The use of Thermal Energy Storage t

The use of Thermal Energy Storage to meet mankind’s needs is not new. Many ancient civilizations harvested ice from frozen rivers and stored it for all sorts of cooling requirements. Buildings were built with huge amounts of internal and external mass to dampen the diurnal ambient temperature swings. The domestic hot water storage tank is the most common use of TES which has achieved essentially 100% market penetration. With the ever increasing importance in demand reduction and stability of our electric grids, Thermal Energy Storage is once again becoming a valuable technology for air-conditioning buildings. However, what is also becoming apparent are the major “Green” benefits of shifting electric usage to off-peak hours, which include lowering source energy usage and emissions.

For decades Off- Peak Cooling (OPC) systems that use thermal energy in the form of ice or chilled water, have been used in the commercial cooling market place. The 80’s and 90’s were the developmental years of the technology with over 6,000 installations around the world. The industry has matured, refining design methods and producing systems that are reliable and cost effective which have the ability to:

• Reduce Peak Demand at most critical time 20-40%
• Reduce installed cost up to 10% [1]
• Reduce consumer’s energy costs 10-20% [2]
• Reduce energy usage at the building up to 14%
• Reduce source energy usage at power plant 8-34%[3]
• Reduce emissions up to 50%
• Increase Load Factor of Generation up to 25%

• Provide operational flexibility

The value of storage is most clearly shown by what it does for the Load Factor (average load divided by the peak load) of the building. Figure 1a[4] represents a building that is designed with many energy efficiency features which we are all familiar with, integrated together into an effective holistic design. Figure 1b is the same building with a storage system added. Though the site energy use for each of the two designs will be essentially the same, the Load Factors are substantially different (53% vs. 88%).

Figure 1. Load Factors change dramatically by adding storage, thereby lessening impact on Grid (Society)

With the simple addition of storage to all buildings needing air-conditioning, 40% fewer power plants and smaller transmission and distribution lines would be required. (Obviously this is only the case with utility grids that peak in the summer months which is increasingly the case around the world.) This highlights one major reason why cool storage has not made major penetration into the market place. In almost all “energy efficiency” building benchmarking systems around the world, Building Load Factor is not measured or reported; therefore little importance is placed on it. Utilities have tried to get the message across with rates which are normally much less expensive at night because of the simple laws of supply and demand. A simple example shows why the variation in day and night costs will continue, even though in a few areas of the world this is not the case now.

Assuming four buildings (non-storage buildings) that have a peak load of 1 megawatt, a utility would need a 4 megawatt (MW) generator and would sell 8,000MWh a year. If the peak demand of the 4 buildings where reduced by 20% to 0.8MW each (storage can actually take off 40% of a building’s peak), utility could supply another similar building and now be selling 10,000MWh/year with the same generator. This equals 25% increase in generator Load Factor which is a huge savings to a utility. Although there are some small areas that have “flat rates” because they have overbuilt the electric supply, it is only a matter of time before large differences between day and night prices are ubiquitous[1]. Therefore, for the building owner, the simple reason to use Off-Peak Cooling is that it costs them less to cool their building, however the benefits extend much further than that, as will be explained later.