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.     

Solar thermodynamic plants for cogenerative industrial applications in southern Europe

The paper deals with the preliminary design and optimization of cogenerative solar thermodynamic plants for industrial users. The considered plants are all based on proven parabolic trough technology, but different schemes have been analyzed: from a conventional configuration with indirect steam cycle and a heat transfer fluid such as synthetic oil or molten salts, to a more innovative arrangement with direct steam generation in the solar field. Thermodynamic parameters of the steam cycle have been optimized considering some constraints due to the heat requirements of the user, leading to a preliminary design of the main components of the system and an estimation of costs. Resulting net electric efficiency is about 10% for conventional synthetic oil plant, while 13% for innovative molten salts and DSG.A comparison with conventional solar thermodynamic systems for electricity production and photovoltaic power plants shows the economic and energetic benefits of the cogenerative solution. Cost of electricity for solar plant is cheaper of about 20 €/MWh than conventional solar power application. Moreover, heat recovery allows to achieve a further 50% of CO₂ emission savings compared to reference solar plants for only electricity production.     

Transient performance of direct steam generation solar power plants

Parabolic trough power plants are currently the most commercial systems for electricity generation. In this study, a transient numerical simulation of a solar power plant was developed by using direct steam generation (DSG) technology. In this system, condensate water from a Rankine cycle is pumped directly to solar parabolic trough collectors. The pressurized water is heated and evaporated before being superheated inside the solar collectors and directed back to the steam turbines, where the Rankine cycle is a reheated‐regenerative cycle. The plant performance with saturated steam production is compared with the performance of a superheated plant. A mathematical model of each system component is presented, with the solar power cycle modeled by the TRNSYS‐17 simulation program. Annual transient performance, including plant power and efficiency, is presented for both plants. As expected, the power of the superheated plant outperforms the saturated plant by approximately 45%, whereas the efficiency decreases by approximately 10%. Furthermore, the power of such plants is considerably improved under the weather of Makkah, 22.4°N, and it is approximately 40 MW for both the spring and autumn seasons. The annual generated energy is approximately 8062 MWh. The levelized electricity cost (LEC) was estimated for both the DSG and the corresponding synthetic oil plants. The DSG plant has an approximately 3% higher LEC than a synthetic oil plant with heat storage and an approximately 11.2% lower LEC than an oil plant if the plant has no storage. Copyright © 2016 John Wiley & Sons, Ltd.     

Unified model of solar thermal electric generation systems

In this paper a unified model of a solar electric generation system (SEGS) is developed using a thermo-hydrodynamic model of a trough collector combined with a model of a traditional steam power-house. The model evaluates thermal properties, steam flow rate and pressure drop in a direct steam generation (DSG) or an oil based collector field. The SEGS’s performance in different collector field power-house arrangements is evaluated. Long term performance is implemented by integrating the unified model in TRNSYS using hourly radiation data from the currently operating SEGS site in California. The performance of SEGS under Jordanian radiation conditions is studied by evaluating the yearly delivered energy from a collector field at different sites.     

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.

Performance of a direct steam generation solar thermal power plant for electricity production as a function of the solar multiple

This paper describes the influence of the solar multiple on the annual performance of parabolic trough solar thermal power plants with direct steam generation (DSG). The reference system selected is a 50 MW_e DSG power plant, with thermal storage and auxiliary natural gas-fired boiler. It is considered that both systems are necessary for an optimum coupling to the electricity grid. Although thermal storage is an opening issue for DSG technology, it gives an additional degree of freedom for plant performance optimization. Fossil hybridization is also a key element if a reliable electricity production must be guaranteed for a defined time span. Once the yearly parameters of the solar power plant are calculated, the economic analysis is performed, assessing the effect of the solar multiple in the levelized cost of electricity, as well as in the annual natural gas consumption.     

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.     

Comparative system analysis of direct steam generation and synthetic oil parabolic trough power plants with integrated thermal storage

Parabolic trough power plants are currently the most commercially applied systems for CSP power generation. To improve their cost-effectiveness, one focus of industry and research is the development of processes with other heat transfer fluids than the currently used synthetic oil. One option is the utilization of water/steam in the solar field, the so-called direct steam generation (DSG).Several previous studies promoted the economic potential of DSG technology (Eck et al., 2008b, Price et al., 2002, Zarza, 2002). Analyses’ results showed that live steam parameters of up to 500 °C and 120 bars are most promising and could lead to a reduction of the levelized electricity cost (LEC) of about 11% (Feldhoff et al., 2010). However, all of these studies only considered plants without thermal energy storage (TES).Therefore, a system analysis including integrated TES was performed by Flagsol GmbH and DLR together with Solar Millennium AG, Schott CSP GmbH and Senior Berghöfer GmbH, all Germany. Two types of plants are analyzed and compared in detail: a power plant with synthetic oil and a DSG power plant. The design of the synthetic oil plant is very similar to the Spanish Andasol plants (Solar Millennium, 2009) and includes a molten salt two-tank storage system. The DSG plant has main steam parameters of 500 °C and 112 bars and uses phase change material (PCM) for the latent and molten salt for the sensible part of the TES system. To enable comparability, both plants share the same gross electric turbine capacity of 100 MWel, the same TES capacity of 9 h of full load equivalent and the same solar multiple of the collector field of about two.This paper describes and compares both plants’ design, performance and investment. Based on these results, the LEC are calculated and the DSG plant’s potential is evaluated. One key finding is that with currently proposed DSG storage costs, the LEC of a DSG plant could be higher than those of a synthetic oil plant. When considering a plant without TES on the other hand, the DSG system could reduce the LEC. This underlines the large influence of TES and the still needed effort in the development of a commercial storage system for DSG.     

Comprehensive assessment of line‐/point‐focus combined scheme for concentrating solar power system

The line‐/point‐focus combined scheme for concentrating solar power (CSP) system is proposed. For solar field, the parabolic trough (PT) or linear Fresnel (LF) is used as the line‐focus preheating and evaporation stages while the solar tower is used as the point‐focus superheating and reheating stages. The combined schemes benefit from the high concentration ratio of point‐focus technology and low cost of line‐focus technology. Particularly, the combined scheme guarantees the concentrated solar thermal energy matching the temperature requirement of steam generation process with less exergy loss. Performance and economic assessments have been performed for 50 MW_e CSP system with two of the combined schemes, ie, PT (synthetic oil, SO) + Tower (molten salt, MS) and LF (direct steam generation, DSG) + Tower (DSG), as well as existing single schemes being the references, ie, PT (SO), LF (DSG), Tower (MS), and Tower (DSG). The comparative results show that the combined schemes are superior to liner‐focus schemes in efficiency and to point‐focus schemes in capital cost and scalability. Specifically, the PT (SO) + Tower (MS) system suggests the favorable potential in practical application with the highest annual net solar‐to‐electrical energy conversion efficiency of 16.07% and the reasonable levelized cost of electricity (LCOE) of 16.121 US cent/(kW·h). This work provides an alternative guidance for future development of the CSP technology.     

Life cycle assessment study of solar PV systems: An example of a 2.7 kWp distributed solar PV system in Singapore

In life cycle assessment (LCA) of solar PV systems, energy pay back time (EPBT) is the commonly used indicator to justify its primary energy use. However, EPBT is a function of competing energy sources with which electricity from solar PV is compared, and amount of electricity generated from the solar PV system which varies with local irradiation and ambient conditions. Therefore, it is more appropriate to use site-specific EPBT for major decision-making in power generation planning. LCA and life cycle cost analysis are performed for a distributed 2.7 kW_p grid-connected mono-crystalline solar PV system operating in Singapore. This paper presents various EPBT analyses of the solar PV system with reference to a fuel oil-fired steam turbine and their greenhouse gas (GHG) emissions and costs are also compared. The study reveals that GHG emission from electricity generation from the solar PV system is less than one-fourth that from an oil-fired steam turbine plant and one-half that from a gas-fired combined cycle plant. However, the cost of electricity is about five to seven times higher than that from the oil or gas fired power plant. The environmental uncertainties of the solar PV system are also critically reviewed and presented.     

A novel power block for CSP systems

Concentrating Solar Thermal Power (CSP) and in particular parabolic trough, is a proven large-scale solar power technology. However, CSP cost is not yet competitive with conventional alternatives unless subsidized. Current CSP plants typically include a condensing steam cycle power block which was preferably designed for a continuous operation and higher operating conditions and therefore, limits the overall plant cost effectiveness and deployment. The drawbacks of this power block are as follows: (i) no power generation during low insolation periods (ii) expensive, large condenser (typically water cooled) due to the poor extracted steam properties (high specific volume, sub-atmospheric pressure) and (iii) high installation and operation costs.In the current study, a different power block scheme is proposed to eliminate these obstacles. This power block includes a top Rankine cycle with a back pressure steam turbine and a bottoming Kalina cycle comprising another back pressure turbine and using ammonia-water mixture as a working fluid. The bottoming (moderate temperature) cycle allows power production during low insolation periods. Because of the superior ammonia-water vapor properties, the condensing system requirements are much less demanding and the operation costs are lowered. Accordingly, air cooled condensers can be used with lower economical penalty. Another advantage is that back pressure steam turbines have a less complex design than condensing steam turbines which make their costs lower. All of these improvements could make the combined cycle unit more cost effective. This unit can be applicable in both parabolic trough and central receiver (solar tower) plants.The potential advantage of the new power block is illustrated by a detailed techno-economical analysis of two 50 MW parabolic trough power plants, comparing between the standard and the novel power block. The results indicate that the proposed plant suggests a 4-11% electricity cost saving.     

Recent Developments in Integrated Solar Combined Cycle Power Plants

Global concern for depleting fossil fuel reserves have been compelling for evolving power generation options using renewable energy sources. The solar energy happens to be a potential source for running the power plants among renewable energy sources. Integrated Solar Combined Cycle(ISCC) power plants have gained popularity among the thermal power plants. Traditional ISCC power plants use Direct Steam Generation(DSG) approach. However, with the DSG method, the ISCC plant’s overall thermal efficiency does not increase significantly due to variations in the availability of solar energy. Thermal Energy Storage(TES) systems when integrated into the solar cycle can address such issues related to energy efficiency, process flexibility, reducing intermittency during non-solar hours. This review work focuses and discusses the developments in various components of the ISCC system including its major cycles and related parameters. The main focus is on CSP technologies, Heat Transfer Fluid(HTF), and Phase Change Material(PCM) used for thermal energy storage. Further, study includes heat enhancement methods with HTF and latent heat storage system. This study will be beneficial to the power plant professionals intending to modify the solar-based Combined Cycle Power Plant(CCPP) and to retrofit the existing Natural Gas Combined Cycle(NGCC) plant with the advanced solar cycle.     

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.     

Integrated solar combined cycle system with steam methane reforming: Thermodynamic analysis

The present paper considers an integrated solar combined cycle system (ISCCS) with an utilization of solar energy for steam methane reforming. The overall efficiency was compared with the efficiency of an integrated solar combined cycle system with the utilization of solar energy for steam generation for a steam turbine cycle. Utilization of solar energy for steam methane reforming gives the increase in an overall efficiency up to 3.5%. If water that used for steam methane reforming will be condensed from the exhaust gases, the overall efficiency of ISCCS with steam methane reforming will increase up to 6.2% and 8.9% for β = 1.0 and β = 2.0, respectively, in comparison with ISCCS where solar energy is utilized for generation of steam in steam turbine cycle. The Sankey diagrams were compiled based on the energy balance. Utilization of solar energy for steam methane reforming increases the share of power of a gas turbine cycle: two-thirds are in a gas turbine cycle, and one-third is in a steam turbine cycle. In parallel, if solar energy is used for steam generation for a steam turbine cycle, than the shares of power from a gas and steam turbine are almost equal.     

Exergetic analysis of solar concentrator aided natural gas fired combined cycle power plant

This article deals with comparative energy and exergetic analysis for evaluation of natural gas fired combined cycle power plant and solar concentrator aided (feed water heating and low pressure steam generation options) natural gas fired combined cycle power plant. Heat Transfer analysis of Linear Fresnel reflecting solar concentrator (LFRSC) is used to predict the effect of focal distance and width of reflector upon the reflecting surface area. Performance analysis of LFRSC with energetic and exergetic methods and the effect, of concentration ratio and inlet temperature of the fluid is carried out to determine, overall heat loss coefficient of the circular evacuated tube absorber at different receiver temperatures. An instantaneous increase in power generation capacity of about 10% is observed by substituting solar thermal energy for feed water heater and low pressure steam generation. It is observed that the utilization of solar energy for feed water heating and low pressure steam generation is more effective based on exergetic analysis rather than energetic analysis. Furthermore, for a solar aided feed water heating and low pressure steam generation, it is found that the land area requirement is 7 ha/MW for large scale solar thermal storage system to run the plant for 24 h.     

Highly integrated post‐combustion carbon capture process in a coal‐fired power plant with solar repowering

While post‐combustion carbon capture (PCC) technology has been considered as the ready‐to‐retrofit carbon capture solution, the implementation of the technology remains hampered by high costs associated with the large energy penalty incurred by solvent regeneration. This paper presents a highly integrated PCC process for a coal‐fired power plant with solar repowering that features significantly enhanced energy efficiency. Validated process models are developed for the power, capture, and solar thermal plants and simulated in a model superstructure to evaluate the possible improvements in power plant energy efficiency and power output penalty reductions. A 660‐MW power plant is taken as the case study. Three cases are used in this simulation analysis: (a) base case consisting of 660‐MW power plant integrated with a PCC plant, (b) the base case extended to incorporate solar repowering, and (c) a highly integrated case that extends on the previous case to include CO₂ gas compression unit heat integration. This study also highlights and discusses the role and interaction of various PCC and solar plant variables (e.g., solar field size, steam extraction flow rate, and twin LP turbine pressures) in the integration with power plant parameters. In particular, the power plant deaerator conditions play an important role in determining the total solar thermal energy required from the solar plant, thus dictating the solar field size. Copyright © 2015 John Wiley & Sons, Ltd.     

Biomass gasification in district heating systems – The effect of economic energy policies

Biomass gasification is considered a key technology in reaching targets for renewable energy and CO₂ emissions reduction. This study evaluates policy instruments affecting the profitability of biomass gasification applications integrated in a Swedish district heating (DH) system for the medium-term future (around year 2025). Two polygeneration applications based on gasification technology are considered in this paper: (1) a biorefinery plant co-producing synthetic natural gas (SNG) and district heat; (2) a combined heat and power (CHP) plant using integrated gasification combined cycle technology. Using an optimisation model we identify the levels of policy support, here assumed to be in the form of tradable certificates, required to make biofuel production competitive to biomass based electricity generation under various energy market conditions. Similarly, the tradable green electricity certificate levels necessary to make gasification based electricity generation competitive to conventional steam cycle technology, are identified. The results show that in order for investment in the SNG biorefinery to be competitive to investment in electricity production in the DH system, biofuel certificates in the range of 24–42 EUR/MWh are needed. Electricity certificates are not a prerequisite for investment in gasification based CHP to be competitive to investment in conventional steam cycle CHP, given sufficiently high electricity prices. While the required biofuel policy support is relatively insensitive to variations in capital cost, the required electricity certificates show high sensitivity to variations in investment costs. It is concluded that the large capital commitment and strong dependency on policy instruments makes it necessary that DH suppliers believe in the long-sightedness of future support policies, in order for investments in large-scale biomass gasification in DH systems to be realised.     

Performance analysis of an Integrated Solar Combined Cycle using Direct Steam Generation in parabolic trough collectors

The contribution of solar thermal power to improve the performance of gas-fired combined cycles in very hot and dry environmental conditions is analyzed in this work, in order to assess the potential of this technique, and to feature Direct Steam Generation (DSG) as a well suited candidate for achieving very good results in this quest. The particular Integrated Solar Combined Cycle (ISCC) power plant proposed consists of a DSG parabolic trough field coupled to the bottoming steam cycle of a Combined Cycle Gas Turbine (CCGT) power plant. For this analysis, the solar thermal power plant performs in a solar dispatching mode: the gas turbine always operates at full load, only depending on ambient conditions, whereas the steam turbine is somewhat boosted to accommodate the thermal hybridization from the solar field.