ENERGY EFFICIENT THERMAL ENERGY STORAGE FOR DX AIR CONDITIONING

Transcription

1 ENERGY EFFICIENT THERMAL ENERGY STORAGE FOR DX AIR CONDITIONING Paul Kuhlman Ramachadran Narayanamurthy Ice Energy Inc Eastman Park Dr.; Suite B Windsor CO ice-energy.com Introduction The growing demand for peak electrical power and increasingly congested electrical transmission lines is the primary driver of excessive emissions caused by peak electrical energy generation. Thermal energy storage(tes) systems are now available for direct expansion air conditioning, which not only shift peak energy, but are also energy efficient. This paper addresses: Energy efficient Thermal Energy Storage (TES) for DX air conditioning systems Peak energy use and its impact on power plant emissions Technology Description Ice Energy Inc. s Ice Bear system is designed to alleviate the electrical grid loading during the peaking hours by attacking the primary source of the peaking, namely, air-conditioning equipment. There are two distinct air-conditioning markets large buildings and facilities cooled by water (or a water-glycol mixture), and smaller buildings and residences cooled directly by refrigerant. In all cases, the ultimate heat sink is refrigerant, but in the first case, a secondary heat exchange is used to provide cold water to cool buildings. Of the entire floor space in the United States, 48% is cooled by refrigerant-based systems and 32% by water-based systems. There is already a well established market with proven economics for energy storage in water/glycol based systems. But due to the complexity of refrigerant management, there are no known commercially available refrigerant-based TES systems other than Ice Energy s Ice Bear. The Ice Bear system utilizes unique technology for efficient energy storage for refrigerant-based airconditioning systems. The heart of the Ice Bear System is the Refrigerant Management System (RMS). There are three main subcomponents to the Ice Bear System; the Tank, the Heat Exchanger, and the RMS. The Tank is doubly insulated with an inner and outer skin of High Density Cross-linked Polyethylene. Between the two layers is a 4-6 thick foam insulation made of BASF Type AF-0306, injected to fill the entire space between the two layers. The shell material is designed for high UV resistance, expected to last for more than 70 years, and the foam provides enough insulation to keep a tank of ice at 120 F outside temperature for more than 30 days. The heat exchanger is composed of helical coils of copper tubing from the premier tubing supplier in the country, Wolverine Tube Inc. It is designed to minimize the amount of copper (and hence cost and refrigerant) while covering the entire volume with equal ice thickness for maximum efficiency. The Refrigerant Management System (RMS) is designed with a liquid overfeed system that is widely recognized within the refrigeration industry as a very efficient technology. The functions accomplished by the RMS include: 1. Provide refrigerant metering (flow regulation) which is matched to the cooling load (ice formation) required. This is accomplished by a patented metering device. 2. Provide oil return through a patented oil distillation heat exchanger 3. Feed refrigerant liquid to both the ice tank heat exchanger during the ice make process and to the pump during cooling

2 4. Circulate refrigerant with a refrigerant pump (another unique component not found in standard refrigeration systems) to the evaporator coils The unique combination of these various components provides the high efficiency of the Ice Bear System that cannot be replicated with other technologies, such as glycol-water systems. The Ice Bear System s integrated controller uses a sophisticated microprocessor-based design, thus increasing functionality at reduced cost. The refrigerant pump being used for the production models is a 115 V pump and consumes less than 100 W. The Ice Bear System is designed to replace units up to 7.5 Tons during on-peak hours which may extend for up to 6 hours. The design is flexible with regards to loading, and there are almost no part-load losses as with standard air-conditioning systems. The Ice Bear System consumes a maximum of 300 W during cooling, which is 20 times less power consumption that standard air-conditioners. It also has the option of stepping down the power consumption during low load time by turning off some components. The Ice Bear System can be operated in various configurations based on the building load requirements. For instance, in buildings requiring larger amounts of cooling (10 or 12.5 Tons), the Ice Bear System can be operated in combination with a 5 Ton condensing unit to deliver up to 12.5 Tons, saving both peak and total energy. In situations requiring greater dehumidification, the Ice Bear System can provide greater energy efficiency using the cold water in the tank. The most popular configuration for peak shifting moves more than 90% of air-conditioner power consumption off-peak. The Ice Bear System can be applied to a wide variety of different commercial building types, including office buildings, shops, restaurants, and sports centers, as well as larger residential buildings. Filter Heating Coi/Furnace Evaporator Coil Conventional Air Handler Fan Conventional Outdoor Condensing Unit Refrigeration Management System Ice Storage Tank Ice Storage Unit Figure 1 Ice Bear Line Diagram Figure 1 shows a simple line diagram of an Ice Bear System installation with a split system, where it is installed between the air-conditioning unit and the indoor coil. Figure 2 compares the operating conditions of an Ice Bear with a standard DX cooling system. During Ice Make, the condensing unit (compressor and condenser) operates at a lower suction and discharge pressure than conventional cooling, thereby reducing demand and total energy consumption. During Ice Melt, there is no significant pressure differential across the loop as the motive force is a small pump with no expansion device.

3 Standard Cooling 45 F Ice Melt 46 F Standard Cooling 115 F Ice Make 25 F Ice Melt 48 F, 96 PSI Ice Make 90 F Figure 2 Ice Bear Connection Drawing The Ice Bear System has three main modes of operation: Ice Melt: This is the cooling mode when the refrigerant pump is operated to provide building cooling. The condensing unit is turned off in this mode. This mode is normally operated during the peak hours of the day. Ice Make: The condensing unit is operated to freeze the water in the Ice Bear System to store cooling capacity. This mode is normally operated at night. Direct Cooling: This mode allows usage of the Ice Make condensing unit to provide cooling instead of using stored ice. In the majority of cases, direct cooling is applied when the Ice Bear System is the only cooling system for a space, but at a time when it not desirable or possible to use the ice (see submodes below). During direct cooling, the energy consumption for the Ice Bear System includes the power consumption of the Ice Bear System plus the power consumption of the condensing unit. In this configuration, the user avoids the first cost of purchasing and installing an additional condensing unit. There are 3 sub-modes for direct cooling: Ice Make Cooling: This occurs in cases where there is a requirement for cooling during the preferred time for Ice Make, and there is no other system to accommodate the load. In this mode, the condensing unit stays on continuously for the duration of Ice Make, but whenever there is a call for cooling the refrigerant pump is turned on providing refrigerant to the cooling coil. When the call for cooling stops, so does the refrigerant pump, and the cold refrigerant provided by the condensing unit reverts to cooling the water and making ice. Ice Save Cooling: This mode is used usually for the mid-morning hours, between the end of ice make and the beginning of ice melt. Ice Melt is not used in order to save the ice for the peak hours of the day. In this mode, when there is no call for cooling, all systems are turned off. When the thermostat calls for cooling, both the refrigerant pump and the condensing unit are turned on at the same time. The pump re-routes the cold refrigerant generated by the condensing unit to the cooling coil for room cooling. Both systems turn off when the call for cooling stops. Ice Exhausted Cooling: This mode serves to cool the building at the end of the day when all the ice melt is completed but the room still requires cooling. Proper design ensures that this mode is not needed during the peak hours of the day. This mode operates similar to Ice Save Cooling mode with regards to the condensing unit and the Ice Bear System, except that there is no ice or

4 cooling capacity left in the tank. This mode also serves as a backup cooling mode in case of an equipment malfunction that results in insufficient ice being produced on the previous night. Performance Data Electrical Data: Voltage: 115 V and 208/230 V Current: Up to 5 A at 115 V during cooling and up to 25 A at 208/230 V during ice-make Cooling Performance: Maximum Capacity: 50 Ton-Hours Net Usable Latent Capacity: 42 Ton-Hours Max. Load Capacity: 7.5 Tons Estimated Power Consumption: Maximum of about 50 KWH over hours for a complete ice make on a peak summer day, and 35 KWH on a typical summer day. Ice Bear and Emissions Reduction Ice Energy s storage technology is applied to off-the-shelf residential split and commercial rooftop refrigerant-based air conditioning systems. The use of energy storage has the potential to reduce NOx air emissions by shifting power generation from peak daytime hours to off-peak nighttime hours. Additionally overall energy savings are realized by running the condensing unit continuously at its maximum heat transfer rate, eliminating inefficient start-stop cycle losses and by running the condensing unit to store energy during the relatively cooler evening hours. During the day when a standard thermostat calls for cooling, a fractional horsepower 300 watt refrigerant pump circulates liquid refrigerant to a standard evaporator coil. Shifting the demand for powering air conditioner compressors from daytime to nighttime hours shifts the electricity needed from higher-emitting power plant generating units that are typically used to meet peak loads to lower-emitting generating units that are used to meet off-peak, or nighttime, loads. Furthermore, this shift (reduction) in generation and emissions occurs at the particular time hot, summer daytime hours when ozone levels are likely to be highest. Ice Energy Inc. asked the environmental and energy consulting firm E3 Ventures to investigate and quantify Ice Bear related emissions reductions in a specific air district. The study 1 investigated the time related energy use of DX air-conditioning systems and the emissions characteristics at the generating sources serving a specific Air Quality Management District, in this case Sacramento California, (SMAQMD) and the primary electrical generation utility for the district, Sacramento Metropolitan Utility District (SMUD). The study then compared the emissions changes that would occur if the Ice Energy Ice Bear were used. More precisely, the study established SMUD energy needs during summer peak energy demand periods with the emissions characteristics of generating sources used to serve SMUD energy needs during off-peak (nighttime) periods. For generation sources in the Sacramento area, the study determined and compared the emissions characteristics of sources that are used during peak hours (in particular, the summer peak load period of 11:00am to 7:00pm) and those that are used during non-peak hours (10:00pm to 6:00am). And finally, the study calculated the reduction in NOx emissions resulting from equipping an air conditioning unit with the Ice Bear load-shifting technology. 1 The Air Quality Benefits of Ice Energy s Energy Storage Technology In Sacramento, California; E 3 Ventures, Inc. 780 Simms Street, Suite 210 Golden, Colorado 80401; September 1, 2005

5 The starting point for evaluating Ice Bear s potential air quality impacts in the Sacramento area is understanding historical generation patterns associated with the area s electricity demand. Data provided by SMUD give a basic understanding of the general types of generation sources supplying power to the area in recent years. Figure 3 shows the historic importance of contract power and market purchases to meet energy demands in Sacramento. 60% 50% 40% 30% 20% 10% 0% Jan 03 Feb 03 Mar 03 Apr 03 May 03 June 03 July 03 Aug 03 Sept 03 Oct 03 Nov 03 Dec 03 Jan 04 Feb 04 Mar 04 Apr 04 May 04 June 04 July 04 Aug 04 Sept 04 Oct 04 Nov 04 Dec 04 Contract purchases Market purchases SMUD hydro SMUD cogen SMUD CT peaker SMUD PV/wind Figure 3: SMUD power sources as percent of total power supply ( ) In the period, SMUD facilities accounted for 33 percent of the agency s power supplies, and purchased power (both market and contract) accounted for 67 percent. The figure also illustrates the importance of cogeneration facilities. SMUD-operated cogeneration facilities, which include Campbell Soup, Carson Ice and Proctor & Gamble SCA and Carson, supplied 20 percent of SMUD s power during this period. 2 Importantly, in 2006 the new 500MW Cosumnes Power Plant will begin generating electricity. The capacity of the new Cosumnes power plant is equal to about 40 percent of SMUD s off-peak load (typically at or below 1,200 MW) and will significantly reduce the amount of power purchased by SMUD during offpeak periods. In addition, Cosumnes could be expanded in the future with another similar size unit i.e., another 500 MW unit. The methodology to evaluate the emissions savings of Ice Energy s technology focuses on developing estimates of 1) the emissions characteristics of generating sources used primarily to serve SMUD energy needs during summer peak energy demand periods (daytime periods associated with high cooling demand) and 2) the emissions characteristics of generating sources used to serve SMUD energy needs during offpeak (nighttime) periods. The emissions savings associated with Ice Energy s shifting of energy use from peak to off-peak times can then be estimated by comparing the difference in emissions characteristics between the generation sources that provide power during these different time periods. 2 Note that Figure 3 is based on total generation across all hours of the day. This information is useful for illustrating the historic importance of power purchased by SMUD and the role of cogen facilities. However, in order to evaluate the benefits of the Ice Bear TES application, it is necessary to understand how generation patterns vary between peak and off-peak periods of the day.

6 The geographic region selected for analysis includes Sacramento and adjacent counties, which include Amador, Contra Costa, El Dorado, Placer, San Joaquin, Solano, Sutter, and Yolo counties. Figure 4 illustrates the power plants in this nine county region. 3 Facilities are classified by their average capacity factor during the six month ozone seasons (April through September) of 2001, 2002 and Capacity factors play a key role in the analysis methodology, as discussed below. Sutter Placer El Dorado Yolo Sacramento Amador Capacity factor Solano <40% >40%, <70% >70% Contra Costa San Joaquin Figure 4 Electric generating units in Sacramento region 3 The rationale for selecting this multi-county region is three-fold. First, emissions from power generation sources in this nine county region are likely to directly impact air quality in Sacramento. Secondly, the region contains a significant number of power plants (approximately 70) that have a combined generating capacity far in excess of SMUD s peak demand. Finally, although sources outside these counties may provide power to Sacramento during both peak and off-peak periods, purchases from distant sources are likely to occur predominantly during off-peak times because sources used to meet peak loads tend to be sources that are located near demand pockets. Therefore, the selection of this relatively constrained region provides a conservative estimate of emissions impacts during peak and off-peak periods. 3 The available latitude/longitude information for almost all of these units referred to the closest major city or some other central location rather than the actual location of the unit. Therefore, a number of units were collocated at the same point. For illustration purposes, many of the generating units have been manually dispersed in the counties.

7 Critical Considerations The methodology for estimating the emissions characteristics of the two categories of generating sources (those serving load primarily during peak periods and those serving baseload needs) recognizes four key factors in the Sacramento area: 1. Gas-fired power is expected to continue to be critical in satisfying future peak electricity demand (i.e., summertime air-conditioning peaks) and, specifically, to meet changing energy needs at the margin during peak periods. 2. SMUD s new Cosumnes power plant will begin generating power in 2006 and will significantly reduce the amount of power SMUD purchases. 3. With the exception of green power purchases, SMUD s power purchase contracts are not tagged to specific generating facilities. Therefore, precise NOx emissions rate estimates for SMUD s purchased power (either during peak or off-peak periods) cannot be developed. 4. To account for purchased power, estimates of the emissions performance of that portion of SMUDs energy portfolio can be developed using reasonable assumptions regarding the types and locations of facilities likely to be supplying power purchased by SMUD during peak and off-peak periods. Given the considerations above, the methodology focuses on identifying and calculating an average NOx emissions rate for natural gas-fired units whose capacity factors indicate that they are likely to be used during periods of peak demand, and comparing that rate to the average NOx emissions rate of units whose capacity factors suggest that they are likely sources of nighttime power. 4 Calculated capacity factors and emissions rates are based on monthly generation and emissions data for 2001, 2002 and 2003 as reported in CEC s Power Plant database. For each reported unit, an average capacity factor is calculated based on that unit s total generation during the six-month period April through September (corresponding to the increased cooling demand) of 2001, 2002 and This multiyear averaging approach minimizes the impact of any unusual disruptions in power generation that may have occurred in a single month. (A similar analysis of a three month ozone season June through August has yielded results similar to the six month analysis.) For this analysis, gas-fired units with a capacity factor below 40 percent during the six-month period are assumed to be called on in meeting peak (daytime) demands in the warm summertime months. All units with a capacity factor of 70 percent or higher during the six-month period are assumed to be used in meeting off-peak (nighttime) demand during the summertime months. Based on information provided by SMUD, the new Cosumnes plant is assumed to be operated in a baseload manner. (Attachment A provides a list of units included in the analysis.) Results The analysis finds that generating sources used primarily during peak periods have an average NOx emissions rate of lb/mwh, while sources used to serve baseload energy needs at night have an 4 Hydroelectric facilities that provide power during peak periods were not included in the analysis of the emissions rate of facilities serving peak load. These facilities were excluded because, according to SMUD, their operations are unlikely to be changed by shaving peak period demand with TES technologies such as Ice Bear. TES is unlikely to affect their operations because they operate at low variable costs and therefore are called on first when available. On the other hand, the operating pattern of natural gas-fired facilities (from which SMUD purchases considerable amounts of power during peak periods) with higher variable costs are most likely to be affected from demand reductions that might be associated with TES technology. This assumption is consistent with assumptions made in the CEC report analyzing TES technologies, which focused strictly on changes in natural gas generation patterns.

8 average emissions rate of lb/mwh. The emissions savings of lbs/mwh (a 56% reduction in the NOx emissions rate) is multiplied by the amount of energy shifted by a typical Ice Bear unit to estimate the emissions savings. Additional emissions benefits are also calculated based on the overall energy conservation savings, which is 4 percent in the Sacramento area according to other analyses conducted for Ice Energy. In combination, the analysis estimates an overall NOx emissions savings from a typical Ice Bear installation of about 6 g/day. Generating sources used to meet demand for power emissions rate* (lbs/mwh) Off-peak demand (10:00pm 6:00am) Peak demand (11:00am 7:00pm) * weighted average emissions rate for generating sources used to meet demand for the period Calculation of Ice Energy s air quality benefit The difference between the daytime NOx emissions rate (i.e., the calculated peak generating source emissions rate) and the nighttime rate (i.e., the baseload generating source emissions rate) can be multiplied by the amount of energy shifted from peak to off-peak using Ice Energy s energy storage technology to estimate the emissions reduction attributable to the technology. Assuming that the air conditioning unit without an Ice Bear unit requires 5 kw of energy on average during the peak time of the day (e.g., 11 a.m. to 7 p.m.) but only 300 W of power with an Ice Bear energy storage unit then the difference, 4.7 kw, is multiplied by the time period (e.g., 8 hours) to determine, in this case, 37.6 kwh, which is then multiplied by the daytime-nighttime emissions rate difference ( =0.339 lb/mwh) to determine the NOx emissions reductions. 5 (5 kw 300 W) x (8 hrs) x ( lbs/mwh) = 4.7 kw x 8 hrs x lb/mwh = lb/day, or 5.8 g/day* [*The conversion to g/day is to facilitate comparisons with typical emissions information, such as the daily emissions of a motor vehicle in this area.] There is also an absolute energy savings (and emissions savings) associated with Ice Energy s energy storage technology that can be added to the above estimate. In the Sacramento area, the addition of an Ice Bear energy storage module is expected to improve overall energy efficiency by 4 percent when compared to a air conditioner alone 6. Thus, the additional emissions reduction would be the nighttime rate (0.264 lbs/mwh) times the load and time period (5 kw x 8 hour period) times the percent savings in efficiency (4 percent). The calculation is as follows: (0.264 lbs/mwh) x (5 kw x 8 hrs) x (4%) = 0.19 grams/day The total emissions savings attributable to an Ice Bear energy storage module based on the methodology used herein is about 6 g/day. In addition, the air quality benefit might actually be larger since the nighttime NOx emissions would be expected to have minimal contribution to ozone smog. Thus, the air quality benefit of this example application could be as high as about 11 g/day if the nighttime NOx emissions are assumed to have no role in ozone formation. Ice Bear & Site Energy Savings 5 The 5 kw load during the daytime period represents a simplified pattern of electricity use during this period. More precise data on the hourly shift in load (kw s) can result in a more precise estimate of the emissions reduction when applying an Ice Bear energy storage module. 6 Preliminary Energy Analysis of the Ice Bear System: Comparisons with Conventional Package DX Systems, Architectural Energy Corporation, June 12, 2004

9 The Ice Bear system utilizes a very efficient patented liquid overfeed refrigerant management system for heat transfer during ice-make and ice-melt operations. Condensing unit efficiency gains due to nighttime versus daytime ambient operating temperatures and the elimination of cycling losses result in a net energy efficiency of the Ice Bear system over conventional DX. Laboratory tests have indicated that in areas of the world where the average (cooling season) diurnal ambient temperature swing is 15F or greater, net site energy consumption is reduced. Improved dehumidification, typical DX over-sizing, DX capacity degradation with temperature, condensing unit aging, and roof top temperature factors further accentuate the site energy savings provided by Ice Bear technology.