How can electricity grids successfully incorporate high (50%+) penetrations of PV and wind?

Question

Although most of the renewable energy currently consumed by the world can be turned on and off at will pretty easily (hydro and biomass), there are two potentially huge supplies, wind and PV, that are more restrictive: the variations in their output are mostly caused by external factors: the weather, and the time of day.

On an electricity grid, power generation and consumption are pretty much simultaneous: there's virtually no capacitance, and transmission happens at the speed of light (in copper, aluminium,...).

Wind and solar are pretty easy to incorporate into the grid, if they form a relatively small (long-term average) proportion of generation. As a crude rule-of-thumb, that's a value at or below their capacity factors, so say 25% for onshore wind, 45% for offshore wind, and 10-20% for PV depending on latitude. At those levels, the challenge might be just that we need to build some extra transmission infrastructure to bypass bottlenecks that arise because the old grid configuration was built around old unsustainable generation sources.

How can higher proportions of exogenously-variable renewables, say 30%+ for onshore wind, 45%+ for offshore, 15%+ for PV, be accommodated onto the electricity grid cost-effectively?

Answer 1

There are two parts to the answer.

One is enlarging the geographic area of capture of wind and solar across diverse climates, to smooth out the exogenous variations. The other is is storage.

Span more climates

Back in the early noughties, Gregor Czisch built a massive optimisation model to look at how to balance a 100% renewables grid at lowest cost, across various geographic areas, as part of his Dipl.Ing degree. His thesis is freely available in German, and the English translation can be bought from the IET. His other papers (some in English, some in German) are freely downloadable.

The model found that increasing the area across which solar and wind were captured, and transmitting that power anywhere across the enlarged area, was one of the cheapest ways to incorporate very high levels of wind and solar into the grid.

Since then, the concept that he and Gregor Giebel developed, of an international supergrid, has really caught on, and the number and size of international electricity connectors (aka interconnectors) has been growing.

Most of Czisch's scenarios, and most subsequent 100% renewables scenarios developed by others, combine this supergrid with storage.

Energy Storage

A lot of people immediately think of grid storage as being something where you put electricity in, and that later get almost all that electricity out again. But that round-trip electricity storage is only one type of useful storage for the grid.

Storage balances the grid by time-shifting something to match supply and demand moment-by-moment. Having storage somewhere in the chain between the capture of the primary energy, and delivery of the energy service demand, achieves that.

Storing energy somewhere in the electricity supply chain

Let's look at the electricity supply chain. I'll tag each of the links in the chain with a number, for reference.

Capture of primary energy
                  ↓ 1
Primary electricity generation
                  ↓ 2
Electricity delivery
                  ↓ 3
Consumption of the energy service.

We can introduce storage at any of those links, to balance the grid.

1. Biomass in a silo sits at link 1. The primary energy has been captured from the sun, locked up in carbohydrates in the plant matter, and that plant matter is then stored until electricity is needed, at which point it is burnt, and the heat is used to drive a turbine. Similarly, water in the upper reservoir of a storage hydro plant is also energy storage at link 1.

2. Round-trip electricity storage, such as pumped hydro storage, chemical batteries, flywheels, and ultra-capacitors, sits at link 2.

3. Once the electricity has been delivered, it has to be consumed. But that doesn't necessarily mean that the end-use of the energy has to happen at that time. It does in some cases - for example if you want light, it's no easier to store pure light than it is to store pure electricity, so electricity consumption and use of light happen pretty much simultaneously. However, most of our domestic energy consumption is for space heating, space cooling, water heating, and water cooling. And it's very easy to store warmth or coolth, for use later. So it's pretty easy to build lots of storage at link 3. Particulary when heating (and maybe cooling) services are provided by shared infrastructure, such as a district heating scheme, where mutli-fuel sources including electricity can be harnessed, and storage of hot water and cold water can be centralised, reaping huge economies of scale.

Some different types of energy storage for the grid, illustrated:

sources: [1],[2],[3]

Answer 2

There are three broad approaches to dealing with variable generation:

1. Diversity of supply: Both through building a grid covering a wide area (see EnergyNumbers' answer for details) and by using different sources of generation. As an example of the latter, consider a grid powered by both wind and solar sources: these are likely to provide peak output at different times, since sunny and windy conditions do not tend to go together[1], and will thus suffer less from the variability of each than a grid powered entirely from either one. Wind and wave are also an interesting combination, since one tends to precede the other.

2. Storage: See EnergyNumbers' excellent answer.

3. Demand response: Do you care which exact times your fridge is on, or do you just care that it stays below a given temperature? Do you need your electric car's battery charged right now, or do you just need it to be ready before morning? When you put a load of washing in the washing machine to run overnight, do you care whether it runs immediately, or would you be content for it to run "sometime during the night", with a "finished by" time? There could be a lower electricity cost associated with the latter options, and the network operator could activate your fridge/car/washing machine when there is spare capacity in the system. Demand response has existed for some time in cruder forms, sometimes limited to heavy industry, but modern technology may allow its further development.

Spinning standby: Of course, while these measures can be sufficient most of the time, it is not possible to guarantee that supply will always be able to meet demand, and so it will still be normal to have a "spinning standby" of generation that can be quickly activated when the storage runs out, most likely from thermal generators powered by gas or biofuels. Such capacity that can be rapidly powered up or down is known as "dispatchable" generation. It is worth noting that this is not a new concept associated with renewables; already, there must be sufficient standby capacity available to cope with the largest single generator on the grid unexpectedly disconnecting.

The question specifically asks about cost-effectiveness, and that is difficult to comment on. All of the measures mentioned have their costs, as well as their benefits, and these should be considered when evaluating the optimum mix of generation.

[1] I'm sure there are some climates where they do. But the general point remains that diversity in sources is good.