The HVAC industry can learn a lot from dual-energy hybrid-car designs. Looking at the numbers, it quickly becomes apparent why manufacturers promote hybrid vehicles and why they are becoming increasingly popular. Hybrid cars provide performance similar to or greater than standard designs while being more fuel-efficient. Consumers get V-6 power with four-cylinder fuel economy. Along with efficiency greater than that of conventionally powered vehicles comes the added benefits of lower operating costs and reduced emissions for smaller carbon footprints. Of course, horsepower and miles per gallon are not the whole story. As with buildings, first cost, maintenance, and overall life-cycle value should be part of the analysis as well.
Conventional vs. Hybrid Systems
What can the HVAC industry learn from hybrid-vehicle design? Can a hybrid cooling system provide similar performance with less horsepower? Can it provide lower operating costs or extra power when it's needed — when loads or energy costs are high? How does a hybrid cooling system compare with conventional systems in terms of life-cycle costs?
Conventional systems produce cooling “just in time,” providing only what is needed when a load requires cooling. Just-in-time cooling production causes heavy demands on the electrical grid during peak cooling-season periods. Peak demand usage and consumption are expensive. Conventional systems must be sized for peak cooling loads to maintain occupant comfort at all times. Because conventionally sized cooling systems seldom operate at full load, overall efficiency suffers.
By definition, a hybrid system must provide output from two sources. A hybrid car uses an internal-combustion engine and stored energy to power an electric motor. A cooling system could combine a chiller with energy storage. Two energy sources — on-peak and off-peak electricity — typically are generated with different fuels and prices because of market conditions.
Most utilities have a “half-off sale” at night because generation is more efficient and demand is low. When hybrid cooling is employed, even a utility with standard demand rates can offer operating-cost savings to a daytime-peaking facility. Let's examine a typical utility's general-service rate: an energy charge of 4.5 cents per kilowatt-hour and a demand charge of $13 per kilowatt per month. A typical commercial facility will peak during the day, setting its demand charge for the month, while energy charges are the same day or night. When a facility uses energy storage for cooling, its utility rates typically include one demand charge that is applied when the customer sets the peak demand, day or night. Some utilities may have a daytime and a nighttime demand charge. In such cases, the nighttime demand charge is almost always cheaper than the daytime demand charge, and savings can be achieved. Additionally, if a utility bases charges on ratchet demand, savings are even greater. For this case, however, let's see the effect typical demand charges have on chiller operations utilizing on-peak electricity.
A conventional chiller system requiring 1,000 tons will have a demand of approximately 800 kw:
1,000 tons × 0.8 kw per ton (including chiller and pumps) = 800 kw
The on-peak cooling demand charge is $10,400 per month:
800 kw × $13 per kilowatt per month = $10,400 per month
The approximate energy usage for the chiller — assuming 10 hr of operation per day, 22 days per month, and a diversity of 75 percent — is 132,000 kwh:
1,000 tons × 10 hr × 0.8 kw per ton × 22 days × 75 percent diversity = 132,000 kwh
The demand contribution to on-peak energy costs is 7.9 cents per kilowatt-hour:
$10,400 per month in demand costs ÷ 132,000 kwh per month = 7.9 cents per kilowatt-hour
Therefore, the blended on-peak electricity costs are 12.4 cents per kilowatt-hour, which is almost twice the cost of a 4.5-cent off-peak kilowatt-hour:
4.5 cents per kilowatt-hour for energy charges + 7.9 cents per kilowatt-hour for peak demand charges = 12.4 cents
A ton-hour of on-peak cooling will cost 10 cents:
1 ton × 0.8 kw per ton × 12.4 cents per kilowatt-hour = 10 cents
An off-peak ton-hour will cost 4 cents:
1 ton × 0.9 kw per ton × 4.5 cents per kilowatt-hour = 4 cents
Per this calculation, energy storage with a standard demand rate and no time-of-day energy-charge provision realized savings of 60 percent. The chiller's ice-making system uses a little more energy at night, but the lower rate more than makes up for the difference. Savings are even greater if the electric rate includes a time-of-day provision.
But is energy storage efficient? Is more energy used to make the ice? The measurements previously mentioned were recorded via a building's electric utility meter, but what happens at the power plant? American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Technical Committee (TC) 6.9, Thermal Storage, conducted a research project and wrote a report that analyzed different energy-storage strategies and applications to understand how source-energy consumption is affected. 1 Source energy is the energy used to generate electricity.
Two utility types were used in TC 6.9's research project, one with coal-generation bias and another with a coal/natural-gas-generation mix.
The project's summary report noted that when storage systems were utilized: “Source-energy reductions were generally on the order of 10 percent. Global-warming impact reductions were also on the order of 10 percent.” Therefore, “Thermal-energy-storage systems should be promoted as an environmentally beneficial technology. These systems have been historically touted as beneficial in terms of operation cost. This study suggests that the economic benefits can be accompanied by environmental ones.”
With a generational focus on sustainable forms of energy, more nighttime electricity will be generated from clean renewable sources, such as wind. Therefore, market influences eventually may create a greater differential between daytime and nighttime electricity prices.
Conventional Cooling System
Figure 1 shows a peak day-cooling-load profile of 1,000 tons. A typical conventional design calls for three 400-ton chillers, providing a 20-percent safety factor for unexpected loads while still providing 80 percent of cooling capacity on a design day when one chiller is out of service. Conventional designs require cooling towers to support 1,200 tons of chillers. The type of conventional system described will have excess capacity for unexpected loads and provide an acceptable level of redundancy.
Hybrid Cooling System
A comparable hybrid cooling system can be designed with any level of safety or redundancy. A diversity factor applied to a peak load also can be applied to an energy-storage design. If a project needs N+1 design, a storage system can be designed to provide the same level of redundancy.
Let's look at a comparable hybrid cooling system with energy storage consisting of two 400-ton chillers and 3,500 ton-hours of storage. Figure 2 compares the excess design-day capacity of the previously mentioned conventional system with that of the hybrid cooling system. The hybrid cooling system more than covers the design-day cooling-load profile and has the ability to supply an even larger peak capacity. Each system adequately provides the peak cooling-load safety factor.
Cooling systems rarely operate at design day or full load. Figure 3 shows how a hybrid cooling system typically operates on a design day without the added burden of excess capacity requirements. Chiller output demand is limited to 500 tons (most days don't qualify as design days, so most of the time only one chiller will operate during the day), and energy storage provides the remaining capacity. Assuming system efficiency with pumps and chillers is 0.75 kw per ton, 375 kw will be shed from the grid when the conventional cooling system is utilized at peak, resulting in decreased billing demand and/or the opportunity to purchase off-peak electricity.
These systems are comparable in peak capacity and safety factor; however, the hybrid system has one fewer chiller, smaller cooling towers, and smaller electrical distribution equipment and will cost less to operate and maintain. Like a hybrid car, the hybrid cooling system provides similar or better performance with two “right-sized” energy sources.
What happens if a chiller fails? Figure 4 compares each system's available capacity when a chiller is out of service. A chiller failure in the conventional system results in an available capacity of 800 tons of cooling. If a chiller in the hybrid cooling system fails, another chiller is available to charge the storage at night, augmenting the ice chiller's cooling capacity during the day.
The conventional system will not meet the load for 6 hr on a design day, the area above the dashed line. The hybrid system will not meet capacity for 3 hr, represented by the yellow bars. Because most days are not design days, these systems should be able to cool a facility at a level acceptable for most owners, even with one chiller out of service.
Many case studies show hybrid cooling systems cost about the same as conventional cooling systems in new construction. However, it can be difficult to obtain real installation costs for specific equipment because companies rarely provide a breakdown of specific project costs. Therefore, let's look at the information available for 2008 ASHRAE Technology Award winner Fossil Ridge High School in Fort Collins, Colo., which placed first in the institutional-buildings category.
The school was built for $130 per square foot, approximately the same cost incurred with the school district's conventionally designed schools. A careful design strategy enabled the school district to utilize a hybrid cooling system without incurring costs above those of a conventional system. At 296,000 sq ft, the school has the capacity for 1,800 students. Its hybrid cooling system consists of a “right-sized” 250-ton air-cooled chiller and 1,200 ton-hours of ice storage with a cooling-season demand of 428 kw, or 1.44 w per square foot. The school's energy costs are about one-third less than other existing conventional high-school designs of similar size.
The costs of replacing chiller tons with energy-storage capacity are similar. On average, a chiller-ton's installed costs are between $1,000 and $1,500. These costs might be higher or lower, depending on geographical location. Ice energy storage costs approximately $130 to $180 per ton-hour on average. Partial storage designs typically need 6 to 8 ton-hr of cooling for each chiller ton being replaced, while full storage designs require more.
Projects seeking Leadership in Energy and Environmental Design certification will need to undergo a rigorous life-cycle-cost analysis that models weather, building performance, and equipment performance to obtain more accurate information.
Just as hybrid cars are becoming popular because they are more efficient, produce fewer greenhouse-gas emissions, and cost less to operate, so are hybrid cooling systems. No longer are hybrid cooling systems experimental, for large users, or only affordable to a very few.
Hybrid cooling systems are proving themselves to be affordable and efficient without giving up comfort or reliability. They can be applied in small facilities, such churches and K-12 schools, as well as larger facilities, such as college campuses.
Because the national focus is moving away from fossil fuels toward intermittent renewable energy, success and implementation will rely heavily on all forms of energy storage to ensure a reliable electric grid that is ready to meet demand when consumers need electricity.
Hybrid cooling systems with ice storage will be a vital part of the switch to renewable energy and a smart grid. Owners who have storage systems will be able to take advantage of programs and pricing opportunities that help minimize cooling costs while decreasing environmental impacts.
Reindl, D., Gansler, R., & Jekel, T. (n.d.) Simulation of source energy utilization and emissions for HVAC systems. Atlanta: ASHRAE.
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North American sales manager for CALMAC Manufacturing Corp., Paul Valenta, LEED AP, is responsible for marketing and sales of ice storage in North America, Central America, and the Caribbean. A member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the Association of Energy Engineers, he has an electrical-engineering degree from the University of Nebraska and has been in the HVAC industry for 24 years. He can be reached via e-mail at email@example.com.