UC Berkeley

The result of several years of analysis of Distributed Concentrating Solar Combined Heat and Power (DCS-CHP) systems is a design that is predicted to convert sunlight to heat at 8-10% solar-electric efficiency while simultaneously capturing ~60% of that initial sunlight as usable heat (at 100ºC). In contrast to similarly sized photovoltaic systems in the U.S. that cost ~$7.90/Watt of generator rated peak electrical output, in mass production the proposed collector and generator system sized at 1-10kW would cost ~$3.20/Watt electricity and ~$0.40/Watt-thermal, allowing adjustment of heat and electrical output on demand. The proposed system would revolutionize distributed energy generation in several ways: 1) by enabling rapid dissemination via avoidance of production limitations of photovoltaics such as expensive and limited materials, 2) by efficient local production of both heat and power to offset more greenhouse gases at a lower cost than other renewable energy technologies, and 3) by democratizing the means of electricity production; putting the power in the hands of the consumer instead of large utilities. With over 5.4 GW of photovoltaics added globally in 2008 and 20GW of solar thermal, there is a large proven market for solar energy. With widespread market penetration, this system would reduce greenhouse gas and criteria pollutant emissions from electricity generation and heating for a significant portion of the developed and developing world, including those in remote locations with no connection to an electric grid.

Chapter 1 begins with analysis of the relative demand for electricity and heat in California, showing that, on average, demand matches production of a theoretical solar CHP system. Then, we explore the economic and technological impetus for a solar powered combined heat and power Rankine cycle, showing cost and performance modeling in comparison to photovoltaics across varying conditions, and concluding that solar CHP generated electricity is comparable to PV electricity in terms of cost "per peak watt". Chapter 2 shows results of a life cycle analysis of DCS-CHP in comparison to other renewable and non-renewable energy systems including its global warming potential (80 gCO₂eq/kWh electric and 10 gCO₂eq/kWh thermal), levelized electric and thermal energy cost ($0.25/kWh electric and $0.03/kWh thermal), and cost of solar water purification/desalination ($1.40/m³). Additionally the utilization of water for this technology is shown to be much less than fossil-fuel based thermal power plants. Chapter 3 explores the expander as an enabling technology for small solar Rankine cycles and presents results of physical testing and characterization of a rotary lobe expander prototype developed by a small Australian company, Katrix, Inc. Testing of this expander using compressed air yields an impressive isentropic efficiency of 80-95% at pressure ratios of 6-11 making the rotary lobe expander a leading choice for DCS-CHP systems, if operation on steam is successful and reliability issues can be addressed.

Chapter 4 concludes with a description and analysis of a DCS-CHP system using simulation software developed in Matlab for the purpose of system analysis and optimization. This chapter focuses on the selection of an appropriate expander and collectors, the two key enabling technologies for DCS-CHP. In addition to discussion of the rotary lobe expander, we present a novel collector device appropriate for solar CHP systems that eliminates high pressure articulate joints, a typical failure point of solar concentrating systems. Given the results of the rotary lobe expander testing, comparison to previous expander modeling work, and comparison of the predicted cost and efficiency of the collectors analyzed, an appropriate technology version of the DCS-CHP system is selected and overall system performance is predicted across 1020 sites in the US.