04-11-2016, 03:43 PM
I mentioned the start up company Terrajoule in post #175. I came across some recent information on their work. See the following video: https://www.youtube.com/watch?v=bHlAohwtLcs . The audio quality of the video leaves a lot to be desired, but the discussion is very interesting. As a recap, know that Terrajoule is using reciprocating piston steam engines in a solar thermal stationary power generation system with very inexpensive energy storage that permits 24/7 operation up to full rated power. The prototype plant they have in operation is using an insulated steel pressure vessel (a large propane storage tank) filled with water as their energy storage vessel. However, the next system is to make use of steel piping with SOIL taken from the site used as the primary thermal mass. The purpose is two fold: (1) to achieve somewhat higher temperatures in their storage (since piping has higher pressure ratings than a large tank), and (2) to store additional energy using soil (an even more cost effective means for energy storage). As before, the piping will be filled with saturated water under pressure. BTW, the use of piping permits somewhat higher pressures and temperatures, and also increases the surface area/volume ratio to allow for effective heat exchange with the surrounding soil (note that the thermal mass is contained ABOVE GROUND - it is NOT buried - rather, soil is taken from the site as a very cost effective thermal mass and used to store most of the heat). The general trend of the discussion is that the physics and economics of solar thermal power generation with thermal storage makes it cost effective at relatively small scale. It turns out there are many dis-economies of scale for these systems. This conclusion implies a return to steam engine technology that is far more efficient at the smaller scales. Also, the argument is for decentralized solar thermal since steam engines are excellent for meeting highly variable power demands and without suffering a loss in efficiency, and there are also opportunities for cogeneration in these settings. In short, solar thermal + steam engine at small to medium scale solves the problems better than alternatives. Note the smallest size of Terrajoule's modular system is 50 KW peak output with 20 KW average (continuous) output. 3S Power is currently looking to power rural towns and farms in regions that see a combination of high solar insolation and high electricity prices with the two best candidate regions being Australia and Chile.
* There is a particularly interesting discussion of the efficiency of steam engines vs. steam turbines as a function of scale, or power output starting at 10:40.
Also mentioned during the video is a start up company based in Seattle that is working on converting existing diesel engines to steam engines. The company web site is here: www.practicalsteam.com. Unfortunately for applications considered in this thread, they are targeting medium scale systems with power levels similar to those targeted by Terrajoule (i.e. no micro scale systems suitable for a household). Still, I like the trend - it seems the benefits of steam engine systems are starting to become recognized.