Elsevier

Solar Energy

Volume 176, December 2018, Pages 589-603
Solar Energy

Optimal start-up operating strategies for gas-boosted parabolic trough solar power plants

https://doi.org/10.1016/j.solener.2018.10.059Get rights and content

Highlights

  • Concentrating solar power plants experience daily start-up and shut-down procedures.

  • The heating rates of the components are limited by thermo-mechanical constraints.

  • The model and validation of a gas-boosted concentrating solar power plant is presented.

  • The optimization of the start-up strategy is presented for two different seasons.

  • High steam generator constraints increase the operating flexibility of the plant.

Abstract

Concentrating solar power plants are taking an increasing share in the renewable energy generation market. Parabolic trough is one of such technologies and the most commercially mature. However, this technology still suffers from technical challenges that need to be addressed. As these power plants experience daily start-up procedures, the optimal performance in transient operation needs to be considered. This paper presents a performance based modelling tool for a gas-boosted parabolic trough power plant. The objective of the paper is to define an optimal operational strategy of the power plant start-up procedure with the aim of minimizing its fuel consumption while at the same time maximizing its electric energy output, taking into account all the thermo-mechanical constraints involved in the procedure. Heating rate constraints of the steam generator and the booster heater, and the steam turbine start-up schedule were considered. The simulation model was developed based on a power plant located near Abu Dhabi, and was validated against real operational data with a maximum integral relative deviation of 4.3% for gross electric energy production. A multi-objective optimization was performed for a typical operating week during winter and spring weather conditions. The results suggest that in order to minimize the fuel consumption and at the same time maximize the electric energy production, an evaporator heating rate of 6 K/min is an optimal value both for winter and spring conditions.

Introduction

The concentrating solar power (CSP) technology shows an increasing trend in capacity installations around the globe. One of the key reasons for this, is the possibility to integrate such technology with relatively cheap ways of storing thermal energy, hence allowing it to decouple the electric energy production from the solar input (International Energy Agency, 2014). Parabolic trough power plants (PTPPs) are the most mature and economically viable plants among the CSP technologies. They account for 85% of the current capacity installed (Groupe Reaction Inc., 2014, Khetarpal, 2016) and 80% considering the power plants currently planned to be installed (Estela et al., 2016). However, they still face problems both at technical and economic levels. From a technical stand-point, the intrinsic fluctuating nature of the solar irradiation causes operating challenges such as daily start-up procedures and frequent variations in loads, which some components of the plant are not fully designed to endure. From an economic perspective, CSP technologies are still not fully competitive with respect to traditional technologies such as gas or coal power plants. A way to improve the operating flexibility and the economic feasibility of PTPPs is to optimize the power block operation by maximizing its flexibility towards fluctuating loads and cyclic daily start-up procedures (Mancini et al., 2011). By doing so, it is possible to harvest as quickly as possible the solar irradiation, hence maximizing its electric energy production and profitability.

One of the key aspects to improve the technical performance of CSP plants is to increase the rate at which the plant can load-up in order to harness the solar energy quickly. On the other hand, in order to preserve the lifetime of certain components, the ramp-up rate is limited by thermo-mechanical constraints (Ferruzza et al., 2017). With regards to the power block this is especially true for the steam generator system (SGS) and the steam turbine (Ferruzza et al., 2017). For the former, the heating rate at which it can experience a temperature increase is limited by the thermo-mechanical stresses on the thick walled components and junctions such as the steam drum, super-heater headers and T or Y junctions in the steam pipelines (Dzierwa et al., 2016, Dzierwa and Taler, 2014, Taler et al., 2015b). Generally, the component limiting the ramp-up rate in the evaporator is the steam drum, which is designed as a high-pressure vessel, with a large diameter and consequently thick walls. The start-up procedure of the component is intended to reach as rapidly as possible nominal conditions for mass flow rates, pressure and temperature. In the case of the steam turbine, the shaft thickness is the main limiting aspect regarding thermal stresses. Therefore, in order to avoid excessive thermal stresses it is desirable to keep the temperature difference between the steam and the turbine metal as low as possible (Topel et al., 2015a, Topel et al., 2015b, Topel et al., 2017).

In order to achieve maximum responsiveness of the power plant towards a change in power load or insolation it is essential that all the components are able to start quickly thus enabling the CSP plants to start harvesting the incoming solar energy as soon as possible. However, there might be limiting factors for one component, which might reduce the required heating rate for another. For example, if the receiver or solar field are the limiting factors, there is no need for the SGS to be able to start up at a faster rate than that of the solar field. From a yearly perspective and optimization point of view, it might happen that a lower constraint is actually needed either for the steam turbine or the SGS. On the other hand, having for instance components like the SGS exceeding such optimal point might allow for more flexibility in the operational strategies of the power plant.

Considering previous work available in literature, research has been performed on modelling and evaluation of the performance of PTPPs with both oil and molten salts as heat transfer fluid, with and without gas-fired backup. For instance, Boukelia et al., 2017, Boukelia et al., 2015 investigated this by modelling specifically the power block in Ebsilon professional (STEAG, 2012) and evaluated the optimal levelised cost of electricity (LCOE) by means of artificial neural network algorithms implemented in Matlab. This, however, was done without considering detailed start-up constraints or different operating strategies. Biencinto et al. (2016) performed modelling of PTPPs both with nitrogen and Therminol-VP as heat transfer fluids. The model of the solar field was validated in detail against real plant data, while the overall model was compared with SAM (System advisor model) (Biencinto et al., 2014). In this case, the model was used to compare the annual yield of the two configurations. Bonilla and Jose (2012) modelled a direct solar steam generator PTPP using object-oriented modelling and calibrated it by comparing the model results with plant data from CIEMAT-PSA (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas – Plataforma Solar de Almería) by means of genetic algorithm based multi-objective optimization. The model focused mainly on the solar field detailed modelling and not on the overall power plant. Blanco et al. (2011) developed a model in the Wolfram mathematical software and compared the results to available power plant data. In this case, the power block was not modelled in detail, but used thermal efficiency correlations as function of the thermal input. Another example can be found in the work performed by Al-Hanaei et al. (2016), in which the authors developed a model of the Shams I power plant. The model did not consider the details of the power block and a validation was not presented in the paper. Detailed models can be also found in the research works presented by Sun et al., 2015, Li et al., 2017a, Li et al., 2017b, Li et al., 2017c, in which the authors developed a multi-dimensional model to address optical, hydraulic and thermo-elastic issues during the operation of a direct steam generator (DSG) parabolic trough collector. Even though the authors addressed the thermo-elastic problems and monitoring, they did not integrate the findings in the definition of an optimal start-up strategy.

In general, it may be claimed that many simulation tools are available to perform CSP plants design and performance evaluations. System Model Advisor (SAM) from NREL (National Renewable Energy Laboratories, USA) (Blair et al., 2008, Gilman et al., 2008, Price, 2003), Greenius (Dersch et al., 2011) from DLR (Deutschen Zentrums für Luft- und Raumfahrt) and Solergy from Sandia National laboratories (Stoddard et al., 1987) are commonly known tools in the CSP community. However, papers including detailed comparisons of simulation results with operational data of existing power plants are scarce. Concerning the start-up limitations, studies have been performed at component level. As for steam turbines, Topel et al., 2015a, Topel et al., 2015b, Topel et al., 2017 studied the thermo-mechanical limitations on steam turbines due to thermal stresses and start-up procedures. Concerning the steam generator system, González-Gómez et al., 2017a, González-Gómez et al., 2017b analysed such constraints for the heat exchangers and employed dynamic models for the stress evaluation. At system performance level, Topel et al., 2015a, Topel et al., 2015b, Spelling et al., 2012 considered the impact of increasing the turbine flexibility with regards to the power plant performance. In a previous study, the authors of this paper analysed the mutual interdependencies between the turbine and steam generator and the impact of their constraints on a parabolic trough power plant performance (Ferruzza et al., 2017). However, no previous study addressed the optimization of the start-up operational strategy of a parabolic trough solar power plant considering thermo-mechanical constraints related to the steam generator, heat exchangers and steam turbine. Specifically, there are no studies available in literature that aim at lowering the fuel consumption of such plants by optimizing the start-up operating strategy.

In this paper, a hybridized PTPP with a gas-fired booster located near Abu Dhabi is considered. The plant is also integrated with an additional heat transfer fluid heaters. The objective of the paper is to define an optimal operational start-up strategy of the power plant start-up procedure with the aim of minimizing its fuel consumption while at the same time maximizing its electric energy output, taking into account all the thermo-mechanical constraints involved in the procedure. This was done by taking into consideration the evaporator and booster heater heating rate constraints to verify how a dynamic performance oriented design for such components could lead to a higher flexibility from an operational standpoint. The optimal range for these constraints in order to satisfy the aforementioned objective were determined. The numerical model was thoroughly validated considering the steady state and transient performances using two sets of operational data of a power plant located near Abu Dhabi.

In Section 2 the paper presents the methods used to model the plant and validate it against operational data. It summarizes the constraints taken into account in the start-up strategy and dynamic operation and the implementation of the operation of the power plant in the control logic. Lastly, it presents the multi-objective optimization routine implemented. Section 3 presents the results of the validation, and afterwards the results and discussion of the multi-objective optimization performed for two different weather conditions. Section 4 outlines the conclusions and final remarks.

Section snippets

Methods

The modelling of the PTPP was carried out in DYESOPT, a tool able to perform power plant steady state nominal design, performance evaluation and techno-economic calculations. The tool has been previously developed and validated at KTH, Royal Institute of Technology, Stockholm (Guédez, 2016, Spelling, 2013). Fig. 1 illustrates the logic flow of information and calculations within the tool, where the grey and black boxes represent the inputs and outputs of the model respectively. In order to

Validation of the model

This section presents the validation both for steady state at design point/nominal load and for the dynamic performance. Tables 3 and 4 present the result of the validation of the main parameters and mass flow rates, respectively, at steady state nominal load. As the values are confidential, the validation is presented in terms of normalized parameters. The mass flow rates and power were normalized with the nominal values of steam mass flow rate and gross power (see Table 1, Section 2.3).

The

Conclusions

A detailed model was used to find the optimal start-up operational strategy of a gas-boosted parabolic trough power plant. The model was developed in DYESOPT – a techno-economic tool for dynamic performance evaluation of power plants. The power block part of the model was developed accounting for the heating rate constraints of the steam generator system, booster heater and the steam turbine start-up control strategy. The model was validated both at steady-state and dynamic operating

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