Impact of Ground Albedo on the Performance of PV ...

21 - 22 May 2015, CELJE, Slovenia

Impact of Ground Albedo on the Performance of PV Systems and its economic analysis Y. Kotak1*, M. S. Gul1, T. Muneer2, S. M. Ivanova3 1 Heriot Watt University, Edinburgh, U.K Edinburgh Napier University, Edinburgh, U.K 3 University of Architecture, Civil Engineering and Geodesy, Sofia, Bulgaria 2

[Corresponding Author indicated by an asterisk *]

Abstract - Several factors influence the energy generation of Photovoltaic (PV) installations. A ground albedo value of 0.2 is widely accepted and is used in modelling of PV systems. Foreground surfaces may have different albedo and hence a constant albedo value is unsuitable. This paper presents the potential for increasing the power output by incorporating higher albedo horizons for PV facades. Presently obtained results show that using a high-reflectance material for foreground may increase the PV output by a third. A discussion is also provided on the type of materials that are available to enhance the albedo and their cost effectiveness.

Keywords: ground albedo, PV systems, reflected radiation, high-reflectance materials. I.

Introduction

Energy is the key element required for sustainable development and prosperity of the society. In present scenario, out of all renewable resources, solar power generation is one of the most rapidly growing sources. It has several advantages like environmental advantage, government incentives, and flexible location, etc. [1]. Currently more than 0.6% of global electricity demand is fulfilled by solar PV and its deployment is highly increasing in recent years [2]. However, to gain maximum yield, a substantial amount of time and money is being invested on designing and modelling of solar PV energy systems. Several factors such as the type of PV modules, ground albedo (ground reflectance), and building azimuth influence the output of PV implementations. The total incident radiation on a surface such as a PV panel (vertical or inclined) is the sum of beam, sky-diffuse and ground reflected radiation (albedo) from various surfaces. In general albedo is referred as ratio between ground's reflected radiation to the global incident radiation [3]. Despite the fact that the reflectivity of snow-covered ground could be as high as 0.9, it is customary to assume a constant value of ground reflectance of 0.2 for temperate and humid tropical localities and 0.5 for dry tropical localities [4]. Practically, a solar PV module mounted on an inclined roof faces a combination of varying ground surfaces such as road, grass, trees, other buildings, etc. This paper illustrates the difference in outcome of PV module (a) by considering a constant albedo value of 0.2 of complete horizon and (b) by taking into account the different albedo values for different foreground surfaces (the actual scenario). A brief discussion is also presented on the type of materials that are available to enhance the ground reflectance and their cost effectiveness. II.

Background Research

A. PV System and albedo The optimum output of PV system relies upon the appropriate selection of location for the installation with a comprehensive understanding of electrical and environmental factors. It is prerequisite to have the accurate knowledge of the radiation components (i.e. direct beam, diffuse and ground reflected) incident on PV module, for installation of solar PV systems [5]. According to reference [6], ground reflected albedo is a significant parameter and may reach up to 100 W/m2 for some locations [7]. Reference [8] proposed that different surfaces have different albedos and accurate estimation of ground reflected radiation would require knowledge of foreground type. 7th International Conference on Solar Radiation and Daylight SOLARIS 2015, website: http://solaris2015.com/


Researchers have reviewed that, oceans, lakes, and forests reflect relatively small fractions of the incident sunlight and have low albedos. Snow, sea ice, and deserts reflect large fractions of the incident sunlight and have higher albedo [9-11]. Nevertheless, for most calculations, when ground albedo measurements are not available, it has been customary to use the average value of 0.2, which describes the reflective properties of bare ground, free of snow [12]. Reference [13] measured the actual ground reflected radiation and compared with the Liu and Jordan value of 0.2 [12] for Geneva and concluded that the constant value 0.2 is too high and should be abandoned. In addition, reference [14] has also claimed that the constant value is unsatisfactory and unrealistic. B. Radiation Exchange The energy exchange between two black surfaces A1 and A2, each maintained at different temperatures can be solved by equation (1), which is known as view factor i.e. reciprocity relation. It is also known as configuration factor, radiant shape factor and angle factor.

Q1 - 2 = A1F12 (Eb1 - Eb2) = A2 F21(Eb1 - Eb2)

(1)

Equation 2 can be generalised for any two surfaces m and n, forming equation 3.

AmFmn = AnFnm

(2)

In terms of radiation exchange, the cases which are applicable to building services are (a) perpendicular surface: surfaces that are perpendicular to each other and (b) inclined/tilted surface: surfaces that are inclined to each other. However, the problem that persists in both the cases is, both the surfaces (ground surface and PV module) should share a common edge, which is not always practically possible. Hence, the only solution for realistic approach is to solve the inclined surface equation (3) by partial analytical and partial numerical. In addition, partial numerical approach is also required because last part of equation (3) is unsolvable integral. F1−2 = − +

(

)

sin 2Φ ⎡ ⎛π ⎞ 2 ⎛ B − A cos Φ ⎞⎤ 2 2 −1 ⎛ A − B cos Φ ⎞ ⎟ + A 2 tan −1 ⎜ ⎟⎥ ⎢ AB sin Φ + ⎜ − Φ ⎟ A + B + B tan ⎜ Φ 4πB ⎣ 2 sin B ⎝ ⎠ ⎝ ⎠ ⎝ A sin Φ ⎠⎦

⎡ B 2 (1 + C)⎤ ⎡ A 2 (1 + A 2 )cos 2 Φ ⎤ ⎫⎪ sin 2 Φ ⎧⎪⎛ 2 ⎞ ⎡(1 + A 2 )(1 + B 2 )⎤ 2 + A 2 ln⎢ ⎨⎜ 2 − 1⎟ ln⎢ ⎥ + B ln⎢ cos 2 Φ ⎥ ⎬ 2 ⎥ 4πB ⎪⎩⎝ sin Φ ⎠ ⎣ 1+ C ⎦ ⎣ C(1 + B )⎦ ⎣ C(1 + C) ⎦ ⎪⎭

⎛ 1 ⎞ C ⎛ 1⎞ A ⎛ 1⎞ ⎟ tan −1 ⎜ ⎟ + tan −1 ⎜ ⎟ − tan −1 ⎜⎜ ⎟ π π π B B A B ⎝ ⎠ ⎝ ⎠ ⎝ C⎠ ⎡ sin Φ sin 2Φ ⎛ A cos Φ ⎞ ⎛ B − A cos Φ ⎞⎤ AD⎢tan −1 ⎜ + ⎟ + tan −1 ⎜ ⎟⎥ D D 2πB ⎝ ⎠ ⎝ ⎠⎦ ⎣

+

+

1

cos Φ πB

∫

B

0

(3)

⎡ ⎞⎤ ⎛ ⎞ ⎛ z cos Φ ⎟ + tan −1 ⎜ A − z cos Φ ⎟⎥dz 1 + z 2 sin 2 Φ ⎢tan −1 ⎜ ⎟ ⎜ ⎟ ⎜ 2 2 2 2 ⎢⎣ ⎝ 1 + z sin Φ ⎠⎥⎦ ⎝ 1 + z sin Φ ⎠

III.

Methodology

To analyse the impact of ground surface reflectivity on a PV system, the proposed numerical techniques is tested with a real case study of PV facade at Edinburgh Napier University, Merchiston Campus building. Fig. 1 is the street view of the PV facade at Edinburgh Napier University [15].

7th International Conference on Solar Radiation and Daylight SOLARIS 2015, website: http://solaris2015.com/


Fig. 1 Edinburgh Napier University PV facade With the help of Google earth software, 14692m2 of front facing horizon was selected. It comprises varying ground surfaces like (a) roof area of 3766 m2, (b) road area of 3041 m2 and (c) grass with tree-top area of 7885 m2. Further, based on the Google earth image, total ground area was transformed into block diagram and on the basis of type of foreground each patch was assigned with its individual albedo value. Thereafter, a numerical approach was incorporated to obtain the amount of reflected-radiation from ground to PV facade. To carry out the calculation, a VBA code was written within a Microsoft Excel Software. IV.

Results and discussion

For the analysis for the ground-reflected radiation that is incident upon the PV modules, a horizontal irradiation of 800 W/m2 was assumed. By considering the traditional approach of keeping ρ as a constant value of 0.2, the reflectedenergy contribution from the total area of 14,682m2 near to PV facade, is calculated as 45.2W/m2. The current approach is to use the actual albedo values of each topographies. The average albedo value for road surface is 0.2, trees and grass is 0.24 and roof tiles is 0.35. The resulted reflected-radiation value is 61.8W/m2. This outcome can be enhanced by applying the highly reflective white cool paint on rooftop surfaces (3766 m2). Considering the average albedo value of 0.7 and keeping the rest topographical details of actual albedo values. Then, the reflected-radiation component increases to 92W/m2. By integrating the use of white cool paint to 3766 m2, the increased reflection will generate an excess 626kWh/annum of solar PV electricity. Considering the cost of electricity as £0.12/kWh, this will provide an extra income of £75/annum. Considering the life time of PV plant as 20 years, the extra energy generated will be 12.5MWh. Taking into account the coat of paint for every five year, the life-time cost of paint will be £393 [16]. Overall, there will be minor profit of £454 in 20 years. V.

Conclusion

This article presents the numerical approach for the estimation of ground-reflected radiation for any viewing surface. The analysis of present case study proves that, there is a gain of 37% by considering the actual albedo values of individual topography, as compared to a constant albedo 7th International Conference on Solar Radiation and Daylight SOLARIS 2015, website: http://solaris2015.com/


value of 0.2 for complete horizon (proposed by Liu and Jordan). However, applying white cool paint to a quarter of total ground surface area would generate the extra energy of 12.5MWh over 20 years of life time with a small profit of £454.

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AUTHORS Y. Kotak is a PhD student at Heriot Watt University, Edinburgh Campus, Riccarton, Currie EH14 4AS, U.K (e-mail: [email protected]). M.S. Gul is with Heriot Watt University, Architectural Engineering, Edinburgh Campus, Riccarton, Currie EH14 4AS, U.K. (e-mail: [email protected]) T. Muneer is with Edinburgh Napier UNiversity, Merchiston Campus, 10 Colinton Road, Edinburgh, EH10 5DT, U.K (e-mail: [email protected]) S.M. Ivanova PhD, is with the University of Architecture, Civil Engineering and Geodesy, 1 Hristo Smirnenski Blvd, Sofia, 1046 BULGARIA (e-mail: [email protected]).

Manuscript received by 10 April 2015 Published as submitted by the author(s).

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