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Energy storage has long been recognized as a highly desirable and valuable component of electric power systems, whether for utility-scale or small to large distributed generation (DG) systems. Specifically, energy storage can greatly enhance the economics of DG or combined heat and power (CHP) installations by flattening power supply/demand and better matching electrical and useful thermal outputs.
Conventional wisdom has often assumed that energy storage is rather elusive, challenged by technical development and high capital costs. The reality is quite different. Large-scale energy storage applications, specifically as cool thermal energy storage (TES), have been widely employed for years, providing energy storage in both demand-side and supply-side applications and using fully developed performance-guaranteed technology deployed at highly attractive capital costs.
Many technologies can and have used energy storage, including pumped hydro, compressed air energy storage, flywheels and electro-chemical batteries. Each has inherent characteristics related to cost, efficiency and readiness for deployment in various sizes. As one example of a developing energy storage technology, a planned energy storage system was described in Power Engineering magazine in an article entitled “AEP Substation To Get Commercial-Scale Energy Storage System” (October 2005, page 68). The stated capacity of this first-of-its-kind “MW-class” energy storage system was 1.2 MW of peak shaving capacity and 7.2 MWh (1.2 MW for six peak hours/day) of stored energy. A 15-year projected life (4,000 to 5,000 charge-discharge cycles) was listed at 90 percent of full energy capacity. The project was described as a pioneering application. No cost figures were presented.
To gain perspective, note that other energy storage technologies have long been fully commercialized, not only in this MW-scale project size, but also in a multi-MW-scale and even in multi-tens-of-MW-scale project sizes. A specific example is the use of cool thermal energy storage technology. This technically-proven and commercially-mature technology has been applied successfully in hundreds of large demand-side applications over the past 20 years. It has also seen significant and increasing application as energy storage for power generation when used as a complement to turbine inlet cooling (TIC) on combustion turbine (CT) power plants in both DG and utility applications. And where DG systems use both CTs and cooling systems (for example, in district energy or combined cooling, heat and power systems) there is further synergy. Beyond the already significant benefits of energy storage and management lies the ability to integrate cooling and TES for hot weather power enhancement of the CT via TIC.
Although only one example of demand-side TES and one example of supply-side TES are presented here, many such applications are successfully deployed worldwide. Both examples illustrate the beneficial performance and economic impacts of TES in typical usage.
Demand-Side TES - In Orlando, Fla., a district cooling network provides chilled water for air-conditioning of a convention-exhibition center, associated convention hotel facilities and a nearby high-tech manufacturing facility. In the absence of TES, an additional 10,000-ton chilled water plant would have to be added to the existing chiller plants. Instead, a 160,000 ton-hour chilled water TES system was installed capable of meeting peak loads of 20,000 tons for eight peak hours/day. Load management impact is 15 MW of peak shaving capacity and 120 MWh (15 MW for eight peak hours/day) of stored energy. A minimum 30-year projected life (at least 6,000 charge-discharge cycles) is expected at 100 percent of full energy capacity.
The installation was a typical commercial application of the TES technology. Thermal performance of the TES element is backed by the supplier in accordance with project specifications. The fully installed capital cost of the TES element (including foundation, tank, paint, insulation, internal flow diffusers, fittings and thermal performance guarantee) was $3 million in 2002, equating to a unit cost of $150/ton or $19/ton-hr of cooling capacity (or $200/kW or $25/kWh of electric capacity). Compared to the conventional non-storage cooling system that would otherwise have been required, the entire TES tank and system was installed at a net capital cost savings of more than $5 million, with no demand-side management incentive from the local electric utility. The installation provides an operating cost saving of more than $500,000 per year.
Supply-Side TES - Hot weather produces high inlet air temperatures in combustion turbines, which in turn produces dramatically higher heat rates and lower power output. A CT power plant in the Middle East, with 10 GE 7EA turbines, each with 75 MW of nominal capacity at ISO conditions, was adversely affected by hot weather. To improve performance, a turbine inlet cooling system was retrofitted to improve output and heat rate at a lower capital cost than adding turbine capacity.
In lieu of installing a 31,773-ton chiller plant sized to meet the peak TIC load (but which would have reduced power enhancement due to large parasitic power losses), a 190,637 ton/hour chilled water TES system was installed. The chilled water system is capable of meeting full peak loads for the six peak hours of the day. Also installed was an 11,188-ton chilled water plant that operates only during the 18 daily non-peak hours. Maximum load management impact of the TES element is 48 MW of peak shaving capacity and 288 MWh (48 MW for six peak hours/day) of stored energy.
A minimum 30-year projected life is expected at up to 100 percent of full energy capacity, depending on actual daily ambient temperatures. That life is expected to include at least 8,000 charge-discharge cycles. The installation was a typical commercial application of the TES technology. Performance of the entire TIC system, including the TES element, is backed by the TIC system supplier in accordance with project specifications. The combined TIC-TES system provides a 30 percent (180 MW) increase in net power plant output during design-day weather conditions and does so at an installed unit capital cost that is well below half that required for an equivalent conventional simple cycle gas turbine power plant installation.
The fully installed capital cost of the TES element (including foundation, tank, paint, insulation, internal flow diffusers, fittings and thermal performance guarantee) was less than $3 million in 2004, equating to a unit cost of less than $100/ton or less than $15/ton-hour of cooling capacity (or approximately $65/kW or $11/kWh of electric capacity). Furthermore, the entire TES system was installed at a net capital cost savings of more than $10 million compared to the cost of a conventional non-storage TIC system that would otherwise have been used.
Studies performed on utility grid power plant fleets (in regions as diverse as California, Texas and Wisconsin) documented reductions (often in the range of 15 to 20 percent or more) in both power plant unit fuel consumption (Btu/kWh) and atmospheric emissions (lbs/MWh) of pollutants and greenhouse gases. This resulted from shifting energy use from on-peak periods (when high heat rate peaking plants are on the margin) to off-peak periods the preceding night (when the marginal plants have much lower heat rates and much lower emission rates).
Energy storage (of whatever technology type) can and does benefit the energy marketplace for generation. T&D and demand-side load management. Benefits can include peak-shaving power capacity, increased system reliability, enhanced power quality, low unit capital cost capacity, improved economics for generators and for energy users and improvements to power plant fuel energy consumption and emissions.
No single energy storage technology is ideal for all situations. However, options afforded by the use of cool thermal energy storage should not be ignored when policy-makers, power producers and energy users are exploring and implementing projects. This is especially true for large-scale, low unit cost, sensible heat storage technology used for demand-side management or supply-side turbine inlet cooling in DG and utility applications.
Research and demonstration of advanced energy storage technologies is appropriate. But technologies that are already thoroughly proven, commercialized and economically attractive should be preferentially implemented in the marketplace on both the demand and supply sides with the full support of regulators and decision-makers.
John Andrepont is founder and president of The Cool Solutions Company, provider of consulting services in thermal energy storage, district cooling and turbine inlet cooling. For more than 25 years he has been involved in hundreds of large TES installations, dozens of DC developments and large and small TIC installations on five continents. He has bachelor’s and master’s degrees in mechanical engineering from Rensselaer Polytechnic Institute and has patented more than a dozen inventions.
Power Engineering May, 2006
Author(s) :
John Andrepont
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