1. Introduction
Concentrated solar power (CSP) offers a potential path towards reducing carbon emissions from centralized power plants. Unlike photovoltaics, which convert solar energy directly to electricity, CSP utilizes the available solar thermal energy to drive conventional power cycles, such as the steam Rankine cycle [
1]. CSP can be easily integrated with thermal energy storage (TES), which is advantageous compared to other energy systems that are limited to battery storage [
2,
3]. Because of these aspects, research and development of CSP has received a lot of attention in recent years.
Like battery storage with solar photovoltaics, TES allows for the solar energy to be stored for dispatch at a later time, generally when demand is higher [
1,
2,
4]. TES is far less expensive than battery storage typically used to offset transient photovoltaics or other intermittent sources. Considerable work has been carried out to investigate TES integration into CSP plants [
5,
6,
7,
8] and there is widespread literature on the benefits and types of TES used in CSP plants [
9,
10,
11]. Such work has been carried out with a goal of improving design and performance of TES systems such as enhancement of heat transfer when TES is charged and discharged in various configurations. Example works involve simulations of storage systems, exergy and economic analysis of their charging cycles, and validating these models with physical systems [
12,
13]. The configurations of CSP and TES systems can vary considerably. Currently, a two-tank storage system integrated into a parabolic trough collector (PTC) CSP plant is the most common configuration, both in physical and modeled systems [
8]. However, novel configurations are still being developed even in PTC plants, and in recent years packed-bed storage has seen an increase in focus due to feasible integration into gas-driven Brayton cycles [
14,
15].
CSP can be easily hybridized with other energy sources, such as fossil fuels, due to equipment commonality. Through fossil fuel hybridization, the two energy sources operate in a synergistic way to produce power. This translates to more reliable operation of solar thermal plants, as well as increasing dispatchability of the converted energy to the grid during times of intermittent solar energy. Literature involving hybridization of CSP systems with natural gas is also quite abundant [
6,
16,
17,
18]. Hybridization offers an additional path forward to enhance solar energy utilization in CSP systems, as it has been shown to increase solar-to-electric efficiency (STE) relative to solar-only plants. STE is the marginal efficiency of converting available solar energy to electricity [
19,
20]. For a solar-only plant, plant operation may lower heat transfer and working fluid flow rates at times of lower solar activity so that the design temperatures can be met. Lower flow rates result in off-design operation and can lead to stagnation and dips in overall plant efficiency. For air turbine plants where air acts as both the heat transfer and working fluid, hybridization sustains turbine firing temperatures without having to lower the flow rate of the heat transfer fluid during hours of lower sunlight [
19,
21]. The sustained flow rates mitigate thermal losses in the solar collector and result in higher STE versus plants operating via only solar energy. Additionally, hybridization continuously maintains design temperatures during hours of intermittent solar activity or during delays caused by storage discharge. This steady temperature control further contributes to overall system performance and solar energy utilization [
22]. A solar-tower, hybrid combined cycle plant has been proposed in literature [
23,
24,
25] and a more common solar-tower Brayton plant has been developed at the pilot level [
26,
27,
28] and modeled extensively [
5,
18,
29]. Studies involving tower-driven plants have seen an increase in interest due to their high concentration ratios, which can realize the high operating temperatures needed to drive the Brayton cycle and combined cycle plants [
14]. Both systems require temperatures greater than what PTC plants can realize, unless the integrated solar combined cycle (ISCC) is considered. Within ISCC systems, CSP compliments operation of the bottoming Rankine cycle, which can be realized using a PTC system [
30,
31].
While physical improvement of TES and hybrid design is vital for development, operation and automation of CSP plants represent substantial efforts for studies involving TES integration and hybridization. Advanced process control (APC) has been applied to common CSP equipment to improve operation [
21,
32,
33]. In all CSP systems, the collector exit temperature is a primary control variable [
34]. Camacho et al. have worked extensively to apply several APC methods, such as model predictive control, to regulate exit temperatures in trough collectors [
35]. Similarly, cascade control has also been used to control the exit temperature of such collectors [
36]. In recent years, many APC has been applied to automate heliostat field operation for central tower receivers, with such studies focused on controlling solar flux distribution at the receiver and maximizing energy collected by the tower receiver [
34]. These control schemes typically involve manipulation of heliostat orientation to maintain solar flux on the receiver to regulate the collector exit temperature. If the temperature becomes too high, mirror orientations are modified to direct irradiance away from the receiver. In tower systems, these control schemes monitor the position of the sun and shifting of the heliostats to distribute high solar fluxes to the collector surface, where a heat transfer fluid absorbs the solar irradiance [
37].
While significant work has focused on component-level operation of CSP plants, such as control of collection temperature, there remains much opportunity to improve CSP performance by focusing on systems-level control and operation. Such holistic methods can help to improve solar penetration by employing sophisticated control algorithms to leverage TES integration and hybrid operation [
38]. Of interest in this work is to employ holistic automation to realize a high solar fraction in a hybrid power plant. The inclusion of the aforementioned storage is not enough to realize higher solar fractions. Smart, holistic automation must be utilized to achieve this goal [
6,
21,
39]. This is done by designing plant configurations and developing control algorithms that can direct excess solar energy to heat sinks, such as the packed-bed TES considered in this study, to better harness excess solar energy without having to direct it away from the collector. In the system presented herein, a recycling configuration is proposed as an alternative means to control receiver exit temperature by increasing thermal capacity of the heat transfer fluid during periods with high solar activity. The recycle stream can redirect excess energy to the packed bed storage system to be dispatched at a later time. This study would be the first to develop such an operation of a hybrid solar-combined cycle power plant with energy storage with a goal to increase solar fraction under a peaking power production schedule. This is achieved by systematically changing the plant configuration as well as the control schemes and analyzing the advantages and disadvantages of each configuration. This work demonstrates that by employing systems-level automation, the solar fraction can be significantly boosted while maintaining collection temperature control. The proposed control schemes also demonstrate tight power control, while diversion of solar energy away from the heliostats, as proposed by previous research, is avoided altogether.
From here on the paper is organized as follows.
Section 2 describes the various plant configurations developed and studied: a base-case configuration, a configuration utilizing a recycle stream to control receiver temperature, a configuration utilizing the recycle stream and storage bypass to enhance stored energy dispatch. Examples of system dynamics are also presented here with context to the control scheme of the recycling configuration. The model equations for these configurations are then presented in
Section 3.
Section 4 discusses the dynamics of each scheme with regards to robust control and solar energy utilization. This section highlights a study to maximize the solar fraction of the plant and test the plant’s ability to handle large amounts of solar input. Additionally, an economic analysis of the final plant configuration is presented in comparison to the base-case power plant. Finally,
Section 5 presents the conclusion of the study.
2. Overview of Plant Configurations
The hybrid solar-combined cycle plant has a capacity of roughly 200 MW between Brayton and Rankine cycles. Three configurations for plant operation are presented here (
Figure 1,
Figure 2 and
Figure 3). The control scheme shown in
Figure 1 is used as a basis for the plant. Ambient air is compressed and passed along to a central tower receiver (CTR) where a heliostat field directs concentrated sunlight to heat flowing air during hours of sunlight. When there is no solar activity, the air bypasses the receiver. Downstream of the collector, air passes through a packed-bed storage system. Then, the air passes through a combustion chamber and the hot flue gas is then directed to a gas turbine to produce power. The turbine firing temperature is controlled continuously by manipulating the natural gas flow into the combustion chamber. The net power of the plant is controlled by manipulating the inlet guide vane (IGV) angle of the compressor, which dictates air flow through the system on a volumetric basis. The IGV angle also controls the exit temperature of the CTR. Because the IGV controls both the receiver exit temperature and net power, a high-value selector (HVS) control is employed. The HVS continuously selects the highest IGV angle input between the power and CTR temperature control loops. The primary objective of the HVS controller is to maintain the power setpoint but it can override that setpoint, so collector temperature does not exceed design values.
The operation of the base-case plant allows for some flexibility in plant operation, but at some point, the amount of solar energy input exceeds the thermal capacity of the air heat transfer fluid. This results in off-design temperatures within the receiver even with the HVS override. To mitigate this and to harness the excess solar energy utilizing the physical sinks in the system, a recycling configuration is proposed, as shown in
Figure 2. Recycling increases the air flow and thus the thermal capacity of the heat transfer fluid in the receiver. This configuration presents a possible means to control CTR exit temperature as an alternative to redirecting excess solar energy to ambient heat (essentially losing solar energy). This solution to temperature control developed in this study is similar to previous research focusing on temperature control in parabolic trough systems through fluid flow manipulation [
40].
Figure 4 shows the dynamics of the HVS-recycle control scheme for the recycling configuration. Initially, as direct normal irradiance (DNI) increases throughout the day (
Figure 4A), the recycle stream (
Figure 4B) turns on to control the receiver exit temperatures (
Figure 4D) by increasing mass flow through the tower. Once the recycling temperature, or storage exit temperature, reaches 1000 K (
Figure 4E), the recycle stream turns off. When the recycle loop turns off, the temperature control of the HVS may override the power control (
Figure 4C) if the solar energy still available would result in temperatures higher than design conditions. The temperature setpoint of the recycle and HVS control loops, shown in
Figure 4D, must be offset so that they do not interfere with one another; that is, the setpoint of the temperature from recycling is lower than the HVS control so that the HVS does not control collector temperature while recycling occurs.
With the increased thermal capacity of the flowing air, an increase in field size or an increase in solar activity results in an increased solar fraction while also maintaining power and temperature of the system. Without a recycle stream, the base-case configuration is not capable of maintaining both power and collector temperature. However, at some point of incrementally increasing field size, the introduced solar energy begins to exceed the thermal capacity of airflow even with the additional recycling capacity. Thus, excessive temperatures are realized in the collector. While there is still substantial solar activity left in the day, the recycle stream is no longer active, as the temperature of the recycle stream has reached its limit of 1000 K. Additionally, as the storage charges, recycle stream steadily increases in temperature at the TES exit, which in turn elevates the temperature entering the tower, making it harder for additional flow to maintain the temperature setpoint. To mitigate the rate at which the recycling temperature rises, a bypass of the TES is implemented in conjunction with recycling. This plant configuration is seen in
Figure 3. The bypass operates using on/off control logic tied to the recycle stream:
If the recycle loop is active to keep receiver temperature at 1300 K, the bypass stream is active. The air flow due to IGV angle bypasses the storage and the recycle stream charges the storage.
If the recycle loop is inactive, the bypass stream is inactive as well. The intake air (which represents the total air flow when not recycling) passes through the TES to discharge any stored energy. Otherwise, if there is no stored energy, the storage is bypassed. The recycle loop turns off in one of two ways:
The temperature of the recycle loop (storage exit temperature) reaches 1000 K. This prevents already heated air from entering the receiver. If this is the case and there is still solar activity which would result in excess temperatures, the HVS operates in temperature control.
The receiver operates below the 1300 K setpoint and temperature control is not needed.
The bypass allows for only a fraction of the air flow to charge the TES while the recycle stream is active, which results in a lower TES charging rate and rise of TES exit temperature. This results in a longer period that the recycle stream can be active to control temperature and absorb excess solar energy. Without the bypass, the flow from both recycle and inlet charge the TES and the elevated exit temperature is realized far more quickly.
In all plant configurations, the operation of the bottoming steam cycle is identical: the steam cycle operates by controlling the temperature of the steam leaving the heat recovery steam generator (HRSG). The power from the Rankine cycle is not directly controlled but is estimated from the flow needed to maintain a temperature setpoint of steam at the HRSG outlet. Prior to the steam cycle, the flue gas undergoes auxiliary firing to reach an operating temperature of 820 K [
41].
To observe the benefits of TES within the system and to more realistically represent variable grid demand where the load is higher during evenings, a peaking power operation is proposed and used as a basis for this study. Under this schedule, the power is set at 100 MW from 11 p.m. to 11 a.m. At 11 a.m., the on-peak power setpoint is set at 175 MW for the next 12 h. The 175 MW is not quite at capacity so that there is room for the IGV angle to increase and control temperature control if needed.
For the recycling configuration, a block diagram of the power plant is seen in
Figure 5. This diagram shows the feedback control loops implemented in the recycling plant and is meant to show the flow of information within the control algorithm. The diagram omits the receiver and TES bypass streams for simplicity purposes.
5. Conclusions
Investigation of a CSP-hybrid plant design and operation is undertaken to achieve very high solar fractions. A first-principles model of a tower-driven CSP hybridized with a combined cycle power plant is developed in Matlab/Simulink. The hybrid plant contains packed-bed thermal energy storage. The study presents a systematic approach where modifications are made to the configuration of the plant and the performance of each configuration is discussed in depth. Such an approach to achieve a high solar fraction has not been applied in CSP literature. Proposed plant configurations operate using a novel recycling configuration to control receiver exit temperature. Receiver exit temperature and power control are system performance metrics of particular interest. A base-case plant is shown to maintain power control and mitigation of temperatures at smaller field sizes. In cases with excessive solar energy available, represented here by increasing the field size in lieu of increased solar irradiance, the base-case cannot maintain a power setpoint, as the HVS-override must increase flow to maintain receiver temperature. The recycling operation is implemented to control temperature independently, resulting in greater reliability in power control. Additional flexibility observed in the proposed recycle scheme results in a yearly solar fraction over 30% when employing a peaking power load. This results in a 4.9% improvement in solar fraction relative to the base-case plant. At subsequently larger field sizes, the recycle control faces issues with inability to simultaneously control collector exit temperature and net power. Therefore, a bypass of storage is implemented to reduce storage charging rate and allow for longer recycle times. Introduction of a bypass allows for flexibility to install larger heliostat fields, which results in a higher solar fraction, tight control, and improved solar-to-electric efficiency. The system utilizing a bypass exhibits a 6% improvement in solar fraction when compared to the plant lacking a TES bypass. By implementing a drastic peaking power load schedule, solar fractions as high as 70% are realized for the final plant configuration. Lastly, an economic analysis shows that by implementing a recycling and TES bypass operation, the LCOE of such a power plant can be reduced by over 4% despite an increase in overall capital cost of the plant. To improve LCOE, further reduction of capital cost is necessary, specifically the capital cost of solar collection equipment. Operation of the plant at baseload can also lead to further reduced LCOE for the hybrid plant.