Two-zone Building Research Articles

Effect of the position of the PCM on the thermal behavior of a multilayer wall of a building in summer period

In Tunisia, this sector will occupy the first place in terms of energy consumption in a few years. To remedy this, we must move towards renewable energy sources that partially or totally replace fossil fuels such as oil, coal, etc., and developing ways to make the building more energy efficient in order to save and reduce energy consumption while working to ensure better comfort for the occupant. This work deals with a numerical study based on the nodal method using software (DIGITAL Visual FORTRAN95) to predict the effect of integrating a PCM layer on the thermal behavior of a two-zone building. The effect of the location of the PCM in the wall on its thermal behavior is demonstrated, and the thermal performance of the PCM is demonstrated by considering energy indicators based on the indoor air temperature and the indoor and outdoor surface temperature of the building. It was concluded that the incorporation of PCM into the building envelope increased its thermal inertia, directly influencing the perception of thermal comfort. This indicates that the use of PCM can contribute to improved thermal performance and a more comfortable indoor environment.

Data-driven estimation of time-dependent solar gain coefficient in a two-zone building with synthetic occupants: Two B-splines integrated grey-box modeling approaches

Solar gains are a significant share of the heat input to the buildings’ interior energy balance. Therefore, it is important to estimate the dynamic details of solar gains precisely for predictive works, such as in model predictive control (MPC). Effective solutions to gauge the dynamic details of solar gains are still lacking. To fill this gap, a data-driven B-splines integrated grey-box modelling technique has been presented recently, and its potential to estimate dynamic solar gains based on the limited size of low-frequency on-site data has been demonstrated in a simplified one-zone building case, with a single level on/off heating input signal. Its robustness is examined in a two-zone building with heating inputs of synthetic occupants, in this study. Two thermal zones are established to mimic day and night zones in a test building, and corresponding synthetic occupancy profiles and thermostat systems have been implemented in each zone. This study shows that the B-spline integrated grey-box model is useful in this more realistic case considering the impacts of occupants, but demands more advanced modellings. Specifically, two updated solutions on B-splines integrated grey-box modelling are offered, although challenges in the model selection are noted, mainly attributed to the low statistical significance of solar gain, which had fortunately almost no impact on the effectiveness of the proposed techniques to characterize solar gain dynamics. The reported challenge could be further investigated in the future based on more case studies.

An improved explicit scheme for whole-building hygrothermal simulation

Implicit schemes require important sub-iterations when dealing with highly nonlinear problems such as the combined heat and moisture transfer through porous building elements. The computational cost rises significantly when the whole-building is simulated, especially when there is important coupling among the building elements themselves with neighbouring zones and with HVAC (Heating Ventilation and Air Conditioning) systems. On the other hand, the classical Euler explicit scheme is generally not used because its stability condition imposes very fine time discretisation. Hence, this paper explores the use of an improved explicit approach - the Dufort-Frankel scheme - to overcome the disadvantage of the classical explicit one and to bring benefits that cannot be obtained by implicit methods. The Dufort-Frankel approach is first compared to the classical Euler implicit and explicit schemes to compute the solution of nonlinear heat and moisture transfer through porous materials. Then, the analysis of the Dufort-Frankel unconditionally stable explicit scheme is extended to the coupled heat and moisture balances on the scale of a one- and a two-zone building models. The Dufort-Frankel scheme has the benefits of being unconditionally stable, second-order accurate in time O(dt^2) and to compute explicitly the solution at each time step, avoiding costly sub-iterations. This approach may reduce the computational cost by twenty, as well as it may enable perfect synchronism for whole-building simulation and co-simulation.

Three-dimensional steady-state ground heat transfer for multi-zone buildings

A closed-form analytical solution is presented for steady-state ground heat transfer from buildings with multiple rectangular floors at different temperatures. The heat transfer results obtained using the analytical solution were found to be in close agreement with those obtained using a commercial numerical simulation software package for a two-zone building. For the case study, the ground heat transfer calculation using the analytical solution is several orders faster than the numerical simulation method. The analytical solution provides a fast and accurate calculation method for the steady-state ground heat transfer components for multi-zone buildings.

Contaminant ingress into multizone buildings: An analytical state-space approach

The ingress of exterior contaminants into buildings is often assessed by treating the building interior as a single well-mixed space. Multizone modelling provides an alternative way of representing buildings that can estimate concentration time series in different internal locations. A state-space approach is adopted to represent the concentration dynamics within multizone buildings. Analysis based on this approach is used to demonstrate that the exposure in every interior location is limited to the exterior exposure in the absence of removal mechanisms. Estimates are also developed for the short term maximum concentration and exposure in a multizone building in response to a step-change in concentration. These have considerable potential for practical use. The analytical development is demonstrated using a simple two-zone building with an inner zone and a range of existing multizone models of residential buildings. Quantitative measures are provided of the standard deviation of concentration and exposure within a range of residential multizone buildings. Ratios of the maximum short term concentrations and exposures to single zone building estimates are also provided for the same buildings.

Simultaneous solutions of coupled thermal airflow problem for natural ventilation in buildings

Natural and hybrid ventilation can be sustainable building ventilation strategies, where airflow is driven naturally by thermal buoyancy and/or wind forces other than pure mechanical means. The simulation and design of these systems need to consider the combined impact of thermal and airflow transport behaviors. The numerical solution of such combined thermal airflow problems often employs a segregate and iterative approach. Either the air temperatures in the thermal problem or the air pressures in the airflow problem are solved separately with the other parameter known from a previous iteration. The newly solved parameters are then substituted successively into the other calculation. For highly coupled thermal airflow problems, the segregate method can cause solution fluctuation or even divergence when relaxation factors are not carefully selected to avoid abrupt changes of air parameters in the successive substitution procedure. This article investigated two nonsegregate methods to solve thermal and airflow problems simultaneously. In the fully simultaneous method, air temperatures and pressures for all rooms of a building are solved simultaneously using a single Jacobian matrix. In the semi-simultaneous method, a Jacobian matrix for the air temperature and pressure of one room is solved when air temperatures and pressures of other rooms are kept as constants. The same procedure is then repeated for each room of a building. In both cases, relaxation factors are not required. The simultaneous solution methods are demonstrated for a two-zone building with thermal buoyancy-driven flows and validated for an experimental study of combined wind and buoyancy forces in a light well. It was shown that the simultaneous solvers provide stable solutions without using any relaxation in both cases. The predicted results also agree reasonably well with the experimental data.

Calibration of HVAC equipment PID coefficients for energy conservation

The combination of proportional–integral–derivative (PID) coefficients for a set of equipment in a heating, ventilating, and air conditioning (HVAC) system has an impact on the overall system energy consumption as well as the ability for the system to maintain temperature setpoints. A simple calibration methodology is discussed where successive optimization on the set of proportional, integral, and derivative coefficients is performed to reduce the energy consumption of the system. The calibration methodology discussed here is applied to a two-zone building for a summer design day. The results show that calibration of proportional coefficients can reduce the system energy consumption by up to 29% and can improve meeting temperature setpoints by up to 45%. Successive calibration of integral coefficients can increase the energy savings up to 35% and can improve meeting temperature setpoints by up to 52%. The successive calibration of derivative coefficients has a negligible impact on the energy conservation and the ability to meet temperature setpoints. The re-calibration of proportional coefficients with the new values of integral and derivative coefficients yields an additional 2.3% increase in energy savings.

Nonlinear Dynamic Analysis of Natural Ventilation in a Two-Zone Building: Part A—Theoretical Analysis

The nonlinear behavior and solution multiplicity of airflow and natural ventilation in multi-zone buildings are significant for building ventilation and smoke control design. A two-zone building with four openings is considered in this study. Nonlinear dynamic behaviors are analyzed, and we report on the fixed points (steady-state solutions) and their stability and provide a description of the bifurcation of the fixed points and an estimation of the separating curve between two stable fixed points. Different flow modes were identified with the same physical and boundary conditions. A multi-zone airflow and thermal coupled program was also used to evaluate the results. There was good agreement between the numerical and theoretical solutions, except that the unsteady-state solutions could not be predicted by the multi-zone program. These results may have significant implications for multi-zone modeling of natural ventilation and smoke or contaminant spread in buildings, as well as allow us to gain a good understanding of physical flow behavior and solution multiplicity.

Nonlinear Dynamic Aalysis of Natural Ventilation in a Two-Zone Building: Part B—CFD Simulations

Computational fluid dynamics (CFD) is a widely accepted design and analysis tool for building ventilation. This paper presents a simple method using commercial CFD software to obtain the solution multiplicity characteristics of laminar and turbulent indoor airflows. The method is known as the two-way continuation simulation, which has been developed for identifying multiple solutions in computational fluid dynamics. We consider airflows in a simple two-zone building with four openings, where one of the zones has a constant heat source, modeling the heat or pollutant generation and spread in buildings governed by natural stack forces. Using the new two-way continuation simulation method, we describe the flow variation and physical mechanisms at different heights in the two zones for different Rayleigh numbers. The results show that two steady solutions with the same boundary conditions and building geometry can be obtained step-by-step in the simulation process. Flow multiplicity can also be found as the height ratio of the two zones increases or decreases. As an important physical parameter, the Rayleigh number can also have some significant influences on the flow bifurcation or variation. Compared with conventional CFD simulation, the two-way continuation simulation method can effectively identify the possible existence of multiple solutions of indoor airflows and pollutant spread with different initial conditions. One shortcoming of the two-way method is that it cannot identify unstable solutions.

Some examples of solution multiplicity in natural ventilation

This paper shows that under certain conditions, multiple solutions for the flow rate exist in a natural ventilation system, induced by the non-linear interaction between buoyancy and wind forces. Under certain physical simplifications, the system is governed in steady state by a non-linear algebraic equation or a system of equations. Three examples are given here: a single-zone building with two openings, a channel with two end openings, and a two-zone building with two openings in each zone. Analytical and numerical solutions are presented. It is shown that in all three cases the flow rate exhibits hysteresis. These results have significant implications for multi-zone modelling of natural ventilation and smoke spread in buildings. An experimental investigation using a small-scale model in a water tunnel confirms that two steady-state solutions exist for a single-zone building.

Nonlinear Least-Squares Minimization Applied to Tracer Gas Decay for Determining Airflow Rates in a Two-Zone Building

We developed a method based on tracer gas decay measurements to quantify the airflow rates, including the interzonal airflows, in a two-zone building: different tracer gases were simultaneously pulse-injected into each of the two zones and the evolution of the gas concentrations in each zone was measured; theoretical concentration profiles obtained by solving dynamic material-balance equations for two coupled, well-mixed zones were fit to the experimental data using nonlinear least-squares minimization; and estimates of the airflow rates were iteratively refined until a best fit was achieved between the model and the data. We conducted experiments validating the method in two full-sized rooms of a test house. Airflows were controlled using blowers, and mixing was ensured by the use of fans. Airflow rates inferred by the tracer gas technique agreed with imposed airflow rates within an average absolute error of 8%. Results are also reported for two experiments conducted in the same structure under uncontrolled conditions. Goodness-of-fit tests revealed no statistically significant differences between measured tracer gas concentrations and theoretical concentration profiles constructed using the least-squares parameter estimates.

Temperature control of multizone indoor spaces based on forecast and actual loads

This paper presents the design of a multivariable controller for zone temperature control. A two-zone building with a fan-coil heating system is considered and a multi-input controller is designed. The controller tracks the reference trajectories which are generated based on forecasted outdoor temperature and at the same time regulates the zone temperatures in response to the actual variations in outdoor temperatures. Results showing the closed-loop responses of the system under realistic operating conditions are given.

An Energy Delivery Efficiency Index to Evaluate Simultaneous Heating and Cooling Effects in Large Commercial Buildings

HVAC systems of large commercial buildings consume energy in excess of the sum total of the building loads. This excess energy use is due to the fact that a single air-handler unit in a HVAC system, having to provide conditioned air at different supply temperatures to different zones in the building, can do so only by resorting to either (a) a certain amount of mixing of cold and hot air streams as in dual-duct systems or (b) to terminal reheating in single-duct systems. This mixing of cold and hot air streams or terminal reheating result in an energy penalty which can be minimized by say, converting a constant air volume (CAV) system to a variable air volume (VAV) system, but it cannot be entirely eliminated. This paper proposes an index, called the Energy Delivery Efficiency (EDE), which characterizes this penalty and rates the energy performance of HVAC systems on an absolute scale. We develop the mathematical basis of the EDE approach for both one-zone and two-zone buildings, that allows determining the variation of the ideal EDE with outdoor temperature for a specific building. Year-long measured whole-building cooling and heating energy use data from two retrofitted buildings are finally used to illustrate differences between actual EDE plots of CAV and VAV systems, how they compare with the ideal EDE of a two-zone building, and how the approach can provide diagnostic insights into HVAC system efficiency in specific buildings.

Temperature and humidity control of indoor environmental spaces

The temperature and humidity control of indoor environmental spaces is studied. A constant volume heating and humidification system consisting of a boiler, a heating coil, ductwork and a fan, and a two-zone building is considered. The temperature and humidity control models are developed and multivariable controllers are designed. The closed loop system responses subject to step disturbances in outdoor temperatures and relative humidity are given. Also, results showing a typical-day operating performance of the closed loop system are given.