用于太阳能超临界CO2布雷顿循环的流态化颗粒换热试验与模拟

搭建30 kW浅层多级流态化颗粒换热试验台,在约1.5倍临界流化速度、换热器采用直管管束逆流形式布置时颗粒侧换热系数可达590～860 W/(m2·K).采用双欧拉流体模型对流化床内水平埋管管束换热进行数值模拟,模拟结果与试验结果偏差在10％以内.利用析因设计与线性回归模型研究颗粒粒径、颗粒导热系数和流化气体速度对流态...     

A novel thermally driven pump and its test in a supercritical CO2 loop system

In this paper, a novel thermally driven pump is introduced, and its test in a closed supercritical CO₂ loop cycle system is studied. The thermally driven pump is a refrigerant‐circulating pump composed of two expansion tanks, which has less electricity consumption and better reliability compared with conventional mechanical pumps. Experimental results showed that the system efficiency of the present thermally driven pump in the supercritical CO₂ loop can be enhanced compared with that using the mechanical feed pump. In addition, it was found that the system efficiency with the thermally driven pump decreases with the outlet temperature of condenser. The thermal energy utilization efficiency of the loop system with the thermally driven pump can be increased when the condenser has higher efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermodynamic analysis of representative power generation cycles for low‐to‐medium temperature applications

This study is focused on the analysis of representative thermodynamic cycles for power generation at low‐to‐medium temperatures (with the highest cycle temperature from 450 to 700 K). The natural working fluid of carbon dioxide is selected for the current tests and comparisons with suitable operation ranges. Energy balance and exergy loss models are established and applied to 10 selected representative thermodynamic cycles. One modified efficiency parameter is also defined for better comparison of performances, which has considered the effects of both specific thermodynamic process and cycle complexity. Based on the modified efficiency parameter, it is found that Rankine cycle yields the highest performance at 450–500 K among the 10 representative cycles, while regenerative Brayton cycle shows better behavior for 550–700 K. Detailed behaviors and optimal principals of regenerative Brayton cycles are also identified and compared in this study. Also, a new cycle is also proposed in this study, which combines the advantages of Rankine cycle and Brayton cycle. The new cycle is proved to have better work output potential but higher system complexity factor. In addition, based on the thermodynamic analysis, possible future directions of low‐to‐medium temperature power cycles are summarized. It is hoped that the results can be of help for related power generation system designs. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of thermodynamic cycles for concentrated solar power plants

Solar thermal power is a promising ‘green’ technology that could contribute significantly – in countries where it may be applicable due to available resources – towards meeting the 2020 and 2050 targets for the free energy production of emissions [Viebahn, P., Lechon, Y., and Trieb, F., 2011. The potential role of concentrated solar power (CSP) in Africa and Europe – a dynamic assessment of technology development, cost development and life cycle inventories until 2050. Energy Policy, 39 (8), 4420–4430]. Especially for the regions where solar radiation is significant, the technology of concentrated solar power (CSP) plants seems to have a great potential, once cost-related issues are resolved. The thermodynamic process, on which the component design of the plant is based, plays a significant role in the optimisation of the efficiency of the derived configuration. This paper aims to present a route for the design of thermodynamic cycles for a CSP, starting from the simplest processes and heading towards more complicated ones. For a reference output capacity, the obtained efficiencies are presented, illustrating the technical benefits of shifting to more advanced thermodynamic processes.     

Theory and performance analysis of a new heat engine for concentrating solar power

The objectives of this paper are to introduce a new heat engine and evaluate its performance. The new heat engine uses a gas, such as air, nitrogen, or argon, as the working fluid and extracts thermal energy from a heat source as the energy input. The new heat engine may find extensive applications in renewable energy industries, such as concentrating solar power (CSP). Additionally, the heat engine may be employed to recover energy from exhaust streams of internal combustion engines, gas turbine engines, and various industrial processes. It may also work as a thermal‐to‐mechanical conversion system in a nuclear power plant and function as an external combustion engine in which the heat source is the combustion gas from an external combustion chamber. The heat engine is to mimic the performance of an air‐standard Otto cycle. This is achieved by drastically increasing the time duration of heat acquisition from the heat source in conjunction with the timing of the heat acquisition and a large heat transfer surface area. Performance simulations show that the new heat engine can potentially attain a thermal efficiency above 50% and a power output above 100 kW under open‐cycle operation. Additionally, the heat engine could significantly reduce CSP costs and operate in open cycles, effectively removing the difficulties of dry cooling requirement for CSP applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of solar energy powered transcritical cycle using supercritical carbon dioxide

In this paper, the performance of solar energy powered transcritical cycle using supercritical carbon dioxide for a combined electricity and heat generation, is studied experimentally. The experimental set‐up consists of evacuated solar collectors, pressure relief valve, heat exchangers and CO₂ feed pump. The pressure relief valve is used to simulate operation of a turbine and to complete the thermodynamic cycle. A complete effort was carried out to investigate the cycle performances not only in summer, but also in winter conditions. The results show that a reasonable thermodynamic efficiency can be obtained and COP for the overall outputs from the cycle is measured at 0.548 and 0.406, respectively, on a typical summer and winter day. The study shows the potential of the application of the solar energy powered cycle as a green power/heat generation system. Copyright © 2006 John Wiley & Sons, Ltd.     

Thermodynamic analysis of the CO2‐based Rankine cycle powered by solar energy

Using carbon dioxide as working fluid receives increasing interest since the Kyoto Protocol. In this paper, thermodynamic analysis was conducted for proposed CO₂‐based Rankine cycle powered by solar energy. It can be used to provide power output, refrigeration and hot water. Carbon dioxide is used as working fluid with supercritical state in solar collector. Theoretical analysis was carried out to investigate performances of the CO₂‐based Rankine cycle. The interest was focused on comparison of the performance with that of solar cell and those when using other fluids as working fluids. In addition, the performance and characteristics of the thermodynamic cycle are studied for different seasons. The obtained results show that using CO₂ as working fluid in the Rankine cycle owns maximal thermal efficiency when the working temperature is lower than 250.0°C. The power generation efficiency is about 8%, which is comparable with that of solar cells. But in addition to power generation, the CO₂‐based solar utilization system can also supply thermal energy. Copyright © 2007 John Wiley & Sons, Ltd.     

A preliminary experimental investigation on characteristics of natural convection based on solar thermal collection using supercritical carbon dioxide

In this paper, an experimental work is carried out to investigate the characteristics of solar thermal collection using supercritical CO₂. This solar thermal conversion is based on supercritical CO₂ natural convection, which is much easily induced because a small change in temperature can result in large change in density close to the critical point. In addition, its critical temperature is 31.1°C and low enough to be easily reached in the low‐temperature solar thermal conversion system. The obtained results show that the supercritical CO₂ flow rate is smooth curve and not affected by the sudden variation of the solar radiation. The solar thermal conversion operation process can be divided into three periods: starting‐up, transition, and stable period. When the system reaches the stable period, the CO₂ flow rate will keep at a high value even if the solar radiation stays at a low level. It is also found that the smaller local solar radiation variation is, the better ability of keeping the flow rate near the peak level the supercritical CO₂ fluid owns. It is also found that a small pressure difference can drive a supercritical CO₂ flow with high flow rate. Furthermore, high solar thermal conversion efficiency is found at a high mass flow rate and under operation pressure near the critical point. Copyright © 2012 John Wiley & Sons, Ltd.     

含CSP电站的风光火储联合外送系统优化配置

为解决中国西北地区的新能源消纳问题,提出一种含CSP电站的风光火储联合外送系统优化配置方法。首先,根据风光火储的互补特性建立含CSP电站的风光火储联合外送系统的典型架构,并分析其工作机制;其次,结合拉丁超立方抽样以及基于时序自相关和互相关的时序重构、时序组合和场景择优,生成考虑时序自相关和互相关性的随机场景集;接着,以计及碳排放成本的系统收益和系统新能源消纳能力最优为多目标,构建含CSP电站的风光火储联合外送系统多目标优化配置模型,并通过增广ε-约束法结合商用软件CPLEX进行求解;最后,以西北某地区2025年电网规划数据构造的算例进行仿真,验证了该配置方法能有效提高系统收益,促进风光消纳,提高系统电力送出能力。     

An experimental investigation on characteristics of supercritical CO2‐based solar Rankine system

In this paper, the characteristics of the supercritical CO₂‐based Rankine cycle powered by solar energy are experimentally investigated. In order to study the controlling factors of system performances, in the experimental setups an electrical heating section as well as an evacuated solar collect is utilized. Also, corresponding heat transfer measurements of the supercritical CO₂ fluid in the heating section are conducted. Results show that the collecting efficiency will increase with the CO₂ mass flow rate. The increase in solar radiation and the decrease in condensation temperature in the cycle both can lead to the increase in CO₂ mass flow. It is also found that the CO₂ fluid flow in the heating section is not fully developed and the Local Nusselt number decreases along the flow direction of the testing pipe. The influences of pressure as well as other controlling factors on heat transfer are also analyzed in detail in this paper. Copyright © 2010 John Wiley & Sons, Ltd.     

Preliminary feasibility evaluation of solar thermal power generation in India

Results of a preliminary techno-economic appraisal of solar thermal power generation at three locations in India are presented. The study uses System Advisor Model developed by NREL, USA. The results of the study provide useful insight into (a) selecting appropriate reference direct normal irradiance for design of solar thermal power plants, (b) identifying suitable combinations of solar multiple and hours of thermal energy storage and (c) cost reduction potential. The parabolic trough technology is used for exemplifying the procedure for this purpose. The estimated levelised unit cost of electricity is in the range of Rs (US$1=Indian rupees 51.66 on 5 October 2012) 16–21 per kWh for the most likely range of input parameters. The results also indicate possibility of about 30% reduction in unit cost of electricity by year 2021.     

Innovative biomass to power conversion systems based on cascaded supercritical CO2 Brayton cycles

In the small to medium power range the main technologies for the conversion of biomass sources into electricity are based either on reciprocating internal combustion or organic Rankine cycle engines. Relatively low energy conversion efficiencies are obtained in both systems due to the thermodynamic losses in the conversion of biomass into syngas in the former, and to the high temperature difference in the heat transfer between combustion gases and working fluid in the latter. The aim of this paper is to demonstrate that higher efficiencies in the conversion of biomass sources into electricity can be obtained using systems based on the supercritical closed CO₂ Brayton cycles (s-CO₂). The s-CO₂ system analysed here includes two cascaded supercritical CO₂ cycles which enable to overcome the intrinsic limitation of the single cycle in the effective utilization of the whole heat available from flue gases. Both part-flow and simple supercritical CO₂ cycle configurations are considered and four boiler arrangements are investigated to explore the thermodynamic performance of such systems. These power plant configurations, which were never explored in the literature for biomass conversion into electricity, are demonstrated here to be viable options to increase the energy conversion efficiency of small-to-medium biomass fired power plants. Results of the optimization procedure show that a maximum biomass to electricity conversion efficiency of 36% can be achieved using the cascaded configuration including a part flow topping cycle, which is approximately 10%-points higher than that of the existing biomass power plants in the small to medium power range.     

聚光太阳能发电技术应用与前景

分析了聚光太阳能发电三大技术（线性聚光系统、碟/引擎系统、电力塔系统）以及热能储存系统,阐述了其结构、工作原理与研究方向,比较了这三大技术之间的经济技术性能,介绍了适合我国太阳能辐射量大的边远地区碟/引擎系统的应用,展示了太阳能热发电技术的应用前景及对节能减排的贡献。     

Dual Loop line-focusing solar power plants with supercritical Brayton power cycles

This study is focused on proposing the combination of a Dual Loop solar field, with Dowtherm A and the Solar Salt as heat transfer fluids in parabolic or linear Fresnel solar collectors, coupled to supercritical Carbon Dioxide (s-CO₂) Brayton power cycle. The Dual-Loop justification relies on gaining the synergies provided by the different heat transfer fluids properties. The oils advantages are related with the operating experience accumulated in numerous solar power plants deployed around the World, assuring the commercial equipment availability. Also the pipes metal corrosion with oil is much lower than with molten salt. The pipes material cost saving is significant with the oil alternative. The thermal oil main constraint is imposed by the maximum operating temperature (around 400 °C) for avoiding chemical decomposition and degradation, stablishing the plant threshold efficiency 37% due to Carnot principle. On the other hand the Solar Salt mixture (60%NaNO₃40%KNO₃) maximum operating temperature goes up to 550 °C, but the freezing point is stablished around 220 °C requiring pipes and equipment electrical heating for avoiding salts solidification at low temperature. Regarding the balance of plant, the s-CO₂ power cycle is the most promising alternative to the actual Rankine power cycle for increasing the plant energy efficiency, reducing the solar collector aperture area and minimizing the equipment dimensions and civil work. Three Brayton cycles configurations with reheating were assessed integrated with the line-focusing Dual-Loop solar field: the simple Brayton cycle (SB), the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling (RCMCI). The power cycle operating thermodynamic parameters (split flow, reheating pressure, mass flow and pressure ratio) were optimized with unconstrained multivariable algorithms: SUBPLEX, UOBYQA and NEWUOA. The main conclusion deducted is the significant efficiency improvement when adopting the s-CO₂ Brayton cycle in comparison with the Rankine legacy solution. The Dual-Loop solar field integrated with a Rankine cycle provides a gross efficiency around 41.8%, but when coupling to s-CO₂ Brayton RC or RCMCI the plant efficiency goes up to ≈50%. It was also demonstrated the beneficial effect of increasing the total heat exchangers (recuperators) conductance (UA) for optimizing the Brayton cycles efficiency and minimizing the solar field aperture area for a fixed power output, only limited by the minimum pinch point temperature in heat exchangers.     

Performance of a flat-plate solar thermal collector using supercritical carbon dioxide as heat transfer fluid

Energetic and exergetic performance analyses of flat-plate solar collector using supercritical CO₂ have been done in this study. To take care of the sharp change in thermophysical properties in near critical region, the discretisation technique has been used. Effects of zonal ambient temperature and solar radiation, fluid mass flow rate and collector geometry on heat transfer rate, collector efficiency, heat removal factor, irreversibility and second law efficiency are presented. The optimum operating pressure correlation has been established to yield maximum heat transfer coefficient of CO₂ for a certain operating temperature. Effect of metrological condition on heat transfer rate and collector efficiency is significant and that on heat removal factor is negligible. Improvement of heat transfer rate is more predominant than increase in irreversibility by using CO₂. For the studied ranges, the maximum performance improvement of flat-plate solar collector by using CO₂ as the heat transfer fluid was evaluated as 18%.     

Comparison of supercritical CO2, liquid CO2, and solvent extraction of chemicals from a commercial slow pyrolysis liquid of beech wood

Innovative extraction methods with supercritical CO₂ and liquid CO₂ have been employed to obtain value-added chemicals from a slow pyrolysis liquid. Sequential solvent extraction with hexane and acetone was carried out for comparison. Pyrolysis liquid was first adsorbed on silica (SiO₂) with weight ratios SiO₂:oil of 100:40 and 100:80. Pyrolysis liquid and extracts were mainly characterized by GC–MS/FID, elemental analysis, and water content. Results show that scCO₂ extraction is mainly controlled by dissolution at the first 3 h during a 6-h extraction period and by combination of dissolution and diffusion at later extraction periods. Around 60–65% of the CO₂ and hexane extracts could be identified by GC compared to 49% of the starting pyrolysis liquid. GC data confirmed that, CO₂ extraction effectively enriched both non-aromatics and aromatic compounds. Hexane extracts contained lower contents of organic acids. Hexane enabled a complete extraction of aromatics. Chemical composition of extracts from scCO₂ and liquid CO₂ are very similar. Extraction with scCO₂ and liquid CO₂ proves to be an effective and innovative pre-treatment process for the production of chemicals from pyrolysis liquid.     

太阳能辅助二氧化碳热泵热水系统实验研究

利用实验的方法,研究了太阳辐照度、外界气温和风速、初始水温、蒸发器出口温度和压力等对太阳能辅助二氧化碳热泵热水系统运行状况和COP的影响。实验结果表明,系统COP随初始水温的升高而增大;太阳辐照度、外界温度和风速对热泵系统性能的影响主要体现在对系统循环水温的影响;在一定范围内,蒸发压力和蒸发温度越高,热泵系统的COP越大。     

Effect of heat extraction optimization with supercritical carbon dioxide on transcritical power cycle

An experimental and thermodynamic analysis was conducted to explore the match in operating conditions for the heat extraction of supercritical CO₂ and the CO₂ transcritical organic Rankine cycle (CTORC). The results revealed that in the optimal conditions of the experiment, the difference between the pseudocritical temperature and the inlet temperature ( ΔT _{pc − in} ) was <10 K and T _b/T _pc (ratio of the bulk temperature to the pseudocritical temperature) was ≤1 (ideal scenario: T _b/T _pc = 1). Furthermore, the heat transfer and fluid flow of CO₂ as well as the CTORC system performance at the optimal T _b/T _pc could be simultaneously improved with respect to those at ΔT _{pc − in} < 10 K. The peak values of system efficiency for the inlet temperature of the expander of 100°C and 150°C were 5.1% at 12.5 MPa and 8.0% at 17 MPa, with the corresponding T _b/T _pc being 1.24 (T _pc of 55.9°C) and 1.45 (T _pc of 70°C), respectively. Consequently, to simultaneously improve the heat transfer, fluid flow and system efficiency, T _pc of the supercritical CO₂ in the CTORC should be sufficiently high to approach half the inlet temperature of the expander for obtaining an optimal T _b/T _pc at a low condensing temperature.     

Thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement for a low‐grade heat source

A thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement using a low‐grade industrial waste heat is presented. Under the identical operating conditions, the reheat cycle is compared to the non‐reheat baseline cycle with respect to the specific net power output, the thermal efficiency, the heat exchanger area, and the total capital costs of the systems. Detailed parametric effects are investigated in order to maximize the cycle performance and minimize the system unit cost per net work output. The main results show that the value of the optimum reheat pressure maximizing the specific net work output is approximately equal to the one that causes the same expansion ratio across each stage turbine. Relative performance improvement by reheat process over the baseline is augmented with an increase of the high pressure but a decrease of the turbine inlet temperature. Enhancement for the specific net work output is more significant than that for the thermal efficiency under each condition, because total heat input is increased in the reheat cycle for the reheat process. The economic analysis reveals that the respective optimal high pressures minimizing the unit heat exchanger area and system cost are much lower than that maximizing the energy performance. The comparative analysis identifies the range of operating conditions when the proposed reheat cycle is more cost effective than the baseline. Copyright © 2012 John Wiley & Sons, Ltd.