Cheap Heliostats and Small Scale Power Generation

Hello. This page describes ideas for cheap heliostats and plans for using them for small scale power generation.


I'm relocating the whole kit and kaboodle to a blog – click here to go there (including how to build one).


Context:
Solar power solutions must compete, without subsidy, with the current state of the art of conventional power generation in order to provide a compelling alternative and ultimately to impact global warming. For this reason, the single most important design driver for solar power systems must be cost. At every decision point, cost should drive the design.

Design concept: small heliostat arrays, thermal storage and stirling engines
The design I'm exploring is a small array of heliostats, which reflect sunlight to a thermal storage container filled with molten salt. A stirling engine is connected to the thermal storage to produce electricity. The goal is a total system cost of $10,000, with an ROI of perhaps 8% (derived from $800/year power savings and a 10 year amortization). It may be possible to derive other value from the stored heat, such as home heating or hot water. Nothing about the design is particularly unusual. The question is only how cheaply it can be accomplished. That is the engineering challenge. The reason for posting this design is to participate the loose internet community which does open design on solar power generation topics.

Heliostats: The design is an array of small heliostats, each around a meter square, each individually controlled. The concept is similar to eSolar's design – they use large arrays of small, flat mirrors which track individually (they are controled centrally). I lean to small mirrors also, to reduce motor requirements and wind load, however, my design requires a very slight parabolic curve and so would require bent reflectors or mirrored injection molded plastics. The correct heliostat choice may turn out to be larger (raw-solar has an interesting implementation) but this decision should be made on a purely practical basis – best price/performance ratio wins. The reason a slight parabola is required in this design is that the common target of all the heliostats is a single point – specifically the focal point of secondary reflector which redirects the collected sunlight to a thermal storage container.
Using assumptions of 1,000 watts of sunlight per square meter available for 5 hours a day, 300 days a year (the US Southwest), and an ultimate conversion efficiency of 15%, 30 square meters of collection area (mirrors) would be required to collect enough energy to power a reasonably efficient home (about 7,000kW-hr a year). Under ideal conditions (zero losses due to reflection or storage), in a single day, 30m
2 receives 150kW-hr of energy (1kW x 5 x 30). So the capacity of the thermal storage must be about 150kW-hr. 2 cubic meters of molten salt can store 130kW-hr.
[insert pic] A crude picture:

Control and solar tracking: For the moment, I am prototyping with an arduino microcontroller and servo motors. Ultimately, I think the logic portion could reside on a simpler (cheaper) chip set, or if the tracking logic matured, it could be converted to a dedicated chip. (This is a product will arise in coming years if designs which leverage small, tracking heliostats become popular). The motors should probably be steppers (cheaper than servos) that are highly geared so low power motors can be used for fairly good power and precision.
The tracking algorithm could either be calculated dynamically at each heliostat, or pre-calculated and loaded into memory so the logic at the heliostats is reduced to to keeping time and checking a look-up table once a minute or so to see if it should move a step . The algorithm needs to calculate the sun's position and then factor in the heliostat's position relative to the target. Calculating the suns position is the hard part but fortunately many smart people have worked on this already. Some of the existing solar positioning algorithms (link) are already written in C and optimized for microcontrollers. Another challenge is maintaining precision time over long periods, or to write logic such that clock errors tend to cancel themselves out rather than accumulate.
Two large challenges here are making the whole motion control system both accurate and cheap, and, developing test mechanisms (to be able to check that individual heliostats are on target).

Thermal storage: Thermal storage has been used in several large scale power tower designs. Usually, it means large holding containers of molten salt (salts which are liquid at temps of 200-500 centigrade) which have good thermal storage properties. It seems to have worked well with enough, but with engineering challenges related to piping it around (piping and fittings must tolerate high temps and the salt tends to solidify if it gets too cool). The proposed design (not well developed) is a 2 cubic meters container, (which equals roughly 530 gallons, 3,360kg / 7,400 lbs), essentially a large water heater filled with salt. It would sit on the ground and the secondary mirror would be above it, reflecting collected sunlight into the salts. Specialized piping is minimized because conversion of heat to power is done at the same place as thermal storage. Collected heat could be used for conversion into electricity via stirling engine, or for household heat. My plan is literally to try and find a hot water heater with a stainless steel tub and some insulation, put some salt in it, heat it up and see what happens.
Adding thermal storage provides a few advantages (most obviously, power when the sun isn't shining). First, stirling engines are expensive, and stored heat would allow them to operate through a larger part of the day (20+ hours) which would help amortize the cost of these devices better. Energy storage with photovoltaic cells is typically done through batteries, which introduces another element of cost to those systems. Second, there may be other uses for the stored heat, such as for household heating, hot water, desalinization, cooking, small scale steam conversion, etc. Third, the high temps that molten salts reach should be enough for efficient use of stirling engines which work best at high temperature differences. Also, it may be that a stirling engine's heat transfer could be optimized for the application – for example, special plugs that mate to a stirling that would make heat transfer easier, or tubing which winds through the thermal storage container that the stirling clamps on to. In general, effective thermal storage de-couples the solar collection problem from the power conversion (or other use) problem so that each could be optimized separately rather than being a monolithic design.
The big design challenge here (aside from getting the right materials) is safety. There is a great deal of concentrated heat in the mirror systems and stored heat in the container. People and property must be protected from the concentrated heat – what is being reflected to the secondary mirror and reflected down into the storage container. The design must tolerate component failure and still preserve safety.

Power conversion – stirling engine: The question of whether a 1kW-3kW stirling can be volume manufactured and sold for a reasonable cost (a few thousand dollars) is open. Infinia Corp is the closest that I know of but perhaps there are others. The design described here turns on the ability to convert 400+ degree (centigrade) heat into grid ready electricity at perhaps a 20% efficiency for something under $5,000. Whether this will ever happen is anyone's guess. Another player in this field is Stirling Energy Systems, which is trying a 25kW Stirling with Sandia Labs [link]. It's possible their engine could be paired with a cheaper collection system and thermal storage. If no one is able to solve the problems of small scale thermal storage and a relatively inexpensive stirling (or micro-steam turbine or some other approach), then the design direction of small scale concentrated solar thermal is probably closed. Companies like eSolar or Ausra doing large scale projects may still have good solutions.
One appeal of using a stirling (or equivalent) device for conversion from heat to electricity is that it brings the design problem away from PV which currently has problems of limited production, high cost, possible material sourcing issues, patent protections, etc.

Other issues, errors, problems and existential crises:
- Estimates of cumulative losses include... 1) cosine losses of 8% (the mirror doesn't face the sun directly), 2) reflection losses of 14%, 3) storage loss (5%), 4) conversion loss (80%).
- Small scale CST probably isn't the most efficient form of solar power but it has the advantage of competing against retail prices (12 cents a kW-h vs 6 cents a kW-h wholesale costs), and is cheap enough that it could be implemented by individuals. For this reason, it might provide an accessible environment for innovation.

Links:
- Redrok
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Another nice link collection:
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eSolar
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Infinia
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Raw-Solar
- T
hermal storage calculations

Contact info: brendan at whatsnewla dot com.
I'll try to add to this page as/if progress is made.
The page that used to be here, with designs for cheap heliostats using domed fresnel lenses is now here. I've abandoned that design direction.