Long-wave Radiation Exchange Research Articles

Validation of CFD models of urban microclimates under high temperature and humidity conditions during daytime heatwaves in dense low-rise areas

In the midst of the ongoing climate crisis, there is a growing need to study the effects of climate on humans, especially those of the microclimates that surround humans and their activities. Various models have been developed to simulate microclimates, but simulation models for densely populated areas experiencing heat waves have shown an issue with simulating temperatures that were too high compared to actual measurements. To overcome these limitations, this study aimed to develop and validate CFD model based on STAR-CCM+ in order to more accurately simulate the microclimate of dense residential areas. To improve the realistic simulation capability found in existing models, the model was designed to include solar radiation, humidity, ground radiation, longwave radiation exchange, heat storage and evaporation due to various urban surfaces, evapotranspiration from vegetation, convection, and heat transfer from buildings. For validation, comparisons were made between the simulation results and the actual measurements during this time. The results showed that the R2 value of the model developed in this study increased from 0.86 to 0.91, indicating that the model was able to reproduce the measured results of the target site well. To avoid overfitting the model, the measurement results of the Han River AWS, another AWS operated by the Korea Meteorological Administration near the target site, were input. It was confirmed that the model reproduced the actual measurements of five locations in the target site well. The R2 of temperature and humidity ranged from 0.96 to 0.98, and wind speed ranged from 0.64 to 0.81.

Exploring the effect of longwave radiation exchange on the energy balance of building façades in subtropical regions

The energy balance of the building façade affects the accuracy of building energy prediction. Although algorithms for energy balance have been applied in building energy simulation software, the longwave radiation exchange of building façades is often considered in a simple (implicit) way. This study proposes the concept of ambient equivalent temperature for quantifying the longwave radiation emitted by other buildings and trees in the surrounding environment. A more detailed radiation transfer model was developed to assess the energy balance of the longwave radiation of building façades. The input parameters of the model were simplified and the applicability of the model was improved by fitting the ambient equivalent temperature and meteorological parameters. Through quantitative and qualitative analysis, the effect of view factor parameters on the model output was assessed, and a universal value for the sky view factor (SVF) was determined to calculate the longwave radiation exchange of the building façade. The calculation accuracy of the longwave radiation was 71.4% higher in the simplified SVF model than in the original model. These findings provide a more accurate method for calculating the longwave radiation energy balance of building façades and have implications for building energy prediction and building energy efficiency design.

Dairy Cow Thermal Balance Model During Heat Stress: Part 1. Model Development

Highlights Equations for a modified steady-state thermal balance model solved with a spreadsheet are described. The modified model describes heat exchange between lactating cows and the surrounding environment. New relations were used for tissue insulation, sweat rate, longwave radiation, and convective heat exchange. A companion paper compares model results to published body temperatures, respiration rates, and skin temperatures. Abstract. Dairy cow heat stress impacts cow well-being, reduces milk yield, and leads to economic losses. Understanding heat stress mechanics supports ongoing and future efforts to mitigate heat stress. The purpose of this project was to modify a steady-state heat transfer model developed by McGovern and Bruce (2000) by incorporating work by Berman (2005), McArthur (1987), Turnpenny et al. (2000a,b), Thompson et al. (2014), Gwadera et al. (2017), two new empirical relations for tissue insulation and sweat rate, and a new solution method that allowed for overlapping changes in heat exchange. The modified model describes heat exchange between a lactating cow and the environment through respiration, convection, sweating, and shortwave and longwave radiation. This article describes the process-based model equations, compares results from the two new empirical relations used to published work, and presents the inputs and results for a cow on pasture in sunlight. The modified model, which can be solved with a spreadsheet, provides insight into factors and processes that affect lactating cow heat exchange. A companion paper compares the modified model results with published average measured body temperatures, respiration rates, and skin temperatures and unpublished body temperature data for cows on pasture in the sunshine. Keywords: Body temperature, Dairy, Heat stress, Lactating cow, Respiration rate, Thermal balance model.

Calculation of heating and cooling energy loads at the district scale: Development of MoDEM, a modular and technologically explicit platform

The rationalization of energy at the district scale requires the development of numerical tools capable of simulating not only the energy demand, but also energy loads at such a scale. However, such simulation remains difficult because of the computational cost and lack of reliable data. A common solution to both these problems is to simplify the model, generally at the expense of accuracy. To overcome these problems, a specific modeling and simulation platform called Modular District Energy Model (MoDEM) is developed. This platform enables the calculation of the heating and cooling load curves of each building, integrating the interactions between the buildings taking into account longwave radiative heat transfers between buildings external surfaces. Moreover, with different levels of models included in its calculation engine, MoDEM also enables the testing of modeling assumptions. MoDEM can also handle the geographical information system geometry and detailed solar radiation calculation. To validate the application of MoDEM, a district in Paris is modeled, comprising 900 flats built in the 1930s. The results demonstrate that 1) various models have little impact on energy consumption and 2) some physical phenomena usually not modeled appropriately, such as the longwave radiative exchange between buildings, can no longer be neglected while considering the load curve. It is worth mentioning that MoDEM is based on a co-simulation layer allowing parallel computing. From a broader point of view, this specific feature enables the easy and computationally efficient integration of various models like, for instance, specific electricity consumption, renewable energy production, grid management simulation and energy storage. • A Modular District Energy Model (MoDEM) was developed. • MoDEM can be adapted to various situations. • The district-scale model can reduce simplification and integrate statistical data.

Solar greenhouses: Climates, glass selection, and plant well-being

Solar greenhouses are currently the most energy-intensive agricultural sector. In literature, there is no worldwide mapping of solar greenhouse performance under different climate scenarios. This study analyzes the performance of a Venlo solar greenhouse for 48 localities around the world. Solar greenhouses are mainly made of a transparent envelope and the effect of the direct and diffuse component of solar radiation impacts the internal plant well-being. This study aims to identify the best solution of a transparent envelope on locations with different latitudes and evenly distributed around the globe. The simulations are carried out using TRNsys, considering different thermal phenomena three-dimensional shortwave and longwave radiative exchange, airflow exchanges, presence of lamps with their exact 3D position, ground, plant evapotranspiration, and convective heat transfer coefficients. A total of 336 simulations are performed in the free-floating regime. A new index for the identification of the best glass solutions based on annual average deviation is defined. For all climates, the best glass solutions work better in winter than in summer. The optimal choice of the glass must be combined with effective scheduling of openings for natural ventilation to avoid internal overheating phenomena.

Validation of a CFD model for the evaluation of urban microclimate at high latitudes: A case study in Trondheim, Norway

The urban microclimate is a rapidly evolving field of research gaining increasing interest from public authorities and researchers. However, studies at high-latitude cities are scarce and researchers primarily focus on summerly overheating. This study focuses on the validation process of a CFD model that applies the 3D URANS approach with the realisable k-ε turbulence model at a highly complex urban area in Trondheim, Norway (63.4° N) during autumn. The CFD model features a polyhedral grid of the urban environment, including geometrically explicitly modelled buildings and trees in the area of interest. Furthermore, solar radiation, longwave radiation exchange, heat transfer from the buildings, heat storage in the urban surface, and the thermal effects of evapotranspiration from trees and grass surfaces are considered. The CFD model is validated with experimental results from a network of five mobile and one reference weather stations in the study area, providing hourly-averaged measurements for wind speed, wind direction (only reference weather station) and air temperature for two 48-h periods from September 27–28 and October 19–20. The results show that the CFD model is well able to reproduce the measured conditions at the area of interest with a mean R2 of 0.60, 0.63, and 0.96 for wind speed, wind direction and air temperature, respectively, at the reference weather station. It will be used in future studies, including the analysis of the impact of urban microclimate on buildings’ energy performance, outdoor thermal and pedestrian wind comfort.

An accurate thermal model for the PV electric generation prediction: long-term validation in different climatic conditions

Solar radiation incident on photovoltaic modules only partly directly convert into electricity; the rest is converted into heat that increases the module layers’ temperature. In order to quantify both the output power and the electrical efficiency, the knowledge of the temperature profile is essential. This study proposes a transient one-dimensional thermal model of photovoltaic modules which provides the temperature distribution across the panel thickness, used to predict the electricity production under variable operating weather conditions. The model was implemented and validated considering the module back surface temperature and the produced electric power measured in an experimental set-up located at the University of Calabria (Italy). A more detailed evaluation of the long-wave radiative heat exchange between the front glass cover and the external environment is considered, employing experimental sky temperatures data. Different formulations of the heat transfer coefficient were tested to provide more accurate results. To show the reliability of the model predictions over a wide range of operating conditions, the validation was conducted considering several days of each season with different meteorological situations. The accuracy of the model was proved by statistical parameters showing the excellent agreement between the predicted and measured temperatures and power outputs.

Dynamic Analysis of the Natural and Mechanical Ventilation of a Solar Greenhouse by Coupling Controlled Mechanical Ventilation (CMV) with an Earth-to-Air Heat Exchanger (EAHX)

Greenhouse crops represent a significant productive sector of the agricultural system; one of the main problems to be addressed is indoor air conditioning to ensure thermal well-being of crops. This study focuses on the ventilation analysis of solar greenhouse with symmetrical flat pitched roof and single span located in a warm temperate climate. This work proposes the dynamic analysis of the greenhouse modeled in TRNsys, simultaneously considering different thermal phenomena three-dimensional (3D) shortwave and longwave radiative exchange, airflow exchanges, presence of lamps with their exact 3D position, ground and plant evapotranspiration, and convective heat transfer coefficients. Several air conditioning systems were analyzed, automatic window opening, controlled mechanical ventilation systems (CMV) and horizontal Earth-to-Air Heat Exchanger (EAHX) coupled with CMV, for different air volume changes per hour. In summer, the exploitation of the ground allows having excellent results with the EAHX system, reducing the temperature peaks of up to 5 °C compared to the use of CMV. In winter, it is interesting to note that, although the EAHX is not the solution that raises the temperature the most during the day, its use allows flattening the thermal wave more. In fact, the trend is almost constant during the day, raising the temperature during the first and last hours of the day.

Complete greenhouse dynamic simulation tool to assess the crop thermal well-being and energy needs

Greenhouse technology is an important energy consumer sector representing an indispensable solution for modern methods of crop production. In the greenhouse envelope and system design phase, thermodynamic performance simulation tools are required. The greenhouse simulation is still a very complex task despite many building tools are available in the literature.This work aims to overcome this gap by proposing a reference methodology to accommodate the building TRNSYS software for a greenhouse able to consider simultaneously different thermal phenomena with detailed modelling of: dense volume discretization, 3D shortwave and longwave radiative exchange, air flow exchanges, presence of lamps with their exact 3D position, ground and plant evapotranspiration, and convective heat transfer coefficients.A standard hourly simulation of the one-zone greenhouse was validated with another recognized tool, showing excellent agreement throughout the year. The main parameters affecting the greenhouse thermal balance were investigated in both a free-floating and continuous regime. The investigation has shown that a standard simulation is accurate to only reproduce the thermal response in a free-floating regime; instead, the detailed simulation has led to overall cooling and heating energy needs in the continuous regime, respectively, of 51.4 kWh/m3 and 49.1 kWh/m3, avoiding to obtain very high errors.

Modeling Thermal Interactions between Buildings in an Urban Context

Thermal interactions through longwave radiation exchange between buildings, especially in a dense urban environment, can strongly influence a building’s energy use and environmental impact. However, these interactions are either neglected or oversimplified in urban building energy modeling. We developed a new feature in EnergyPlus to explicitly consider this term in the surface heat balance calculations and developed an algorithm to batch calculating the surrounding surfaces’ view factors using a ray-tracing technique. We conducted a case study with a district in the Chicago downtown area to evaluate the longwave radiant heat exchange effects between urban buildings. Results show that the impact of the longwave radiant effects on annual energy use ranges from 0.1% to 3.3% increase for cooling and 0.3% to 3.6% decrease for heating, varying among individual buildings. At the district level, the total energy demand increases by 1.39% for cooling and decreases 0.45% for heating. We also observe the longwave radiation can increase the exterior surface temperature by up to 10 °C for certain exterior surfaces. These findings justify a detailed and accurate way to consider the thermal interactions between buildings in an urban context to inform urban planning and design.

Snowpack determines relative importance of climate factors driving summer lake warming

AbstractMountain lakes experience extreme interannual climate variation as well as rapidly warming air temperatures, making them ideal systems to understand lake‐climate responses. Snowpack and water temperature are highly correlated in mountain lakes, but we lack a complete understanding of underlying mechanisms. Motivated by predicted declines in snowfall with future temperature increases, we investigated how surface heat fluxes and lake warming responded to variation in snowpack, ice‐off, and summer weather patterns in a high elevation lake in the Sierra Nevada, California. Ice‐off timing determined the phenology of lake exposure to solar radiation, and was the dominant mechanism linking snowpack to lake temperature. The relative importance of heat loss fluxes (longwave radiation, latent and sensible heat exchange) varied among wet and dry years. Declines in snowpack and ice cover in mountain systems will reduce variability in lake thermal responses and increase the responsiveness of lake warming to atmospheric forcing.

System design and feasibility of trigeneration systems with hybrid photovoltaic-thermal (PVT) collectors for zero energy office buildings in different climates

Zero or plus energy office buildings must have very high building standards and require highly efficient energy supply systems due to space limitations for renewable installations. Conventional solar cooling systems use photovoltaic electricity or thermal energy to run either a compression-cooling machine or an absorption-cooling machine in order to produce cooling energy during daytime, while they use electricity from the grid for the nightly cooling energy demand. With a hybrid photovoltaic-thermal collector, electricity as well as thermal energy can be produced at the same time. These collectors can produce also cooling energy at nighttime by long-wave radiation exchange with the night sky and convection losses to the ambient air. Such a renewable trigeneration system offers new fields of applications. However, the technical, ecological and economical aspects of such systems are still largely unexplored.In this work, the potential of a PVT system to heat and cool office buildings in three different climate zones is investigated. In the investigated system, PVT collectors act as a heat source and heat sink for a reversible heat pump. Due to the reduced electricity consumption (from the grid) for heat rejection, the overall efficiency and economics improve compared to a conventional solar cooling system using a reversible air-to-water heat pump as heat and cold source.A parametric simulation study was carried out to evaluate the system design with different PVT surface areas and storage tank volumes to optimize the system for three different climate zones and for two different building standards. It is shown such a systems are technically feasible today. With a maximum utilization of PV electricity for heating, ventilation, air conditioning and other electricity demand such as lighting and plug loads, high solar fractions and primary energy savings can be achieved.Annual costs for such a system are comparable to conventional solar thermal and solar electrical cooling systems. Nevertheless, the economic feasibility strongly depends on country specific energy prices and energy policy. However, even in countries without compensation schemes for energy produced by renewables, this system can still be economically viable today. It could be shown, that a specific system dimensioning can be found at each of the investigated locations worldwide for a valuable economic and ecological operation of an office building with PVT technologies in different system designs.

Monitoring the Effective Ambient and Sky Temperature Based on Infrared Sensor for Advanced Thermal Calculations

Building performance simulations and advanced thermal analysis are becoming the basis for a well-established practice of a building sector. This approach requires many input data which are typically not available on site. Apart from already well-practiced climate variable quantities, such as ambient temperature, solar radiation and parameters of wind, more complex data are needed for advanced building thermal analysis. One of those is based on longwave radiation level. A pyrgeometer is a device that measures longwave radiation part of whole thermal radiation phenomena. This can be determined based on sky or effective ambient temperature monitoring. Secondly, both variables might be approximated by infrared sensors as an applicable option in the calculation of longwave radiation heat exchange between the external surface and the ambient building environment. The paper presents data obtained both by pyrgeometer and infrared sensor corresponding to their mutual comparison to demonstrate its application when longwave radiation exchange needs to be calculated or analyzed in advanced. Integrating of the infrared sensor with aim to monitor the effective ambient and/or sky temperature enables its applicability as an alternative, integrated and less cost consuming method towards the monitoring by commercial pyrgeometer.

A multilayer mean radiant temperature model for pedestrians in a street canyon with trees

We introduce a multilayer model to estimate mean radiant temperature (MRT) and evaluate the pedestrian thermal comfort in a street canyon. This multilayer MRT model (MMRT) is suitable for urban streets with varying building and tree heights. The model simulates shortwave and longwave radiation exchange for each urban element and area-weighted view factors, then finally obtains MRT of pedestrians on the sidewalk. Probability density profiles of buildings and trees enable the consideration of urban vertical heterogeneity. Furthermore, Monte Carlo ray tracing (MCRT) allows the model to evaluate the radiation transfer in complex urban areas. We verify the effectiveness of MCRT and the probabilistic density profile approach. A sensitivity test conducted in Seoul on September 1, 2017 using the MMRT reveals that MRT can be reduced by 23 °C as the tree leaf area density increases from 0 to 1, and by 18 °C as the tree height increases from 0 m to 12 m in 1300 LST. The model controls urban form and pavement parameters as well as tree parameters. We aim to use this model to compare diverse MRT mitigation strategies and confirm the best strategy for thermal-friendly street design.

Influence of PV-powered thermoelectric surfaces for user-individual radiative cooling on the cooling energy demand of buildings

This paper discusses a newly developed personal cooling device called Thecla and its influence on a subject’s thermal sensation and comfort as well as the energy demands of cooling buildings. Thecla is a thermoelectric cooling partition that provides a cooled surface using longwave radiation exchange to cool a user’s skin and clothing. The impact of this device on the skin temperatures and the resulting impact on the personal thermal sensation and comfort is determined through a human subject study and numeric comfort and sensation model simulations. The cooling capability of Thecla is quantified by the term “corrective delta”, which describes the highest possible temperature increase from a neutral thermal condition for which Thecla could still maintain thermal comfort. The amount of cooling energy saved because of admissible higher set temperatures is assessed.

Coupling of physical phenomena in urban microclimate: A model integrating air flow, wind-driven rain, radiation and transport in building materials

Building materials play an important role in the absorption, transport and storage of heat and moisture in the built environment. A fully-integrated urban microclimate model is proposed, which solves for wind flow and for the transport of heat and moisture in the air and building materials. The model includes long-wave and short-wave radiative exchange between surfaces and the distribution of wind-driven rain intensity. Transport in air and building materials are coupled in such a way that the steady Reynolds-averaged Navier-Stokes (RANS) is solved iteratively with the unsteady heat and moisture transfer in building materials. The proposed approach provides the information required for analyzing different contributions of convective cooling, sensible heat transfer due to rain, evaporation, in addition to the thermal storage throughout the day. This approach is demonstrated with a case study investigating the impact of rain deposition on an isolated three-dimensional street canyon lined with porous building materials. The study shows different rate of evaporation and duration of evaporative cooling for a change in wind speed during a rain event. The distribution of wind-driven rain particularly influences the spatial and temporal distribution of surface and air temperatures. A significant influence of neighboring surfaces is found on the surface temperatures.

Accuracy analysis and improvement of the Blind Enclosure Model to calculate the longwave radiative heat transfer for a façade with louver blinds

Longwave radiative heat exchange plays an important role in heat transfer of building facades with exterior louver blinds, which is a well-known solution for constructing energy-efficient buildings and the good indoor thermal-visual environment. However, errors may arise in the calculations for this process, due to the simplification of surroundings by the Blind Enclosure Model (BEM), which is the most common method to determine the view factors for a façade with louver blinds. In this study, we used field measurements to determine boundary conditions, and estimated longwave radiative heat exchange results of BEM by numerical calculations. It indicates the net radiative heat gain of the building façade will be underestimated by the BEM, for its view factors calculation is inaccurate, and thus, the BEM does not provide a conservative prediction for load and energy consumption of HVA&C system. Then, we established a combined method of street canyon geometry and crossed string algorithm (named SCCS method), in order to improve the view factor determination. The difference between this method and numerical simulation is less than 5% in the majority of cases, representing an improvement compared to the BEM.

Impact of radiation exchange between buildings in urban street canyons on space cooling demands of buildings

There is worldwide a trend towards urbanization, and cities try to improve their sustainability by saving natural resources and energy, and to mitigate the impact of climate change. Therefore growing cities need to optimize the energy demand of their new and existing buildings. There are a large number of urban scale phenomena that influence the energy demand of buildings. One of the most important influencing phenomena is the shortwave solar and longwave radiation exchange within an urban environment. It was demonstrated that the influence of neighbouring buildings has to be considered in detail to predict correctly the building energy demand. In this study, space cooling demand of stand-alone buildings is compared with buildings in an urban street canyon configuration. Detailed radiation models for the solar and longwave radiation are used to determine the radiation exchange between the buildings. The results show a strong influence of neighbouring buildings on the space cooling demands. This influence is strongly dependent on the shading device control strategy incorporated in the building energy management. Counterintuitively the space cooling demands are in some cases are higher for low values of solar irradiance, because the shading device control directs the devices to remain open during longer time periods when irradiance values are low, leading to higher total solar gains through the windows.

Impact of external longwave radiation on optimum insulation thickness in Tunisian building roofs based on a dynamic analytical model

In Tunisia, the building sector is considered as a major issue of energy consumption. A special attention should be drawn to improve the thermal quality of the building envelope with real consideration of the Tunisian climate specificity. One of the most effective measures is the roof insulation. Therefore, the present study is concerned with the determination of the optimum insulation thickness and the resulting energy savings and payback period for two typical roof structures and two types of insulation materials. An efficient analytical dynamic model based on the Complex Finite Fourier Transform (CFFT) is proposed and validated in order to handle the nonlinear longwave radiation (LWR) exchange with the sky. This model provides a short computational time solution of the transient heat transfer through multilayer roofs, which could be a good alternative to some numerical methods. Both heating and cooling annual loads are rigorously estimated and used as inputs to a life-cycle cost analysis. Among the studied cases, the most economical one is the hollow terracotta-based roof insulated with rock wool, where the optimum insulation thickness is estimated to be 7.9cm, with a payback period of 6.06years and energy savings up to 58.06% of the cost of energy consumed without insulation. The impact of the LWR exchange component is quantified and the results show its important effect on the annual transmission loads and, consequently, on optimum insulation thickness. A sensitivity analysis shows the efficiency of cool roofs in the Tunisian climate context, where the cooling energy cost benefits outweigh the wintertime penalty. Comparison of CFFT results with those of sol–air Degree-Hours (DH) shows that optimum insulation thickness and energy savings are overestimated and payback period is underestimated using the latter model. The proposed CFFT model could be an efficient tool for the design and the energy analysis of building envelope components in various climatic locations.

Accuracy analysis of longwave sky radiation models in the MZELWE module of the ESP-r program

Sky radiation models play an important role in the longwave radiation exchange between the opaque surfaces of buildings and their surroundings. The article is focused on an accuracy analysis of sky models implemented in the MZELWE simulation module of the ESP-r program. Dynamic outdoor tests and spectral laboratory methods were used for the validation of thermodynamic procedures in the given simulation module to demonstrate its applicability to calculating the accuracy level of models simulating longwave radiation phenomena. Based on high and low-emissivity properties, the presented issue focuses on vertical opaque walls in relation to highly insolated and weakly insulated building components for which longwave radiation exchange has been observed to be of most significance. Clear sky climatic conditions were predominantly applied for the purpose of validation at a Central European location. Among all the analyzed longwave sky radiation models implemented in the ESP-r program of the MZELWE module, only Clarke's model provides an applicable result with regard to the overall heat flow measured. The effects of the sky models were compensated by the greater thermal performance of the building components, for which the influence of the longwave radiation exchange on the resulting heat flows was significantly reduced.