March is a "shoulder" month in most cities. Extreme weather conditions are rare and none were evident this year. However, a number of cities experienced a significant shortfall of economizer "free cooling" hours, resulting in higher-than-normal cooling costs for the month. The greatest deficiencies were reported in the Northeast, including Philadelphia, New York, Washington, Boston, and Baltimore.

In fact, waterside economizer hours in Philadelphia were down 43%, or 232 hours. That's almost 10 fewer days of free cooling than the city normally experiences in March. On the upside, facilities located in Detroit and Minneapolis benefited from considerably higher-than-normal economizer hours, resulting in lower March cooling costs.

Low economizer hours usually coincides with lower energy consumption for outdoor air ventilation heating and humidification systems. This holds true in the data reported for the Northeastern cities mentioned above. It also makes sense that Philadelphia saw the biggest drop in heating consumption of 24% and humidification water consumption of 32%. Conversely, March heating and humidification costs were markedly higher than normal in Detroit and Minneapolis.

How to Calculate Heat Recovery Savings

Designing a heat recovery system into a project is no longer a rare occurrence due to utility rebate incentives and a wider selection of equipment types. In addition, some states have adopted energy codes that require heat recovery on every new project. As a result, manufacturers are quickly developing many different heat exchanger technologies in response to this growing market.

When confronted with the task of evaluating the payback associated with heat recovery systems, many engineers will turn to the manufacturer's representative for support. This is not always the best approach because the results presented by the rep will likely be skewed to favor a particular type of system. Results can vary widely depending on the accuracy of the data used and the assumptions made. Engineers should always take the time to perform the analysis themselves.

This can be done fairly easily by setting up a spreadsheet that can be adapted for use on any heat recovery project. In this and upcoming issues, we will develop this spreadsheet together. And when we're done, you will be able to perform your own analysis with confidence and generate your own version of the data you see in Figure 2. Figure 2 is a summary of the potential air-to-air heat recovery cost savings for 2001. The assumptions taken to achieve these results will be discussed in more detail as we move ahead.

Here is Your First Assignment

Obtain a copy of the TMY2 hourly data set from the National Renewable Energy Laboratory (NREL, This data is the same data used by many energy programs and is free. If you use an energy program, such as Tracer, DOE, or E20-II, see if you can pull the hourly temperature and humidity text files out of the program. Another option is to purchase a copy of BINMAKER. This program allows you to generate the text files you will need and offers some other neat features as well. You can purchase it online. See you next month! ES

EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.