ASHRAE Journal - October 2010 - (Page 8)
Visions of Computers of the Future
Cleaning up my library, I found the booklet “Computer Applications for Air-Conditioning Engineers,” which is a collection of symposium papers presented at ASHRAE’s semiannual meeting in 1968. I am amused about some of the statements, but I am astonished to find several visionary predications. In the introduction: • “Although many air-conditioning engineers may never directly use a computer, methods for calculating heatingcooling loads…may be the result of computer programs.” In “Computers in a Consulting Engineer’s Office”: • “There is a gap between the effective computer user and the non- or ineffective computer user, and the gap is widening at an ever-increasing rate.” • “Most equipment manufacturers would like you to believe they can make a programmer out of everyone in your organization with an afternoon of training…it is something else to write the program that can really serve you and run efficiently.” • “The computer isn’t going to be sideline; it is going to be an integral part of an organization and needs to be understood by everyone.” In “Computer Capabilities in Air-Conditioning Problems”: • “The engineer needs to know what assumptions…the program is using…to have a…feeling of the accuracy of the answers.” I completed my first programming course in FORTRAN in 1968. I feel young engineers, who grow up with the personal computer, should also get a feeling of the tremendous development in IT during those 40+ years. Robert Guery, Life Member ASHRAE, Switzerland tion on site by a steam engine or steam turbine. This is nothing but combined heat and power (CHP). Therefore, if we install steam boilers in buildings, steam must be used first in high-exergy applications and followed by space heating and domestic hot water applications. Otherwise, there is no point to generate steam upstream for building heating. After all, a condensing boiler has higher first-law efficiency. Furthermore, small gas engines are commonly used in condensing boilers, using natural gas to produce electric power and then provide waste heat for comfort. These are known as micro-CHP systems. In addition, district energy systems, waste heat technologies, and solar/geothermal systems are needed to replace fossil fuels for heating and cooling. The debate should go beyond steam boilers to how to use fossil fuel exergy in the best possible manner, and how to replace fossil fuels with sustainable resources in present and future buildings. Birol Kilkis, Ph.D., Fellow ASHRAE, Ankara, Turkey
ashrae.org October 2010
Exergy and Steam Heat
The letters to the editor regarding May’s “Water & Energy Use in Steam-Heated Buildings” by Ian Shapiro seem to debate whether steam boilers are a good choice for building heating. I do not think this issue may be resolved conclusively based on water use, thermal efficiency, and heat transfer characteristics. According to the first law of thermodynamics, a condensing boiler may have a higher thermal efficiency than a steam boiler. Yet the comparison stops here. We need to look at the second law of thermodynamics. The second law deals with the useful work potential (exergy) of a given amount or flow of an energy source. Consider a natural gas-fired condensing boiler. Natural gas is a high-exergy resource. Its flame temperature is in the order of 1800°C in a typical combustion process. According to the Carnot cycle, almost 85% of a given amount of natural gas may be used as useful work in many temperature-cascaded applications (power generation, process steam, process heat). If we directly use natural gas for building heating, all upstream useful work opportunities (and sometimes downstream application opportunities) are destroyed. As a result, about 95% of the available exergy of the natural gas is destroyed. This is due to the striking imbalance between source and demand exergy. Therefore, with a condensing boiler, the exergy efficiency is about 5%, although its first law (thermal) efficiency may peak at 97%. Alternatively, heating could use low-exergy sources at temperatures as low as 50°C. The interim conclusion shows a better exergy balance is essential by introducing other useful applications upstream from building heating. If we use natural gas to generate steam, which is a high-exergy source, it appears that a better exergy balance is established using high-exergy natural gas to generate high-exergy steam. But, using steam only for building heating returns us to square one. Steam is a high-exergy source used for a low-exergy demanding application, which generates a similar exergy imbalance as in the first example. So, a steam boiler has the same low-exergy efficiency. The only difference is that exergy destruction takes place one step downstream in the process. Therefore, according to the second law of thermodynamics, if fossil fuels are involved, the debate may not be based on simply whether to use a steam boiler instead of a condensing boiler, etc. The common sense of exergy shows us that once steam is produced, it should at least be used first in electric power genera8 ASHRAE Journal
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