1. Introduction
Due to the declining patronage of nuclear energy and volatile petroleum and natural gas prices, coupled with the rising global temperature, predominately due to the atmospheric build-up of CO
2, nations at large are opting for and considering renewable energy technologies for their power generation. Solar energy, in particular, is seen as an extremely viable option, especially in areas with good solar insolation [
1]. Solar thermal energy for electricity generation is typically referred to as Concentrated Solar Power (CSP) [
2]. CSP can be a driving force in the cause of reducing CO
2 emission, thereby contributing to reducing and limiting the global temperature increase. Of the existing types of CSP, power tower systems are one of the most promising solar thermal technologies. This is mainly due to their ability to offer higher temperatures and, hence, higher efficiencies [
2,
3,
4,
5,
6,
7].
In power tower systems, the heliostat field is one of the essential subsystems. This is due to its significant contribution to the plant’s total investment cost: about 40%–50% of the plant’s cost is attributed to the heliostat field [
8,
9,
10,
11,
12]. The field contributes equally to the plant’s overall power losses of about 40% [
8,
13,
14,
15,
16]. It has hence become essential to ensure that the field layout is optimal at collecting energy from the sun. The design and optimization of the heliostat field is hence an active area of research, with new field improvement processes being actively investigated. Several methods have been proposed in the literature to improve heliostat field efficiencies and reduce losses, either by improving through optimization or by suggesting new heliostat field layout patterns and configurations entirely.
Several literatures that focus on heliostat field layout patterns and configurations are available. In Noone et al.’s [
8] work, for example, a spiral field pattern inspired by disc phyllotaxis is introduced, which is applied in heliostat field design and optimization. By redesigning the layout of the PS10 field using the algorithm, an improvement in the optical efficiency and a reduction in the land area utilized is witnessed. E. Carrizosa et al. [
17] presented a pattern-free heliostat field layout distribution style, obtained by the simultaneous optimization of both the heliostat field (heliostat locations and number) and the tower (tower height and receiver size). Cadiz et al. [
10] presented shadowing and blocking optimization procedures for a variable-geometry heliostat field. The variable-geometry concept explored by the author allows the possibility of minimizing the cosine losses by rotating the entire field. In a similar vein, Mohammed Aldulaimi and MS Soylemez [
14] suggested a new heliostat field layout arrangement by identifying heliostats with low optical efficiency and increasing their heights in a bid to curb blocking losses and hence increase the total annual field efficiency. Emilo Carrizosa [
18] also suggested some alterations in the field by considering a field with different heliostat sizes. Mani Yousefpour Lazardjan [
19] presented a tool developed at Solar-Insitut Julich (SIJ) primarily for the optimization of a novel micro-heliostat concept. In a novel and unconventional heliostat field layout design, Danielli et al. [
20] developed the concatenated micro-tower (CMT). In this configuration, dynamic receivers mounted on arrays of small towers enable heliostats in mini subfields to direct sunlight with minimal cosine losses, thus improving the field’s overall optical efficiency. N. Cruz et al. [
21] also developed an algorithm using a genetic algorithm that generates a continuous pattern-free field layout. The algorithm developed, using parallelization, provides a solution to the conceptual complexity and high computational cost associated with pattern-free heliostat field optimization. Arrif et al. [
22] also developed an algorithm which is based on the bee colony optimization method to optimize the case study site of the PS10 CSP power plant in Spain. The proposed algorithm, the Artificial Bee Colony Algorithm (IABCA), finds the best position for each heliostat on the field in which the maximum efficiency for both the heliostat and field is reached within a limited area. L Deng et al. [
23] proposed a new pattern for the heliostat field layout: a rose pattern based on the classic radial staggered configuration. The pattern divides the radial staggered configuration into six sectors, and some of those sectors are then optimized separately using advanced differential equation algorithm, thus increasing the optimization variables. In another unconventional heliostat field layout design, additional towers, each having its receiver mounted atop, are introduced in the field.
Although it is only recently that the multi-tower setup has attracted much interest in the research community, the configuration has been explored in the past. In 1999, Romero et al. [
24] pointed out that centralized large solar power tower plants are at odds with the increasing shift towards a distributed-energy setup and could hence face future deployment difficulties. They proposed and analyzed how small tower fields could be integrated into a Modular Integrated Utility Systems (MIUS) approach, in order to fully exploit the advantages of a distributed-energy setup in a community. In 2002, Schramek and Mills [
25] proposed a Multi-Tower Solar Array (MTSA) system, which consists of a collection of solar towers densely grouped together, thereby allowing for partial overlapping of the heliostats in the field, and hence allowing greater utilization of the solar radiation falling on the unused ground area in the field. In a more recent work, in 2012, Augsburger and Favrat [
26] proposed a method in which each individual heliostat is instantaneously aimed at a receiver in a multi-tower setup, following an aim selection criterion. The thermo-economic performance of a three-tower heliostat field was then evaluated using a model. In another work, eSolar and Inc. (B&W PGG) [
27] investigated the use of small heliostats with multiple receivers and towers [
27]. They proposed a configuration with 14 molten salt power towers, for a 100 MWe (net) power block that is capable of delivering a 75% capacity factor. Tyner and Wasyluk [
28] presented a follow-through on the conceptual design previously developed by eSolar and Inc., where several trade studies were carried out in order to arrive at the optimally cost-effective system configuration for the multi-tower setup. The concept proposed involves replicating the field, without scaling or redesign, in order to meet the capacity required. In another work by Pasha Piroozmand and Mehrdad [
29], an iterative algorithm was developed in order to obtain the optimum instantaneous efficiency of the heliostat in the field when selecting the tower which radiation will be reflected onto in a two-tower field set up, so as to maximize the annual optical efficiency of the field. As a case study, the authors used Particle Swarm Optimization (PSO) to optimally design a two-tower spiral patterned field along the east-west line before redesigning the field using the iterative method. The authors here noted that issues such as field layouts and aiming strategies need to be further investigated in order to achieve a more optimized and comprehensive multi-tower system. In 2018, Vast Solar, an Australian company engaged in CSP research, developed and commissioned a 1.1MWe pilot plant utilizing a modular solar array field [
30]. Each of the five modular arrays in the field has a dedicated tower in which the Heat Transfer Fluid (HTF) is heated at the receiver. The multiple towers are connected to a central thermal storage unit. The company is already planning to go further by developing a 30MW commercial demonstration project in Australia using the modular array field.
In all the multi-tower configurations reviewed, each tower has its own heliostat field, which replaces an entire field with surrounding heliostats in smaller units until the capacity required is met. In this paper, a different architecture of the multi-tower configuration is investigated. The configuration explored, which provides an alternate viewpoint to the usual mainstream multi-tower configuration, involves adding an auxiliary tower to an existing surrounding field. The paper initially begins by defining the models used for the development of a conventional heliostat field. After model validation, a 50 MWth solar field was simulated in Nigeria, with the objective function being the Levelized Cost of Heat (LCOH). An auxiliary tower was then added onto the existing field, and its effects investigated. The results from the optimized multi-tower configuration were then compared with conventional fields at different thermal field powers, in order to determine the optimum transition size from a single field to a multi-tower field.
4. Results and Discussion
The results from the optimization process are highlighted in
Table 6. In order to provide a comparative description, the results from the multi-tower field are compared to the results from an optimized single-tower conventional field. The results are all tabulated in
Table 6.
As shown in
Table 6, the additional tower for the multi-tower field results in a marked increase in the optical field efficiency value. A 3.64% increase in the mean annual field efficiency and a 2.94% increase in the design point efficiency is observed when compared to the results obtained in a conventional field setup. The most considerable improvement in optical efficiency is seen in the cosine efficiency value. This is primarily because the additional tower in the field provides an alternate aim point for the heliostats with the least reflecting efficiency.
The LCOH, however, is higher in the new configuration. This indicates that the benefits due to the increment in the optical efficiency values and annual energy output do not outweigh the cost of installing an additional tower and receiver.
In the new configuration, the number of heliostats aiming at the one auxiliary tower changes through the months and through the day (see
Figure 10). At the design point date, April 20th, a total number of 317 heliostats aim at the auxiliary tower at solar noon (
Figure 10a). The number of heliostats aiming at the auxiliary tower at solar noon peaks in January and December, when the sun’s position is low, making it difficult for the ‘weak’ heliostats to reflect radiation onto the main central tower without incurring enormous cosine losses (
Figure 10a). On the other hand, during sunshine hours, and at around solar noon, a reduced number of heliostats aim at the auxiliary tower (
Figure 10b). This is predominately due to the lesser cosine losses from the heliostats aiming at the main central tower, hence the central tower becomes a preferred target.
The computed thermal power rating of the auxiliary tower was 11.51 MWth. The main central tower, which caters to the bulk of the heliostats in the field, now has a computed thermal rating of 38.28 MWth.
In
Figure 11a,b, the month-on-month variation of the total energy output and mean efficiency values for both the conventional field and the multi-tower field is shown. A marked improvement in the mean efficiency value was observed from January to December in the two models shown in
Figure 11b. In
Figure 11a, it is worth mentioning that the dip witnessed during months 6–8 is a result of the poor DNI values, as a result of cloud cover during that period (from NiMet data).
A more explicit demonstration of the effect of the multi-tower field is shown in
Figure 12. The mean annual efficiency field layout for the conventional system and the one additional tower field is seen in
Figure 12a,b. The change in shading matrices of the optical losses model from the conventional field to the multi-tower field is shown in
Figure 12c–i.
The entire optimization for the multi-tower field was initially made for a 50 MWth field. A broader range of thermal power was examined, so a more critical analysis of the effect of a multi-tower field can be observed.
Figure 13 shows the LCOH results from both the conventional field model and the multi-tower field model at various-power thermal field values.
Results from
Figure 13 clearly show the promising LCOH trend of multi-tower fields at higher thermal power figures and larger fields. In larger fields, a higher number of weaker heliostats are witnessed in the field, making the need for and use of an additional tower all the more critical. The heliostats at the weaker region of a multi-tower field are provided with an additional tower to reflect the sun’s radiation, thereby considerably cutting down on cosine, spillage and attenuation losses which in turn gives rise to higher total energy output. At a certain point, as seen in the trend, the multi-tower field continuously provides a lower LCOH value compared to a conventional field of similar thermal power. This is seen explicitly at the 400 MWth range in the one auxiliary tower configuration, where expanding the conventional field in order to attain a higher thermal field output becomes less effective due to the significant optical losses gained as a result of the size of the field.
5. Conclusions
In power tower systems, the heliostat field is one of the essential subsystems, comprising 40%–50% of the plant’s total investment cost and of about 40% of the plant’s overall power losses. Different field configurations are therefore being investigated. Multi-tower systems provide an alternative approach in which the heliostat field efficiencies can be increased. In this paper, a different architecture of the multi-tower configuration is investigated. The configuration explored, which provides a different take to the usual mainstream multi-tower configuration, involves adding an auxiliary tower to an existing surrounding conventional field.
As a case study, the multi-tower configuration was applied in Katsina, Nigeria, and the field parameters were optimized for 50 MWth field power. To identify the position in which the auxiliary tower was sighted, the mean annual field efficiency of the 50 MWth conventional field was computed. The results clearly showed that the southern region, which had heliostats with huge cosine losses, had the least mean annual efficiency value of heliostats. The presence of the auxiliary tower provided an overall increase in the system efficiency of the field by reducing some of the losses entailed in a conventional single-tower setup, by providing the ‘weaker’ heliostats in the region a more favourable tower to target. A reduction in attenuation, spillage and cosine losses by 0.55%, 4.88% and 6.63%, respectively, was observed in the multi-tower configuration. This led to an overall increase in the mean annual efficiency of the field by 3.64%.
With most heliostats on the multi-tower configuration targeting the main central tower in the 50 MWth field, the auxiliary tower’s contribution to the LCOH becomes limited. This can be seen in the small increment of 3020MWht recorded in the thermal field energy output. The LCOH is thus higher in the new configuration. This indicated that the size of the field limits the potential contribution of the auxiliary tower at 50 MWth. Similarly, the benefits due to the increment in the optical efficiency values and annual energy output do not out-weigh the cost of installing an additional tower and receiver.
In larger fields, a more significant number of weak heliostats are witnessed. The poorly reflecting heliostats are provided with an additional tower to reflect the solar radiation to, thereby considerably cutting down on some optical losses, which in turn gives rise to higher energy output. A continuous reduction in the LCOH for larger fields is seen as a result. At a thermal field power rating of 400 MWth, the multi-tower configuration provides a higher LCOH compared to a conventional field with similar power.
The growth and development of power tower systems has seen larger systems, up to 150MWe, being built around the world. The multi-tower configuration provides a viable alternative way in which such large power tower systems can be built, by potentially providing a lower LCOH and higher plant efficiency. The configuration could also be applied in existing fields by updating or retrofitting existing conventional fields and adding auxiliary towers. The study here provides a quick overview of auxiliary towers in a multi-tower field configuration. In further studies, the effect of the field configuration on the whole plant, especially on the storage subsystem, can be investigated. Furthermore, techniques for developing the field by reconfiguring the field layout to reflect the multi-towered setup can also be studied in future work.