海洋温差能发电循环系统热力学分析

对海洋温差发电朗肯循环系统进行分析,分别研究蒸发压力、氨水浓度、冷热海水温度等参数对系统热效率和?效率的影响,对比分析纯氨和二氟一氯甲烷工质系统热效率和?效率,为海洋温差能开发及商业级发电系统研建提供技术支撑。结果表明:系统热效率和?效率与密切相关,氨水浓度的影响较小;纯氨质量流量远小于二氟一氯甲烷,且其热效率和?效率均大于二氟一氯甲烷系统,是海洋温差发电系统较为理想的循环工质。     

海洋温差发电系统热力学及热经济性分析

以热力学基本原理为基础,建立了海洋温差发电系统仿真模型,对比分析了亚临界状态下R717、R134a和R600三种工质系统在约束蒸发器窄点温差条件下优化目标函数随蒸发温度的变化规律。结果表明:蒸发温度越高,不同系统换热器的热负荷以及冷、热海水泵功率越小,最佳蒸发压力和工质泵功率越大;不同系统的热效率和单位换热面积输出电量与蒸发温度的相关性较大,随蒸发温度的增加近似线性递增。蒸发器的换热面积与循环工质种类的相关性较小,但冷凝器的换热面积与循环工质种类的相关性较大。R717循环更接近于卡诺循环,R717的系统热效率最大,热负荷及泵功率最小,且其热经济性目标函数值在合适的范围内,是海洋温差发电系统较为理想的循环工质。研究结果可为海洋温差发电系统的设计、试验及设备选型提供理论参考。     

海洋温差能发电研究现状及展望

阐述海洋温差能发电OTEC(ocean thermal energy conversion)技术的发展历程,分析国内外海洋温差能发电技术的难点和研究热点。重点介绍了闭式循环系统、开式循环系统、卡琳娜循环系统、上原循环系统、国海循环系统等典型温差能发电循环系统的工作原理及装置特点,指出了海洋温差能发电系统今后的研究热点和发展方向。     

海洋温差能朗肯循环非公沸混合工质理论研究

在海洋温差能发电系统中,氨工质的做功能力较强,缺点是湿工质,且有毒、易燃,安全性较差。文章根据热力学性质互补原则,以R227ea/ammonia混合物作为研究对象,对其应用于海洋温差能朗肯循环的性能进行理论研究。研究结果表明:非公沸混合工质在相变时存在温度滑移,这有利于减少换热不可逆损失;随着混合工质中R227ea质量分数的增加,混合工质的毒性和可燃性降低;当R227ea的质量分数为0.15时,混合工质的热效率达到最大值,为3.1%。     

基于He-N2混合工质的闭式Brayton循环系统及设备的优化设计

对以He-N_2混合气体为工质的闭式Brayton循环系统进行了优化分析,获得循环最高效率下系统各组成设备的性能参数。以此优化结果为目标,设计了轮机和换热器的初步方案。以纯氦气工质时轮机和换热器的成本为基准,利用设备成本与几何参数之间的近似关系,获得不同氦气摩尔分数下轮机和换热器的相对成本。通过Brayton循环中轮机与换热器的成本权重分配,得到不同氦气摩尔分数下Brayton循环的相对总成本。结果表明:当氦气摩尔分数为50%～60%时,存在一个相对总成本的最小值,与纯氦气时相比,其相对总成本降低了12%左右,这对于闭式Brayton循环系统工质的选择具有一定的指导意义。     

华东沿海海洋温差发电系统的优化设计

海洋温差能是一种可再生的绿色能源,储藏量大,资源稳定。海洋温差发电是利用深层、表层海水的温度差,以高温海水为热源,使液态工质气化推动发电机发电,以低温海水为冷源,使气态工质液化的不断循环的过程。基于能源的可持续发展考虑,可以利用风能、太阳能等可再生能源来优化设计海洋温差发电系统。华东沿海海域有着丰富的太阳能和风能资源,利用太阳能可以提高表层海水与深层海水的温差,利用风力转化装置可以提高和调整汽轮机的转速,保证发电系统持续稳定的发电。利用太阳能、风能对海洋温差发电系统进行优化设计,不仅避免和解决了当前海洋温差发电技术上的一些难点,还扩大了应用温差能资源的海域范围。     

低温热能发电的研究现状和发展趋势

低温热能种类繁多,数量巨大,利用这部分能源意义重大。介绍了低温热能发电技术的研究现状和发展趋势。低温热能发电技术主要应用于太阳能热电、工业余热发电、地热发电、生物质能发电、海洋温差发电等方面。现阶段低温热能发电的研究重点有:工质的热物性和环保性能、循环优化研究;提高低温热能发电效率的研究,包括混合工质循环、Kalina循环、回热、氨吸收式动力制冷循环等;基于有限时间热力学的系统最优控制等方面的研究。     

亚临界有机朗肯循环系统的热经济分析与优化

以系统发电成本(electricity production cost,EPC)为评价指标,对用于回收工业锅炉烟气余热的有机朗肯循环(ORC)系统进行了热经济分析与优化。结果表明,随着蒸发器和冷凝器节点温差的增大,系统发电成本先减小、再增大,即存在一组最优的蒸发器和冷凝器节点温差使发电成本最小。分别以纯工质R245fa和R236ea、非共沸混合工质R141b/RC318和乙烷/丁烷为循环工质,得到了最小发电成本时有机朗肯循环系统的最优工作参数,以及对应的系统净输出功、热效率和火用效率。     

海洋温差能发电热力循环技术进展

受传统化石能源日趋枯竭和环境污染的影响,海洋能作为一种清洁可再生能源得到了广泛关注。海洋温差能作为海洋能的重要组成部分,其储量和可转化电能巨大,且发电波动小、能量密度高,积极开发海洋温差能资源对实现科技兴海战略具有重要意义。海洋温差热力循环是海洋温差能开发利用的关键技术,其循环效率的高低直接决定了海洋温差发电系统的技术和经济性。本文综述了海洋温差能发电热力循环技术研究现状,对其基本原理及形式、热力循环构架、循环工质和热力学分析方法进行了详细阐述,并对海洋温差能发电热力循环技术进行了深入分析和展望。     

海洋温差能开发利用高效热交换技术

海洋温差能储量巨大,全球总量约400亿kW,我国温差能资源十分丰富,达3.67亿kW。海洋温差能清洁可再生、发电波动小、能量密度高,积极开发海洋温差能资源对实现科技兴海战略具有重要意义。高效热交换技术是海洋温差发电的核心技术。由于海洋温差发电过程中海水和工质流量大、换热器介质进出口温差小,开展高效热交换技术适应性分析十分必要。从高效热交换技术出发,对其换热形式进行综述,阐述并分析其在海洋温差能开发利用中的适应性,并对其发展方向进行展望,为海洋温差能发电工程示范和应用提出指导性建议。     

Experimental investigation of a vapor compression heat pump used for cooling and heating applications

The present work aims at evaluating the performance characteristics of a vapor compression heat pump (VCHP) for simultaneous space cooling (summer air conditioning) and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of evaporator water inlet temperature (14:26 °C) and condenser water inlet temperature (22:34 °C). R134a was used as a primary working fluid whereas water was adopted as a secondary heat transfer fluid at both heat source (evaporator) and heat sink (condenser) of the heat pump. Performance characteristics of the considered heat pump were characterized by outlet water temperatures, water side capacities and coefficient of performance (COP) for various operating modes namely: cooling, heating and simultaneous cooling and heating. Results showed that COP increases with the evaporator water inlet temperature while decreases as the condenser water inlet temperature increases. However, the evaporator water inlet temperature has more effect on the performance characteristics of the heat pump than that of condenser water inlet temperature. Actual COP of cooling mode between 1.9 to 3.1 and that of heating mode from 2.9 to 3.3 were obtained. Actual simultaneous COP between 3.7 and 4.9 was achieved.     

Westinghouse OTEC power systems

OTEC R&D in the U.S. has been focused mainly on the closed cycle with ammonia as the working fluid. The open cycle offers a number of advantages, including cost competitiveness. The two important features are in turbine protection in case of load loss and in the absence of evaporator biofouling. The Westinghouse open-cycle concept departs from earlier approaches which locate deaeration ahead of the flash evaporator. Westinghouse proposes to allow all noncondensibles to flow into the condenser. This paper summarizes the main features of both the closed- and open-cycle concepts and provides systematic discussion of performance features and cost.     

On the selection of working fluids for OTEC power plants

This paper analyzes the effect of three different working fluids (ammonia, propane, and freon-114) on the size of OTEC heat exchangesrs and system performance. Seven different combinations of shell-and-tube heat exchangers are considered. For each combination, a simple computer model of the OTEC power system is used to compare the three fluids. The comparison is made on the basis of A/W_net, where A is the total heat transfer area (evaporator plus condenser) and W_net is the net power output of the plant. Overall, ammonia is shown to be the best fluid (i.e. it yields the lowest value of A/W_net), although in some cases only by a small margin. The thermophysical property that gives ammonia its general superiority is its relatively high thermal conductivity. The paper also discusses heat exchanger design problems associated with liquid entrainment and boiling liquid superheat.     

Conceptual design for combined ocean thermal energy conversion using computational fluid dynamics and heat balance analysis

To overcome the limited efficiency of ocean thermal energy conversion (OTEC), particularly in the mid-latitudes, combined OTEC (C-OTEC) could use power extracted from the latent heat of a power plant condenser. Past research in South Korea has demonstrated the feasibility of a 10 kW C-OTEC system using R134a as a working fluid. As the next phase, a 200 kW C-OTEC demonstration facility with a thermal efficiency of greater than 3% is proposed. This paper presents the engineering design process for kW-scale C-OTEC within a 100 MW-scale thermal power plant. The design process is divided into two stages. First, to predict patterns in steam flow to a connected external evaporator with a porous medium, computational fluid dynamics are calculated. The results show a conservative margin suitable for the conceptual design. Second, an iterative heat balance simulation method simultaneously evaluates the heat balance analysis of the C-OTEC design and the thermal impact of the existing power plant. The design stages are then integrated in terms of heat transference capacity.     

A study on the design and analysis of a heat pump heating system using wastewater as a heat source

In this study, the compression heat pump system using wastewater, as a heat source, from hotel with sauna was designed and analyzed. This study was performed to investigate the feasibility of the wastewater use for heat pump as a heat source and to obtain engineering data for system design. This heat pump system uses off-peak electricity that is a cheap energy compared to fossil fuel in Korea. For this, the charging process of heat into the hot water storage tank is achieved only at night time (22:00–08:00). TRNSYS was used for the system simulation with some new components like the heat pump, which we create ourselves.As a result, it was forecasted that the yearly mean COP of heat pump is about 4.8 and heat pump can supply 100% of hot water load except weekend of winter season. The important thing that should be considered for the system design is to decrease the temperature difference between condenser and evaporator working fluids during the heat charging process by the heat pump. This heat pump system using wastewater from sauna, public bath, building, etc. can therefore be effectively applied not only for water heating but also space heating and cooling in regions like as Korea.     

A study on the new separate heat pipe refrigerator and heat pump

A new separate heat pipe refrigerator and heat pump is suggested based on the general three temperature thermal jet refrigerator and heat pump cycle. Sub-cooled hot water or other appropriate liquid heated by low grade heat sources forms the hot end and another heat pipe containing evaporator and condenser ends, adiabatic section of two-phase ejector and throttling tube is as the cold end of the separate heat pipe system. Performance relations for the thermal jet refrigerator and heat pump of such system is analyzed and a method of thermodynamic performance analysis is recommended. Primary prediction shows the feasibility of such heat pipe system for cold and warm water supply.     

Analysis of optimization in an OTEC plant using organic Rankine cycle

This study quantified the effects of evaporation temperature, condensation temperature, and the inlet- and outlet-temperature differences of deep cold seawater and warm seawater on the performance of an ocean thermal energy conversion (OTEC) plant using an organic Rankine cycle (ORC), and also investigated the optimal operations required for the performance. A finite-temperature-difference heat transfer method is developed to evaluate the objective parameter, which is the ratio of net power output to the total heat transfer area of heat exchanger in the system, and R717, R600a, R245fa, R152a, and R134a were used as the working fluids. The optimal evaporation and condensation temperatures were obtained under various conditions for maximal objective parameters in an OTEC system.The results show that R717 performed optimally in objective parameter evaluation among the five working fluids, and that R600a performed better than other fluids in thermal efficiency analysis. The optimal seawater temperature differences between the inlet and outlet of the evaporator and condenser are proposed. Furthermore, the influences of inlet temperatures of warm and cold seawater in the ORC are presented for an OTEC plant. The simulation results should enable the performance of an ORC system to be compared when using various organic working fluids.     

Simulated production of electric power and desalination using Solar‐OTEC hybrid system

Ocean thermal energy conversion (OTEC) is an electric power generation method that utilizes temperature difference between the warm surface seawater and cold deep seawater of ocean. As potential sources of clean‐energy supply, OTEC power plants' viability has been investigated. However, The OTEC system has problems of low efficiency and high investment cost because the temperature difference between the surface and the deep sea is small and it has a long pipe line and high pumping cost for using cold deep water. Therefore, in this present study, the OTEC system is combined with a solar system. It evaluated the thermodynamic performance of Solar‐OTEC Convergence System for the simultaneous production with electric power and desalinated water. The performance analysis of Solar‐OTEC Convergence System was carried out as the fluid temperature, saturated temperature difference and pressure of flash evaporator under equivalent conditions. The results showed that the performance of solar‐open OTEC system is the highest at the flash evaporator pressure of 10 kPa. At this time, the system efficiency, electric power and desalination production enhancement ratios were approximately 3.9, 13.9 and 5.1 times higher than that of the base open OTEC system respectively. Also, the performance of solar‐hybrid OTEC system is the highest at the inflow fluid temperature of evaporator of 80 °C. The system efficiency, electric power and desalination production enhancement ratios were approximately 3.5, 3.5 and 14.5 times higher than that of the base hybrid OTEC system. Copyright © 2016 John Wiley & Sons, Ltd.     

Analyzing the pressure-difference phenomenon between the condensing and boiling sections in a heat pipe cooling system

The heat pipe cooling system in this study consists of a flat evaporator, a condenser, and rising and falling tubes with water as working fluid. The working fluid has different water levels inside the two components. This is due to the vapor pressure deficits of the evaporation section and condenser section. This paper utilizes condensing and boiling pressure-difference theory and measures the temperature of the condenser wall to develop a theoretic model for the water level deficit inside the thermal module. Results indicate that the working fluid infiltrates the condenser and indirectly verifies the phenomenon leading to the different water levels inside the cooling system. Moreover, the water level height difference theory presented in this study may reduce the length of the condenser by 3.14 cm.     

A distributed model of a space heat pump under transient conditions

A prototype heat pump was designed and tested, as means of active thermal management for electronics packages to be used on stratospheric balloon missions. The evaporator worked as a cold plate to absorb heat dissipated by the electronics, while the condenser rejected heat primarily by radiation to the rarified environment. To predict the transient performance of the heat pump under varying environmental temperature and cooling load conditions, a dynamic model of the heat pump is created with a graphical user interface (GUI). The simulation of the evaporator and condenser are fully transient and the components are segmented, whereas the compressor and expansion device are lumped models and assumed to be at quasi-steady state. A detailed model for the mass and energy conservation in the two heat exchangers is presented. The spatial and temporal variation of temperature and mass flow rate in the heat exchangers are predicted. Several types of transient conditions such as step changes of the space temperature and cooling load, system start-up, shutdown, and cycling, are studied. The space temperature, cooling load, compressor power, mass flow rates of the compressor and expansion device, pressures and refrigerant charges of the condenser and evaporator, and temperature distribution in the heat exchangers are dynamically displayed on the GUI. The simulation results are compared with experimental data for step changes in the cooling load and show good agreement in terms of trends. Copyright © 2002 John Wiley & Sons, Ltd.