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Abstract Algae have been considered as a promising biodiesel feedstock. One of the major factors affecting large-scale algae technology application is poor wintering cultivation performance. In this study, an integrated approach is investigated combining freshwater microalgae Chlorella zofingiensis wintering cultivation in pilot-scale photobioreactors with artificial wastewater treatment. Mixotrophic culture with the addition of acetic acid (pH-regulation group) and autotrophic culture (control group) were designed, and the characteristics of algal growth, lipid and biodiesel production, and nitrogen and phosphate removal were examined. The results showed that, by using acetic acid three times per day to regulate pH at between 6.8 and 7.2, the total nitrogen (TN) and total phosphate (TP) removal could be increased from 45.2% to 73.5% and from 92.2% to 100%, respectively. Higher biomass productivity of 66.94 mg L−1 day−1 with specific growth rate of 0.260 day−1 was achieved in the pH-regulation group. The lipid content was much higher when using acetic acid to regulate pH, and the relative lipid productivity reached 37.48 mg L−1 day−1. The biodiesel yield in the pH-regulated group was 19.44% of dry weight, with 16–18 carbons as the most abundant composition for fatty acid methyl esters. The findings of the study prove that pH adjustment using acetic acid is efficient in cultivating C. zofingiensis in wastewater in winter for biodiesel production and nutrient reduction.

Abstract A coiled-pipe exchanger was employed to extract heat rapidly at relatively high temperatures from a 90-litre hot-water charged tank, the water being initially at a temperature of approximately 80°C. The free-convective movements of the water around the outside of the coiled pipe (immersed in the store) were due to buoyancy forces induced by colder water being forced through the heat-exchanger's pipe. For the heat-exchanger orientations tested, the maximum effectiveness, with respect to the quality of the heat extracted was achieved (i) by having the axis of the coiled heat-exchanger arranged horizontally with its inlet at the lowest level; and (ii) with the lower rate tested (=6·6 litre/min) of water being passed through the heat-exchanger's pipe, partly because this led to a lower rate of disruption of the stratification of the water within the store.

The multi-effect distillation with thermal vapour compression (MED-TVC) desalination system is efficient to produce freshwater. The steam ejector performance is not fully understood as the phase transition has been ignored in many studies. The present work develops a two-phase condensing flow model to assess the steam ejector performance considering nonequilibrium condensation phenomena. The transition of the flow structure from an under-expanded flow to an over-expanded flow in the steam ejector is investigated in detail. We present that the maximum Mach number can reach 4.02 in the under-expanded flow, which is weakened to 2.88 in the over-expanded flow. The steam undergoes several expansion-compression processes in the steam ejector in the under-expanded flow, which induces the formation and evaporation of massive droplets. In the over-expanded flow, the steam is compressed and then expanded after leaving the primary nozzle and the condensation process is not observed in mixing and constant sections. The increasing suction chamber pressure significantly improves the entrainment ratio while leading to an increasing entropy loss coefficient. The entrainment ratio is improved from 0.25 for the under-expanded flow to 1.69 for the over-expanded flow, while the entropy loss increases from 0.081 for the under-expanded flow to 0.29 for the over-expanded flow. This indicates that the transition of the flow structure from an under-expanded flow to an over-expanded flow can entrain more steam from the last effect while causes more entropy losses in a steam ejector for the MED-TVC desalination system.

An innovative small dynamic water desalination plant was developed and tested under laboratory conditions. The system is a combination of a heat pipe evacuated tube solar collector, conventional condenser and novel fluid piston converter. Saline water is boiled and turned into vapour in the manifold of the solar collector. A small fraction of the solar energy supplied to the plant is used to drive the fluid piston converter. Oscillations of the fluid piston periodically change the volume and pressure in the plant. For the duration of approximately half of the periodic cycle the pressure in the plant drops below the atmospheric level causing flash boiling of saline water in the manifold of the solar collector. Generated vapour is turned into fresh water in the condenser which is surrounded by a cooling jacket with saline water. The flash boiling effect improves the fresh water production capacity of the plant. Additionally, the fluid piston converter drives a pump which provides lifting of saline water from a well and pumps this through the cooling jacket of the condenser to a saline water storage tank. This tank replenishes saline water in the manifold of the solar collector. Experimental investigations demonstrated the saline water self-circulation capability of the plant and increase in the fresh water production compared to the static mode of operation. Experimental data was also used to calibrate the mathematical model of the plant. Comparison of theoretical and experimental information demonstrates that the model accurately predicts the performance of the plant. The proposed novel system with greater fresh water production capacity has a simple design and is easy to manufacture using low cost materials and therefore can be mass deployed for small scale saline water pumping and desalination across different regions with the relatively high solar radiation and shortage in the drinking water supply.

Abstract This paper investigates reactions of mercury (Hg) compounds in effluents of the wet flue gas desulfurization (FGD) process during waste water treatment. Hence, a concept for the controlled desorption and immobilization of Hg is introduced. The aim is to create a highly concentrated sink for Hg for further processing. Experiments are carried out with a continuously operated lab-scale wet FGD system and a batch-wise operated alkalization reactor for the treatment of synthetic and real waste water samples. By aeration of the liquid phase, the controlled desorption of Hg during the alkalization step of the waste water treatment process is enabled. The Hg-rich exhaust air is directed to an activated carbon fixed bed adsorber. It is demonstrated, that Hg is emitted in its elemental form (Hg0). Thus, a chemical reduction of dissolved Hg2+ compounds takes place prior to Hg0 desorption to the gas phase. Mechanisms for the reactions are proposed, identifying SO32− and OH− as electron donors. Linear dependency of Hg0 formation on SO32− and OH− concentration indicate first order dependencies of reaction kinetics. Decreasing concentration of Hg0 in the exhaust air for increasing Cl− concentration is observed. The results show exponential dependence of Hg0 desorption on temperature and stirring speed. The mass flow of desorbed Hg0 remains constant for variation in aeration flow rate. Thus, the application of low air flux is beneficial in terms of energy demand and for the purpose of creating a highly concentrated sink for Hg in the process. The concentration decrease of Hg2+ in the waste water is proportional to the savings in terms of precipitating agent consumption of further processing steps. Finally, the concept developed prevents unnoticed Hg desorption during waste water treatment and increases sustainability and plant safety.

Abstract The effect of molecular diffusion of salt is not only significant for the smoothing process during the pond's establishment, but also for its operation. An analysis is presented to calculate the rate of salt transport within an isothermal liquid column, according to the molecular diffusion process. Salt-transport measurements in laboratory pond models provide experimental data in good agreement with the theoretical predictions.

Abstract This work aims at developing a comprehensive methodology defining the influence of the proper feedstock management, number and location of gasification-power plants, and process configuration based on the thermodynamic performances of the proposed system under different scenarios. The feedstock was citrus peel from local citrus processing factories. A three-step model was adopted: best site criteria decision tool, location-allocation analysis by minimising energetic transport costs, energy/exergy and emissions analysis. This model was applied to three process scenarios. In the first, we considered a wet biomass (65% water content) dried at a local plant using cogenerated heat; in the second, the biomass dried prior to transportation (15% water content) used to power the combined heat and power plant. The cogenerated heat fed an Organic Rankine Cycle (ORC) unit. In the third, the cogenerated heat is used for district heating. The scenario analysis showed that the regional potential for producing renewable electricity from citrus peel gasification is 66.7–74.6 GWh. Results showed that the Energy Return on Investment (EROI) is

We determine the environmental impact of different biodiesel production strategies from algae feedstock in terms of greenhouse gas (GHG) emissions and non-renewable energy consumption, we then benchmark the results against those of conventional and synthetic diesel obtained from fossil resources. The algae cultivation in open pond raceways and the transesterification process for the conversion of algae oil into biodiesel constitute the common elements among all considered scenarios. Anaerobic digestion and hydrothermal gasification are considered for the conversion of the residues from the wet oil extraction route; while integrated gasification–heat and power generation and gasification–Fischer–Tropsch processes are considered for the conversion of the residues from the dry oil extraction route. The GHG emissions per unit energy of the biodiesel are calculated as follows: 41 g e-CO2/MJb for hydrothermal gasification, 86 g e-CO2/MJb for anaerobic digestion, 109 g e-CO2/MJb for gasification–power generation, and 124 g e-CO2/MJb for gasification–Fischer–Tropsch. As expected, non-renewable energy consumptions are closely correlated to the GHG values. Also, using the High Dimensional Model Representation (HDMR) method, a global sensitivity analysis over the entire space of input parameters is performed to rank them with respect to their influence on key sustainability metrics. Considering reasonable ranges over which each parameter can vary, the most influential input parameters for the wet extraction route include extractor energy demand and methane yield generated from anaerobic digestion or hydrothermal gasification of the oil extracted-algae. The dominant process input parameters for the dry extraction route include algae oil content, dryer energy demand, and algae annual productivity. The results imply that algal biodiesel production from a dried feedstock may only prove sustainable if a low carbon solution such as solar drying is implemented to help reducing the water content of the feedstock.

Abstract A novel process is presented to generate electricity from low-grade heat by combining a Reverse Electrodialysis membrane with an Adsorption desalinator in a closed-loop system. A Reverse Electrodialysis membrane generates electricity by controlled mixing of two salt solutions of different concentrations. An Adsorption desalinator restores the initial salt gradient by utilising low-grade heat for the separation. In this study the process is designed from optimising the salt and material selection to the development of the real system application. Energy and exergy efficiencies of the proposed system show the potential of this novel renewable energy technology. The efficiencies of 227 salts with a range of different valences and 10 adsorption materials have been investigated over a large number of system parameters. The results show that the optimised system can achieve an exergy efficiency of up to 30%. Moreover, high salt concentrations do not significantly increase the specific energy consumption of the Adsorption desalinator, which allows operating the Reverse Electrodialysis membrane at the optimal salt concentrations.