Wind-powered Membrane Desalination Feasible with Minimal Energy Storage

Reverse osmosis (RO) is a desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> process that uses pressure to force high salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> water through a semi-permeable membrane. Since the production rate depends on the pressure difference, which in turn depends on the power supplied, the fuel of choice is energy-dense coal<!–[if supportFields]> XE “coal” <![endif]–><!–[if supportFields]><![endif]–> and oil. However, not only are fossil fuels unsustainable, but polluting as well. Park et al. (2011) studied the possibility of powering an RO plant with renewable wind power. The advantage to using wind power is that there is a steady supply of wind along coastal areas where desalination plants are most commonly placed. However, without energy storage as a buffer between the wind turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> and the plant, changes in wind speed and direction translate directly into changes in RO pressure and flow rate. While changes in power supply are expected with any power source, this is particularly troublesome for wind energy as wind flux<!–[if supportFields]> XE “flux” <![endif]–><!–[if supportFields]><![endif]–> typically changes by 12% per second, compared to the 1% change on average of solar flux. Park et al. tested their RO model with brackish feedwater using both a programmable power supply and an actual wind turbine. Issues arose such as a maximum power output of 300 W from the turbine, system shutdown under low wind speeds, salt diffusion across the membrane during shutdown, and a wind speed threshold for feedwater with a high osmotic pressure. However, the authors found that desalination performance under wind conditions of more than 7.0 m/s and turbulence of less than 0.4 was similar to that of steady state conditions, thereby concluding that directly connected wind-powered desalination is feasible with energy buffering to prevent system shutdown. —Erin Partlan
Park, G., Schäfer, A., Richards, B., 2010. Renewable energy powered membrane technology: The effect of wind speed fluctuations on the performance of a wind-powered membrane system for brackish water desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–>. Journal of Membrane Science 30, 34–44.

Park et al. used a test bench model of a reverse osmosis desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> system. They examined variables of average wind speed, oscillations in wind speed, and turbulence. Most situations were created with a programmable power supply and then verified using a wind turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> in a wind tunnel. Two brackish water concentrations were used–2750 and 5500 mg/L NaCl–and testing occurred under wind speeds ranging from 3.7–8.7 m/s. Using the power supply, they tested steady state wind speeds (no turbulence or oscillation) and programmed oscillating wind speeds with a turbulence intensity of 0.4 (0.6 being extreme fluctuations and 0.0 being no fluctuation).
They found that under steady state conditions, the optimal power outputs were 120 W for low concentration feedwater and 180 W for high concentration feedwater. The maximum power output for their experimental turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> was 300 W as safety mechanisms were activated under high wind speeds. They also found that while all wind speeds produced permeate flows with acceptable salt concentrations using low concentration feedwater, high concentration feedwater required a minimum of 120 W (corresponding to a steady state wind speed of approximately 5.3 m/s). Under oscillating conditions, they found that low wind speeds with low frequency oscillations produced the lowest permeate flows. This was caused by system shutdown due to low membrane pressure as a direct result of low power supply. They found that shutdown occurred at power outputs of less than 40 W for three seconds. Also, due to the low wind speeds, the system had difficulty restarting, thus further reducing productivity under these conditions. Another issue with system shutdown was the diffusion of salt across the membrane, resulting in a raised permeate salt concentration. Due to this diffusion, the maximum shutdown period for high concentration feedwater is three minutes as the permeate will reach unacceptable salt concentrations at this point. To re-achieve permeate salt concentration within two minutes after a shutdown period, 240 W was needed for high concentration feedwater compared with 120 W for low concentration feedwater. In contrast, high frequency oscillations did not permit shutdown even during low wind speeds as the pressure always returned quickly and thus did not differ greatly from steady state conditions.
The third variable tested was turbulence, measured by the amplitude of the oscillating wind speeds. The authors found that wind conditions of more than 7.0 m/s and turbulence of less than 0.4 adequately resembled steady state conditions. At less than 7.0 m/s, the system was able to reach low enough pressures to reach shut-off. In the extreme case, the osmotic pressure of the high concentration feedwater combined with the low power supply of low wind speeds resulted in zero permeate production.
Lastly, the authors used a wind turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> inside a wind tunnel to perform verification tests. They found that the wind speed did not always correspond with the power output due to complexities in the system, though the membrane pressure still depended directly on the power output. However, the system performance still compared well with the steady state test results using the power supply. The authors were also able to demonstrate an exponential decay in the membrane pressure after system shutdown, thus providing an explanation for the buffer time during low wind speeds. In addition, the wind tunnel tests displayed a 50% production loss with large wind speed fluctuations. However, the authors note that this type of turbulence is not typical and only observed in extreme conditions. They also note that large power fluctuations should not be significantly detrimental as their test membrane has been used for over 250 hours under extreme turbulence. Overall, the authors conclude that wind-powered membrane desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> is feasible despite any drawbacks.
We saw that both wind and solar power are feasible options for powering desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–>, though in different applications. We also saw that technology is constantly changing, both in the manipulation of existing processes to the invention of new ones. However, desalination is not a standalone process. Issues such as the ethical disposal of concentrated brine or the effect of environmental conditions are just some of the concerns that arise in operating a desalination plant.

Heat-absorbing Materials Useful for Increased Solar Still Efficiency

The solar still is the simplest form of solar-powered desalination. It uses the mechanisms of evaporation and condensation—the same processes used other forms of distillation—to purify the water. In a solar still, water is kept in an airtight container. As the water heats up, it evaporates and becomes water vapor. The lid of the still serves as the condenser to transform the purified water vapor back into water and the water slides down the slope of the lid to a collection point. Murugavel et al. (2010) built and tested a solar still with a roof-like glass lid, shallow basin, and insulation. They investigated the effects of various insulating and heat-absorbing materials on the efficiency of the still, since operation of the still depends on the amount of water evaporated, which depends on the amount of heat added to the water, among other things. The materials tested here included rocks, brick, metal, and cloth. The results of their testing showed that a ¾ inch layer of quartzite rock on the bed of the still performed the best. Murugavel et al. also performed theoretical calculations using energy balances and heat transfer equations to determine the theoretical efficiency possible for the chosen parameters. While the quartzite rock performed the best in tests, the actual efficiency was still nowhere near the theoretically possible one.—Erin Partlan
Murugavel, K., Sivakumar, S., Ahamed, J., Chockalingam, K., Srithar, K., 2010. Single basin double slope solar still with minimum basin depth and energy storing materials. Applied Energy, 87, 2, 514–523.
Murugavel et al. built and tested a solar still in Kovilpatti, India. They crafted a basin from mild steel plate, created a glass cover with a north and south slope, and insulated it with glass wool. In testing, a minimal water depth of 0.5 cm was used. Measurements were taken of the influx and outflux of water and of the temperature of the body of water and the water vapor. Also, atmospheric conditions were monitored to ensure that factors were controlled between test days. From incident solar radiation and ambient temperature data, the authors conclude that this is a valid assumption. The materials used to collect extra heat on the basin were ¼ inch quartzite rock, ¾ inch quartzite rock, ¼ inch washed stones, 1½ inch cement concrete pieces, 1¼ inch brick pieces, mild steel trimmings, and a light black cotton cloth. Multiple trials were run with each material, and while the overall productions hovered around 3.5 L/day of water, the ¾ inch quartzite material performed slightly better than the rest at 3.66 L/day of water.

In the theoretical testing, the authors use thermodynamic equations to model the heat influxes and outfluxes of the system. They note that they are novel in their approach as they use a variable term for the transmittance of solar energy through the glass cover, a term usually assumed to be constant. The resulting equations in their modeling are expressions for the instantaneous and overall water production of the solar still. However, when the parameters from the test of the ¾ inch quartzite rock are used, it was found that four-fold increase in the production rates was theoretically possible. While the authors note several areas of discrepancy—the change in water volume and depth over time, a higher proportion of water vapor inside the still, and differences in the absorptivity of the different testing materials—these results imply that the heat-absorbing material used has a minimal impact on improved efficiency, and that instead, focuses should be made on improving the design of the solar still itself to better utilize the incident energy.