Why solar cells failed Bahrain

The design of Bahrain’s stunning wind-powered buildings makes more sense than you might initially think. The three wind modules supported on bridges between these twin towers are projected to cover some 11-15 percent of the buildings electricity needs, and this week the turbines turned in tandem for the first time. Now, the building’s engineers are tweaking the system to optimize their output.

As the first (allegedly) building-integrated installation of wind turbines, the Bahrain World Trade Center is a landmark in green design. Electricity generation was involved in the design discussion from the start, and some people—like me—wondered why an eco-conscious building in the Middle East would pick wind over solar as their renewable of choice.

There are a couple reasons, and they both reflect on a central part of renewable energy policy today. First of all, Bahrain is an island in the middle of the Persian Gulf, so wind is actually available in abundance. But more interestingly, I hadn’t thought much about the effect of very high temperatures on the resilience of solar cells. Those thin semiconductor slivers, for all that they’re optimized for sunshine, are also quite fragile, and that’s something that I tend to forget.

A recurring conversation in the renewables world centers on the challenge of finding site-specific solutions: it’s unlikely that we’ll ever have a fossil-fuel equivalent going forward that will be as ubiquitous as oil, so we need to develop solutions that meet the needs of different geographies. Not only is there no one-size-fits-all renewable, we also need to embrace a paradigm shift in urban planning. To build a green structure or system, you need to think about its energy needs from the very first steps. And some things would seem obvious—for example, that in the sunny Middle East, solar would always be the answer.

But that wasn’t the case here, because the building designers found that the extreme heat conditions would cause both decreases in efficiencies and long-term damage to the cells.

The photovoltaics rule of thumb is that for every 1 degree Celsius increase in temperature, the efficiency of the module—its power output—decreases by half a degree. Solar cell efficiencies are rated at a base temperature, often 77 degrees F (25 degrees C), so any performance decreases are observed at temperatures higher than that benchmark.

It’s not a crippling condition, because even if the solar modules perform below their peak capabilities, the sun intensity and greater number of sunny days in certain parts of the world would make up for the losses from cells overheating. But at a certain point the heating turns into irreversible deterioriation, and finding passive methods of doing PV cooling remains an ongoing area of research. The hotter the cells, the worse their circuits perform. Friction within the circuits increases, causing more heat loss, which increases the local heating in a feedback loop. I remember once seeing a guy spritzing his solar cells with a thin stream of water on a particularly hot day and wondering why he was cleaning his cells—it turned out he was just trying to lower their temperatures by a few degrees. Rather than hiring lots of guys with spritzers, Bahrain opted for wind turbines.

So the designers shaped the towers into wing-like forms to help funnel the wind between them, and they navigated a new set of problems to perfect the function of the turbines. The wind moving between the two buildings is at a higher velocity than the wind around the buildings, due to the Venturi effect, which explains how a fluid, such as air, must move faster and decrease its pressure to get through a constricted area. And some things remain to be seen, such as how well the turbines will cope with dust settling on them, as you can see in the last picture here.