CSP as Energy Storage

Another way to look at CSP, as a Stored Energy System that complements PV, rather than competing with it.

Electricity is a bit of a strange beast, kind of like the Internet, in that generating power does not need to be concentrated in one place. For example, if a big PV plant only outputs while the sun shines, some other plant, perhaps wind, gas, or stored energy hundreds of kilometers away can take up the slack when it gets dark. Since thermal storage is the main intrinsic advantage of CSP compared to PV, this geographic/power source decoupling makes many other non-obvious possibilities practical.

We all know that solar and wind have a big problem: you can't rely on them. The key to this is energy storage - store it up for when the sun goes down or the wind dies. There are lots of storage schemes, the main problem being that it is difficult and expensive to store electricity. Heat is easier.

The heat storage of CSP may be an economical storage system, in fact, why not just use them as storage systems, only generating electricity when the lower cost PV and wind systems are dark? This would entail making the molten salt storage much larger than currently planned (usually about 8 hours storage). The molten salt retains its heat for a long time, I recall seeing something like a 1 degree per day loss in a hot tank. If you make the storage big enough, it could run the turbines for days, even in the rain, making up for some of those dark PV panels and stopped wind turbines. Acting as a renewable storage scheme may make CSP much more valuable, rather than simply competing with PV.

As to how much energy can be stored, I don't think there are any practical limits, just economic ones. The 8 hour storage was chosen because this is about how long the Spanish stay awake after the sun goes down. The molten salt (special low melting point salt) is pumped from a cold (~250C) insulated tank, heated using the oil in the solar troughs and pumped into the hot (~500C) tank. When you need electricity, you reverse the flow and heat up water for the turbine. A dedicated "peaking" CSP with massive thermal storage like this would only generate electricity at high $ value times, and storing heat at other times for when it is needed.I think this may be practical, but who knows? It would compete with other storage schemes like pumped hydro, CAES and flow batteries. Someone would have to run the numbers. Are you listening Flagsol/Solar Millenium?

I do know, that these storage systems are massive. The Grenada plant is two 50MW plants side by side (to comply with the 50MW maximum for the Spanish Feed in law). For 8 hours of storage, this is 2X50X8= 800MWHours - that's a big battery! Of course it cost a lot too, something like 150Million euros each.

Also, I wonder if the resulting system would be simpler than the current one that runs the boiler and the storage in parallel. If all you want to do is run the boiler off the stored heat, you could just have one heat exchanger for the oil to salt transfer, and one other one for the salt to water transfer. Currently the Flagsol system in Spain uses a more complicated system that heats both the water and the salt in parallel so the steam turbine can run off solar and/or stored heat and mixes and matches thermal flows to the solar output and needed thermal storage.

I envision a solar complex that uses cheap PV for generating electricity during the day, coupled with a CSP Energy Storage (CSPES?) system for making the complex fully dispatchable, even during night and cloudy days. I know, right now, PV isn't cheap, but someday it will be! This scheme uses both technologies for what they do best - PV for daytime electricity, and CSP for energy storage.

How about wind - would it help for wind? Wind blows somewhat unpredictably, so, yes, any storage system can help even out the spikes and troughs. Use your CSP to absorb energy from the sun and store it. Then when the wind dies, use the stored heat to run the steam generator. The only issue would be that you will still probably throw away the wind energy at night, when no-one is there to use it. Overall though, CSP is probably better as a storage system than as a direct solar system!

New Economics of V2G

I was just reading this blog of an interview with AltairNano CEO Alan Gotcher at

http://www.autobloggreen.com/2007/05/07/autobloggreen-qanda-altairnano-ceo-alan-gotcher/

He says their NanoSafe batteries are showing 100% charge/discharge cycles of over 25,000 times.
Well, 25,000 cycles certainly changes the economics of Vehicle To Grid (V2G)! Here is a blog all about V2G:
http://www.insidegreentech.com/node/990

Basically, V2G is using an Electric Vehicle's battery to help sink "excess" electricity, like nighttime wind for example. Buy power cheap at night, and use it during the day. V2G is the idea that, if you don't use all of your electricity during the day for driving, sell some of it back to the grid at high daytime prices - the old "buy low, sell high" trick – but, will it pay for that Tesla Roadster?

The main problem with the V2G idea was that the batteries would die after 1000 cycles or so, so you would kill your expensive batteries after only a year or two with V2G - a losing proposition. However, with the Nano Safe going 25,000 cycles, you can cycle the battery as many times as you want, without hurting it.

So, with 25,000 cycles for the NanoSafe (longer than the car will last probably), V2G immediately becomes a solution to many vexing renewable and other grid problems. Currently, the electric grid has very little storage capability. This means that there needs to be enough power plants to supply the peak use times, such as summer days. At night, these power plants are idle, waiting for people to wake up, a huge waste of capital resources. If there was a big storage system (a giant “battery”), then these plants could work all night too, filling up the “battery” of the storage, which could then discharge during the day and evening when demand is highest. Renewables are even worse, with unpredictable and unreliable power output. With storage, wind at night, and excess solar during the day can be saved for high demand (high $$) periods, making renewables more competitive. The reason the electric grid has very little storage is because storage is expensive. See http://electricitystorage.org/index.html for lots of info on large scale grid storage.

Fortunately (or unfortunately) cars are big energy consumers. Typically, a house uses 938 kwh per month (about 30 kwh/day) according to the EIA. EVs seem to run about 35 - 60 kwh storage (Tesla Roadster – 56kwh Phoenix SUT 35kwh). So, your EV car battery could run your house all day long if needed. If you multiply this by the number of cars in the US and the world, you find that all these big EV batteries can add up to a huge grid storage system.

So, what does this mean to you, the consumer? Some electric companies like PGE, are offering “Time of Use” rates, as low as 6cents/kwh at night, and up to 25 cents during summertime days. This is a big price differential. So, how much does 30kwh of power cost? Well, at 6 cents, $1.80. At 25 cents, $7.50. So, if you could charge up at night, then use (or sell to the utility) the electricity in the day, in a year, this might save you $650, not a whole heckuva lot, but enough to pay maybe for one payment on that Roadster. These numbers are definitely back of the envelope, but serve for illustration.

In addition, there are be some other compelling reasons you might like to go V2G with your NanoSafe EV. First you are encouraging the use of wind and solar by giving them the storage capability they need to time shift their output to times when people really need it. Secondly, instead of building big new coal fired power plants, the existing ones can instead run 24/7. Third, this is basically free money, given the 25,000 cycles of the Nano Safe batteries. Cycle cost is no longer an issue, instead, if you don't do V2G, you are simply throwing money away.