We discuss the technical and economic feasibility of a low-cost distributed solar-thermal-electric power generation technology based on the use of a solar thermal collector (STC)in conjunction with a free-piston Stirling engine.The solar thermal collector is to be comprised of low-concentration nonimaging concentrators and absorbers with spectrally selective coatings.The Stirling engine converts moderate temperature heat to electricity by way of integrated electric generation.In spite of its relatively low conversion e .ciency,the proposed system can be a cost-e .ective alternative to solar photovoltaic (PV)modules,as discussed in the project description. The system is conceived to operate with collector temperatures in the range of 125 .C to 150 .C, which is consistent with the use of stationary solar thermal collectors employing low-concentration nonimaging re .ectors.Thus,the system avoids the costs and maintenance issues associated with tracking collectors based on high concentration ratio concentrators. We take the view that cost e .ectiveness of solar electric technologies should be judged by output power per dollar rather than by e .ciency or other technical merits.This view re .ects the observation that there are vast untapped siting opportunities in both urban and rural regions of the world.Research into photovoltaics has been focused on improving e .ciency because there has been no signi .cant decrease in the inherent cost of silicon wafer area.However,the proposed solar-thermal-electric system is designed for fabrication out of low-cost materials.A collector is built of glass,aluminum,copper,and insulation,while engines and generators are primarily steel, aluminum,copper,and plastics.In high-volume manufacturing,the cost of the proposed system will be determined by the weight of its bulk materials.This study of solar-thermal-electric systems involves searching for a more cost-e .ective balance between system e .ciency and materials cost. Broader Impacts Energy independence and diversi .cation through the use of renewable dis- tributed sources is essential for the future of our society.In the project description,we make a case that the proposed technology will cost on the order of $1 per Watt of peak output power.As such, this technology would represent at least a .ve-fold improvement over photovoltaic technology in terms of cost.A simple estimate of energy cost using the proposed technology based on an average of only four hours per day of operation over a modest ten year lifetime indicates a cost of about $0.066/kW-hr.Although not equal to the lowest costs for electricity,this is certainly an energy cost that makes the technology very valuable when considering the additional advantages of its nature.Namely,this is a distributed renewable energy source with an availability that is fairly well matched to peak demand times.Further,this technology is well suited to distributed deployment in an analogous manner to that of photovoltaic generation.As such,it will be valuable for remote sites where utility infrastructure is non-existent or too costly as well as for residential and business sites as an alternative to photovoltaics. The PI has been active in integrating research into new and existing classes,and plans to con- tinue this in conjunction with the proposed work.The PI also actively seeks to include underrep- resented student groups in his projects.UC Berkeley hosts the SUPERB (Summer Undergraduate Program in Engineering Research at Berkeley,www.coe.berkeley.edu/cues/superb/index.html) program which targets underrepresented and minority students.The PI plans to actively recruit in the SUPERB program for students to participate in the proposed project during the summer periods. Intellectual Merit A direct strategy for enabling a renewable energy source by seeking to min- imize the cost-to-output-power ratio is outlined.We analyze the cost of an integrated system including a solar thermal collector and a low-temperature Stirling engine.Novel multi-phase free- piston Stirling machine designs are proposed along with rigorous analysis techniques for their design and evaluation.Speci .cally,a Stirling machine with no mechanical linkages and only three moving parts is discussed.This is a design that inherently permits for a long maintenance-free lifetime and for very low frictional losses. 1
