Rooftop Area Research Articles

Methods for determining the uncertainty of population estimates derived from satellite imagery and limited survey data: a case study of Bo city, Sierra Leone.

This study demonstrates the use of bootstrap methods to estimate the total population of urban and periurban areas using satellite imagery and limited survey data. We conducted complete household surveys in 20 neighborhoods in the city of Bo, Sierra Leone, which collectively were home to 25,954 persons living in 1,979 residential structures. For five of those twenty sections, we quantized the rooftop areas of structures extracted from satellite images. We used bootstrap statistical methods to estimate the total population of the pooled sections, including the associated uncertainty intervals, as a function of sample size. Evaluations based either on rooftop area per person or on the mean number of occupants per residence both converged on the true population size. We demonstrate with this simulation that demographic surveys of a relatively small proportion of residences can provide a foundation for accurately estimating the total population in conjunction with aerial photographs.

Analysis of spatial fixed PV arrays configurations to maximize energy harvesting in BIPV applications

This paper presents a new approach for efficient utilization of building integrated photovoltaic (BIPV) systems under partial shading conditions in urban areas. The aim of this study is to find out the best electrical configuration by analyzing annual energy generation of the same BIPV system, in terms of nominal power, without changing physical locations of the PV modules in the PV arrays. For this purpose, the spatial structure of the PV system including the PV modules and the surrounding obstacles is taken into account on the basis of virtual reality environment. In this study, chimneys which are located on the residential roof-top area are considered to create the effect of shading over the PV array. The locations of PV modules are kept stationary, which is the main point of this paper, while comparing the performances of the configurations with the same surrounding obstacles that causes partial shading conditions. The same spatial structure with twelve distinct PV array configurations is considered. The same settling conditions on the roof-top area allow fair comparisons between PV array configurations. The payback time analysis is also performed with considering local and global maximum power points (MPPs) of PV arrays by comparing the annual energy yield of the different configurations.

Household electricity use, electric vehicle home-charging and distributed photovoltaic power production in the city of Westminster

In this paper we investigate household electricity use, electric vehicle (EV) home-charging and distributed photovoltaic (PV) power production in a case study for the city of Westminster, London. Since it is economically beneficial to maximize PV power self-consumption in the UK context the power consumption/production patterns with/without introducing EV home-charging on the household level is investigated. Additionally, since this might have an effect on the electricity use on an aggregate of households a large-scale introduction of EV charging and PV power production in the entire city of Westminster is also investigated. Household electricity consumption and EV home-charging are modeled with a Markov-chain model. PV power production is estimated from solar irradiation data from Meteonorm for the location of Westminster combined with a model for photovoltaic power production on tilted planes. The available rooftop area is estimated from the UK map geographic information database. EV home-charging increases the household electricity use mainly during evening with a maximum during winter whereas PV produces power during daytime with maximum during summer. On the household level this mismatch introduces variability in power consumption/production, which is shown to be less prominent for the large-scale scenario of the entire city of Westminster.

Spatial statistical analysis and simulation of the urban heat island in high-density central cities

The urban heat island (UHI) is a mounting problem in built-up areas, leading to increased temperatures, air pollution, and energy consumption. This paper explores the urban determinants of the UHI, using two-dimensional (2-D) and three-dimensional (3-D) urban information as input to spatial statistical models. The research involves: (a) estimating land surface temperatures, using satellite images, (b) developing a 3-D city model with LiDAR data, (c) generating urban parameters with 2-D/3-D geospatial information, and (d) conducting spatial regression analyses. The data are captured over three grids of square cells – 480m, 240m, and 120m – and characterize the center of Columbus, Ohio. The results show that solar radiations, open spaces, vegetation, building roof-top areas, and water strongly impact surface temperatures, and that spatial regressions are necessary to capture neighboring effects. The best regression is obtained with the general spatial model (GSM), which is then used to simulate the temperature effects of different greening scenarios (green roofs, greened parking and vacant lots, vegetation densification) in the center of Columbus. The results demonstrate the potential of such models to mitigate the UHI through design and land-use policies.

도시열섬 적응능력 제고를 위한 옥상녹화 중점지역 선정 방안

The improvement of adaptation capability against heat island (ACHI) by greening buildings is considered as an important measure to cope with a climate change. This study aimed to select the most appropriate zones for green roof initiative in case study sites, Bucheon, Anyang, and Suwon Cities and to investigate the characteristics of buildings for greening to improve ACHI. Relative ACHI for each lot was estimated from 0 to -9, assuming that it decreases with the distance from green space and waterbody. Low adaptation capabilities were mostly shown in the old urban blocks with dense low-rise buildings and lack of green space. Three blocks with the lowest ACHIs were chosen as a green roof initiative zone in each city. They are largely residential areas including low-rise buildings such as single, multi-household houses, townhouses, 5 or lower story apartments and few are industrial areas crowded with small factory buildings. The areas of building roof available for greening are 8.8% within the selected zones in Bucheon City, 5.3% in Anyang City, and 4.9% in Suwon City. As it were, 25.2~41.7% of the roof top areas are available for greening in these zones. It means that roof top areas of <TEX>$25,000{\sim}120,000m^2$</TEX> can be used for greening within the selected zones of <TEX>$0.64{\sim}1.65km^2$</TEX> to improve ACHI. The approach and results of the study are significant to provide a logical basis and information on location, scale, effect, and target figure of greening as a measure to cope with climate change.

Lifetime performance of semi-transparent building-integrated photovoltaic (BIPV) glazing systems in the tropics

Abstract The use of semi-transparent BIPV as a form of on-site renewable energy generation in energy efficient sustainable buildings is increasing. Its popularity is due to its contribution towards zero-(or even plus) energy buildings. In urban cities where the buildings have limited rooftop area but large facade areas, adopting semi-transparent BIPV windows is an alternative for conventional windows. The semi-transparent BIPV windows not only solely generate electricity but also affect the buildings' electricity usage through daylighting and heat gain/loss. In tropical climates where it is hot and humid all year round, the importance of window facade elements is even more pertinent due to high cooling load from the solar heat gains. This paper examines the life cycle environmental and economic performance of commercially available semi-transparent BIPV modules for window application under the tropical conditions of Singapore. Energy simulations, previously performed, were adopted to conduct a life cycle assessment to determine the long term performance in terms of energy and carbon emissions, as well as cost considerations. The EPBT and EROEI for the modules ranged from 0.68 to 1.98 and 11.72 to 34.49 respectively. After considering government subsidies, some modules cost lower than conventional windows, while half of the remaining modules, achieved payback periods of 1.1–13.1 years. These performance indicators were used to form a decision-making tool to assist architects and building designers in the selection of BIPV modules for windows during the early design stages.

Novel Approach for Rooftop Detection Using Support Vector Machine

A new method for detecting rooftops in satellite images is presented. The proposed method is based on a combination of machine learning techniques, namely, k-means clustering and support vector machines (SVM). Firstly k-means clustering is used to segment the image into a set of rooftop candidates—these are homogeneous regions in the image which are potentially associated with rooftop areas. Next, the candidates are submitted to a classification stage which determines which amongst them correspond to “true” rooftops. To achieve improved accuracy, a novel two-pass classification process is used. In the first pass, a trained SVM is used in the normal way to distinguish between rooftop and nonrooftop regions. However, this can be a challenging task, resulting in a relatively high rate of misclassification. Hence, the second pass, which we call the “histogram method,” was devised with the aim of detecting rooftops which were missed in the first pass. The performance of the model is assessed both in terms of the percentage of correctly classified candidates as well as the accuracy of the estimated rooftop area.

Improved Sustainability of Water Supply Options in Areas with Arsenic-Impacted Groundwater

The supply of water for rural populations in developing countries continues to present enormous problems, particularly where there is arsenic contamination in the groundwater, as exists over significant parts of Bangladesh. In response, improvements in the sustainability of water supplies are feasible through the use of a combination of water sources wherein rainwater harvesting is employed for a portion of the year. This can potentially reduce the duration of the year during which arsenic-contaminated groundwater is utilized. As demonstrated, a rainwater cistern volume of 0.5 m3 in the Jessore district area of Bangladesh can provide rainwater for periods averaging 266 days of the year, which allows groundwater at 184 µg/L arsenic to be used as a water supply for the remainder of the year. This dual supply approach provides the body burden equivalent to the interim drinking water guideline of arsenic concentration of 50 µg/L for 365 days of the year (assuming the water consumption rate is 4 L/cap/day for a family of five with a rainwater collection area of 15 m2). If the water use rate is 20 L/cap/day, the same cistern can provide water for 150 days of the year; however, although this is insufficient to supply water to meet the body burden equivalent guideline of 50 µg/L. Results are provided also for different rooftop areas, sizes of cisterns and alternative arsenic guidelines [World Health Organization (WHO) and Bangladeshi]. These findings provide useful guidelines on supply options to meet sustainability targets of water supply. However, they also demonstrate that the use of cisterns cannot assist the meeting of the 10 µg/L WHO target arsenic body burden, if the arsenic contamination in the groundwater is high (e.g., at 100 µg/L).

Small Wind Turbines in the Built Environment: Influence of Flow Inclination on the Potential Energy Yield

Increasing interest is being paid by architects, project developers and local governments to understanding where small wind turbines can effectively be exploited to provide delocalized power in the built environment. The wind conditions in the rooftop area of buildings in urban locations are, however, very complex and the real adaptability of wind turbines to these environments is not yet tested both in terms of real producibility and of structural compatibility with the building themselves. In these installations, in particular, the flow that incomes on the rotor is often inclined with respect to the horizontal direction due to the interaction with the building façade and the roof. A correct estimation of the impact of an inclined flow on the performance of horizontal-axis wind turbines, therefore, becomes a very relevant issue to correctly predict the potential energy yield of a machine. To this purpose, a simulation code based on a blade element momentum (BEM) approach was developed and validated by means of experimental data found in the literature. The code was then used to evaluate the energetic suitability of a small-size wind turbine installation in the rooftop of a building in a conventional European city. A numerical computational fluid dynamics (CFD) analysis was carried out to characterize the flow field in the rooftop area of different buildings. The flow velocity modulus and direction were calculated for several oncoming wind profiles: The results were projected into an available wind power curve in the rooftop of the building. The effective energy-yield capabilities were then corrected using the model for the flow inclination as a function of the specific flow conditions in the rooftop area. The results were finally exploited to analyze the energy-oriented feasibility of an installation in a similar context.

Evaluation of residential rainfall harvesting systems in Jordan

A field study was conducted to evaluate the potential of rooftop rainfall harvesting (RRH), determine the smallest tank volume and the potential contribution of RRH to the national water budget. The potential water volume from rooftops nationwide is 14.7 million cubic metres per year (MCM/year), which comprises 6% of the domestic national water budget. Public water supply rates decreased for areas practicing RRH. The number of underground tank (cisterns) increased in high rainfall areas indicating adaptation behaviours for securing water. Smallest cistern volume charts were constructed for each governorate in relation to consumption rate and rooftop area. An immediate and nationwide awareness and legislative programmes are needed to spread RRH among the whole population of Jordan.

Rooftop photovoltaic (PV) systems: a cost–benefit analysis study of industrial halls

Rooftop photovoltaic (PV) systems can be readily deployed on industrial halls with relatively large rooftop area. Feed-in tariff above the base price of electricity is offered in many countries to subsidize the high initial investment of PV systems. In order to fully capitalize the benefit of the feed-in tariff, the investigation of the actual performance of PV systems under case-specific conditions is very important. With building energy simulation, this paper explores the cost-benefit of implementing PV systems of different capacities for a few different cases of industrial halls. The impact of various economic parameters is also investigated.

Assessment of Residential Rain Barrel Water Quality and Use in Cincinnati, Ohio1

AbstractThe collection, storage, and reuse of rainwater collected in rain barrels from urban rooftop areas assists municipalities in achieving stormwater management objectives and in some areas also serves as an adjunct resource for domestic water supplies. In this study, rainwater reuse and levels of select microbial indicators were monitored for six residential rain barrels located in the Shepherd Creek watershed of Cincinnati, Ohio. Water from rain barrels typically had poor microbial quality and was used for watering indoor and outdoor plants. Rain barrel water chemistry was slightly acidic, exhibited wide ranges in conductivity, turbidity, and total organic carbon (TOC) concentrations and gave no evidence of the presence of cyanobacterial microcystin toxins. Selected microbial water‐quality indicators indicated that counts of total coliform and enterococci were consistently above U.S. Environmental Protection Agency standards for secondary recreational contact water‐quality standards. Residential rain barrels can provide water appropriate for low‐contact reuses (such as plant watering), although there may be transient periods of high levels of indicator bacteria in the collected water.

Potential for rooftop photovoltaics in Tokyo to replace nuclear capacity

In 2010, nuclear power accounted for 27% of electricity production in Japan. The March 2011 disaster at the Fukushima Daiichi power station resulted in the closure of all of Japan’s nuclear power plants and it remains an open question as to how many will reopen. Even before the loss of nuclear capacity, there were efforts in Japan to foster the use of renewable energy, including large scale solar power. Nuclear power plants in Japan provided more than just base-load by storing energy in large scale pumped hydroelectric storage systems, which was then released to provide some peaking capacity. If this storage were instead coupled to current generation rooftop solar systems in Tokyo, the combined system could help to meet peak requirements while at the same time providing ∼26.5% of the electricity Tokyo used to get from nuclear output, and do so 91% of the time. Data from a study of rooftop space and a 34 yr data set of average daily irradiance in the Tokyo metropolitan area were used. Using pumped hydroelectric storage with 5.6 times this rooftop area could completely provide for TEPCO’s nuclear capacity.

The Potential of Rainwater Harvesting: A Case of the City of Windhoek, Namibia

Windhoek, Namibia’s capital has experience for more than 25 years of novel approaches in integrated water management largely driven by the scarcity of water in the area. Notably absent in their approaches however, is the rooftop rainwater harvesting which is regarded as one of the viable alternative sources of water for domestic use. This paper assesses the potential economic benefits for rooftop rainwater harvesting for the City of Windhoek. The rooftop areas from four representative formal suburbs of Okuryangava (low income) in the north, Academia (middle income) in the south-central, Pioneers Park (middle income) in the south-west and Ludwigsdorf (high income) in the east, were estimated from high resolution satellite images captured from Google Earth. These estimates were used to extrapolate for the potential amount of rainwater that can be harvested in an average rainy season (i.e. December to April) in the study area. The estimated harvestable amount for each residential area was developed using a simple model that incorporates total rooftop area and estimated rainwater. The derived figure was then expressed in terms of cost per unit prices charged by the City of Windhoek. Pioneers Park attests to have the highest (134 m3) potential harvestable rainwater per household, while Okuryangava is estimated to harvest the least amount of 36 m3 per raining period. Given the high density of erven, however, Okuryangava has a potential to harvest approximately 920484 m3 of water per hectare, ranking this suburb second after Academia, which stands at 1120716 m3. This is a significant amount of water effectively taken from rainwater rooftop that could also provide justifiable saving to residents if used instead of tap water. It is therefore recommended that the City of Windhoek actively promotes rooftop rainwater harvesting for the benefit of residents and also as a measure to reduce storm-water runoff due to urban development.

Viability of Living Roof Systems in Albuquerque, New Mexico

This paper addresses the viability of living roof systems to function as a building eciency technology in the climatic conditions of the Southwestern United States. Results are drawn from an analysis of the thermal performance and water requirements of an extensive living roof located on the School of Architecture and Planning's George Pearl Hall at University of New Mexico in Albuquerque. Measurements taken over a three- month period from June 2010 to August 2010 revealed that the living roof system was, on average, 5.45 - C cooler at the soil-membrane interface as compared to the surface temperature of an analogous roof on the same building. To operate without supplemental aquifer derived water, native grass planted living roof systems in Albuquerque, New Mexico require a rooftop rainwater catchment area equal to half the area of the living roof with an appropriately sized rainwater storage system.

A method of micrositing of wind turbine on building roof-top by using joint distribution of wind speed and direction, and computational fluid dynamics

Urban wind turbines are recommended for installation on a building roof-top to capture more wind energy. It is critical to decide an exact location for the wind turbine installation on the roof-top area. In this paper a joint probability density function of wind speed and direction is proposed as a statistical model for wind distribution, and a CFD approach is taken to predict the wind acceleration effect by the geometry of the building. Singly Truncated Normal Weibull (TNW) PDF mixture (TNW-PDF) is used as the wind speed distribution function, and von Mises mixture distribution is utilized for the wind direction distribution. These two functions are jointly tested to accurately describe the wind database. The method proposed by Johnson and Wehrly is used to obtain angular-linear distributions for this purpose. The inevitable acceleration and secondary effects of building’s roof-top upon the wind distribution is assessed quantitatively by means of the CFD technique. The CFD simulation provides time-averaged wind speeds of various directions with their calculated probabilities at a specific location over the roof top. Thereby the micrositing method is suggested for determination of optimal location for the installation of small wind turbine in the urban roof-top environment.

Community-scale assessment of rooftop-mounted solar energy potential with meteorological, atlas, and GIS data: a case study of Guelph, Ontario (Canada)

Forward-thinking governments recognize that local renewable resource use is crucial to the resilience of communities and are developing and implementing community energy plans (CEPs). Guelph, Ontario, (Canada) included solar energy in its CEP, but the question of resource potential remained. The primary goal of this work was the assessment of Guelph’s solar energy potential with arrays of grid-tied, rooftop-mounted photovoltaic (PV) and hot-water (HW) panels. This work relies on atlas and meteorological data with insolation modeling to evaluate the solar resource and incorporates geographic information system data for rooftop area quantification. The assessment applies the performance characteristics of commercially available PV and HW panels and systems to estimate the upper-limit potential for electricity and hot-water production. The modeling results closely matched the values provided by two solar resource atlases for multiple orientations and time periods. Extensive modeling led to the development of sunflower diagrams, radial plots representing the variation of insolation on planar surfaces with orientation and inclination. The optimum orientation for annual beam and sky-diffuse insolation occurred at 18° east of south with a slope of 30°. Deviation from the optimum by 15° resulted in losses of less than 5%. Assuming horizontal rooftops, Guelph could provide 1,496 GWh of electricity or 5,360 GWh[e] of heat, over 90% or twice the respective demands for 2005. Including the variation of rooftop orientation and inclination, Guelph could achieve 2,950 GWh of electricity or 10,580 GWh[e] of heat, approaching double and quadruple the respective 2005 demands.

Can cities become self-reliant in energy? A technological scenario analysis for Cleveland, Ohio

This study applied the concept of local self-reliance in energy for a North American city, Cleveland, Ohio. Results revealed that while nearly all of Cleveland’s energy is imported, there is potential to meet the demand entirely using local, renewable sources of energy. Four scenarios were constructed. The first scenario included planned renewable energy developments: the 20MW Municipal Solid Waste to Energy (MSWE) facility by Cleveland Public Power and a 20MW offshore wind project by the Lake Erie Energy Development Corporation (LEEDCo). These resulted in 1% energy self-reliance for the city. The second scenario, in which the contribution of offshore wind power was increased to 1000MW (LEEDCo’s target for 2020), solar power was provided using 10% efficient solar panels on one-quarter of all rooftop area and biodiesel was produced on one-half of the city’s vacant land using algae of moderate productivity, produced 17.6% self-reliance. In Scenario 3, the level of energy self-reliance jumped to 70% when LEEDCo’s 2030 target of 5000MW was considered together with solar panels of 20% efficiency and high-productivity algae. In Scenario 4, 100% self-reliance was attained by further increasing offshore wind power. The analysis also revealed that enhanced energy self-reliance could result in $28.7M to $1.76bn being retained in Cleveland annually. Although consumer behavior, market forces, and political dynamics can shape the production and use of energy, it is concluded that cities have the potential to substantially increase their energy self-reliance, which may bolster their economic resilience and reduce their ecological footprint.

Microeolic turbines in the built environment: Influence of the installation site on the potential energy yield

Abstract Generic proposals for an effective integration of renewable energy sources in the urban environment are frequently carried out by project developers, local governments and media, although an in-depth knowledge of the technical and energetic limitations is often missing. In particular, the installation of small wind turbines on the rooftops of tall buildings is considered to represent an attractive solution thanks to the supposed possibility of exploiting local flow accelerations induced by the building facades. The real feasibility of this scenario has, however, yet to be proved, both in terms of real energy harvesting and of compatibility of the machines with a densely populated area. In this study, a critical examination of the flow conditions on the rooftop of a building in an urban environment has been carried out by means of CFD simulations. The main goal of the analysis was the assessment of some general criteria to evaluate the convenience of a microeolic turbine installation on the roof of a selected building as a function of both its geometrical features and those of its upwind building along the prevailing wind direction. In each configuration, the flow velocity and skew angle on the rooftop area of the installation building were calculated for different incoming wind profiles and compared to their levels in the undisturbed flow. Finally, an energy-based comparison between the flow potentials in the investigated environment is provided and some general tendencies are outlined.

Feasibility analysis of a Darrieus vertical-axis wind turbine installation in the rooftop of a building

The renewed interest that is being paid by architects, project developers and local governments to small-size wind turbines is mainly connected to the attractive prospects of future applications in the urban environment; the delocalized power production of these systems could indeed provide an effective answer to both the growing demand for renewable energy and the increased attention in buildings with a sustainable and low-energy design. In particular, Darrieus vertical-axis wind turbines (VAWTs) are being considered as one of the most attractive solutions due to their low visual impact, the reduced acoustic emissions and their better response to a turbulent and skewed oncoming flow. The feasibility of this scenario has, however, to be proved yet; in particular, doubts are still connected to the real producibility in a complex terrain like the urban one and to the compatibility of microeolic machines with a densely populated area. On these assumptions, the aim of this work is to critically evaluate the energetic suitability of a Darrieus VAWT installation in the rooftop of a building in a reference European city. With this goal in mind, a numerical CFD analysis was carried out to characterize the flow field in the rooftop area of buildings with different shapes and geometrical proportions: the flow velocity modulus and direction were calculated for different oncoming wind profiles and the results were projected into a net available wind distribution in the rooftop of each building. As a second step, in order to provide a reliable estimation of the real functioning of the turbine in the investigated environment, a specific numerical model has been developed to account for the effects of a skewed flow on the power performance of the Darrieus rotor. The results of these analyses were finally combined and synthesized in an energy-oriented study to evaluate the feasibility of a rooftop installation.