Dynamic Simulation of an Office Building in Two Different Softwares

Transcription

1 Dynamic Simulation of an Office Building in Two Different Softwares Joana Filipa Das Neves Cartas * * joana.cartas@gmail.com Abstract: This project has the goal of performing a comparative analysis of the simulated consumptions, electric and thermal energy, between two dynamic simulation softwares: EnergyPlus with DesignBuilder as an interface and TRACE 700. The models, developed in each software, were validated through the comparison between the simulated results and the measurements done by the electric and thermal energy distribution company, in which the invoices are based. In the end the results were compared and the electric equipment ratio was calculated in order for the simulated results to be ± 10% of the real consumption obtained through the invoices. The field work allowed the rigorous definition of the constructive elements of the building thermal envelope, lighting ratios, occupation, HVAC and Air Handling System, as well as all kinds of schedules. The only variables that were left undefined were the electric equipment ratios (e.g. computers, copy machines, etc.), working as an adjustment parameter between the simulated results and the real consumptions. Both models were, according to the options available in each software, created in an equivalent way. The main differences and similarities between the two softwares were evaluated, which lead to the identification of constrains, relating to the options available in both softwares, to the creation of equivalent complex models of the same building. The results obtained in each software with equivalent options selected were compared, and more simulations were made in order to determine the influence of some variants such as load calculation methods. Keywords: EnergyPlus, DesignBuilder, Trace 700, dynamic simulation, energy consumption 1. Introduction According to the Portuguese legislation in building energy consumptions (RSECE), when a user wants to perform a building dynamic simulation in order to access the energy consumptions, it must choose a software that is validated by the ASHRAE Standard. According to this standard, all the softwares validated produced similar results in equivalent situations, for several test cases. The only inconvenient is that the situations analyzed were very simple in terms of geometry and HVAC system, when compared to a real building, for example a large office building. This project intended to explore the usability of two different softwares, both validated by the ASHRAE Standard and compare the results between them. The softwares analyzed were EnergyPlus with DesignBuilder interface and TRACE 700. Several important differences were found, such as the creation of the geometry and the HVAC system, all of this combined resulted in significant constrains in creating an equivalent model in both softwares. Some work had already been done in this area and two reports were particularly important during the research for this project, (Roriz & Gonçalves, 2003) and (Roriz & Silva, 2008).

2 In (Roriz & Gonçalves, 2003) the problems concerning the use of dynamic simulation in the energy assessment as established by the Portuguese legislation are addressed. This study approaches the difficulties in creating a realistic model for the building being studied, and the impact of the simplifications that needs to be done in terms of geometry. Two different approaches are made concerning the complex geometry of the real building, and the cooling and heating loads are compared for both. What comes out of this is that depending on what simplification the user decides to make, different results will be obtained. This conclusion is even more important when we think that there is no specific guideline in the legislation, for most of the assumptions that need to be done in the simulation process. In the second report, (Roriz & Silva, 2008), the impact of the load calculation method chosen within the same simulation software, is exposed. Once again it becomes clear that according to the method chosen by the user, different results will be obtained, that can ultimately influence the energy level of the building in terms of the limits imposed by the Portuguese legislation. This conclusion also contributes to understand the importance of defining a guideline for the calculation done in the simulation process. Another study is also an important reference for this subject, in (Crawley, Hand, Kummert, & Griffith, 2005) several comparisons are made between the main characteristics and options of twenty of the most used simulation softwares. Among this softwares are EnergyPlus and TRACE Cooling and heating load calculation methods HVAC had its major development in the XX century, with large contributions of people such as Willis Carrier, Reuben Trane, James Joule, and Sadi Carnot. In 1894, the ASHRAE foundation was created, and major developments were accomplished in the decades that followed. With the increasing growth of the HVAC industry, the need to estimate correctly the cooling and heating loads arouse. Untill then, an HVAC project had a major probability of under/over estimating the size of the system, with the consequence of endear the final product. According to ASHRAE Handbook - Fundamentals - SI Units, cooling and heating loads result from different energy transfer processes such as conduction, convection and radiation. Some of the parameters that may influence the thermal loads are: External: Walls, roofs, windows, skylights, doors, partitions, ceilings, and floors; Internal: Lights, people, appliances, and equipment; Infiltration: Air leakage and moisture migration; System: Outside air, duct leakage and heat gain, reheat, fan and pump energy, and energy recovery; In 1967 the TETD/TA method appears for the first time in ASHRAE Handbook. This method not only allowed the calculation of thermal load through conduction, but also the heat gains through radiation, using timeaverage values. This procedure however didn t treat the building envelope time delay effect very well. Energy absorbed by walls, floor, furniture, etc., contributes to space cooling load only after a time lag, some of this energy is still present and reradiating even after the heat sources have been switched off or removed.

3 Still in 1967 Mitalas e Stephenson develops the Thermal Response Factor Method that led to the Transfer Function Method that appears for the first time in the ASHRAE Handbook in This method can incorporate fairly well the time delay affect. In the 70 s the ASHRAE RP 138 (Rudoy & Duran, 1975) was the base for the CLTD/SCl/CLF method (Rudoy & Duran, 1979). In the next decade two research projects by ASHRAE were determinant for the development of the cooling and heating load calculation methods. RP (Sowell & D.C.Chiles, 1985) and RP (Sowell, Harris, & McQuiston, 1988). In 1996 ASHRAE launches the RP 875 (Pedersen, Fisher, & Richard J. Liesen, 1997) with the main goal of replacing the former calculation methods with the Heat Balance Method, the most comprehensive method of all, that can be used as a simplified version in a spreadsheet, known as the Radiant Time Series Method (RTS). These last two methods, HBM and RTS are now the main calculation methods and are a part of the 2009 version of the ASHRAE Handbook Fundamentals. 3. Using EnergyPlus/DesignBuilder and TRACE 700 One of the main differences between EnergyPlus and TRACE 700 is the existence of the DesignBuilder interface that allows the use of EnergyPlus from a definition of a 3D model with flexible geometry features, unlike TRACE 700. Both softwares allow the user to define in equivalent ways parameters such as building construction elements, occupation, lighting and equipment ratios and schedules. TRACE 700 as more limitations in the definition of schedules than EnergyPlus/DesignBuilder, because it does not allow daily variations (Monday, Tuesday, etc.). The most important differences between the two softwares, besides the creation of the geometry as was mentioned before, are: Weather Files Shading Elements Non conditioned areas HVAC systems The type of weather file used in each software is different, EnergyPlus uses and epw extension file and TRACE 700 uses a TMY extension file. A TMY can be converted into an EPW with the use of equest software. ( The user should however be careful because the TRACE weather library consists mainly of weather files for load calculation, that do not performs an annual calculation, using only typical days. The shading elements in TRACE 700 are pretty limited when compared to EnergyPlus/Design were the user can create 3D elements with all sorts of geometry. In TRACE 700 one can only use pre-defined elements and cannot combined them. The TRACE version used in this project only allows the definition of the conditioned areas, unlike EnergyPlus that allows the definition of unconditioned areas and their internal gains, such as lighting and equipment, giving a more correct treatment to the impact of these areas in the temperatures of conditioned ones. The differences between softwares in terms of the HVAC system definitions are even greater, not only between EnergyPlus and TRACE 700 but also between EnergyPlus and its interface DesignBuilder. For example, when creating a fan coil system in DesignBuilder, such as the one that is present in this case study, the user should be aware that the software does not automatically create a central air handling unit that supplies pre conditioned air to the terminal units. This procedure should be done in EnergyPlus main file with idf extension, and can be significantly difficult. EnergyPlus despite of a user-friendly environment provides a flexible and comprehensive way of creating a complex HVAC system, requiring however extensive knowledge from the user.

4 4. Case Study The case study is a 16 story office building located in Lisbon. Figure 1 3D model created in DesignBuilder The building has 16 stories above the ground with office spaces and restaurants and 4 underground parking stories. This study focused only in the office floors (3-15) because they had individual energy meters (electric and thermal). All the information concerning the building envelope, lights, equipment, HVAC system and occupation profile (schedules, etc.) were obtained in loco. The building HVAC system consists of a 4 pipe fan-coil (FC) system served by a refrigerating fluids distribution company. The air treatment is done by a central air handling unit that supplies pre-conditioned air to the terminal units (FC). 5. Results The results presentation can be divided in three parts: EnergyPlus/DesignBuilder, TRACE 700, and comparison between them. First the electric simulated consumptions were analyzed and compared to the invoices and then the same was done for the thermal consumptions. Table 1 presents this comparison of the annual electric consumptions simulated with EnergyPlus that are consistent with the real consumptions obtained through the invoices. Total Real Total Floor Electric Electric Simulation/Invoices consumption Consumption (%) (Simulated) (Invoices) kwh kwh 103% kwh kwh 103% kwh kwh 100% kwh kwh 96% kwh kwh 93% 08 Vacant kwh kwh 103% kwh kwh 101% kwh kwh 101% kwh kwh 94% kwh kwh 94% kwh kwh 95% kwh kwh 102% Total kwh kwh 97% Table 1 EnergyPlus annual electric results and real consumptions In Table 1 we can observe that the electric consumptions are consistent with the invoices, not only the total value, but also the for each floor. Since EnergyPlus allows the user to separate the central (AHU) and terminal (FC) cooling and load consumption, the results are presented in Table 2. Total Cooling Total Heating Central AHU kwh t kwh t Floors 3-P kwh t kwh t Total simulated kwh t kwh t Real consumption kwh t kwh t (invoices) Simulation/Invoices (%) 93 % 35% Table 2 - EnergyPlus annual thermal results and real consumptions In Table 2 we can see that the total simulated cooling consumptions are consistent with the invoices but the heating is not. One of the reasons that may explain this is the fact that in heating period even though the internal loads (lights, equipment and occupation) would guarantee the theoretical comfort temperature, the occupants still choose to increase heating. This would

5 result in heating consumptions larger than the real needs. It is also important to refer that the invoices for the heating consumptions were not uniform in the three year period that we analyzed due to problems in the meters. This could mean that the thermal results we are comparing the simulation with may not be totally correct, which in any case does not invalidate the purpose of this project which was to compare the results obtained with two different softwares. Having in mind that TRACE 700 does not allow the definition of the non-conditioned areas, these areas were analyzed with EnergyPlus. Their results were separated so that they can be summed to the results obtained through TRACE 700 and then the total could be compared to the invoices. Table 3 presents the comparison between the TRACE model results and the real consumptions. Despite having the same inputs that had been used in EnergyPlus it can be observed that it does not provide electric consumptions that verifies the condition of ± 10% of the real consumptions. Floor Electric consumption (Simulated) Real Electric Consumption (Invoices) P03-P kwh kwh * Simulated consumption / Real consumption 85 % Table 3 TRACE annual electric results and real consumptions * This result includes the electric consumption regarding the ventilation of the central AHU This could be due to the differences found while creating the HVAC system and the shading devices for example, that made really difficult to create an equivalent model. The same analysis that was made for EnergyPlus in Table 2, was also made foe TRACE 700, the total cooling and heating results are presented in Table 4. Total Cooling Total Heating Simulated Invoices Simulated Invoices kwh t kwh t kwh t kwh t 108% 19% Table 4 - TRACE annual thermal results and real consumptions One of the disadvantages of TRACE 700 is the inability to separate the cooling and heating consumption of the central AHU and the terminal units (FC). In Table 4 we can see the same situation with the heating consumptions that has been identified in EnergyPlus, probably for the same reason. Next, the impact of the options available in TRACE 700 was evaluated, first in terms of the load calculation method and later in terms of the treatment for the nonconditioned areas. The initial simulation was made using the HBM for cooling option and CLTD for heating and considering a constant temperature for the non conditioned areas within the building. Figure 2 presents the deviation in the energy consumption for ventilation, heating and cooling obtained for the different methods, compared to the use of HBM for cooling. Ventilation (FC + AHU) Centralized Total Heating Centralized Total Cooling 3% RP % 1% -8% -0.15% CEC-DOE2-5% 8% TETD-PO 0.56% 6% 6% TETD-TA2 0.47% 5% -3% -0.13% CLTD-CLF -1% 6% TETD-TA1 0.41% 5% Figure 2 Deviations on the HVAC consumptions according to the cooling load calculation method

6 We can see in Figure 2 that the choice of the cooling load calculation method can lead to differences as high as 8 % in the cooling results. These results provide a confirmation of the statement done in (Roriz & Silva, 2008) about the lack of a more specific guideline for the simulation process. The same analysis was made changing the heating load calculation method and keeping the cooling load calculation in the HBM. In this case the differences obtained were below 0.2%, which makes this option of little interest since it has almost no impact in the final results. Since, as it was already mentioned, TRACE 700 gives the user the possibility of choosing between several cooling and heating calculation methods and different ways of treating the influence of the non conditioned areas within the building, several simulations were performed in order to access the impact of this choices. Figure 3 presents a comparison of the HVAC consumption according to the different options, and the initial case (constant temperature). In Figure 3 we can see that this option can have a great impact in the HVAC consumptions. Differences up to 15% in cooling loads and 12% in heating loads can be found according to treatment of non-conditioned areas selected by the user. So even inside the same software, the user can produce different results for the same building and system, according to the options selected. The initial case where the constant temperature option was selected used the temperatures for the non- conditioned areas obtained in the EnergyPlus simulation. These temperatures increased as the floors were getting higher, which is consistent with the vertical connection with the stairs and elevators. At last a comparison was made between the consumptions obtained with EnergyPlus and TRACE 700, the results are presented in the Figure 3. EnergyPlus Trace 783 MWh 676 MWh Ventilation (FC + AHU) Centralized Total Heating Centralized Total Cooling -11% Interior Mass 3% -10% 445 MWh 438 MWh 231 MWh 225 MWh 202 MWh 169 MWh 167 MWh 92 MWh -15% -6% Hourly OADB 12% Lighting Equipment Centralized Total Cooling Centralized Total Heating Ventilation (FC+AHU) -3% Prorated -4% 4% Figure 3 Comparison of the results obtained with EnergyPlus and SineFit -1% 1% 0.2% TRACE 700 Figure 3 Deviations on the HVAC consumptions according to the non-conditioned areas calculation method In Figure 4 we can see that the lightning and equipment consumption is pretty much the same in both softwares as was expected, the small differences that exist can be due to the difference in the annual calendars between softwares. Since the case study is an office building,

7 small differences in holidays and weekends, can justify this variation. The major differences refer to the HVAC consumptions. TRACE presents more total cooling, up to 14% more and less total heating (-45%). In consequence of the differences in the total cooling and heating consumptions, EnergyPlus has a higher ventilation consumption, 17% more. One of the major differences when creating the HVAC system in both softwares, was the definition of the central air handling unit, TRACE 700 fan coil template already includes this, but EnergyPlus does not. In result a AHU had be manually created in EnergyPlus with the help of the support forums, since this subject does not come directly referred in the manual. When the AHU was finally created in EnergyPlus, one difference between the two softwares still remained. In Trace 700 the fresh air is supplied directly to the back of the fan coil unit, but in EnergyPlus the solution found was to supply the fresh air directly into the zone and let the fan coil unit to treat the remain of the load. This situation alone can lead to different HVAC consumptions in the end. 6. Conclusion The first analysis should be made to the simulation process itself, and its integration in the Portuguese building energy consumption legislation (RSECE). The fact that the building energy indicator is based on a computer model that sometimes needs to have several approximations, in order to be created, can lead to incoherence s. As we saw before, it is not always simple to create a realistic version of a building geometry and it s HVAC system. The Portuguese legislation includes two types of simulations; a real simulation based on the actual utilization of the building (schedules, occupation, etc.) and a nominal one, based on the profiles defined in RSECE. This nominal simulation basically uses the same model that the real simulation, and changes the schedules, occupation and equipment ratios etc. to the ones defined in RSECE. It is therefore influenced by the detail applied in the model. The invoices serve to compare the results obtained with the simulation in real conditions to the real consumptions of the building. Here it is important to distinguish between the fact that the results simulated and the invoices may be similar but that doesn t necessarily means that the model is correctly defined. A good example of this is the fact that the HVAC system of this case study is a fan coil system with a central AHU, the ventilation of the terminal and the central units must be correctly defined so that the results in nominal conditions can express both of them. Since the building energy indicator applies a weather correction to ventilation that is directly responsible for the conditioning of the spaces, and leaves the rest untouched according to region of the country where the building is located, it is extremely important for the HVAC system to be well defined. The next issue is that the user might find more or less difficulties creating a building model according to the software that he chooses. Also it is important to be aware of the differences that can be found inside the same software. As we saw in TRACE 700, the treatment of adjacent non-conditioned areas, alone, can influence significantly the HVAC consumptions according to the method used.

8 7. References [1] Adene - Agência Para a Energia. (s.d.). RSECE Decreto-Lei nº 79/2006. Lisboa: Diário da Républica. [2] ASHRAE ASHRAE Handbook - Fundamentals - SI Units. American Society of Heating, Refrigerating, and Air-Conditioning Engineers. [3] Crawley, D. B., Hand, J. W., Kummert, M., & Griffith, B. T. (2005). Contrasting the capabilities of building energy performance simulation programs. Department of Energy of the USA, University of Strathclyde, University of Wisconsin. [4] Mitalas, G. P. (1973). Transfer Function Method of Calculating Cooling Loads, Heat Extraction & Space Temperature. OTTAWA: ASHRAE JOURNAL. [8] Rudoy, W., & Duran, F. (1975). RP Development of an improved cooling load calculation. ASHRAE. [9] Sowell, E., & D.C.Chiles. (1985). RP Characterization of Zone Dynamic Response for CLF/CLTD Tables. ASHRAE. [10] Sowell, E., Harris, S. M., & McQuiston, F. C. (1988). RP Development of Expanded Wall, Roof and Zone Classifications for Cooling Load Calculation Methods. ASHRAE. [11] Spitler, J., Fisher, D., & Pederson, X. (1997). The Radiant Time Series Cooling Load Calculation Procedure. ASHRAE. [12] Spitler, J., McQuiston, F., & Lindsey, K. (1993). The CLTD/SCL/CLF Cooling Load Calculation Method. ASHRAE. [5] Pedersen, C. O., Fisher, D. E., & Richard J. Liesen. (1997). Development of a Heat Balance Procedure for Calculating Cooling Loads. ASHRAE. [6] Roriz, L., & Gonçalves, A. (2003). Os problemas da utilização de métodos de simulação de cargas térmicas e consumo energético na auditoria energética para verificação dos Requisitos Energéticos dos edifícios. Obtained in February 10, 2011, from [7] Roriz, L., & Silva, O. (2008). Efeitos da metodologia aplicada na simulação energética de edifícios. Obtained in February 10, 2011, from