In the previous post, I came to the sad conclusion that harnessing sunlight with PV panels is not feasible as a reliable, green power source for a 24/7 data centre. It is at best an auxiliary power source, or a boost to the primary source during daylight hours.

This is because nature is not very good at storing electricity. But nature is good at storing other types of energy, and is very good at storing heat. And this is where Concentrated Solar Power comes in. There’s a detailed wiki article here; the short story is that CSP uses multiple mirrors to reflect the sun’s heat to a small point. That point in turn heats up molten salts, which in turn super-heat water to drive a conventional steam turbine.

The downside is that where PV has a single conversion – a photon knocks an electron free – CSP has multiple conversions – from solar radiation to heat to kinetic motion which drives a turbine and finally, via induction in an alternator, to electricity. Hence, although each of these steps can be very efficient, the theoretical limit on efficiency is going to be lower. However, as even domestic solar water heaters are 70% efficient, we’re starting at a much higher baseline than PV.

The joy of this is that rather than battle nature by trying to store electricity, CSP opens the possibility of storing the heated molten salts in a tank for release during the dark hours of the night. Insulating a tank is much easier than persuading molecules to host itinerant electrons. And, as I keep saying, there’s more to greenness than efficiency. Building a tank to contain hot stuff requires far fewer nasty chemicals than batteries. In addition, the tank’s life time is more or less indefinite, whereas batteries die.

Given these tensions, the overall viability of CSP is still up in the air. Predicted efficiencies for CSP are below theoretical ones, and the costs are 2-3 times those of PV panels (albeit, PV panels without the battery packs). So this is a technology that’s still not quite ready for the big time. But because energy is stored as heat, CSP offers the promise of 24/7 solar power without the horrors of batteries.

The wiki article appears to be 2-3 years out of date. More up to date information is on the National Renewable Energy Laboratory website. This suggests another problem, which is that the power densities from CSP just aren’t in the same league as those from PV panels. The site has an index of projects here, and most CSP plants cover large areas.

And this comes back to the nature of solar radiation: even if we recover up to 60%-70% of insolated energy, and even if we find ways of storing solar energy overnight that are 80% efficient, we still need a rainy-day buffer. Put this altogether, and the 1,350W / m2 diminishes fast. 70%*80% = 56%. Add a rainy-day buffer of a day or two, and we’re down to 30%. That’s 400W / m2. A typical data centre will consume 4kW / rack. After allowing for the usual factors, each rack takes about 4-5 m2 of space, so say 1kW / m2. That means for every 1 m2 of IT load, we need 2.5m2 of solar capture. That’s a 2.5:1 ratio at best. In practice, it’s going to be much worse.

Hence we find ourselves on a trajectory back to off-setting.

But before we go there, perhaps we’re solving the wrong problem. And that’s for the next post,

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