Figure 2. Typical ice thermal storage schematic.
The Baltimore State Office Center has installed a new vault that secures the equivalent of millions of dollars. Located 20 ft deep, this vault does not hold gold bouillon or stock certificates; it holds 11,000 lb (5.5 tons) of ice coils that translate into substantial energy savings for the Baltimore State Office Center. The new ice storage vault is part of a $13 million retrofit project to replace 30-year-old equipment, improve efficiency and comfort levels, and generate $1 million annually in energy savings.

With deregulation affecting utility rates in different ways all over the nation, the Baltimore State Office Center isn't the only facility looking to decrease costs. Engineers, contractors, and endusers nationwide are seeking methods to leverage utility rate structures and reduce escalating expenses. Fortunately, ice thermal storage technology provides a proven way to put the freeze on cooling costs.

Figure 1. Sample first-cost comparison for a 400-ton system.

Opening the vault

Ice thermal storage is the process of generating and storing ice at night to cool a building the next day, which results in lower kW demand and energy usage. The California Energy Commission estimates an average of 12% fewer kWh are used with ice thermal storage. With a low cost of operation and the potential for the lowest first cost, ice thermal storage offers an energy-saving technology to accommodate new changes and trends in the electric power industry.

The three functional areas of the electric power industry - generation, distribution, and transmission - face increasing challenges that typically translate into rising costs. For generation, it is becoming harder to meet the traditional industry norm of 15% capacity above demand. Brownouts can start being a concern at 10% above capacity. In 1980, the industry exceeded the norm with 30% excess capacity but this has steadily dropped to 14% in 2000. Some forecasts predict less than 10% average excess capacity in 2005.

Plans are in the works for new power plants, but analysts are uncertain that they will survive the tangle of financing and regulations. Overall, North American generating capacity is marginal with areas such as the Pacific Northwest, California, Arizona, New England, and New York already experiencing shortfalls.

Secondly, the industry now faces transmission gridlock as activity has expanded outside of local/regional transactions. In many areas, transmission lines cannot carry any more power, and they cannot be upgraded fast enough. In 1995, there were only 25,000 transactions when power left local/regional areas and was sold. Conversely, in 1999, there were more than 2 million transactions with power moving outside of regional lines. The number of transactions will continue to increase as deregulation takes effect more widely. Areas with current transmission problems include San Diego, Dallas/Fort Worth, New York City, and Chicago.

Lastly, on the distribution end, local systems are having increasing difficulty handling peak loads. These electric demand overloads have caused blackouts in New York City, Chicago, and Detroit.

All of these challenges can affect the cost of energy. And, the trend toward pricing has turned to "real time." With hourly real-time pricing, utilities charge according to fluctuating market demands, which usually means high summer daytime rates and lower summer nighttime rates. Fortunately, many utilities are now willing to negotiate a cost-effective usage package with facilities that leverage energy-saving technologies to even out demand.

Putting costs on ice

Ice thermal storage allows users to take advantage of new rate structures by shifting the bulk of cooling energy needs to nighttime and helping to balance 24-hr usage. This means that users can secure better contracts with utilities because they have flattened their load profile and reduced their kW demand with ice thermal storage.

Additionally, ice thermal storage systems for commercial buildings decrease supply water temperature to 36°F, which results in as much as a 25% increase in operating energy savings over a traditional chiller-cooling tower system.

Furthermore, ice systems use smaller components than traditional cooling systems, resulting in even more operating cost savings and lower first costs. These types of ice systems feature smaller chillers and cooling towers, reduced pump and pipe sizes, and less connected hp.

By designing a system around 24-hr/day chiller operation, the size of the chillers and cooling towers required for an ice system is significantly reduced compared to conventional chillers and cooling towers sized for the instantaneous peak load. For example, a partial-storage ice design includes chillers that provide approximately 60% of the peak cooling load. The ice storage system handles the balance of the cooling requirement.

In a 400-ton peak cooling load system (Figure 1), ice storage reduces the nominal capacity of the chiller and cooling tower from 400 tons to 200 tons with associated savings of $73,500 by allowing users to take advantage of the low temperatures available with ice.

Pump and pipe sizes are also reduced in an ice storage system. Users realize substantial savings with the chilled-water distribution loop when the system design incorporates reduced flow rates that result from using a larger temperature range in the water loop. Use of a 14° temperature range instead of a conventional 10° temperature range results in a reduction of pipe size from 8 in. to 6 in. This reduction in pipe size corresponds to a $60,000 cost savings for the pipe.

Condenser water pipe sizes decrease due to lower flow requirements for the smaller chiller. Using 3 gpm/ton, the condenser water piping drops from 8 in. on a conventional system to 6 in. for the ice storage system. This results in cost savings of $8,000. Pump savings due to reduced chilled water and condenser water flow rates also decrease, and in this example, calculate to $3,000.

As the size of major components of the mechanical system drop, the hp associated with these components falls, too. Continuing the above example, total connected hp falls by 190 hp, resulting in savings for transformers, starters, and wiring of approximately $28,000 with total system savings of $172,500. Ice thermal storage requires some additional components such as the ice thermal storage units, ethylene glycol, heat exchanger, and concrete slab, which total $137,600. In this 400-ton example, the ice thermal storage system nets nearly a $35,000 first-cost savings or almost $90/ton.

Ongoing operating costs also decrease with an ice storage system. For example, with lower hp, ongoing operating costs can drop by up to 50%. With a 400-ton system, endusers realize annual operating cost savings of $13,240 based on a $10/kW demand charge and usage charges of $0.06/kWh peak and $0.03/kWh off peak. Ice thermal storage systems pass along optimum operating cost savings with a proper system design and strategy.

Photo 1. Located 20-ft deep, this ice thermal storage vault at the Baltimore State Office Center holds 11,000 lb of ice coils that translate into substantial energy savings for the center.

Design for efficiency

There are many different ice thermal storage designs, and engineers continue to develop new ones. However, the two basic operating strategies are full storage and partial storage. Full storage systems build enough ice during the night to serve the entire on-peak cooling requirement. This strategy, which is demand- or usage-charge driven, shifts the largest amount of electrical demand and results in low operating costs. However, due to larger storage requirements, full storage systems have a higher upfront cost.

Partial storage systems, on the other hand, offer the lowest first cost design as well as low operating cost. This system builds enough ice during the night to serve part of the on-peak cooling requirement. In this design, ice supplements the chiller, allowing use of smaller and more efficient chillers. Modular thermal storage systems (Figure 2) are designed specifically for partial storage, internal melt applications. With this efficient operating strategy, the chiller precools warm return glycol before it passes through the steel heat exchanger and is cooled indirectly with the melting ice.

Locating the ice system upstream, rather than downstream, from the chiller offers additional benefits. First, when operating at higher supply temperatures to precool the glycol, the capacity at the chiller increases. Second, the efficiency (kW/ton of refrigeration [TR]) of the chiller also improves. This closed, pressurized loop reduces pumping hp requirements. The design eliminates the need for a heat exchanger between the thermal storage unit and the cooling system when glycol is circulated to the air conditioning system.

Locking in reliability

As a proven technology, ice thermal storage systems supply the desired reliability for high air conditioning availability. With conventional systems, installing multiple chillers offers redundancy. In the event of a mechanical failure of one chiller, the second chiller supplies limited cooling capacity. The maximum available cooling for the conventional system would, with one chiller out of service, be only 50% on a design day.

Most ice storage systems utilize two chillers in addition to the ice storage equipment. Two chillers provide approximately 60% of the required cooling on a design day while the ice storage provides the remaining 40% of the cooling capacity. In the event that only one chiller is available to provide cooling during the day, up to 70% of the cooling capacity is available. The one operable chiller supplies 30% of the cooling requirement, while the ice provides up to 40%. Based on typical hvac load profiles and ASHRAE weather data, 70% of the cooling capacity would meet the total daily cooling requirements 85% of the time.

Higher system reliability also translates to lower maintenance. In an ice thermal storage system, all parts are smaller than those in conventional systems, which reduces maintenance, parts, and labor. Ice coils themselves have no moving parts and essentially require no maintenance. An ice inventory sensor requires adjustment twice annually. Water in the tank and glycol in the ice coils only need an annual analysis. With minimal maintenance, keeping the system operating in an environmentally friendly manner is easier, too.

Green in the vault

By reducing energy consumption and using electricity at night, ice thermal storage systems provide environmentally friendly operation. New energy-efficient chillers, water-cooled equipment, and ice storage can significantly decrease energy consumption. Electricity generated at night generally has a lower heat rate (fewer Btu/kW produced) and therefore lowers CO2 emissions and lessens the potential for global warming.

Everyone benefits from environmentally friendly cooling, and many types of facilities are well suited to ice thermal storage applications. In particular, new construction of office buildings, schools, hospitals, district cooling plants, and manufacturing and process facilities are ideal. Any application over 100 tons provides an opportunity for savings via ice thermal storage, with the exception of apartment and condominium buildings. Additionally, retrofits involving ice thermal storage allow designers to add more capacity without increasing electric requirements or equipment, incorporate existing series or parallel equipment into new designs, and solve problems with existing water and air systems.

For many retrofits, ice thermal storage allows the enduser to decrease chilled water temperature in the loop, adding as much as 50% more cooling capacity to piping. The system can also lower water temperature and drop the leaving air temperature, which increases capacity utilizing the same duct and fan systems. Overall, designers can unlock some pretty "cool" ideas from the ice storage vault. ES

Load Profile Lowdown

A daily load profile is the hour-by-hour representation of cooling loads for a 24-hr period. For ice storage systems, designers select chillers based on the ton-hours of cooling required and a defined operating strategy. Thermal storage systems provide flexibility for varying strategies as long as the total ton-hours selected are not exceeded. This is why designers must provide an accurate load profile for an ice storage system.

The Air-Conditioning & Refrigeration Institute (ARI) Guideline T establishes for cool storage equipment the minimum information required for supplier-specified thermal performance data, and selection software should reflect this third-party recommendation. For preliminary equipment selection, designers need to know the estimated building peak cooling load and duration of the cooling load to use such software.