Tuesday, October 26, 2010

Small epiphany

I realized today that an operating ‘stat like v2.3 is a proof of concept for a design that would be built differently. If mass manufactured, it would probably be a plastic frame, motors and hardware rather than the current angle iron-based system.

The design questions that the proof of concept needs to address include:
1) Is the system mechanically practical?
Does the center point bearing mirror weight and allowing full X/Y motion work? Can the mirror be positioned as needed by the other two points? Can the other points provide the positional stability needed for a large mirror in wind? Are the current motors (small gear motors, not steppers or servos) usable? Does the system provide adequate accuracy? What mirrors would ultimately be practical for heliostats (lifecycle cost analysis)? Etc.

2) Is the system electronically practical?
Can an open source microcontroller & motor controller group handle a residential size array (2-20) of heliostats cheaply and reliably? Can a power distribution system work cheaply and reliably? Can additional sensors adding capabilities be added easily? (Example 1: heat detection placed at PV cells that cuts out stats above a certain temperature. Example 2: additional positioning sensors at the stats). Can the whole system be matured enough to make installation relatively simple? Etc.

3) Is the overall system viable?
What is the true final cost per square meter after mechanical and electronic corrections are incorporated? What types of installations are possible or attractive? What is the ROI in actual installations? Can installations be made simple enough so almost anyone could do it? Are there economic niches where the system could flourish? Are there installations where it could compete economically with conventional power generation? Etc.

The system is essentially a base and frame that holds the components in fixed relationships to one another paired with some microcontroller logic and directions on how to assemble and run the thing. It would be cheaper and more consistent to do this in a piece of blow molded plastic that fixes all the special relationships, provides mounts and protection for motors, wires and rods, can be filled with concrete or sand for weight, all while simplifying installation and improving reliability. Less modular, yes, but more practical.

The exercise of building it out of off-the-shelf parts is valuable. But if the exercise proves a success, the end product (what you could buy in a hardware store, or manufacturer yourself) will look different.

Version 2.4, little fixes

Another configuration of motor mount. An issue with the other was that if the threaded rod drifted out of its path, it put stress on the motor. The function of defining the path for the threaded rod needed to be separated from the motor mount. Hence, the second angle iron 8" below the first. Nice thing about using angle iron is you know the holes are aligned relative to the center rod. You can also see another 1/2 cinder block which I added because the first angle iron was a bit whippy from being cantilevered out so far.

The eyelet nicely prevents the t-rod from turning, but one of the purposes of the in-line ball joint was to handle the angle change (and associated length change) when the mirror is at larger angles. The fix is hardware that pivots in only one axis, like this, but I can't find it cheaply. They're about $3 each in large quantities, but that's still pretty pricy. Working on it.

Sunday, October 17, 2010

Parts list: $100 per square meter

In general, these are retail prices but without taxes or shipping. There are no costs for installation and profit. It's not meant to be perfect. In earlier calculations I've gotten into the $80 per square meter range and I still think that's possible.

My vision is something like an in-lawn sprinkler system. Central control is in the garage, 10-20 mirrors would be a typical residential installation. A home owner could do it themselves, or hire someone local who could install it all in a day or two and make $500 or so. 10-20 square meters of mirror isn't enough to power your house but if the ROI was 5%-10% it would attract some interest. A mid-tier application might be 100 heliostats generating about 10kW at peak, making the owner about $2K/year for a capital investment of $15K-$20K, so 10%+ return. A larger application would be an open source 'power tower' type configuration with thermal storage and power generation from steam turbines.

Note: For the figures above, the value of generated power assumes available solar energy of 1000 watts per square meter, with the conversion to electricity operating at 10% efficiency (100 watts per square meter). It assumes 5 hours a day of sun, 300 days a year, so this really only works for the US Southwest and similar climates. Power is considered to be worth .12/kW-hour -- whether used or sold back to the utility.

V 2.3, an update. Finally.

Hola amigos, it's been a long time since I rapped at ya. The I video below is a quick update, and this post will provide detail.

So, the main news of the past months is the arrival, assembly and tweaking of the Makerbot. Assembling it is fun, but the tweaking takes awhile. And, it tends to break a lot so you have to fix it all the time. But, the MK5 plastruder, which arrived in September has greatly stabilized it and now it is a prototyping tool rather than a troubleshooting time sink. There is a great on-line community and I give the product, company and community a big thumbs up. They also just came out with a new product, essentially the Cupcake v2, which is called the Thing-O-Matic. The leap they are attempting with the thing-o-matic is automation -- the fabrication of many parts, unattended. Using the Cupcake made me re-think the design. The initial intent was to make something that others could assemble themselves with off the shelf parts. It's clear now that automated custom part fabrication will soon be commonplace and there is not a reason to exclude custom parts from a public domain/open source design. If custom plastic pieces can make the heliostat work better, assemble easier, facilitate upgrades, and simplify repairs and maintenance, they should be included.

So once the gears were done (pleasingly easy) the next element I focused on was the motor, mount, and threaded-rod component. The new motor is the GM17 from Solarbotics, mentioned earlier. It's got good torque and takes about 1/3 less power (253mA stall current at 3V vs 400mA) than the GM2 used previously. I wanted a mount that would fix the motor in place using simple connections to the angle iron. Another bracket holds the threaded rod in place relative to the motor.

First pic shows the mount and the motor. You can see an embedded notch which receives a nut that holds the motor mount to the angle iron. Easy to do on a Cupcake and it makes for a simple installation.
The motor fits into the mount like this...

Then the whole thing mounts to the angle iron like this. You also see the top bracket, which holds the spur gear down (so it doesn't float up the rod) and provides the pathway for the threaded rod.
It's not perfect but I like the direction. Motor is kept securely in place without having to drill into the metal of the angle iron, it's fixed with simple screws. The motor also keeps it's place relative to the spur gear and the threaded rod. Installation takes a couple minutes.

This was the piece that convinced me that plastic pieces are a net positive even though you can't easily buy them at the store. I'll upload the files. Once they're mature enough, anyone with a makerbot could make them, or, there are many on-line production shops that could fabricate them. It should be noted that in some cases the parts are made to accomodate constraints in the build capabilities of the Cupcake. C'est la vie. Ultimately all the parts (except the heavy stuff like cinderblock, concrete and a mirror) could be bought on-line as a kit. There is nothing proprietary so costs will be low.

Next, a parts list and posting of source files. It's not there yet. This is first assembly, it needs many adjustments, and many improvements are tempting. But, it shows some promise and there is now (or will soon be) enough detail that others can take the design and run with it if they wish.

Version 2.3, first assembly