Szabolcs Varga

Characterisation of Thermal Diode Panels For Use in the Cooling Season in Buildings

Thermal diode panels, incorporating heat pipes, were tested under cooling season conditions. Their thermophysical properties were evaluated by measuring temperature distributions in an experimental test facility and by using numerical simulation together with an optimisation procedure. The method allowed the quantification of thermal characteristics for both operating modes: forward and backward heat transfer. Forward heat transfer led to an apparent thermal conductivity up to five times, the one for backward mode.

Evaluation of the use of artificial neural networks for the simulation of hybrid solar collectors

Abstract In the last decade, Artificial Neural Networks (ANNs) have been receiving an increasing attention for simulating engineering systems due to some interesting characteristics such as learning capability, fault tolerance, speed and nonlinearity. This article describes an alternative approach to assess two types of hybrid solar collector/heat pipe systems (plate heat pipe type and tube heat pipe type) using ANNs. Multiple Layer Perceptrons (MLPs) and Radial Basis Networks (RBFs) were considered. The networks were trained using results from mathematical models generated by Monte Carlo simulation. The mathematical models were based on energy balances and resulted in a system of nonlinear equations. The solution of the models was very sensitive to initial estimates, and convergence was not obtained under certain conditions. Between the two neural models, MLPs performed slightly better than RBFs. It can be concluded that similar configurations were adequate for both collector systems. It was found that ANNs simulated both collector efficiency and heat output with high accuracy when “unseen” data were presented to the networks. An important advantage of a trained ANN over the mathematical models is that convergence is not an issue and the result is obtained almost instantaneously. #Contributed by the Organizing Committee for the First International Exergy, Energy and Environment Symposium (IEEES-1). Paper presented at IEEES-1, Izmir, Turkey, 13-17 July 2003. Manuscript received by IJGE on 2003-12-04; final revision received on 2004-02-17. Corresponding guest editors: I. Dincer and A. Hepbasli.

Ventilation terminals for use with light pipes in buildings : a CFD study

A CFD modelling approach was developed for studying natural ventilation in combined light/vent pipes. Both stack and external wind effects were considered. Practical details of models are presented. Different pipe terminals were considered and the results obtained allow a comparison between them for different wind velocities and angles.

Experimental and Numerical Analysis of Natural Ventilation With Combined Light/Vent Pipes

Abstract Both experimental and numerical procedures were developed for studying natural ventilation in combined light/vent pipes. An experimental tracer gas technique was used to measure ventilation rates in a test cell. Two different pipe terminals were simultaneously used, dividing the cell in two identical volumes, in order to allow a direct comparison of their performance. A 3D numerical model was developed using ansys5.5/flotran . Experimental and numerical results are shown and compared. It is concluded that pipe terminals significantly enhance ventilation, that one type of terminal has a better performance and that the numerical model underestimates measured flows.

Combining light pipe and stack ventilation — some development aspects

The work aims to develop a light-vent unit combining light pipe technology and passive stack ventilation by utilising the light pipe as an exhaust stack. This system can be further integrated with a novel design of heat pipe to heat or cool the building. The work contains several aspects related to the use of light pipes and stack ventilation. A study of the statistical information on the main building types and their morphology in several European countries has helped to determine the most suitable regions for this technology. It seems, that the most potential building types are educational, office and retail buildings. In order to enhance the natural stack ventilation and to prevent reverse flow, a wind terminal is installed on the top of the light-vent pipe. Several types of terminals have been investigated. The best performing ones, a variant of the h-pot and the umbrella type have been investigated further. The umbrella type gives slightly higher air flows but both types have a positive effect on the functioning of the ventilation. Also the development of a light collector is included in the work. Results of modelling have shown, that for the concept to be economically feasible, a well performing collector is essential. It seems, that static collectors are not able to guide enough light into the pipe and that a one-axis or two-axis tracking system is needed.

Development and performance of an advanced ejector cooling system for a sustainable built environment

Ejector refrigeration is a promising technology for the integration into solar driven cooling systems because of its relative simplicity and low initial cost. The major drawback of such a system is associated to its relatively low coefficient of performance (COP) under variable operating conditions. In order to overcome this problem, an advanced ejector was developed that changes its geometrical features depending on the upstream and downstream conditions. This paper provides a short overview of the development process and results of a small cooling capacity (1.5 kW) solar driven cooling system using a variable geometry ejector. During the design steps, a number of theoretical works have been carried out, including the selection of the working fluid, the determination of the geometrical requirements and prototype design. Based on the analysis, R600a was selected as working fluid. A prototype was constructed with two independent variable geometrical factors: the area ratio and the nozzle exit position. A test rig was also assembled in order to test the ejector performance under controlled laboratory conditions and to elaborate a control algorithm for the variable geometry. Ejector performance was assessed by calculation of cooling cycle COP, entrainment ratio and critical back pressure. The results show that for a condenser pressure of 3 bar, an 80% increase in the COP was obtained when compared to the performance of a fixed geometry ejector. Experimental COP values varied between 0.4 and 0.8, depending on operating conditions. Currently the cooling cycle is being integrated into a solar driven demonstration site for long term “in situ” assessment.

A Trnsys simulation of a solar-driven ejector air conditioning system with an integrated PCM cold storage

In this paper, the development of a TRNSYS model, for the simulation of a solar driven ejector cooling system with an integrated PCM cold storage is presented. The simulations were carried out with the aim of satisfying the cooling needs of a 140 m3 space during the summer season in Tunis, Tunisia. The system is composed of three main subsystems, which include: a solar loop, an ejector cycle and a PCM cold storage tank. The latter is connected to the air-conditioned space. The influence of applying cold storage on the system performance was investigated. It was found that the system COP increased compared to a system without cold storage. An optimal storage volume of 1000 l was identified resulting in the highest cooling COP and highest indoor comfort (95% of the time with a room temperature below 26°C). The maximum COP and solar thermal ratio (STR) were 0.193 and 0.097, respectively.