KEYWORDS: Solar thermal energy, Solar energy, Photovoltaics, Solar concentrators, Compound parabolic concentrators, Solar cells, Aluminum, Temperature metrology, Manufacturing, Copper
Due to their high relative cost, solar electric energy systems have yet to be exploited on a widespread basis. It is believed in the energy community that a technology similar to photovoltaic (PV), but offered at about $1/W would lead to widespread deployment at residential and commercial sites. This paper addresses the investigation and feasibility study of a low-cost solar thermal electricity generation technology, suitable for distributed deployment. Specifically, we discuss a system based on nonimaging solar concentrators, integrated with free-piston Stirling engine devices incorporating integrated electric generation. We target concentrator-collector operation at moderate temperatures, in the range of 125°C to 150°C. This temperature is consistent with use of optical concentrators with concentration ratios on the order of 1-2. These low ratio concentrators admit wide angles of radiation acceptance and are thus compatible with no diurnal tracking, and no or only a few seasonal adjustments. Thus, costs and reliability hazards associated with tracking hardware systems are avoided. Further, we note that in the intended application, there is no shortage of incident solar energy, but rather it is the capital cost of the solar-electric system that is most precious. Thus, we outline a strategy for exploiting solar resources in a cost constrained manner. The paper outlines design issues, and a specific design for an appropriately dimensioned free-piston Stirling engine. Only standard low-cost materials and manufacturing methods are required to realize such a machine.
A self-aligned reduced mask count micromachined polysilicon on nitride (SAMPSON) surface micromachining process is introduced. The self-alignment fabrication concept enables rapid fabrication, improves yields, and reduces parasitic capacitance of MEM devices. For many MEM devices the SAMPSON process results in a one mask savings over more conventional approaches. As a demonstration of the fabrication technique a burst-proof surface micromachined polysilicon resonant pressure sensor is fabricated using the SAMPSON process. This pressure sensor does not require a sealed cavity. It is also insensitive to mechanical property variations of the structural material, rendering its response essentially temperature independent.
We present the concept of utilizing micro-electromechanical (MEM) energy storage for power conversion. A capacitor and inductor-free step-up converter based on mechanical resonance and silicon strain energy is introduced. In this circuit, a single MEM device is used to generate and supply high voltages, suitable for driving other MEM devices. A test converter using vertical polysilicon based resonators as energy storage elements is fabricated and tested, demonstrating the basic step-up function.
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