Solar Powered Bike

Solar Roof Layout

2011-10-10T09:13:00.000-07:00

It's been a while since I posted updates. I still read and respond to comments on this blog. However, I've been busy starting up my new company providing solar layout and shading analysis services for PV installers and contractors.

116 Miles Today

2009-10-03T19:59:00.000-07:00

A personal record. I estimate about 10 miles of this was on solar power, 50-60 miles on charge stored in the batteries (including a 3 hour recharge at a solar-powered facility using plug-in AC battery chargers) and the rest was good old fashioned pedaling. My cruising speed was usually between 20 and 30 miles per hour, depending on the terrain and headwind/tailwind. Here are some numbers for those of you who like numbers:

Total Distance: 186.9 km (116.1 mi)
Moving Time: 5:46
Average Moving Speed: 32.3 km/h (20.1 mi/h)
Max Speed: 66.6 km/h (41.4 mi/h)
Min Elevation: -76 m (-249 ft)
Max Elevation: 197 m (646 ft)
Elevation Gain: 1433 m (4702 ft)
Max Grade: 23 %
Min Grade: -14 %

Created by My Tracks on Android.

How to build a solar powered bike

2009-08-16T11:41:00.000-07:00

The original version of this "how to" article, written about a year ago, included gems like "save up money to live on for six months" and "quit your job." While this advice may be applicable to the no-compromise high-performance long-range solar bike project I'm still working on, it's pretty useless to the novice who is just starting to think about how to "add some solar" to their electric bike. Maybe you don't even have a bike but the thought of having a solar-charged electric assist sounds like a good idea and you just need some tips on where to get started and how much such a thing might cost. This article is for you.

The recumbent bike with the yellow tailbox you see pictured to the left is still my main focus but I've recently completed a second bike to use when the recumbent is disassembled for repair or upgrades. The new bike is a traditional upright mountain bike with an Xtracycle conversion kit. I've installed a hub motor and used batteries I had lying around from an earlier project. The photos show some proposed solar panel mounting locations. This baby is perfect for hauling groceries and large packages. With a couple of optional accessories it can even haul a ladder or a surfboard.

The middle column below shows exactly what I built. The left and right columns are suggestions for how to build a less expensive version or a deluxe version to match your budget.

	$ Budget Version	$$ My Version	$$$ Deluxe Version
	Electric bike conversion
	~ US$ 500	~ US$ 1,300	~ US$ 1,800
Bike	$0 if you convert an older bike you already own or get donated for this project. A mountain or hybrid bike with 7 speeds or less in the rear is ideal but with sufficient determination and ingenuity almost any bicycle can be pressed into service.	$325 New Raleigh Mojave 3.0 mountain bike. I considered converting my trusty Surly Long Haul Trucker for this project but it had the wrong kind of handlebars (throttle wouldn't fit), wrong size cassette (8 speed instead of 7 max needed for the hub motor) and 700c wheels instead of 26" I wanted for good hill-climbing torque. I also considered a used bike on Craigslist but concerns about getting a stolen bike and time and money spent getting a lemon into shape lead me to a new entry-level mountain bike from my local bike shop.	$600 for a new mountain/hybrid bike with better quality components. Don't get a rear suspension if you're using the Xtracycle conversion and don't spend more than $600 to get a lighter bike since you're going to be adding motor and batteries which will make it heavy no matter how light the frame. Do spring for disc brakes because this baby will have a top speed over 30 mph and will weigh over 60 lbs plus rider. Sooner or later you will need to make an emergency stop and you will be glad you have disc brakes when that moment arrives.
Motor	$350 bargain kit on eBay including hub motor, wheel, motor controller, throttle, wiring accessories (compared with middle option: less power, lower top speed, less durable, no watt-hour meter to measure remaining range in batteries). See also Amped Bikes	$690 for 600W geared BMC V2-S hub motor kit including motor controller, throttle and Cycle Analyst watt-hour meter	$800 for 600W geared BMC V2-S hub motor kit including motor controller, throttle and Cycle Analyst watt-hour meter. This is price is my best guess. You may find a better deal.
Batteries	$100 36V 10Ah SLA (sealed lead acid) battery pack plus charger. Heavy and don't last long but they're cheap and recyclable.	$250 approx value of used LiFePO4 48V 10Ah battery I had from a previous project (bought new on eBay 2 years ago for $600)	$400 LiFePO4 48V 20Ah battery pack on eBay (August 2009 pricing)
Battery range	Up to 20 miles without pedalling (varies greatly by rider weight, speed and terrain)	Up to 30 miles with no hills, no pedalling, few stops, keeping speed under 15 mph	Up to 60 miles with no hills, no pedalling, few stops, keeping speed under 15 mph
Top speed	15-20 mph	34+ mph (California legal limit is 20 mph)	up to 40 mph with smooth high-pressure tires, flat, smooth road and rider weight under 180 lbs
	Solar conversion
	~ US$ 200	~ US$ 850	~ US$ 1,700
Mounting rack	$100 for a heavy duty bike rack for mounting your batteries and solar panels. It should be rated for at least 50 lbs (~25 kg).	$450 Xtracycle FreeRadical/Longtail kit affords plenty of space for mounting 2 or 3 solar panels. I'm still working out the details of how to attach the panels. Check back in a few weeks for updates.	$750 Xtracycle conversion including FreeRadical/Longtail kit plus KickBack stand, WideLoader and LongLoader accessories for carrying passengers and large loads with greater ease
Solar panels	$100 for suggested minimum 30 watts at $3/watt on eBay. Good for 10-15 solar miles/day in the summer. Selecting solar panels for your bike.	$300 approximate value of prototype solar panels I acquired under special circumstances (I work in the solar industry)	$777 for three 24 watt SunWize Sol-Charger modules for a solar range of 30+ miles/day in the summer
Charge controller	Not needed if you use SLA batteries and several small solar panels wired in series and facing the same way. For example three 12V 10 watt solar panels can be wired in series to get 36V and connected directly to a 36V SLA battery pack. It's not ideal but it works and it's cheaper than getting a charge controller.	$100 Solar Converters CV 12/24-2PV solar charge controller. Takes nominal 12V solar input (up to about 20V) and delivers optimal charging voltage for 24V SLA battery pack (CC/CV, tapering off to 28.8V). Limited to 2 amps. This isn't ideal for LiFePO4 batteries but it seems to work just fine.	$200 Solar Converters 5 amp 12V input, 36V output charge controller. They have a model with an adjustable output voltage which may be a better choice for your expensive LiFePO4 battery pack.
	Not included: Mounting hardware for solar panels and batteries.

How to do it even cheaper?

Walmart and Target sell electric bikes starting at US$300. That price includes the bike, motor and battery. If you are considering a bike in this price range think about the compromises in quality that had to be made to get the price so low.

If you think my budget version above is still too expensive and you have access to junk yard parts and a well-equipped workshop, you may consider building your own electric bike from scratch. Using old bicycle parts and almost any 200 to 1000 watt DC motor, a moderately skilled Jack or Jane of all trades with ample determination can build an electric bike on a shoestring budget. Batteries can be re-purposed from just about any source - old cordless tools, backup power supplies for computers, slightly used SLA's from medical equipment, even old car batteries may be enough to get you started and can fuel your enthusiasm until you save up for a proper LiFePO4 battery pack and charger.

The only item on the list that doesn't have a free or cheap alternative are the solar panels. I don't know of any sources for used solar panels. Here in California, they are phasing out the old emergency roadside call boxes so maybe you can find if the old solar panels from these call boxes are being sold at auction somewhere? Searching for used solar panels on eBay yielded a handful of scam auctions trying to sell used panels for $5 or more per watt. Keep in mind that you can get new panels on eBay for less than $3 per watt.

Original post (April 2008)

Save up enough money to live on for at least 6 months. It is important that you do this before you proceed to step 2.
Quit your job or arrange to take an extended sabbatical leave. You won't have time for your job once this project gets under way. You did follow step 1 first, right?
Next, you'll need to decide between an old fashioned safety bicycle and a recumbent bicycle.

The safety bicycle was invented in 1885 by John Starley and hasn't changed much since. The "safety" part refers to the improvement it represented when compared with the penny-farthing bicycles that preceded it. It doesn't offer much opportunity to add large solar panels so you'll be limited to a 20-30 watt panel or about 10-15 solar miles per day of charging in the sun. You may be able to bring that figure up to 50-75 watts if you add a trailer. The advantage of this approach is that you can leave the trailer at home on cloudy days.

The recumbent holds every world record for human powered travel in terms of distance and speed (over 80 mph) thanks to it's efficient design. More importantly, it will allow you to mount 100-150 watts of solar panels directly on the bike. Based on my road tests, this would give you 40-60 solar miles per day in the summer. If you design your panels as front and rear fairings, you can actually improve the aerodynamics of your bike and help offset some of the added weight.

Select a motor and battery. I use the Electro-Portal E-4 kit but there are lots of other options available. You will need a battery to store the energy produced by your panels when you're not using the motor, such as when you're stopped or when you're going downhill or just pedaling on a straight stretch of road. The battery will also provide the extra bursts of power you'll want when you're climbing a hill or starting from a dead stop. Start with an inexpensive SLA (sealed lead-acid) battery and upgrade to a lighter, longer-lasting LiFePO4 battery later on. The initial cost is higher but they last longer so the life-cycle cost is the same or even lower.
You're going to have to design and build your own panels. Standard rooftop solar modules are designed to withstand impact by 1 inch hail at 50 mph, not to mention 30+ years of wind, sun and rain. They have to be very tough and durable to survive the extreme environments in which they're used. This is accomplished by framing the module with tempered glass and aluminum. These materials are heavy and don't belong on a bicycle. You may also be tempted by light, flexible thin-film solar modules. They can be rolled up and are virtually unbreakable but they suffer from a low efficiency, meaning your panel would need to be twice as big to get the same amount of power. Since you want to get the most power possible from the smallest panel possible, you're going to need to build your own panel from scratch. Given enough time and patience, anyone can build a solar panel. I didn't know anything about solar power when I started. You should start by purchasing this DIY solar guide from Stephan Hughes. Stephan is a great resource for do-it-yourselfers. [Update: May 20, 2008 - Stephan's eBay store appears to be offline. My email asking about this change has gone unanswered. Contact me via the comment link below if you're looking for resources for building your own panel.]
Start a blog and tell the whole world about your crazy adventure.

Fake solar bike exposed

2009-08-02T12:09:00.000-07:00

Bottom line: These "solar panels" are fake. Even if such wheel-disc solar panels were possible, they would require nearly 20 hours of charging for every 1 hour of riding.

I've been contemplating publishing my impressions of this "solar powered" bike since I started this blog over a year ago. I've held back because I didn't think I could talk about it without coming across as petty and jealous. In the interest of fair play, I'll start by discrediting myself and confessing that I'm cranky about this project being the top Google result for the query term solar bike. I don't think it deserves this level of attention.

Here are two major design problems the inventor would need to overcome to build a working prototype.

How do you transfer power from the moving solar cells in the wheel to the stationary forks? Magnetic induction? Inefficient and heavy. Brushed commutator? Inefficient, vulnerable to dirt and moisture. Hub motors solve this problem by having the wires attached through a hollow stationary axle and the permanent magnet parts of the motor (armature) rotate around the electromagnetic windings. This doesn't work if your current source is moving and your batteries are stationary. And it certainly doesn't work if you have a quick-release mechanism on the wheels like the bike pictured. The skewers in the center of the hollow hubs are right where the wires would have to be. Now that I think about it, the bike pictured above cannot have a hub motor hiding in the wheel (as the inventor claims) because hub motors cannot use quick-release levers.
Where do you get photovoltaic thin film which can be cut into a circle? I'm not saying it couldn't be manufactured but I attended the InterSolar trade show in San Francisco three weeks ago. 444 exhibitors showing off the latest solar technology and I didn't see one product which can be cut like this. In the Discovery Channel video on this page, the inventor holds a real PowerFilm thin film cell in his hand but the shiny metallic film in the wheels appears to be cheap reflective mylar held together with scotch tape. But it was on TV so it must be real, right?

Over two years have passed since this tantalizing photo surfaced and the website still doesn't offer to sell anything resembling the bike with the shiny wheel discs. The owner appears to be offering a cheap Chinese e-bike with a 200 or 300 watt front hub motor and a 5 watt solar panel strapped to the rear rack for a bargain price of $1800. A comparable ebike sans solar sells at Wallmart for under $300. Something tells me the shiny wheel idea didn't work out.

But let's be generous and assume that our intrepid inventor somehow overcame these two engineering hurdles. The numbers show that this idea still doesn't work. For the purposes of this exercise, I'll define "work" as having a design where the solar charging adds enough value to the non-solar enhanced ebike to justify the added cost and expense.

Let's start by calculating the surface area of the wheel discs. Measuring one of my 26" wheels, the inner diameter is 21". The area of a circle is πr² so we have 3.14 x (21 inches/2)² = 346 in² = 0.223 m². I'll be charitable and pretend that the parts of the panel obscured by the front fork, chain stay and seat stay don't reduce the panel's output.

At 1000 W/m², we would have 1000 x 0.223 = 223 watts theoretical output at 100% efficiency. Thin film modules have efficiencies from 3.8% to 5.9% (see the full-screen table in this post for details). Using the best-case scenario of 5.9% this gives us 223W x 5.9% = 13 watts per wheel or 26 watts total. Note that I am not counting both sides of each wheel because only one side can face the sun at any given time.

Now, let's consider the vertical orientation of these solar panels. Using the U.S. Department of Energy redbook data on flat-plate solar collectors for my locale (San Francisco Bay Area), we get an average of 2.7kWh/m²/day for a 90° tilt collector facing South. That means our shiny wheel discs would produce 2700 Wh x 0.223 m² x 5.9% panel efficiency x 2 wheels x 90% charging efficiency = 64 Wh/day. At an estimated 20 Wh/mile for a gearless hub motor on flat ground without pedalling, this gives us a whopping 3 miles on an all-day solar charge. Maybe twice that if you pedal.

If you lay the bike flat on the ground (0° tilt) from sun up to sun down in the summer, you get an average of 7.1 kWh/m²/day or 7100 Wh x 0.223 m² x 5.9% panel efficiency x 2 wheels x 90% charging efficiency = 168 Wh/day. Good for about 8.4 miles.

So there you have it, 3-8 extra miles per day under average charging conditions. You'll get a little more on clear, sunny days and a lot less on cloudy, rainy days. Put another way, you need 15-20 hours of charging for every 1 hour of riding. If this was your only solar option, your money would be much better spent on upgrading to slightly larger and/or more efficient batteries and skipping the shiny wheel discs. Luckily, there are many other solar options out there.

Security warning: The ordering page on this site is not secure (does not use SSL). Unacceptable. Never order anything online if you don't see HTTPS at the beginning of the page address.

Best solar panels for your solar powered bike

2009-07-19T21:31:00.000-07:00

I've recently been asked to recommend solar modules for use with a solar powered electric-assist bicycle project. It's not the first time the question has come up so I decided to organize some of my old research, do some new checking around. Here's my ranked list of about 20 different solar modules which may be suitable for a bike. They are ranked by weight, cost and size.

View full screen | download as MS Excel file

Short Answer

Get the SunWize Sol-Charger SC24-12V. It looks rugged, is relatively light, small and is priced reasonably well for this kind of module. Here is the spec sheet in PDF. I don't have any hands-on experience with this product but if anyone out there takes my advice, please let me know how it works out for you.

Long Answer

There's a confusing array of solar products out there. Most of them making the same claims about being "high efficiency" and whatnot which makes it difficult to sort them out. I approached this problem by identifying the following criteria:

Weight If you're putting a solar module on your house or boat, you're probably not very concerned with how much it weighs.* But if you're going to be hauling it around on your bicycle it really makes a difference when your batteries run out and you need to pedal home up a long steep hill. I ranked all the different modules I found by how many watts you get for each lb/kg of module you have to haul around and found a huge difference between the best and worst examples. At the top of the list was the PowerFilm 28W rollable thin-film module with nearly 16 watts per pound (34 watts per kg) but at 3.75% efficiency it was the worst of the bunch in terms of size. A lower efficiency module needs needs more surface area than a higher efficiency module to produce the same amount of energy.
Cost If cost is your primary concern you should have no trouble finding solar modules for under $3/watt on eBay. The trade-off is that your module will weigh about 4 times as much as the winner in the weight category above. If you're carrying 100W of solar panels, this difference adds up to 18 extra pounds. This is because the high volume production lines that make these lower prices possible churn out solar modules built around a big piece of heavy tempered glass surrounded by a chunky aluminum frame. It's a cost-effective solution for use on a roof but is far from ideal on a bicycle. If after reading this you are still considering glass/aluminum framed panels, pay extra attention to how you mount them to your bike. You will need to protect them from vibrations and mechanical shock.
Size Solar real estate is scarce on a bicycle so the more watts per square foot/meter your module can produce, the better. The highest efficiency modules available are very large 200+ watt Sanyo N-series HIT modules and SunPower's slightly less efficient modules. Both are about 16-18% efficient and too big and heavy for bicycles. If you can get your hands on some Sanyo or SunPower cells and build your own lighter module, that would give you the best of all worlds.

I hope some of this was helpful. Leave comments if you would like clarification on any of this and I'll be happy to update the post.

* Yes, I know about concentrated point loading and rafter spans in light frame construction.

Solar Bike FAQ

2009-05-30T17:25:00.000-07:00

What is the solar panel for?

Would you believe it's for charging my iPod? Mostly it's charging a battery that runs a motor. Think human-electric hybrid. On long trips, I only use it for extra help up hills. On short trips, I use it to boost my speed and reduce pedaling effort. Imagine commuting 10 miles without breaking a sweat. Imagine riding a bike which pulls its own weight up hills so that it feels weightless.

Do you really have to pedal?

While it's possible to ride shorter distances on flat ground without pedaling, I wouldn't want to. I started this project because I like cycling but I don't like climbing steep hills. This is still a bicycle. If I didn't want to pedal, I would have built a solar electric scooter.

Did you build it yourself?

Yes and no. The bike frame is an HP Velotechnik Street Machine GTe touring recumbent. Most of the other components, like the motor kit, are off-the-shelf items. The lightweight, aerodynamic solar electric module is a one of a kind custom job I designed and built.

Can you charge the batteries while pedaling?

I get this one a lot. In theory, it is possible. In reality, it takes a lot of effort to generate a useful amount of electricity. Imagine riding a bike with the brakes on all the time. How fun would that be?

But what about regenerative breaking down hills and in stop-and-go traffic?

Regenerative braking (using the motor as brakes and using the generated energy to charge the batteries) has attained near-mythical status as a source of free, non-polluting energy. It's like the Holy Grail of energy conservation. Regenerative braking makes sense on an electric car because a car is heavy and has a lot of momentum. Because a bicycle is so much lighter, regenerative braking would only add 2-3% to the battery's range.

The added cost and weight simply aren't justified. In my case, I'm using a mid-drive motor with a freewheel on it which allows me to pedal without using the motor or motor without pedaling or do both at the same time. It's a very efficient, safe set-up and has very little motor drag when pedaling. However, the wheels cannot turn the motor. To add regenerative braking I would have to change to a completely different drive system where the motor is integrated into the wheel hub.

BionX makes an electric bike conversion kit with a regenerative braking option. It's easier to implement this on a hub motor rather than the mid-drive motor I'm using. For the technically-minded, here's a great article with lots of numbers for a more comprehensive analysis: Regenerative braking and electric bicycles (PDF) .

What's with the funny looking bike?

Oh, the recumbent? In the earliest days of this design, I envisioned a large, flat solar panel with the rider sitting in a hole in the middle. Here's a sketch of that idea. I though I would need a low, stable three-wheeled platform to carry such a large panel. I figured the panel would need to be that big because I read that a cyclist uses 100 to 200 watts of energy while pedaling and I imagined being able to cruise all day without getting tired. I have since figured out that I don't need a 200 watt panel but that I do need a battery. The panel placement and design have undergone a least a hundred changes since the start of this project. Eventually, I may build a custom bike using carbon fiber composite construction to make it lighter.

How fast can it go?

52 mph (84 km/h) top speed coasting downhill
15-30 mph (24-48 km/h) pedaling with power assist on flat ground
20 mph (32 km/h) batteries driving motor, flat ground, no pedaling (this is the legal limit in California)
8 mph (13 km/h) projected top speed on solar power alone in full sun, no batteries, no pedaling

What's the range?

34 miles (55 km) on a full charge using just the batteries
20-25 miles (30-40 km) a day on solar power with my current solar module (cloudless day in Northern California in May)
40-60 miles (65-95 km) a day on solar power after I finish my next module
Up to 150 miles (240 km) a day starting with full batteries, given ideal sun conditions all day between June and August with my current 40 watt module in the rear and a second 60 watt module in the front (under construction). All of these numbers assume mostly flat terrain. Hill climbing can cut all of these numbers in half.

Are you an engineer?

Nope. Would you believe first semester engineering school drop-out? Basically, I'm just stubborn. I refuse to believe it couldn't be done.

How much did it cost?

About US$ 0.25/mile, assuming it lasts 20,000 miles. I have put 3,100 miles on it so far. Put another way, I've spent enough on research and development to buy two or three really nice carbon fiber road bikes.

Don't see your question here? Drop me a note in the comments.

Welcome Googlers

2008-06-14T11:42:00.000-07:00

Since I've put up this blog, it has received a steady trickle of visitors from Google searches related to solar / electric / recumbent bicycles. I find this fascinating reading and thought I would share the list of search terms some of you are using to find this site. Here's a list of the top searches based on how long each person spent on the site. The map to the right shows which cities visitors are from.

how to build recumbent bike
solar power bicycle build your own
electric bike solar
solar bike conversion
mid drive electric bicycle motors
recumbent bicycle solar panels & kits
tadpole trike mid-drive setup
solar powered recumbent
recumbent bicycle solar
how to make solar powered bicycles

For those of you who are considering a solar powered bicycle project, I recommend starting with my entry on How to Build a Solar Powered Bike. Drop me a comment. I'm curious to hear about your projects.

Estimating the Daily Range of a Solar Bicycle

2008-05-25T06:48:00.000-07:00

Given that most visitors to this blog seem to be interested in building a solar powered bicycle, I thought it might be helpful to share some information about how you can evaluate the range of such a vehicle based on the solar resources available in your area. My interest in building a solar bike is to extend my daily range on a long bike tour (think cross country) so I want to know "how many solar miles per day?" Here's how we figure that out.

We will need to know the efficiency of our motor. I measured mine by using a Watt's Up power meter (the Cycle Analyst will also work) while riding around a flat parking lot without pedaling until my battery's management system (BMS) cut out. I went 17 miles and used 217 Wh of energy so my motor's efficiency in this scenario was 12.8 Wh/mile (8.0 Wh/km).

Under more typical riding conditions where I'm pedaling most of the time and going over hilly terrain, my power consumption is about 8-12 Wh/mile (5-8 Wh/km) depending on how hard I'm pedaling and how steep the hills are. If you weigh more than me your power needs will increase. If your riding involves a lot of stops your power needs will increase. You will need to run these tests for yourself to get reasonably accurate figures but you can use my numbers as a starting point for your planning.

Next, we will need to assess how much sunshine is available where we will be riding. This chart shows year-round insolation data for the United States. I live in Northern California which is in the light green band between the yellow area in the southwest and the darker green along the Canadian border on this chart. Don't worry about what the colors mean. I've included this chart so you can get a rough idea if you can expect more or less sunshine where you live. Phoenix, Arizona gets more sunshine than I do. Chicago, Illinois gets less. Here's a global insolation map.

While this map is useful for a quick visual assessment, it doesn't give us the numbers we will need to figure out our solar miles per day. For that, we will need to look at the Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors. In the United States, the National Renewable Energy Laboratory (NREL) has been collecting solar radiation data at 239 sites spread across all 50 states from 1961 to 1990. The reports include average monthly sunshine for each site and they take into account weather like clouds and fog. The amount of information available in these reports is overwhelming but I'll show you which figures are useful for our solar bike project.

Let's look at the report of Sacramento, California. We will use this site instead of San Francisco because they're both at the same latitude but Sacramento's weather is more like the weather in Oakland where I live. The San Francisco numbers are a little lower because local microclimates create pockets of fog which reduce the amount of available sun. The numbers I've highlighted show the average number of sun hours per day. This is not the same as number of daylight hours. These numbers take the peak sunshine at solar noon and tell us the equivalent number of such hours per day. You get much less power early in the morning and late in the evening but the total output for the day measured in "sun hours" means "as if the sun had been shining at full intensity for this number of hours."

This is very useful because now we can take my solar panel's peak power output (41 watts in full sun) and multiply it by the number of sun hours per day to get watt-hours (Wh) per day. Please keep in mind that we're not using the panel's theoretical rated output but actual power measured between my charge controller and my battery on a hot day. This panel would probably be rated at about 55 watts at STC but we're only interested in actual, real-world output.

So we have 41 watts in peak sun times 7.2 sun hours in May for a 0° tilt panel for a total daily estimated production of 295 Wh. My actual measurements with the Watt's Up meter confirm this number (based on multiple 50 mile trips). This reflects energy collected per day while riding. To account for occasional shading from trees, buildings, and the rider's body casting a shadow on the panel, we'll subtract 20% from this figure. To account for battery efficiency losses (we have to put in 220 Wh to get out 200 Wh), we'll subtract another 10%. This gives us 207 Wh/day while riding.

On a tour, I may spend half the day riding and half the day with the bike parked in the sun. When it's parked, I can tilt the panel and turn the bike so that the panel is perfectly perpendicular to the sun (dual-axis tracking). My power generation will increase by 45 to 115% compared with 0° tilt, depending on the time of year. The NREL data shows 10.2 sun hours for dual-axis tracking in May. This would give me 41W x 10.2 hrs/day = 418 Wh/day. Subtracting 10% for less-than perfect tracking (maybe we only adjust the panel once per hour) and 10% for battery losses gives me 334 Wh/day during parked "charging mode."

So if I ride half the day and park half the day on my tour, I will generate 207/2 + 334/2 = 271 Wh/day. On flat ground, without pedaling, this translates to 21 solar miles per day. If I add the effect of hills and my pedaling efforts into the equation (8-12 Wh/mile), we have 23-34 miles per day. Adding the power from 2 batteries charged overnight, we get 60-90 miles/day. Adding a second panel (@ 60 watts) we're up to 1128 Wh/day or 94 to 141 miles/day.

Keep in mind that these numbers represent average monthly and yearly figures including all foul weather days with little or no sun. If you're a hard core commuter who rides rain or shine, your numbers should closely resemble these averages. However, if you're a fair-weather rider who selectively avoids days with little sun your averages should look better than these numbers.

Here's how all this looks in a spreadsheet. You can also access the interactive version of this spreadsheet and plug in your own numbers. Some figures may vary from the article above because I update the spreadsheet as I gain additional road test data and better understanding of solar-assist cycling.

Sourcing parts

2008-04-23T12:23:00.000-07:00

One of the major challenges in this project has been finding sources for some of the materials I've needed. To date, I've done 4,887 Google searches and accumulated 237 bookmarks. In addition to the cycling specialty shops and solar energy suppliers you might expect, I've gleaned insight and ordered materials from an assortment of highly specialized industries and niche hobbies I never knew existed. Here's a list of some of my favorites:

Aerospace - satellite-grade solar cells (Emcore via eBay)
Fire performance - as in breathing and juggling fire. Bought Kevlar tape to make an abrasion-resistant border around the solar module (Bearclaw Manufacturing)
Sculpture - fiberglass mesh for first solar module (Compleat Sculptor)
Auto racing - aluminum rivets
Boat building - marine-grade wiring and electrical components
Remote controlled hobby airplanes - power meter/fuel gauge (RC Electronics)
Veneer woodworking - vacuum bagging supplies (Veneer Supplies)
Star Wars replica costume builders - plans for vacuum forming machine (TK560)

Imagine all of these people at a cocktail party trying to make small talk.

Expert Help Wanted

2008-04-15T21:08:00.000-07:00

I'm currently working on a problem with composite core construction. Are you an expert in this area? Can you point me in the right direction? WARNING: I'm about to seriously geek out on you here. If you don't understand what I'm talking about, this entry is not for you.

How can I maximize the strength to weight ratio in a fiberglass/Baltek Mat layup?

Here's the layup: Tap Marine Grade Epoxy with slow hardener (143), 2 mm Baltek Mat with 3.7 oz "S" glass on either side using a hand layup. Pressing it between two boards while it cures gives excellent strength but at 0.40 lb/sq ft it seems needlessly heavy and resin rich. Same layup under vacuum bagging, Mylar on one side, polyester peel ply and breather/bleeder on the other at 5" Hg drops the weight down to 0.23lb/sq ft but the stiffness suffers and there's noticeable compression (30-40%) of the core material. I'm using this as the structural support for the next solar module and I'm looking for maximum stiffness and minimum weight. Carbon fiber is a little too spendy at $40-60/yd. As a composites novice, am I really going to see performance gains if I go with carbon?

Genesis

2008-04-13T12:00:00.000-07:00

Here's an older slideshow with more information about the genesis of this project. I put this together in October of 2007 so it's a little out of date. If you want to read the captions you can pause the auto-play feature and advance through the slides manually.