J. Facao - Academia.edu (original) (raw)

Papers by J. Facao

International Journal of Low-Carbon Technologies, 2006

The thermal behaviour of a plate heat pipe solar collector was analysed numerically and experimen... more The thermal behaviour of a plate heat pipe solar collector was analysed numerically and experimentally. The numerical model is based on energy balance equations assuming a quasisteady state condition. The major simplification was that the temperature in the heat pipe was considered to be uniform and equal to the saturation temperature. This assumption is not far from the truth, since heat pipes are considered as isothermal devices. A small-scale solar collector, with an aperture area of about 0.1 m 2 , was experimentally tested during the Summer season in Porto. Two types of tests were made: the first was the determination of the instantaneous efficiency curve and the second was the determination of the collector time constant, a measure of its thermal inertia. Results showed a collector optical efficiency of 64% and an overall loss coefficient of 5.5 W/(m 2 K), for a non-selective surface coating. There was a good agreement between numerical and experimental results.

International Journal of Low-Carbon Technologies, 2006

The use of solar thermal collectors for electricity production is a way to contribute to the Port... more The use of solar thermal collectors for electricity production is a way to contribute to the Portuguese objective of reaching 39% of electricity production from renewable energy sources, until 2010. This is also in accordance with the objectives of the European Union and the Kyoto Protocol. The system in analysis is powered by solar energy and supplemented by a natural gas boiler, specially for periods when solar radiation is low. Use of the system would result in significant savings in primary energy consumption and a reduction in CO2 emissions to the environment. The solar collectors are of the heat pipe type and hybrid: they act as a boiler economizer, as boiler exhaust gases circulate below the absorber plate, increasing the energy input and collector efficiency. The behaviour of a combined heat and power cycle producing 6 kW of electricity was simulated. The heat rejected in the cycle condenser can be used for space/water heating or cooling of buildings. Several refrigerants have been considered for the cycle and methanol presented the best performance. The contribution of solar energy (solar fraction) was evaluated for the climatic data of Lisbon (Portugal), for two applications: a pool complex and an office building. The energy and economic potential of the system was compared to the conventional alternative.

International Journal of Low-Carbon Technologies, 2010

A trapezoidal cavity receiver for a linear Fresnel solar collector is analysed and optimized via ... more A trapezoidal cavity receiver for a linear Fresnel solar collector is analysed and optimized via ray-trace and computational fluid dynamics (CFD) simulations. The number of receiver absorber tubes and the inclination of lateral walls in the cavity are checked with simplified ray-trace simulation. The CFD simulation makes possible to optimize cavity depth and rock wool insulation thickness. The simulated global heat transfer coefficient, based on primary mirror area, is correlated with a power-law fit instead of a parabolic fit. The correlation results are compared with heat transfer coefficients available for linear Fresnel collector prototypes.

International Journal of Low-Carbon Technologies, 2008

In this work, three solar assisted thermodynamic cycles for a micro-cogeneration system with a po... more In this work, three solar assisted thermodynamic cycles for a micro-cogeneration system with a power output of 5 kW are studied. The thermodynamic cycles are based on the Organic Rankine Cycle (ORC) and the operating temperatures of solar thermal collectors are 80°C, 100°C-150°C and 200°-250°C, for cycles 1, 2 and 3, respectively. The main work objective was to model the selected cycles for optimisation according to the temperature range. The performance of several fl uids was evaluated from a thermal and an economical point of view. The integration of the micro-cogeneration system with solar thermal collectors was evaluated and solar fractions were calculated for the climatic conditions of Almeria (Spain), Tunis (Tunis) and Cairo (Egypt).

Numerical Heat Transfer, Part A: Applications, 2008

The present work is focused on the computational analysis of heat and mass transfer in an indirec... more The present work is focused on the computational analysis of heat and mass transfer in an indirect contact cooling tower. The main objectives of the study are to contribute to the understanding of heat and mass transfer mechanisms involved in the problem and to check the possibility of making use of a commercial computational fluid dynamics (CFD) code for simulating mass and heat transfer phenomena occurring in indirect cooling towers. The CFD model uses as boundary conditions the temperatures of the tubes obtained by a correlation model developed by Mizushina. The available mass transfer correlations for indirect cooling towers are presented and compared with a correlation obtained from CFD simulations. The assumption of analogy between heat and mass transfer is also discussed.

International Journal of Green Energy, 2004

In the last decade, artificial neural networks (ANNs) have been receiving an increasing attention... more 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 non-linearity. This paper 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 non-linear 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.

International Journal of Energy Research, 2000

A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experi... more A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experimentally. The thermal e$ciency of the cooling tower was measured for di!erent air #ow rates, water #ow rates, spray #ow rates and wet bulb air temperatures. CFD was also used to predict the thermal performance of the cooling tower. Good agreement was obtained between CFD prediction and experimental measurement.

Applied Thermal Engineering, 2000

A new closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested. E... more 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 coecients. Existing thermal models for this type of cooling tower were found to predict well thermal performance, if the above correlations are used.

Applied Thermal Engineering, 2004

Cooling towers are increasingly used in buildings as a component of environmental cooling systems... more Cooling towers are increasingly used in buildings as a component of environmental cooling systems. When compared with industrial towers, cooling towers with lower capacities and smaller dimensions can be used for this application, with inlet water temperatures ranging from 22 to 25°C. Correlations for heat and mass transfer coefficients were obtained experimentally for a small-size indirect cooling tower, with a nominal cooling capacity of 10 kW. A detailed analysis of the mass transfer coefficient and spray heat transfer coefficient correlations is made, including the effect of the error in their evaluation on tower efficiency. The new correlations are compared with existing ones and results of their application to a simplified cooling tower model are shown.

gas, 2002

This paper presents a numerical study of a hybrid flat-plate solar collector using heat pipes. Th... more This paper presents a numerical study of a hybrid flat-plate solar collector using heat pipes. The hybrid collector uses solar radiation and hot gases (gas burner) as energy inputs. Model results were obtained and the hybrid collector efficiency was quantified for all possible operating conditions.

International Journal of Energy Research, 2000

*Correspondence to: Sa!a Ri!at, Institute of Building Technology, School of the Built Environment... more *Correspondence to: Sa!a Ri!at, Institute of Building Technology, School of the Built Environment, University of Nottingham, Nottingham NG7 2RD, UK RE-mail: Sa!a.Ri!at@nottingham .ac.uk Contract/grant sponsor: Commission of the European Union (DG XII).

International Journal of Low-Carbon Technologies, 2006

The thermal behaviour of a plate heat pipe solar collector was analysed numerically and experimen... more The thermal behaviour of a plate heat pipe solar collector was analysed numerically and experimentally. The numerical model is based on energy balance equations assuming a quasisteady state condition. The major simplification was that the temperature in the heat pipe was considered to be uniform and equal to the saturation temperature. This assumption is not far from the truth, since heat pipes are considered as isothermal devices. A small-scale solar collector, with an aperture area of about 0.1 m 2 , was experimentally tested during the Summer season in Porto. Two types of tests were made: the first was the determination of the instantaneous efficiency curve and the second was the determination of the collector time constant, a measure of its thermal inertia. Results showed a collector optical efficiency of 64% and an overall loss coefficient of 5.5 W/(m 2 K), for a non-selective surface coating. There was a good agreement between numerical and experimental results.

International Journal of Low-Carbon Technologies, 2006

The use of solar thermal collectors for electricity production is a way to contribute to the Port... more The use of solar thermal collectors for electricity production is a way to contribute to the Portuguese objective of reaching 39% of electricity production from renewable energy sources, until 2010. This is also in accordance with the objectives of the European Union and the Kyoto Protocol. The system in analysis is powered by solar energy and supplemented by a natural gas boiler, specially for periods when solar radiation is low. Use of the system would result in significant savings in primary energy consumption and a reduction in CO2 emissions to the environment. The solar collectors are of the heat pipe type and hybrid: they act as a boiler economizer, as boiler exhaust gases circulate below the absorber plate, increasing the energy input and collector efficiency. The behaviour of a combined heat and power cycle producing 6 kW of electricity was simulated. The heat rejected in the cycle condenser can be used for space/water heating or cooling of buildings. Several refrigerants have been considered for the cycle and methanol presented the best performance. The contribution of solar energy (solar fraction) was evaluated for the climatic data of Lisbon (Portugal), for two applications: a pool complex and an office building. The energy and economic potential of the system was compared to the conventional alternative.

International Journal of Low-Carbon Technologies, 2010

A trapezoidal cavity receiver for a linear Fresnel solar collector is analysed and optimized via ... more A trapezoidal cavity receiver for a linear Fresnel solar collector is analysed and optimized via ray-trace and computational fluid dynamics (CFD) simulations. The number of receiver absorber tubes and the inclination of lateral walls in the cavity are checked with simplified ray-trace simulation. The CFD simulation makes possible to optimize cavity depth and rock wool insulation thickness. The simulated global heat transfer coefficient, based on primary mirror area, is correlated with a power-law fit instead of a parabolic fit. The correlation results are compared with heat transfer coefficients available for linear Fresnel collector prototypes.

International Journal of Low-Carbon Technologies, 2008

In this work, three solar assisted thermodynamic cycles for a micro-cogeneration system with a po... more In this work, three solar assisted thermodynamic cycles for a micro-cogeneration system with a power output of 5 kW are studied. The thermodynamic cycles are based on the Organic Rankine Cycle (ORC) and the operating temperatures of solar thermal collectors are 80°C, 100°C-150°C and 200°-250°C, for cycles 1, 2 and 3, respectively. The main work objective was to model the selected cycles for optimisation according to the temperature range. The performance of several fl uids was evaluated from a thermal and an economical point of view. The integration of the micro-cogeneration system with solar thermal collectors was evaluated and solar fractions were calculated for the climatic conditions of Almeria (Spain), Tunis (Tunis) and Cairo (Egypt).

Numerical Heat Transfer, Part A: Applications, 2008

The present work is focused on the computational analysis of heat and mass transfer in an indirec... more The present work is focused on the computational analysis of heat and mass transfer in an indirect contact cooling tower. The main objectives of the study are to contribute to the understanding of heat and mass transfer mechanisms involved in the problem and to check the possibility of making use of a commercial computational fluid dynamics (CFD) code for simulating mass and heat transfer phenomena occurring in indirect cooling towers. The CFD model uses as boundary conditions the temperatures of the tubes obtained by a correlation model developed by Mizushina. The available mass transfer correlations for indirect cooling towers are presented and compared with a correlation obtained from CFD simulations. The assumption of analogy between heat and mass transfer is also discussed.

International Journal of Green Energy, 2004

In the last decade, artificial neural networks (ANNs) have been receiving an increasing attention... more 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 non-linearity. This paper 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 non-linear 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.

International Journal of Energy Research, 2000

A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experi... more A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experimentally. The thermal e$ciency of the cooling tower was measured for di!erent air #ow rates, water #ow rates, spray #ow rates and wet bulb air temperatures. CFD was also used to predict the thermal performance of the cooling tower. Good agreement was obtained between CFD prediction and experimental measurement.

Applied Thermal Engineering, 2000

A new closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested. E... more 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 coecients. Existing thermal models for this type of cooling tower were found to predict well thermal performance, if the above correlations are used.

Applied Thermal Engineering, 2004

Cooling towers are increasingly used in buildings as a component of environmental cooling systems... more Cooling towers are increasingly used in buildings as a component of environmental cooling systems. When compared with industrial towers, cooling towers with lower capacities and smaller dimensions can be used for this application, with inlet water temperatures ranging from 22 to 25°C. Correlations for heat and mass transfer coefficients were obtained experimentally for a small-size indirect cooling tower, with a nominal cooling capacity of 10 kW. A detailed analysis of the mass transfer coefficient and spray heat transfer coefficient correlations is made, including the effect of the error in their evaluation on tower efficiency. The new correlations are compared with existing ones and results of their application to a simplified cooling tower model are shown.

gas, 2002

This paper presents a numerical study of a hybrid flat-plate solar collector using heat pipes. Th... more This paper presents a numerical study of a hybrid flat-plate solar collector using heat pipes. The hybrid collector uses solar radiation and hot gases (gas burner) as energy inputs. Model results were obtained and the hybrid collector efficiency was quantified for all possible operating conditions.

International Journal of Energy Research, 2000

*Correspondence to: Sa!a Ri!at, Institute of Building Technology, School of the Built Environment... more *Correspondence to: Sa!a Ri!at, Institute of Building Technology, School of the Built Environment, University of Nottingham, Nottingham NG7 2RD, UK RE-mail: Sa!a.Ri!at@nottingham .ac.uk Contract/grant sponsor: Commission of the European Union (DG XII).