Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.     

Automatic control of a 30 MWe SEGS VI parabolic trough plant

For the 30 MWe SEGS VI parabolic trough collector plant, one task of a skilled plant operator is to maintain a specified set point of the collector outlet temperature by adjusting the volume flow rate of the heat transfer fluid circulating through the collectors. For the development of next generation SEGS plants and in order to obtain a control algorithm that approximates an operator’s behavior, a linear model predictive controller is developed for use in a plant model. The plant model is discussed first in this work. The performance of the controller is evaluated for a summer and a winter day. The influence of the control on the gross output of the plant is examined as well.     

Applied research concerning the direct steam generation in parabolic troughs

With levelized electricity costs (LEC) of 10–12 USCts/kWh the well-known SEGS (Solar Electric Generating Systems) plants in California are presently the most successful solar technology for electricity generation [Price and Cable (2001) Proc. ASME Int. Solar Energy Conf. Forum 2001]. The SEGS plants apply a two-circuit system, consisting of the collector circuit and the Rankine cycle of the power block. These two-circuits are connected via a heat exchanger. In the case of the Direct Steam Generation (DSG) in the collector field [Zarza et al. (2001) Proc. Solar Forum 2001, Washington], the two-circuit system turns into a single-circuit system, where the collector field is directly coupled to the power block. This renders a lower investment and higher process temperatures resulting in a higher system efficiency. Due to the lower investment and the higher efficiency a reduction of the LEC of 10% is expected when the DSG process is combined with improved components of the solar collectors [Zarza (2002) DISS Phase II Final Report, EU Contract No. JOR3-CT98-0277]. Within the European DISS (Direct Solar Steam) project the feasibility of the direct steam generation has been proven in more than 3700 operation hours. Steam conditions of 100 bar and 400 °C have been demonstrated. This paper presents the main scientific results of the DISS project that aims at the investigation and demonstration of the DSG process in parabolic troughs under real solar conditions.     

Conceptual design and techno-economic assessment of integrated solar combined cycle system with DSG technology

Direct steam generation (DSG) in parabolic trough collectors causes an increase to competitiveness of solar thermal power plants (STPP) by substitution of oil with direct steam generation that results in lower investment and operating costs. In this study the integrated solar combined cycle system with DSG technology is introduced and techno-economic assessment of this plant is reported compared with two conventional cases. Three considered cases are: an integrated solar combined cycle system with DSG technology (ISCCS-DSG), a solar electric generating system (SEGS), and an integrated solar combined cycle system with HTF (heat transfer fluid) technology (ISCCS-HTF).This study shows that levelized energy cost (LEC) for the ISCCS-DSG is lower than the two other cases due to reducing O&M costs and also due to increasing the heat to electricity net efficiency of the power plant. Among the three STPPs, SEGS has the lowest CO₂ emissions, but it will operate during daytime only.     

Saturated steam process with direct steam generating parabolic troughs

The direct steam generation (DSG) in parabolic trough collectors is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi Megawatt range. The European DISS project has proven the feasibility of the direct steam generation under real solar conditions in more than 4000 operation hours. Within the European R&D project INDITEP the detailed engineering for a pre-commercial DSG solar thermal power plant with an electrical power of 5 MW is being performed. This small capacity was chosen to minimise the risk for potential investors.In regards to DSG solar thermal power plants, only steam cycles using superheated steam have been investigated so far. The paper will investigate the advantages, disadvantages, and design considerations of a steam cycle operated with saturated steam for the first time. For near term applications, saturated steam operated DSG plants might be an interesting alternative for power generation in the small capacity range due to some specific advantages:

• Simple set up of the collector field.

• Proven safe collector field operation.

• Higher thermal efficiency in the collector field.

Control scheme for direct steam generation in parabolic troughs under recirculation operation mode

Electricity production using solar thermal energy is one of the main research areas at present in the field of renewable energies, these systems being characterised by the need of reliable control systems aimed at maintaining desired operating conditions in the face of changes in solar radiation, which is the main source of energy. A new prototype of solar system with parabolic trough collectors was implemented at the Plataforma Solar de Almería (PSA, South-East Spain) to investigate the direct steam generation process under real solar conditions in the parabolic solar collector field of a thermal power plant prototype. This paper presents details and some results of the application of a control scheme designed and tested for the recirculation operation mode, for which the main objective is to obtain steam at constant temperature and pressure at the outlet of the solar field, so that changes produced in the inlet water conditions and/or solar radiation will only affect the amount of steam produced by the solar field. The steam quality and consequently the nominal efficiency of the plant are thus maintained.     

Analytic modeling of a solar power plant with parabolic linear collectors

An analytic model for a solar thermal electric generating system with parabolic trough collectors was developed. The energy conversion of solar radiation into thermal power along the absorber tube of the parabolic collector is studied, taking into consideration the non-linearity of heat losses and its dependence on the local temperature. The coupling between the collector and the thermodynamic cycle is made up of three heat exchangers, yielding the characteristic temperatures of the cycle. The conventional Rankine cycle is treated as an endo-reversible Carnot cycle, whereby the mechanical and electric power is calculated. For comparison, we refer to the Solar Electric Generating System VI (SEGS VI), installed in the Mojave desert-CA, whose solar field is composed by LS2 parabolic trough collectors. We simulated the efficiency curves of collectors LS2 with evacuated and non-evacuated absorbers and compared with experimental results. A second simulation was carried out to estimate the optimum quantity of non-evacuated LS2 collectors in series in a collectors’ row, when friction losses along the absorber tubes are considered. Also, the performance of a 30 (MWe) power plant, composed of 50 rows with 16 LS2 collectors in series (total 800 collectors) was simulated. Three fields of different collectors were considered, the first field with evacuated absorbers, the second with non-evacuated absorbers and the third with bare absorbers. Finally, the output power of the plant is analyzed as a function of the evaporation temperature of the water-vapor fluid. A large maximum of the overall cycle efficiency is found for evaporation temperatures around 320 °C. Good agreement is obtained when comparing the results of this model with experimental data belonging to the Solar Electric Generating Systems (SEGS) installed in the Mojave desert. The analytic model developed combines simplicity, precision and flexibility, making it an attractive tool for simulation and design of solar power stations.     

Dynamics and control of parabolic trough collector loops with direct steam generation

Parabolic trough power plants with direct steam generation technology are a promising option for the production of electricity from renewable energy resources. For the layout of the collector field and the design of the control system the knowledge of the short-time dynamics is of essential importance. To study the dynamic behaviour a transient non-linear simulation tool is developed based on the Modelica language. The impact of over-all and local shadings of an 1000 m collector loop is simulated. Different feed water control systems are developed, implemented and evaluated. It turns out that in addition to the liquid level control of the buffer tank a fast feedback of the actual steam production is necessary for a good performance. For the case of steam temperature control by an injection cooler, it is shown that feedforward control significantly reduces the temperature deviations compared to a simple proportional–integral controller. The reaction of the controlled loop is simulated under a varity of irradiation disturbances. A key finding is that the orientation of the collector field relative to the moving clouds has large impact on the peak values during transients.     

Analysis of an innovative direct steam generation‐based parabolic trough collector plant hybridized with a biomass boiler

Direct steam generation (DSG) is the process by which steam is directly produced in parabolic trough fields and supplied to a power block. This process simplifies parabolic trough plants and improves cost effectiveness by increasing the permissible temperature of the working fluid. Similar to all solar‐based technologies, thermal energy storage is needed to overcome the intermittent nature of solar. In the present work, an innovative DSG‐based parabolic trough collector (PTC) plant hybridized with a biomass boiler is proposed and analyzed in detail. Two additional configurations comprising indirect steam generation PTC plants were also analyzed to compare their energy and exergy performance. To consider a wide range of operation, the share of biomass input to the hybridized system is varied. Energy and exergy analyses of DSG are conducted and compared with an existing indirect steam generation PTC power plants such as Andasol. The analyses are conducted on a 50 MW regenerative reheat Rankine cycle. The results obtained indicate that the proposed DSG‐based PTC plant is able to increase the overall system efficiency by 3% in comparison with indirect steam generation when linked to a biomass boiler that supplies 50% of the energy.     

Control concepts for direct steam generation in parabolic troughs

A new prototype parabolic-trough collector system was erected at the Plataforma Solar de Almería (PSA) (1996–1998) to investigate direct steam generation (DSG) in a solar thermal power plant under real solar conditions. The system has been under evaluation for efficiency, cost, control and other parameters since 1999. The main objective of the control system is to obtain steam at constant temperature and pressure at the solar field outlet, so that changes in inlet water conditions and/or in solar radiation affect the amount of steam, but not its quality or the nominal plant efficiency. This paper presents control schemes designed and tested for two operating modes, “Recirculation”, for which a proportional-integral-derivative (PI/PID) control functions scheme has been implemented, and “Once-through”, requiring more complex control strategies, for which the scheme is based on proportional-integral (PI), feedforward and cascade control. Experimental results of both operation modes are discussed.     

Steam temperature stability in a direct steam generation solar power plant

Direct steam generation (DSG) is one alternative to the current oil-based parabolic trough solar thermal power plants. Within the German research project ITES, the dynamic behavior of a DSG collector field and the interactions with the conventional power block are assessed in detail. A transient solar field model developed by DLR is used to simulate the steam temperature behavior. Artificial irradiance disturbances as well as real irradiance data are used as input to the system. The resulting main steam temperature gradients are then analyzed by Siemens considering the standards for steam turbines.This paper presents the transient simulation results of the steam temperature as well as the corresponding results of the steam turbine analysis. It is found that the occurring temperature gradients are challenging for a safe turbine operation, if a conservative control system is used. Therefore, the use of an additional thermal inertia to stabilize the steam temperature is suggested. Its impact is also analyzed and discussed in this paper.     

太阳能中温集热利用制蒸汽系统性能研究

构建了一套供热功率为310kW的太阳能中温集热利用制蒸汽系统,建立了系统主要模块——太阳能集热器与热変换器的热力学模型,研究了变工况下太阳辐射强度、凝水回收比、环境温度对系统效率和供热功率的影响,探讨了不同运行参数条件下集热温度与系统性能之间的关系。研究结果表明:增大辐射强度对系统性能提升显著;回收凝水对系统效率的影响不大,但对制热功率的提升较为明显;系统性能随环境温度升高呈先上升后下降的趋势;系统存在最佳集热温度,最佳集热温度随辐射强度和环境温度的增大而升高。     

Comparison of Linear Fresnel and Parabolic Trough Collector power plants

The Linear Fresnel Collector (LFC) technology is currently being commercialised by several companies for the application in solar thermal power plants. This study compares the electricity generation costs for LFC and Parabolic Trough Collector (PTC). PTC is the most commercial CSP technology to date and is therefore regarded as the benchmark. For reasons of comparability, direct steam generation is assumed for both LFC and PTC.For the LFC, cost data comparable to typical CSP plant sizes are hardly available. Therefore, the break even cost – referring to aperture-specific collector investment – is determined, where cost-parity of the electricity generation with a PTC reference plant is reached.This study varies the assumptions on collector performance and operation and maintenance costs to reflect different designs of LFC technologies. The calculations were carried out using cost and hourly simulation performance models. Depending on the assumptions, the costs for a linear Fresnel collector solar field should range between 78 and 216 €/m² to reach cost-parity at assumed reference solar field costs of 275 €/m² for the PTC.The LFC principle of arranging the mirrors horizontally leads to lower aperture-related optical efficiency which must be compensated by lower cost per m² of aperture compared to PTC. The LFC is a collector with significant cost reduction potential, mainly due to cheaper mirrors and structural advantages.The presented cost and performance targets shown in this study must be met by LFC technology developers to reach the PTC benchmark.     

Dynamic modeling and simulation of a solar direct steam‐generating system

A dynamic model of solar parabolic trough collectors has been developed and solved by explicit Euler's method to predigest partial differential equations to ordinary differentials in this paper. Different working conditions of the collector structure and thermal parameters have been considered in the model. The simulated results are validated using the selected real test data on typical summer and winter days, and the steam‐generating process from unsaturated water to superheated steam has been studied in the startup simulation. The disturbance analysis validates the model's dynamic responses and confirms the relations between output steam characters and solar radiation, inlet water temperature, flow rate and collectors' area. The results indicate the possible demarcation point of superheated steam generating in different working conditions. Therefore, the model can be used to analyze the performance of the solar direct steam‐generating system. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal energy storage for direct steam generation

Parabolic trough power plants with direct steam generation are a promising option for future cost reduction in comparison to the SEGS type technology. These new solar thermal power plants require innovative storage concepts, where the two-phase heat transfer fluid poses a major challenge. A three-part storage system is proposed where a phase change material (PCM) storage will be deployed for the two-phase evaporation, while concrete storage will be used for storing sensible heat, i.e. for preheating of water and superheating of steam. A storage system with a total storage capacity of approx. 1 MW h is described, combining a PCM module and a concrete module. The storage modules have been constructed for testing in a DSG-test facility specially erected at a conventional power plant of Endesa in Carboneras (Spain). Commissioning of the storage system started in May 2010; testing under real steam conditions around 100 bar will begin in August 2010.     

Modelling, optimisation and performance evaluation of a parabolic trough solar collector steam generation system

A parabolic trough collector (PTC) system used for steam generation is presented in this paper. PTCs are the preferred type of collectors used for steam generation due to their ability to work at high temperatures with a good efficiency. The modelling program developed called PTCDES is used to predict the quantity of steam produced by the system. The flash vessel size, capacity and inventory determines how much energy is used at the beginning of the day for raising the temperature of the circulating water to saturation temperature before effective steam production begins. Optimisation of the flash vessel presented here uses a simplified version of the program PTCDES. System performance tests indicate that the modelling program is accurate to within 1.2% which is considered very adequate. Finally, the theoretical system energy analysis is presented in the form of a Sankey diagram. The analysis shows that only 48.9% of the available solar radiation is used for steam generation. The rest is lost either as collector or thermal losses.     

Pre-adsorption of low-grade waste steam for high-temperature steam generation by using composite zeolite and long-term heat storage salt of MgSO4

Adsorptive heat transformer is a promising technology for waster heat recovery and global energy conservation. A novel cyclic adsorption heating system based on direct contact heat exchange method has been established for the purpose of high-temperature steam generation from hot water. Pre-adsorption is originally proposed before generation phase to enhance the system performance with composite zeolite 13X and MgSO₄ in the open-loop adsorption heating system. Composite zeolite is prepared by impregnation method. Experimental results show steam with temperature higher than 200°C is generated from inlet water at 72.0°C. During regeneration phase, dry air at 130°C and relative humidity of 7.37% is employed. Gross temperature lift is 95.0°C to 103°C for different pre-adsorption conditions. The effective steam generation time with pre-adsorption temperature at 90.0°C is prolonged by 27.4%. Meanwhile, the mass of steam is elevated by 16.2% compared with the cycle without pre-adsorption. Exergy coefficient of performance is upgraded by 14.7% and specific heating power for steam generation is increased by 16.0%. The pre-adsorption operation achieved the goal of recovery of low-grade waste steam on adsorbents to enhance the subsequent high-temperature steam generation. After pre-adsorption operation, the packed bed reaches adsorption and thermal equilibrium more quickly during generation phase. Thus, dynamic steam generation is significantly intensified and then system performance is improved correspondingly.     

Modeling analysis on solar steam generator employed in multi-effect distillation (MED) system

Recently the porous bilayer wood solar collectors have drawn increasing attention because of their potential application in solar desalination. In this paper, a thermodynamic model has been developed to analyze the performance of the wood solar collector. A modeling analysis has also been conducted to assess the performance and operating conditions of the multiple effect desalination (MED) system integrated with the porous wood solar collector. Specifically, the effects of operating parameters, such as the motive steam temperature, seawater flow rate, input solar energy and number of effects on the energy consumption for each ton of distilled water produced have been investigated in the MED desalination system combined with the bilayer wood solar steam generator. It is found that, under a given operating condition, there exists an optimum steam generation temperature of around 145°C in the wood solar collector, so that the specific power consumption in the MED system reaches a minimum value of 24.88 kWh/t. The average temperature difference is significantly affected by the solar heating capacity. With the solar capacity increasing from 50 kW to 230 kW, the average temperature difference increases from 1.88°C to 6.27°C. This parametric simulation study will help the design of efficient bilayer wood solar steam generator as well as the MED desalination system.     

DSG太阳能槽式集热器的热性能研究

采用数值计算的方法分别对稳态条件下直接产生蒸汽(DSG)太阳能槽式集热管中单相水区、饱和相区和干蒸汽相区的吸收管温度沿周向的分布进行了研究,在此基础上建立了集热器热损模型,并分析了流体温度、质量流量及工作压力对集热管中不同相区热损的影响.结果表明:影响集热器热损的关键因素是流体温度,随着流体与环境温差的增大,集热管中各相区的热损增加;流体的质量流量和工作压力对集热器热损的影响不大.