MEASUREMENT OF THE HEAT ISLAND ABOVE AN

ALUMINIUM SMELTER

I. G. PIGGIN

Natural Systems Research, 25 Burwood Road, Hawthorn, 3122 Australia

(Received in final form 18 March, 1982)

Abstract. Soundings of vertical profiles of atmospheric temperature were conducted above an aluminium smelter and anearby paddock during twonights when stableconditions induced nocturnal radiation inversion formation. On both nights the sky was cloudless, the surface wind speed was generally less than 2 m s- ‘, and process heat produced surface temperatures at the smelter up to 6 C” higher than the paddock. This temperature excess decreased in magnitude up to a height of approximately 100 m, after which the protiles became similar. In addition, the inversion (4-6 C” increase in lowest 100-200 m) which formed above the paddock on these nights was prevented from occurring in the lowest 100 m above the smelter, where unstable temperature lapse rates prevailed.

1. Introduction

The presence, strength and vertical extent of the heat island effect generated above Alcoa’s aluminium smelter at Point Henry was investigated to provide an indication of source depth for the modelling of atmospheric dispersion of fluorides.

Figure 1 shows the layout of the aluminium smelter operated by Alcoa of Australia Limited at Point Henry, 6 km east-northeast of Geelong in Victoria. In 1980 the smelter had three potlines and a rated production capacity of 165 000 tonnes yr - ‘. Each potline comprised a pair of potrooms, 270, 330 and 390 m long. The six potrooms were 18 m high, 22 m wide, 18 m apart, and encompassed an area of 7.35 ha. In total there were 368 pots or electrolytic cells for reduction of alumina, 104 in the first potline, 126 in the second and 138 in the third. The power demand for the three potlines totalled 300 MW, and some 50% of the energy consumed was emitted as heat. Sensible heat loss from the smelter is calculated to have been around 200 mW hr cm-’ each hour of the day, roughly twice the global radiation and thrice the net radiation at noon on a clear day in summer at Geelong.

2. Method

It was proposed to conduct soundings on nights when a high pressure system was centred over or to the east of Victoria and the forecast was for clear skies and light, variable winds. These conditions maximize the occurrence of strong nocturnal radiation inversions and the effects of heat evolved from the smelter on the temperature profile in the lower atmosphere.

On two nights when the synoptic situation and forecast were favourable, a series of vertical soundings was carried out at Point Henry, alternatively over the smelter (based

Boundary Layer Meteorology 23 (1982) 3 17-322. 0006-83 14/82/0233-03 17$00.90. Cop.vright 0 1982 by D. Reidel Publishing Co.. Dordrecht, Holland, and Boston, U.S.A.

318 I. G. PIGGIN

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weSmeltsr Sounding Site \

Fig. I. Location and layout of Alcoa aluminium smelter at Point Henry.

in the courtyard between the third and fourth potrooms) and in an open paddock some 3 km to the southeast (Figure 1).

On the second night, 10-l 1 November, 1980, vertical profiles of temperature and wind speed and direction were measured with a tethersonde system manufactured by Atmospheric Instrumentation Research Incorporated, Boulder, Colorado. This instru- ment has a temperature resolution of 0.1 Cc, a pressure resolution of 0.1 nib, and wind speed and direction resolutions of 0.1 m s ’ and 1 ’ respectively.

MEASUREMENT OF THE HEAT ISLAND ABOVE AN ALUMINIUM SMELTER 319

Only temperature data from the second night are presented here since less sophisti- cated equipment (a balloon-borne thermistor-transmitter) was used on the first night (21-22 October, 1980); nevertheless, the results from both expeditions were in accord.

3. Results and Discussion

Reproduced in Figure 2 are Bureau of Meteorology surface synoptic charts at noon on the days before and after the night (lo-11 November, 1980) when soundings were conducted. Figure 3 is a copy of the anemograph for the same night from the Dines anemometer operated by the Geelong Harbour Trust at the Signal Station at Point Henry (Figure 1). The sensor on this instrument is located 27 m above sea level and 19 m above ground level.

Fig. 2. Surface synoptic charts at noon on 10 and 11 November, 1980.

With a high pressure system located over the coastline of New South Wales, conditions were ideal for nocturnal radiation inversion formation on the night of lo- 11 November, 1980. The sky was clear throughout and the wind speed measured at the Signal Station was less than 2 m s-’ for most of the time between 23.00 and 05.00 D.S.T. (Daylight Saving Time or 1 hr in advance of Eastern Standard Time). Sunset was at 20.06 D.S.T. and sunrise at 06.05 D.S.T.

Twenty-one soundings were conducted in runs of two, three or four, with three series above each of the smelter and the paddock encompassing twelve hours between 21.00 and 09.00 D.S.T. To facilitate comparison, Figure 4 gives one temperature profile selected from each of the six series. Also plotted on Figure 4 is the temperature profile to 2000 m, measured by the 09.00 D.S.T. radiosonde ascent at Laverton (40 km north- east of Point Henry).

Comparison in Figure 4 of successive soundings above the paddock and the smelter shows the following features:

Fig.

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MEASUREMENT OF THE HEAT ISLAND ABOVE AN ALUMINIUM SMELTER 321

300

280

260

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220

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180

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Temperature (‘C)

TEMPERATURE I’C)

Soundmg Smelter Paddock

8 2124 - 2142 2346 - 0001

- @ 0259 - 0311

___---__ 63 0437 0448

0646 - 0655

. . . . . . . . . . . . . . . . . 0824 - 0831

Fig. 4. Atmospheric temperature profiles at Point Henry, above Alcoa smelter and nearby paddock, 011 night of lo- 11 November, 1980.

(i) A temperature excess (up to 6 C” at the surface) occurred in the atmosphere over the smelter compared with the nearby paddock, extending to a height of approximately 100 m, after which the profiles tended to become similar.

(ii) The lower levels of the atmosphere did not exhibit the normal trend of cooling throughout the night. As the wind backed from between southeast and south before midnight to northwesterly after 02.00 D.S.T., the shift from maritime to continental airflow caused the temperature to rise. More pronounced warming occurred in the lower levels after sunrise (which was at 06.05 D.S.T.).

322 1. G. PIGGIN

(iii) The nocturnal radiation inversion which formed above the paddock displayed a temperature increase of 4; Cc in the lowest 120 m in sounding 2A and 6 Cc in the lowest 220 m in sounding 4A.

(iv) Soundings above the smelter showed that the heat emission prevented occurrence of inversion conditions in the lowest 100 m or thereabouts, where unstable temperature lapse rates prevailed.

(v) The temperature fluctuations recorded just above the smelter during soundings 3B and 5A were probably caused by passage of the sensor through heat plumes from more distant potrooms.

4. Conclusions

During two stable nights, measurement of temperature profiles above an aluminium smelter have indicated the presence of a heat island up to approximately 100 m high with a temperature excess of up to 6 C3 at the surface. Mixing conditions within this unstable zone immediately above the smelter would therefore be enhanced in comparison with the surrounding atmosphere which is unaffected by artificial heating.

The extent and strength of the heat island effect are likely to have approached their maximum values during the extremely stable conditions prevailing on the monitored nights; however, further measurements are necessary to ascertain the behaviour of the heat island under a range of stabilities.

Acknowledgements

These measurements were commissioned by Alcoa of Australia Limited through their consultants, Kinhill Planners Pty. Ltd., whose project director was Trevor Fuller. Thanks are due to Dr Manuel Nunez and Greg Broszczyk, Geography Department, University of Tasmania, for provision and operation of their tethersonde.