Current technology makes central heating and cooling plants unnecessary
Editor's note: This is Part 1 in a two-part series. See Rethinking Central Utility Plants, Part 2
Times have changed and so have the buildings in which we live, work, and teach. They have become more energy-efficient because of better facades and more sophisticated HVAC, electrical, and automation systems. But what about the central plants that produce the heating and cooling delivered to these buildings? How have they evolved over the years? Are they serving us in an efficient, sustainable manner?
Many universities are making a big push to reduce their facilities' carbon footprints but relegating the assignment to those who operate and maintain their central plants. Although those individuals most likely have implemented capital-improvement projects that have reduced operating and maintenance costs, the greater question is whether the plants still are relevant in a world increasingly concerned with global warming, fossil-fuel dependence, and economic competitiveness. (The use of cogeneration and thermal-energy storage are exceptions, but will not be addressed in this column.)
CENTRAL HEATING PLANTS
Certain HVAC trends, such as decoupling ventilation systems from space-heating and cooling systems, eliminating reheat for comfort-cooling applications, minimizing fan energy, and using warmer chilled water and cooler heating hot water, impact how central-plant equipment relates to the buildings it serves. These trends are necessary to achieve the building energy savings our society and government desire.
Campus waste is most evident at a central heating plant, where anachronistic boilers burn large quantities of fossil fuel at a modest thermal efficiency. Some energy goes up the stack, some goes down the drain, and some is radiated and distributed in an inefficient manner. Many heating plants are located in cold-weather climates where coal once was or still is used. Saturated steam is generated at 80 psig or more and distributed via tunnels to buildings, where the pressure is reduced. The steam then is used in heating coils or radiators or, more commonly, converted to hot water for space heating. Problems associated with these types of systems involve:
- Thermal efficiency
Efficiency suffers partly because a central boiler plant must generate a heating medium, such as steam, at a higher temperature or pressure than a smaller distributed plant. High-delivery temperatures and pressures compensate for thermal and friction losses that occur en route to the heating load. But by generating a heating medium at a higher temperature, the central plant misses out on potential thermal efficiencies, such as those that can be realized with a condensing boiler.
- Radiation and convective losses
Steam boilers have large surface areas that constantly radiate energy when operating or idle. Simple math demonstrates that a boiler estimated to have convective and radiation losses of 1 percent at full load will have 2-percent losses at half load and 4-percent losses at one-quarter load.
- Distribution losses
CENTRAL COOLING PLANTS
Campus piping distribution systems experience thermal and friction losses. Central steam plants require large networks of tunnels, shallow trenches, and direct-buried piping systems to deliver energy to remote points throughout a campus. Friction losses require boilers to produce higher steam pressures than typically needed at remote buildings. Higher pressures create higher distribution temperatures and thermal losses as well as lower boiler thermal efficiencies.
- Condensate losses
Steam boilers require high-quality water for longevity. Consequently, operating personnel go to great lengths to ensure feedwater quality by treating it expensively. Boilers turn the water to steam and send it to buildings, where it is used for space heating, domestic water heating, and processes, such as humidification. In an ideal steam-boiler system, all of a boiler plant's condensate returns make another trip through the system. Condensate then can be lost at faulty steam traps, via aging leaky piping systems, and through poor practices, such as allowing receivers to vent to the atmosphere.
- Blowdown losses
Because steam systems lose condensate, they require makeup water. The water leaves behind impurities when it evaporates. Over time, the impurities must be “blown down” a drain, taking energy with them. Often, this blowdown is mixed with cold water before being sent to the sewer system, wasting more resources.
If central heating plants are so inefficient, why are they utilized? Once regarded as inexpensive, coal used to be a popular heat source. Systems that utilized coal were major pollutants, creating black soot that covered the landscape. Therefore, it was imperative to locate campus heat sources away from occupied areas, such as classrooms, libraries, dormitories, administrative offices, and patient rooms.
The Clean Air Act of 1963 and its amendments in 1970, 1977, and 1990 caused the U.S. Environmental Protection Agency to tighten restrictions on large campus boiler plants. Costly emissions control and monitoring equipment was installed on coal plants, while other boiler plants were converted to fuel oil and natural gas. Many steam radiators still are operating in campus buildings, and, unfortunately, windows are kept open for much of the year to compensate for a lack of temperature control. All of this has increased the cost of owning, operating, and maintaining central heating plants.
Central cooling plants also have proliferated over time. In the past, chillers were large and inefficient and tended to surge when unloaded at less than 80 percent of full-load capacity. As chillers aged, owners were forced to replace them directly or construct a central chilled-water plant that could serve a number of buildings. Central plants were able to have smaller-than-required installed capacities by replacing every building's chillers because not all buildings would have simultaneously peaking cooling loads.
Because large centrifugal chillers operate at a higher efficiency when fully loaded, central plants could operate more efficiently by virtue of their enhanced opportunity to keep their chillers fully loaded. Couple that with the fact many campuses already had steam tunnels in which chilled-water pipe could be installed, and the economics of the central plant were favorable. However, central chilled-water plants have their drawbacks, such as:
- Distribution losses
Distribution losses come in two varieties: friction and thermal. It is not uncommon to see cold pipes in extremely hot steam tunnels or buried in soil warmed by the summer sun. I know of campuses at which direct-buried chilled-water return lines are uninsulated and bell-and-spigot connections that have a catalogued fluid leakage rate are used. Some campuses extend expensive steam-tunnel systems just to accommodate chilled-water-plant expansion. A central plant may produce chilled water at much colder temperatures than required by the majority of end users just to satisfy the needs of a small load, such as a surgical suite.
- Central-plant distribution and other problems
Low-delta-T syndrome has been a plague to many campuses that have been connecting buildings to central chilled-water loops without first upgrading the buildings to conserve chilled water. Campus planners also make the mistake of not thinking through the hydraulic decoupling of buildings from central loops. A common approach is to use a heat exchanger for isolation, which requires a chilled-water plant to produce water a couple of degrees colder than otherwise required, thereby decreasing chiller-plant efficiency and increasing thermal losses.
The refrigeration industry has made great technological strides to advance chiller efficiency. Chillers truly can unload to match building loads with favorable coefficients of performance. Smaller machines now have excellent efficiencies approaching that of the largest centrifugal chillers. Economical and efficient chillers in smaller sizes were not available until water-cooled screw chillers were adapted and refined for HVAC applications.
Chiller manufacturers also have made great strides in accommodating colder condensing temperatures and fitting chillers with variable-frequency drives. Another development is the use of magnetic bearings on small centrifugal compressors, which offer excellent part-load efficiencies. Now, small distributed plants can operate as efficiently as large central plants.
Next month, the author will discuss the use of renewable energy sources for heating and cooling.
A vice president with Advanced Engineering Consultants, Carl C. Schultz, PE, LEED AP, has 20 years of experience designing mechanical systems for hospitals, laboratories, universities, prisons, data centers, and large office complexes. Additionally, he has extensive experience designing central steam, high-temperature-hot-water, and chilled-water plants. He can be contacted at email@example.com.