Whole-building Energy Research Articles

Building envelopes toward energy‐efficient buildings: A balanced multi‐approach decision making

The worldwide environmentally friendly trend has focused the last decade on emphasizing the value of energy conservation and reducing the carbon footprint in buildings, achieving zero energy buildings. Presently, viable building techniques, in developing countries, are insufficient to achieve zero energy buildings. Thus, authorities should implement policies mandating new developments and renovations to establish “energy-efficient buildings”; nevertheless, design solutions should be properly evaluated and assessed preceding execution. The conservation of energy without jeopardizing human comfortability is a huge challenge for any designer. Occupants are less interested in making a major investment to save some expenses over the next two decades, especially nowadays that energy is still affordable. Therefore, improved indoor environmental conditions are perhaps another important parameter toward energy-efficient buildings. Through dynamic simulations, this study examines the energy efficiency and thermal comfort achieved by integrating retrofitting strategies in an institutional building in three different American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) hot climate zones represented by three cities of Egypt (Aswan, Cairo, and Alexandria). The built-up baseline model is validated using actual energy usage data. The validated baseline model is then subjected to local sensitivity analysis to determine the driving parameters influencing the building's energy demand. The study at hand focuses on a broader perspective on sustainability. With a multiapproach decision-making methodology based on the most recent measures, the study highlights the outcomes through an environmental–economic assessment and indoor thermal comfort depending on experts' weighting, responses, and recommendations. The outcomes postulate that the implementation of reflective paints solutions would achieve the highest percentages of whole-building energy savings with 21%, 19%, and 17% for Aswan, Cairo, and Alexandria, respectively, improving indoor thermal comfort levels.

A synthetic building operation dataset

This paper presents a synthetic building operation dataset which includes HVAC, lighting, miscellaneous electric loads (MELs) system operating conditions, occupant counts, environmental parameters, end-use and whole-building energy consumptions at 10-minute intervals. The data is created with 1395 annual simulations using the U.S. DOE detailed medium-sized reference office building, and 30 years’ historical weather data in three typical climates including Miami, San Francisco, and Chicago. Three energy efficiency levels of the building and systems are considered. Assumptions regarding occupant movements, occupants’ diverse temperature preferences, lighting, and MELs are adopted to reflect realistic building operations. A semantic building metadata schema - BRICK, is used to store the building metadata. The dataset is saved in a 1.2 TB of compressed HDF5 file. This dataset can be used in various applications, including building energy and load shape benchmarking, energy model calibration, evaluation of occupant and weather variability and their influences on building performance, algorithm development and testing for thermal and energy load prediction, model predictive control, policy development for reinforcement learning based building controls.

Comparative environmental and economic evaluation of green roofs under Mediterranean climate conditions – Extensive green roofs a potentially preferable solution

Green roofs constitute a prominent urban green infrastructure solution and are considered to be able to enhance urban sustainability and resilience. Existing literature, however, lacks a holistic evaluation of their life cycle impacts. In this study, a comparative environmental and economic life cycle analysis between green and flat roofs under Mediterranean climate conditions was conducted, examining all green roof types (extensive, semi-intensive, intensive), two types of drainage layer (granular and synthetic) for each one, all green roof layer materials, life cycle stages (production of materials, use and maintenance, disposal) and their quantifiable costs and benefits, aiming to address this research gap. Dynamic whole-building energy demand simulations were also conducted in its context, which additionally investigated the effect of various factors on green roofs' energy saving potential. Thermally insulated green roofs provided a small improvement in the energy consumption for heating and cooling (up to 8.30% and 3.50% respectively) but significant reductions in total life cycle energy consumption (8–31%), CO2 emissions (24–32%) and waste production (15–60%) rendering them a cleaner environmental option compared to flat roofs. However, they also produced a very important increase in total life cycle water consumption (279–835%). The extensive green roof types were the ones to perform better in all these indexes. They were also the only type to constitute a better economic choice than a flat roof for the private owner in a low discount rate (0.25%) scenario, both with or without public benefits. As this study shows, the green roofs' potential is not limited only to the improvement of the buildings’ energy efficiency, as numerous studies in the field of energy efficiency highlight, but compared to flat roofs, expand to reduce the embodied energy, greenhouse gas emissions and material waste. By means of environmental footprint, they are superior to flat roofs when only energy and emissions are considered, while in regions where water availability is a key environmental factor, this superiority is in question, together with their economic feasibility in countries where this technology is not financially supported.

Modelling double skin façades (DSFs) in whole-building energy simulation tools: Validation and inter-software comparison of a mechanically ventilated single-story DSF

Double skin façades (DSFs) have been proposed as responsive building systems to improve the building envelope’s performance. Reliable simulation of DSF performance is a prerequisite to support the design and implementation of these systems in real buildings. Building energy simulation (BES) tools are commonly used by practitioners to predict the whole building energy performance, but the simulation of the thermophysical behaviour of DSFs may be challenging when carried out through BES tools. Using an exhaust-air façade case study, we analyse and assess the reliability of four popular BES tools when these are used to simulate a DSF, either through available in-built models or through custom-built representations based on zonal models. We carry out this study by comparing numerical simulations and experimental data for a series of significant thermophysical quantities, and we reflect on the performance and limitations of the different tools. The results show that no tool is outstandingly better performing over the others, but some tools offer better predictions when the focus is placed on certain thermophysical quantities, while others should be chosen if the focus is on different ones. After comparing the different models’ limitations and challenges, we conclude that BES tools can simulate the performance of DSF systems over long periods. However, their use alone is not recommended when the simulation’s scope is to replicate and study short-term phenomena and dynamic aspects, such as sizing the building’s HVAC system.

Recent Developments in the Design of Vertical Borehole Ground Heat Exchangers for Cost Reduction and Thermal Energy Storage

AbstractGround source (geothermal) heat pumps (GSHPs) can meet the thermal demands of buildings in an energy-efficient manner. The current high installation costs and long payback period limit the attractiveness of GSHP installation in the United States. Vertical borehole ground heat exchangers (VBGHEs), which are commonly used in GSHP systems, contribute most to the cost premium of GSHPs. Reducing the cost of VBGHEs could help increase market penetration of GSHP systems. This paper reviews recent developments for VBGHEs, including improvements in borehole heat transfer and borehole field layout, integration with thermal energy storage, and new design tools. Improvements in the borehole design and materials are more likely to be justified when the ground has high thermal conductivity. Integrating thermal energy storage can provide additional value to the GSHP system, especially when flexible electric demand at buildings becomes more valuable. Advanced design tools for VBGHEs that account for the thermal response of irregularly shaped borehole fields and that are more closely integrated with whole-building energy simulation programs may facilitate more innovations and optimization of GSHP system designs.

Evaluating potential benefits of air barriers in commercial buildings using NIST infiltration correlations in EnergyPlus

According to the U. S. Department of Energy (DOE), infiltration accounts for 6% of the energy use and $11 billion in energy cost for U. S. commercial buildings. One strategy to reduce infiltration in commercial buildings is to provide more supply airflow than return and exhaust in order to “pressurize the building”. DOE has developed EnergyPlus models of several prototype buildings which assume that pressurization results in system-on infiltration rates that are 75% less than the system-off rates. However, airflow simulations of these buildings using the CONTAM multizone airflow software showed that pressurization reduced infiltration by an average of 44% only. To improve the infiltration rates calculated by the EnergyPlus models of prototype buildings, CONTAM infiltration rates were used to develop coefficients that can be input into EnergyPlus. CONTAM captures the effects of wind, temperature difference, and system operation on infiltration rates. Coefficients were developed for 11 prototype buildings, eight cities, and two levels of building envelope airtightness. Comparisons of the predicted infiltration rates were made between using the DOE prototype model inputs and the NIST infiltration correlations. Using the NIST correlations resulted in an average HVAC-EUI (HVAC-related energy use intensity) savings of 6% or 1.4 kBtu/ft2 due to airtightening. These results indicate that the effects of infiltration on HVAC energy use are important and that infiltration can and should be better accounted for in whole-building energy modeling.

Minimization of energy consumption by building shape optimization using an improved Manta-Ray Foraging Optimization algorithm

For the optimization of envelope characteristics also the shape of the building, an optimization–simulation technique is applied in this paper. To obtain the best values of all related variables for the minimization of energy consumption in residential buildings, an improved Manta-Ray Foraging Optimizer is considered as the optimization algorithm. Also, for whole-building energy simulation, RIUSKA is used. The optimization parameters are the area and type of the windows, foundation, wall and roof insulations, level of infiltration, orientation, and thermal mass. Various forms of the building including rectangle, trapezoid, T-shape, H-shape, cross, L-shape, and U-shape are studied. The model optimization process takes fewer computation time and expense. Moreover, the utilized technique implements fully proper in comparison to the particle swarm, approximating very close to the optimum in less than 50% of the simulations The lowest life cycle cost is achieved by the buildings with trapezoid and rectangle form for five various climatic conditions. Also, the minimum variation from the optimum to the worst is observed by trapezoid and rectangle. The change in the values of the life cycle cost is lower than 4.5%.

Energy and Greenhouse Gas Savings for LEED-Certified U.S. Office Buildings

In this work, we present results from the largest study of measured, whole-building energy performance for commercial LEED-certified buildings, using 2016 energy use data that were obtained for 4417 commercial office buildings (114 million m2) from municipal energy benchmarking disclosures for 10 major U.S. cities. The properties included 551 buildings (31 million m2) that we identified as LEED-certified. Annual energy use and greenhouse gas (GHG) emission were compared between LEED and non-LEED offices on a city-by-city basis and in aggregate. In aggregate, LEED offices demonstrated 11% site energy savings but only 7% savings in source energy and GHG emission. LEED offices saved 26% in non-electric energy but demonstrated no significant savings in electric energy. LEED savings in GHG and source energy increased to 10% when compared with newer, non-LEED offices. We also compared the measured energy savings for individual buildings with their projected savings, as determined by LEED points awarded for energy optimization. This analysis uncovered minimal correlation, i.e., an R2 < 1% for New Construction (NC) and Core and Shell (CS), and 8% for Existing Euildings (EB). The total measured site energy savings for LEED-NC and LEED-CS was 11% lower than projected while the total measured source energy savings for LEED-EB was 81% lower than projected. Only LEED offices certified at the gold level demonstrated statistically significant savings in source energy and greenhouse gas emissions as compared with non-LEED offices.

The impact of vertical vegetation on thermal performance of high-rise office building facades in Mediterranean climate

Nowadays, vertical greenery systems are considered as a promising solution for improving energy efficiency, while providing environmental and socio-economic benefits. However, a literature gap regarding their quantitative effects on building’s thermal performance exists. The main aim of this paper is to initiate an analytical approach towards evaluating the greenery effectiveness in reducing the energy loads and improving the indoor thermal comfort, contributing to the current body of knowledge. The main novelty and scientific significance consist of evaluating different design strategies of green facade and shading systems, widely implemented on building envelopes, even though no previous study has provided scalable results regarding their efficiency. Several design variables including WWR (window-to-wall ratio), LAI (leaf area index), and plant height are selected for a comprehensive whole-building energy performance analysis. The results highlight the efficiency of vegetation strategies on opaque walls achieving a significant range of energy reduction from 9% up to 11% for 50% window-to-wall ratio scenarios and from 3% up to 6% for 70% window-to-wall ratio scenarios, with 1 °C up to 2 °C indoor air and mean radiant temperature increase in the winter period. Additionally, the impact of vertical greenery effectiveness on a fully glazed building scenario, rarely mentioned in previous studies has been proved, providing a considerable reduction of 4.7 °C indoor air temperature and 9 °C radiant temperature in the summer period, ranging from 14% up to 34% energy savings. The performed simulation results generate a fundamental framework towards early design decision-making stages.

Theoretical Minimum Thermal Load in Buildings

Summary Building cooling and heating accounts for a large portion of total global energy use and requires commensurate amounts of resources, which contribute significantly to global warming. Traditionally, addressing this issue has meant improving the efficiency of equipment supplying the thermal energy, reducing envelope heat transfer, and reducing air infiltration. However, this approach is already reaching practical limits. In this perspective, we explore (1) how to reduce thermal load in buildings theoretically and (2) how to achieve that reduction and dramatically lower the energy required to support building loads practically. First, we discuss our framework developed for calculating the theoretical minimum thermal load (TMTL) in buildings. Our analysis shows that current thermal loads in buildings are more than an order of magnitude higher than the TMTL. We also introduce an approximate formula to calculate energy savings from zonal control of thermal load, which shows that the majority of zonal control benefits can be achieved with fewer than 10 zones. Then, we discuss pros and cons of various approaches and strategies to achieve the TMTL. We conclude our perspective with some longer-term R&D ideas, such as thermally adaptive clothing and thermal storage to help approach the TMTL, while providing the additional benefit of interacting with the renewable grid of the future.

Using Smart-WiFi Thermostat Data to Improve Prediction of Residential Energy Consumption and Estimation of Savings

Energy savings based upon use of smart WiFi thermostats ranging from 10 to 15% have been documented, as new features such as geofencing have been added. Here, a new benefit of smart WiFi thermostats is identified and investigated; namely, as a tool to improve the estimation accuracy of residential energy consumption and, as a result, estimation of energy savings from energy system upgrades, when only monthly energy consumption is metered. This is made possible from the higher sampling frequency of smart WiFi thermostats. In this study, collected smart WiFi data are combined with outdoor temperature data and known residential geometrical and energy characteristics. Most importantly, unique power spectra are developed for over 100 individual residences from the measured thermostat indoor temperature in each and used as a predictor in the training of a singular machine learning models to predict consumption in any residence. The best model yielded a percentage mean absolute error (MAE) for monthly gas consumption ±8.6%. Applied to two residences to which attic insulation was added, the resolvable energy savings percentage is shown to be approximately 5% for any residence, representing an improvement in the ASHRAE recommended approach for estimating savings from whole-building energy consumption that is deemed incapable at best of resolving savings less than 10% of total consumption. The approach posited thus offers value to utility-wide energy savings measurement and verification.

Residential Natural Gas Demand Response Potential during Extreme Cold Events in Electricity-Gas Coupled Energy Systems

In regions where natural gas is used for both power generation and heating buildings, extreme cold weather events can place the electrical system under enormous stress and challenge the ability to meet residential heating and electric demands. Residential demand response has long been used in the power sector to curtail summer electric load, but these types of programs in general have not seen adoption in the natural gas sector during winter months. Natural gas demand response (NG-DR) has garnered interest given recent extreme cold weather events in the United States; however, the magnitude of savings and potential impacts—to occupants and energy markets—are not well understood. We present a case-study analysis of the technical potential for residential natural gas demand response in the northeast United States that utilizes diverse whole-building energy simulations and high-performance computing. Our results show that NG-DR applied to residential heating systems during extreme cold-weather conditions could reduce natural gas demand by 1–29% based on conservative and aggressive strategies, respectively. This indicates a potential to improve the resilience of gas and electric systems during stressful events, which we examine by estimating the impact on energy costs and electricity generation from natural gas. We also explore relationships between hourly indoor temperatures, demand response, and building envelope efficiency.

Risk-informed multi-criteria decision framework for resilience, sustainability and energy analysis of reinforced concrete buildings

ABSTRACT With recent advancement in energy-based sustainable design of building structures, the need for inclusive yet practical models to integrate resilience and sustainability is increasingly recognized. This paper integrates structural seismic resilience and sustainability assessment methods with whole-building energy simulation techniques to present a new comprehensive decision model for the design of buildings. Risk-based multi-attribute utility theory and analytic hierarchy process are used to develop a multi-criteria decision-making (MCDM) framework considering various economic, social and environmental criteria involved in design of buildings. The model is implemented on a number of RC buildings, and the influence of building configuration on environment, seismic performance, and energy consumption is studied. It is found that shear wall ratio plays a significant role in both seismic loss and energy consumption of RC buildings. Increasing the shear wall ratio effectively reduces the direct monetary loss and downtime as well as energy consumption.

An Enhanced Vertical Ground Heat Exchanger Model for Whole-Building Energy Simulation

This paper presents an enhanced vertical ground heat exchanger (GHE) model for whole-building energy simulation (WBES). WBES programs generally have computational constraints that affect the development and implementation of component simulation sub-models. WBES programs require models that execute quickly and efficiently due to how the programs are utilized by design engineers. WBES programs also require models to be formulated so their performance can be determined from boundary conditions set by upstream components and environmental conditions. The GHE model developed during this work utilizes an existing response factor model and extends its capabilities to accurately and robustly simulate at timesteps that are shorter than the GHE transit time. This was accomplished by developing a simplified dynamic borehole model and then exercising that model to generate exiting fluid temperature response factors. This approach blends numerical and analytical modeling methods. The existing response factor models are then extended to incorporate the exiting fluid temperature response factor to provide a better estimate of the GHE exiting fluid temperature at short simulation timesteps.

Performance Simulation and Analysis of Occupancy-Based Control for Office Buildings with Variable-Air-Volume Systems

Variable-air-volume (VAV) systems are used in many office buildings. The minimum airflow rate setting of VAV terminal boxes has a significant impact on both energy consumption and indoor air quality. Conventional controls usually have the terminal’s minimum airflow rate at a constant (e.g., 30% or more of the terminal design airflow rate), irrespective of the occupancy status, which may cause problems, such as excessive simultaneous heating and cooling, under ventilation, and thermal comfort issues. This paper examines the potential of energy savings from occupancy-based controls (OBCs). The sensed occupancy information, either occupant presence or people count, is used to determine the airflow rate of terminal boxes, the thermostat setpoints, and the lighting control. Using EnergyPlus, a whole-building energy modeling software, the energy savings of OBC strategies are evaluated for representative existing medium office buildings in the U.S. The simulation results show that the conventional OBC, based on occupant presence sensing, can save 8% of whole-building energy use in Miami (hot climate) for systems without air-side economizer and about 13% in both Baltimore (mixed climate) and Chicago (cold climate). Comparatively, the advanced OBC, based on people counting, can save 8% in Miami to 23% in Baltimore for systems with economizers. The outdoor-air fraction of the supply air from air-handling units significantly affects the potential energy savings from the advanced OBC strategy. In addition to energy savings, the advanced OBC satisfies the zone ventilation during all occupied hours over the whole year.

Going Beyond Code: Monitoring Disaggregated Energy and Modeling Detached Houses in Hawai‘i

Hawai‘i was the first state to establish a statewide 100% renewable energy goal to be met by 2045. To address the potential impact of new residential construction, a team conducted field studies and developed whole-building energy models of detached houses in Hawai‘i. The goal was to inform the state, a major developer, and the Hawai‘i design community on how current residential design could be modified to meet the newly adopted energy codes and renewable energy goals. Seven houses with air-conditioning were monitored for disaggregated energy use, temperature, and humidity for one year. The building simulation software BEoptTM v.2.8 was used to conduct parametric analyses and cost/energy optimizations to evaluate design options. The energy-optimized model for this hot-humid climate included the following: increasing the efficiency of the air-conditioning system; utilizing ceiling fans with occupancy sensors while increasing the thermostat set-point; a light-colored exterior finish and roof; a radiant barrier; longer eaves; and Energy Star appliances. The energy-optimized model reduced the annual site energy by approximately 47.1% compared to the model that minimally complied with the International Energy Conservation Code 2015. A 4.5-kW grid-tied photovoltaic array would achieve a net-zero annual site energy value.

Energy cost savings and expected payback for Re-tuning the controls of US Army buildings

The Headquarters Department of the Army sponsored a study to determine the national impact of deploying the Re-tuning™ (“Re-tuning” is a trademark of the US Department of Energy) methodology in five building types that account for over 40% of the Army’s conditioned building stock. Re-tuning is a systematic process that improves operational efficiency and reduces energy consumption at no or low cost through the building automation system by correcting operational problems that plague buildings. The study relied on successful demonstration of the Re-tuning methodology at four pilot US Army installations that informed a holistic effort, including simulating 12 individual Re-tuning energy efficiency measures (EEMs) and six packages of EEMs in five selected Army building prototype models that represent 311 million ft2 (28.9 million m2) of the Army’s conditioned floor space. Both the baseline buildings and the packages were customized to capture the expected outcomes of Re-tuning a diverse set of buildings. The study highlighted the benefit of individual Re-tuning EEMs and economics of implementing packages of EEMs in applicable Army buildings across 16 climates and two building vintages. The average whole-building energy savings ranged from 10.8 to 40.5% by building type, with the company operations facility building having the highest value proposition from Re-tuning. In addition, the study revealed that all large office buildings in the Army are economical to Re-tuning. The total modeled cost savings potential for Re-tuning the five building types across the US Army is $204/1000 ft2 ($220/100 m2) or $64 M annually. This cost savings represent 5.6% of all the Army’s energy expenditures.

A Comprehensive Analysis of Energy and Daylighting Impact of Window Shading Systems and Control Strategies on Commercial Buildings in the United States

Commercial buildings consume approximately 1.9 EJ of energy in the United States, 50% of which is for heating, cooling, and lighting applications. It is estimated that windows contribute up to 34% of the energy used for heating and cooling. However, window retrofits are not often undertaken to increase energy efficiency because of the high cost and disruptive nature of window installation. Highly efficient window technologies would also need shading devices for glare prevention and visual comfort. An automated window shading system with an appropriate control strategy is a technology that can reduce energy demand, maintain occupant comfort, and enhance the aesthetics and privacy of the built environment. However, the benefits of the automated shades currently used by the shading industry are not well studied. The topic merits an analysis that will help building owners, designers and engineers, and utilities make informed decisions using knowledge of the impact of this technology on energy consumption, peak demand, daylighting, and occupant comfort. This study uses integrated daylight and whole-building energy simulation to evaluate the performance of various control strategies that the shading industry uses in commercial office buildings. The analysis was performed for three different vintages of medium office buildings at six different locations in United States. The results obtained show the control strategies enabled cooling energy savings of up to 40% using exterior shading, and lighting energy savings of up to 25%. The control strategies described can help building engineers and researchers explore different control methods used to control shading in actual buildings but rarely discussed in the literature. This information will give researchers the opportunity to investigate potential improvements in current technologies and their performance.

Energy-saving potential of 3D printed concrete building with integrated living wall

Large-scale concrete 3D printing and digital construction has brought enormous potential to expand the design space of building components (e.g., building envelope) for the integration of multiple architectural functionalities including energy saving. In this research, a modular 3D printed vertical concrete green wall system – namely the 3D-VtGW, was developed. The 3D-VtGW envelope was assembled with prefabricated (3D printed) multifunctional wall modular elements, which serves as the enclosure of the building as well as the backbone for a green wall system to improve building’s energy efficiency. Using this design concept and large-scale concrete 3D printing, a prototype commercial building was built in Nanjing, China. To quantify the energy-saving potential of the 3D-VtGW system, a thermal network model was developed to simulate the thermal behavior of buildings with 3D-VtGW system and for thermal comfort analysis. Whole-building energy simulation was carried out using Chinese Standard Weather Data (CSWD) of Nanjing, China. The simulation results indicate that the building with 3D-VtGW exhibited prominent potential for energy saving and improved thermal comfort. The integrated greenery system in 3D-VtGW largely reduces wall exterior surface temperature and through-wall heat flux via the combined effects of plant shading, evapotranspiration, and heat storage from soil. This study presents the immense opportunities brought by digital fabrication and construction to extend the design space and function integration in buildings.

Effects of Rooftop Photovoltaics on Building Cooling Demand and Sensible Heat Flux Into the Environment for an Installation on a White Roof

Abstract Photovoltaic (PV) panels are commonly used for on-site generation of electricity in urban environments, specifically on rooftops. However, their implementation on rooftops poses potential (positive and negative) impacts on the heating and cooling energy demand of buildings, and on the surrounding urban climate. The adverse consequences can be compounded if PV is installed on top of an otherwise highly reflective (“white”) rooftop. This study investigates these impacts on a test building in Tempe, AZ, by directly measuring the temperature of all involved surfaces. These measurements are supplemented by whole-building energy simulations to model the energy implications for archetypical residential and retail buildings. This includes calculations of the ratio of the energy demand penalty to electricity generation as well as the net sensible heat flux to the ambient environment. Results indicate that the summertime cooling energy penalty due to blockage of outgoing longwave radiation can be 4.9—11.2% of the PV electricity generation. The addition of PV to the white roof resulted in a small decrease in the computed sensible heat flux at night, but a daytime increase in sensible flux by more than a factor of 10 (from less than 25 W/m2 for the white roof alone, to more than 250 W/m2 when PV is added to the roof). This study highlights the potential unintended consequences of rooftop PV under certain conditions and provides a broader perspective for building designers and urban planners.