Project Overview and Pre-Retrofit System Diagnosis
A Marriott hotel in Shanghai is located in Pudong New Area, with a building area of approximately 42,000 m², featuring 380 guest rooms and supporting facilities including a banquet hall, conference center, and food and beverage outlets. The hotel opened in 2006, and the original air-conditioning system was equipped with 4 screw chillers (cooling capacity 800 kW each, COP 4.8), with a total installed capacity of 3,200 kW. By 2022, the units had been in operation for 16 years, exhibiting multiple issues such as decreased compressor efficiency, severe heat exchanger fouling, and aging control systems. The measured COP of the units had dropped to 3.8–4.2, approximately 15%–20% below the rated value. Annual cooling electricity consumption was approximately 1.65 million kWh, accounting for 38% of the hotel's total electricity consumption. According to Clause 5.2.1 of GB 50189 Energy-Saving Design Standard for Public Buildings, the COP of water-cooled screw chillers shall not be lower than 4.5 (energy-saving evaluation value); the existing units no longer meet the code requirements. In accordance with GB/T 17981 Economic Operation of Air-Conditioning Systems, energy-saving retrofit shall be carried out when the operating efficiency of the air-conditioning system falls below the specified value. In March 2022, the hotel commissioned a professional energy service company to conduct a 3-month energy consumption audit and system diagnosis. The diagnosis adopted methods including hourly energy consumption monitoring, field measurement of chilled water/cooling water temperature and flow, and verification of unit performance curves, comprehensively evaluating the actual operating status of the system. The diagnostic conclusions showed that the comprehensive COP (SCOP) of the existing system was 3.6, far below the industry advanced level of 5.5–6.5 for similar hotels. The main problems included: poor part-load performance of the units (the hotel operates in the 40%–70% load range for more than 70% of the year, and the COP of screw chillers in this range is only 3.2–3.8), poor cooling water temperature control accuracy due to the absence of variable-frequency drives on cooling tower fans, fixed-frequency operation of both chilled water pumps and cooling water pumps unable to match load variations, and hydraulic imbalance caused by outdated terminal equipment control strategies.
Retrofit Scheme Design and Technology Selection
Based on the energy consumption audit results, the retrofit scheme design followed the energy-saving requirements of GB 50736 Design Code for Heating Ventilation and Air Conditioning of Civil Buildings and GB 50189, while incorporating the technical standards of Marriott International Design Standards (MIDS). The core of the scheme was to replace the original 4 screw chillers with 3 magnetic bearing centrifugal chillers, each with a cooling capacity of 1,000 kW and a total installed capacity of 3,000 kW (6.25% lower than the original system, due to accurate matching of actual load demand). The magnetic bearing units adopt Danfoss Turbocor compressors equipped with magnetic bearing technology, eliminating mechanical friction losses, with a full-load COP of 6.5 and an IPLV (Integrated Part Load Value) of 14.2. Supporting retrofit scope included: all chilled water pumps and cooling water pumps replaced with variable-frequency control, equipped with ABB ACS880 series variable frequency drives; variable-frequency drives added to cooling tower fans, adopting a wet-bulb temperature-based optimized control strategy developed by Tsinghua University; an EMS energy management system added to achieve full-parameter monitoring and intelligent group control of the cooling source system; dynamic balancing valves added to the chilled water piping to optimize hydraulic balance. In the technical and economic comparison, Scheme A (magnetic bearing scheme) and Scheme B (high-efficiency screw scheme) were subjected to a life cycle cost (LCC) comparative analysis. Scheme A had an initial investment of approximately 5.2 million yuan (including equipment, installation, commissioning, and civil works), while Scheme B had an initial investment of approximately 3.8 million yuan. However, Scheme A had an annual operating cost of approximately 820,000 yuan, versus approximately 1.18 million yuan for Scheme B. Over a 10-year period, the total cost (including maintenance) of Scheme A was approximately 14.2 million yuan, versus approximately 16.2 million yuan for Scheme B, with Scheme A saving approximately 2 million yuan. Scheme A had a payback period of 3.2 years and an internal rate of return (IRR) of 21.5%. Considering energy efficiency level, Marriott Group's sustainability goals, and carbon emission reduction requirements, the magnetic bearing scheme was ultimately selected.
Retrofit Implementation Process and Key Technical Difficulties
The retrofit project commenced in September 2022 and was implemented in two phases to ensure the hotel remained operational during the retrofit. Phase 1 (September–November 2022, non-cooling season): dismantling screw chillers No. 2 and No. 3 and their ancillary equipment, installing magnetic bearing chillers No. 1 and No. 2, and renewing the corresponding piping and valves; retrofitting variable-frequency control for cooling tower fans; installing EMS energy management system hardware. Phase 2 (January–March 2023): dismantling screw chillers No. 1 and No. 4, installing magnetic bearing chiller No. 3; completing the replacement of variable-frequency pumps and piping retrofit; and system linkage commissioning. Multiple technical difficulties were encountered during construction: limited plant room space — the original 4 screw chillers occupied approximately 120 m², and although the new units were reduced to 3, their dimensions were larger (4.5 m × 2.0 m × 2.5 m each), requiring re-planning of equipment layout. The solution adopted an L-shaped compact arrangement, removing an abandoned water tank in the plant room to release space. Regarding piping retrofit, the original chilled water pipes were DN300 steel pipes, and pipe changeover had to be completed without shutting off water or interrupting cooling supply. Line-Tap technology and a double-flange blind plate isolation scheme were adopted for sectional construction. Regarding vibration control, the magnetic bearing units generate high-frequency vibration during operation (approximately 20,000–40,000 rpm), requiring special vibration isolation measures. The unit base adopted a double-layer vibration isolation scheme of spring isolators plus inertia base, and double-sphere rubber flexible joints were used for pipe connections. Regarding control system integration, the magnetic bearing units use the Modbus RTU communication protocol and needed to be integrated with the hotel's existing Honeywell EBI building control system. By developing a dedicated gateway protocol converter (BACnet/IP to Modbus), unit data uplink and remote control were achieved. During commissioning, it was found that the magnetic bearing units had a large current surge at the instant of startup (approximately 2 times the rated current), which caused coordination problems with the circuit breakers of the original power distribution system. After analysis, the original C-curve circuit breakers were replaced with D-curve circuit breakers, and the startup parameters of the soft starters were adjusted.
Energy-Saving Effect Verification and Operating Data Analysis
After the retrofit was completed, a 12-month energy consumption tracking test was conducted (April 2023 to March 2024). The test was based on GB/T 17981 "Economic Operation of Air Conditioning Systems" and the ASHRAE Guideline 14 Measurement and Verification (M&V) protocol, using Option C (whole facility method) to verify energy savings. Comparison of operating data before and after the retrofit showed significant energy savings. Annual cooling electricity consumption dropped from 1.65 million kWh before the retrofit to 1.188 million kWh, saving 462,000 kWh, an energy savings rate of 28%. Calculated at Shanghai's commercial electricity price of 0.95 yuan/kWh, annual operating cost savings are approximately 439,000 yuan. If variable-frequency energy savings from the pump group and cooling tower (approximately 52,000 yuan/year) are included, total energy-saving benefits are approximately 491,000 yuan/year. The system's comprehensive SCOP increased from 3.6 before the retrofit to 6.2, an improvement of 72%. The cooling plant EER (Energy Efficiency Ratio) increased from 2.9 to 5.1. In the 25%-75% load range (accounting for approximately 73% of annual operating hours), the magnetic bearing chiller unit's COP remained between 8.5-12.0, significantly better than the screw chiller unit's 3.0-4.0 in that range. Specifically: after variable-frequency operation of the chilled water pumps, transmission energy consumption dropped from 28.5 kWh/10,000 RT·h to 16.2 kWh/10,000 RT·h, a decrease of 43%. After variable-frequency operation of the cooling water pumps, energy consumption dropped from 22.8 kWh/10,000 RT·h to 14.5 kWh/10,000 RT·h, a decrease of 36%. After variable-frequency control of the cooling tower fans, energy consumption dropped from 6.5 kWh/10,000 RT·h to 4.1 kWh/10,000 RT·h, a decrease of 37%. In terms of equipment reliability, the magnetic bearing chiller unit experienced no unplanned downtime during the operating period, with an average availability rate of 99.8%. The unit's refrigerant R134a charge is 35% lower than that of the screw chiller unit, and the magnetic bearing chiller unit uses a hermetic compressor, greatly reducing the risk of refrigerant leakage. Carbon reduction effect: annual electricity savings of 462,000 kWh, calculated at Shanghai grid carbon emission factor of 0.5703 tCO₂/MWh, reducing annual carbon emissions by 263.5 tCO₂. Marriott International listed it as a demonstration project for energy-saving retrofits in the Asia-Pacific region and shared the case at the 2024 Marriott Global Sustainability Conference. The retrofit project also received the 2023 Shanghai Excellent Energy-Saving Retrofit Project Award.