Ice-storage systems

Consulting engineers must provide cooling designs that will work in a variety of conditions from low loads to unexpected heavy loads on peak design days. Therefore, safety factors become a consideration of risk, first cost and operating cost. Is there a better choice than to install a 20% larger chiller for a facility to occasionally meet unusually high or unexpected loads? That extra chiller capacity with its larger support equipment can result in lower energy efficiency and increased maintenance costs with poor return-on-investment (ROI) because oversized cooling systems that quickly cycle on and off to meet set points might not dehumidify the air properly. In addition to operational inefficiency, poor indoor air quality can be a side effect of oversizing. Is there a better choice?

Consider a building that has a peak cooling requirement of 1,000 tons on a design day. One 1,000-ton chiller would do the job on most days, but this choice provides no safety factor for days warmer than ASHRAE design—days with unexpected loads—or for future growth. Additionally, should the chiller fail, no cooling would be available. Designing the system instead with two 500-ton chillers would not provide extra safety factor capacity, but would provide half capacity should one chiller fail. The U.S. General Services Administration directs engineers to design three equal-size chillers that total 120% of the buildings peak load. In this case, three 400-ton chillers would be required to provide 20% safety factor and 80% capacity should one chiller be unavailable.

A cooling load profile from a popular HVAC modeling program (see Table 1) for a 1,000-ton building in Chicago shows that 87% of the time the load is less than 800 tons, or 66% of the recommended GSA installed chiller capacity. Seventy-six percent of the time the cooling load is less than 700 tons, or just under 60% of the GSA chiller installed capacity. The investment in 200 tons of safety factor capacity, including the larger chillers, cooling towers, fans, pumps and electrical equipment, results in a higher cost and connected electrical load for capacity that is seldom used and has minimal ROI.

Should one chiller become unavailable, the conventional selection of three 400-ton chillers provides 80% capacity. This is a reasonable operational risk because most operating hours have capacities less than 80% design-day load.

Figure 1 shows a cooling load profile for a 1,000-ton design day load profile for an office building. The conventional scenario would be to install three 400-ton chillers to supply safety factor and redundancy. Figure 1 also shows the chiller potential over the required cooling load profile the building requires.

Now compare the conventional-chiller-only system with a partial-ice-storage system, keeping in mind that safety factor and redundancy are requirements of the ice storage system as well. For this example, the third chiller or “safety factor chiller” is replaced with ice storage. Two 400-ton ice making chillers and 3,900-ton hours of storage are selected for the system. The ice storage provides the safety factor and redundancy just as the third chiller did in the conventional design. Cooling towers and all chiller support equipment are now sized for 800 tons. The potential load profiles are compared in Figure 1. Notice that the ice storage system is capable of a larger peak capacity than the conventional system if needed. In this case the safety factor is comparable.

With the ice storage system, on a night before a design day, the chillers would charge the ice tanks using less expensive, more efficiently generated electricity (assuming there is a price difference in electrical consumption and demand, according to the time of day). During the daytime, one chiller would handle the first 400 tons of cooling and the ice storage would supplement cooling for any loads above 400 tons (see Figure 2).

One chiller would be off during the day, lowering the use of expensive on-peak energy consumption. The safety factor would help to pay for itself by reducing the demand associated with 600 tons of cooling and the support equipment designed for 1,200 tons. Safety factor investments associated with conventional designs cannot provide operational choice (time factor) or operational savings such as partial-storage safety factor designs offer.

If the demand cost is $10.00/kW, the demand reduction alone can lower operating costs $3,360 every month in the summer season. More energy savings are achieved with low-flow high delta T designs, smaller cooling towers and smaller pumps. (600 tons x 0.56 kW/ton x $10/mo = $3,360)

But what if one main system component fails? Assume, for this first example that a chiller fails on the conventional design and the ice fails to be made on the ice storage system. Each system would be able to provide 800 tons of cooling.

For the next example assume that one chiller fails on the conventional design and that one chiller also fails on the ice storage system design (see Figure 3). One ice chiller can make ice for a longer period of time, meeting the complete load for most of the day. With some creative control using pre-cooling and chill water temperature reset, the building could be usable for most of a peak day and certainly all day for a non-design day.

In a right-sized cooling system the equipment and ancillary components operate more efficiently at full load more often. So a right-sized partial ice storage design can provide similar safety factor and redundancy compared to conventional designs while improving efficiency, demand, connected load, and operating costs. Lowering operating costs can help earn LEED NC points in the Energy and Atmosphere section. Rightsizing with partial ice storage can reduce the impact of peak-time comfort cooling on the environment. A California study showed that power is generated and transmitted more efficiently at night, saving valuable natural resources. More efficient generation means lower greenhouse gas emissions as well.

What about the installed cost?

If there is room for ice tanks, the cost per ton is very similar. A chiller with a cooling tower or an air-cooled machine is about $450 per ton, which is very similar to the cost-per-ton of ice storage.

How much room do you need?

Typical designs call for the ice storage to supply about 30% of the cooling required on a design day.

Full storage space required

70 sq. ft. / tank 500 sq.ft./ ton x 20 ton/ tank

= 0.70% of conditioned space

Partial storage (30% of peak) = 0.70% x 33% = 0.23%

The equations demonstrate that on average the space required for a partial ice storage system is relative to the water heater space required in a typical home. They also show that 0.23% of the conditioned space is required for an ice storage footprint. In the partial ice storage example space was required for the ice tank farm but space was not needed for the third chiller and associated cooling tower required by the conventional system.

Because cooling systems seldom operate at full load, utilities struggle to plan and provide for increasing demand. This forces designers to create more sustainable buildings, and also forces the HVAC design paradigm to change regarding capacity safety factor and redundancy. Instead of adding 20% chiller capacity to a design day building peak load, reduce chiller capacity by at least 20% and add ice storage for safety factor. The ice storage is practically free and provides lower operating costs with increased efficiency, lower environmental impact and operational flexibility.

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