Days 43 & 44: class storage

I’ve fallen behind in my blogging and will attempt to catch up.  It’s a bit of a task, because we covered a lot of ground.  I hope I can be forgiven lumping some things together.

One of the issues that frequently crops up in power generation is that the demand for energy is rarely static.  I made mention of this fact in my last few posts, describing how Iceland uses geothermal power to provide heat as well as electricity.  The demand for hot water used to heat homes increases in the winter and declines in the summer.  You can see the cycle vividly in this chart:

Image credit: Orkuveita Reykjavíkur.
Image credit: Orkuveita Reykjavíkur.

Each spike in water production (and each dip in water level) corresponds to winter months, where the demand for hot water is at its height.  The demand for electricity undergoes similar cycles, although on shorter scale: people tend to use electricity more when they’re awake, so demand declines at night and picks up again with each new day.

Cycles are good.  They’re predictable and easy to plan for.  If you’re building a system to supply energy, it would be ideal to have a constant demand: you’d always know how much you needed to supply, and you could build power plants and other equipment to meet that need.  Cycles are nearly as good, provided they remain steady.

Here’s the problem, though: the cycles don’t remain steady.  We might have a colder-than-average day or two in Reykjavik one week, or there might be an unusual demand for electricity because a football match in Hafnarfjörður was delayed and people got home later than usual.  This can be a problem.

Power production facilities need time to start up and to shut down.  They also need time to increase or decrease their rate of production.  How much time depends on what sort of power you’re talking about (electricity versus thermal power, for example) and how you generate it.   But however you generate power, you always want to be able to match supply to demand.  People don’t like it when their houses go cold and dark.  Businesses – and power-reliant critical facilities like computer centers, air traffic control centers, and hospitals – really don’t like it.

You also need to be able to dispose of excess power when demand declines.  If you’re pumping more hot water to Reykjavik than the residents need, the water still has to go somewhere.  If you’re supplying electricty, overvoltage can burn out sensitive components.  You want to match the supply to the demand as closely as possible.

Image credit: Iceland School of Energy.
Image credit: Iceland School of Energy.

This sounds like it should be an easier problem to deal with – just drop the output as soon as demand drops – but it isn’t always so simple.  There’s going to be a certain amount of lag between the time demand begins to change and the time you become aware of the change.  A complicating factor is that certain kinds of generation systems have to operate at minimum levels if they’re to function at all; you have to maintain this base load or take the system offline altogether.  That means you’re more likely to get caught short if demand suddenly picks up again.*

So you need to be able to store energy: enough to absorb any excess until you can throttle back a bit on production, and to meet any surge in demand long enough to bring your power generation to a level that matches it.

Storage devices are sometimes categorized according to their specific energy and specific power.  Specific energy describes how much work can be performed by the stored energy; specific power describes the rate at which power can be discharged.  Looking at some common electrical storage devices, we see that as a class, batteries can store a lot of power, so their specific energy is high; but they don’t discharge the power very quickly, so their specific power is low.  Capacitors can’t store as much, but can discharge what they have very quickly.  But you can build big batteries (or arrays of batteries) and big capacitors, and use them in tandem to help match output to demand.  If you have excess output, put it in storage; if you have excess demand, draw down the batteries and the capacitors.  Simple.

But what happens when there’s a big mismatch?  Suppose a generator fails entirely, or a natural disaster cuts the flow of energy from one of your plants?  Or, to be less apocalyptic, suppose you’re running on solar power and the sun goes down?

There are lots of strategies for storing massive amounts of power.  On the southern Japanese island of Okinawa, for example, the Yanbaru Seawater Pumped Storage Power Station holds over 560,000 cubic meters of seawater in an artificial reservoir on a hillside about 150 meters (roughly 500 feet) above sea level.

Yanbaru power station reservoir, Okinawa. Image Credit: Japan Agency of Natural Resources and Energy.
Yanbaru power station reservoir, Okinawa. Image Credit: Japan Agency of Natural Resources and Energy.

The reservoir is connected to a Francis turbine that can produce 30 megawatts of electrical power until the water stored in the reservoir is depleted.  At full capacity, the station can supply power for about 5-6 hours – after which the imbalance in supply and demand will become a problem again.  But for large short-term mismatches of supply and demand (as can happen in a place like Okinawa which is prone to violent storms), the system works quite well: when you need more power than you can otherwise supply, drain the water.  When you have more power than demand, pump the water back uphill.

Yanbaru turbine. Image credit: Fujihara et al, Hitachi Review.
Yanbaru power station’s Francis turbine. Image credit: Fujihara et al, Hitachi Review.

Solar power plants can deal with the fall of night by pumping energy into a suitable storage medium, often nitrate salt.  The salt is melted during the day, when the reflected heat of the sun is at its height, and heat is drawn from the molten salt at night to main a steady supply of energy.

Image credit: Iceland School of Energy.
Image credit: Iceland School of Energy.

You can even heat water and pump it into aquifers or boreholes to store energy and then draw it back out to provide heat to homes – sort of an artificial geothermal system.  A community in Alberta, Canada, completed in 2007, uses this method to store thermal energy for its residents.

Schematic of thermal storage system in Drake's Landing, Alberta, Canada. Image credit: Iceland School of Energy.
Schematic of thermal storage system in Drake Landing, Alberta, Canada. Image credit: Iceland School of Energy.

The upshot of all of this is that running a power system isn’t just a matter of spinning a turbine or lining up photovoltaic cells in the desert; there are other factors to be considered as well.  But the problems are solvable, and new solutions are being developed every day.

*And if you’re thinking there’s equipment wear and tear associated with surging and crashing your capacity to try to match demand – you’re right.

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