1. 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.
Abstract
Replacing 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 thermodynamic
efficiency, a lower levelized cost of electricity (LCOE) and a lower environmental impact due to smaller
collector surface needed and lower water consumption for a given amount of energy produced. Based on the
premise of lower LCOE, solar thermal – biomass hybrid power plants operating at areas in low irradiation
conditions could fulfill a minimum requirement of profitability, enabling the solar trough technology to
broaden its geographical boundaries tower upper latitudes [3]
Despite these advantages, and despite the fact that the Spanish Royal Decree 661/2007 includes an
advantageous specific feed in tariff that would enable a reasonable return on investments, only one hybrid
power 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 in
Spain before 2014. The lack of maturity of biomass markets could be the bottle neck preventing this
technology from taking off.
The purpose of this study is to present a rigorous analyze of these facts in order to settle objective criteria to
choose this technology in front of the parental technologies (solar thermal and biomass technologies). To
fulfill this goal, tailor-made thermodynamic and economic models have been developed to simulate the
behavior of parabolic trough power plants hybridized with biomass boilers. Results are discussed in terms of
thermodynamic efficiency, LCOE and environmental impact.
Keywords: solar thermal biomass hybrid power plant; parabolic trough; optimization
1. Introduction
The usual size of solar – thermal power plants being built in Spain at present is 50 MWe since this is the
upper limit that the Spanish regulatory framework has set for any solar thermal power plant to be covered by
the Spanish special regimen. On the other hand, 92% of the 852.4 MWe currently under operation in Spain
are based on the parabolic trough technology, the most mature and lowest cost solar thermal technology
available today [6]. 53% of this operating power is provided with thermal energy storage (TES) in order to
prevent the operation from its inherent variable behavior; it is expected that this percentage will grow within
the next decade. For this reason, the development of efficient and cost-effective thermal storage systems is
crucial for the future development of concentrating solar power [2]. Different scientific works have been
carried 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 standard
solar 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 the
solar thermal technology to become competitive in a liberalized market.
2. Performance model
Magtel R&D has recently finalized a research work aimed to set up the basis for the optimized design of
Solar Thermal Power Plants, being co-financed by the Spanish Ministry of Innovation and Technology under
the Torres Quevedo Program (Project Reference PTQ-08-03-06366) and the Holding Company Magtel. In

2. this context, a model has been developed to simulate the behavior of parabolic trough solar thermal power
plants. 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 and
power module.
The solar module calculates the heat transferred to a heat transfer fluid (HTF) by means of a model that takes
into account the position of the sun, and estimates the thermal behavior of the collector field with a horizontal
north-south axis of rotation. The quality of the solar resource is evaluated hourly in function of an optical
incident angle modifier. The irradiation historical data have been collected from the Energy Plus data base. In
this 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 has
been developed based on energy balances. It has been assumed that the combustion system operates in a
nominal condition at all times; hence, in the absence of solar irradiation, it has been assumed that the plant
will 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 based
on sensible heat storage by liquid media in an indirect storage system. Hourly, the excess of thermal energy is
estimated by comparing the amount of heat available in the field of collectors with the energy demanded by
the power block and the storage capacity of the TES. An average value has been assumed for the efficiency
of the charge – discharge cycles.
The simulation model of the power system is based on benchmarks of steam turbines. The model calculates
the power generation hourly according to the thermal energy supplied by the solar field, TES, and biomass
systems. Depending on the nominal thermal energy demanded by the power cycle, the model calculates the
gross power plant and evaluates the parasitic losses of the system.
Once adjusted and validated the performance model, a series of parametric studies were undertaken to
determine the optimum solar field required to minimize the cost of energy. Two different designs have been
analyzed: a thermal solar power plant with 1,000 MWht TES capacity (case PT HTF 156.50.1000.0) and a
thermal solar power plant integrated with a biomass boiler (hybrid design, case PT HTF 69.43.0.50). The
economic model is presented in the next section.
3. Economic model
The economic analysis has been carried out following a cost model based on the revenue requirement
approach [1], developed by the Electric Power Research Institute, once adapted for use in solar thermal
power plants by the authors of this paper. With this approach, the cost of electricity is calculated trough the
following four steps: estimate the total capital investment; determine the economic, financial, operating, and
market input parameters for the detailed cost calculation; calculate the total revenue requirement, and
calculate the levelized cost of energy.
The total capital requirement is calculated as the sum of fixed-capital investment (FCI, direct and indirect
costs) and other outlays. Table 1 shows the breakdown of fixed-capital costs used in this study. Direct costs
include purchased-equipment costs, purchased-equipment instalation, piping, instrumentation and controls,
and electrical components and materials. Purchased equipment costs is estimated by the relation
ܺ௒ ‫ן‬
‫ܥ‬௉ா,௒ ൌ ‫ܥ‬௉ா,ௐ ൬ ൰
ܺௐ
This ecuation allows the purchase cost of an equipment item (‫ܥ‬௉ா,௒ ) at a given capacity or size (as expressed
by the variable ܺ௒ ) to be calculated when the purchased cost of the same equipmen item (‫ܥ‬௉ா,௪ ) at a different
capacity or size (expressed by ܺௐ ) is known. Current market prices of the items in an commercial parabolic
trough solar thermal power plant with two-tank molten thermal storage been promoted by Magtel have been
used. In the absence of cost information, an scaling exponent α value of 0,7 has been used [8]. In all the
cases, offsite and indirect costs have been estimated as percentages of the direct costs.

3. CET CCP HTF CET CCP HTF
Units 156.50.1000.0 69.43.0.50
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 investment
Other outlays comprise the startup costs, working capital, cost of licensing, research and development, and
allowance of funds used during construction. An average value of 5,48% of the FCI has been used for the
two cases analized.
CET CCP HTF CET CCP HTF
Units 156.50.1000.0 69.43.0.50
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 costs
Table 2 shows the breakdown of operation and maintenance (O&M) costs. These are broken into categories
of annual fixed O&M costs (labor, maintenance and supervision) and annual variable O&M costs (natural gas
consumption and other operating suplies –water-). Values for maintenance and supervision costs have been
estimated 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

4. Once obtained FCI and O&M costs, the revenue requirements were calculated. Table 3 shows the most
relevant 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 is
taken as 6.6%. Based on these premises, a capital recovery factor equal to 0.07757 has been calculated.
4. Results and discussion: features and drawbacks
The most outstanding feature emerging from this study is that integrating a biomass boiler into a parabolic
trough solar thermal power plant could be a very interesting choice aimed to improve the sustainability of
solar thermal power plants. In this context, it is possible to infer that the higher the efficiency of a solar
thermal power plant, the lower the LCOE and the lower the environmental impact of a given solar thermal
power 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 HTF
156.50.1000.0 corresponds to a 50 MWe 156 loop parabolic trough solar thermal power plant with integrated
1.000 MWht TES; an annual power utility equal to 181,005 MWh has been estimated for this case when
operating at the location suggested. On the other hand, case PT HTF 69.43.0.50 corresponds to a hybrid
power 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 a
29.51 MWt biomass boiler fulfilled such annual power production. A can be observed in this figure, the
hybrid case appears to result in lower cost (right side) and in higher efficiencies than the reference case.
20
19 PT HTF 69.43.0.50 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 69.43.0.50
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 69.43.0.50.
Table 4 compares the main performance design, overall system and economic parameters for the two cases
just presented. According to this study, replacing part of the solar field with a biomass boiler has led a
227,018 m2 solar field instead of the 510.000 m2 solar field associated to the reference case, leading a lower
environmental impact in terms of surface occupation. This reduced solar field, integrated with a biomass
boiler 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 installed
power 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 the
same amount of power utility as in the other two cases analyzed. Under this assumption, the biomass
consumption of the hybrid design (case 2) is roughly 30% of that for the biomass power plant (75,927 t
compared with 225,000 t), and the cooling water consumption at the condensing tower is 50% (0.6343 Hm3
instead of 1.11 Hm3).

5. Case 1: Case 2: Case 3:
PT HTF 156.50.1000.0 PT HTF 69.43.0.50 Biomass Power plant
PERFORMANCE 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 MWt
PERFORMANCE 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 h
PERFORMANCE 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 69.43.0.50 Case 2: PT HTF 156.50.1000.0
COEL 3,239 €/kW 12.98 c€/kWh 6,186 €/kW 19.22 c€/kWh
On 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€/kWh
Other 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€/kWh
Total capital costs 3,239 €/kW 5.62 c€/kWh 6,186 kW 12.37 c€/kWh
O&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 69.43.0.50

6. Table 5 shows that the cost estimation for the power utility delivered by the hybrid design (12.98 c€/kWh) is
one third the cost estimation for the reference design (19.22 c€/kWh). Two are the main reasons justifying
this difference in prices: a lower contribution of solar field on site cost (0.806 c€/kWh instead of 1.805
c€/kWh), and a lower contribution of the combustion system when compared to TES (0.1 c€/kWh instead of
0.51 c€/kWh). In general, capital costs contribute to LCOE with lower figures due to a lower capital
requirement 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 in
this upper figure, integrating a biomass boiler into a solar thermal power plant would lead lower costs of
electricity for the whole range of power utilities analyzed when compared to the reference case. In
consequence, the smaller hybrid size analyzed (rated 4 MW) would be profitable, being the capital
requirement accessible to small and medium sized firms, balancing the distribution of national wealth, and
contributing 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 the
equivalent hybrid case (PT HTF 69.43.0.50). As it can be observed, the hybrid case has been designed under
the assumption that the biomass boiler is in steady-state operation throughout the year, leading a higher
number of equivalent full load operation hours (4,169 h instead of 3,620 h; see Table 4). Another feature
from 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 an
inconvenience for the hybrid designs: the challenge of ensuring the biomass supply throughout the plant
economic life could jeopardize financial support from banks and investors. However, this fact will become an
advantage of the hybrid design if we take into account that the biomass consumption of biomass power plants
is almost three times higher than in the case of hybrid designs (see table 4).

7. Fig.3. Daily performance curves

8. 5. Conclusions
The most outstanding feature emerging from this study is that integrating a biomass boiling system into a
parabolic trough solar thermal power plant could be a very interesting choice aimed to improve the
sustainability of solar thermal power plants. In this context, it is possible to infer that the higher the efficiency
of a solar thermal power plant, the lower the LCOE and the lower the environmental impact of a given
energy 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 be
accessible to small and medium sized firms, balancing the distribution of national wealth, and contributing to
a distributed renewable energy system. Other features of the hybrid designs are a higher number of
equivalent 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 drawback
for hybrid designs. However, this fact becomes an advantage of hybrid designs if we take into account that
the biomass consumption of biomass power plants is almost three times higher than in hybrid designs.
Acknowledgements
The authors would like to thank to Magtel and the Spanish Ministry of Innovation and Technology’s Torres
Quevedo program for their support of this work.
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