Roof mounting site analysis for micro-wind turbines

Building-integrated micro-wind turbines are promising low-cost renewable energy devices. However, the take-up of micro-wind turbines in high density suburban environments is still very limited due to issues such as: a) low wind speeds; b) high turbulence intensity; and c) the perception of potentially high levels of aerodynamic noise generated by the turbines. The wind flow field above the roof of buildings in this environment is different to that over flat terrain or around isolated buildings. The effect of the local suburban topology on the wind speed and turbulence intensity fields in a given locality is therefore an important determinant of the optimal location of micro-wind turbines. This paper presents a numerical study of above roof wind flow characteristics in three suburban landscapes characterized by houses with different roof profiles, namely: pitched roofs, pyramidal roofs and flat roofs. Computational Fluid Dynamic (CFD) technique has been used to simulate the wind flow in such environments and to find the optimum turbine mounting locations. Results show how the wind flow characteristics are strongly dependent on the profile of the roofs. It is found that turbines mounted on flat roofs are likely to yield higher and more consistent power for the same turbine hub elevation than the other roof profiles.     

Thermal behavior of curved roof buildings exposed to solar radiation and wind flow for various orientations

In this study air flow, solar radiation and heat transfer from a two dimensional curved roof with north-south and east-west faced are determined and results are compared with flat roof for the same size and orientation. Comparison are performed for their corresponding roof surface temperature, and heat flow for several roof rim angles and also for various wind flow velocities, as well as for different wind directions. Turbulence is modeled by RNG k–ε method and solar radiation distribution over the roof is determined based on an appropriate model applicable to hot arid regions of Iran. Solar radiation is calculated based on the summation of beam and diffuse radiation and ground reflected radiation. For certain inside roof temperature, over all heat transfer to the building is determined with day time for various wind flows and arc shapes and results are compared with flat roof. It was found that various wind flow condition over the vaulted roof makes substantial difference on the convection heat transfer coefficient and finally on the rate of heat transfer to the building with respect to flat roof. Based on heat transfer simulation, roof temperature, heat transfer convection coefficient and heat flow though the vault for different roof arrangement and flat roofs have been determined and advantages of specific vault orientation and wind direction are specified.     

Low‐cost mounting arrangements for building‐integrated wind turbines

Micro‐generation is being widely promoted as a way for householders in the UK and elsewhere to take part in ‘the Green Revolution’. Building‐integrated wind turbines (BIWTs) provide a way to do this, enabling people to reduce their contribution to the problems of both climate change and decreasing fossil fuel availability. Although energy yields from BIWTs for many householders have been shown to be low, there are still situations where such turbines can make a useful contribution to electricity generation, e.g. in windier areas and for isolated detached buildings. The standards for the installation of BIWTs are still being developed including those for the safe mounting of turbines on domestic buildings. This paper investigates the current trend for mounting small wind turbines on the walls of domestic premises and compares this with an approach which uses roof timbers. It identifies the main characteristics of building construction which affect the integrity of such installations. European and British standards have been used to calculate wind and gravitational loads. Finite element models are used to derive working stresses and, hence, some basic principles of good design. The likely costs of wall and roof mounting are then compared. Installation and health and safety issues are also examined briefly. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation on the feasibility and enhancement methods of wind power utilization in high-rise buildings of Hong Kong

This paper reports the investigation results of wind power application in buildings. It is found that the concentration effect of buildings and the heights of buildings could enhance wind power utilization by increasing the wind speed by 1.5–2× and wind power density by 3–8× under the given simulation conditions. The wind aerodynamics and wind flows over the buildings are investigated based on local meteorological data and local high-rise building characteristics. This paper concludes that wind power utilization in high-rise buildings in Hong Kong is feasible theoretically, and some effective enhancement methods are proposed based on the simulation results, such as making full use of the heights of buildings and the concentration effect of buildings, and choosing optimal shape of building roof. However, to receive the highest potential wind energy resource and avoid turbulent areas, the tool of Computational Fluid Dynamics (CFD) has to be used to model the annual wind flows over buildings to help analyze, locate, and design wind turbines in and around buildings.     

Barriers to implement extensive green roof systems: A Hong Kong study

Air pollution problems caused from the development of infrastructures are getting serious, in which air flow is reduced and heat is trapped among high-rise buildings. In order to mitigate these problems, various methods have been developed in previous studies. Extensive green roof has been identified as one of the most important means to mitigate these problems and implement sustainable development principles in the building features. Governments world-wide have been introducing various policies and regulations for promoting extensive green roof particularly for building projects. However, the existing buildings in many large cities such as Hong Kong display few extensive green roof features. Hong Kong is one of the most densely populated cities with many high-rise buildings. This paper examines the major barriers encountered in promoting extensive green roof systems for the existing buildings in Hong Kong. Case study approach is adopted to investigate how and why the barriers can hinder the implementation of extensive green roof features. Research results show that lack of promotion and incentives from governments and the increase maintenance cost are identified as the top barriers to the implementation. The paper concludes by providing further suggestions and actions that can help mitigate these existing barriers.     

Cooling by evaporation of flowing water over a hollow roof

This paper discusses the thermal modelling of the technique for cooling buildings by means of open evaporation of water over the roof. A comparative study of cooling by means of (i) a roof pond, (ii) a water spray and (iii) moving water over the roof is presented. The influences of parameters such as wind speed, relative humidity and water flow velocity on the performance of the system are numerically examined.     

Performance and safety of rooftop wind turbines: Use of CFD to gain insight into inflow conditions

The installation of small and medium-size wind turbines on the rooftops of high buildings has been often suggested by architects and project developers as a potential solution for achieving sustainable energy in building design. In such locations, however, because of the presence of buildings and other adjacent obstructions, wind is normally turbulent, unstable and weak, in terms of direction and speed. The use of wind turbines in the built environment poses challenges to overcome, including energy yield reduction due to lower mean wind speeds in urban areas, and environmental impacts because of their close vicinity to people and property.There is a need to understand the inflow wind conditions for a small wind turbine in the built-environment. A resource assessment of the potential wind turbine site in the built environment can determine the wind characteristics including zones of wind acceleration, recirculation, blocking and channelling. This knowledge is crucial for input into the design process of a small wind turbine to accurately predict blade fatigue loads and ensure that it operates safely, and performs optimally in the environment.Computational Fluid Dynamics (CFD) is a useful method to model wind flows in order to perform a resource assessment for the application of small wind turbines in a manner that requires less time and investment than a measurement campaign. This paper presents the results of research using a CFD code to model wind flows over the roof of a building and assesses the possibility of combining a CFD package with wind atlas software to form a wind energy resource assessment tool for the application of small wind turbines on the roof of a building. Experimentation with the model shows that the results are particularly sensitive to building height and shape, roof shape, wind direction, and turbine installation height and location. The results will be used to help develop a recommended practice of wind resource assessment in the built environment, in an international collaborative effort via the International Energy Agency Task 27.     

建筑外形对建筑顶面风力机微观选址的影响

针对城市建筑环境内的风能应用问题,利用CFD方法对4种不同建筑外形的高层建筑物顶面风场湍流特征进行研究,开展建筑外形对风向变化的敏感性分析,并分析4种建筑物顶面风速、湍流强度随高度的变化规律,确定4种建筑外形的建筑顶面风力机的合理安装位置和高度,结果表明：具有圆润曲线外形的2类建筑更利于风力机的安装,风力机的安装高度可更低;建筑物的长轴和短轴越接近,顶面越有利于风力机的安装;4种建筑外形的建筑顶面安装风力机时仅考虑U/U₀≥1的有利安装高度H_u即可保证风力机的有效输出功率和运行安全;无盛行风向情况下,4种建筑物的中心区域更有利于风力机安装,风力机的安装高度最低,圆柱体、椭圆体、正方体及长方体建筑物顶面中心区域风力机最低安装高度分别为1.05H、1.09H、1.11H及1.14H。     

Investigations of a building‐integrated ducted wind turbine module

So far, wind energy has not played a major role in the group of technologies for embedded generation in the built environment. However, the wind flow around conventional tall buildings generates differential pressures, which may cause an enhanced mass flow through a building‐integrated turbine. As a first step, a prototype of a small‐scale ducted wind turbine has been developed and tested, which seems to be feasible for integration into the leading roof edge of such a building. Here an experimental and numerical investigation of the flow through building‐integrated ducting is presented. Pressure and wind speed measurements have been carried out on a wind tunnel model at different angles of incident wind, and different duct configurations have been tested. It was confirmed that wind speeds up to 30% higher than in the approaching freestream may be induced in the duct, and good performance was obtained for angles of incident wind up to ±60°. The experimental work proceeded in parallel with computational fluid dynamics (CFD) modelling. The geometry of the system was difficult to represent to the required level of accuracy, and modelling was restricted to a few simple cases, for which the flow field in the building‐integrated duct was compared with experimental results. Generally good agreement was obtained, indicating that CFD techniques could play a major role in the design process. Predicted power of the proposed device suggests that it will compare favourably with conventional small wind turbines and photovoltaics in an urban environment. Copyright © 2002 John Wiley & Sons, Ltd.     

Wind environment at a roof-mounted wind turbine on a peaked roof building

Knowledge of the wind resource above peaked roofs is necessary to determine whether installing small wind turbines on low-rise peaked roof buildings is feasible. The wind characteristics at a representative peaked roof barn in southern Ontario, Canada were investigated using a boundary layer wind tunnel and computational fluid dynamics. Field measurements at the barn were collected using sonic anemometers and compared with the simulation results. Wind speed amplification was confined to a region immediately above the roof and was relatively low for wind energy purposes. The presence of nearby trees or buildings adversely impacted wind speed amplification. Considering only wind-related factors, the placing of micro-wind turbines on roof peaks may be warranted. However, if sufficient space is available, it is recommended to place small turbines on a tower rather than on the peaked roof of a low-rise building.     

A technical review of building-mounted wind power systems and a sample simulation model

Small scale wind turbines installed within the built environment is classified as micro generation technology. This paper reports the investigation results of wind power application in buildings. First, general information is given for common type of wind turbines are used on buildings. Second, the wind aerodynamics and wind flows over the buildings are investigated based on local meteorological data and local building characteristics. However, to receive the highest potential wind energy resource and avoid turbulent areas, the tool of Computational Fluid Dynamics (CFD) has to be used to model the annual wind flows over buildings to help analyze, locate, and design wind turbines on and around buildings. Three different sample models for buildings and rural residential areas are explained with CFD models.     

Estimating the potential yield of small building‐mounted wind turbines

The wind profile in the urban boundary layer is described as following a logarithmic curve above the mean building height and an exponential curve below it. By considering the urban landscape to be an array of cubes, a method is described for calculating the surface roughness length and displacement height of this profile. Firstly, a computational fluid dynamics (CFD) model employing a k‐? turbulence model is used to simulate the flow around a cube. The results of this simulation are compared with wind tunnel measurements in order to validate the code. Then, the CFD model is used to simulate the wind flow around a simple pitched‐roof building, using a semi‐logarithmic inflow profile. An array of similar pitched‐roof houses is modelled using CFD to determine the flow characteristics within an urban area. Mean wind speeds at potential turbine mounting points are studied, and optimum mounting points are identified for different prevailing wind directions. A methodology is proposed for estimating the energy yield of a building‐mounted turbine from simple information such as wind atlas wind speed and building density. The energy yield of a small turbine on a hypothetical house in west London is estimated. The energy yield is shown to be very low, particularly if the turbine is mounted below rooftop height. It should be stressed that the complexity of modelling such urban environments using such a computational model has limitations and results can only be considered approximate, but nonetheless, gives an indication of expected yields within the built environment. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of building height and roof effect on the air velocity and pressure distribution around the detached houses in Turkey

Heat losses from buildings occur to a great extent through external walls. As compared to other types of buildings detached houses have fewer stories constructed in open places with less environmental shelter against wind. In urban and rural settlements, the wind effect on houses with roof remains as an important subject of study. More specifically, the role of roof as an insulator and the effect of roof on forced convection from buildings need to be considered in this context. In this study, we have investigated the potentials of roofs for reducing the wind effect on the detached houses during winter months in Kayseri, a midsize city in Turkey located at 38.44°N and 35.29°W Turkey.For this purpose, four different detached house scenarios are taken into account. They are (i) without roof and one storey, (ii) with roof and one storey, (iii) without roof and two storey and, (iv) with roof and two storey. For each scenario, air velocity and pressure distributions in front and back sides of the house are investigated using finite volume methods with FLUENT software. The evaluation of velocity and pressure distributions reveals the importance of roof in reducing the wind effect on houses.     

晶硅光伏系统在高层建筑屋顶的应用探讨

晶硅光伏系统在高层屋顶上应用较为普遍,但由于受到风荷载、运输、加固、荷载、经济性等各方面的影响,在我国高层建筑中,目前应用还不广泛。本文主要通过对高层建筑屋顶上建设光伏系统各类影响因素的计算和量化分析,剖析高层建筑采用晶硅光伏系统的合理性,并最终给出相应建议。     

Thermal modeling of integrated roof air heater for passive solar space heating of a non-air-conditioned building

This communication presents the periodic heat transfer analysis for solar space heating of an unconditioned building with an integrated roof air heater. The system consists of an air duct within the roof such that the air is continuously or intermittently forced to circulate the cooler room air through the inlet of the air duct. Time dependence of the air flow is represented by a step function of time for daily operation and, hence, has been expressed as a Fourier series in time. The analysis takes into account air ventilation, ground heat conduction and furnishings. The effects of depth of the air duct from the outer surface of the roof and the magnitude and duration of air flow rate on indoor air temperature have been studied for a typical cold winter day in Delhi. It is seen that a time dependent air flow through the duct is desirable from the point of view of increasing the indoor air temperature in the case of a bare roof. However, in the case of a blackened and glazed roof, continuous air flow is needed for increasing the room air temperature. The results are desirable from the point of view of efficient space heating of solar passive buildings.     

建筑体对下游垂直轴风力机气动性能影响研究

为有效利用城市风能,提高风力机运行效率,需对建筑体下游风力机位置分布开展研究。采用计算流体力学方法分析不同建筑体结构下游各位置处风速及风力机气动性能。结果表明：建筑体对自由来流的阻塞、加速与偏转作用可有效提高下游部分位置处风速,提升风力机气动性能;圆形建筑体对下游流场影响较小,各位置处平均风速接近自由来流;相比之下,三角形与四边形建筑体下游风速波动较为剧烈,平均风速较高,风力机转矩较圆形建筑体下游风力机的有较大提升;对于相同外廓建筑体,立式矩形较大的受风面积可扩大其背风低压区范围,有效提高下游流场风速,较卧式矩形建筑体具有更好的聚风效果。     

建筑密度对建筑物群内风能利用的影响

针对建筑物群内风能应用问题,采用CFD方法,对建筑密度分别为26%、20%、18%、16%、14%的5种建筑物群周围风速和湍流强度特征开展研究,分析建筑密度对建筑物群内风力机合理安装位置的影响方式。结果表明：在低于1.5H高度范围内,建筑物群的建筑密度越大,同一安装高度上适合于安装风力机的区域就越大,即越有利于建筑物群内风能的应用;在高度高于1.5H后,建筑密度对建筑物群内风力机安装位置的影响消失;无论建筑密度大小,在低于1.2H的高度范围内,建筑物群内不适合安装风力机;在高度高于1.45H后,可优先考虑将风力机安装于建筑物群内中间一排建筑物顶面,在建筑物顶面可优先将风力机安装于拐角位置;5种建筑密度的建筑物群内只考虑风速要求即可确定风力机的合理安装位置。     

Thermal regulation of photovoltaic cladding

In the use of photovoltaic cladding for walls and roofs of buildings, the PV components can experience temperatures high enough to reduce the electrical output well below the rated value. It is shown that reductions in temperature of up to 20 K can be obtained by heat transfer to an air flow induced by buoyancy in a duct behind the PV component, with a significant increase in the electrical output and reduction of heat gain into the building.Practical PV roof installations have been implemented, on the basis of measurements with a full-scale roof specimen. A numerical model is used, which enables basic design data for PV walls and roofs to be obtained, such as the effects of the depth of the cooling duct on the thermal and electrical performance of the system. The model predictions are confirmed by measurements on a purpose-built test rig and simulations using CFD.     

Sustainable urban energy: Development of a mesoscale assessment model for solar reflective roof technologies

Buildings and other engineered structures that form cities are responsible for a significant portion of the global and local impacts of climate change. Consequently, the incorporation of building design strategies and materials such as the use of reflective roof materials, or ‘cool’ roofs, are being widely investigated. However, although their benefits for individual buildings have been studied, as yet there is little understanding of the potential benefits of urban scale implementation of such systems. Here we report the development of a new methodology for assessing the potential capacity and benefits of installing reflective roofs in an urbanized area. The new methodology combines remote sensing image data with a building energy computer simulation to quantify the current rooftop reflectivity and predict the potential benefits of albedo improvement. In addition to the direct electricity savings, cool roof systems reduce peak electrical demand in the month of August when the peak demand is at its highest in the case study area. Environmental benefits associated with lowering greenhouse-gas emissions are also substantial. The new methodology allows the calculation of payback periods to assist planners to evaluate the potential economic benefits of the widespread installation of cool roof systems.     

Vertical axis resistance type wind turbines for use in buildings

Renewable energy generation in the urban environment has been receiving an increased attention over recent years due to the proximity with the point of use. Building integrated wind turbines are an interesting option in this respect. However, due to technical as well as architectural barriers, the uptake of wind energy converters into buildings has been rather limited. This paper analyses the oldest known form of wind energy converter, the Sistan type windmill, and discusses modern adaptations of this drag force type energy converter for building integration. It is shown that design improvements can lead to an increase of the theoretical efficiency of a drag force type rotor to about 48% (conservative) or 61% (optimistic). Initial experiments with a scale model have shown that efficiencies higher than 40% can be achieved. The integration of the proposed design into buildings is related to current building integrated wind turbine types and demonstrated architecturally.