Ford E250 Van covered with Solar Panels

Solar panels on the roof of a vehicle is nothing new. RV owners have been doing it for years. While we did not set out to set any records we may quite possibly have the largest surface area every covered with solar panels on the roof of a Ford van. The panels actually extend six inches beyond each side of the van and run the entire length of the roof from front to back.

mobile solar charge controller and power inverter

About our Solar Setup:

Components we used to generate, store and produce usable 110V electricity.

* Solar Panel(s) Three Suntech STP260/Vb1 260W 24V panels. On the web at $675.00 each
* Charge Controller – Morningstar TriStar Charge Controller 45A 12/24/48 VDC, TS-45 $140
* Xantrex Prowatt SW600 True Sine – Pure Sine Wave Inverter 600W $150.00
* Two Crown CR200 Deep Cycle 6 Volt Batteries. $125 each
* Random circuit breakers, thick 2 AWG gauge wire and wire connectors. $100
* Panel mounting materials. Roof racks worth approximately $300
* Total system cost = $2965.00

When I first started working on my camping van solar panels were not at the top of my wish list but they were certainly something I was interested in. Anyone who really knows me is familiar with the fact that I love to create gadgets and tinker with anything electrical or mechanical. When I was in my early 20′s camping without electricity was the norm, primarily because at that time the only things we had that needed batteries was our Sony Walkman and flashlights. Times change and I’m now usually dragging around a laptop, cell phone, Nikon camera, Sony video camera and for this trip a 22inch flat screen TV with hundreds of movies preloaded onto a Western Digital media player the size of beer can.

When I first started my research on what I would need to add solar power to the van I often referenced Handy Bob’s Solar Blog. Bob has lived in a solar camper for over seven years so I assumed he knew what he was talking about. Like most things these days technology changes quickly and if I knew then what I know now I would have done things slightly differently. One of the things I did not expect was such generosity from Clean Energy USA a Delaware Solar Installer. My good friend and travel companion Jeremy Bell and I built the website for Clean Energy USA and I always hinted to Dave Preston (our primary contact at CEUSA.com) that I would be happy to accept payment in panels or do a trade for services in exchange for Solar Panels. During a random Google Chat I asked Dave if they had any spare panels lying around the shop and he said they did. A few days later I showed up to pick up the panels and was amazed to find that they were each over six feet tall and nearly four feet wide. Not knowing much about Solar I had no idea what amount of electricity these solar panels would produce but my thinking was better to have too much than too little. One of the things I hate about math and engineers is that even the experts I talked to could not tell me what I could or could not definitely run or how long I could run electronic devices with these panels because of all the variables such as amount of sun per day, shadows falling on the panels, angle of the panel in relationship to the sun, length and thickness of the wires used to transfer the energy, size of the batteries and what condition they were in, make and model of the charge controller as well as make and model of the power inverter. I had people tell me I would not be able to run a 22” TV off the limited power storage of two 6 volt deep cycle batteries, They were obviously wrong because on many nights I would watch up to six hours of TV without putting a dent in the battery bank charge. When in doubt trial and error has always worked for me in the past. If we could make this work it would be great, if not I could always add additional equipment until it performed as I needed.

Incredible views near the top of Hurrah Pass 4740 ft. Moab Utah

Other things I was told that proved to be unnecessary:
That I needed to attach my house batteries (solar storage batteries) to my vans alternator via a battery isolator. While this seems to make good sense, we found it to be unnecessary. The solar panel generated all the power we needed even on overcast days. Attaching them to the alternator would have charged the batteries anytime we ran the vans gas engine but as I said time was short, I did not want to spend the additional two hundred dollars for the battery isolator and then spend another afternoon of running additional wiring. We got lucky and the sun provided all the energy we needed and in the end as you will soon read the solar panels outperformed the vans alternator.

Mounting the panels on the roof of a house is one thing, mounting them on the roof of a van that would be traveling at 65 – 80 mph for two weeks was something entirely different. I graciously accepted the aluminum support beams from Clean Energy USA that they usually use to mount a panel to a roof but from there on all of the connections were non standard and I’m sure voided any warranty the panels originally came with. A trip to Lowes and $50 of stainless steel fasteners with lock nuts and a day and a half of labor produced the mount you see in the van pictures. Just hours before we left on the trip I was still fabricating and attaching the air deflector mounted in front of the first panel.
To be completely honest Jeremy and I left on our journey before the Solar Panels and many of the other parts were wired or connected. Basically we pulled out of Delaware with the panels mounted on the roof and a pile of high tech hardware and wiring sitting in a box in the back of the van. Around 11:00pm our first evening on the road at a truck stop in Pennsylvania I located fuses for the wiring that I could not locate in Southern Delaware. It would not be till day two of the trip that we got around to making all the connections and mounting the hardware to the panel, batteries, charge controller and power inverter at a rest stop on the side of a road in Indiana.

Jeremy Bell Wires up the Tri-star charge controller

The biggest unforeseen problem we ran into was that the panels were rated at 24 volt whereas our batteries and power inverter were set up for a 12 Volt system. The charge controller we had on hand could charge 12, 24 or even a 48 volt system but it was not set up to handle the amount of voltage that three of these panels in Series would have produced, it would however have worked if the three panels were run in Parallel but this would have taken another 60 feet of wiring and additional holes drilled in the side of the van. For another $600 I could have purchased a different charge controller from Morningstar that would have handled all the current from three panels wired in Series.

charging in series - charging in parallel

I had planned on running our two 6 Volt golf cart batteries in Series which would double the output of the batteries to 12 Volt (6v + 6v) . To have a 24 volt system we would have needed two additional 6 volt batteries at a cost of about $120 each and 65 lbs apiece. A 24 Volt system would also require a different and more expensive 24 Volt Power Inverter.

What we ended up doing was connecting a single 24 volt solar panel to our charge controller set for 12 Volt. This had the unfortunate result of cutting the solar panels rated power in half giving us only 130 watts of power to our system but we would find in the following weeks that 130 watts was more than enough to handle what we threw at it. As sad as it may seem, two of the panels on the roof of the Van were not harnessed to perform their task and the one we did use was restricted to only giving us ½ of its rated output. For all the work it took to install the three panels, two of them went 100% unused throughout the entire trip. Live and learn….

When I purchased the new batteries they were given to me with about ½ charge already on them. Jeremy and I completed our Solar Installation on the roadside by about 2:00 in the afternoon and only had another three hours of daylight before the sun went down. That afternoon on the road we recharged our laptops, cell phones and even watched a few full length movies that evening on the 22” LCD TV all courtesy of the suns rays that we collected in those few hours.

The following days were actually pretty uneventful as far as the solar system was concerned. It just worked and worked well. We would run down the batteries to about 12.5 Volts at night watching movies and using our laptops and by noon the next day they would be charged back up to around 14.7 Volts ready for our next evenings electrical harvest of 4 – 6 hours of movie watching and surfing the net as well as charging other random electronic devices throughout the day.

It was not until my second week on the road that something unexpected happened where my use of the solar system would actually do more than just charge my entertainment and communications systems. After a long day of driving fire roads in Moab Utah I returned to my campsite after the sun went down to watch movies and read a book. I had over the last couple days begun the habit of running a cable from the 22” TV to the car stereo for stunning surround like theater sound. Approximately ½ way through the movie the sound died and I thought I had killed my Van Starter battery from running the car stereo. No big deal, I turned off the van ignition switch and finished watching my movie without sound from the car stereo and went to bed. The next morning I woke up and because the starter battery was a $200 Optima Yellow Top I expected it to have enough juice left in it to turn over the Van starter motor and start the engine. No such luck. The starter battery was dead. Solar Batteries to the rescue! I did not have jumper cables long enough to go from the vans starter battery (up front in the engine bay) back to our house batteries so rather than move 130lbs of solar batteries to the front of the van I removed the 40lb starter battery and brought it around to where the jumper cables would reach. I fired up my camping stove and made a pot of coffee while I waited for the solar batteries to transfer some of their charge to the stater battery. Not knowing how long it would take to transfer enough power to the starter battery I went on a four hour hike. When I returned I used my voltage meter to check the charge and all three batteries were at nearly 14 volts, more than enough to start the van. After returning the stater battery to its spot under the hood I stuck in the key and she started right up. Tragedy adverted, saved by solar. It would have cost an arm and a leg to get a tow truck to rescue me 30 miles down a forest service fire road 50 miles from civilization. I believed the problem was remedied and I would simply not use the car stereo to enhance my late night movie watching experience. Little did I know the minuscule power drain from the stereo was not the real culprit. Early in the morning the following day as I drove down a two lane highway somewhere between Utah and Colorado I noticed the starter battery voltage as indicated on the vans instrument cluster slowly seem to decrease. At first I thought I was imagining it and even put a piece of tape where the needle indicator was to see if it was truly moving. The tape did not lie, the voltage was decreasing. A healthy car battery while driving will usually have a voltage near 14 volts, my battery had about 9 volts. It was still early in the morning and I decided to pull over, make some coffee and try the solar battery to starter battery trick a second time. After only an hour the starter battery again had a nice healthy charge of nearly 14 volts. I re-installed it and was back on my way. Now paying more attention to the battery voltage than the road before me I watched as the voltage would again drop about two volts for every 20 miles driven. I turned off everything electrical in the van including the radio, fans and lights. About 30 miles into this I found myself on a very steep decent following an extremely slow moving semi truck. About ¼ of a mile from the bottom of the hill I ran so low on starter battery electricity that the van engine died.

The van with a dead battery rolling down a steep mountain in Colorado

I was still rolling downhill due to gravity but all of the vans gauges showed ZERO. Zero gas, Zero speed, Zero temperature and Zero oil pressure and Zero battery voltage. Luckily for me there was a scenic river pullout at the bottom of the hill where the van came to a rest. Either I had a bad battery or my alternator was toast. Seeing as though the starter battery was top of the line and only two weeks old I suspected the alternator which had nearly 150,000 miles on it.

6 volt + 6 volt in series = 12 volt 200 Amp Hours

I took out the starter battery again to recharge it from the solar batteries and fired up the laptop. Searching Google with a 3G connection hotspot from My Droid X Phone the nearest auto parts store was nearly 100 miles away. I called the number to make sure they were still in business and had the part in stock. Fortunately for me they had the part in stock and were open till 10:00pm. I told the auto parts lady on the phone that I loved her and that she had saved my life. Now all I had to do was figure out a plan to get the van another 100 miles. A charged starter battery would get me about 30 miles before I had to stop and recharge it again from the Solar batteries.

Charging the Optima Yellow Top Starter Battery

The good news in Colorado is that every twenty miles there are plenty of scenic places to stop for an hour or two to look around while the battery recharged. By the end of the day the van was nearly to its destination which was the Advanced Auto Parts store in Montrose Colorado. I was so close to the store that I threw caution to the wind and skipped my final recharge. I swear I hit every red light for the next ten minutes and had every slow driver in Montrose pull out in front of me. I pulled into the auto parts store with less than 9 volts displaying on the meter and not enough to restart the van if I had wanted to. I pulled the starter battery out for another recharge and walked into the store. The sales clerk who I professed my love to was no longer there, instead I got Ralph, who was bright enough to pull the right part and charge my credit card but took forever. I swear it took him 30 minutes to process the transaction.

Stuck at the bottom of a hill. You can see the Yellow Optima inside the side door charging from the solar batteries

I’m thankful that I was a boyscout and always took the motto “Be Prepared” literally. I had a pretty good set of tools and the Ford E Van repair manual with me, my only problem now was that I was parked in a seedy part of town and it was beginning to get dark. I popped the hood and began removing the air filter housing and fan belt to access the alternator when a friendly drunk came by and offered to hold my flashlight for me. I told him “You know I’ll be at this for a while, it will probably take me a good 20 minutes” he said “I don’t care, I ain’t got nothing better to do tonight”. Approximately 20 minutes later I had the old alternator removed, the new one installed and after returning the other van parts to their appropriate locations held my breath and turned the key. The van started right up and showed approximately 14.5 volts on the meter. 15 minutes later the meter still showed 14.5 V with no drop. Problem solved! I thanked my friend with the whiskey breath and was again on my way. In two weeks and 5000 miles of road that was the only problem I experienced with the van.

What would I do differently?

The system we had was thrown together at the last minute and some of it even after the last minute. We did not have a functional solar collecting system until our second day on the road and I had no idea we would be given 24 volt rather than 12 volt panels. Beggars cannot be choosers and the 24 volt panels were 1000% better than nothing and we were able to make them do everything we needed them to.

The three panels were massive overkill. One 260 watt Solar Panel would be more than enough. The fact that the panel was a 24 volt panel actually hurt our performance rather than helped. If we had a 12 volt 260 watt panel we could have used every one of those 260 watts, as it was we only used half its rated capacity because of the limited size of our battery array. If we had gone with a single 130 watt 12 Volt panel it would have been half the size and half the weight.

24 Volt vs. 12 Volt.
24 volt is great for residential and commercial grid tied systems. For a mobile van or RV based system my opinion is that a 12 volt system is a better option seeing as though so many auto accessories come from the factory to run on 12 volt such as small refrigerators, lights, fans, small TV’s and car stereos.

To convert the system I have now to run 24 volt I would need to purchase a 24 volt power inverter such as the Go Power GP-SW1500-24 1500-Watt Pure Sine Wave Inverter
($555), two additional 6 volt batteries ($240), and if I wanted to harness the full power of the three 240 watt 24V solar panels in series I would need an upgraded charge controller similar to the Mornigstar TS-MPPT-60 ($480)

What would I run with a more powerful system in the wild? A microwave oven might have been a nice addition and a 12 volt fridge would be great in warmer weather, as it was I was late fall early winter camping and all of my food stayed pretty cold on its own.

We were limited by our power inverter more than our system size for running some larger appliances. Power inverters come in all shapes and sizes. They can be as little as 5 Watt and as large as 5000 Watt they come in two common configurations the first is modified sine the second is pure sine. Modified sine inverters are a good deal less expensive and they can run 95% of the appliances you would typical plug into a wall, but it can damage some sensitive electronic equipment. Pure Sine is a cleaner form of electricity and won’t damage your $3000 laptop or $1500 video camera. For the $200 I spent on a 600 watt pure sine wave inverter I could have purchased a 3000 watt modified sine inverter. The 3000 watt inverter could certainly run larger appliances but I would worry about plugging expensive electrical appliances into it. If money were not an object I would simply purchase the largest pure sine inverter I could find. If money is limited and you still want to run larger appliances you could purchase two separate inverters with one for sensitive electronics and the other for higher wattage appliances like microwave ovens and hair dryers. A quick note when dealing with power inverters. You get what you pay for. I’ve used a few of the $50.00 300 watt inverters that sell at places like Wall Mart and Home Depot and they typically last less than three months.

Notes on Batteries:
Trojan (not the condom manufacturer) used to be the battery of choice for affordable, reliable solar power storage and depending on who you talk to may still be. When I started to call around however I was told that Crown made better batteries for nearly half the price. I also could not find a dealer of Trojan Batteries in Southern Delaware because they had all switched to selling Crown. Being on a limited budget on this trip I went with two Crown CR200 6 Volt 200 AH batteries wired in Series. With my limited power requirements they were more than enough and even able to charge the Vans starter battery over six times in one afternoon. Typical battery life for batteries like these can range from as little as three years to as high as six years if properly maintained. Weighing in at 65lbs each these batteries are not light. When charging, Deep Cycle batteries vent hydrogen gas which is not toxic to humans but is explosive and can replace oxygen in confined spaces leading to asphyxiation. Our quick solution? Leave a window open and the vans vent on high. If we had more time we would have built a vent from the battery box to the exterior of the vehicle. I like to say “Safety Third, enough with the fear. Full Speed Ahead!” and luckily for me I experienced no explosions or death by asphyxiation on this trip;)

Installation Notes:

I am not electrical genius but I can follow simple directions. According to much I’ve read some people cannot even follow simple directions. Where many Solar systems fall short is their failure to use the proper size wire. Both my charge controller and power Inverter came with charts describing what minimum size wire to use for a certain length run of cable. Basically the thicker the wire and shorter the run from charge controller to batteries and from batteries to power Inverter the better. I wanted to use 0 AWG gauge wire which is nearly ½ inch thick , but could not locate any or the cable connectors to fit in southern Delaware so I went with the thickest that Lowes had in stock which was 2 AWG which is still larger than ¼ thick. The directions that came with my inverter said to use nothing less than 4 AWG so I was still larger than their minimum requirements (lower number = thicker). We also went with extremely short cable runs of less than three feet each to minimize voltage drop. Why do I mention this? I’ve heard stories of people using small wire sizes and running lengths of 20 feet or more in their RV’s and wondering why their batteries never fully charge and why they can only run their appliances for short times before their batteries die. By following simple instructions Jeremy and I were able to create an efficiently running system with minimal power loss.

To sum up, if you are looking to add a small solar system to your Van, RV or End of the world 7.3 liter Bio Diesel Bug Out Vehicle (currently in the works) I highly recommend the following equipment.

* One 130 Watt 12 Volt Solar Panel Approximately $400 on Ebay.
* Two Crown 230AH 6 Volt Batteries Approximately $120 each. Most Golf Cart supply companies will have them in stock. Trojan T105’s are another similar more expensive alternative.
* An inexpensive charge controller like the Tristar TS-45 $140
* Your choice of 12 Volt power inverters. They range in price from $25.00 to over $1000 and you get what you pay for. Anything in the $300 and up range should be decent quality.

You should be able to keep your total price under $1000 for a system capable of doing what ours was able to which includes running small TVs, computers, small power tools, fans, radios, charging other automotive batteries and similar tasks. Just don’t expect it to produce enough juice to keep you warm at night or cool on a scorching hot summer day.

Related posts:

1. Sine Wave Inverter and Inverter/Charger compact G3 series
2. Eltek Valere Brings Solar Power to Wireless and Broadband Telecom Networks
3. Nissan and Showa Shell partner to test solar quick charge system

Ben from VictoryGasworks.com the Gasifier Designer.

Related posts:

1. Chemrec Gasifier Reaches 10,000 Operating Hours
2. Former Ford factory in Wixom could be used by alternative energy companies
3. Ford finds buyer for Wixom plant, will be used as renewable energy park

140.7 degree solar reflector

I first came up with the idea of building my own Solar Trough while researching a topic for a client on youtube.  The video I found suggested cutting a piece of 8″ pvc tube in half, lining it with a reflective surface and aiming the concentrated sunlight at whatever you wanted to heat.   I had everything I needed sitting in my garage and following the directions from youtube,  I had one constructed in less than half an hour.  Initial testing resulted in a maximum temp of 140 degrees.  Not bad, but nothing  spectacular.  As a test I also spray painted a 2 liter soda bottle black and filled it with water.  In the same sunlight in less than an hour it achieved a maximum temperature of 120 degrees.  For all the effort of building the PVC reflector I was only getting 20 degrees hotter than I was by heating up water with sunlight in a black bottle.  After going back to the drawing board I realized the PVC tube is a circle (duh) and what I really wanted was a parabolic shape (thanks for wasting my time youtube guy!)
Second Attempt:

300 + Degree Homemade Parabolic Solar Trough

My math is a little rusty but a quick search on the internet revealed a number of sites that provided me with the proper calculations to draw a parabola. Rather than bore you I’ll cut right to the chase. Again I had everything I needed in my garage to construct my new design.  The 4′ x 4′ Parabolic Trough took me about an hour to build and initial testing provided over 300 degrees in less than three minutes of direct 10:00am sunlight.   That’s enough to boil water (212 degrees), cook a hamburger or provide all the hot water a family could ever need as long as the sun is out.

Conditions on the test date:  90 degrees ambient air temperature, clear and sunny.  Location – Lewes Delaware USA.  Date August 12 2009.

300 Degrees is a good deal more than 140 degrees so I would consider this second test a huge success.  With some slight modifications I should be able to reach somewhere between 400 and 500 degrees.

Materials:
4′ x 4′ piece of plywood $12.00
three  2 x 4′s $9.00
1 box of 2″ screws – $5.00
4′ x 4′ piece of 1/8″ wall paneling. Lowe’s or homedepot sell them for about $10
4′ x 4′ sheet of mylar $5.00
Can of spray glue $5.00
4′ length of 1″ Copper Tube $5.00

Grand Total $51.00 ( I had all of these items lying around as scrap so it cost me nothing. )

Possible Improvements:

High Temp Thermometer. Currently my thermometer maxes out at around 300 Degrees.
High Temp black paint for the Copper tube.
Better Mylar reflector. My Mylar was very thin and difficult to attach without creating wrinkles.

Coming Soon:
I plan on adding a pump and some plumbing to this design to see how quickly it can heat 30 gallons of water.
If stationary testing provides the desired results I may add solar tracking to it at a later date.
In the winter months  running glycerin through the plumbing to a small radiator could easily heat a small cabin while the sun is out.

To do List:
Search Ebay for high temperature pumps.
Search the web for a tempering valve to control  water temperature.
Search the web for solar tracking devices.
Search the web for a long Pyrex tube to encase the copper tube inside of to trap generated heat.
Locate insulation that will not melt at 500 degrees.

Related posts:

1. New Solar Parabolic Mirror Design
2. Abengoa To Integrate Solar Thermal System with Coal Plant
3. SkyFuel to Install SkyTrough Collectors at Cogentrix Facility

My solar tracking mirror array or “death ray” as it is affectionately referred to by my friends is actually a heliostat. A heliostat is technically any device that tracks the movement of the sun, but most often the term refers to a device that orients a mirror to reflect sunlight continuously onto a specific target.

My heliostat consists of an array of 7 mirrors, each 1?x4? in size mounted to a 4?x8? plywood backing reinforced by a 2×3 frame to prevent warping. The frame is supported by a welded steel gimbal mount allowing rotation about both horizontal and vertical axes. Each axis is equipped with a stepper motor and leadscrew to adjust the position. In spite of the size of the heliostat, relatively small motors can be used because the system is well balanced. Using two stepper motor controllers wired to a laptop inside my home, I can control the orientation of the mirror array to reflect sunlight continuously onto a specific target on the north side of my home.

Heliostats are not a new concept. They have been used for very large projects such as two 10MW solar thermal electric power plants, Solar One and Solar Two. See The Solar Project for more details. I’m also aware of their use in some residential water heating applications; however, I have not seen any reference to their use for heating a home’s interior by reflecting sunlight directly into the home as I am doing.

Heliostats provide both light and heat (much like the sun itself… go figure). On warm days, I target a relatively small office window, providing plenty of natural lighting for the occupants (normally electric lighting would be used all day). On cold days I target the glass sliding door of my bedroom. The door is large enough to accept all the sunlight reflected from the heliostat. On a clear day at solar noon, this amounts to just under 2kW of power, and lights up the room like nobody’s business. It’s quite a surreal experience waking up to sunlight streaming in through a north facing window. One of the benefits I hadn’t anticipated is the device’s use as a solar alarm clock.

Gallery

Here’s an image of the front of the heliostat. This is about all you ever see from inside the house. One thing that surprised me about the project was how pretty it turned out. People who see it for the first time often don’t realize they’re looking at mirrors.

This is the back of the array showing the gimbal mount and the motors. Note the gimbal mount is fixed to a 2×4 with a point cut on the end that I simply pounded into the ground with a sledge hammer. This is just a prototype. Were I to install it permanently, I’d use something a little more rigid (the 2×4 tends to wobble a bit in the wind).

Here is a closeup of the drive system showing the stepper motors and lead screws. The stepper motor controllers are built into the same black boxes that contain the stepper motors. I build my own stepper motor controllers (my business is motion control, so this was the easiest solution for me) but they could have been purchased off-the-shelf too. Because the array itself acts as a roof, I haven’t found it necessary to provide much in the way of water proofing, though this may prove to be an issue over the long term. I do keep the lead screws well greased so they don’t rust

Here is an image of the initial software I wrote quickly in VB for Applications in MS Excel. For initial testing, I just calibrated manually by adjusting the mirror on target at a few different times during the day. I then matched a 3rd order polynomial to the calibration points to calculate proper motor position for any time of day. Every 10 seconds, the computer sends an instruction to each motor controller to move to the calculated position. This type of calibration doesn’t account for seasonal variation so I had to re-calibrate every few days as the heliostat drifted off target. Eventually I added all the proper math to calculate desired motor position based on the linkage geometry and solar position so the system doesn’t require any adjustment. See the software section below for more details.

Here are two images showing what it looks like near mid day when I’m targeting the upstairs office window.

This is a picture of a smaller solar project, my solar hot dog cooker, with the heliostat visible in the background, targeting the sliding glass doors of my bedroom at the right of the image.

And last but not least, here’s yours truly, standing next to my solar house cooker to give a better idea of scale.

Parts

This article isn’t meant to be a detailed set of instructions for building a heliostat. I built mine with parts that were available or easily sourced by me, which may be very different from the parts available to others. However, since a few people have asked where they can get the components, I will provide some information.

Support Frame

The support frame for mounting the mirror in such a way to allow rotation about horizontal and vertical axes was welded from 1? x 1? square tubing with 1/16? wall thickness as well as some round and flat bar. Below is a rough sketch showing the way I cut and assembled the pieces.

Motors

The motor I used for turning the heliostat left/right is an ordinary size 17 rotary stepper motor with a piece of 1/4-20 threaded rod attached to it’s shaft. This is the lower of the two motors in the images. A suitable motor is Jameco part number 155460. This is a little weak for the application, though it is adequate. For a little more thrust, try Jameco part number 238538. The price for either of these is under $25/ea US. These are not the exact motors I used but they are the least expensive I’ve found that are available to the average consumer.

You could use the same part number for the motor that tilts the heliostat up/down, but I found it more convenient to use a motor with non-rotating leadscrew. This is essentially a motor with a threaded hole through it’s center and a matching leadscrew. Rotating the motor pulls the leadscrew in or out (assuming the leadscrew is prevented from rotating by some means). A suitable motor is EAD Motors part number L1MGE-H12XX-0. This part number is for just the motor with no leadscrew. The leadscrew required is an ACME 1/4-20 leadscrew, which I already had (you cannot use 1/4-20 threaded rod). Since the average joe probably doesn’t have ACME 1/4-20 leadscrew lying around, you’d likely need to order a different part number that includes a length of leadscrew. Price for this motor is around $70 US.

Note the “tilt” motor is a lot more expensive than the “turn” motor, so if you’re on a tight budget, you might consider using a rotating leadscrew for both axes. If you do you’ll have to change design a bit. In the current design, I mount both motors to the T-frame. If you use a motor with rotating leadscrew for the “tilt” motor, you should mount that motor to the back of the mirror array and mount the leadnut to the T-frame. Then as the leadscrew extends through the nut, it won’t interfere with anything else.

Controllers

I used my own controller designed for my work (my company specializes in precision motion control products). This controller isn’t available as a stand alone product. It’s built-in to various products we sell. However many stepper motor controllers can be purchased off the shelf. To work with the above motors, you will want something capable of driving a 12V bipolar stepper motor with up to about 750 mA per phase. You also want something that can be interfaced to a computer via serial port, USB, or some other means.

A suitable controller that I’m aware of is the Allmotion EZ10EN. It sells for $175 US each. You can also try a Google Product Search on Stepper Motor Controller.

Mirrors

The guiding principle behind my mirror sourcing efforts was “cheap cheap cheap”. For this type of application, precision and/or flatness is not much of a concern. I found some 1?x4? mirrors on sale for around $8 each and snapped them up. I’ve since seen similar mirrors go for as little as $5 each. Mine are glass, which is not the most durable material (an array of mirrors may make a tempting target for any kid with a slingshot). If you can find mirrors made of acrylic or polycarbonate or another more durable material at a reasonable price, go for it.

Software

I automated my heliostat by adapting this program which is an Excel workbook with custom functions for calculating dawn, sunrise, solar noon, sunset, dusk, solar azimuth, and solar elevation. It’s based on formulas from http://www.srrb.noaa.gov/highlights/sunrise/azel.html. This program only calculates solar position so I had to adapt it to calculate the mirror position required (essentially aiming the mirror exactly half way between the solar vector and the target vector).

Update 2009-03-02: In the comments below you can also find reader contributed code in BASIC and C for calculating solar position and mirror position based on time and target position. See the comments for more information.

Performance

Power

On a clear day, the solar power available at the earth’s surface is about assuming a collector oriented towards the sun. Where I live (around the 49th parallel) the sun reaches a maximum of about 60° above horizontal at mid day in the summer. The sliding doors of my bedroom are about 20° above horizontal, relative to the heliostat. Therefore, when the heliostat is targeting the bedroom, the mirror surface is about 20° ((60°-20°)/2) from perpendicular to incoming rays. Therefore, the captured area of sunlight is about times the area of the mirror array. The area of the mirror array is .

Therefore, the power available to the array is:

The array is constructed of common glass mirrors which typically reflect around 90% of the light that hits them. Ordinary window glass typically reflects about 4% per surface (ie it transmits 96% per surface). A double-paned glass door has 4 glass surfaces (two for each pane). Therefore, the transmitted power can be calculated as:

Efficiency

The efficiency is simply the power transmitted divided by the power available.

Note that flat panel solar water heaters (which are the most common method of active solar heating) have an efficiency of only 30-40%; less than half that of a very simple heliostat. In addition, most flat panel collectors or passive solar heating methods don’t track the sun, and therefore the “power available” per m^2 of collector area diminishes more rapidly on either side of mid-day. A heliostat, on the other hand, can capture early morning and late evening light, and multiple heliostats can be used to reflect light through a single, relatively small, north facing window. For these reasons, heating a home by this method could be an even more efficient process than passive solar heating through large south facing windows. A home designed to take advantage of such a solar heating system would have multiple heliostats reflecting sunlight through a single relatively small north facing window onto a thermal storage and distribution medium (ex a brick wall or a large water tank) inside the home.

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