Whole-building Simulation Program Research Articles

Model-driven development of durable and scalable thermal energy storage materials for buildings

The energy impact of integrating phase change materials (PCMs) in buildings for thermal energy storage has been modeled by various whole-building simulation programs, demonstrating that PCM incorporation can reduce energy consumption, provide grid flexibility and resilience, and reduce CO2 emissions. The models assume that the PCMs are in perfect operating condition and underestimate the impact of actual phase change behavior (e.g., enthalpy curve shape) on thermal load shifting in practical deployment. In this paper, we bridge the gap between theory and practice when evaluating the energy impact of PCMs by using a model-driven approach to develop durable thermal energy storage materials with desired phase change properties. For ease of integration, we fabricate shape-stabilized PCMs (ss-PCMs) by encapsulating solid-liquid polyethylene glycol (PEG) consisting of different molecular weights within mesoporous magnesium oxide (MgO) matrices. Learning from the modeling results, we manipulate phase change properties such as peak melting temperature and temperature glide of PEG-MgO ss-PCMs during the synthesis process to achieve desired properties. As such, the energy density is maximized within the optimum operating temperature range, which is critical to boosting energy efficiency. Compared to a case with no PCM, a layer of PEG-MgO ss-PCM integrated into the wall provides a 50% reduction in the peak load and also exhibits a repeatable phase change behavior for up to 1000 thermal cycles without leakage, showing durability of this material. We also show that this lab-scale synthesis process is easy to be scaled up by 100 times for a demonstration of large-scale industrial production. The synthetic tunability of transition temperature of ss-PCMs also extends their applicability beyond buildings.

Thermal model of heat transfer in a PCM multilayer construction using Moving Mushy Volume Approach – verification, validation and sensitivity analysis

A new model for predicting the thermal performance of a thin layer of phase change material enclosed in a multilayer structure has been developed, verified, and validated. Numerical verification includes heat transfer and solar radiation processes, while only heat transfer was considered for experimental validation. The model considers thermo-optical properties of the PCM layer using a Moving Mushy Volume Approach. The basic assumption of the proposed approach is to divide the PCM domain into sub-volumes and determine their physical state separately, which allows varying the parameters in time. Each sublayer can be in a solid, liquid, or semi-transient state (mushy) where the level of latent heat and the solid–liquid ratio is determined by a dimensionless temperature-dependent melting function. Hence, it is appropriate for use in whole-building simulation programs, where it can be applied to more precise determination of heat transfer within a transparent PCM structure.

Numerical and Experimental Study on the Performance of Thermoelectric Radiant Panel for Space Heating.

The purpose of this study is to investigate the suitable operation and performance of a thermoelectric radiant panel (TERP) in the heating operation. First, the hypothesis was suggested that the heating operation of TERP can operate without a heat source at the cold side according to theoretical considerations. To prove this hypothesis, the thermal behavior of the TERP was investigated during the heating operation using a numerical simulation based on the finite difference method. The results indicated that it is possible to heat the radiant panel using a thermoelectric module without fan operation via the Joule effect. A mockup model of the TERP was constructed, and the numerical model and hypothesis were validated in experiment 1. Moreover, experiment 2 was performed to evaluate the necessity of fan operation in the heating operation of TERP regarding energy consumption. The results revealed that the TERP without fan operation showed the higher coefficient of performance (COP) in the heating season. After determining the suitable heating operation of the TERP, prediction models for the heating capacity and power consumption of the TERP were developed using the response surface methodology. Both models exhibited good R2 values of >0.94 and were validated within 10% error bounds in experimental cases. These prediction models are expected to be utilized in whole-building simulation programs for estimating the energy consumption of TERPs in the heating mode.

A simplified PEM fuel cell model for building cogeneration applications

A simplified model of a polymer electrolyte membrane (PEM) fuel cell has been suggested for simulating packaged commercial fuel cell systems. PEM fuel cell systems are used in building cogeneration applications because of their high efficiency, low transmission loss and pollution, flexible scalability, and low noise. In conventional cogeneration applications, optimization techniques are utilized to size a system and determine proper operational strategies using simulations. To evaluate the performance of a building cogeneration system, a fuel cell model should be concise but accurate to allow its implementation in a whole-building simulation program. Some existing models are appropriate for building applications, but they have some limitations in modeling when a commercial packaged fuel cell system is used. To overcome these problems, a simplified fuel cell model is suggested for commercial packaged fuel cells by adding some new variables and validating through experimental and published data. This model is relatively simple compared to other models but can be easily utilized in some limited cases with performance predictions.

Development procedure of an air-source heat pump base-case simulation model for a code-compliant residential building

Computer simulation is widely used for analyzing building energy performance. A building simulation model is often verified by comparing the simulation results of the same building using different simulation programs, and thus developing a same building simulation model for different programs is inevitable for the comparative verification. This study proposed a step-by-step input parameter change procedure to develop a residential building model that complies with the 2009 International Energy Conservation Code (IECC). By implementing the procedure, this study developed the code-compliant residential air-source heat pump base-case model using two whole-building simulation programs: DOE-2.1e and eQUEST. The simulation results from DOE-2.1e and eQUEST at each step in the procedure were compared. To evaluate the accuracy and comparability of the final base-case model developed with the procedure, this study compared the simulation results of the same base-case model using Residential Energy Services Network (RESNET) accredited programs. The comparison showed that the differences in the annual total site energy use among the simulation programs were well-matched within 4.7%.

Energy savings analysis of fuel-cell microgeneration systems with ground source heat pumps in load-sharing buildings

This paper presents the potential energy savings of implementing a combination of fuel-cell microgeneration (FCMG) systems and ground source heat pump (GSHP) systems in load-sharing buildings. The energy modeling and simulation technology is used to evaluate the effectiveness of FCMG systems in buildings. There are a number of simulation programs to evaluate the performance of buildings with various electric, mechanical and thermal systems. However, it is still a challenge to model and simulate the FCMG systems using the existing whole-building simulation programs. This paper first overviews the current technology of simulation modeling of FCMG, and then presents the results of energy savings analyses obtained from the FCMG systems.

A frequency-domain regression method for estimating building foundation heat transfer

In this article, the Interzone Temperature Profile Estimation solutions are first utilized to carry out a frequency analysis for the foundation heat transfer to determine the effect of indoor air and ambient air temperature fluctuations on the variation of the foundation heat loss/gain. Then, a frequency-domain regression-based method is used to develop transfer functions for ground-coupled surfaces. The regression frequency-domain method is tested and validated using one-dimensional heat transfer for above-grade walls. It is found that the proposed method provides a more effective alternative than numerical methods to estimate conduction transfer function coefficients for building foundations and thus can be easily implemented in whole-building simulation programs.

Techno-economic assessment and optimization of Stirling engine micro-cogeneration systems in residential buildings

Micro-cogeneration offers numerous potential advantages for the supply of energy to residential buildings in the sense of improved energy efficiency and reduced environmental burdens. To realize these benefits, however, such systems must reduce energy costs, primary energy consumption, and CO 2 emissions relative to conventional heating systems. In this paper, we search for optimized strategies for the integration of a Stirling engine-based micro-cogeneration system in residential buildings by comparing the performance of various system configurations and operational strategies with that of a reference system, i.e. hydronic heating and a low temperature gas boiler in standard and passive house constructions located in different climates. The IDA-ICE whole-building simulation program is employed with the Stirling engine micro-cogeneration model that was developed by IEA/ECBCS Annex 42. In this way the dynamic effects of micro-cogeneration devices, such as warm-ups and shutdowns, are accounted for. This study contributes to the research by addressing hourly changes in the fuel mix used for central electricity generation and the utilization of thermal exhaust through heat recovery. Our results suggest that an optimally operated micro-cogeneration system encompassing heat recovery and appropriate thermal storage would result in a 3–5% decrease in primary energy consumption and CO 2 emissions when compared to a conventional hydronic heating system. Moreover, this configuration is capable of delivering annual savings in all the combinations of electricity and fuel price between 0.05 and 0.15 € kW h −1. As can be expected, these results are sensitive to the electrical energy supply mix, building type, and climate.

Energy and CO 2 emissions performance assessment of residential micro-cogeneration systems with dynamic whole-building simulation programs

Micro-cogeneration, also termed micro combined heat and power (MCHP) or residential cogeneration, is an emerging technology with the potential to provide energy efficiency and environmental benefits by reducing primary energy consumption and associated greenhouse gas emissions. The distributed generation nature of the technology also has the potential to reduce losses due to electrical transmission and distribution inefficiencies and to alleviate utility peak demand problems. Detailed MCHP models for whole-building simulation tools, developed in Annex 42 of the International Energy Agency (IEA) Energy Conservation in Buildings and Community Systems Programme, have been used to conduct a performance assessment study for a number of micro-cogeneration systems and residential buildings. Annual non-renewable primary energy (NRPE) demand and CO 2-equivalent (CO 2-eq) emissions were determined by simulation for different cogeneration technologies, namely natural gas-fuelled solid oxide (SOFC) and polymer electrolyte membrane fuel cells, Stirling and internal combustion engines. These were compared to the reference system with a gas boiler and electricity supply from the grid. A ground-coupled heat pump system was also analysed for comparison. The cogeneration units were integrated in single and multi-family houses of different energy standard levels. Two different electricity generation mixes were considered: European mix and combined cycle power plant (CCPP). For the MCHP devices, detailed dynamic component models as well as simplified performance map models were used, developed and calibrated with either results from laboratory experiments or with manufacturer data. The simulations were performed using the whole-building simulation programme TRNSYS, using domestic hot water and electric demand profiles specified in IEA Annex 42. Combinations of three demand levels were analyzed. In NRPE demand, for the European electricity mix, most MCHP systems offered reductions (up to 34%) in comparison with the gas boiler reference system and crediting the electricity exported to the grid. For the CCPP electricity generation mix, the largest NRPE reductions resulted for the ground-coupled heat pump systems (up to 29%). The maximum reduction with a cogeneration system was 14%. In terms of CO 2-eq emissions, most cogeneration systems offered reductions for the European electricity mix (up to 22%). However, maximum reductions resulted for the heat pump system (23%). For the CCPP mix, maximum reductions by far again resulted for the heat pump systems (up to 29%). The maximum reduction for a cogeneration system was achieved with the ICE system in the single-family house (14%).

Impact of Solar Model Selection on Building Energy Analysis for Kuwait

This paper summarizes the results of a comparative analysis for four models utilized to predict solar radiation in Kuwait City, Kuwait. The four models include the Kasten model, Zhang and Huang model, Muneer model, and neural network based model. The analysis was based on hourly measured solar data for Kuwait City. The measured hourly solar radiation data are obtained for the year 1994 and include global, direct, and diffuse solar radiations. Nonsolar weather data for the same year and site are obtained from the US National Climatic Data Center (NCDC). Weather files suitable for building energy simulation are developed using measuered data as well as predictions from the four solar models. A series of simulation analysis to determine the impact of solar model selection for the weather file on the energy uses predictions from a whole-building simulation program using office buildings in Kuwait. The results of the validation analysis and the simulation evaluation indicate that Zhang and Huang model is suitable for the predicting hourly solar radiation suitable for energy analysis of buildings in Kuwait.

A Model for Simulating the Thermal and Electrical Production of Small-Scale Solid-Oxide Fuel Cell Cogeneration Systems within Building Simulation Programs

A new model for predicting the thermal and electrical performance of solid-oxide fuel cell (SOFC) cogeneration devices for residential buildings has been developed and demonstrated. This is a system-level model that considers the thermodynamic performance of all components that consume energy and produce the thermal and electrical output of the SOFC-cogeneration device. The model relies heavily upon empirical information that can be acquired from the testing of coherent systems or components and is designed for operation at a time resolution that is in the order of minutes. Hence, it is appropriate for use in whole-building simulation programs, where it can be applied to assess the energy and greenhouse gas emission benefits of this nascent technology.

Modeling radiant heating and cooling systems: integration with a whole-building simulation program

One of the primary goals of the radiant heating and cooling community for the past two decades has been the fair and accurate comparison of radiant space conditioning systems with conventional forced air systems. The desire of the community was to give architects and engineers the ability to compare the energy consumption of different system types while requiring the systems to produce equivalent thermal environments. Unfortunately, radiant systems, though relatively simple in concept, are relatively complex from a modeling standpoint. A rigorous model must include fundamental laws of thermodynamics and properly account for heat transfer due to radiation, convection, and conduction. In addition, the model must do this in a generic way within a program that is capable of modeling buildings of any size and type and their associated HVAC systems. Such challenges are not easily overcome and require careful attention to detail as well as a solid foundation from which to build. This paper summarizes a model which addresses these concerns and provides a radiant system model within a popular building energy simulation program.

Whole-building simulation with symbolic DAE equations and general purpose solvers

In building simulation, as in several other domains, traditional monolithic simulation codes are still in dominance over simulators based on symbolic equations in a general modeling language. Introduced in 1998, IDA Indoor Climate and Energy has become the first widely spread thermal building performance simulator based on the new technology. Developing a full-fledged dynamic whole-building simulation program is a formidable endeavor in any setting and since the first beta version in 1997 a number of lessons have been learned. The paper shares some of these experiences concerning general program structure, models, performance, integration, GUI design and issues specifically linked to equation-based modeling using a variable timestep differential-algebraic (DAE) solver.

Applying the building energy simulation test (BESTEST) diagnostic method to verification of space conditioning equipment models used in whole-building energy simulation programs

Validation of building energy simulation programs consists of a combination of empirical validation, analytical verification, and comparative analysis techniques. An analytical verification and comparative diagnostic procedure was developed to test the ability of whole-building simulation programs to model the performance of unitary space-cooling equipment that is typically modeled using manufacturer design data presented as empirically derived performance maps. This procedure is based on the International Energy Agency (IEA) building energy simulation test (BESTEST) diagnostic method and systematically tests whole-building energy simulation software by comparing results from such software to analytical solutions that were developed for the test cases. Field trials of the new procedure were conducted by researchers from nations participating in the IEA Solar Heating and Cooling (SHC) Programme Task 22, using a number of detailed hourly simulation programs from Europe and the US, including: CA-SIS, CLIM2000, ENERGYPLUS, PROMETHEUS, TRNSYS-TUD, and two versions of DOE-2.1E. This article also includes discussion about simulation validation methodologies.