TCS Daily


Let the Sun Shine In

By Sallie Baliunas - July 11, 2002 12:00 AM

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

In our last installment we discussed solar energy -- directly converting sunlight to electricity by the use of devices called photovoltaic cells. These cells can be arrayed in panels to combine power from individual cells.

One significant drawback of solar energy is that without an attached storage device, a photovoltaic system supplies electricity only in sunlight. Nighttime and times of poor weather mean that solar panels alone produce electricity intermittently.

Another drawback of solar panels is that large arrays of them are required to produce significant amounts of power because sunlight is dilute near the surface of the earth. The extensive area footprint of panels would itself be environmentally punishing. In contrast to the progressive shrinkage of computer chips as technology advances, the sunlight's fixed output prevents an evolution toward ever-shrinking panels that would ameliorate the environmental impact. Even at an impossible 100% efficiency, panels can never be scaled down to an environmentally benign area that directly deliver city-sized electrical needs.

What's In Storage? Solar Two

But what if some of the energy converted from sunlight using solar panels could be stored and then transformed into electricity for later use, thereby boosting the usefulness of solar power?

One way to capture and store energy from the sun is with curved mirrors that focus sunlight on a medium that holds heat produced by the concentrated sunlight. Solar Two in the Mojave Desert concentrates sunlight with an array of more than 1900 parabolic mirrors to raise the temperature of molten salt held in a tank atop a central 300-foot tower to 1050 degrees F. Even after sunset the heat of the still-hot molten salt can be drawn to run a generator.

Another feature of Solar Two's power tower system increases the amount of heat delivered to the molten salt over the course of a day. The collecting mirrors are mounted on tracking devices that aim the mirrors at the sun throughout the day. Not only does the sun's position sweep across the sky throughout the day, but also the arc described by the sun each day changes day by day through the year. For example, in the Northern Hemisphere the sun reaches a lower maximum height above the horizon in winter than in summer, owing to the tilt of the earth's axis of rotation away from the perpendicular to the plane of the earth's orbit about the sun. The mirrors are controlled to follow the sun as it arcs through the sky.

Contrast the complexity of Solar Two with solar arrays on roofs of homes in residential areas. These arrays demand simplicity and so in general they neither track the sun nor include energy-storing devices. A solar power tower increases the cost and complexity over fixed-panel systems by adding tracking and storage capabilities and there would be higher capital and maintenance costs for the tower system. But more electricity could be produced. In some circumstances, the increased complexity and cost of a power tower system over a fixed panel system may make economic sense.



Solar Two's ability to store heat for later use increases the flexibility of a solar electricity system because it can deliver electricity for a while after sunset. For example, according to the Department of Energy, on a September day the power output of the turbine reached a peak by around noon, and continued for about two hours beyond the time the collectors no longer receive significant energy. The time of the turbine's main output was matched to the time of peak demand for power, so the power from the system is a timely addition to the available supply of electricity on the grid.

Location, Location, Location

Solar Two's location in the Mojave Desert is ideal for siting a solar tower installation because of the locally intense sunshine on many days of the year. In contrast, Kotzebue, Alaska receives roughly half the intensity of sunlight averaged over the year than does the Mojave Desert. Koztebue's solar resources are so feeble because the sun's rays arrive obliquely, making Koztebue a much less effective site for collecting solar energy than the Mojave.

The performance figures for Solar Two provide instructive estimates on the effectiveness of generating industrial and city-sized power demands. Howard Hayden's book, The Solar Fraud: Why Solar Energy Won't Run the World, gives the facts on solar and other renewable technologies.

For example, if Solar Two were scaled upward in power output to deliver 1000 Megawatts -- the typical size of a conventional coal-fired utility -- then the hypothetical army of sun towers would occupy an expansive 127 square miles! Let's put that in perspective. If we were to build a solar array in Vermont to deliver electricity to Pennsylvania's approximately 12 million residents, how much land would need to be dedicated in the Green Mountain State? Accounting for the weaker intensity of sunlight (Vermont's solar resources are poorer than the Mojave Desert's), the answer is about 6,100 square miles - or two-thirds of Vermont's land area.

That, however, is a minimum estimate, because it does not factor in the loss through transmission lines as the electricity speeds to distant Pennsylvania. An added problem is that the electricity supplied by solar towers would not be available for most of the night, because the technology of storage capacity is not yet sufficient for running more than a few hours beyond sunset. A present-day solar tower system would actually not be able to supply all the electricity for Pennsylvania on a 24-hour basis. Traditional, reliable sources of generating electricity, like uranium or coal plants, would still be necessary to supply power on demand and around the clock.

Cost-Effectiveness

Another important consideration when it comes to developing solar energy is the cost of a solar system when weighed against both the effectiveness of the system and its environmental footprint. Hayden describes another solar program in the Mojave, SEGS (Solar Electric Generating System) with tracking mirrors and a heat storage system that differ from those of Solar Two. SEGS uses parabolic mirrors laid in rows, or troughs, that track the sun to heat oil contained in pipes running along the foci of the mirrors. That heated oil ultimately runs a generator.



Compared to Solar Two, SEGS has a smaller but still environmentally demanding footprint - about one-fourth the area for a similar power output, or approximately 1500 square miles. The wholesale cost of SEGS electricity is higher than the national average, despite its subsidies. Another environmental and maintenance cost comes from keeping mirror reflectivity high in order to attain good efficiency; SEGS' 21 million square feet of mirror surface must be washed every few days.

Solar power is by nature intermittent and dilute and with current technology is not cost-effective for running the machinery of a major modern economy. Solar power may work for some distributed, boutique uses, although not generally at rates that compete with the cost of traditional supplies of electricity. Some uses may be cost effective, for example, in isolated areas distant from the grid and for applications with low power demands.

Augmenting solar capability with storage devices, and back-up sources of power, is essential to supply electricity on demand. Even in the best locations - like the Mojave - solar power cannot replace large-scale utilities run on, for example, coal, uranium or hydropower. For all of these reasons, solar power will likely remain a marginal source of power in the next decade or so.

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