Solar Concentrator Reviews

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Review of cost predictions by Gang Xiao and Response by Marcin

See the work of Gang Xiao - http://wims.unice.fr/xiao/solar/index.html

Hi,

This computation is not quite correct. I am doing a better calculation; I will let you know when it is OK.

Here I would like to give you my opinion on the cost analysis. I don't expect you to believe me right now; I only hope that you remember my estimations when you've learned it the hard way via a prototype whose efficiency falls short of your expectations.

Your cost computation on the linear fresnel concentrator is fundamentally flawed. It is a general understanding within the solar concentrator industry that the main cost of a concentrator is not the raw materials but the manufacturing cost meeting the high optical precision requirement. If you put a value of zero for the manufacturing cost in your cost analysis, it does not make much sense.

To my knowledge, my trough design is the only method that can avoid this fatal correlation between cost and precision. That's the reason why my cost is low. If your design is based on known technology, you can only reduce cost by sacrifying precision and hence efficiency, and you will end by getting a product with worse performance/cost ratio.

In fact, the cost to pay by using flat slat mirrors is exactly that the precision requirement is several times higher for a given concentration ratio. I've told you that you need a precision of plus minus 2 mrad, or 0.1 degree. At this level, everything is non-negligeable: manufacturing (fixation) error, deformations by gravity, wind, aging, etc. Your design should take all this into account, and calculate carefully incorporationg error margins. Of course, the cost will inevitably grow.

For example, using a simple screw to fix the mirror on the rod is much too coarse. The screws will get loose over time, and it is impossible to refasten on the field towards a precision of 2 mrad.

Such a precision requirement needs precision manufacturing (fixation) equipments and trained personnel, so you cannot put labor cost to 0 by counting on unexperienced volonteers. And I have made a preliminary cost analysis for you, according to the industrial standard. My estimation is about 200$/m^2 plus installation cost, in line with commercial products. What I am not sure is whether you will be able to get the same quality standard as a commercial product.

On the other hand, the steam engine net electricity output will be single digit if you use direct steam. My next precise calculation will show that it is around 8%. So the overall efficiency is less than 4%, so the net electricity cost is more than that of optimally tilted photovoltaic panels, if you add shade loss, end loss, cosine effect, as well as the miss of diffuse light. Your claim of 1$/W is much too exaggerated.

In fact, electricity-only solar thermal technology is only viable for large scale power plants, with high quality concentrators and sophisticated turbines and so on. But this market is for the time being neither for you nor for me: it is not a technical problem but a social one. It is big money affair and political lobbying, we have no access to it.

We can only look at small scale applications. But even with my trough design that is much cheaper and of much higher performance, I don't dare advocate for small scale electricity-only applications. This is not economically competitive. At small scale, CHP is the only competitive solution.

It is here that the compactness of my design shows its great advantage. CHP means that it must be installed near heat users, because long hot water pipes mean huge heat losses. Ideally, installations for individual homes. For this, linear fresnel is downright impossible, because of its minimal practical size. You need a minimal field size of tens of square meters to minimize endlosses. How can a family do to consume the huge quantity of collected heat? In most places, the only real "market" is winter home heating. However, linear fresnel collectors simply don't work during winter due to a too low sun angle. The cosine effect is too high for flat north-south installations, and shade loss is too important for flat east-west installation. The incoming energy will even not make up for heat losses. I call this phenomenon "solar hibernation". The only solution is to make tilted installations, but that's explosion in cost. In most residential areas, the height of a tilted linear fresnel collector would simply exceed what is allowed by the legislation. And you will have hard time fighting wind load and so on.

Response to Critique

Yes, but how many industrial players are working with flat arrays? That coudl be the secret to low cost - via DIY - as the intelligence is taken out of the manufacturing process and put into the advanced electronic controls. Isn't it true that field adjustment of alignment, slat by slat, will be a foolproof way to obain the required alignment?

The only trick is - a properly stiff mounting structure for the slats. That does not appear to be an intractable problem, and these guys from the UK claim to have done it at low cost -

http://blog.opensourceecology.org/?p=446 We are allowing for 50 cent per watt labor costs.


Are you saying that oil heat exchange is the answer?


The electronically valved steam engine is reportedly superior to turbines in the sub 500 hp power scale.

What do you think of the potential of a system like Nolaris? (http://nolaris.ch/17-0-Our-Technology.html)

Marcin

Further Communication

>Why is it not possible to have a secondary reflector at the collector tube, with nonimaging optics, for another factor of 3 of concentration onto a 2" tube, whe we start with 6" slats? >Total theoretical limit becomes 48x concentration.


This was my unit confusion. In theory, what you do is possible (and it's the limit), but this only gives you 16x concentration not 48x. The geometric concentration ratio means mirror surface devided by by the receiving surface, so you should calculate with the circumference of the tube.

In practice, you have to incorporate error margins and increase the tube diameter (or circumference) by 20% or so, because reflections will never be exact. That's why I give you a practical concentration ratio of 13 times.

However, with more careful calculations taking into account the sun's angular diameter, I am afraid that 20% is not enough. It should rater be 30%-35% or so. That's very bad. If you give me all your design diameters (gaps between the slats, distance between the slats and the receiver), I can tell you how this computation should be done.



>We are aiming for a working temperature of about 350C, and we will have to do as many slats as are necessary to achieve that. Then, the question is, how much power will be available to the steam engine, if we optimize for a temperature of 350C.


You need a much higher concentration ratio to reach that temperature. At least 20 times geometric. This would mean roughly at least 30 slats and a surface error of 1 mrad. If you don't believe me that this is impossible, just try it.

And you have to insulate the receiver with vacuum technology. Glass wools and double panes won't be enough to check conductive and convective thermal losses.

On the other hand, 350C with big turbine can give you up to 30% net heat-to-electricity efficiency. This is the same for the big trough power plants. Do you know any existing linear fresnel powe plant? If yes, please read their cost report. If no, there is certainly a reason.


Marcin Jakubowski wrote:

>Yes, but how many industrial players are working with flat arrays? That coudl be the secret to low cost - via DIY - as the intelligence is taken out of the manufacturing process and put into the advanced electronic controls. Isn't it true that field adjustment of alignment, slat by slat, will be a foolproof way to obain the required alignment?


My own experience tells me that this kind of argument won't convince anybody. Even showing the prototype won't be enough. You have to be able to sell your product with the promised price.

Now there is no fundamental IP in the linear fresnel design. What makes the industrial prices is mainly the real manufacturing cost.

Field adjustment means heavy equipment. The big troughs align to 3 mrad precision, and heavy and special equipments are used for that. Did you notice Sandia making big hypes on an invention that improves this process? This is definitely not suitable for DIY. Don't forget that your adjustment should last 20 years.

One of the reason why people use big troughs is that the alignment (adjustment) costs too much. So big troughs mean less adjustment, hence economy.

>The only trick is - a properly stiff mounting structure for the slats. That does not appear to be an intractable problem, and these guys from the UK claim to have done it at low cost -

>http://blog.opensourceecology.org/?p=446


I've seen that. No data to show what is their performance and quality, so that doesn't mean much.

>>If you put a value of zero for the manufacturing cost in your cost analysis, it does not make much sense.

>We are allowing for 50 cent per watt labor costs.

What it corresponds for one slat? What is key to me is how you plan to do to meet the 2 mrad error rate. And what's your calculation with wind deformation and so on. This is not field adjustable without heavy equipment and a proper design!

>>To my knowledge, my trough design is the only method that can avoid this fatal correlation between cost and precision.

>Tell me more about your key to success.

The key is to obtain the desired shape entirely thru natural elastic deformation of flat material. The problem is, whenever you try to force the material to follow your predefined shape, you get a cost to precision correlation. But natural deformation can give the high precision with low cost, if you know how to induce the deformations. There are quite some mathematical computations behind the seemingly simple construction.

>>On the other hand, the steam engine net electricity output will be single digit if you use direct steam.

>Are you saying that oil heat exchange is the answer?

No, the key is temperature hence concentration ratio. Of course, with high temperature, you will have to switch to oil, for otherwise the pressure would be too high.

>>In fact, electricity-only solar thermal technology is only viable for large scale power plants, with high quality concentrators and sophisticated turbines

>The electronically valved steam engine is reportedly superior to turbines in the sub 500 hp power scale.


Don't buy into people's commercial hypes! Always ask the question of performance to price ratio.

I am doing some detailed steam engine calculations. It seems that it's better than I first thought; but I haven't finished yet.

>Shade loss fraction may increase, but can be potentially mitigated by sufficient inter-slat spacing.

That's too quick. Increasing inter-slat spacing proportionally reduce concentration ratio or reduce the error tolerance, either one or the other, or both.

>This may work in winter - and it can definitely work under the assumption of an optimally-matched steam engine. At the very worst, there may be a limited range of performance in winter. At the very least, winter operation could afford space heating.

>What do you think of the potential of a system like Nolaris? (http://nolaris.ch/17-0-Our-Technology.html)

No idea because there is no technical information nor cost. There are too many things like that on the internet, most of them are vaporware.

You know that a person with marketing talent may successfully sell a technically inferior product. This kind of website is part of such talent.

I once contacted a trough manufacturer who told me that his product costs only less than 200 euros per m^2. I never know his exact price, but from his ads, I can guess that it's probably over 1000. Be careful.

I myself am suffering from this situation. Nobody believed my cost analysis. So now I try to make the product and sell it for the promised price, with promised quality. I suspect that even this is not enough. I will have to make the product test by third party and so on.

Your project will be in the same situation. Marketing hypes can bring some unexperienced volunteers, but knowledgable people will only be convinced if you show them concrete product with complete third party test data. In between, if you don't have the capability to fix every technical detail, you will fail.

With respect to your project, I am in a different situation. I can guess what are the problems you will encounter. For the time being, you haven't convinced me that you will be able to solve them. So I am not very optimistic about your linear fresnel. Of course, if you finally succeed, I will only be happy with that. I am not really considering you as a competitor: our technologies are somewhat complementary.

Getting Interesting

On Tue, Feb 24, 2009 at 3:11 PM, XIAO Gang <xiao@unice.fr> wrote:

>Marcin Jakubowski wrote:

>>Have you ever tried to align a single reflector onto a receiver tube? I don't see why that is overly difficult, if one has closed loop feedback.

>Can you image how many calculations I have done, and how many tests I have made?

Show me the data. There would be no need for me to do the experiments if you provide some data.

>I know what is realistic and what isn't.

>>Now, our strategy is to sacrifice performace at the gain of absolute lowest cost. We believe that this point is a winner for overall cost competitiveness.

>Except that accumulation of performance loss will end at a worse performance/cost ratio.

That may or may not hold true, depending on the details of the situation. Yes, typically lower performance means higher cost/performance, but if cost is reduced more than performance is reduced, then we may have a winner. That is not impossible, as you suggest.

>The whole picture is linked. I am certain that many people have had the same idea. The linear fresnel design existed at least since 1970s.


We didn't have cheap tracking electronics until the 10 years or so back, though.

>The commercial products are usually optimized for the performance/cost ratio. If you can break this optimization without non-trivial inventions, all these engineers are good for nothing.

Correct.

>Well I don't say that's impossible, but the chances are rare. In most of the time, you will be reiterating errors already encountered by others.

Exactly. That's why before any experiments, I am and will seek the advice of those who have more experience. That's the nature of the open source effort.

>XIAO Gang (肖刚) http://wims.unice.fr/xiao/

More Issues

>>>>Have you ever tried to align a single reflector onto a receiver tube? I don't see why that is overly difficult, if one has closed loop feedback.

>>>Can you image how many calculations I have done, and how many tests I have made?

>>Show me the data. There would be no need for me to do the experiments if you provide some data.

>I am giving your the compiled data, but you are sceptical right now. For example, that 2 mrad precision is not a DIY affair. But here the test is easy to mount.

If we have a secondary nonimaging optics reflector shroud (like in Powerfromthesun.net) in front of the collector tube for 2x secondary reflection, then at 10 meter separation from reflectors, we can accept 6 mrad presision.

With individual slat motors and 60 pitch gear of 2" diameter for controlling slat rotation gets you 2 mrad accuracy.

What is so difficult about implementing this in practice?

I'm interested in hearing more about your experimental procedure with which you obtained your precision results so I can assess your statements properly.

>>>The whole picture is linked. I am certain that many people have had the same idea. The linear fresnel design existed at least since 1970s.

>>We didn't have cheap tracking electronics until the 10 years or so back, though.

>It's still not yet zero today. So what is your cost? Where is the schematics, parts list, cost addup, programming and so on? By the way, do you know that you should use stepper motors? Brush DC motors don't last long.

Don't have any of it. All I know is that we can use cheap off-shelf open source controllers like Arduino, tracking algorithms can be implemented, and if need be, we'll tool up for fabricating our own stepper motors if that turns out to be prohibitive. I don't see any a priori reasons that make these challenges intractable. You must remember that we will go as deep into developing tooling as needed to achieve required cost predictions.

>>>Well I don't say that's impossible, but the chances are rare. In most of the time, you will be reiterating errors already encountered by others.

>>Exactly. That's why before any experiments, I am and will seek the advice of those who have more experience. That's the nature of the open source effort.

>Sorry but when I look more closely at your design, I've just got the impression that it's not yet a question of performance/cost: it simply won't work. Too naive.

>Secondary reflector. How would you do to protect the metal mirror surface? Plastics would melt down, glass would crack under this temperature. Left unprotected, the metal surface would oxide in just a few days, under the accelerated oxidation of high temperature. Other people put the secondary mirror under evacuated environment, but that's again not a DIY affair, and the cost is probably prohibitive if you can find somebody making it.

I don't know yet. If we can't solve this issue, we will have to go to additional reflector slats to make up for this.

>Glass cover of the receiver. During our trough test, people once cracked a glass cover with a temperature gradient of only about 70C. If you want to reach 200C for your receiver, you have to put margins and test it under 250C or even 300C. At this temperature gradient, even a borosilicate glass of the highest grade won't resist. You'll need the kind of special glass found in kitchen ustensils and ovens, but you have to incorporate the proper price tag into your cost estimate (it's going to be expensive).


What is the going price estimate for this glass? We may have to produce this glass ourselves it it is not cheap off-shelf.

>Or you should switch to evacuated tubes,

Evacuated tubes - I don't think that will have the same cost performance as non-evacuated conditions.

>or reduce your temperature objective to less than 150C. Of course, 150C won't give you much electricity. If you want 300C or so, evacuated receiver is a must.

>It's going to be a subtle problem. When there is end loss, the receiver is half hot half cold. If you don't take care, even a heat resistant glass cover could crack in the middle. That's one of the reasons behind receiver tube breaks in the big trough power plants. Of course, people know the solutions, but these are trade secrets.

>And there are norms for solar collectors: wind resistance, hail resistance, lifetime, etc. If your product doesn't respect these norms, it's no better than a toy. So for example you have to compute/test the mechanical behavior of your slats/receiver under the wind pressure of 120km/h, as well as the deformation extent under 60km/h, etc. The low profile is not an excuse.

Agreed.

Marcin

Mirror Testing

Hi,

By curiosity, I've just made a small mirror test for you.

I used a high quality vanity mirror which I picked for earlier optical experiments. Excellent surface flatness.

The mirror is of width 13cm, with an almost normal angle to the sun. At a distance of 5m, the image scattering reaches 21cm. That's just the image width, not counting positionning errors. Therefore for this distance and with slats of 15cm wide, the minimal receiver width should be 23cm, or the reflected light will be partly lost. Adding inevitable error margins, 25cm or 26cm is a strict minimum. That's even worse than what I calculated. With 16 slats, the concentration ratio won't exceed a single digit.

Adding slats is no big improvement, because the distance should increase, and the image wider. So a wider receiver has to be used, and the concentration ratio almost remains the same.

An even bigger trouble is with deformation. My mirror is only of 20cm long. At this length, a torsional force of only a few dozens of grams is enough to deform the image by several centimeters. Now even a mild wind force is of 10kg/m^2, and at equal tortional force, the deformation is proportional to the square of the length!

Try it for yourself.

Steam Engine Efficiency

I have done the preliminary engine efficiency calculations. It is for classical piston engines, but incorporating design improvements such as high cylinder expansion ratio, cylinder heating and electronic valve control. Other engine options are definitely excluded, for it is impossible to do the same level of efficiency optimization on these fancy engines.

The style is hard to understand, for it is primarily just my private memo. With this, I know how an efficient engine can be designed, but I won't go to design it right now. It is not yet my priority. If someone is interested in putting this calculation into practice, I can help.

The big point is that a triple expansion home CHP engine can reach or exceed 20% net electric efficiency. That is enormous; commercial big organic rankine turbines only reach 18% under more favorable temperature conditions. Of course, this is not going to be easy. Huge development effort is required, and one should probably borrow some advanced auto engine technologies. But a reduced objective of 17% or so should be much easier.

Enjoy!

--

XIAO Gang (肖刚) http://wims.unice.fr/xiao/



Water characteristics

Enthalpy data: http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html http://www.thermexcel.com/english/tables/index.htm

For steam, gamma=Cp/Cv=1.32

Notations:

V0= steam volume before expansion. P0= steam pressure before expansion. t = pressure difference between the hot and the cold sides. r = volume expansion ratio within the cylinder. This should be controlled

      by an electronic valve.

T_out = theoretical temperature of the steam exiting the cylinder after

      expansion, assuming perfect adiabatic expansion and no condensation.

Within one engine cycle, there are two periods where work is done by the steam: the injection period when the hot side of the piston has pressure equal to the maximal one, and the expansion period when the steam injection is cut off, and the steam remaining within the cylinder expands.

The expansion is assumed to be adiabatic. In reality it is not, hence efficiency will be lower than the theoretic one.

During the injection period, the work is done by constant pressure against the low pressure at the other side of the piston, so the work equals

      V0*P0*(1-1/t).

During the adiabatic expansion period, the pressure is proportional to 1/V^gamma where V is the volume, so the total work is the integration

      V0*P0*(1/x^gamma-1/t)dx, from 1 to r.

Practical losses: friction 5%, generator 15%, parasitics 5%, fluid loss 10%. Overall 69%.

Water pump efficiency: 80% (mechanical drive).

Possible improvement: heat up the cylinder to high temperature. This can dry up the partial condensation, as well as can improve expansion efficiency, because the expansion will somewhat approach the isothermal process. But this may create difficulties for the lubricating oil.

If the cylinder heating is not done or is not efficient, the whole efficiency computation may suffer from a loss of 20% or more.

Cylinder heating should be efficient for multi-stage engines, by heating stages except the last one. It is not clear whether heating single-stage engines will be efficient. The situation is quite intricate, and can only be determined via practical tests.

We assume a maximal expansion ratio of r=5. In practice, this probably requires a high precision standard for the cylinder manufacturing. The piston compression ratio should not be lower than 15.

All examples are given for CHP cases, with condensed cooling side towards 60C.

Note. The calculations are approximative, especially for the high end. Steam nearing boiling point and at high temperature is not ideal gas, and data are hard to find for non-saturated steam.


Examples.

60C-170C. P0=5bar, V0=0.384m^3/kg, t=5/0.2=25, enthalpy difference=2795-251=2544kJ/kg. For r=4: output=0.384*500*(0.96+1)=376kJ/kg, pump consumption = 1.25*0.5=0.6kJ/kg, net work 376kJ/kg, raw efficiency=14.8% (10.2% net electricity)

Comment: this case is of very low requirement, with a simple construction and requiring low solar concentration ratio, but the efficiency is too optimistic for a single stage engine, due to condensation and heat exchange in the cylinder. 8% is more realistic. For prototypes and starting products.

60C-250C. P0=10bar, V0=0.238m^3/kg, t=10/0.2=50, enthalpy difference=2930-251=2679kJ/kg. For r=5: output=0.238*1000*(0.98+1.18)=514kJ/kg, pump consumption = 1.25*1=1kJ/kg, net work 513kJ/kg, raw efficiency=19.1%. (13.2% net electricity)

Comment: this is close to optimum for a single expansion home engine. 250C should be more or less the limit of what legislation will allow for a home, 10bar is reasonable, and increasing pressure for single stage does not give much more efficiency. But again the efficiency is probably too optimistic.

60C-250C. P0=15bar, V0=0.158m^3/kg, t=15/0.2=75, enthalpy difference=2910-251=2659kJ/kg. First stage: r=5, t=10, with steam reheating to 230C. T_out=523K/10^0.24=28C. Average heat capacity=2kJ/kg.K, reheating energy input 404kJ/kg, total energy input 3063kJ/kg. Second stage: r=4, t=7.5, T_out=503K/7.5^0.24=37C. First stage output: 0.158*1500*(0.9+0.86)=417kJ/kg. Second stage output: 1.52*150*(0.87+0.72)=363kJ/kg. Pump consumption = 1.25*1.5=2kJ/kg, net output 778kJ/kg, raw efficiency=25.4%. (17.5% net electricity)

Comment: this should be the standard product. Double expansion with steam reheat, that's more serious. But the cylinders are easier to construct with a smaller r which can be further reduced without much penalty on the efficiency. The efficiency is competitive with commercial organic rankine systems. The last stage should use a slightly lower compression ratio to get a T_out closer to the cooler temperature. Cylinder heating of the first stage is a must.

60C-250C. P0=25 bar, V0=0.0952m^3/kg, t=25/0.2=125, enthalpy difference=2879-251=2628kJ/kg. Equal triple expansion stages: r=3.3 each, t=5 each. First stage T_out=523K/1.47=83C, reheated to 230C. Average heat capacity=2kJ/kg, reheating energy 294kJ/kg. Second stage T_out 513K/1.47=76C, reheated to 230C. Average heat capacity=2kJ/kg, reheating energy 308kJ/kg. Third stage T_out=493K/1.47=62C. Total energy input 3230kJ/kg. First stage output: 0.0952*2500*(0.8+0.53)=317kJ/kg. Second stage output: 0.457*500*(0.8+0.53)=304kJ/kg. Third stage output: 2.28*100*(0.8+0.53)=303kJ/kg. Pump consumption = 1.25*2.5=3kJ/kg, net output 924kJ/kg, raw efficiency=28.6%. (19.7% net electricity)

Comment: this is the high end home product. With triple expansion and double reheating, the cost is going to be higher, and it will require significant development efforts. Net efficiency might hopefully exceed 20%. Note that the Carnot efficiency is 35.6%; the raw efficiency is quite close.

40C-350C. P0=100bar, V0=0.026m^3/kg, t=100/0.07=1400, enthalpy difference=2900-163=2737kJ/kg. Reheated triple stage. r=6 each, t=11.2 each. T_out=623K/1.8=73C. Average heat capacity=2.1kJ/kg.K, reheating energy input 2*582kJ/kg, total=3901kJ/kg. Output=3*0.026*10000*(0.91+0.92)=1427kJ/kg, pump consumption = 1.25*10=13kJ/kg, net work 1415kJ/kg, raw efficiency=36.6%.

Comment: this is to compare with first generation trough solar power plants who use 100 bar turbines. The efficiency calculation is in conformity with what is obtained by the turbines, but the computations here contain errors due to some guessed data values.


Reflectors and Collector

Marcin Jakubowski wrote:


   If we have a secondary nonimaging optics reflector shroud (like in Powerfromthesun.net) in front of the collector tube for 2x secondary reflection, then at 10 meter separation from reflectors, we can accept 6 mrad presision.


How did you get this 6 mrad result?

Every mrad error in the mirror results in 2 mrad displacement of the reflected image, because mirror reflection is a symmetry. Therefore 6 mrad corresponds to an image error of 12 mrad, which over 10 meters is 12 cm. Plus minus, you have to add a margine of 24 cm to the width of the receiver, on top of other margines. What will remain to your concentration ratio?


   With individual slat motors and 60 pitch gear of 2" diameter for controlling slat rotation gets you 2 mrad accuracy.
   What is so difficult about implementing this in practice?


I am not your evaluator. If you don't believe me, try with an experiment. That's what you should do anyway.


   I'm interested in hearing more about your experimental procedure with which you obtained your  precision results so I can assess your statements properly.


No I have never tried lfr. But my experiments on the trough give me intuitions on what an mrad means.


      It's still not yet zero today. So what is your cost? Where is the
      schematics, parts list, cost addup, programming and so on? By the
      way, do you know that you should use stepper motors? Brush DC
      motors don't last long.


   Don't have any of it. All I know is that  we can use cheap off-shelf open source controllers like Arduino, tracking algorithms can be implemented, and if need be, we'll tool up for fabricating our own stepper motors if that turns out to be prohibitive. I don't see any a priori reasons that make these challenges intractable. You must remember that we will go as deep into  developing tooling as needed to achieve required cost predictions.


Once again, I am not your evaluator. I would simply say that if you publish a cost analysis that misses the cost of an important component, it is your own credibility that is lost in the eyes of knowledgable people. Such a people might one day to be asked for opinion about your project, and you would be hard hit. Take care!


      Secondary reflector. How would you do to protect the metal mirror
      surface? Plastics would melt down, glass would crack under this
      temperature. Left unprotected, the metal surface would oxide in
      just a few days, under the accelerated oxidation of high
      temperature. Other people put the secondary mirror under evacuated
      environment, but that's again not a DIY affair, and the cost is
      probably prohibitive if you can find somebody making it.


   I don't know yet. If we can't solve this issue, we will have to go to additional reflector slats to make up for this.


No more slats cannot help. But your earlier design with multiple tubes can.


      Glass cover of the receiver. During our trough test, people once
      cracked a glass cover with a temperature gradient of only about
      70C. If you want to reach 200C for your receiver, you have to put
      margins and test it under 250C or even 300C. At this temperature
      gradient, even a borosilicate glass of the highest grade won't
      resist. You'll need the kind of special glass found in kitchen
      ustensils and ovens, but you have to incorporate the proper price
      tag into your cost estimate (it's going to be expensive).
   What is the going price estimate for this glass? We may have to produce this glass ourselves it it is not cheap off-shelf.


No idea at all. Even no idea whether you can find any that easily. And if you think that you can produce it yourself, try producing an ordinary flat glass first, before talking about the high tech one. This is not reasonable.

I suggest that you try to find more information on the heat resistant glasses. You will have a lot of work to do, testing samples and so on. Be prepared to the reality that many claimed "heat resistant" glass samples won't live up to your requirements. Much trouble down the road, but that's much better than if you spend a huge amount of money, get up the whole thing, only to see that it fails.

Ausra Comparison

Hi,

I have come across some documents about the Ausra lfr. Here is the compiled information that I have been able to get.

1. Performance/cost. They do not give exact cost values, but third party revelations and Ausra cost analyses show that the cost per m^2 of the collectors is within 50%-60% of that of big troughs. In absolute terms, it is a bit more than 200$.

On the other hand, the efficiency of the lfr can be estimated to be around 30% less of that of big troughs. So the net benefit in performance/cost is 20%-30%. This is highly competitive, but a beginning technology usually will expect cost overruns during the field deployment. Everything depends on the extent of the cost overruns, but the competition between the two technologies is well engaged.

However, in no case this one will be able to compete with my technology. Mine has a cost 80%-90% less than the big troughs, while having a higher efficiency. It's much too far away.

2. Concentration ratio. One has only to notice that Ausra uses working temperature of 285C, instead of 400C for the big troughs. This is a clear indication that the equivalent concentration ratio of clr is much less than the big troughs.

3. Secondary reflector. From the pictures, you can notice that the receiver tube is evacuated, and the secondary reflector is mounted outside the evacuated receiver tube. The temperature outside an evacuated receiver tube is not very high, so it will be not hard to mount the secondary reflector. However, this setup leads inevitably to some optical losses.

4. Slats. With a careful examination of the pictures, you can see that the mirrors are curved. This is the trick: I have explained that flat mirrors can only give you single digit concentration ratio, well below what is needed for 285C working temperature. They apparently have managed to find a good balance between the manufacturing precision and the cost. However, this one is tricky, and you cannot do it yourself unless they are willing to supply the mirrors to you: slats at different locations should have different curvatures!

Regards,

Towards a Steam Engine

It seems that you are really up to make things go. Great. You can count on my participation.


Marcin Jakubowski wrote:

   Hello.
   Can you tell me about manufacturing requirements of your system?
   What is the feasibility of manufacuring at different scales? What volume of production is optimal, and what is the smallest feasible production facility?


Wait a minute. My design is a modular system, with several possible objectives. What is your objective? Residential water heater? Residential electricity? Residential electricity 24h/24? Medium size CHP? Large scale power plant?

Don't try to do everything at the same time. Fix one objective. Then we could make up a plan.


   On the steam engine, here's a plan, so let me know what you think. I want to develop an open source steam engine.
   1. Write up specifications for steam engine
   2. Submit for bidding
   3. Fund it
   4. Have fabricator visit Factor e Farm and we docuement fabrication process
   5. We replicate fabrication in one day as proof of replicability
   What do you think of this?


Well I don't know how this has worked. For the time being, the biggest unknown for me is who can take charge of which step.

What I can do (and will promise to do):

Write up the specifications after the objective is determined (size of the engine, degree of sophistication, temperature range).

Make fundamental design calculations.

Check design details.

Locate cheap Chinese fabricators for series fabrication once the replicability is proved.

Sell the product to some extent (I will need it myself).

On the other hand, I cannot promise to do all the detailed designs.

I suppose that you can coordinate the whole plan?

Summary of Compact Linear Reflector Discussions

For the linear fresnel reflectors, the key point is error margins. If 15cm flat mirror slats are used, the receiving width should be at least 15cm, secondary reflector or not.

But that's not enough, because error margins have to be added. There is the sun's angular diameter which theoretically is 9mrad, but my test shows that in practice it's rather 12-14mrad after a mirror reflection.

Then there are various manufacturing/installation errors, inevitable. These errors will accumulate. As a rule of thumb, in order to have a good chance that the accumulated error does not exceed a global margin, every individual error margin should be kept within 1/3 of the global margin.

Let's suppose that the global margin is 10mrad (plus minus 5 mrad), and the optical distance is 5m. 10 mrad over 5m, the distance margin on the receiver is 5cm (plus minus 2.5cm).

On the one hand, this makes a receiver width of 27cm, so the concentration ratio is 9 times. Adding cosine effect etc., the equivalent concentration ratio is about 7-7.5 times. At this ratio, the practical working temperature cannot exceed 150C, or the thermal loss would be too important. Steam pressure: 3 bars. Espected steam engine net efficiency: 7%. Very poor performance.

On the other hand, every individual error should be kept within plus minus 1.6 mrad (0.8mrad for mirrors), or plus minus 8mm for the receiver. Including:

Receiver positionning, receiver straightness, receiver gravity bending, receiver wind deformation, chassis assembly precision, slat prositionning, mirror precision, tracking precision, mirror gravity bending, mirror wind deformation, mirror torsion, etc. All these errors should remain within the range after years of use.

Several of these error margins seem very hard to achieve. So the global margin of 10 mrad is probably even not enough, and the concentration ratio should be lowered even more. In any case, if one does not take extreme care with all these errors, big trouble is ahead when realising the design.

For a concentration ratio of 7 times and an engine efficiency of 7%, that does not worth the pain. So I seriously propose that you put aside this lfr option unless you are sure about how these error magins can be controlled.

Now there is something I have just forgotten before, that I now propose to you: there is a "downgraded" version of my collector that is patent-free. In fact, the construction is partly described in an expired (abandonned) patent WO80/02604, partly released by me into the public domain thru my 2 documents "on closed parabolic trough" and the diy trough document. With this design, you can easily get a geometric concentration ratio of 10 times, with a cost that certainly is lower than lfr. I have already tested it in an earlier diy model that is not published. The construction is foolproof, and for the design, you can simply take the diy trough document and ignore the adjustment part.

And this one is enough for the starting steam engine version (170C/5 bar).

Of course, this downgraded version is not very interesting when you progress in the project. But its existence is an insurance, assuring you that the pending patent will be licensable, with resonable license fees. (I've already said somewhere that the license will be free for non-profit organizations.)