June 01, 2012, CLIMA 2012 - III National Congress on Climate Change - FCT UNL, Lisbon
WIND ENERGY DRM TECHNOLOGY FOR POTABLE WATER SUPPLY A Cape Verde Example Rita Sousa1, Maria João Martins2 1: University of Minho,
[email protected] 2: Ecovisão,
[email protected]
Abstract: In developing countries water and energy needs are a priority and an everyday concern. Typical methodologies respond to water and energy supply problems through independent solutions. In this paper we present an innovative technology called DRM, “Dutch Rainmaker” that allows a common approach for both issues. DRM is a windmill technology that produces fresh water either from air, saline or brackish water using wind energy. It is a patented technology, a simple and robust system with low maintenance and infrastructural costs. Therefore it may be installed in areas with differing climate conditions. DRM is, in its essence, a mitigation and adaptation technology, although here we consider GHG emissions mitigation issues only. Our study evaluates a plant to be installed in Cape Verde. It regards the production of fresh water from saline water, where water is directly harvested from salty water without using fossil fuel energy, namely diesel, usually necessary for generators in Cape Verde’s islands. It has an expected average water production of 60m3/day/windmill. Results show the increase of local population with access to safe water; higher water consumption per capita; an increase in the penetration of renewable energy in the country, and consequently savings in CO2 emissions; and lower water costs for consumers are expected.
KEYWORDS: Desalination, wind power, GHG emissions, Cape Verde.
I. Introduction In developing countries fresh water supply, in adequate quantity and quality, is currently a challenge, mostly because of infrastructure limitations and shortage of the water resource itself. A possible solution to meet future consumption is provided by seawater, since the oceans represent approximately 97% of the water on the globe. On the other hand, surface water appears to be mostly brackish water, also useful if treated (Showers, 2002). In consequence, the desalination of brackish water and seawater presents itself as an immediate possibility to tackle the above referred shortage.
Regarding desalination, various processes have been developed within evaporation and membrane separation methods. The difficulty with their application is that all associated technologies require a generous amount of energy to desalinize water. This energy is usually electric power, in the case of developing countries such as Cape Verde sometimes fuelled by diesel generators. It is also of relevance to note that scientific studies on the water sector usually aim exclusively at issues regarding adaptation to climate change, mostly because of its immediate importance to human life (Muller, 2007). Nevertheless, as we show in this paper, water supply also allows for GHG emissions mitigation actions, thus contribut-
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ing to adaptation-mitigation integration at policy level, as it has been advocated as a better balanced solution (Klein et al., 2005; Mata and Budhooram, 2007; Swart and Raes, 2007). In fact, regarding greenhouse gas emissions, the water supply systems are responsible for direct (through fuel burning) and indirect (electricity use) emissions, in water capture and water treatment stages. Under this framework, the aim of this paper is to present a new technology for the direct production of fresh water from seawater or brackish water, or even from air, using wind power as an alternative to electric power used in conventional technologies, allowing for greenhouse gas emissions reductions, i.e. local mitigation measures. In chapter 2 we describe Cape Verde’s needs for fresh water, in chapter 3 we look at desalination processes and in particular we describe the DRM technology and its advantages for small islands communities, in chapter 4 we report our example in Cape Verde, including expected projects results. We conclude the analysis in chapter 5.
II. Cape Verde water supply availability and associated energy use The Republic of Cape Verde is a ten-island country, with an area of 4000km2. The Cape Verdean landscape is not particularly green because of its characteristics of almost no rainfall (average 227mm/year). Global freshwater availability in Cape Verde, strongly influenced by the climate, is therefore a problem. In this country we find a chronic shortage of water (less than 1m3/inhabitant/year). Urban development and consequent growth of water needs worsen the situation. Although access to drinking water from several sources has grown in the last years, there are still major gaps regarding supplies from fountains, national network and other sources. According to the 2010 Census, only 30% of the population has
public water network connection (supplied by Electra, the national water company), 45% collect water at water fountains and 25% from other sources (wells and water springs) (Chart 1). Water transport is carried out mainly by women and children, on foot.
Chart 1 - Population with access to safe water and sanitation (Source: Association of Commerce, Industry and Tourism, Cape Verde).
A significant fraction of the population does not yet have access to regular supplies. The water network provides only irregular supplies for urban population and in rural areas the situation is worse due to scattered communities and difficult access to many of them (IMF, 2008). In short, water supply and overall water resource management have been a challenge in Cape Verde, in terms of supply availability and cost to families. Taking into account the geographical and climatic characteristics of the Cape Verde islands and the consequent lack of fresh water sources and rain, it is understandable that the country has become increasingly dependent on seawater desalination. This process supplies 88% of water needs in Cape Verde (Chart 2).
Chart 2 - Cape Verde water sources, 2003-2007 (Source: National Agency for Economic Regulation, Cape Verde).
In 2008, 91.6% of Electra’s water supply came from desalination plants (table 1):
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Table 1 - Electra water production (m3) in 2008.
Process
N. plants
Total production
Multi Effect Distillation
1
194 002
Reverse Osmosis Mechanical Vapour Compression
8
3 556 430
5
132 509
Total desalinated water Total subterranean water
Desalination processes Common desalination processes
Evaporation and membrane separation are the main methods on which conventional desalination technologies are based. Others include crystallization, ion exchange, but their representation is currently negligible (He and Yan, 2009). Evaporation methods include Multi Stage Flash (MSF), Multi Effect Distillation (MED), both used in combination with power plants, and Vapour Compression Distillation, working on the principle of a heat pump. On the other hand, the most common membrane separation processes are Reverse Osmosis (RO) and Electro-dialysis (ED) (Figure 1). MSF and RO are the leading processes around the world with MSF representing 93% of evaporation methods and RO 88% of membrane separation (He and Yan, 2009).
Multi Effect Distillation (MED) Vapour Compression Distillation (VCD)
Desalination Processes Membrane Separation
353 682
The use of alternative energy sources, like wind and solar, comes now into view as a needed viable solution. The Government has already stated the interest in an integrated management of water resources, including aspects regarding a greater use of renewable energy to produce water The goal is to have 50% of water desalination powered with renewable energy by 2020 (INGRH, 2007).
III.A.
Evaporation
3 882 941
Desalination is an energy intensive activity and can therefore be directly linked to the production of electricity. In Cape Verde desalination is responsible for the use of 10% of the electrical power generated (IMF, 2008).
III.
Multi Stage Flash (MSF)
Reverse Osmosis (RO) Electro-Dialysis (ED)
Figure 1 – Most common desalination processes.
Cost-efficiency of all technologies using the above referred processes were analysed in several research studies and, in general, evaporation processes proved to be more energy intensive than membrane processes (Einav et al., 2003). In fact, considering that energy is an important input in desalination, RO has been recognized as the most efficient process, and hence, has become the favourite (Einav et al., 2003). Studies show that the production of 1m3 of water requires 3.5 to 4.5 kWh in a RO typical plant (Einav et al., 2003). However, land use, impact on groundwater and on the marine environment, including the composition of discharge brines and noise pollution are also aspects to be considered when evaluating the impact of a desalination plant. Typically, RO plants have higher discharge concentrations of salts, and are noisier than MSF plants (Einav et al., 2003).
Regarding the use of alternative energies in desalination plants, solar, wind and nuclear have become more competitive in recent years (He and Yan, 2009). Since solar evaporation plants or solar energy powered RO systems require large implementation areas, wind power is more adequate to islands, for it does not require much land. Studies have demonstrated the technical feasibility of RO and mechanical VC wind-powered desalination (Forstmeier et al., 2007), and cost evaluation has also been assessed for RO plants (GarcíaRodríguez et al., 2001). Next step technology in desalination involves a wind powered flash evaporation process and is
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described in the following chapter. This technology was considered to be the best desalination solution over RO technologies given the conditions of the Cape Verdean municipality chosen.
III.B.
pected maximum yield is 80m3/day depending on local climatic conditions. In Table 1 we summarize the DRM water-to-water technical specifications.
DRM technology
DRM water-to-water specifications The concept of DRM technology is a modification of the traditional electricity generating windmill.
Figure 2 - DRM windmill for water treatment.
In this “water-to-water” technology, the generated energy in the wind turbine is used directly to drive a closed loop heat cycle which results in water treatment by evaporation and condensation (Figure 3). Wind energy (1) is used to directly drive a compressor (2) that compresses a coolant, in this case ammonia (NH3). The compression results in heat generation that is used for flash evaporation (5) of the saline or brackish water. After this, the compressed ammonia is expanded (4) resulting in a temperature decrease. The water vapour generated in the evaporation stage can subsequently be recovered by extracting heat using the cooled ammonia (6). Since wind energy is directly used, it increases considerably the process efficiency when compared to others where fuel is needed to previously power an engine. Depending on the source, treated water will be comparable to demineralised water and may undergo post-treatment processes so that it may be used as drinking water (water-to-water). The ex-
Figure 3 - Process and detail of the water-to-water DRM technology.
Table 2 - Technical specifications of a DRM windmill.
Installed power Wind speed (model) Wind speed(minimum) Production (average) Production (maximum) Specific energy use (water pumping) Rotor diameter Area swept by blades N. of blades Tower height Tower weight
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Value 100 13 2,5 50 – 60 80 2,15
Unit kW m/s m/s m3/day m3/day kWh/m3
18 – 21 40278 3 37 25
m m2 blades m ton
This technology may also be applied in an air-towater process: since the technology is based on a heat cycle it also allows the recovery of water from air by cooling it, resulting in moisture condensation.
DRM Technology advantages for Cape Verde purposes As previously referred, other desalinization processes are commonly used, as RO or MSF. However, their use of renewable energy sources is still very limited. Wind-powered DRM solves the energy problem of desalination without requiring a large amount of wind. This is the most important advantage of DRM technology. Other advantages include: a reduced need for infrastructures; a simple, robust and low-maintenance system with a high lifespan (up to 20 years); reduced maintenance and operating costs; possibility of application in areas of diverse climatic characteristics. In fact, depending on the existing wind, one or two water-to-water windmills have the capacity to provide irrigation water to a community of 2000 people. Considering a desalination plant to be implemented on an island with small communities DRM technology has both environmental and economic advantages when compared to RO. State of the art RO requires electric power, usually from network or diesel generators, although latest developments include experimental wind powered RO plants. DRM is a simple system that only works on wind. Regarding the process, RO requires a prefiltering and a frequent membrane substitution. It also originates a concentrate containing high levels of chlorine, copper and antiscalants that may have impacts on the ecosystem when discharged into the aquatic environment, as mentioned above. DRM does not require the use of antiscalants nor generates a concentrate.
IV.
Application of DRM water-towater in Cape Verde
IV.A.
Project phases, location and population
This project aims to demonstrate the potential of DRM technology application in Cape Verde, through the installation of 4 windmills for production of drinking water from seawater and brackish water. The rural area is a prime target for implementation of the project because of its needs regarding basic infrastructure and access to water. It is expected that the 4 windmills provide safe water for 6000 people. The project will undergo seven actions, starting with a validation study regarding the chosen location. Then on a second phase, all equipment and structures will be implemented. Actions three, four and five will occur simultaneously and comprise the operation and monitoring, and awareness raising and training activities. Finally, the project will be transferred to local authorities (action six), and to conclude, a seventh action consists in consulting support after transfer. This is a pioneering project to be replicated in the future in other municipalities and / or industrial units.
IV.B.
Expected results and indicators
There are two specific objectives of this project: the supply of drinking water using renewable energy and the capacitation of the population and technicians involved. Increased access to potable water for the local population is the first expected result. The second regards the energy used in collecting this water: increased supply of potable water using renewable energy. The third result is stimulation on consumption, representing an increase in m3 of water consumed per capita. In places where access to potable water is limited, DRM will reduce the need for water collection at water fountains, while at the same time reducing
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the need to build infrastructures for water supply from already existing desalination plants. In Cape Verde the water consumption from fountains is about 15L/person/day and from its national network of 35L/person/day. We consider that people will have access to larger quantities of potable water (3rd result) at an estimated 40L/person. This results in the supply of potable water to 6000 people, with the installation of 4four windmills (1st and 3rd results). We set up a comparison between the implementation of 4 DRM windmills and the installation of a new RO desalination plant. The considered rural population has no access to the water network and is dependent on water supply from fountains or water tankers, provided by the municipality. For model purposes we compared the implementation of these 4 towers to the best alternative which would be implementation of a new RO desalination plant, as demonstrated in chapter 3. Specific conventional energy consumption associated to the DRM windmills is associated to water pumping and amounts to 2.15kWh/m3. Recalling that RO plants have an energy intensity of 4kWh/m3, then savings may amount to 1.85kWh/m3 in a worst case scenario, where electric power must be provided by the grid for water pumping. Considering UNCTAD indications for African electricity emission factors, Cape Verde’s power sector emits 0.629kCO2eq/kWh. So, our 1.85kWh/m3 savings may be translated into 1.16kCO2eq/m3. In this situation, this four tower project totalizes savings of 279kCO2eq/day, or 102tCO2eq/year. Considering that wind is a renewable energy source with almost no emissions in greenhouse gas accounting principles, we may consider further developments in DRM technology that will allow water pumping to be fuelled by the tower energy itself. In this case, total savings will amount to 4kWh/m3 regarding the electricity use by a RO plant. In this case, savings would sum 603kCO2eq/day, or 220tCO2eq/year. These values are associated with water consumption by 6000 people.
Further analysis must take place in order to calculate emissions savings from the substitution of water tankers by DRM technology. We currently have no data to perform this study, but knowing that a diesel fueled vehicle/tanker emission factor is higher than the electricity grid emission factor, carbon savings will exceed the values presented above. A fourth result of this DRM project is an increased penetration of renewables in the country. Considering that in average a 100kW wind tower produces about 75000kWh of energy per year, depending on the wind and tower itself, this fourth result is automatically fulfilled. Finally, Cape Verde is considered to have very high water prices, because of its scarcity. Electra prices are about 3-4$/m3 of water. Next step analysis will include water tariff calculus with the purpose of reducing water costs for families. This will be the project last result: a reduction in the price of water.
V. Conclusions In this paper we present a pilot application of DRM water-to-water, a new desalination technology, in a rural municipality in Cape Verde. This technology operates on wind power, and because of this it is highly competitive with state-of-the art RO desalination processes, for small communities. In this “water-to-water” technology, the generated energy in the wind turbine is used directly to drive a closed loop heat cycle which results in water treatment by evaporation and condensation. Since wind energy is directly used, it increases considerably the process efficiency when compared to others where fuel is needed to previously power an engine. This project aims to demonstrate the potential of DRM technology application in Cape Verde through the installation of 4 windmills to provide safe drinking water for 6000 people, using renewable energy, and capacitating the population and technicians involved. The increase of m3water per capita consumption from 15 L/day/person to
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40L/day/person is also an expected result. Lastly, this project will save 102tCO2eq/year or even 220tCO2eq/year if technology developments will allow the use of wind energy for water pumping. More studies are needed on emission of water supply by auto-tanks, and on water tariff. Although we recognize that this sector has a small impact on emissions when compared to transports or residential sectors, we also consider water to be a crucial sector for human living, so all possible mitigation measures in this sector should be implemented. The use of renewable energy in desalination is therefore an important step for sustainable growth and development in small islands developing countries.
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VI.
References
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