Membrane distillation as a thermal conductivity measurement device

A large number of geothermal exploration wells exist in the US that don’t produce the quantity and quality of heat required for power generation with an Organic Rankine Cycle (ORC). At temperatures under 150 °C, power generation is currently not cost effective. Membrane distillation (MD) can desalinate water at temperatures as low as 50 °C and thus may enable production of clean water from low grade heat provided by these abandoned wells. Moreover, if the distillate can be used in evaporative cooling of air-cooled geothermal plants, the efficiency and thus revenue of these plants can be substantially augmented. This narrative motivated the geothermal office of US Department of Energy (DOE) to fund a collaborative research between the National Renewable Energy Laboratory (NREL) and the Colorado School of Mines (CSM) to map the potential application of geothermal MD deslination in the US and design, build, and test a pilot-scale MD unit at two major geothermal power plants in California and Nevada. In the first phase of the project we are selecting most suitable MD membranes. The performance of 15 different MD membranes from 6 different manufacturers was analysed over a wide temperature range (40-70 °C). Most of these manufacturers don’t make dedicated MD membranes, but make microfiltration membranes for water and air purification with the adequate pore size and hydrophobicity for MD. Due to the different application of these membranes, some manufacturers don’t report important membrane characteristics, or use different methods to determine essential parameters (e.g., pore size and porosity) needed for modeling of MD performance. None of them report vapour fluxes or thermal conductivity. Therefore, a model was developed that does not require the knowledge of the pore size, the porosity, or the thermal conductivity of the membrane. We found that thermal efficiency is fairly constant for a wide range of salinities and temperature differences across the membrane. The Schofield method [1] was adapted to incorporate the thermal efficiency instead of the thermal conductivity, pore size, and porosity. Thermal efficiency is often not reported in MD literature, but it can have an important impact on the efficiency and hence the cost effectiveness of the process for geothermal desalination. The membranes tested exhibited thermal efficiencies from 15 to 55 %. In other words, up to 85% of the heat input was lost for thermal conduction through the membrane. Because a relation was derived between the thermal conductivity and thermal efficiency, MD can effectively be used as a thermal conductivity measurement device. References: [1] R.W. Schofield, A.G. Fane and C.J.D. Fell, JMS, 33 (1987) 299-31

Salinity gradient energy: Assessment of pressure retarded osmosis and osmotic heat engines for energy generation from low-grade heat sources

Development of clean energy technologies that maximize efficiency and minimize resource consumption is a necessary component for a clean and secure energy future. The osmotic heat engine (OHE) is a closed-loop, membrane based process that utilizes low-grade heat and salinity-gradient energy between two streams for electrical energy generation. The OHE couples pressure retarded osmosis (PRO), an osmotically driven membrane process, with membrane distillation (MD), a thermally driven membrane process. In PRO, water permeates via osmosis through a semi-permeable membrane from a low concentration feed stream into a higher concentration brine (draw solution). The permeate stream becomes pressurized on the high concentration side of the membrane and a mechanical device (e.g., turbine generator set) is used to convert the hydraulic pressure to electrical energy. The MD process utilizes low-grade heat to reconcentrate the diluted brine from the PRO process and to produce a deionized water stream; these streams are then resupplied to the PRO process in the OHE. High power-density (power generated per unit area of membrane) of the PRO membrane is essential to maximize the efficiency and minimize the capital and operating costs of the OHE. Likewise, high separation efficiency is needed in the MD process to effectively reconcentrate the diluted draw solution. Thus, robust PRO membranes that can support high pressure, have high water flux, low reverse salt flux, low structural parameter, and a good membrane support structure are essential. The MD process must also be able to withstand high operating temperatures (\u3e 60 ºC) and feed water concentrations, and have low pore wetting propensity. Additionally, the use of highly soluble ionic organic and inorganic draw solutions can increase PRO power densities while achieving high MD water fluxes, thus increasing efficiencies and decreasing costs of OHE. Please click Additional Files below to see the full abstract

Hybrid Pressure Retarded Osmosis–Membrane Distillation System for Power Generation from Low-Grade Heat: Thermodynamic Analysis and Energy Efficiency

We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal relative flow rate that maximizes the overall energy efficiency of the PRO-MD system for given working temperatures and concentration. In the case of unlimited mass and heat transfer kinetics, the energy efficiency of the system can be analytically determined based on thermodynamics. Our assessment indicates that the hybrid PRO-MD system can theoretically achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and a working solution of 1.0 M NaCl. When mass and heat transfer kinetics are limited, conditions that more closely represent actual operations, the practical energy efficiency will be lower than the theoretically achievable efficiency. In such practical operations, utilizing a higher working concentration will yield greater energy efficiency. Overall, our study demonstrates the theoretical viability of the PRO-MD system and identifies the key factors for performance optimization

Evaluating energy consumption of air gap membrane distillation for seawater desalination at pilot scale level

This study aimed to optimise an air gap membrane distillation (AGMD) system for seawater desalination with respect to distillate production as well as thermal and electrical energy consumption. Pilot evaluation data shows a notable influence of evaporator inlet temperature and water circulation rate on process performance. An increase in both distillate production rate and energy efficiency could be obtained by increasing the evaporator inlet temperature. On the other hand, there was a trade-off between the distillate production rate and energy efficiency when the water circulation rate varied. Increasing the water circulation rate resulted in an improvement in the distillate production rate, but also an increase in both specific thermal and electrical energy consumption. Given the small driving force used in the pilot AGMD, discernible impact of feed salinity on process performance could be observed, while the effects of temperature and concentration polarisation were small. At the optimum operating conditions identified in this study, a stable AGMD operation for seawater desalination could be achieved with specific thermal and electrical energy consumption of 90 and 0.13 kW h/m3, respectively. These values demonstrate the commercial viability of AGMD for small-scale and off-grid seawater desalination where solar thermal or low-grade heat sources are readily available