Editor's note: This is Part 2 in a two-part series. See Rethinking Central Utility Plants, Part 1
Part 1 of this series discussed the inefficiencies of central heating and cooling systems. Although there can be advantages to central utility plants, such as the opportunity for cogeneration and thermal-energy storage (TES), these strategies rarely are employed.
RENEWABLE ENERGY FOR HEATING AND COOLING
Thermodynamics demonstrates that heat should be transferred from one body to another with the lowest-possible temperature difference to utilize energy effectively. This currently is not possible with central steam or chilled-water plants. If heat-transfer fluids must be transported from one building to another, temperatures should be as close to ambient as possible, minimizing thermal losses in the distribution system. For example, water could be circulated through a heat-pump loop at temperatures between 35°F during winter and 105°F during summer.
University of Ontario Institute of Technology
Three North American universities benefit from alternative renewable-energy sources for heating and cooling:
The University of British Columbia Okanagan
The University of Ontario Institute of Technology (UOIT) in Oshawa, Ontario, Canada, has a 2,000-ton borehole TES system consisting of nearly 400 geothermal wells that extend 600 ft below a landscaped quad, creating a closed loop that transfers heat from the campus during summer and stores it in the ground for use during winter. This is a relatively new campus, and payback is expected in four to six years.
The Richard Stockton College of New Jersey
The University of British Columbia Okanagan in Kelowna, British Columbia, Canada, has an open-loop geo-exchange system that utilizes a vast aquifer with a year-round temperature of 51°F. The water is pumped out of the system, and heat is extracted from the water during winter and rejected during summer. The water then is re-injected into the system with pressurized wells or through a rapid-infiltration lagoon. The system replaced an aging gas-fired heating plant, and the additional $1 million in capital costs is expected to be offset by an anticipated $100,000 per year in energy savings.
The Richard Stockton College of New Jersey in Pomona, N.J., has a closed-loop geothermal system similar in size to the UOIT system with 1,741 tons of installed water-source-heat-pump capacity serving 440,000 sq ft of floor space. The system draws on 400 wells that were installed 425 ft under a 4-acre parking lot in 1990. The system serves nearly 120 heat pumps and has a projected payback of less than four years, with an annual savings of $330,000 on an investment of $1.2 million. Because of reduced peak electrical demand, the college was able to secure an $800-per-ton rebate from its utility company.
MOTIVATIONS FOR DEVELOPMENT
Encouraged by the success of the initial project, the college embarked on the construction of an aquifer TES system, which is an open loop that draws from six wells located in two clusters approximately 1,000 ft apart. The system is characterized as a seasonal cold storage system because it stores energy by chilling ground water during winter for use during summer. Additional capacity is met by a cooling tower when temperature and humidity conditions are favorable. The loop utilizes heat exchangers to isolate water in the aquifer from the buildings' refrigeration equipment. The campus's heating needs are met by the original closed-loop system supplemented by conventional boilers.
Reasons behind the development of such innovations in building-service-system design include:
- Social and political pressure
Concern is growing over the impact buildings and campuses have on the environment, resulting in voluntary efforts to curb negative effects. Rating systems, such as the U.S. Green Building Council's Leadership in Energy and Environmental Design Green Building Rating System, and legislation, such as the Energy Policy Act of 2005, are designed to foster innovation and reduce energy consumption.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is acting aggressively on this front with ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, which continues to raise the bar on building efficiency. It has been proposed that buildings meeting Standard 90.1-2010 will be 30-percent more efficient than those built to Standard 90.1-2004 specifications. Most states have building codes that adopt this energy standard.
- Facility trends
A small portion of the building-services industry realizes that heating and cooling systems will have to change to meet challenges associated with energy efficiency and carbon neutrality. The industry will have to move away from variable-air-volume systems because they use large amounts of fan energy and rely on reheat for occupant comfort.
Some future systems will utilize warmer chilled water and cooler hot water, while others will decouple ventilation from space cooling and heating functions. These changes will allow the use of open-ground-water, closed-geothermal-loop, diurnal-storage, and solar-thermal systems to feed thermoactive slabs, chilled beams, and water-source heat pumps for building heating and cooling.
Spurred by social and political pressure, the HVAC industry is undergoing major changes made possible by technological advances, which will result in reduced energy consumption and greenhouse-gas emissions. Central heating and cooling plants and their vested interests stand in the middle of this path to change. Hope for such plants may include the utilization of cogeneration to maximize thermal efficiencies, as well as the use of TES to maximize the effectiveness of power generation and the electric grid. More research should be done in all of these areas, and open debate needs to be fostered and encouraged to determine the relative economic merits of each viable approach.
However, utilizing the earth as a heat source and sink, coupled with the use of distributed heating and cooling plants, may make more sense for many campuses. Another benefit of the earth-coupled approach is the elimination of cooling towers, which use large amounts of water through evaporation and drift and require chemical treatment that ends up down the drain. Cooling towers also can be noisy, do not last long, create tower plume, and have been associated with Legionnaires' disease.
For previous Engineering Green Buildings columns, visit www.hpac.com.
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 protected].