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Hybrid concentrated photovoltaic and thermal power conversion at different spectral bands
Solar Energy 76 (2004) 591–601 www.elsevier.com/locate/solener
Hybrid concentrated photovoltaic and thermal power conversion at different spectral bands Akiba Segal *, Michael Epstein, Amnon Yogev Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel Received 16 July 2003; received in revised form 18 November 2003; accepted 3 December 2003 Communicated by: Associate Editor Manuel Romero-Alvarez
Abstract The option to use the beam down optics of a solar tower system for large-scale and grid-connected concentrated photovoltaic (PV) cells is examined. The rationale is to use this system to split the solar spectrum. Part of the spectrum can be utilized for PV cells. For instance, but not limited to, mono-crystalline silicon cells can convert the 600–900 nm band to electricity at an efficiency of 55–60%. The rest of the spectrum remains concentrated and it can be used thermally to generate electricity in Rankine–Brayton cycles or to operate chemical processes. Two optical approaches for a large-scale system are described and analyzed. In the first concept, the hyperboloid-shaped tower reflector is used as the spectrum splitter. Its mirrors can be made of transparent fused silica glass, coated with a dielectric layer, functioning as a band-pass filter. The transmitted band reaches the upper focal zone, where an array of PV modules is placed. The location of these modules and their interconnections depend on the desirable concentration level and the uniformity of the flux distribution. The reflected band is directed to the second focal zone near the ground, where a compound parabolic concentrator is required to recover and enhance the concentration to a level depending on the operating temperature at this target. In the second approach, the total solar spectrum is reflected down by the tower reflector. Before reaching the lower focal plane, the spectrum is split and filtered. One band can be reflected and directed horizontally to a PV array and, in this case, the rest of the spectrum is transmitted to the lower focal plane. To illustrate the feasibility of these options, commercial silicon cells with antireflective coating, intended to operate under concentrated solar radiation in the range of 200–800 suns, were chosen. The results show that 6.5 MWe from the PV array and 11.1 MWe from a combined cycle can be generated starting from a solar heat input of 55.6 MW. 2003 Elsevier Ltd. All rights reserved. Keywords: Concentrated PV; Solar tower; Beam down optics
1. Introduction Operating photovoltaic (PV) cells under concentrated solar radiation have been studied extensively during the last three decades. The rationale behind this concept is that lenses or other optical concentrating elements are less expensive than solar cells. This statement should be proved to be accomplished and analyzed from time to
*
Corresponding author. Tel.: +972-8-934-2935; fax: +972-8934-4117. E-mail address: [email protected] (A. Segal).
time, in view of the cost reduction of PV cells. It is not only the cost of the optical elements, but also the more complex structure and drive mechanism required in concentrating systems to track the sun, which have to be considered. In addition, concentrated cells are more expensive than the flat plate module cells, because they must handle a higher current. Likewise, the assemblage of the concentrator modules is more intricate, since these are exposed to a higher solar incident flux and excess of heat. An excellent summary review is presented by Luque (1993). A recent report of a 480 kWp system, using single-axis tracking with linear focusing and PV cells operating at an average concentration ratio of about 40,
0038-092X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2003.12.002
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A. Segal et al. / Solar Energy 76 (2004) 591–601
asserts a considerable cost reduction compared to flat plate systems (Arboiro et al., 1998). In order to decrease further the cost, a concentration ratio of 300 was demonstrated in a PV system with single-axis tracking (Hein et al., 2003). To achieve this high concentration by means of a paraboloidal trough mirror, an additional three-dimensional second stage was applied, consisting of compound parabolic concentrators (CPCs). The spectral selectivity applied to a hybrid quantum or thermal solar system can improve the overall conversion efficiency of concentrated solar energy. The solar spectrum can be separated into spectral bands or windows matched to photoquantum processes and the balance used for photothermal conversion. The basic approaches of spectrally selective beam splitters using dichroic filters and liquid absorption filters were described by Osborn et al. (1986). Operation of multi-band gap concentrator cells, with a spectrum-splitting filter at 450–525 suns, has been reported by James et al. (1979). The splitting of the solar spectrum into different spectral bands for pumping lasers and operating solar PV cells, using a hyperboloidal dichroic mirror, which was a part of a Cassegrainian concentrator, was described by Yogev et al. (1996a). With a suitable design optimization, 60–70% of the sunlight to electricity conversion efficiency can be achieved (Penn, 2002). Yogev et al. (1996b) demonstrated the potential use of a large-scale, gridconnected solar concentrating system with spectrum splitting. The study by Yogev et al. (1996b) analyzes a beam down or tower reflector optics comprising a reflector made of dielectric mirrors with a beam-splitting ability and a negligible absorption coefficient. According to this concept, part of the solar energy collected by a heliostat field at a specific band gap,
optimized for the PV cells, can be utilized at a conversion efficiency as high as 60%, the balance of the power being available in a concentrated manner for thermal application, such as power production or the performance of chemical reactions. The present work provides a detailed assessment and analysis of the tower reflector optics as a basis for a large-scale concentrating PV system with beam splitting. Various geometries and configurations are analyzed and optimized.
2. The optical system To illustrate the current concept, an optical system comprising a field of heliostats surrounding a tower with a hyperboloidal reflector (Segal and Epstein, 1999) and having an aim point at 130 m above ground is considered. The layout of the heliostats is optimized, as well as the position of the hyperboloidal mirror, which constitutes the tower reflector, between its two foci. The upper focus coincides with the field aim point and the lower focus is located at the entrance plane of a group of seven identical secondary concentrators placed at 21 m above the level of the heliostats (Segal and Epstein, 1999, 2000) (Fig. 1). This illustration assumes a heliostat field consisting of 783 heliostats of 95.5 m2 each, according to the layout depicted in Fig. 2. Their layout is optimized with respect to maximization of the average energy delivered yearly by each heliostat in the field. This field is capable of delivering about 46 MW on the tower reflector at the design point (spring equinox, noon). In this example, the hyperboloidal mirror has an outside radius of 23.8 m. The field is asymmetric around the tower reflector as
Aim point First focal point Hyperboloidal mirror
Second focal point
Secondary concentrators
Heliostat field
Receiver
Heliostat field
Fig. 1. The principle of tower reflector optics.
A. Segal et al. / Solar Energy 76 (2004) 591–601
593
(m) 400
South-North Axis
300
200
100
0
-100
-200 -300
-200
-100
0
100
200
300 (m)
West-East Axis Fig. 2. Optimized layout of the heliostat field for the examples detailed in Tables 1 and 2.
shown in Fig. 2. Therefore, the radius of the hyperboloid at the South side is shorter than in the North side. The total area of this mirror is about 1250 m2 . In order to assure the verticality of the ground CPCs, the hyperboloidal axis is tilted at a small angle of 1.8, relative to the vertical (Segal and Epstein, 1999). The asymmetry of the heliostat field not only influences the shape of the tower reflector and its inclination but also the location and position of the PV array as described later. Two optional configurations of a selective mirror have been studied. In the first case, the tower reflector itself is a selective mirror. In this case, an array of PV cells is placed close to the area of the aim point. In the second case, the tower reflector is a regular mirror and somewhere between its location and the entrance plane of the ground CPCs, a selective paraboloidal mirror is placed. This selective mirror reflects about 30% of the radiation having a desirable range of wavelengths in a lateral direction, compared to the rays reflected by the tower reflector down to the CPCs. In this case, the array of the PV cells is positioned in the focal area of the selective paraboloidal mirror. The advantages and disadvantages of these two configurations are further analyzed.
3. The tower reflector as a selective mirror When the hyperboloidal tower reflector is used as a selective mirror, it transmits a part of the solar spectrum matching best the specific PV cells, e.g., for monocrystalline silicon, in a wavelength range of 600–900 nm.
At this range, a simple dielectric, selective mirror can be applied, although the peak efficiency of silicon cells is around 1 micron wavelength. As a result, the spectral emissive power, calculated according to the American Society for Testing and Materials (ASTM) (1999), is 30% of the total power in the spectral band of 600–840 nm. Three types of losses have been considered: (i) slope error of the hyperboloidal mirror due to imperfection of its geometrical shape was assumed as 1%, (ii) absorption by the mirror was assumed as 5% (the tower reflector’s mirrors are made of back silvered borosilicate glass with 95% reflectivity since they are exposed to concentrated light. The reflectivity can be improved to 98% using enhanced silver coatings), and (iii) about 5% of the rays transmitted through the mirror were absorbed by the metal structure that supports the selective reflector. The path of the rays originated at the heliostats and transmitted by the selective tower reflector reaching the focal zone has been analyzed. Since the field is asymmetric toward the North–South direction, the projection of the ray path in this plane has an apparent focal plane tilted with an angle of about 11 with respect to the horizontal plane (Fig. 3a), while in the West–East direction, the field is symmetric and the focal plane is horizontal (Fig. 3b). The flux distributions at these two planes are presented in Fig. 4a and b. This illustrates that the positioning of the PV array requires careful matching of the flux distribution. There is also a slight difference in the flux peak levels of these planes (2400 kW/m2 in Fig. 4a vs. 2200 kW/m2 in Fig. 4b). In addition, the section shown in Fig. 4b is slightly inferior in the total power collected in the circles
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Fig. 3. (a) Pattern of rays transmitted through a selective tower reflector on their path to the focal plane. Cross-section through yOz plane (Oz axis vs. South–North axis). (b) Pattern of rays transmitted through a selective tower reflector on their path to the focal plane. Cross-section through xOz plane (Oz axis vs. West–East axis).
having the same radius. The peak fluxes calculated at the focal plane are too high for direct exposure of PV cells. A possible solution is the displacement of the PV cells from the focal plane. For example, advancing the plane of the PV cells by 1.5 m before the focal plane in parallel to the xOy plane (i.e., at a height of 128.5 m) can result in a lower peak flux, as shown in Fig. 5. In this case, the peak flux is reduced from 2400 to about 800 kW/m2 . A peak flux of about 800–850 kW/m2 can be tolerated by the cells. The issue of non-uniformity is negligible in this case, since the size of the image is roughly confined in 6 · 9 meters rectangular while the size of the cell or a module of cells is small and the interconnection of several cells to achieve the desirable voltage requires relatively a small portion of the image area. An important question is to verify that the image does not move with the time during the day and the season. Fig. 6 shows the flux distribution at the same position of the plane of the PV cells (placed 1.5 m in front of the focal
plane) during representative hours and days. It can be seen that the peak level of the flux profile varies depending on the corresponding insolation but its position is unchanged. It can also be seen that beyond a radius of 4 m from the center of the image, the concentration can be considered as too low for its exploitation by the PV cells. The 4-m radius confines more than 90% of the power transmitted by the tower reflector. These findings confirm that by appropriate interconnections of the PV modules the issue of non-uniform illumination is in principal solved and the penalty on the total efficiency of the PV array is small. Results of the calculated output for the above example at the design point are presented in Table 1. From the total power that hits the heliostats, the 20.5% arrived at the PV cells is available for conversion at about 60% to electricity. In addition, it can be observed that from the power reaching the entrance plane of the CPCs, about 5% are lost and 49.6% can be utilized for thermal pur-
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Fig. 4. (a) The solar flux profile at the focal plane, cross-section parallel to the xOy plane, in the case of tower reflector as a selective mirror. (b) The solar flux profile at the focal plane, cross-section parallel to yOz plane, in the case of tower reflector as a selective mirror.
poses in the ground receivers. Consequently, a total optical efficiency of 70% can be achieved by this system. The evaluation of the total electric power that can be obtained at the design point and the contribution of the PV system in this specific illustration are summarized in Table 1. 4. A regular tower reflector and an additional paraboloidal mirror as a selective mirror The second configuration considered in this study conceives a paraboloidal mirror placed between the
tower reflector and the ground CPC. In this case, the selective mirror reflects the part of the spectrum delivered to the PV cells and transmits the part that is used for the thermal application. The reflected rays are concentrated in a focal plane positioned laterally to the vertical path of the rays reflected by the tower reflector (Fig. 7). The size of this paraboloidal mirror must be large enough to intercept most of the rays that could be collected by the ground CPCs in the absence of this mirror. Over-sizing this mirror could help to collect also the spillage around the CPCs. However, calculations show
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Fig. 5. Calculated solar flux distribution at a plane situated 1.5 m before the focal plane of the heliostats and parallel to the xOy plane.
Fig. 6. Solar flux distribution and power vs. radius of the PV array at representative hours during the year, for the situation shown in Fig. 5.
that this is not worthwhile. Therefore, the size of the paraboloidal mirror is calculated to be 470 m2 for the above basic example. Similar to the previous concept, three types of losses have been considered with respect to the paraboloidal beam splitter: (i) geometrical imperfection of the surface of the paraboloidal mirror, (ii) absorption in this mirror (both result in about 6% of the solar power delivered by the tower reflector), and
(iii) about 5% from the rays transmitted by the mirror are obstructed by the support structure of the paraboloidal mirror. The location of the center of the paraboloidal mirror was selected at 7 m above the entrance plane of the CPCs (28 m above the level of the heliostats). Determining the position of the parabolic mirror is a result of several competing considerations. It is desirable to place
A. Segal et al. / Solar Energy 76 (2004) 591–601 Table 1 Systematic energy balance for the case of the hyperboloidal beam splitter (design point––March 21, noon) Energy balance
kW
%
Hitting the heliostats Directed to the tower reflector Spillage around the tower reflector Hitting the tower reflector: Transmitted through the tower reflector Absorbed before reaching the plane of the PV cells Arriving at the plane of the PV cells: Spillage around the PV cells Hitting the PV cells (R ¼ 4 m) Absorbed by the tower reflector Reflected by the tower reflector to the ground CPCs Attenuation between the tower reflector and the CPCs Arrived at the entrance plane of the CPCs
55,635 47,323 718 46,605 13,304
100.0 85.1 1.3 83.8 23.9
658
1.2
12,646
22.7
1239 11,407 2345 30,956
2.2 20.5 4.2 55.6
556
1.0
30,400
54.6
it close to the CPCs entrance to enable the PV array to be installed at lower level. This, however, increases the size of the mirror if the majority of the rays reflected from the tower reflector should be intercepted. On the other hand, raising the mirror can expose it to higher radiation intensity, resulting in overheating. The
597
paraboloidal mirror is oriented in a way that a ray reflected vertically from the center of the tower reflector to the center of the paraboloidal mirror can be directed to a predetermined side focal point. An important characteristic of the optics of this paraboloidal mirror is its strong off-axis orientation. The incidence angle of the rays reflected by the tower reflector is between 38 and 45, and therefore, the achievable concentration level at the side focal plane is not high. A large range of focal distances has been investigated. The criterion for selecting the best focal distance was the position of the focal plane, which can be satisfactory far from the path of the edge rays reflected from the tower reflector to avoid obstruction and the flux distribution at this focal plane. Fig. 7 depicts schematically the calculated position of the focal plane, relative to other optical elements, which was found to be the most suitable case. It can be seen that this plane obstructs only a very small part of rays (less than 0.1%), which falls within the spillage around the entrance of the CPCs. Fig. 8a–c show the path of rays reflected by the paraboloidal selective mirror and projected on the planes of the coordinate system. The focal plane is tilted 28 in the South–North direction and 4–5 in the West– East direction. The above procedure reflects the vast amount of calculations required to select the suitable size and position of the parabolic reflector and the associated position and orientation of the PV array. The solar flux finally obtained at the selected focal plane of the paraboloidal selective mirror has a peak of 800 kW/m2 (Fig. 9). Since the size of the image is roughly 6 · 9 meters, the non-uniformity of the flux is again not an important issue, as explained previously. Fig. 10 shows the flux distribution at this focal plane for different representative hours along the year. The curves in Fig. 10 show that the position of the flux profile remains almost unchanged during the year. The changes are primarily due to solar irradiation. The energy balance for the design point (March 21, noon) is presented in Table 2. From the total power that hits the heliostat field, 20.6% arrived at the PV cells and 46.7% can be used for the thermal purposes in the ground receivers (5% optical losses were assumed in the ground concentrators). The advantage of this optical arrangement is that the PV cells can be placed only at about 40 m height instead of 128 m in the previous analyzed case, on a separate and relatively small mounting structure.
5. Ground receivers Fig. 7. Schematic layout of a beam down tower, reflector with a parabolic beam splitter and PV array located laterally.
Tables 1 and 2 show the power reaching the entrance plane of the ground concentrators at the design point,
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Fig. 8. (a) The projections of rays reflected by a paraboloidal beam splitter on the three planes of the coordinate system. The coordinates system of the paraboloidal mirror is translated and rotated, and thus, the plane xOy is tangent to the paraboloidal vertex. (b) The projections of the rays on the plane xOz (West–East axis vs. Oz axis). (c) The projections of the rays on the plane yOz (South– North axis vs. Oz axis).
which is 30.4 and 28.8 MW, respectively. This power can be utilized for various purposes, e.g., power generation or performance of chemical processes. The treatment of the specific receiver in each application is beyond the scope of this study. The optics of the tower reflector, including the relationship between the location of the reflector, its size and the dimensions of the ground CPC, has been analyzed in detail by Segal and Epstein (2000). The image obtained at the lower focal plane of a tower reflector is always magnified compared to the image that could be obtained at the aim point. A meticulous process of optimization provides the position (height) of the tower reflector applicable to the specific purposes. In the
above example, 95% of the power reaching the lower focal plane is confined in a circle having a radius of 8.5 m. The view angle of this beam is less than 26. This indicates that a concentrator having an entrance radius of 8.5 m and an acceptance angle of 13 will be most suitable. However, such a concentrator (CPC) is practically prohibitive, since it will have a height of 45 m. Possible solutions to this hurdle have been described in the literature, e.g., the use of a cluster of smaller concentrators. A possible arrangement of such a cluster is one central and six peripheral units (Segal and Epstein, 1999). Mechanically, a simple entrance geometry that can cover almost the entire image area confined in a certain
A. Segal et al. / Solar Energy 76 (2004) 591–601
100 2 kW/m
8
South-North Axis (m)
599
6
400 700 200
4
600
Focal plane The target plane tilted o with the angle of 28 with the horizontal plane 2 (flux in kW/m ) Total power collected 12.36 MW R=3m P= 8.41 MW R=4m P= 10.48 MW R=5m P= 11.59 MW
500 300 2 kW/m
2
-4
-2
0
2
4
West-East Axis (m) Fig. 9. Solar flux distribution on the focal plane of the paraboloidal selective mirror at the design point (March 21, noon).
Fig. 10. Solar flux distribution and power input on a lateral target vs. radius of the PV array at representative hours along the year.
radius by seven concentrators, are seven hexagons (Fig. 11) (Segal and Epstein, 2003). Each of these is circumscribed in a radius of 3.21 m, have an exit radius of 0.72 m and a non-truncated height of 17 m. Fig. 11 shows ray tracing in all of these seven concentrators and the power at the entrance of each concentrator for the design point. The values of the power shown in Fig. 11 are related to the case of the tower reflector splitter, as summarized in Table 1. The hexagonal cross-section necessary to cover the entire area can be transformed after some distance in the concentrator to a circular section, or can remain as a hexagonal crosssection until the exit. In the circular case, the extra losses due to rejection and additional reflections are negligible,
and in the hexagonal case, they can be accounted for 1.5–2% of the power entering the concentrator. Table 3 summarizes the power and losses from each of the concentrators for the case of the hexagonal cross-section. A better conception in this case is to bend six sheets of the reflective mirror, thus, the edge rays of each sheet will be positioned on the mathematical profile. The results presented in Table 3 are based on this conception for a simplified practical construction of a CPC. From the results shown in Table 3, it can be seen that the total calculated optical efficiency of the part of the spectrum that reaches the ground receiver and is available for the thermal application is 48.9% (from solar to the receiver, at the design point).
600
A. Segal et al. / Solar Energy 76 (2004) 591–601
Table 2 Systematic energy balance for the case of the paraboloidal beam splitter (design point––March 21, noon) Energy balance
kW
%
Hitting the heliostats Directed to the tower reflector Spillage around the tower reflector Hitting the tower reflector: Absorbed by the tower reflector Reflected by the tower reflector Attenuation between the tower reflector and the CPCs Arriving at the paraboloidal selective mirror: Absorbed by the paraboloidal mirror Reflected and arrived at the plane of the PV cells: Spillage around the plane of the PV cells Hitting the plane of the PV cells Arriving at the entrance plane of the ground CPCs
55,635 47,323 718 46,605 2316 44,289 545
100.0 85.1 1.3 83.8 4.2 79.6 1.0
43,744
78.6
2625
4.7
12,341
22.2
892
1.6
11,449
20.6
28,778
51.7
6. Conclusions The feasibility of adding existing technologies of beam splitting and PV cells, operating with concentrated light at a specified spectral band, to high temperature applications, concerning the unique optics of a tower reflector, was studied. This probability shows that beam splitting can be performed either by using a semitransparent tower reflector or on the way down from the reflector to the ground secondary concentrator. In the
first case, the tower reflector transmits the PV band and in the second option, a paraboloidal diverting mirror reflects this band. Each option has its advantages and weaknesses, which have to be further analyzed with economical tools. The spectral band selected for mono-crystalline silicone PV cells is in the range of 600–900 nm and the average operating flux is of 500 suns. Under these conditions, the cost of the PV modules are almost negligible and the major cost items are related to additional heliostats, to conversion of the tower reflector to a spectral filter and to the cooling requirements of the PV modules. However, the concept analyzed in this paper is certainly not limited to the use of mono-crystalline silicone PV cells. Other types of PV cells that can handle the high electrical currents generated under the above concentrations, and are chemically stable, can be used as well. For instance, thin film PV modules could be promising candidates. Beam splitters based on a dielectric bandpass filter with a negligible absorption are available at a relatively low cost. Assessing the total conversion efficiency of the solar plant as described in the example analyzed in this paper for generating of electricity in both PV and thermally via combined cycle, shows that above 32% can be achieved. Needless to say that this is a very encouraging figure in spite of the additional optical losses associated with the current concept. In the case that a solar thermal plant is used, e.g., for chemical applications, the addition of a PV system could be very promising from an economical viewpoint, compared to other PV large-scale options, which, certainly, deserves further economic investigation. In this case the PV array can flexibly designed to provide all the local electrical needs of a stand-alone solar chemical
Fig. 11. Rays traces at the entrance plane of the ground concentrators (CPCs).
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Table 3 Energy balance for the ground concentrators (design point––March 21, noon) Energy balance
Power arrived at the entrance plane of the CPCs Spillage around the CPCs Power entering the CPCs Central CPC Peripheral CPC P1 Peripheral CPC P2 Peripheral CPC P3 Peripheral CPC P4 Peripheral CPC P5 Peripheral CPC P6
Power input
Losses by absorption and rejection (kW)
Net power entering the receiver (kW)
Net % of total power reaching the heliostats
kW
%
30,400
100.0
54.6
1474 28,926 13,496 2553 2560 2529 2602 2606 2580
4.9 95.1 44.4 8.4 8.4 8.3 8.6 8.6 8.5
2.6 52.0 22.8 4.3 4.3 4.3 4.4 4.4 4.4
plant or can be designed also to export the surplus of electricity. References American Society for Testing and Materials (ASTM), 1999. Solar spectral irradiance: air mass 1.5. Terrestrial Reference Spectra for Photovoltaic Performance Evaluation. Available from
811 148 146 149 157 159 155
12,685 2405 2411 2380 2445 2449 2425
Luque, A., 1993. Coupling light to solar cells. In: Prince, M. (Ed.), Advances in Solar Energy, vol. 8. American Solar Energy Society, Boulder, CO, pp. 161–230. Osborn, D.E., Chendo, M.A.C., Hamdy, M.A., Luttermann, F., Jacobson, M.R., Macleod, H.A., Swenson, R., 1986. Spectral selectivity applied to hybrid concentration systems. Solar Energy Materials 14 (3–5), 299–325. Penn, J.P., 2002. High concentration spectrum splitting solar collector. US Patent, 6,469,241, October 22, 2002. Segal, A., Epstein, M., 1999. Comparative performances of tower top and tower reflector central solar receiver. Solar Energy 65, 206–226. Segal, A., Epstein, M., 2000. The optics of the solar tower reflector. Solar Energy 69 (Suppl.), 229–241. Segal, A., Epstein, M., 2003. Solar ground reformer. Solar Energy 75, 479–490. Yogev, A., Appelbaum, J., Oron, M., Yehezkel, N., 1996a. Concentrating and splitting of solar radiation for laser pumping and photovoltaic conversions. Journal of Propulsion and Power 12 (2), 405–409. Yogev, A., Krupkin, V., Epstein, M., 1996b. Solar energy plant. US Patent, 5,578,140, November 26, 1996.
Hybrid concentrated photovoltaic and thermal power conversion at different spectral bands Akiba Segal *, Michael Epstein, Amnon Yogev Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel Received 16 July 2003; received in revised form 18 November 2003; accepted 3 December 2003 Communicated by: Associate Editor Manuel Romero-Alvarez
Abstract The option to use the beam down optics of a solar tower system for large-scale and grid-connected concentrated photovoltaic (PV) cells is examined. The rationale is to use this system to split the solar spectrum. Part of the spectrum can be utilized for PV cells. For instance, but not limited to, mono-crystalline silicon cells can convert the 600–900 nm band to electricity at an efficiency of 55–60%. The rest of the spectrum remains concentrated and it can be used thermally to generate electricity in Rankine–Brayton cycles or to operate chemical processes. Two optical approaches for a large-scale system are described and analyzed. In the first concept, the hyperboloid-shaped tower reflector is used as the spectrum splitter. Its mirrors can be made of transparent fused silica glass, coated with a dielectric layer, functioning as a band-pass filter. The transmitted band reaches the upper focal zone, where an array of PV modules is placed. The location of these modules and their interconnections depend on the desirable concentration level and the uniformity of the flux distribution. The reflected band is directed to the second focal zone near the ground, where a compound parabolic concentrator is required to recover and enhance the concentration to a level depending on the operating temperature at this target. In the second approach, the total solar spectrum is reflected down by the tower reflector. Before reaching the lower focal plane, the spectrum is split and filtered. One band can be reflected and directed horizontally to a PV array and, in this case, the rest of the spectrum is transmitted to the lower focal plane. To illustrate the feasibility of these options, commercial silicon cells with antireflective coating, intended to operate under concentrated solar radiation in the range of 200–800 suns, were chosen. The results show that 6.5 MWe from the PV array and 11.1 MWe from a combined cycle can be generated starting from a solar heat input of 55.6 MW. 2003 Elsevier Ltd. All rights reserved. Keywords: Concentrated PV; Solar tower; Beam down optics
1. Introduction Operating photovoltaic (PV) cells under concentrated solar radiation have been studied extensively during the last three decades. The rationale behind this concept is that lenses or other optical concentrating elements are less expensive than solar cells. This statement should be proved to be accomplished and analyzed from time to
*
Corresponding author. Tel.: +972-8-934-2935; fax: +972-8934-4117. E-mail address: [email protected] (A. Segal).
time, in view of the cost reduction of PV cells. It is not only the cost of the optical elements, but also the more complex structure and drive mechanism required in concentrating systems to track the sun, which have to be considered. In addition, concentrated cells are more expensive than the flat plate module cells, because they must handle a higher current. Likewise, the assemblage of the concentrator modules is more intricate, since these are exposed to a higher solar incident flux and excess of heat. An excellent summary review is presented by Luque (1993). A recent report of a 480 kWp system, using single-axis tracking with linear focusing and PV cells operating at an average concentration ratio of about 40,
0038-092X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2003.12.002
592
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asserts a considerable cost reduction compared to flat plate systems (Arboiro et al., 1998). In order to decrease further the cost, a concentration ratio of 300 was demonstrated in a PV system with single-axis tracking (Hein et al., 2003). To achieve this high concentration by means of a paraboloidal trough mirror, an additional three-dimensional second stage was applied, consisting of compound parabolic concentrators (CPCs). The spectral selectivity applied to a hybrid quantum or thermal solar system can improve the overall conversion efficiency of concentrated solar energy. The solar spectrum can be separated into spectral bands or windows matched to photoquantum processes and the balance used for photothermal conversion. The basic approaches of spectrally selective beam splitters using dichroic filters and liquid absorption filters were described by Osborn et al. (1986). Operation of multi-band gap concentrator cells, with a spectrum-splitting filter at 450–525 suns, has been reported by James et al. (1979). The splitting of the solar spectrum into different spectral bands for pumping lasers and operating solar PV cells, using a hyperboloidal dichroic mirror, which was a part of a Cassegrainian concentrator, was described by Yogev et al. (1996a). With a suitable design optimization, 60–70% of the sunlight to electricity conversion efficiency can be achieved (Penn, 2002). Yogev et al. (1996b) demonstrated the potential use of a large-scale, gridconnected solar concentrating system with spectrum splitting. The study by Yogev et al. (1996b) analyzes a beam down or tower reflector optics comprising a reflector made of dielectric mirrors with a beam-splitting ability and a negligible absorption coefficient. According to this concept, part of the solar energy collected by a heliostat field at a specific band gap,
optimized for the PV cells, can be utilized at a conversion efficiency as high as 60%, the balance of the power being available in a concentrated manner for thermal application, such as power production or the performance of chemical reactions. The present work provides a detailed assessment and analysis of the tower reflector optics as a basis for a large-scale concentrating PV system with beam splitting. Various geometries and configurations are analyzed and optimized.
2. The optical system To illustrate the current concept, an optical system comprising a field of heliostats surrounding a tower with a hyperboloidal reflector (Segal and Epstein, 1999) and having an aim point at 130 m above ground is considered. The layout of the heliostats is optimized, as well as the position of the hyperboloidal mirror, which constitutes the tower reflector, between its two foci. The upper focus coincides with the field aim point and the lower focus is located at the entrance plane of a group of seven identical secondary concentrators placed at 21 m above the level of the heliostats (Segal and Epstein, 1999, 2000) (Fig. 1). This illustration assumes a heliostat field consisting of 783 heliostats of 95.5 m2 each, according to the layout depicted in Fig. 2. Their layout is optimized with respect to maximization of the average energy delivered yearly by each heliostat in the field. This field is capable of delivering about 46 MW on the tower reflector at the design point (spring equinox, noon). In this example, the hyperboloidal mirror has an outside radius of 23.8 m. The field is asymmetric around the tower reflector as
Aim point First focal point Hyperboloidal mirror
Second focal point
Secondary concentrators
Heliostat field
Receiver
Heliostat field
Fig. 1. The principle of tower reflector optics.
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593
(m) 400
South-North Axis
300
200
100
0
-100
-200 -300
-200
-100
0
100
200
300 (m)
West-East Axis Fig. 2. Optimized layout of the heliostat field for the examples detailed in Tables 1 and 2.
shown in Fig. 2. Therefore, the radius of the hyperboloid at the South side is shorter than in the North side. The total area of this mirror is about 1250 m2 . In order to assure the verticality of the ground CPCs, the hyperboloidal axis is tilted at a small angle of 1.8, relative to the vertical (Segal and Epstein, 1999). The asymmetry of the heliostat field not only influences the shape of the tower reflector and its inclination but also the location and position of the PV array as described later. Two optional configurations of a selective mirror have been studied. In the first case, the tower reflector itself is a selective mirror. In this case, an array of PV cells is placed close to the area of the aim point. In the second case, the tower reflector is a regular mirror and somewhere between its location and the entrance plane of the ground CPCs, a selective paraboloidal mirror is placed. This selective mirror reflects about 30% of the radiation having a desirable range of wavelengths in a lateral direction, compared to the rays reflected by the tower reflector down to the CPCs. In this case, the array of the PV cells is positioned in the focal area of the selective paraboloidal mirror. The advantages and disadvantages of these two configurations are further analyzed.
3. The tower reflector as a selective mirror When the hyperboloidal tower reflector is used as a selective mirror, it transmits a part of the solar spectrum matching best the specific PV cells, e.g., for monocrystalline silicon, in a wavelength range of 600–900 nm.
At this range, a simple dielectric, selective mirror can be applied, although the peak efficiency of silicon cells is around 1 micron wavelength. As a result, the spectral emissive power, calculated according to the American Society for Testing and Materials (ASTM) (1999), is 30% of the total power in the spectral band of 600–840 nm. Three types of losses have been considered: (i) slope error of the hyperboloidal mirror due to imperfection of its geometrical shape was assumed as 1%, (ii) absorption by the mirror was assumed as 5% (the tower reflector’s mirrors are made of back silvered borosilicate glass with 95% reflectivity since they are exposed to concentrated light. The reflectivity can be improved to 98% using enhanced silver coatings), and (iii) about 5% of the rays transmitted through the mirror were absorbed by the metal structure that supports the selective reflector. The path of the rays originated at the heliostats and transmitted by the selective tower reflector reaching the focal zone has been analyzed. Since the field is asymmetric toward the North–South direction, the projection of the ray path in this plane has an apparent focal plane tilted with an angle of about 11 with respect to the horizontal plane (Fig. 3a), while in the West–East direction, the field is symmetric and the focal plane is horizontal (Fig. 3b). The flux distributions at these two planes are presented in Fig. 4a and b. This illustrates that the positioning of the PV array requires careful matching of the flux distribution. There is also a slight difference in the flux peak levels of these planes (2400 kW/m2 in Fig. 4a vs. 2200 kW/m2 in Fig. 4b). In addition, the section shown in Fig. 4b is slightly inferior in the total power collected in the circles
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Fig. 3. (a) Pattern of rays transmitted through a selective tower reflector on their path to the focal plane. Cross-section through yOz plane (Oz axis vs. South–North axis). (b) Pattern of rays transmitted through a selective tower reflector on their path to the focal plane. Cross-section through xOz plane (Oz axis vs. West–East axis).
having the same radius. The peak fluxes calculated at the focal plane are too high for direct exposure of PV cells. A possible solution is the displacement of the PV cells from the focal plane. For example, advancing the plane of the PV cells by 1.5 m before the focal plane in parallel to the xOy plane (i.e., at a height of 128.5 m) can result in a lower peak flux, as shown in Fig. 5. In this case, the peak flux is reduced from 2400 to about 800 kW/m2 . A peak flux of about 800–850 kW/m2 can be tolerated by the cells. The issue of non-uniformity is negligible in this case, since the size of the image is roughly confined in 6 · 9 meters rectangular while the size of the cell or a module of cells is small and the interconnection of several cells to achieve the desirable voltage requires relatively a small portion of the image area. An important question is to verify that the image does not move with the time during the day and the season. Fig. 6 shows the flux distribution at the same position of the plane of the PV cells (placed 1.5 m in front of the focal
plane) during representative hours and days. It can be seen that the peak level of the flux profile varies depending on the corresponding insolation but its position is unchanged. It can also be seen that beyond a radius of 4 m from the center of the image, the concentration can be considered as too low for its exploitation by the PV cells. The 4-m radius confines more than 90% of the power transmitted by the tower reflector. These findings confirm that by appropriate interconnections of the PV modules the issue of non-uniform illumination is in principal solved and the penalty on the total efficiency of the PV array is small. Results of the calculated output for the above example at the design point are presented in Table 1. From the total power that hits the heliostats, the 20.5% arrived at the PV cells is available for conversion at about 60% to electricity. In addition, it can be observed that from the power reaching the entrance plane of the CPCs, about 5% are lost and 49.6% can be utilized for thermal pur-
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Fig. 4. (a) The solar flux profile at the focal plane, cross-section parallel to the xOy plane, in the case of tower reflector as a selective mirror. (b) The solar flux profile at the focal plane, cross-section parallel to yOz plane, in the case of tower reflector as a selective mirror.
poses in the ground receivers. Consequently, a total optical efficiency of 70% can be achieved by this system. The evaluation of the total electric power that can be obtained at the design point and the contribution of the PV system in this specific illustration are summarized in Table 1. 4. A regular tower reflector and an additional paraboloidal mirror as a selective mirror The second configuration considered in this study conceives a paraboloidal mirror placed between the
tower reflector and the ground CPC. In this case, the selective mirror reflects the part of the spectrum delivered to the PV cells and transmits the part that is used for the thermal application. The reflected rays are concentrated in a focal plane positioned laterally to the vertical path of the rays reflected by the tower reflector (Fig. 7). The size of this paraboloidal mirror must be large enough to intercept most of the rays that could be collected by the ground CPCs in the absence of this mirror. Over-sizing this mirror could help to collect also the spillage around the CPCs. However, calculations show
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Fig. 5. Calculated solar flux distribution at a plane situated 1.5 m before the focal plane of the heliostats and parallel to the xOy plane.
Fig. 6. Solar flux distribution and power vs. radius of the PV array at representative hours during the year, for the situation shown in Fig. 5.
that this is not worthwhile. Therefore, the size of the paraboloidal mirror is calculated to be 470 m2 for the above basic example. Similar to the previous concept, three types of losses have been considered with respect to the paraboloidal beam splitter: (i) geometrical imperfection of the surface of the paraboloidal mirror, (ii) absorption in this mirror (both result in about 6% of the solar power delivered by the tower reflector), and
(iii) about 5% from the rays transmitted by the mirror are obstructed by the support structure of the paraboloidal mirror. The location of the center of the paraboloidal mirror was selected at 7 m above the entrance plane of the CPCs (28 m above the level of the heliostats). Determining the position of the parabolic mirror is a result of several competing considerations. It is desirable to place
A. Segal et al. / Solar Energy 76 (2004) 591–601 Table 1 Systematic energy balance for the case of the hyperboloidal beam splitter (design point––March 21, noon) Energy balance
kW
%
Hitting the heliostats Directed to the tower reflector Spillage around the tower reflector Hitting the tower reflector: Transmitted through the tower reflector Absorbed before reaching the plane of the PV cells Arriving at the plane of the PV cells: Spillage around the PV cells Hitting the PV cells (R ¼ 4 m) Absorbed by the tower reflector Reflected by the tower reflector to the ground CPCs Attenuation between the tower reflector and the CPCs Arrived at the entrance plane of the CPCs
55,635 47,323 718 46,605 13,304
100.0 85.1 1.3 83.8 23.9
658
1.2
12,646
22.7
1239 11,407 2345 30,956
2.2 20.5 4.2 55.6
556
1.0
30,400
54.6
it close to the CPCs entrance to enable the PV array to be installed at lower level. This, however, increases the size of the mirror if the majority of the rays reflected from the tower reflector should be intercepted. On the other hand, raising the mirror can expose it to higher radiation intensity, resulting in overheating. The
597
paraboloidal mirror is oriented in a way that a ray reflected vertically from the center of the tower reflector to the center of the paraboloidal mirror can be directed to a predetermined side focal point. An important characteristic of the optics of this paraboloidal mirror is its strong off-axis orientation. The incidence angle of the rays reflected by the tower reflector is between 38 and 45, and therefore, the achievable concentration level at the side focal plane is not high. A large range of focal distances has been investigated. The criterion for selecting the best focal distance was the position of the focal plane, which can be satisfactory far from the path of the edge rays reflected from the tower reflector to avoid obstruction and the flux distribution at this focal plane. Fig. 7 depicts schematically the calculated position of the focal plane, relative to other optical elements, which was found to be the most suitable case. It can be seen that this plane obstructs only a very small part of rays (less than 0.1%), which falls within the spillage around the entrance of the CPCs. Fig. 8a–c show the path of rays reflected by the paraboloidal selective mirror and projected on the planes of the coordinate system. The focal plane is tilted 28 in the South–North direction and 4–5 in the West– East direction. The above procedure reflects the vast amount of calculations required to select the suitable size and position of the parabolic reflector and the associated position and orientation of the PV array. The solar flux finally obtained at the selected focal plane of the paraboloidal selective mirror has a peak of 800 kW/m2 (Fig. 9). Since the size of the image is roughly 6 · 9 meters, the non-uniformity of the flux is again not an important issue, as explained previously. Fig. 10 shows the flux distribution at this focal plane for different representative hours along the year. The curves in Fig. 10 show that the position of the flux profile remains almost unchanged during the year. The changes are primarily due to solar irradiation. The energy balance for the design point (March 21, noon) is presented in Table 2. From the total power that hits the heliostat field, 20.6% arrived at the PV cells and 46.7% can be used for the thermal purposes in the ground receivers (5% optical losses were assumed in the ground concentrators). The advantage of this optical arrangement is that the PV cells can be placed only at about 40 m height instead of 128 m in the previous analyzed case, on a separate and relatively small mounting structure.
5. Ground receivers Fig. 7. Schematic layout of a beam down tower, reflector with a parabolic beam splitter and PV array located laterally.
Tables 1 and 2 show the power reaching the entrance plane of the ground concentrators at the design point,
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Fig. 8. (a) The projections of rays reflected by a paraboloidal beam splitter on the three planes of the coordinate system. The coordinates system of the paraboloidal mirror is translated and rotated, and thus, the plane xOy is tangent to the paraboloidal vertex. (b) The projections of the rays on the plane xOz (West–East axis vs. Oz axis). (c) The projections of the rays on the plane yOz (South– North axis vs. Oz axis).
which is 30.4 and 28.8 MW, respectively. This power can be utilized for various purposes, e.g., power generation or performance of chemical processes. The treatment of the specific receiver in each application is beyond the scope of this study. The optics of the tower reflector, including the relationship between the location of the reflector, its size and the dimensions of the ground CPC, has been analyzed in detail by Segal and Epstein (2000). The image obtained at the lower focal plane of a tower reflector is always magnified compared to the image that could be obtained at the aim point. A meticulous process of optimization provides the position (height) of the tower reflector applicable to the specific purposes. In the
above example, 95% of the power reaching the lower focal plane is confined in a circle having a radius of 8.5 m. The view angle of this beam is less than 26. This indicates that a concentrator having an entrance radius of 8.5 m and an acceptance angle of 13 will be most suitable. However, such a concentrator (CPC) is practically prohibitive, since it will have a height of 45 m. Possible solutions to this hurdle have been described in the literature, e.g., the use of a cluster of smaller concentrators. A possible arrangement of such a cluster is one central and six peripheral units (Segal and Epstein, 1999). Mechanically, a simple entrance geometry that can cover almost the entire image area confined in a certain
A. Segal et al. / Solar Energy 76 (2004) 591–601
100 2 kW/m
8
South-North Axis (m)
599
6
400 700 200
4
600
Focal plane The target plane tilted o with the angle of 28 with the horizontal plane 2 (flux in kW/m ) Total power collected 12.36 MW R=3m P= 8.41 MW R=4m P= 10.48 MW R=5m P= 11.59 MW
500 300 2 kW/m
2
-4
-2
0
2
4
West-East Axis (m) Fig. 9. Solar flux distribution on the focal plane of the paraboloidal selective mirror at the design point (March 21, noon).
Fig. 10. Solar flux distribution and power input on a lateral target vs. radius of the PV array at representative hours along the year.
radius by seven concentrators, are seven hexagons (Fig. 11) (Segal and Epstein, 2003). Each of these is circumscribed in a radius of 3.21 m, have an exit radius of 0.72 m and a non-truncated height of 17 m. Fig. 11 shows ray tracing in all of these seven concentrators and the power at the entrance of each concentrator for the design point. The values of the power shown in Fig. 11 are related to the case of the tower reflector splitter, as summarized in Table 1. The hexagonal cross-section necessary to cover the entire area can be transformed after some distance in the concentrator to a circular section, or can remain as a hexagonal crosssection until the exit. In the circular case, the extra losses due to rejection and additional reflections are negligible,
and in the hexagonal case, they can be accounted for 1.5–2% of the power entering the concentrator. Table 3 summarizes the power and losses from each of the concentrators for the case of the hexagonal cross-section. A better conception in this case is to bend six sheets of the reflective mirror, thus, the edge rays of each sheet will be positioned on the mathematical profile. The results presented in Table 3 are based on this conception for a simplified practical construction of a CPC. From the results shown in Table 3, it can be seen that the total calculated optical efficiency of the part of the spectrum that reaches the ground receiver and is available for the thermal application is 48.9% (from solar to the receiver, at the design point).
600
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Table 2 Systematic energy balance for the case of the paraboloidal beam splitter (design point––March 21, noon) Energy balance
kW
%
Hitting the heliostats Directed to the tower reflector Spillage around the tower reflector Hitting the tower reflector: Absorbed by the tower reflector Reflected by the tower reflector Attenuation between the tower reflector and the CPCs Arriving at the paraboloidal selective mirror: Absorbed by the paraboloidal mirror Reflected and arrived at the plane of the PV cells: Spillage around the plane of the PV cells Hitting the plane of the PV cells Arriving at the entrance plane of the ground CPCs
55,635 47,323 718 46,605 2316 44,289 545
100.0 85.1 1.3 83.8 4.2 79.6 1.0
43,744
78.6
2625
4.7
12,341
22.2
892
1.6
11,449
20.6
28,778
51.7
6. Conclusions The feasibility of adding existing technologies of beam splitting and PV cells, operating with concentrated light at a specified spectral band, to high temperature applications, concerning the unique optics of a tower reflector, was studied. This probability shows that beam splitting can be performed either by using a semitransparent tower reflector or on the way down from the reflector to the ground secondary concentrator. In the
first case, the tower reflector transmits the PV band and in the second option, a paraboloidal diverting mirror reflects this band. Each option has its advantages and weaknesses, which have to be further analyzed with economical tools. The spectral band selected for mono-crystalline silicone PV cells is in the range of 600–900 nm and the average operating flux is of 500 suns. Under these conditions, the cost of the PV modules are almost negligible and the major cost items are related to additional heliostats, to conversion of the tower reflector to a spectral filter and to the cooling requirements of the PV modules. However, the concept analyzed in this paper is certainly not limited to the use of mono-crystalline silicone PV cells. Other types of PV cells that can handle the high electrical currents generated under the above concentrations, and are chemically stable, can be used as well. For instance, thin film PV modules could be promising candidates. Beam splitters based on a dielectric bandpass filter with a negligible absorption are available at a relatively low cost. Assessing the total conversion efficiency of the solar plant as described in the example analyzed in this paper for generating of electricity in both PV and thermally via combined cycle, shows that above 32% can be achieved. Needless to say that this is a very encouraging figure in spite of the additional optical losses associated with the current concept. In the case that a solar thermal plant is used, e.g., for chemical applications, the addition of a PV system could be very promising from an economical viewpoint, compared to other PV large-scale options, which, certainly, deserves further economic investigation. In this case the PV array can flexibly designed to provide all the local electrical needs of a stand-alone solar chemical
Fig. 11. Rays traces at the entrance plane of the ground concentrators (CPCs).
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Table 3 Energy balance for the ground concentrators (design point––March 21, noon) Energy balance
Power arrived at the entrance plane of the CPCs Spillage around the CPCs Power entering the CPCs Central CPC Peripheral CPC P1 Peripheral CPC P2 Peripheral CPC P3 Peripheral CPC P4 Peripheral CPC P5 Peripheral CPC P6
Power input
Losses by absorption and rejection (kW)
Net power entering the receiver (kW)
Net % of total power reaching the heliostats
kW
%
30,400
100.0
54.6
1474 28,926 13,496 2553 2560 2529 2602 2606 2580
4.9 95.1 44.4 8.4 8.4 8.3 8.6 8.6 8.5
2.6 52.0 22.8 4.3 4.3 4.3 4.4 4.4 4.4
plant or can be designed also to export the surplus of electricity. References American Society for Testing and Materials (ASTM), 1999. Solar spectral irradiance: air mass 1.5. Terrestrial Reference Spectra for Photovoltaic Performance Evaluation. Available from
811 148 146 149 157 159 155
12,685 2405 2411 2380 2445 2449 2425
Luque, A., 1993. Coupling light to solar cells. In: Prince, M. (Ed.), Advances in Solar Energy, vol. 8. American Solar Energy Society, Boulder, CO, pp. 161–230. Osborn, D.E., Chendo, M.A.C., Hamdy, M.A., Luttermann, F., Jacobson, M.R., Macleod, H.A., Swenson, R., 1986. Spectral selectivity applied to hybrid concentration systems. Solar Energy Materials 14 (3–5), 299–325. Penn, J.P., 2002. High concentration spectrum splitting solar collector. US Patent, 6,469,241, October 22, 2002. Segal, A., Epstein, M., 1999. Comparative performances of tower top and tower reflector central solar receiver. Solar Energy 65, 206–226. Segal, A., Epstein, M., 2000. The optics of the solar tower reflector. Solar Energy 69 (Suppl.), 229–241. Segal, A., Epstein, M., 2003. Solar ground reformer. Solar Energy 75, 479–490. Yogev, A., Appelbaum, J., Oron, M., Yehezkel, N., 1996a. Concentrating and splitting of solar radiation for laser pumping and photovoltaic conversions. Journal of Propulsion and Power 12 (2), 405–409. Yogev, A., Krupkin, V., Epstein, M., 1996b. Solar energy plant. US Patent, 5,578,140, November 26, 1996.