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.     

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.     

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.     

Cooling potential of ventilated PV façade and solar air heaters combined with a desiccant cooling machine

Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library, near Barcelona. The system is designed to utilise surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling. A desiccant cooling machine was installed on the library roof with an additional solar air collector and connected to the existing ventilated PV façade and PV sheds. The desiccant cooling cycle is a novel open heat driven system that can be used to condition the air supplied to the building interior. Cooling power is supplied to the room space within the building by evaporative cooling of the fresh air supply, and the solar heat from the PV-solar air heating system provides the necessary regeneration air temperature for the desiccant machine. This paper describes the system and gives the main technical details. The cooling performance of the solar powered desiccant cooling system is evaluated by the detailed modelling of the complete cooling process. It is shown that air temperature level of the PV-solar air heating system of 70 °C or more can be efficiently used to regenerate the sorption wheel in the desiccant cooling machine. A solar fraction of 75% can be achieved by such an innovative system and the average COP of the cooling machine over the summer season is approximate 0.518.     

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%.     

Unglazed coloured solar absorbers on façade: Modelling and performance evaluation

Integrating solar collectors as building elements is one of the most promising way to decrease the cost of the delivered energy and to increase the architectural acceptance of energy self-supplying buildings.For that purpose, steel-made coloured unglazed solar absorbers as facade elements were investigated. Besides the questions of building integration, selective layer, durability and industrial production, it is essential to study the potential yield of such devices and to optimize the design and sizing of the complete heating system. Simulations were carried out using the TNRSYS program in different European climates.Results show that the absorbers reach appreciable efficiency for domestic water preheating but rather low yields when connected to the space heating circuit. Sensitivity of the quantity of saved energy towards the sizing parameters (absorbance, emissivity, orientation, wind, surface, radiator outlet temperature) was analysed. Based on a large set of simulations, general formulas completed by a European map of constants required by said formulas were established that allow calculating the absorbers yield without running complete simulations.General conclusion is that the best use of façade absorbers is water preheating in urban (wind-sheltered) environment.     

Experimental assessment of the performance of an active transparent façade during actual operating conditions

Results of an extensive measurement campaign performed on an active transparent façade during actual operating conditions are presented. The main aims of the research were: to assess the actual façade performance, both in terms of energy savings and enhanced comfort conditions, to obtain more detailed knowledge of its thermofluid dynamic behaviour and to highlight the weak points of this relatively new technology that still requires further improvement. The analysed component consists of a transparent mechanically ventilated façade integrated with an HVAC system. The façade is used as the exhaust outlet of the HVAC system. The temperatures, heat fluxes and air velocities in the ventilated façade were continuously monitored, over a period of 2 years, using a monitoring system with 34 sensors. In the paper, attention is focused on the measurement techniques that were adopted and on the critical analysis of the experimental data.     

Simulation of façade and envelope design options for a new institutional building

This paper presents a simulation case study of façade and envelope preliminary design options for the new Engineering building of Concordia University in Montreal. A major principle of the analysis was to create a high quality building envelope in order to optimally control solar gains, reduce heating and cooling energy demand and reduce electricity consumption for lighting, while at the same time maintain a comfortable and pleasant indoor environment. The stated approach of the design team was to aim for an energy-efficient building, employing innovative technologies and integrating concepts such as daylighting and natural ventilation. Detailed energy simulations were therefore performed from the early design stage, in order to present recommendations on the choice of façade, glazings, shading devices, lighting control options, and natural ventilation. Integrated thermal studies, a daylighting analysis and the impact of the above on HVAC system sizing were considered. Simulation results showed that, using an optimum combination of glazings, shading devices and controllable electric lighting systems, the energy savings in perimeter spaces can be substantial. Perimeter heating could be eliminated if a high performance envelope is used. The building is currently being commissioned.     

Modeling, design and thermal performance of a BIPV/T system thermally coupled with a ventilated concrete slab in a low energy solar house: Part 2, ventilated concrete slab

This paper is the second of two papers that describe the modeling and design of a building-integrated photovoltaic-thermal (BIPV/T) system thermally coupled with a ventilated concrete slab (VCS) adopted in a prefabricated, two-storey detached, low energy solar house and their performance assessment based on monitored data. The VCS concept is based on an integrated thermal-structural design with active storage of solar thermal energy while serving as a structural component - the basement floor slab (∼33 m²). This paper describes the numerical modeling, design, and thermal performance assessment of the VCS. The thermal performance of the VCS during the commissioning of the unoccupied house is presented. Analysis of the monitored data shows that the VCS can store 9-12 kWh of heat from the total thermal energy collected by the BIPV/T system, on a typical clear sunny day with an outdoor temperature of about 0 °C. It can also accumulate thermal energy during a series of clear sunny days without overheating the slab surface or the living space. This research shows that coupling the VCS with the BIPV/T system is a viable method to enhance the utilization of collected solar thermal energy. A method is presented for creating a simplified three-dimensional, control volume finite difference, explicit thermal model of the VCS. The model is created and validated using monitored data. The modeling method is suitable for detailed parametric study of the thermal behavior of the VCS without excessive computational effort.     

Thermal performance of a naturally ventilated cavity wall

Cavity walls are often proposed in the building envelope design as a solution for improving the thermal comfort of the occupants and reducing the adverse condensation effects on the building fabric. Although the behaviour of a non‐ventilated cavity wall is well‐known, more studies are required when cavity ventilation is allowed. In order to consistently predict the thermal behaviour of a naturally ventilated cavity wall, a convective model based on the integral equations of motion and enthalpy was developed and applied in the present study. The model is presented as a combination of two limiting cases of a steady laminar flow into the channel gap: fully developed flow and boundary layer flow. Conduction effects across the system are also included through a proper limiting case and then combined with the convective model. In addition a numerical CFD model was developed that provides solution for free convective flow configurations between two parallel conducting vertical walls. For comparison purposes, some test cases were simulated with the two models and a general good agreement was found between results. Finally, the integral model was applied to assess the thermal performance of a ventilated cavity wall for winter and summer conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Heating energy savings potential from retrofitting old apartments with an advanced double-skin façade system in cold climate

Apartments account for over 60% of total residential buildings and consume a significant portion of primary energy in South Korea. Various energy efficiency measures have been implemented for both new apartment constructions and existing apartment retrofits. Old apartment structures have poor thermal performances, resulting in a high energy consumption. The South Korean government initiated retrofitting projects to improve the energy efficiency in old apartments. Apartment owners typically replace old windows with high-performance windows; however, there is still a demand for better and more innovative retrofit methods for a highly improved energy efficiency. This paper proposes an advanced double-skin façade (DSF) system to replace existing balcony windows in old apartments. Considering the cold climate conditions of Seoul, South Korea, it mainly discusses heating energy savings. Three case models were developed: Base-Case with existing apartment, Case-1 with typical retrofitting, and Case-2 with the proposed DSF system. The EnergyPlus simulation program was used to develop simulation models for a floor radiant heating system. A typical gas boiler was selected for low-temperature radiant system modeling. The air flow network method was used to model the proposed DSF system. Five heating months, i.e., November to March, and one representative day, i.e., January 24, were selected for detailed analysis. The main heat loss areas consist of windows, walls, and infiltration. The results reveal that the apartment with the DSF retrofit saves 38.8% on the annual heating energy compared to the Base-Case and 35.2% compared to Case-1.     

Numerical evaluation of a PEM fuel cell with conventional flow fields adapted to tubular plates

In this research a 3D numerical study on a PEM fuel cell model with tubular plates is presented. The study is focused on the performance evaluation of three flow fields with cylindrical geometry (serpentine, interdigitated and straight channels) in a fuel cell. These designs are proposed not only with the aim to reduce the pressure losses that conventional designs exhibit with rectangular flow fields but also to improve the mass transport processes that take place in the fuel cell cathode. A commercial computational fluid dynamics (CFD) code was used to solve the numerical model. From the numerical solution of the fluid mechanics equations and the electrochemical model of Butler-Volmer different analysis of pressure losses, species concentration, current density, temperature and ionic conductivity were carried out. The results were obtained at the flow channels and the catalyst layers as well as in the gas diffusion layers and the membrane interfaces. Numerical results showed that cylindrical channel configurations reduced the pressure losses in the cell due to the gradual reduction of the angle at the flow path and the twist of the channel, thus facilitating the expulsion of liquid water from the gas diffusion layers and in turn promoting a high oxygen concentration at the triple phase boundary of the catalyst layers. Moreover, numerical results were compared to polarization curves and the literature data reported for similar designs. These results demonstrated that conventional flow field designs applied to conventional tubular plates have some advantages over the rectangular designs, such as uniform pressure and current density distributions among others, therefore they could be considered for fuel cell designs in portable applications.     

Energy and exergy performance assessment of a novel solar-based integrated system with hydrogen production

In this study, a new solar power assisted multigeneration system designed and thermodynamically analyzed. In this system, it is designed to perform heating, cooling, drying, hydrogen and power generation with a single energy input. The proposed study consists of seven sub-parts which are namely parabolic dish solar collector, Rankine cycle, organic Rankine cycle, PEM-electrolyzer, double effect absorption cooling, dryer and heat pump. The effects of varying reference temperature, solar irradiation, input and output pressure of high-pressure turbine and pinch point temperature heat recovery steam generator are investigated on the energetic and exergetic performance of integration system. Thermodynamic analysis result outputs show that the energy and exergy performance of overall study are computed as 48.19% and 43.57%, respectively. Moreover, the highest rate of irreversibility has the parabolic dish collector with 24,750 kW, while the lowest rate of irreversibility is calculated as 5745 kW in dryer. In addition, the main contribution of this study is that the solar-assisted multi-generation systems have good potential in terms of energy and exergy efficiency.     

Energy supply performance and environmental load‐reduction effect of a cogeneration system for an apartment building using distributed heat storage technology

In order to make distributed generation systems for apartment buildings economically viable, it is essential to develop an efficient and low‐cost heat supply system. We are developing a new cogeneration system (Neighboring CoGeneration system: NCG system). The key concept of this system is to install a heat storage unit for the hot water supply, floor heating, and bath heating in each house, and to connect the heat storage units by a single‐loop hot water pipe. The system leads to time leveling of the total heat supply and reduced installation costs. Furthermore, it is expected that the cogeneration can operate according to electrical demand because of the large heat storage capacity of the system. In this study, a dynamic simulation model is developed to evaluate the performance and environmental load‐reduction effect of the NCG system for 50 households. The results show that the NCG system can supply sufficient heat for peak demand in winter and reduce annual CO₂ emissions by 23% on average. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 42(8): 745–757, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20376     

Energy performance of a hybrid space-cooling system in an office building using SSPCM thermal storage and night ventilation

Thermal performance of a hybrid space-cooling system with night ventilation and thermal storage using shape-stabilized phase change material (SSPCM) is investigated numerically. A south-facing room of an office building in Beijing is analyzed, which includes SSPCM plates as the inner linings of walls and the ceiling. Natural cool energy is charged to SSPCM plates by night ventilation with air change per hour (ACH) of 40 h⁻¹ and is discharged to room environment during daytime. Additional cool-supply is provided by an active system during office hours (8:00-18:00) necessary to keep the maximum indoor air temperature below 28 °C. Unsteady simulation is carried out using a verified enthalpy model, with a time period covering the whole summer season. The results indicate that the thermal-storage effect of SSPCM plates combined with night ventilation could improve the indoor thermal-comfort level and save 76% of daytime cooling energy consumption (compared with the case without SSPCM and night ventilation) in summer in Beijing. The electrical COPs of night ventilation (the reduced cooling energy divided by fan power) are 7.5 and 6.5 for cases with and without SSPCM, respectively.     

Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit

The thermal performance of a prototype solar cooker based on an evacuated tube solar collector with phase change material (PCM) storage unit is investigated. The design has separate parts for energy collection and cooking coupled by a PCM storage unit. Solar energy is stored in the PCM storage unit during sunshine hours and is utilized for cooking in late evening/night time. Commercial grade erythritol was used as a latent heat storage material. Noon and evening cooking experiments were conducted with different loads and loading times. Cooking experiments and PCM storage processes were carried out simultaneously. It was observed that noon cooking did not affect the evening cooking, and evening cooking using PCM heat storage was found to be faster than noon cooking. The cooker performance under a variety of operating and climatic conditions was studied at Mie, Japan.     

A numerical investigation of the effect of sinusoidal temperature on mixed convection flow in a cavity filled with a nanofluid with moving vertical walls

The aim of this study is to investigate numerically the effect of sinusoidal temperature on mixed convection flow in a cavity filled with nanofluid and moving vertical walls by using a new temperature function, where thermal heating takes the form of the sinusoidal temperature; and could be found in various natural processes and industries such as solar energy, and cooling of electronic components. The heating is concentrated in the center and then distributed to both ends at different values of Rayleigh numbers, Reynolds numbers, and volumetric fractions of nanoparticles ranging from 1.47 × 10³ to 1.47 × 10⁶, 1 to 100, and 0 to 0.1, respectively. The impact of nanoparticle size on thermal characteristics and hydrodynamics was considered and evaluated. From the results, the volume fraction concentration of nanoparticles affects the flow shape and thermal performance in the case of a constant Reynolds number. Moreover, the effect of nanoparticles decreases with the increase of the Reynolds number. Besides this, with increasing the volume percentage of nanoparticles, the rate of heat transmission increases. It is worth noting that the presence of nanoparticles results in height convective heat transfer coefficient. On the other hand, the thickness of thermal boundary layers decreases with increasing Rayleigh number. The current investigation found that the (sinusoidal) temperature change significantly affects heat transfer.     

CFD analysis of two‐phase cavitating flow in a centrifugal pump with an inducer

One of the most undesirable phenomena encountered in the operation of a centrifugal pump is cavitation. It causes structural damage, vibration, and blockage of mass flow, leading to a drop in performance and life of the pump. This study addresses cavitation modeling of a single‐stage centrifugal pump and aims at minimizing cavitation by introducing an inducer upstream of the impeller. Furthermore, it aims at understanding different multiphase modeling schemes by a computational fluid dynamics software and its variation from single‐phase flows. The results from the numerical model are first validated against standard experimental data to check the credibility of the model. After validation, a single‐phase analysis is performed for a wide range of operating conditions. Subsequently, the Schnerr‐Sauer cavitation model is invoked and a multiphase analysis is carried out for the same. The results obtained shows that the inducer is effective in reducing the amount of cavitation for a substantial number of operating conditions. The effectiveness of the inducer is calculated, and a 96% effectiveness is observed at the best efficiency point. Furthermore, data from single‐phase and multiphase analysis are compared, and a method based on absolute pressure is proposed, which can provide results with significant accuracy without the need for expensive computation. Finally, Zwart‐Gerber‐Belamri model is used for cavitation modeling, and the behavior of the scheme is compared with Schnerr‐Sauer model. The pump parameters are compared, and the obtained results show close similarity between the two models.     

Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store

A domestic-scale prototype experimental solar cooling system has been developed based on a LiBr/H₂O absorption system and tested during the 2007 summer and autumn months in Cardiff University, UK. The system consisted of a 12 m² vacuum tube solar collector, a 4.5 kW LiBr/H₂O absorption chiller, a 1000 l cold storage tank and a 6 kW fan coil. The system performance, as well as the performances of the individual components in the system, were evaluated based on the physical measurements of the daily solar radiation, ambient temperature, inlet and outlet fluid temperatures, mass flow rates and electrical consumption by component. The average coefficient of thermal performance (COP) of the system was 0.58, based on the thermal cooling power output per unit of available thermal solar energy from the 12 m² Thermomax DF100 vacuum tube collector on a hot sunny day with average peak insolation of 800 W/m² (between 11 and 13.30 h) and ambient temperature of 24 °C. The system produced an electrical COP of 3.6. Experimental results prove the feasibility of the new concept of cold store at this scale, with chilled water temperatures as low as 7.4 °C, demonstrating its potential use in cooling domestic scale buildings.     

Effect of non-uniform θ-temperature distribution on a receiver of an evacuated tubular collector on its performance parameters

A performance of an evacuated tubular collector (G.E. design) fixed at the focus of a compound parabolic concentrator is investigated. In the G.E. design, heat is transmitted to the circulating fluid inside a U-tube. The U-tube is in contact with the receiver only on a line along the length of the receiver. This results in a non-uniform temperature distribution on the receiver in the θ-direction. The effect of the non-uniform temperature distribution on the performance parameters of the collector, viz. overall heat loss coefficient, plate efficiency factor and heat removal factor, has been studied. The results are presented in the form of a graph.