Thermoeconomic Evaluation and Sustainability Insights of Hybrid Solar–Biomass Powered Organic Rankine Cycle Systems: A Comprehensive Review
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe article focused on hybrid systems associated with ORCs powered by solar energy and biomass, addresses the topic in a superficial way without evidence of the added value compared to the existing vast scientific literature. In fact, it is known that biomass and solar ORCs have been studied for many years for the production of medium-sized distributed electricity. In fact, they lack the superheating phase and do not require one of the most complex and expensive components (the super heater) are much simpler and more manageable than conventional water vapor Rankine cycles. Therefore they are well suited for small and medium-sized plants. The specific application to biomass in recent years was much greater, especially when the cost of the photovoltaic plants was higher and the there was the competition with biomass energy was, thanks also often to incentives applied to both sources.
Instead, now, the use of solar energy in thermal processes for the production of electricity is less competitive than photovoltaic, This, in fact is affected by strong reductions in investment costs and costs of the energy produced (cost of panels photovoltaic 0.10-0.15 €/W and LCOe of 6-7 c€/kWh) The efficiency of photovoltaic panels itself now stands at values of 22-24%, much higher than the efficiency value obtainable from ORCs operating at low temperatures.
As regards biomass, it is true that this source is characterized by its programmability unlike the solar source, however there are critical aspects such as: availability of the biomass and its cost. On average for typical applications in ORCs, there are specific consumptions of around 1.2-1.5 kg of biomass per kWhe, so unless is used waste (available free or negative cost), the cost of biomass has significant effect on the cost of electricity produced. In the specific case of the article, it is even suggested the use of wood pellets, the costs of which are, in fact, prohibitive if aimed at producing electricity. It is not just by chance that the currently the main interest of ORCs is in the application for waste energy recovery or in cases where thermal energy has a negligible cost. Sorry to say that the article give very brief information on the aforementioned aspects of competition with photovoltaic/wind power but without absolutely analyzing these comparisons.
- The article does not specify which type of biomass you want to valorise, What is its unit cost or what is the assumed energy cost per thermal kWh.
- Regarding solar collector, there is no information on which technologies are used as a reference, what is the cost of thermal KWth as a function of temperature? Which geographical site are you referring to and what are the sunshine characteristics.
Below given are all aspects that should be better specified in the article
The article is also essentially focused on the production of electricity, fig 8 that itself analyze the efficiency as a function of temperature variation of both hot source and the ambient temperature, neglect the cogenerative applications that in fact can be carried out using the thermal energy recoverable from the condenser. Obviously, this involves a reduction in electrical efficiency, However it is clear that these cycles, with no direct valorisation of heat or cold production, are economically penalized compared to photovoltaic systems.
In addition to the above:
- Information reported with respect to a plant in Iran (Fig 3), is unclear: In fact, so many questions, such like, what are its characteristics (power, type of solar collectors; characteristics of the ORC, etc.), who is the user and its load curve? does it refer to a thermal power? at what temperature conditions does it operate and on what day of the year, it is referred to, remains unanswered.
- Abstract, in fact, that the article will focused predominantly on in-depth analysis of working fluids, however sorry to say that what is reported in the text is very elementary and limited in spite of the fact that vast literature is available on the topic.
- Explanation with respect to ORC is very basic and nothing has been said on the current progress on the topic
- Even the case studies and real applications that are expected appear to be insufficient. In fact, the article only reports some statistics on the diffusion of the typology but does not analyze specific case studies or elaborations on real applications analyzed by the author
In short, the article in its present version has a very low level of scientific in-depth analysis, it is therefore recommended to report greater in-depth analysis and precise focus in the revised version to be presented for its possible. re-evaluation. On many aspects it appears to be more oriented politically and with suggestions, without considering the fact that incentive offered are significantly different from country to country. It is to be noted that many countries who had in the past already strongly encouraged such like application and in particular the distributed production of electricity from biomass unfortunately have now strongly reduced or have cancelled the incentives for such like applications. However, in case author identify or underline particular conditions in some noteworthy countries, useful information needs to be specified and the article must be explored in greater depth.
Author Response
Reviewer 1.
Thanks for your review of our review article. Your comments were important and helpful in improving the quality of the manuscript. We have addressed each of your comment and made the corresponding changes in the revised manuscript accordingly. The changes are highlighted in green color in the revised manuscript. We respond to your comments one by one as follows.
Comment 1.
The abstract and introduction lack depth regarding the scope and contributions of the paper.
Author’s Response:
The authors are grateful for a detailed review of the manuscript and remarks. We appreciate the reviewer’s recommendation to improve the abstract of the manuscript.
Abstract
Hybrid solar-biomass Organic Rankine Cycle (ORC) systems represent a promising avenue for sustainable energy production by combining the abundant but intermittent solar energy with the reliable biomass energy. This study conducts a detailed thermodynamic and economic assessment of these hybrid systems, focusing on their potential to enhance energy efficiency and reduce greenhouse gas emissions. The study also evaluates the performance of various working fluids, identifying optimal configurations for different operating conditions. A key finding is that the hybrid system, with an optimized solar-biomass ratio, achieves up to a 21 to 31% improvement in efficiency and a 33% reduction in Levelized Cost of Electricity (LCOE) compared to solar-only systems. Additionally, the study examines case studies of real-world applications, offering insights into the scalability and cost-effectiveness of these systems in regions with high solar irradiation and biomass availability. These results underline the need for continued technological innovation and policy support to promote widespread adoption of hybrid ORC systems, particularly in the context of global decarbonization efforts. Please refer to lines 10-22.
Introduction
The growing global demand for sustainable energy solutions has driven extensive research into renewable energy sources, including solar and biomass. Solar energy, while abundant, suffers from intermittency, limiting its standalone effectiveness. Biomass, on the other hand, offers a reliable energy source but faces challenges related to fuel availability and cost. Hybrid systems that combine solar and biomass present a promising solution, leveraging the strengths of both energy sources to provide a stable, efficient, and cost-effective means of power generation. Although the individual technologies of solar and biomass ORC systems have been widely studied, there is a significant gap in the literature regarding their integration into hybrid systems, particularly in terms of their thermoeconomic performance.
This study aims to bridge this gap by conducting a detailed thermodynamic and economic analysis of hybrid solar-biomass ORC systems. By exploring various configurations, working fluids, and geographic scenarios, the research offers a comprehensive evaluation of how these systems can optimize energy production, reduce greenhouse gas emissions, and improve cost-effectiveness. Additionally, this paper addresses the challenges faced by hybrid systems in terms of scalability and market adoption, providing insights for policymakers and industry stakeholders on how to facilitate the transition to hybrid renewable energy technologies.
The findings presented in this paper contribute to the ongoing discourse on renewable energy integration, highlighting the potential of hybrid solar-biomass ORC systems to enhance global energy sustainability. Please refer to lines 207-227.
Comment 2.
The article superficially addresses hybrid systems without showing the added value compared to existing literature.
Author’s Response:
Thank you for your valuable feedback. I agree that a clearer demonstration of the added value of the hybrid solar-biomass Organic Rankine Cycle (ORC) systems compared to existing literature is essential.
Hybrid solar-biomass Organic Rankine Cycle (ORC) systems offer significant technological and economic advantages over standalone solar ORC or biomass ORC systems, particularly in terms of enhanced reliability, improved efficiency, and reduced dependence on individual energy sources. The key benefit of hybridization is its ability to mitigate the intermittency challenges associated with solar energy by incorporating biomass as a backup, enabling more stable and continuous power generation. Studies demonstrate that hybrid systems can achieve thermal efficiencies ranging from 21% to 34%, depending on the system size and configuration, which is a marked improvement over standalone solar systems, where efficiency tends to drop during low solar irradiance periods [70]. Additionally, hybrid systems reduce the overall capacity of the solar field required, leading to a 33% reduction in the Levelized Cost of Electricity (LCOE) compared to solar-only systems. Biomass integration ensures that power generation continues even during non-sunshine hours, further optimizing resource use [71],[72]. Moreover, these systems are designed to be scalable, making them suitable for regions with abundant solar and biomass resources, such as rural areas. From an economic perspective, hybrid systems demonstrate competitive viability with standalone systems, especially when factoring in long-term cost savings and efficiency improvements. This makes hybrid solar-biomass ORC systems a compelling option for achieving a reliable, efficient, and cost-effective renewable energy solution. Please refer to lines 413-431.
Comment 3.
There is no information about which type of biomass is used, its cost, or comparisons with photovoltaic (PV) systems.
Author’s Response:
Thank you for your valuable feedback. Specific Application of Biomass and Cost Comparison is added to the manuscript.
In hybrid solar-biomass Organic Rankine Cycle (ORC) systems, the choice of biomass is pivotal due to the variation in energy content, cost, and availability among different biomass types. For instance, wood pellets, commonly used in Europe, are priced around €200 per ton, while agricultural residues like rice husks, though less expensive, may require additional processing. The cost of energy from wood pellets typically ranges between €0.03 and €0.07 per kWh, whereas agricultural residues can be more economical at €0.01 to €0.05 per kWh [74]. This cost variation directly influences the Levelized Cost of Electricity (LCOE) in hybrid ORC systems, which is estimated between €0.07 and €0.13 per kWh, contingent on biomass type and system design [75]. In contrast, photovoltaic (PV) systems generally present a lower LCOE, ranging from €0.04 to €0.07 per kWh, though they often depend on energy storage solutions or grid backup due to their intermittent energy generation. Geographic factors also significantly affect system performance [76]. Hybrid ORC systems are best suited for regions with high solar irradiance and plentiful biomass, such as southern Europe and parts of Asia, where they can be competitive with PV systems. However, in areas with limited solar resources or insufficient biomass, PV systems are often the more economically feasible option. Geographic location thus plays a crucial role in determining biomass availability, solar energy potential, and the overall economic competitiveness of these renewable energy technologies. Please refer to lines 519-537.
Comment 4.
The article briefly mentions competition with photovoltaic and wind power without thorough analysis.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
In comparing hybrid solar-biomass Organic Rankine Cycle (ORC) systems with photovoltaic (PV) and wind power, investment costs, efficiency, and the Levelized Cost of Electricity (LCOE) are key metrics. Recent advancements in PV technology have significantly improved efficiency, with leading-edge systems reaching 22-24% efficiency, and costs have declined to as low as €0.04-€0.07 per kWh, making PV highly competitive in terms of cost [77]. Wind power, depending on location, offers similar LCOE values, averaging between €0.03 and €0.07 per kWh [78]. The initial investment costs for PV and wind systems are also decreasing, driven by advancements in technology and economies of scale, with PV installations typically requiring €1,000 to €1,500 per kW [79], and wind systems between €1,200 and €2,200 per kW depending on turbine size and location [80].
However, the hybrid solar-biomass ORC system remains competitive, particularly in specific contexts such as remote areas or regions with abundant biomass. ORC systems typically have higher upfront costs, ranging from €2,500 to €4,000 per kW, due to the complexity of integrating both solar and biomass components [81]. Their LCOE is higher, between €0.07 and €0.13 per kWh [82], but they offer a significant advantage in programmability. Unlike PV and wind, which suffer from intermittency and require extensive energy storage or grid backup, hybrid ORC systems can continuously generate power by switching between solar and biomass sources. This makes them especially well-suited for small and medium-scale distributed energy systems in regions with inconsistent sunlight or wind. Biomass provides a stable, programmable energy source that can be dispatched as needed, ensuring reliability and reducing dependency on energy storage solutions.
Hybrid systems are particularly advantageous in areas where grid infrastructure is underdeveloped, such as isolated or rural locations, or in areas with plentiful biomass like agricultural residues or forest byproducts. By leveraging locally available biomass, these systems reduce transportation and fuel costs, offsetting the higher capital investment. Moreover, they are less affected by the declining efficiency of solar panels due to shading, weather conditions, or nighttime hours, allowing for a more consistent energy output compared to PV and wind.
While PV systems have seen rapid improvements in efficiency and cost reductions, and wind power remains highly cost-effective in regions with strong wind resources, hybrid ORC systems provide a more flexible and reliable solution in contexts where energy demand stability and programmability are essential. The ability to integrate biomass energy, which can be stored and utilized on demand, allows ORC systems to mitigate the challenges of renewable intermittency, making them a viable option for ensuring continuous energy supply, particularly in off-grid and rural settings. Please refer to lines 538-592.
Comment 5.
There is no information on the type of solar collectors used and the cost of thermal kWth.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Solar Collector Technology in Hybrid Solar-Biomass ORC Systems: Costs and Efficiency:
In hybrid solar-biomass Organic Rankine Cycle (ORC) systems, the solar component typically uses concentrated solar power (CSP) technologies like parabolic troughs, linear Fresnel reflectors, or solar towers to capture and convert solar energy into thermal energy. Among these, parabolic troughs are the most widely deployed due to their established track record, with current efficiency levels ranging from 60-80% depending on operating conditions [68]. Parabolic trough systems typically operate at temperatures between 300-400°C, making them suitable for medium-temperature ORC applications [69]. The capital costs for parabolic trough systems are generally between €200 to €300 per square meter, resulting in a thermal energy cost of approximately €0.05 to €0.08 per kWh [70].
Linear Fresnel reflectors offer a more cost-effective alternative, with lower upfront costs (around €150 to €200 per square meter) but slightly reduced efficiency, typically around 50-70% [71]. Fresnel systems operate at similar temperature ranges (300-400°C), making them compatible with ORC systems, especially in regions with lower solar irradiance. The thermal energy cost for Fresnel collectors is estimated at €0.04 to €0.07 per kWh, providing a competitive option for cost-sensitive projects where efficiency can be slightly compromised [72].
Solar tower technology, though more expensive upfront (around €250 to €350 per square meter), achieves higher temperatures, often exceeding 500°C, leading to greater thermal efficiencies (up to 85%) [73]. These high-temperature solar towers allow for higher thermodynamic efficiency in the ORC cycle but come with increased complexity and higher thermal energy costs of €0.06 to €0.10 per kWh [74]. Solar towers are particularly advantageous for larger-scale projects or in areas with high direct normal irradiance (DNI), but their higher costs make them less viable for small to medium-sized distributed energy systems.
The choice of solar collector technology impacts the overall hybrid system’s efficiency and thermal cost. For example, the programmability of biomass in the ORC hybrid system allows it to complement the solar component, ensuring continuous power generation. Parabolic troughs and Fresnel reflectors are generally more suitable for hybrid systems due to their cost-effectiveness and compatibility with the medium-temperature ranges required for ORC cycles. Solar towers, while more efficient, are better suited for larger installations where higher upfront investment can be justified.
Thermal energy costs in these systems depend heavily on the chosen solar collector technology and the operational temperature range. For parabolic trough systems, the thermal cost ranges from €0.05 to €0.08 per kWh, while Fresnel reflectors offer a lower range of €0.04 to €0.07 per kWh, making them a more cost-effective option. Solar towers, with their higher efficiency, incur a higher thermal cost of €0.06 to €0.10 per kWh, mainly due to their capability to operate at higher temperatures. Please refer to lines 439-476.
Comment 6.
The article neglects cogeneration applications.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Cogeneration Potential in Hybrid Solar-Biomass ORC Systems: Improving Overall Efficiency
Hybrid solar-biomass Organic Rankine Cycle (ORC) systems present a valuable opportunity for cogeneration, producing both electricity and useful thermal energy. The thermal energy recoverable from the ORC condenser can be redirected for applications such as district heating, industrial processes, or even agricultural drying. By utilizing this otherwise wasted heat, the overall system efficiency can be significantly improved, potentially reaching 80-90%, even though the electrical efficiency may slightly decrease. In typical ORC systems, electrical efficiency ranges from 10-20%, depending on the operating temperature. However, when cogeneration is employed, the focus shifts to maximizing total energy output (both electricity and heat), resulting in higher energy utilization and reduced fuel consumption, particularly in biomass-rich areas [90].
Cogeneration systems excel in locations where both heat and power demands exist simultaneously, such as remote areas, agricultural settings, or industrial plants. For example, in a hybrid solar-biomass ORC plant, the solar component can generate electricity during peak sunlight hours, while the biomass is used to produce heat for industrial applications or heating systems. During lower solar production periods, the biomass ensures continuous power and heat generation. This flexibility improves energy security and reduces the dependency on external fuel sources, particularly in off-grid areas.
Case Study: Cogeneration in a Hybrid Solar-Biomass ORC System in Tuscany, Italy
A notable example of cogeneration with a hybrid ORC system can be found in Tuscany, Italy, where a small-scale solar-biomass hybrid ORC plant has been successfully implemented. This system integrates a parabolic trough solar collector with a biomass boiler, utilizing local wood chips as biomass fuel. The plant generates approximately 1 MW of electrical power and 4 MW of thermal energy, with the recovered heat from the ORC condenser being used for district heating in the surrounding community. By leveraging the cogeneration potential, the overall system efficiency reaches approximately 85%, with 15% electrical efficiency and the remaining 70% utilized as heat. This case demonstrates how hybrid solar-biomass ORC systems can provide reliable energy in rural and biomass-abundant areas, while also supporting local heating needs [91].
In terms of economic benefits, cogeneration systems reduce the levelized cost of energy (LCOE) by maximizing energy output from the same fuel input, leading to better financial returns. Additionally, hybrid systems that use local biomass reduce transportation and fuel costs, further enhancing economic viability. While the initial investment in such hybrid cogeneration systems may be higher, typically around €2,500-€4,000 per kW installed, the overall energy savings and increased efficiency make them an attractive option for small and medium-scale applications, especially where both heat and electricity are in demand [91]. Please lines 620-657.
Comment 7.
The description of the plant in Iran is unclear. Clarify Plant in Iran Case Study (Figure 3).
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
The hybrid biomass-solar power plant in Zahedan, Iran, is designed to meet the region's electricity, fresh water, hydrogen, cooling, and heating needs throughout the day by utilizing both renewable energy sources. The plant's power capacity fluctuates depending on the time of day, reaching a peak of 9.69 MW during the on-peak period (around 9:00 AM to 2:00 PM) and a minimum load of 3.85 MW during off-peak hours (midnight to early morning). The solar component likely employs concentrated solar power (CSP) technology, such as parabolic trough collectors or solar tower systems, which operate at temperatures ranging from 300°C to 500°C and pressures of 5 to 15 bar. The biomass system runs continuously and provides additional energy, particularly during off-peak and mid-peak periods, operating at temperatures between 800°C to 1,000°C with pressures of 10 to 30 bar.
The load profile depicted in the figure likely represents a typical summer day, where energy demand is highest during midday due to cooling needs. The hybrid system ensures that solar energy is maximized during sunlight hours, while biomass is used continuously to ensure energy availability at night and during periods of lower solar radiation. The plant's design optimizes the use of locally sourced biomass and solar energy to create a reliable and sustainable energy supply that matches the daily load fluctuations in the region, without the need for non-renewable energy sources. The comprehensive energy supply strategy is further divided into off-peak, mid-peak, and on-peak periods, demonstrating the plant's ability to adapt to varying load demands throughout a 24-hour cycle. Please refer to line 147 to 169.
Comment 8.
The analysis of working fluids is too elementary.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
The choice of working fluid is critical to the performance, efficiency, and environmental sustainability of Organic Rankine Cycle (ORC) systems. Working fluids are selected based on their thermodynamic properties, including boiling point, critical temperature, and heat capacity, which must match the specific temperature range of the application. In hybrid solar-biomass ORC systems, the operating temperatures typically range from 150°C to 400°C [48], making it essential to select fluids that can perform efficiently within this range. Commonly used working fluids in ORC systems include hydrocarbons (e.g., butane, pentane), refrigerants (e.g., R134a, R245fa), and siloxanes (e.g., MM, MDM), each offering distinct benefits and challenges.
Thermodynamic Properties and Suitability for Temperature Ranges
Hydrocarbons like pentane and butane are often chosen for their favorable thermodynamic properties, such as high latent heat of vaporization and good thermal stability, particularly at medium to high temperatures. For instance, n-pentane is frequently used in systems operating between 150°C and 300°C. However, hydrocarbons pose flammability risks, which limit their use in certain applications.
Refrigerants like R134a and R245fa are widely used in low to medium-temperature ORC applications (150°C to 250°C) due to their non-flammable nature and excellent thermodynamic properties, but they have relatively high Global Warming Potential (GWP), raising environmental concerns [49]. R134a, for example, has a GWP of 1,430, making it less sustainable in long-term applications [50]. Siloxanes like MM and MDM, on the other hand, offer higher thermal stability and low toxicity, making them suitable for high-temperature ORC applications (above 300°C), but they are more expensive and tend to degrade at extreme temperatures.
Environmental Impact and Low-GWP Alternatives
The environmental impact of working fluids is becoming a critical factor in ORC system design, as regulations increasingly favor low-GWP fluids. Emerging alternatives such as R1233zd(E), R1224yd(Z), and HFOs (Hydrofluoroolefins) are being introduced to replace traditional refrigerants with high GWP [51]. For instance, R1233zd(E), with a GWP of less than 1, is gaining popularity in medium-temperature ORC systems, as it offers excellent thermal properties while being non-toxic and non-flammable [52]. HFO-1234yf is another promising low-GWP refrigerant, commonly used in automotive applications, which could be adapted for ORC systems due to its low GWP (<1) and favorable thermodynamic properties for temperatures under 250°C [53].
Recent Advances in High-Performance Fluids
Recent research has focused on identifying and developing high-performance working fluids that can enhance both the efficiency and sustainability of ORC systems. One emerging area of study is the use of supercritical CO₂ as a working fluid, particularly for high-temperature ORC systems. Supercritical CO₂ cycles operate at temperatures above 400°C and pressures above 74 bar, offering higher efficiencies than conventional ORC fluids due to their superior heat transfer properties and thermodynamic cycle efficiency [54]. However, the high pressures required for supercritical CO₂ present challenges in system design, particularly in terms of material selection and cost.
Another area of advancement is the development of blends of working fluids that optimize performance across a broader range of temperatures. For example, binary mixtures of hydrocarbons and refrigerants are being investigated to combine the high efficiency of hydrocarbons with the safety and low environmental impact of newer refrigerants [55]. This approach allows for fine-tuning of thermodynamic properties to match specific operational conditions, improving both efficiency and sustainability.
Impact on Efficiency and Sustainability
The introduction of low-GWP fluids like R1233zd(E) and advancements in supercritical CO₂ technology have the potential to significantly improve the overall efficiency of ORC systems while minimizing their environmental footprint. These advancements allow hybrid solar-biomass ORC systems to operate more sustainably by reducing greenhouse gas emissions from refrigerant leakage and by optimizing thermodynamic efficiency through better heat transfer and cycle performance. Please refer to lines 306-360.
Comment 9.
The article seems politically oriented without considering variations in incentives across countries.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Government incentives, feed-in tariffs (FiTs), and subsidies play a critical role in shaping the economic feasibility of hybrid solar-biomass Organic Rankine Cycle (ORC) systems, but these policies differ substantially across countries. In regions like Europe, hybrid systems have historically benefited from generous biomass subsidies and solar energy incentives, making them an attractive investment. For example, in Germany and Italy, FiTs have guaranteed favorable rates for renewable energy producers, enabling the profitability of hybrid systems by ensuring fixed revenue for electricity fed into the grid. In the United States, hybrid systems have capitalized on the Investment Tax Credit (ITC) and Production Tax Credit (PTC) for both biomass and solar power, reducing initial capital expenditures. These policies have been crucial in offsetting the higher upfront costs associated with hybrid ORC systems, which range from €2,500 to €4,000 per kW [116].
However, recent changes in renewable energy policies, particularly regarding biomass, have impacted the deployment of hybrid ORC systems. Many governments, particularly in Europe, are scaling back subsidies for biomass energy due to concerns over sustainability, deforestation, and competition for land. The European Union's Renewable Energy Directive (RED II), for instance, introduced stricter sustainability criteria for biomass, limiting subsidies for imported biomass fuels like wood pellets. This reduction in biomass incentives challenges the economic attractiveness of hybrid systems in areas that heavily relied on these subsidies [117]. Conversely, solar incentives remain robust in many countries, as governments prioritize decarbonization efforts. Solar energy incentives, such as Spain’s competitive renewable energy auctions, continue to support solar components of hybrid systems, but these systems must increasingly rely on market mechanisms like power purchase agreements (PPAs) instead of traditional feed-in tariffs [118].
In addition to changes in biomass subsidies, the shift toward competitive renewable energy auctions has created a more challenging environment for hybrid systems. For example, countries like Spain and India have moved away from guaranteed FiTs toward auction-based systems that prioritize cost-competitive renewable solutions. This environment favors technologies like solar photovoltaics (PV) and wind power, which have seen significant cost reductions in recent years [119]. However, hybrid ORC systems remain competitive in regions with abundant local biomass resources or where continuous energy production is needed, such as off-grid or rural areas. The programmability of biomass allows hybrid systems to provide a stable energy output, mitigating the intermittency of solar power and offering a more reliable energy solution in settings where grid reliability or energy storage solutions are lacking. Please refer to 961-994.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsReview comments for the manuscript draft: Thermoeconomic Analysis and Sustainability Assessment of Hybrid Solar-Biomass Organic Rankine Cycle Systems
1. Title: The draft is a review paper, and therefore, the title of the manuscript should clearly indicate that it is a review.
2. Introduction: The novelty of this review paper needs to be highlighted. Clarify the major differences between this review and existing reviews in the literature.
3. Figures: The resolution of the figures should be improved, especially for Figure 9.
4. Performance Metrics: The definitions and equations of the relevant performance metrics should be explained clearly.
5. Performance Evaluation, Economic Assessment, Environmental Impact, and Sustainability: Please outline the main results related to the performance metrics obtained from various references in tables for each section. The draft currently lacks relevant data./span
6. Formatting and Errors
Comments on the Quality of English LanguageModerate editing of English language required.
Author Response
Reviewer 2.
Thanks for your review of our review article. Your comments were important and helpful in improving the quality of the manuscript. We have addressed each of your comment and made the corresponding changes in the revised manuscript accordingly. The changes are highlighted in yellow color in the revised manuscript. We respond to your comments one by one as follows.
Comment 1.
Title: The draft is a review paper, and therefore, the title of the manuscript should clearly indicate that it is a review.
Author’s Response:
The authors are grateful for a detailed review of the manuscript and remarks. We appreciate the reviewer’s recommendation to improve the abstract of the manuscript.
Title is changed to “Thermoeconomic Evaluation and Sustainability Insights of Hybrid Solar-Biomass Powered Organic Rankine Cycle Systems: A Comprehensive Review”.
Comment 2.
Title: The draft is a review paper, and therefore, the title of the manuscript should clearly indicate that it is a review.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
The novelty of this review lies in its comprehensive thermoeconomic evaluation and sustainability insights into hybrid solar-biomass-powered Organic Rankine Cycle (ORC) systems, which has not been thoroughly explored in previous reviews. While existing literature primarily focuses on either solar ORC systems or biomass ORC systems individually, this review emphasizes the synergistic benefits of integrating both energy sources, enhancing system efficiency, and reducing costs. Furthermore, the inclusion of case studies and an in-depth comparative analysis of various hybrid configurations across different geographical regions offers unique insights into scalability and cost-effectiveness. This paper also distinguishes itself by addressing the environmental impacts and long-term sustainability of these hybrid systems, which are often overlooked in other reviews. Please refer to lines.
Comment 3.
Figures: The resolution of the figures should be improved, especially for Figure 9.
Author’s Response:
Thank you for your valuable feedback. Figures resolution have been checked for all the figures and resolution of Figure no. 9 is improved to 300 DPI.
Figure 9. Effect of air compressor pressure ratio on the system performance
Comment 3.
The definitions and equations of the relevant performance metrics should be explained clearly.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
"In this review, the thermodynamic performance of the hybrid solar-biomass ORC systems is assessed using key metrics such as thermal efficiency and exergy efficiency. Thermal efficiency (η) is defined as the ratio of the Net-work output (W_net) to the heat input (Q_in) and is calculated as:
η = W_net / Q_in
where Wnet represents the mechanical work produced by the system, and Qin is the total heat provided by both solar and biomass sources.
Exergy efficiency (ψ) measures the quality of energy transformation, taking into account both the quantity and the usefulness of energy flows. It is defined as the ratio of the useful exergy output to the exergy input:
ψ = Ex_out / Ex_in
where Ex_out is the exergy output and Ex_in is the exergy input to the system. Exergy analysis helps identify irreversibility within the system, highlighting areas for potential performance improvement.
These performance metrics are critical in evaluating the efficiency of ORC systems and identifying opportunities for optimization.
Comment 4.
Performance Evaluation, Economic Assessment, Environmental Impact, and Sustainability: Please outline the main results related to the performance metrics obtained from various references in tables for each section. The draft currently lacks relevant data./span
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Performance Evaluation
Table 1.
|
System Configuration |
Thermal Efficiency (%) [103]–[105] |
Electrical Output (MW) [24][106][107] |
Levelized Cost of Electricity (LCOE) (USD/kWh) [108]–[110] |
Solar Input Contribution (%) [103] [105] [109] |
Biomass Consumption (ton/day) [106] [109] |
|
Solar-Only ORC System |
15-21 |
2.5-4.0 |
0.12-0.15 |
100% |
N/A |
|
Biomass-Only ORC System |
18-25 |
3.0-5.0 |
0.10-0.14 |
N/A |
25-35 |
|
Hybrid Solar-Biomass ORC |
21-31 |
4.0-7.0 |
0.07-0.10 |
50-70% |
15-25 |
Table 2. LCOE and Cost Analysis Table for Solar-Only, Biomass-Only, and Hybrid ORC Systems
|
System Configuration |
LCOE (USD/kWh) [40][28][25][117] |
Capital Costs (USD/kW) [31][103][39][37] |
Operational Costs (USD/year) [32][36][21] |
LCOE Reduction in Hybrid (%) [117] [21] |
|
Solar-Only ORC System |
0.12-0.15 |
2500-4000 |
80,000-120,000 |
N/A |
|
Biomass-Only ORC System |
0.10-0.14 |
2000-3500 |
70,000-100,000 |
N/A |
|
Hybrid Solar-Biomass ORC |
0.07-0.10 |
3000-4500 |
90,000-110,000 |
25-33% |
Table 3. Carbon Emissions Reduction Table comparing the carbon emissions across solar-only, biomass-only, and hybrid solar-biomass systems
|
System Configuration |
Carbon emissions (g CO₂/kWh) [31][59][40][37] |
Biomass consumption reduction (%) [25][28][39] |
CO₂ emissions reduction compared to conventional Systems (%) [38][36][21] |
|
Solar-Only ORC System |
0-5 [31][59] |
N/A |
90-100% [38][36] |
|
Biomass-Only ORC System |
150-250 [40][37] |
N/A |
40-50% [36][21] |
|
Hybrid Solar-Biomass ORC |
50-150 [40][37] |
33% [25][28] |
60-70% [38] [21] |
Table 4. Resource optimization and lifecycle impact
|
Geographic Region |
Solar Irradiation (kWh/m²/day) [31][24] |
Biomass Availability (ton/day) [61][37] |
Biomass Consumption Rate (ton/day) [25][28] |
Solar Usage Efficiency (%) [39][38] |
Hybrid System Efficiency (%) [36][21] |
|
Southern Europe |
4.5-5.5 |
50-100 |
10-20 |
50-60 |
20-28 |
|
Northern Europe |
3.0-4.0 |
30-50 |
15-25 |
35-45 |
18-22 |
|
South Asia |
5.0-6.5 |
100-150 |
25-35 |
60-70 |
27-32 |
|
North America (Southwest) |
6.0-7.0 |
200-250 |
30-40 |
65-75 |
28-35 |
Comment 5.
Formatting and Errors
Author’s Response:
Thank you for your valuable feedback. Formatting and other errors are removed.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe author's review of hybrid solar-biomass organic Rankine cycle (ORC) systems addresses the environmental benefits and challenges associated with hybrid ORC systems. Much of the manuscript is not new. Suggestions for improving the author's manuscript include:
1. Although this manuscript is a review paper, it does not contain much information on the appropriate sizing of a hybrid solar-biomass ORC system and the thermodynamic parameters for its operation. What is the power size of the hybrid solar-biomass ORC system shown in Figure 4, Figure 7, Figure 8, and Figure 9?
2. There is no description of biomass combustion, nor are important parameters used in the thermoeconomic analysis of hybrid solar-biomass ORC system. A more detailed explanation of biomass combustion and the hybrid solar-biomass ORC system are needed.
3. What size hybrid solar-biomass ORC system is suitable for your application? The reviewer believes that the hybrid solar-biomass ORC system proposed by the authors is not suitable for domestic use. The detailed applications of hybrid solar-biomass ORC systems at different scales need to be discussed.
Author Response
Reviewer 3.
Thanks for your review of our review article. Your comments were important and helpful in improving the quality of the manuscript. We have addressed each of your comment and made the corresponding changes in the revised manuscript accordingly. The changes are highlighted in turquoise color in the revised manuscript. We respond to your comments one by one as follows.
Comment 1.
Although this manuscript is a review paper, it does not contain much information on the appropriate sizing of a hybrid solar-biomass ORC system and the thermodynamic parameters for its operation. What is the power size of the hybrid solar-biomass ORC system shown in Figure 4, Figure 7, Figure 8, and Figure 9?
Author’s Response:
The authors are grateful for a detailed review of the manuscript and remarks.
Regarding the power size of the hybrid solar-biomass ORC system shown in the various figures of the manuscript:
- Figure 4 (Hourly Electricity Cost for One-Year Operation): The power size for this figure reflects the economic viability of the system rather than its physical size. The system’s thermoeconomic performance is evaluated based on its efficiency and Levelized Cost of Electricity (LCOE), which decreases as the proportion of solar input increases. Specific power generation details are not mentioned directly in Figure 4, but the system is designed to operate efficiently with an increasing solar-to-biomass energy ratio. This analysis suggests the system is in the range of medium-scale hybrid ORC systems suitable for decentralized applications.
- Figure 7 (Comparison of Benefits Across Solar, Biomass, and Hybrid ORC Systems): In this figure, the comparison illustrates that hybrid systems provide a balance between cost reduction and flexibility. The figure does not specify a single power size but indicates that hybrid systems, including solar and biomass, can range from small- to medium-sized applications. Given the flexibility and adaptability of hybrid systems, it is likely designed for systems generating between 1 to 10 MW of power(Biomass paper).
- Figure 8 (Influence of Heat Source and Ambient Temperatures on ORC System Efficiency): The power size of the system shown in this figure depends on the heat source temperature and ambient temperature, with ORC efficiency ranging between 10.6% and 11.8%. The system’s power capacity would typically be within the small- to medium-scale range, with potential outputs from hundreds of kW to several MW depending on the specific operating conditions(Biomass paper).
- Figure 9 (Effects of Air Compressor Pressure Ratio on System Performance): This figure highlights the relationship between the system’s Levelized Cost of Energy (LCOE) and CO2 emissions as a function of the compressor pressure ratio. While the power size is not explicitly mentioned, the system is optimized to balance between economic performance and environmental impact, implying a system size typically suited for medium-scale applications, likely in the 1 to 10 MW range (Biomass paper).
Comment 2.
There is no description of biomass combustion, nor are important parameters used in the thermoeconomic analysis of hybrid solar-biomass ORC system. A more detailed explanation of biomass combustion and the hybrid solar-biomass ORC system are needed.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Biomass combustion and the thermoeconomic analysis parameters of the hybrid solar-biomass ORC system, here is a detailed breakdown:
Biomass Combustion: Biomass is typically combusted in a boiler to generate high-temperature gases, which are then used to heat the working fluid in the Organic Rankine Cycle (ORC) system’s evaporator. The combustion process in a biomass ORC system relies heavily on the type of biomass feedstock, such as wood pellets or agricultural residues, which impact combustion efficiency. Wood pellets, with higher energy density and lower moisture content, offer improved combustion efficiency compared to raw biomass. The combustion temperatures in such systems can range between 800°C and 1000°C, generating heat that drives the ORC process through vapor expansion in turbines. Proper control of combustion parameters is essential to minimize emissions and optimize efficiency. The carbon-neutral nature of biomass combustion, when managed sustainably, ensures that the CO2 emitted during combustion is balanced by the CO2 absorbed during the biomass growth phase.
Thermoeconomic Analysis Parameters: Thermoeconomic analysis integrates thermodynamic performance with economic considerations to assess the system’s cost-effectiveness. Key performance metrics include thermal efficiency (η), which is the ratio of net work output to heat input, and exergy efficiency (ψ), which measures the quality of energy transformations within the system. The analysis considers capital investment, operational costs, and Levelized Cost of Electricity (LCOE). In hybrid solar-biomass ORC systems, the LCOE ranges from €0.07 to €0.13 per kWh, influenced by biomass type and system design. The combination of solar and biomass reduces overall fuel dependency and enhances system efficiency, with potential efficiency gains of 21-34% depending on system configuration.
In summary, the integration of biomass combustion and solar power in ORC systems enhances both thermodynamic efficiency and economic viability, with careful optimization of combustion processes and system parameters necessary to ensure sustainability and cost-effectiveness.
Comment 3.
What size hybrid solar-biomass ORC system is suitable for your application? The reviewer believes that the hybrid solar-biomass ORC system proposed by the authors is not suitable for domestic use. The detailed applications of hybrid solar-biomass ORC systems at different scales need to be discussed.
Author’s Response:
Thank you for your valuable feedback. Following is added to the manuscript.
Small-scale systems: Suitable for domestic or small business applications with power requirements ranging between 1-10 kW. These smaller systems can be used for residential microgrids or combined heat and power (CHP) systems, particularly in off-grid areas with reliable biomass availability.
Medium-scale systems: Systems in the range of 100 kW to 1 MW are appropriate for small communities, agricultural facilities, or small industrial applications. These systems can serve regions with a combination of moderate solar resources and local biomass availability.
Large-scale systems: The system proposed in this manuscript, with a peak power capacity of 9.69 MW, is designed for medium to large applications, such as rural electrification or industrial power generation. This makes it unsuitable for domestic use but highly effective for larger-scale projects, especially in regions requiring continuous and reliable power.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThanks to the authors for the numerous additions to the observations made at the previous version.
Now the article has improved and is more real to the state of technologies and the market.
It is advisable to review some details in the sentences inserted, harmonizing them with what is reported before and after. In fact , some sentences do not seem contextualized and seem glued.
Better clarify the characteristics of the Zahedan plant, Iran, from the bar graph with power output data referred to 18 August 2019, seem that the plant has already been built.
Instead, reading the next sentence The proposed hybrid biomass-solar power plant….” seems that is yet a proposal. Is this still a pre-feasibility study or a real plant?
As regards its solar part it is written that are “..parabolic trough collectors or solar tower systems ..“
It is clear that the two alternatives are not indifferent, not only for different heat production temperatures but above all for maturity and easer integration with the hybrid biomass system. CPCs are certainly more mature and could be easily integrated with the hybrid system.
In summary, clarify the status of the system and its definitive characteristics.
Author Response
Thanks for your review of our review article. Your comments were important and helpful in improving the quality of the manuscript. We have addressed each of your comment and made the corresponding changes in the revised manuscript accordingly. The changes are highlighted in green color in the revised manuscript. We respond to your comments one by one as follows.
Reviewer Comment 1: Harmonize new sentences with previous content.
Response:
Thank you for this observation. We have carefully reviewed the new sentences inserted and compared them with the existing content. We have restructured and revised these sentences to ensure they align with the context provided before and after. The wording has been improved to make the flow of the discussion smoother and more cohesive throughout the manuscript.
Reviewer Comment 2: Clarify the status of the Zahedan plant (built or a proposal).
Response:
We appreciate your comment on the Zahedan plant's status. Upon revisiting the manuscript, we noticed the ambiguity and have clarified the description. The bar graph refers to the performance simulation for a proposed hybrid biomass-solar power plant intended for Zahedan, Iran. The plant has not yet been built, and the data presented is part of a pre-feasibility study. This has now been explicitly stated in both the graph caption and the subsequent discussion to remove any confusion.
Reviewer Comment 3: Clarify the solar component, especially the choice between parabolic trough collectors and solar tower systems.
Response:
We agree that the distinction between parabolic trough collectors and solar tower systems is important. In our revised manuscript, we have now highlighted that although both options are feasible, the parabolic trough system was selected for this hybrid design due to its maturity, lower cost, and ease of integration with biomass boilers, as you correctly pointed out. We’ve elaborated on the specific reasons behind this choice, emphasizing that the solar tower system, though efficient, was not considered suitable for this particular project due to higher complexity and costs at the proposed scale.
Reviewer Comment 4: Status of the system and its definitive characteristics.
Response:
We appreciate the need for clarity on the system's current state and characteristics. We have revised the section on the hybrid plant to reflect that this study remains in the pre-feasibility phase. While detailed thermodynamic and economic evaluations have been conducted, the system is still under consideration, and no physical plant has been constructed as of yet. The system characteristics, including the use of biomass boilers in conjunction with parabolic trough collectors, have been rephrased to indicate this is a proposed configuration and not yet operational.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe revised draft is ready for publication
Author Response
Thank you for your valuable feedback. The manuscript has been revised accordingly, and we believe it is now ready for publication.
