Hey Infinia! Use modular mirrors!

You gotta love Infinia’s PowerDish. Solar thermal generating instant residential spec A/C. It is a better-sized version of the SES attempt at Sandia. And as someone from Detroit, I love their interest in leveraging the manufacturing expertise there.

But they should consider modular mirrors. Check the specs on the PowerDish: 1900lbs with a 4.7m/15’ diameter mirror (call it 16m² or 175ft²). That’s big. I’m betting the installed cost of the dish sans motor is a lot more than $1600 ($100/ m²). Plus, if it breaks, it’s off line, whereas with 20 modular mirrors, if one breaks, it goes off line and you’ve only lost 5% of your light collection capacity.

Mirror curvature (and wind shear) could be addressed with modular heliostats. Ignoring a 1m² hole in the middle of your parabola, you’d have 8 1m² identical parabolic-slice mirrors in your first ring and 16 1m² mirrors in your second ring. You only need two mirror shapes. Injection molded curved plastic with PVD reflective coatings anyone? Include a few holes to reduce wind strain.

And let’s face it, that engine is designed for household use (1φ, 240V A/C). What you really want is for 1,000,000 homeowners to buy it and install it at their house. Is the hesitation releasing the IP into the wild before you’ve scaled?

Longer term, it just makes more sense to decouple the light collection from the power conversion. The power conversion component is where the value is added. Light collection should be a commodity. A monolithic design which ties these functions together reduces the chance that the power conversion part will be a success. And I hope Infinia is a resounding success.

V 2.5, catching up

The base: this new base sets the relations between the rods within the cinderblock and concrete instead of using angle irons that rest on top of the cinderblock. To make it, pre-cut holes in the pieces of wood above and below the cinderblock. Fit tubes into the holes for the shafts that the threaded rods move through, and wedge in the 1/2” rod which provides the main mirror support. Then, pour concrete around all of it and put the wood piece on top to help set positions as the concrete dries. The result is the two shafts are firmly held in position relative to the center support. This version used metal tubing for the shafts, but I think that tubing can be eliminated. PROs: cheap, sturdy, no custom parts; solid shaft path; wood creates a platform for motor mounting and simplifies prototyping. CON: the points may end up too close to each other and require too much force from itty bitty motors. If need be, there are larger CMU’s (concrete masonry blocks) but the one’s I’m using are extremely common and cheap.

Manufacturability: the new base is easier to make. There is a template which can be laid on to pieces of wood so you can pre-drill holes for the shafts and motors. Having a standard piece allows the rods to be fixed in place before concrete is poured and keeps them plumb and square. The wood provides a platform for mounting motors and other items. Later the wood platform could be plastic but for now wood makes it easier for others to play with the design if they wish.

Other: threaded rods are up-ticked to 3/8” to make the whole thing more sturdy. The way the motor interfaces to the platform and spur gear is simpler. I’m using store-bought clevis & pushrod pairs but this whole linkage (including the interface to the mirror) still needs improvement. My current thinking is to use a saddle joint for the weight bearing point which would allow movement in two axis but not rotation, but this is still a challenge.

Electronics: I’ll do a separate post when ready but briefly… Current approach is to put a motor controller at each ‘stat. Also, I’m driving the motors with 9V supply to make their work at the easier end of their power range so they are less stressed (motor failure is an area of worry) and also to combat voltage drop. Connections between the Arduino and each stat will be by Ethernet type cable because it’s fairly standard. Also, it’s mass produced and fairly cheap.