Expected Output and cleaning

Tony Sirna from the Dancing Rabbit eco-village in NE Missouri?, asks about efficiency, production, yearly variance.

Hey Tony,

For predicting direct solar from PV output (though it is better than nothing), probably is not anymore accurate than looking at solar map. Solar concentration requires completely direct light (casting a sharp shadow) to work. So PV definitely has the advantage of working in diffuse light as you point, though PV has the disadvantage of become less efficient the hotter it becomes (where solar thermal will produce the most power). So the primary maps are probably the best bet, though they can be off -+ 20 % depending on the micro-climate. Nasa has maps of the amount of direct solar radiation around the world, what are your coordinates?

Yes, definitely battery storage has some good advantages. However, every person or community I’ve seen relying only on batteries do power intensive activities (washing machine, power tools) only when there is direct power, relying on batteries only for lighting and electronics which consume relatively little. If you want total on-demand power then the amount of batteries required multiplies greatly and quickly becomes too expensive.

However you mention "electrical and thermal storage costs are in a similar range". Are you refering to high-temperature storage? At the industrial scale high temp thermal storage is feasible, there is even a plant recently opened that has 24 hr power generation, but it’s at a huge scale, in the desert, and subsidized. Scaling such technology downward has significant problems since at the large scale the proportion of surface area of the storage medium with respect to volume is favourable. At the small scale there is far more surface area with respect to volume and so the storage medium cools faster. Again, when I say storage I’m talking about over 6 hours at hundreds of degrees (+300 C).

Questions needing answers ...

1. Whats the timeline for having an automatic tracking heliostat built at Factor-E?

Tracking shouldn’t really take all that much time since the technology is mature and Factor-E. It’s not the development priority since a cost effective reflector is the pre-condition for success. Automatic tracking simply adds value to that. For the reflector itself the goal is to start building in September.

2. Whats the expected efficiency of converting solar energy to thermal?

Solar thermal can create a huge heat difference and so is extremely efficient at transferring that heat to a energy capture medium, such as steam, Solar Fire is about 80% total (rays to vapour). Then there are always pipe losses, about 5 percent with good insulation. If you want to convert to mechanical power this is between 10 - 20 % efficient at the relatively small scale. So, in terms of efficiency to generate electricity solar thermal can be expected to be about 10-15% efficient as is PV. The advantage are that it’s cheaper to build, can be built locally, and can provide far higher quality thermal waste heat (as PV must be kept cool it can only produce warm water as a by-product), and of course can also provide high quality thermal heat for thermal applications, which is where there is a huge gain, as a thermal application has no electric conversion loss and so can run at 80% efficiency.

Purely in term of electricity, the two technology are fairly even. However, when there is a thermal need (and 85 percent of domestic and industrial energy consumption is thermal) then solar thermal becomes 4-5 times cheaper.

3. I’m trying to estimate thermal production capacity at our site. As mentioned above there are so many factors that come into play (even positioning, cleanliness of PV cells) that the correlation would be fairly weak. Solar map will be far more accurate.

4. Do the mirrors on the heliostat really require daily cleaning as is mentioned on tinytechindia.com? Is there any way to avoid that labor intensive process?

I’ve built solar machine in a half dozen countries and the requirement of cleaning is definitely proportional to amount of pollution. In Canada for instance we cleaned the mirror every month or so since rain water generally cleaned them effectively and dut build up is very slow. In southern Mexico we cleaned every week or so as there’s not much smoke pollution but more dust than Canada. In Rajkot every day is required, but your snot is also pitch black.

There’s an easy way to find out. Simply place a mirror or piece of glass outside and check with your finger every day how much dust builds up.

Also, at Tinytech we only spray washed the mirrors everyday, which was enough. However, the minerals would build up drying to white rings, which we would remove with vinegar about every 4-5 days. Of note is that we used well water which is not at all ideal. If you experiment on your test mirror with rain-water or river water if you have it, I think you’ll find the build up extremely slowly and spray washing would work for a long time, if you depend on well water then by testing you’ll get a good idea of how much you’ll actually have to clean the mirrors by hand.

5. What help is needed on the heliostat project?

Thanks for asking this question. Help will definitely be ... very helpful! Right now I’m finishing the drawings and dimensions of the system built in India, the SFP32, with only obvious improvements based on my experience on the ground there. So, thus far the job has been to document progress so far which was solitary. But as soon as this is done, within a week or so. The documentation so far is at

http://www.solarfire.org/Solar-Fire...

After the SFP32 documentation is complete the improvement/refinement phase begins. There’s a practical and theoretical side to this. The practical side is definitely where the most impact can be made by non-solar experts or non-programmers. Specifically, practical experience on the best way to set the mirrors is a crucial component, both in terms of ease and accuracy.

The ultimate design I will be proposing to E-farm will be with wood for the rows where a whole is drilled directly into the wood at the correct angle.

I hope all this helps.

Also, do you mind if I post this conversation for those that are perhaps asking the same questions?

Best regards

Our coordinates are:

latitude: 40.338694 longitude: -92.097402

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Parameters for Sizing and Pointing of Solar Panels and for Solar Thermal Applications:

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