Solar-aided Power Generation System Research Articles

Exergy analysis of off-design performance in solar-aided power generation systems: Evaluating the rationality of using part-load operation solution of coal-fired plants

Solar aided coal-fired power generation (SAPG) technology has been proven to be an effective way of renewable energy utilization. However, the efficiency of coal-fired units declines at partial loads, and solar inputs may further force the system to deviate from its design condition. In particular, part-load operation solutions for coal-fired units (PLOS) are conventionally applied to the SAPG system, a practice whose appropriateness has not yet been thoroughly evaluated. This paper, therefore, selects a 330 MW SAPG plant as a case study to conduct an exhaustive exergy analysis under off-design conditions. The findings indicate that the governing stage of the steam turbine and the slip-pressure operation are the main contributors to the efficiency degradation at partial loads. In addition, solar input resulted in significant changes in key operating parameters, such as exhaust flow rate and the pressure of each turbine stage. Merely adopting the PLOS for SAPG systems results in a marked increase in standard coal consumption by 0.99, 2.82, and 3.63 g/kWh at 50 %, 40 %, and 30 % load, respectively. However, the introduction of solar energy presents the potential to operate the unit more efficiently, particularly when valve point conditions are optimally adjusted. For instance, upon input of 28 MW solar thermal power, the #3 valve point condition shifted from a load of 321 MW–330 MW, achieving a reduction in standard coal consumption by 0.96 g/kWh, compared to the design condition of the original coal-fired unit. This study underscores the irrationality of directly applying PLOS from coal-fired units to SAPG system without tailored adjustments for solar integration.

An efficient and low-cost solar-aided lignite drying power generation system based on cascade utilisation of concentrating and non-concentrating solar energy

A new solar-aided power generation system is proposed. It is based on the unique characteristics of non-concentrating and concentrating solar energy applied to lignite drying. In the new system, solar energy cascade utilisation is achieved by the two-stage solar drying of lignite. Solar irradiance, especially diffused irradiance, can be efficiently used in the new system at a relatively low cost using a non-concentrating solar collecting device. High solar energy conversion efficiency is achieved through solar-integrated lignite drying. For a 600-MW base unit, the solar energy in the new system increases electric power by 26.8 MWe; the solar-to-electricity conversion efficiency is 35.83 %. The low-cost non-concentrating solar collecting device has outstanding thermal performance. The net annual revenue of the new system is 63.45 million CNY with a solar-generated electricity cost of 0.346 CNY/kWh. The problem of solar fluctuation is resolved by dried coal storage. The new system achieves significantly better performance than the conventional system by efficiently utilising diffused solar irradiance during cloudy days.

On the use of thermal energy storage in solar-aided power generation systems

The solar-aided power generation (SAPG) technology has been proven to be one of the most efficient ways to integrate solar thermal energy into coal-fired power plants. An open question is whether to integrate the SAPG plant with a thermal energy storage system (TES). Conventionally, most SAPG plants are not designed to include TESs due to high costs and challenges related to the intermittent nature of solar energy. To address this question, a realistic case study utilizing a 330MWe SAPG plant is investigated in this paper. The simulation shows that when the SAPG plant uses a TES with a proper regulation strategy, the energy efficiency of the SAPG can be increased by 2.5% and the levelized cost of electric energy (LCOE) can be reduced by 0.27 cents/kWh. A well-regulated TES does not deteriorate the economic performance of the SAPG plant here and has a positive impact on its operational safety (especially under sudden irradiance change). To the best of authors’ knowledge, this is the first time to study the operational strategies of TES (based on the operating characteristics of the SAPG systems) and to combine them with the TES’s size optimization to maximize the techno-economic performance of the SAPG plant.

Optimisation of Solar Aided Power Generation plant with storage system adopting two non-displaced extraction steam operation strategies

Solar Aided Power Generation (SAPG) is one of cost-effective methodologies of integrating solar heat into fossil fuel fired power plant. For such system, solar heat used to displace extraction steam to high grade extraction steam is the best choice. However, strategies to adjust low grade extraction steam would also influence the plant performance. Two strategies exist: steady temperature (ST) and steady mass flow (SMF) strategies. In present study, the impact of storage integration on the SAPG plant adopting two strategies has been investigated. For the SAPG plant with storage system, the plant using ST strategy would have higher technical and economic performance than that using SMF strategy. With the increase of the solar multiply, the advantage of the ST strategy over the SMF strategy would be decreased. However, the increase of storage capacity can alleviate this trend, especially in high radiation regions. The Levelized cost of electricity (LCOE) of the SAPG plant with storage system would be greatly promoted in low solar multiple cases. When the solar multiple increases to about 2, the LCOE of the SAPG system with and without storage capacity becomes convergence. The results give a useful guide for operation of the SAPG plant with storage system.

Research on the Performance of Solar Aided Power Generation System Based on Annular Fresnel Solar Concentrator

A solar-aided power generation (SAPG) system effectively promotes the high efficiency and low cost utilization of solar energy. In this paper, the SAPG system is represented by conventional coal-fired units and an annular Fresnel solar concentrator (AFSC) system. The annular Fresnel solar concentrator system is adopted to generate solar steam to replace the extraction steam of the turbine. According to the steam–water matrix equation and improved Flugel formula, the variable conditions simulation and analysis of the thermo-economic index were proposed by Matlab. Furthermore, in order to obtain the range of small disturbance, the method of partial replacement is used, that is, the extraction steam of the turbine is replaced from 0 to 100% with a step size of 20%. In this work, a SAPG system is proposed and its thermo-economic index and small disturbance scope are analyzed. The results show that the SAPG system is energy-saving, and the application scope of small disturbance is related to the quantity of the extraction steam and evaluation index.

An efficient solar-aided waste heat recovery system based on steam ejector and WTA pre-drying in solar/lignite hybrid power plants

An efficient solar-aided waste heat recovery system based on steam ejector and WTA pre-drying was proposed. In the proposed system, solar energy collected by parabolic trough collector is used to generate a little high-grade steam, inducing steam waste heat in steam ejector and producing large amount of medium-grade drying steam. By absorbing waste heat in steam ejector and integrating with coal pre-drying, the solar energy could be considered amplified in the steam ejector and converted into the dried coal’s heating value. The results reveal that, for a typical 1000 MW lignite-fired power plant, 46.6 MWth of steam waste heat could be recovered efficiently by collecting 21.6 MWth of solar energy. The proposed system could produce 36.2 MWe of additional power with 1.62% of thermal efficiency increase. The cost of solar generated electricity of the proposed system is 0.028 USD/kWh, which is significantly lower than that of the conventional parabolic trough concentrating solar plants and solar-aided power generation system. The off-design performance reveals that the proposed system could solve the fluctuation of solar energy by dried coal storage.

Optimization of solar field layout and flow velocity in a solar-aided power generation system

The solar-aided power generation (SAPG) system is an efficient way to integrate solar thermal energy into the normal coal-fired power plant. This work constructed a hydraulic model of a parabolic trough solar field and updated its thermodynamic evaluation algorithm. Moreover, this work aimed to optimize the solar field layout and flow velocity of heat transfer fluid (HTF) in view of net solar-to-electricity efficiency, which deducted pump power consumption from the solar-generated electricity. Furthermore, the non-uniformity of the outlet HTF temperature in each loop was found on the basis of the proposed model. This non-uniformity threatened the safety of the solar field. The number of loops should be reduced, and the flow velocity of HTF should increase to alleviate such non-uniformity. The case study based on a typical 330 MW SAPG system indicated that the optimal solar field layout consisted of 24 loops with eight solar collector assemblies in each loop. The optimal average flow velocity of HTF was 2.33 m/s. The corresponding net solar-to-electricity efficiency was 22.83%, and the net solar-generated electricity was 8.4984 MW.

A new solar share evaluation method of solar aided power generation (SAPG) system by tracing exergy flows and allocating exergy destruction

Solar thermal and coal are two major energy sources in solar aided power generation (SAPG) systems. Reasonably distinguishing the solar shares of the total electricity output is a problem that must be solved. This study modeled the energy balance, exergy balance and solar exergy balance of the key subsystems in an SAPG system. Meanwhile, this study also designed a new solar share evaluation method by tracing solar exergy flows and allocating exergy destruction. How and how much solar exergy was transferred from the sun to electricity could be theoretically estimated by analyzing the distributions of solar share and solar exergy destruction. The proposed method considered energy level differences between solar energy and coal without assuming plant efficiency or solar shares in each energy stream to be constant.The proposed method could not only be used to calculate solar-generated electricity on the point of exergy transfer and destruction, but also be further used for excavating the potential of solar share expansion.

Stabilizing operation of a solar aided power generation (SAPG) plant by adjusting the burners’ tilt and attemperation flows in the boiler

Solar Aided Power Generation (SAPG) technology has been proved to be one of the most efficient ways to integrate solar energy into a coal-fired power plant. However, previous studies on SAPG systems have mainly focused on their design and design optimization. There are few studies on their operation. In an SAPG plant, solar heat is used to replace the extracted steam to preheat feedwater. However, the introduction of solar energy inevitably affects the steam flow rate through the boiler, especially when the SAPG plant is in fuel-saving operation mode, thus affecting the boiler's operation. Namely, the steam parameters at the boiler outlets would change with various solar inputs if no control mechanism were applied. A mechanism for adjusting the burners' tilt and attemperation flows in the boiler is therefore proposed in this study, in order to stabilize the temperature of the steam at the outlet of the boiler. To simulate the effectiveness of the mechanism, a transient model of the SAPG plant, including a series of modules for key components in the plant, has been developed in this study. The simulation results in a case study of a 300 MWe SAPG plant, showing the proportion of heat that was absorbed in the boiler was redistributed due to the solar input, without adjusting the burners' tilt and attemperation flows, resulting in temperature drops of superheated/reheated steam when in a fuel-saving mode. However, the simulation in the case study also shows that by swinging the burner tilt angle up to 22.7° and adjusting the attemperation flows in the super-heaters and re-heaters in the boiler, the superheated/reheated steam temperature can be restored from 523 °C and 510 °C respectively back to 543.0 °C, the desired temperature.

A technique to avoid two-phase flow in solar collector tubes of the direct steam generation system for a solar aided power generation plant

In a traditional solar aided power generation (SAPG) plant, problematic two-phase flow may be present in the solar collector tubes which can lead to burst pipes. Within the SAPG plant, it is proposed that a flash tank be applied. By using a flash tank and mandating its inlet pressure by controlling the booster pump, the preheating and overheating collectors can each be ensured to have single-phase flow. To assess the economic and performance criteria of adding a flash tank, a simulation model using is developed and using a unified matrix equation technique. This method is used to assess the effect of the flash tank on a case study 300 MW SAPG plant. Two modes of SAPG operation are considered. In the ‘single-generation mode’ only steam is introduced to the fossil-fuelled plant. In the ‘co-generation mode’ both steam, from the overheating collector, and water, from the flash tank, are introduced to the fossil-fuelled plant. The ‘co-generation mode’ is found by the authors to be the marginally better performing mode. By applying a flash tank in the SAPG system, the problem of unstable two-phase flow in solar collector tubes can be effectively overcome thereby improving the viability of implementation of retrofitted solar aided power generation to fossil-fuelled plants.

An evaluation method of solar contribution in a solar aided power generation (SAPG) system based on exergy analysis

Solar aided (coal fired) power generation (SAPG) is the synergy of solar thermal energy and fossil power plant technologies by using solar thermal energy to replace extraction steam to preheat feed water. SAPG combines environmental benefits of the former and large capacity, high efficiency and reliability of the latter. However, there are no appropriate criteria for evaluating the thermal performance of the system now. In this paper, a new evaluation method of SAPG system is proposed based on the exergy analysis and the characteristics of exergy flows crossing the boundary of the system. In the established evaluation method, a new ideal cycle is constructed in the perspective of external heat sources, and the difference of solar proportion among the exergy input, work output and exergy output in the same subsystem is considered. With the evaluation method, the contribution proportion of solar energy in a SAPG system can be determined. To illustrate applications of the new method, a parabolic trough solar field aided 600MW coal-fired power generation system is discussed as a case study.

Analysis of a solar-aided coal-fired power generation system based on thermo-economic structural theory

Solar-aided coal-fired power generation system is the integration of solar energy and conventional coal-fired power system. The first stage extraction from turbines was replaced by solar energy to heat feedwater, so that the replaced high pressure steam can continue to work in Rankine cycle. Thermo-economic structural theory was applied to perform a comparative study between the fuel-saving mode and power-boosting mode of SAPG (solar aided power generation) system. The physical and productive structures of systems were built to describe the interactions of all components in the system. Then thermo-economic costs of each component were calculated on the basis of exergy analysis. The results showed that compared to coal-fired power generation system, coal consumption rate is decreased by 15.04 g/kWh in fuel-saving mode. The power output is 57.2 MW higher in power-boosting mode. The unit thermo-economic cost of electricity is increased by 16.9–21.6% due to the large investment of solar collector field. Although the cost of solar energy is considered free, the thermo-economic cost of solar energy was found to be very high because of this large investment. Analysis of the condenser suggested that the exergy cost of condenser is all of irreversibility and thermo-economic cost of condenser is made up of irreversibility and investment.

The development of a thermo-economic evaluation method for solar aided power generation

In this paper, a method is derived for evaluating the thermo-economic performance of solar aided power generation (SAPG) hybrid systems. These systems can achieve emissions reduction and energy savings compared to conventional coal-fired power generation systems. In the discussed model of SAPG hybrid system, various stages of heating feed-water in a conventional coal-fired power plant steam Rankine system are replaced by steam injected from solar direct steam generation (DSG). In order to determine the thermo-economic viability of SAPG compared to conventional power generation, convenient methods are required for evaluating the performance of SAPG systems. Therefore, an analysis method is developed which involves defining a generalized steam–water distribution matrix equation. This matrix approach simplifies the traditional thermal calculation and has a one-to-one correspondence with the system structure. As a case study, the proposed method is successfully used to solve the case of a 600MW power generation system with the integration of DSG. A comparison of the case study system with (SAPG) and without (non-SAPG) the integration of the solar DSG shows thermo-economic benefits to the hybridised system. If the state parameters are taken as variables, this method can be used to study the effects of changes in system, equipment, operation, and load on the thermal economy of the integrated system.

Performance of a solar aided power plant in fuel saving mode

Integrating solar energy into conventional fossil-fuel fired power plant has been proven to be an efficient way to use solar energy for power generation purpose. In a solar-aided power generation (SAPG) system without storage system, owing to the instability of solar radiation, the turbine and the boiler work under off-design conditions most of time. In this paper, a modified simulation model is established to analyze the off-design performance of SAPG system. In order to speed up the calculations of the model, improved Matrix Thermal Balance Equation (MTBE) approach is used. A solar-aided feedwater heating of a 330MW coal-fired power generation unit in fuel-saving operation mode is discussed as a case study. The results show that, in a SAPG system, when HP extraction steam is partly replaced, with the increasing solar energy being introduced, the temperature of exhaust flue gas decreases; boiler efficiency improves and standard coal consumption rate decreases. With more solar heat introduced the temperature of superheated and reheated steam keep constant at first and then decrease rapidly. However, in real project, the temperature drop should be no more the permissible limits, which should be considered. The results obtained in the current study could provide a promising approach to find the balance between coal saving and safe operation of the SAPG system.

Comparison in net solar efficiency between the use of concentrating and non-concentrating solar collectors in solar aided power generation systems

Solar Aided Power Generation (SAPG) technology offers the promise of a higher energy efficacy and lower carbon emissions. In virtually all pilot-scale SAPG projects, concentrating solar collectors (e.g. parabolic trough collectors) are used to produce solar heat with a temperature ranging from 300 °C to 350 °C, to replace the feed water pre-heating in high pressure feed-heaters (HPH) in regenerative Rankine steam cycles. Although concentrating solar collectors offer a higher solar thermal efficiency, whether the use of this type of solar collector in a SAPG system can still offer both higher net solar thermal and net solar to power efficiencies remains unknown. This paper investigates the performance of an SAPG with either concentrating collectors or non-collectors (evacuated tube collectors). It was found that non-concentrating collectors are able to generate more power through the SAPG mechanism and exhibit higher net solar efficiencies than concentrating collectors on the per unit land area basis. This outcome suggests that the use of non-concentrating collectors in SAPG systems is more efficient than the use of concentrating collectors.

Multi-point and Multi-level Solar Integration into a Conventional Coal-Fired Power Plant

Solar-aided power generation (SAPG) is capable of integrating solar thermal energy into a conventional thermal power plant, at multi-points and multi-levels, to replace parts of steam extractions in the regenerative Rankine cycle. The integration assists the power plant to reduce coal (gas) consumption and pollution emission or to increase power output. The overall efficiencies of the SAPG plants with different solar replacements of extraction steam have been studied in this paper. The results indicate that the solar thermal to electricity conversion efficiencies of the SAPG system are higher than those of a solar-alone power plant with the same temperature level of solar input. The efficiency with solar input at 330 °C can be as high as 45% theoretically in a SAPG plant. Even the low-temperature solar heat at about 85 °C can be used in the SAPG system to heat the lower temperature feedwater, and the solar to electricity efficiency is nearly 10%. However, the low-temperature heat resource is very hard to be used for power generation in other types of solar power plants. Therefore, the SAPG plant is one of the most efficient ways for solar thermal power generation.