Dueling Mirrors: How Many Recursions?

Many old-style barber's shoppes had opposing walls of mirrors that allowed someone sitting in the chair to see an "infinite" number of images of himself, receding into the distance. I am reminded of the dueling mirrors by an idea from a Salerno province high school physics professor to oppose two parabolic mirrors in order to magnify the heat concentrating effect of incident sunlight. Here is the professor's idea:

The design consists of two parabolic mirrors arranged face-to-face. Sunlight first hits the larger mirror and reflects to the smaller mirror placed a short distance away. Then the light from the smaller mirror reflects back, this time being focused into the vertex of the larger mirror. By confining sunlight into this small region, scientists can ideally trap solar radiation. The sunlight is stored in a blackbody, which consists of a cavity with perfectly reflecting inner walls.

"Through a sunlight trap system, solar radiation is first concentrated in a small region of space and then sent into a blackbody, where it can be stored (not for an arbitrary long time, though) for a variety of uses," De Luca told PhysOrg.com. "For example, after having trapped sunlight in a cavity with perfectly reflecting inner walls, what we call a blackbody, one can think of heating water enclosed in a container placed inside the cavity itself. Other uses of this concept are also conceivable." _PO

The European researchers (U. of Salerno) want to use the idea to generate solar thermal electricity, and possibly to desalinate seawater -- among other ideas.

I cannot help but wonder how many times one could repeat the trick, using increasing numbers of opposing parabolic mirrors? Why stop with only 2 mirrors? Each successive nested "zozzaroid" would be smaller than its predecessor, which places limits on your nested array. Alignment would be critical, but could be easily achieved using visible targets or electronic devices. How hot could you make the "black body" target with 6 opposing mirrors? With 12? What about using "fresnel" "parabolic" mirrors for space saving?

The basic idea is similar to a reflector telescope, which "amplifies" the incident starlight from a large area into the small eyepiece optic. It is unlikely that Newton (or even Archimedes) anticipated these particular uses of dueling reflectors.

Hybrid California Power Plant 24 h Renewable

Two new hybrid power plants are to be built near Coalinga, California, to provide roughly 106 MW of renewable power 24 hours to an energy starved US state. It will be powered by the sun in the daylight, and by bio-gas powered steam at night.

The two planned solar thermal-biomass hybrid power plants - the first of their kind - will be managed by Martifer subsidiary San Joaquin Solar LLC and will provide enough power for nearly 75,000 homes in northern and central California. They combine solar thermal technology with steam turbines powered by gas made from locally available biomass (agricultural waste and livestock manure.)

When the sun is shining during peak hours, it will just be the solar facility. As the sun sets, biomass will be available to support the solar generation, and then at night the biomass will run purely on its own. - Andrew Byrnes, project developer __Biopact

This use of bioenergy as an after-hours backup power source for solar power plants is quite logical. Biomass and biofuels are simply another form of solar energy, containing their own built-in storage.

This is only one small step toward moving power generation back into the golden state from neighboring states that are less fastidious. Most of the new energy plants in California over the past 20 years are of an intermittent, non-baseload nature--solar and wind. That means that during long spells of energy shortages, one could not always count on California utilities to reliably provide needed service. As more ingenious ways of using bioenergy come online, expect it to find more of a place across North America--even in California.

Can You See the Tiny Dots On the Map?

Image Source

The dots cover the land area necessary to provide the current energy needs of the human world, given only 8% energy efficiency of the solar to electricity process used. Solar thermal gets about 30% efficiency, and most photovoltaics get close to 10% or more.Solar thermal energy is an abundant resource most constant and plentiful in the near-equatorial desert regions of the world--such as North Africa. Europeans hungry for more energy are looking south to the African desert for electricity that may allow Europe to limp forward despite a suicidal KyotoII gesture being pushed forward by EU bureaucrats.

A small modular solar thermal plant such as this might produce roughly 25 MW of electrical power. If you could also utilise the waste heat from the plant, your total energy production including useful heat would at least double.

A large power grid spanning huge distances of North Africa, the Arabian peninsula, and parts of Europe, would allow the abundant solar resource of the Sahara to be exploited by wealthier and more productive Europeans to the north.H/T Treehugger (from Spiegel) via Peswiki

Excerpted from Al Fin Energy

Solar Thermal Kicking PV's Arse

Solar thermal energy plants have advantages over photovoltaics (PV) plants. Thermal energy is easier to store than electrical energy, for matching production to load. In addition, solar thermal is more efficient than PV. And, electricity from solar thermal plants can be produced by steam turbine driven generators, in three phase AC--just like in the most common form of electrical power systems. Eleven solar thermal companies are currently vying for desert space in the US Southwest.

California’s Mojave desert sure is getting crowded as more and more companies are raising money in an effort to build large solar power plants to harness the sun’s heat. Yesterday, eSolar said it had raised a massive $130 million from the likes of Google and Idealab for its small-scale solar thermal designs, while Stirling also said it had raised $100M from Irish renewable energy developer NTR for its SunCatcher solar thermal dishes. And this morning Infinia, which makes Stirling engines for solar thermal, said it had raised $57 million in a second round of funding. __Read more at earth2techfor information about 11 solar thermal companies

eSolar has been quite successful in raising funding, and has developed a modular design to match the power plant to the available land and required power loads.

Based on eSolar’s 33 MW pre-fab form-factor, the company’s modular design translates to minimal land requirements and are tailored to fit local resources and produce a low environmental footprint, favoring a straightforward sitting and permitting process. Various locations with multitude interconnection options mean that eSolar can deliver more clean, carbon free power where it is needed; close to the cities and towns where it is consumed.

“The eSolar power plant is based on mass manufactured components, and designed for rapid construction, uniform modularity, and unlimited scalability,” said Asif Ansari, CEO of eSolar. “Rather than over-engineering the solution, eSolar’s smart scalable solar architecture targets what we see as the four key business obstacles facing the sector: price, scalability, rapid deployment, and grid impact.”

eSolar’s approach to power plant design revolves around a tiered delivery model, beginning with a 25 MW base unit, called a module, consisting of several thermal receiver towers, each with a field of heliostats. These modules are replicated as many times as necessary to fit specific power requirements. ___ImpactLab

The rush is on to cash in on abundant solar energy in the desert regions of the US, in Spain, and soon the Persian Gulf countries. Solar thermal plants range from the 25 MW eSolar module up to 500 the MW installation being installed by Stirling Energy Systems, which can be expandable to over 850 MW eventually.

Someday, PV will be ready to compete with solar thermal. But that day will not appear within the next 15 or 20 years.

And if you need a lot of baseload energy--in the multi-GW range--nuclear is still your best bet for large scale baseload energy, other than coal.

The US is unfortunate in having what appears to be an incompetent US Congress at this time, on energy matters. The current US Congress seems determined to choke US energy sources and to create a large scale economic crisis in the US economy. I say the Congress "seems determined" to do this. If there is a better explanation for the extreme incompetence of the US Congress on energy matters, I am open to hearing it. Perhaps the Congress is simply stupid. That is possible. I prefer stupid over criminal.

Solar Thermal: The More Mature Older Solar Sister

Of the "Solar sisters", PV and Thermal, Thermal is clearly the more mature and ready for primetime. Why? Because Thermal solar contains its own storage.

...Some of the new plants will feature systems that allow them to store heat and generate electricity for hours after sunset..

..The workability of solar thermal power was established in the 1980s, when developers in California built a series of plants in the Mojave Desert, eventually reaching 354 megawatts of capacity. A megawatt is enough electricity to run 1,000 room air-conditioners at once.

The California plants grew more sophisticated and costs shrank as the project progressed. But then the price of a competing fuel, natural gas, collapsed in the 1990s and building new solar plants became uneconomic.

...solar plants do tend to produce peak power during the hottest part of the day, when demand is highest and electricity is costly, so at certain times they are already competitive with plants using natural gas. And they have an advantage over the other widely available form of renewable power, wind turbines: they are more predictable.___NYTimes__via_KurzweilAI.net

Heat is currently easier to store for later use than is electricity. With photovoltaic, when the sun is gone the power is gone. With solar thermal, the heat can be stored and used for several hours after sundown, until people go to bed.

Al Fin has argued for the superiority of solar thermal over PV for years. Incorporating phase change materials in thermal storage systems should extend heat storage through most of the night.

Solar Energy--Even When the Sun Is not Shining

The rate of solar energy intercepted by the earth is about 5,000 times greater than the sum of all other energy sources, but less than 0.5 percent is represented in the kinetic energy of the wind, waves and in photosyntheticstorage in plants. The amount of the solar energyintercepted by earth is only one thousandth of one millionof the total released energy in the sun. Source.

Using photovoltaics, you are limited to an average of six hours per day of usable energy production, minus time for bad weather. Often the periods of heaviest usage of energy are times that the sun does not shine.

One way around that problem is to place photovoltaic panels in earth orbit, well above the shadow of the earth. Generated power can be beamed to earth by microwave, and collected by large rectenna farms on the surface. Here is a link to a blog, Power From Space, devoted to this topic.

Another method is to collect the sun's energy while it shines, and store the energy for later use. Unfortunately, this method does not collect nearly as much energy as the orbiting solar satellites, but the sun provides so much extra energy to earth that it will suffice. What is the best method of storing solar energy?

Batteries are not a good choice for utility-scale storage, because the energy density of batteries is too low, given their high cost and short lifetimes. The only possible exception in terms of current battery technology would be redox flow cells. In five or ten years, redox flow cells might be ready for the challenge.

Electricity is hard to store at the present time. But energy comes in many forms, and is convertible from one form to another. With present technologies, the best form of solar energy storage is thermal storage--heat. Below are several links providing more information about thermal storage.

Wiki
Purdue
Open Directory Thermal Energy Links
Ionic Thermal Storage
Dissertation on Phase-Change Heat Storage Systems

Thermal storage is a type of energy averaging. Rather than being forced to use all the six hours of sunlight at one time, the energy can be stored and used over the entire 24 hour period. Solar Ponds are one form of thermal storage. OTEC is another, and in that sense the ocean itself could be thought of as a huge thermal system.


Phase change thermal storage will eventually become an important strategy for energy conservation in residential and commercial building construction. An incredible amount of useful heat can be stored inside materials that undergo phase changes at appropriate temperatures. Utilities are experimenting with large solar ponds, phase change thermal storage, and other methods of using stored heat to drive a heat engine, to generate electricity over the entire 24 hour period, adjustable to demand.

Eventually, for electric power purposes, redox flow cells and other newer electrical storage methods will eliminate the need for thermal-electric conversion losses. But humans need heating and cooling to live and work comfortably, and in that sense thermal storage will always be useful for human buildings and infrastructure.