OS2 FAQ

1. Why are you going with a linear system, the linear Fresnel reflectors?

In short, Ausra is doing it, and they are to date the most successful solar thermal concentrator electric enterprise concept. For the longer answer - simplicity and low-to-the-ground mounting structure combine to the lowest cost possible. Other concentrator systems, such as dish systems or power tower heliostats - have much larger structural costs. Note that we are talking about the most effective way to concentrate a lot of sun onto a small target. The key to that is minimizing structure costs. The linear Fresnel system accomplishes this.

2. Why are you going with flat collectors if parabolic collectors are able to focus sun to a finer line? After all, Ausra has curved collectors, and they are the best in the business.

Correct on your point. We are optimizing our system for a smaller scale, on the order of KW instead of MW. At this scale, it is more difficult to bend small mirrors of 6 or 12" width. Ausra mirrors are almost 6 feet across. Moreover, because we are not bending our mirrors, we are hoping for lower cost as well. As a third optical detail, you must consider that parabolic mirrors produce defocusing through the day, as the distance of solar rays from the reflectors to the collector tube changes throughout the day. One must draw this out on a piece of paper to see that this is the case for an E-W oriented array.

3. What is the concentration ratio of your system?

We have 16 slats for 16x concentration, with a parabolic shroud on the collector tube for another 3x concentration - for a total of 48x concentration. The collector tube is 2" wide.

4. Can you attain your 650K (377C) temperature with 48x concentration?

Yes. It is a matter of the balance of how much energy enters and leaves the collector tube.

5. How much power can you put into the steam engine at 650K?

We have about 38kW entering the collector tube - see Solar_Collector_Losses_Summary. We have radiation, conduction, and absorptance losses on top of that. This balance allows for 21kW to enter the steam engine, where we assume that all power that does not leave the collector tube is absorbed to heat the water working fluid?

6. Why are you using water as a working fluid? Aren't there more favoarable choices?

Water is the safest, most accessible, and easiest to work with fluid. 70% of power generation in the USA uses water as the working fluid - coal and nuclear power plants. The high heat of vaporization indicates that water is an energy dense working medium. Other fluids may have other advantages.

7. Why are you using a steam engine? That's an outdated technology.

Actually, the modern steam engine has many advantages. Performance close to diesel engines is expected with steam engines that have electronic steam injection, which optimizes the power that can be extracted from steam. This modern evolution of the steam engine makes it a heat engine of choice for applications under 500 hp - and it appears that Combined Heat and Power systems under that power range will use steam engines. Additionally, steam engines can be used with any fuel - from sun, wood, oil, biofuel, and anything else - as it is an external combustion engine. It is also a quiet engine.

8. What happened to the Boundary Layer Turbine as the heat engine choice?

Boundary Layer Turbine, or the Tesla Turbine, as the initial candidate for the heat engine in this system. As exotic as a turbine may seem – its performance is significantly lower than steam engines, which have 90% mechanical efficiency. We switched to the steam engine. Initially, it appeared that Tesla turbines would be easier to fabricate, but it turns out that these turbines don’t have a long lifetime, unless exotic materials are used for the blades.

9. What is the market for distributed solar concentrator power in the US?

The market for electrical energy in the united States is over $200 billion in total sales, of which 40 billion is for rural areas. The market for the solar power generator is at least the rural areas, plus suburbia. Space considerations may limit applications in cities. An average household can be saving about $1000 per year, once the solar turbine generator is paid for. At $3k to 6k cost - that is a 3-6 year payback period

10. Can you achieve the 16% efficiency in a simple, one-cylinder steam engine?

Yes, this is proven Steam Engine Efficiency for the temperature conditions in question.

11. You'll never be able to produce a 5 hp steam engine for $250.

Think about it this way. Commercial gasoline engines of this size cost $160 - and they are much more complex, though they are 1/3 the weight. Commercial steam engines of this size go for $1400 at the least. Scrap steel to fabricate the steam engine costs $10 for the 100 pounds required to build this steam engine. As such, the question becomes, is $240 sufficient to cover the labor required to fabricate this engine, based on digital fabrication techniques of metal casting and CNC machining? We believe that this is just a matter of proper tooling. If the investment for proper tooling is made, then there is no question that effective production can be attained. This is known as mass production. Thus, the question boils down to whether a large number of investors would be willing to fund such tooling, in a community supported production model of open source production. We believe the answer to the last question is yes, because of the large attraction of solar power applications and at-cost production according to the type of business model that we're developing at Factor e Farm.

12. What is the Solar Turbine?

We have begun the solar power generator project with the Boundary Layer Turbine as the heat engine choice, so we called the project the Solar Turbine project. Now we are calling it the Open Source Solar Power Generator project - or Open Solar for short. We have found out that the boundary layer turbine (BLT), or Tesla turbine, does not have the efficiency or longevity of steam engines. In fact, BLTs are typically 3 times less efficient than steam engines. They also fail due to stresses under steam conditions, unless exotic materials are uses for the turbine. We then examined bladed turbines and slotted disk turbines, but these don't have efficiency as high as the steam engine on the sub-500 hp range.