Net Zero Energy Building

Based on the Department of Energy definition for zero energy buildings a net zero energy building is an energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy. Reducing building energy consumption in new building construction or retrofit can be accomplished through various paths, including integrated design, energy efficiency retrofits, reduced plug loads and energy conservation programs. Reduced energy consumption makes it simpler and less expensive to meet the building’s energy needs with renewable sources of energy. ZEB energy accounting would include energy used for heating, cooling, ventilation, domestic hot water (DHW), indoor and outdoor lighting, plug loads, process energy and transportation within the building. Vehicle charging energy for transportation inside the building would be included in the energy accounting. On-site renewable energy may be exported through transmission means other than the electricity grid such as charging of electric vehicles used outside the building.
In order to focus efforts and resources to reduce residential energy consumption and greenhouse gas (GHG) emissions an accurately designed strategy with specific goals is required. For this purpose, first the Solar Building Research Network (SBRN) was established (2005-2010) followed by the Smart Net-zero Energy Buildings Strategic Research Network (SNEBRN) with Canadian government funding to bring together Canadian researchers to develop a framework for converting existing and newly built Canadian houses into net/near zero energy buildings.
A subproject was defined under the umbrella of these two networks to evaluate the techno-economic feasibility of retrofit options in the Canadian context and to develop strategies and guidelines that would lead to converting the Canadian housing stock into NZEBs. Canada has numerous regions that exhibit unique climatic, geographical and economic characteristics. Furthermore, the availability and price of fuels and energy sources are also diverse. Consequently, the housing stock in each region exhibits unique characteristics in terms of construction, envelope and HVAC systems and equipment, as well as primary and secondary GHG emissions due to end-use energy consumption. This high level of diversity requires unique approaches, policies and strategies to achieve, encourage and support the conversion of existing buildings into net zero energy buildings (NZEB). So far, a wide range of retrofit options including envelope modifications such as glazing and window shading upgrades, as well as installation of solar domestic hot water (SDHW) systems, phase change material (PCM) thermal energy storage, internal combustion engine (ICE) and Stirling engine (SE) based cogeneration systems, solar combisystem and air source heat pump were studied.
The Drake Landing Solar Community (DLSC) is a master planned neighbourhood in the Town of Okotoks, Alberta, Canada that demonstrate successful integration of high efficient and renewable energy technologies into Canadian houses. The first of its kind in North America, DLSC is heated by a district system designed to store abundant solar energy underground during the summer months and distribute the energy to each home for space heating needs during winter months. The system is unprecedented in the World, fulfilling ninety percent of each home’s space heating requirements from solar energy and resulting in less dependency on limited fossil fuels.

Building Performance Simulations

Building performance simulation is one of the most powerful tools to tackle growing concerns regarding the design and operation of built environment. Building performance simulation is multidisciplinary, problem oriented and wide in scope. It assumes dynamic boundary conditions and is normally based on numerical methods that aim to provide an approximate solution of a realistic model of complexity in real world.
ESP-r represents state of the art building performance simulations that considers heat transfer, air flow in the zone and between zones, water flow in hydronic systems, electric power and primary energy sources flow, moisture transfer and illumination. ESP-r employs a finite difference control volume approach for energy simulation. Building domain is discretized into control volumes and each control volume include finite difference nodes. Control volumes represent a wide range of building components such as air volume in thermal zones, opaque and transparent structures, solid-fluid interfaces and plant components. Governing equations are discretized based on the Crank-Nicolson finite difference method for all nodes in the control volume. The system of algebraic equations is solved using a simultaneous direct solution approach based on matrix partitioning and Gaussian elimination in each time step. The system of linearized mass balance equations is solved using the Newton-Raphson iterative method. ESP-r has been validated by a vast amount of available simulation data published in literature.
Various models for high efficient and renewable energy systems including solar combisystem, ICE and SE cogeneration systems, air source heat pump and building integrated PV and thermal systems are developed in ESP-r. The models were used to evaluate energy consumption, GHG emissions and annual cost energy cost of households.

Housing Stock Modeling

Residential sector is the third largest energy consumer in Canada with close to 17% of the energy consumption and 16% of greenhouse gas (GHG) emissions. Therefore, efforts to achieve energy savings and GHG emission reductions in Canada must address the residential sector. There are a variety of approaches and technologies to achieve net zero energy (NZE) status for existing buildings and communities. Their feasibility is impacted by parameters such as build ing size, envelope, and HVAC equipment as well as climate and geological characteristics, community density, primary energy availability and mix, and economic conditions. The impacts of these parameters and retrofit choices on the energy performance of buildings and communities are highly inter-related and complex. By strategically applying complementary technologies, it is possible to develop feasible approaches to achieve NZE status at the building as well as community levels.
To assess the feasibility of retrofit measure in a house energy consumption, GHG emissions and energy cost before and after retrofit should be analyzed. In a case of massive implementation of energy efficient retrofits housing stock model is an effective tool to accurately predict the system behavior. The bottom-up technique is based on the results of performance simulation of individual houses. The regional and national energy consumption and GHG emissions is evaluated by extrapolating results of a set of individual houses that represent the residential sector. Thus, bottom-up method is the sole option to evaluate the impacts of integrating new technologies in residential sector. The Canadian Hybrid Residential End-Use Energy and GHG Emissions Model (CHREM) was developed based on bottom-up method through the SBRN. CHREM capabilities is then expanded during the SBRN and SNEBRN to evaluate techno-economic impacts of several energy efficient retrofits including glazing modifications, window shading upgrades, PCM for thermal energy storage, SDHW heating system, IC and SE engine based cogeneration and solar combisystem in the CHS.
CHREM consists of six components that work together to provide predictions of the end-use energy consumption and GHG emission of the CHS. These components are:
  • The Canadian Single-Detached & Double/Row Housing Database (CSDDRD),
  • A neural network model of the appliances and lighting (AL) and DHW energy consumption of Canadian households,
  • A set of appliance, lighting and domestic hot water load profiles representing the usage profiles in Canadian households,
  • A high-resolution building energy simulation software (ESP-r) that is capable of accurately predicting the energy consumption of each house file in CSDDRD,
  • A model to estimate GHG emissions from marginal electricity generation in each province of Canada and for each month of the year,
  • A model to estimate GHG emissions from fossil fuels consumed in households.

Computational Fluid Dynamics (CFD)

Hydrogen purification through water-gas shift (WGS) is a preferable option in fuel processing for hydrogen fuel cells. A three-dimensional single channel model was developed to simulate the behavior of a water-gas shift micro reactor. A water-gas shift reaction rate was utilized to simulate the surface reaction of Pt/TiO2 catalyst. The feed gas composition was the outlet of a typical auto-thermal reforming (ATR) reactor. Numerous parametric studies were conducted to investigate the effect of feed gas temperature, gas space velocity and channel length on water-gas shift micro reactor performance. The study resulted in an optimum water-gas shift micro reactor design.
In another study a three-dimensional model was developed to simulate the behavior of a single-channel three-way catalytic converter. The flow regime was assumed to be steady and laminar, and the channel walls were considered as isothermal. A multi-step, global heterogeneous reaction mechanism with 16 reactions and 11 species was used to enhance the accuracy of the results. The chemical reactions were assumed to occur only on the reactor walls. The effect of the feed temperature on the conversion efficiency of the main pollutant components was studied. The light-off temperature for the stoichiometric A/F was found to be about 530 K for CO, NO and UHC, and 425 K for H2 conversion. The model was also applied to predict the effect of reactor length and inlet mixture space velocity on the conversion efficiency at two different temperatures. By using the same kinetics a well-stirred, unsteady model was also developed to identify the sensitivity of the multi-step kinetic mechanism to the mixture composition. The effect of mole fraction variation of each species on the conversion of other mixture components was investigated.
The CFD codes were developed based on the finite volume method using the SIMPLE algorithm for the coupling of pressure and velocity domains. To linearize the convective terms the power-law scheme was utilized, and a central difference scheme was applied to linearize the diffusive terms. The algebraic system of equations were solved with the Jacobian point-to-point iteration method, except for pressure correction equation which was solved by exploitation of the Gauss-Seidel iteration method. Furthermore, the reaction kinetic rate equation was linearized for each species. The developed models were validated against available experimental data for stoichiometric operating conditions.

Proton Exchange Membrane (PEM) Fuel Cell

Direct Methanol Fuel Cell (DMFC) is a PEM fuel cell that consumes methanol directly as the fuel source. A three dimensional model was developed to simulate a vapor feed DMFC. Flow was assumed to be steady and fully developed at channel exit and the walls were adiabatic.
Governing equations include conservation of mass, momentum, energy and species as well as electrochemical equations. Proper source terms were added in porous medium and the Darcy’s equation was considered in catalyst and gas diffusion layer (GDL).
Meyers and Newman Tafel like equation was used to simulate electrochemical reactions in DMFC. Dissolved water equation was considered in the investigation and dissolved water distribution was obtained in the membrane. Methanol cross over was simulated with the consideration of methanol full consumption at the membrane and cathode catalyst layer interface. Effects of electro osmotic drag and diffusion were considered in methanol cross over model.
The cell was simulated continuously through different layers. So, satisfying boundary conditions at walls, flow channel inlet and outlet of both anode and cathode were only required. A Cartesian orthogonal grid with appropriate stretch at high gradient points was used. Central difference and power law methods were used for discretization of diffusive terms and convective terms, respectively. SIMPLE algorithm was used for coupling of velocity and pressure field. An in house FORTRAN code was developed to solve governing equations. The results were validated against available experimental results in literature. A base line simulation was conducted and obtained results were analyzed.
A series of case studies were conducted to investigate effects of temperature, pressure, relative humidity at anode inlet at constant flow rate condition and stoichiometry ratio, GDL porosity and membrane thickness at constant stoichiometry ratio condition at flow channel inlet. Polarization curve and power density curve were studied at different cases.