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Sw4mq

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Posts: 6
Reply with quote  #1 
Hello all, I’m completely new to this forum; but I wasn’t sure who else to turn to because this is far from my area of expertise. I was wondering if someone could debunk this CSP tower concept I thought up on the way home from work so I can stop thinking of it.

The benefits and drawbacks of solar thermal towers are well known. I came up with this in an effort to reduce the costs associated with the heliostats in order to allow smaller csp tower plants.

A normal solar power tower uses a large array of constantly moving heliostats in order to keep the focal point on the central collector as the sun moves across the sky. These heliostats and their solar tracking systems collectively make up a very expensive portion of the overall plant (50%), and generally have high operational & maintenance costs due to all the moving parts spread across the entire plant. Recently we are seeing more proposals like ganged heliostats in order to cut down on the number of moving parts, with savings estimates of 30% upfront costs.

What if instead of having a large array of MOVING heliostats (keeping the focal point centered), the heliostat array didn’t move at all, rather - the central collector moved in order to position itself over the now constantly moving focal point.

I’m imagining a central tower taking the form of a commercially available spider boon lift. Not only would it require less overall power, but it would also have the ability to be packed up and moved on a moments notice. The heliostat price would drop considerably as they are no longer moving (they are at this point just a circle of mirrors), and O&M costs would drop (as there is now only one moving part in the entire plant). The exact location of the focal point at any given time can be easily calculated, and I expect there is an array arrangement that would limit focal point diffusion.

I’m aware there are a number of engineering challenges that need to be overcome, especially as it relates to the collection system used and what form it would take to capture and store the heat (it’s definitely possible the new problems this concept creates are more expensive than the savings associated with static heliostats).

Someone tell me nicely why this won’t ever work.

stmbtwle

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Reply with quote  #2 
Interesting, something like the Arecibo radio telescope?

It might work, but I think the engineering required would be challenging... and expensive.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Sw4mq

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Reply with quote  #3 
Exactly! Although I was thinking something like a mechanical arm in the center moving the collector around a flat field rather than a parabolic dish; although suspending it in mid air (like the Arecibo observatory) via steel cables and moving it by changing the cable length that way would certainly allow a collector with high weight and mass; hell you could probably suspend an entire hydrogen reactor and tanks with that. 
Gordy

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Reply with quote  #4 
Interesting idea, if you can dream it up it can be built. As you are probably aware most of those sites have refinery sized phase change heat storage tanks, So a flexible plumbing system needs to be addressed to follow the tower/collector.
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Gordy,
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stmbtwle

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Reply with quote  #5 
I think trying to support and move a heavy collector assembly above a stationary reflector while possible would be an engineering, financial and maintenance nightmare.  At least with the stationary tower and movable mirrors, all the heavy, expensive stuff doesn't move, while the relatively lightweight mirrors are on the ground and accessible.  The myriad mounts are all identical, accessible, and should be easy to maintain. 
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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Sw4mq

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Reply with quote  #6 
2" wire rope has an individual breaking strength of 320,000lbf (safe load strength of 64,000lbf). Common 1/2" steel cable sits at around a break strength of 23,000lbs. So supporting whatever you want and moving it above some kind of area by reeling in and letting out rope length from three towers isn't much of a problem (these cables can be set to reel out and reel in automatically to align the collector above the focal point within acceptable margins of error by taking into account sun position, mirror arrangement, and cable lengths via incorporated software systems).

The current class of widely used heliostats can make up 50% of the total plant cost, a large portion of which is due to the individual mounts and tracking systems. That cost is significantly reduced (as well as associated O&M costs) by keeping the reflectors stationary on the ground. 

I think the engineering, financial, and maintenance nightmare you alluded to could come into play when designing the collection system itself that Gordy referred to in a previous post.

Do the increased collector costs offset the heliostat savings? Most definitely probably not, but this is still a fun exercise to think about 😉
stmbtwle

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Reply with quote  #7 
Agree completely. The collector itself is only part of the cost. You will still need the mirrors and their mounts will have to be adjustable. You'll still need the tracking system and computer, only now it's moving the collector instead of the heliostats. The only saving will be in the drive units for the heliostats, and that may or may not offset the additional cost of the collector suspension/drive/plumbing.

Still, it is an interesting exercise.

In my OPINION I think solar thermal electric is on the way out anyway, and will eventually be replaced by much simpler PV.
http://www.tampaelectric.com/company/ourpowersystem/powergeneration/bigbendsolar/

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Sw4mq

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Reply with quote  #8 
Yeah I think so too; there’s been a lot of talk recently about solar thermal hydrogen, as it’s apparently the most efficient way to make hydrogen given the temperatures are high enough:


http://www.solarpaces.org/csp-efficient-solar-split-h2o-hydrogen/


However not sure how big the market is going to be with battery tech always increasing and energy density always getting better.

One of the refineries I contract at (I’m a process engineer in the Texas oil industry) uses a LOT of hydrogen in their plant to lower the sulfur produced in their diesel. I suppose a solar thermal plant could find a sizable market supplying local hydrogen to them, or maybe as fuel in the space industry.

That being said the above concept could probably only be used in a solar thermal electric capacity as solar thermal hydrogen would add complexity to the moving collector.
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