Elsevier

Applied Energy

Volume 279, 1 December 2020, 115839
Applied Energy

Low Concentrating Photovoltaics (LCPV) for buildings and their performance analyses

https://doi.org/10.1016/j.apenergy.2020.115839Get rights and content

Highlights

  • Three Low Concentrating Photovoltaic (LCPV) systems have been developed for building retrofit.

  • Ray trace simulations are verified with the indoor experimentation.

  • Daily, monthly and seasonal variation of predicted energy output is presented.

  • The effect of cell temperature on energy generation is presented.

Abstract

Low concentrating photovoltaic technologies (LCPV) for building application offer viable solutions in improving the conversion efficiency of solar cells leading to an improved electrical output per unit cell area required when compared to conventional solar photovoltaic modules. The current study explores the feasibility of different geometrically equivalent LCPVs designed for building application through indoor experimental characterisation and analytical investigations. LCPV concentrator geometries were designed and simulated to predict optical efficiency at various truncation levels and range of angles of incidence using ray trace module in COMSOL Multiphysics version 5.3. The geometric concentration ratios of LCPVs investigated Compound Parabolic Concentrator (CPC), V-Trough and Asymmetric Compound Parabolic Concentrator (ACPC) with geometric concentration ratios of 1.46, 1.40, and 1.53 respectively. These prototypes were manufactured and their electrical conversion efficiency in conjunction with crystalline silicon (c-Si) solar photovoltaic cells were measured using OAI Trisol Class AAA solar simulator. Analytical model developed in the present study predicts the annual energy output generated and payback period for the LCPVs compared to an equivalent area of conventional flat module. Theoretical modeling results have showed that Asymmetric Compound Parabolic Concentrator (ACPC) with mono-crystalline silicon cells (m-Si) have generated highest energy output per unit area of 177 kWh/m2 as compared to the other configurations which make it economically viable for building retrofit with a predicted payback period of 9.7 years.

Introduction

Buildings account for 40% of Europe’s total energy consumption and 36% of CO2 emissions [1]. It has been reported that 75% of total existing European buildings are energy inefficient with 35% of them over 50 years old [2]. Hence, harnessing solar energy to generate power using wall/roof mounted photovoltaic (PV) systems offer an opportunity to enhance their energy efficiency. It also reduces their environmental impact for PV panels are noise free and emit no emissions during operation. No wonder, the cumulative solar PV installation capacity across Europe was 26.77 GW in 2018 and estimated to pick up significantly from 2019 to 2024 to 50.39 GW [3].

The amount of power generated and electrical conversion efficiency is dependent on the insolation (type and amount) available on PV cells. Optical concentrators can increase radiation intensity on the solar cell surface, which can reduce the amount of semiconductor material used in manufacturing PV panels, thus reduction in the overall PV module cost. High concentrating photovoltaic (HCPV) technologies account for >90% of the global installed capacity though all CPV technologies with geometric concentration ratio Cg > 10 require solar tracker, which uses a proportion of the power generated, substantially increases the capital and maintenance costs and adds weight to the panel structure [4], [5]. On the contrary, low concentrating photovoltaic (LCPV) technologies with Cg ≤ 10 are simpler in design, can harness a large part of solar spectrum whilst requiring no or a simple tracking mechanism requiring zero or minimum maintenance. The concentrators with Cg ≤ 3 with a capability to even harness a significant amount (factor of inverse of Cg) of the diffuse solar radiation incident on the aperture are strong candidates for deployment in the geographic regions experiencing high diffused solar radiation component [6]. Clearly, these are best suited for building integration or retrofitting and have been investigated for several years by many research groups globally.

Compound parabolic concentrator (CPC) based CPV systems due to their ability to harness diffuse radiation gripped a lot of attention since they were first reported in 1974 [6]. Hasan et al. [7] have compared the electrical performance of actively cooled CPC PVT collectors using p-Si solar cells with a non-concentrating p-Si solar panel. Results have showed that CPC with p-Si solar cells produced 62.5% higher than a non-concentrating p-Si solar panel. Baig et al. [8] have reported a three-dimensional crossed compound parabolic concentrator (3.6x) with flat m-Si PV cells, which achieved a power ratio of 2.67 when compared to a flat PV panel. A novel mirror symmetrical dielectric totally internally reflecting concentrator was reported to achieve a concentration ratio of 13.45x, for radiation incident within its half acceptance angle, when compared to a non-concentrating solar cell [9]. Shaltout et al. [10] studied a dual axis tracked V-Trough concentrator in conjunction with p-Si and a-Si solar cells. Sangani and Solanki [11]  experimentally measured a 44% higher electrical energy produced by a seasonally tracked 2-sun V-Trough concentrator than a non-concentrating counterpart. Baig et al. [12] and Elminshawy et al. [13] have reported water-cooled V-Trough concentrators with c-Si PV technology. Recently, Hadavinia and Singh [14] through an experimental study reported that a CPC with geometric concentration ratio of 2.7 generating 2.4% higher power than a geometrically equivalent V-Trough. These studies have shown CPC and V-Trough based LCPV systems in conjunction with c-Si solar PV technologies have several advantages as compared to the conventional flat panel devices in terms of a higher electrical conversion efficiency and an effective use of the roof space. However, their high performance is limited to within narrow acceptance angles these have been originally designed for, sacrificing optical concentration at angles of incidence outside this range.

Zachropolous et al. [15] have reported a comparative three-dimensional optical analysis of both symmetric and asymmetric dielectric non-imaging concentrators for building applications. Later, Mallick et al. [16], [17] and Mallick and Eames [18] experimentally found asymmetric compound parabolic concentrator (ACPC) with a flat m-Si PV module producing 62% more power than a flat PV module. Lu et al. [19] reported PCM cooled m-Si solar cells in conjunction with a truncated ACPC (2x) producing 10% higher electrical energy than a concentrator with no heat sink at rear end.

This paper presents a validated comparative assessment of energy output and payback period for three geometrically equivalent LCPV optical concentrator in conjunction with low cost c-Si cells. This demands a combined computer and lab test-based investigations into geometrically equivalent LCPV geometries in conjunction with low cost c-Si technologies focused on building integration or retrofit.

The measured difference among LCPV performances has been found to be a complex phenomenon as it involves several independent parameters that should be considered simultaneously. LCPV designs were developed and their optical performance at various angles of incidence predicted using ray-tracing tool in COMSOL Multiphysics. Led by the optical analysis three concentrator prototypes - ACPC (1.53x), CPC (1.46x) and V-Trough (1.40x) - were manufactured and tested in conjunction with commercially available c-Si solar cells under simulated solar conditions. The daily, monthly and annual energy output and payback period of the LCPV modules are presented with a view to identify the best CPV module for fitting onto buildings, new or existing, by employing an in-house analytical tool developed in the study.

Section snippets

V-trough concentrator

A V-Trough concentrator consists of two flat reflectors inclined at an angle (θT) to the axis normal to the receiver as shown in Fig. 1. Two flat mirrors focus solar irradiance incident on the aperture onto the receiver. Critical parameters governing the ray acceptance in a V-Trough design consist of trough angle (θT), concentration ratio (Cg) and the height of concentrator (Hv) [20].

Compound parabolic concentrator (CPC)

A symmetric compound parabolic concentrator (CPC) consists of two parabolic reflectors with equal half

Ray trace modelling

Optical analysis of LCPV designs in this research involved CPC, V-Trough and ACPC geometries using ray optics module in COMSOL Multiphysics. The optical analysis of the LCPV designs have been simulated by considering direct irradiation as a function of angle of incidence (AoI) on the aperture of the concentrator as the diffused radiation is not directional therefore independent of AoI. The design of the reflector walls has been simplified by using parametric curve in COMSOL which solved the

CPV systems description and construction

For optical analysis, all three different LCPV geometries investigated were, truncated at different levels whilst keeping the same receiver width of 33 mm. For ACPC, a full height concentrator with a geometric concentration ratio (Cg) of 2.82 and half acceptance angles (θa) of 0° and 60° was considered. The full height ACPC was truncated to one-third height, which reduced the geometric concentration by 54.3%. Similarly, a CPC with θa = 30° and Cg = 2 truncated to 1/3rd of its full height with a

Model for the electrical power generation

A computer model has been developed to predict the techno-economic viability of the LCPV panels and the flat PV modules. In the model, the solar irradiation data (global and diffused) and climate file for London has been employed from the design reference year [27]. Beam, isotropic diffuse and the diffused radiation reflected from neighbouring and received on the aperture of the PV panels have been considered [28].

The electrical output and thermal resistance of the various solar PV technologies

Electrical conversion efficiency measurements

The measured electrical conversion efficiencies (ηele) of the m-Si and p-Si solar cells in conjunction with the CPV prototypes are shown in Fig. 12. Performances of solar cells under one-sun condition are also shown. Solar radiation intensity of 1000 ± 2 W/m2 supplied by the solar simulator were employed for the tests. Cells were tested first under one-sun and then in conjunction with the developed optical concentrators. Measurements have shown that m-Si cell in conjunction with the CPC panel

Conclusions

In this study, the potential of LCPV panels using low cost c-Si solar cells has been analysed by investigating the technical and economic performances.

LCPV concentrator geometries were designed and simulated to predict ray acceptance at range of angles of incidence using ray trace module in COMSOL Multiphysics.

The manufactured prototypes were tested under OAI simulator and measurements have shown that ACPC achieved 42.2% higher electrical efficiency for m-Si cell than a non-concentrated m-Si

CRediT authorship contribution statement

Ranga Vihari: Conceptualization, Methodology, Writing- Original draft preparation, Visualization, Investigation, Software and experimental validation. Harjit Singh: Conceptualization, Supervision, Project administration. Maria Kolokotroni: Supervision, Project administration.

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.

Acknowledgement

This project has received funding from the European Union’s H2020 research and innovation programme under grant agreement no 768576.

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