To wrap up our analysis of the total annual data for 2002, let’s take a look at how the economizer and heat recovery data compared to 2001 as shown in Figure 2. The airside economizer hours are the total number of hours that the dry bulb temperature was below 55 degrees F. The waterside hours are the number of hours that the wet bulb temperature was below 40 degrees. The heat recovery savings represents the total savings (sensible + latent) for both heating and cooling based on a thermal effectiveness of 75%.

Of the reporting cities, Seattle recorded the highest number of airside economizer hours, followed by Minneapolis, Denver, Detroit, and Chicago. Seattle is a good example of a city where airside economizers make a lot of sense but waterside systems may not. Approximately 62% of the year in Seattle was below 55 degrees dry bulb on average. But only 25% of the year was below 40 degrees wet bulb (only about 7% for operation during typical business hours).

Minneapolis recorded the highest number of waterside economizer hours at 4,443, 11% higher than in 2001. Detroit also saw a jump of 16% compared to 2001 — or an additional 561 hours of waterside economizer cooling. These differences are significant when evaluating utility costs. For example, for a constant 750-ton waterside economizer load in Detroit, the cooling costs were $42,000 lower in 2002 (at 1.0 kW/ton and $0.10/kWh) compared to 2001, simply because of the difference in available waterside hours.

For much of the country, the 2002 total winter heat recovery savings did not deviate all that much from 2001 (+5% for the most part). However both years were well below normal in cities like Washington, Philadelphia, and New York. In Washington, the 2002 heating heat recovery savings were 19% lower than normal or $1.11/cfm compared to a normal of $1.37/cfm (at 80% efficiency and $0.75/therm).

However, the reported heating total (sensible + latent) heat recovery data includes humidification cost savings, which obviously would only apply if a humidifier is used. If not, the only actual heating cost savings would be sensible. But does that mean you should only consider an enthalpy wheel for applications with humidifiers? Probably not, because enthalpy wheels are usually more efficient than sensible systems and they do recover some moisture from the space, resulting in better comfort and reduced static electricity.

Cooling heat recovery is more volatile due to wide swings in humidity levels from year to year. In 2002, the total cooling heat recovery savings in Miami was up by 18% compared to 2001, and up 31% compared to normal, resulting in a cooling cost savings of $1.67/cfm compared to a normal of $1.28/cfm (at 1.0 kW/ton and $0.10/kWh).

Evaluating heat recovery data can be a little tricky though. Lower heat recovery savings doesn’t necessarily translate to higher utility costs because lower heat recovery simply means milder weather. Heat recovery data is also useful for simple payback analysis confirmation.

Most projects today are designed with economizer cooling or heat recovery systems or both. These systems can have a significant impact on utility costs and their performance can vary greatly from year to year due to the weather. So, it’s always a good idea to take the weather into consideration when determining the feasibility of installing a new system or analyzing utility costs associated with an existing system. ES

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