【作者】 李晶；

【导师】 季杰； 裴刚；

【作者基本信息】 中国科学技术大学 ， 热能工程， 2011， 博士

【摘要】 结合太阳辐照分散性强、能流密度低、易于转化为中低温热源的物理特性,本文提出低倍聚焦集热与有机朗肯循环(organic Rankine cycle, ORC)相结合的太阳能中低温热发电系统。与大规模、高聚焦比的太阳能高温热发电方式相比,太阳能中低温热发电通过无须复杂跟踪装置的低倍聚焦复合抛物面集热器(compound parabolic concentrator, CPC)获取热能,集热效率高且可有效利用漫射辐照。ORC循环由于工质低沸点的特性,在中低温条件下可以获得较高的蒸汽压力,推动膨胀机做功,其中低温热功转换性能优于水蒸气朗肯循环。ORC与CPC集热器相结合将是实现低成本、规模化太阳能热发电利用的有效途径之一。系统初投资少,建设周期短,风险小,加上易模块化的特点,其技术容易成熟。同时太阳能ORC热发电系统的冷凝端可实现热水及暖气输出,100-250℃的集热温度可实现热能驱动的制冷循环,中低温相变蓄热技术易于实现系统的稳定持续运行。这些特点有利于形成基于太阳能的冷热电综合供能系统。目前ORC主要与工业余热、地热等热源相结合,在工业余热、地热领域的应用已具备一定基础。许多学者也对ORC的重要部件膨胀机进行了研究,但是采用的膨胀机通常为螺杆膨胀机或涡旋膨胀机。这些膨胀机大多是在压缩机的基础上进行结构改造,绝热等熵效率不高,且缺乏小型有机工质膨胀机内部流动及热功转换等基础问题的深入研究。在太阳能集热方面,过去很长一段时期相关研究主要在高温(300℃以上)和低温(100℃以下)集热领域,太阳能中温光热利用(100-250℃)只在最近几年时间才引起较为广泛的重视,并得到了快速发展。尽管太阳能中温集热技术具有很强的应用前景,但相关的应用基础问题研究相对滞后。本文围绕100-250℃温度范围内太阳能ORC热发电应用基础问题开展研究。拟解决的问题主要包括:1)传统太阳能热发电系统正常运行时,换热介质从集热器获得热量并通过蒸发器把热量传递给有机工质。蒸发器中存在巨大的传热不可逆损失,是ORC效率低于卡诺循环效率的重要原因。蒸发器中可用能的损失占整个ORC可用能总损失的比例达70%以上。巨大的传热不可逆损失导致了高的换热介质平均工作温度,与之相应的是较低的太阳能集热效率。2)膨胀机是太阳能ORC热发电的关键部件之一,目前商业化膨胀机大多为MW级,且采用的做功介质通常为水蒸气或空气,因此已有的膨胀机流动及热功转换关系式是基于水蒸气或空气物性的。相同温度条件下,有机工质不论是粘度、密度、压力、绝热指数还是声速等都与水蒸气或空气有着巨大差异,在新的物性参数作用下,传统大功率膨胀机热功转换效率的经验关系式有可能不具备适用性。工质泵的加压也存在新的科学问题,泵机械运动的不可逆损失将导致有机工质流入泵时温度上升,有机工质常温下高汽化压力容易产生气蚀现象,并导致泵机械效率下降或断液等问题。为此,本文建立了系统的辐照集热-相变蓄热-热力循环三者之间的参量耦合及能量匹配数学模型,对系统结构及运行方式、有机工质、光热光电性能等方面进行了数值优化,并利用初步搭建的实验平台对kW级小型有机工质膨胀机、工质泵及ORC循环进行了测试和性能验证。论文的主要创新点为:1)设计了两级集热结构,集热器传热介质与有机工质通过双循环进行换热,有效降低了蒸发器内可用能的损失。2)在两级集热的基础上,提出了平板集热器与CPC集热器联合工作的思想,提出了平板集热器比例的优化方法,设计了系统优化运行方案。3)设计了内蓄热结构,在维持太阳能ORC热发电稳定性的同时减少了传热介质与相变材料之间的二次换热。并进一步设计了两级蓄热结构,减少蓄热及放热过程的可用能损失。4)将太阳能ORC热发电与光伏发电相结合,设计太阳热发电/光伏发电复合系统,并对其性能进行了研究。5)依靠课题组自主设计与搭建的实验平台,基于高效kW级小型涡轮膨胀机,创新性对小型ORC系统进行了实验研究。更多还原

【Abstract】 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.更多还原