• Energy Research
• 6. Clean water close

Energy plays an important role in the water sector. In fact, energy consumption by this sector accounts for nearly 44% of municipalities’ energy costs in developed countries. To minimize adverse environmental and human health impacts wastewater can undergo three treatment levels before discharge or reuse. Since it is an energy intensive process, wastewater treatment represents the largest share of water-related electricity consumption in developed countries. According to the literature, there is a potential for improvement and energy savings in wastewater treatment facilities. Indeed, International Energy Agency states that, the energy consumption in the water sector can be reduced by 15% if this potential is harnessed. Consequently, energy audits can provide valuable information about those facilities, allowing to assess their energy performance and to identify energy-saving opportunities. In this paper the results of internal energy audits conducted in two “small” wastewater treatment plants, located in the North of Portugal, are presented and analyzed. The results, 13% and 22% savings, show that simple energy audits, which can be implemented by company members, should not be considered as a cost, but rather as a step to a more efficient energy use, reducing energy costs and environmental impacts.

Abstract Given their increasing number and the implementation of more energy-intensive treatment methods, wastewater treatment plants (WWTPs) expect to increase energy consumption. In addition, climate change presents new challenges to the operation of these facilities, thus being critical to understand how to improve their energy performance and environmental sustainability while ensuring the quality of service provided. This paper reviews the latest publications on the energy performance of municipal WWTPs, particularly on the different phases of the treatment process and the impacting factors. The contextual and underlying factors that influence energy performance were identified, categorized, and analyzed through a broad survey. The most significant factors are the plant size, load factor (plant capacity utilization), and dilution factor. The implementation of anaerobic-anoxic-oxic systems is considered suitable in sensitive areas requiring high pollutant and nutrient removal rates, presenting, in some cases, moderate energy consumption (0.267 kWh/m3), being similar to the conventional activated sludge (0.269 kWh/m3). In more stringent effluent quality requirements, such as wastewater reuse, membrane bioreactors are advised, despite higher consumption (0.33 kWh/m3). Energy improvements can also be achieved by implementing automatization, inverters, and strategies that increase flexibility and adaptability in the operational process. Lastly, given the multidimensional characteristics of the WWTPs assessment, further improvements may be identified if the energy performance of these plants is compared using holistic and multi-criteria approaches, integrating multiple inputs and outputs simultaneously.

Abstract The optimization and assessment study of a thermal energy adsorption storage system is presented. The system integrates an adsorption heat storage module in a conventional hot water storage tank of a solar thermal system, operating with the silica-gel/water adsorption pair. The system was modeled using TRNSYS® and MATLAB®, and was previously assessed and improved through a set of parametric tests for each main component. In this work, the GenOpt® optimization software was used to obtain the optimal performance of the whole system. It is found that a slender and lengthy adsorber with a large number of thin fins, a thick and lengthy condenser, and an evaporator with a large number of lengthy tubes improve the system’s performance, by increasing the heat transfer areas and the adsorbent mass. The performance also improves by controlling the adsorber-condenser valve only through the system’s pressure and opening the evaporator-adsorber valve at the hot water setpoint temperature. The optimized system presents a 16% saving in annual backup energy consumption compared with a similar conventional storage system, thus validating the results of the previous segregated parametric study. This optimized system operates at the highest performance with the same configuration in different locations/climates, as only the inclination of the solar collector affects the results: larger inclinations improve the system’s performance, by favoring its operation in Winter. Results present this system as a promising solution to increase the energy storage capacity of solar thermal systems, and potentially of systems using other primary energy sources.

Abstract The dynamic model of a heat storage adsorption device is presented. The adsorption module operates with the silica-gel/water pair and is capable of storing the thermal energy received from the hot water of the storage tank where it is immersed, to give it back later as adsorption heat. The module is applied to a solar thermal energy system and assessed through a set of parametric tests. It is found that higher condenser lengths and larger pre-heating water tank volumes always improve the system’s performance. For a selected fixed heat exchange area, smaller evaporator tube diameters are found to improve the system’s performance, while reducing the number of tubes of a settled diameter has a negative effect. It is also found that the system’s performance tends to decrease by increasing the main tank’s volume, thus requiring even larger adsorbers for larger tanks. Throughout this exploratory study, the adsorption system always presents higher performances when compared with a similar conventional storage system (up to 16% savings in annual backup energy), showing promising perspectives for the overall optimization and application studies, and presenting an attractive solution to increase the thermal storage capacity of solar thermal systems.

Abstract Owing to environmental problems arising from the use of refrigerants, first the ozone depletion and later the global warming, various refrigerants have been banned and their phase-out deadlines established through some international protocols, the Montreal and Kyoto Protocols being two of the most famous of such agreements. This paper intends to make a characterization of the most currently used refrigerants in the Portuguese food industry, through a sample of 148 companies gathered in a nationwide survey. It also aims to analyse not only the environmental impacts of these refrigerants, but also the restrictions that these companies may incur according to the EU Regulation No. 517/2014, such as maintenance and service bans or cap and phase-down on the supply. Finally, according to other research and tests reports, some possible replacements for two of the most commonly used refrigerants (R404A and R22) which are being phased down, are summarized. According to the research, there are some satisfactory mid-term and long-term alternatives available for R22 and R404A that have much lower GWP, allowing systems to achieve higher efficiencies and in turn, lower environmental impacts during their service life.

Abstract The dynamic model of an adsorber unit used as thermal energy storage device immersed in water is presented. The system operates with the silica-gel/water pair and is capable of storing the thermal energy received from the surrounding water ( e . g ., excess heat input from a hot water storage tank), in order to give it back later to the water as adsorption heat. The model was developed following a lumped parameter approach implemented in MATLAB ® code. The performance of the absorber unit was assessed by a set of parametric tests under different geometric configurations and temperature conditions. The mass of adsorbent was found to have a higher impact on the thermal energy exchange than the surface contact area between metal and adsorbent. An improved finned adsorber, with 27 internal longitudinal fins and 120 external annular fins, resulted in a heat output to the water 2.3 times higher than with a similar finless adsorber. Moreover, the evaporation temperature effect was found to be much higher than the condensation temperature effect. This device seems to be an attractive solution to include, for instance, in solar hot water systems in order to fulfill the thermal energy needs during periods of low solar radiation.