Water, Water Nowhere – Implementing Solar Thermal in a Desert Environment

The world’s deserts beckon. 

Having previously been ‘off the radar’ of the large-scale power generation industry, deserts are now as hot from a project development standpoint as, well, the deserts themselves.  Multi-gigawatt visions abound, most notably the DESERTEC Initiative in North Africa.  And it’s a potent vision: exceptional solar resources, abundant land, and – in the case of North Africa and the Levant – rapidly growing home markets and close proximity to rich European power markets.

And almost no sustainable natural water supply.  And that’s a big problem, because parabolic trough solar thermal facilities – the technology most likely to be deployed in the near-term in these environments – require a great deal of water, both to create steam to drive steam turbines and for regular cleaning of the large mirrors that form the parabolic trough. 

For example, a typical parabolic trough plant with wet cooling uses approximately 800 gallons/MWh, comprised of 780 gallons for evaporation and water make-up and 20 gallons for mirror washing.  Change to dry cooling – at the expense of increased capital costs and decreased efficiency – and a facility still requires approximately 80 gallons/MWh for make-up and mirror washing.  For a 100 MW facility operating 14 hours per day (i.e. producing 1,400 MWh per day), that’s over one million gallons of water per day; change to dry cooling and that 100 MW facility still consumes more than 100,000 gallons of water per day.

The US Department of Energy summarizes the mirror cleaning issue this way:  “For example, reduced reflectivity of a solar mirror due to soiling can lead to an 8%–12% drop in performance between cleanings. The issue of soiling and cleaning must be dealt with before CSP plants are deployed on a massive scale in low-water desert environments.” 

So how do we square this circle?  What innovations are needed in solar thermal technology so that the deserts’ potential can be realized?

Looking ahead, next generation receiver tube technologies have the potential to minimize or eliminate the water necessary to drive a steam turbine:  for example, solid-state thermoelectric, where electricity is generated in the receiver tube itself.  (Another highly touted technology – direct steam generation – doesn’t change the water equation significantly and creates additional operational issues, as the impurities that are typically concentrated in a boiler and removed by blow-down will have to be removed in the water pre-treatment process – a much more difficult task given low concentrations of impurities.)

Solid-state thermoelectric and other reduced water consumption solar technologies – including solar towers, like those under development by eSolar and BrightSource – deserve rapid development and deployment for this very reason.

Mirrors will still require washing, however, in any mirror-based technology, whether trough or tower.

Increased receiver tube and energy conversion efficiencies should (over time) reduce required mirror area per MW.  Larger improvements, however, will come from the reduction of the mirror’s cleaning requirements in the first place.  That means coatings designed to slough off dust and dirt, maintaining maximum reflectivity with minimum washing and, therefore, water use.  (Further into the future, it may be possible to reduce or eliminate cleaning needs through electrostatic technologies.)

Coatings may seem a somewhat mundane – and indeed, mostly invisible – linchpin for the realization of the deserts’ potential for near-term, emissions-free power generation (and the ambitions of DESERTEC and the like) compared to soaring towers, massive mirrors, complex receiver tubes, and humming heat exchangers and steam turbines.  Without them, though, those visions of multi-gigawatt, emissions-free power may remain just that: visions.

Let’s get innovating.