Blue Energy: Electricity Production from Salinity-Gradients by Reverse
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Blue Energy: electricity production from salinity gradients by reverse electrodialysis Jan Willem Post Thesis committee Thesis supervisor Prof. dr. ir. C.J.N. Buisman Professor of Biological Recycling Technology Sub-department of Environmental Technology Thesis co-supervisor Dr. ir. H.V.M. Hamelers Assistant professor Sub-department of Environmental Technology Other members Prof. Dr. A.A. Broekhuis, University of Groningen Prof. Dr. M.A. Cohen Stuart, Wageningen University Prof. Dr. L. Diels, University of Antwerp / VITO Prof. Dr. C. Kroeze, Open University / Wageningen University This research was conducted under the auspices of the Graduate School SENSE (Socio-Economic and Natural Sciences of the Environment). Blue Energy: electricity production from salinity gradients by reverse electrodialysis Jan Willem Post Thesis submitted in partial fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. dr. M.J. Kropff, in the presence of the Thesis Committee appointed by the Doctorate Board to be defended in public on Tuesday 3 November 2009 at 4 PM in the Aula. Jan W. Post Blue Energy: electricity production from salinity gradients by reverse electrodialysis, 224 pages. Thesis, Wageningen University, Wageningen, NL (2009) With references, with summaries in Dutch and English ISBN 978-90-8585-510-1 Abstract Salinity-gradient energy or Blue Energy is a promising renewable energy source for the future. Estimates from literature predicted coverage of over 80% of the current global electricity demand when applied in all river mouths. From thermodynamic calculations it can be derived that each m3 of river water can yield 1.4 MJ when mixed with the same amount of sea water. Two techniques are available to convert Blue Energy into electricity: pressure- retarded osmosis and reverse electrodialysis. For further research in this thesis, the latter was selected. From a review we concluded it has better prospects for river mouths regarding power density, energy recovery, fouling behavior and process economy. Until now, it has been investigated generally with a focus on obtained power, without taking care of the energy recovery. In this thesis, we emphasized the aspect of energy recovery. In our opinion, this is the most critical factor to success. We were the first to obtain a significant energy recovery of over 80%. Another important issue of this thesis is how the system will behave in practice, i.e., when it is applied to feed waters with different chemical compositions and biological activity. We investigated different operations and designs in relation to biofouling. Regarding the operations, a periodically applied feed water reversal hampers the biofouling significantly (extending the operational period with a factor 4). Regarding the design, a proof of principle was given of a newly designed spacer-free stack. This design leads to better performances (power density and energy recovery) and is less sensitive to biofouling. Based on this work, we defined requirements for membrane development and stack design (in relation to pre-treatment and friction losses). We also re-examined the economic feasibility and the global and national prospects of Blue Energy. Keywords: Salinity-gradient energy, Blue Energy, reverse electrodialysis, pressure-retarded osmosis, power density, energy recovery, biofouling Voor onze zonen: Richard, Jonathan en Marnix Table of contents 1 Introduction to this thesis 9 2 Evaluation of pressure-retarded osmosis and reverse 31 electrodialysis 3 Energy recovery from controlled mixing of salt and fresh 71 water with reverse electrodialysis 4 Influence of multivalent ions on power production with 93 reverse electrodialysis 5 Prevention of biofouling by flow reversal in reverse 117 electrodialysis stacks 6 Proof-of-principle of a reverse electrodialysis stack without 147 spacers 7 Technical and economic prospects of reverse electrodialysis 161 8 Global and national prospects of salinity-gradient energy 187 Summary and discussion / Samenvatting en discussie 205 Post Scriptum 219 1 Introduction to this thesis Salinity-gradient energy or Blue Energy is a promising renewable energy source for the future. In densely populated delta areas, where rivers with a low salinity flows in to the saline sea, the potential is enormous. Estimates from literature predicted coverage of over 80% of the current global electricity demand. This means a potential reduction of 40% of global energy- related greenhouse gas emissions. In this introduction to the thesis, we provide a short overview of the thermodynamic base of salinity-gradient energy (Gibbs’ free energy of mixing) and the investigated conversion technology (reverse electrodialysis). We present the aspects that primarily influence the technical and economic potential of Blue Energy, resulting in the aim and outline of the thesis. Chapter 1 1.1 Blue Energy or salinity-gradient energy The current energy production is largely based on fossil fuels and is not only characterized by (i) vulnerability and (ii) imminent scarcity, but also by (iii) the emission of greenhouse gasses as a consequence of the combustion of fuels. It is very probable that this human fossil-fuel burning is the biggest contributor to climate change. Each of these three concerns should provide enough motivation for drastically reducing the burning of fossil fuels. This should be done by reducing the energy demand (consumption) and by changing the energy supply (sources and production). In order to meet the respective concerns, the energy supply should be based on (i) a locally available alternative energy source, (ii) a renewable energy source, and (iii) an environmental-friendly non- combustion energy conversion. In this thesis an interesting, but hardly known, alternative renewable and environment-friendly energy source is investigated: salinity-gradient energy, or according to the research and development program in The Netherlands: “Blue Energy”. Salinity-gradient energy is a renewable energy source that was recognized already in the 1950s [1]. It was mentioned that besides the gravitational potential, the natural runoff in coastal areas has a huge physical-chemical potential. This potential is the result of the salinity-gradient between the mainly-fresh runoff (river mouths) and the receiving mainly-saline reservoirs (seas and oceans). When a river runs into a sea, spontaneous mixing of fresh and salt water occurs. This natural process is irreversible; no work is attained from it. However, if the mixing is done (partly) reversibly, work can be obtained from the mixing process. In literature [2, 3] it was assumed that from each cubic meter of river water 2.3 MJ of work could be extracted. According to Norman [2]: The tremendous energy flux available in the natural salination of fresh water is graphically illustrated if one imagines that every stream and river in the world is terminated at its 10 Introduction to thesis mouth by a waterfall 225 m high… For the Netherlands, a geographically low-lying flat country with about 27% of its area and 60% of its population located below sea level [4], this provides an extremely interesting opportunity to harvest energy from the estuary of two important European rivers, which together with their distributaries form the Rhine-Meuse- Scheldt delta. As an example, keeping in mind the words of Norman, the 30- km long Afsluitdijk that dams up the river mouth of the river IJssel (a distributary of river Rhine) becomes comparable to a huge power dam of over 200 m high, see Figure 1. The 1,100-km2 Lake IJssel (the artificial estuarine reservoir of river IJssel) becomes comparable to an enormous energy reservoir with over a billion m3 storage capacity (assuming a level difference of only 1 m). The globally available power in form of salinity gradients has been estimated in the 1970s (on the basis of average ocean salinity and annual global river discharges) to be between 1.4 and 2.6 TW [5, 6]. Compared to other forms of marine energy, salinity gradient resources are in the same Figure 1: The salinity-gradient energy potential makes the 30-km long Afsluitdijk in The Netherlands comparable to the 221 m high Hoover Dam in Nevada and Arizona (USA). 11 Chapter 1 order as wave energy or thermal gradients and are 100 times higher than those of tidal energy [7]. Or, 1.4 TW (12,279 TWh/y) should be able to satisfy over 80% of the current global electricity demand (15,746 TWh/y [8]). In 2006, coal-fired generation accounted for 41% of world electricity supply [8]. Due to expected high prices for oil and natural gas, the coming decades the coal-fired generation capacity will expand, particularly in nations that are rich in coal resources, which include China, India, and the United States. Annual coal-fired generation is projected to double from 7,400 TWh in 2006 to 9,500 TWh in 2015 and 13,600 TWh in 2030 [8]. Replacing current and planned coal-fired power plants with salinity power plants (Figure 2) could reduce global greenhouse gas emissions by 10 Pg 10 CO2-eq/year (~ 10 tonnes/year), when calculated with standard emission factors [9, 10] and an efficiency for coal-fired power plants of 40%. This means a potential reduction of 40% of current global energy-related greenhouse gas emissions (40% of 28.9 Pg CO2-eq/year [11]). Figure 2: Replacing current and planned coal-fired power plants with salinity power plants has the promise to reduce greenhouse gas emissions. 12 Introduction to thesis These substantial numbers for the theoretical potential justify the research efforts in the field of salinity-gradient energy or Blue Energy in general, and the topics addressed in this thesis in particular. In this introduction to the thesis, we provide a short overview of the thermodynamic base in section 1.2 and the investigated conversion technology, called reverse electrodialysis, in section 1.3. In section 1.4 the aim and outline of the thesis are presented.