How should the cool storage and heat storage scheme for a hotel air-conditioning system be selected? What are the respective advantages of ice storage and chilled water storage?
I am an HVAC expert. The selection of cool storage and heat storage schemes for hotel air-conditioning systems is a key part of the design, directly affecting initial investment, operating costs, plant room floor area, and the hotel's flexibility in responding to peak-valley electricity pricing and load fluctuations. According to Section 8.7 of the Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings GB 50736-2012 and the Technical Specification for Cool Storage Air-Conditioning Engineering JGJ 158-2008, the design of energy storage systems must comprehensively consider the local electricity pricing policy, building load characteristics, site conditions and economics. Below I analyse in detail the scheme selection and advantages of ice storage and chilled water storage.
Scheme Selection Principles
Hotel loads are characterised by large day-night fluctuations, a high proportion of cooling load in summer, and the continued need for cooling in some areas (such as guest rooms) at night. When selecting a cool storage scheme, the following steps should be followed:
1. Load analysis: According to Article 8.7.1 of GB 50736-2012, the hourly cooling load on the design day should be calculated to determine the cool storage capacity. For example, a five-star hotel has a peak cooling load of about 5000 kW and a night-time (23:00-07:00) cooling load of about 1500 kW; the cool storage system needs to balance the peak-valley difference.
2. Electricity pricing policy: Taking advantage of the peak-valley price difference (e.g., a peak-valley ratio of 3:1 or above), priority should be given to schemes with a high cool storage ratio. Ice storage has a high cool storage density and is suitable for projects with limited space; chilled water storage has a low initial investment and is suitable where large water tanks are available.
3. Site and equipment: Ice storage requires consideration of ice tanks and a glycol system, while chilled water storage requires a large-volume chilled water storage tank. Hotel basements are usually tight, so ice storage is more common.
Advantages of Ice Storage
Ice storage uses off-peak electricity at night to make ice and melts the ice during the day to supply cooling; the cool storage density is about 50-60 kWh/m³ (chilled water storage is only 7-12 kWh/m³). According to Article 4.2.2 of the Technical Specification for Cool Storage Air-Conditioning Engineering JGJ 158-2008, ice storage systems may adopt an upstream series or parallel arrangement of the chiller.
- Advantages:
- Space saving: For the same cool storage capacity, the volume of ice storage is only 1/5-1/3 of that of chilled water storage. For example, a hotel requiring 10000 kWh of cool storage needs only about 200 m³ of ice tanks with ice storage, whereas chilled water storage would require a 1000 m³ tank, which may require additional basement structure.
- Low-temperature air supply potential: Ice storage can provide low-temperature chilled water at 1-4℃, supporting low-temperature air supply systems (supply air temperature 4-6℃), reducing duct size and fan energy consumption, and complying with the provisions of Article 7.4.2 of GB 50736-2012 on supply air temperature difference. A hotel in South China adopting ice storage + low-temperature air supply saved 15% of duct area.
- Operating flexibility: Ice storage can be full storage or partial storage, adapting to the characteristics of high daytime and low night-time loads in hotels. For example, ice is melted to supply cooling during the daytime peak, while at night the chiller supplies cooling directly or makes ice.
- Case: A five-star hotel in Shanghai (total building area 80000 m²) adopted an ice storage system with an ice storage tank capacity of 8000 kWh, making ice at night and melting ice during the day; combined with screw chillers, the operating electricity cost was reduced by 35%, with an investment payback period of about 4 years.
Advantages of Chilled Water Storage
Chilled water storage uses the sensible heat of water (temperature difference 6-10℃); the cool storage density is low, but the system is simple. According to Article 8.7.3 of GB 50736-2012, chilled water storage may adopt a naturally stratified or labyrinth design.
- Advantages:
- Low initial investment: Chilled water storage requires no glycol system, heat exchanger or complex controls, and the equipment cost is 20-30% lower than that of ice storage. For example, an economy hotel (cooling load 2000 kW) adopting chilled water storage saved about RMB 500,000 in initial investment.
- Simple maintenance: The system has no phase-change process and no risk of ice blockage, so the maintenance workload is small. Chilled water storage equipment (such as storage tanks and pumps) has a long service life and a low failure rate.
- Applicable to multiple conditions: Chilled water storage can be used for both cool storage and heat storage (e.g., combined with a heat pump system), suiting the winter heating demand of hotels. For example, a hotel in North China used its chilled water storage tank also as a heat storage tank, storing heat at night in winter and supplying heat during the day, saving boiler operating costs.
- Case: A conference hotel in Beijing (building area 60000 m²) adopted a chilled water storage system with a 1000 m³ storage tank (temperature difference 8℃), connected in series with the chillers; the operating electricity cost was reduced by 25%, with an investment payback period of about 3 years.
Scheme Comparison and Selection Recommendations
| Parameter | Ice Storage | Chilled Water Storage |
|--------------|---------------------------|---------------------------|
| Cool storage density | 50-60 kWh/m³ | 7-12 kWh/m³ |
| Initial investment | Higher (equipment + glycol system) | Lower (tank + simple equipment) |
| Floor area | Small (about 1/5) | Large (requires a large tank) |
| Operating efficiency | Low ice-making efficiency (COP about 3.5) | High storage efficiency (COP about 5.0) |
| Applicable scenarios | Limited space, large peak-valley price difference | Idle space available, initial-investment sensitive |
Selection recommendations:
- Tight space, large peak-valley price difference (>3:1): Give priority to ice storage. For example, a city-centre hotel with limited basement area can save plant room space with ice storage.
- Large tank available, initial-investment sensitive: Give priority to chilled water storage. For example, the hotel has a fire water tank or landscape water tank that can be retrofitted for use, or the owner prefers a low-risk scheme.
- Comprehensive consideration: In actual projects, the load characteristics (e.g., the proportion of night-time cooling load in the hotel), equipment selection (e.g., whether the chillers are compatible with ice making) and local climate (e.g., southern China requires year-round cooling, where ice storage is better) must be taken into account.
Concluding Summary
The selection of cool storage and heat storage schemes for hotel air-conditioning systems should be based on load characteristics, electricity pricing policy and site conditions: ice storage is suitable for projects with limited space and a large peak-valley price difference, and can significantly save plant room area and support low-temperature air supply; chilled water storage is suitable for projects that are initial-investment sensitive and have idle water tanks, with a simple system and convenient maintenance. In actual design, Section 8.7 of GB 50736-2012 and JGJ 158-2008 must be strictly followed, and the optimal scheme determined through technical and economic comparison.
The above are reference opinions. The actual situation of the project still needs to be further evaluated and implemented in combination with specific operating conditions, site conditions and other factors.