On the Influence of Thermal Mass and Natural Ventilation on Overheating Risk in Offices

Free cooling strategies are gaining importance in design practice due to the increased risk of overheating in well-insulated buildings with high internal loads such as offices. The state of the art highlights that the most efficient passive solution for indoor temperature stabilization and control is the integration of thermal mass with an optimized ventilative cooling profile to enhance the thermal cycle of heat storage. Due to its cyclical behavior, thermal mass effects are difficult to predict and quantify with the traditional steady-state approach to building thermal performance. Dynamic thermal simulations help to assess a building’s behavior under transient situations, including the thermal mass influence. However, building codes usually include thermal simulations based on standard assumptions: typical meteorological year (TMY), standard occupancy, standard daily-based lighting and appliances profiles, and standard weekly-based occupancy. Thus, when assumptions change, the actual behavior of the building may vary consistently from the predicted conditions. In this paper, we focused on the ability of thermal mass to contrast the influence of variations from the standard assumptions, especially in relation to climate and ventilation profiles. The results show the necessity of encompassing different risk scenarios when evaluating a free cooling solution performance. Among the different scenarios simulated, natural ventilation misuse shows greater influence on the thermal indoor environment, especially if coupled with low thermal mass

Life cycle efficiency ratio: a new performance indicator for a life cycle driven approach to evaluate the potential of ventilative cooling and thermal inertia

Building envelope design has gained importance as a means to reduce heating and cooling demand related to a building’s operational phase. However, in high internal load buildings, such as offices, internal gains can easily lead to overheating. Thermal inertia (TI) and night ventilation have a great potential for reducing heat loads and temperature. However, their influence is difficult to predict due to the complex nature of the TI phenomenon, which is related to the interactions of multiple factors such as architecture, building physics and external conditions. Moreover, TI efficacy has often been studied in relation to energy savings or temperature analysis, overlooking other aspects implicated in buildings’ efficiency, such as the embodied energy involved. This paper presents a multidimensional approach to evaluate ventilative cooling and thermal inertia as a sustainable strategy to improve building performances. To that end, several scenarios of night ventilation strategies have been applied to the case study of an experimental double-office room placed in Fribourg (Switzerland). Based on this monitoring, a dynamic software simulation tool has been calibrated and used to analyze the energy savings potential and the life-cycle performance of TI. A new ratio index has been introduced to easily evaluate the life cycle efficiency. The results show the importance of balancing operational and embodied impacts when evaluating a design choice. Although high TI levels have great benefits on reducing the cooling loads, the results are completely different when a life cycle assessment is applied. Natural ventilation coupled with middle levels of TI have been identified as the best strategy to optimize the building’s energy and environmental performances, without compromising indoor temperatures

Novel methodology and platform for NZEB renovation of residential buildings

Renovation of the existing building stock is a vital part of reaching upcoming energy savings and CO2 emission targets. The European Union (EU) continuously publishes directives and guidance to support the transition of existing buildings to nearly-Zero Energy Buildings. A method for calculation of cost-optimal levels for minimum energy performance was introduced in 2012. Its aim is to compare and select different renovation alternatives, based on energy savings and global costs. It has been successfully applied on a package level; however, its complexity has restricted it from being used for comparing renovation alternatives between single components with the same and different functions. This paper presents a methodology for determination of a simplified value linking economical and efficiency parameters on a component level. The value allows for fast overview of cost-benefit of different renovation alternatives between components and systems. It serves a decision-making aid for compilation of renovation packages, which are further evaluated with the cost-optimal approach. The paper also introduces novel refurbishment assessment platform that can assist decision makers with fast compilation of refurbishment packages incorporating key aspects of the presented methodology. The current functionality of the platform is showcased at the end of the paper by a case study

Optimal design and operation of a building energy hub: A comparison of exergy-based and energy-based optimization in Swiss and Italian case studies

With growing concerns on global warming, exergy-based design methods for energy hubs (EHs) in the urban context have been recently investigated to promote more rational and efficient use of energy sources. This study aims to compare exergy-based multi-objective optimization for energy hubs with two primary energy-based methods. The comparison has been performed for Italy and Switzerland, two countries with diverse markets and national electricity production mixes, to indicate the generalizability of our findings. An apartment building in Vevey, Switzerland, was selected to provide domestic hot water demand and structural data to the space heating demand dynamic simulation, for which different TRY weather data have been used for the two countries. Once a superstructure for the energy supply system had been defined, a MILP framework was developed, minimizing a weighted sum of exergy and cost. Using different weights for the two objectives, a Pareto frontier was obtained for each scenario, defining the best possible trade-off solutions between economic and exergetic objectives. The same optimization methodology was performed using total or non-renewable primary energy as an objective. The use of a boiler and PV panels is preferred when primary energy-based methods are applied, while the use of heat pumps and solar thermal panels is preferred when exergy-based method is applied. As a result, the exergy-based method seems to be the most effective as the carbon intensity of the electricity decreases, providing solutions with lower CO2 emissions and reasonable costs in the future when national electricity production will be gradually decarbonized. In addition, a sensitivity analysis of the exergy method was carried out to analyze the influence of key parameters such as energy prices and energy demand variation on the optimized energy system. In addition, the renovation scenarios of the case study building were presented, revealing different optimal setups of the energy hub as the weight of investment costs increases

Ventilative Cooling Design Guide

status: Published onlin