Cost Implications of Combined Power Generation and Seawater Desalination by OTEC Dominic Michaelis In 1881, Jacques Arsene d?Arsonval patented the idea of using the 20°C difference of temperature between the warm oceans? surface and the cool waters 1000 meters below these to produce energy. A brilliant follower of his, both pupil and friend, Georges Claude, inventor of the neon tube and liquefied air, in 1930, built the first experimental OTEC system, but instead of using a working fluid such as ammonia, he introduced the idea of using the surface water itself as the working fluid, evaporating it in a vacuum chamber to create desalinated vapour that would activate a turbogenerator, producing electricity, the desalinated vapour, then being condensed by heat exchangers cooled by the cold water drawn from the depths, delivering desalinated water. This became known as the open cycle OTEC. Since then, some small scale experimental plants have been built, the best known being the 210 kW open cycle plant in Hawaii, designed and monitored by the NEHLA laboratories, with Dr Luis Vega, who has written in depth about open cycle OTEC (1). A 1 MW plant is being built by a Japanese/Indian consortium. No large scale OTEC platform has been built, delivering 100 MW or more, although a few are planned. It is important, at this early stage, to establish a credible allowable budget for an open cycle OTEC platform, taking into account both the value of electrical energy delivered, and the value of desalinated water produced by other means , such as reverse osmosis. This would allow OTEC project costings to be carried out and their competitivity and suitability assessed against a realistic cost yardstick. The cheapest electrical generation plants are those that burn fuel, some gas turbogenerators reaching efficiencies of 50%. But these fossil fuel plants, whether burning coal, petroleum, or gas, inevitably contribute to global warming and to the greenhouse effect and are prey to unpredictable fuel cost variations. Nuclear electrical power generation is seen as ?clean? when one forgets that it also rejects some 50% of its heat into rivers , seas, or the atmosphere, that it carries with it nuclear waste disposal and environmental risks, together with its vast decommissioning ?hidden? costs, but seems to be the choice taken by most nations to provide for their ever growing electrical needs. Nuclear power costs It therefore seems reasonable to take the cost of nuclear power generation as the comparative base cost of OTEC electricity generation plants, expressed in $/kW installed. An idea of present day costs of nuclear plants is given by the Olkiluoto 3 power plant in Finland, scheduled to go on line in 2009, which will be the first EPR or European Pressurized Reactor built (2). The construction will be a joint effort of French Areva and German Siemens AG through their common subsidiary Areva NP. The electrical power output of the plant will be 1600 MW and will cost about ?3 billion. This is equivalent to 1875 Euros per kW, which, at today?s rate of exchange of 1 Euro = $1.2713 (1 July 2006 ) gives a figure of $2383 per kW. This is definitely the top end of nuclear plant costs. The Westinghouse AP-1000, scaled-up from the AP-600, has now received final design approval from the NRC and is expected to gain full design certification. Capital costs are projected at $1200/kW. (2005) (3). A base cost of $1500 / kW, between the two examples given, but much closer to the lower figure, therefore seems a reasonable figure to set as a comparative standard for OTEC power generation systems. OTEC desalination values Open cycle OTEC systems produce large quantities of valuable desalinated water. A project by Sea Solar Power, on a ship, is designed to produce 100 MW of electricity. Calculations demonstrate that the system will also produce 120 million litres of desalinated water/ day (4). I MW of open cycle OTEC generated electricity will produce 1,2 million litres of desalinated water/ day. This figure will be taken, although work carried out by OCEES indicates that this figure can be considerably improved, to give up to 2,8 million litres of desalinated water per MW per day (4a). The two principal methods of desalination are Multistage Flash (MSF) desalination, requiring large costly thermal energy inputs, and Reverse Osmosis (RO) which needs an associated power plant to force the sea water through a set of osmotic membranes. Multistage Flash distillation costs about $1 per 1,000 litres, Reverse Osmosis costs about half that amount (5). For a variety of reasons, in many cases where fossil fuels are costly, Reverse Osmosis is often selected. A recent RO example has been selected, to gauge its cost so as to give a value to the desalination ?by product? of OTEC per MW. A Reverse Osmosis desalination plant is being built in Perth, Australia, designed to provide 140 million litres of water per day. Its cost is $210 M. ( First assessment 2002 ) Suez Degremont are the contractors. It is planned for a 25 year life. The sum, to cover these 25 years of running and maintenance is 160 $M, bringing the total contract value to $370 M This is an overcost factor of 1,75, compared to the initial cost of $210M. The given figure excludes the necessary associated power plant, a 24 MW wind farm, and its high capital and yearly maintenance costs (6). A 200 MW OTEC plant, taken as a standard example, will generate 240M litres of desalinated water per day. This is an equivalent volume to that of a large tanker. Considering the initial contract sum only of $210M for Perth RO producing 140M litres a day, when upgraded as though it were producing 240M litres per day, the OTEC 200 MW corresponding value would be $360M. This figure, divided by 200 000 to reduce from 200 MW to 1 kW, represents a system benefit of $1800/ kW installed, $300 above the nuclear base cost of $1500/ kW installed. The cost of a dedicated 24 MW plant is not included. Conclusions The first conclusion is that an OTEC platform can justify a budget of over twice the base cost of nuclear, set at $1500/ kW installed, to include 1kW of power installed, and the desalination value of $1800/kW ( Perth figure) associated with that kW, a total figure of $3300/ kW. The budget for a 200 MW OTEC plant would be $660M. ( This is the cost of 5 French rafale fighters or 1 US stealth bomber! Energy independence must be a national priority and could be better achieved by OTEC than by conventional forms of military might.) If the 25 year maintenance and running costs are taken into consideration, the Perth figure needs to be multiplied by the factor of 1,75 previously referred to, giving a total cost benefit of $3150 /kW, over twice the base nuclear cost. These costs are relevant because, in open cycle OTEC, the running and maintenance are an integral part of the power generation process, and imply no significant overcost, all the more so because of its relative simplicity compared to a Reverse Osmosis plant. Figures available for running, maintenance and fuel of nuclear plants are comparatively low, and will be ignored, given the large capital cost variations. The second conclusion is that, if running and maintenance of Reverse Osmosis plants over 25 years are included with the capital costs, the OTEC platform can justify a budget of 1kW at $1500 / kW, plus the derived figure of $3150, a total of $4650 / kW; over three times the base cost of nuclear at $1500 / kW The third conclusion is that, if the desalinated water output can be doubled, to 2,4M litres per day / MW, or more, as has been calculated by OCEES, then the desalination figure is doubled proportionally, the OTEC platform budget jumping to $1500 / kW, plus $3150x2 or $6300 / kW, a total of $7800 / kW, just over 5 times the base cost of nuclear. Rather than a power generation plant with a desalination capacity, it then becomes a desalination plant with a power generation capacity. Although over 100 years old, OTEC is in its infancy. OTEC desalination values may increase, as may power output with techniques such as Mist Lift.(7 ) Renewable alternatives for the world?s ever growing needs must be sought, OTEC representing a possible source of clean energy on a nuclear scale. It also represents a stepping stone towards the hydrogen economy. It is hoped that these guide OTEC platform budgets will help those working on OTEC to be able to present a financially credible tool to assess their work, and encourage energy and water firms, funding organisations and governments to look at OTEC more favourably. Thanks are due to Michel Gauthier for his help in establishing the basic figures of OTEC desalination. www.clubdesargonautes.org
Dominic Michaelis References:
(1) OTEC open cycle overview by Dr Luis Vega, NELHA.
(2) EPR Finland at $1611/kW
(3) Westinghouse AP-1000 Nuclear ? Light Water ? Reactor.
(4) 100 MW OTEC ship planned by Sea Solar Power to give 120 M litres/day. That is
1,2 M litres/ day/ MW
(4a) OTEC open cycle modified to give up to 2,8 million litres of desalinated water
per MW per day.
(5) Multistage Flash distillation costs about $1 per 1,000 litres, Reverse Osmosis
costs about half that amount
(6) PERTH /AUSTRALIA
(7) Mist Lift OTEC |