COLLECTIONS

Documents published in Scipedia

• Hydraulic management drives heat budgets and temperature trends in a Mediterranean reservoir

  E. Moreno-Ostos, R. Marcé, J. Dolz, J. Ordóñez, J. Armengol

  International Review of Hydrobiology (2008). Vol. 93 (2), pp. 131-147

  
  

  Abstract

  By contrast to the more regular and predictable temperate lakes, heat budgets and temperature dynamics in Mediterranean reservoirs are characterized by a marked interannual variability. In the present paper, the heat content, annual Birgean heat budget (ABHB), and thermal structure of Sau Reservoir were examined during a period of hypolimnetic withdrawal between 1980 and 1985, and during a period of withdrawal at intermediate depths between 1996 and 2003. The two study periods were also characterized by a wide range of stored water volume fluctuations. Results were used to develop and validate an empirical model to predict annual and monthly heat dynamics statistics and mixed layer depth as a function of hydraulic management parameters such as water volume and selective withdrawal depth. During the hypolimnetic withdrawal period elevated ABHB and deep mixed layer depths were recorded in the reservoir, which behaved as a heat trap. By contrast, intermediate depth withdrawal promoted a shallower and more stable thermocline, thus increasing the cold hypolimnetic water volume and decreasing heat content and ABHB. The study reveals that hydraulic management constitutes the main driver of the heat and thermal dynamics in reservoirs with multiple withdrawal outlets. By contrast with the increasing temperature trends recorded in many natural lakes, the hydraulic management in Sau Reservoir induced a progressive reduction in water temperature and heat content in the system, thus partially counteracting the possible deleterious effects of global warming. Our intensive study in a single, highly‐dynamic ecosystem constitutes a new approximation to the study of thermal structure and heat dynamics in water bodies.

  Abstract
  By contrast to the more regular and predictable temperate lakes, heat budgets and temperature dynamics in Mediterranean reservoirs are characterized by a marked interannual [...]

  • 1
  •  read

• Analysis of pressures on a stepped spillway

  M. Juny, E. Bladé, J. Dolz

  Journal of Hydrology (2008). Vol. 46 (3), pp. 410-414

  
  

  Abstract

  The pressure profiles on the horizontal and vertical step faces of a stepped spillway in the fully developed skimming flow region were measured. These observations allow to provide generalized equations that enable designers to analyze the amount and the location of negative pressures along the steps. The step locations subjected to negative pressure are also discussed including an analysis of the duration of the negative pressures. On the horizontal step faces, negative pressure were only observed on their upstream half for dimensionless discharges larger than dc/h = 1.3. On the vertical step faces, only the region close to the adjacent horizontal face was not subjected to negative pressure

  Abstract
  The pressure profiles on the horizontal and vertical step faces of a stepped spillway in the fully developed skimming flow region were measured. These observations allow to [...]

  • 2
  •  read

• Dynamic thermal simulation of a solar chimney with PV modules

  J. Martí-Herrero, M. Heras-Celemin

  pp. 891-896

  
  

  Abstract

  The issue of this paper is to present theoretical results for a solar chimney with thermal mass, where the glass surface is replaced by photo-voltaic (PV) modules. A portion of the heat ab-sorbed by the PV modules is dissipated to the air channel in convective form, and it exchanges radiation heat with the concrete wall. These cooling phenomena for the PV modules im-prove their efficiency with a lower working temperature. Both phenomena are heating proc-ess to the air and the concrete wall, that produce natural ventilation. The solar chimney is sup-posed to be isolated from any building. The re-sults obtained are an average air mass flow rate around 0.02 kg/s along the day and night, and a maximum PV temperature of 321 K.

  Abstract
  The issue of this paper is to present theoretical results for a solar chimney with thermal mass, where the glass surface is replaced by photo-voltaic (PV) modules. A portion [...]

  • 1
  •  read

• Dynamic physical model for a solar chimney

  J. Martí-Herrero, M. Heras-Celemin

  Solar Energy (2007). Vol. 81 (5), pp. 614-622

  
  

  Abstract

  The aim of this research is to investigate the theoretical usefulness of a solar chimney with thermal inertia applied to the Mediterranean climates, offering nocturnal ventilation benefits. A mathematical dynamical model is proposed to evaluate the energy performance of a solar chimney with 24 cm concrete wall as storage surface for solar radiation. The results obtained with the proposed model are coherent with several models response and experiments reported on solar chimneys. As well, the difference of the proposed model to others is the incorporation of an unsteady state and the inclusion of thermal inertia. The results show that for a 2 m height and width of air channel of 14.5 cm, 0.011 kg/s air mass flow rate is obtained for 450 W/m2. The 24 cm thickness concrete wall, reaches its greater temperature 2 h later with respect to the maximum ambient temperature, maintaining its temperature over the beginning of the night, so nocturnal ventilation is achieved. The model shows the interest in continuing investigating on this cooling techniques and to built a solar chimney with thermal inertia for future experimental research.

  Abstract
  The aim of this research is to investigate the theoretical usefulness of a solar chimney with thermal inertia applied to the Mediterranean climates, offering nocturnal ventilation [...]

  • 9
  •  read

• Transfer of low-cost plastic biodigester technology at household level in Bolivia

  J. Martí-Herrero

  Livestock Research for Rural Development (2007). Vol. 19 (12), Article, 192

  
  

  Abstract

  The familiar low cost biodigestores are an important tool for urban and rural development. These natural systems fulfill the criteria of appropriate technology, producing biogás and improved natural fertilizer, through the anaerobic digestion of organic remainders. Also the health and hygiene of the families, as well as of the cattle, affect positively, being a sustainable handling environmentally of remainders that otherwise are polluting. The potential of this technology in Bolivia is very high, and once it has already been developed to cover the three different regions (Altiplane, valley and jungle), validated like development tool and accepted by the families, it is precise `to popularize the technology', so to make it accessible to the greater number of people. In this article a review to the development of the biodigestores in Bolivia, its adaptation to high altitudes, as well as the learned lessons is made, to in the end comment the first steps that already are occurring to popularize this technology.

  Abstract
  The familiar low cost biodigestores are an important tool for urban and rural development. These natural systems fulfill the criteria of appropriate technology, producing [...]

  • 6
  •  read

• Reduced hydraulic retention times in low-cost tubular digesters: Two issues

  J. Martí-Herrero

  Biomass and Bioenergy (2011). Vol. 35 (10), pp. 4481-4484

  
  

  Abstract

  Low-cost tubular digesters have been implemented in several developing countries. One of the problems reported from field surveys is that biogas generation that does not meet the user’s expectations. This report provides two reasons for the discrepancy between the biogas generation rate estimated in the design phase and the actual rate measured after construction, due to a lower final hydraulic retention time (HRT). The hydraulic retention time is normally determined from the liquid volume calculated from the cylindrical shape of the bag and not from the trench dimensions. The result is a reduction in HRT of 6%–51%, depending on the dimensions of the trench recommended by various authors. Another factor that is not normally considered is the influence of the biogas pressure on the liquid level inside the digester which negatively affects the liquid volume of the digester, reducing HRT by as much as 15%.

  Abstract
  Low-cost tubular digesters have been implemented in several developing countries. One of the problems reported from field surveys is that biogas generation that does not meet [...]

  • 2
  •  read

• Towards thermal design optimization of tubular digesters in cold climates: A heat transfer model

  T. Perrigault, V. Weatherford, J. Martí-Herrero, ´. Poggio

  Bioresource Technology (2012). Vol. 124, pp. 259-268

  
  

  Abstract

  A cold climate, low cost, tubular digester is monitored and temperatures from different parts of the slurry, greenhouse, and adobe walls are presented, discussing the thermal performance of the digester. The slurry exhibits a vertical gradient of 6 °C, with a mean value of 24.5 °C, while the ambient temperature varies from 10 °C to 30 °C, showing the efficiency of the system as a solar heat collector with thermal inertia. A simple time-dependent thermal model is developed using inputs of solar radiation, wind velocity, ambient temperature, and digester geometry. The model outputs include temperatures of the slurry, the biogas, its holding membrane and the greenhouse air, wall and cover. Radiative, convective and conductive heat transfer phenomena are considered between all system elements. The model has 0.47 °C (2%) standard error for the average slurry temperature. This model can be used to predict the influence of geometry and materials on the performance of the digester.

  Abstract
  A cold climate, low cost, tubular digester is monitored and temperatures from different parts of the slurry, greenhouse, and adobe walls are presented, discussing the thermal [...]

  • 2
  •  read

• Scour of rock due to the impact of plunging high velocity jets Part I: A state-of-the-art review

  E. Bollaert, A. Schleiss, J. Dolz

  Journal of Hydraulic Research (2007). Vol. 41 (5), pp. 853-856

  
  

  Abstract

  This paper presents the state-of-the-art on methods to estimate rock scour due to the impingement of plunging high velocity water jets. The following topics are addressed: empirical formulae, semi-empirical and analytical approaches, determination of extreme pressure fluctuations at plunge pool bottoms and, finally, the transfer of these pressure fluctuations in joints underneath concrete slabs or rock blocks. Available methods on rock scour have been thoroughly investigated on their ability to represent the main physical-mechanical processes that govern scour. This reveals lack of knowledge on turbulence and aeration effects, as well as on transient pressure flow conditions in rock joints. These aspects may significantly influence the destruction of the rock mass and should be accounted for in scour evaluation methods. Their relevance has been experimentally investigated by dynamic pressure measurements at modeled plunge pool bottoms and inside underlying one-and two-dimensional rock joints. Test results are described and discussed in Part II of this paper.

  Abstract
  This paper presents the state-of-the-art on methods to estimate rock scour due to the impingement of plunging high velocity water jets. The following topics are addressed: [...]

  • 4
  •  read

• A methodological approach to the reconstruction of the 1949-2000 water temperature series in the Ebro River at Escatrón

  J. Prats, R. Val, J. Armengol, J. Dolz

  Limnetica (2007). Vol. 26 (2), pp. 293-306

  
  

  Abstract

  Water temperature is one of the main variables influencing freshwater organisms. In recent years, increasing water temperature trends have been observed in freshwater ecosystems worldwide. To assess the importance of such increases, they need to be put into context by studying long-term water temperature variability. However, it is often difficult to obtain a sufficiently long data series. This problem may be overcome by using data collected by different entities in different periods of time. However, this can lead to data comparability problems. In this paper, we present the process of reconstructing the annual mean water temperature series for the Ebro River at Escatron using data measured by the Escatr ´ on power station, the Ebro Hydrographic ´ Confederation and the Flumen research group. The data sources used different measurement methodologies and frequencies. Therefore, particular attention was paid to analysing the bias of the annual mean water temperature estimations calculated from the available historical data. A resampling methodology applied to 10-minute water temperature data was performed to assess the quality of the different estimators used to calculate the annual mean water temperature. Only one of the three estimators used could be considered to be unbiased. Another estimator could be corrected by estimating its bias. As a result, it was possible to reconstruct a water temperature series from 1949 to 2000, with a gap in the period 1978-1995. The series shows that there was an increase of 2.3◦C in the period 1955-2000.

  Abstract
  Water temperature is one of the main variables influencing freshwater organisms. In recent years, increasing water temperature trends have been observed in freshwater ecosystems [...]

  • 2
  •  read

• Pressures on a stepped spillway

  M. Juny, E. Bladé, J. Dolz

  Journal of Hydraulic Research (2007). Vol. 45 (4), pp. 505-511

  
  

  Abstract

  This paper presents an analysis of the pressure field on the steps of a stepped spillway on an RCC dam. The fundamental component of this study is a series of pressure measurements obtained in the fully-developed zone of skimming flow regime along a reduced-size stepped spillway model constructed at the Department of Hydraulic, Maritime and Environmental Engineering Laboratory of the Technical University of Catalonia (UPC). A mathematical model was fitted to the pressure profiles on the horizontal and vertical faces of the steps. This equation allows designers to estimate the possible negative values of pressure on the vertical and horizontal faces of the steps in the fully-developed zone of skimming flow regime.

  Abstract
  This paper presents an analysis of the pressure field on the steps of a stepped spillway on an RCC dam. The fundamental component of this study is a series of pressure measurements [...]

  • 2
  •  read