Solar Updraft – Inefficient but Effective

Posted in Uncategorized at 5:24 pm by Administrator

If you are a student of renewable energy you probably know the statistics regarding solar potential.  The earth receives solar radiation that is many thousands of times the energy requirements of the human race.  So energy supply is not the problem.  The efficiency with which we can capture that energy, the variability of sunlight received at any specific location and the requirement to have energy at night are the main obstacles to implementing a sustainable energy regime based upon solar power.

Much progress has been made over the past 20 years using a variety of solar technologies.  The most common technology employed today is called photo-voltaic (PV) which uses an electro-chemical reaction to convert light directly into electricity.  Historically this technology was very expensive but in recent years prices have dropped dramatically to the point where consumer solar panels are widely used to provide electricity for camping, battery charging, and other uses.

Rooftop residential PV installations have exploded in jurisdictions such as Germany and Hawaii where the regulatory environment is designed to promote a public policy goal of increasing electricity generation from renewable sources.  This strategy has generated not only electricity but also plenty of issues for local grid operators as passing clouds cause drops in PV output of as much as 60% in a few minutes.

Another solar technology referred to as Concentrated Solar Power (CSP) involves the use of optic lenses or mirrors to focus visible light onto receptors in order to generate very high temperatures.  The first and still the world’s largest CSP plant (with a rated capacity of 354 MW) was built in the Mojave desert in California between 1984 and 1990 (http://www.nexteraenergyresources.com/pdf_redesign/segs.pdf).  Since that time the technology has been refined and deployed extensively in Spain.  CSP plants use the concentrated solar energy to heat a fluid which in turn is then used to boil water for conventional steam turbines.

A large CSP facility is currently under construction in Gila Bend, Arizona.  The Solana plant (http://www.abengoasolar.com/web/en/nuestras_plantas/plantas_en_construccion/estados_unidos/#seccion_1) will generate up to 280 MW of power not only during daylight hours but also for up to 6 hours after sunset through the use of a molten salt energy storage system.  Because CSP plants generate electricity using steam turbines the variability issues associated with PV solar power are also eliminated.  The only drawback to this type of facility is cost which will come out to something like $5/watt of capacity compared to $1/watt for PV.

There are some variations on concentrated solar which do not involve the use of steam generators but instead generate power in smaller units directly at the location where the solar rays are producing heat.  Those technologies have not yet been deployed in large-scale production applications and will be the subject of my next blog.

A third potentially significant way to tap solar energy is a technology referred to as a solar updraft tower.  I would characterize this technology as a “Black Swan” because it has not, as yet, been demonstrated in a commercially viable operation despite many plans and proposals.

The solar updraft tower generates electricity by creating a large temperature difference from the bottom to the top of a vertical tower.  This, in turn, produces a vertical updraft which can be used to drive a wind turbine.

A pilot plant was operated in southern Spain from 1982 to 1989. This project was very successful in terms of the research data provided and clearly demonstrated that the technology could be used to consistently provide power.

The power generation did not drop off to zero at sunset but functioned at about 10% of capacity for several hours after sunset because of the dependence on a temperature differential rather than on the presence of direct sunlight. In fact, models indicate that by covering the area under the “greenhouse” with black, water-filled plastic containers the power production cycle could be substantially smoothed out with the facility producing power well into the late evening.

Finally, because the air and soil beneath the “greenhouse” act as a massive heat sink this type of facility is also not subject to the very dramatic short-duration variability that is associated with PV.

The major technological barrier that has so far prevented a successful commercial implementation of this technology is the height of the tower required to produce utility-scale electricity.  As an example, the towers being proposed for Arizona by Eviromission are planned to be more than 800 meters (half a mile) high.  The construction costs and associated engineering challenges associated with a structure this tall are formidable to say the least.

In my opinion this is a situation where “keeping things simple” might lead to success.  The original drawing of this concept contained in the 1954 book “Engineer’s Dream” by Willy Ley showed a large glass roof at the foot of a mountain acting as the solar collector.  The updraft tunnel in this illustration was a concrete tube built up the side of the mountain.

Updated technology could provide a workable solution.  Instead of a concrete tube, a series of hoops constructed from simple PVC tubing could be placed up the side of a mountain, covered by PTFE fiberglass fabric such as that used to provide roofs for sports facilities (an example being the Talisman Centre in Calgary).

Rather than have a single generator station located at the top of the mountain a number of large modified tunnel ventilation fans could be located at regular intervals up the slope (for example see the fans sold by Alphair).

With the right combination of technologies solar updraft could work.  It has the lowest efficiency of any solar power technology but offers the significant advantages of a smooth and reliable power production curve that extends well into the evening, matching electricity usage patterns.  It is mechanically very simple and in the right geographic setting could be very competitively priced.

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