Simulation of Source Energy Utilization and Emissions for HVAC Systems. A report ASHRAE TC 6.9 in response to the 991-TRP workstatement.

Transcription

1 Simulation of Energy Utilization and Emissions for HVAC Systems A report ASHRAE TC 6.9 in response to the 991-TRP workstatement submitted by Douglas T. Reindl, Ph. D, P.E. Director of Research University of Wisconsin-Madison Robert A. Gansler Associate Researcher University of Wisconsin-Madison Todd. B. Jekel, Ph. D. Assistant Scientist University of Wisconsin-Madison

2 Executive Summary Two different building types were used to investigate the source energy requirements and emissions attributable to various space conditioning systems serving facility cooling loads. The hourly electrical and gas requirements for various types of space conditioning systems were traced back to the point where fuel is extracted from the earth (i.e. the source). All electrical consumption was assumed to be generated at the last utility plant (marginal) dispatched to meet the network load. Two different utilities, with different generation mixes and marginal dispatch plans formed the basis to assess the source energy requirements and emissions associated with electrical usage. Marginal unit dispatch was determined on a lowest operational cost basis, provided with the utility data. The space conditioning technologies that comprised this study were: Electric chiller directly meeting the load Electric chiller with chilled water storage Electric chiller with ice storage Electric chiller with ice storage and cold air distribution Natural gas direct-fired double-effect absorption chiller directly meeting the load The site gas and electric consumption of the equipment for each of the above options was determined on an hourly basis. This included both primary energy consumption (chiller) and auxiliary system energy consumption (fans and pumps). Based on the energy trajectories (which traces the energy from its source to its site) of the equipment tied with the fuel source, the source energy and emissions characteristics (of,,,,, and particulates) were determined for each of the technologies. A global warming index (GWI) was developed to combine all of the emission constituents to a common metric. On an annual basis, thermal energy storage systems had lower source energy requirements and resulting emissions as compared to a base case of an electric direct chilling system. Emission results in all categories were lower than the base. Absorption systems tended to have the lowest emissions in all other equipment categories except for. Absorption systems had the highest source energy consumption, the highest emissions and the highest GWI. i

3 Table of Contents 1 INTRODUCTION Background Research Objectives Research Scope 2 2 SOURCE ENERGY PERSPECTIVE 2 3 MARGINAL METHODOLOGY 2 4 UTILITY DATA 3 5 HEAT RATE DEVELOPMENT 6 6 ENERGY TRAJECTORIES Distribution Natural Distribution Natural Natural for Coal for Natural Equipment 10 7 BUILDING SIMULATION 10 8 COOLING SYSTEMS System Basics Conventional System Chilled Water Storage System Static Ice Storage System Absorption System 18 i

4 8.6 Condensing Systems 20 9 OPERATION RESULTS Office Building School Building CONCLUSIONS REFERENCES 38 APPENDIX A Monthly Marginal Plant Frequency Plots APPENDIX B Process Emission Characteristics APPENDIX C Monthly Energy Results ii

5 1 Introduction 1.1 Background Absorption cooling and thermal energy storage is a proven technology for commercial and industrial applications. These technologies have benefited both end-users and utilities. Both absorption and thermal energy storage can reduce aggregate electric demand during peak periods. Thermal energy storage provides for the potential reduction on first cost and almost certain reduction in operating costs, as compared to a conventional system without storage meeting the same building cooling loads. The benefit to utilities comes in the form of reduced electric demand and improved load factors. Thermal energy storage has been used for many years as an important component of utility demand side management programs. While demand side management programs are not as prevalent as they once were, thermal energy storage systems continue to be economically justified in many applications. Thermal energy storage systems typically "produce cooling capacity" during the nighttime hours by charging a storage medium. During the daytime, that cooling capacity is discharged to meet the building's cooling loads. As such, a portion (and in some cases, the entirety) of the building's cooling load, and thus the building's energy consumption for cooling, can be shifted from daytime to nighttime hours. There will be some storage losses with both ice and chilled water storage and there is an energy penalty for creating ice. When cold air distribution is used in conjunction with ice thermal energy storage systems, the energy penalty for producing ice can be mitigated. The environmental impact attributable to the electricity consumed for cooling is linked to the output of the generating utility. The emissions and fuel requirements of an electric utility will vary over the course of the day, over the time of the year, and based on the characteristics of the various types of generating plants dispatched to meet the aggregate system electric demand. Utilities typically dispatch plants to meet the system load in the most economic manner possible; that is, they will run the least cost (which are generally the most energy efficient) plants first and then successively add higher cost (less efficient) plants as the aggregate demand warrants. During the day, a utility will likely have to bring on the less efficient equipment in order to meet their peak demand. At night, a utility will be able to meet the extra electric demand by running their more efficient "baseload" plants. Another contributing factor to inefficiencies in delivering energy to a site is transmission and distribution losses. These losses are typically proportional to utility grid load, resulting in higher losses during the daytime and reduced losses during the nighttime hours. In terms of resource impacts, more efficient plants means plants with lower heat rates. 'Heat rate' is defined as the required Btu fuel input to a plant to produce a kwh of electricity. A low heat rate means that the plant requires low amounts of fuel in order to produce a kwh of electricity. Lower heat rates typically also translate to lower emissions per unit of electricity produced. It is necessary to understand the equipment and technologies that are used deplete resources and affect the environment. Operation of thermal energy storage systems may prove to be beneficial by capitalizing on the differences between day time and night time electric generation. By tracing back the site energy required to provide cooling at the building, various space conditioning systems can be evaluated to assess their potential to be a environmentally and resource friendly. Analysis will show that thermal energy storage systems can realize these benefits. 1.2 Research Objectives The objective of this research project is to determine the environmental impact associated with temporal end-use energy consumption for various space options serving different facility types. The environmental impact will be quantified in terms of source energy consumed and emissions produced as a result of operating end-use space conditioning equipment. A summary of the tasks executed in the course of this research project include: 1. Perform a literature search to identify assess current work in the area of source energy utilization. 2. Gather data from two different electric utilities having numerous types of generation plants. 3. Characterize the emission and source energy requirements as a function of electric load for the generating equipment of each utility. 1

6 4. Develop hourly cooling load profiles for a school and a large commercial office building. 5. Determine the source energy consumption and emissions produced as a result of space conditioning the two buildings being served by a number of different cooling options: Electric chiller directly meeting the load Electric chiller with chilled water storage Electric chiller with ice storage Electric chiller with ice storage and cold air distribution Natural gas direct-fired double-effect absorption chiller directly meeting the load 1.3 Research Scope The work completed during the course of this research project determines the source energy requirements and emissions associated with the space conditioning of two different buildings each supplied by two different electric utility generation profiles. 2 Energy Perspective The term "source energy" can be defined in a number of different ways. The term is certainly different than "site energy," which is commonly understood to be the energy actually consumed at the building. " energy," by comparison, means tracking and accounting for the energy consumed from the point where fuel is extracted from the earth (the source) to the site where energy is ultimately converted to cooling. One of the first attempts at establishing a methodology for characterizing source energy in various processes was performed by Melcher, et al. (1976). The study developed "energy trajectories" for various fossil fuel energy systems. The "energy trajectories" were part of a "net energy" analysis. This type of analysis attempted to encompass the fossil fuel steps of extraction, processing, and delivery as well as to estimate the energy flows attributable to providing the materials used in constructing the equipment required in each step. It did not take into account any seasonal effects in distribution efficiency. A more recent study undertaken by Tabors Caramanis & Associates for the California Energy Commission (1996) specifically looked at the source energy impacts of thermal storage. The study attempted to determine at the potential source energy impacts attributable to a large number of thermal storage systems operating within two California utilities - Southern California Edison and Pacific & Electric. As a part of their study, they looked at two different methods of assessing source energy and emissions - an incremental method and a marginal method. Their incremental approach utilized a modified average heat rate and emissions characteristic for a utility mix. The marginal method used a "system lambda" to account for the marginal cost per kwh to serve an increased or decreased load. The Tabors study analyzed three different building types - a hospital, a large office, and a small office. The study considered full-shift systems, and they further presumed that there would no net site kwh savings from thermal storage systems. The study indicated source energy savings on the order of 30% using their incremental method and savings on the order of 10% using the marginal method. A previous study by the authors was undertaken by the authors (Reindl, et al, 1995). This study looked at the source energy consumption for an office building being served by a single electric utility. The analysis showed that thermal energy storage technologies resulted in lower source energy consumption as well as lower carbon dioxide emissions. 3 Marginal Methodology The source energy and emissions associated with comfort cooling depend on the energy use at the site. It will also depend on the energy losses due to transmission and distribution as well as the type of electrical generating equipment on-line at a given time. As such, this site energy is traced back to the origin. The development of these trajectories is discussed in Section 6. 2

7 The question is how to trace the energy back to the source. One approach is to determine the mix of plants operating during a given hour. The site energy consumed could be expressed as a fraction of the entire utility system load. The source energy and emissions associated with that site energy-use could be calculated based on that fraction and knowing the composite energy consumed and emissions produced from all plants in the entire utility service territory. The analysis conducted during this project considered an alternative approach. The building space conditioning system electrical demand is assumed to be served by the last piece of generating equipment dispatched, i.e. the marginal plant. All of the source energy and emissions were considered to be a fraction of those related to that marginal plant. In order to pinpoint the marginal plant for a given hour, it is necessary to know what plants are on-line in that given hour. Utilities typically operate their least cost (more efficient) generating equipment to meet their base load, and then progressively higher cost (less efficient) units are dispatched as the load demands. Actual details of hourly dispatch information was not available from either of the utilities employed in this study; however, operation cost information, heat rates, fuel source, and plant size for each plant in the generation mix were available. With this information in hand, the dispatching was assumed to occur on the basis of operating cost. Plants were dispatched, beginning with the least cost unit, until the system load was met. The last unit dispatched was the marginal unit for that hour. 4 Utility Data It was hoped that this investigation would utilize data from a number of electric utilities in a number of different climates. However, there was great difficulty in obtaining this sort of information that had been deemed sensitive in today's increasingly competitive utility marketplace. Data was obtained from a number of utilities providing electricity in Wisconsin. Useful data was distilled data from two of these utilities - Madison & Electric (MG&E) and Wisconsin Electric Power Company (WEPCO). Table 1 shows the characteristics of the MG&E plants, in increasing order of operation cost; Table 2 does the same for the WEPCO plants. Type Capacity NO X Heat Rate (MW) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (Btu/kWh) Nuclear ,841 Coal 113 2, ,370 Coal 111 2, , , , , , , , , ,625 Coal 23 3, , , ,625 Coal 50 2, , , ,625 Coal 49 2, , , , , , , , , , , , , , ,000 Table 1: Plant Dispatch Sequence and Characteristics - MG&E 3

8 Type Capacity NO X Heat Rate (MW) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (lbs/mwh) (Btu/kWh) Nuclear ,977 Nuclear ,988 Coal 580 2, ,028 Coal 580 2, ,028 Coal 97 2, ,005 Coal 305 1, ,449 Coal 280 1, ,476 Coal 258 1, ,911 Coal 260 1, ,865 Coal 57 2, ,909 Coal 85 2, ,264 Coal 84 2, ,274 Coal 25 3, ,847 Coal 37 3, ,383 Coal 58 2, ,905 Coal 80 2, ,802 Coal 80 2, ,544 Coal 62 2, ,704 Coal 70 2, ,834 Coal 64 2, ,928 Coal 84 2, ,681 Coal 83 2, ,684 Coal 81 2, ,689 Coal 70 2, ,337 Coal 82 2, ,582 Coal 80 2, , , , , , , , , ,359 Table 2: Plant Dispatch Sequence and Characteristics - WEPCO As the tables above display, these two utilities were characterized by different generation mixes. By combining this information with annual utility system load data on an hourly basis, the marginal plant for every hour could be determined. Figure 1 and Figure 2 show the frequency of the type of plant on the margin for the two utilities. Monthly plots, on an hourly basis, are included in Appendix A. 4

9 Hours Coal Month Figure 1: Marginal Plant Frequency for MG&E Hours Coal Month Figure 2: Marginal Plant Frequenccy for WEPCO 5

10 5 Heat Rate Development The raw data for the two utilities was not in completely compatible format. Emissions were expressed in terms of pounds of pollutant per MWh delivered to the plant boundary. One of the utilities (MG&E) had heat rate information at three different levels (low, medium, and high loads) for each plant; the other (WEPCO) only had an average heat rate for each plant. For the MG&E data, a weighted average heat rate for each plant was determined. The average heat rate was compared to the reported heat rates at the given load levels. Since the plants had data reported at various low and medium levels, distributions of heat rates as a function of part-load ratio could be developed. These heat rates were then expressed as a fraction of the average heat rate and as a function of the plant's part-load ratio. This was done for both types of marginal power plants coal and gas. Regressions were performed to establish a part-load curve for each type of generation. The polynomial curves are presented below in Figure Heat Rate/Average Heat Rate PLR Coal Figure 3: Heat Rate Adjustment Factors for Different Power Plant Types PLR stands for the Load Ratio, and it represents the ratio of actual plant demand to its capacity. 6 Energy Trajectories The energy trajectories that were developed by Reindl, et al (1995) were incorporated for the different types of generation and distribution. Note that there are only energy trajectories for coal and gas turbine plants since neither utility ever had a nuclear plant functioning as the marginal unit. 6.1 Distribution Characterizing the transmission and distribution losses on an hourly basis is quite difficult. A previous study by the authors (Reindl, et al, 1995) used information from TU Electric (1993). Expressed in terms of percent, the losses are characterized by: Losses = X 2 [1] X = kw/kw peak [2] where kw is the hourly electrical demand and kw peak is the peak electrical demand over the year. 6

11 The energy and emission trajectory for electrical distribution is shown in Figure 4. CO,, Power Plant Transmission & Distribution Electric Equipment Net 1 kw delivered (kw/kw peak ) 2 Consumed: (kw/kw peak ) 2 Figure 4: Transmission and Distribution Trajectory For the overall energy and emission trajectory, the values shown in Figure 4 would be added to the end of the trajectory of the given power plant type. 6.2 Natural Distribution As mentioned previously, the Melcher (1976) analysis did not take into account seasonal effects to the distribution efficiency of the natural gas pipeline. The delivered volume of gas is much higher in the winter than in the summer. Therefore, it is reasonable to expect that the overall process efficiency would vary over the course of the year. EIA (1992) data was used in order to characterize the efficiency of the gas distribution process. The data was tabulated monthly for various points along the process. The efficiency was defined for the purposes of this study as: Efficiency = E delivered /E overall, consumed [3] E delivered = E consumed - E plant, fuel - E pipeline, fuel [4] E consumed = E produced + E store, extract + E supplement + E import - E store, add - E export [5] The EIA data contained values collected from the years An average value was generated for each month of the year, and the subsequent average curve was fitted to a trigonometric functional form. Efficiency = cos(2z) [6] Z = (month-1)/12 [7] Thus, the gas distribution varies over the course of the year, although it is considered constant for a given month. 7

12 Efficiency 94% 93% 92% 91% 90% 89% 88% 87% 86% Month Average Figure 5: Distribution Efficiency 6.3 Natural Natural gas is used for two different purposes in this investigation. Natural gas is used as a fuel source for directfired equipment at the site; it is also used for electrical power generation. An illustration of the energy and emission trajectory for natural gas from the source (point where fuel is extracted from the ground) to the end of the delivery pipeline is shown in Figure 6. This is a representation of the process at a given point in time with a given distribution efficiency. CO = x lb = x 10-6 lb = x 10-9 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-9 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-8 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-8 lb = X 10-9 lb Process Input Natural 1.16 Btu or Oil Well Btu Gather Btu Btu Btu Processing Distribution 1 Btu Process Energy Output Natural Process Emission Output Consumed: Btu Consumed: Btu Physical loss: Btu Consumed: Btu Fuels & : Btu Consumed: Btu CO = x 10-8 lb = x 10-4 lb = x 10-6 lb = X 10-8 lb Figure 6: Illustration of Energy and Emission Trajectory for Natural The "principal energy" flows are depicted by the double lines connecting the various stages of the process. As the illustration shows, 1.16 Btu of natural gas energy content are required to deliver 1 Btu of natural gas to the point of consumption. Above and below each step are shown the external energy and resulting emissions associated with that step. These baseline energy flows and emissions were obtained from Melcher (1976) but varied on an hourly basis for the present investigation. 8

13 The processes involved in delivering natural gas begins at the wellhead of an oil or natural gas well. The losses at the wellhead consist of spills, leaks, flaring, etc. In the gathering process, the gas is collected from the various wells in a wellfield and piped to the processing plant. At the processing plant, the gas is dehydrated and prepared for introduction into the pipeline. The largest losses occur during the distribution phase. In the illustration shown in Figure 6, there is an overall efficiency (output/input) of 86%. 6.4 Natural for The previous section described the overall process for natural gas to the consumption point. When natural gas is used for electrical power generation, an extra process step of generation needs to be added to the trajectory. The heat rate and emission data will depend on the power plant on the margin during the given hour. Figure 7 couples the natural gas trajectory with the electrical power production as well as the transmission and distribution. In this case, Btu of natural gas is required to produce 1 Btu of electricity to the site. The power plant in this example has a heat rate of 11,887 Btu/kWh. CO = x lb = x 10-6 lb = x 10-9 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-9 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-8 lb = X 10-9 lb CO = x lb = x 10-6 lb = x 10-8 lb = X 10-9 lb CO = x 10-8 lb = x 10-4 lb = x 10-6 lb = X 10-8 lb Process Input Natural Btu or Oil Well Btu Gather Btu Processing Btu Distribution Btu Electrical Production Process Energy Output 1 Btu Process Emission Output Consumed: Btu Consumed: Btu Physical loss: Btu Consumed: Btu Fuels & : Btu Consumed: Btu CO = x 10-8 lb = x 10-4 lb = x 10-6 lb = X 10-8 lb Consumed: Btu Figure 7: Energy and Emission Trajectory for a Natural Power Plant 6.5 Coal for Coal is the other fuel source that supplies power plants on the margin. For this particular example power plant, Btu of natural gas is required to produce 1 Btu of electricity supplied; the power plant had a heat rate 10,529 Btu/kWh. 9

14 CO = x lb = x 10-6 lb = x 10-9 lb = x 10-8 lb CO = x 10-8 lb = x 10-4 lb = x 10-6 lb = x 10-6 lb Process Input Coal Extraction Loss Btu 5% Btu Btu Surface Mine Coal Plant Process Energy Output Net 1 Btu Process Emission Output Physical Loss: Btu Fuel & : Btu CO = x 10-8 lb = x 10-4 lb = x 10-6 lb = x 10-6 lb Internal Consumption: Btu Figure 8: Energy and Emission Trajectory for a Coal Power Plant 6.6 Natural Equipment The combustion of fuel in natural gas equipment produces a variety of emissions. Table 3 shows the emissions associated with the natural gas cooling equipment that was used in this study. Emission Level (lbs/btu) 1.16 x x 10-8 Table 3: Emission Characteristics for Natural Equipment 7 Building Simulation The annual hourly building loads were simulated using PowerDOE, a windows simulation program that uses DOE-2.2 as the calculation engine. The weather data used for the load determination was TMY2 data for Madison, WI. An office building was used as the first building to be included in this study. The 11-story building covered 506,000 ft 2 of floorpsace. It had a peak load of 980 tons. The cooling loads were calculated with PowerDOE and reported along with ambient conditions. The office building constructions were chosen to be ASHRAE Wall Group 17 and Roof Group 11. The windows were specified to be clear, double-pane glazing that accounted for 40% of the gross wall area. The size of the building was chosen to have approximately a 1000 ton (3520 kw t ) load on the peak day. The resulting building area was 506,000 ft 2 (47,000 m 2 ) with dimensions of 200 ft (61 m) by 230 ft (70 m) and 11 stories with 12 ft (3.7 m) floor-to-floor height. The internal load densities and assumptions for the office are shown in Table 4. The bulk of the daily occupancy is from 8:00am until 5:00pm. The lights were scheduled to be on from 1-hour prior to 1-hour after occupancy. The equipment was scheduled to reflect occupancy. 10

15 Sensible Load Latent Load Load Density Peak Diversity Btu/h (W t ) Btu/h (W t ) Lighting 1.5 W/ft 2 / (16.1 W/m 2 ) 0.90 Equipment 1.5 W/ft 2 / (16.1 W/m 2 ) 0.50 Occupants 150 ft 2 /person (14 m 2 /person) (73) 200 (59) Table 4: Internal load description for the office building. Load profiles for both conventional (55 F / 13 C) supply air and cold air distribution (44 F / 6.7C) were calculated for the office building. The air distribution system was a variable air volume (VAV) system with a supply fan static pressure drop of 4.0 in H 2 O (995 Pa) and 4.25 in H 2 O (1,060 Pa) for conventional and cold air respectively. Both conventional and cold air systems had a return fan with a static pressure drop of 0.5 in H 2 O (124 Pa). The thermostat setting was 74 F (23 C) during occupied hours and 82 F (28C) during unoccupied hours. The occupied setpoint is imposed one hour prior to occupancy. The fans are required to be on during occupied hours and allowed to cycle-on during the unoccupied hours to maintain the setup thermostat setting. Outside air ventilation is provided at 15 cfm/person (7.1 l/s/person) peak occupancy during occupied hours Load (tons) F Air 44F Air Hour Figure 9: Design Day Load Profile - Office 11

16 Load (tons) Cumulative Frequency Figure 10: Load Cumulative Frequency Distribution - Office The second building modeled was a school. The school building model dimensions and constructions were entered from actual bid microfiche data for a rural high school. The wall and roof constructions are best represented with ASHRAE Wall Group 15 and Roof Group 1 respectively. The building is single-story, approximately 110,000 ft 2 (10,200 m 2 ) and occupied by 1,040 persons. The building has a gross exterior wall area of 52,500 ft 2 (4,880 m 2 ) and 2,184 ft 2 (200 m 2 ) of clear, double-pane windows. It is assumed that the school operates all year round. The design day peak cooling load is 200 tons (700 kw t ). Table 5 shows the internal loads and assumptions for the school. Sensible Load Latent Load Load Density Peak Diversity Btu/h (W t ) Btu/h (W t ) Lighting 1.5 W/ft 2 / (16.1 W/m 2 ) 0.90 Equipment 0.5 W/ft 2 / (5.4 W/m 2 ) 0.90 Occupants 1,040 persons (72) 180 (53) Table 5: Internal load description for the school building. Just as with the office building, load profiles for both conventional (55 F / 13C) supply air and cold air distribution (44 F / 6.7C) are generated for the school building. The air distribution system is variable air volume (VAV) with a supply fan static pressure drop of 4.0 in H 2 O (995 Pa) and 4.25 in H 2 O (1,060 Pa) for conventional and cold air respectively. All other aspects of the system and its control are identical to the office building. 12

17 Load (tons) Hour 55F Air 44F Air Figure 11: Load Profile - School Load (tons) Cumulative Frequency Figure 12: Load Cumulative Frequency Distribution - School 8 Cooling Systems 8.1 System Basics Five different central plant options were investigated direct cooling electric chiller, double-effect direct-fired absorption chiller, chilled water storage, ice storage, and ice storage with cold air distribution. Models developed at the HVAC&R Center were utilized in the simulation of the central plant energy consumption. The PowerDOE simulations provided the fan energy usage. The thermal energy storage systems were operated as a single chiller combined with storage. The chiller would serve to meet off-peak loads as well as to recharge storage. The intent was to minimize chiller and storage size. The 13

18 sizing of these systems was dependent upon the timing and duration of the on-peak period. Two different on-peak periods were investigated. The electric chillers were modeled as water-cooled with a cooling tower (for the office building and the school) and air-cooled with a condensing unit (for the school only). The compressor capacity and power requirements were based upon a polynomial curve fit of manufacturer data. The functional forms are given below: Capacity = C 1 + C 2 *SST + C 3 *SST 2 + C 4 *SST 3 + C 5 *SDT + C 6 *SDT 2 + C 7 *SDT 3 + C 8 *SST*SDT 2 + C 9 *SST 2 *SDT [8] Power = D 1 + D 2 *SST + D 3 *SST 2 + D 4 *SST 3 + D 5 *SDT+ D 6 *SDT 2 + D 7 *SDT 3 + D 8 *SST*SDT 2 + D 9 *SST 2 *SDT [9] The capacity is expressed in tons and the power in brake horsepower. SST is the saturated suction temperature ( F), and SDT is the saturated discharge temperature ( F). The C and D coefficients were developed from a least squares fit of performance data. The compressor capacity is also corrected for prevailing levels of subcooling and superheat. The performance of the entire chiller plant - compressor, condenser, and evaporator - is calculated hourly by balancing all components based upon current load and ambient conditions. The part-load performance of the chiller is modeled by typical unloading characteristics of a positive displacement compressor. The same part load performance is used for all electric chillers. Figure 13 shows chiller unloading performance as the fraction of full load power as a function of part load ratio. Fraction of Full Load Power Load Ratio Figure 13: Compressor -Load Performance (adapted from BLAST Technical Manual) 8.2 Conventional System The system schematic of the non-storage direct chilling system is shown in Figure 14. In this configuration, the chiller only operates to meet the instantaneous space cooling load. The chiller plant was sized to meet the maximum hourly cooling load. The chiller leaving water temperature is controlled to be 6 F less than the cooling coil leaving air temperature. The cooling coil was modeled to operate with a fixed approach of 8.1 F. There was a water-side temperature difference of 10 F across the coil. The system operated at approximately 2 gpm/ton for the chiller. The flow rate to the coil was dependent on the load requirements. The chiller side pump operated with a head of 25 feet and a pump 14

19 efficiency of 65%. The load side pump operated with a minimum head of 10 feet, but this value was dependent on the flow rate to the load. The pump efficiency was 65% as well. Since there is only one chiller sized to meet the peak load used in each building type, the chiller runs at part load for most hours of the year. For each building type, there were some hours with very small loads compared to the maximum load which would result in very inefficient operation. To avoid having these hours of very inefficient operation skew the results, the chiller was controlled to let the cooling load carry over until the integrated load was at least 10% of its capacity. Chiller Load Figure 14: Non-Storage System Arrangement 8.3 Chilled Water Storage System The chilled water storage system functioned similarly to the non-storage system. An electrically-driven chiller served in conjunction with the chilled water storage tank to meet the space cooling loads. The storage tank was a stratified chilled water tank with a design temperature differential of 20F (40F supply; 60F return) 15

20 Chiller Stratified Chilled Water Storage Load Figure 15: Chilled Water System Arrangement Since the tank is assumed to be exposed to the outside environment, thermal losses were taken into account by the following equation: Q aux = UA (T ambient - T tank ) [10] Where Q aux is the tank loss in tons, UA is the overall tank loss coefficient (0.221 ton/ F), T ambient is the ambient dry bulb temperature ( F), and T tank is the average tank water temperature ( F). The chiller operates to charge the tank during off-peak hours. A deadband of 100 ton-hours was applied to chiller operation for charging. During on-peak hours, storage operates in conjunction with the chiller to meet the cooling load. For the full-shift systems, storage would meet the load exclusively during the on-peak hours. For the load leveling systems, storage would supply the supplemental cooling that could not be met the chiller directly. There was temperature difference of 20 F across the cooling coil. The system operated at approximately 2 gpm/ton for the chiller. The flow rate to the coil was dependent on the load requirements. The chiller-side and the load-side pumps were operated identically to the direct chilling case. For each building type, there were some hours with very small off-peak loads compared to the maximum load. If the tank were already fully charged, the single chiller would run very inefficiently to meet those small loads. To avoid having these hours of very inefficient operation skew the results, the tank was controlled to meet those offpeak cooling loads. 8.4 Static Ice Storage System The static ice systems used an arrangement that was similar to the chilled water systems. The major difference was that the working fluid was a secondary coolant consisting of a 25% (by weight) mixture of ethylene glycol. The storage tanks were based on a 190 ton-hour internal-melt ice-on-coil tank. 16

21 For charging, the chiller operated by delivering the coolant to the storage tanks at a temperature below the freezing point of water (32 F). The performance of the ice storage tanks was calculated using the Jekel (1991) effectiveness model. This model determines the effectiveness of charging and discharging as a function of the flow rate through the tank, the flow inlet temperature, and the tank's state of charge. Load Load-Side Pump Chiller-Side Pump Chiller Storage Figure 16: Static Ice System Arrangement The model predicts the effectiveness of charging and discharging as a function of the secondary coolant flow rate, the secondary coolant inlet temperature, and the state of charge of the of the storage tank. = (T b,in - T b,out )/(T b,in -T ice ) [11] where T b,in is the tank secondary coolant inlet temperature( F), T b,out is the tank secondary coolant outlet temperature ( F), at T ice is 32 F, the freezing temperature for the water in the tank. This represents the actual energy transfer as a portion of the maximum possible energy transfer. For charging, the brine temperature leaving the chiller was set downward to 24F. If a cooling load occurred offpeak, the chiller would try to meet both the load and charge storage. If the chiller entering brine temperature rose and the chiller leaving brine temperature rose above 32F, the storage tanks would be bypassed and the chiller would reset to meet the load solely. If the cooling load is greater than the chiller capacity, the additional load will be met by utilizing the storage. The system used a 9 F approach for 55 F supply air; there was an approach for 6 F approach for 44 F supply air. In both cases, there was temperature difference of 16 F across the cooling coil. The system operated at approximately 17

22 2 gpm/ton for the chiller. The chiller-side and the load-side pumps were operated identically to the direct chilling case. The chiller operated to charge the tanks during off-peak hours. During on-peak hours, storage would operate in conjunction with the chiller to meet the cooling load. For the full-shift systems, storage would meet the load exclusively during the on-peak hours. For the load-leveling systems, storage would supply the supplemental cooling that could not be met the chiller directly, i.e. chiller-priority control. For each building type, there were some hours with very small off-peak loads, as compared to the maximum load. If the tanks were already fully charged, the single chiller would run very inefficiently to meet those small loads. To avoid having these hours of very inefficient operation skew the results, the tanks were controlled to meet those offpeak cooling loads. 8.5 Absorption System The absorption system was modeled based upon a water-cooled direct-fired double-effect absorption chiller. Figure 17 shows the arrangement for the absorption system. Absorption Chiller Load Figure 17: Absorption System Arrangement The full- and part-load performance of the absorption system was developed from manufacturer data. Figure 18 displays the part load performance characteristics of the absorber based on two different entering condensing water temperatures. Note that the fraction of fuel input decreases in an almost linear fashion with decreasing load fraction. When compared to the data from a more recent study (Energy Center of Wisconsin, 1996) shown in Figure 19, the absorption system in this study was credited with much better part-load performance that possibly warranted. The control strategy for the absorption chiller was identical to that of the conventional electric chiller. The chiller is meant to only operated to meet the instantaneous space cooling load. The chiller plant was sized to meet the maximum hourly cooling load. 18

23 Fraction Fuel Input F ECWT 85 F ECWT Load Fraction Figure 18: Absorption -Load Performance Figure 19: Absorption COP at -Load The cooling coil was modeled to operate with a fixed approach of 6 F. There was temperature difference of 10 F across the coil. The flow rate to the coil was dependent on the load requirements. The load side pump operated with a minimum head of 10 feet, but this value was dependent on the flow rate to the load. The condenser pump operated with 40 feet of head. The pump efficiency for both of these pumps was 65% as well. In addition to the gas requirements to fire the absorber, the equipment also had electrical usage to its auxiliaries. The power required for the cooling tower fans was assumed to be proportional to the total heat rejected through the tower with a proportionality constant of kw/ton of heat rejected. Since there was only one chiller for each building type, the chiller would run at part-load for most hours of the year. For each building type, there were some hours with very small loads, as compared to the maximum load. The single chiller would run very inefficiently to meet those small loads. To avoid having these hours of very inefficient 19

24 operation skew the results, the chiller was controlled to let the cooling load carry over until the integrated load was at least 10% of its capacity. 8.6 Condensing Systems The analysis looked at two different type of condensing for the electric chillers: water-cooled condensing with a cooling tower and air-cooled condensing with a condensing unit. The water-cooled condensing was used for both building types (school and office) while the air-cooled condensing was used only for the school building. The water-cooled condenser was also based on a curve fit of manufacturer data. The model calculated an approach temperature (i.e., the difference between the cooling tower leaving water temperature and the outside air wet bulb temperature) based upon the outside air wet bulb temperature and the range. The range is based upon the difference between the cooling tower entering water temperature and the cooling tower leaving water temperature. An effectiveness model was used to characterize the evaporator performance. The cooling tower model demanded an entering temperature of condenser water to the chiller of 85 F. Electrical energy usage for the pumps and fans was estimated to be kw/ton. The air-cooled condensing unit was modeled with a 15 F temperature difference between saturated condensing temperature and outside air dry bulb temperature. In order to maintain a minimum of 80 psi of pressure difference between the condenser and the evaporator, a minimum condensing temperature of 80 F was established. The auxiliary energy consumption for the condensing unit was estimated to be kw/ton 9 Operation The operation of the non-storage systems, electric chiller-only and absorption chiller, is very straightforward. When a load exists, the chillers run to meet the load. The only exception to this was when the loads were very small. For the thermal storage systems, the operation was based upon three different strategies. Two were full shift strategies corresponding to two different on-peak periods. One scenario was to shift all of the space cooling loads from 12 noon through 8 PM; the other was to shift all cooling loads from 8 AM through 5 PM. During these hours, only the hydronic system (including storage) and fan systems operate to meet cooling loads. During all other hours, the chiller is allowed to run in order to meet cooling loads and to charge the storage. The third strategy was a load-leveling chiller-priority. In this strategy, the chiller always operates in order to meet the load. Storage would be called upon only when the cooling load exceeds the available capacity of the chiller. The sizing of the equipment was done to meet the cooling loads on the peak day. The peak days profile for the cold air systems were different than those for the other systems. In terms of the storage scenarios, the systems were sized to minimize both chiller and storage size. The sizing criteria was such that the available storage capacity never went below 20% of the nominal capacity. These three strategies impacted the sizing of the chiller and storage for the two different buildings. The sizing results are shown in Table 6 and Table 7. 20

25 System Nominal Chiller Capacity (tons) Nominal Chiller Efficiency (kw/ton) Icemaking Chiller Capacity (tons) Icemaking Chiller Efficiency (kw/ton) Nominal Storage Capacity (ton-hours) Electric Chiller 1, Absorption 1, COP On-Peak 12 PM - 8 PM Chilled Water Storage ,400 Ice Storage 1, ,300 Ice Storage with Cold Air 1, ,440 On-Peak 8 AM - 5 PM Chilled Water Storage ,900 Ice Storage 1, , ,200 Ice Storage with Cold Air 1, , ,480 Load Level Chilled Water Storage ,000 Ice Storage ,090 Ice Storage with Cold Air ,750 Table 6: System Sizes - Office Building System Nominal Chiller Capacity (tons) Nominal Chiller Efficiency (kw/ton) Icemaking Chiller Capacity (tons) Icemaking Chiller Efficiency (kw/ton) Nominal Storage Capacity (ton-hours) Electric Chiller Absorption COP On-Peak 12 PM - 8 PM Chilled Water Storage ,200 Ice Storage ,900 Ice Storage with Cold Air ,09 0 On-Peak 8 AM - 5 PM Chilled Water Storage ,900 Ice Storage ,850 Ice Storage with Cold Air ,850 Load Level Chilled Water Storage ,100 Ice Storage Ice Storage with Cold Air Table 7: System Sizes - School Building, Water-Cooled 21

26 System Nominal Chiller Capacity (tons) Nominal Chiller Efficiency (kw/ton) Icemaking Chiller Capacity (tons) Icemaking Chiller Efficiency (kw/ton) Nominal Storage Capacity (ton-hours) Electric Chiller Absorption COP On-Peak 12 PM - 8 PM Chilled Water Storage ,200 Ice Storage ,520 Ice Storage with Cold Air ,900 On-Peak 8 AM - 5 PM Chilled Water Storage ,100 Ice Storage ,850 Ice Storage with Cold Air ,850 Load Level Chilled Water Storage ,000 Ice Storage Ice Storage with Cold Air Table 8: System Sizes - School Building, Air-Cooled 10 Results This section brings together the building and central plant models with the energy and emission trajectories developed for the utility power plants. The energy and emission values for the electric-based equipment serving the cooling load incorporates the entire energy and emission trajectory for the marginal plant in any given hour of the year. The energy and emissions produced as a result of the operation of gas-fired equipment is taken into account by the natural gas trajectory Office Building Using the conventional system (electric chiller) as a base, all of the thermal storage technologies resulted in lower source energy requirements. This was true for both utility generation mixes. While thermal storage could result in higher site energy consumption (Ice Storage: On-Peak 8 AM - 5 PM), source energy requirements were always lower. This was also true for the emissions associated with energy use. Thermal storage technologies always had lower emissions than the base case. 22

27 System (kbtu e ) (kbtu) (kbtu g ) Energy (kbtu) Electric Chiller Base 1,133,000 17,419, ,419,500 Absorption 648,851 9,945,072 11,788,170 13,296,440 23,241,512 On-Peak 12 PM - 8 PM Chilled Water Storage 978,186 14,253, ,253,130 Ice Storage 1,110,537 16,229, ,229,230 Ice Storage with Cold Air 1,065,168 15,534, ,534,370 On-Peak 8 AM - 5 PM Chilled Water Storage 1,021,997 14,773, ,773,180 Ice Storage 1,163,846 16,873, ,873,130 Ice Storage with Cold Air 1,085,039 15,690, ,690,000 Load Level Chilled Water Storage 940,510 14,320, ,320,430 Ice Storage 974,073 14,939, ,939,230 Ice Storage with Cold Air 930,165 14,297, ,297,260 Table 9: Energy Usage - Office Building, MG&E System (% of base) (% of base) Energy (% of base) Electric Chiller Base 100% 100% 100% Absorption 57% 57% 133% On-Peak 12 PM - 8 PM Chilled Water Storage 86% 82% 82% Ice Storage 98% 93% 93% Ice Storage with Cold Air 94% 89% 89% On-Peak 8 AM - 5 PM Chilled Water Storage 90% 85% 85% Ice Storage 103% 97% 97% Ice Storage with Cold Air 96% 90% 90% Load Level Chilled Water Storage 83% 82% 82% Ice Storage 86% 86% 86% Ice Storage with Cold Air 82% 82% 82% Table 10: Energy Usage Versus Base - Office Building, MG&E 23

28 System (kbtu e ) (kbtu) gas (kbtu g ) Energy (kbtu) Electric Chiller 1,133,000 13,695, ,695,100 Absorption 648,851 7,840,596 11,788,170 13,296,440 21,137,036 On-Peak 12 PM - 8 PM Chilled Water Storage 978,186 11,611, ,611,280 Ice Storage 1,110,537 13,182, ,182,110 Ice Storage with Cold Air 1,065,168 12,629, ,629,630 On-Peak 8 AM - 5 PM Chilled Water Storage 1,021,997 12,140, ,140,560 Ice Storage 1,163,846 13,818, ,818,980 Ice Storage with Cold Air 1,085,039 12,865, ,865,930 Load Level Chilled Water Storage 940,510 11,376, ,376,650 Ice Storage 974,073 11,799, ,799,690 Ice Storage with Cold Air 930,165 11,286, ,286,390 Table 11: Energy Usage - Office Building, WEPCO System (% of base) (% of base) Energy (% of base) Electric Chiller Base 100% 100% 100% Absorption 63% 62% 127% On-Peak 12 PM - 8 PM Chilled Water Storage 86% 85% 85% Ice Storage 98% 96% 96% Ice Storage with Cold Air 94% 92% 92% On-Peak 8 AM - 5 PM Chilled Water Storage 90% 89% 89% Ice Storage 103% 101% 101% Ice Storage with Cold Air 96% 94% 94% Load Level Chilled Water Storage 83% 83% 83% Ice Storage 86% 86% 86% Ice Storage with Cold Air 82% 82% 82% Table 12: Energy Usage versus Base - Office Building, WEPCO 24