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Vol. 232 No. 3 |
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HENRY TERRELL, CONTRIBUTING NEWS EDITOR
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Solar power enhances recovery in old oil fields
Eighteenth-century gentlemen scientists discovered that sunlight could produce remarkably high temperatures under the right circumstances. When light passed through glass, it could be trapped inside a box as heat. In the 1770s, a Swiss naturalist named Horace de Saussure built a sturdy wooden box, which he lined with black cork on the inside and packed with wool on the outside, then sealed the opening with three layers of glass. When angled toward the sun, the inside of the box heated quickly to well above the boiling point of water. Actually, as long ago as ancient Greece, scholars had found that parabolic mirrors, such as are used in Newtonian telescopes, could be made to boil water by focusing the sun’s rays on a metal bucket. If the bottom of a reflective trough is parabola-shaped, the sun’s heat can be focused on a pipe, in as long a length as you can build.
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Parabolic reflectors are constructed of light, thin material. They pivot around the receiver, heating water to create steam. Courtesy of GlassPoint.
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Commercial applications. Many people are surprised to find that most solar energy is not photovoltaic, but direct, heating water to steam and driving a turbine (or, more usually, heating a medium which then heats water to steam). Experiments along these lines had been conducted since the 1800s, but the first commercial-scale solar power plants in the US were built in the early 1980s in response to the oil embargos and price spikes of the previous decade. These plants were built exactly where you’d expect—where the sun is bright and the land is cheap, California’s Mohave Desert. This area has some of the world’s highest insolation (that’s with an O), or the amount of solar energy received per unit of area.
In addition to the parabolic trough, there are the heliostat, or power-tower, systems that use arrays of mirrors to focus sunlight on a receiver at the top of a tower. These produce the highest temperatures—some use molten salt as a transfer medium. They have the advantage that—unlike the trough systems that require natural gas backup—so much heat is stored that steam generation can continue at night or in cloudy weather.
How much heat do you really need? What these large-scale systems have in common is that they all try to get the medium as hot as feasible. The more heat, the more power. But if the goal is just to produce steam, requirements are less stringent. In steamflood, what you need is not superheated salt but steam. One company, GlassPoint Solar, has built the first field-deployed solar-powered steamflood in an oil field for Berry Petroleum, in Kern County, California.
The Kern County Solar Pilot 1 addresses one of the biggest problems with solar arrays—wind. A typical 20-mph desert wind creates hundreds of times the force of gravity on a system where precision is crucial, so it must be built sturdy. And a real storm could end up destroying very expensive equipment.
Another problem is dirt. The Mohave is a very clean place, but nevertheless the acres of solar arrays have to be hosed down regularly or their efficiency plummets. In parts of the Middle East, such as Oman, a regular cycle of dust and dew means the reflectors must be washed daily.
Glass houses. The Kern County design addresses these problems by deploying the parabolic troughs inside a greenhouse. This not only protects the system from wind, so it can be made of much lighter materials, but the air inside is kept filtered and dry so there is no dust accumulation. Moving it all indoors, as it were, makes it cheaper and faster to build.
“We were able to take existing technology and build a completely new type of parabolic trough that is made with ultra-light reflective materials. It uses first-surface reflectors and not back-surface reflectors,” GlassPoint Senior VP John O’Donnell told World Oil. With a conventional mirror, a silver layer in back of a layer of glass, the light must pass through the greenhouse glass, the mirror glass and then back through the mirror glass before it can reach the target. With front-surface reflectors, light passes through glass once.
“In our case, once the light passes through one layer of glass, we can use higher-quality reflective material,” O’Donnell said. “Now the target can be fixed, because we have an ultralight reflector that can pivot around it. We can create steam directly. We’re using exactly the same tubing and materials as conventional gas-fired steam generators, and because everything is inside, we can deploy these things in the oil field.”
The greenhouse panels do collect dirt, of course, but washing flat panels is an easier task. Long agricultural experience with greenhouse design has resulted in automated washing systems that use minimal water. Glass cannot be made perfectly transparent (even in principle) but the glass used in the project allows 81% of the light to reach the reflectors.
The Solar Pilot 1 is a hybrid system of solar and natural gas. For this particular pilot, the solar array is too far from the injection wells to provide steam directly, so water is heated to sub-boiling and fed to gas-fired steam generators. This arrangement reduces gas usage by about 20%. In a larger system, and with most of the steam injected during the day (variable-rate steaming) the savings in gas can potentially reach 80%.
Money, money. Economics will determine the success or failure of solar steamflooding. “Instead of being in the $12/MMBtu range like other solar technology, we’re in the $2.50 to $3/MMBtu range,” O’Donnell said. “We are below the market price of natural gas in the US, and at or below market price in the Middle East. It’s the economic advantage that is going to drive this technology in the oil field, not mandates or restrictions on CO2.”
Once installed, solar steam takes couch change to operate, so the EOR in a field can be run longer, potentially increasing reserves. If you get past the odd-sounding idea of using solar energy to produce oil, it makes an elegant sort of sense. 
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