Nick Raaum wrote:Marcus, also I am not certain if the costs to a low pressure heat exchanger would prohibitive, the reason is that though a greater surface area would be required, the pressures are negative, so the high safety factors and very conservative boiler code that governs dangerous high pressure steam would go away as the system would be relatively failsafe (hole in a line results in air in leakage and system shutdown not explosion). This would also mean that though the volume would increase the thickness would decrease, and reduce the amount of mass used on a sq surface area basis relative to a higher pressure heat exchanger. Also radiative losses could be a non issue if "boiler" was essentially a large pipe inside of a pipe were the flame was completely contained within and steam flowed inbetween the walls, though yes higher volume would equal higher convective losses. I need to do the math to quantify those.
I am also not certain about too low of efficienices, when I do the math for a rankine cycle like this without any regeneration operating at 1000 deg F I get over 30% mechanical output efficiencies, that iseems pretty decent for a cycle operating on 75% solar energy.
The prime mover is indeed the sticking point, so what I'd propose is to run the system at scales large enough that you could buy industrial scale Low Pressure Turbine cycles and condensers.
Nick Raaum wrote:Marcos, Well coal plants are being retired and being pushed into retirement quite frequently, you could land a semi suitable LP turbine for less than $100/kW of course that still is a fraction of overall system cost.
Nick Raaum wrote:I still think there is a way to build an efficient steam driven thermal engine at a small scale, perhaps open sourcing, and CNC machining will drive down barriers to producing turbines on a small scale. If not then there is always modernizing the steam engine. The biggest barrier to an efficient steam engine is figuring out how to lubricate at high temps as that puts a limit on cycle efficiency.
Nick Raaum wrote:My interest in this idea has nothing to do with me wanting to go off grid, I know how to do that. What I am seeking is a system that is non dependent on fossil fuel, simple enough to be manufactured and maintained regionally, but still produces enough surplus to have something beyond a subsistence economy. PV and wind do not do this, they are still nursing off of the industrial teat so to speak, and it is unclear (EROEI) if they will ever get off of them. If you do the math on biomass you know that won't work, so it really suggests to me that integrated solar thermal systems really might be an important approach to evaluate.
Nick Raaum wrote:You are right, now silver bullet, if anything intelligent regional adaption is the silver bullet. Your CSP Brayton engine is a great example of this, should be a killer cycle in the clear southwest, but not a winner in the northwest. You somehow tripped a memory I have from some late night googling of a guy who was building a solar brayton cycle from piston engines (one acted as the compressor the other the exander) can't find it, but anyways maybe some DIY potential on that one...Also while I'm recalling google anamolies have you seen this solar evaporation cycle ? http://en.wikipedia.org/wiki/Barton_evaporation_engine