Economic potential analysis of photovoltaic integrated shading strategies on commercial building facades in urban blocks: A case study of Colombo, Sri Lanka

Highlights

• Evaluates PV integrated shading strategies in the urban context of Colombo, Sri Lanka.

• Provides design guidelines for commercial buildings in Colombo for optimised façade PV electricity generation.

• Establishes optimised PV integrated shading strategy for all blocks.

• Determines most relevant urban compactness indicators for the urban context of Colombo.

Abstract

Building integrated photovoltaics (BIPV) are becoming a viable solution for clean on-site energy production and utilisation. In tropical climates, although rooftops are ideal for photovoltaic (PV) module integration, the available area may be insufficient to meet building energy demand due to the increase in high-rise urban buildings, causing a requirement for the utilisation of facades. However, the high solar elevation angle means that facades are unfavourably oriented towards receiving incident irradiation. Also, the issue exists of high solar heat gains into built spaces. This paper evaluates the utilisation of horizontally inclined PV integrated shading strategies to combat these issues based on the urban context of Colombo, Sri Lanka. Various strategies are evaluated in terms of their inclination angles and the distance between installations, and urban blocks in Colombo are analysed in terms of how they affect the solar potential in the urban canyon. The results are analysed in terms of economic potential to determine the optimised installation strategies based on urban block type. The results suggest that installations inclined at 30° at a distance-to-length ratio of 4 provide the greatest economic viability in this context.

Introduction

The conservation of conventional energy sources and utilisation of non-conventional renewable energy (NCRE) sources has become a matter of utmost in recent years. The cost of energy has risen due to the consumption and exhaustion of fossil fuel energy. A renewable energy resource with great potential is solar energy, and could become the prime source of electricity in the world by 2050 [1]. As of recent, building forms in urban contexts have been changing to be more high-rise, thus further increasing the necessity for energy. The construction sector of Sri Lanka is on the rise and currently consumes 35% of the national energy consumption [2,3]. The country has been moving towards a green economy after realising the need to change to clean energy, and policies are being implemented in order to reduce the country’s reliance on imported fossil fuels [4]. The tropical island is situated just north of the equator and receives plentiful solar irradiation all year round. Building integrated photovoltaics (BIPV) are an ideal solution in this context, as they are capable of producing electricity on-site, which cuts down losses caused by transformation and transmissions through grid lines. Furthermore, they are a renewable and carbon dioxide neutral energy system [5]. The use of BIPV in urban areas allows for the capture and transformation of solar energy on-site, by using building envelopes for PV module integration. BIPV application on the facades of modern buildings has become common in countries at higher latitudes [[6], [7], [8], [9], [10]] due to the favourable inclination towards solar irradiation and the expansive area available for module installation. Office buildings are mostly suited for BIPV applications as the panels produce electricity roughly during the same hours that the building operates [11]. Constructions with fully glazed facades are not usually recommended for tropical locations [12], even when used with materials of low emissivity, unless they are incorporated with the addition of shading strategies. Tropical regions are more suited for rooftop photovoltaic installations, but high-rise buildings may be unable to meet the building energy demand in this way as the rooftop area may be insufficient. This brings about a need to optimise PV integration on facades in the tropical context.

An efficient solution to combat this issue is the utilisation of PV integrated shading strategies (PVIS). This can be done the installation of horizontally inclined shading strategies with integrated PV modules that will be better oriented to receive incident solar irradiation, whilst also blocking high solar heat gains through building fenestrations.

In the past decades, attention has been veering towards attempting to assess the amount of solar irradiation that is incident on building envelopes and evaluate the potential for active and passive solar heating [[13], [14], [15], [16]]. Active systems include electrical and mechanical devices which convert solar energy to heat and electricity, such as PV systems for electricity generation and solar thermal systems for space and water heating. Whereas passive systems make use of the building design in order to receive or block solar radiation, thus decreasing internal heating or cooling loads. In addition to building-scale evaluations, there is also a necessity to assess how urban form and function could affect solar radiation on building envelopes, due to heavy shadow effects and reflections caused by neighbouring buildings. Studies have previously been conducted on solar potential based on urban form [13,[17], [18], [19], [20], [21]] from building and neighbourhood to urban scale [14,[22], [23], [24]]. These have been based on investigating the effects of various horizontal and vertical layouts of built forms on solar potential and daylight availability [18], and the effects of various parameters indicating urban form and density, including, but not limited to, site coverage, plot ratio, and building density [25]. Likewise, a few studies have focused on evaluating solar potential in existing urban layouts [26], but were based on characteristic building forms with little or no variation. Others considered only residential buildings [20,27], whilst several were conducted to present design guidelines for urban spaces by suggesting optimised building shapes and urban layouts for the utilisation of solar irradiation [25]. However, many of these studies have primarily been focused around using characteristic and generic urban layouts in order to explore the effects of urban form on solar potential [28], but have not been applied to real case studies. In addition, although stand-alone buildings have been analysed in detail, due to the undesirable effects of neighbouring buildings and mutual shading, urban blocks are not as capable in capturing as much solar irradiation as stand-alone buildings are. Therefore, the study of the effects of urban form on solar potential is an area of increasing interest [29].

To the authors’ knowledge, the research that has been carried out in reference to evaluation of the proposal PV integrated shading strategies in order to analyse them in terms of building energy consumption and economic potential is lacking. In terms of the evaluation of solar potential, Izquierdo et al. [30] proposed a hierarchy to analyse it in terms of physical, geographical, technical, economic, and social potential. Much of the research carried out in the past has concerned physically available solar radiation, the geographically available area for PV implementation, and even technical potential. However, studies on economic and social potential are lacking. The economic analysis of BIPV systems usually only take into consideration the PV generation capabilities [31], and a requirement exists for a comprehensive energy analysis that considers both PV generation capabilities and changes in building energy consumption.

A main concern in the built environment of Colombo is the swift spread of urban sprawl due to rapid urbanisation [32]. Combinations of unplanned urban development and lack of zoning plans have caused random development in urban areas resulting in randomised urban forms throughout the city, with no quantifiable form or function. Urban blocks in Sri Lanka tend to be of mixed function, where residential, commercial, government, and industrial buildings coexist in the same urban space. It is difficult, therefore, to conduct studies in the city based on urban block function. One of the most commonly used indicators of urban form is urban compactness [29]. However, research related to assessing the ample effects of urban compactness on solar potential are limited in the context of the real built environment. Compactness has been studied in many ways, but knowledge as to how compactness of existing neighbourhoods affects the solar potential of the buildings is lacking [20,21,33]. Furthermore, in the developed world, such as in North America and Europe, there are greater amounts of available funding for the planning and implementation of renewable energy projects to meet clean energy targets [[34], [35], [36]], and the research and execution of urban-scale solar energy projects have been carried out over many years now [37], contrary to developing countries like Sri Lanka. In addition, it was only recent declared as a developing country, and limited government funding is available for the popularisation and utilisation of BIPV. Tropical regions such as Sri Lanka are actually optimal regions for the harnessing of solar energy, but their potential is not efficiently utilised due to the lack of economic resources present for the execution of large-scale projects. It is therefore important to realise this potential by making the idea of solar power economically viable to project planners and consumers. It is imperative that PV installations are designed to be as economically efficient as possible in order to minimise unnecessary expenditure in developing regions.

This paper attempts to assess the solar potential available in the urban canyon based on urban form in Colombo in terms of urban compactness indicators. Ten random blocks in the city are assessed in order to determine their respective urban compactness indicators, and how the urban form affects the solar potential of the central building. Further studies are carried out in order to determine how the urban form affects the solar irradiation incident upon the PV integrated shading strategies to establish the most efficient installation method based on urban form. The PVIS strategies are then assessed in terms of their PV generation capabilities and how they affect the building energy consumption of the central buildings. The economic potential is calculated for the different PVIS strategies based on the urban block. The final results from this paper provide design guidelines for a global optimised PV integrated shading strategy that is applicable to all blocks.