Thermal Energy Storage: Putting
on SiteBy John E. Flory, Loren W. McCannon, Stan Tory,
Donald L. Geistert, and James PattersonA recent study coordinated by the California Energy Commission shows how stored-cooling applications provide both environmental and competitive benefits in a summer-peaking market.As California prepares for a more competitive electric future, the California Energy Commission (CEC) is taking another look at some key customer technologies. One CEC program, known as Opportunity Technology Commercialization (OTCOM), carries a mission to boost market penetration of energy technologies that offer "compelling energy, environmental, diversity, and economic
development benefits." OTCOM selected thermal energy storage (TES) as just such a promising technology. As defined here, TES denotes a chiller system operated during the night to store energy for air-conditioning use during the day. Traditionally, building owners have employed TES to trim power costs by reducing peak-demand charges. But TES offers other benefits for both energy users and suppliers.
To address market barriers that might stand in the way of TES applications, OTCOM organized a collaborative of TES users, utilities, governmental agencies, consultants, and TES manufacturers. As a first step, the collaborative enlisted a study to measure the potential impacts of TES in California. The highlights of that study, Source Energy and Environmental Impacts of Thermal Energy Storage, are presented here.
The study split the analysis of the energy use by TES into two components:
s Source energy use: Units of fuel (in Btus) required at the power plant source to supply one kilowatt-hour of energy. (The study presumed that shifting generation away from the peak would reduce the energy needed to supply each kilowatt-hour.)
s Site energy use: Electric energy (in kilowatt-hours) required to provide a ton-hour of cooling at the customer site.
To analyze the source energy use of TES, the study focused on the two largest electric utilities in California (em Pacific Gas and Electric Co. (PG&E) and Southern California Edison Co. (SCE) (em which together supply almost three-fourths the electricity used in the state. It employed two cost analysis methods: the "incremental energy method" (the standard planning method used in California1) and a variation termed the "marginal plant method."2 (The primary difference is that the incremental energy method captures the fuel savings from reduced need for "unit commitment" (em i.e., committing a power plant unit to run much of the day to be available to meet daily peak demand.)
The results showed that the source energy savings for a particular TES system at a particular building depend on a number of factors, including:
s Building usage: the building's normal air conditioning usage pattern without TES (em e.g., What percent of the cooling is summer vs. winter, or day vs. night?
s Thermal system: the design and operating strategy for the
TES system (em e.g., Is the TES storage tank sized so that the air-conditioning compressor runs only at night (full storage) or all day (partial storage)?
s Utility's fuel mix: the characteristics of the utility supplying the electricity (em e.g., How much hydroelectric power is available?
s Cost tracking method: the method used3 (in particular, whether the savings from reduced power plant "unit commitment" are included, as under the incremental energy method described above).
In many TES installations in California, 40 to 80 percent of the annual kilowatt-hours of electricity used for air conditioning can be shifted from day to night. For such installations, the incremental energy method showed significant source energy savings. Using that method, the savings for each kilowatt-hour of shifted energy use ranged between 36 to 43 percent for SCE, and between 20 to 30 percent for PG&E. The savings indicated by the marginal plant method were lower, but still significant (em from 12 to 24 percent for SCE and 8 to 10 percent for PG&E. Thus, even if some TES systems use more kilowatt-hours altogether than conventional air conditioning, they can still yield a net savings in "source energy."
In fact, if TES could achieve a
20-percent market penetration by 20054, it would save enough source energy from load-shifting only (ignoring kilowatt-hour impacts) to supply the energy needs of all the electric cars running on California highways in 2005, as projected by the CEC.
At the customer site, TES systems can improve energy efficiency to a significant degree
compared with conventional air-conditioning systems. Although early TES systems squandered more kilowatt-hours than conventional systems, recent systems consume as much as 12-percent fewer kilowatt-hours than conventional cooling systems. These efficiencies present attractive alternatives to the 20- to 50-percent energy penalties inflicted by conventional utility storage technologies such as pumped storage for hydroelectric generation.
When energy savings at the customer site are combined with those at the power plant source, again assuming a 20-percent market penetration, TES could save enough energy to supply over a third of the new air-conditioning load that the CEC projects for California by 2005.
At the power plant source, TES can reduce air emissions significantly. Indeed, in California, where natural gas serves as the fuel of choice for the marginal power plant, the extent of reductions in power plant emissions (on a percentage basis) is comparable to energy savings from TES.5 Assuming a 20-percent market penetration by 2005, TES could save 260,000 tons of CO2 annually statewide. Just as important, it could save about 1.6 tons of nitrogen oxide (NOx) per day in the South Coast Air Quality Management District (SCAQMD), which encompasses the greater Los Angeles area. These NOx savings are equivalent to the emissions from almost 100,000 cars.
TES can also help reduce combustion air emissions at the customer's building site. SCAQMD, in fact, explicitly identifies thermal storage as an option for reducing site emissions.6
In addition, TES can help in the transition to air-conditioning refrigerants without chloroflorocarbons (CFCs). When existing chillers are converted to a non-CFC refrigerant, for example, their effective cooling capacity may be reduced. Some key facility managers envision TES making up the difference. In addition, partial-storage TES systems often can require half as much chiller capacity, which means half as much refrigerant.
Several other aspects of TES work to make energy suppliers and building owners more competitive.
For energy suppliers, for example, the marginal cost of serving a customer's air-conditioning load can be decreased by 30 to 50 percent. In addition, electric utilities are about 5 times more capital intensive than other manufacturing businesses per dollar of revenue. Improving the customer's load factor by 30 to 50 percent with TES can mean a significant reduction in financing requirements and financial exposure.
California financing requirements could shrink by a billion dollars for a transmission and distribution system, and comparable savings in generation capacity may be possible. Finally, TES's ability to lower a customer's average price makes TES an effective customer retention tool.
California building owners can also benefit by paying lower costs for energy. In addition, using TES for chilled water storage tanks could lower fire insurance premiums. Some commercial facilities managers believe that TES could provide the best tool available for reducing power costs in a restructured electricity industry. And, because TES increases property value, the building owner can often obtain more external financing on a project and use less of the developer's own cash.
Finally, the building owner can increase revenues with TES: Cold-air distribution systems allow building owners to offer more floors of leasable space and, hence, greater revenues.
Given the energy savings and other benefits of TES, several policy actions become feasible. First, government agencies could nominate TES as a priority energy-efficiency or demand-side management (DSM) program in state energy resource policy decisions. TES has demonstrated energy and emissions savings consistent with DSM objectives but, unlike most energy-efficiency measures, TES also improves load factor significantly and provides cost savings that render both energy users and suppliers more competitive.
Second, building standards could be modified to compare the energy efficiencies of alternative cooling technologies. State public utility commissions could reexamine source energy comparisons of alternative systems, including the opportunities of TES systems. In addition, as in Switzerland,7 the building code could encourage designers to lower building peak demands with TES.
Importantly, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has put its energy-efficiency Standard 90.1 forward for public review. In that draft, ASHRAE is considering using an average electricity cost for all kilowatt-hour use that does not recognize the time-varying nature of electricity cost and source energy efficiencies. Such a standard would significantly reduce a building owner's options to
control cost in an increasingly competitive electricity market. ASHRAE members have voiced significant concern over this draft.
Third, government agencies charged with emissions monitoring could recognize TES as an effective emissions-control measure. The SCAQMD has already done so.8 Other air districts could follow suit. Many California air districts would benefit from encouraging TES as a control measure for power-plant emissions.
Finally, TES could be promoted as a priority cooling system option in "environmental partnerships" with key energy-user groups. One such group could involve "sister" governmental agencies of the CEC, including local, state, and possibly, federal government agencies. Another group could include businesses involved as "environmental partners."
For example, the U.S. Environmental Protection Agency has attained considerable success in recruiting business "environmental partners" for its "Energy Star" programs, such as Green Lights. This program has committed a number of business partners in California to installing high-efficiency lighting in 90 percent
of their floor space over a 5-year period when the internal rate of return exceeds 20 percent. California could develop a "Competitive Electricity Environmental Partnership" program for TES modeled along the same lines. Such a partnership would position California businesses to benefit from a competitive electricity market and help clean the air as well. Alternatively, perhaps TES could be included as a priority cooling technology in the second phase of the Energy Star program (em which moves from lighting to heating and air-conditioning system improvements.
TES is now poised for full
commercialization. Institutional policies, such as those identified above, should be pursued to increase TES's market penetration. t
John E. Flory is principal in charge of the Western Division of Tabors Caramanis and Associates and lead author of the TES Collaborative's report, Source Energy and Environmental Impacts of Thermal Energy Storage. Loren W. McCannon is the founder and president of the International Thermal Storage Advisory Council and editor of the Council's newsletter. Until recently, Stan Tory managed the Thermal Energy Storage Program for PG&E and represented the utility within the Collaborative. Donald L. Geistert represented SCE's interests with the TES Collaborative, as SCE's lead TES engineer. James Patterson is an associate energy specialist with the California Energy Commission. He coordinates the TES Collaborative and researches other new technologies.
1. The "incremental energy" method applies the state's official method (as used by the California Public Utilities Commission, CEC, and utilities) for marginal-cost calculations and resource planning (including demand-side management programs such as TES) to the state's official guidelines for source energy analysis to develop time-differentiated source energy impacts. These impacts allow the determination of source energy savings by shifting electricity usage from day to night with TES. For more details see the report, Source Energy and Environmental Impact of Thermal Energy Storage.
2. The "marginal plant" method is similar to the state's official method with one exception. It has a different way of computing marginal-source fuel use in different time periods.
3. The method used in this study assumes that TES is operated under conventional time-of-use (TOU) rates. Some studies have found that thermal storage, operating with intelligent control systems under hourly varying real-time pricing (RTP), almost doubles the utility's marginal energy cost savings (and presumably source energy savings) as compared to thermal storage operating under conventional TOU rates. (See, B. Darayanian, L. K. Norford, and R.D. Tabors, ORTP Based Energy Management Systems: Monitoring, Communication, and Control Requirements for Buildings under Real-Time Pricing." ASHRAE Transactions 1992, V.98, Pt.1) (American Society of Heating, Refrigeration, and Air Conditioning Engineers). The CPUC recommends RTP as the dominant pricing in a competitive or restructured electric power industry. Therefore, the source energy savings of TES under the increasingly more common RTP could be significantly higher than the source energy savings reported here.
4. An interest Pacific Gas & Electric Study, "Offpeak Cooling Market Potential Study, conservatively estimates 20 percent as an achievable market penetration for TES.
5. The difference in emissions costs for day vs. night is greater than the difference in source energy use; however, some of that difference is due to emissions being generated in different air basins. Conservatively, then, emissions reductions must at least equal the source energy and site energy savings combined - 20 to 40 percent per kilowatt-hour of annual cooling energy (using the "incremental energy" method), depending on the TES system application.
6. These savings occur when a heat-recovery storage system is used with a cool storage system to use heat from the chillers in lieu of a separate boiler.
7. For example, the Geneva Electric Utilities article 117A requires that any building over 10-Kw demand be designed to limit the maximum needed power by cutting excessive thermal charges." Moreover, the designs reviewed by a commission must analyze the possibility of thermal storage and waste-heat recovery.
8. South Coast Air Quality Management District, 1994 Air Quality Management Plan, Appendix IV-A, Stationary Source Control Measures, "Area Source Credit Program for Commercial and Residential Combustion Equipment [NOx]"
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