Lowering Peak Demand with Ice Storage
As expected, recent high electric cost made us focus considerable attention on thermal storage technologies. New York city has always had high electric demand during peak season (July and August) and building operators try many methods to reduce the peak electric demand to avoid demand charges. Cooling is a major contributor to peak electric demand, but it also represents one of the few areas where load management methods are not widely applied. It is time to pay attention to thermal storage systems, well designed thermal storage system will effectively and efficiently reduce electrical demand, exploit time-of-day rates and remain totally transparent to a building’s occupants. In this article, we will try to explain the types of ice storage systems and applications. We will also explain what kind of systems can use ice storage, design challenges, where to locate them, and financial benefits.
There are many types of storage systems, but chilled water-based ice storage systems always stand out to provide efficient design and operation. Ice storage system is a relatively basic concept, during off peak hours chilled water system operates to make ice and during peak hours ice can be used to meet the load. Using ice during peak hours will reduce the chiller load and chilled water system initial size. This approach will save tremendous electric cost and chilled water system initial capital cost.
Designing Ice Storage Systems
There are two types of ice storage systems, both systems use glycol mix for making ice. Chillers circulate glycol mix below freezing point through the ice storage tank and make ice during off peak hours.
For an ice storage system, we commonly describe chiller capacity in two modes a conventional daytime cooling capacity and a nighttime, ice-making capacity, which is typically 65% to 70% of the daytime value.
First system is called partial storage, basically ice capacity is only meeting the portion of the peak demand, chillers must run while burning ice to make up the capacity. But chillers would not be running full capacity since ice storage would meet the partial cooling load.
The second type of system is the full ice storage system, where chillers are completely shut down during ice burning mode. This system requires more capital or initial investment. Please remember, as an engineering approach we use ton-hr for sizing the ice storage systems. The total integrated cooling load (ton-hours), must be met by the chiller over its entire operating period, with appropriate capacity adjustments for different conditions
How do engineers know which system is the best approach for the building? The first step is to prepare full energy model. Load calculation can also be used but reports will only show the tonnage and will not show the financial benefits. Engineers will calculate ton-hr demand for the building and size the storage tanks to avoid peak demand charges. It depends on the building operation and the region; tank capacity may need to meet 8 hours of cooling or partial cooling while the chiller plant is operating.
For each of the approaches we might consider, minimum chiller capacity based on energy model that can supply all of the required cooling. The minimum chiller capacity is now defined in terms of its daytime load. The minimum storage capacity will equal to the total ton hours less the daytime chiller capacity contribution. Some approaches may use larger than minimum chillers that allow the use of either more or less storage, but the required storage capacity will still be accurate as long as the actual daytime chiller contribution is properly described.
Total Load (ton-hrs) = Chiller day capacity (ton-hrs) + chiller after hours capacity (ton-hrs)
Both chiller day capacity (ton-hrs) and chiller after hours capacity (ton-hrs) comes from energy modeling or load calculations.
Of course, the simplest approach is to use full ice storage system design to meet the peak demand load. This is clearly the most expensive of both options and is most common where extended payback periods are acceptable or where incentives or rebates are offered.
With this basic explanation, imagine a commercial building in NYC, occupied during business hours and pays for peak demand in August and July. Meanwhile chillers are just running low load at night. The benefits of using ice storage system will be substantial if we use ice storage system and make ice at night and use ice during daytime.
Challenges for Implementing Ice Storage Systems
So why the storage systems are not common. Designing and implementing ice storage systems have three challenges
· Sizing and designing the storage system correctly based on building operation
· Chiller efficiency during ice making
· Control system reliability
· Locating the storage systems. This is mainly NYC specific challenge.
All these challenges have improved past twenty years. Engineers can model the buildings accurately and select more accurate chiller and storage, chillers are far more efficient and controls systems are more advanced and reliable. Locating the storage systems in the NYC underground parking garages is becoming more common.
Of course, designing and implementing ice storage systems is not a simple task. One of the big challenges’ engineers face is the chiller efficiencies. Glycol mix will reduce the chiller efficiency and low leaving water temperatures will impact the chiller operation. the manufacturer may recommend that flow through the storage equipment be in the same direction for charge and discharge. The series arrangement automatically accomplishes this while a parallel arrangement necessitates a change in flow path as the system cycles between charge and discharge. As “full storage” systems are fairly straightforward in selection and application, our focus will be on “partial storage” techniques, where controlling the contribution of chiller and storage are critical to system economy and comfort.
Ice Storage Chiller Plant Example
We will discuss traditional 6,000 ton chiller plant operation and benefits. This chiller plant is in NYC and completed in 2016. Building is 3.5 million sqft, scope included upgrading the existing standard-duty chillers with one electric drive and one steam turbine, totaling 6,000 tons of chilled water The project included high-efficiency chiller replacements, the installation of a thermal energy storage system, and the implementation of a building automation system
· Energy Savings: 2 GWh/year
· Summer Peak Demand Reduction: 2.1 MW
· Carbon Footprint Offset: 36 million lbs. CO2
· Building Operational Cost Savings: $2.5 million/year
· Internal Rate of Return: 12.8%
· Awarded $942,000 in incentives from the NYSERDA Existing Facilities Program
For the past 30 or more years most central chilled water plants have been built without any ice storage systems or considering the peak demand. This approach is completely wrong. Sizing systems for the peak cooling load without considering the thermal storage and peak demand is wasting capital and poor capital planning.
Some of the questions we heard over the years from the operators
· Chiller plant does not make ice or goes through the ice quickly. This is very common problem, mainly because of the undersized tank or poor control sequences.
· Our demand charges are still occurring. This is another common complain, main reason for not meeting the peak demand is either building usage has changed or initial tank sizing is not done correctly.
All these questions are directly related to controls and detailed analysis of the building.
Although the solution is simple, most of the building operators and engineers are not familiar with ice storage systems. Lowering the peak demand for a chilled water based building is directly related to chiller plant efficiency and load. Before changing the existing chillers with new, performing energy analysis and calculating ice storage options is very simple.
If you want to lower your chiller plant peak demand, contact us. We will analyze it and provide you options.
YEC Engineering has designed many chiller plant projects in NYC and other states.
Questions about MEP engineering and our services. Contact us
Baltu Yorkos, PE