ARTICLE pubs.acs.org/est
Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance David A. Vermaas,†,‡ Michel Saakes,† and Kitty Nijmeijer*,‡ † ‡
Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands Membrane Science and Technology, University of Twente, Research, P.O. Box 217, 7500 AE Enschede, The Netherlands
bS Supporting Information ABSTRACT: The mixing of sea and river water can be used as a renewable energy source. The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED). This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output. Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance. The generated (gross) power densities (i.e., generated power per membrane area) are larger for smaller intermembrane distances. A maximum value of 2.2 W/m2 is achieved, which is almost double the maximum power density reported in previous work. In addition, the energy efficiency is significantly higher for smaller intermembrane distances. New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design. In that case power outputs of more than 4 W per m2 of membrane area at small intermembrane distances are envisaged.
’ INTRODUCTION The salinity difference between salt water and fresh water can be used to generate renewable energy. This salinity gradient power is available from the change in Gibbs energy when fresh and salt water mix to a brackish solution; for example at locations where river water flows into the sea. The global runoff of river water into the sea has a potential to generate 2.4 TW1 of salinity gradient power. This huge amount of power exceeds the prospected global electricity demand for 2011, which is 2.3 TW.2 Several techniques are proposed to capture salinity gradient power.1,37 Reverse electrodialysis (RED)1,3,4,8 and pressure retarded osmosis (PRO)5,6 are most cited in literature. RED facilitates the transport of positive and negative ions present in the water through selective ion exchange membranes. PRO uses membranes that allow only water to pass, creating a pressure difference that can be converted into electrical energy. Although the theoretical potential is equal for both technologies, Post et al.9 concluded that RED is more favorable for power generation from sea and river water, because the power density (i.e., generated power per membrane area) was expected to be higher for RED in that case and this technology was considered less sensitive to fouling of the membranes. Although power densities reported in literature are currently higher for PRO,5 RED is considered as a viable candidate to generate energy from salinity gradients. Modeling data show that much higher power densities in RED are possible8,10,11 by optimizing the flow rates and intermembrane distance. The present research focuses on power generation from seawater and river water using RED to test this hypothesis. r 2011 American Chemical Society
A RED device consists of an alternating series of cation exchange membranes (CEMs) and anion exchange membranes (AEMs), stacked with alternately salt water and fresh water flowing between these membranes. The salinity difference on either side of the membrane generates ion transport through the ion exchange membranes, resulting in a net charge transport. At the electrodes this ionic charge transport is converted into electrical energy by a reversible redox reaction. To save electrode area in a large-scale application, a sequence of multiple CEMs and AEMs can be stacked between the two electrodes (i.e., anode and cathode). To make RED a commercially attractive renewable energy source, the gross power density should reach a value of at least 2.2 W/m2.12 The highest reported gross power density so far is 1.2 W/m2.13 The design of the RED stack as used in previous experiments is predominantly based on its reverse application, electrodialysis (ED), where an electric current is applied to desalinate water or recover dissolved salts. Because ED has a different aim, its optimal design is significantly different from the design preferred in RED. For example, high flow velocities, which require a thick feedwater compartment, are desired in ED to reduce salt depletion in the boundary layers adjacent to the membranes. In RED, where ions move in the direction of the concentration gradient, depletion of salt is not an issue and the Received: April 14, 2011 Accepted: July 7, 2011 Revised: June 30, 2011 Published: July 07, 2011 7089
dx.doi.org/10.1021/es2012758 | Environ. Sci. Technol. 2011, 45, 7089–7095
Environmental Science & Technology
ARTICLE
Table 1. Characteristics of Spacers As Used in This Research type
thickness specified (μm)
thickness measured (μm)
ratio mesh size/wire diameter
open area (%)
porosity (%)
Sefar 03-90/49
60
61 ( 1
2.31
49
71
Sefar 03-160/53
100
101 ( 1
2.62
53
70
Sefar 03-300/51
200
209 ( 2
2.46
51
67
Sefar 06-700/53
485
455 ( 6
2.64
53
75
optimal thickness of the water compartments will be smaller. Consequently, power densities obtainable in RED can be significantly increased by tuning and improving the design of the RED stack toward the specific application. The feedwater compartments, and more specifically the river water compartments with their low salt concentrations, have a large contribution to the internal resistance of the RED system.4,15 Thinner compartments, i.e., smaller intermembrane distances, will reduce this resistance and consequently increase the obtained power densities. Previous work shows that a RED stack with an intermembrane distance of 200 μm generates more than twice the power density obtainable from the same stack with an intermembrane distance of 500 μm.3,4 Model calculations for intermembrane distances smaller than 200 μm indicate that higher power densities are possible.10,11,14 A disadvantage of small intermembrane distances is the large hydraulic friction of the feedwater in the compartments and extra pretreatment to avoid fouling. The energy spent on pretreatment to prevent fouling is considered relatively small for intermembrane distances in previous research (
