Nonlinear model predictive control methodology for efficiency and durability improvement in a fuel cell power system

The main contribution of this work is the improvement of the efficiency of a PEMFC power system while guaranteeing conditions that also improve its durability. Adopting the NMPC scheme with the distributed parameter model and the nonlinear observer, the efficiency of the PEMFC-based system can be maximized guaranteeing at the same time the appropriate internal gas concentration profiles to avoid global and local hydrogen and oxygen starvation and proper membrane humidification.Peer ReviewedPostprint (author's final draft

Forecasting investment: A fishing contest using survey data

This paper assesses the usefulness of business surveys as a source of information for investment developments in Portugal. This will be achieved by what will be named a “fishing contest”, where the “participants” are bridge models, models based on principal components (derived from standard and non-standard methods), and models built with the outcome of partial least squares regressions. All models, based on quarterly data, are estimated using a general-to-specific approach and are designed to produce 1 to 4 out-of-sample direct forecasts. The accuracy of these forecasts is then compared with the one of autoregressive processes. The empirical evidence indicates that, in general, there is always a participant in the fishing context that produces a lower out-of-sample Root Mean Squared Error (RMSE) than the one associated with the autoregressive benchmark. In most cases, the combination of autoregressive processes with each participant reduces the RMSE further. A striking outcome is the relative accuracy of bridge models.

Energy optimisation and controllability in complex distillation columns

To extend the knowledge of complex distillation arrangements for the separation of ternary mixtures and make them closer to implementation in industry is the motivation of this thesis work. Some design, operation, and control aspects are analysed, having the conventional distillation arrangements as comparison basis. The DWC is a complex distillation arrangement very attractive in terms of energy and cost savings. However, the DWC require large columns in order to be economically advantageous. Therefore, this arrangement is more attractive when the energy cost is high compared to the trays cost. From rigorous simulations of different separation examples, some synthesis rules have been obtained.Designs that minimise the distillation cost have been given special importance. DWC design procedures reported in the literature were based on only two decision variables. A new procedure that uses three decision variables for design optimisation has been proposed. It has been seen that the use of three decision variables is important to avoid excluding possible optimum designs.The control of the DWC is more complex than the control of the other distillation arrangements because the DWC has more operation degrees of freedom. The approach in this work has been to solve the DWC control by levels: stabilisation, composition control, and optimising control. When the manipulated variables for DWC stabilisation control are the distillate and the bottoms flowrate, which is the most common stabilisation control structure in the literature, the DWC presents high directionality and interaction. Besides, this stabilisation control structure is not appropriate to the DWC because the DWC has typically high reflux ratios. When the manipulated variables for stabilisation control are the reflux flowrate and the boilup, the DWC has better controllability and larger stability margins. Linear analysis tools are useful to select the set of manipulated variables for the DWC composition control. The preferred set of manipulated variables and the controllability of the corresponding control structure depend on the nominal operation. The DWC has two extra operation degrees of freedom that permit an operation optimisation. This optimisation is used to minimise the boilup. At a non-optimal operation, the controllability is improved. Therefore, a trade-off appears between operation optimisation and controllability. Changes in the design of the DWC can be used to improve its controllability. High directionality is a problem associated to DWC that may be improved using a large number of trays. On the other hand, DWC optimal designs have an equilibrated distribution of distillation effort between sections which, if broken, may also present improve the controllability. As found for operation, also for design, leaving optimal conditions permits to improve the controllability. Alternatively, for the composition control of the DWC, Dynamic Matrix Control is analysed. It is found that DMC performs worse than the diagonal feedback control strategy. The DWC extra operation degrees of freedom can be used dynamically for optimising control. Due to a marked directionality found in the response surface, DWC operation may be kept close to optimal operation using only one of the operation degrees of freedom. Optimising control of the DWC through the feedback control of a variable that characterises the optimal operation is possible. Some measurable variables are able to maintain optimal conditions with certain accuracy.The controllability of the DWC is compared to the controllability of other distillation arrangements. In general, the complexity of a distillation arrangement makes its energy efficiency better and its controllability worse. However, important exceptions have been found: - The DWC may give important energy savings as well as the best controllability if it is operated at non-optimal conditions. - When the manipulated variables for stabilisation are internal flows (reflux rates and boilups), the controllability of the DWC is better than the controllability of the other distillation arrangements. The DWC controllability is found to be more interesting for long columns. Therefore, the DWC needs long columns to be really attractive in terms of energy as well as in terms of controllability. In the literature, DWC controllability was misevaluate. In this thesis work, it is seen that, thanks to the arrangement complexity, the DWC may give at the same time energy savings and a good controllability, what makes it very attractive. To know the influence of the non-modelled detail and to obtain experimental results for the DWC is the most important step in the way to DWC implementation in industry

Mesocotyl elongation in Digitaria sanguinalis during seedling development

The mesocotyl is an embryonic organ present in Poaceae that plays an important role in seedling emergence. The elongation of this first internode contributes decisively to the coleoptile reaching the soil surface. This study examines the process of mesocotyl elongation under controlled conditions in three caryopsis collection sites of Digitaria sanguinalis (L.) Scop. originating from Spain (Barcelona and Girona) and Argentina that may have two patterns of germination: radicular or coleoptilar. The frequencies of the two germination patterns varied significantly depending on the origin. Light inhibited the elongation of the mesocotyl drastically, resulting in maximum lengths of 3.5mm, while in darkness the maximum length was 57mm. The time-course evolution displayed under dark conditions was quite similar for all sites of origin and both germination patterns; the growth rate ranged from 0.23 to 0.30mmh21. Within localities, caryopses with a coleoptilar pattern of germination showed a lower growth rate than those with a radicular one.Postprint (author's final draft

Farms' technical inefficiencies in the presence of government programs

We focus on determining the impacts of government programs on farms’ technical inefficiency levels. We use Kumbhakar’s stochastic frontier model that accounts for both production risks and risk preferences. Our theoretical framework shows that decoupled government transfers are likely to increase (decrease) DARA (IARA) farmers’ production inefficiencies if variable inputs are risk decreasing. However, the impacts of decoupled payments cannot be anticipated if variable inputs are risk increasing. We use farm-level data collected in Kansas to illustrate the model.Just and Pope production function, Agricultural and Food Policy, Farm Management,

Effects of policy instruments on farm investments and production decisions in the Spanish cop sector

Our paper asses the impacts of the partially decoupled (PD) scheme, implemented during the 1990s and first half of the 2000s in the framework of the Common Agricultural Policy (CAP), on on-farm investment as well as on other production decisions. The Spanish COP sector was taken as a case study due to its economic and political relevance. The empirical analysis is applied on farm-level data from the Farm Accountancy Data Network (FADN), observed from 2000 to 2004, based on. We use a reduced-form application of the dual model of investment under uncertainty and a system of censored and non censored equations is estimated. PD payments are found to increase short-run production and to generate a statically significant increase in the investment in farm assets. Results also show the importance of assessing the effects of PD payments in a dynamic framework as the one applied in this paper.farm investments, Common Agricultural Policy, decoupling, production., Agricultural and Food Policy, Food Consumption/Nutrition/Food Safety,

The Determinants of Survival of Spanish Consumers Fronting the BSE Crisis

The impact of food scares on meat consumption has been traditionally investigated by estimating food demand systems using aggregated time series. Only a few have considered micro data but none of them has tried to quantify consumers’ reaction to food scares and the speed of such reactions. In this study we apply duration analysis techniques with the aim of analysing the effect of different explanatory variables on both the risk of reducing beef consumption and the timing of this reduction. Our results suggest that the maximum hazard occurs during the few months after the occurrence of the food crisis and then the reducing consumption hazard tend to diminish. Moreover, economic factors such as prices and income could be considered as the most determinant factors of the survival of the Spanish consumers facing the BSE crisis while other socioeconomic characteristics such as the age, the gender, etc., have a small, if any, effect on the occurrence and the speed of beef consumption reduction indicating a quite homogenous reaction among Spanish consumers to BSE crisis. These results provide interesting insights about how policy makers could orientate food policies in order to recover consumption after a food scare.Food scare, BSE, duration analysis, reaction timing, Spain, Food Consumption/Nutrition/Food Safety, Livestock Production/Industries, C41, D1,

Non-parametric and Parametric Modeling of Biodiesel - Sunflower Oil - Crude Oil Price Relationships

Multivariate local linear regression and parametric error correction models are applied to assess price linkages and price transmission patterns between food and energy prices in Spain. Weekly biodiesel, sunflower and crude oil prices observed from November 2006 to October 2010 are used in the empirical analysis. Results suggest the existence of a long-run equilibrium relationship between the three prices studied. Biodiesel is the only variable that adjusts to deviations from this long-run parity. Local linear regression techniques show that the speed of adjustment of biodiesel prices is higher when biodiesel is cheap than when it is expensive. Energy prices are also found to influence sunflower oil prices through the short-run price dynamics.Price transmission, local linear regression, biodiesel, Spain, Demand and Price Analysis, Resource /Energy Economics and Policy,