Question for Smurf

[TjamesX]

Smurf,

I have a quick question for you regarding Bell Bay gas fired powerstation in Tassie. I have a report that infers Bell Bay powerstation had a load factor of 40% (ie used 40% of the time) in 03/04.

I thought this was high - does this sound right to you????? I'm assuming this is the only Gas fired generator in Tassie??

any help would be great
**and if anyone else knows anything about it, fell free to add **

Cheers
TJ

(PS I sent via PM but, I'm not sure if you've read it)

 

[mit]

You are right that it is large for peaking gas plants (normally around 20%) but SA have a lot gas plants and they have a much higher load factor than that so it could be a mid load station. Interesting what effect BassLink will have on Tassie generation.

MIT

 

[TjamesX]

Thanks mit,

Yeah I believe it is used just as a back up for Hydro in tassie, but only very rarely. I was under the assumption that Tassie had a lot of spare capacity and thats why Basslink was built - so some spare could be applied to VIC/SA. So that would contradict Bell Bay having such a high usage factor??

 

[Smurf1976]

There are basically 3 reasons why Bell Bay power station (BBPS) operates. BBPS is the only fossil fuel power station in Tasmania and is gas-fired. Prior to 2002 it operated using heavy fuel oil.

1. The Tasmanian system is energy constrained unlike all mainland electricity systems. That is, there is a limit to the ability to run the installed generating plant which depends, over the long term, on water inflows to storage.

Unlike most hydro-electric systems, the Tasmanian system was built as a bulk baseload energy source rather than as a peak load system (such as the Snowy). The emphasis has always been on total energy generated rather than peak power output.

A range of ratings is possible for the energy production capability of the system since rainfall is not known with certainty. There is thus a band of capacity with a nominal firm output value determined by the accepted probability of failure. Based on long term meteorological data the net system yield across all power stations is 10250 GWh at present and this will rise to around 10400 over the coming years due to minor plant efficiency improvements.

HOWEVER at the 10250 GWh (average output 1170MW over the year) level there is a 50% probability of failure since it is by definition the average of inflows. This is unacceptable for reasons best explained by stating that some two thirds of Tasmania's exports relate directly to industry established on the basis of what was known locally as hydro-industrialisation and that practically all Tasmanians cook and heat water with electricity (and over half keep warm that way too).

At a 2% probability of failure the stand alone (without BBPS) rating of the system is an average output of 1065 MW or 9330 GWh per annum and this rises to 1105 MW or 9680 GWh with BBPS (or an equivalent) available as backup.

If the acceptable probability of failure is increased then the firm rating will also increase and vice versa. At average output of about 900 MW (7900 GWh) the risk of failure approaches zero. A great deal of energy would be lost to spill at these lower output levels although the system would easily withstand any credible drought scenario.

Hydro Tasmania is also pursuing the development of large scale wind farms with one approximately half completed (operating) and another planned. These will have a combined installed capacity of about 270 MW.

Since the hydro system is energy constrained rather than capacity constrained, the energy from wind contributes to overall system yield and is thus a complete alternative to building a new traditional power source. This is in contrast to the mainland states where generation is capacity constrained and thus gains minimal benefit from wind energy. The two wind farms ought to add somewhere in the order of 800-850 GWh to net system yield when completed thus giving a total system yield of around 11200 GWh from both hydro and wind. This compares to load (including transmission losses) of about 10800 GWh per annum although load is steadily rising (with a couple of significant individual load increases quite likely from major industry).

Since late 1997 Tasmanian electricity load (energy load NOT peak power demand) has consistently exceeded the system firm rating and has continued to rise. This period has coincided with a period of below average rainfall which has seen system storage drop from a high of 86% in 1997 down to 22% earlier this year.

The current storage level is 40.4% versus 44% this time last year. Since this time of year is normally the seasonal peak levels are likely to fall significantly over the coming months and, depending on inflows, should be somewhere between 15% and 30% when Basslink enters service.

Under these conditions BBPS is and has been operating as a bulk baseload energy source to reduce hydro generation on average and thus conserve water. Over time this leads to higher storage levels (via reduced outflows) than would otherwise be the case. A similar run of intense operation of BBPS occurred during 1989-91 for the same reason of low storage levels.

Since demand for electricity exceeds hydro+wind yield it is necessary to make up the energy balance using BBPS (or net imports via Basslink once it is operating) otherwise storages would fall to zero even with average inflows. Drought has simply increased the need to run BBPS and 3 gas turbines (108MW in total) are being installed on a temporary basis at BBPS to boost capacity. These also are intended for baseload operation as a bulk energy source to boost overall system yield despite such turbines usually being associated with peak load operation. Note that the existing machines at BBPS are steam turbines with gas-fired boilers, NOT gas turbines.

It is important to understand that in order to boost overall system yield the timing of BBPS operation is not critical. The effect is largely the same at 2am as it is during a period of peak demand. The aim is to conserve water and when this occurs is generally not particularly critical.

2. The Tasmanian system obtains two thirds of its total system yield from run-of -river and small / medium storage generation.

During an intense dry spell (common in Autumn) the run-of river generation will largely cease. Hydro power stations associated with the major storages would operate baseload at full output during such times but it is still necessary to release water elsewhere in the system to meet peak loads since only about 60% of total demand (energy basis) can be drawn directly from the major storages.

This situation may cause the medium storages to become excessively depleted during Autumn despite levels in the major storages remaining at an acceptable level which does not in itself justify running BBPS. BBPS is then operated in an intermediate merit order level to reduce drawdown of the small and medium storages.

In order to "kill two birds with the one stone" where there is a need (as at present) to run BBPS to support total system yields it is done so as to run at sustained 100% load during Autumn thus overcoming any potential problems with the smaller storages running out. Since the smaller storages are drawn down to target levels to avoid spill during the wet season (winter-spring) this may mean backing off output from the major storages somewhat. But assuming that they are at low levels this is desirable. If they weren't low then BBPS would be backed off (or shut down altogether) instead. It all depends on actual inflows.

The seasonal depletion of the medium storages is the greatest single threat to short and medium term supply of electricity in Tasmania and is thus something which requires constant careful management. Simply maintaining storages at a high level by minimising outflows wouldn't work since that would only increase drawdown of the major storages which are long term energy inflow constrained. This would lead to a collapse of the system (total depletion of storage) in due course and thus is to be avoided.

So it's a complex balance of trying to avoid spill (lost energy) but also avoid running any storage dry (since that means the associated plant can not run which reduced peak generating capacity). A difficult task given the unpredictability of the weather and small size of many of the storages (topography precluded building the dams higher as a solution).

The overall operation of the system is thus somewhat "fuzzy" with all power stations being at times baseload, sometimes peak load and sometimes operating in the middle. It all depends on water availability and power demand.

The installed capacity at the hydro power stations was determined to meet energy production requirements NOT peak loads. That is, at the smaller storages it was sized to run flat out during a wet winter with the storage slowly rising (but ideally not spilling). The minimum size of generating plant which could use all of the water - as I said the emphasis was always on energy rather than peak power. At the major storages the size of the generating plant was determined by the need to produce baseload energy during dry conditions when output elsewhere drops off. Again the consideration was water management to get the highest total output rather than an emphasis on peak power.

However, the need to shift production around according to water availability combined with the high overall load factor in Tasmania (average load is 70% of peak load annually and in Summer the average is over 80% of the (low compared to Winter) peak load).

This combined with the fact that Tasmanian load is low during Summer afternoons when mainland load peaks enables Tasmania to export peak power via Basslink PROVIDED THAT it imports sufficient energy at other times to maintain an overall balance between inflows and outflows.

Due to the problem of Tasmanian load (plus exports) significantly exceeding hydro system inflows there will be an ongoing need for imports via Basslink and/or generation at BBPS to make up the difference.

Load in Tasmania peaks around 8am during Winter with a seccondary peak around 6pm in Winter. Even in Summer the peak is around 8am with a relatively flat load during the rest of the day. Mid-afternoon is in fact an off-peak period whereas in the other states it is the absolute peak. The reasons beign climate, method of water heating, space heating and cooking plus the large portion of constant heavy industrial load (4 industries take 50% of the load and heavy industry as a whole takes nearly thirds).

BBPS may also operate to support the smaller storages during maintenance outages of power stations drawing from the major storages. These are necessarily timed during the wet season when inflows to the smaller storages are highest.

3. The third reason for operating BBPS is to provide peak generating capacity or support the Northern part of the system during transmission outages.

 

[Smurf1976]

Regarding Basslink (had to post this separately due to post length limit) the original "need" is to import to Tasmania since the state is short on electricity due to the energy constrained nature of the system. Since Tas has adequate peak generating capacity when Vic/SA supply is tight, some of the electricity can be sent back thus giving benefit to both sides of Bass Strait and making the overall project far more economic.

So, power flows both ways according to short term demand but under present conditions Tasmania will import more than it exports. Sustained high rainfall, development of new generation in Tas or a fall in load would change that.

Actual operation will be determined by prices in the National Electricity Market. In brief, when storage is low then prices are high and vice versa. Present Tasmanian spot prices are high due to the low storage level and under these conditions the flow of power would predominantly be from Vic to Tas except when prices in Vic are high (due to shortage) in which case the flow would reverse.

 

[TjamesX]

Thanks Smurf, much appreciated

All i can say is - comprehensively answered!!!

the energy industry is complicated to say the least....

 

[mit]

not really it has only taken me 25 years of working in the industry to understand it :