Solar Power Generator Distillation

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(in progress)


  • Intro to Solar Power Genreator - an important part of Global Village Construction Set
  • Motivation
  • There is enough solar power to make it a viable power source, in wide use
  • This power can be concentrated
  • Why doesn't everybody use solar power and take $1000 off their bills per year?
  • Solar thermal concentrators have the greatest promise of delivering affordable solar power
  • Solar power is too expensive
  • Solar thermal concentrator power (STC) is becoming affordable
  • If solar power were cheaper, it could provide energy in tropics and most of temperate zones
  • It is the cost per watt that matters for feasibility, not efficiency
  • We have a program for delivering $1-1.5/watt solar power generators for the home scale
  • This is hard to believe, but we have a detailed program and assumptions
  • Assumptions are laid out clearly.
    • Community Supported Manufacturing business model
    • 40% overall reflector/collector efficiency from solar gain to heat absorbed by water in boiler tube
    • 12% steam cycle efficiency out of 40% theoretical maximum (including boiling, feedwater, steam feed into steam engine, losses)
    • 95% mechanical efficiency steam engine
    • 95% efficient electric generator, coupled directly to steam engine

Today’s episode is on the Solar Power Generator (note on name). This is one of the most important projects of the Global Village Construction Set, on a par with the Open Source Fab Lab, and CEB press/Tractor, in terms of productive power. The Solar Power Generator is a solar thermal concentrator (STC) electrical power system. Conceptually, this system is quite simple. Heat from the sun is concentrated by mirrors onto a collector. The heat boils water in the collector tube, and the resulting steam is used to run a steam engine. 70% of the energy in the United States is generated by steam power in large power plants, which use fuel to boil water and run turbines for electrical generation.

We are presenting this video to explain why we think that the Solar Power Generator is feasible on a small scale. We aim to explain it in lay terms – to the point that you could understand the potential and the issues involved.

Everyone loves solar energy. But where is its mass adoption? It’s not there. Why not? It is not that there is not enough solar power. Did you know that the solar power coming to the Earth is 8000 times larger than the total energy use on the entire Earth? This means that there is plenty of solar energy, but the trick is to capture that energy into a useful form.

You may have heard that solar energy is ‘too dispersed’ to harness it for electrical power generation. That’s where solar concentrators are used to concentrate solar power to a more usable form. Concentrated sun has plenty of power per unit area. You can see videos on YouTube that show a solar concentrator melting right through steel.

To summarize what we have so far – there is plenty of solar energy, and it can be concentrated effectively to make it usable for electricity generation. Or, it can be used in its dilute form by using photovoltaic (PV) panels. So why doesn’t everybody have a solar collector, or photovoltaics, and power their house by solar energy, and save an average of $1000 per year in electricity costs, if you are talking about the United States?

The reason is that harnessing solar power is too expensive by present standards. This applies to photovoltaic PV power and solar concentrators. This is changing, as PV panels are down to under $3 per installed watt, at liquidation prices, and solar concentrator power plants are presently down to $1/ installed watt (I DON’T HAVE A REFERENCE HERE. in the desert areas of the USA. This price is comparable to the cost of coal power plants, the main electrical source in the USA.

Solar thermal concentrator (STC) power is actually economically feasible today, in sunny areas. There are large, megawatt scale utility solar thermal concentrator power plants in the desert regions of the USA, such as the company Ausra. These are past the prototype or demonstrator stage – these actually work in today’s economies. At the same time - there is no economically-feasible, distributed power, STC generation option on the kilowatt scale – or at the scale of individual homes. We are speaking on purely economic terms, not in terms of ecological values.

To reframe our discussion to this point – if solar power were less costly, it would be economically feasible. This is a tautology, but it needs to be pointed out to state that there are no technical barriers to adoption. The barriers are those of cost. Moreover, if solar power were sufficiently less expensive, it would also be feasible in areas with less sun than the deserts – areas where people typically think that solar power is not feasible. This is important, because it reveals that solar power could be feasible in most of the temperate zone as well.

The argument of dismissing small scale solar concentrators by virtue of low efficiency is void – the efficiency does not matter in practice if the cost of the power system is low. The figure of merit is not percent efficiency, but cost per watt of power delivered. The higher efficiency systems may have a lower cost per installed watt – but that does not have to be the case. Thus, if we could develop a low-cost solar concentrator system, it has a chance of widespread adoption. Positive environmental effects assist in this adoption. To repeat- it is not efficiency, but cost per watt that matters.

We are here proposing a design and program for producing cost-effective, off-grid solar concentrator electric systems as an open source initiative. We have a basic system with a cost of $1-1.5/watt – where the former involves grid intertie and the latter involves off-grid energy storage.

We are aware of the immediate critique of this proposition, and possible premature dismissal by the experts. We know that this cost figure is highly suspect to just about any critic with basic knowledge of solar concentrator power.

However, we are not relying on the several assumptions inherent in such critique.

First, we are not using an industrial business model, which has 4-6 fold cost increases over the true cost (materials and labor of production) built into the industrial model in the form of development costs, financing, overhead, stockholder profit, inventory, transportation, and other costs. We are proposing a post-industrial model of Community Supported Manufacturing.

We are proposing at-cost production – where the consumer pays only for materials and labor. Basic analysis of industrial economics reveals the 4-6 price increase above – this is not in question. So then next question is, how does one attain at-cost production and still be able to ‘make a living?’ We will expand on the CSP business model in a dedicated episode. For now, we will just show the case example for the solar power generator.

There are several keys to the success of this project. Community Supported Manufacturing is one of them. The second one is simplicity. Then there are several technical improvements.

Simplicity: First, we take the simplicity. We xplain the system, solar angles, and how the design relates to cost.

Our basic system configuration be seen on the Wiki. The system that we are presently considering a system based on individual, flat, East to West oriented mirror reflectors. The reflector array itself is parallel to the ground and low to the ground. We are using high-iron glass mirrors, a 2” collector tube, and a steam engine as the heat engine of choice.

The most important part of simplicity is the low cost of the mounting structure. Since the reflectors are close to the ground, they require very little structure – as opposed to parabolic dishes, where most of the cost of a parabolic system is in the structure itself. Our system has only 1/4 of the cost in structure. Such parabolic dishes must have structural strength for wind loads. Think of it this way – building a large parabolic dish is like building a house, and costs at least $100 per square meter (1) for flat heliostats, in structure, but more typically, it is several hundreds of dollars for parabolic dishes (3) . Heliostat Power Towers (4) are viable today, though we are not aware of any parabolic power systems that have passed the feasibility stage.

The flat arrays that we are talking about cost about $30 per square meter (2) of reflector, of which $20 is the mirrors.

For our first prototype, we will do a 60’x20’ array, where there are 16 individual, flat 6” wide reflectors. This array thus has Mirror prices are $2 per square foot for high-iron glass mirrors.

Continuing on the simplicity – we chose flat reflectors because they are easiest to mount, and no fabrication is needed to make curved structures. Mirrors come flat out of a factory. We are also using good old glass mirrors – because these have a proven track record of 50 or more years. No other film or reflector has that track record.

For the heat engine, we are using the good old steam engine. We’ve considered the Tesla turbine, but found documentation of 30% efficiencies, while the steam engine is 90%. Documented bladed turbines are 50% as efficient as steam engines at the under 500 kW range scale (5). We’ve had a number of people ask us about the Tesla turbine – which has a certain charm to it, and features great simplicity on the surface. That’s what attracted us to the Tesla turbine in the first place. Closer examination leads us to conclude that the charm is only superficial, because efficiencies are low and advanced materials are required to make these turbines last. The fact is, there is no better heat engine for tapping energy on a small scale than the simple steam engine.

Going further on the simplicity – we are using water as the working medium. It is low cost and environmentally sound. We know no other working fluid that is as easy to work with or as environmentally friendly. Remember that 70% of the electricity in the USA is generated by steam.

The final note on simplicity must be made on flat mirrors. These are tracked on individual motors, not a mechanical linkage for the 16 slats. This is feasible because of low-cost electronics. We understand that using a mechanical linkage does not provide the fine accuracy necessary for pointing all the slats on the collector tube. Moreover, it appears that the grouped reflector design that we considered is proprietary.

Thus we will use 16 slats of 6” width and 60 foot length, and the collector tube will have a shroud that does a secondary, 3x concentration onto a 2” solar boiler tube. All together, this is about 50x concentration.

Some other details to consider surround the E-W orientation. This means that there are edge losses in the morning and evening, as the collector tube is the same length. You have to note that as sun moves throughout the day, the solar angle to the reflectors changes – and that’s why daily tracking is necessary for the reflectors. This also accounts for seasonal variation.

Why not avoid edge losses and orient the reflectors N-S? Because the North end would have to be lifted way up in the air if the slats are to be perpendicular to the sun. That would introduce the heavy structural costs that we aim to avoid by keeping the reflectors close to the ground.

You can see all the losses in the system explained here. These total 60% in the solar collection and heat transfer prior to steam engine.

The elegance of the system comes from several features. One is active tracking of each slat with a feedback sensor, so that each slat is aligned with the collector tube. We understand that trying to program in the correct motion, without using feedback, is problematic.

Elegance comes from several improvements. One is optimizing engine efficiency by electronically-controlled valves or injectors that admit steam in a controlled fashion – to optimize efficiency of the steam engine. The amount of sun varies in the day – so power variations must be accounted for actively to optimize steam engine efficiency under different conditions. This is the difference between generating useful power, or not generating any power, when the sun is low or there are partial clouds.

Another development is gravity water feed into the collector tube to eliminate a feedwater pump. Valves require energy on the scale of watts, while pumps require energy on the order of hundreds of watts. This could be the difference, under conditions of low light – between generating net power versus using up all the power to pump feedwater.

The third development is streamlining the production of steam engines down to $250 for 3 kW engines. The basic materials for such are only $20 if you start from scrap metal – so the challenge is making digital fabrication and casting to work as promised.

The 4th improvement is to produce our own solenoid valves for steam entry. Skip Goebel of Sensible Steam communicated that including such valving will bring steam engines close to the perforamance of diesels, and about 5 times as efficient as steam turbines in the sub-500 hp range.(6). The commercial solenoid steam valves are presently $300 for the 3KW steam engine of interest.

The 5th improvement will be electronically-controlled feedwater flow rate, on top of the feedwater timing.

Losses include 60% loss of the reflectors and collectors combined, and we assume a steam engine efficiency of 12% - for an overall efficiency of 5% - to go from 60kW of solar intercept to 3 kW of usable electricity.

Our phases in this project are to start with building 1 slat. We will explore spot welding of a light frame – by using off-shelf, light gauge metal angles. When we can document our price prediction of under $30/m square, we’ll move on to making 16 slats. Nick is working on the OS steam engine.

The points of development are basically to put the entire structure together. Right now we have a concept, and we believe all aspects of the concept are either sound or optimal for cost reduction. Let us know if you disagree with any of our approach. The program is simply to test whether we can achieve all the cost predictions. Then we have a 3kW system for $1/watt in materials. This would be the DIY cost – which would have a payback time of 3 year for grid-intertie operation. It would be about $1.5/watt for a turnkey system fabricated in the CSM model. Based on historical steam engine performance, we are aiming for 18-20% steam cycle efficiency, where the thermodynamic maximum is around 40% for those conditions. In this case, the price of the system could drop back down to $1/watt for turnkey systems. Then, add another $1k for a battery backup via 100% recyclable lead acid batteries for off-grid power, and we have a system for about $1.3/watt – or $6k – which pays back for itself in 6 years at today’s electricity consumption rates. We are also predicting a turnkey system cost of $2400 for an evolved version which includes the CSM model plus technological recursion or import substitution, but that’s beyond the scope of the present discussion, and you can read a few more notes on this at the wiki.

The market for electricity (7) is ( is over $200 billion in total sales, and 40 billion for rural areas. The market for the solar power generator is at least the rural areas and much of suburbia, though space considerations may limit applications in cities. An average household can be saving about $1000 per year (8), once the solar turbine generator is paid for.

References: (1) (2) (3) needs reference (4) (5) (6) df (7) (8)