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A conceptual design of solar boiler
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Solar Energy 83 (2009) 1713–1722 www.elsevier.com/locate/solener
A conceptual design of solar boiler Javier Mun˜oz, Alberto Aba´nades *, Jose´ M. Martı´nez-Val Universidad Polite´cnica de Madrid, ETSII, J.Gutie´rrez Abascal, 2, 28006 Madrid, Spain Received 1 September 2008; received in revised form 22 May 2009; accepted 12 June 2009 Available online 21 July 2009 Communicated by: Associate Editor Robert Pitz-Paal
Abstract State-of-the-art concepts for solar thermal power systems are based on parabolic trough, tower or parabolic disks either heating molten salts, mineral oil, air or generating steam. We propose in this paper, a conceptual design of a solar boiler. This concept comes from the conventional thermal power plants boiler, with the difference that the heat comes from mirrors that concentrate the solar radiation on wall-type array of solar collectors, instead of coming from fuel flames and hot gases. In our preliminary performance, analysis of this innovative solar boiler applied to electricity production, we have found that overall efficiency of the conversion from direct solar irradiation energy to electricity is above 20%, which is comparable to the value of parabolic trough and central tower technologies. Besides that, the concept seems very robust and could overcome some drawbacks derived from pressure losses, control complexity and material thermo-mechanical stress. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Concentrated solar energy; Conceptual design; Electric power generation; Sustainable development
1. Introduction and background Electricity generation in solar thermal power plants is a fundamental point in the quest to Sustainable Development (EC, 2004). There is a sizeable fraction of the continental surface with very high solar radiation flux for a large number of hours (PS10, 2008). It is true that most of this land corresponds to deserts which are far away from regions of high energy consumption rates, but ultra-high voltage transmission lines can help overcome that drawback. On the other hand, some regions with high-level solar radiation are much close to intensively populated areas, as in the southern part of Europe and in the southwest of the United States. In summary, solar thermal electricity seems very suited to match the requirements of a fraction of the World population, if fossil fuel prices continue to climb
*
Corresponding author. Tel.: +34 913364268; fax: +34 913363079. E-mail addresses: [email protected], [email protected] (A. Aba´nades). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.06.009
(as expected from increasing demand and stiffness in the producing fields) and CO2 emission must be limited. There is a large variety of solar power plant configurations for electricity generation, mainly depending on the solar light concentration configuration, absorbing collector arrangements, and working fluids, both in the collectors and in the thermodynamic cycle (CIEMAT, 2006). The most classical structure is based on parabolic trough collectors using mineral oil as high-temperature coolant, and using a Rankine steam cycle in the balance of power. This cycle is activated in a central water boiler heated by the oil coming from the solar field (Frier et al., 1998). Many variations can be devised for this objective. In particular, the concentration of the solar radiation onto a central receiver mounted at the top of a tower. There are different receiver configurations (Buck et al., 2006). As an example, it can be a cavity-type collector with an optional transparent window for minimizing heat losses. Radiation losses are minimized by reducing the window aperture. Conduction losses area is usually negligible, because of isolation blankets around hot parts. Other configurations can
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eliminate the window, just be based on an external tube arrangement (Solar One), on volumetric receivers (SOLGATE project). The first commercial plant of this technology has recently started its operation in the south of Spain (PS10, 2008). One of the possibilities to activate a Rankine steam cycle is to directly generate steam inside the tube of parabolic trough collectors, what increase plant efficiency by eliminating the steam generator required when using other heat transfer fluids. A large experimental facility of this type has been built in Plataforma Solar de Almerı´a (Spain) (Zarza et al., 2004; Valenzuela et al., 2006). Projects for building a power plant of this type are under study. A totally different proposal is introduced in this paper. The configuration presented here is based on the structure of coal-fired power plants, where steam is produced in a large array of vertical tubes heated by the combustion products (B&W, 2005). In our case, the array of tubes will be formed by a set of absorbing modules on the wall of a tower properly placed in the solar field (where the concentrating mirrors are). Of course, there will be a main difference between both structures. The coal-fired boiler is a large box encircling the burning volume, and the solar boiler has to be open to receive the concentrated sun radiation. A sketch of the power plant proposed is depicted in Fig. 1. The solar filed is shown in a very simplified style, but it is important to note that the total collector surface is much larger, in width and height, than the standard cavity receiver in a central tower scheme (Abengoa, 2008). Our
rationale was inspired in the ‘‘do not heat at temperatures higher than you need” principle, so reducing the technological requirements for the materials maximizing energy and exergy efficiency. This fact conveys a degree of freedom to select the focusing spot of each mirror in a more flexible way. The lower part of the collectors would act as preheater and boiler, and a steam separator would be placed at a suitable height, in order to recirculate the liquid water and to send the steam to the overheater, which would obviously be at the highest part of the circuit. From a qualitative point of view, there are pros and cons in this type of power plant, either in relation to direct steam generation in trough collectors or in relation to central cavity receivers. As compared to trough collectors, steam generation follows in a much better way natural rules when it is produced in vertical tubes. Boiling inside horizontal tube is a rather complex situation, with a phase separation between the liquid layer in the bottom and the steam phase occupying partially the tube cross section. Slip ratio between both phases (with very different speeds) adds some complexity to the picture. Liquid recirculation becomes mandatory, and poses control difficulties (Valenzuela et al., 2006). The standard vertical array of the boiler tubes has been proven for more than one century which is the simplest, most efficient and most robust way to generate steam. Besides that, pumping power will be much lower in our case for the same level of absorbed thermal power. It goes without saying that it is much simpler to make the photons
1 2 3 4 5 6 7 8 9 10 11 12 13 Fig. 1. Solar boiler conceptual scheme.
Boiler body First heating grid Solar field Condensed pump Recirculation pump Down pipe Insulation Phase separator Solar boiler outlet Vapour dryer Second heating grid Overheated vapour pipe Laying of foundations
J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
run (until reaching a central boiler) than making the steam run along the very long distances of the solar field of parabolic trough tubes and connecting headers and pipes. In relation to steam generation in a central cavity receiver, our proposal seems to have much higher heat losses. This is true for the same level of temperatures, but temperatures and heat loads in the solar boiler collectors will be lower, or much lower, than those inside their cavity receiver. Hence, losses can be kept low enough in our case, and the collector efficiency can therefore be high enough. As a matter of fact, the heat loads and the temperatures inside the cavity receiver can be too high (Alvarez et al., 2005) for the objective of feeding a Rankine steam cycle of standard values, for instance 4 MPa and 690 K of overheated steam. If 4 MPa is chosen as working pressure to generate steam, the boiler will be slightly above 523 K, which is the saturation temperature. It is worth pointing out a fundamental point for understanding the moderate values of the radiation losses: currently available coating (used, for instance, in trough collectors tubes) have a very high solar absorptivity and very low thermal emissivity at temperatures below 800 K. In fact, those values can be 0.9 and 0.1, respectively (Alvarez et al., 2005; Mills, 2004). It means that radiation losses, depending on e T4, will be modest if the temperatures are not too high. This is also a key point in this design option that is intended to propose a first-generation solar power plant with an overall efficiency well above 20% (Martin and Berdahl, 1982). In Section 2, a brief description of the solar boiler concept is presented. Its thermal performance is calculated in Section 3, for a small prototype unit, although the methodology can be applied to a plant of higher power. The analysis of the performance of the facility is discussed in Section 4. Conclusions and future work are discussed in Section 5. 2. Description of a power solar boiler facility A sketch of a solar boiler for electricity production is shown in Fig. 2, where the boiler coupled to a Rankine cycle is shown. The main steps in this scheme are: A first heating module (Preheating and boiler), where slightly subcooled water enters the boiler at point 1, and reaches boiling until a certain vapour fraction at point 2. A phase separator, that removes liquid saturated water (point 4-b) to the mixer and saturated water-vapour (point 4-c) to the second heating phase. A second heating phase (superheating) to produce superheated steam at point 5. A steam use phase with the implementation of a water– steam Rankine cycle with classic steam turbines and condenser. A receiver module for the solar boiler is depicted in Fig. 3. It mainly consists in a transparent cover inside
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which there is a medium vacuum level, in order to protect the absorber selective coating of the quick degradation that it can suffer at high-temperatures in air presence (Alvarez et al., 2005). The absorber is an array of tubes inside a high-conductivity metallic plate coated by a high-absorptivity, low emissivity material (the same coating of standard trough collector tubes). The tube diameter and the pitch between tubes can be optimized in a given design according to some relevant variables, as the heat load flux (which depends on the solar concentration) the water/ steam conditions (speed and title) and the working pressure. In this conceptual design, the absorbing wall will be formed by an arrangement of modules of this kind with its geometry dependant on the heliostat footprint or heliostat size. In a first approach for a general design, we have chosen a solar irradiation surface power density of about 74.4 kW/ m2, with an operating pressure of 4 MPa (what implies boiling at 523 K) with a 20% of vapour title at the exit of the boiling section and an overheating of 150 K to obtain the adequate vapour quality at turbine outlet, 89.5%, when the Rankine cycle chosen has two extractions (with iso-enthalpyc steps). In order to have proper thermalhydraulic conditions in the boiler, the inner tube diameter was fixed at 2.4 cm without separation pitch. Steam quality is one of the key design options to fix in this concept. In our preliminary analysis it will be seen that the higher the title in the steam separator, the better performance is obtained. Nevertheless, this limit must be fixed after a detailed parametric study, including experimental evidence in mock-ups, in order to avoid the dry-out inside the boiler tubes, what would be catastrophic for their integrity. As a reference value, the height of the first stage collectors (preheating plus boiling) has been fixed in 20 m. The width of the collectors has been adjusted for having the required electric output in the generator. In a first example of a small size prototype, this value has been fixed in 4.6 MWe, for a total thermal power absorption in the facility of 20 MWth. Similarly, the height of the overheating section (second stage of collector, after the steam separator) was fixed in 10 m, the width being also adjusted to the required final power. It goes without saying that both types of collector can be made in modular arrangements, both in length and span. That option is almost mandatory from the point of view of structural robustness and simplicity in the dilation joints, but it is out of the scope of this paper. 3. Thermal model of the facility A design tool has been developed using the Engineering Equation Solver code (EES, F-Chart, 2005) to analyse the conceptual design of this solar boiler concept. An analytical model to describe the heat transfer and hydraulic phenomena in the solar reception surface has been done,
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1 2 3 4-a 4-b 5 6 7 8 9 10
Boiler grid inlet Boiler grid outlet Phase separator Re-heat grid inlet Mixer hot inlet Re-heat grid outlet Turbine inlet Turbine outlet Condenser outlet Pressurization pump Mixer
Fig. 2. Thermal schematics of a solar boiler facility.
1 2 3 4
Heating tubes Dilatation absorber Glass cover Thermal insulation
Fig. 3. A horizontal cross-section of the solar boiler collector.
what include features as the treatment of the radiation losses, heat conductivity trough the radiation absorbing module arrangement in function of its geometry, hydrodynamic analysis of the single and two-phase flow in the tubes and the water-vapour thermodynamic cycle. A very important part of our modelling effort has been focused in the description of the thermal hydrodynamic behaviour of the two-phase flow into the module tubes. There are significant differences between the physical phenomena involved in a solar boiler in comparison with a combustion boiler. The utilization of solar radiation as main energy source and the open energy absorption surface requires a special analysis of the radiation processes to estimate accurately the total amount of energy that can potentially be used for steam production. The flexibility given by a suitable concentration system allows a homogeneous treatment of the radiation flux in the absorbing surface.
Such thermal treatment is rather different in the case of a combustion chamber where the flame is controlled by the burners and radiation exchange has certain heterogeneity as flame characteristics and radiation view factors are variable along the boiler wall. Another important difference is the close nature of a combustion chamber in opposition to the radiation exchange with the environment in our concept. This fact makes the thermal efficiency behaviour completely different as it will be dominated by conduction losses in a combustion chamber and by radiation losses in the case of our solar boiler, what implies a much stronger dependence on the temperature in the latter. The energy balance in the absorbing surface of the solar boiler can be briefly denoted by: Qsun ¼ Qreflected þ Qloss;convection þ Qloss;radiation þ Quseful
ð1Þ
where Qsun is the solar radiation that is illuminating the solar absorbing surface, corresponding to the direct radiation that is reflected by the heliostat field. Such energy can be either reflected, or lost by the collector area of the modules, or absorbed. Thermal losses trough the support structure has been evaluated as less than 1.5% in parabolic though collectors (Forristal, 2003). Thermal insulated structures will have even less losses in our case, being negligible for the thermal model developed for this conceptual design. The absorbed energy and the losses that are produced by convection and radiation, adding the cooling effect of the heat transfer fluid (water/steam) determines the temperature level and the energy efficiency in the boiler itself that can be given, in function of the enthalpy increase in the heat transfer fluid (Quseful) by: gboiler ¼
Quseful Qsun
ð2Þ
J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
ð3Þ
where system pumping and recirculation pumping power will be include in the evaluation of the cycle efficiency. This term (gte) will be evaluated in this section with our EES thermal model of the boiler and the cycle. The radiation losses from the tube absorbing wall structure are one of the critical issues and can be estimated in function of the sky temperature (Tsky) and ambient dew temperature (Tdew) (Martin and Berdahl, 1982), being Ts the absorbing surface and Ta the ambient temperatures: Qloss;radiation ¼ As r e
T sky
ðT 4s
T 4sky Þ
¼ As hrad ðT s T a Þ h ¼ T a 0:711 þ 0:0056 ðT dew 273Þ i1 2 4 þ0:000073 ðT dew 273Þ
ð4Þ
ð5Þ
The behaviour of the flow inside the heating tubes can be described as in a combustion boiler with well-known phenomenological correlations as the Lienhard expression for the film heat transfer coefficients (Lienhard, 2006) based on Kandlikar work (Kandlikar and Nariai, 1999). Concerning pressure drop, Martinelli–Nelson correlations (Tong and Tang, 1997) has been implemented in our model for two-phase flow while in single-phase flow we use the Gnielinski correlation (Gnielinski, 1976). The overall heat transfer coefficient that takes into account tube conduction and tube to water/steam should be maximised versus the convection and radiation processes, using a suitable absorbing coating in the modules. In our model, free convection is assumed in the external surface of the module arrangement and the combination of radiation–convection heat transfer mechanism has been modelled for the thermal interaction between heated absorbing surface and the glass cover. We have validating our model evaluating the efficiency of available data form commercial flat plate and other parabolic trough collectors (Forristal, 2003; Moss and Brosseau, 2007) with discrepancies below 1%. The result of our model validation with available parabolic trough data, working at a comparable temperature range of the solar boiler, is shown in Fig. 4, giving enough confidence in our tools for the calculation of the solar boiler behaviour. The results of our simulation are shown in the following figures. The first heating stage (preheater and boiler) has been analysed fixing a group of modules forming an arrangement with a height of 20 m and formed by 303 3.3 cm outer diameter tubes with a set of module arrangement width of 10 m. The surface heat flux given by the concentrating optics has been fixed to 74.4 kW/m2. In Fig. 5, we can see the temperatures reached by the water
Temperature (C)
gte ¼ gboiler gcycle
Collector outlet temperature 450 400 350 300 250 200 150 100 50 0 860
AZTRAK data Solar Boiler Model
880
900
920
940
960
980
1000
Solar radiation (W/m2) Fig. 4. Validation of our solar boiler model vs. AZTRAK data (Moss and Brosseau, 2007).
versus tube height. A temperature difference below 20 K is found between the external surface of the tube and the internal flow. The glass outer temperature is also shown, being important for the evaluation of the losses by free convection, and kept in the order of 330 K. The maximum temperature in the coating is limited to less than 750 K, less than the maximum operating temperature than can be reached in the coating (Kumar, 1997). We have made a thermal–mechanical stress analysis to assess the mechanical integrity of the absorbing tubes. Assuming a stainless steel AISI302 type, with a Young modulus of 19 1010 Pa, a conductivity of 19 W/m-K, a Poisson module of 0.29 and a linear thermal expansion coefficient of 1.79 105 K1, our 1.5 mm thickness tube can withstand a 18 K temperature difference between internal and external surface. In Fig. 6, we can see how our heat flux density is well below safety limits, taking for granted the integrity of the system under long-term operation. The overall energy balance in the receiver surface can be seen in Fig. 7, where the energy fluxes in this first heating section are shown. The high film coefficient in the boiling section makes much higher thermal transfer rates to the tube flow, minimising the total losses that include radiation and convection to the environment. Fig. 8 shows the energy efficiency evolution along the reception surface height. The analysis of the superheating section has been done to assess the maximum temperature that will achieve the 560
External surface tube
520
Temperature [K]
The efficiency of the thermodynamic cycle (gcycle), must also be taken into account, in order to have an overall thermo-electrical conversion efficiency of the facility (gte) expressed as:
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Fluid mean temperature 480 440 400 360
Glass outer temperature
320 0
2
4 6 8 10 12 14 16 Preheater & Boiler height [m]
18
Fig. 5. Temperatures in the preheater and boiler region vs. height.
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20
0.757
18
ΔΤ max
0.7565
Efficiency [-]
16
Δ T [C]
14 12 10
ΔΤ
8
0.756
Energy Efficiency 0.7555 0.755
6
0.7545
4 2 50000
0.754 95000
140000
185000
230000
275000
0
4 8 12 16 Preheater & Boiler height [m]
qA [W/m2] Fig. 6. Maximum temperature difference between internal and external tubes surface (in blue) admisible by the stainless steel considered (AISI302) and temperature difference for the boiler tubes operating condition (540 K). It can be seen that with a lower thermal power density the operation is more safety in terms of tubes thermal stress (hoop stress). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
coating, that in our design, with a 150 K steam superheating, is estimated below 750 K (Fig. 9). This temperature is below the acceptable operating temperature that the coating will achieve (Rubbia, 2006). Energy efficiencies are slightly reducing along the tube height as a consequence of the increasing radiation losses as the surface temperature rises. In the meantime, thermo-mechanical conversion efficiency of the cycle (gc) increases due to the higher temperature of the steam. The overall plant efficiency will be a compromise between the solar boiler efficiency, whose tendency will be dominated by radiation losses and decreasing with temperature, and the thermodynamic cycle efficiency that will increase with steam temperature. This fact implies that the optimal temperature of superheated steam in a solar boiler applied to electricity production should be limited (see Figs. 9 and 10). The maximum steam temperature reached in our case is 690 K (417 °C), the working pressure 40 bar, but the mixer pressure is 42.4 bar due of the pressure drop in first heating, and the condensation pressure is 7385 Pa, correspond-
Fig. 8. Energy efficiency along the first heating section.
ing with 40 °C. The yield of this power cycle taking into account turbine, condenser, two extractions, alternator and pressurization pump is near of 32.6%, including enthalpy drops in turbine and pumps inlet-outlet, the pumping and turbine isentropic efficiencies (0.65 and 0.83, respectively) and the alternator efficiency (0.97). The sketch of the Rankine cycle can be seen in Fig. 11, and the T-s evolution in Fig. 12. In Table 1 are tabulated the main operating points in the water/steam cycle. The turbine iso-entrophic efficiency has been calculated with the Spencer – Cotton – Cannon method (Spencer et al., 1974), taking into account humidity and vapour mass flow (or power) corrections. 4. Performance of the facility The system efficiency in our conceptual facility can be evaluated taking into account the heliostat efficiency, the field efficiency, the boiler efficiency and the block of power efficiency (see Fig. 13). gsystem ¼ greflection gfield gboiler gcycle
0.8 2
Power Flux [kW/m ]
2
Power Flux [kW/m ]
Total
Absorbed
60
Gain
50 40 30
Reflected
20 10 0 02
ð6Þ
The reflection efficiency is referred to the heliostat performance, depending on heliostat reflectivity, being evaluated as 0.92 (valid value for this article purposes, less
80 70
4
6
8
10
12
14
Preheater & Boiler height [m]
16
18
20
0.7
Total losses
0.6 0.5 0.4
Radiation losses
0.3
Convection losses
0.2 0.1 02
Rear losses 4
6
8
10
12
14
16
Preheater & Boiler height [m]
Fig. 7. Energy fluxes in the module arrangement vs. height.
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800
0.835
750
Tubes surface Efficiency
0.83
700 650
Efficiency [-]
Temperature [K]
1719
600 550
Fluid mean temperature
500 450
0.825 0.82 0.815
Glass outer surface
400
0.81
350 1
2
3
4 5 6 7 8 Overheater height [m]
9
10
1
2
3
4 5 6 7 8 Overheater height [m]
9
10
Fig. 9. Temperatures along the superheating section.
Fig. 10. Energy efficiency in the superheating section.
than others considered in the literature, Segal and Epstein, 2001; Ferna´ndez-Reche et al., 2006). Field efficiency is a parameter that will depend in our case on several assumptions, as the cosine factor, shadows and blocking, air transmittance and spillage. In our case, optical analysis has some differences respect to central tower optical analysis as we our sun rays will fly in parallel 2-D mode, instead of focused 3-D mode. In our case, every row of modules at the absorbing wall will be illuminated by a fixed set of
heliostats, given flexibility to optimize incident angle by setting a fix inclination of the modules depending of the position of the heliostats. The cosine factor will be the most important contribution to the field efficiency that we have estimating in 0.82, including shading, blocking losses and spillage. The boiler efficiency has been obtained by our thermal model, taking into account the glass cover and selective coating optic performance (absortivity, emissivity) and
Fig. 11. A sketch of the flow and energy diagram.
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Steam IAPWS
700
5 100 bar
600
40 bar
T [K]
2
500 13 11
400
4 6
3
1
12 7
10 8
300
9
0,2
0,6
0,4
0,8 7385 Pa
200
0
1100 2200 3300 4400 5500 6600 7700 8800
s [J/kg-K] Fig. 12. Simplified T-s diagram of the thermodynamic cycle.
Table 1 Water/steam characteristics at selected stages based on Fig. 12. Point
Temperature (K)
Pressure (bar)
Enthalpy (J/kg)
1 2 3 4 5 6 7 8 9 10 11 12
507.7 523.5 523.5 523.5 690.7 522.5 377.2 313.2 313.2 377.2 378.0 442.8
42.40 40.00 40.00 40.00 39.97 9.29 1.17 0.07 0.07 1.17 39.69 42.40
1.01E+06 1.09E+06 1.43E+06 2.80E+06 3.26E+06 2.94E+06 2.63E+06 2.32E+06 1.68E+05 4.36E+05 4.43E+05 7.20E+05
The main parameters of a solar boiler application to electricity production are given in Table 2. The main figures report an overall facility efficiency of 20%, referred as total electrical output to direct radiation in the heliostat field in our reference design (Fig. 13), comparable with existing solar technologies for electricity generation, with a foreseen reduction in cost (Table 3). Some optimization work is in progress to increase this efficiency. Such optimization would be focused on a better material selection of the absorbing coating, heliostat field optimization or an innovative collector design to diminish radiation losses in the absorbing structure. Additionally, higher power units would be considered, up to 50 MWe or so. On the other hand, storage and hybridization are major technological options for solar energy development. Future work should be shed some light about the applicability of hybridization and storage of this concept, although no major show-stoppers are foreseen.
Quality (–) 0.20
0.98 0.89
the heat transmission between steel tubes and fluid at each stage, and stated to be 0.84. Concerning the cycle efficiency, we estimate 0.326 according with the analysis shown in the previous section.
5. Conclusions and future work A conceptual design of a solar boiler has been proposed in this paper. This concept comes from the conventional thermal power plants boiler, with the difference that the heat comes from mirrors that concentrate the solar radiation on its external absorbing side at open air, instead of fuel flames heating its internal tube grid. The results of our conceptual design show that plant performance of 20% is achievable, comparable with foreseen figures in the most advanced solar concepts, but with less foreseen operational and technological constraints and limits, either in control of the instabilities of the solar radiation and its effects, pumping power, dry-out risks, or thermal overloads of the structural materials. The system seems to be very
Fig. 13. Power flow diagram in the solar boiler.
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Table 2 Main parameters of the solar boiler plant. Features Solar design radiation (W/m ) Power flux (kW/m2) (Boiler/Overheater) Net electric power (MWe) Energetic efficiency (%) Environment temperature (°C) Environment humidity (%) Unit number Mirrors surface (m2) Tower height (m) (boiler + overheater) Thermal power (MWth) Boiler efficiency (%) Collectors surface (m2) Cycle Turbine number Iso-enthalpic extractions number Fluid Temperatures (°C) condenser/turbine inlet Net conversion efficiency (%) Iso-entrophic turbine efficiency Iso-entrophic pumps efficiency Electro-mechanical turbine efficiency Electro-mechanical pump efficiency Electro-mechanical alternator efficiency Cycle pumping power (kWe) Recirculation pumping power (kWe) Selective coat absortivity Selective coat emissivity Mirrors reflectivity Mirrors tracking mistake per axis (mRad) Preheating and boiling tubes Tube diameter Preheating and boiling length/width (m) Superheating tubes Tube diameter Superheating length/width (m)
Design conditions
Heliostat field (technical features) Receptor (technical features)
Thermo-electric conversion
Optical properties
Grill
a
Value 2
800 74/52 4.5 20 20 66 726 39.6 20 + 10 17.4 0.84 260 (200 + 60) Rankine 1 2 Water/vapour 40/417 32,56 0.83a 0.65 0.98 0.98 0.97 36,4 2,9 0.9 0.1 0.92 1.2 303 100 20/10 222 3= 00
4
10/6
Turbine iso-entrophic efficiency obtained through the Spencer – Cotton – Cannon method (Spencer et al., 1974).
Table 3 Solar technologies comparison.
Power Operation temperature Nominal efficiency Commercial state Technological risk Available storage Hybrid design
Parabolic through
Central receipt
Solar boiler
Parabolic dishes
30–80 MW 390 °C 20% Avaliable Low Limited Yes
10–200 MW 565 °C 23% Available Low–Medium Yes Yes
4–50 MW 417 °C 20% Conceptual Low–Medium Yes Yes
5–25 kW 750 °C 29.40% Proto-Demo High Batteries Yes
robust, and no major technological show-stoppers have been envisaged in our first analysis. Note that this concept could use standard materials already available. The main point in our development is related to a better use of the concentrated radiation in order to reduce exergy losses. This feature, and technological robustness, could imply competitive levelized energy costs of electricity production, but additional work is needed to evaluate the plant performance as a detailed lay-out of the heliostat field and the behaviour of the system under transient conditions and to optimise its design windows, in variables such as radia-
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Kumar, D., 1997. An overview of solar thermal devices based solar water heating systems & the necessity of using solar selective coating. Renewable Energy 10 (2/3), 355–361. Lienhard, J.H., 2006. A Heat Transfer Textbook, third ed. Phlogiston Press, Cambridge, Massachusetts. Martin, M., Berdahl, P., 1982. The thermal radiance of clear skies. Solar Energy 29 (4), 299–314. Mills, D., 2004. Advances in solar thermal electricity technology. Solar Energy 76, 19–31. Moss, T., Brosseau, D., 2007, Testing Capabilities NSTTF (AZTRAK) Rotating Platform, Sandia National Laboratories, Albuquerque, New Mexico, USA. PS10, 2008. PS10. Available from:. Rubbia, C., 2006, Advances in Solar Energies: Looking into the Future. In: 13th Biennial SolarPACES Symposium, 20–23 June 2006, Sevilla, Espan˜a. Segal, A., Epstein, M., 2001. The optics of the solar tower reflector. Solar Energy 69 (Suppl. 6), 229–241. Spencer, R.C., Cotton, K.C., Cannon, C.N., 1974. A method for predicting the performance of steam turbine generators 16,500 kW and larger. Based on ASME Paper 62-WA-209, revised July 1974, reprinted by the General Electric Company. Tong, L.S., Tang, Y.S., 1997. Boiling Heat Transfer and Two-Phase Flow, second ed. Springer, Berlin. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2006. Control scheme for direct steam generation in parabolic troughs under recirculation operation mode. Solar Energy 80 (1), 1–17. Zarza, E., Valenzuela, L., Leo´n, J., Hennecke, K., Eck, M., Weyers, H.D., Eickhoff, M., 2004. Direct steam generation in parabolic troughs: final results and conclusions of the DISS project. Energy 29 (5–6), 635–644.
Solar Energy 83 (2009) 1713–1722 www.elsevier.com/locate/solener
A conceptual design of solar boiler Javier Mun˜oz, Alberto Aba´nades *, Jose´ M. Martı´nez-Val Universidad Polite´cnica de Madrid, ETSII, J.Gutie´rrez Abascal, 2, 28006 Madrid, Spain Received 1 September 2008; received in revised form 22 May 2009; accepted 12 June 2009 Available online 21 July 2009 Communicated by: Associate Editor Robert Pitz-Paal
Abstract State-of-the-art concepts for solar thermal power systems are based on parabolic trough, tower or parabolic disks either heating molten salts, mineral oil, air or generating steam. We propose in this paper, a conceptual design of a solar boiler. This concept comes from the conventional thermal power plants boiler, with the difference that the heat comes from mirrors that concentrate the solar radiation on wall-type array of solar collectors, instead of coming from fuel flames and hot gases. In our preliminary performance, analysis of this innovative solar boiler applied to electricity production, we have found that overall efficiency of the conversion from direct solar irradiation energy to electricity is above 20%, which is comparable to the value of parabolic trough and central tower technologies. Besides that, the concept seems very robust and could overcome some drawbacks derived from pressure losses, control complexity and material thermo-mechanical stress. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Concentrated solar energy; Conceptual design; Electric power generation; Sustainable development
1. Introduction and background Electricity generation in solar thermal power plants is a fundamental point in the quest to Sustainable Development (EC, 2004). There is a sizeable fraction of the continental surface with very high solar radiation flux for a large number of hours (PS10, 2008). It is true that most of this land corresponds to deserts which are far away from regions of high energy consumption rates, but ultra-high voltage transmission lines can help overcome that drawback. On the other hand, some regions with high-level solar radiation are much close to intensively populated areas, as in the southern part of Europe and in the southwest of the United States. In summary, solar thermal electricity seems very suited to match the requirements of a fraction of the World population, if fossil fuel prices continue to climb
*
Corresponding author. Tel.: +34 913364268; fax: +34 913363079. E-mail addresses: [email protected], [email protected] (A. Aba´nades). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.06.009
(as expected from increasing demand and stiffness in the producing fields) and CO2 emission must be limited. There is a large variety of solar power plant configurations for electricity generation, mainly depending on the solar light concentration configuration, absorbing collector arrangements, and working fluids, both in the collectors and in the thermodynamic cycle (CIEMAT, 2006). The most classical structure is based on parabolic trough collectors using mineral oil as high-temperature coolant, and using a Rankine steam cycle in the balance of power. This cycle is activated in a central water boiler heated by the oil coming from the solar field (Frier et al., 1998). Many variations can be devised for this objective. In particular, the concentration of the solar radiation onto a central receiver mounted at the top of a tower. There are different receiver configurations (Buck et al., 2006). As an example, it can be a cavity-type collector with an optional transparent window for minimizing heat losses. Radiation losses are minimized by reducing the window aperture. Conduction losses area is usually negligible, because of isolation blankets around hot parts. Other configurations can
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eliminate the window, just be based on an external tube arrangement (Solar One), on volumetric receivers (SOLGATE project). The first commercial plant of this technology has recently started its operation in the south of Spain (PS10, 2008). One of the possibilities to activate a Rankine steam cycle is to directly generate steam inside the tube of parabolic trough collectors, what increase plant efficiency by eliminating the steam generator required when using other heat transfer fluids. A large experimental facility of this type has been built in Plataforma Solar de Almerı´a (Spain) (Zarza et al., 2004; Valenzuela et al., 2006). Projects for building a power plant of this type are under study. A totally different proposal is introduced in this paper. The configuration presented here is based on the structure of coal-fired power plants, where steam is produced in a large array of vertical tubes heated by the combustion products (B&W, 2005). In our case, the array of tubes will be formed by a set of absorbing modules on the wall of a tower properly placed in the solar field (where the concentrating mirrors are). Of course, there will be a main difference between both structures. The coal-fired boiler is a large box encircling the burning volume, and the solar boiler has to be open to receive the concentrated sun radiation. A sketch of the power plant proposed is depicted in Fig. 1. The solar filed is shown in a very simplified style, but it is important to note that the total collector surface is much larger, in width and height, than the standard cavity receiver in a central tower scheme (Abengoa, 2008). Our
rationale was inspired in the ‘‘do not heat at temperatures higher than you need” principle, so reducing the technological requirements for the materials maximizing energy and exergy efficiency. This fact conveys a degree of freedom to select the focusing spot of each mirror in a more flexible way. The lower part of the collectors would act as preheater and boiler, and a steam separator would be placed at a suitable height, in order to recirculate the liquid water and to send the steam to the overheater, which would obviously be at the highest part of the circuit. From a qualitative point of view, there are pros and cons in this type of power plant, either in relation to direct steam generation in trough collectors or in relation to central cavity receivers. As compared to trough collectors, steam generation follows in a much better way natural rules when it is produced in vertical tubes. Boiling inside horizontal tube is a rather complex situation, with a phase separation between the liquid layer in the bottom and the steam phase occupying partially the tube cross section. Slip ratio between both phases (with very different speeds) adds some complexity to the picture. Liquid recirculation becomes mandatory, and poses control difficulties (Valenzuela et al., 2006). The standard vertical array of the boiler tubes has been proven for more than one century which is the simplest, most efficient and most robust way to generate steam. Besides that, pumping power will be much lower in our case for the same level of absorbed thermal power. It goes without saying that it is much simpler to make the photons
1 2 3 4 5 6 7 8 9 10 11 12 13 Fig. 1. Solar boiler conceptual scheme.
Boiler body First heating grid Solar field Condensed pump Recirculation pump Down pipe Insulation Phase separator Solar boiler outlet Vapour dryer Second heating grid Overheated vapour pipe Laying of foundations
J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
run (until reaching a central boiler) than making the steam run along the very long distances of the solar field of parabolic trough tubes and connecting headers and pipes. In relation to steam generation in a central cavity receiver, our proposal seems to have much higher heat losses. This is true for the same level of temperatures, but temperatures and heat loads in the solar boiler collectors will be lower, or much lower, than those inside their cavity receiver. Hence, losses can be kept low enough in our case, and the collector efficiency can therefore be high enough. As a matter of fact, the heat loads and the temperatures inside the cavity receiver can be too high (Alvarez et al., 2005) for the objective of feeding a Rankine steam cycle of standard values, for instance 4 MPa and 690 K of overheated steam. If 4 MPa is chosen as working pressure to generate steam, the boiler will be slightly above 523 K, which is the saturation temperature. It is worth pointing out a fundamental point for understanding the moderate values of the radiation losses: currently available coating (used, for instance, in trough collectors tubes) have a very high solar absorptivity and very low thermal emissivity at temperatures below 800 K. In fact, those values can be 0.9 and 0.1, respectively (Alvarez et al., 2005; Mills, 2004). It means that radiation losses, depending on e T4, will be modest if the temperatures are not too high. This is also a key point in this design option that is intended to propose a first-generation solar power plant with an overall efficiency well above 20% (Martin and Berdahl, 1982). In Section 2, a brief description of the solar boiler concept is presented. Its thermal performance is calculated in Section 3, for a small prototype unit, although the methodology can be applied to a plant of higher power. The analysis of the performance of the facility is discussed in Section 4. Conclusions and future work are discussed in Section 5. 2. Description of a power solar boiler facility A sketch of a solar boiler for electricity production is shown in Fig. 2, where the boiler coupled to a Rankine cycle is shown. The main steps in this scheme are: A first heating module (Preheating and boiler), where slightly subcooled water enters the boiler at point 1, and reaches boiling until a certain vapour fraction at point 2. A phase separator, that removes liquid saturated water (point 4-b) to the mixer and saturated water-vapour (point 4-c) to the second heating phase. A second heating phase (superheating) to produce superheated steam at point 5. A steam use phase with the implementation of a water– steam Rankine cycle with classic steam turbines and condenser. A receiver module for the solar boiler is depicted in Fig. 3. It mainly consists in a transparent cover inside
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which there is a medium vacuum level, in order to protect the absorber selective coating of the quick degradation that it can suffer at high-temperatures in air presence (Alvarez et al., 2005). The absorber is an array of tubes inside a high-conductivity metallic plate coated by a high-absorptivity, low emissivity material (the same coating of standard trough collector tubes). The tube diameter and the pitch between tubes can be optimized in a given design according to some relevant variables, as the heat load flux (which depends on the solar concentration) the water/ steam conditions (speed and title) and the working pressure. In this conceptual design, the absorbing wall will be formed by an arrangement of modules of this kind with its geometry dependant on the heliostat footprint or heliostat size. In a first approach for a general design, we have chosen a solar irradiation surface power density of about 74.4 kW/ m2, with an operating pressure of 4 MPa (what implies boiling at 523 K) with a 20% of vapour title at the exit of the boiling section and an overheating of 150 K to obtain the adequate vapour quality at turbine outlet, 89.5%, when the Rankine cycle chosen has two extractions (with iso-enthalpyc steps). In order to have proper thermalhydraulic conditions in the boiler, the inner tube diameter was fixed at 2.4 cm without separation pitch. Steam quality is one of the key design options to fix in this concept. In our preliminary analysis it will be seen that the higher the title in the steam separator, the better performance is obtained. Nevertheless, this limit must be fixed after a detailed parametric study, including experimental evidence in mock-ups, in order to avoid the dry-out inside the boiler tubes, what would be catastrophic for their integrity. As a reference value, the height of the first stage collectors (preheating plus boiling) has been fixed in 20 m. The width of the collectors has been adjusted for having the required electric output in the generator. In a first example of a small size prototype, this value has been fixed in 4.6 MWe, for a total thermal power absorption in the facility of 20 MWth. Similarly, the height of the overheating section (second stage of collector, after the steam separator) was fixed in 10 m, the width being also adjusted to the required final power. It goes without saying that both types of collector can be made in modular arrangements, both in length and span. That option is almost mandatory from the point of view of structural robustness and simplicity in the dilation joints, but it is out of the scope of this paper. 3. Thermal model of the facility A design tool has been developed using the Engineering Equation Solver code (EES, F-Chart, 2005) to analyse the conceptual design of this solar boiler concept. An analytical model to describe the heat transfer and hydraulic phenomena in the solar reception surface has been done,
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1 2 3 4-a 4-b 5 6 7 8 9 10
Boiler grid inlet Boiler grid outlet Phase separator Re-heat grid inlet Mixer hot inlet Re-heat grid outlet Turbine inlet Turbine outlet Condenser outlet Pressurization pump Mixer
Fig. 2. Thermal schematics of a solar boiler facility.
1 2 3 4
Heating tubes Dilatation absorber Glass cover Thermal insulation
Fig. 3. A horizontal cross-section of the solar boiler collector.
what include features as the treatment of the radiation losses, heat conductivity trough the radiation absorbing module arrangement in function of its geometry, hydrodynamic analysis of the single and two-phase flow in the tubes and the water-vapour thermodynamic cycle. A very important part of our modelling effort has been focused in the description of the thermal hydrodynamic behaviour of the two-phase flow into the module tubes. There are significant differences between the physical phenomena involved in a solar boiler in comparison with a combustion boiler. The utilization of solar radiation as main energy source and the open energy absorption surface requires a special analysis of the radiation processes to estimate accurately the total amount of energy that can potentially be used for steam production. The flexibility given by a suitable concentration system allows a homogeneous treatment of the radiation flux in the absorbing surface.
Such thermal treatment is rather different in the case of a combustion chamber where the flame is controlled by the burners and radiation exchange has certain heterogeneity as flame characteristics and radiation view factors are variable along the boiler wall. Another important difference is the close nature of a combustion chamber in opposition to the radiation exchange with the environment in our concept. This fact makes the thermal efficiency behaviour completely different as it will be dominated by conduction losses in a combustion chamber and by radiation losses in the case of our solar boiler, what implies a much stronger dependence on the temperature in the latter. The energy balance in the absorbing surface of the solar boiler can be briefly denoted by: Qsun ¼ Qreflected þ Qloss;convection þ Qloss;radiation þ Quseful
ð1Þ
where Qsun is the solar radiation that is illuminating the solar absorbing surface, corresponding to the direct radiation that is reflected by the heliostat field. Such energy can be either reflected, or lost by the collector area of the modules, or absorbed. Thermal losses trough the support structure has been evaluated as less than 1.5% in parabolic though collectors (Forristal, 2003). Thermal insulated structures will have even less losses in our case, being negligible for the thermal model developed for this conceptual design. The absorbed energy and the losses that are produced by convection and radiation, adding the cooling effect of the heat transfer fluid (water/steam) determines the temperature level and the energy efficiency in the boiler itself that can be given, in function of the enthalpy increase in the heat transfer fluid (Quseful) by: gboiler ¼
Quseful Qsun
ð2Þ
J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
ð3Þ
where system pumping and recirculation pumping power will be include in the evaluation of the cycle efficiency. This term (gte) will be evaluated in this section with our EES thermal model of the boiler and the cycle. The radiation losses from the tube absorbing wall structure are one of the critical issues and can be estimated in function of the sky temperature (Tsky) and ambient dew temperature (Tdew) (Martin and Berdahl, 1982), being Ts the absorbing surface and Ta the ambient temperatures: Qloss;radiation ¼ As r e
T sky
ðT 4s
T 4sky Þ
¼ As hrad ðT s T a Þ h ¼ T a 0:711 þ 0:0056 ðT dew 273Þ i1 2 4 þ0:000073 ðT dew 273Þ
ð4Þ
ð5Þ
The behaviour of the flow inside the heating tubes can be described as in a combustion boiler with well-known phenomenological correlations as the Lienhard expression for the film heat transfer coefficients (Lienhard, 2006) based on Kandlikar work (Kandlikar and Nariai, 1999). Concerning pressure drop, Martinelli–Nelson correlations (Tong and Tang, 1997) has been implemented in our model for two-phase flow while in single-phase flow we use the Gnielinski correlation (Gnielinski, 1976). The overall heat transfer coefficient that takes into account tube conduction and tube to water/steam should be maximised versus the convection and radiation processes, using a suitable absorbing coating in the modules. In our model, free convection is assumed in the external surface of the module arrangement and the combination of radiation–convection heat transfer mechanism has been modelled for the thermal interaction between heated absorbing surface and the glass cover. We have validating our model evaluating the efficiency of available data form commercial flat plate and other parabolic trough collectors (Forristal, 2003; Moss and Brosseau, 2007) with discrepancies below 1%. The result of our model validation with available parabolic trough data, working at a comparable temperature range of the solar boiler, is shown in Fig. 4, giving enough confidence in our tools for the calculation of the solar boiler behaviour. The results of our simulation are shown in the following figures. The first heating stage (preheater and boiler) has been analysed fixing a group of modules forming an arrangement with a height of 20 m and formed by 303 3.3 cm outer diameter tubes with a set of module arrangement width of 10 m. The surface heat flux given by the concentrating optics has been fixed to 74.4 kW/m2. In Fig. 5, we can see the temperatures reached by the water
Temperature (C)
gte ¼ gboiler gcycle
Collector outlet temperature 450 400 350 300 250 200 150 100 50 0 860
AZTRAK data Solar Boiler Model
880
900
920
940
960
980
1000
Solar radiation (W/m2) Fig. 4. Validation of our solar boiler model vs. AZTRAK data (Moss and Brosseau, 2007).
versus tube height. A temperature difference below 20 K is found between the external surface of the tube and the internal flow. The glass outer temperature is also shown, being important for the evaluation of the losses by free convection, and kept in the order of 330 K. The maximum temperature in the coating is limited to less than 750 K, less than the maximum operating temperature than can be reached in the coating (Kumar, 1997). We have made a thermal–mechanical stress analysis to assess the mechanical integrity of the absorbing tubes. Assuming a stainless steel AISI302 type, with a Young modulus of 19 1010 Pa, a conductivity of 19 W/m-K, a Poisson module of 0.29 and a linear thermal expansion coefficient of 1.79 105 K1, our 1.5 mm thickness tube can withstand a 18 K temperature difference between internal and external surface. In Fig. 6, we can see how our heat flux density is well below safety limits, taking for granted the integrity of the system under long-term operation. The overall energy balance in the receiver surface can be seen in Fig. 7, where the energy fluxes in this first heating section are shown. The high film coefficient in the boiling section makes much higher thermal transfer rates to the tube flow, minimising the total losses that include radiation and convection to the environment. Fig. 8 shows the energy efficiency evolution along the reception surface height. The analysis of the superheating section has been done to assess the maximum temperature that will achieve the 560
External surface tube
520
Temperature [K]
The efficiency of the thermodynamic cycle (gcycle), must also be taken into account, in order to have an overall thermo-electrical conversion efficiency of the facility (gte) expressed as:
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Fluid mean temperature 480 440 400 360
Glass outer temperature
320 0
2
4 6 8 10 12 14 16 Preheater & Boiler height [m]
18
Fig. 5. Temperatures in the preheater and boiler region vs. height.
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20
0.757
18
ΔΤ max
0.7565
Efficiency [-]
16
Δ T [C]
14 12 10
ΔΤ
8
0.756
Energy Efficiency 0.7555 0.755
6
0.7545
4 2 50000
0.754 95000
140000
185000
230000
275000
0
4 8 12 16 Preheater & Boiler height [m]
qA [W/m2] Fig. 6. Maximum temperature difference between internal and external tubes surface (in blue) admisible by the stainless steel considered (AISI302) and temperature difference for the boiler tubes operating condition (540 K). It can be seen that with a lower thermal power density the operation is more safety in terms of tubes thermal stress (hoop stress). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
coating, that in our design, with a 150 K steam superheating, is estimated below 750 K (Fig. 9). This temperature is below the acceptable operating temperature that the coating will achieve (Rubbia, 2006). Energy efficiencies are slightly reducing along the tube height as a consequence of the increasing radiation losses as the surface temperature rises. In the meantime, thermo-mechanical conversion efficiency of the cycle (gc) increases due to the higher temperature of the steam. The overall plant efficiency will be a compromise between the solar boiler efficiency, whose tendency will be dominated by radiation losses and decreasing with temperature, and the thermodynamic cycle efficiency that will increase with steam temperature. This fact implies that the optimal temperature of superheated steam in a solar boiler applied to electricity production should be limited (see Figs. 9 and 10). The maximum steam temperature reached in our case is 690 K (417 °C), the working pressure 40 bar, but the mixer pressure is 42.4 bar due of the pressure drop in first heating, and the condensation pressure is 7385 Pa, correspond-
Fig. 8. Energy efficiency along the first heating section.
ing with 40 °C. The yield of this power cycle taking into account turbine, condenser, two extractions, alternator and pressurization pump is near of 32.6%, including enthalpy drops in turbine and pumps inlet-outlet, the pumping and turbine isentropic efficiencies (0.65 and 0.83, respectively) and the alternator efficiency (0.97). The sketch of the Rankine cycle can be seen in Fig. 11, and the T-s evolution in Fig. 12. In Table 1 are tabulated the main operating points in the water/steam cycle. The turbine iso-entrophic efficiency has been calculated with the Spencer – Cotton – Cannon method (Spencer et al., 1974), taking into account humidity and vapour mass flow (or power) corrections. 4. Performance of the facility The system efficiency in our conceptual facility can be evaluated taking into account the heliostat efficiency, the field efficiency, the boiler efficiency and the block of power efficiency (see Fig. 13). gsystem ¼ greflection gfield gboiler gcycle
0.8 2
Power Flux [kW/m ]
2
Power Flux [kW/m ]
Total
Absorbed
60
Gain
50 40 30
Reflected
20 10 0 02
ð6Þ
The reflection efficiency is referred to the heliostat performance, depending on heliostat reflectivity, being evaluated as 0.92 (valid value for this article purposes, less
80 70
4
6
8
10
12
14
Preheater & Boiler height [m]
16
18
20
0.7
Total losses
0.6 0.5 0.4
Radiation losses
0.3
Convection losses
0.2 0.1 02
Rear losses 4
6
8
10
12
14
16
Preheater & Boiler height [m]
Fig. 7. Energy fluxes in the module arrangement vs. height.
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J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
800
0.835
750
Tubes surface Efficiency
0.83
700 650
Efficiency [-]
Temperature [K]
1719
600 550
Fluid mean temperature
500 450
0.825 0.82 0.815
Glass outer surface
400
0.81
350 1
2
3
4 5 6 7 8 Overheater height [m]
9
10
1
2
3
4 5 6 7 8 Overheater height [m]
9
10
Fig. 9. Temperatures along the superheating section.
Fig. 10. Energy efficiency in the superheating section.
than others considered in the literature, Segal and Epstein, 2001; Ferna´ndez-Reche et al., 2006). Field efficiency is a parameter that will depend in our case on several assumptions, as the cosine factor, shadows and blocking, air transmittance and spillage. In our case, optical analysis has some differences respect to central tower optical analysis as we our sun rays will fly in parallel 2-D mode, instead of focused 3-D mode. In our case, every row of modules at the absorbing wall will be illuminated by a fixed set of
heliostats, given flexibility to optimize incident angle by setting a fix inclination of the modules depending of the position of the heliostats. The cosine factor will be the most important contribution to the field efficiency that we have estimating in 0.82, including shading, blocking losses and spillage. The boiler efficiency has been obtained by our thermal model, taking into account the glass cover and selective coating optic performance (absortivity, emissivity) and
Fig. 11. A sketch of the flow and energy diagram.
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Steam IAPWS
700
5 100 bar
600
40 bar
T [K]
2
500 13 11
400
4 6
3
1
12 7
10 8
300
9
0,2
0,6
0,4
0,8 7385 Pa
200
0
1100 2200 3300 4400 5500 6600 7700 8800
s [J/kg-K] Fig. 12. Simplified T-s diagram of the thermodynamic cycle.
Table 1 Water/steam characteristics at selected stages based on Fig. 12. Point
Temperature (K)
Pressure (bar)
Enthalpy (J/kg)
1 2 3 4 5 6 7 8 9 10 11 12
507.7 523.5 523.5 523.5 690.7 522.5 377.2 313.2 313.2 377.2 378.0 442.8
42.40 40.00 40.00 40.00 39.97 9.29 1.17 0.07 0.07 1.17 39.69 42.40
1.01E+06 1.09E+06 1.43E+06 2.80E+06 3.26E+06 2.94E+06 2.63E+06 2.32E+06 1.68E+05 4.36E+05 4.43E+05 7.20E+05
The main parameters of a solar boiler application to electricity production are given in Table 2. The main figures report an overall facility efficiency of 20%, referred as total electrical output to direct radiation in the heliostat field in our reference design (Fig. 13), comparable with existing solar technologies for electricity generation, with a foreseen reduction in cost (Table 3). Some optimization work is in progress to increase this efficiency. Such optimization would be focused on a better material selection of the absorbing coating, heliostat field optimization or an innovative collector design to diminish radiation losses in the absorbing structure. Additionally, higher power units would be considered, up to 50 MWe or so. On the other hand, storage and hybridization are major technological options for solar energy development. Future work should be shed some light about the applicability of hybridization and storage of this concept, although no major show-stoppers are foreseen.
Quality (–) 0.20
0.98 0.89
the heat transmission between steel tubes and fluid at each stage, and stated to be 0.84. Concerning the cycle efficiency, we estimate 0.326 according with the analysis shown in the previous section.
5. Conclusions and future work A conceptual design of a solar boiler has been proposed in this paper. This concept comes from the conventional thermal power plants boiler, with the difference that the heat comes from mirrors that concentrate the solar radiation on its external absorbing side at open air, instead of fuel flames heating its internal tube grid. The results of our conceptual design show that plant performance of 20% is achievable, comparable with foreseen figures in the most advanced solar concepts, but with less foreseen operational and technological constraints and limits, either in control of the instabilities of the solar radiation and its effects, pumping power, dry-out risks, or thermal overloads of the structural materials. The system seems to be very
Fig. 13. Power flow diagram in the solar boiler.
J. Mun˜oz et al. / Solar Energy 83 (2009) 1713–1722
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Table 2 Main parameters of the solar boiler plant. Features Solar design radiation (W/m ) Power flux (kW/m2) (Boiler/Overheater) Net electric power (MWe) Energetic efficiency (%) Environment temperature (°C) Environment humidity (%) Unit number Mirrors surface (m2) Tower height (m) (boiler + overheater) Thermal power (MWth) Boiler efficiency (%) Collectors surface (m2) Cycle Turbine number Iso-enthalpic extractions number Fluid Temperatures (°C) condenser/turbine inlet Net conversion efficiency (%) Iso-entrophic turbine efficiency Iso-entrophic pumps efficiency Electro-mechanical turbine efficiency Electro-mechanical pump efficiency Electro-mechanical alternator efficiency Cycle pumping power (kWe) Recirculation pumping power (kWe) Selective coat absortivity Selective coat emissivity Mirrors reflectivity Mirrors tracking mistake per axis (mRad) Preheating and boiling tubes Tube diameter Preheating and boiling length/width (m) Superheating tubes Tube diameter Superheating length/width (m)
Design conditions
Heliostat field (technical features) Receptor (technical features)
Thermo-electric conversion
Optical properties
Grill
a
Value 2
800 74/52 4.5 20 20 66 726 39.6 20 + 10 17.4 0.84 260 (200 + 60) Rankine 1 2 Water/vapour 40/417 32,56 0.83a 0.65 0.98 0.98 0.97 36,4 2,9 0.9 0.1 0.92 1.2 303 100 20/10 222 3= 00
4
10/6
Turbine iso-entrophic efficiency obtained through the Spencer – Cotton – Cannon method (Spencer et al., 1974).
Table 3 Solar technologies comparison.
Power Operation temperature Nominal efficiency Commercial state Technological risk Available storage Hybrid design
Parabolic through
Central receipt
Solar boiler
Parabolic dishes
30–80 MW 390 °C 20% Avaliable Low Limited Yes
10–200 MW 565 °C 23% Available Low–Medium Yes Yes
4–50 MW 417 °C 20% Conceptual Low–Medium Yes Yes
5–25 kW 750 °C 29.40% Proto-Demo High Batteries Yes
robust, and no major technological show-stoppers have been envisaged in our first analysis. Note that this concept could use standard materials already available. The main point in our development is related to a better use of the concentrated radiation in order to reduce exergy losses. This feature, and technological robustness, could imply competitive levelized energy costs of electricity production, but additional work is needed to evaluate the plant performance as a detailed lay-out of the heliostat field and the behaviour of the system under transient conditions and to optimise its design windows, in variables such as radia-
tion concentration, working pressure, and total power. For the moment, our analysis points out that the overall efficiency with this concept, for a given radiation field, is higher than the calculated values of other types of solar power plants. References Abengoa, 2008. ABENGOA Solar web page. Available from:
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