Table C: Comparison of WLTOC for different energy efficient class transformers Transformer

Energy Class losses

+8 x 0.0625) x365 Purchase price (PP)

WLTRC* Over 25 years

WLTOC over 25 years per duty Tx

kWh losses per year per duty Tx

CO2 emissions on losses over 25 years per duty Tx (assumed cost only)

PP + WLTRC

0.4 x kWh losses x 25 (**Assume 0.40 kg/kWh)

£14,000

£35,587.5 £190,237.5

£239,825 £11,000

£43,800 £221,945

£8,500

£50,917.5 £266.335.0

£7,000

£62,962.5 £332,917.5

£276,745 £317,752 £395,880 36.13MWh 42.52MWh 50.76MWh 63.34MWh 361.32 T 425.92 T 507.60 T 633.41 T

* WLTRC is the Whole life total running cost = TLTOC-PP ** It is assumed that within the next 25 years the UK will have achieved a higher than expected CO2 emission reduction target; the carbon content of the grid electricity is therefore assumed to be lower than 0.59kg/kWh on average. An arbitrary 0.4kg/kWh figure is adopted for the purpose of illustrating the Whole-life total ownership cost (WLTOC) design approach in here for a more conservative result.

Table D: Comparison of WLTRC and CO2 emissions for the fictitious building adopting different energy efficient class transformers

Transformer

Purchase price (PP) for 3 off Tx

WLTRC Over 25 years

WLTOC CO2

Energy Class losses

(assumed cost only – professional fees included)

25 x (cost of 3 x No load + 2 x On Load losses)

PP + WLTRC emissions on losses over 25 years *See note for Table C (**)

0.4 x kWh losses x 25 *Assume 0.40 kg/kWh

A £42,000 £487,237

£529,237 779 T

B £33,000 C £25,500 D £21,000 £575,290 £669,412 £840,710

£608,290 £694,912 £861,710 920 T

1,071 T 1,345 T

The WLTOC for two 100% rated duty and standby transformers is summarised in Table C for different energy efficient class transformers. Since the system has three transformers,

the overall WLTRC and CO2 emissions of the transformers for this building can be shown to be (see Table D): Table D gives the practical realisation

results of optimising the sub-function (

Λ

WLTRC CO emissions f Tx size op hours CO kg kWh life time yrs load factor no load and on load losses tariffs pp

( 2 _ )) =

( _ , _ , 2 _ / _

, _ Even if we reduce the average electricity

tariff to £0.18 per kWh for the 25 years whole- lifetime period, the payback for a one-off marginal capital cost of £12,000 to purchase the more energy efficient B class transformer will take no more than five years. It can be said that under the right circumstances – that is, where a client can invest upfront to cut CO2 and reduce future electrical bills – the

50 CIBSE Journal June 2011 _ _

, _ _ , ,

electrical building services engineer (say, with a class B transformer) can provide the building with a minimum return on investment of £10.61k and 17.82T CO2 per annum. Not only will this achieve a much better energy and CO2 rating than any renewables that can be purchased at £12,000 at the time this article is written, it is also the most reliable and sustainable solution for the life of the building.

, )

Conclusion This article has shown how electrical building services engineers can use the WLTOC design approach to deliver the best value electrical supply design scheme in terms of both cost and CO2 emissions- reduction in the long term. Based on the same optimisation technique, engineers can apply a similar calculation to select more energy efficient motors, cables3

and lighting

installations together to achieve a much- improved carbon and energy rating for the building. The supply grid outside the building will also receive knock-on benefits of a reduction

www.cibsejournal.com A (W)

Ao=650 Ak=6000

£1,423.5 £7,609.5

B (W)

Bo=800 Bk=7000

£1,752 £8,877.8

C (W)

Co=930 Ck=8400

£2,036.7

D (W)

Do=1150 Dk=10500

£2,518.5 £10,653.4 £13,316.7

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