Low Cost Low Tech Solar Thermal Engine

I've been fascinated by air engines in general and stirling engines in particular for there potential for solar electrical energy production.  Though in studying these systems a couple of drawbacks stick out. 

1. Low energy density per unit of material used.
2.  Correspondingly difficulty in manufacturing the large tight tolerance cylinder that is required to make up for said low energy density.


I've also observed that though concentrated solar will produce far superior efficiencies, its associated complexities and poor performance in all but the clearest skies lead to less than stellar $/kwHr figures. 

To address these issues how about a passive solar stirling cycle with a bit of a twist?  The twist being that the passive absorber is an air diaphragm that is also the working piston.
Here is a rough schematic of this idea.

 The air cylinder could maybe be a canvas fabric to withstand heat with a plastic outer layer for zero porosity.  The material requirements really would be basic as the engine would not see more than 5 psi and 250 deg.  However in order to develop useable amounts of power the cylinder would be large, as would the force transfered onto the hydraulic piston. At even low temperatures this force would be significant, for a 9' diameter cylinder at 3 psi roughly 27,000lbs would be transfered to the piston. To handle this force transfer I was thinking steel cables could be woven in a concentric pattern on the outside of the air cylinder.  These steel cables would then connect to a center steel collar to transfer the force to the hydraulic piston.   This way force concentration is transfered to the steel cables, and the fabric only endures around 5psi max.  The hydraulic cylinder will then act as both the power piston and the displacer piston.



Here are a few quick calcs on efficiency.  Assuming 150-160 deg average temp, 55 deg heat rejection, and 55% of carnot ideal, we'd get something like 8.9% efficient engine, perhaps 6-7% is even more realistic.  Not good by normal heat engine standards but look what might the $/kwhr metric look like?

Intro

I'm starting this blog under the premise that the energy infrastructure we are currently committed to will no longer remain viable in the future. We are obviously fossil fuel dependent and our continued reliance on these forms will continue to flare geopolitical instability, marked economic dispartities (due to the highly centralized nature of fossil energies), and of course environmental problems. Perhaps though greater than any of these individual problems is the overall emergence of an increasingly brittle energy infrastructure. One only needs to look at how vulnerable both electric grids, and oil pipelines have proven to be in countries of instability to realize this point. Though in the US we probably needn't worry about these direct attacks (yet) we do need to worry about the indirect results. In a world of tight energy interdependence these problems will affect us all.

I hope to initiate a pragmatic discussion of what realistic sorts of possibilities we can develop. I'll be sharing some general approaches and ideas I've come up with you in hopes that through the power of networks we can perhaps create a couple of useful emergent ideas.

First though let me share my philosophy and approach to the problem. Since highly centralized energy infrastructures seem vulnerable to both breakdowns and economic control and that renewable energy is by very nature distributed I am biased towards localized renewable energy systems.  Given that neither small wind or PV systems seem to have proven succesful in this arena I am trying to really think outside the box.  My constraints are:

1. Needs to be able to reproduce locally, therefore readily available and abundant materials must be used in construction.

2. Cost per kwHr produced (not cost/kW installed) is most important economic metric.

3. Should not rely on other potentially unstable centralized technologies.

4.  Construction must be accomplished through readily available means (cnc mills, lathes ect).

Those constraints are not final of course, but do set the stage for explaining some of the energy system ideas that follow.  So that being said lets brainstorm a little on low cost solar thermal systems.