The Hill School, referred to as Hill, was founded in 1851 as The Family Boarding School for Boys and Young Men. The school enrolled 25 boys in its first year and since then has increased annual enrollment to 500 boys and girls.
Throughout the years, the school has benefited from the leadership and forethought of its overseers and benefactors. The school’s initial investments in infrastructure have yielded significant dividends, both in avoiding major utility expenses and in limiting maintenance to the out-of-sight central heating and cooling plants. The network of underground utility tunnels affords flexibility to the changing campus heating and cooling needs without disturbing the school’s green spaces or architectural appeal.
In 1954, the school built a 200-ton chilled brine plant to serve an outdoor ice skating rink. In 1989, realizing that the cooling plant was not being utilized during the summer, and with the desire to provide cooling to an increasing number of buildings on campus, a 1,400-ton-hour ice building storage chest was added to the chilled brine cooling plant. Chilled water piping was installed in the campus utility tunnels so that the chilled water could be distributed to serve campus buildings during the cooling season. The cooling plant produced ice only during times when electrical demand was not measured, thereby not increasing the school’s summer electrical demand rate and cost.
During the day, when cooling was required, the ice was melted and pumped to several buildings. This peak shaving technique resulted in a relatively flat summertime electrical usage pattern, resulting in low electrical energy costs. When Pennsylvania electrical rates were deregulated in 2009, the school was able to leverage flat electrical demand into a low electrical cost contract rate. Despite the convenience of the low rate, there were days when the ice ran out and buildings became warm and humid, thus highlighting the need for a new solution.
The Value of a Report
In 2010, The Hill School trustees wanted to address several HVAC issues holistically. They commissioned a study and report to address several key areas of concern related to the existing campus mechanical and plumbing systems. Gannett Fleming, a global infrastructure firm headquartered in Harrisburg, PA, conducted interviews with the school’s staff; surveyed the existing site conditions; examined existing drawings; developed multiple engineering solutions; provided construction schedules, probable design, and construction and life cycle costs; and made recommendations for addressing the HVAC issues.
The resulting report provided the trustees with a document that functioned as a master plan for the campus heating and cooling and related distribution systems. It was used to prioritize resources, develop capital budgets, and strategically plan for future campus projects and energy conservation strategies. Perhaps most importantly, the study identified that there was insufficient incoming electrical service capacity to operate three 200-ton chillers simultaneously. Increasing the electrical service size would be a considerable expense.
In its report to the trustees, Gannett Fleming evaluated five cooling plant systems:
- Hybrid electric and natural gas-driven chillers with ice storage
- Hybrid electric and natural gas-driven chillers with ice storage using the existing indoor ice rink brine chillers
- All engine-driven chillers with ice storage
- All engine-driven chillers with chilled water production only
- Decentralized cooling that abandoned the cooling plant and utilized local cooling at each building
Challenges and Solutions
The report addressed the competing priorities and challenges for each cooling plant system. The team had to consider restrictions, such as limiting construction to times when students were not on campus while not exceeding the available funds and providing system redundancy. They had to eliminate the low delta-T syndrome and come up with a solution to the inability to operate three electrically driven chillers simultaneously without increasing the incoming electrical service capacity, which came with a significant up-front cost.
“The report was key,” explained Don Silverson, The Hill School’s treasurer. “We were planning to make a significant investment in infrastructure, and presenting the board of trustees with a report that methodically established priorities, phases, and bidding showed them that we had done our homework. We didn’t go to the board with a problem; we went to them with a problem and a solution, which is important when you’re talking about millions of dollars.”
Gannett Fleming recommended and then designed a hybrid system that included electric and natural gas engine-driven chillers and ice thermal storage cooling plant. This allowed The Hill School to produce ice at night when the regional electrical grid is not at peak demand, operate low energy cost natural gas engine-driven chillers during the day, and melt ice for the additional cooling as needed. The new cooling plant consists of a chilled water distribution loop to the campus buildings and an ethylene glycol ice water loop with a pair of plate and frame heat exchangers between the closed loops.
On the glycol ice water loop is a 200-ton air cooled chiller, 200-ton water cooled engine-driven chiller, and 1,750 latent ton-hours of ice storage. Any of the cooling devices may provide cooling to the chilled water distribution loop or recharge the ice tank. The maximum ice melting rate is 265 tons, the pair of heat exchangers is rated at 535 tons, and the peak cooling load is 400 tons.
Typical daily cooling operation is to base load the 200-ton engine-driven chiller and melt ice as necessary to meet the campus chilled water demand above 200 tons. In this scenario, an average of 375 tons throughout a 10-hr day can be provided without operating electric chillers. The air cooled chiller typically is used to recharge the ice tanks at night and the engine-driven chiller is reserved for evening/nighttime cooling, but they also may be operated conversely.
This triad of cooling devices provides multiple use redundancy, where each cooling device is more than just functional redundancy. A high-efficiency water-cooled engine-driven chiller saved the school from having to increase its campus electrical service size. The lower capital cost air cooled chiller, operated at night for quick ice building, kept the project on budget, and the ice storage tanks provided electrical demand control. All three together provide redundant cooling capacity.
Two 20-horsepower chilled water distribution pumps with VFDs modulate to maintain minimum supply/chilled water pressure differentials. The chilled water piping connection to each building is the primary chilled water piping in a primary/secondary piping system with decoupled by-pass. A control valve at each building chilled water supply modulates to maintain a fixed temperature difference between the supply and return chilled water temperature to maximize the chilled water distribution piping capacity. This decreases pumping costs, eliminates low delta-T syndrome, assures that the buildings are using chilled water efficiently, and prevents the first buildings on the system from using all the chilled water.
This temperature differential value (delta-T) is between 10?F and 14?F, depending on the design parameters and condition of the individual building HVAC chilled water system. Over time, as the HVAC equipment is replaced, chilled water coils will be replaced with coils with larger delta-Ts. This will allow the building chilled water delta-T set-point to be increased, thereby increasing distribution piping cooling capacity.
Distribution flow meters and temperature sensors allow the school to constantly monitor the campus’ actual cooling tonnage and determine if they are losing pumping and chiller efficiency by a reduction in the difference between chilled water supply and return temperatures.
When the electricity markets base electrical pricing on real-time electrical demand, the Hill School will be ready to leverage its ability to control the electrical demand at the cooling plant. This will be accomplished by staging the electrical chiller, engine-driven chiller, and melting ice regardless of the cooling load to maintain a constant/market desired electrical demand that results in lower campus-wide electrical rates. Campus-wide, the summertime electrical usage for 2013 was 8% lower than the cooling plant renovation usage during 2011, and there was superior building comfort.
Building Information Modeling
The design team recognized early on that developing building information modeling (BIM) for the cooling plant upgrades and the steam distribution system renovations was the best way to thoroughly and accurately document the required work. Through the use of Revit® MEP®, 3-D views were used in construction documents to better describe the complex piping and equipment arrangement and to improve design coordination. The use of Revit also reduced engineering time when changes in the cooling plant layout were made during design to keep the capital cost within the funds available.
Ready for the Future
By keeping a close eye on the cost estimates during design and providing bid alternates, the school was able to maximize its purchasing power without exceeding available funds. Remaining funds were available for other school improvements. By selecting qualified contractors, all construction schedules were met, and there was no disruption to the students or staff. The Hill School now has sufficient cooling capacity, system redundancy, system efficiency, and the ability to control electrical demand to carry them well into the future.
Low delta-T syndrome occurs when the temperature difference between the chilled water supply and chilled water return degrades over time. It increases pumping energy directly by causing an increased flow for the same transported cooling quantity. For chiller operation, pending chiller plant arrangement, low delta-T primarily increases chiller operating cost by requiring a second chiller to operate in order to keep the primary flow equal to or greater than the secondary flow in a primary/secondary pumping system. When the primary flow exceeds the secondary flow, the secondary return water mixes with the primary supply water, thereby raising the secondary chilled water temperature. If raised too much, dehumidification ceases to occur at the air handing equipment.
Prior to this chilled water plant renovation, two 40 HP constant volume chilled water distribution pumps were utilized.