- 2070 Magh · 8 marks
Electronic ballast replacement savings with ToD tariff
Short answer
Replacing 500 tubes with electronic ballasts saves 3.5 kW and 23.44 kVA. Annual energy saving Rs 58,765 (day tariff), demand saving Rs 59,080; total Rs 1.18 lakh. Payback ≈ 1.27 years.
In an industry, there are 100 numbers of 1×36 watt fluorescent tubes and 200 numbers of 2x 36 watt fluorescent tubes with copper ballast. The p.f. with copper ballast is very poor and is below 0.5. Also, losses in that ballast are very high. Replacing the electronic ballast can avoid these two problems. By laboratory test conducted on 36W fluorescent tube followings are finding: (a) with cu. Ballast power consumed 44W and p.f. is 0.5 b) with electronic ballast power consumed 37 watt and p.f 0.9. Calculate the energy cost saving and payback period with electronic ballast; if operating hours are 8 hours/day and electronic ballast cost is NRs 300 per piece, energy tariff is: 6 AM - 6 PM, Rs.5.75 paisa 6 PM - 11 PM, Rs.6.55 paisa 11 PM - 6 AM, Rs.4.00 paisa And kVA charge per month Rs. 210.00 paisa
Assumptions: one ballast per tube (electronic ballast cost Rs 300 per tube), lights run 8 h/day for 365 days, and all 8 h fall in the 6 AM – 6 PM band (day shift, Rs 5.75/kWh). The lights are on during the monthly peak, so the kVA saving reduces the demand charge.
Number of tubes:
100 × (1×36 W) = 100 tubes
200 × (2×36 W) = 400 tubes
Total N = 500 tubes
Power and kVA per tube:
| Ballast | kW per tube | pf | kVA per tube |
|---|---|---|---|
| Copper | 0.044 | 0.5 | 0.044/0.5 = 0.0880 |
| Electronic | 0.037 | 0.9 | 0.037/0.9 = 0.0411 |
Total load:
Copper: P = 500 × 44 = 22.0 kW, S = 44.00 kVA
Electronic: P = 500 × 37 = 18.5 kW, S = 20.56 kVA
Saving: ΔP = 3.5 kW, ΔS = 23.44 kVA
Annual energy cost saving:
ΔE = 3.5 kW × 8 h × 365 = 10 220 kWh/year
Cost saving = 10 220 × 5.75 = Rs 58 765 /year
Annual demand charge saving:
= 23.44 kVA × 210 × 12 = Rs 59 080 /year
Total saving and payback:
Total saving = 58 765 + 59 080 = Rs 117 845 /year
Investment = 500 × 300 = Rs 150 000
Payback = 150 000 / 117 845 = 1.27 years ≈ 15.3 months
If the 8 h were instead in the evening peak (Rs 6.55/kWh), the energy saving would be 10 220 × 6.55 = Rs 66 941 and payback ≈ 1.19 year; the conclusion is the same.
Other benefits: lower line current (less cable and transformer loss), no flicker or hum, and longer tube life with electronic ballasts.
Answer: Saving ≈ Rs 1.18 lakh per year (Rs 58 765 energy + Rs 59 080 demand); payback ≈ 1.27 years.
✓ Answer checked by Claude Opus 5.5 (high effort)
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