TCS Daily


Solar Delusions

By Sallie Baliunas - May 21, 2002 12:00 AM

Editor's note: This article is the second in a series.

Solar power proponents tout sunlight as an energy source that is abundant, free of noxious pollutants and carbon dioxide emission. They claim that if only sunlight were harnessed, plenty of clean, inexpensive and abundant energy would be available to improve the human condition while preventing environmental degradation.

When asked why the fantastical promise of solar power over the last several decades has not led to very much of it -- less than 0.1% of total energy supplied in the United States -- Ralph Nader in an interview could only explain, "Because Exxon doesn't own the sun."

Nader and I agree on one implication of his statement: capitalism works. Beyond that, Nader ignores some down-to-earth realities about converting the sun's energy for human use.

Sunny Promises

People want electricity when they want it. Electricity cannot be stored; it must be generated and delivered as needed. Flicking "on" a light switch instructs the local power distribution system to locate and deliver electricity that courses from power plants through a grid of miles of wires to light the bulb in a fraction of a second. In the case of power from fossil fuels -- which at present supply about 70% of the U.S.'s electricity needs -- those fuels are burned to generate the electricity at nearly the moment the switch is flicked. No ready power generation; no light.

Can't sunlight do that job just as well?

As we learned in the last column, energy can be neither created nor destroyed, only transformed. To get electricity from sunlight, humans must do a lot of work to transform and deliver electricity to their homes and businesses. That work is a major barrier in cost-effectiveness of solar electricity compared to the current price of conventional sources of electricity.

Taking In The Rays

How do we transform sunlight? This episode focuses on fixed photovoltaic cells, which transform sunlight for local use. (Another way to transform sunlight is to concentrate and store its heat so it can create electricity through a generator -- solar thermal power. And hydroelectric and wind power owe their power to the sun. Those energy sources will be covered in another installment.)

Photovoltaic cells are remarkable semiconductor devices, producing an electrical current when sunlight strikes them. For example, my decades-old calculator works when light from the sun or a lamp shines on its blocky gray solar cell.

Most cells are manufactured from silicon and can convert up to about 20% of the sunlight illuminating them to electrical energy. The more expensive the cell, the more electricity it will yield. Even higher efficiencies may be possible with more exotic and expensive alloys employed as semiconductors, e.g., indium gallium arsenide nitride.

Highly efficient photovoltaic cells are excellent for space application, where a smaller size and lighter weight is favored over higher cost. But even in space, where sunlight is undimmed by clouds and atmosphere, the real estate required for useful applications of solar arrays is enormous.

To illustrate, consider the panels of ganged-together solar cells for operating the International Space Station that are, according to NASA, the largest electricity-generating arrays in space. They cover eight wings spanning more 32,000 square feet -- nearly three quarters of an acre. Even so, the more than 250,000 solar cells deliver a theoretical peak power of only 246 kilowatts - in the sunlit portion of the 90-minute orbit.

The director of NASA solar system space exploration says of planet-exploring satellites, "We currently operate with a light bulb's worth of power on board," which can limit science experiments. For spacecraft in deep space, faint sunlight means that solar panels are prohibitive in terms of size and weight. Expectations are for nuclear power systems aboard spacecraft to provide the kilowatts for improved science in deep space exploration.

The limitation of photovoltaics in space is mirrored on Earth with even greater trade-offs. Even the least expensive panels are relatively quite expensive. According to the Federal Energy Management Program, photovoltaic solar systems average about 25 to 50 cents per kilowatt-hour at remote locations, over a system lifetime of 20 years. The national average of conventional power delivered from the grid costs 4 to 8 cents per kilowatt-hour. More power can be had when the panels track the sun, rather than being fixed. The ability to track the sun adds to capital and upkeep costs. For now, less efficient and fixed systems will be favored, but they require more square footage for light collection.

At Home with Solar Arrays

Here's a practical example showing the impracticality of operating a fixed-panel system in New England.

A home clothes dryer uses about 5,000 watts (5 kW) of energy and takes about one hour to dry one batch of laundry. Now, my New England neighbors might save resources (money, most importantly) by hanging laundry outdoors. The downside is New England's weather: one must have a back-up plan in case of, e.g., freezing temperatures, blizzards or rain. An alternative would be to use the waste heat from a home furnace to dry clothing hung near the furnace.

The inconvenience is unappealing to many people. Could photovoltaics run the clothes dryer? Practically speaking, no, because sunlight shines feebly and intermittently in New England.

Accounting for both day and night, seasons throughout a year, and incidence of clear weather, a typical New England yard receives about 15 Watts of sunlight per square foot. There is no changing that -- it is the flux from sunshine that one can expect, on average, in New England.

So to run just the clothes dryer off sunshine would take about of 3,300 square feet of cells if the system delivers electricity at a good, 10% efficiency. True, the times of peak sunlight could deliver enough electricity to operate the dryer with less area of solar cells, but then the inconvenience of planning to work only during peak power - as in drying clothes in the air - returns.

In short, the diluteness and intermittency of sunlight means that solar collecting devices require land area, storage devices and back-up sources of electricity. On-demand electricity is convenient, and solar panels alone cannot provide it.

To operate more electrical appliances at the same time -- like the furnace, hot water heater, air conditioner, lights or computer -- would require ever more thousands of feet of panels. At night, nothing would run, unless there were significant energy storage capabilities. And what about those majestic cedar, sugar maple, ash, hickory, alder, beech, white pine, oak and giant sequoia trees? To keep shadows at bay during the day, those tall trees that sequester carbon and provide woodland habitat for animals from hawks to fishers would have to be chopped down so they do not block sunlight falling on thousands of square feet of panels.

The expanse of panels - much larger than the area of the typical home's south-facing roof - would need the support of sturdy steel and concrete structures to survive outdoor hazards. New England experiences hurricanes with winds over 100 miles per hour, tornadoes (a July 9, 1953, twister killed 90 people in Worcester, Mass., within one minute), heavy snowfalls of three to five feet, hailstorms and, although on very rare occasions, earthquakes (a magnitude 6 shock struck Cape Ann in 1755 and 6.5 in central New Hampshire in 1638). The panels also will need to be kept clean with periodic washing. Other drawbacks: in hot weather, the panels are less efficient, and as they age, their efficiency declines.

Still, couldn't solar panels offset some electrical use from other sources, so that, for example, less coal would be used to generate electricity, and wouldn't that be better for the environment?

Solar arrays may be economically worthwhile at isolated, sunny sites, or for small demand at peak sunlight times, far from the electrical grid. But on the electrical grid, solar arrays are not yet cost-attractive. The capital cost of installed panels (on a roof) is about $10 per watt -- $30,000 for 3kW, still not enough to run the clothes dryer. Conventional electricity from the grid is available at costs of roughly one-fifth to one-tenth that of the solar panel power, so the homeowner won't recover the capital costs even in the sunniest climates over a system's 20-year lifetime of collecting "free sunlight."

Thus small solar panel systems aren't likely to draw a lot of customers to reduce the need for non-renewable sources like coal. As for larger systems that might provide economies of scale to reduce costs, they have a major cost in the land needed for arrays. The problems homeowners face with small arrays compound when one looks at a hypothetical system that might serve communities.

Consider solar energy to serve Pennsylvania's 12 million people. How much land would be needed for solar panels at current energy usage, assuming the panels deliver 10% of the incident sunlight as electricity, averaged around the year, day and night? The answer: about 1,100 square miles snuggled together in one massive ecosystem-robbing swath, consuming a land area equal to an eighth of Vermont - all of it on land that is clear cut and of necessity kept bald.

That 1,100 square miles doesn't include the land needed to accommodate more panels to make up transmission losses, for service roads, for buildings and high power transmission lines, and for the inevitable storage devices when the sun sets.

The footprint of a massive "clean" photovoltaic facility serving any large community's energy needs would raise serious environmental issues all its own. That is one reason that even as photovoltaics may have a small niche market in rural areas distant from existing electricity lines, they for now appear incapable of delivering the power for a 21st century nation.

Next: Solar Towers

So, fixed solar arrays appear to be too environmentally costly to replace a typical 1,000-megawatt utility plant. The cost of photovoltaic cells may drop and their efficiency increase, but the sun is too feeble to make a substantial increase in capacity, without the technological breakthrough of inexpensive storage devices.

Could solar thermal towers that concentrate and store the sunlight, be less costly? The answer, again, is, "No," as we shall see in the next installment.
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