Energy Storage - options

There is huge potential in the energy storage business and only the hydro companies are buying and selling. The price of generating electricity varies throughout the day. Electricity can be bought and sold a few hours later for double the price.

At present there are peaking gas turbines on standby to cope with peak loads. They are on standby fully manned for only a few hours a year. It should be far cheaper to store cheap electricity than to have standby power stations. 

These graphs are for daily and monthly variations in price. 

Price spikes are greatest in bad weather, air conditioners in summer and heaters in winter.

In the daily price the variation is from -$100 to +$10,000 /MWh. That is minus, they pay to dispose of excess electricity.

An opportunity surely?

Energy can be stored in many ways:

Kinetic energy - flywheel

Potential energy - hydro or compressed air,

Heat capacity - hot salt or graphite

Latent heat - ice or steam

Magnetic field - superconducting coil.

Attraction between electrons and protons - capacitor, batteries, chemicals, fossil fuels, etc

Energy densities

If we look at this list of energy densities we can see the enormous energy in nuclear reactions in which the inner core of atoms fuse together or break up.

In the chemical energy department, hydrogen reigns supreme. The problem with hydrogen is it is not very dense so you can’t get hold of much.  It is almost impossible to compress and liquefy. The best way to hold it is to combine it with something such as carbon. Methane is one carbon with 4 hydrogens, and because of this holds a lot of energy. Coal has half the energy, then there are the carbohydrates such as sugar, starch and wood.

Batteries in comparison have an uphill battle because they store very little energy. 50 times less than petrol.  However it is better than it looks. Petrol engines are only about 15% efficient whereas electric batteries and motors are 80%, five times as efficient. An electric vehicle can re-charge the batteries while slowing down, or going downhill.

So we can say a Lithium sulfur battery will be about one quarter the capacity of petrol.

There are many batteries under development, and often weight is not an issue.

The interesting thing is the energy of molten salt. It can store as much energy as a good battery. It is used to store solar thermal energy.

Wikipedia: Table of more energy densities

Choosing which storage process

When choosing the most suitable method the things to consider are:

cost per MWhr,

energy lost in the return trip, and

energy loss over time

space and geography (hills, rivers, caverns, cheap land, etc)

For example:  Storing heat energy is 20 to 100 times more cost-effective than storing electricity in batteries, with energy-conversion efficiencies already exceeding 90%

Time interval of energy storage

A flywheel evens out the piston strokes of a reciprocating engine, so the storage time can be in thousandths of a second.

An ultra capacitor evening out voltage fluctuations may be in hundredths of a seconds.

Wave energy may need to store pulses over several seconds.

A hydro company will want to buy cheap electricity, and sell it a few hours later at peak.

A solar thermal plant will need to store energy overnight.

A biofuels company will want a fuel to store energy until the customer uses it in their vehicle.

The following text and charts come from Electricity Storage Association at www.electricitystorage.org/technology/storage_technologies/technology_comparison

This website is excellent in that it puts the various options in perspective. It covers technologies available now rather than in the future.

Large -scale stationary applications of electric energy storage can be divided in three major functional categories:

Power Quality. Stored energy, in these applications, is only applied for seconds or less, as needed, to assure continuity of quality power.

Bridging Power. Stored energy, in these applications, is used for seconds to minutes to assure continuity of service when switching from one source of energy generation to another.

Energy Management. Storage media, in these applications, is used to decouple the timing of generation and consumption of electric energy. A typical application is load leveling, which involves the charging of storage when energy cost is low and utilization as needed. This would also enable consumers to be grid-independent for many hours.

Although some storage technologies can function in all application ranges, most options would not be economical to be applied in all three functional categories.

Source - Electricity Storage association

SIZE AND WEIGHT

 For mobile energy storage size and mass are important. In a power station the main limitation is cost and reliability

Metal-air batteries have the highest energy density in this chart. However, the electrically rechargeable types, such as zinc-air batteries, have a relatively small cycle life and are still in the development stage.

The energy density ranges reflect the differences among manufacturers,  product models and the impact of packaging.

CAPITAL COSTS

While capital cost is an important economic parameter, it should be realized that the total ownership cost (including the impact of equipment life and O&M costs) is a much more meaningful index for a complete economic analysis. For example, while the capital cost of lead-acid batteries is relatively low, they may not necessarily be the least expensive option for energy management (load leveling) due to their relatively short life for this type of application.
The battery costs in this chart have been adjusted to exclude the cost of power conversion electronics. The cost per unit energy has also been divided by the storage efficiency to obtain the cost per output (useful) energy.
Installation cost also varies with the type and size of the storage. The information in the chart and table here should only be used as a guide not as detailed data.

LIFE EFFICIENCY

Efficiency and cycle life are two important parameters to consider along with other parameters before selecting a storage technology. Both of these parameters affect the overall storage cost. Low efficiency increases the effective energy cost as only a fraction of the stored energy could be utilized. Low cycle life also increases the total cost as the storage device needs to be replaced more often. The present values of these expenses need to be considered along with the capital cost and operating expenses to obtain a better picture of the total ownership cost for a storage technology.

PER-CYCLE COST

Per-cycle cost can be the best way to evaluate the cost of storing energy in a frequent charge/discharge application, such as load leveling.
This chart shows the capital component of this cost, taking into account the impact of cycle life and efficiency. For a more complete per-cycle cost, one needs to also consider O&M, disposal, replacement and other ownership expenses, which may not be known for the emerging technologies.
It should be noted that per-cycle cost is not an appropriate criterion for peak shaving or energy arbitrage where the application is less frequent or the energy cost differential is large and volatile.
Updated April 2009