Julian Gonzalez-Ayala

University of Salamanca · Department of Applied Physics

Floating photovoltaic energy is an emerging solution to the need for decarbonization of the current society. It is currently in the early stages of implementation, so there are not many previous experiences to standardize decision-making and the most relevant operating parameters such as, the tilt angle of a fast as in conventional photovoltaics. I...

Absorption carbon capture is currently the most commercialized technology and deemed as the vital solution to balance continued use of fossil fuels and carbon emission reduction. Nevertheless, its high energy cost remains the major concern for wide-scale application. Consequently, it is of great significance to address this issue by analyzing the u...

The extraordinary thermal-to-electricity conversion efficiency of thermally regenerative electrochemical cycle triggers interest in its reverse counterpart, namely thermally regenerative electrochemical refrigerator (TRER), a promising alternative to conventional cooling devices. Nevertheless, due to three fundamental obstacles, the practically fea...

Previous research has suggested that molecular energy converters such as ATP synthases, ion pumps, and cotransporters operate via spatially separate pathways for free energy donor and acceptor reactions linked by a protein molecule. We present a chemical kinetics model based on these works, with the basic assumption that all molecular energy conver...

A pumped heat energy storage (PHES) system based on a Rankine cycle for supercritical working fluids, such as carbon dioxide and ammonia, accounting for the irreversible latent and sensible heat transfers between the working fluid and the storage liquid medium, as water or thermal oil, is analyzed. The model also includes several parameters such as...

In the present paper, a general non-combined model of three-terminal refrigerator is established based on the low-dissipation assumption. The relation between the optimized cooling power and the corresponding coefficient of performance (COP) is analytically derived, according to which the COP at maximum cooling power (CMP) can be further determined...

Classical equilibrium thermodynamics provides, in a general way, upper Carnot bounds for the performance of energy converters. Nevertheless, to suggest lower bounds is a much more subtle issue, especially when they are related to a definition of convenience. Here, this issue is investigated in a unified way for heat engines, refrigerators, and heat...

A model for a pumped thermal energy storage system is presented. It is based on a Brayton cycle working successively as a heat pump and a heat engine. All the main irreversibility sources expected in real plants are considered: external losses arising from the heat transfer between the working fluid and the thermal reservoirs, internal losses comin...

A multi-objective and multi-parametric optimization of a Pumped Thermal Electricity Storage system based on Brayton cycles is presented by the calculation of different Pareto fronts and the associated Pareto optimal sets for energetic and design analysis, respectively. A large range of internal and external irreversibilities and the thermodynamic p...

A two-stage sodium thermal electrochemical converter is updated. • Main irreversible losses are considered and the coupling of two stages is optimized. • The maximum efficiency and power output density increase 17.5% and 40.6%. • The optimum selection criteria of main parameters are supplied. • The optimum energetic space is given by the Pareto fro...

A thermodynamic model for a steady state pumped heat energy storage in liquid media is presented: it comprises a coupled Brayton-like heat pump and heat engine cycles connected to a cryogenic liquid and a hot molten salt by counter-flow heat exchangers. The model considers non-isothermal heat transfers between the working fluid and the liquid media...

The stability of endoreversible heat engines has been extensively studied in the literature. In this paper, an alternative dynamic equations system was obtained by using restitution forces that bring the system back to the stationary state. The departing point is the assumption that the system has a stationary fixed point, along with a Taylor expan...

The performance characteristics, operation, and design strategies of a class of thermally driven heat pumps are investigated due to their important roles in the efficient utilization of low-grade thermal energy. In order to establish a more generic thermodynamic model of thermally driven heat pumps mainly including absorption, adsorption, and eject...

In the present paper the possibility of an energetic self-optimization as a consequence of thermodynamic stability is addressed. This feature is analyzed in a low dissipation refrigerator working in an optimized trade-off regime (the so-called Omega function). The relaxation after a perturbation around the stable point indicates that stability is l...

A working fluid performs a Brayton cycle that is fed by a heat input from a solar power tower and from a combustion chamber, which burns natural gas. This hybrid system is described by a complete model that includes all the main losses and irreversibility sources (optical and thermodynamic). Numerical implementation and validation is performed base...

The local stability of a weakly dissipative heat engine is analyzed and linked to an energetic multi-objective optimization perspective. This constitutes a novel issue in the unified study of cyclic energy converters, opening the perspective to the possibility that stability favors self-optimization of thermodynamic quantities including efficiency,...

A relationship between stability and self-optimization is found for weakly dissipative heat devices. The effect of limited control on operation variables around an steady state is such that, after instabilities, the paths toward relaxation are given by trajectories stemming from restitution forces which improve the system thermodynamic performance...

In order to investigate the performance of a class of thermally driven refrigerators, usually driven by low-grade thermal energy, a generic thermodynamic model of three-heat-source refrigerator without involving any specific heat-transfer law is put forward by adopting low-dissipation assumptions. Based on the proposed model, the analytical express...

In order to utilize the low-grade thermal energy efficiently, a more realistic model of thermally regenerative electrochemical cycles system with continuous power output is proposed in which the heat transfer irreversibility, external heat leakage and non-ideal regeneration losses are taken into account. Besides, the symmetry of cells, which is nec...

Local stability of maximum power and maximum compromise (Omega) operation regimes dynamic evolution for a low-dissipation heat engine is analyzed. The thermodynamic behavior of trajectories to the stationary state, after perturbing the operation regime, display a trade-off between stability, entropy production, efficiency and power output. This all...

In the present paper we study the connection between local stability and energetic properties in low-dissipation heat engines operating in the maximum-power and maximum-compromise (Ω) regimes. We consider two different feedback regulatory pathways: 1) one in which restitutive forces linearly depend on the deviations from the stationary values of th...

We present a molecular dynamics simulation of a two-dimensional Carnot engine. The optimization of this engine is achieved through the velocity of the piston, allowing not only the optimization of power output but also some other figures of merit involving entropy production. The maximum power and maximum ecological efficiencies are computed. It is...

The connection between Carnot-like and low-dissipation refrigerators is proposed by means of their entropy generation and the optimization of two unified, compromise-based figures of merit. Their optimization shows that only a limited set of heat transfer laws in the Carnot-like model are compatible with the results stemming from the low-dissipatio...

In this work we address the stability of a low dissipation (LD) heat engine (HE) under maximum power conditions. The LD system dynamics are analyzed in terms of the contact times between the engine and the external heat reservoirs; which determine the amount of heat exchanged by the system. We study two different scenarios that secure the existence...

Since the decade of 1980’s, a connection between a family of maximum-work reversible thermal cycles and maximum-power finite-time endoreversible cycles has been established. The endoreversible cycles produce entropy at their couplings with the external heat baths. Thus, this kind of cycles can be optimized under criteria of merit that involve entro...

In this paper, a comparison between two well-known finite time heat engine models ispresented: the Carnot-like heat engine based on specific heat transfer laws between the cyclic systemand the external heat baths and the Low-Dissipation model where irreversibilities are taken intoaccount by explicit entropy generation laws. We analyze the mathemati...

For a low-dissipation heat engine model we present the role of the partial contact times and the total operational time as control parameters to switch from maximum power state to maximum $\Omega$ trade-off state. The symmetry of the dissipation coefficients may be used in the design of the heat engine to offer, in such switching, a suitable compro...

The main unified energetic properties of low dissipation heat engines and refrigerator engines allow for both endoreversible or irreversible configurations. This is accomplished by means of the constraints imposed on the characteristic global operation time or the contact times between the working system with the external heat baths and modulated b...

In this work we analyze the deviations of reversible cycles (for both heat engines and refrigerators) from the corresponding Carnot cycle operating between the same extreme temperatures, and deviations of irreversible cycles from their corresponding reversible realization while putting emphasis on the corresponding losses. The endoreversible models...

It is well established that at early times, long before the time of radiation-matter equality, the universe could have been well described by a spatially flat, radiation only model. In this letter we consider the whole primeval universe as a black body radiation (BBR) system in an $n-$dimensional Euclidean space. We propose that the $\left(3+1\righ...

In 1978, Landsberg proposed an elegant way of obtaining the inequality between arithmetic and geometric mean by using the first and second laws of thermodynamics. This result opened a debate on the logic legitimacy of this procedure to obtain some mathematical truths. Although this discussion can not be considered completed, the Landsberg approach...

In this work, we present the generalization of some thermodynamic properties of the black body radiation (BBR) towards an n-dimensional Euclidean space. For this case, the Planck function and the Stefan–Boltzmann law have already been given by Landsberg and de Vos and some adjustments by Menon and Agrawal. However, since then, not much more has bee...

This paper discusses the possibility of using the Joule-Brayton cycle to determine the accessible value range for the coefficients a and b of the heat capacity at constant pressure Cp, expressed as Cp=a+bT (with T the absolute temperature) by using the Carnot theorem. This is made for several gases which operate as the working fluids. Moreover, the...

We propose a new connection between maximum-power Curzon-Ahlborn thermal cycles and maximum-work reversible cycles. This linkage is built through a mapping between the exponents of a class of heat transfer laws and the exponents of a family of heat capacities depending on temperature. This connection leads to the recovery of known results and to a...

It is common in many thermodynamic textbooks to illustrate the Carnot theorem through the usage of diverse state equations for gases, paramagnets, and other simple thermodynamic systems. As it is well-known, the universality of the Carnot efficiency is easily demonstrated in a temperature-entropy diagram, which means that the Carnot efficiecy is in...

In this paper we present an alternative proof of the so-named g-lemma of finite-time thermodynamics. It is shown that g-lemma does not impose any restriction on the heat transfer laws or reflect any kind of criterion on whether a heat transfer law proposal could be valid in nature or not, however, the possibilities to work with new heat exchange mo...