Solar to electrical energy conversion system Development

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Solar to electrical energy conversion system
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Goals, purpose and design criteria

  • Convert solar energy to electrical energy.
  • Have an operating and capital cost which is the lowest that we can manage while meeting other design criteria. Current working goal in sight is 1$ per peak watt in capital material costs.
  • We still have to decide exactly how to factor in capital labor costs (e.g. $ per hour equivalent). We still have to decide how we want to factor in running material and labor costs, too, to have uniform values for evaluating expected system costs for various designs.
  • Be designed in accordance with OSE Specifications, including lifetime design, environmentally benign, etc.
  • The exact power output and size is not specified because the main performance parameter is the cost per peak watt. (Assuming reasonable efficiency under other than peak insolation conditions - maybe we should be more specific like the kWh gathered in the worst 3 day period of the typical meteorological year for Columbia, Missouri (using TMY3 data) divided by the total system cost (if you are off grid that is how you might size the system, and therefore what determines the capital cost). However it may change relatively little so peak watt is okay and more convenient if less than perfect.
  • The components should be considered in the context of the rest of the OSE enterprise and larger goals. Most of these are explicitly stated in the OSE specifications, but the implications are not always obvious. For example, the choice to take the current piston steam engine approach was influenced by the fact that the same design can be scaled up for use in future versions of the powercube causing it to meet the modularity part of the OSE specifications better. (The powercube project has as one of it's stated goals the evolution towards a biomass powered powercube.)
Similarly, if the light-electric component for a prospective design is a low temperature differential Stirling engine, or photovoltaics that would interfere with it's potential for being re-used in the biomass co-generation unit project.
Specifically, we would like to be able to re-use the the solar collector or parts thereof for:

- Process heating for chemical engineering (see chemical engineering category for specific processes under consideration)

- Producing heat for heating homes, greenhouses and other buildings, hot water, etc. in winter (this can be waste heat after electricity production)

Specifically, we would like to be able to re-use the conversion (light or heat to electric) unit for: - A biomass cogeneration unit

- A Biomass powered powercube (a hydraulic powerplant)

- Either in reverse as a heat pump or to power: air conditioners or refrigeration or cryocooling (for liquefying air to extract pure gasses or produce LN2 etc.) units. (less important)

- As a water pump, either in reverse or to power one. (less important)

  • The end use of the system is to part of the electrical power supply for a ~200 person community based around the GVCS, which is part of the OSE specifications. If the cost goals are met it will also be highly useful outside of this context, such as powering an off grid home. The current prototype I is expected to have a peak output in the 3 kW range, which is in the right range for powering a single home with one unit for an efficient home and 2 for a typical one.
If anyone has more to add, please do.

History and overview of the project

OSE, at the main Factor e Farm facility, has been working on the project for the last 3 years or so, off and on. It was originally called the Solar Turbine Project because the most promising approach for the heat-electric conversion was thought to be a boundary layer turbine (A Tesla turbine). This approach was later abandoned after it was found that a Tesla turbine was found to be impractical due to low mechanical efficiency and that it is actually not that easy to manufacture.

To improve the modularity, both the solar collector and the heat-electrical power conversion unit have to some degree been separated and this is evident on the wiki.

The expertise of the Steam Automobile Club of America (SACA) has been drawn on, with Marcin going to some of their meetings, and some experts contributing to the steam engine design.

Recently, a collaboration has been struck with the people at to use the collector and absorber they have developed. See blog posts in the prototyping work documentation section.

Current state of progress

As of June 19 2011 the design stage for prototype one is finished and prototyping has begun. It is to combine:

  • a P32 solar concentrator and absorber of the design developed by with an automatic tracking system and other peripherals with,
  • A piston based steam engine developed through the (general purpose) steam engine project.

Completion is expected in the next 6 months or so.

  • The wiki still needs work to organize all the design documents produced, they should all be included under the solar turbine category or subcategories thereof.

Performance, specs and cost estimates for prototype I

Cost: need to compile from existing design docs Maintenance cost/year, materials:

Maintenance cost/year, labour (low skilled person-hours):

Maintenance cost/year, labour (highly skilled person-hours):

Capital materials cost:

Capital labor costs (highly skilled person-hours):

Capital labor costs (low skilled person-hours):

Cost of any non-flexfab production equipment:

Total production equipment requirements:

Definition of cost terms:

labour (low skilled person-hours): Done by people with less than a week training for almost anyone? (95 percent of population?).

labour (highly skilled person-hours): Skilled persons need more than 1 year of training in a certain field compared with 95% of people? how is this normally defined?.

materials cost: Funds that must be transferred to entities outside of the producer's organization. So, for now we cannot make mirrors so it would be $1.5 per sq foot or so. As we gain the capacity to make mirrors, but not the raw glass or aluminum, that will become a labor cost and some per-kg materials costs.

Cost of any non-flexfab production equipment: In the case where equipment is used for multiple purposes it get harder to estimate the capital cost of the production equipment, and since with flex-fab that should be the norm. But this is any capital cost for dedicated equipment that has relatively little use elsewhere in the 200 person community - including things made with the flexfab equipment.

Total production equipment requirements: Which machines used etc. including parts of the replab and any other tools that are eventually included in the flexfab set. That way we have some idea of what is required at least even if it is hard to put a dollar figure on it because the equipment is re-used for other purposes. The minutes on each machine per unit produced would also be nice to know, where possible.

Performance and specs:

Area: 32 Sq meters collector area (P32)

Thermal energy collected at about 550 degrees output temperature at standard test conditions ( same as for photovoltaic panels, 1 kW insolation, 1 air mass spectrum etc.): 16 Kw.

Efficiency overall at 550 deg C of first-gen steam engine engine (heat-to mechanical): 23% (26% with later refinements).

Efficiency overall at 550 deg C of first-gen steam engine engine (heat-to electrical): 22% (95% efficient inverter-generator)

Output temperature at peak: 40 Deg C.

Net output at peak: 3.5 kW electric.

Net electrical energy gathered on TMY3 June 15, in Columbia, Missouri:

Net energy gathered on TMY3 January 15, in Columbia, Missouri:

We need to decide as a group how we are going to evaluate costs of a particular system that we are looking at so they can be compared easily, as we will need this for other tools too. What category should I put this under? Gregor 21:38, 27 June 2011 (PDT)

Existing design work

This section and the linked pages contains all existing design documents that have been produced during the design work as of June 21, 2011. Documents produced after this point should be included in the right categories. I'm excluding blog posts because they are not part of the design process.

have to remove this list, better to do it through the categories, the problem is that there is a lot of stuff in those categories that has nothing to do with this project. The pages that do should be under solar turbine and it's subcategories (subcategories can also be subcategories of other categories at the same time, so steam engine can be under both energy and solar turbine), and the stuff in solar turbine that is not relevant to current efforts should be moved to subcategories "historical" and "low relevance information" or similar. Would be nice to have better wiki management tools to avoid loading each page etc. which is really slow on my computer gregor

More relevant to current efforts:

see also there are too many for me to go through there.

Better wiki management tools or a Wikian team would really help to get things categorized better and keep them categorized.

All new pages on this project should be placed in the category "Solar Turbine" or a subcategory thereof from now on. Simply add [[category:Solar Turbine]] or a subcategory of Solar Turbine somewhere on the page. In addition to any other categories they are in. This will prevent the problem of stuff being scattered everywhere from re-appearing all over again as new pages are produced. Pages can be under more than one category, see wiki maintenance.

Forum posts:

Documentation on prototyping work so far

Less relevant to current prototyping:

More relevant to current prototyping: Recent blog posts laying out the current happenings:

  • The solar concentrator prototyping specific to this project has not yet started as of june 21, 2011. The P32 solar concentrator system and absorber/steam generator is a finished product of the Solar Fire organization, but we still need to add automatic tracking and mirror cleaning, then combine it with the engine.

Prototype II

It has been suggested by Gregor that we try a community-exploration and design approach for the design or initial decision making of prototype II. This may help to improve the openness and collaboration of the process, and efficiently harness the efforts of more people. Gregor is currently working on getting the groundwork to start with this approach up on the wiki, including this page.

Requirements: -Draw a clear line around the whole body of design work and the complete design and/or judging criteria that relates to the project. That does not mean it is fixed in place, just that it can be found easily.

-End decisions are made by knowledgeable peers to give confidence.

-Preferably commit to dates for decision making

-decisions only based on the design criteria stated, and all the design work which is in the designated place is openly accessible.

Anticipated advantages:

- The limits on the review process causes openness and collaboration to be integral to the process and gives people confidence they have all the existing work in front of them.

-Explicitly states what is going on so people know that they are welcome to add their arguments.

-Unlike doing it through the forum emphasizes the not-just-talking-about-it part, and allows much better flexibility.

- Produces products like documents that can be taken and used rather than a discussion only.

-Makes it clear all options are under due consideration to ensure no concerns about high quality contributions getting drowned.

It can be scaled from a single person to many people. I don't think it will interfere with existing approaches (as of june 24 2011). Essentially this is already being done to a limited degree through the wiki and time to make it into an explicit collaboration approach.

Having looked at pivotal I think this sort of thing has a very important place at the beginning of new projects, when there is a lot of uncertainty about the best way to proceed, and no single person or small team can illuminate very well due to limited knowledge. This harnesses a large number of viewpoints, knowledge and experience. With enough eyeballs all problems are shallow. The rest can of course be done through the Agile approach using Pivotal.

The length of time before the review process can be as long or short as desired, this does not necessarily have to entail a long process.

Community exploration for the design of Prototype II

There are often a lot of suggestions and criticisms of a project like this, and for good reason sometimes. There is such a large number of ways of doing it and there are so many people that have tried disparate approaches.

There are a ton of good ideas to be explored and this section and linked pages are where we can explore them collaboratively. If you make a new page make sure you add it to the solar turbine category of a subcategory thereof by putting [[category:Solar Turbine]] on the page so it gets categorized and does not get lost.

The process is as follows: We have about 6 months until prototype I is complete, tested, and building of prototype II can begin. We don't have to wait for the test data though as we have estimates of the cost etc. already so we can start in on designing prototype II now and make good use of time to provide the best design for prototype II.

1. Anyone can add any valid argument to the pages. Argument does not imply qualitative or necessarily low quality. Some arguments are a lot stronger than others - an anonymous opinion with no context is not very good. Some quantitative estimates to answer questions that have come up is good. A promising design sketch of a system you have in mind is very good. Let's try to keep the signal to noise ratio high. Certainly the information gathered from prototype I will be an interesting argument.

If there is copyrighted material it should be be summarized and shared with the community to whatever extent is legal, e.g. put your email address up so people can ask you for a copy, which is legal under fair use. If you can't even share it like this then it cannot be included as information must be readily available to other contributors.

Sections and pages are divided up as needed into subpages etc. when they get too long.

2. Some time after prototype I is tested and the data from it has been processed into the design process we will hold a vote as a community on options for how to proceed. No new additions can be added after voting starts. The vote will be open for 3 days only. This will be done through the wiki so a number of more detailed results can be extracted, e.g. the edit history of voters can be correlated with the votes if desired.

This is an information gathering tool for the community and the Design Review Team.

4. After the vote the Review Team will read through the arguments on the page and 1 link/reference deep, and come to a conclusion about how to proceed. If there are completed designs one will probably be chosen. If there are not, then a decision will be made for which design to finalize.

The decision will be based ONLY on the arguments presented under the solar turbine category and subcategories and links/references 1 deep from the pages, information within a defined area of the Pivotal system, their existing knowledge, and no more than a few hours additional work on the arguments. So make sure that if you have an argument, you add it in it's complete form by the beginning of the vote process or it cannot be considered. And only based on the stated design/judging criteria/goals, and no others. This ensures that they too are well defined and openly known to everyone to whatever extent they can be (they may change a bit as time goes on if seriously disruptive information is encountered). I do not think this will interfere with the Agile process, only be complementary too it.

This makes sure that everything is in one place, and lightens the workload on the review team. It also makes sure contributors know they have everything in front of them to whatever extent possible so they can work together effectively. So for example comments on the forum or whatever are not included unless they are linked to as described above. It will not always be possible, and the design requirements may actually change slightly as time goes on, in accordance with the Agile-like development approach we are currently trying.

The Review team has yet to be formed. It will consist of knowledgeable peers, engineers etc. who we trust to come to the best conclusion based on the arguments.

This has yet to be ratified by the community and leadership as on June 21, 2011.

  • Negative but constructive criticism is inevitable and necessary. If someone unfairly criticizes something or is exaggerating just point that out without escalating things and remember the Review Team are knowledgeable peers who will spend the time needed to come to the right conclusions, not be mislead by hollow arguments.
  • When you add a single comment or question instead of modifying a block of text or something, sign your additions with ~~~~ and your username will be appended automatically.
  • Remember, we are focusing on existing technology, so e.g. thermochips (google) are interesting but still in development technology. A fair way to distinguish is probably to look at the uncertainty and expense of getting to an easily producible, practical, working unit. Exactly where to draw the line is a matter of debate, but just keep it in mind.
  • We are not just talking, we are moving towards the best complete design, together, in conjunction with the work being done through the Pivotal system.


Organic Rankine based

The point of using an organic fluid in a rankine engine instead of steam is that:

  • it has different thermodynamic properties which can lead to higher efficiencies (? double check)
  • the vapor pressure variation vs. temperature variation can be much higher, allowing practical operation at lower temperature differentials.
  • and that the heavier the fluid molecules are, the slower a turbine expander has to spin. The centrifugal forces at the RPMs involved is a major limiting factor in turbine design, so the lower the better.

I know there are a lot of existing designs with trough collectors and ORC engines, including some practical ones that have been developed using engine turbocharger turbines as the expanders. I know some people from MIT did one to power hospitals in India. They can be made with off the shelf refrigeration parts. Those two options would

They can also be made with liquid ring pumps as the expanders, according to wikipedia. The liquid in the pump can be high temperature hydraulic fluid, which is good to 400 deg. C. Such pumps are really easy to build and last a long time. gregor gregortheinventor

Steam based

Wha tabout a totally linear steam engine? Linear alternator, and hydrodynamic bearings using water (tap off of the condensate return pump maybe) for the linear bearings. Easier to make a large expansion ratio in a singel stage than with crankshafts. Mounted vertically to avoid the static axial load on the piston rod.

Electric valves: Suppose you take a ball valve. Make it so it can rotate continuously. Now attach a small brushless motor or stepper motor. Make the hole through the ball very small and likewise with the input/output holes so that during a full rotation, onl ya very small fraction of the time is the valve "open". Have it rotate at the same frequency as the cycle frequency of a steam engine. Thus the valve opens and closes at just the right frequency. But it opens and closes non-abruptly and the full-open has a limited open area. Although that is similar to a bump valve. It also does nto have to be a ball. Suppose it was a cylinder and rot with a slot in it instead, coudl be easier to make. A roller bearing with a slot in it might make a good valve bit thing. It can be made longer to make the full-open position have less constriction whithout affecting the open-time. Thus higher abruptness could be acheived... What about the steam trapped in the slot? probably not a problem?

Problems: the actuator has to be kept below the curie point of the material used, probably not hard. Come to think of it, the water would condense in any non-heated-by-the sun parts, an this coudl be taken advantage of a s a handy lubricant.

But the whole point is to be able to change the open time as a fraction of cycle tiem which requires that the outlet valve area be changed somehow. That could be added in some way, but migth end up mor ecomplex than a similar variable-bump valve thing (but if the wear issues are too hard to solve with bump valves maybe it woud lbe useful).

The key thing is the recognition that the timing stroke-to-stroke does not really change and therefore some sort of continuously rotating actuator can be used without the inertia being a problem.gregorgregortheinventor

Stirling based

There's kinematic, which is what almost everyone thinks of when you say stirling because that is what the historical designs invented by Stirling were. But Stirling is a cycle, and there are also Free piston stirling engines. Dish-stirling is a proven commercial technology and almost invariably uses FPSEs.

Also, thermoacoustic engines are very interesting. You might say they are an undeveloped technology, but I think it depends on the exact design. Wikipedia says ring-shaped ones get 30% overall at combustion temperatures (the highest they can operate at) and while the wikipedia artice cautions they are experimental, I say if you can do it once of course you can do it again, we just need to track down the people who made it, the articles published etc. The question is how long they will last, and how much they cost per watt, etc. I remember reading that there is one group of people developing a 1KW one for dish-sterling but they said it was the first time it had been done and they were still working out the issues, so this may be straying into the new technology zone.

To make a kinematic engine last, you can just put all the load bearing surfaces away from the hot area so normal lubricants can be used and last practically forever (a sealed bearing unit lasts many many years) , you can use gas bearings in the hot areas, and maybe use ceramic parts.

The whispergen is a cogen unit that supposedly lasts a very long time no maintenance, and I don't know if they have yet another way to make it last. Most cogen units seem to use FPSEs, which last because they use a spring to support any loads and the parts otherwise never touch. The details can be divided apparently into two aproaches (need to add link to the forum post "infomation about linear free piston stirline engines relevant to OSE purposes" gregorgregortheinventor

I just remembered seeing some FPSEs based entirely on metal diaphragms, will try to find the document later. No pistons to machine, no planar springs. There may be major performance downsides though. gregor

Collector-focussed design

Trough collector

What about an extruded plastic collector? Greenhouse plastic carbonate panels only last about 20 years BUT they are transparent. If you need to metallize the material why not metallize the whole surface to effectively protect it from UV degradation? Then we could use unstabilized recycle plastic resin of whatever sort we wish

In the prior work section some of the pages looked at plastic metallized films and apparently they do not last more thn 7 years, but if you metallize both sides, why on earth not? You sure that's not just planned obsolescence or something? Can't be heat, microbial action . Maybe scratches or something, the metallized surface gets scratched, plastic underneath exposed to UV then that's it. gregorgregortheinventor

Array of flat heliostatic mirrors

The solar fire people already proved this works at about 50% efficiency at peak insolation (1Kw per sq meter presumably) apparently including their absorber design. What is the cost estimate for the P32 collector materials-only? gregorgregortheinventor

Static mirrors moving focus

This is being used on a lot of largeish scale systems with steam turbines as the engine, but those are really big turbines that can get good rim speeds at reasonable RPM. See the turbines section of the expanders section.

Here's an idea: long sheets of aluminum, on a roll a bit more than half a meter wide, not foil but more like aluminum pop can sides but a little thicker (ever notice how springy and resilient that alloy is?). Polished to a mirron on one side. Now with a single machine, pulverize the earth at the installation site, maybe mix in some stabilizer like cement if needed, then pack it down like earthcrete or a compressed earth brick into, how can I describe this... I shoudl draw a picture, but for now, an array of straight sided dunes maybe half a meter tall, which extend in a semicircle shape, and are concentric semicircles or whole circles. Add the aluminum to the correct side of the dune and the array forms a fresnel reflector. The bottom edge of the aluminum may need to be buried, and it may need to be achored to the dunes or something to protect against wind damage.

So that's the

Factors that coudl mitigate performance to below the ideal:

has to be surrounded by a fence, which adds cost or peopl/animals would step on it.

- scratching of the alimunum? - have to deal with rainwater runoff somehow, add complexity. - how to move the focus, coudl be mounted on a moving boom with the whole engine at the end of the boom (eliminates transmission losses) or maybe hung from say 3 chains.

Okay, now what if it was a ground-hugging array of fully (all sides) aluminized plastic injection-molded peices so the earthworks can be skipped? The number of differrently shaped peices that need to be molded woudl be limited to much less than the total number in the array (just a pie slice copied multipe times). Hm. See potential reflector materials. Glass plates could be used to, and it might at least save on structural costs. In that case they are heavy enought they could be just be put in place by hand maybe.

To get the dunes in a good circle shape the boom could be used to guid the shaping of the dunes.

Static mirrors non-imaging optics

There was a TEd talk with some retired physcist in a mcarthur grant that developed a system with neither movingmirrors nor moving absorber, everything stayed still with a clever arrangement of the mirrors. Of course the concentration ration would not be very high probably, but the relationship betwee net output power vs. output temperature of an absorber is what really matters there and is dependent on other factors as well, maybe if the absorber was vacuum insulated and used reflective coated glass as mentined in the existing design work it woudl achieve a good performance level? gregorGregortheinventor

More general information and discussion

Reflector materials

As mentioned in the existing work, normal glass (high-iron is normal I think) mirrors last a very long time. How much do they cost? Says $1.5 USD per sq foot, can we get them for less in bulk? How hard are they to produce locally, though, too?

The probelm with plastic is that even with uv-stabilizing additives it won't last long enough (polycarbonate greenhouse stuff lasts about IIRC 20 years) will it? But it doesn't have to be transparent if aluminized on the front, so if say carbon poweder or charcoal dust was added to the surface or bulk of the material that would produce a material that would retain structural strength, as the UV cannot get beyond the surface. That way recycled plastic can be used. The problem beign that the surface woudl still degrade. The surface degradation would only be a problem at the reflector surface though, which is covered with aluminum anyway, as long as the Al layer does nto get scratched too much (how thick can it be?).

If the reflector is first-surface, i.e. has the Al in front, is scratching a problem due to dust etc.? Coould be a major lifetime limiting issue. If that were true maybe a hard alloy can overcome it? If not glass is practically the only option for long life? gregorgregortheinventor


  • liquid ring pumps
  • so called gearrotors, same mechanism as a crescent gear pump, there is a company developing one Brayton engine based on this that they claim 50% overall for at combustion temperatures.
  • worm compressors used in reverse
  • other types of gear pumps or roots blowers (similar if you think about it, the 2 vanes can be many different shapes) note that to use it like an expander not a compressor you have to drill a hole in the face of it, not just use it in the same way as a compressor.
  • claw compressor like mechanism
  • linear piston, no crankshaft just a linear alternator
  • Impulse turbine and Hero turbine. A hero turbine works optimally with the rim velocity the same as the fluid stream, so they are never used in steam. An impulse turbine turns at only half the speed as the gas stream, optimally. It also keeps the high temperatures away from the bearings, hero migh ttoo, not sure, depends on where expansioin occurs. gregorgregortheinventor


An impulse turbine would really be the cat's meow for brayton or rankine and maybe other cycles. from steam engine/gregor it seems like the machining accuracy is not the biggest deal, it is the high rim speed and material strength limitations/centrifugal force!

These can be mitigated to some degree with multistaging. Having seen pictures of ORC turbine based single household sized cogeneration units (so sized about right for our purposes though I didn't check the efficiency of them IRC there are no working fluids except water that can handle much above 450 deg C or so and stil lbe liquid at room temperature, so that migh tproduce an efficiency hit right there, maybe there are such fluids though, what about bromine or other elements?) and they seemed to be quite small diameter, so I wonder how they do it, maybe a lot of multistaging.

Might it be possible to make a large diameter turbine of more than a meter dia?

What happens to efficiency when it is operated at less than optimal speed? Maybe the hit is acceptable?

What is the temperature and pressure the blades and casing are subject to? Do not confuse steam turbines with gas turbines, in steam the expansion of the steam occurs in the de laval nozzle of course before ti gets to the turbine, so the pressures and temperatures should be substantially lower than the input steam involved, which is very good as it meant no special materials needed. gregorgregortheinventor

Related pages

See the energy category page for more pages on the project. Remember this project has been going on a long time and many of the pages have little applicability to the current development effort(check the date in the edit history). Remember, see the rules for a definition of where arguments must be located to be included in the review process.