OCR Text |
Show A number of recoverable reject heat sources are available in industry. Generally, they can be classified into three general categories: gas streams, water streams, and product cooldown. A recent survey (8) of industries in the Pacific Northwest indicates that gas streams contain the greatest amount of reject heat. The results of this study also point to the wide temperature range over which industrial reject heat is available (Table 1). This supply temperature is important when designing a TES system since higher storage temperatures generally require more costly storage equipment. For example, water/steam systems are generally competitive up to about 175°C. However, at temperatures above 175°C rock storage systems are generally the economic choice. Also, when sources are available that span a fairly large temperature band, a staged storage system may be employed to fully utilize the reject energy source. Source temperature5 although inportant, is not the only design consideration. Perhaps more important is the use for the stored energy. These uses generally fall into four general categories: raising process steam, raising steam for electrical generation, feedwater heating, and preheating of combustion air or raw materials. These uses can be classified into three broad temperature ranges: process steam and feedwater heating, 100- 200°C; electrical power production 400-600°C; and preheating and drying with a useful temperature range of 100-1000°C. Clearly, there is little advantage in storing heat at 600°C when the use calls for energy at 200°C. Thus, the TES design is controlled more by the use than the supply. Storage time also has a significant bearing on the economics of the application. The frequency of use affects the justifiable cost oi the TES system since frequent use results in larger energy and economic savings per unit of. storage capacity (hence per dollar of capital outlay.) Peripheral equipment such as heat exchangers and thermal transport piping are sometimes crucial. Exhaust gases are frequently corrosive and contaminated with particulate matter thus presenting heat exchange problems. Often the sources of reject heat and the potential uses are not in close proximity. Therefore, thermal transport can be a limiting factor. Most of these considerations, of course, must be resolved on a case-hv-case basis. However, it is important to note that a number of opportunities exist where reject energy can be recovered and via a TES unit be reused for 8-5 |