Unsteady-state exergy analysis on two types of building envelopes under time-varying boundary condition

In the built environment, the thermal exergy behavior is very sensitive to the change of environmental temperature, because the temperature difference between the reservoir and a system of interest is small. Moreover, the transient characteristics become very important for the building envelope, which is primarily affected by the environmental temperature changes and has a relatively large heat capacity. Most of the exergy analyses have been performed under steady-state assumption. However, it may miss some important details of the transient process. Thus, when the transient transfer process becomes important, the unsteady-state exergy analysis should be conducted. In this study, we propose complete energy, entropy, and exergy equations in their partial differential forms. By solving them numerically, we examined the transient exergy process inside the building envelope composed of concrete and insulation layers under time-varying boundary condition. Using this new methodology, we can improve the temporal and spatial resolution of the exergy analysis and thus provide more complete information about exergetic behavior

Comparison of Lattice Boltzmann Method and Finite Volume Method with Large Eddy Simulation in Isothermal Room Flow

Lattice Boltzmann method (LBM), as a new computational fluid simulation method, has aroused widespread attention in recent decades within engineering practice. LBM with large eddy simulation (LBM-LES) model is commonly used in predicting high Reynolds flow, and is considered to have a prediction accuracy similar to traditional finite volume method (FVMLES). Nonetheless, a systematic discussion on the accuracy of LBM-LES, and its consistency with FVM-LES, in indoor turbulent flow situations, is still insufficient. In this study, simulations of an indoor isothermal forced convection benchmark case (from IEA Annex 20) are implemented by using both LBM-LES and FVM-LES, with the aim of comparing the accuracies of LBM-LES and FVM-LES, in indoor turbulent flow situations. A comparison of their relative computation speeds, and parallel computation performances, is also implemented. The results show that LBM-LES can achieve the same level of accuracy as FVM-LES, in indoor turbulent flow situations; however, more refined meshes are required. Compared with FVMLES, half size grids are required for LBM-LES to approach similar levels of accuracy, meaning that the meshes of LBM-LES are approximately eight times as large as FVM-LES. The computation speeds of both LBM-LES and FVM-LES scale well, with the increase in the number of computation cores in one node. Their computation speeds (with the same accuracy) approach a similar level; however, the parallel computation speed of the LBM-LES speed can be larger than FVM, owing to its superior parallel speedup performance

Development of TPRT (Thermal Performance-Response Test) for borehole heat exchanger design

To obtain the effective thermal conductivity and borehole thermal resistance required to design borehole heat exchangers (BHEs), thermal response tests (TRTs) are usually conducted. Although many advanced TRT methods have been proposed, most TRTs cannot directly provide the actual thermal performance of an installed BHE. Because many uncertainties exist in constructing even conventional BHEs, examining the transient heat exchange rate allows inspection of the construction quality; the rate can also be used as a reference value for design. To determine the actual heat exchange rates of BHEs, it is necessary to conduct thermal performance tests (TPTs) under a constant inlet fluid temperature. However, TPT requires expensive equipment, including a water tank and a complex control system; thus, generally only TRT is conducted. To overcome the existing problems of TPT, in this study, we proposed a thermal performance-response test (TPRT) that combines TRT and TPT. This method involved the construction of a cost-effective TPRT apparatus by adding only a general PID controller and a solid-state relay to an existing TRT apparatus. Using the apparatus constructed by the proposed method, two TPRTs were conducted to confirm the performance of the apparatus and the validity of the TPRT method. Additionally, by defining the new parameter of the unit heat exchange rate, one potential simple and reliable design method for BHEs was explored

Case-study of thermo active building systems in Japanese climate

Thermo active building systems (TABS) have been applied in office buildings as a promising energy efficient solution in many European countries. The utilization of building thermal mass helps to provide high quality thermal environments with less energy consumption. However, the concept of TABS is entirely new in Japan. This paper introduces and evaluates TABS under Tokyo weather conditions to clarify the potential of use TABS in Japan. Cooling capacity of thermo active building systems used in an office building was evaluated by means of dynamic simulations. Two central rooms of the office were selected for the analysis. Six water control strategies were studied and two of those were found reasonable and suitable for TABS use in Tokyo. These two strategies are: free-cooling using underground heat exchanger combined with TABS and free-cooling with desiccant dehumidification system. For these two cases, the operative temperature was between 22-27 °C during 97∼99% of the occupation time. The operative temperature drift was less than 4 °C per day. The pump running time was 7 hours per day and the cooling power of the TABS was 36 W/m2 floor area. For those free-cooling cases, the average supply water temperature was 20 °C, which shows that free-cooling is achievable using underground heat exchangers even considering the temperature increase of the ground during cooling season

Analysis of the Relationship between Changes in Meteorological Conditions and the Variation in Summer Ozone Levels over the Central Kanto Area

An increasing trend in ground-level ozone (O3) concentrations has recently been recognized in Japan, although concentrations of ozone precursors, nitrogen oxides (NOx), volatile organic compounds (VOCs) and nonmethane hydrocarbons (NMHCs) have decreased. In this paper, the relationship between meteorological factors (temperature and wind speed) and ground-level ozone concentrations in the summer over the central Kanto area of Japan was examined using both statistical analyses and numerical models. The Fifth-Generation NCAR/Penn State Mesoscale Model (MM5) and the Community Multiscale Air Quality (CMAQ) model were employed in this study. It was found that there is a close relationship between meteorological conditions and ground-level ozone concentrations over the central Kanto area. In summer, up to 84% of the long-term variation in peak ozone concentrations may be accounted for by changes in the seasonally averaged daily maximum temperature and seasonally averaged wind speed, while about 70% of the recent short-term variation in peak ozone depends on the daily maximum temperature and the daily averaged wind speed. The results of numerical simulations also indicate that urban heat island (UHI) phenomena can play an important role in the formation of high ozone concentrations in this area

Predictive Control Strategies based on Weather Forecast in Buildings with Energy Storage System: A Review of the State-of-the Art

Energy storage systems play a crucial role in decreasing building energy consumption during peak periods and expanding the use of renewable energies in buildings and communities. To have a high system performance, the energy storage system has to be properly controlled while maintaining a comfortable thermal environment for the occupants. However, defining the optimal charging period for a storage system may be difficult since storage systems address issues with conflicting needs between cost saving and thermal comfort. Moreover, with the increase of the use of renewable energies, the complexity increases with the consideration of the renewable energy production. As a result, the decision process should be able to predict both loads and renewable energy production in order to increase the storage system efficiency. This necessity explains the increasing interest during the last decade for predictive control, i.e., control system considering the forecasting. This paper reviews the recent advancements in building predictive control with energy storage system. Special attention is paid to its limitations and abilities

Numerical Analysis of Outdoor Thermal Environment in City Block Area Using Coupled Simulation of Convection and Radiation -Part 2 CFD Study on of Outdoor Thenna1 Environment sing Modified κ-ε model -

In this study, velocity, temperature and humidity fields in urban area were analyzed using CFD technique developed by the authors. Spatial distribution of SET* is calculated in order to assess pedestrian comfort from the CFD predictions. It was darified that 1) revised κ-ε model can predict the velocity field and temperature field more accurately　than does the stndard κ-ε model， 2) greening is effective for relaxation of thermal environment on the urban area in summer，3) coupled simulation of radiation and convection with incorporating of moisture transport is very powerful tool for eva1uating the outdoor environment

Numerical Analysis of Outdoor Thermal Environment in City Block Area Using Coupled Simulation of Convection and Radiation -Part 2 CFD Study on of Outdoor Thenna1 Environment sing Modified κ-ε model -

In this study, velocity, temperature and humidity fields in urban area were analyzed using CFD technique developed by the authors. Spatial distribution of SET* is calculated in order to assess pedestrian comfort from the CFD predictions. It was darified that 1) revised κ-ε model can predict the velocity field and temperature field more accurately　than does the stndard κ-ε model， 2) greening is effective for relaxation of thermal environment on the urban area in summer，3) coupled simulation of radiation and convection with incorporating of moisture transport is very powerful tool for eva1uating the outdoor environment

CFD Analysis of flowfield around a high-rise building using various revised k-ε models

The flowfield around a high-rise building model placed within the surface boundary-layer was analyzed using various revised k-ε models, and these results were compared with the result of DSM and the experimental data. The standard k-ε model could not reproduce the reverse flow on the roof. This drawback was corrected by all revised k-ε models and DSM tested here. However the revised k-ε models except for the model proposed by Durbin overestimated the reattachment length behind the building in comparison with the standard k-ε model. Among the computations using various turbulence models compared here, the result with the Durbin’s revised k-ε model showed the best agreement with the experiment