Energy-Saving Retrofit Solutions for Cooling and Heating Source Systems
The cooling and heating source system is the core of hotel air-conditioning energy consumption, accounting for approximately 45%-55% of total air-conditioning energy consumption. Therefore, retrofitting the cooling and heating source system is the top priority of energy-saving retrofits. According to the energy efficiency requirements in Section 5.2 of GB 50189 Energy-Saving Design Standard for Public Buildings, chillers that have been in operation for more than 15 years are recommended for energy efficiency assessment, and replacement should be considered when COP is below 4.5. Typical cooling and heating source retrofit solutions include: (1) Magnetic levitation centrifugal chiller replacement solution: Magnetic levitation centrifugal chillers adopt magnetic bearing technology, with no mechanical friction loss in the compressor; full-load COP can reach 6.0-7.0, and part-load (30%-70%) COP can be maintained at 9.0-12.0, significantly better than conventional screw chillers (full-load COP 5.0-5.5, dropping to 3.5-4.5 at part load). Taking a five-star hotel retrofit project in Shanghai as an example, the original 4 screw chillers (installed in 1998, COP 4.8) were replaced with 3 magnetic levitation centrifugal chillers (cooling capacity 1,200kW each, IPLV 14.0); the annual comprehensive COP of the refrigeration plant increased from 4.2 to 7.8, saving approximately 580,000 kWh of electricity per year, with an energy-saving rate of 38% and an investment payback period of 4.2 years. (2) Variable-frequency air-cooled heat pump replacement of gas boilers: For heating systems, replacing gas boilers with variable-frequency air-cooled heat pumps can achieve significant energy savings. The COP of air-cooled heat pumps ranges from 3.0 to 4.5 (depending on outdoor temperature), while the thermal efficiency of gas boilers is only 85%-92%. Taking a hotel in Xi'an as an example, before the retrofit the gas boiler consumed 420,000 m³ of gas per year; after the retrofit, variable-frequency air-cooled heat pumps (COP 3.5) were adopted, with annual electricity consumption of 1.08 million kWh, equivalent to a 35% reduction in standard coal consumption and a 42% reduction in annual operating costs. (3) Ground-source heat pump system retrofit solution: For hotels with available site conditions, a ground-source heat pump system can be added to handle the base load. Ground-source heat pumps can achieve a COP of 5.0-6.0; compared with conventional electric cooling plus gas boiler solutions, annual operating costs are reduced by 30%-50%. However, note that the initial investment for ground-source heat pumps is relatively high (approximately 1.5-2 times that of conventional systems), and the site conditions for the ground heat exchanger must be evaluated. (4) Natural cooling source utilization retrofit: During transition seasons and winter, cooling towers can be used for direct cooling (Free Cooling) with chillers shut down. The cooling tower cooling solution requires additional plate heat exchangers and switching valve groups; when the outdoor wet-bulb temperature is below 10°C, the cooling tower circulating water is used to directly provide chilled water to the air-conditioning system through the plate heat exchanger. Article 8.7.8 of GB 50736 recommends the use of natural cooling technology in severe cold and cold regions. Equipment selection principles for cooling and heating source retrofits: give priority to equipment with energy efficiency grade 1; Article 5.2.1 of GB 50189 requires that chiller COP shall not be lower than the grade 3 energy efficiency limit, but five-star hotels should select grade 1 energy efficiency products; the Marriott MIDS standard requires the comprehensive COP of chillers to be no less than 6.3.
Variable-Frequency Retrofit and Hydraulic Optimization of Distribution Systems
The energy consumption of distribution systems (pumps and fans) accounts for approximately 20%-30% of the total air-conditioning system energy consumption; variable-frequency retrofits can reduce distribution energy consumption by 40%-60%. According to Articles 8.5.7 and 8.5.8 of GB 50736 Code for Design of Heating Ventilation and Air Conditioning for Civil Buildings, air-conditioning water systems should adopt variable-flow operation, and pumps should adopt variable-frequency speed control. The core technical contents of variable-frequency retrofit of distribution systems are as follows: Chilled water pump variable-frequency retrofit: install variable-frequency drives on existing fixed-frequency pumps and convert them to variable-speed pumps. The control strategy adopts the most unfavorable loop differential pressure control method, with a differential pressure sensor installed at the terminal equipment farthest from the pump, and the differential pressure setpoint generally set at 1.0-1.2 times the design-condition differential pressure. After variable-frequency operation, pump energy consumption decreases with the cube of flow (Q∝n³), i.e., when flow drops to 80%, energy consumption is only 51.2%. In actual operation, the annual average frequency of hotel air-conditioning chilled water pumps is 30-40Hz, and pump energy consumption is approximately 40%-60% of fixed-frequency operation. Cooling water pump variable-frequency retrofit: variable-frequency control of cooling water pumps must be combined with the cooling tower operating strategy. The control target is set to maintain a constant condenser leaving water temperature of the chiller (e.g., 37°C) by adjusting the cooling water pump frequency and cooling tower fan speed. Note that the cooling water pump frequency should not be lower than 30Hz; otherwise, uneven water distribution in the cooling tower may occur. Regarding head calculation, the actual resistance curve of the most unfavorable loop must be recalculated before the retrofit to determine the minimum operating frequency and minimum flow limit after variable-frequency conversion. Fan variable-frequency retrofit: install variable-frequency drives on supply fans of air-handling units and fresh air handling units, with variable-frequency regulation based on CO₂ concentration, return air temperature or supply air temperature. The minimum frequency is generally set at 20Hz, corresponding to a minimum air volume of 40%. Installing CO₂ concentration sensors enables demand-controlled ventilation (DCV), automatically reducing fresh air volume when indoor CO₂ concentration is below the setpoint (e.g., 800ppm). The annual energy-saving rate of fan variable-frequency retrofits is typically 40%-60%. The following technical points should be noted for pump variable-frequency retrofits: (1) Select high-quality brands of variable-frequency drives (such as ABB ACS880, Siemens G120 series), and the harmonic distortion rate should meet the requirements of the IEEE 519 standard. (2) Variable-frequency drives must be equipped with EMC filters to prevent electromagnetic interference from affecting the normal operation of the BAS system. (3) Both cooling water pumps and chilled water pumps should be provided with bypass pipes and minimum flow control valves to ensure the minimum flow requirement of chillers when variable-frequency pumps operate at low speed. The Marriott MIDS standard requires that hotel air-conditioning pumps and fans all adopt variable-frequency control and be provided with bypass devices.
Terminal Equipment and Heat Recovery System Retrofit
Terminal equipment retrofits and heat recovery system upgrades are important supplementary elements of hotel HVAC energy-saving retrofits, further tapping energy-saving potential on the basis of chiller/heat source and distribution system retrofits. According to Article 4.2.14 of GB 50189 Energy-Saving Design Standard for Public Buildings, fresh air systems shall be equipped with heat recovery devices, with heat recovery efficiency not lower than 60%. The main technical contents of terminal equipment retrofits include: (1) Fan coil electric valve upgrade: replace original manual valves or solenoid valves with electric control valves or PICV dynamic balancing valves. PICV can automatically maintain constant flow at terminal equipment, eliminating over-flow or under-flow caused by hydraulic imbalance. After installing PICV, terminal equipment flow deviation can be reduced from ±15% to ±5%, improving indoor temperature uniformity while reducing pump energy waste caused by over-flow. Retrofit economics: the PICV retrofit cost per fan coil is approximately 200-400 yuan (including valve and installation); for a hotel with approximately 600-800 guest rooms, the total retrofit cost is approximately 150,000-250,000 yuan, with an annual electricity saving rate of approximately 5%-8%. (2) Air handling unit cooling coil replacement: cooling coils of old air handling units typically have low heat transfer efficiency and high air resistance; replacing them with high-efficiency copper tube + aluminum fin cooling coils (fin spacing 2.0-2.5mm, hydrophilic coating treatment) can improve heat exchange efficiency by 15%-25% and reduce air resistance by 10%-20%. (3) Fresh air heat recovery device installation: for hotels whose original design did not include heat recovery, install rotary wheel or plate-fin total heat recovery devices. Installation requirements: the heat recovery device should be installed in the fresh air handling unit section or between the fresh air and exhaust air outlets of an independent fresh air system; the fresh air bypass duct cross-section shall be designed for 100% fresh air volume. Selection parameters: total heat recovery efficiency ≥65%, sensible heat efficiency ≥70%, air-side resistance ≤200Pa. The payback period for installing heat recovery devices is typically 2-4 years. (4) Cooling tower free cooling retrofit: add plate heat exchangers and changeover valves to utilize natural cooling from cooling towers during shoulder seasons and winter. Plate heat exchanger capacity shall be selected at 30%-50% of cooling load; changeover valves shall be electric butterfly valves. Retrofit payback period is approximately 2-3 years. (5) Radiant terminal installation: install radiant cooling/heating terminals in large spaces such as lobbies to reduce the supply air volume of convective air conditioning and lower fan energy consumption. Radiant terminals require higher supply water temperature (16-18°C for cooling), allowing chillers to operate at higher evaporating temperatures and improving COP. Terminal retrofits must address hotel operation interference during construction; phased and zoned construction is recommended, preferably during the hotel's low season. Marriott MIDS standards require that all newly installed terminal equipment shall have independent regulation capability and shall be linked with the BAS system.
Intelligent Control System and Energy Management Platform Upgrade
The intelligent control system and energy management platform (EMS) are key to coordinating various energy-saving retrofit measures and achieving overall system energy efficiency optimization. According to the relevant requirements of GB/T 34136 Technical Specification for Building Energy Management Systems and GB 50314 Standard for Design of Intelligent Building, hotel HVAC system energy management shall cover four major functions: energy consumption monitoring, optimized control, abnormal alarm and energy-saving assessment. The main contents of intelligent control system upgrade include: (1) Energy management platform (EMS) construction: install smart electricity meters (accuracy class 0.5S), cooling/heating meters (accuracy class 1) and flow meters on key equipment such as chillers, pumps, air handling units and fresh air handling units to establish a sub-metering system. The EMS platform shall have functions including real-time data acquisition (acquisition interval ≤5 minutes), historical data storage (at least 2 years), energy efficiency analysis (automatic calculation of EER, COP, IPLV and other indicators), abnormal alarm (automatic push notification when limits are exceeded) and energy-saving reports (daily, weekly and monthly reports). (2) Chiller plant group control optimization: optimize chiller start/stop sequence, number of operating units and set points according to outdoor temperature and humidity, indoor load demand and electricity price periods. Control strategies include: determining the optimal operating combination according to the IPLV curve, resetting chilled water supply temperature based on cooling water inlet temperature (for every 1°C increase in chilled water supply temperature, unit COP improves by approximately 2%-3%), nighttime free cooling mode, etc. (3) BAS linkage control: deeply integrate the HVAC system with the building automation system (BAS) to achieve linkage control of HVAC with lighting, shading, fresh air and other systems. For example, when a guest room is unoccupied, the RCU system automatically adjusts the room temperature set point to energy-saving mode (26-28°C in summer, 18-20°C in winter), and restores it to comfort mode when the guest checks in (24°C in summer, 22°C in winter). The payback period for intelligent control systems is generally 2-3 years, with an annual energy saving rate of approximately 10%-20%. GBT 50314 Standard for Design of Intelligent Building has clear classification requirements for intelligent system configuration of star-rated hotels. Marriott MIDS standards require that the communication protocol of hotel HVAC control systems shall adopt BACnet/IP or Modbus TCP to achieve seamless data integration with upper-level management platforms.