Which of the following is poised to become the energy mainstay for the twenty-first century: (a) coal, oil, or natural gas - fueled generating plants; (b) hydroelectric generators; (c) nuclear plants; or (d) systems built around clean renewables, such as geothermal systems, solar systems, wind turbines, or biomass/biogas?

Obviously all of these probably will figure into the mix, but only one is correct. It will be outlined and substantiated as follows (no peeking!). First, though, meet a company claiming not only to know the answer but also to have the wherewithal to make it happen and - perhaps most critically of all - to be able to muster the data and demonstration models to persuade governments and financiers they're right.

Stirling Energy Systems (SES), a project management and systems integration company, was founded in 1996 with the aim of becoming a major player in global energy. Toward that end, Chief Executive Officer David Slawson busily has been teaming SES with strategic partners, comprising a major Swedish defense contractor called Kockums AB; the United States Department of Energy (USDOE), which has underwritten SES's testing and development projects; Sandia National Laboratories, which is scrutinizing and testing SES's technology; Schuff Steel, the largest steel fabricator in the US; and Vesta Wind Systems, a leading provider of wind turbine technology.

Now, from all of this, can you guess what Slawson and SES's answer to the energy riddle is?

It's solar heat - not solar photovoltaic output but heat from the sun, captured to fuel generators. Harnessed heat will run both distributed gensets and centralized grid-connected generators - potentially on a massive scale - while serving niches in between. If Slawson and SES succeed in their aspirations, concentrated solar heat will become the worldwide energy lynchpin for decades to come. Eventually it will supplant fossil fuels and other old-tech relics. SES's heat-powered gensets - easily scalable, versatile, affordable, and extensible - will usher in an era of distributed, cogenerated, and centralized power in a well-balanced hybrid.

Now why is solar heat the correct answer? As is well known, solar heat is clean, quiet, and inexhaustible. Hence, as the technology for capturing it has advanced, solar heat's natural appeal has been ripening into real potential. The question was never whether solar would be tapped but when, how, and how much. Photovoltaic cells - though well known and fully established - probably aren't the answer yet; they're expensive and so far not sufficiently productive. What's relatively unknown outside the field of solar research - but what Slawson's company knows very well and owns rights to - is a system for concentrating solar rays for heat and transforming heat into a lot of electric power. SES currently is perfecting such technology, which, in ongoing field and lab tests, has yielded the highest solar-energy efficiency of any known methodology, year after year for 17 years running.

This superior hardware is actually not one product but two: the genset and the apparatus that concentrates solar rays combined into one extraordinary integrated system. Just how extraordinary is it? Realistically, says Slawson, it someday could fulfill the lion's share of domestic electrical needs for the US, in a manner both environmentally friendly and very cheap. As the following points from Slawson and Chief Financial Officer Bob Liden show, one way or another solar heat is in the lead position to achieve the spectacular potential for solar energy that enthusiasts have long anticipated.

About 150 years ago amid the industrial revolution, a Scottish engineer named Robert Stirling created an ingenious engine based on the novelty of harnessing external combustion. Heat from outside the engine - alternating with cooling of an enclosed working gas (typically hydrogen) - produces oscillating pressure waves inside the engine's two-stroke piston, and this turns a crankshaft.

Naturally, Stirling's engine's components have since been refined and improved. Currently the state-of-the art engine patent belongs to Kockums AB, which installs them in submarines of the Swedish Navy. In the mid-1990s, Kockums inked a deal with SES to allow for building a version of the Stirling in the US under a licensing and partnership agreement for powering gensets; Stirling Energy Systems LLC was launched at the same time.

Even after more than a century, Robert Stirling's invention remains the most energy-efficient heat-powered engine ever designed. This makes it a sort of romantic maverick, in technological terms. Too, it offers a seductive combination of features not found in internal-combustion machines. Slawson describes Stirling engines as follows:

• They can use an almost unlimited variety of fuel sources. Unlike gas- or diesel-burning motors, Stirlings can run on virtually any conventional combustible substance and on many unconventional ones, as long as they make heat; these include gasoline, wood, fuel oil, natural gas, propane, and - better still - used motor oil, landfill gas; "digested" feedlot manure, waste liquids, dirty, used, or low-octane fuels and gases, and assorted combustible waste byproducts. You name it, says Slawson. "They're omnivorous."
• They're quiet and clean. Indeed they are very clean and almost muted compared to the racket and exhaust emissions of internal combustion. Because the Stirling's fuel burns outside the engine block, combustion can be completed fully, unlike the case with the quick, bang-bang-bang explosive design of internal combustion, which emits hydrocarbon waste.
• They suffer no corrosion. Because the fuel and combustion never get inside, maintenance needs are minimal, and a well-made Stirling engine can continue for many years of continuous reliable operation.
• They're as versatile as any other genset. Although Stirlings probably never will power conventional vehicles, they easily can supplant diesel or other engines used in typical power-generation settings. "Virtually anywhere you can install a diesel generator, a fuel cell, or a microturbine, you can install a Stirling genset," notes Slawson.

With such beguiling features, why aren't Stirlings already powering everything in sight? There are several reasons, the main one being the complexity of their components, such as a high-temperature heater head, a regenerator system, and a gas-cooling system. Compared with pistons and valves, "they're relatively more expensive and trickier to make" under current demand conditions, notes Liden. "Essentially they're hand-built Lamborghinis."

Also, even if Stirlings were someday mass-produced, you probably would never see them available in a wide range of horsepower sizes. SES currently produces only two, a 10-kW model and a 25-kW model (with capabilities for making a 75-kW model when the anticipated market materializes). The 10-kW model has a theoretical potential role in off-grid applications. On the whole, though, the realistic, maximum practical size for Stirling gensets is around 75 kW. They can be chained in series to achieve several hundred kilowatts, but even at this maximum, the Stirling's marketability will be limited to only light industrial and small residential applications.

Slawson and Liden, however, have much bigger plans.

Now the best-case future for Stirling gensets won't involve either oddball or conventional fuels but rather will use heat beaming down from the sun and focus this onto the genset heat head by a second marvel of technology multiplied massively. Think not of isolated generators but of large tracts blossoming with neatly aligned, manmade parabolic reflector dishes. Think of big solar energy farms with groves of dishes numbering in the thousands. Dubbed the "Suncatcher," the dish is a story in itself. This highly sophisticated technology emerged from 30 years of research and development conducted by McDonnell Douglas, Kockums AB, and USDOE, at an estimated combined cost of $400 million. The effort resulted in the most efficient solar-to-electric technology ever developed. Operating on the same principle as a satellite TV dish, the Suncatcher's 82 faceted mirrors (38 ft. in total diameter) focus heat energy onto a genset heater head cantilevered 24 ft. away. Don't touch it. The warm sunlight is magnified to about 1,300¡F. A motor adjusts the mirrors' positioning throughout the day to follow the sun. The steady heat drives a generator yielding a consistent 20 - 25 kW of power all day long.

These numbers are not merely theoretical either. The solar-powered dish/Stirling combination has been field-tested and measured every year since the company's inception. To date, it has logged more than 25,000 hours of on-sun operating time (half a dozen years of continual use) and yielded more than 270 MWh of electric energy. Its up-time power availability exceeds 95%. Peak efficiency has reached 29.4%. Currently a two-year trial at the University of Nevada at Las Vegas (UNLV) is finding the 25-kW solar genset capable of yielding "a consistent 25 kilowatts of power uniformly during daytime hours, assuming bright sun," reports Bob Boehm, director of UNLV's Center for Energy Research. Wintertime wattage has dipped only slightly. Rival solar technologies "all have their beauties and drawbacks," notes Boehm, whose research unit busily is comparing head-to-head the Stirling with advanced photovoltaics. "The beauty of the Stirling, in contrast to [photovoltaics]," he says, "is that the heat approach doubles the efficiency of conventional [photovoltaics], which is only 8% to 15% efficient, compared to the dish/Stirling's nearly 30% [efficiency]." But he concedes that photovoltaics' big plus is that they usually don't require moving parts unless they're equipped with a sun-tracking element, such as the Suncatcher.

On that note, one dish improvement that Boehm and SES attained at the UNLV site was a fine-tuning of the tracking system. Originally this used a clocklike mechanism, but greater precision was achieved by replacing it with a computerized, self-correcting optimizer. This is typical of refinements that result from field tests.

Slawson and Liden list five more advantages that the solar heat strategy has over various kinds of photovoltaics. First, it is readily scalable, unlike some competitors; second, it can be harnessed for both distributed generation and as a central power plant; third, it is easily extensible to meet increasing demand; fourth, unlike some solar technologies, it requires minimal water; and fifth, it is easier to site and arguably is more environmentally friendly, in the sense that the prospective solar "farms" would be planted in the Mojave desert or other remote spots, with no leveling or defoliation required.

Best of all though in the larger-scale solar farm scenario, a dish/Stirling explosion will offer a large price advantage over rivals, in per-kilowatt-hour cost. Current testing indicates that a "grove" of 12,000 dishes would yield about 300 MW. That output will run a lot of air conditioners during the summer peak. Multiply this to a larger scale, however, and you arrive at a still-rosier scenario. The company calculates that an 11-mi.² solar farm planted in a particular barren desert in southern California would supply as much electricity on an annual basis as the Hoover Dam does.

A 25-kW dish/Stirling genset at UNLV consistently meets output expectations in summer and winter.

At first, such an installation would supplement aging fossil fuel power plants, but in rapid order, solar gensets would render the old guard obsolete.

Solar farm ownership could be retained either by a utility company or by independent power producers selling the "juice" for grid distribution. Over time in this model, North America would wind up with all-solar, pollution-free power.

And again, in a realistic best case, this would come at a very cheap rate. Specifically, Liden calculates that a utility would need to charge only about $0.05 - $0.06/kWh for the first 15 years of farm operation. This would enable investors to recoup their entire outlay within that period. The rate is very competitive too. Currently, for example, spot market prices for peak-demand power range much higher, to about $0.25 - $0.30, Slawson finds. Granted, price competition from cheap wind turbine producers, charging just $0.04 - $0.06, or from biogas plants, charging only $0.03 - $0.05, would undercut a big solar farm. Neither of these laudable renewable sources, however, can even come close to meeting the peak energy demand of the Southwest.

At any rate, after the 15-year payback period in which utilities are billed at the $0.05 - $0.06 rate structure, the solar gensets would continue producing electricity for the remainder of the estimated 25-year life of the farm - at a rate of only $0.015. "That's basically the cost of routine maintenance," notes Liden. For the duration of the project, an investor's annualized internal rate of return would come to about 18%, under the given assumptions.

What will happen to you and me when the wholesale price of power falls to just $0.015/kWh? That's an exciting and intriguing proposition. Suddenly it becomes realistic to talk about solar farms devoted to producing liquid hydrogen through electrolysis - thus yielding a cheap, clean, and endlessly renewable fuel to run next-generation automobiles. If power costs were only $0.02/kWh, liquid hydrogen could be retailed at just $1.50/gal. Fossil fuels eventually would be completely replaceable.

Moreover, with factories producing thousands of Stirling gensets serving to cultivate these big solar farms, unit manufacturing costs would drop sharply to a point making them very competitive with other gensets. This, in turn, would open up many more affordable distributed-generation and other off-grid applications. Remember that a Stirling isn't hooked on fossil fuels; it can run on anything that burns - even methane transmuted from manure. Animal feedlots, dairy farms, and landfills are obvious genset candidates here. Although the technical challenges of achieving this methane "alchemy" are considerable "and may not be commercially appealing to a lot of farmers," Liden concedes, the prospect nevertheless is promising enough that SES recently won a research grant from the California Energy Commission to explore this mucky avenue. SES is partnering with the Inland Empire Utilities Agency to launch a pilot program at a feedlot in Ontario, CA, beginning in mid-2004.

Other Stirling genset prospects in this scenario might include sites producing unusable hydrocarbon liquids, such as dirty motor oil, waste hydrogen, or other combustible waste byproducts.

Standalone, distributed, solar-powered Stirling gensets also would be lower-priced and suitable for potential off-grid jobs at agricultural pumping stations in the Southwest, at Native American tribal power stations, or for village power needs around the developing world. The power output in these cases would be stored in batteries so a system would be able to meet both daytime and nighttime demand.

One key to advancing the dish/Stirling's potential will be to multiply demonstration projects in the near term, similar to the previous, current, and pending ones in Las Vegas (25 kW), Albuquerque, NM (80 kW), northern Arizona (500 - 1,000 kW), and Johannesburg, South Africa (25 kW). Continued good results will lead to contracts for commercially viable projects both in the US sunbelt and in sunlit regions of the developing world.

Achieving the next big leap - scale solar farming - will hinge on these steppingstones and ultimately on whether Slawson and SES can persuade utilities, energy regulators, state and federal legislatures, and investors to share the solar energy vision. Sun power always will be clean and abundant, but to achieve its real promise, it will need access to markets committed to transitioning to renewable energy.

Faceted mirrors focus and magnify solar heat, enabling the generator to yield steady power.

For the near term, fossil fuels (still the largest industry on Earth) wield plenty of influence, but ultimately market forces will prevail; according to recent DOE estimates, in order to meet future power needs in the US, we'll need to increase our energy capacity an average of 20,000 MW every year for the next two decades. That's a total of 400,000 MW of new capacity. How will we get it? Gorging ourselves on coal, oil, natural gas or hydroelectric, nuclear, or even geothermal energy will not be enough because each of these alternatives carries limits or poses various problems.

Meanwhile the utilities of the Southwest face a yearly summer demand surge. This is increasingly expensive and problematic for all. At the moment the sun is shining brightest, the kilowatt-hour rates are peaking too. The solution becomes obvious. Solar energy - cheap, clean, and abundant - inevitably will assume center stage in the Southwest. From this base, it might well advance to the lead role in satisfying a voracious energy appetite worldwide.

Slawson concludes, "All of the technology needed to do this is off the shelf. Nothing new is going to be needed. Nothing has to be reinvented. It's already developed; it's interconnected now. The power-conditioning hardware is available to adapt to the solar gensets. What we're trying to do as a company now is to get our product ready so that as markets mature in, say, 2005, we're ready to roll out with solar gensets. And then we'll go right into these markets."

La Mesa, CA—based writer DAVID ENGLE specializes in construction-related topics.

DE - March/April 2004