Performance of wedge-shaped luminescent solar concentrators employing phosphor films and annual energy estimation case studies

Highlights

• Wedge-shaped luminescent solar concentrators with phosphor films are characterized.

• Lab performance as a function of illumination’s polar and azimuthal angle is obtained.

• Performance in applications is predicted using measured solar irradiance data.

• Annual energy produced is assessed at two locations for eastern and southern walls.

• More than 30% performance gain relative to traditional solar cells is demonstrated.

Abstract

Luminescent solar concentrators (LSCs) are solar devices that focus sunlight through red-shifted internal reflection and have been proposed as a potential alternative to traditional photovoltaic (PV) panels. Such systems also have potential applications as multi-functional building envelope materials. This paper investigates the performance of wedge-shaped LSCs employing inorganic luminescent phosphors that demonstrate enhanced performance at incidence angles that conventional solar panels do not use effectively. This behavior is investigated experimentally through the current-voltage (I–V) characteristics of solar cells attached to wedge-shaped LSCs illuminated at angles covering the entire range of possible insolation conditions. Similar experiments were also performed on solar cells to determine the power ratio, or the power produced by solar cells within an LSC normalized to the power produced by bare solar cells exposed to identical insolation conditions. Using laboratory measured power ratio and recorded direct normal irradiance data for two case studies in Phoenix AZ and Albany NY, the annual energy production for wedge-shaped LSCs mounted on vertical walls due to direct-beam irradiance was determined. The energy produced by solar cells within vertically installed LSCs is between 20 and 40% above annual energy produced by vertically installed solar panels, depending on location and orientation (facing east or south). Moreover, depending on orientation, wedge-LSC power production peaks at traditionally suboptimal times for a solar panel. In all, these results demonstrate the potential for wedge-shaped LSCs as a power harvesting building envelope, an architectural solution emerging in response to net-zero energy building legislation.

Introduction

Renewable energy accounted for 24% of global electricity generation in 2016 and is rapidly closing the gap on coal, currently the largest source of electricity generation [1]. Much of this is thanks to growth in photovoltaic (PV) solar energy, which added 74 GW in 2016 alone (as compared with a 57 GW increase in energy production from coal). Additionally, the International Energy Administration (IEA) projects that PV will add 438 GW from 2017 to 2022, accounting for 48% of growth in renewable electricity capacity [1].

These statistics indicate that there is a steadily growing demand for solar energy. Thus, the use of new solar energy options may see increased adoption. One such option is building-integrated photovoltaics (BIPV), which are PV solutions deployed within the building envelope. BIPV products are multifunctional, providing the traditional functionality of the building envelope, while additionally producing energy [2]. While BIPV remains a niche market, increasingly aggressive renewable energy policies have continued to fuel speculation relating to their growth [3]. This is demonstrated in part by a widespread drive to create net-zero energy buildings, or buildings that produce as much energy as they consume. The European Union recently passed legislation requiring all new buildings to be “Nearly Zero Energy Buildings” by 2020 [4]. In the United States, California has required all new residential buildings be net-zero by 2020, with the same requirement for commercial buildings following in 2030 [5]. Thus, BIPV are attracting interest due to these stringent energy policies and the abundance of surface area available in the built environment. Confirming this idea, a recent study found that utilizing BIPV alone could account for 32% of the EU’s power demand [6]. Accordingly, research in this area is growing [[7], [8], [9]].

One of the technologies considered for BIPV is the incorporation of luminescent solar concentrators (LSCs) into the building envelope. LSCs consist of a transparent plastic sheet incorporating absorbing-emitting materials with solar cells attached to the edge(s). Light is concentrated and guided to the edge(s) through internal reflection and red-shifted due to luminescent emission [10,11]. LSCs have shown the ability to accept light effectively at a variety of incidence angles without the need for tracking systems, an advantage mentioned frequently in literature, although not explored extensively in practice [12,13]. While LSC geometry is most commonly planar, wedge-shaped LSCs (depicted in Fig. 1) have been theoretically shown to exhibit an enhanced ability to capture sunlight when polar incidence angles are moderate (20–40°) [14]. Additionally, most LSCs that have been reported employ organic dyes or quantum dots as a luminescent material [15,16]. Although less commonly researched, LSCs with inorganic phosphors have also been reported recently. These materials are a stable alternative to quantum dots and organic dyes, which are degraded by ultraviolet radiation. While the use of inorganic phosphors can result in significant scattering losses [17], previous research regarding planar LSCs has indicated that the placement of phosphor films on the bottom surface of the LSC minimizes the negative impact of scattering [18].

Planar LSCs employing inorganic phosphors have achieved an overall efficiency of 7.7% at a polar incidence angle of 70°, improving linearly from a more modest overall efficiency of 3.8% at 0° [18]. While wedge-shaped wave guides have been a subject of research [16,19], the current-voltage characteristics of wedge-shaped LSCs have not been experimentally demonstrated yet.

This paper investigates wedge-shaped LSCs incorporating films of inorganic phosphors and reports the experimentally measured power ratio of these systems relative to a solar cell alone for representative combinations of polar incidence angle θ_i and azimuthal incidence angle φ_i. Based on experimentally measured power ratios, the annual performance of such devices is predicted using recorded annual direct beam solar irradiance for two case studies, namely deployment on vertical walls facing south and east in Phoenix, AZ and Albany, NY. LSC performance has rarely been characterized relative to sun position [20], so these case studies seek to show the unique behavior of solar cells within wedge-shaped LSCs relative to bare solar cells throughout the year and demonstrate their potential as BIPV.