Techno-Economic Assessment of an Innovative Small-Scale Solar-Biomass Hybrid Power Plant

Citation: Vélez Godiño, J.A.; Torres García, M. Techno-Economic Assessment of an Innovative Small-Scale Solar-Biomass Hybrid Power Plant. Appl. Sci. 2023,13, 8179. https://doi.org/10.3390/app13148179 Academic Editor: Andreas Sumper Received: 13 June 2023 Revised: 10 July 2023 Accepted: 11 July 2023 Published: 13 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Techno-Economic Assessment of an Innovative Small-Scale Solar-Biomass Hybrid Power Plant JoséAntonio Vélez Godiño 1,* and Miguel Torres García2 1Departamento de Máquinas y Motores Térmicos, Escuela Superior de Ingeniería, Universidad de Cádiz, Avda. Universidad de Cádiz, n◦10, 11519 Puerto Real (Cádiz), Spain 2Departamento de Ingeniería Energética, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Seville, Spain *Correspondence: [email protected] Abstract: The current context of the climate emergency highlights the need for the decarbonization of the energy sector by replacing current fossil fuels with renewable energy sources. In this regard, concentrating solar power (CSP) technology represents a commercially proven alternative. However, these types of plants are associated with high production costs and difficulties in controlling production during temporary variations in solar resource availability. In order to minimize these drawbacks, this study proposes the hybridization of CSP technology with direct biomass combustion, with the particularity of an innovative process scheme that does not correspond to traditional series or parallel configurations. This paper focuses on the techno-economic evaluation of this novel configuration in a small-scale power plant. To achieve this, both solar resource and biomass production, which are dependent on the selected location, were analyzed. Additionally, the plant was characterized from both technical and economic perspectives. The obtained results allowed for the characterization of the Levelized Cost of Energy (LCOE) based on various parameters such as the size of the solar field and biomass boilers, as well as limitations on biomass consumption. Keywords: renewable energy; CSP; biomass; hybrid power generation; LCOE 1. Introduction The need to limit the impact of the power generation sector on the environment is clearly identified in the UN’s Sustainable Development Goals, which propose actions in the areas of energy, the atmosphere, and climate change [ 1 ]. Currently, there is evidence of a nearly linear relationship between cumulative greenhouse gas (GHG) emissions and global temperature variation [ 2 ]. In such a scenario, there has been a growing interest in the development and application of cleaner and more sustainable power generation technologies [ 3 ]. Renewable energies constitute a technically feasible alternative, allowing for the simultaneous reduction of GHG emissions. Unlike other renewable technologies such as wind or photovoltaic, concentrating solar power (CSP) plants are currently not economically competitive compared to fossil fuel-based power generation due to their high production costs [ 4 , 5 ]. Additionally, CSP technology has an added disadvantage in managing electricity production due to the high variability of solar resources, which necessitates costly thermal energy storage systems [ 6 ]. However, there is a wide consensus in the scientific community that one of the most promising alternatives for reducing the costs of CSP plants is hybridization with other lowercost generation technologies [ 7 ], such as direct biomass combustion, which is considered carbon-neutral [ 8 , 9 ]. The cost reduction attributed to CSP-biomass hybridization stems from the shared use of infrastructure and common equipment [ 10 ]. In addition to the aforementioned production cost reduction, hybridization contributes to improving the dispatchability of electricity generation compared to conventional CSP plants, which are more or less subject to the randomness of solar resources [ 11 , 12 ]. Furthermore, other Appl. Sci. 2023,13, 8179. https://doi.org/10.3390/app13148179 https://www.mdpi.com/journal/applsci Appl. Sci. 2023,13, 8179 2 of 16 additional advantages derived from CSP-biomass hybridization include improved thermal cycle efficiency and reduced environmental impact due to the smaller solar field size and lower water consumption [13]. While most publications regarding CSP-biomass hybridization refer to theoretical studies [ 14 – 16 ], the most notable milestone in this field is the Termosolar Borges project in Spain. This 22.5 MW capacity plant, operational since 2012, was the first commercial plant to use biomass from agricultural residues to complement solar resource generation through parallel hybridization [ 17 ]. The Brønderslev plant in Denmark, although smaller in size (4 MW), is also noteworthy [18]. Economic analysis is a fundamental part of any feasibility study, especially when addressing projects of an innovative nature [ 19 ]. In the field of power generation plants, the Levelized Cost of Energy (LCOE) serves as the benchmark indicator for comparing different technologies [ 20 , 21 ] and as a guide for design optimization [ 22 , 23 ]. However, the LCOE has limitations, such as not taking into account the dispatchability of electricity production when evaluating certain renewable technologies [ 24 , 25 ]. In the specific case of CSP-biomass hybridization, noteworthy economic analyses were conducted by Milani et al. [26] and Bai et al. [27]. The objective of this study was to assess the economic viability of a CSP-biomass hybrid plant based on an innovative series/parallel-process scheme. The hypothetical power plant analyzed in this work had a capacity corresponding to a small scale, with a chosen gross generation capacity of 2 MW. This study was structured into three parts. First, the materials and methods applied in this work, including the influence of the selected location, technical plant characterization, and economic analysis details, are presented. Next, the obtained results are analyzed, encompassing daily simulations, annual balances, and the calculation of the LCOE. Finally, the conclusions drawn from this analysis are presented. 2. Materials and Methods 2.1. Plant Location Characterization The development of the feasibility analysis for the hybrid plant under study required the characterization of both the available solar resources and the biomass production. However, these two parameters are strongly dependent on the selected plant location. Therefore, the decision made regarding the chosen location for this study is justified below. While selecting a location for a conventional power plant is always a critical decision, the complexity of this decision increases in the case of a CSP-biomass hybrid power plant. This added complexity arises from the need for high availability of both of the two primary energy sources used for electricity generation. In other words, in the case of the plant under study, high levels of both annual irradiation and biomass production had to be ensured. For this reason, the region of Andalusia (southern Spain) was selected as an optimal location for a plant of this type. From the perspective of solar resources (Figure 1), the most attractive areas in Andalusia are the Guadalquivir Valley (western Andalusia) and the province of Almería (eastern Andalusia), both of which have annual direct normal irradiation values exceeding 1900 kWh/m2. Appl. Sci. 2023, 13, x FOR PEER REVIEW 3 of 17 Figure 1. Average daily global horizontal irradiation (kWh/m 2 ) [28]. On the other hand, Andalusia is a region with a high capacity for biomass generation with an estimated annual production of 20.1 million tons, encompassing sources such as agriculture, forestry, industry, energy crops, and urban and industrial waste. However, the technical characterization of the hybrid plant required the selection of a specific type of biomass, as this aspect greatly influences the design of critical components such as biomass boilers. Agricultural biomass represents 33% of the total energy potential of biomass produced in Andalusia, with olive crop residues (2.5 million tons/year) being particularly significant. However, due to its lower chlorine content, which allows for higher working temperatures in the boilers, the chosen biomass type corresponded to forest residues. This type, consisting of residues from the maintenance (pruning and cutting) of forest stands mainly composed of pine (pinus), eucalyptus (eucalyptus), and oak (quercus), has an annual production of 1.3 million tons, with the province of Huelva (western Andalusia) contributing 50% of it (Figure 2). Figure 2. Forest biomass production in Andalusia (green: pinus, orange: eucalyptus, brown: quercus) [29]. The combined analysis of available solar resources and biomass led to the conclusion that the western region of Andalusia combines high levels of both resources. For this reason, a site corresponding to the latitude 37.25° north and longitude 6.50° west was chosen as the hypothetical plant location. Once the exact location of the hybrid plant under study was determined, a typical design year was developed based on the information available in the NSRDB (NREL) [30,31], which provides representative values of various meteorological variables. Among these variables, direct normal irradiance (DNI) is worth noting, and its annual evolution is shown in Figure 3. The typical design year had a resolution of 10 min, as can be observed Figure 1. Average daily global horizontal irradiation (kWh/m2) [28]. Appl. Sci. 2023,13, 8179 3 of 16 On the other hand, Andalusia is a region with a high capacity for biomass generation with an estimated annual production of 20.1 million tons, encompassing sources such as agriculture, forestry, industry, energy crops, and urban and industrial waste. However, the technical characterization of the hybrid plant required the selection of a specific type of biomass, as this aspect greatly influences the design of critical components such as biomass boilers. Agricultural biomass represents 33% of the total energy potential of biomass produced in Andalusia, with olive crop residues (2.5 million tons/year) being particularly significant. However, due to its lower chlorine content, which allows for higher working temperatures in the boilers, the chosen biomass type corresponded to forest residues. This type, consisting of residues from the maintenance (pruning and cutting) of forest stands mainly composed of pine (pinus), eucalyptus (eucalyptus), and oak (quercus), has an annual production of 1.3 million tons, with the province of Huelva (western Andalusia) contributing 50% of it (Figure 2). Appl. Sci. 2023, 13, x FOR PEER REVIEW 3 of 17 Figure 1. Average daily global horizontal irradiation (kWh/m 2 ) [28]. On the other hand, Andalusia is a region with a high capacity for biomass generation with an estimated annual production of 20.1 million tons, encompassing sources such as agriculture, forestry, industry, energy crops, and urban and industrial waste. However, the technical characterization of the hybrid plant required the selection of a specific type of biomass, as this aspect greatly influences the design of critical components such as biomass boilers. Agricultural biomass represents 33% of the total energy potential of biomass produced in Andalusia, with olive crop residues (2.5 million tons/year) being particularly significant. However, due to its lower chlorine content, which allows for higher working temperatures in the boilers, the chosen biomass type corresponded to forest residues. This type, consisting of residues from the maintenance (pruning and cutting) of forest stands mainly composed of pine (pinus), eucalyptus (eucalyptus), and oak (quercus), has an annual production of 1.3 million tons, with the province of Huelva (western Andalusia) contributing 50% of it (Figure 2). Figure 2. Forest biomass production in Andalusia (green: pinus, orange: eucalyptus, brown: quercus) [29]. The combined analysis of available solar resources and biomass led to the conclusion that the western region of Andalusia combines high levels of both resources. For this reason, a site corresponding to the latitude 37.25° north and longitude 6.50° west was chosen as the hypothetical plant location. Once the exact location of the hybrid plant under study was determined, a typical design year was developed based on the information available in the NSRDB (NREL) [30,31], which provides representative values of various meteorological variables. Among these variables, direct normal irradiance (DNI) is worth noting, and its annual evolution is shown in Figure 3. The typical design year had a resolution of 10 min, as can be observed Figure 2. Forest biomass production in Andalusia (green: pinus, orange: eucalyptus, brown: quercus) [ 29 ]. The combined analysis of available solar resources and biomass led to the conclusion that the western region of Andalusia combines high levels of both resources. For this reason, a site corresponding to the latitude 37.25 ◦ north and longitude 6.50 ◦ west was chosen as the hypothetical plant location. Once the exact location of the hybrid plant under study was determined, a typical design year was developed based on the information available in the NSRDB (NREL) [30,31] , which provides representative values of various meteorological variables. Among these variables, direct normal irradiance (DNI) is worth noting, and its annual evolution is shown in Figure 3. The typical design year had a resolution of 10 min, as can be observed in Figure 4, which displays the temporal evolution of DNI for the four days corresponding to the two equinoxes and the two solstices. Lastly, the typical design year revealed an annual direct normal irradiation of 2150 kWh/m 2 with an uncertainty of less than 10%, making it highly suitable for the implementation of a CSP plant. Appl. Sci. 2023, 13, x FOR PEER REVIEW 4 of 17 in Figure 4, which displays the temporal evolution of DNI for the four days corresponding to the two equinoxes and the two solstices. Lastly, the typical design year revealed an annual direct normal irradiation of 2150 kWh/m2 with an uncertainty of less than 10%, making it highly suitable for the implementation of a CSP plant. Figure 3. Direct normal irradiance (DNI, W/m2) of the design year. Figure 4. Hourly evolution of direct normal irradiance (DNI, W/m2) for 4 representative days. Regarding the impact of the hybrid plant’s location on the availability of forest biomass, it is worth noting that within a radius of 25 km, a production of 146 kton/year was quantified. The maximum distance of 25 km chosen as a reference was based on the fact that biomass, unlike traditional fossil fuels, has a large volume, which directly affects transportation costs [32]. 2.2. Technical Characterization Below are the most characteristic parameters of the plant. In terms of the power cycle, since the gross electrical power of the plant was 2 MW, it is worth noting that a non-re- generative Rankine cycle without intermediate reheating was employed. Table 1 summarizes the main characteristics of the Rankine cycle considered. DAY #79 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #172 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #266 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #355 01020 TIME (h) 0 500 1000 DNI (W/m2) Figure 3. Direct normal irradiance (DNI, W/m2) of the design year. Appl. Sci. 2023,13, 8179 4 of 16 Appl. Sci. 2023, 13, x FOR PEER REVIEW 4 of 17 in Figure 4, which displays the temporal evolution of DNI for the four days corresponding to the two equinoxes and the two solstices. Lastly, the typical design year revealed an annual direct normal irradiation of 2150 kWh/m2 with an uncertainty of less than 10%, making it highly suitable for the implementation of a CSP plant. Figure 3. Direct normal irradiance (DNI, W/m2) of the design year. Figure 4. Hourly evolution of direct normal irradiance (DNI, W/m2) for 4 representative days. Regarding the impact of the hybrid plant’s location on the availability of forest biomass, it is worth noting that within a radius of 25 km, a production of 146 kton/year was quantified. The maximum distance of 25 km chosen as a reference was based on the fact that biomass, unlike traditional fossil fuels, has a large volume, which directly affects transportation costs [32]. 2.2. Technical Characterization Below are the most characteristic parameters of the plant. In terms of the power cycle, since the gross electrical power of the plant was 2 MW, it is worth noting that a non-re- generative Rankine cycle without intermediate reheating was employed. Table 1 summarizes the main characteristics of the Rankine cycle considered. DAY #79 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #172 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #266 01020 TIME (h) 0 500 1000 DNI (W/m2) DAY #355 01020 TIME (h) 0 500 1000 DNI (W/m2) Figure 4. Hourly evolution of direct normal irradiance (DNI, W/m2) for 4 representative days. Regarding the impact of the hybrid plant’s location on the availability of forest biomass, it is worth noting that within a radius of 25 km, a production of 146 kton/year was quantified. The maximum distance of 25 km chosen as a reference was based on the fact that biomass, unlike traditional fossil fuels, has a large volume, which directly affects transportation costs [32]. 2.2. Technical Characterization Below are the most characteristic parameters of the plant. In terms of the power cycle, since the gross electrical power of the plant was 2 MW, it is worth noting that a non-regenerative Rankine cycle without intermediate reheating was employed. Table 1 summarizes the main characteristics of the Rankine cycle considered. Table 1. Main characteristics of the Rankine cycle. Parameter Value Units Live steam temperature 400 ◦C Live steam pressure 41 bar Live steam flow rate 2.8 kg/s Turbine isentropic efficiency 0.85 - Condenser pressure 0.1 bar Feedwater temperature 105 ◦C Next, the parameters corresponding to the solar field of the parabolic trough collectors are presented. The solar field consisted of loops of 4 collectors with a length of 150 m each, and their main characteristics are shown in Table 2[ 33 ]. The collectors were aligned along the north–south axis. Although the final size of the solar field was determined through a techno-economic optimization process, it was initially considered that the design DNI of the solar field was 850 W/m 2 and was the optical efficiency corresponding to solar noon on the summer solstice. The heat transfer fluid used in the solar field was a synthetic oil. Specifically, a eutectic mixture of 73.5% diphenyl oxide and 26.5% diphenyl was considered [ 34 ]. Due to the temperature limitations of the heat transfer fluid in the boundary layer of the absorber tube, a minimum DNI of 150 W/m 2 was required for the operation of the solar field. The analyzed plant did not have a thermal energy storage system, as one of the aims was to demonstrate the production management capability derived from the proposed process scheme. Appl. Sci. 2023,13, 8179 5 of 16 Table 2. Main characteristics of the of the parabolic trough collectors. Parameter Value Units Aperture width 5.77 m Aperture area 817.5 m2 Focal length 1.71 m Absorber radius 3.5 cm Concentration factor 82.43 - Peak optical performance 0.73 - Fouling factor 0.90 - In terms of the hybridization configuration for steam generation, the solar steam generator (SSG) consisted of shell-and-tube heat exchangers, while the boilers used for the proposed plant were based on the cooled grate burner technology. Given that the proposed hybrid scheme involved operating the biomass boilers at partial load, it was necessary to characterize their performance. Within the range from their nominal power to 75% load, the biomass boilers are capable of generating steam at nominal conditions, with only the mass flow rate being modified. In the range from 75% to 50% load, in addition to reducing the steam mass flow rate, the boilers are unable to generate steam at nominal conditions. Instead, it was assumed that there is a linear decrease in temperature from the nominal temperature (at 75% load) to the saturation temperature (at 50% load). Finally, within the range from 50% load to the minimum load (20%), the biomass boilers generate saturated steam. The design inputs for the biomass boilers were completed with the assumption of a combustion efficiency of 88%, an excess air factor of 30%, and a furnace exit gas temperature (FEGT) of 700 ◦ C. This temperature could be reached because the chosen biomass type was forest residue, which has a lower heating value of 14.63 MJ/kg on a dry basis. The main decision regarding the technical characterization of the plant in this study was determining the hybridization configuration of the two technologies used for steam generation. Traditionally, hybrid plants have been based on either series or parallel designs. Below are some of the most relevant advantages and disadvantages of these two types of hybrid configuration: • The main advantage of series hybridization is that it allows for an increase in the live steam temperature of the Rankine cycle, which contributes to improving the thermal efficiency of the plant [ 35 ]. Additionally, this compensates the limitation of the maximum allowable temperature of the synthetic oil employed as heat transfer fluid in current parabolic trough solar power plants. One disadvantage of series hybridization is that the biomass boiler and the solar steam generator must operate simultaneously, limiting the operating time of the hybrid plant and linking its operation to the availability of solar radiation. This does not improve the dispatchability of the energy supplied to the electrical distribution grid and negatively affects the profitability of the plant. • Although parallel hybridization designs do not improve the efficiency of the hybrid plant compared to conventional parabolic trough solar plants, this configuration allows the hybrid plant to operate during periods of null or low solar radiation [ 36 ]. This improvement has a very positive influence on the dispatchability of the electrical supply, decoupling it from the random nature of solar resources. Additionally, the increase in operating hours of this configuration has a positive impact on the profitability of the plant. While initially considering only the possibility of choosing between series or parallel hybridization between the solar steam generator and the biomass boiler, a more complex configuration was ultimately adopted, consisting of a mixed series/parallel hybridization (Figure 5). It can be observed that the SSG was supported by a biomass boiler in parallel (Boiler 2). In this way, Boiler 2 compensated for fluctuations in solar resource availability. Additionally, an additional biomass boiler (Boiler 1) was proposed in the series with the Appl. Sci. 2023,13, 8179 6 of 16 SSG and Boiler 2 to improve the quality of the generated steam. The combustion products used in the superheater of Boiler 1 came from a furnace, which needed to be cooled. For this reason, the furnace of Boiler 1 also acted as an evaporator located in parallel with the evaporators of the SSG and Boiler 2. Finally, the thermal energy from the combustion products at the outlet of the superheater of Boiler 1 was utilized in an economizer located upstream of the SSG and Boiler 2. This achieved higher process efficiency by reducing the stack temperature of the flue gases in Boiler 1. The proposed configuration, although more complex than traditional hybridizations, simultaneously allowed for control of the dispatchability of the power plant and for the improvement of the live steam parameters, resulting in improved thermal performance. Furthermore, the proposed hybridization scheme enabled a significant increase in the operating hours of the hybrid plant compared to a conventional CSP plant, without the need for expensive thermal energy storage systems. Lastly, compared to conventional biomass plants, the proposed hybrid plant reduced biomass consumption, thereby reducing its global carbon footprint. Although the present study did not consider it, in the future, the possibility of extending the hybridization to include biomass treatment will be evaluated, for instance, through the use of solar-based drying technologies as proposed by Kumar et al. [37]. Appl. Sci. 2023, 13, x FOR PEER REVIEW 7 of 17 Figure 5. Process diagram of the proposed power plant (water/steam side). The following economic analysis took into account different designs of CSP-biomass hybrid power plants, all based on the previously described concept of mixed hybridization. The various designs considered variations in the size of the solar field, characterized by the Solar Multiple (SM); as well as the thermal power of Boiler 2, characterized by the Biomass Fill Fraction (BFF). The SM represents the ratio between the surface area actually considered for the solar field and the surface area of the solar field that, under the design conditions (850 W/m 2 , noon of the summer solstice), provides the nominal thermal power. On the other hand, the BFF corresponds to the percentage of electrical power that can be achieved by operating solely with biomass in the complete absence of solar radiation. 2.3. Economic Analysis The economic viability analysis of the hybrid power plant proposed in this study was based on the assessment of the LCOE, commonly used in evaluating the profitability of power production technologies [38,39]. The suitability of this parameter lies in comparing the present value of investment costs (CAPEX), operation and maintenance costs (OPEX), and financing costs with the annual energy production [40]: 𝐿𝐶𝑂𝐸  𝐿𝐶𝐶 ∑𝐸 󰇛1𝑑󰇜   (1) where the term LCC corresponds to life cycle cost, E is the annual energy production (assumed constant throughout the project’s lifetime), N is the project’s lifetime period, and d is the discount rate considered. In turn, the LCC is obtained from the initial investment (I) and the present value of the annual operation and maintenance costs (C&), which also include costs associated with project financing, insurance, and taxes: 𝐿𝐶𝐶 𝐼 𝐶 &  󰇛1𝑑󰇜   (2) The analysis of the LCOE allows for the determination of a theoretical constant price throughout the project’s lifetime (EUR/kWh). The CAPEX includes costs such as development, consenting, engineering, procurement, construction, and commissioning, while the OPEX encompasses both direct and indirect costs [41]. Both the investment required for the plant’s execution and the operation and maintenance costs (Table 3) were characterized using the System Advisor Model [42]. The main financial and tax parameters considered for the LCOE analysis are presented in Table 4. Figure 5. Process diagram of the proposed power plant (water/steam side). The following economic analysis took into account different designs of CSP-biomass hybrid power plants, all based on the previously described concept of mixed hybridization. The various designs considered variations in the size of the solar field, characterized by the Solar Multiple (SM); as well as the thermal power of Boiler 2, characterized by the Biomass Fill Fraction (BFF). The SM represents the ratio between the surface area actually considered for the solar field and the surface area of the solar field that, under the design conditions (850 W/m 2 , noon of the summer solstice), provides the nominal thermal power. On the other hand, the BFF corresponds to the percentage of electrical power that can be achieved by operating solely with biomass in the complete absence of solar radiation. 2.3. Economic Analysis The economic viability analysis of the hybrid power plant proposed in this study was based on the assessment of the LCOE, commonly used in evaluating the profitability of power production technologies [ 38 , 39 ]. The suitability of this parameter lies in comparing the present value of investment costs (CAPEX), operation and maintenance costs (OPEX), and financing costs with the annual energy production [40]: LCOE =LCC ∑N t=1Et (1+d)t (1) where the term LCC corresponds to life cycle cost, Et is the annual energy production (assumed constant throughout the project’s lifetime), N is the project’s lifetime period, and Appl. Sci. 2023,13, 8179 7 of 16 d is the discount rate considered. In turn, the LCC is obtained from the initial investment ( I0 ) and the present value of the annual operation and maintenance costs ( CO&M ), which also include costs associated with project financing, insurance, and taxes: LCC =I0+ N ∑ t=1 CO&M (1+d)t(2) The analysis of the LCOE allows for the determination of a theoretical constant price throughout the project’s lifetime (EUR/kWh). The CAPEX includes costs such as development, consenting, engineering, procurement, construction, and commissioning, while the OPEX encompasses both direct and indirect costs [ 41 ]. Both the investment required for the plant’s execution and the operation and maintenance costs (Table 3) were characterized using the System Advisor Model [ 42 ]. The main financial and tax parameters considered for the LCOE analysis are presented in Table 4. Table 3. Cost assumptions for economic analysis. Parameter Value Units Site improvement 30 EUR/m2 Solar field 175 EUR/m2 HTF system 75 EUR/m2 Biomass system 1750 EUR/kWe Power cycle 1100 EUR/kWe Balance of plant 400 EUR/kWe Contingency 10 % EPC cost 10 % O&M fixed cost 66 EUR/(kW·yr) O&M variable cost (excl. biomass) 5 EUR/MWh Table 4. Financial and tax conditions. Parameter Value Units Annual inflation rate 2.5 % Income tax rate 25.0 % Annual loan interest 4.0 % Debt fraction 70.0 % Plant life cycle 30 years As shown in Table 4, no incentives such as subsidies or tax exemptions were considered. Additionally, the depreciation of assets followed a linear model applied over the plant’s lifetime. Furthermore, the debt return period varied for each analyzed case. Although its impact was not significant, the objective was to minimize the duration of this period while ensuring positive cash flows at all times. An annual availability of 95% and parasitic losses of 10% were assumed. Regarding biomass consumption, which constituted the main operating and maintenance costs of the hybrid plant, a cost of EUR 65/ton was considered. This cost included the acquisition, preprocessing (chipping), and transportation of forest biomass to the plant. To conclude with the characterization of the applied methodology, Figure 6shows a workflow that summarizes the different stages that were addressed in carrying out this work. Figure 6shows the input data, the most significant calculation processes, the decisions and, finally, the results generated. Appl. Sci. 2023,13, 8179 8 of 16 Appl. Sci. 2023, 13, x FOR PEER REVIEW 8 of 17 Table 3. Cost assumptions for economic analysis. Parameter Value Units Site improvement 30 EUR/m 2 Solar field 175 EUR/m 2 HTF system 75 EUR/m 2 Biomass system 1750 EUR/kWe Power cycle 1100 EUR/kWe Balance of plant 400 EUR/kWe Contingency 10 % EPC cost 10 % O&M fixed cost 66 EUR/(kW·yr) O&M variable cost (excl. biomass) 5 EUR/MWh Table 4. Financial and tax conditions. Parameter Value Units Annual inflation rate 2.5 % Income tax rate 25.0 % Annual loan interest 4.0 % Debt fraction 70.0 % Plant life cycle 30 years As shown in Table 4, no incentives such as subsidies or tax exemptions were considered. Additionally, the depreciation of assets followed a linear model applied over the plant’s lifetime. Furthermore, the debt return period varied for each analyzed case. Although its impact was not significant, the objective was to minimize the duration of this period while ensuring positive cash flows at all times. An annual availability of 95% and parasitic losses of 10% were assumed. Regarding biomass consumption, which constituted the main operating and maintenance costs of the hybrid plant, a cost of EUR 65/ton was considered. This cost included the acquisition, preprocessing (chipping), and transportation of forest biomass to the plant. To conclude with the characterization of the applied methodology, Figure 6 shows a workflow that summarizes the different stages that were addressed in carrying out this work. Figure 6 shows the input data, the most significant calculation processes, the decisions and, finally, the results generated. Figure 6. Workflow corresponding to the applied methodology. Figure 6. Workflow corresponding to the applied methodology. 3. Results and Discussion 3.1. Simulation As mentioned earlier in Section 2.2, for the development of this analysis, different designs of hybrid plants were considered, varying both the SM and the BFF. Given the relatively small size of the proposed plant in this study, only two realistic values of BFF were considered. This is due to the technical difficulty of implementing biomass boilers with cooled grate burners on a very small scale. Therefore, two alternatives were analyzed regarding the size of Boiler 2, one corresponding to a BFF of 100% and another to a BFF of 50%. Regarding the size of the solar field, SM values from 0.5 to 2.5 were considered in intervals of 0.5. Figures 7and 8show the hourly evolution of both the biomass consumption and gross electric power generated for the same four representative days previously shown in Figure 4. Both figures assume SM = 1. However, Figure 7considers BFF = 100%, while Figure 8considers BFF = 50%. The analysis of Figure 7(BFF = 100%) highlights the complete independence of the generated power from the solar resource, even on days with high variability such as days #172 and #355. By regulating the mass flow rate of the biomass consumed in Boiler 2, the steam turbine can maintain operation at its nominal power. It can be observed that during periods of null or low solar resource availability, biomass consumption increases, compensating for the deficit of thermal energy from the solar field. When the BFF is reduced to 50% (Figure 8), notable differences compared to Figure 7are observed. The first difference is the inability to maintain the steam turbine operating at full load during periods of null or low solar resource availability, resulting in a decrease in gross electric power generated to 1 MW (BFF = 50%). Additionally, it is identified that when the thermal power provided by the solar field drops below a certain value, biomass consumption decreases. This decrease corresponds to the reduction in the steam turbine load to 50%, as mentioned earlier. Appl. Sci. 2023, 13, x FOR PEER REVIEW 9 of 17 3. Results and Discussion 3.1. Simulation As mentioned earlier in Section 2.2, for the development of this analysis, different designs of hybrid plants were considered, varying both the SM and the BFF. Given the relatively small size of the proposed plant in this study, only two realistic values of BFF were considered. This is due to the technical difficulty of implementing biomass boilers with cooled grate burners on a very small scale. Therefore, two alternatives were analyzed regarding the size of Boiler 2, one corresponding to a BFF of 100% and another to a BFF of 50%. Regarding the size of the solar field, SM values from 0.5 to 2.5 were considered in intervals of 0.5. Figures 7 and 8 show the hourly evolution of both the biomass consumption and gross electric power generated for the same four representative days previously shown in Figure 4. Both figures assume SM = 1. However, Figure 7 considers BFF = 100%, while Figure 8 considers BFF = 50%. The analysis of Figure 7 (BFF = 100%) highlights the complete independence of the generated power from the solar resource, even on days with high variability such as days #172 and #355. By regulating the mass flow rate of the biomass consumed in Boiler 2, the steam turbine can maintain operation at its nominal power. It can be observed that during periods of null or low solar resource availability, biomass consumption increases, compensating for the deficit of thermal energy from the solar field. When the BFF is reduced to 50% (Figure 8), notable differences compared to Figure 7 are observed. The first difference is the inability to maintain the steam turbine operating at full load during periods of null or low solar resource availability, resulting in a decrease in gross electric power generated to 1 MW (BFF = 50%). Additionally, it is identified that when the thermal power provided by the solar field drops below a certain value, biomass consumption decreases. This decrease corresponds to the reduction in the steam turbine load to 50%, as mentioned earlier. Figure 7. Hourly evolution of biomass consumption (kg/s) and gross electric power generated (MW) for the four representative days (SM = 1, BFF = 100%). Figure 7. Hourly evolution of biomass consumption (kg/s) and gross electric power generated (MW) for the four representative days (SM = 1, BFF = 100%). Appl. Sci. 2023,13, 8179 9 of 16 Appl. Sci. 2023, 13, x FOR PEER REVIEW 10 of 17 Figure 8. Hourly evolution of biomass consumption (kg/s) and gross electric power generated (MW) for the four representative days (SM = 1, BFF = 50%). 3.2. Annual Balances After simulating the daily behavior of the different analyzed plant configurations (with varying SM and BFF values), annual balances were conducted for both the gross electric energy generated and the biomass consumption by combining the typical design year (Section 2.1) and the technical characterization (Section 2.2). However, the mentioned annual energy and biomass balances not only depended on the combination of SM and BFF considered in each analyzed case but also on the existence of limits on the amount of biomass consumed. This was because it is common for current legislation to limit the maximum fraction of energy generated from biomass [43], which is cheaper than energy generated from CSP technology. For this reason, two extreme situations were identified for each considered plant. The first corresponded to the absence of limits on biomass consumption. In this case, the mixed hybrid plant could generate during periods of no solar radiation, with the only limit on biomass consumption derived from the BFF and the maximum allowable hours of annual operation. The second extreme case corresponded to a situation where biomass could only be consumed as a complement to solar resources. In other words, in the second case, biomass could not be used for nighttime operation (direct biomass combustion without solar input). In addition to the mentioned extreme cases, intermediate limitations on biomass consumption were considered. Specifically, limits from 40% to 75% were considered, with increments of 5%, although not all analyzed plant configurations could reach all these intermediate limits due to reaching one of the aforementioned extreme situations. Figures 9 and 10 show the annual biomass consumption (kton/year) and gross electric energy generated (GWh/year), respectively, as a function of the SM and the biomass consumption limit for the two considered BFF values. Firstly, it is worth noting that for any of the displayed graphs, the upper white area is delimited by the extreme case corresponding to the maximum allowable hours of annual operation. On the other hand, the lower white area corresponds to the limit derived from the impossibility of biomass consumption during nighttime operation. In both figures, it can be observed that within the analyzed ranges, both biomass consumption and energy generation increase with increasing SM or the limit of the maximum fraction of energy generated from biomass, regardless of the BFF value. The increase in both variables with increasing SM is not only due to the Figure 8. Hourly evolution of biomass consumption (kg/s) and gross electric power generated (MW) for the four representative days (SM = 1, BFF = 50%). 3.2. Annual Balances After simulating the daily behavior of the different analyzed plant configurations (with varying SM and BFF values), annual balances were conducted for both the gross electric energy generated and the biomass consumption by combining the typical design year (Section 2.1) and the technical characterization (Section 2.2). However, the mentioned annual energy and biomass balances not only depended on the combination of SM and BFF considered in each analyzed case but also on the existence of limits on the amount of biomass consumed. This was because it is common for current legislation to limit the maximum fraction of energy generated from biomass [ 43 ], which is cheaper than energy generated from CSP technology. For this reason, two extreme situations were identified for each considered plant. The first corresponded to the absence of limits on biomass consumption. In this case, the mixed hybrid plant could generate during periods of no solar radiation, with the only limit on biomass consumption derived from the BFF and the maximum allowable hours of annual operation. The second extreme case corresponded to a situation where biomass could only be consumed as a complement to solar resources. In other words, in the second case, biomass could not be used for nighttime operation (direct biomass combustion without solar input). In addition to the mentioned extreme cases, intermediate limitations on biomass consumption were considered. Specifically, limits from 40% to 75% were considered, with increments of 5%, although not all analyzed plant configurations could reach all these intermediate limits due to reaching one of the aforementioned extreme situations. Figures 9and 10 show the annual biomass consumption (kton/year) and gross electric energy generated (GWh/year), respectively, as a function of the SM and the biomass consumption limit for the two considered BFF values. Firstly, it is worth noting that for any of the displayed graphs, the upper white area is delimited by the extreme case corresponding to the maximum allowable hours of annual operation. On the other hand, the lower white area corresponds to the limit derived from the impossibility of biomass consumption during nighttime operation. In both figures, it can be observed that within the analyzed ranges, both biomass consumption and energy generation increase with increasing SM or the limit of the maximum fraction of energy generated from biomass, regardless of the BFF value. The increase in both variables with increasing SM is not only due to the greater contribution of thermal energy from the solar field but also, given a biomass consumption limit, a higher solar production enables a greater generation of electricity from direct biomass combustion, which increases both the biomass consumption Appl. Sci. 2023,13, 8179 16 of 16 27. Bai, Z.; Liu, Q.; Gong, L.; Lei, J. Investigation of a solar-biomass gasification system with the production of methanol and electricity: Thermodynamic, economic and off-design operation. Appl. Energy 2019,243, 91–101. [CrossRef] 28. Mapa de Radiación Global de Andalucía. Agencia Andaluza de la Energía, Junta de Andalucía. Available online: https://www. agenciaandaluzadelaenergia.es/Radiacion/vermapas.php (accessed on 9 June 2023). 29. Agencia Andaluza de la Energía, Junta de Andalucía. La Biomasa en Andalucía. 2016. Available online: https://www. agenciaandaluzadelaenergia.es/biomasa/biomasa/init.do?prefix=/biomasa&name=potencial (accessed on 9 June 2023). 30. Wilcox, S.M. National Solar Radiation Database 1991–2010 Update: User’s Manual (No. NREL/TP-5500-54824); National Renewable Energy Laboratory (NREL): Golden, CO, USA, 2012. 31. Sengupta, M.; Xie, Y.; Lopez, A.; Habte, A.; Maclaurin, G.; Shelby, J. The national solar radiation data base (NSRDB). Renew. Sustain. Energy Rev. 2018,89, 51–60. [CrossRef] 32. Dominguez, J.; Marcos, M.J. Análisis de la producción potencial de energía con biomasa en la región de Andalucía (España) utilizando sistemas de información geográfica. Cybergeo Eur. J. Geogr. 2000. [CrossRef] 33. Bellos, E.; Tzivanidis, C. Analytical expression of parabolic trough solar collector performance. Designs 2018,2, 9. [CrossRef] 34. Solutia Technical Bulletin 7239115C. Available online: https://www.therminol.com/products/Therminol-VP1 (accessed on 6 June 2023). 35. Peterseim, J.H.; Tadros, A.; Hellwig, U.; White, S. Increasing the efficiency of parabolic trough plants using thermal oil through external superheating with biomass. Energy Convers. Manag. 2014,77, 784–793. [CrossRef] 36. Algieri, A.; Morrone, P. Thermo-economic investigation of solar-biomass hybrid cogeneration systems based on small-scale transcritical organic Rankine cycles. Appl. Therm. Eng. 2022,210, 118312. [CrossRef] 37. Kumar, B.; Raj, A.K.; Szepesi, G.; Szamosi, Z. A conspectus review on solar drying of wood: Regional and technical contrivances. J. Therm. Anal. Calorim. 2023,2023, 1–25. [CrossRef] 38. Talavera, D.L.; Pérez-Higueras, P.; Ruíz-Arias, J.A.; Fernández, E.F. Levelised cost of electricity in high concentrated photovoltaic grid connected systems: Spatial analysis of Spain. Appl. Energy 2015,151, 49–59. [CrossRef] 39. Idel, R. Levelized full system costs of electricity. Energy 2022,259, 124905. [CrossRef] 40. Peterseim, J.H.; White, S.; Tadros, A.; Hellwig, U. Concentrated solar power hybrid plants, which technologies are best suited for hybridisation? Renew. Energy 2013,57, 520–532. [CrossRef] 41. Bosch, J.; Staffell, I.; Hawkes, A.D. Global levelised cost of electricity from offshore wind. Energy 2019,189, 116357. [CrossRef] 42. System Advisor Model, Version 2022.11.21 (SAM 2022.11.21); National Renewable Energy Laboratory (NREL): Golden, CO, USA, 2022. 43. Srinivas, T.; Reddy, B.V. Hybrid solar–biomass power plant without energy storage. Case Stud. Therm. Eng. 2014 ,2, 75–81. [CrossRef] 44. Kearney, D.; Herrmann, U.; Nava, P.; Kelly, B.; Mahoney, R.; Pacheco, J.; Cable, R.; Potrovitza, N.; Blake, D.; Price, H. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. J. Sol. Energy Eng. 2003,125, 170–176. [CrossRef] 45. Răboacă, M.S.; Badea, G.; Enache, A.; Filote, C.; Răsoi, G.; Rata, M.; Lavric, A.; Felseghi, R.A. Concentrating solar power technologies. Energies 2019,12, 1048. [CrossRef] 46. Pantaleo, A.M.; Camporeale, S.M.; Miliozzi, A.; Russo, V.; Shah, N.; Markides, C.N. Novel hybrid CSP-biomass CHP for flexible generation: Thermo-economic analysis and profitability assessment. Appl. Energy 2017,204, 994–1006. [CrossRef] 47. Oyekale, J.; Heberle, F.; Petrollese, M.; Brüggemann, D.; Cau, G. Biomass retrofit for existing solar organic Rankine cycle power plants: Conceptual hybridization strategy and techno-economic assessment. Energy Convers. Manag. 2019 ,196, 831–845. 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