Progress in heliostat development
Introduction
Concentrated solar thermal (CST) energy is a promising renewable energy technology capable of large scale electricity production and industrial process heating, usually incorporating energy storage. In a solar tower plant, moving mirrors called ‘heliostats’ track the sun in two axes and reflect the sun’s rays onto a ‘receiver’ at the top of a tower (Fig. 1). The receiver absorbs the radiation and supplies thermal energy via a working fluid at a temperature of typically 300–700 °C. For power towers incorporating energy storage, the working fluid is also a heat storage medium (e.g. molten salt), and is stored in tanks to allow power generation upon demand. Alternatively, the energy received by the solar tower plant may be used for providing heat to a thermochemical process, such as the production of synthetic transport fuels.
A photovoltaic (PV) power plant currently provides electrical energy at a lower cost than a concentrating Solar Power (CSP) plant; however, storage of electrical energy is in general more expensive than storage of thermal energy. Therefore, PV plants are more suitable for power supply during sun hours and CSP plants during the night and in cloudy conditions. A combination of both PV and CSP is seen as a promising solution for future power supply: “The cost of solar technologies are falling so quickly that within a few years the combination of solar PV and solar towers with storage will be able to compete directly with base load fossil fuels” (Padmanathan, 2015). An important advantage of CSP compared with PV is that during construction a high fraction of labour and equipment is sourced locally, which is especially attractive for developing countries.
Examples of industrial processes that could be driven by solar tower plants are cement production (Gonzáles and Flamant, 2014) and enhanced oil recovery (CSP today, 2013b). Solar tower systems can also supply heat to thermal processes at 550 °C or below, although 500 °C have been achieved by some small trough demonstration projects as well. Many industrial processes are designed for higher working temperatures, which are provided by fossil fuel burners. To incorporate solar input it is sufficient to replace only the burner with a solar receiver and the rest of the plant stays almost unchanged. With further reduction of the cost of concentrated solar systems, applications for solar thermal industrial processes will become economically viable.
The heliostats represent 40–50% of the cost of a power tower plant, so they must be relatively low cost for the cost of energy from the plant to be competitive with that of fossil fuels (Mancini et al., 2000). It was shown by Gary et al. (2011) that to achieve a levelised cost of electricity (LCOE) of 0.10 USD/kWh the heliostats must cost no more than 120 USD/m2. The heliostats must cost about 75 USD/m2 if the target LCOE is 0.06 USD/kWh (Gary et al., 2011). To achieve these targets, innovative designs and solutions regarding the complete heliostat concept and its components, are needed. Furthermore, the dimensions of heliostats must be selected to minimise manufacturing and installation costs. This requires accurate estimation of the wind loading on both operating and parked heliostats to allow structurally efficient heliostat designs to be developed with good optical performance characteristics, while avoiding structural failure.
Mancini et al. (2000) collected data from eight different commercial heliostats that were available on the market. They presented a general description of heliostats and their cost structure based on the information provided by the manufacturers of the heliostats.
In July 2006, a two-day workshop was held at the National Solar Thermal Test Facility (NSTTF) in Albuquerque, New Mexico, to discuss heliostat technology and to identify solutions for technology improvement. Approximately 30 heliostat experts and manufacturing companies from the United States, Europe, and Australia participated in this workshop. After the workshop a team of six experts developed a price estimate for current heliostats and evaluated the price-reduction potential solutions for the future heliostats. The results of this study were published in a SANDIA report (Kolb et al., 2007).
Pfahl (2014) presented an overview of 48 approaches for heliostat cost reduction in a tabular form and discussed their main advantages and disadvantages, giving example reference cases. The review was intended to serve as a base for the development of new low-cost heliostat concepts.
One of the objectives of the European project STAGE-STE is the development of a low cost heliostat. At the commencement of the program, the state-of-the-art of heliostat technology and the specifications were discussed and published (Téllez et al., 2014). The report presents potential solutions for cost reduction consistent with the required functional specifications, and a review of heliostat deployment worldwide.
Similarly, the Australian Solar Thermal Research Initiative (ASTRI) carried out a Heliostat Cost Down Scoping Study (Coventry et al., 2016, Coventry and Pye, 2013) as a first step in a heliostat cost-reduction project.
In the following sections, the current level of knowledge in heliostat technology about different aspects of heliostat design and manufacturing will be discussed. This review paper is arranged according to the following topics:
- 1.
Static and dynamic wind loads.
- 2.
Heliostat dimensioning.
- 3.
Heliostat components.
- 4.
Heliostat designs.
- 5.
Canting.
- 6.
Manufacturing and assembly.
- 7.
Qualification.
- 8.
Heliostat field layout.
- 9.
Mirror cleaning.
- 10.
Cost.
Section snippets
Relevant wind properties
Heliostats are exposed to the atmospheric conditions prevailing on the field. They experience aerodynamic forces caused by wind that can lead to a mechanical failure if they are not accounted for in the design. At extreme wind speeds, the loads can lead to a failure by exceeding the maximum stress that the heliostat structure is designed to sustain. In addition, fluctuating wind forces may result in fatigue failure due to flow-structure interaction and resonance.
Emes et al. (2015) showed that
Heliostat dimensioning
In the following, the main decisive aspects for heliostat dimensioning considering wind loads are discussed:
- 1.
Heliostat size.
- 2.
Tracking accuracy.
- 3.
Deformations during operation.
- 4.
Storm wind loads.
Heliostat components
In this section, the main heliostat sub-functions and their related components are described (Pfahl, 2014).1. Reflecting sunlight: mirrors 2. Fixing mirror shape: mirror support structure 3. Ground connection: pylon and foundation 4. Offset determination: control 5. Rotation of mirror panel: drives
Heliostat designs
Different combinations of single heliostat sub-functions describe a complete heliostat design. In the following section, current heliostat designs known to the authors are presented with focus on the special design features of each heliostat. The designs are divided into two groups: commercial heliostats and possible next generation heliostats. The amount of details given differs and depends on the information that was available to the authors.
Canting
Yellowhair and Ho (2010) reviewed several options for canting which are summarised in tabular form by Coventry et al. (2016, paragraph 13.4) along with some general information about canting. Téllez et al. (2014, paragraph 2.7) split the canting methods “into two broad categories, mechanical and optical. Mechanical methods (such as through the use of gauge blocks or inclinometers) involve pre-calculation of the facet canting angles required and manual measurement and adjustment of each facet to
Heliostat manufacturing and assembly
Usually, details about the manufacturing process are a well-kept secret of the companies and not much public information is available. An exception is the manufacturing of the heliostats for Ivanpah. A video of the assembly process is available online (BrightSource Energy, 2017). Additionally, some general information about heliostat manufacturing is given by Coventry et al. (2016, paragraph 7). They summarise: “It has been estimated that as much as 80% of the cost of product development and
Heliostat qualification
Heliostat testing and qualification, e.g. (Thalhammer, 1979; King and Arvizu, 1981; Mavis, 1988; Strachan and Houser, 1993; Weinrebe et al., 1996; Monterreal et al., 1997), has a long history in CSP, because it allows the designer to estimate and optimise performance. It is an integral part of heliostat design and development (phase 1), manufacturing (phase 2), commissioning (phase 3) and operating heliostat fields (phase 4). An overview of the four phases and their different measurement and
Methods and tools for heliostat field layout
The layout of a solar tower plant, i.e. the definition of the number and positions of the heliostats and the size and position of the receiver on top of the tower, is a problem with almost indefinite degrees of freedom. That is why, since the early 1970s the development of the solar tower technology came along with the creation of numerical models and computer codes for heliostat field analysis and layout. These codes simulate the concentration of the sunlight by modelling the reflected image
Mirror cleaning
The efficiency of CSP plants depends on the reflectance of the concentrating mirrors. Their reflectivity can be greatly reduced by soiling - the reversible process of particle and dust adhesion to surfaces. The parameter to quantify soiling-induced reflectance loss is called cleanliness (ξ). It is defined as the ratio of the reflectivity (ρ) of a solar reflector relative to its reflectivity in the clean state (ρcl):ξ changes due to time and site dependent on influences such as dust
Heliostat cost
In a review of costs in 2013, Coventry et al. (2016, paragraph 16) summarised the results of several heliostat cost studies. The cost of heliostats for high production rates at that time were estimated to be in the range of 150–200 USD/m2. Although there is little published about current heliostat costs, there is evidence both anecdotally and based on significantly lowered LCOE bid prices (e.g. SolarReserve’s Copiapo plant in Chile in August 2016 at 63/MWh), that heliostat costs have been
Summary and outlook
The amount and variety of creative ideas to drive heliostat field cost down is impressive. 21 heliostat designs based on many different approaches for cost reduction were presented. For the heliostat field layout, canting, control, qualification, and cleaning a huge amount of literature is available with smart solutions. Significant advances have been made in recent years to the understanding of wind loads, the impact of turbulence, and dynamic wind loading impact on heliostats. It is clear
Acknowledgment
We acknowledge Gregory Kolb for his valuable comments for improvement of the paper.
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