Office Chiller Retrofit, Star Rating and Demand Response
A 15,000 m2, 12-storey office building has annual electricity use of 2,850 MWh. HVAC accounts for 1,650 MWh. The central plant has two 350 TR constant-speed centrifugal chillers (COP 4.6 at design) and one 200 TR screw chiller. The measured peak cooling load is 520 TR, but the annual average is 230 TR. Chilled-water pumps are constant speed; AHUs have two-speed fans. The building is in a composite climate and has single glazing with a SHGC of 0.65. The owner wants to replace one 350 TR chiller with a 400 TR magnetic-bearing VSD chiller (COP 6.8 at rating, 0.35 kW/TR at 40% load) and claim a 30% HVAC saving. The building also has CO2 levels of 1,200 ppm in meeting rooms at design occupancy. The utility tariff is Rs 8.5/kWh with a demand charge. Management asks for a technical review before approving the chiller.
1. Critique the 30% HVAC saving claim. Use the load data to explain part-load and sizing effects.
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Installed capacity is 900 TR versus a measured peak of 520 TR and an average of 230 TR. Replacing one 350 TR machine with 400 TR raises capacity and may increase cycling. Annual HVAC 1,650 MWh over 8,760 h = 188 kW average; at 230 TR average, cooling is 809 kW, so average COP is about 4.3, consistent with the existing plant. A new chiller rated COP 6.8 at full load and 0.35 kW/TR at 40% can save, but only on the load it carries. If the 400 TR machine carries 230 TR average, it runs at 58% and may be around 0.40–0.45 kW/TR, not 0.35. A 30% saving is 495 MWh, requiring average kW to fall by about 56 kW; possible but not guaranteed. Need an hourly load profile and chiller sequencing; keep the 200 TR screw for low load. Claim savings from simulation, not rated COP.
2. The owner wants a 5-star labelled chiller. Explain why the label alone does not guarantee savings and rank the chiller, pump VFDs, CO2-based ventilation and SHGC glazing.
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A star label is based on standard rating conditions; it does not guarantee savings at the building’s part-load profile, chilled-water temperature or condenser conditions. Compare certified part-load data and sequencing, not just the star count. Rank pump VFDs and CO2-based ventilation first: lower capex, reduced parasitic and fan/coil energy, and CO2 control from 1,200 ppm toward 800–1,000 ppm at part occupancy saves outdoor-air load while protecting IAQ. Add chiller sequencing and commissioning. Chiller replacement comes next if the existing machine is near end of life; right-size to 300–350 TR VSD rather than 400 TR and use the 200 TR screw for low load. Glazing has long payback and tenant disruption; try solar-control film or shading first. Do not claim chiller savings before fixing chilled-water flow and controls.
3. Design M&V and demand-response participation for the building.
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Use IPMVP Option B for the chiller and pumps: separate meters for kW, supply/return temperatures, flow and runtime; baseline 12 months and adjust for cooling degree-days and occupancy. Use Option C for whole-building total. For demand response, enroll the new chiller and AHUs in a 2-hour curtailment; establish the baseline from similar non-event days, not a fixed setpoint. Pre-cool before 18:00 if the tariff allows. Verify CO2 stays below about 1,000 ppm during curtailment; if not, reduce curtailment. Report both energy and peak kW savings, because demand-charge savings may dominate. Keep the star-rated equipment baseline separate from DR savings to avoid double counting.