First vs second order magnetocaloric material for thermomagnetic energy conversion

International audienceWe estimate the power and efficiency of a thermal energy harvesting thermodynamic Brayton cycle using a first and second order magnetocaloric materials as active substance. The thermodynamic cycle was computed using a simple thermal exchange model and an equation of state deduced from a phenomenological Landau model. For the first and second order materials, narrow and high frequency cycles are optimum and give similar performances. Considering technological issues hindering the increase of frequency, we introduced a more detailed approach where we take into account the time needed to switch the material between two heat reservoirs. We show that the first order material equation of state leads thermodynamic cycle shape keeping it closer to the optimum cycle. Conditions to improve the performance of second order materials are discussed. In addition, we infer key remarks for prototype design regarding the power density and efficiency reachable in different configurations

Continuity and boundary conditions in thermodynamics: From Carnot's efficiency to efficiencies at maximum power

[...] By the beginning of the 20th century, the principles of thermodynamics were summarized into the so-called four laws, which were, as it turns out, definitive negative answers to the doomed quests for perpetual motion machines. As a matter of fact, one result of Sadi Carnot's work was precisely that the heat-to-work conversion process is fundamentally limited; as such, it is considered as a first version of the second law of thermodynamics. Although it was derived from Carnot's unrealistic model, the upper bound on the thermodynamic conversion efficiency, known as the Carnot efficiency, became a paradigm as the next target after the failure of the perpetual motion ideal. In the 1950's, Jacques Yvon published a conference paper containing the necessary ingredients for a new class of models, and even a formula, not so different from that of Carnot's efficiency, which later would become the new efficiency reference. Yvon's first analysis [...] went fairly unnoticed for twenty years, until Frank Curzon and Boye Ahlborn published their pedagogical paper about the effect of finite heat transfer on output power limitation and their derivation of the efficiency at maximum power, now known as the Curzon-Ahlborn (CA) efficiency. The notion of finite rate explicitly introduced time in thermodynamics, and its significance cannot be overlooked as shown by the wealth of works devoted to what is now known as finite-time thermodynamics since the end of the 1970's. [...] The object of the article is thus to cover some of the milestones of thermodynamics, and show through the illustrative case of thermoelectric generators, our model heat engine, that the shift from Carnot's efficiency to efficiencies at maximum power explains itself naturally as one considers continuity and boundary conditions carefully [...]

Termodinamičke osnove termokemijskih energetskih sustava i gorivnih članaka

This research treats power optimization for energy converters, such as thermal, solar and electrochemical engines (fuel cells). A common methodology is developed for the assessment of power limits in thermal systems and fuel cells. Thermodynamic analyses lead to converter efficiency and limiting power. Steady and dynamic systems are investigated. Static optimization of steady systems applies the differential calculus or Lagrange multipliers, dynamic optimization of unsteady systems uses variational calculus and dynamic programming. The primary result of the first is the limiting value of power, whereas that of the second is a total generalized work potential. The generalizing quantity depends on the thermal coordinates and a dissipation index, h, i.e. the Hamiltonian of the problem of minimum entropy production. The advanced thermodynamics, of an irreversible nature, implies stronger bounds on work delivered or supplied than the classical reversible work. It is shown how various analytical developments can efficiently be synthesized to quantitatively evaluate power limits in thermal systems and fuel cells of a simple topology (without countercurrent flows).Ovo se istraživanje bavi optimizacijom snage sustava za pretvorbu energije poput termičkih, solarnih i elektrokemijskih (gorivni članci). U radu je razvijena jedinstvena metoda procjene granice snage u termičkim sustavima i gorivnim člancima. Termodinamičkim analizama dolazi se do učinkovitosti sustava za pretvorbu i granične snage. Istražuju se stacionarni i nestacionarni sustavi. Za statičku optimizaciju stacionarnih sistema primjenjuju se diferencijalni račun ili Lagrangeovi faktori; dinamička optimizacija nestacionarnih sustava koristi varijacijski račun i dinamičko programiranje. Rezultat prvog je ograničavajuća vrijednost snage dok je rezultat drugog ukupni poopćeni potencijal rada. Poopćenje ovisi o termičkim koordinatama i indeksu disipacije, h, npr. Hamiltonov operator problema minimalne entropije. Razvijena termodinamika nepovratnih sustava implicira čvršće granice na potrošeni ili predani rad nego što je to kod termodinamike povratnih procesa. Pokazano je kako različite analize mogu efikasno biti sintetizirane u svrhu kvantitativne procjene granica snage u termičkim sustavima i gorivnim člancima jednostavne topologije (bez protustrujnih tokova)

Local stability of an endoreversible heat pump with linear phenomenological heat transfer law working in an ecological regime

AbstractBased on the optimal ecological performance parameters of a heat pump with linear phenomenological heat transfer law between working fluid and heat reservoirs, the local stability analysis of the endoreversible heat pump working in an ecological regime is studied. The steady state of the heat pump working at the maximum ecological function is steady. After a small perturbation, the system state exponentially decays to steady state with either of the two relaxation times. The effects of temperatures of heat reservoirs and heat transfer coefficients on the local stability of the system are discussed. Distribution information of phase portraits of the system is obtained. It is concluded that both the energetic properties and local stability of the system should be considered for designing the real heat pumps

Correlation-powered Information Engines and the Thermodynamics of Self-Correction

Information engines can use structured environments as a resource to generate work by randomizing ordered inputs and leveraging the increased Shannon entropy to transfer energy from a thermal reservoir to a work reservoir. We give a broadly applicable expression for the work production of an information engine, generally modeled as a memoryful channel that communicates inputs to outputs as it interacts with an evolving environment. The expression establishes that an information engine must have more than one memory state in order to leverage input environment correlations. To emphasize this functioning, we designed an information engine powered solely by temporal correlations and not by statistical biases, as employed by previous engines. Key to this is the engine's ability to synchronize---the engine automatically returns to a desired dynamical phase when thrown into an unwanted, dissipative phase by corruptions in the input---that is, by unanticipated environmental fluctuations. This self-correcting mechanism is robust up to a critical level of corruption, beyond which the system fails to act as an engine. We give explicit analytical expressions for both work and critical corruption level and summarize engine performance via a thermodynamic-function phase diagram over engine control parameters. The results reveal a new thermodynamic mechanism based on nonergodicity that underlies error correction as it operates to support resilient engineered and biological systems.Comment: 22 pages, 13 figures; http://csc.ucdavis.edu/~cmg/compmech/pubs/tos.ht

Economic predictions for heat mining : a review and analysis of hot dry rock (HDR) geothermal energy technology

The main objectives of this study were first, to review and analyze several economic assessments of Hot Dry Rock (HDR) geothermal energy systems, and second, to reformulate an economic model for HDR with revised cost components.A general evaluation of the technical feasibility of HDR technology components was also conducted in view of their importance in establishing drilling and reservoir performance parameters required for any economic assessment (see Sections 2-5). In our review, only economic projections for base load electricity produced from HDR systems were considered. Bases of 1989 dollars ($) were selected to normalize costs.Following the evaluation of drilling and reservoir performance, power plant choices and cost estimates are discussed in Section 6. In Section 7, the six economic studies cited earlier are reviewed and compared in terms of their key resource, reservoir and plant performance, and cost assumptions. Based on these comparisons, we have estimated parameters for three composite cases. Important parameters include: (1) resource quality--average geothermal gradient (oC/km) and well depth, (2) reservoir performance--effective productivity, flow impedance, and lifetime (thermal drawdown rate), (3) cost components--drilling, reservoir formation, and power plant costs and (4) economic factors--discount and interest rates, taxes, etc. In Section 8, composite case conditions were used to reassess economic projections for HDRproduced electricity. In Section 9, a generalized economic model for HDR-produced electricity is presented to show the effects of resource grade, reservoir performance parameters, and other important factors on projected costs. A sensitivity and uncertainty analysis using this model is given in Section 10. Section 11 treats a modification of the economic model for predicting costs for direct, non-electric applications. HDR economic projections for the U.S. are broken down by region in Section 12. In Section 13, we provide recommendations for continued research and development to reduce technical and economic uncertainties relevant to the commercialization of HDR

Analysis of information systems for hydropower operations

The operations of hydropower systems were analyzed with emphasis on water resource management, to determine how aerospace derived information system technologies can increase energy output. Better utilization of water resources was sought through improved reservoir inflow forecasting based on use of hydrometeorologic information systems with new or improved sensors, satellite data relay systems, and use of advanced scheduling techniques for water release. Specific mechanisms for increased energy output were determined, principally the use of more timely and accurate short term (0-7 days) inflow information to reduce spillage caused by unanticipated dynamic high inflow events. The hydrometeorologic models used in predicting inflows were examined to determine the sensitivity of inflow prediction accuracy to the many variables employed in the models, and the results used to establish information system requirements. Sensor and data handling system capabilities were reviewed and compared to the requirements, and an improved information system concept outlined