’t have a solution to either of these problems.  If I did, I wouldn’t be blogging, I’d be swimming in my money bin a la Scrooge McDuck.  Fortunately, there are researchers working on both energy creation and storage.  In this post, I’d like to mention just a few of the more interesting techniques, past and present, for the storage of energy.

Ultracapacitors

Alright, I’ve got to mention this one first, since it comes from my alma mater.  Researchers at Rensselaer Polytechnic Institute (RPI) have just received $2 million from the National Science Foundation (NSF) to help develop new ceramic materials for energy storage.

Maxwell Ultracaps

Now supercapacitors such as those pictured above, have been around for a number of years.  However, they’re still very expensive and although they offer impressive power densities (i.e. the ability to charge and discharge rapidly), they can’t match the energy density (the quantity of energy stored per unit volume) of batteries.

Fortunately, this new research into nanostructured capacitors may yield better results.  According to Doug Chrisey, a professor in RPI’s materials science department (one place I rarely dared to visit as a student), these new capacitors will be smaller, lighter, and more efficient than batteries.  How will this be accomplished?  By constructing a capacitor from extremely thin layers composed of “a mix of ferroelectric nanopowder and low-melting, alkali-free glass.”  Sounds intriguing…  Sadly, it’ll be years before the results of this research are known and available to consumers.

Antimatter

Now here’s an approach to energy storage that’s even further off into the future.  It may also be completely impractical.  Nevertheless, it’s still really cool.  See, when matter and antimatter collide, they release an extraordinary quantity of energy – approximately 10,000 times the energy produced by nuclear fission.  Yea, wow indeed.

Visual Approximation of a Matter-Antimatter Collision

So the idea here is to produce a bit of antimatter, store it as fuel, and then use it as needed.  You’d only need to store a few nanograms of antimatter to power your Tesla Roadster.  The trouble comes in production.  It’s expensive.  And slow.  Not to mention difficult to store.  Extrapolating from data on the 2004 production of antiprotons at CERN, to obtain just one gram of antimatter would require $100 quadrillion and 100 billion years.  And despite the vast amounts of energy invested in antimatter production thus far, the sum of all antimatter ever created would only provide enough energy to light a bulb for a few minutes.  Better luck next century guys.

Flywheels

When you think of a flywheel, you probably picture a heavy metal disc of some sort (I think there might be a joke in there someplace).  At first glance, it doesn’t seem like much of an energy storage device.  But you’d be wrong about that.  With enough speed and inertia, flywheels can store over 100kWh as rotational kinetic energy.  They can also be “charged” and “discharged” quite rapidly (on the order of minutes).  Here’s one example which stores roughly 500Wh and can source up to 1kW, the NASA G2 flywheel:

NASA's G2 Flywheel

To reduce frictional losses in the G2, the flywheel itself is suspended within a vacuum on magnetic bearings.  It can be rotated at up to 60,000RPM.  In terms of energy and power density, flywheels can actually out-perform most batteries.  The problem?  Cost, as usual.

Now the “charging” of such a device is performed by powering a motor which spins up the flywheel.  To “discharge” the flywheel, a generator (which could be the same motor used in spin-up) is used to convert that stored rotational energy back into electricity.

One interesting application of flywheel energy storage is the Incredible Hulk roller coaster at Universal in Orlando, FL.  In order to power the coaster’s initial uphill launch, the ride uses several motor-generator sets attached to large flywheels to store and release enormous amounts of energy.  Without these flywheels to provide this initial boost of power, Universal would have needed to build an entirely new electrical substation (or risk brownouts every time the ride launched).

Flywheels have also been used to provide energy for vehicles, such as the Gyrobus, as early as the 1940s.  Prototype charging stops were installed in Switzerland in the early 1950s, but the idea never really caught on.  Gyrobusses were limited to distances of about 6km at speeds of up to 60km/h.  They also had issues with gyroscopic forces tending to tip the vehicles.  However, research is still proceeding on flywheels for use in electric trains.  A 133kWh flywheel developed at UT Austin can accelerate an electric locomotive from rest all the way up to cruising speed.

One final interesting note.  I haven’t been able to confirm this, but I’ve heard that a large flywheel, used to provide pulse power to a particle accelerator, once failed catastrophically (i.e. exploded).  The resultant shockwave was detected hundreds of miles away on seismographs.  This is one problem you won’t have to worry about with supercaps…

Anyway, I’d love to hear your thoughts on energy storage and production, so feel free to leave a comment below.  Thanks!

The Doom Box: Part II

Well after completing the first version of my oddly-named “Doom Box” I set out to quantify its performance. Just how much energy was I collecting with my solar panels? How much was I using? This part of a project is, for me, the most satisfying. Is there anything better than logging, graphing, and analyzing tons of data? 😛

So again, I turned to LabVIEW to help collect and display this data. Information was gathered via a serial connection to my electronics, then parsed into HTML using a very simple template I’d created. The lovely charts shown below are generated by a bit of JavaScript code provided by JS Charts (thank you guys!). This is an example of data collected during a partly cloudy day this past January (1/21/2010):Solar Monitor WebpageThe graphs of voltage and net power indicate the photovoltaic (PV) panels hit peak output just before 10AM (600 minutes from midnight) on this particular day. Past this point net power rapidly dropped, most likely because of cloudiness. For this system, net power indicates just how quickly energy is being stored (positive) or withdrawn (negative) from the batteries. The graph of output power indicates the wattage being drawn by loads attached to the system. In this case, a 25W compact fluorescent bulb was turned on at 5:30PM and turned off about an hour later. The text at the top of the screen also displays ambient temperature, the setpoint of a Z-Wave thermostat, the state of the controller’s three MOSFET outputs (M1-M3), and the state of one Z-Wave switch (E1).

Now, the site you see above is no longer active. Sadly, the system has been offline for a few months and will likely not be back in service for another month or two. It’s not that there’s anything wrong with the box, it’s just that I no longer have space to setup my PV panels. I am, however, in the process of making a new controller for the system. Details on that project will be coming soon.

So admittedly, that status page isn’t the most attractive thing in the world. However, the formatting was chosen so that the data would present well on my BlackBerry Storm:

BlackBerry Storm Displaying Status PageYou may have noticed two links on this page: “Webcam” and “Control Panel.” As I mentioned, these are no longer current. However, when the system was running, the PC in charge was linked to a webcam as well as a Z-Wave USB transmitter. The webcam allowed me to see what sort of nasty weather was going on outside my window. It was also used as a day/night sensor which would automatically turn on lights each evening.

WebcamThe control panel link brought you to a LabVIEW-generated page that let you remotely adjust the thermostat, turn on lights, and control the state of the controller’s MOSFETs. This turned out to be a really neat feature, particularly when used with my BlackBerry. The front panel of the LabVIEW VI behind all of this craziness is shown here:

Solar Monitoring and Control VIThis VI was responsible for quite a lot: data logging, HTML generation, FTP uploading, system control, webcam monitoring, scheduling, and even the sending of summary performance emails at the end of each day. If anyone is interested in seeing the LabVIEW code behind this setup, feel free to comment or contact me (the code is rather messy).

So what did I learn from all of this? Location, location, location. The system’s PV panels were located on my east-facing balcony and spent all but three hours each day in shade. As you can imagine, this didn’t allow me to capture very much solar energy – only about 300Wh on a good day. I’m going to make sure that wherever I live next has a good southern exposure (unless I wind up in the southern hemisphere). Thanks for reading!

Reviving a Failed Inverter

AIMS 180W InverterIn my last post I discussed the building of a solar-powered “reverse UPS” called the Doom Box (a name given for, really, no good reason). To briefly recap, its purpose was to capture, store, and convert solar energy into AC power. This AC output was provided by either a direct connection to the mains or, when solar power was available, an AIMS 180W pure sine wave inverter (specifically model number PWRI18012S, shown in the image at right).

Well after about six months of daily operation (with an average power output of 35W), my inverter bit the dust. To be precise, its 12VDC input suddenly turned into a dead short. Now, if you short out a big lead-acid battery, bad things can happen (burning insulation, melted wires, explosions). This is why I always (mostly) put fuses in series with my batteries. So because of these fuses, when this inverter failed, it wasn’t all that exciting. All I heard was a small pop from the fuse and that was it. Thinking that this was just a temporary problem, I replaced the fuse, removed the AC load, re-enabled the inverter, and pop – another fuse blown.

As a first test, I got my multimeter into the system and checked that the inverter’s input was in fact internally shorted. The next step was to remove it from the system. If you’ve seen the pictures from my last post, you’ll know this was no easy task. However, after disentangling it from the rest of the components, I popped it open and had a look:

AIMS 180W InverterAIMS 180W InverterYes, that assembly really is as sloppy as it looks. It appears that components were just shoved in at random. Seriously, it’s amazing that this thing ever worked. The two TO-220 package ICs (which are linear regulators) in the middle of the device are actually insulated from the toroidal inductor by a thin piece of paper. So much for build quality.

Well enough ranting – the question was, can this be fixed? I’d already likely voided my warranty by cutting off the cigarette-lighter plug, so returning the unit would be difficult. I started to poke around looking for anything that had obviously failed (e.g. burst capacitors – these are fairly common culprits). It turns out both of the input MOSFETs had shorted out. I’m not sure what caused this, but I suspect they were fairly low-quality components to begin with, and repeated thermal cycling led to failure.

AIMS 180W Inverter - Replaced MOSFETsSo to make a long story short, I replaced the failed MOSFETs (indicated by the red arrow in the image above) with similar, but higher-rated devices from Fairchild Semiconductor (FDP8860). By higher-rated I mean that these new transistors were rated for greater currents (80A) and had lower on-resistance (less than 3 mΩ). As a result the new MOSFETs should actually be more efficient and thus run cooler. So my messy little AIMS inverter is now up and running once more.

The Long Story: How to Identify Failed MOSFETS

You might be wondering, just how did I know which transistors had failed? Well many times, when a transistor fails and shorts out, it rapidly becomes very hot – so hot that its case can actually melt, crack, burst, glow red, or all of the above, as seen here:

Failed IC

(Note that the IC in the 14-DIP package shown above is not a MOSFET, but a dual MOSFET gate driver – I may discuss its failure in a future post.) Now, sometimes the damage to a failed device is much less obvious. I’ve seen a number of failed MOSFETs that appear to have one or two small droplets of water on their surface after failure. It’s not actually water, but some melted component of the package.

However, if you’re taking appropriate precautions and fusing your power source, it’s likely that your failed device will shown no physical signs of failure whatsoever. This is because after the device shorts, the fuse immediately blows and prevents catastrophic heating. This was the case with my inverter: the MOSFETs I replaced did not appear damaged in any way. So how did I find them, out of the multitude of devices crammed into that tiny case? Simple, actually – I measured the drain-source resistance on each device:

MOSFET SymbolBefore I continue, a little background is in order. A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) works because voltage applied between the gate and source (Vgs) creates a small electric field inside the device. This electric field causes charge carriers to be pulled into the region between the drain and source. As a result, the resistance between drain and source drops as Vgs increases. So, if there is no voltage applied between the gate and source, and if the gate has been discharged (this happens over time, but can be accelerated by a short to the source), the resistance between drain and source should be infinite (or at least in the megaohm range).

So one way to verify that a MOSFET is working properly is to touch the positive probe of your multimeter to the drain pin (typically the middle pin on a TO-220 package) and the negative lead to the source pin (typically the rightmost pin). The meter should register at least several tens of kΩ, if not MΩ. If you’re getting a low resistance, try shorting the gate and source pins to remove and residual gate charge. If the resistance is still low (typically less than 100 ohms), the MOSFET is likely dead shorted. Of course, if the MOSFET is still in a circuit, your meter may be measuring the resistance of some other component. One thing you might try with the TO-220 package (pictured above) is to clip only the source lead before you test the device. Then you should be able to safely test the device. If it seems to be working, just dab a bit of solder over the cut lead.

One last thing you may wish to verify is the internal body diode (seen in the schematic symbol above) between the source and drain. Many multimeters have settings to measure the voltage drop across diodes. However, if your meter does not have this function, a simple resistance check will do. Our earlier test of resistance between the drain and source should have indicated very high resistance. However, if you reverse the polarity of your meter’s leads (positive → source and negative → drain), you should now read a very low resistance due to the body diode (if all is well).

Well, that’s my preferred technique for testing MOSFETs – if you have further suggestions or tips please feel free to comment! Thanks!

ΩΩ

The Doom Box: Part I

I’ve long had a fascination for renewable energy – be it from wind, solar, or hydro-electric sources. Perhaps it’s just my penchant for penny-pinching, but the idea of free and (virtually) unlimited energy is something I find quite appealing. So, in February of 2009 (wow, has it been that long?) I bought my first SUN 90W, 18VDC photovoltaic (PV) panel:

Solar Panel (with voltmeter)

(As a side note, if that looks like a stock image, that’s because it is. However, it really is a picture of my very own panel and voltmeter. Stock photography is another hobby of mine which I may post on later.)

So, what did I have in mind for this 90W powerhouse? A reverse-UPS of course! What is a UPS, you ask? Well first off, UPS stands for uninterruptible power supply. You typically find these connected to computers, particularly server computers. Their purpose is to continue supplying power in the event of an outage. Essentially they’re fancy battery backup systems that can be switched on almost instantly in the event of a main power failure. Actually, some UPS systems run their inverters continuously and only rely on the mains input to keep their internal batteries charged.

Now most uninterruptible power supplies power their loads via a connection to the mains (120VAC) until an outage occurs. At this point the load is switched over to battery power. My plan for a reverse-UPS was just the opposite. I intended to power loads via solar and battery power until the battery reached a certain level of discharge (~70% of full charge so as to extend the life of my battery), then automatically switch over to mains power.

To accomplish this, I first bought a 63Ah deep-cycle lead acid battery and a SunSaver 15A MPPT charge controller from the Alternative Energy Store. While the MPPT feature on the charge controller did cost me a bit more, I think it’s a big plus, particularly for cloudy winter weather. So far I’ve been very impressed with the SunSaver’s performance:

SunSaver 15A MPPTThe next step was to obtain an AC inverter. I considered making one myself (as well as the charge controller), but opted to go COTS for convenience and safety. I may have it backwards, but I generally feel more comfortable using professionally-made devices when it comes to dealing with high voltage and current. (But of course I also fused just about every wire in the system – a very good choice, as you’ll see later.) I also wanted a pure sine-wave inverter for efficiency and smooth SSR switching (discussed later).

So with my battery-backed solar-powered AC source, all I needed was a means of switching between that and line AC. Paralell solid-state relays (SSRs) seemed to be the best solution because of their speed and easy of use. All I needed was an accurate and reliable way of controlling them. For that, I chose a simple 8-bit AVR microcontroller, the ATMega48. This is a great little MCU that can handle plenty of digital I/O and up to eight analog inputs. One of those inputs was wired up to monitor the battery. Several digital inputs went to switches and buttons, and two digital outputs went to the SSRs.

Switching from line AC to inverter AC was actually quite simple. Once you remove the digital “ON” signal from the SSR, it takes at most half an AC cycle to turn off (this is because its current must reach zero before turn-off occurs; see TRIAC). So, for a 60Hz AC source (standard here in the US), that means 8.3ms. To be safe, I went ahead and doubled that figure. So, to switch between relays, the MCU only needs to turn one off, wait about 16ms, then turn another on. Of course, the consequences of getting that timing wrong could be painful, so I was sure to thoroughly stress-test my code and electronics before running a powered test. So far, no fuses have been blown by crossed sources.

Here are a few images of the original completed system (before any revisions):

Doom Box (1)

Doom Box (2)

Yes, it’s made of wood. Pine, if I recall correctly. And yes, wood is flammable. I don’t plan on taking advantage of that fact. The cooling fan is there to keep everything well below flash point. It’s also intended to vent any hydrogen gas that may be given off by the lead-acid battery. Wood does have one plus though: it’s non-conductive.

Below is a close-up of the front panel, where all the user interaction happens. The large switch on the top left connects the solar panel to the charger. The bottom left connects the battery to the rest of the system (inverter, microcontroller, etc.). The left black button allows the user to force the battery to operate even if it’s below 70% of full charge. The right button allows the user to manually command line AC output if desired.

Doom Box (3)

Here are a couple shots with the lid removed. The system measures about 16″ x 12″ x 14″ and, with battery in place, weighs about 40lbs. I don’t remember exactly why we started calling it the “Doom Box” – perhaps it just sounded amusing. I suppose it is a potentially dangerous box to go sticking your hands in…

Doom Box (4)

Doom Box (5)I’m happy to report that the doom box worked quite well for a number of months. I strapped my 90W solar panel to a lawn chair, weighed that down with a cheap sack of marble chips, and put the whole thing out on my east-facing balcony. The AC output was connected up to an old laptop I run as a server. It draws about 35W continuously. So whenever the sun was out and the battery fresh, the laptop was powered entirely by the doom box. Once the sun moved out of view and the battery discharged to 70%, the box automatically switched the laptop over to line AC.

Unfortunately, given its location, the solar panel only saw direct sunlight for a few hours each morning, but that’s the best I could do at the time. As such the laptop was only off the grid for about 6-8 hours each day. I was also quite surprised at the difference in performance between direct sunlight and cloudy days. Very cloudy days cut down my power input by about 80-90%.

Well since this initial build I’ve modified the MCU to support a greater level of monitoring. Current sensors are now in place and the entire system is programmed to log and report its status via website. I’ve also added a second solar panel and battery. But that’s another post for another time. Stick around for dramatic stories of inverter failure!

Part II of this article may be found here!