A novel stationary concentrator to enhance solar intensity with absorber-only single axis tracking
Introduction
Under the looming threat of global warming and irreversible climate change, harvesting sunrays for useful energy production is one of the cleanest path towards a sustainable progress. The most mature technology for the utilization of the solar energy is the employment of the photovoltaic panels which convert sunrays directly into electricity [[1], [2], [3], [4], [5]]. The performance of the photovoltaic conversion process depends critically on the intensity of the solar beam as incident on the panel surface. The relative position and hence the direction of the solar beam, change during the diurnal motion of the sun as well as due to the change in the declination during the annual revolution of the earth. As the absorption is maximum for the normal incidence of the solar beam, the panels are often provided controlled motion to track the motion of the sun [[6], [7], [8]]. Nevertheless, tracking of the sun can improve the incidence up to the maximum limit of the direct normal irradiance (DNI) which measures the intensity on a plane placed normal to the solar beam. To enhance the intensity beyond this limit, optical concentration of the beam is essential [9]. The application of the concentrator reduces the requirement of large area for the collector. It indirectly allows the designer to use a more efficient material without any augmentation of the cost. However, concentration optics relies on the precise direction of the sun rays and hence necessitates tracking of the sun. Therefore, the state of the art systems rely on employing both the routes (i.e. tracking and concentration) simultaneously to maximize the solar incidence [10,11]. For any such tracking system, inclusion of the concentrator optics in the tracked assembly always introduces the issues of accuracy (due to the relative motion between the concentrator and the absorber) and increased parasitic power consumption due to the overall bulkiness.
Concentration of the solar beam can be achieved either by the principle of refraction or by the principle of reflection. The reflection based concentrators include parabolic trough, parabolic dish, heliostat field with a central receiver tower etc. The refraction route relies on the directional deviation of the sunrays at the interface of two media with different refractive indices and works in the principle of the common magnifying glasses. The material requirement of such lens is minimized by the use of interface with discontinuous slopes which are called Fresnel lens after its discoverer [12]. During the design of the Fresnel lens based concentrators, both the average intensity and the uniformity of the intensity distribution are considered. The performance of the concentrator is estimated in terms of the concentration ratio (CR), the ratio between the average intensity with and without the concentrator. A typical cylindrical compound Fresnel concentrator provides an average CR of 33 for an incident angle of 1.5° [13]. However, modular Fresnel lens provide CR of 100–121 at the absorber with 20% variation in intensity for normal incidence [14,15]. Also, a curved Fresnel lens can provide a uniform CR of 900 only for 0.8° incident angle of the light beam [16].
As the concentration optics is critically sensitive to the direction of the input beam, tracking of the whole combination of the absorber and the concentrator is necessary for effective day-long operation. The concentrator optics along with the absorber is often bulky and therefore requires considerable amount of parasitic power during the operation. The effective gain due to the tracking and concentration gets offset due to the necessity of the operational power. Single and dual axis tracking systems without concentrator augment the energy collection by approximately 30% and 44%, respectively [[17], [18], [19]]. A state of the art dual axis tracking system without a concentrator consumes 6% of total generated power in a clear sunny day however it may increase up to 40% under the adverse weather conditions such as cloudy days [20].
From the above discussion, it is clear that the tracking as well as beam-concentration has significant benefit in photovoltaic energy harvesting while a collateral drawback is the augmentation of the parasitic power consumption. Therefore, a standard route to optimize between two counter-acting effects is to separate the concentration optics from the tracking mechanism. As the concentrator is not moved, it is called a static or stationary concentrator.
A stationary concentrator, which includes a non-imaging Fresnel lens with a secondary concentrator and a diffuser, was designed by Leutz et al. [21] For this optical arrangement, non-tracking operation is possible only for the incident angle in the window of ±22.5° and it provides an average concentration ratio (CR) of 2.37. Consequently, another static concentrator is designed and optimized for photovoltaic cells using ray tracing simulation. The optics consists of two Fresnel lenses, one convex and the other concave, and it is capable of concentrating sunlight without any active tacking system. The system can accept the radiation falling within ±23.5° of incidence angle. The system is adjusted twice annually and it results an average concentration ratio in the range of 1.94–2.86 [22]. The other option of the non-imaging concentrator is through the use of compound parabolic concentrators (CPC) which also suffers from the similar limitations of small range of incidence-angle and low concentration ratio [23,24].
Clearly, the effectiveness of the stationary concentrators is restricted within a specified range of incidence angle. Moreover, the parasitic power consumption of the tracking mechanism is much higher if the concentrator is moved along with the collector due to the cumulative weight [20]. Therefore, in the current study, we present a novel solar collection system where the concentrator is staionary and only a small collector is tracked with a miniscule parasitic power. We start with the details of the proposed geometry and its working principle. The simulation tool is validated against the analytical ray tracing. The parametric optimization of the geometry leads to the chosen dimensions which maximize the average intensity on the collector and minimize the non-uniformity of the intensity distribution. The enhancement in diurnal as well as annual basis are then reported to prove its usefulness in the future designs of the PV based collection systems.
Section snippets
Proposed concentrator geometry
In the literature, previous attempts of working with non-tracked (i.e. static) concentrators showed limitation of being useful only for a narrow range of incidence angle. If concentration is to be obtained throughout the day, tracking the sun is indispensable. Nonetheless, a tracker requires higher parasitic power when the optical arrangments and components associated with the concentrator are required to be moved along with the absorber. Here we propose a geometry which only requires movement
Geometry optimization
The intensity on the absorber is dependent on a number of geometrical variables. The symmetry of the optics to nullify the effect of the diurnal motion of the sun is obtained by the choice of a cylindrical geometry of the concentrator. The annual variation due to the earth’s declination is adjusted partially by choosing an optimum angle for the axis of the cylindrical static concentrator. The optimum inclination angle of concentrator with horizontal is |δ-ϕ| that considers the effect of earth’s
Discussions
The underlying idea of the current work is to obtain a mechanism which can provide concentration of the solar beam without the associated difficulties such as continuous two-axis tracking of a bulky concentrator optics along with the absorbing collector. The typical approaches in this direction are based on either the use of multiple Fresnel lens or through the compound parabolic collectors (CPC) [34]. Both of these methods are effective only for a narrow range of incidence angles. The
Conclusions
The current study is motivated from the reduction of parasitic power consumption by the tracker which is achieved through decoupling the concentration optics from the tracking of the sun. The idea is to keep the concentrator static while the absorber alone tracks the diurnal movement of the sun. We have designed a static cylindrical concentrator by using discrete prisms and tracking the performance metrics through the ray tracing module of COMSOL 5.4. At first, the optical tool is validated
CRediT authorship contribution statement
Manoj Kumar Sharma: Data curation, Formal analysis, Validation, Visualization, Writing - original draft. Jishnu Bhattacharya: Conceptualization, Funding acquisition, Supervision, Resources, Writing - review & editing.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgement
The authors gratefully acknowledge the financial support from Indo-US Science and Technology Forum under the project IUSSTF/WAQM-Water and Air Quality Project-IIT Kanpur/2017 (project number: IUSSTF/ME/2017379B). Manoj Kumar Sharma is thankful to the Ministry of Human Resources and Development (MHRD), Government of India for the scholarship. The COMSOL forum is acknowledged for the continual help during the execution of the optical module. The probing comments and suggestions from the anonymous
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