Optimization And Test Of Low-medium Temperature Solar Thermal Electric Generation Using Organic Rankine Cycle

The organic Rankine cycle (ORC) is combined with a compound parabolic concentrator (CPC) of small concentration ratio in this study. Solar heat is collected at temperatures below 250°C and converted into work by the ORC. Compared with high-temperature solar power generation on using high-concentration collectors for heat collection and steam Rankine cycle for power conversion, low-medium solar thermal electric generation offers many advantages, including the following:(1) The ORC is one of the most favorable and promising techniques in low-temperature applications. The ORC demonstrates higher efficiencies during cooler ambient temperatures, immunity from freezing at cold winter nighttime temperatures, and the adaptability to semi-attended or unattended operations. In the case of a dry fluid, the ORC can be used at lower temperatures and does not require superheat. The ORC can be easily modularized and used in conjunction with various heat sources. The feasibility of ORC technology is reinforced by the high technological maturity of most of its components; this maturity stems from their extensive use in refrigeration applications. The advantage of the ORC for low-temperature heat sources is obvious because of the more limited (in comparison to steam) volume ratio of the working fluid at the turbine outlet and inlet. This ratio can be further downsized to an order of magnitude for organic fluids than for water; hence, it enables the use of simpler and cheaper turbines.(2) CPC collectors with smaller concentration ratios can accept a large proportion of incident diffuse radiation on their apertures, and direct it without tracking the sun. Therefore, the cost of collectors can be reduced.(3) Low-medium solar thermal electric generation enables scaling to smaller unit sizes. The interest for small-scale ORC is now growing. Small-scale production of electricity at or near customers' homes and businesses could improve the reliability of power supply. And heat demand can be fulfilled by domestic heating, which results in an increase in the overall energy conversion efficiency of ORC. The size of the ORC plant is limited by the low energy density of heat sources. And the size of the ORC plant is also limited by the availability of energy consumers. Many applications in residential areas only require several to tens of kWe for pumping, refrigerator and air conditioning. (4) The technology of heat storage in the temperature range of 100–250°C is much easier to realize compared with high-temperature heat storage. Many kinds of phase change materials (PCMs) can be used for the proposed system; these include paraffin, magnesium chloride hexahydrate, erythritol and galactitol.The ORC has been successfully applied in general low-grade heat utilization. And substantial improvements have been made in ORC technology in the past decade. However, this technology is primarily applied in waste heat recovery, geothermal plants, and biomass power plants. The integration of ORC and solar collectors has attracted limited attention. Meanwhile research on CPC collectors for medium temperature applications is currently expanding because solar heat ranging from 100–250°C can be utilized directly for industrial processes, desalination, solar cooling, solar thermal power, and so on. Although the CPC have large market potential, basic research should be carried out to explore the application of low-medium temperature solar thermal electric generation.The specific scientific issues that the study aims to address are as follows:(1) The application and performance of the ORC have been investigated by previous researchers; however, most of the investigations were focused on the ORC with a basic configuration, wherein a large sum of exergy is lost in the evaporator because of the unmatched temperature between the organic fluid and the conduction oil.(2) Although small-scale ORC units in power ranges below 100 kW exhibit significant potential for electricity and heat supply produced at or near the site of consumption, the feasibility of small-scale expanders has yet to be demonstrated. The scroll expander appears to be a good candidate for the expansion device of small-scale ORC systems but most of the employed scroll expanders are obtained by modifying existing compressors. A turbo-expander may offer advantages such as its compact structure, small size, light weight, and stability. However, there is little thorough and comprehensive investigation on the performance of small-scale expanders, especially turbo-expanders.(3) The choice of working fluid for the ORC is critical because the fluid must have not only thermophysical properties that match the application, but also adequate chemical stability at the desired working temperature. Particularly, the collector efficiency will be influenced directly by the thermophysical properties of the working fluid. The working fluid selection for the proposed system differs from that for the ORC applied in waste heat recovery, biomass, or geothermal power generation. This paper focuses on the basic scientific issues related to the low-medium temperature solar thermal electric generation using the ORC and the CPC. The identification of the performance advantages and disadvantages of small-scale expanders and pumps with organic working fluids , the optimization of the thermodynamic cycle and working fluid selection at a range of 100–250°C are conducted. The paper has the following innovative features:1. The proposed system combines the advantages of the ORC, non-tracking CPC, and heat storage of PCMs.2. Two-stage collectors with two thermal oil cycles are proposed to reduce the heat transfer irreversibility between the organic fluid and oil.3. Preheat on use of the non-concentrated collector is proposed. And the preheat temperature is optimized.4. Two-stage heat storage units with different PCMs are adopted to improve the heat collection efficiency.5. A novel hybrid solar electricity system with ORC and PV cells is designed and performance simulation is carried out.6. A specially designed and manufactured turbine is innovatively applied to the solar ORC system. Preliminary testing of the ORC performance is conducted. The performance advantages and disadvantages of small-scale expander and pumps with R123 are identified.