Ice storage systems offer a versatile and energy-efficient solution for cooling, especially during periods of high cold demand or power outages. However, to maximize efficiency and performance, it is essential that the system is sized to the specific requirements of the existing cooling plant. This requires a detailed understanding of the cooling strategy, application requirements, load time, load profiles, peak load requirements and space constraints.
This article explains the key factors for designing and dimensioning an ice storage system to ensure optimum cooling performance while achieving cost and energy savings.
1. Selection of the right cooling strategy: Full storage vs. partial storage
The first step in dimensioning an ice storage system is to define your cooling strategy. Based on the load profile of your process, the strategy determines how much cooling capacity your system requires and how much ice needs to be stored to meet the demand.
Full storage
In a full storage strategy, the ice storage system is designed to meet all cooling requirements. This strategy is ideal for systems that want to completely avoid peak electricity costs or cool during extended power outages without being dependent on the grid.
Advantages:
- Full independence from the grid at peak times
- Maximum savings on energy costs
- Energy storage at times when electricity prices are low
- Utilization of surplus capacities
- Redundancy capability
- High efficiency in cooling generation
- Reduction in CO2 emissions
Partial storage
In a partial storage strategy, the ice storage system is designed to cover only a portion of the load, while the chiller provides the rest. This strategy can reduce energy demand during peak periods and requires less storage capacity.
Advantages:
- Energy storage at times of low electricity prices
- Utilization of surplus capacities
- High efficiency in cooling generation
- Reduction in CO2 emissions
- Smaller system size compared to a full-load strategy
- Lower initial investment
The choice between full and partial storage depends on the energy cost structures of the cooling system, the cooling requirements, and whether grid independence during peak periods is critical.
2. Application: Adapting the system to your industry
Different industries and applications have different cooling requirements, and the sizing of your ice storage system depends on the type of system and its operational needs.
Cooling requirements of frequent applications:
- Data centers: Require precise and continuous cooling to maintain safe operating temperatures for servers. Redundancy and reliability are critical here.
- Hospitals: Require uninterrupted cooling for patient care and sensitive equipment. Redundant systems with backup power are often required.
- Office buildings: Usually have a pronounced daily load profile, so partial storage systems are used more frequently as they can significantly reduce peak cooling loads.
- Industrial plants: Often have large, constant cooling requirements, making them ideal for full storage strategies to reduce operating costs.
- Dairies: The delivery of milk in the morning and evening results in short-term peak loads. These can be covered with an ice store.
- Agriculture: Agricultural businesses often have large PV systems. The feed-in tariffs for older systems are coming to an end. The available PV electricity can be used to cool agricultural goods such as vegetables and dairy products.
Each application has its own load profile and operating characteristics, so understanding the specific requirements of the system being expanded is key to proper sizing.
3. Loading time: How much time is available to make the ice?
The charging time, i.e. the time window available to produce ice, is a critical factor when dimensioning your system. Ice storage systems are designed to store cooling energy during off-peak hours, usually at night when electricity is cheaper and demand is lower.
Key factors:
- Energy off-peak: How long are the off-peak hours in your region? A shorter off-peak window requires a more efficient chiller to produce enough ice to meet cooling demand.
- Energy pricing: If electricity prices are highly dependent on the time of day, make sure the ice storage system can be charged quickly to maximize savings during off-peak hours.
By carefully considering charging time, a system can be sized to be not only effective but also energy optimized.
4. Load profile: Understanding the daily and seasonal cooling requirements
Your system's load profile describes the pattern of cooling demand over the course of a day, week, or year. It is important to analyze both peak and average loads to ensure that the ice storage system can be properly sized.
Key parameters:
- Peak load: Knowing the peak load is necessary to determine the capacity of the ice storage (size of the heat exchanger).
- Daily capacity: To determine the size of the storage tank, it is necessary to know the required daily capacity. This information can be obtained from the load profile.
- Temperature: What are the supply (flow and return) temperatures? Knowing the operating temperature is necessary to determine the discharge capacity.
5. Space restrictions: Designing a system that fits
Available space is another important factor in determining the size and type of ice storage system. Ice storage tanks require space for housing.
- Modular Tanks: Modular ice storage tanks can be installed in smaller units, providing more flexibility in limited spaces. Modular ice storage tanks are ideal for retrofitting into existing systems.
- Custom storage in on-site tanks: Custom ice storage tanks can be integrated into on-site storage facilities, such as a concrete tank. This option allows maximum flexibility in shape and size.
- Container systems: are particularly suitable for outdoor installation.
The space requirements should be defined at an early stage of the planning concept and should be coordinated with all the parties involved in the planning process.
6. Design and dimensioning examples for ice storage tanks
Example 1: Full load ice storage
An ice storage system is to be provided for the cooling supply of an office building. The load profile has been determined according to VDI 1946 and is available for design. When designing the system, the capacity of the chiller should be kept as low as possible.
Table 1.1 : Load profile according to VDI 2078
Time | Cooling load | Time | Cooling load | Time | Cooling load | Time | Cooling load |
|---|---|---|---|---|---|---|---|
1 | 20kW | 7 | 100kW | 13 | 540kW | 19 | 60kW |
2 | 20kW | 8 | 160kW | 14 | 520kW | 20 | 40kW |
3 | 20kW | 9 | 300kW | 15 | 500kW | 21 | 20kW |
4 | 20kW | 10 | 400kW | 16 | 440kW | 22 | 20kW |
5 | 20kW | 11 | 500kW | 17 | 380kW | 23 | 20kW |
6 | 40kW | 12 | 300kW | 18 | 140kW | 24 | 20kW |
The load profile results in a peak load of 540 kW. Further design is based on the assumption that the hourly cooling energy is equal to the hourly maximum multiplied by one hour. The total cooling energy for the day can be determined from Table 1.1. Knowing the total cooling energy, the minimum capacity to meet the daily load is calculated by dividing by 24 hours. In this case 191.6 kW. As a recommendation, we would select a capacity of 200 kW and be able to generate the required capacity of 4,600 kWh in 23 hours. Table 1.1 is completed with the capacity of the chiller. Subtracting the demand from the generator capacity gives the surplus and shortfall. The sum of the shortfall, in our case 2,200 kWh, must be covered by the ice storage system.
Table 1.2: Load and power profile example 1
Time | Demand | Cooling capacity | Coverage |
|---|---|---|---|
1 | 20 kWh | 200kW | 180 kWh |
2 | 20 kWh | 200kW | 180 kWh |
3 | 20 kWh | 200kW | 180 kWh |
4 | 20 kWh | 200kW | 180 kWh |
5 | 20 kWh | 200kW | 180 kWh |
6 | 40 kWh | 200kW | 160 kWh |
7 | 100 kWh | 200kW | 100 kWh |
8 | 160 kWh | 200kW | 40 kWh |
9 | 300 kWh | 200kW | -100 kWh |
10 | 400 kWh | 200kW | -200 kWh |
11 | 500 kWh | 200kW | -300 kWh |
12 | 300 kWh | 200kW | -100 kWh |
13 | 540 kWh | 200kW | -340 kWh |
14 | 520 kWh | 200kW | -320 kWh |
15 | 500 kWh | 200kW | -300 kWh |
16 | 440 kWh | 200kW | -240 kWh |
17 | 380 kWh | 200kW | -180 kWh |
18 | 140 kWh | 0kW | -140 kWh |
19 | 60 kWh | 200kW | 140 kWh |
20 | 40 kWh | 200kW | 160 kWh |
21 | 20 kWh | 200kW | 180 kWh |
22 | 20 kWh | 200kW | 180 kWh |
23 | 20 kWh | 200kW | 180 kWh |
24 | 20 kWh | 200kW | 180 kWh |
Total: | 4,600 kWh | Required | 2,200 kWh |
Table 1.3: Material data
| Material data from literature |
| |
|---|---|---|
| Specific heat capacity of water 0°C | J/kgK | 4,181 |
| Specific heat capacity 30% glycol at 0°C | J/kgK | 3,660 |
| Melting heat of ice | J/kg | 332 |
| Density of water at 0°C | kg/m³ | 999.8 |
| Density of ice | kg/m³ | 918 |
| Density of glycol at 0°C | kg/m3 | 1,053 |
| Specific heat capacity of ice -2°C | J/kgK | 2,220 |
Using the determined values and material data, the ice storage system is designed according to VDI Guideline 4657 Bald 2, Planning and Integration of Energy Storage Systems in Building Energy Systems, Thermal Energy Storage Systems, Chapter 9, Dimensioning / Design of Storage Systems, Section 9.3 Dimensioning / Design of a PCM Storage System (Ice Storage System). The guideline recommends a storage utilization factor of 0.7 for manufacturer-independent design.
Design: The values determined from the load profile in Table 1.1 (Table 1.2 and the material data from Table 1.3) are used to design the ice storage tank.
Table 1.4: : Ice storage tank design example 1
| Name | Value |
|---|---|
| Theoretically required capacity | 2,220 kWh |
| Storage utilization rate from VDI 4657 Sheet 2, Section 9.3 | 0.7 |
| Theoretical volume | 26,223 l |
| Volume actually required | 37,461 l |
| Inside dimensions | |
| Selected height | 2.2 m |
| Selected width | 2.2 m |
| Calculated depth | 7.7 m |
| Outside dimensions | |
| Wall thickness | 0.15 m |
| Height | 2.50 m |
| Width | 2.50 m |
| Depth | 8.04 m |
For the conditions shown in the example and a cooling demand of 2,220 kWh, the sp.ICE-20' ice storage unit with 2,195 kWh provides sufficient cooling capacity.
The dimensions of the sp.ICE-20' are
Length: 6,058 mm, Width: 2,438 mm, Height: 2,591 mm
Technical specifications of sp.ICE storage tanks
Example 2: Partial-load ice storage
As an alternative to example 1, the cooling work should be covered by photovoltaic power during the day if possible. For the evening and night hours, cooling should be provided by an ice storage system. The PV electricity is available with sufficient power in the period from 10 am to 6 pm.
Load profile: The load profile corresponds to the profile in Table 1.1.
Table 2.1
Time | Demand | Cooling capazity | Coverage |
|---|---|---|---|
1 | 20 kWh |
| -20 kWh |
2 | 20 kWh |
| -20 kWh |
3 | 20 kWh |
| -20 kWh |
4 | 20 kWh |
| -20 kWh |
5 | 20 kWh |
| -20 kWh |
6 | 40 kWh |
| -40 kWh |
7 | 100 kWh |
| -100 kWh |
8 | 160 kWh |
| -160 kWh |
9 | 300 kWh |
| -300 kWh |
10 | 400 kWh | 575kW | 175 kWh |
11 | 500 kWh | 575kW | 75 kWh |
12 | 300 kWh | 575kW | 275 kWh |
13 | 540 kWh | 575kW | 35 kWh |
14 | 520 kWh | 575kW | 55 kWh |
15 | 500 kWh | 575kW | 75 kWh |
16 | 440 kWh | 575kW | 135 kWh |
17 | 380 kWh | 575kW | 195 kWh |
18 | 140 kWh |
| -140 kWh |
19 | 60 kWh |
| -60 kWh |
20 | 40 kWh |
| -40 kWh |
21 | 20 kWh |
| -20 kWh |
22 | 20 kWh |
| -20 kWh |
23 | 20 kWh |
| -20 kWh |
24 | 20 kWh |
| -20 kWh |
Total: | 4,600 kWh | Required | 1,020 kWh |
The values obtained and the material data are used to design the ice storage tank according to Example 1.
Table 2.2: Ice Storage Tank Design Example 2
| Name | Value |
|---|---|
| Theoretically required capacity | 1,020 kWh |
| Storage utilization rate from VDI 4657 Sheet 2, Section 9.3 | 0.7 |
| Theoretical volume | 12,048 l |
| Volume actually required | 17,212 l |
| Inside dimensions | |
| Selected height | 2.2 m |
| Selected width | 2.2 m |
| Calculated depth | 3.6 m |
| Outside dimensions | |
| Wall thickness | 0.15 m |
| Height | 2.50 m |
| Width | 2.50 m |
| Depth | 3.86 m |
The requirement for a capacity of 1,020 kWh can be safely met with a sp-ICE-10' ice storage unit with a capacity of 1,280 kWh.
The dimensions of the sp.ICE-10' are
Length: 2,991 mm, Width: 2,438 mm, Height: 2,591 mm
Example 3: Thermal energy storage
Excess power is to be used to store energy for a local cooling network. At night, 1,000 kW of electricity is available between 11 pm and 4 am. The EEP for the chiller was determined to be 4.5. The ice storage system is to be installed underground in a concrete tank.
Time | Energy load | Cooling power |
|---|---|---|
1 | 4,500 kW | 4,500 kWh |
2 | 4,500 kW | 4,500 kWh |
3 | 4,500 kW | 4,500 kWh |
4 | 4,500 kW | 4,500 kWh |
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24 | 4,500 kW | 4,500 kWh |
| Required | 27,000 kWh |
The ice storage tank is designed using the values determined and the material data from Example 1.
| Name | Value |
|---|---|
| Theoretically required capacity | 27,000 kWh |
| Storage utilization rate from VDI 4657 Sheet 2, Section 9.3 | 0.7 |
| Calculation | |
| Theoretical volume | 318,923 l |
| Volume actually required | 455,604 l |
| Inside dimensions | |
| Selected height | 4.0 m |
| Selected width | 10.0 m |
| Calculated depth | 11.8 m |
Required concrete tank dimensions: H x W x D = 4.2 x 12.0 x 12.0 m
Conclusion: Dimensioning for Optimal Efficiency and Performance
Proper sizing of an ice storage system requires a holistic approach that takes into account the cooling strategy, specific application requirements, charging time, load profiles, peak load requirements, and space requirements.
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