Nathan Mendes

Moisture effects on conduction loads

Abstract The effects of moisture on sensible and latent conduction loads are shown by using a heat and mass transfer model with variable material properties, under varying boundary conditions. This model was then simplified to reduce calculation time and used to predict conduction peak load (CPL) and yearly integrated wall conduction heat flux (YHF) in three different cities: Singapore (hot/humid), Seattle (cold/humid) and Phoenix (hot/dry). The room air temperature and relative humidity were calculated with the building energy simulation program DOE-2.1E. The materials studied were aerated cellular concrete (ACC), brick (BRK), lime mortar (LMT) and wood. It is shown that the effects of moisture can be very significant and that simplified mathematical models can reduce the calculation time with varying effects on accuracy.

A new mathematical method to solve highly coupled equations of heat and mass transfer in porous media

Heat and mass conservation equations in porous media are coupled and, in general, solved, iteratively, by using the values of temperature and moisture content from previous iteration to calculate source terms. This is the traditional mathematical method and numerical stability is only ensured for small time steps, depending on the source terms magnitudes. This is specially important, when material properties have strong variations with moisture content. This paper presents an unconditionally stable numerical method, conceived accordingly to a new methodology, which considers: (i) linearization of the term giving the vapor exchanged at the boundaries in terms of temperature and moisture content and (ii) introduction of a new generic algorithm to solve, simultaneously, the governing equations, for each time step. Numerical stability of these two methods are compared and it is shown that, in addition to avoid numerical unstability for arbitrary time steps and material properties, convergence is always quickly reached, in the presently proposed calculation method.

Combined Heat, Air and Moisture (HAM) Transfer Model for Porous Building Materials

In the building science area, mathematical models are developed to provide better indoor thermal comfort with lower energy consumption. Although the fact moisture and air transfer can strongly affect the temperature distribution within constructions, whole-building simulation codes do not take into account the convective air transport in porous materials. In this way, this article presents a heat, air, and moisture (HAM) transfer model based on driving potentials of temperature, air pressure, and water vapor pressure gradients for consolidated porous material in both pendular and funicular states. The solution of the set of governing equations has been simultaneously obtained using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three potentials. To conclude, results are presented showing the impact of convective terms on the HAM transfer through a two-layer porous building envelope.

PMV-Based Predictive Algorithms for Controlling Thermal Comfort in Building Plants

The present paper is focused on thermal comfort control for building occupants. Thermal comfort is addressed here by the use of PMV index for such measurement. Based on PMV, two predictive strategies characterized by having terminal constraints are proposed and compared. The first is based on generating a temperature set-point signal that optimizes the building (single zone) internal PMV value. The second includes the PMV model in the controller prediction computations, generating a non-linear PMV model having Wiener structure. In both cases, the linear part of the model is built by using Laguerre basis. Simulation results, conducted with actual climate data, illustrate the performance of the thermal comfort control algorithms.

Cooperação tecnológica universidade-empresa para eficiência energética: um estudo de caso

The central aim of this article is to demonstrate the development and characteristics of an interinstitutional relationship with a view to perfecting existing technology in reducing the consumption of energy in household refrigerators and also to show the adequacy of the case studied for the most recent preconceived ideas concerning the development of Etzkowitzs Triple Helix. The study refers to the technological cooperation between universities and industries and the presentation of a successful experience in which the joint action of individual institutions led to technological gains for both parties. The data was collected from individual in depth interviews with the research coordinators at the both company and the university. The results obtained showed not only the efficiency of the joint research but also the success of the cooperation and the identification of the practice of new positions raised in recent studies. Thus, by studying the experience of these institutions in technological cooperation, we can see that the productivity and efficiency of the cooperation between the university and the company open up possibilities for contributing to the technological development of the country.

THERMAL COMFORT BASED PREDICTIVE CONTROLLERS FOR BUILDING HEATING SYSTEMS

Abstract This work is focused on indoor thermal comfort control problem in buildings equipped with HVAC (Heating Ventilation and Air Conditioning) systems. The occupants thermal comfort is addressed here by a comfort zone in the psychometric chart and the PMV (Predict Mean Vote) index. In this context, three control algorithms are proposed by using only-one-actuator system associated to a heating equipment. The methods are based on the model predictive control scheme and on the improvement of indices related to occupants thermal comfort sensation. Simulation results – obtained by using the weather data file for the city of Curitiba, Brazil – are presented to validate the proposed methodology in terms of room air temperature, relative humidity and PMV control.

Stable explicit schemes for simulation of nonlinear moisture transfer in porous materials

Implicit schemes have been extensively used in building physics to compute the solution of moisture diffusion problems in porous materials for improving stability conditions. Nevertheless, these schemes require important sub-iterations when treating nonlinear problems. To overcome this disadvantage, this paper explores the use of improved explicit schemes, such as Dufort–Frankel, Crank–Nicolson and hyperbolization approaches. A first case study has been considered with the hypothesis of linear transfer. The Dufort–Frankel, Crank–Nicolson and hyperbolization schemes were compared to the classical Euler explicit scheme and to a reference solution. Results have shown that the hyperbolization scheme has a stability condition higher than the standard Courant–Friedrichs–Lewy condition. The error of this schemes depends on the parameter τ representing the hyperbolicity magnitude added into the equation. The Dufort–Frankel scheme has the advantages of being unconditionally stable and is preferable for nonlinear transfer, which is the three others cases studies. Results have shown the error is proportional to . A modified Crank–Nicolson scheme has been also studied in order to avoid sub-iterations to treat the nonlinearities at each time step. The main advantages of the Dufort–Frankel scheme are (i) to be twice faster than the Crank–Nicolson approach; (ii) to compute explicitly the solution at each time step; (iii) to be unconditionally stable and (iv) easier to parallelize on high-performance computer systems. Although the approach is unconditionally stable, the choice of the time discretization remains an important issue to accurately represent the physical phenomena.

Accurate numerical simulation of moisture front in porous material

When comparing measurements to numerical simulations of moisture transfer through porous materials a rush of the experimental moisture front is commonly observed in several works shown in the literature, with transient models that consider only the diffusion process. Thus, to overcome the discrepancies between the experimental and the numerical models, this paper proposes to include the moisture advection transfer in the governing equation. To solve the advection-diffusion differential equation, it is first proposed two efficient numerical schemes and their efficiencies are investigated for both linear and nonlinear cases. The first scheme, Scharfetter-Gummel (SG), presents a Courant-Friedrichs-Lewy (CFL) condition but is more accurate and faster than the second scheme, the well-known Crank-Nicolson approach. Furthermore, the SG scheme has the advantages of being well-balanced and asymptotically preserved. Then, to conclude, results of the convective moisture transfer problem obtained with the SG numerical scheme are compared to experimental data from the literature. The inclusion of an advective term in the model may clearly lead to better results than purely diffusive models.

Comparative Analysis of Response-factor and Finite-volume based Methods for predicting Heat and Moisture Transfer through Porous Building Materials:

Many of the now well-known building energy simulation programs use the response factor method developed in the early 1970s by Stephenson and Mitalas. These are TRNSYS, EnergyPlus, Blast, and DOE-2, to name but a few. Others, such as PowerDomus, ESP-r, and BSim, perform finite-volume or finite-difference calculations to solve the heat and mass transfer through the building envelope. These two different approaches are known to have strengths and weaknesses. The main objective of the present exercise is to compare the prediction of both methods. A two-step procedure is employed here. The first deals with the pure thermal problem, i.e., without moisture calculation. Three different cases of increasing complexity are studied and compared to analytical solutions. The second step focuses on the moisture problem alone by comparing the responses obtained with a two-layer buffer storage model and a finite-volume discretization for moisture transfer. Results show that time step values are determinant even for pure thermal cases where the classical value of 1 h can lead to notable errors. For problems with moisture sorption in the wall, it has been shown that grid refinement is a very decisive parameter, while the time step has to be set, to unusually small values, to achieve a good response.

University-industry technological cooperation for energy efficiency: a case study

The central aim of this article is to demonstrate the development and characteristics of an inter-institutional relationship with a view to perfecting existing technology in reducing the consumption of energy in household refrigerators and also to show the adequacy of the case studied for the most recent preconceived ideas concerning the development of Etzkowitzs Triple Helix. The study refers to the technological cooperation between universities and industries and the presentation of a successful experience in which the joint action of individual institutions led to technological gains for both parties. The data was collected from individual in depth interviews with the research coordinators at the both company and the university. The results obtained showed not only the efficiency of the joint research but also the success of the cooperation and the identification of the practice of new positions raised in recent studies. Thus, by studying the experience of these institutions in technological cooperation, we can see that the productivity and efficiency of the cooperation between the university and the company open up possibilities for contributing to the technological development of the country.