The Benefits of Controlling Your Power Demand

In today’s competitive business world, it is in every facilities manager’s interest to look for cost-reduction opportunities. Not only does it make sense for business, but it is also good for the environment. More and more organizations are looking to promote rational energy use, and some even utilize government incentives to attract interest from major consumers with solar, wind, and heat recovery investments.

However, some areas of opportunity that are often overlooked require a small investment or none. One of these opportunities involves looking at your company’s electrical power consumption profile and potential energy-saving features your HVAC equipment may already have. In addition, the tariffs your utility company imposes could include other opportunities for cost reductions.

Let’s look at a six-story building occupied by a company of approximately 1700 employees providing CAD design and shared services for the medical device sector. The building was under a commercial electric power tariff (TCO).

The objective was to find and justify a change of tariff, under the assumption that the change would not only lower the building’s monthly energy bill but also accommodate its power profile regarding maximum demand. The utility company offered a medium voltage tariff (TMT) based on maximum demand that could offer significant savings if the appropriate conditions could be met.

The first step is to analyze the building’s power profile.

The existing TCO power and energy tariff is as follows:

Actual TCO Billing Tariff

• Energy Cost ($/kWhr) $ 0.123
• Power Cost ($/kW) $ 19.20
• Public Lighting $ 297.74
• Fire Department Tax $ 3.76

The TMT tariff in consideration is:

Proposed TMT Billing Tariff

• Energy Cost Night ($/kWhr) $ 0.037
• Energy Cost Valley ($/kWhr) $ 0.052
• Energy Cost Peak ($/kWhr) $ 0.103
• Power Cost Night ($/kW) $ 8.170
• Power Cost Valley ($/kW) $ 12.870
• Power Cost Peak ($/kW) $ 18.088
• Public Lighting $ 297.74
• Fire Department Tax $ 3.76

Because the proposed TMT tariff charges significant fees for maximum demand throughout the day, it is important to control peak power demand. Otherwise, the overall monthly bill could be significantly higher than expected. One very important aspect of the TMT tariff is the different costs associated with power and energy during specific periods of the day. These periods are daily and segmented in the following manner:

• Valley: 06:00 – 09:59 and 12:31 – 17:29
• Peak: 10:00 – 12:30 and 17:30 – 20:00
• Night: 20:01 – 6:00

When we compare the billing of a typical month using both tariffs, we can determine if the proposed TMT tariff is feasible. We will use November 2018 as our test month.

By using simple integration methods, we can determine the energy and maximum power demands during the three different periods mentioned previously and calculate the monthly bill if we were under the TMT tariff. The actual billing for the month was $35,214. Using the TMT tariff for the November power profile, the result would have been the following:

Proposed TMT Billing Tariff

• Energy Cost Night $ 2,852.59
• Energy Cost Valley $ 4,481.32
• Energy Cost Peak $ 5,381.47
• Power Cost Night $ 2,741.66
• Power Cost Valley $ 5,561.88
• Power Cost Peak $ 7,950.20
• Public Lighting $ 297.74
• Fire Department Tax $ 3.76
• Total: $ 29,270.62

These results show a potential savings of $5,943.38 per month or approximately $71,320.56 per year.

Because the TMT tariff is heavily weighted on power and less on energy and charges more during certain periods of the day, it is imperative to monitor and control maximum power demand. Otherwise, a lack of control could cause high costs that will cancel out any forecasted savings.

Let’s say we wanted to analyze how a chiller and different floors contribute to the overall building’s power profile. At first glance, it can seem that the chiller is the most important contributor—not only the highest but also because the chiller’s power profile seems to be in unison with the overall building’s profile. To confirm this hypothesis, we can run a fast Fourier transformation and a correlation analysis of the chillers and the building’s power profile.

Not only does the Fourier analysis tell us the chiller is the most significant power contributor, but it also shows the chiller has a 24hr/day cycle–same as the building’s profile. Regarding the chiller’s correlation with the building’s power profile, we can run a standard statistical correlation analysis to test the differences between the profiles.

Using nine days of data with power data at 15-minute intervals, we can estimate the correlation by comparing the chiller’s power profile as the input and the building’s power profile as the output. The statistical correlation of 0.7 confirms that the chiller’s power demand is an important contributor to the overall building’s power profile. Thus, if we can control the chiller’s peak demand, we can control the building’s peak demand.

We now know the chiller is our major focus. As with most commercial and industrial chillers, these have current limiters as part of the features embedded in their controls. The default setting is commonly set at “OFF.” This feature can be set from 25 to 100 percent cooling capacity, limiting the current the chiller consumes.

Historical data shows that 90 percent of the time, the chiller was running at approximately 50 percent of its cooling capacity, with occasional peaks at 75 percent. Based on the historical data, we set the current limiter at 65 percent.

After running the chiller at this setting for seven days in the summer season, we obtained the results detailed in Figure 4. By enabling the current limiter, the maximum chiller running capacity decreased from 72 to 61 percent compared to the previous month.

This is a 20 percent overall decrease in maximum cooling demand, which will translate into an approximate $1,000/month additional savings using the TMT tariff. No negative cooling effect could be detected in the building regarding effects on its occupants or equipment.

In summary, by changing the utility tariff and identifying key power demand contributors, such as the chiller in this case, we can obtain substantial savings.