Optimization of the performance of double-façades with integrated photovoltaic panels and motorized blinds

Double-façades with integrated photovoltaic panels may be employed to generate electricity, thermal energy and for daylighting. A theoretical study of double-façades with integrated photovoltaics (PV) and motorized blinds is presented, which investigates the effect of various design parameters in order to maximize the conversion of solar radiation to useful energy. Two configurations of the façade with a lower section with integrated PV and an upper Vision (viewing) section with motorized blinds, are examined. A one-dimensional finite-difference thermal model is developed, with an algorithm that iteratively determines which convective heat transfer coefficient correlation to use for each surface inside the cavity using expressions that consider system characteristics and temperature distribution. When PV modules are installed in the middle of the cavity, air flows on both sides, increasing PV section overall (thermal-electric) efficiency by about 25%, but lowers electricity generation by 21%. Integrating 0.015 m long, 0.002 m wide fins to the PV back plate leads to a similar increase in efficiency without compromising electricity generation. Placing the blind in the middle of the cavity increases the Vision section efficiency by 5%. Using this approach to optimize performance can lead to combined thermal-electric efficiencies of over 60%.     

Design and overall energy performance of a ventilated photovoltaic façade

In this study, a theoretical ventilated photovoltaic (PV) façade, which functions as a pre-heating device in winter and a natural ventilation system in summer and reduces PV module temperatures, was analysed. The interrelationship between an optimum proportion of transparent window (and an opaque PV module) to the total façade area, and the variables relevant to the energy performance was assessed. The design parameters under consideration have been categorised according to climate, building characteristics, façade configurations and PV system elements. One outcome of this investigation is a new index, effectiveness of a PV Façade (PVEF), that has been developed to evaluate the overall energy performance of a PV façade with regard to the proportion of useful daylight that may displace the use of electric lighting, and the electricity generated by the PV modules to the heating and cooling energy consumption within a building. In conclusion, the electricity generation and the factors, affecting the ventilation performance of a ventilated PV façade, have been presented.     

Façade solar collectors

Rational use of energy in buildings leads to a concept of active energy façades such as transparently insulated massive walls, solar thermal or PV façades, advanced glazings for daylighting purposes or double ventilated façades. The paper is concerned with the façade-integrated solar thermal collectors concept for water heating in the existing building stock in the Czech Republic (panel and brick blocks of flats), which are ready for major renovations. Thermal behavior of façade collectors compared with standard roof-located collectors in solar DHW systems was investigated. Façade solar collectors should have an area increased by approximately 30% to achieve the usual 60% solar fraction compared with conventional roof solar collectors with a 45° slope. Further increases in the solar fraction above 70% lead to a required area comparable with roof collectors but with less stagnation periods compared with roof collectors. Application of façade solar collectors affects the indoor comfort in buildings in a reasonable range. Indoor temperatures increase by no more than 1 K in all investigated configurations. Building behavior is not strongly affected by façade collectors when sufficient insulation layers are present.     

Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade

The term “open-joint ventilated façades” refers to a building system in which coating material (metallic, ceramic, stone or composite) is hanged by means of a metallic-frame structure to the exterior face of the wall, creating an air cavity between wall and slabs. The coating material is placed in an arrangement of slabs and a series of thin joints from slab to slab to allow the surrounding air to enter and leave the cavity all along the wall. In addition to aesthetic and constructive reasons, the main interest in open joint ventilated façades is their ability to reduce cooling thermal loads. This is achieved by the buoyancy effect induced by solar radiation inside the ventilated cavity, where the air can enter or leave freely through the joints. This paper focuses on the phenomena produced on a typical open joint ventilated façade, and the comparison of its energy performance with that of a conventional sealed air cavity façade. The thermo fluid-dynamic behaviour of both systems has been analysed with CFD techniques and the results of the 3D simulations conclude that open-joint ventilated façades can help to achieve important energy savings in climates with hot summers and mild winters.     

Performance of PV-Trombe wall in winter correlated with south façade design

PV-Trombe wall (PVTW) is a novel version of Trombe-wall. Photovoltaic cells on the cover glazing of the PVTW can convert solar radiation into electricity and heat simultaneously. A window on the south façade can also introduce solar heat into the room in the winter season. Experiment has been conducted to study the temperature field of a building with both southern facing window and the PVTW. A dynamic numerical model is developed for the simulation of the whole building system. The temperature of the indoor air is found to be vertically stratified from the measurement. The nodal model is adopted to calculate the temperature profile in the room. The simulation results are in good agreement with the experimental data. The different south façade designs affect the thermal efficiency of the PVTW significantly from the numerical simulation. With a southern facing window, the thermal efficiency of the PVTW is reduced by 27% relatively. The increase of PV coverage on the glazing can reduce the thermal efficiency of the TW by up to 17%. By taking account of electric conversion, the total efficiency of solar utilization is reduced by 5% at most while the glazing is fully covered with PV cells. The electric conversion efficiency of the PVTW achieves 11.6%, and is slightly affected by south façade designs.     

Effect of louver shading devices on building energy requirements

External louvers are increasingly used to provide solar protection for building glazed surfaces. In this work, a general study of the effect of louver shading devices applied to different façades of a building is carried out, for different locations (latitudes). Building energy requirements for a building in the cooling and heating seasons is quantified for different window and louver areas, under climatic conditions of Mexico (Mexico), Cairo (Egypt), Lisbon (Portugal), Madrid (Spain) and London (UK). Also, operative and indoor temperatures were calculated through simulations using TRNSYS software, whereas the model for the shading geometry study was solved with EES software. Both horizontal and vertical louver layouts were considered. The results show that the integration of louver shading devices in the building leads to indoor comfortable thermal conditions and may lead to significant energy savings, by comparison to a building without shading devices.     

Quantifying the potential of automated dynamic solar shading in office buildings through integrated simulations of energy and daylight

The façade design is and should be considered a central issue in the design of energy-efficient buildings. That is why dynamic façade components are increasingly used to adapt to both internal and external impacts, and to cope with a reduction in energy consumption and an increase in occupant comfort. To gain a complete picture of any façade’s performance and subsequently carry out a reasonable benchmarking of various façade alternatives, the total energy consumption and indoor environment need to be considered simultaneously. We quantified the potential of dynamic solar shading façade components by using integrated simulations that took energy demand, the indoor air quality, the amount of daylight available, and visual comfort into consideration. Three types of façades were investigated (without solar shading, with fixed solar shading, and with dynamic solar shading), and we simulated them with various window heights and orientations. Their performance was evaluated on the basis of the building’s total energy demand, its energy demand for heating, cooling and lighting, and also its daylight factors. Simulation results comparing the three façade alternatives show potential for significant energy reduction, but greater differences and conflicting tendencies were revealed when the energy needed for heating, cooling and artificial lighting were considered separately. Moreover, the use of dynamic solar shading dramatically improved the amount of daylight available compared to fixed solar shading, which emphasises the need for dynamic and integrated simulations early in the design process to facilitate informed design decisions about the façade.     

Potential application of a centralized solar water-heating system for a high-rise residential building in Hong Kong

There is a growing, government-led trend of applying renewable energy in Hong Kong. One area of interest lies in the wider use of solar-energy systems. The worldwide fast development of building-integrated solar technology has prompted the design alternative of fixing the solar panels on the external façades of buildings. In Hong Kong, high-rise buildings are found everywhere in the urban districts. How to make full use of the vertical facades of these buildings to capture the most solar radiation can be an important area in the technology promotion. In this numerical study, the potential application of a centralized solar water-heating system in high-rise residence was evaluated. Arrays of solar thermal collectors, that occupied the top two-third of the south and west façades of a hypothetical high-rise residence, were proposed for supporting the domestic hot-water system. Based on typical meteorological data, it was found that the annual efficiency of the vertical solar collectors could reach 38.4% on average, giving a solar fraction of 53.4% and a payback period of 9.2 years. Since the solar collectors were able to reduce the heat transmission through the building envelope, the payback was in fact even shorter if the energy saving in air-conditioner operation was considered.     

Energy analysis of façade-integrated photovoltaic systems applied to UAE commercial buildings

Developments in the design and manufacture of photovoltaic cells have recently been a growing concern in the UAE. At present, the embodied energy pay-back time (EPBT) is the criterion used for comparing the viability of such technology against other forms. However, the impact of PV technology on the thermal performance of buildings is not considered at the time of EPBT estimation. If additional energy savings gained over the PV system life are also included, the total EPBT could be shorter. This paper explores the variation of the total energy of building integrated photovoltaic systems (BiPV) as a wall cladding system applied to the UAE commercial sector and shows that the ratio between PV output and saving in energy due to PV panels is within the range of 1:3–1:4. The result indicates that for the southern and western façades in the UAE, the embodied energy pay-back time for photovoltaic system is within the range of 12–13 years. When reductions in operational energy are considered, the pay-back time is reduced to 3.0–3.2 years. This study comes to the conclusion that the reduction in operational energy due to PV panels represents an important factor in the estimation of EPBT.     

Implications of urban settings for the design of photovoltaic and conventional façades

This study investigates the impacts of urban settings on the energy and environmental performances of office space with a conventional and two photovoltaic (PV) integrated facade alternatives. The urban factors under consideration are orientation of buildings and streets and degree of obstruction. PV electrical and thermal models are integrated into an existing energy simulation tool, the LT model, to calculate hourly PV electricity production and thermal performance of a ventilated PV façade in the urban environment. The predictions of PV output are compared with the monitoring data and show a good agreement. After the energy and CO₂ emission consequences of the building and urban factors are identified, the appropriateness of the urban PV façade in relation to obstruction in three locations (Oslo, Cambridge and Milan), each representing northern, central and southern Europe, is presented. Finally, efficient façade types with optimal glazing ratios in urban context for three locations are discussed.     

A prototype photovoltaic/thermal system integrated with transpired collector

Building-integrated photovoltaic/thermal (BIPV/T) systems may be utilized to produce useful heat while simultaneously generating electricity from the same building envelope surface. A well known highly efficient collector is the open-loop unglazed transpired collector (UTC) which consists of dark porous cladding through which outdoor air is drawn and heated by absorbed solar radiation. Commercially available photovoltaic systems typically produce electricity with efficiencies up to about 18%. Thus, it is beneficial to obtain much of the normally wasted heat from the systems, possibly by combining UTC with photovoltaics. Combination of BIPV/T and UTC systems for building facades is considered in this paper - specifically, the design of a prototype façade-integrated photovoltaic/thermal system with transpired collector (BIPV/T). A full scale prototype is constructed with 70% of UTC area covered with PV modules specially designed to enhance heat recovery and compared to a UTC of the same area under outdoor sunny conditions with low wind. The orientation of the corrugations in the UTC is horizontal and the black-framed modules are attached so as to facilitate flow into the UTC plenum. While the overall combined thermal efficiency of the UTC is higher than that of the BIPV/T system, the value of the generated energy - assuming that electricity is at least four times more valuable than heat - is between 7% and 17% higher. Also, the electricity is always useful while the heat is usually utilized only in the heating season. The BIPV/T concept is applied to a full scale office building demonstration project in Montreal, Canada. The ratio of photovoltaic area coverage of the UTC may be selected based on the fresh air heating needs of the building, the value of the electricity generated and the available building surfaces.     

Photovoltaic collectors efficiency according to their integration in buildings

A model for building integrated photovoltaic systems has been developed and implemented in a dynamic simulation tool. This tool takes into account the thermal interactions between the PV collector and the building. The influence of the type of integration upon the PV collector efficiency has been evaluated and hybrid PV/air collectors have been studied. An overall efficiency is defined, including the production of electricity and heat. A case study has been performed on two different typical buildings. In the case of a multi-crystalline silicon PV collector integrated on the roof of a single family house located in Paris, the efficiency of unventilated PV modules fixed on the roof is 14%. If the PV collector is used to preheat the ventilation air, the efficiency reaches 20%. A proper building integration also improves the environmental balance of PV technologies over their life cycle.     

光伏/光热-地源热泵联合供热系统运行性能研究

为解决太阳电池的发电效率随温度升高而下降以及地源热泵系统供热引起的土壤热失衡问题,以典型居住建筑的光伏/光热-地源热泵（PV/T-GSHP）联合供热系统为研究对象,基于TRNSYS软件,采用土壤温度、地源热泵机组季节能效比、光伏发电效率和太阳能保证率为评价指标,对该联合供热系统进行运行性能分析。研究结果表明：夏热冬冷地区（以长沙为例）太阳能保证率相对较高,PV/T组件面积为满屋顶最大化安装（900 m²）时,第20年末土壤温度相比初始地温仅升高0.8 ℃,热泵机组季节能效比约为5.1,太阳能保证率为97.0%~98.7%;不同气候地区的太阳能保证率与PV/T组件面积和建筑全年累计供热量有关,通过定义单位建筑全年累计供热量PV/T组件面积指标,得到中国不同气候地区的太阳能保证率与该指标的耦合关系,回归方程的决定系数R²为0.983,得出在已知建筑全年累计供热量和太阳保证率设计目标值的条件下所需PV/T组件面积的计算方法。PV/T-GSHP联合供热系统的全年运行能耗显著小于平板太阳能集热器-地源热泵联合系统（最小降幅为沈阳,49.7%）,远小于空气源热泵（最小降幅为石家庄,79.8%）和燃气壁挂炉（最小降幅为沈阳,65.1%）。     

Cost-optimal energy performance measures in a new daycare building in cold climate

New municipal service buildings must be energy effective, and cost-optimality is one of the criteria for selecting the suitable energy performance improvement measures. A daycare building in a cold climate was studied by means of simulation-based, multi-objective optimisation. Using a genetic algorithm, both target energy use and life-cycle cost of the selected measures were minimised. It was found that extensive insulation of the building envelope is not a cost-optimal method to reduce the daycare building energy use. Improving energy efficiency of the ventilation system, utilising solar energy on-site and employing a light control strategy are preferable ways of improving the building energy performance. Ground-source heat pump is a more cost-optimal heating system for the daycare building than district heating. The cost-optimal sizing of the heat pump is small, only 28% of the required maximum heating power.

Abbreviations: AHU: air handling unit; CAV: constant air volume; COMBI: comprehensive development of nearly zero-energy municipal service buildings; COP: coefficient of performance; DH: district heating; DHW: domestic hot water; EPBD: energy performance of buildings directive; EU: European Union; FINVAC: Finnish Association of HVAC Societies; GSHP: ground-source heat pump; HRU: heat recovery unit; IDA ICE: IDA Indoor Climate and Energy; LED: light-emitting diode; MOBO: multi-objective building optimisation tool; NSGA-II: Non-dominated Sorting Genetic Algorithm II; nZEB: nearly zero-energy building; PV: photovoltaic; TRY: test reference year; VAV: variable air volume; ZEB: zero-energy building     

Available remodeling simulation for a BIPV as a shading device

A building integrated photovoltaic system as a shading device is used as an application and remodeling model. This study applies the simulation program SOLCEL and the computational fluid dynamics method to cases with solar irradiance components analysis and a ventilated double façade remodeling of the BIPV. For the validation of the theoretical work, experimental results of the Samsung Institute of Engineering and Construction Company building are used with a wind velocity of the weather data of Suwon area, Korea, where the real building is located. A photovoltaic system can be used only to generate electricity, but if a photovoltaic module can be used as an element of a double envelope, it could be more useful at the point of view of renewable energy usage and night insulation. Increase of PV module surface temperature is negative for power generation by installing PV module as an element of double envelope. A reasonable combination between renewable energy usage and power generation should be well analyzed for better usage of natural energy to design a BIPV.     

Aspects and improvements of hybrid photovoltaic/thermal solar energy systems

Hybrid photovoltaic/thermal (PV/T or PVT) solar systems consist of PV modules coupled to water or air heat extraction devices, which convert the absorbed solar radiation into electricity and heat. At the University of Patras, an extended research on PV/T systems has been performed aiming at the study of several modifications for system performance improvement. In this paper a new type of PV/T collector with dual heat extraction operation, either with water or with air circulation is presented. This system is simple and suitable for building integration, providing hot water or air depending on the season and the thermal needs of the building. Experiments with dual type PV/T models of alternative arrangement of the water and the air heat exchanging elements were performed. The most effective design was further studied, applying to it low cost modifications for the air heat extraction improvement. These modifications include a thin metallic sheet placed in the middle of the air channel, the mounting of fins on the opposite wall to PV rear surface of the air channel and the placement of the sheet combined with small ribs on the opposite air channel wall. The modified dual PV/T collectors were combined with booster diffuse reflectors, achieving a significant increase in system thermal and electrical energy output. The improved PV/T systems have aesthetic and energy advantages and could be used instead of separate installation of plain PV modules and thermal collectors, mainly if the available building surface is limited and the thermal needs are associated with low temperature water or air heating.     

Numerical determination of adequate air gaps for building-integrated photovoltaics

The efficiency of photovoltaic (PV) devices is approximately inversely proportional to the cell temperature and the air gap of PV modules over or beside a building envelope can facilitate ventilation cooling of building-integrated photovoltaics. The effect of gap size on the performance of one type of PV module (with dimensions 1209 × 537 × 50 mm) in terms of cell temperature has been determined numerically for a range of roof pitches and panel lengths under two different settings of solar heat gains. It has been found that under constant solar heat gain, the air velocity behind PV modules due to natural convection in general increases with roof pitch angle. For a given location where solar heat gain varies with inclination from horizontal plane, however, the air velocity increases up to a pitch angle of about 60 degrees and then decreases with increasing roof pitch. The mean and maximum PV temperatures decrease with the increase in pitch angle and air gap. The mean PV temperature also decreases with increasing panel length for air gaps greater than or equal to 0.08 m, whereas the maximum PV temperature generally increases with panel length but decreases when the length of a roof-mounted panel increases from two modules to three modules and the air gap is between 0.1 and 0.11 m. Without adequate air circulation, overheating of PV modules would occur and hot spots could form near the top of modules with potential cell temperatures over 80 °C above ambient air temperature under bright sunshine.     

Coupling of thermoelectric modules with a photovoltaic panel for air pre‐heating and pre‐cooling application; an annual simulation

Thermoelectric (TE) modules are possible reversible pre‐cooling and pre‐heating devices for ventilation air in buildings. In this study, the opportunity of direct coupling of TE modules with photovoltaic (PV) cells is considered. This coupling is evaluated through a numerical simulation depending on the meteorological conditions of Chambéry, Alpine region in France, and on the cooling or heating use of the TE modules, through annual energy and exergy efficiencies. For the considered conditions, TE module performances are of the same order as the ones of the vapour compression heat pumps, with a TE coefficient of performance higher than 2 for low values of input DC current. The PV–TE coupling efficiency varies between 0.096 and 0.23 over the year, with an average value of 0.157. Evolutions of the exergy effectiveness of PV and TE elements follow the same trends as the corresponding energy efficiencies but with steeper variations for the coupling exergy yield that varies between 0.004 and 0.014, with an annual average value of 0.010. The direct PV–TE coupling does not seem to be a sustainable option for the summer cooling purpose particularly. A case study with indirect coupling under a warm climate is considered and shows that the use of TE devices could be efficient in housing to ensure summer thermal comfort, but the corresponding necessary PV area would induce a high investment. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental study of a mechanically ventilated double-skin façade with venetian sun-shading device: A full-scale investigation in controlled environment

The aim of this article is to present results of an experimental campaign performed on a full-scale facility provided with a double-skin façade. The behaviour of this architectural concept is tested under controlled climatic conditions. A summer case is scrutinised under different configurations: variation of the airflow through the double-skin façade and different angle of the solar shading device. This paper describes the experimental conditions, as well the test facility and the tested façade element. The results show the temperatures of the test cell and the façade and how they depend on the climatic conditions and the sun-shading device blade angles. One objective of this research was to measure and provide extensive data set detailing air and surface temperatures on the double-skin façade, together with airflow rates and air velocities. The experiments are fully described so that the results can be used for the validation of numerical models dealing with ventilated double-skin façades with venetian sun-shading device.     

一种多功能光伏发电系统的可行性研究

通过在光伏组件的背面连接了一个热电转换模块,形成一个光伏一热电混合模块,从而将光伏组件工作过程中产生的废热转换成电能的同时又降低了光伏组件的温度,进而提高了光电转换效率。将光伏一热电模块与百叶有效结合,从而实现了室内采光、通风及节约空间等多种功能。同时,为了提高光伏组件的入射太阳辐射,引入了可调节的抛物型双面聚焦板,减少了太阳能电池板的面积,从而减少了太阳能发电的成本。