CSP engineers are seeking ways to reduce water use so they can overcome one of the greatest challenges in locating power plants in desert regions.
By Jason Deign
The latest images emerging from BrightSource Energy are breathtaking. As Ivanpah, the world’s largest solar thermal plant, reaches the halfway point in its construction, the developer’s press photos begin to convey the truly immense scale of the project.
An August update on the construction work shows how more than 48,000 heliostats, in excess of 90% of a 53,500 total, are already in place at the plant’s Unit 1 solar field. A further 8,000 are on the ground at Unit 2, and power block building activity is underway at Unit 3.
Another detail that comes across clearly from the images is that there is not much water about. As with practically all CSP plants in the world, the location for Ivanpah was selected on the basis of having clear skies year-round. It is in a desert.
And that poses a challenge for engineers, because although CSP makes great use of a desert’s abundant solar resource, it also requires a fair amount of another resource that is in short supply: water.
Generally speaking, CSP plants require significant amounts of water in order to cool steam turbines and to wash heliostats. Both functions are fairly essential but can be mitigated if necessary.
For the turbines, there is always the option of using dry cooling to reduce water use. This is more expensive than wet cooling, but may be the order of the day in places where water is at as much of a premium as power.
According to a June 2009 Congressional Research Service paper titled ‘Water Issues of Concentrating Solar Power (CSP) Electricity in the US Southwest’: “Water supply is an issue for locating any thermoelectric power plant, not only CSP.
“Why is there concern specifically about the CSP water footprint? CSP facilities using wet cooling can consume more water per unit of electricity generated than traditional fossil fuel facilities with wet cooling.”
The report adds: “Options exist for reducing the freshwater consumed by CSP and other thermoelectric facilities.
“Available freshwater-efficient cooling options, however, often reduce the quantity of electricity produced and increase electricity production costs, and generally do not eliminate water resource impacts.”
This means project developers need to give careful consideration to power cooling design in relation to the sites selected for CSP.
In the United Arab Emirates, for example, Masdar Power was able to save costs and stick with wet cooling for its 100MW SHAMS 1 parabolic trough development thanks to the proximity of a sewage treatment plant that could supply waste water.
Another advantage of the site chosen is that it is less exposed to airborne particulates than Madinat Zayed, the first location considered.
This means the need for mirror washing is approximately halved, to once a week, although it is still roughly twice the frequency required for California.
BrightSource, in any case, is aiming to minimise water consumption as much as possible at Ivanpah, with an air-cooled condenser system that reduces water usage by more than 90% compared to conventional wet cooling systems.
In fact, Andrew Beaux and Simon Pouet, project engineer and project manager, respectively, at the direct steam generation Fresnel reflector plant developer Solar Euromed, assert that developing projects with dry cooling can reduce water consumption by up to 95%.
And there is research underway to cut that level even further.
Valeriano Ruiz Hernández, the president of the Spanish Renewable Energy Advanced Technology Centre, CTAER, says: “The idea is to diminish or reduce to zero the water the plants use. You can do it with air, but that penalises the performance.”
Currently, notes Ravi Yadav, project manager for alternative energy at the analyst firm GlobalData, the problem with dry cooling is: “It hasn’t been used on a massive commercial scale.
“Most of the developers have not tried it out; a part of [CSP] is still through wet cooling. Technically speaking there seems to be no issues, it is more a matter of much implementation has gone on.”
That could change thanks to developers such as BrightSource. And the challenges involved in saving water could ultimately be worth overcoming in order to help CSP break into new markets.
For example, says Patricio Castro, a former United Nations Industrial Development Organisation adviser: “The use of dry-cooled CSP plants with molten-nitrate thermal energy storage is a natural for the big copper mines in the Atacama Desert.
“Nobody seems to have made inroads into this opportunity. Copper mines run there on expensive diesel fuel to make copper concentrates for export. Chuquicamata, La Escondida and Tesoro should be able to use at least 300MWe units each.”
If the mining company owned the CSP plant for private consumption then it would not be liable for value-added sales tax, he states, and there would be no transmission losses. That could make CSP a highly desirable power source.
Provided developers can resolve the water-use issue, of course.
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