- Varela Díez, Fernando

## Persona: Varela Díez, Fernando

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0000-0002-3564-1639

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Varela Díez

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Fernando

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Publicación Using Fourier series to obtain cross periodic wall response factors(Taylor & Francis, 2023-11-22) Theirs, Eduardo; Sánchez Orgaz, Susana; Varela Díez, Fernando; González Gaya, CristinaMás... Wall periodic response factors are a very usual calculation method of transient heat transfer through building envelope elements (walls, roofs …) in steady periodic conditions, used in popular heat load calculation procedures as ASHRAE’s RTS method [Spitler, Jeffrey D., Daniel E. Fisher, and Curtis O. Pedersen. 1997. “The Radiant Time Series Cooling Load Calculation Procedure.” ASHRAE Transactions 103 (2): 503–515]. This response factors, time sampled heat flux responses of a multi-layer wall to a 24h-periodic unit triangle function, can be obtained by means of multiple methods: Laplace’s method, state space method, frequency domain methods, etc. These methods are numerical since there is no analytical way of obtaining these response factors. The aim of this work is, taking advantage of the periodic nature of excitations, use Fourier series to represent boundary conditions, and this way find an easier and less computationally demanding procedure to calculate these response factors. Additionally, the convergence of these Fourier series will be analysed to determine the minimum set of frequencies needed to ensure a fixed admissible error for wall periodic response factors.Más... Publicación Analysis and comparison of Integrated Solar Combined Cycles using parabolic troughs and linear Fresnel reflectors as concentrating systems(Elsevier, 2015-11-12) Abbas, Rubén; Rovira de Antonio, Antonio José; Barbero Fresno, Rubén; Montes Pita, María José; Varela Díez, FernandoMás... This paper compares the annual performance and economic feasibility of Integrated Solar Combined Cycles (ISCC) using two solar concentration technologies: parabolic trough collectors (PTC) and linear Fresnel reflectors (LFR). Integration of solar energy to the steam turbine of a combined cycle gives some advantages: the first one is the fuel saving due to the solar contribution and, additionally, the second one is that this contribution takes place especially in highly insolated periods with high ambient temperatures, when conventional combined cycles decrease their power rate and work with decreased efficiency. Previous works showed the convenience of ISCC using PTC and direct steam generation in locations with severe climatology. Besides, LFR technology is currently considered as a good option for reducing the cost of concentrating solar power. Thus, in the present work both concentrating technologies are studied and compared. Solar contribution is only used for evaporating water, increasing the production of the high pressure level of the steam generator. Two locations, Almeria and Las Vegas, are selected for the annual analyses. Results show that the proposed evaporative configurations increase the annual performance. Also, the thermal contribution is higher with PTC, but LFR may improve the economic feasibility of the plant.Más... Publicación A New Thermodynamic Model to Approximate Properties of Subcritical Liquids(MDPI, 2023-06-29) Sánchez Orgaz, Susana; González Fernández, M. Celina; Varela Díez, Fernando; Rodríguez Laguna, JavierMás... In order to obtain the thermodynamic properties of compressed liquids, it is usual to consider them as incompressible systems, since liquids and solids are well represented by this thermodynamic model. Within this model, there are two usual hypotheses that can be derived in two different submodels: the strictly incompressible (SI) model, which supposes a constant specific volume 𝑣=𝑣0, and a more general model, called temperature-dependent incompressible (TDI) model, which relates a specific volume to temperature, 𝑣=𝑣(𝑇). But, usually, this difference ends here in the thermal equation of state, and only the SI model was developed for caloric and entropic equations. The aim of this work is to provide a complete formulation for the TDI model and show where it can be advantageously used rather than the SI model. The study concludes that the proposed model outperforms the traditional model in the study of subcritical liquid. One conceivable utilization of this model is its integration into certain thermodynamic calculation software packages (e.g., EES), which integrate the more elementary SI model into its code for certain incompressible substances.Más... Publicación A new method for calculating conduction response factors for multilayer constructions based on frequency–Domain spline interpolation (FDSI) and asymptotic analysis(Elsevier, 2017-08-01) Sanza Pérez, Javier; Chicote, Manuel Andrés; Velasco Gómez, Eloy; Varela Díez, FernandoMás... Conduction heat transfer through building construction elements is one of the main components of space heating and cooling loads, and, thus, one of the key aspects when planning sustainable energy designs in the building sector. The Response Factors (RF) method sets the base for related dynamic calculations implemented by most well-known Building Energy Simulation (BES) software, and it represents a research topic of present interest. In this regard, this work introduces a new method for calculating conduction Response Factors in building multilayer constructions, based on the definition of an approximated wall model through Frequency–Domain Spline Interpolation (FDSI) and asymptotic analysis. Its conceptual development as well as first validations comparing with existing methods from previous literature are presented. Finally, as a result of applying a table-lookup approach and the possibility of pre-calculating most of the involved operations, an accurate, fast and easy-to-code algorithm is obtained, which constitutes a promising alternative to improve the current state-of-art calculation procedures.Más... Publicación A solar air-cooled high efficiency absorption system in dry hot climates: Reduction of water consumption and environmental impact(VINČA Institute of Nuclear Sciences, 2018) Lizarte, Raquel; Palacios Lorenzo, María Esther; Blanco Marigorta, Ana María; Marcos del Cano, José Daniel; Varela Díez, FernandoMás... A solar cooling system with an optimized air-cooled double-effect water/LiBr absorption machine is proposed as a sustainable alternative to meet cooling demands in dry hot climates. This system allows eliminating the cooling towers in those regions of the planet where water is scarce. This work analyses the environmental benefits of this air-cooled system, as well as its environmental foot-prints, compared to a solar water-cooled single effect. In this regard, a methodology has been applied to calculate the annual saving in water consumption produced in a case study: a hospital located in Almería, in South of Spain. Further-more, the reduction in energy consumption and CO2 emissions is also quantified since this machine can be driven by solar energy and with higher efficiency than those of single effect.Más... Publicación Comparison of Different Technologies for Integrated Solar Combined Cycles: Analysis of Concentrating Technology and Solar Integration(MDPI, 2018-04-25) Sánchez, Consuelo; Abbas, Rubén; Muñoz Antón, Javier; Ortega, Guillermo; Rovira de Antonio, Antonio José; Valdés Fernández, Manuel Tomás; Barbero Fresno, Rubén; Montes Pita, María José; Muñoz Domínguez, Marta; Varela Díez, FernandoMás... This paper compares the annual performance of Integrated Solar Combined Cycles (ISCCs) using different solar concentration technologies: parabolic trough collectors (PTC), linear Fresnel reflectors (LFR) and central tower receiver (CT). Each solar technology (i.e. PTC, LFR and CT) is proposed to integrate solar energy into the combined cycle in two different ways. The first one is based on the use of solar energy to evaporate water of the steam cycle by means of direct steam generation (DSG), increasing the steam production of the high pressure level of the steam generator. The other one is based on the use of solar energy to preheat the pressurized air at the exit of the gas turbine compressor before it is introduced in the combustion chamber, reducing the fuel consumption. Results show that ISCC with DSG increases the yearly production while solar air heating reduces it due to the incremental pressure drop. However, air heating allows significantly higher solar-to-electricity efficiencies and lower heat rates. Regarding the solar technologies, PTC provides the best thermal results.Más...