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Status: [7th Sept] solid draft

ELECTRICITY STORAGE

Introduction

IF mass storage of electrical energy were easy and cheap, then the problem of integrating large-scale renewables into the energy sector would be greatly simplified. The variability of supply could simply be buffered in storage and drawn down as needed to meet demand.

This page has two purposes: the primarily OzEA project -- of modelling electrical supply meeting demand in a system with high renewable penetration -- requires specification of the system storage capabilities, and this page provides the background discussion on this specific point. This page is also a place for exploring more generally the possibilities for large scale and economically feasible storage in electricity systems looking into the medium term.

The short answer is that Hydro Electricity is the only mass electricity storage that we have available. Other possibilities, including thermal storage within concentrated solar thermal plants, may develop but remain speculative at this time.

Storage and Demand Management

Conceptually 'Storage' has two halves; the first half is the sinking of electricity into a storage medium, and the second is 'on demand' availability of electricity from the storage medium. It is usual to think of Electricity Storage as encompassing both of these halves in a single mechanism or process, however, processes that achieve either can be relevant here.

For example, hydro electricity (as distinct from Pumped Storage Hydro) stores energy in the form of water from rain contained in a dam at a height: this is storage of energy that is available as electricity on demand, but does not take electricity to 'charge'. Conversely, the use of plentiful electricity to displace the use of gas (in a furnace or some other heating process) productively uses that electricity, but not in a way that makes it available for future use. Compressed air storage for use with gas turbines (as below) is a similar displacing mechanism. In short, sometimes the need is to 'put' electricity somewhere useful; sometimes the need is to 'get' extra electricity in order to balance supply against demand.

The further one delves into variations on the basic concept of storage, the more the term Demand Management becomes applicable. We make no hard distinction here, but do develop these areas as separate discussions.

Pumped Storage Hydro

Pumped Storage Electricity is when water is pumped from a lower dam into a higher dam to 'charge' a hydro system where the water otherwise runs downhill through turbines to generate electricity. As a rule of thumb this process gives back around 80% of the electricity put in.

Australia currently has around 20 GWh of pumped storage hydro capacity, principally associated with the Tumut 3 System as part of the Snowy Mountains Scheme, and also the Wivenhoe - Splityard Creek system near Brisbane. So far as OzEA is aware there are no serious proposals to build further large PSH capacity in Australia.

Straight Hydro

There are, however, tens of thousands of GWh of straight Hydro power in Australia when the dams are full, and the various Hydro facilities can collectively provide around 7 GW of power supply. The total capacity (perhaps in excess of 30,000 GWh) is not an especially useful number; what matters is the total amount of power that can be reliably drawn from these resources year after year, which in turn would involve an analysis of long-term rainfall patterns. As an expedient we consider the amount of currently used Hydro power that can be displaced by other sources (predominately Wind), and take such displacement as 'storage' for the purposes of OzEA analysis. Work to provide an estimate of this 'storage' is pending, however it is noteworthy that Tasmania uses significant amounts of Hydro power.

Compressed Air Energy Storage (CAES)

Energy can be stored in the form of a compressed gas, however, there are some complexities to appreciate here. Like hydro power, compressed air energy storage can IN PRINCIPLE approach 100% efficiency; however, unlike hydro power it is problematic in practice to achieve high efficiencies. Pumping water (an essentially incompressible fluid) up a hill incurs frictional losses in the pipes, and any losses associated with the engineering realities of the turbines. With a compressible fluid (i.e. a gas) the process is more complex - and we briefly outline the physics in a non-mathematical way.

Physically, the temperature of a gas is related to the (square of the) average velocity of the individual gas molecules. The act of compressing a gas tends to involve hitting the molecules 'up the pipe', and thus the molecules themselves obtain higher velocities resulting in the gas overall obtaining an increased temperature -- in addition to an increased pressure. It is this increase in temperature that is wasteful because the heated gas, like a cup of coffee, tends to return to the temperature of the surrounding environment by dissipating low grade waste heat. In this way the pressure of the gas once it has settled (cooled) in storage is less than the pressure at the time of compression. These losses can be reduced by being as gentle as possible with the gas in the compression process (as slowly as possible, and in a staged manner). Thus, a trade-off exists between efficiency on the one hand, and the engineering complexity and cost on the other.

[fc - Aug 2010] I'm struggling to get the physics of the decompression straight in my head, possibly because I'm thinking in terms of ideal rather than real gases. Am fairly sure that there is "adiabatic cooling" of the gas as pressure is released, thus creating further losses. The point does not bear further time on clarification at this stage, although I'd be grateful if anyone can give a tidy wrap up.

One particular implementation of CAES is to use it in combination with Generating Gas Turbines (and this combination is sometimes what is meant by "CAES"). Gas turbines pre-compress the air-fuel mixture (and use perhaps 40% of the energy generated to do this), but with a source of compressed air at a suitable pressure this parasitic load on the turbines can be removed. In this way electricity can be usefully stored; however, and so far as we understand, there is no extra power from the gas turbine - just a reduction in the gas needed to power it.

Vessels for storing usefully large amounts of compressed air present a particular difficulty, usually requiring a convenient underground cavern such as an abandoned mine. It could also be possible to use 'large underwater balloons' for compressed air storage, where at a depth of around 600m the necessary pressure is provided by the water column rather than the strength of the container. Such proposals are very much at the research end of the development spectrum.

High Temperature Thermal Storage

The concentrated rays from a Solar Thermal plant are converted into heat when they are absorbed by the medium at the focal point, and this heat drives a steam turbine or similar heat engine to produce electricity. These two steps can be separated by storing the heat in a suitable medium (e.g. molten salts, large graphite blocks) that is suitably stored / insulated. In this way solar thermal plants can generate electricity into the evening. The length of time that heat can be usefully stored in this way is an economic as much as an engineering problem.

Note that it would be a particularly inefficient storage mechanism to use electricity to generate heat for later use to generate electricity. The reason is that extracting useful work from heat is a fundamentally inefficient process (as per the Carnot Limit), depending strongly on how high the temperature is.

Hydrogen and other Chemical Transformations

It is not difficult to 'make' hydrogen from water and electricity. It is somewhat more complicated to capture and store large amounts of hydrogen. Using hydrogen in a Fuel Cell to generate electricity is efficient but requires expensive platinum catalyst. Burning hydrogen is an easier but much less efficient way of recovering some of the electricity invested. To the best of our knowledge the use of hydrogen for large-scale energy storage remains at a research level.

In addition to Hydrogen there are many ways of storing electrical energy as chemical energy. While this topic is doubtless deserving of more exploration than given here, OzEA is not currently aware of any chemical storage mechanism or proposal that is likely to provide significant storage capacity in the Australian electricity market any time soon. Please let us know of any proposals or technologies that might reasonably be part of the medium term development of the energy sector.

Overall

Storage of electrical energy can take many forms, but the provision of inexpensive and large-scale electricity storage appears for now to be limited to hydro-electricity. In particular this includes the currently existing 20 GWh of Pumped Storage Hydro, and the displacement of hydro power that is currently used for Tasmanian baseload (this latter amount being on 'The List' for calculation). The use of these resources in OzEA models of electricity supply will require attention to the locations and capacities of the individual components.

Beyond these Hydro resources, the potential for the development of further storage capacity within the Australian Electricity System remains opaque. We can only give focus to storage mechanisms that have serious potential to be large scale and low cost in the next decade or so. However, this page is open to general discussion and the sharing of details in relation to all sorts of storage technologies or possibilities.

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fc - August 2010