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Tiny Solar Thermal Power Plant Solves Gigantic Salt Problem

02 Apr

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We’ve been getting all excited about Ivanpah and Crescent Dunes, two new gigantic concentrating solar thermal projects in California and Nevada. Now let’s turn our attention to a third solar thermal plant at the Panoche Water and Drainage District in California.

This one, from a company called WaterFX, is different in terms of scale (miniature compared to the first two), solar energy collection (its concentrators are troughs, not flat panels), and thermal platform (it uses a form of mineral oil, not molten salt).

solar thermal desalination

Solar Power Desalination Solves Part Of the Problem…

The plant is actually a modular, integrated system that WaterFX calls the Aqua4™ Concentrated Solar Still, and it’s designed to solve a problem bedeviling the water desalination field: what to do with all the leftover salty brine after you’ve extracted all the fresh water?

That problem is bound to keep growing as already-stressed global water resources keep shrinking, forcing farmers and other users to tap formerly overlooked sources including seawater, brackish groundwater, and various forms of wastewater.

One key element in the problem is that advanced desalination typically uses a high-pressure process called reverse osmosis, which requires a tremendous amount of energy, adding to the global carbon burden aside from adding to costs.

The energy factor can be partly solved by using solar power for desalination, along with other forms of clean, renewable energy.

However, that still leaves the salt problem. According to WaterFX, typical reverse osmosis operations only recover about half their input in the form of freshwater. The other half is a saltwater brine that needs to be further processed or transported offsite for environmentally responsible disposal.

To get reverse osmosis from the 50 percent recovery rate to a more concentrated byproduct would help ease disposal costs but that would require more pressure, which requires more energy, making the whole system less cost-effective.

The resource recovery route also involves additional expense, since the highly diluted brine would have to undergo additional steps to achieve a concentration that makes extraction cost-effective.

…And Here’s A Solution For The Other Part

One Aqua4 module is designed to produce about 65,000 gallons of freshwater daily, with a relatively small footprint of just 160 x 40 feet.

The system basically consists of common off-the-shelf components including a 400 kW trough-shaped solar thermal collector that focuses energy on a pipe through which mineral oil runs.

The heated oil goes to a heat pump to ramp up efficiency, which goes to a distillation system that yields a freshwater condensate and a concentrated brine.

The system also includes a thermal energy storage component so it can continue to operate at night, but that’s not what caught our attention.

What did catch our eye is the potential for resource recovery from spent brine, helping to make the operation cost-effective.

Here’s how it works on the agricultural drainage water of Central Valley, which is typically discarded as an unsuable byproduct of irrigation. The salinity level for drainage water in that region can range higher than the content of seawater.

The solar power plant provides emission-free energy that can be cheaper than fossil fuels, helping to keep energy costs down.

The distillation part of the process achieves a recovery rate of more than 93 percent while working far more quickly than natural evaporation and simple stills.

The concentration of salt in this solution is more than 20 percent by weight, enabling a cost-effective solids extraction process. The idea is that since you’re dealing with far smaller volumes of brine, you don’t need the kind of high-volume equipment (and energy input) that would be required when the recovery rate is only 50 percent.

Now, About That Salt…

Right now the project is in phase one, with the equipment humming along nicely and plans afoot for expansion.

Phase 2 is where the rubber will hit the road in terms of resource recovery. Due for completion this fall, phase 2 is expected to demonstrate that resource recovery can from the brine can be managed with minimal environmental impacts, if any.

The recoverable products include gypsum and calcium compounds that are widely used in the building industry for drywall, plaster, and cement.

Also present in the brine from this particular drainage area are magnesium salts, which are used in the medical industry, selenium (a health supplement), nitrates (fertilizer), and boron, best known for its use in bleach and pyrotechnics among many other uses.

Boron is also coming into its own in high-efficiency electronics and cutting edge solar technology, and for the record, selenium is also used in electronics as well as glass making, so altogether the region could be looking at a new high-value, job-creating industry in tandem with its agriculture base.

WaterFX also points out that the re-use of local groundwater reduces carbon emissions related to long-distance transportation of freshwater supplies, and it could also help alleviatewater resource competition issues  that are sure to intensify unless more sustainable solutions are adopted.

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