Impact of HRSG characteristics on open volumetric receiver CSP plant performance
Introduction
The use of open volumetric receivers (OVRs) with near-ambient pressure air as the heat transfer fluid (HTF) in Rankine-based concentrating solar power (CSP) plants, offers a number of potential benefits over more traditional HTFs, such as molten salt or water/steam. Air is freely available and non-hazardous, it doesn’t undergo a phase transition in the temperature range concerned, it is chemically stable at high temperatures, and its low heat capacity permits accelerated plant start-up owing to lower HTF network thermal inertia. In addition, thermal storage and fuel hybridisation can easily be incorporated into the HTF circuit. The general operation of a central receiver-type OVR CSP plant is outlined in the work of Fricker (1985) and shown in Fig. 1.
When sufficient solar energy is available from the plant’s heliostat field, ambient air is entrained by and drawn through the receiver, in which it is convectively heated to the specified receiver outlet temperature. This hot air is then passed via insulated ducting to the heat recovery steam generator (HRSG), where it is used to produce steam at the conditions required by the steam turbine. The exhaust air exiting the HRSG is then driven by a blower back to the receiver via a return duct, where a portion of it is entrained with ambient air back through the HTF circuit. The incorporation of packed bed thermal energy storage, comprising media such as crushed rock, provides a number of valuable benefits. These include the damping of solar energy transients or the temporal shifting of power production to electricity demand periods that may not coincide with the available solar resource.
The development of open volumetric receiver technology has been ongoing since the late 1970s, with pioneering work undertaken in North America (Sanders Associates Inc., 1979), and subsequently in Europe in the early 1980s (Fricker, 1985). As detailed in Ávila-Marín (2011), a number of research and development programmes have since ensued, broadly associated with either metallic or ceramic receiver absorber materials. Notable activities in this regard include the development and testing of the Sulzer (Becker and Sánchez, 1989), Phoebus-TSA (Meinecke et al., 1994) and Bechtel (Hellmuth, 1995) metallic receivers, and the HiTRec (Hoffschmidt et al., 2003) and SolAir (Téllez et al., 2002) ceramic receivers. Subsequent research has investigated, for example, improved characterisation of absorber heat transfer (Fend et al., 2004a) and fluid dynamics (Wu et al., 2010), improved receiver air re-entrainment (Marcos et al., 2004), and the development of ceramic foam absorbers and optimised absorber architectures (Fend et al., 2004b), amongst other topics.
Open volumetric receiver CSP plant technology has not yet experienced commercial implementation as a consequence of various factors, including the technology’s comparative immaturity in the context of the more-established CSP technologies. Nonetheless, the technology has been given serious consideration for commercial implementation previously, and is now being operated at a pre-commercial level in Germany. In 1990, based on the outcomes of the Phoebus-TSA project, the technical and economic feasibility of the development of a 30 MWe plant to be located in Jordan was established (Fricker, 1990). In preparation for this development, successful testing of a 2.5 MWth OVR and an associated HTF circuit incorporating thermal storage was then undertaken at the Plataforma Solar de Almería (Häger, 1994). Later, the initial 10 MWe PS10 plant design was presented, which employed an atmospheric air HTF circuit and improved OVR technology (Osuna et al., 2000). Prior to construction however, the PS10 design was altered, with steam generation instead being accomplished directly in a water/steam receiver. Most recently, the AlSol OVR CSP research plant has been proposed by a consortium led by Kraftanlagen München for construction in Algeria (Koll et al., 2011).
The first and thus far only OVR CSP plant to be constructed and operated is the Solar Power Tower Jülich, a 1.5 MWe pre-commercial demonstration plant built in Germany (Hennecke et al., 2009), based on Solar receiver technology. The Jülich plant, which reflects the technology’s state-of-the-art, has been a significant development as it provides a first-time opportunity for overall, long-duration plant performance to be measured.
Various aspects of OVR CSP plant performance modelling have been addressed thus far in associated literature. Ahlbrink et al. (2009a) employed the STRAL heliostat field raytracing tool coupled to the Dymola simulation platform to numerically investigate OVR behaviour and heliostat tracking procedures during transient plant operation. Additional details concerning the use of STRAL and Dymola in conjunction with the MATLAB/Simulink platform for detailed, overall OVR plant simulation are provided by Ahlbrink et al. (2009b). Koll et al. (2011) described an investigation of different gas turbine hybridisation approaches for the AlSol OVR plant on the basis of annualised simulations, although little information is provided concerning modelling methodology.
Alexopoulos et al. (2011) detailed the development of a model library to support the performance modelling of OVR plants in the MATLAB/Simulink environment, and employed the library to evaluate the performance of various gas turbine-hybridised OVR plants, co-fired with biogas. Coelho et al. (2012) investigated the thermodynamic and economic performance of OVR plant hybridisation via biomass-firing. Various plant configurations with and without thermal storage were simulated on an annual basis, using an assemblage of the HFLCAL, Ebsilon and Excel software packages. Ertl (2012) conducted an exergoeconomic analysis of OVR plant technology, benchmarking it against parabolic trough technology and in the context of solar-only and hybridised operation. A combination of HFLCAL, Ebsilon, EES and Excel was employed for simulation purposes.
Rau et al. (2014) reported on the simulated performance of four variations of a gas turbine-hybridised OVR plant studied in support of the AlSol project. The variants considered were characterised by the operational strategy and heliostat field size employed, and simulations were undertaken using MATLAB/Simulink in conjunction with the model library alluded to in Alexopoulos et al. (2011). Coelho et al. (2014) examined the impact of solar field multiple, storage capacity and control strategy on the optimisation of a 4 MWe OVR CSP plant on the basis of LCOE minimisation. Simulations were undertaken using a combination of HFLCAL, Ebsilon and Excel. As an extension of this work and utilising the same simulation tools, Coelho et al. (2015) investigated the thermodynamic and economic performance of a hybridised 4 MWe OVR CSP plant co-fired with various biomass fuels.
Over and above established challenges facing CSP technology, two specific challenges faced by OVR CSP technology are the improvement of receiver thermal performance and the minimisation of exergy destruction in the HRSG. Ávila-Marín (2011) provides a comprehensive review of volumetric receiver technology which illustrates that while open volumetric receiver technology can clearly achieve high outlet temperatures, the technology is compromised by thermal efficiencies that are lower than those associated with established central receiver technologies. A contributing factor to this reduced performance is the difficulty in achieving the classical “volumetric effect” in open volumetric receivers. The volumetric effect relies on absorber porosity to permit the subsurface penetration and absorption of solar radiation. In theory, it is possible for the absorber to be given tailored optical characteristics that enable more radiation to be absorbed beneath the surface than at the surface. This in turn permits the absorber surface to be kept at a lower temperature than that of the heat transfer fluid leaving the receiver, resulting in a significant reduction of receiver thermal losses.
Although the improvement of receiver performance has been the subject of appreciable research work, relatively little attention has been given to the matter of HRSG performance improvement. Buck et al. (2006) proposed a novel dual receiver concept for Rankine-based OVR CSP plants. The concept divides the production of superheated steam between an OVR and a conventional tubular water/steam receiver. The water/steam receiver is used to perform evaporation, whilst the volumetric receiver is responsible for the preheating and superheating duties via a dedicated HRSG. This approach significantly reduces exergy loss in the HRSG as solar heat is used here only for sensible heating. The dual receiver concept was further investigated by Eck et al. (2006), in a study that drew comparisons between the performances of a dual receiver plant and an OVR-only plant; each sized to generate 100 MWe. For both cases, design-point and annual performance was assessed in the context of throttled and sliding pressure part-load operation. In these studies, the dual receiver plant was predicted to perform appreciably better than the OVR equivalent – in part, due to the increased Second Law efficiency of the modified HRSG configuration. Despite the enhancement in plant performance, the authors noted that the concept faces particular complexities related to operational control and thermal storage.
In relation to the traditional OVR plant concept, there is a distinct lack of literature exploring plant performance characteristics in the context of the energetic coupling that occurs between the receiver and the water/steam plant via the HRSG. As a case in point, in conventional combined cycle plants, raising the gas-side inlet temperature of an HRSG increases steam production for a given gas mass flow rate, thus raising exergy utilisation (Kehlhofer et al., 2009). In such plants, the gas-side inlet temperature is limited by the gas turbine exhaust temperature, which is in turn limited by the maximisation of the combined cycle efficiency. In an OVR CSP plant, the gas-side inlet temperature can be increased above this limit by raising the receiver outlet temperature, but increased receiver outlet temperatures lead to higher receiver thermal losses and correspondingly lower exergetic performance. Clearly, within the bounds of technology limitations, an optimum operating point for the coupled receiver-HRSG system must therefore exist.
A question that has not been specifically addressed in published literature is how HRSG configuration and operating parameters affect overall plant performance, and in particular, which characteristics allow for the best utilisation of solar energy for a given plant configuration. The results presented here answer these questions by evaluating the impact of HRSG operating characteristics on the performance of a 100 MWe OVR central receiver plant. Investigations are carried out on the basis of coupled OVR/HRSG/water/steam cycle models implemented and solved in the EES© programming environment (Klein, 2013). Single and multi-pressure HRSG arrangements with and without reheating are examined. In specific terms, sensitivities to the variation of receiver outlet temperature, air return strategy, air return ratio, HRSG pinch-point temperature difference, deaerator outlet temperature and duct velocity are evaluated and discussed.
Section snippets
Plant and simulation details
The plant considered in this study is a hypothetical dry-cooled 100 MWe OVR CSP plant in a standard central receiver configuration, featuring a single, external, cylindrical OVR receiver with a surrounding heliostat field layout Thermal storage capability is not considered on the grounds that the study is solely concerned with the steady-state, design-point operation of the plant.
Plant behaviour is simulated using a coupled assembly of subsystem models, representing the OVR, HTF distribution
Impact of receiver outlet temperature
Fig. 6 illustrates the trends in solar-electric efficiency for all air return (AR) configurations as a function of receiver outlet temperature. It can be observed that maximum and minimum receiver outlet temperatures applicable to each configuration vary. At the lower end of the range, a limit exists as a consequence of superheater inlet temperature difference restrictions, whilst at the higher end, limitations are imposed either by economiser outlet temperature difference restrictions or in
Conclusion
The current study addresses the lack of published literature investigating how OVR CSP plant performance is affected by HRSG configuration and operating parameters. The work has presented the design-point performance characteristics of twelve plant configurations, distinguished by the number of water/steam cycle pressure levels and receiver air return strategy employed. A strong dependence of these characteristics on receiver outlet temperature was observed, illustrating the importance of
Acknowledgements
The authors wish to acknowledge with gratitude funding provided by the Fulbright Foreign Student Program and the University of KwaZulu-Natal, in the form of a Competitive Research Grant, in support of this study.
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