Is coal power "dispatchable"? | RenewEconomy

Is coal power “dispatchable”?

We need genuinely dispatchable power stations to complement the growing capacity of wind and solar PV. Coal and other baseload power stations cannot fill that role.

(AAP Image/Julian Smith)

As renewable electricity continues to grow rapidly, the proponents of coal power in federal government and the media are claiming that our electricity system needs ‘dispatchable, baseload’ power stations.

The federal government plans to obtain them either by forcibly extending the lifetimes of clapped-out old coal stations such as Liddell, or by building a new, taxpayer subsidised, coal station.

This, the proponents claim, is necessary for maintaining the reliability of the generating system.

They support this claim by mis-interpreting the following ACCC recommendation: ‘The Australian Government should…enter into low fixed price…energy offtake agreements…for appropriate generation projects that meet certain criteria”. One of the criteria is “to be capable of providing a firm [i.e. dispatchable] product’.

Furthermore, the coal proponents often confuse the concepts of ‘dispatchable’ and ‘baseload’, when in reality these are well-established technical terms with different meanings.

This article acknowledges that we need genuinely dispatchable power stations to complement the growing capacity of variable renewable energy power stations (wind and solar PV) and argues that these can be provided by renewable energy technologies.

It shows that coal and other baseload power stations cannot fill that role.

The definitions

A baseload power station is one that can operate continuously at its rated power (aka generating capacity), except when it breaks down or undergoes planned maintenance.

In Australia, most baseload power stations burn coal to heat water in a boiler. South Australia has a single gas-fired baseload power station, Torrens Island, that’s heading for retirement. Tasmania’s huge hydro storage plays two roles, both baseload and ‘peakload’ (see below).

Baseload power stations are inflexible in operation. They can take from several hours to a whole day to go from cold to full power. Even when hot, they cannot easily and economically vary their outputs rapidly to meet the peaks in demand. T

That’s why the traditional generation mix also has flexible, fast-response, peakload power stations, comprising hydro, open-cycle gas turbines (OCGTs, essentially jet engines), and, in some cases, reciprocating engines (e.g. diesel generators) to handle sudden changes in demand and supply.

Incidentally, OCGTs and diesel generators can burn renewable liquid and gaseous fuels –biofuels and renewable hydrogen and ammonia. In future, hydrogen and ammonia could become commercially available via renewable power to gas. These renewable fuels could then be used as electricity storage or aircraft fuel or injected into the gas grid.

A dispatchable power station is one that can supply power on demand. To do this, it must be controllable to the extent that it can respond promptly and flexibly to sudden changes in supply and demand, both unexpected and predictable.

Dispatchable power stations play a major role in balancing supply and demand. This balance is essential for maintaining the fixed frequency of alternating current and for avoiding blackouts. All dispatchable power stations incorporate some form of storage, whether it be electrical, thermal, mechanical or chemical (i.e. a stored fuel).

Peakload power stations are, at least in theory, dispatchable, but in practice that isn’t always the case.

For instance, during the state-wide South Australian blackout in September 2016, some of the supposedly dispatchable fossil-fuelled peakload generators, that were needed to restart the system, failed. Furthermore, hydro systems, even with very large storages, can be constrained occasionally by drought.

Clearly, there is no perfectly dispatchable power station, just as there is no totally reliable electricity generating system. Therefore, let’s consider different degrees or ranks of dispatchability, based on the major requirements of speed of response and flexibility in operation, and the minor optional requirement of long-term operation.

Degrees of dispatchability
Table 1: Ranked dispatchability of power stations

Table 1 shows the four proposed ranks of dispatchable power stations. ‘Very fast’ response takes place in a fraction of a second; ‘fast’ in several seconds; ‘medium’ in several minutes; and ‘slow’ in several hours to a day.

Additional top-rank (i.e. Rank 1) technologies are not currently feasible for Australia. Additional large dams are ruled out on environmental grounds and hot-rock geothermal is not commercially available.

Rank 2, with a small contribution from Rank 3, are the realistic options. A mix of several types of responder with different response times works best and indeed this is the pathway being followed by South Australia.

Demand response is a fast and flexible first responder, but can only last for about an hour. Batteries too are excellent first responders, but on their own are (and will be for several years) too expensive to provide energy for many hours.

Concentrated solar thermal can respond promptly and run for several hours to overnight, provided its thermal storage has been charged up on a sunny day. Off-river pumped hydro (ORPH) with small upper dams can respond quickly and run for several hours to a day or so.

OCGTs and diesels are slower responders, but can operate for up to several days before fuel costs become limiting. ORPH could replace OCGTs and diesels entirely if environmentally sound dam sites with medium storage capacity were found and became part of the mix.

In Rank 3, when demand is lower than supply, the outputs of wind and solar PV can be reduced rapidly to restore balance. In other words, wind and PV are ‘dispatchable downwards’.

Also, if the need for future temporary supply (e.g. from a spike in demand) can be predicted while wind and solar farms are operating, their outputs can be reduced shortly beforethe expected spike and then increased rapidly for a brief period during the spike, to help buy time for other dispatchable responders to come on-line.

Hourly computer simulation modelling of Australia’s large-scale electricity system with 100% renewable energy, by at least five different research groups, shows that the NEM reliability criterion can be satisfied even though the majority of electricity generation comes from variable renewable sources, namely wind and solar PV.

Furthermore, reliability can be achieved without any baseload thermal power stations and without large hydro.

However, some dispatchable renewable energy power stations from Rank 2 are needed. (In this article, demand response and batteries are included in the category ‘dispatchable renewable energy power station’).

Justification of Rank 4 for coal

To what extent can coal and other baseload power stations be considered dispatchable? They are inflexible in operation and often intermittent during heat-waves when electricity demand for air conditioning is high.

Old coal-fired generating units fail or ‘trip’ quite frequently– also here. On the positive side, once they are operating at the required level, they can often continue for months without interruption.

However, this advantage over the faster and more flexible dispatchable technologies would be rarely used, because demand spikes can be as short as a few minutes and rarely exceed a few hours, while low-generation periods from geographically dispersed wind and solar farms rarely exceed several days.

The gaps of durations ranging from several minutes to several days can be covered by a mix of dispatchable renewables, as outlined above.

This justifies the classification of baseload power stations as Rank 4 in this analysis. To insure against rare events with long duration (weeks), a baseload station could possibly be kept in cold reserve.

A few additional transmission linesto link widely dispersed wind and solar farms would also increase generation reliability.

Federal policy implications

In Australia, some states and territories are in the lead, with policies comprising renewable electricity targets backed up by reverse auctions, power purchase agreements and contracts-for-difference.

A federal government that wishes to take effective action on climate change by facilitating large-scale renewable electricity only needs a few policies and a modest budget:

  • Set renewable electricity targets of at least 60% of total electricity generation by 2030 and 100% by 2040.
  • Allocate additional funds of $4 billion over 4 years to ARENA for a specific tranche of grants for dispatchable renewables, including storage.
  • Allocate additional funds of $2 billion over 4 years to CEFC for a specific tranche of loans for dispatchable renewables, including storage.
  • Abolish the Australian Energy Market Commission (AEMC), transfer its rule-making responsibilities to the Australian Energy Market Operator (AEMO) and ensure that 5-minute settlement commences on 1 July 2019.
  • Mark Diesendorf is Education Program Leader (part-time)  at the Cooperative Research Centre for Low Carbon Living and 
    Honorary Associate Professor,  Environmental Humanities Group, School of Humanities & Languages at UNSW.
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