An idea for Krampus:
Idea: high temperature processing
The problem: Maintaining an ability to make materials requiring high temperature processing when fossil fuels are no longer available and a high technology culture and infrastructure are no longer functional.
Two of the primary materials we currently use in our built infrastructure are cement and steel. Metallic tools and devices are also integral to a technic society. These are very energy intensive materials to make. The idea of using concentrated solar energy is explored in this paper, and a description of a proposed facility and associated technology is described. The scenario where this facility would be appropriate is a future in which a salvage economy would provide raw materials, and knowledge of metallurgy and related technologies have been preserved.
Current status of technology:
The linked article has a good overview of the general concepts involved, and links to information on the two largest facilities built to date. Solar heat concentration is scalable, so one decision to make is what size is needed for the most likely batch or process in mind, and reasonable to build with limited resources in a future energy constrained world.
The solar furnace at Odeillo, France , built in 1969, is currently the largest solar furnace built, and did research for a number of years. http://en.wikipedia.org/wiki/Solar_furnace
Another similar sized furnace was built in Uzbekistan in the 80’s, and is also no longer in service.
They both used modern technology for tracking and process control. Odeillo reaches temperatures of 6330 F, which is much more than needed to melt ferrous metals.
Overall facility description: A high temperature processing facility would be a large investment , with the main support structure for the parabolic concentrator being on the order of 50 meters tall and 50 meters wide. While the future capacity need is uncertain, it is unlikely that very many would be justified. The main components are the array field, the large parabolic concentrator, and the focal receiver. The engineering challenge is to design a solar furnace facility similar to the existing ones, but assemble and operate it in a technology and resource constrained world.
There are other high temperature applications this facility could be used for also, further justifying the effort to build it. For example, photochemical processing, http://www.pre.ethz.ch/publications/0_pdf/books/Solar_Thermochemical_Process_Technology.pdf
photo catalytic processing, and high temperature ceramics
Assumptions on design limitations:
The facility should be designed to enable replication of its constituent components. Smelting and making steel structural members, heating limestone to make lime or cement, and melting sand to make new mirrors are all things that a solar furnace would be able to do.
Sizing the facility for reasonable production levels: Sufficiently high temperature is only part of the sizing question. Sufficient total heat available will define how much mass can be brought to smelting temperature, so maximum rate and total BTU collected in one day will govern the batch size.
Batch processing: Due to the high temperatures needed, it is not envisioned that a solar processor would store heat in some medium for long runs like CSP steam generation plants do now, rather the unit would need to be designed for single day batch runs. This implies batch limits based on the processing time and time to bring the mass up from ambient to processing temperatures. This will be the main variable for sizing the collector field and parabolic concentrator. The Odiello facility has a concentrator of 1800 square meters, and 63 heliostats.
Utilization of unused heat: For periods when insolation is not adequate for smelting or limestone calcination, it might still be collected and used for lower temperature activities and thermal heat storage. A facility like this would lend itself to the concept of collocated industry, where synergies in energy use and material streams are optimized. District heating schemes, common in Europe now, might also be a side benefit.
Location criteria: Locating a solar processing furnace would of course primarily driven by maximum solar insolation. A south facing slope of the site would help with efficient land use and mirror placement. Proximity to raw materials would be important also. Limestone deposits would be needed for making lime or cement, and steel or iron nearby would minimize transport costs. Mining the ruins of Las Vegas might well be a good first location.
Smelting steel: Steel is relatively straightforward to melt, but assuring the ability to obtain the desired metallurgy and mechanical properties would necessitate some ways to get the alloying mix right without modern instrumentation. Reference books explaining early steel works, which relied on visual indications, would need to be secured, studied, and the methods practiced. One advantage of solar heating for smelting scrap is that it does not introduce any impurities into the charge. This link shows that the knowledge to do alloys without modern instruments is available, but needs to be collected and the skills relearned.
example of “low tech” steel making
To effectively control alloy content, scrap and salvage would need to be categorized and then blended to get the intended mixture in the melt. This will require coordination with scrap and salvage businesses.
Solar heating: A key engineering challenge will be figuring out how to direct the solar ray onto the mass with a high efficiency, while controlling and monitoring the melt. Also, a solar field geometry similar to Odiello will have the rays coming in to the focus area horizontally, but a crucible full of melting or molten steel will need to be oriented vertically, with continuous sides in order to contain the liquid.
Making lime and cement: The need for cement and lime as building materials in a future energy decent era would be less than now, but there will be specific applications where it would still be worth the investment of energy. Some buildings in this future might well revert to a long term sustainable design approach based on stone and lime, with high initial effort to build, but with a lifetime measured in centuries. Cement, and resultant concrete, being a rather short lived material, may recede in use.
links on making cement and lime:
This last linked article is specifically about the design parameters and economics of making lime with a solar concentrator.
Allayed necessary technologies:
Making mirrors: A facility like this would need to be self replicating in some respects. One aspect would be making new mirrors for replacements or to build another facility. The following links show methods of making plate glass with relatively simple methods, as well as silvering the glass.
how to make plate glass
silvering glass to make mirrors
Tracking: Without current electronic control systems, based on plc controlled stepper motors, sensors, and machined mechanical components, tracking would need to be done with simpler methods. Clockwork type mechanisms have been used in the past to aim small heliostats, so some design work would need to be done to make a large, strong actuator that could withstand transient wind loads and maintain adequate accuracy. It may well be that a combination of clockwork mechanism with some daily human adjustment would end up being the optimal way to solve “low tech” tracking.
Crucible melting ferrous metals
making refractory materials adequate to contain steel smelting is a challenge that needs research. Current crucible materials are based on very specific chemistry and processing, and I have not yet found a low tech alternative.