SOLAR PARABOLIC TROUGH – BIOMASS HYBRID PLANTS: FEATURES AND DRAWBACKS 1 2 Ángel Moreno-Pérez and Pablo Castellote-Olmo 1 PhD in Industrial Engineering. Mayor: Energy System Optimization. Director. Magtel R&D. Address: Gabriel Ramos Bejarano, 114 – 14014 Córdoba, Spain. +34 957 429 060 2 Industrial Engineer. Major: Energy System Optimization. Senior Research. Magtel R&D.AbstractReplacing melting salt thermal storage system with a biomass green storage system in solar parabolic trough– biomass hybrid power plants is an emerging concept that has these advantages: a higher thermodynamicefficiency, a lower levelized cost of electricity (LCOE) and a lower environmental impact due to smallercollector surface needed and lower water consumption for a given amount of energy produced. Based on thepremise of lower LCOE, solar thermal – biomass hybrid power plants operating at areas in low irradiationconditions could fulfill a minimum requirement of profitability, enabling the solar trough technology tobroaden its geographical boundaries tower upper latitudes Despite these advantages, and despite the fact that the Spanish Royal Decree 661/2007 includes anadvantageous specific feed in tariff that would enable a reasonable return on investments, only one hybridpower plant has been included in the pre-registration required to receive the Spanish feed-in tariff and,therefore, only this hybrid power plant project is expected to be submitted for administrative approvals inSpain before 2014. The lack of maturity of biomass markets could be the bottle neck preventing thistechnology from taking off.The purpose of this study is to present a rigorous analyze of these facts in order to settle objective criteria tochoose this technology in front of the parental technologies (solar thermal and biomass technologies). Tofulfill this goal, tailor-made thermodynamic and economic models have been developed to simulate thebehavior of parabolic trough power plants hybridized with biomass boilers. Results are discussed in terms ofthermodynamic efficiency, LCOE and environmental impact.Keywords: solar thermal biomass hybrid power plant; parabolic trough; optimization1. IntroductionThe usual size of solar – thermal power plants being built in Spain at present is 50 MWe since this is theupper limit that the Spanish regulatory framework has set for any solar thermal power plant to be covered bythe Spanish special regimen. On the other hand, 92% of the 852.4 MWe currently under operation in Spainare based on the parabolic trough technology, the most mature and lowest cost solar thermal technologyavailable today . 53% of this operating power is provided with thermal energy storage (TES) in order toprevent the operation from its inherent variable behavior; it is expected that this percentage will grow withinthe next decade. For this reason, the development of efficient and cost-effective thermal storage systems iscrucial for the future development of concentrating solar power . Different scientific works have beencarried out showing that this concept could improve the economy of parabolic trough plants [4,8].As shown in this paper, solar thermal – biomass hybrid power plants present the advantage over the standardsolar thermal technologies of a higher annual efficiency, a lower LCOE and a lower environmental impact.Due to the fulfillment of these requirements, biomass hybridization could be a step forward necessary for thesolar thermal technology to become competitive in a liberalized market.2. Performance modelMagtel R&D has recently finalized a research work aimed to set up the basis for the optimized design ofSolar Thermal Power Plants, being co-financed by the Spanish Ministry of Innovation and Technology underthe Torres Quevedo Program (Project Reference PTQ-08-03-06366) and the Holding Company Magtel. In
this context, a model has been developed to simulate the behavior of parabolic trough solar thermal powerplants. This model has been programmed and a computer program has been written in Open Fortran;different modules have been developed and liked together: solar module, TES module, biomass module andpower module.The solar module calculates the heat transferred to a heat transfer fluid (HTF) by means of a model that takesinto account the position of the sun, and estimates the thermal behavior of the collector field with a horizontalnorth-south axis of rotation. The quality of the solar resource is evaluated hourly in function of an opticalincident angle modifier. The irradiation historical data have been collected from the Energy Plus data base. Inthis study, a location near Córdoba with coordinates N37.75º W5.053 has been analyzed.The biomass resource in this paper is agricultural residue from olive. A combustion conversion model hasbeen developed based on energy balances. It has been assumed that the combustion system operates in anominal condition at all times; hence, in the absence of solar irradiation, it has been assumed that the plantwill operate part load fed by biomass, transferring its chemical energy into the power block via HTF.A model has been developed to estimate the behavior of a two-tank molten-salt storage system. TES is basedon sensible heat storage by liquid media in an indirect storage system. Hourly, the excess of thermal energy isestimated by comparing the amount of heat available in the field of collectors with the energy demanded bythe power block and the storage capacity of the TES. An average value has been assumed for the efficiencyof the charge – discharge cycles.The simulation model of the power system is based on benchmarks of steam turbines. The model calculatesthe power generation hourly according to the thermal energy supplied by the solar field, TES, and biomasssystems. Depending on the nominal thermal energy demanded by the power cycle, the model calculates thegross power plant and evaluates the parasitic losses of the system.Once adjusted and validated the performance model, a series of parametric studies were undertaken todetermine the optimum solar field required to minimize the cost of energy. Two different designs have beenanalyzed: a thermal solar power plant with 1,000 MWht TES capacity (case PT HTF 156.50.1000.0) and athermal solar power plant integrated with a biomass boiler (hybrid design, case PT HTF 184.108.40.206). Theeconomic model is presented in the next section.3. Economic modelThe economic analysis has been carried out following a cost model based on the revenue requirementapproach , developed by the Electric Power Research Institute, once adapted for use in solar thermalpower plants by the authors of this paper. With this approach, the cost of electricity is calculated trough thefollowing four steps: estimate the total capital investment; determine the economic, financial, operating, andmarket input parameters for the detailed cost calculation; calculate the total revenue requirement, andcalculate the levelized cost of energy.The total capital requirement is calculated as the sum of fixed-capital investment (FCI, direct and indirectcosts) and other outlays. Table 1 shows the breakdown of fixed-capital costs used in this study. Direct costsinclude purchased-equipment costs, purchased-equipment instalation, piping, instrumentation and controls,and electrical components and materials. Purchased equipment costs is estimated by the relation ܺ ן ܥா, ൌ ܥா,ௐ ൬ ൰ ܺௐThis ecuation allows the purchase cost of an equipment item (ܥா, ) at a given capacity or size (as expressedby the variable ܺ ) to be calculated when the purchased cost of the same equipmen item (ܥா,௪ ) at a differentcapacity or size (expressed by ܺௐ ) is known. Current market prices of the items in an commercial parabolictrough solar thermal power plant with two-tank molten thermal storage been promoted by Magtel have beenused. In the absence of cost information, an scaling exponent α value of 0,7 has been used . In all thecases, offsite and indirect costs have been estimated as percentages of the direct costs.
CET CCP HTF CET CCP HTF Units 156.50.1000.0 220.127.116.11 FIXED CAPITAL INVESTMENT Direct costs (DC) 2 Solar field €/m 204 204 2 HTF system €/m 21 20 Power Block €/kW 241 260 Steam generator €/kW 142 150 TES €/kWht 17 - Biomass combustion system €/kWt - 156 Surcharge for offsite costs % of DC 7.96 7.48 Indirect costs Surcharge for indirect costs % of DC 38 38 OTHER OUTLAYS Statup, working capital, licensing, R&D… % of FCI 5.58 5.58 Table 1. Breakdown of fixed capital investmentOther outlays comprise the startup costs, working capital, cost of licensing, research and development, andallowance of funds used during construction. An average value of 5,48% of the FCI has been used for thetwo cases analized. CET CCP HTF CET CCP HTF Units 156.50.1000.0 18.104.22.168 Annual fixed O&M costs Number of persons for plant operation workers 15 15 Number of persons for field maintenance workers 10 10 Annual maintenance % FC 1 1 Annual supervision cost % FC 0.55 0.55 Annual variable O&M costs Annual natural gas consumption MWht 79,056 83 Annual water consumption 3 0.64 0.63 Hm Table 2. Breakdown of O&M costsTable 2 shows the breakdown of operation and maintenance (O&M) costs. These are broken into categoriesof annual fixed O&M costs (labor, maintenance and supervision) and annual variable O&M costs (natural gasconsumption and other operating suplies –water-). Values for maintenance and supervision costs have beenestimated as 1% of FCI, and annual supervision cost as 0.55% of FCI. Price of resources (2011 prices) Price of Natural Gas 2.28 c€/kWht Price of Biomass 49 €/t Price of Water 0.474 €/t Inflation and escalation rates Average inflation rate 3% Average nominal escalation rate of all (except fuel) costs 2% Average nominal escalation rate of fuel costs 1% Plant financing fractions and required returns of capital Rate of Return -project- 6.6% Capital Recovery Factor 0.07757 Table 3. Most relevant market input parameters for cost calculation
Once obtained FCI and O&M costs, the revenue requirements were calculated. Table 3 shows the mostrelevant market input parameters for the calculation. Other assumption were 30 years of plant economic life,straight-line depreciation and net salvage value equal to 0. No taxes have been included. The rate on retun istaken as 6.6%. Based on these premises, a capital recovery factor equal to 0.07757 has been calculated.4. Results and discussion: features and drawbacksThe most outstanding feature emerging from this study is that integrating a biomass boiler into a parabolictrough solar thermal power plant could be a very interesting choice aimed to improve the sustainability ofsolar thermal power plants. In this context, it is possible to infer that the higher the efficiency of a solarthermal power plant, the lower the LCOE and the lower the environmental impact of a given solar thermalpower plant, measured in terms of surface occupation and water consumption.Fig. 1 presents the comparison of efficiency and cost of energy for the two cases analyzed. Case PT HTF156.50.1000.0 corresponds to a 50 MWe 156 loop parabolic trough solar thermal power plant with integrated1.000 MWht TES; an annual power utility equal to 181,005 MWh has been estimated for this case whenoperating at the location suggested. On the other hand, case PT HTF 22.214.171.124 corresponds to a hybridpower plant without TES, operating at the same location and supplying the same amount of power utility.Under these assumptions, a 43 MWe 69 loop parabolic trough solar thermal power plant integrated with a29.51 MWt biomass boiler fulfilled such annual power production. A can be observed in this figure, thehybrid case appears to result in lower cost (right side) and in higher efficiencies than the reference case. 20 19 PT HTF 126.96.36.199 25 18 23 17 21 LCOE, c€/kWh Efficiency, % 16 19 15 PT HTF 156.50.1000.0 14 17 13 PT HTF 156.50.1000.0 15 12 13 11 PT HTF 188.8.131.52 10 11 0 40 80 120 160 200 240 280 0 40 80 120 160 200 240 280 Loops Loops Fig.1. Efficiency and LCOE vs. size of the solar field for cases PT HTF 156.50.1000.0 and PT HTF 184.108.40.206.Table 4 compares the main performance design, overall system and economic parameters for the two casesjust presented. According to this study, replacing part of the solar field with a biomass boiler has led a227,018 m2 solar field instead of the 510.000 m2 solar field associated to the reference case, leading a lowerenvironmental impact in terms of surface occupation. This reduced solar field, integrated with a biomassboiler rated at 39.51 MWht and a steam turbine rated at 43 MWe fulfills the given annual power utility,accounting for 14% of reference case steam turbine power. The consequence of this reduction in installedpower is a lower environmental impact in terms of cooling water consumption.Table 4 also presents typical performance data for a standard biomass power plant (case 3) supplying thesame amount of power utility as in the other two cases analyzed. Under this assumption, the biomassconsumption of the hybrid design (case 2) is roughly 30% of that for the biomass power plant (75,927 tcompared with 225,000 t), and the cooling water consumption at the condensing tower is 50% (0.6343 Hm3instead of 1.11 Hm3).
Case 1: Case 2: Case 3: PT HTF 156.50.1000.0 PT HTF 220.127.116.11 Biomass Power plantPERFORMANCE DESIGN PARAMETERS Parabolic trough surface 510,000 m2 227,018 m2 - Number of loops 156 69 - Biomass annual consumption - 75,927 t 225,000 t Heat duty - 39.510 MWt 111.3 MWtPERFORMANCE OVERALL SYSTEM PARAMETERS Plant output 50 MWe 43 MWe 25 MWe Power utility 181,005 MWh 179,263 MWh 182,500 MWh Annual net plant efficiency 14.52% 18.82% 21% Annual water consumption 0.644 Hm3 0.6343 Hm3 1.11 Hm3 Full load equivalent annual operation hours 3,620 h 4,169 h 7,300 hPERFORMANCE ECONOMIC PARAMETERS LCOE 19.22 c€/kWh 12.98 c€/kWh 10.95 c€/kWh Table 4. Performance design, overall system and economic parameters Case 1: PT HTF 18.104.22.168 Case 2: PT HTF 156.50.1000.0COEL 3,239 €/kW 12.98 c€/kWh 6,186 €/kW 19.22 c€/kWhOn site costs Solar field 1,075 €/kW 0.806 c€/kWh 2,090 €/kW 1.805 c€/kWh HTF system 107 €/kW 0.08 c€/kWh 217 €/kW 0.19 c€/kWh TES system - - 596 €/kW 0.51 c€/kWh Biomass 136 €/kW 0.1 c€/kWh - - Power Block 260 €/kW 0.19 c€/kWh 241 €/kW 0.21 c€/kWh Steam generator 150 €/kW 0.11 c€/kWh 142 €/kW 0.12 c€/kWh BOP 160 €/kW 0.12 c€/kWh 328 €/kW 0.28 c€/kWh Electrical components 108 €/kW 0.08 c€/kWh 206 €/kW 0.18 c€/kWh Total on site costs 1,996 €/kW 1.5 c€/kWh 3,820 €/kW 3.3 c€/kWhOther capital costs Offsite costs 161 €/kW 0.12 c€/kWh 330 €/kW 0.29 c€/kWh Indirect costs 815 €/kW 0.61 c€/kWh 1,567 kW 1.35 c€/kWh Other outlays 101 €/kW 0.06 c€/kWh 149 €/kW 0.13 c€/kWh AFUDC 166 €/kW 0.1 c€/kWh 319 €/kW 0.22 c€/kWh Total other capital costs 1,243 €/kW 0.89 c€/kWh 2,366 kW 1.98 c€/kWh Financial costs - 3.23 c€/kWh - 7.08 c€/kWhTotal capital costs 3,239 €/kW 5.62 c€/kWh 6,186 kW 12.37 c€/kWhO&M costs Natural gas - 1.28 c€/kWh 1.2 c€/kWh Biomass - 3.13 c€/kWh - Other O&M costs - 2.95 c€/kWh 5.65 c€/kWh Total O&M costs 7.36 c€/kWh 6.86 c€/kW Table 5. Breakdown of capital costs and LCOE for cases PT HTF 156.50.1000.0 and PT HTF 22.214.171.124
Table 5 shows that the cost estimation for the power utility delivered by the hybrid design (12.98 c€/kWh) isone third the cost estimation for the reference design (19.22 c€/kWh). Two are the main reasons justifyingthis difference in prices: a lower contribution of solar field on site cost (0.806 c€/kWh instead of 1.805c€/kWh), and a lower contribution of the combustion system when compared to TES (0.1 c€/kWh instead of0.51 c€/kWh). In general, capital costs contribute to LCOE with lower figures due to a lower capitalrequirement of the hybrid design (3,239 c€/kW compared with 6,186 c€/kW).Fig.2 depicts the inter-relation of power utility, cost of electricity and investment. As it can be observed inthis upper figure, integrating a biomass boiler into a solar thermal power plant would lead lower costs ofelectricity for the whole range of power utilities analyzed when compared to the reference case. Inconsequence, the smaller hybrid size analyzed (rated 4 MW) would be profitable, being the capitalrequirement accessible to small and medium sized firms, balancing the distribution of national wealth, andcontributing to a distributed renewable energy system. Cost of Electricity vs. Annual Power Utility 22 L COE (c€/kWh) 20 4 MWe PT HTF 156.50.1000.0 18 16 25 MWe 50 MWe 14 HYBRID DESIGN 12 10 0 40000 80000 120000 160000 200000 Power Utility (MWh) Capital Investment vs. Plant Size Capital investment (M€) 200 7.000 Capital investment 160 6.000 (€/kWe) 120 5.000 80 4 MWe 4.000 40 3.000 2.000 0 40000 80000 120000 160000 200000 Power utility (MWh) Fig.2. Cost of electricity and capital investment vs. plant size for Integrated Parabolic Trough Biomass Boiler. Hybridization level: 50%Fig. 3 shows daily performance curves for both the reference case (PT HTF 156.50.1000.0) and theequivalent hybrid case (PT HTF 126.96.36.199). As it can be observed, the hybrid case has been designed underthe assumption that the biomass boiler is in steady-state operation throughout the year, leading a highernumber of equivalent full load operation hours (4,169 h instead of 3,620 h; see Table 4). Another featurefrom this performance curves is a wiser use of natural gas.The fact that the biomass energy market in Spain is a developing but immature market could be aninconvenience for the hybrid designs: the challenge of ensuring the biomass supply throughout the planteconomic life could jeopardize financial support from banks and investors. However, this fact will become anadvantage of the hybrid design if we take into account that the biomass consumption of biomass power plantsis almost three times higher than in the case of hybrid designs (see table 4).
5. ConclusionsThe most outstanding feature emerging from this study is that integrating a biomass boiling system into aparabolic trough solar thermal power plant could be a very interesting choice aimed to improve thesustainability of solar thermal power plants. In this context, it is possible to infer that the higher the efficiencyof a solar thermal power plant, the lower the LCOE and the lower the environmental impact of a givenenergy system, measured in terms of surface occupation and water consumption.In addition to these advantages, smaller hybrid size could be profitable, so the capital requirement could beaccessible to small and medium sized firms, balancing the distribution of national wealth, and contributing toa distributed renewable energy system. Other features of the hybrid designs are a higher number ofequivalent full load operation hours and wiser use of natural gas.The fact that the biomass energy market in Spain is a developing but immature market could be a drawbackfor hybrid designs. However, this fact becomes an advantage of hybrid designs if we take into account thatthe biomass consumption of biomass power plants is almost three times higher than in hybrid designs.AcknowledgementsThe authors would like to thank to Magtel and the Spanish Ministry of Innovation and Technology’s TorresQuevedo program for their support of this work.References A. Bejan, G. Tsatsaronis, M. Moran (1996). Thermal Design and Optimization. John Willey & Sons, Inc.New York. A. Gil, M. Medrano, I. Martorell, A. Lázaro, B. Zalba, L.F. Cabeza (2010). State of the art on hightemperature thermal energy storage for power generation. Part1 – Concepts, materials and modellization.Renewable and Sustainable Energy Reviews 14, 31 -55. Elsevier.  A. Moreno-Pérez, N. Mesa-Torres (2010). Solar Parabolic Trough – Biomass Hybrid Plants: a cost-efficient concept suitable for places in low irradiation conditions. 17th International SolarPACES Symposiumon Solar Thermal Concentrating Technologies, Perpignan, 2010. B. Kelly, D. Kearney (2004). Thermal Storage Commercial Plant Design Study for a 2-Tank IndirectMolten Salt System. National Renewable Energy Laboratory (NREL/SR-550-40166) F. Rossi, D. Velazquez, R. González (2010). Off-desing behaviour of a solar electric generating systemusing biomass hybridization. 17th International SolarPACES Symposium on Solar Thermal ConcentratingTechnologies, Perpignan, 2010. H. Price (2003). A Parabolic Trough Solar Power Plant Simulation Model. National Renewable EnergyLaboratory (NREL/CP-550-33209) H. Price et al. (2002). Advances in parabolic trough solar power technology. Journal of Solar EnergyEngineering, 124(2), 109 – 125. U. Herrmann, B. Kelly, H. Price (2004). Two-tank molten salt storage for parabolic trough solar powerplants. Energy 29. pp. 883-893. Elsevier. J. López-Carvajal, J.M. Sáenz-Caballos, J.A. Vélez-Godino (2010). Biosol Hybrid Project: solar-thermaltechnology hibridization with biomass combustión in a pilot plant. 17th International SolarPACESSymposium on Solar Thermal Concentrating Technologies, Perpignan, 2010.