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.