Design and performance of a new type of boiler using concentrated solar flux

https://doi.org/10.1016/j.enconman.2021.114835Get rights and content

Highlights

  • Proof of concept of a new solar boiler using concentrated solar flux was made.

  • Experiments were conducted for an inverted conical receiver under film boiling regime.

  • The conical shape of the receiver is submerged upside down in the boiling liquid.

  • Experiments validated the theoretical model of film boiling with a conical receiver.

Abstract

In order to reduce the amount of CO2 emissions associated with industrial boiling, the design of a new solar boiler operating with concentrated solar energy has been proposed and studied in the present work. A lab-scale solar boiler was built to show the feasibility of the concept through experiments, and a thermal model was then developed to study the efficiency of the solar boiler under different boiling conditions. Several peculiarities distinguish this new design from conventional boilers: (i) the use of vertical heating downwards from a receiver placed on top of the boiler, (ii) the conical shape of the receiver oriented upside down and submerged in the liquid, (iii) the solar receiver has a conical cavity on top where vertical solar flux is received, and its lower surface is submerged in boiling fluid, (iv) the direct use of solar energy with optical concentrating facilities for boiling without intermediate heat transfer fluids, and (v) the study of the unconventional film-boiling regime at elevated receiver temperatures which spaces the boiling fluid from the surface of the receiver. The model was validated by the data collected and showed good consistency between predictions and experimental results, with an average error of 3.79% for temperatures and 2.08% for vapor mass flow rate. For the chosen operating conditions studied, the thermal efficiency of the solar boiler varied from 80% to 90%. Such a thermal model can play an important role for the study and design of new thermal processes involving concentrated solar heating for boiling such as desalination.

Introduction

In many industrial processes, boiling is required, for example, in the production of pressurized steam in power generation systems or for concentrating products in the food or chemical industry. A wide range of boilers operating at low pressure and high pressure already exist with proven efficiency and robustness where for the majority of applications however, they involve the use of fossil fuels [47], [48]. In the contemporary context where the reduction of CO2 emissions is an absolute necessity, it is essential to also explore alternative solutions mitigating the massive use of fossil boilers, when possible and relevant [27], [45].

Solar energy, and more particularly concentrated solar energy, is one of the solutions to be considered to replace part of the fossil fuels used for conventional boiling [31]. Indeed, the development of concentrated solar power (CSP) technologies and their application have technically proven the feasibility of using concentrated solar energy to provide a large heat flux which is a requirement in industrial boiling [2], [20], [28] . The strong experience feedback in this technology makes it possible to deduce the strengths and weaknesses of concentrated solar heating when performing boiling.

Desalination is a relevant example where the concentrated solar boiling could play an important role in reducing the use of fossil-based boilers in thermal distillations. Indeed, many desalination plants use thermal processes, such as Multi-Stage Flash distillation (MSF) and Multiple-effect distillation (MED) which represented in 2018, respectively 18 % and 7 % of the global desalination capacity [29]. The concept of concentrated solar boiling can also be incorporated into the design and improvement of current thermal desalination systems which are supplied today, for the most part, by the energy released by the combustion of fossil fuels. In addition, the locations of areas where desalination is necessary for the supply of fresh water are mostly arid regions with high solar potential [42].

Different types of solar receivers have been developed in the CSP sector. The design of these receivers strongly depends on the chosen solar concentration technology. As several forms of concentrators exist (Parabolic trough, Fresnel linear, solar tower, Dish-Stirling), various forms of receiver have been developed. For example, in tubular shape for solar towers [11], [24], or vacuum tube for parabolic trough [9], [44]. But these receivers have been designed for power generation applications. In the majority of cases, boiling does not occur at the receiver; instead, an intermediary fluid is used to collect the energy in the form of sensible heat and bring it to the storage system or boiler; for this reason, the majority of solar receivers in the CSP sector are not solar boilers. This separation between the boiler and the receiver involves the use of thermal fluid, different heat transfer fluids can be used such as synthetic oils or molten salts for the most common ones [51]. The use of a heat transfer fluid has several advantages such as the practical ease of adding high capacity thermal energy storage (TES), which is one of the key elements for the viability of CSP technologies. But preventing the use of heat transfer fluid has also several advantages such as reduced environmental concerns, operating and maintenance costs, and increased overall efficiency by avoiding intermediate exchangers [23]. For this reason, several studies were conducted on direct vaporization in the receiver [15], [19], [32], [50], [54]. Note that today certain industrial installations use the direct steam generation [1], [49], nevertheless these applications are only targeted in the context of power generation.

Outside the context of CSP and power generation, it is interesting to explore the technical feasibility of a direct boiling in solar receiver using concentrated solar flux and its overall efficiency at high receiver temperatures according to the scale of the boiler and the concentrated solar heat flux used. Additionally, to cope with the possibility of boiling corrosive or scaling liquids (e.g. seawater), boiling under film-boiling conditions using solar energy may be reinvestigated due to its potential protective effect [18]. Film-boiling describes a liquid–vapor flow heat transfer in which a vapor film separates the boiling liquid from the heating surface, where due to the lower conductivity of the vapor, the temperature of the heating surface can increase significantly (Jin and Shirvan [56]). Bromley [10] was the first to present an analytical approach for the calculation of heat transfer in this regime. Since then, many works have been carried out to study the phenomenon on simple geometries such as tube and plate [38], [52], [53] and in different orientations, e.g. vertical, horizontal and downward [3], [16] where the resulting correlations are geometry specific.

In this work, the proof of concept of a solar boiler design to directly generate the steam under a solar receiver was carried out using the medium-power solar furnace at the Odeillo laboratory in France (Fig. 1). The medium-power solar furnace provides a substantial concentrated heat flux on a focal zone (point D in Fig. 1). Even though the solar collection and concentration systems are not the focus of this study, it is important to have a global view of the system. On this device, a heliostat (Fig. 1A) permits the direct solar irradiation to be redirected onto a second reflector (Fig. 1C); this reflector then concentrates the flux and orients it vertically. More details concerning this installation are provided in Section 2.1.

The vertical descending concentration presented is similar to that of “Beam-Down” solar concentrators [6]. As the solar fields of solar towers, the “Beam-down” system allows a point focusing of solar energy, to work at high power density and flexible scales of installation. But unlike the solar tower, where the beam is directed upwards to the top of a tower where the receiver is located, the “beam down” technology has a second reflector which allows the beam to be conveyed to the ground level. This helps reducing construction costs compared to a solar tower. In addition, focusing solar radiation at ground level seems to be more practical to integrate it into various processes. These systems have been tested on laboratory scale, such as in Masdar city [36] or in Miyazaki [30] and industrial facilities are currently under development such as in the Yumen Xinneng factory [4] or the San Filippo del Mela project [35].

Because the solar furnaces have a point focus, a conically shaped receiver was chosen (Fig. 2E). The advantage of this shape is that it traps the solar flux in a cavity to avoid both radiation and reflection losses, conical shapes have already been studied for Dish-Stirling installation and have shown good optical performances [12], [55]. The conical cavity in vertical descending flow has also been studied and confirms the good optical results of this geometry [33]. Moreover, because boiling takes place directly in contact with the receiver, on the fluid side, the conical shape permits a larger contact surface for heat transfer.

Several objectives are targeted by this study; the first objective is to provide experimental results and a proof of concept for a solar boiler where concentrated solar radiation is received on a main solar receiver to operate in boiling mode at the level of solar receiver without intermediate heat transfer fluids. The second objective is to present a thermal model, fitted to the specific conical geometry of the solar receiver, to estimate the efficiency of such a boiler and its potentials when using the concentrated downward vertical solar flux and operating under different boiling conditions. Note that in this design, the conical receiver is submerged in the boiling tank and therefore operation under film boiling conditions may space the surface of the receiver from that of the liquid to prevent scaling or corrosion. Consequently, the experimental data are used to estimate the precision of the developed model and to adjust the correlation of the heat transfer under film boiling for the particular conical geometry of the studied solar receiver.

Section snippets

Experimental setup

A lab-scale experimental device was constructed in the Procédés, Matériaux et Energie Solaire (PROMES) laboratory and was supplied with concentrated solar heat from the Odeillo solar furnace. The solar boiler module allowed boiling under concentrated solar heating to be studied. The optical facilities of the Odeillo solar furnace enabled the collection and concentration of solar energy with a downward vertical flux (Fig. 1). A first reflector, made of float low-iron glass with a silver coating,

Boiler experiments under concentrated solar heating

Fig. 6 shows the conical receiver for a single experiment under different boiling conditions. In this experiment, once the solar boiler had been adjusted, regulation shutters were fully open. These images were taken at key moments in the experiment. Regarding these images, it should be noted that they provide a qualitative view of the boiling phenomena and are not intended for quantitative measurements. Indeed, due to the conical geometry and the large variation in the optical coefficient

Conclusions

This study introduced a new boiler module operating with concentrated solar energy. Such a module can be designed at different scales and for different purposes, including for the desalination of seawater. The most important features distinguishing this new boiler and its modeling from conventional boilers are first, the use of vertical heating downward from a receiver placed on top of the boiler where the conical receiver is oriented upside down and it is directly in contact with the boiling

CRediT authorship contribution statement

Dylan Lorfing: Writing – original draft, Methodology, Software, Investigation. Régis Olives: Conceptualization, Methodology, Resources. Quentin Falcoz: Methodology, Conceptualization, Resources, Writing - review & editing, Supervision. Emmanuel Guillot: Resources. Claude Le Men: Resources. Aras Ahmadi: Conceptualization, Methodology, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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