GSA's Cool Coup at the Philadelphia Custom House
The U.S. General Service Administration's (GSA) Philadelphia Custom House saved almost $70,000 in demand payments during 2005-2006, and expects savings of nearly $100,000 (about 15 percent of the facility's annual electricity bill) in 2006-2007, thanks to a study of load management and demand response approaches conducted by the Environmental Energy Technologies Division of Lawrence Berkeley National Laboratory (Berkeley Lab). The study was sponsored by GSA and the Federal Energy Management Program (FEMP). By adopting Berkeley Lab's key recommendation and making a few targeted operational changes (with no capital expenditures), the Custom House has substantially reduced its summer peak demand and thereby a significant portion of its yearly energy costs.
GSA pays roughly $27 per kilowatt (kW)-two to three times the national norm-in demand charges for the 570,000-square-foot Custom House (see Figure 1). The facility is also subject to a demand "ratchet" so that 80 percent of its summer peak power draw (i.e., its highest single 30-minute-interval reading) becomes its minimum billed demand for each of the next eight months (October through May).
Because the Custom House generally experiences a summer period (June - September) peak of about 2,000 kW, GSA is obligated to pay for at least 1,600 kW per month (80 percent of the summer peak, per the ratchet clause), during off-peak months. However, the Custom House is a conventional federal office building with a low load factor and typically peaks at less than 1,000 kW per month from December to March. Thus, the Custom House regularly pays its utility (PECO Energy) about $15,000 per month during these four off-peak months (as well as additional sums in the "shoulder" months of October, November, April, and May) for power it does not draw.
GSA commissioned EETD's Phil Coleman, with partial funding also from FEMP, to research ways to cost-effectively reduce the Custom House's peak demand. The study recommended a "pre-cooling" strategy-inspired by similar efforts led by EETD's Peng Xu-in which, on hot summer days, the chilled water plant would be turned on very early in the morning. Simultaneously, the chilled water valves in the building's roughly 1,000 perimeter induction units would be tripped to a "fail-open" position using solenoid valves (a contribution from EETD's Dave Watson) so that the facility would actually be somewhat over-cooled (as well as de-humidified). The idea was to utilize the circa-1934 building's substantial mass as a thermal storage medium, which could absorb heat and provide cool-temperature radiation throughout the day, mitigating the customary afternoon power peak.
Working with GSA and its operations and maintenance contractor, Brooks Range Contract Services, EETD helped develop a multi-part plan to reduce the building's peak by using the morning pre-cooling strategy combined with an afternoon "demand-limiting" approach. The key elements are:
- If the outside air exceeds 70°F at 2 a.m., one of the facility's 650-ton chillers is turned on and programmed to produce 42°F chilled water;
- All induction unit chilled water valves are set to a full-open position during the early morning;
- At 9 a.m., the chilled water temperature is raised to 46°F, and induction unit control reverts to the tenants (the units have no reheat coils, but controls can be set to "warmer" to reduce or eliminate the flow of chilled water through the units);
- If demand reaches 1,500 kW and is still rising by noon, the chilled water temperature is raised again, to 48°F;
- Only one of the two 650-ton chillers is allowed to operate at any given time.
At the beginning of the summer of 2005, the Brooks Range team executed the strategy manually, using control system overrides for chiller operation and bleeding air out of the pneumatic lines to open the induction unit valves. Once the team gained confidence in the strategy, the building's controls contractor automated it within the energy management control system (installed in 2003 as part of a FEMP Super Energy Savings Performance Contract).
Using this approach, the operations team was able to keep the facility's peak demand down to 1,766 kW during the summer (defined by the PECO tariff as June though September), in contrast to an expected minimum of 2,050 kW. (The previous four summers' average peak had been 2,080 kW; Berkeley Lab researchers conservatively estimated 2,050 kW would have been 2005's peak draw, despite the fact that the summer of 2005 was unusually hot in the mid-Atlantic). (See Table 1).
|Month||Expected Peak (kW)*||Actual Peak (kW)||Billed Peak (kW) **||Peak Reduction (kW)||kW Value|
|June, '05||1,900||1,766||1,766||134||$ 3,410|
|July, '05||2,050||1,692||1,692||358||$ 9,109|
|August, '05||2,050||1,692||1,697||353||$ 8,982|
|September, '05||1,900||1,711||1,711||189||$ 4,809|
|October, '05||1,640||1,604||1,604||36||$ 916|
|November, '05||1,640||1,448||1,448||192||$ 4,885|
|December, '05||1,640||1,015||1,413||227||$ 5,776|
|January, '06||1,640||992||1,413||227||$ 6,134|
|February, '06||1,640||961||1,413||227||$ 6,134|
|March, '06||1,640||953||1,413||227||$ 6,134|
|April, '06||1,640||1,393||1,413||227||$ 6,134|
|May, '06||1,850||1,646||1,646||204||$ 5,512|
|Total Savings||$ 67,934|
GSA benefited directly from the reduced summer demand, saving an estimated $26,000 (see Table 1). GSA also reaped considerable savings during the winter months, because 80 percent of the 1,766 kW summer peak set the winter's minimum demand at 1,413 kW under the terms of the facility's ratchet clause. If the summer demand had reached the expected 2,050 kW, the ratchet figure would have been 1,640 kW. The 227 kW reduction (1,640-1,413) translated to more than $30,000 in savings for the five months of December through April; additional ratchet savings in October, November, and May made for a total annual savings (including the direct summer months' savings) of roughly $68,000.
The EETD researchers expected that there would be a slight-perhaps 5 percent-energy (kWh) penalty from the pre-cooling because some of the coolth generated by the chiller in the early mornings would escape from the building envelope without affecting occupant comfort. Although this effect unquestionably took place, the building's summer electricity usage does not appear to have gone up. Next to the most comparable recent summer (2002), summer 2005's usage was only 0.5 percent higher even though there were 4.3 percent more cooling degree days in summer 2005. Moreover, a regression plot of the four previous summers' kWh consumption against the number of cooling degree days in each revealed that summer 2005's actual consumption was 2 percent less than the model predicted.
Why was consumption lower than expected? One reason might be that, because only one chiller was run at a time, average chiller operation was at much higher load conditions than in previous summers, which increases efficiency. Another small effect might be from the cooling towers, which purge their heat into 70-degree night air more efficiently than into 95-degree sunny daytime conditions.
Despite the impressive savings, GSA was adamant that any decrement in occupant comfort would not be acceptable. Both FEMP and GSA were concerned that the pre-cooling would generate tenant complaints about cold in the mornings, and complaints about heat during the hottest afternoons when GSA used only one chiller as part of the "demand-limiting" strategy. However, "Thermal complaints went down," asserts John Kleaver, the GSA building manager. "The tenants have never been happier," agrees Brooks Range's Rich Ponticelli, head of the operations team that implemented the pre-cooling. GSA's complaint logs corroborate these accounts, showing that hot complaints went down from 41 in 2004 to 26 in 2005 (despite the much hotter summer of 2005). Interestingly, cold complaints went down slightly also, from 10 in 2004 to six in 2005.
In previous summers, the shutting off of the facility's chilled water system from 5:00 each evening until 6:00 the following morning meant that some tenants, particularly early birds, found the building too warm and humid upon arriving for work on the hottest summer days. The pre-cooling strategy appears to have resolved this problem without overcompensating. The 2 a.m. start time seems to have been an effective choice.
In sum, the Custom House's pre-cooling thermal storage experiment was an enormous success. The GSA avoided almost $70,000 in demand charges for the cost of a visit from the controls contractor and two solenoid valves. GSA manager Tom McGarry concluded at a "lessons learned" meeting that the GSA should "declare victory" and commit to repeating the strategy in the summer of 2006. Indeed, though final savings results will not be available until the 2006-7 ratchet period is over (i.e., at the end of May, 2007), GSA was able to suppress demand even further in the summer 2006, from an expected 2,100 kW to 1,684 kW. This 20% reduction is expected to generate savings of roughly $100,000.
For more information, contact:
- Phil Coleman
- (610) 604-0170