This paper has been downloaded from the Building and Environmental Thermal Systems Research Group at Oklahoma State University (www.hvac.okstate.edu) The correct citation for the paper is: Spitler, J.D., D.C. Hittle, D.L. Johnson and C.O. Pedersen. 1987. A Comparative Study of the Performance of Temperature-based and Enthalpy-based Economy Cycles, ASHRAE Transactions. 93(2): 13-22. Reprinted by permission from ASHRAE Transactions (Vol. #93, Part 2, pp. 13-22). © 1987 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.No. 3055A COMPARATIVE STUDY OF THEPERFORMANCE OF TEMPERATURE-BASEDAND ENTHALPY.-BASED ECONOMY CYCLESJ.D. SpitlerASHRAE Associate MemberD.C. Hittle, Ph.D.ASHRAE MemberD.L. Johnson, Ph.D.ASHRAE MemberC.O. Pederson, Ph.D.ASHRAE MemberABSTRACTDeciding whether to use an economy cycle and then whether to use a temperature-based or an en-thalpy-based economy cycle is a problem that many HVAC designers face today. The controversyabout ,which type of economy cycle should be used was demonstrated at a recent ASHILAE meetingwhen one author recommended the use of enthalpy-based economy cycles (Sud 1984) while anotherdiscouraged their use in favor of temperature-based economy cycles (Haines 1984.) This studyquantitatively examines the potential energy savings for two prototypical buildings due toboth types of economy cycles. This was done in a systematic manner in order to give designersthe guidance that they need.Two different buildings, one more core-dominated than the other, were studied at fivedifferent locations, using the Building Loads Analysis and System Thermodynamics (BLAST) pro-gram (Hittle 1979). Two different types of systems, terminal reheat and variable air volume(VAV), were simulated, each with and without economy cycles. For all locations exceptHouston, the temperature-based economy cycle achieves most of the savings that can be achievedby the enthalpy-based economy cycle.INTRODUCTIONThe use of economy cycles is a popular means of saving energy in HVAC systems. Basically, aneconomy cycle involves bringing in excess outdoor air in order to utilize "free" cooling whenthe outdoor temperature is low enough. Two means of deciding when to bring in excess outdoorair were studied. The first is based on a comparison of the outdoor air and return-air dry-bulb temperatures. If the outdoor-air temperature is colder than the return-air temperature,then outdoor air is used to the extent necessary to maintain the mixed-air temperature asclose as possible to the controller’s set point. If the outdoor-air temperature is higherthan the return-air temperature, minimum outdoor air is used. The enthalpy economy cycle issimilar, but it compares the outdoor-air and return-air enthalpies.The subtle advantage of the enthalpy economy cycle is that it keeps the outdoor-airdamper at its minimum position under those conditions when the outdoor temperature is coolerthan the return-air temperature but the outdoor humidity is so high that the enthalpy of theoutdoor air is higher than the enthalpy of the return air. Therefore, we can expect the en-thalpy econom.y cycle to have advantages in climates where it is often moderate but very humidduring the daytime operating hours of the buildings. The enthalpy economy cycle also allowsthe continued use of outdoor air if it is warmer than the return air but so dry as to havelower enthalpy. This may or may not be advantageous and will be discussed later in thepaper. If neither of these conditions occurs frequently, temperature alone will be a suitableindication of enthalpy .....J. D. Spitler is research engineer and C. O. Pedersen is associate professor, Department ofMechanical and Industrial Engineering, University of lllinois, Urbana-Champaign; D. C. Hittleis associate professor of mechanical engineering, Ray Herrick Laboratory, Purdue University,West Lafayette, IN;D. L. Johnson is principal investigator, ES Division, U. S. Army Construction EngineeringResearch Laboratory, Champaign, IL.Temperature-based economy cycles use some type of temperature sensor, such as a platinumresistance temperature detector (RTD). Enthalpy economy cycles require both a temperaturesensor and some type of humidity sensor. Sensors used for temperature are, in general, simp-ler and much more reliable than those used for measuring humidity. Humidity sensors, on theother hand, tend to drift from calibration and have extensive maintenance requirements. Prob-lems with humidity sensors and enthalpy economy cycle controls are discussed by Hittle andJohnson (1985).A study of the effects of sensor errors on building energy performance (Kao and Pierce1983) demonstrated that moderate humidity sensor errors (10 F error in dew point temperature)can cause significant increases (up to 13%) in system energy consumption. An increase system energy consumption of this m~gnitude will negate any additional savings obtained by us-ing an enthalpy-based economy cycle-instead of a temperature-based economy cycle. This find-ing highlights the need for an extremely accurate humidity sensor if one wishes to use an en-thalpy economy cycle.The objective of this study was to determine potential energy savings that could beachieved by temperature-based economy cycles and enthalpy-based economy cycles. The potentialenergy savings were determined through simulation, using the Building Loads Analysis andSystem Thermodynamics (BLAST) program (Hittle 1979).The first step in this study was to select two prototypical buildings. Five locations inthe United States were chosen, and both buildings were simulated in each of the five loca-tions. For each building/location, six systems were simulated: (1) a terminal reheat systemwith no economy cycle, (2) a terminal reheat system with a temperature economy cycle, (3) terminal reheat system with an enthalpy economy cycle, (4) a VAV system with no economy cycle,(5) a VAV system with a temperature economy cycle, and (6) a VAV system with an enthalpy econ-omy cycle. The economy
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