Rather than continuing to build new generation to keep pace with peak demand, wouldn't it be more efficient to use existing generation and store that energy for later use? Capturing the cheaper, more-efficient off-peak electricity and using it when the sun comes up seems like a no-brainer, effectively shifting large portions of peak load to off-peak hours and making it part of the base load. Reshaping the load curve by decreasing peak load and increasing base load improves the utility's capacity factor and, by extension, its financial health.
There are several energy-storage technologies for this scenario, including deep-cycle batteries and thermal energy storage (TES). Being an electric-focused industry, utilities tend to gravitate toward battery technology. Maybe the industry is more comfortable discussing ohms and amps than enthalpy and endothermic processes. But remember, energy is actually measured by British thermal units, not kilowatt-hours. It's time to become familiar with thermodynamics and heat transfer.
Sorry electrical engineers out there, but TES offers both heating and cooling applications. More importantly, deploying TES to offset summer peak demand offers several advantages over deep-cycle batteries. TES is simple to install and easy to maintain, contains no hazardous materials (lead, cadmium or acid), requires no interconnection agreements, generates no back feed to the grid, requires no net metering and can be deployed at the load instead of the source.
TES uses a medium (water, ice or a phase-changing material such as a eutectic salt) to store either heat or cold. For cool TES, refrigeration equipment is used to extract heat from a storage medium to produce chilled water or ice at night, which creates a cold reservoir. During the day, the reservoir provides a cooling capacity for the building instead of using electricity.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (Atlanta, Georgia), 87% of the cooling systems with TES in the U.S. use ice, 10% use water and 3% use eutectic salt as the storage medium. The medium is selected based on system requirements. Chilled water can store and release 1 Btu/lb, ice systems store and release about 144 Btu/lb and eutectic salts store and release 50 Btu/lb. The chilled-water medium, which requires large storage tanks, makes the most economic sense for tall buildings with plenty of floor space. Eutectic salts require a secondary heat exchanger and are more expensive, but they only require 30% to 50% of the space needed for chilled-water systems. Ice-storage systems are slightly more complex than chilled water, but less so than salt. They require 90% less space than chilled-water systems to provide the same cooling capacity and can be easily installed in most commercial or residential buildings.
Storing Energy (Not Electricity)
Utilities, customers, thermal storage companies and the heating ventilating and air-conditioning (HVAC) industry have a tremendous opportunity to work together to effect change. All residential, commercial and industrial customers, regardless of size, have the ability to store energy at night in the form of ice or chilled water, and to use the energy-absorbing properties of melting ice or chilled water to cool their homes, offices and factories from the heat of the day.
Take a look at what legitimate options are really available to utilities and energy users to cut their energy use and/or reduce peak demand. Central air conditioning makes up about 44% of the residential peak demand. Similarly, cooling makes up 43% of the commercial peak. The next-closest category of energy use is lighting, representing 32% for commercial customers and just 12% for residential customers.
So, if the U.S. and energy industry are serious about reducing energy consumption and improving efficiencies, especially on peak demand, expect a lot of action in the conditioned air space. It has the potential for the most-significant impact and makes the most sense for the industry.
Back in the Carter era, utilities first looked into ice-based energy-storage systems. The early prototypes presented problems with control technology and sealing systems that neither the early thermal-storage developers nor the HVAC industry could solve at the time. Thermal-storage technology has evolved dramatically since those early prototype systems. The utility industry has been evolving as well, with time-of-use rates becoming more widespread and giving both customers and the utility a compelling economic incentive to invest in this peak-load-shifting technology.
A good example of chilled-water technology is Chicago's McCormick Place. It has one of the world's largest chilled-water storage systems with an 8.5-million-gal capacity to cool a 5-million-sq-ft exhibition center, an 800-room Hyatt Regency Hotel and several nearby buildings. Another example is the University of Arizona's campus in Tucson, with more than 10 million sq ft of buildings connected to three central plants by 7 miles of distribution tunnels, producing 23,000 tons of chilled water for cooling along with steam and electricity.
Eutectic salt TES systems have been in use since the late 1800s and are getting attention in concentrated solar projects. The National Renewable Energy Laboratory (Golden, Colorado) reports that there are several parabolic power plants under development in Spain using this TES medium.
Cooling With Ice Storage
In addition to the large multistory structures served by chilled-water systems, ice-based energy-storage systems are now available as a practical, cost-effective solution for the majority of small- to mid-sized retail and commercial buildings in this country.
Units also are being developed for residential use. The devices enable utilities and their customers to permanently shift the bulk of the energy required for cooling from peak to off-peak hours by producing ice for the next day's cooling after the sun goes down.
Unlike large-scale TES systems for high-rise buildings and campus applications that require custom engineering and complex installations, distributed TES systems are essentially off the shelf, can be easily and rapidly deployed at scale, and plug right in to the building's existing duct work and airflow system. Perhaps most importantly, ice-based energy-storage systems do not require any behavioral modification on the part of the customer, nor do they adversely affect the customer's comfort or experience.
Major ice-storage systems are being used in commercial buildings throughout North America. One example is the Unicom Thermal Technologies (Chicago, Illinois) ice-cooling facility at State and Adams streets in the heart of Chicago's historic retail district. The cooling system features a subgrade water-distribution network that provides chilled water from ice melt to approximately 50 million ft of existing buildings, using non-CFC-based refrigerants.
For this facility, the architect was Eckenhoff Saunders Architects Inc., the general contractor was Pepper Construction Co. and the consulting firm was Environmental Systems Design Inc., all based in Chicago.
Ice-storage systems like the one described here can be found in most major cities, but they are custom engineered to meet local-district cooling needs. Distributed ice-storage systems are also commercially available and have millions of hours of operation across a wide range of applications, including restaurants, retail, food service, schools, and government and municipal buildings.
To keep tabs on the progress of these customer-side innovations, the Electric Power Research Center (EPRI; Palo Alto, California) opened its Living Lab in Knoxville, Tennessee. The lab tests commercially available customer-side solutions to bring the intelligent grid to the home as well as customer-side technologies that can reduce the country's energy dependence. EPRI has tested a device called the Ice Bear produced by Ice Energy Inc. (Windsor, Colorado).
Unlike traditional air-conditioning systems, with Ice Bear there is no need to run the compressor during peak times. Demand drops to roughly 300 W, which is the load of the fan unit, for a typical six-hour peak period. A fairly typical load might be 10 kW for each rooftop air-conditioning unit, so dropping to 300 W has a significant impact for the building and the grid when aggregated.
By linking individual units through Smart Grid technology, utilities can dispatch these load-shifting devices across a utility's entire distribution system. Extrapolate one Ice Energy device by a thousand, 10,000 or more units — with each removing 10 kW of energy from the peak — and it is easy to see why this technology is integral to utilities looking to improve relations with customers while reshaping load curves.
Time-of-use rates support ice-cooling technology. DTE (Detroit, Michigan) is one of the more-progressive distribution utility companies. DTE has been actively involved in placing distributed generation and energy-saving solutions in its service territory for more than a decade. And since divesting its transmission assets in 2002, the utility naturally gravitates toward local solutions to meet energy needs.
Take a look at the time-of-use rates for energy that DTE posts. Its peak and off-peak rates are $0.0875 per kWh and 0.0210 per kWh, respectively, during the months of June through October. On-peak hours are between 11 a.m. and 7 p.m. Monday through Friday. Delivery charges are an additional $0.04359 per kWh with a monthly service fee of $19 per month.
Thus, a customer who decides to embrace off-peak ice cooling will save $0.061 per kWh. With well-insulated and designed ice-storage systems, a round trip from water to ice and back to water can be accomplished with round trip efficiencies of around 85% to 90%. However, well-designed systems that integrate effectively with building cooling systems can have minimal energy conversion losses and, in fact, may even create energy efficiencies for the building.
So, what it comes down to is that utilities can either build new generation or build virtual generation in the form of energy storage and permanent load shifting. Across the country, state public utility commissions are enabling utilities to be reimbursed for prudent expenses incurred from demand-reduction and energy-efficiency measures, as well as from rate-base and earn-on equipment used to shift energy consumption.
When thermal energy is stored, the balance of the daily ups and downs associated with climate control on the power system is improved. As thermal-driven load is the root cause of peak and needle peaks, it is the obvious first choice for the utility industry to attack.
TES technology is proven and the savings are substantial to both the customer and the utility. Decreasing energy consumption also reduces greenhouse-gas emissions, which is what utilities are trying to do with all the renewable energy being installed on the grid. TES has come of age and will help transform the overall efficiency of the electric grid.