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Solar Power Towers Efficiently Using Brayton Cycle Answered

I want to point out a solar to electric generation concept that has yet to be seen anywhere, even though it originated back during the Carter Administration's ERDA programs of the late 70's. I’m talking about solar power towers that convert solar energy into electricity at the hundreds of mega-watt level. While power towers do exist today, and the world currently does have a handful of them as shown in Fig-1, none use the Brayton Cycle nor can they boast an energy conversion efficiency at the mid to upper thirty percent level.

 A group of engineers got together at a think tank organization called Sanders Associates in Nashua, N.H., several decades ago, and designed a unique Brayton Cycle, 100 MW solar Power Tower concept for generating electricity. This was accomplished under ERDA (Energy Research Development Administration) who gave us a phase-2 follow-up contract that took our phase-1 design and built a working scale model at the 10 KW level. This model was tested at the Georgia Tech Solar Research Facility and "registered" ~37% electric solar conversion efficiency. The system used ambient air as its working fluid, and was to be located in open-spaced desert regions.

Phase-2 was lost to competition using a closed-loop liquid sodium system that boiled water into superheated steam at 900F to run a turbine that generated ~21% overall electric conversion efficiency.  Apparently, at that time ERDA would rather haul water out to the desert than use ambient air to generate electricity? The politics of their decision is beyond reason and clashes with improving the world’s development of green technology energy.  ERDA shut out our better technological performer and safely locked it away for another day! ERDA's official reason for turning us down: "this technology uses excessively high temperatures (2500F versus 900F) that are dangerous to workman maintaining the equipment". But that was back in the 70’s, maybe we’ve learned to deal with high-temp heat by now?


Solar Energy Concept Using Low Pressure Storage

Our solar power tower would collect the sun’s energy by locating its ceramic heat exchanger on top of a tall tower as shown in Fig-1. The tower was located in the center of a field of active sun-searching mirrors (heliostats, Figure-2). These mirrors reflected sunlight onto our ceramic honeycomb heat exchanger, producing a concentrated flux intensity level that heated it to around 2500F. At the same time, low pressure fans generating only a few psi pressure would suck the ambient air through the honeycomb, heating it to just under the 2500F and then passing it through energy storage silos which stored the heat down to ~150F. We purposely designed the energy storage charging phase of our hot air system to work at only a few psi above ambient as a safety feature.

The sun effectively acts as the combustor of our jet engine or Brayton cycle engine. Once the sun heats the air, it passes through heat exchangers consisting of a labyrinth of underground silos that are temperature segregated. These silos receive our 2300F airflow and cool it down to about 150F, transferring this heat into solid salt containers which turn to liquid once they have absorbed sufficient heat. Figure-3 is a schematic of this underground energy storage facility and shows the airflow being heated by a fully charged set of silos containing liquid salt-bricks. This airflow direction is reversed when we charge the silo’s salt-bricks. The bricks are kept in specially insulated, high pressure silos (located underground for added insulation) that store the heat energy at one atmosphere for later use. These underground silos act as our energy storage batteries, and when needed would discharge their heat energy accordingly into the moving airflow. This energy storage concept permitted the generation of electricity at night and during overcast days. Two sets of storage systems are required for continuous operation. One would be charging at low pressure while the other is discharging at high pressure through the Brayton engine to generate electricity.


Electric Energy Generation at High Pressure

Electricity would be created by turning an electric generator at high speed. The generator was turned by running a jet engine connected to it.  The engine’s combustor for heating the air is effectively the sun, hence the name Brayton cycle for generating our solar electricity (Figure-4). The heat from the molten salt containers would increase the energy of the high pressure air coming from the compressor, and would then force it through a typical turbine that turns this energy into high rotational speed to run the generator and make electricity. Our solar jet engine sucks in ambient air using its compressor, as all jet engines do, and blows it through a series of silos at high pressure whose stacked bricks are held at different temperature levels. We start our airflow through a silo held as low as 150F and work our way up to ~2300F as we pass through our last, hottest silo which acts to complete the effective solar combustion process.

This air preheating technique dramatically improves our energy turnover capability and allowed us to convert solar energy into electricity at near 37% efficiency. During our electric energy generation phase, the silos of our Brayton system requires operating at many atmospheres of pressure just as in any jet engine combustor using petroleum-based JP-fuel.      


Again, I'm at a loss as to the intent of this post - are you trying to educate us? Or are you looking for somebody to build one?

If you're just after a discussion, why not start your own blog?

Yes I suppose this is more like a blog that solicits discussions hopefully. I'm sharing my experiences and trying to make folks aware of some amazing technologies that I've seen scuttled simply for political reasons. Yes, I would love to help and build one of these! it'll put a big dent in renewable, green energy, could replace coal burning, and doesn't pollute but for dumping hot air back at around 150F. I thought I was starting a discussion? Seems like this I'bles "forum" gives me a convenient blog site, that I don't think I'm abusing? Is there a better site where I can make my points and have a discussion?

If you're trying to stimulate discussion, make your post shorter, and make your objectives clear - a block of text like that, that reads like a lecture, almost comes across as spam.

Instead of giving all the information in one off-putting block, include links to your sources that the more interested can follow.

Thanks understood.


This "blog-forum" is arranged for folks to exit at depths of interest, but no thought given to entice readers with teasers and links.

This topic is germane to our future energy needs, yet no links exist except my mind and maybe 1976 vaulted unshredded ERDA records (doubtful).

I'm using the Internet's power of viral communication or “social networking” to disseminate/discuss/inform ourselves for a better future. WWW technology has created the era where mass communication melds our world and eliminates sovereign borders.

It’s like DARPA’s “Red Balloon Project” where “social networking” enabled “ourselves” to strategically find 10 red weather balloons, that were publically visible yet discretely planted throughout the US, in just 90-minutes! An unbelievable achievement for mankind yet only a “small step for man”!

I'm not saying "don't post", I'm saying "be aware of your audience".

If you look at the site demographics, the users of this site are young, even compared to the internet as a whole.

That's a "sound bite" audience - keep it short and tantalising, and provide links to service those who are more interested.


You’re right on about sound-bite audience!

True, I’m intuitively trying to appeal to an older group.

It wasn’t overlooked and initially did concern me.

But why not raise the age-bar on I’bles and let it evolve to young and old viruses alike?