Armando C. Oliveira

Solar chimneys: simulation and experiment

The use of solar chimneys in buildings is one way to increment natural ventilation and, as a consequence, to improve indoor air quality. They are similar to conventional chimneys except that the south wall is replaced by a glazing. In order to compare the behaviour of a solar chimney with a conventional one, one of each was built in Porto. Results of measurements carried out in both chimneys are shown in this paper, as well as results of a thermal model specially developed for simulating solar chimneys, taking into account the wind effect. It was concluded that there is a significant increase in ventilation rate with solar chimneys, and that the thermal model predicts with good accuracy the measurements carried out.

Natural refrigerants for refrigeration and air-conditioning systems

In the past, several natural working fluids were used in refrigeration and air-conditioning systems. However, the appearance of CFCs caused a drastic reduction in the utilisation of those fluids. Understanding of the environmental damage of CFCs focused attention on alternative refrigerants. From these, the ones that have minimum (or nil) environmental impact are natural ones. This paper presents a review of the application of the main natural refrigerants, for refrigeration and air-conditioning systems, as an alternative to synthetic new refrigerants (HFCs). Recent research and development in this area is analysed and various cycles are considered.

Thermal behaviour of closed wet cooling towers for use with chilled ceilings

A new closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested. Experimental correlations were obtained for mass and heat transfer coefficients. Existing thermal models for this type of cooling tower were found to predict well thermal performance, if the above correlations are used.

A combined heat and power system for buildings driven by solar energy and gas

A novel hybrid solar/gas system intended to provide cooling/heating and electricity generation for buildings was developed. The system is based on the combination of an ejector heat pump cycle with a Rankine cycle. It is driven by solar energy and supplemented by a gas burner. The system also uses an environmentally friendly refrigerant to have minimal impact on the environment. Results of system computer modelling, prototype tests and economic analysis are reported. The system was judged to be viable and reliable. Technical improvements still have to be achieved to improve system economics.

Thermal performance of a novel air conditioning system using a liquid desiccant

A new air conditioning system using a liquid desiccant and needle impeller rotors has been modelled. Experimental data obtained for different components, i.e., evaporators and absorber, were used in the model. System performance was quantified through the definition of thermal coefficient of performance. Simulation results show the effect of different system parameters: ambient temperature, ambient humidity and heat exchanger efficiency.

Concentrated solar power for renewable electricity and hydrogen production from water—a review

Todays world suffers from an increasing dependence on fossil fuels, either for electricity production, transportation or reagent for the chemical industry. A technological revolution in hydrogen and electricity production is important to support the future needs and lead the world towards a better future. For that, technological and economical barriers have to be broken. Concentrated solar power (CSP) has been proving to be a valid means to start this revolution and produce electricity and hydrogen from completely renewable sources—water and the sun. Although solid steps should be taken to solve the current limitations and increase the technical and economical viability of these projects, there are conditions to begin this revolution using factual bridges from the current fossil technologies to renewable technologies.

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.

Analysis of Energetic, Design and Operational Criteria When Choosing an Adequate Working Fluid for Small ORC Systems

Small cogeneration (CHP) systems may lead to a significant reduction of primary energy consumption and harmful emissions. Low temperature Rankine cycles, that can be assisted by solar energy, are a possible solution for producing combined electricity and useful heat. These cycles usually use an organic working fluid. This study presents an analysis of the energetic, design and operational features, that have to be taken into account when choosing an adequate working fluid for these Organic Rankine Cycles (ORC). When using renewable energies as a heat source, like solar or geothermal, the cycles may operate at temperatures between 120°C and 230°C. A system producing 5 kW of electricity was considered as a basis of comparison. Several fluids were analysed: n-dodecane, water, toluene, cyclohexane, n-pentane, HFE7100, R123, isobutane and R245fa. The organic dry fluids, with a positive slope of the saturated vapor curve in a T-s diagram, are in principle desirable for low temperature applications, simplifying turbine design. The degree to which the fluids are drying, is generally related to their molecular weight or molecular complexity. Practical issues, like thermal stability, toxicity, flammability and cost are considered. The thermodynamic cycle efficiency is also important. The saturated vapor specific volume gives an indication of condenser size, which is related to system initial cost. A super-atmospheric (>100 kPa) saturation pressure eliminates infiltration gases, which is important for operational reasons, because infiltration reduces system efficiency. The degree of superheating was optimized for maximum cycle efficiency, with a quadratic approximation method. This optimization makes it possible to decide if it is better to have saturated vapor or superheated vapor at turbine inlet, for a fixed turbine inlet temperature. For a heat source temperature of 120°C, only toluene and isobutane present a small advantage in superheating. It is difficult to find the best fluid, which has simultaneously: high cycle efficiency, low vapor specific volume at turbine outlet, super-atmospheric saturation pressure, good thermal stability, small environmental impact, small toxicity and no flame propagation. From the point of view of cycle efficiency, n-dodecane presents the best performance. However, this fluid presents the highest saturated vapor specific volume (resulting in a larger condenser) and the smallest condenser saturation pressure (resulting in infiltration of gases). The best candidates for the cycle regarding all the aspects are: toluene, cyclohexane and n-pentane. Comparing the three fluids, toluene presents the highest efficiency, the highest impact in environment and the biggest vapor specific volume. N-pentane presents the smallest cycle efficiency and smallest vapor specific volume, but is the unique fluid with super-atmospheric saturation pressure. Cyclohexane is the fluid with lowest impact in environment.Copyright

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.

A new simplified method for evaluating the thermal behaviour of direct gain passive solar buildings

Abstract A new simplified method for calculating the monthly solar heating fraction of direct gain buildings is presented. Two different operating regimes have been considered: (1) thermostatically controlled temperature, typical of buildings with auxiliary heating systems; and (2) free-floating temperature, typical of buildings without auxiliary heating systems. In the latter case, to quantify the thermal behaviour of the building, a comfort solar fraction is introduced. In this method, the solar fraction is a function of three parameters: (1) the solar/load ratio; (2) the building thermal inertia; and (3) the monthly non-utilizability. Some examples of the application of the new method are presented, as well as comparisons with the results from existing methods.