HVAC systems must counteract weather forces when outdoor temperature

Transcription

1 ASHRAE JOURNAL The following article was published in ASHRAE Journal, March Copyright 1999 American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. New Weather Data For Energy Calculations By Lewis G. Harriman III, Member ASHRAE Donald G. Colliver, Ph.D., P.E. Fellow ASHRAE and K. Quinn Hart, P.E. Member ASHRAE HVAC systems must counteract weather forces when outdoor temperature or humidity moves away from an acceptable range for safety and comfort. Consequently, a clear understanding of weather behavior is helpful for system designers and operators. Shortcomings in that understanding are often at the root of problems involving poor indoor air quality and premature deterioration of buildings and HVAC equipment. Recognizing the economic benefit of avoiding such problems, organizations such as ASHRAE, the U.S. Department of Defense and industry-funded research institutions have recently invested over $1,000,000 in engineering weather data research. This article describes some of the products of that research, a selection that is necessarily limited by space constraints of the magazine. Readers are encouraged to share what they consider to be important sources and uses for weather data by writing letters to the editor for possible publication in future issues. Peak Load Data Chapter 26 of the 1997 ASHRAE Handbook Fundamentals contains the results of ASHRAE research project 890- RP, which defined new peak design conditions for sizing equipment. 1, 15 The revised and expanded information repre- sents a significant advance over the 20 to 40-year-old data contained in the 1993 Handbook, some of which was obtained through graphical interpolation of as little as five years of data. Annual Not Seasonal Extremes Data contained in 1993 and earlier editions were calculated at different times using different methodologies. For example, Canadian cooling data was based on the extreme for July alone, while U.S. data was based on the extreme for the summer season consisting of June, July and August. To ensure uniformity for worldwide calculations, the 1997 data is based on annual extremes rather than seasonal or single-month peaks. For example, earlier handbooks showed the dry bulb temperature exceeded for 1% of the hours during the summer period. Now, the 1997 Handbook shows the temperature which is exceeded for 0.4% of the entire year s observations. Likewise, the old 99% winter seasonal dry bulb for heating has been replaced by the new 99.6% annual value. The percentiles of 0.4, 1 and 2.5% for cooling and 99.6% and 99% for heating were chosen because they yielded values which, for most stations, corresponded closely to the earlier seasonal extremes. So the new annual values are close to, but seldom the same as the old seasonal extremes. The user can expect that variations result more from the different calculation methods rather than from any significant climate change. This new methodology is especially useful in light of ASHRAE s commitments to its international members in population-dense tropical climates. The summer and winter seasons near the equator occur during different months compared to seasons in continental locations like Canada and Argentina. Therefore, annual rather than seasonal calculations are more appropriate for a uniform worldwide methodology. International Coverage Earlier handbooks reflected ASHRAE s intense focus on U.S. and Canadian sites About the Authors Lewis G. Harriman III is director of research at Mason-Grant, Portsmouth, N.H. He is the Chair of the Handbook Subcommittee of ASHRAE Technical Committee 3.5, Desiccant and Sorption Technologies and is the author of The Dehumidification Handbook. Donald G.. Colliver,, Ph.D.,., P.E.E.,., is professor of engineering in the Biosystems and Agricultural Engineering Department of the University of Kentucky in Lexington. He was the principal investigator for ASHRAE Research Project 890, which produced the new climatic design data in the 1997 ASHRAE Handbook Fundamentals. K.. Quinn Hart, P.E.E.,., is the chief mechanical engineer of the U.S. Air Force at the USAF Civil Engineering Support Agency at Tyndall AFB in Panama City, Fla. He was the project officer for the revision of the Department of Defense Engineering Weather Data, USAF Handbook March 1999 ASHRAE Journal 31

2 rather than international locations. For example, the 1993 Handbook contained data for two locations in Rhode Island. That data density is useful. However, at the opposite extreme, the entire country of China, with a population of 1.2 billion people and a land mass larger than the United States, was also covered by data for exactly two locations. The 1997 Handbook reflects the society s intention to become a more international organization. Coverage outside the United States and Canada has expanded from 243 locations to 801. Unfortunately, some previously listed domestic and international locations were dropped from the new handbook. Raw data available to ASHRAE for those locations did not meet new uniform standards for completeness, and/or the length of their periods-of-record. Projects are underway in some parts of the world, notably the Indian subcontinent and Australia, to locate better raw data and to calculate design values for more locations using the internationally consistent methodology developed during the ASHRAE research project. Correct Humidity Extremes Somewhat surprisingly, and possibly because of the emphasis on temperature rather than moisture control, older handbooks did not contain data describing extremes of humidity. The 1993 and earlier Figure 1: Design conditions are useful for sizing equipment. However, hourly records are used for simulating controls and estimating energy use, because design conditions occur less than 1% of the equipment s life. 13 handbooks showed only the average humidity during periods of extreme temperature. Those values do not represent the extreme humidity, which occurs at moderate temperatures during rainstorms or during early morning as dew evaporates. The humidity misimpression created by focusing on high temperature was often quite significant. An example can be seen in the listing for Huntsville, Ala., shown in Figure 2. The high dry bulb temperature is 94 F (34.4 C) with an average coincident wet bulb temperature (MWB) of 75 F (23.9 C). Those peak dry bulb values create the impression that the extreme humidity ratio is 100 gr/lb (14.3 g/kg). In fact, the true peak moisture is much higher at 135 gr/lb (19.3 g/kg), as shown in the 0.4% dew point columns. These columns also show that the peak moisture occurs at an average dry bulb temperature (MDB) of 83 F (28.3 C) rather than at 95 F (35 C). That represents a significant reduction in the sensible heat ratio for a cooling coil which pre-treats outside air, and probably suggests a different equipment selection for such applications. Having correct peak moisture data should allow improvements in equipment Figure 2: New design data for sizing dehumidification and cooling equipment from the 1997 ASHRAE Handbook Fundamentals. 32 ASHRAE Journal March 1999

3 WEATHER DATA and systems for dehumidification. The chapter also suggests using the peak humidity condition as a part-load check point for cooling systems that control humidity as a secondary function. Another potential use for the peak humidity data involves ventilation air, which is usually loaded with two to six times more moisture than sensible heat. 2 Because the enthalpy is highest at the peak moisture condition (except in desert and high-altitude climates), engineers might consider sizing ventilation preconditioning equipment at the peak dew point condition rather than the peak dry bulb temperature. 3 Peak Loads for Extended Periods The handbook describes extremes that are exceeded for 0.4, 1.0 and 2.0% of the 8,760 hours of the year (35, 58 and 175 hours). Because those hours do not occur together, this presents a problem for sizing thermal storage and power storage systems, where the engineer must know the continuous duration of the peak load. Duration is also important for analyzing night setback or off-peak precooling and predrying strategies. Those techniques use the thermal inertia and moisture capacitance of the building to conserve energy or to store cooling or dehumidification effect ahead of expected peak periods. ASHRAE research project 828- RP was completed in 1997 to address these needs. 16. The researchers identified extreme weather patterns, which lasted for one, three, five or seven 24-hour periods in a continuous sequence. Separate sequences are defined for extremes of dry bulb, dew point, enthalpy and low wet bulb depression. Those hourly observations are stored on a CD-ROM, together with a computer program, which displays summaries of the data as well as the actual hourly data for each extreme sequence. The CD-ROM includes data for 381 locations in the United States and Canada. 4 Hourly Data for Troubleshooting and Analysis Although extreme design data allows the engineer to size the equipment, it does nothing to quantify how much energy the system will use in a year, nor does it help the operator understand how the system will respond to changing loads. Hourly weather observations are used for analyzing system behavior during the 99.6% of the hours in a year that the weather does not create the peak loads. New sources for hourly data are available in three formats: current observations, historic observations and historic summaries. Current Hourly Observations for Troubleshooting Engineers, service technicians and building operators constantly analyze and troubleshoot normal problems of HVAC systems. Often, knowing the current and recent weather conditions can help explain the cause of a problem, and lead to a faster solution. Recently, the World Wide Web has become a source for such current observations. Figure 3 shows the results of a query of the hourly weather archive maintained for public use by an industry-funded research Figure 3: Current hourly weather downloaded from a web-based weather archive located at 5 institution. 5 The archive is available at no charge, and contains simultaneous hourly observations of dry bulb temperature, humidity ratio, barometric pressure and wind speed for 240 locations in the United States and Canada. The archive is updated 24 hours a day, 365 days per year, with records beginning on Feb. 1, Any series of hourly observations since that date can be downloaded as a tab-delimited electronic file for importing into spreadsheets or other computer analysis tools. Historic Hourly Observations for Energy Analysis Full-year, 8,760 hour energy analysis is becoming more important than in the past, because computing power is so inexpensive, and because power costs can now vary throughout a single day rather than changing only on a seasonal basis. There are two categories of hourly observations: actual year and typical year. Actual hourly observations are available from many sources, including the U.S. National Climatic Data Center and the Industrial Climatology Division of Environment Canada. 6, 7 Using historic observations, system simulations can be run for a specific year in the past. An engineer could estimate how a new system design would have behaved, for example, in the actual conditions experienced during On the other hand, actual records also reflect the normal problems encountered in large-scale data acquisition. Some records are missing data elements for a few hours. They may also contain zero values, which reflects the fact that the data gathered during those hours was outside the checkpoint boundaries and therefore too inaccurate to include in the record. For many computer programs, missing data elements or unexpected zero values can cause problems. Also, the weather for one particular year is not likely to reflect the average weather over the 15-year life of the system. So many engineers prefer to use typical hourly observations instead of records for a single March 1999 ASHRAE Journal 33

4 specific year. In the U.S. and Canada, popular formats include Typical Meteorological Year (TMY and TMY-2), Weather Year for Energy calculations (WYEC and WYEC-2) and Canadian Weather for Energy Calculations (CWEC). TMY and CWEC records are constructed by examining the weather records for a series of past years, and then calculating how typical are the months in each year. In other words, the records for all Januarys are compared to each other, and the most average January is combined with the most average February, and so on until 12 typical months are assembled in a single 8,760-hour record. The most typical January might be from 1962; February may be from 1976 and so forth. Mathematical smoothing is applied to the data at the beginning and end of each month to avoid abrupt changes in values. Figure 4 shows an example of a partial extract from a TMY-2 record, imported into a spreadsheet. 9 The most recent TMY-2 methodology is well documented 8 and repeatable, and the program that examines the historic records can be adjusted to weight different weather parameters more strongly as it searches for typicality. For example, in 1995 the National Renewable Energy Laboratory constructed 239 TMY-2 records for U.S. locations. 8, 9 In those files, the primary selection criterion was completeness of the solar record, and dry bulb is weighted slightly more strongly than wind speed in the search for typicality. So in those records, the temperature will be slightly more average than the wind speed. The original WYEC records were generated for 56 U.S. cities by ASHRAE research in the 1970s. 10. The methodology reflected the resources of the society at the time. An expert examined actual records. Hand adjustments were made when data was missing, or when just averages rather than observed data were available for a particular variable. Consequently, the methodology is less well documented, and difficult to repeat. The newer WYEC-2 records 10 were constructed for 76 U.S. cities using a methodology similar to that used for the 239 TMY-2 locations. In Great Britain, an Example Weather Year (EWY) data set is available for 15 Figure 4: An extract from a TMY hourly record, imported into a spreadsheet. 9 locations. A complete year of representative weather data for these locations was selected by the Chartered Institution of Building Services Engineers (CIBSE). 18 From the available set of weather years, a smaller group of candidate years was chosen by examining the value of several parameters during each month. Candidate years had values, which were within two standard deviations of the long-term monthly averages for those parameters. The single year selected for each location had values with the minimum total deviation of all parameters from their long-term averages. Typical hourly weather for other European locations are contained in the data set of Test Reference Years (TRY) constructed by the Commission of the European Communities (CEC). 19 These years were compiled using individual months that were selected as average using a methodology similar to that used in the United States and Canada to compile the TMY and CWEC records. Typical hourly weather data for locations outside the United States, Canada and Europe are not easily available in the public domain. As a substitute for typical data, the International Surface Weather Observations (INSWO) set of actual hourly records for 1,500 sites is available through the U.S. National Climatic Data Center. 6 In addition, some commercially available building energy analysis software includes actual 8,760-hour records for Latin American and Asian locations embedded in the program, but these proprietary data are not available as separate Figure 5: A joint frequency table for hours at each temperature and humidity ratio, imported into a spreadsheet. 9 files. 12 Current ASHRAE research is addressing this issue. Typical hourly records for 200 non-north American locations will be published within two years. Summaries of Hourly Data When less precise estimates of energy consumption are adequate, summaries of hourly data are used in place of a full 8,760-hour record. These are available in many forms. The typical day summary format is used in some computer programs published by a major manufacturer of HVAC equipment. 11 Weather data embedded in these programs is not actually 8,760 observations. It is a summary of one or more typical 24-hour days for each month. For example, January might be represented by the same single day, repeated 31 times. Another popular format is the BIN summary. These summaries indicate the number of hours a single variable is within a specified range (the BIN width ). For example, a dry bulb summary indicates the number of hours in which the dry bulb temperature falls between the upper and lower boundary of each BIN (usually between 2 F and 5 F [1 C and 2 C]). Often, a BIN summary will also contain the average value of other variables during the hours in each BIN. For example, the dry bulb summary might contain the average wet bulb for hours in each BIN. Traditional BIN summaries are limited in some respects. They allow reasonable calculation of loads, which depend on the primary BIN variable, but they do not re- 34 ASHRAE Journal March 1999

5 WEATHER DATA flect the loads dependent on any average coincident variable. For example, a dry bulb BIN summary with average wet bulb estimates sensible heat loads, but underestimates moisture loads by 25% to 45%. Recently, computer tools have become available to allow construction of BIN summaries by variables other than dry bulb temperature, 9, 17 but any BIN summary will underestimate the magnitude of loads dependent on average coincident variables. Joint-frequency tables do not have this limitation and sometimes are used as an alternative to BIN summaries. A joint frequency table sorts the hours of the year and places the hour-counts into a two-dimensional matrix. One can visualize a joint frequency table for temperature and humidity as an overlay for a psychrometric chart. Total hours occurring at each combination of temperature and humidity can be placed in a matrix over the appropriate points on the chart. Figure 5 shows such a joint frequency table. 9 The advantage of this type of summary is that two variables, not one, are appropriately quantified for the year. Neither value is the product of averaging. The recent ASHRAE research project revising the peak design data in the Handbook Fundamentals involved the creation of joint frequency tables for all the locations displayed in Chapter 26 of that volume. The society is planning to publish these monthly and annual joint frequency tables of dry bulb/wet bulb, dry bulb/ enthalpy and dry bulb/wind speed in a CD-ROM format during the coming year. 17 Both BIN summaries and joint frequency tables are limited in that they lose the time sequence of climate conditions such as the hour-by-hour history of temperature and solar radiation. Thus, they are not used for estimating any thermal or moisture lag effects in buildings. Figure 6: The new electronic version of the Department of Defense weather data contains graphics, as well as numerical values, allowing a designer to quickly compare climates at different locations. 13 Understanding Weather and Climate Dynamics One of the greatest challenges for a designer is gaining an overall understanding of the climate in an unfamiliar location. In both government and commercial building practice, design services are increasingly centralized to save costs and administrative overhead. But such centralization virtually guarantees that important school-of-hard-knocks knowledge about local climate behavior is less available to the remotely located designer, sometimes with costly consequences. A standard procedure for selecting cooling equipment for a restaurant in Chicago may not scale well for the same restaurant when built in Puerto Rico, with its ten-times larger latent load. A military dormitory design that works well in New Hampshire may not perform well in Alaska. Both are cold climates, but Alaska has farlarger drifting snow loads, and a foundation will crack if the building melts the permafrost. To help the designer gain a rapid, quantified understanding of local climate behavior, the U.S. Department of Defense recently redesigned its Tri-Service Engineering Weather Data Manual, widely known as The Blue Book for the color of its cover. The result has been published in electronic form. Information is available for 511 U.S. locations and 292 international sites. 13 Figure 6 shows one example of the extensive graphics available for each site. By comparing graphics for a new location with the same graphics for a familiar location, a designer can gain a fast understanding of the differences between the two sites. The numbers behind these and other graphics can be exported from the file by cutting and pasting the values into other computer programs for quantitative analysis. Finally, HVAC designers and system operators can often benefit from understanding the dynamic nature of weather behavior. We design at steady-state conditions, but systems are never at the equilibrium conditions we must assume for design. Weather March 1999 ASHRAE Journal 35

6 Ue se I te m D ata Typ e C overag e R emark s Publishe r Sizin izing Equipment 1997 ASHRAE Handbook Fundamentals Sequences of extreme temperature and humidity Long-term Extremes 1,459 U.S. and Int'l Locations 320 U.S. and Canadian Locations All-new data, now includes humidity extremes Extremes with durations of 24, 72, 120 and 168 consecutive hours 1, 9 4 Monitoring and Troubleshooting Installed Equipment Hourly weather archive data Curren urrent Hourly DB and gr/lb 240 U.S. and Canadian Locations Internet archive, with downloadable data 5 TMY-2 Typical meteorological years 239 U.S. Locations with Puerto Rico Includes calculation software for BIN summaries and ventilation loads 9 WYEC-2 Weather years for Energy Calculations 76 U.S. locations Updates 1970-era WYEC files 10 Estimating Long- term Behavior and Energy Consumption CWEC Canadian Weather Year for Energy Calculations EWY Example Energy Year Typica ypical hourly observations 145 Canadian locations 15 locations in Great Britian Can be used to meet provisions of the draft Canadian Natl. Energy Code Actual years, selected typical of the location to be 7 18 TRY Test Reference Years and DRY Design Reference Years 156 locations in Europe, Russia and Turkey Data and formats vary between countries, not all locations available from a single publisher 19 SAMSON Solar and Meteorological Surface Observation Network 237 U.S. locations May include three-hour averages in place of hourly observations 6 Simulatin imulating Equipment Behavior for a Specific Year CWEEDS Canadian Weather for Energy and Engineering Actua ctual hourly observations for specific years 145 Canadian locations File format same as WYEC-2 7 INSWO International Surface Weather Observations 1,500 worldwide locations Often includes hour averages only three- 6 Table 1: Common types and sources of engineering weather data. and internal loads change minute-byminute in highly complex ways. For those who would like to gain a rudimentary understanding of weather dynamics, references are available at low cost to educate the layman. 14 Summary In summary, our industry now has better, more extensive and less expensive engineering weather data than ever before. That data comes to us courtesy of expenditures by United States and Canadian taxpayers, member donations to ASHRAE research and from industry-funded research institutions. Over the next few years, it will be interesting to observe how system designers and equipment manufacturers use this improved information to benefit building owners and occupants. References 1. Climatic Design Information. 1997ASHRAE Handbook Fundamentals, Chapter Harriman, L. G. III, D. Plager, D. Kosar. Dehumidification and cooling loads from ventilation air. ASHRAE Journal 40(11): Harriman, L. G. III New peak moisture design data in the 1997 ASHRAE Handbook of Fundamentals. Proceedings of the Symposium on Improving Building Systems in Hot and Humid Climates. Energy Systems Laboratory, Texas A&M University. 4. ASHRAE Design Weather Sequence Viewer Version Hourly Weather Data Archive Chicago: The Gas Research Institute, 6. Solar and Meteorological Surface Observation Network (SAMSON) National Climatic Data Center of the National Oceanographic and Atmospheric Administration of the U.S. Department of Commerce, 36 ASHRAE Journal March 1999

7 WEATHER DATA Asheville, N.C. (828) Canadian Weather Energy and Engineering Data Sets (CWEEDS) Environment Canada, Atmospheric Environment Service 4905 Dufferin Street Downsview, ON M3H 5T4 (416) TMY-2 Methodology. National Renewable Energy Laboratory of the U.S. Department of Energy. Building Energy Technology Program 1617 Cole Blvd. Golden, CO (303) Typical Meteorological Years (239 U.S. cities). (TMY-2) BinMaker PLUS CD-ROM. Batavia, Ill.: Gas Research Institute (773) WYEC-2 Weather Year for Energy Calculations (76 U.S. cities). ASHRAE. 11. Trace 600 Building Energy Analysis Program. The Trane Company. 12. Hourly Analysis Program (HAP) Carrier Corporation. Syracuse, N.Y.:Software Systems. 13. Department of Defense Engineering Weather Data USAF Handbook (I). Tyndall AFB, Fla. (850) Williams, Jack. The Weather Book (2nd Edition) 1997 Vintage Books, New York: Random House. ISBN Colliver, D. G., Gates, R. S., Zhang, T.F. Burkes and Priddy, T.K Updating the Tables of Design Weather Conditions in the ASHRAE Handbook Fundamentals. 16. Colliver, D.G., R. S. Gates, H. Zhang, and T.K. Priddy Sequences of Extreme Temperature and Humidity for Design Conditions. ASHRAE Transactions 104(1): Colliver, D. G., T. F. Burks, and R. S. Gates ASHRAE Design Weather Data Viewer. CD-ROM. 18. Irving, S. J The CIBSE Example Weather Year. Weather Data and Its Applications A Symposium for Building Service Engineers. London: Charted Institution of Building Services Engineers. CIBSE Test Reference Years TRY, Weather Data Sets for Computer Simulations of Solar Energy Systems and Energy Consumption in Buildings. Commission of the European Communities, Directorate General XII for Science, Research and Development, Brussells, Belgium Please circle the appropriate number on the Reader Ser- vice Card at the back of the publication. Extremely Helpful Helpful Somewhat Helpful Not Helpful Advertisement in the print edition formerly in this space. March 1999 ASHRAE Journal 37