ASHRAE Journal - September 2010 - 29

One of those effects is the amount of oversizing of the grid. The U.S. had 1 trillion watts of electric generation in 2008. However, the average electric use was less than half of that.4 This means we have twice as much generation as we need if we used electricity at a level rate year-round. The cause of this is the usage patterns of buildings. Energy storage within buildings is certainly not new. Thousands of projects around the world have used thermal energy storage (TES) in the form of ice or chilled water, with hundreds of articles describing the advantages and justifications.6,7 Even so, the use of storage has been relatively limited in comparison to the market size and potential. The move toward sustainability and renewable resources will completely change the potential value of TES in buildings, which should, if free market forces prevail, bring greater financial rewards to those who use thermal storage.
Energy Storage Types

that it is downstream of both the transmission and distribution systems and has relatively higher roundtrip efficiencies (addressed later).
Costs

A few different forms of energy storage exist including: potential, kinetic, chemical and thermal. One example of potential energy storage is pumped hydro (PH) where water is pumped up a mountain at night, and the next day the water flows down to run a turbine to create electricity. Other means for storing energy for electrical production Cycle Efficiency include flywheels for kinetic Perhaps the most dramatenergy, and some batteries for ic differentiator for TES is Figure 1: Different types of energy storage systems.7 chemical energy. its relatively high roundtrip Figure 17 shows the differcycle energy efficiency that ent types of energy storage and the size range of storage capacity ranges from 75% to 95%. As with any storage device, losses versus the length of time of discharge for each type of storage are associated with putting the energy in and removing it from system. The figure is divided into three general areas: the lower the device (Figure 3). The thermal losses of ice storage sysleft is for power quality, the upper right for energy management/ tems on a daily basis are less than 1% with similar numbers shifting and bridging power in between. (Some types of thermal for stratified water storage (so 99% thermal efficiency). Nonestorage are used on the “grid side” of the electric meter, but this theless, there is a wide range of cycle efficiencies shown for article focuses on TES, a form of distributed energy storage.) TES. That is because water storage can have cycle efficiency The critical factors of any storage device are application (type of 98%, simply because the lower ambient temperatures make and size), costs, cycle efficiency and longevity. the creation of the cooling more efficient at night, which makes up for the pumping power to transport the energy. This is true Applications for air-cooled chillers making ice9 since the drop in ambient All of the various types (Figure 1) of storage will likely be temperature at night is about the same as the drop in evaporator needed to replace the storage of fossil fuels each in various ap- temperatures to make ice. plications as appropriate. Flywheels, capacitors and specialty For larger ice systems that use high efficiency water-cooled chemical batteries can react instantaneously to provide ancillary chillers, storage cycle efficiency, relative to nonstorage waterservices for regulating power quality (e.g., keeping 60 cycle cooled chiller operation, may be about 25% lower, because the power at 60 cycles). Sodium sulfur (NaS) batteries have been ambient wet-bulb temperature drops much less when compared demonstrated8 to provide bridging capacity in the two to four to the drop in evaporator temperatures required to make ice. hour range. The solutions for storing large quantities of power However, the overall absolute efficiency of a water-cooled for longer durations are pumped hydro, compressed air energy chiller with storage will be better than an air-cooled chiller storage (CAES) and TES. Thermal energy storage is unique in with storage.
September 2010 ASHRAE Journal 29

Figure 27 compares storage technologies used in bridging power and/or energy management applications, and compares costs per kW output versus cost per kWh. The lower left hand quadrant is the lowest cost for both energy and capacity. The integrated thermal storage system is one of the most cost-effective storage options, and delivers benefits to both the transmission and distribution systems. Part of the reason for the large cost range is that when TES is integral to the design of a building cooling solution, other cooling equipment can be downsized or eliminated, reducing the overall capital equipment cost and, thereby, the cost per unit of TES. Naturally, as with all storage technologies, TES is designed for a specific purpose: peak load shifting of inductive motor loads used to provide cooling. The important point here is that it is dramatically less expensive to store cooling than it is to store electrons to create cooling.

ASHRAE Journal - September 2010

Table of Contents for the Digital Edition of ASHRAE Journal - September 2010