Density Straight vegetable oil Diesel engine Combustion a b s t r a c tThe straight use of vegetable oils as fuel in diesel engines entails adjusting several physical properties such as density and viscosity. By adequately heating the vegetable oil before entering the injection system, its physical parameters can reach values very close to that of diesel fuel. Consequently, by properly adjusting the temperature of vegetable oils used as fuel, it is possible to improve their combustion performance, thus avoiding premature engine aging due to incomplete burning. In this study the density and viscosity of several vegetable oils are studied within a wide variety of temperatures. The optimal range of temperatures at which each vegetable oil should operate in order to adjust its properties to those of automotive diesel and biodiesel is then found. Additionally an empirical relationship between the dependence of viscosity with density is presented. Thus, by means of the above-described relationship, through measuring the density of a given oil, its viscosity can be directly deduced.ª 2012 Elsevier Ltd. All rights reserved. IntroductionDiesel engines are being extensively utilized worldwide due to their high economic advantage and durability [1,2]. They have appealing features including robustness, high torque, and lower fuel consumption under certain conditions. According to Moron et al. [3] they are prevalent in sectors such as road and train transport, agriculture, military, construction, mining, maritime propulsion and stationary electricity production. Diesel engines can use several fuel types, including diesel fuel, straight vegetable oils (SVO), biodiesel e transesterified vegetable oil e and short chain alcohols. Diesel engines may also function with hybrid fuels, including SVO mixtures in different proportions with diesel or diesel/ ethanol. At the present time there is an increasing demand for energy, concerns about global warming and a growing interest in renewable energy sources; particularly in biofuels [4,5]. This is due to diminishing reserves and price instability of the world's petroleum fuel. These challenges are in part due to the diesel engines themselves. Consequently, it is an urgent matter to reduce hazardous pollutants that diesel engines emit such as NO x , CO, CO 2 and particulate matter (PM). According to Lee at al.[1] this can be achieved by using new combustion technology, by improving fuel characteristics and or by applying after-treatment technology. It is well known that utilizing biofuels with internal combustion engines may contribute to reduce greenhouse gas emissions [6]. Smallscale produced SVOs are considered attractive options for renewable fuel because of environmental benefits [7]. Smallscale use of vegetable oils is also considered an interesting option because they can be obtained from agricultural or industrial sources with very simple processing. This processing includes cold pressing and refining stages that avoid * Corresponding author. Tel.: þ34 938035300; fax: þ34 938031...

Abstract-Hybrid renewable energy systems (HRES) have been widely identified as an efficient mechanism 6 to generate electrical power based on renewable energy sources (RES). This kind of energy generation 7 systems are based on the combination of one or more RES allowing to complement the weaknesses of one 8 with strengths of another and, therefore, reducing installation costs with an optimized installation. To do so, 9 optimization methodologies are a trendy mechanism because they allow attaining optimal solutions given a 10 certain set of input parameters and variables. This work is focused on the optimal sizing of hybrid grid-11 connected photovoltaic -wind power systems from real hourly wind and solar irradiation data and electricity 12 demand from a certain location. The proposed methodology is capable of finding the sizing that leads to a 13 minimum life cycle cost of the system while matching the electricity supply with the local demand. In the 14 present article, the methodology is tested by means of a case study in which the actual hourly electricity retail

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Hybrid renewable energy systems (HRES) are a trendy alternative to enhance the renewable energy deployment worldwide. They effectively take advantage of scalability and flexibility of these energy sources, since combining two or more allows counteracting the weaknesses of a stochastic renewable energy source with the strengths of another or with the predictability of a non-renewable energy source. This work presents an optimization methodology for minimum life cycle cost of a HRES based on solar photovoltaic, wind and biomass power. Biomass power seeks to take advantage of locally available forest wood biomass in the form of wood chips to provide energy in periods when the PV and wind power generated are not enough to match the existing demand. The results show that a HRES combining the selected three sources of renewable energy could be installed in a rural township of about 1300 dwellings with an up-front investment of US $7.4 million, with a total life cycle cost of slightly more than US $30 million. Such a system would have benefits in terms of energy autonomy and environment quality improvement, as well as in term of job opportunity creation. OPEN ACCESSSustainability 2015, 7 12788