The Numbers

How much electricity can we really produce?

Engineers love numbers. They (the numbers, that is) generally bore people to death, but at times they are necessary for understanding. One of the biggest questions that has been asked is simply, "Can we really generate enough pollution-free electricity to power our businesses and homes?" The calculations below are presented to answer this very important question.

First, the "givens":
In the 48 contiguous states alone, pavements and other impervious surfaces cover 112,610 square kilometers-an area nearly the size of Ohio-according to research published in the 15 June 2004 issue of Eos, the newsletter of the American Geophysical Union. Continuing development adds another quarter of a million acres each year.

Let's do some conversions: 112,610 square kilometers equals 43443.54 square miles. The report was done six years ago, so that means an additional 1.5 million acres have been turned into impervious surfaces. That's an additional 2343.75 square miles, so all told, we have 45787.29 square miles of impervious surfaces. Let's make a conservative assumption that a full 1/3 of that number accounts for rooftops of homes and businesses, which we're not currently interested in. That leaves us with 28962.36 square miles of roads, parking lots, driveways, playgrounds, bike paths, sidewalks, etc., to work with.

If these impervious surfaces were replaced with Solar Road Panels, how much electricity could we produce?

In labs, solar cell efficiency has exceeded 42%, but you can't get your hands on these - at least not yet. For our calculations, we looked up commercially (and cost competitive) available solar panels.

Sunpower claims they will be shipping solar panels with 19.5% efficiency later this year (2010). They're already shipping E18 series panels with 18.5% efficiency, so let's go with what is currently available.

For our calculations, let's use the following assumptions:
. We use solar cells that have an 18.5% efficiency
. We average only 4 hours of peak daylight hours per day (4 x 365 = 1460 hours per year)

Sunpower offers a 230 Watt solar panel rated at 18.5% efficiency. Its surface area is 13.4 square feet. If we covered the entire 28,962.36 square miles of impervious surfaces with solar collection panels, we'd get:

((28,962.36 mi²) x (5280 ft / mi)²) / (13.4 ft²/230W) = ((28,962.36 mi²) x (27,878,400 ft² / mi²)) / (13.4 ft²/230W;) = (807424257024 ft²) / (13.4 ft²/230W) = 13858774560860 Watts or over 13.8 Billion Kilowatts

If we average only 4 hours of peak daylight hours (1460 hours per year), this gives us: 13.8 Billion Kilowatts x 1460 hours = 20,233,810,858,855,600 Kilowatt-hours (or) 20,233 Billion Kilowatt-hours of electricity.

In 2009, we received a contract from the Federal Highway Administration to test some of our theories and to build a crude prototype Solar Road Panel. One of the tests that we conducted was "real world" solar collection.

When you install a solar panel, you have to take into account where you are installing it. The farther north you live, the more you have to angle your panel toward the equator (or more accurately, the sun above the equator). We did our testing in January and February in northern Idaho.

Here we have worst case scenario: our measurements were taken in the dead of winter (sun is at its lowest point of the year) an hour south of the Canadian border at latitude 48.19 degrees. The farthest northern point in the contiguous 48 states is 49.38 degrees near Lake of the Woods, Minnesota. That's 82miles farther north than our location. Conclusion: we would be hard pressed to find a worse time and place to conduct this experiment!

At our northern position (48.19 degrees North), the optimal solar gain angle for our solar panels is 72 degrees. Brownsville, Texans would want to angle their solar panels at 26 degrees. So our southern roads will naturally produce much more electricity than their northern counterparts, as solar intensity maps show.

Unfortunately, we can't angle roads or parking lots. Roads go up and down hills, have banks on curves (going both left and right), and have a typical three percent "crown" (on both sides) to allow stormwater runoff. It's a pretty safe assumption to figure that the national average angle of roads is zero degrees.

We tested two identical solar panels. We mounted one at the recommended 72 degrees for our location and leveled the other one with the horizon (zero degrees) to simulate an average road. We installed a monitoring system to track the data 24/7.

Although the tilted solar panel produced more energy as expected (an average of almost 31 percent more than its horizontal counterpart), we were surprised to see the phenomenon of the horizontal solar panel producing more energy than the tilted panel on certain overcast days. It appears to be similar to getting sunburned on a cloudy day: sunlight is still present, but it is scattered, so the horizontal solar panel is more likely to pick up the scattered photons than the solar panel aimed at the southern horizon.

For fairness, let's subtract 31 percent from our totals since we can't angle roads and parking lots:

20,233 Billion Kilowatt-hours x 0.69 = 13,961 Billion Kilowatt-hours.

While we found no evidence that moonlight or the light from shining stars at night produce energy in solar panels (a common question), we found that headlights did. Although it would be very difficult to measure accurately due to distance, speed, hi/low beams, etc., we found that a small solar panel placed flat on the ground about 10 feet in front of a vehicle with its high beams on produced electricity in otherwise total darkness. So it appears that vehicles driving on the surface at night will be providing a service as well as reaping the benefits.

According to the Energy Information Administration, the United States (all 50) used 3,741 Billion Kilowatt-hours of electricity in 2009 (EIA Electricity Overview, 1949-2009). It's easy to see that the Solar Roadways could produce over three times the electricity that we currently use in the United States. The "lower 48" could produce just about enough electricity to supply the entire world.

What does this do for greenhouse gases?

About 40% of U.S. carbon dioxide emissions stem from the burning of fossil fuels for the purpose of electricity generation. Coal accounts for 93% of the emissions from the electric utility industry.
~US Emissions Inventory 2004

This is where some of the numbers become "fuzzy": as best we can tell, it is estimated that approximately half (different agencies provide different estimates, but the average is about 50%) of the greenhouse gases that are causing global warming come from the burning of fossil fuels (primarily coal) to generate electricity. The Solar Roadway will, therefore, eliminate half of the greenhouse gases currently being produced.

Summary: the Solar Roadway can cut the causes of global warming in half!

What is all this going to cost?

The cost of road materials has shot through the roof. We're told that highway construction materials have gone up 500% over the past five years. Our own Governor Otter (Idaho) shared that the cost of liquid asphalt was $175/ton in December of 2007. By June 2008, it was $480/ton. When I spoke with the governor in July 2008, they were getting bids at over $1000/ton. Asphalt is petroleum-based.

The Federal Highway Administration came out last year with a solicitation calling for a new pavement system that could generate power and pay for itself over its lifetime. That's our goal.

What will it take to make the Solar Roadway financially self-sustaining (the financial benefits over the design life outweigh its initial cost)?

There are multiple ways that the Solar Roadway can generate revenue:

. Through the generation of electricity
. By transporting cleaned water to municipalities or agricultural centers
. By leasing the internal or roadside conduit to entities such as telephone, high-speed
internet, cable TV, etc.
. By selling advertising in the road or in parking lots with the configurable LEDs.
. By charging people or companies to recharge their electric vehicles

Since it would be difficult to guess what could be anticipated with the later four methods of creating return on investment (ROI), we'll focus on the one method for generating revenue that can be calculated: electricity generation.

. The average cost of residential electricity was 12˘/kWh in the U.S. in April 2009.(6)
. The cost of electricity has increased an average of 35 percent from 1989 to 2009.(7)
. Our target cost for a Solar Road Panel is $10,000.00 and our target lifespan is 20 years.

At today's price of 12˘/kWh, a Solar Road Panel would have to generate 10,000/0.12 = 83,333 kWh of electricity over 20 years to pay off its initial cost. That's a worst case scenario: assuming that the price of electricity doesn't increase over the next 20 years (we know it will!).

To generate the 83,333kWh that it would take a Solar Road Panel to pay off its initial cost over a 20 year life span requires an average daily production of 11.4kWh (at today's prices).

By the time it reaches Earth's surface, the energy in sunlight has fallen to about 1,000 watts per square meter at noon on a cloudless day. Averaged over the entire surface of the planet, 24 hours per day for a year, each square meter collects the approximate energy equivalent of almost a barrel of oil each year, or 4.2 kilowatt-hours of energy every day. Deserts, with very dry air and little cloud cover, receive the most sun-more than six kilowatt-hours per day per square meter. Northern climates, such as Boston, get closer to 3.6 kilowatt-hours. Sunlight varies by season as well, with some areas receiving very little sunshine in the winter. Seattle in December, for example, gets only about 0.7 kilowatt-hours per day. It should also be noted that these figures represent the maximum available solar energy that can be captured and used, but solar collectors capture only a portion of this, depending on their efficiency. For example, a one square meter solar electric panel with an efficiency of 15 percent would produce about one kilowatt-hour of electricity per day in Arizona.(8)

This Arizona number is based on 6.8kWh per day. It appears that the U.S. average falls at right about the world average of 4.2kWh of energy per square meter every day (averaged over the entire year). We'll use this number to calculate what efficiency our solar cells will have to perform to achieve our goal of paying off a Solar Road Panel in twenty years at today's cost per kWh.

Our Solar Road Panel is 3.66m by 3.66m (12ft by 12ft), or 13.4m2. So based on the 4.2kWh average per meter squared, our Solar Road Panel should average receiving 56.28kWh of energy per day.

Let's now take our Sunpower 18.5% efficient solar cells and see how they'd do. They could theoretically collect (56.28kWh x 0.185) 10.41kWh per day. At that rate, it would take just under 22 years for the Solar Road Panel to pay itself off. Using solar cells with slightly higher efficiencies will allow us to make our goal of 20 years. Again, this is assuming that the price of electricity will not increase over 2009 rates for the next two decades, and that we don't use any of the other methods of revenue collection - only electricity generation.

One of the great features of the Solar Road Panel is that much of it can be reused. Some components like the solar cells, capacitors, and LEDs will wear out and have to be replaced, but the majority of the panel is reusable. If we began manufacturing today with 18.5% efficient solar cells, and the panels lasted 20 years before the need for refurbishing, the latest (20 years from now) efficiency solar cells would be installed and the Solar Road Panel would produce even more power than before. This will allow the Solar Roadway to keep up with the increase in electricity demand over the years.

In addition, the Solar Roadway replaces our current aging power grid. The Solar Roadways carry power - not from a centralized point like a power station, but from the power-producing grid itself along with data signals (cable TV, telephone, high-speed internet, etc.) to every home and business connected to the grid via their driveways and parking lots. In essence, the Solar Roadways becomes a conduit for all power and data signals.

Comparing apples to oranges:
For an accurate cost comparison between current systems and the Solar Roadways system, you'd have to combine the costs of asphalt roads, power plants, and power and data delivery systems (power poles and relay stations) to be compatible with the Solar Roadway system, which provides all three.

In addition, there is no way to calculate additional savings such as the reduction in costs of vehicle and health insurance (due to lighted night roads, wildlife avoidance systems, snow/ice removal, etc.). Accidents and the loss of life (both human and wildlife) will be drastically reduced upon the Solar Roadway.

Then there is the whole environmental issue: elimination of the fossil fuel plants will take away about half of the CO2 emissions that are known to be contributing to the climate crisis. Providing a means to recharge all-electric cars anywhere along the roadside will open the door for the elimination of the internal combustion engines, which account for most of the other half of the CO2 emissions. With internal combustion engines now obsolete, our dependency on oil - foreign or domestic - will finally be over with.

Conclusion: for roughly the same cost of the current systems (roads and fossil fuel burning electricity generation plants), the Solar Roadways can be implemented. Unlike current road systems, it will pay for itself over time. No more Global Warming. No more power outages (roaming or otherwise). Safer driving conditions. Far less pollution. A new secure highway infrastructure that pays for itself. A decentralized, self-healing, secure power grid. No more dependency on foreign oil.

The real question may be: what will be the cost if we don't implement the Solar Roadways?