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Abstract This report summarizes progress to date on the thermal stability of imidazolium salts being considered for application as heat transfer and thermal storage fluids in solar parabolic trough power systems. Imidazolium salts are a subset of the general class of molten salts. They are termed ionic liquids because many have freezing points at or below room temperature. This class of salts was selected for initial study because there were many examples that were reported to be stable at high temperatures. These reports were usually based on the results of standard thermal gravimetric analysis (TGA) methods. Work by our subcontractor at the University of Alabama and at NREL showed that slow heating rates or when the temperature is held constant for long times resulted in decomposition temperatures that are much lower than those found with the usual TGA methods. We have used a TGA technique that allows calculation of the rates of thermal decomposition as a function of temperature. The results lead us to the conclusion that the imidazolium salts known to be the most thermally stable would not have useful lifetimes above about 200°C. At present this determination is based on the rough approximation that the fluid in a solar trough system experiences a constant, high temperature. Better estimates of the useful lifetime will require a system model that takes into account the time at temperature distribution of a fluid moving through the different components in a solar plant.

A linear correlation between UV-A and 380 nm was developed by means of the TUV 4.1 radiative transfer model. The prediction error of the correlation was evaluated with data from Buenos Aires, Argentina, 2001, and from 2006, Almeria, Spain. Percent random mean square error (RMSE%) was calculated for intervals of 10° of solar zenith angles, ranging 4.75% at 20° to 37.70% at 90° in clear days and 22.16% at 20° to 26.17% at 90° for cloudy days in Buenos Aires Argentina, and 1.27% at 20° to 11.27% at 90° for clear days in Almeria, Spain. Clouded days were not assessed with the data from Spain. In Argentina, the UV-A radiometer is located in a rural area and the 380 nm radiometer is located in an urban area 6 km away. Hence the real error of the proposed model is closer to that found in Spain were both measurements were performed at the same site. The objective of the work is to achieve a simple and precise method to assess UV-A availability for environmental applications of solar energy, particularly for solar water treatment, at any desired latitude.

Abstract Renewable energy sources (RES) coupled to desalination offers a promising prospect for covering the fundamental needs of power and water in remote regions, where connection to the public electrical grid is either not cost effective or not feasible, and where the water scarcity is severe. Stand-alone systems for electricity supply in isolated locations are now proven technologies. Correct matching of stand-alone power supply desalination systems has been recognized as being crucial if the system is to provide a satisfactory supply of power and water at a reasonable cost. The paper covers plants installed since 1990 on the coupling of the two technologies. The main driver promoting the take up of this technology is that water is a limiting factor for many countries in the Mediterranean region. This paper presents the two technologies, RES desalination, and describes the most promising couplings such as PV–reverse osmosis, wind-mechanical-vapor compression, geothermal-multieffect distillation, etc as well as technologies selection guidelines. Also, included applications and lessons learned from specific applications as well as data on the economics. RES for desalination is an important challenge and useful work has been done. However in order to provide practical viable plants, much remains to be done.

Abstract An experimental laboratory-scale setup of a solar-powered electrodialysis (ED) water desalination system is investigated in this study. The introduced ED system maintains a coupled configuration of corrugated membranes for the first time. The corrugated membrane configuration allows for increasing saline water's flow velocity fluctuations, especially near the membrane surfaces. As a result, the flow turbulence and mixing at the membrane surfaces are promoted, and therefore, higher rates of ion exchange can be attained. The acquired concentration of dilute water was measured at the steady-state operation over a diversity of system parameters: input voltage (4–12) volts, flow rates (5–22 mL/s), and feed concentrations (15–35 g/L). The optimal current efficiency (CE) value was obtained at 70% with a flow rate of around 15 m L / s and feed concentration of around 30 g / L . High CE percentage values were obtained (60–70%) within the ED system, which indicates that the process of ions transfer through the exchange membranes is effective even if higher feeding flow and concentrations are applied. Regarding the obtained salt removal (SR) percentage values, the present ED model showed a practical operation scenario where an optimal salt removal value of 35% was achieved within 15 minutes. The findings in this study concluded that the present ED system is superior in desalinating saline water when compared to other ED systems in the literature. The energy demands required to power the current ED system are fully supplied by a photovoltaic solar panel. A larger scale of the current ED setup could be useful in the regions that suffer from the lack of freshwater while potential access to renewable energy sources is available, especially in off-grid areas.

Abstract Compared to solar water heaters, high-temperature solar air heaters have received relatively little investigation and have resulted in few commercial products. However, in the context of a humidification–dehumidification (HD) desalination cycle, air heating offers significant performance gains for the cycle. Heating at a constant temperature and constant heat output is also important for HD cycle performance. The use of built in phase change material (PCM) storage is found to produce consistent air outlet temperatures throughout the day or night. In this study, the PCM has been implemented directly below the absorber plate. Using a two dimensional transient finite element model, it is found that a PCM layer of 8 cm below the absorber plate is sufficient to produce a consistent output temperature close to the PCM melting temperature with a time-averaged collector thermal efficiency of 35%. An experimental energy storage collector with an 8 cm thick PCM layer was built and tested in a variety of weather and operating conditions. Experimental results show strong agreement with model in all cases.

Abstract This study aims to stimulate the discussion on how to optimize a sustainable energy mix from an environmental perspective and how to apply existing renewable energy sources in the most efficient way. Ground-mounted photovoltaics (PV) and the maize–biogas-electricity route are compared with regard to their potential to mitigate environmental pressure, assuming that a given agricultural area is available for energy production. Existing life cycle assessment (LCA) studies are taken as a basis to analyse environmental impacts of those technologies in relation to conventional technology for power and heat generation. The life-cycle-wide mitigation potential per area used is calculated for the impact categories non-renewable energy input, green house gas (GHG) emissions, acidification and eutrophication. The environmental performance of each system depends on the scenario that is assumed for end energy use (electricity and heat supply have been contemplated). In all scenarios under consideration, PV turns out to be superior to biogas in almost all studied impact categories. Even when maize is used for electricity production in connection with very efficient heat usage, and reduced PV performance is assumed to account for intermittence, PV can still mitigate about four times the amount of green house gas emissions and non-renewable energy input compared to maize–biogas. Soil erosion, which can be entirely avoided with PV, exceeds soil renewal rates roughly 20-fold on maize fields. Regarding the overall Eco-indicator 99 (H) score under most favourable assumptions for the maize–biogas route, PV has still a more than 100% higher potential to mitigate environmental burden. At present, the key advantages of biogas are its price and its availability without intermittence. In the long run, and with respect to more efficient land use, biogas might preferably be produced from organic waste or manure, whereas PV should be integrated into buildings and infrastructures.

Soiling Losses for Solar Photovoltaic Systems in California Felipe A Mejia, Jan Kleissl Keywords: Soiling, PV Performance Center for Renewable Resources and Integration, Department of Mechanical and Aerospace Engineering, University of California, San Diego 9500 Gilman Dr., La Jolla, CA 92093, USA Abstract Soiling is the accumulation of dust on solar panels that causes a decrease in the solar photovoltaic (PV) system’s efficiency. The changes in conversion efficiency of 186 residential and commercial PV sites were quantified during dry periods over the course of 2010 with respect to rain events observed at nearby weather stations and using satellite solar resource data. Soiling losses averaged 0.051% per day overall and 26% of the sites had losses greater than 0.1% per day. Sites with small tilt angles (<5 o ) had larger soiling losses while differences by location were not statistically significant. 1. Introduction With the rapid increase in the use of photovoltaic (PV) power in California, which has 47% of the installed PV capacity in the US, the optimal management and analysis of expected performance of PV sites becomes increasingly important. Soiling can have a large effect on efficiency during long droughts[1], which mainly occur during the summer season coincident with the largest solar resource. Dust from air pollution particles, sea salt, pollen, agricultural activity, construction and other anthropogenic and natural sources accumulates on the panels until it is removed either by rain or washing. Research on soiling has primarily been conducted in the middle-east [2] due to the large aerosol loading in the air and the greater abundance of or plans for concentrating solar power plants that are much more affected by soiling. For a concentrating solar power desalination plant in Abu Dhabi, UAE soiling was found to be strongest during sandstorms in the summer season [3]. The transmittance of glass panels after 30 days of exposure in India decreased from 90% to 30% for horizontal and from 90% to 88% for vertical panels [4]. Another more recent study examined the effects of soiling for 250 sites monitored by PowerLight (now SunPower) [1]. Since several of these sites are in areas with frequent rain their study focused on sites in the southwestern United States where long droughts are more common. They also excluded sites with an R 2 value between soiling energy losses and time of less than 0.7 which left a total of 46 sites. Between rain events, soiling losses were found to aggregate linearly with time with an average daily soiling loss of 0.2%. While this paper provides a methodological foundation for analyzing soiling losses, the site selection criteria may have led to an overestimate of soiling losses.

Abstract This article presents the experiences learned using micro-hydro power at the village level. Site evaluation procedure, financing methods, turbine fabrication, and site construction are discussed. Micro-hydro power provides a decentralized energy source for several of the energy-intensive tasks of villagers. Low-head, small volume hydro potential is common in the Zairian countryside. Often a potential site also serves as the village water source, hence it is located near potential beneficiaries of the power. Over the past three decades, a religous NGO in the Ubangi and Mongala Subregions of northwest Zaire has been developing this small hydro potential as part of its technology transfer and village development program. Local materials and knowledge are used as much as possible in construction. Experiences gained constructing a 370 kW hydro-electric site, as well as building water wheels for water pumping has led to the construction of micro-hydro sites using locally made cross-flow turbines. Four water wheel sites and six micro-hydro sites have been built. The hydropower is used to mill flour and hull coffee. One site also generates 220 V electricity, and two others have 12 V generation planned.

Abstract The use of solar ponds is becoming more attractive in today's energy scene. A major advantage of solar ponds over other collectors is the ability to store thermal energy for long periods of time. The solar pond comprises a hydraulic system subject to processes of heat and mass transfer. The design of this system and the related equipment requires a thorough knowledge of the pond heating-up process and expected thermohaline structure within the pond. The current study considers that convection currents in the pond are inhibited by the salinity distribution, and applies a finite difference implicit model in order to investigate the interaction among physical variables represented by various dimensionless parameters. Variables which are included in the analysis comprise the solar radiation input and absorption as it passes through the pond; diffusion and dispersion of heat within the pond; absorption of heat at the bottom of the pond; and withdrawal of heat from layers within the pond. The physical variables generate 3 dimensionless variables associated with the pond's heating-up process. A 4 dimensionless variable is associated with the heat utilization. The analysis represented in this paper concerns the interaction between these dimensionless parameters and its implications.