1、4697 An Air-Conditioning Model Validation and Implementation into a Building Energy Analysis Software Kamel H. Haddad, Ph.D. ABSTRACT An empirical model for a split air-conditioning system with an air-cooled condensing unit and an indoor evaporator unit is presented. The model is intended for hourly
2、 building energy simulations. Major inputs to the model are the cooling capaciq, coeficient ofperformance, andsensible heat ratio at rating conditions. Performance degradation at part-load conditions is accounted for The model is implemented in the ESP-r/H3Ksimulation engine. Model implementation in
3、 ESP- rH3K is validated through comparison to spreadsheet calcu- lations. In addition. model predictions from ESP-r/H3K are compared to results from other simulation programs and analytical solutions contained in an International Energy Agency set of test cases (HVAC BESTEST). The agreement between
4、the results from the simulation model in ESP-r/H3K and those from the HVAC BESTEST is very good. Results also show the importance of software testing and validation in increasing the confidence in simulation results. INTRODUCTION New simulation algorithms are implemented on a regular basis within ne
5、w and existing building energy simulation engines. The accuracy of the results obtained using these new algorithms depends, on the one hand, on the quality of the experimental data, assumptions, and derivations used to deduce the equations describing the model. On the other hand, the accuracy of the
6、 results also depends on the correct imple- mentation of the model into the simulation engine. During the model development and model implementa- tion phases, errors can occur that can lead to inaccurate results when using the simulation tool. As a result, validation of the model and its implementat
7、ion into a building energy simula- tion engine is very important. One validation method is to compare model predictions to experimental results. Another method is to compare results from the model to those from an analytical solution. Model validation can also be done through comparisons to results
8、from other simulation engines. In this paper, a simulation model for a simple split air- conditioning system is developed. The model is then imple- mented into the simulation engine ESPdH3K. The model implementation is validated by comparisons to spreadsheet calculations. In addition, results from t
9、he model implemented into ESP-dH3K are compared to results from other simulation engines and analytical solutions contained in the IEA HVAC BESTEST (Neymark and Judkoff 2002). AIR-CONDITIONING MODEL The model applies to a unitary split air-conditioning system consisting of an air-cooled condensing u
10、nit and an indoor evaporator unit. Mixed air from the conditioned zones and the outdoors flows on the outside of the evaporator coil by an indoor circulation fan. This fan can be placed either upstream or downstream of the evaporator coil. Figure 1 shows the various processes on the psychrometric ch
11、art for the case when the circulation fan is upstream of the coil: 1-2 2-4 4-0 5-4 Sensible cooling through coil 2-5 Latent cooling through coil Sensible heating through circulation fan Sensible + latent cooling through the coil Heating and humidification of the air inside the space State 3 is the a
12、pparatus dew-point temperature. State 1 is the result of mixing return air (state O) from the conditioned Kamel Haddad is a building energy simulation researcher, CANMET Energy Technology Centre, Ottawa, Ontario, Canada. 46 02004 ASHRAE. Table 1. Comparison of Capacity Predictions from Correlation t
13、o Manufacturers Equipment Data 19.4 Figure 1 Representation of the air-conditioning process through the coil on the psychrometric chart. 7.21 7.17 0.55 zones and outdoor air (state O). The figure also shows the line 0-0 for the mixing of return air with outdoor air. Input variables, listed in the “N
14、omenclature” section, to the model are Tdb, wl, Tdbo, SHRAR, VI, QARI, COPA, Qfan and Qtoai, load. 21.7 Correlation for Air-Conditioner Capacity model is at the following AR1 rating conditions: The steady-state cooling capacity QARI input to the Tdbl= 26.7OC Tdbo 35.0C Twb, = 19.4”C This capacity is
15、 the gross cooling capacity of the equip- ment. This is the total cooling capacity at the evaporator coil without correcting for any heat from the circulation fan. The cooling capacity Qc at other operating conditions is given by GC = OAR, XfCAp(TWb2, TdbO) . (1) 7.71 7.73 -0.25 The correction funct
16、ion is based on the correlation used in DOE2.1E (1994) for system RESYS: fCAP(Twb2, Tdb,) = 0.6003404 + 0.0022873 x Twb2 - 0.0000128 x Twbi + 0.0013898 x Tdb, - 0.0000806 x Tdbt + 0.0001412 x Twb, x Tdb, (2) The temperatures in this equation are in OF. The International Energy Agency series of test
17、cases for HVAC models by Neymark and Judkoff (2002) contains manufacturers performance data for an air-conditioning system. The data list values for the total cooling capacity of the equipment at various inlet wet-bulb temperatures and outdoor dry-bulb temperatures. Table 1 lists some of the manufac
18、- turers data contained in the IEA document and the predictions I 17.2 I 6.72 1 6.62 I 1.48 I I I I I 1 obtained using Equations 1 and 2. The percentage difference between the manufacturers data and the predictions from Equation 1 and 2 can be as high as 4.30%. Overall though there is good agreement
19、, indicating that the correlation described by Equations 1 and 2 is acceptable for predicting the capacity of the equipment at off-rating conditions. Correlation for Coefficient of Performance The input CPARItO the model is also at ARI rating condi- tions mentioned previously. The COP, at other oper
20、ating conditions is given by where the correction function is also based on a correlation for system RESYS in DOE2.1E (1994): f:;I:;) . Effectiveness is defined by It is assumed in this equation that the apparatus dew-point temperature T3 is a good estimate for the temperature of the refrigerant. Th
21、is is deemed a good assumption given the low thermal resistance associated with the tube walls of the evap- orator coil. Therefore, we have, Using the bypass factor at rating conditions BFARI, it is possible to estimate the ratio hConJ1IJCp of the heat exchanger. This ratio is then used to find BF a
22、t the new flow rate from Equation 26. Coil Bypass Factor at Rating Conditions As mentioned in the previous section, the quantity hCon_ JiiJCp is to be estimated from the value of the bypass factor at rating conditions BFARI, which is not an input to the model. However, BFARI can be deduced knowing t
23、he sensible heat ratio at rating conditions SHRARI, which is an input to the model. The procedure for deducing BFAMfrom SHRARIis very similar to that outlined previously for deducing SHR knowing BF. Cutoff Inlet Wet-Bulb Temperature for Dry Coil Conditions As mentioned previously, for every inlet dr
24、y-bulb temper- ature to the coil, there is a wet-bulb temperature Twb, above 49 which the coil surface is always wet. The wet-bulb tempera- ture Twb, needs to be determined to decide whether the coil is dry or wet. Given the inlet dry-bulb temperature to the coil, an inlet wet-bulb temperature and t
25、he corresponding humidity ratio are first assumed. Then using these values the sensible heat ratio is determined following the procedure outlined previously. In case the calculated SHR is greater than unity, the assumed wet-bulb temperature and humidity ratio is increased. In the other case, where t
26、he calculated SHR is less than unity, the assumed wet-bulb temperature and humidity ratio are decreased instead. Then the process is repeated again until the calculated SHR is an assumed tolerance away from unity. At this point, the assumed wet-bulb temperature is equal to the cutoff temperature Twb
27、,. Fan Electricity Consumption Assuming that the circulation fan is on when there is a call for cooling only, the electricity consumption of the fan during a time step At is The same expression can be used to determine the elec- tricity consumption of the outdoor condenser fan. Compressor and Outdoo
28、r Fan Electricity Consumption compressor and the outdoor condenser fan is given by The electricity consumption during the time step At of the MODEL IMPLEMENTATION The cooling model described previously is implemented into the energy analysis program ESP-r/H3K. When the prob- lem description has defi
29、ned an air conditioner, a subroutine is called to read all the necessary inputs for the model. Every time step during the simulation, when there is a need for sensi- ble cooling in the conditioned zones, the cooling model subroutine is called. The sensible cooling requirement of the conditioned zone
30、s is passed on to the cooling model subrou- tine. The current implementation is intended to work with a temperature controller specified in each of the conditioned spaces. When meeting the sensible cooling requirement of these spaces, the air-conditioner can also be subjected to a latent load removi
31、ng moisture from the zones. The major steps executed when the cooling model subroutine is called are the following: 1. Determine air properties at inlet of the coil Tdb2 and w2 using Equations 11 and 12. Twb, is then calculated. Effect of outdoor air on inlet conditions to the coil is also accounted
32、 for. 2. 3. 4. 5. 6. 7. 8. 9. Determine bypass factor BF at actual air flow rate using Equation 26. Determine cutoff wet-bulb temperature Twb,. If Twb2 is greater than Twb, then the coil is wet. In this case the total cooling capacity and COP of the coil are deter- mined using Twb, and Tdb,. Otherwi
33、se the coil is dry and the cooling capacity and COP are determined at Twb, and Tdb,. Capacity is determined using Equations 1, 2, and 5. COP is determined using Equations 3,4, and 6. Determine SHR. The sensible heat ratio is equal to 1 when coil is dry. At this time w4 is also determined. Find space
34、 latent load. It is assumed that the space sensible heat ratio is the same as that of the coil. Determine PLR and PLF using Equations 7,8,9, and 10. Determine electricity consumption of air conditioner and indoor circulation fan for the time step using Equations 27 and 28. If there is a latent load
35、at the coil, modi the moisture balance of each of the conditioned spaces to account for moisture removal by the au conditioner. 10. Include any outdoor air flow through the WAC system in the space energy and moishue balance. ALGORITHM VALIDATION The cooling model implemented in ESP-rH3K was vali- da
36、ted in two ways. First, the model was also implemented in an Excel spreadsheet. Hourly energy consumption of the air conditioner and the circulation fan from ESP-r/H3K were compared to those from spreadsheet calculations. This comparison can help uncover any mistakes made during the coding phase of
37、the model. The other part of the validation process was to compare model predictions to results contained in the IEA cooling test cases E100-200 manual (Neymark and Judkoff 2002). These test cases were specifically designed to test the accuracy of a simulation model for a simple unitary vapor-compre
38、ssion cooling system. There are a total of 14 test cases that test different aspects of an air-conditioner simulation algorithm under controlled load and weather conditions. All of the test cases are characterized by steady-state conditions inside and outside the conditioned space. Manufacturers dat
39、a on the performance of the air-conditioner are included in the test manual. The manufacturers data are supposed to be used in generating the results for all test cases. In addition, the test manual specifies the part-load performance degradation equa- tion of the equipment. All 14 test cases are su
40、ch that there is a deterministic analytical solution to each one of them. The base case E100 consists of a near adiabatic rectangu- lar zone. The cooling load in this case is driven by user-spec- ified sensible internal gains. The mechanical system is a split- system, air-cooled condensing unit with
41、 an indoor evaporator coil. The indoor circulation fan is in the draw-through position with respect to the evaporator coil. The characteristics of the 50 ASHRAE Transactions: Research Table 4. Summary of Test Cases E100-140 for Dry Coil Conditions Case E100 Tdb, Tdb, (“C) (OC) Description 22.2 46.1
42、Base case. High PLR E110 1 22.2 1 29: 1 High PLR. Low Tdb, vs E100 E120 26.7 High PLR. High Tdb, vs EI 10 E130 22.2 46.1 Low PLR vs E100 E140 22.2 29.4 Low PLR. Low Tdb, vs E130. LOW PLR vs El 10. Case E150 E160 E165 E170 E180 E185 E190 E195 E200 Table 5. Tdb, (“C) 22.2 26.7 23.3 22.2 22.2 22.2 22.2
43、 22.2 26.7 Tdb, (“C) 29.4 29.4 40.6 29.4 29.4 46.1 29.4 46.1 35.0 Summary of Test Cases E150-200 for Wet Coil Conditions Description High PLR. High SHR. Latent load vs E110. High PLR. High SHR. High Tdb, vs E150 High PLR. High SHR. Low Tdb, and high Tdbo vs E160 Medium PLR. Medium SHR. Low sen- sibl
44、e load vs E 150 High PLR. Low SHR. Low sensible load vs E150. High latent load vs E170. High PLR. Low SHR. High Tdbo vs E180 Low PLR. Low SHR. Low PLR vs E180. Latent load at low PLR vs E140. Low PLR. Low SHR. High Tdb, vs E190. Low PLR vs E185. Latent load at low PLR vs E130. ARi indoor Twbl at ful
45、l sensible and latent loads. mechanical system and the envelope of the zone are kept the same for all test cases. The remaining test cases are developed from the base case E100 by varying the following: Internal sensible gains internal latent gains Outdoor dry-bulb temperature Thermostat setpoint (i
46、ndoor dry-bulb temperature) Tables 4 and 5 contain a description of the test cases. Table 4 is for the test cases for dry-coil conditions and Table 5 is for those with wet-coil conditions. The IEA cooling test cases manual (Neymark and Judkoff 2002) contains results for all the test cases E 100-200.
47、 Results from six simulation programs (CASIS, CLIMA2000, DOE- $ &! $Q (UCASIS UCLlM2OoO UME-2 IE OMERGYPLUS OPROMETFEUS OTRNSYS .Analylytlcal UESP-rIKIK, Figure 2 Comparison of the zone humidity ratio from ESP- r/H3K and from the results in the IEA test case series 100-200. l I . . . . . . . . . . .
48、 . . . . . . . . . . Fgure3 Comparison of space total cooling load from ESP-r/H3K and from the results in the IEA test cases series 100-200. 2. lE, ENERGYPLUS, PROMETHEUS, and TRNSYS) are documented. Results obtained by analytical solutions are also presented. Figures 2 through 7 show results from t
49、he previous simulation programs and the analytical solution for seven of the fourteen test cases. Note that ENERGYPLUS does not provide results for the compressor electricity consumption, which is then not included in Figure 7. Results from ESP-r/ H3K are also shown in Figures 2 through 7. Results for the other test cases are not shown to iimit the size of this paper. Predictions from ESP-r/H3K agree very well with those from the other simulation programs and especially with those from the analytical solution. These results help increase the confi- dence in the fundamental b
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