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Article Excerpt
INTRODUCTION
High performance can best be achieved by improving the efficiencies of all of the energy features of a given building in a vigorous, but balanced, manner. Use of high-performance envelopes can reduce heating energy in most United States' locations to little more than what is needed to heat the ventilation air plus what is needed for night setback recovery heating on a few mornings in the coldest locations. Ventilation air conditioning (heating, cooling, and [de]humidifying) can be largely satisfied by enthalpy exchange with exhaust air. Heating aside, electricity use in well-designed contemporary buildings is roughly equal parts lighting, cooling and air movement, and equipment operated by occupants. Lighting design, fixture efficacy, daylighting, and controls have reduced lighting power densities substantially and progress continues. Office equipment efficiencies have shown similar progress.
Reduction of cooling loads is a straightforward, albeit first-cost intensive, matter of improving window, window-shading, and envelope performance; of recovering ventilation enthalpy; of improving lighting efficiencies; and of reducing end-user equipment loads. To achieve balanced (equal marginal returns) energy efficiency investments, the high-performance building designer must aggressively address both cooling load sources and cooling system efficiency. Cooling system efficiency, in turn, requires attention to both thermodynamic performance and fluid transport performance. That efficient cooling is a significant technical challenge should not deter us. Potential impacts in the world's fastest growing economies are tremendous because growth is projected largely for tropical, subtropical, and desert regions where cooling is a basic need.
This paper will show that substantial improvements in cooling system efficiency can be achieved by integrating low-lift cooling technologies: variable-speed compressor and transport motor controls, radiant cooling with dedicated ventilation air transport and dehumidification, and cool storage. Estimates of cooling energy use and savings potential are developed for a baseline and all feasible combinations of the low-lift cooling technologies.
The three low-lift technologies or technology groups that can be implemented independently are described, and a brief description of why each is of interest is provided.
Radiant Cooling Subsystems (RCSs). RCSs save energy in three ways. First, by achieving a given level of comfort at a higher air temperature, the envelope load is reduced. Second, transport energy is reduced when cooling capacity is delivered by water instead of air. Finally, chilled water temperatures of 15[degrees]C or higher, instead of the 10[degrees]C needed by all-air systems for humidity control, result in higher chiller efficiency. A dedicated outdoor air system (DOAS) is required to provide fresh, dry air because RCSs cannot perform these functions. Splitting the latent and sensible cooling functions in this way results in better control of zone temperature, humidity and air quality, as well as in better cooling efficiency. The variable-air-volume (VAV) distribution system, being the most efficient all-air system used widely in the United States, serves as the baseline technology that RCSs and DOASs replace.
Thermal Energy Storage (TES). TES (intrinsic or discrete) saves energy by shifting cooling load to night and by spreading the load over time. Shifting the load to night reduces the condensing temperature regardless of what kind of chiller is used. Spreading the load over time can further reduce the average difference between chiller condensing and evaporating temperatures ([T.sub.c] - [T.sub.e]) if the refrigerant flow rate can be efficiently modulated. Along with foregoing savings mechanisms comes potential energy penalties in the form of storage heat-exchanger penalties that can lead to lower evaporating temperatures and transport energy penalties associated with discrete storage, which involve essentially doubling the transport costs for each charge-discharge cycle. If sufficient thermal capacitance exists in the building fabric and contents, load shifting can be achieved while avoiding both the heat-exchanger and double transport penalties. When thermal energy storage and load-shifting controls are absent (baseline case), the chiller must exactly satisfy all hourly cooling loads.
Variable-Speed (VS) Compressor and VS Transport. VS compressor and VS transport motor operation save energy by striking a balance between condensing-evaporating temperature difference and transport energy. While transport energy is directly tied to the flow-pressure characteristic of each heat transfer fluid circuit and is a very strong function of pump or fan speed, the trade-off between transport and compressor savings, governed by the condenser and evaporator approach temperature relations, is also very sensitive. The theoretical compressor energy savings from reduced condensing-evaporating temperature difference are significant but can easily be wiped out if per-unit compressor and motor losses increase as the flow rate is reduced. The sensitivity and nonlinearity of these trade-offs become more problematic, and therefore require more rigorous modeling, when the chiller system is modulated over a wide range of capacity. The baseline chiller capacity modulation technology chosen for this assessment is the two-speed chiller, which is roughly equivalent to a chiller with two equally sized compressors.
Low-lift savings are achieved through two distinct mechanisms: improved transport efficiency and improved thermodynamic efficiency. Thermodynamic efficiency is mainly a function of condensing and evaporating temperatures, determined in turn by condenser and evaporator approach temperatures and source ([T.sub.z]) and sink ([T.sub.x]) temperatures. The potential reduction in compressor work per unit cooling capacity is illustrated in the temperature-entropy (T-s) diagram of Figure 1. The areas of the cycle polygons represent compressor work and the areas under the polygons (extending beyond the plot viewport to absolute zero) represent cooling capacity. The large polygon is determined by condensing and evaporating temperatures typical of a conventional chiller and distribution system, while the smaller polygon is determined by condensing and evaporating temperatures typical (for the same daily total load) of the proposed low-lift system. The T-s diagram illustrates not only that compressor work is much smaller for the low-lift system, but that the cooling capacity of the low-lift system is about 10% larger than for the corresponding conventional system with identical compressor, condenser, and evaporator components.
[FIGURE 1 OMITTED]
The paper begins with an overview of the efficient cooling literature. The assessment applies three building performance levels to the same prototypical office building plan. Performance level parameters, including envelope parameters that are indexed to the five climates used in the assessment, are described. The seven combinations of the three low-lift cooling system elements used in the assessment are enumerated and performance maps, based on the baseline and low-lift chiller models developed in the companion paper (Armstrong et al. 2009), are presented. Results of the simulations are reported and discussed. The impact of peak-shifting (i.e., pre-cooling) controls on annual cooling load distribution are presented; these load distributions can be used by chiller manufacturers to guide and optimize designs for variable-speed low-lift equipment. A methodology for estimating potential national energy savings is described, and results of the assessment are presented. The analysis does not attempt to estimate cost-effective potential savings because costs of mass-produced and mass-marketed low-lift components are not yet known. The main contributions of this paper are the thorough exploration of parameter space (i.e., climate, HVAC (1) configuration, balance of system performance), the use of cooling equipment models that are consistent over a wide range of lift and part-load fraction, and results that indicate a very feasible and very promising approach that can have tremendous impact on global warming potential in temperate as well as hot-climate regions of countries now experiencing rapid economic growth.
LITERATURE REVIEW
A wide-ranging literature review was performed to identify efficient cooling technologies. Cooling by natural ventilation was not addressed because of its climate-limited application range, but an effort was made to find all significant research, analysis, and design guidance germane to the low-lift technologies described above. The lack of a rigorous evaluation of chiller efficiency benefits potentially enabled by hydronic radiant cooling is surprising. No quantitative assessments of annual energy benefits for cooling plants designed to modulate efficiently over a wide range of capacity fraction (substantially greater than 2:1) were found, and no assessment of the three low-lift technology elements in combination was found. A summary of the relevant literature is presented below.
Radiant cooling was of interest 50 to 60 years ago when the evolving air-conditioning industry was still in the process of trying out a variety of systems--a process that eventually resulted in dominance of the all-air approach in North America. Current interest (from about 1990) in radiant cooling, initially motivated by fan energy savings, is also seen as a way to reduce ventilation flow rates and to control humidity better. Recent work by proponents has focused on accurately estimating panel capacity, on modeling the interactions of convection and radiation, and on supervisory control strategies of decoupled dehumidification/ventilation and sensible cooling systems that ensure comfort and acceptable indoor air quality (IAQ), while avoiding condensation on panels under all conditions. At least one paper has made credible estimates of fan-energy- and higher-air-temperature-effected savings. The loss of (i.e., ~80% of the normal) air-side free cooling potential has been noted, but the potential national impact across climates and seasons of this loss has not been addressed. The effectiveness of applying water- or refrigerant-side economizers, in conjunction with radiant cooling, is not well understood. The impact of higher evaporator temperatures on chiller performance has not been quantified. Radiant cooling systems are gaining market share faster in Asia and Europe than in the United States (Adlam 1947; ASHRAE 2004b; Ayoub et al. 2006; Baker 1960; Carpenter and Kokko 1998; Chantrasrisalai et al. 2003; Conroy and Mumma 2001; Dieckmann et al. 2004; Feustel and Stetiu 1995; Feustel 1999; Franta 1983; Jeong et al. 2003; Jeong and Mumma 2006; Jiang et al. 1992; Kallen 1982; Kilkis 1993, 2000; Kilkis et al. 1994, 1995; Kochendorfer 1996; Manley 1954; Mumma 2001a, 2001b; Novoselac and Srebric 2002; Shank and Mumma 2001; Shoemaker 1954; Simmonds 1994; Stetiu 1997, 1999; Zweifel and Koschenz 1993).
Work on active core cooling has addressed thermal occupancy conditions such as strong vertical temperature gradients. The considerable challenge of control during diurnal and shorter load transients has been addressed in a few papers. Potential improvements in system efficiency through active core cooling combined with peak-shifting and efficient low-lift chiller equipment have not been quantitatively assessed. Air-side free cooling possibilities are limited in radiant/DOAS systems. Refrigerant-side free-cooling has been mentioned, but details of design and performance were not found in the literature, probably because this design traditionally has had little attraction when used with all-air systems or systems with discrete storage (Ataer and Kilkis 1994; Athienitis and Shou 1991; Hauser et al. 2000; Kallen 1982; Kochendorfer 1996; Koschenz and Dorer 1999; Meierhans 1993, 1996; Michel and Isoardi 1993; Olesen 1997, 2000; Olesen et al. 2000; Simmonds et al. 2000; Strand and Pedersen 1997).
The ability of radiant ceiling panels (RCPs) to cool the building fabric and contents at night will be even more problematic, in terms of capacity, transport energy, and control, than for active core systems. Although no papers or reports were found to directly address the RCP precooling question, there is a body of residential-scale night cooling literature in which the small driving temperature differences involved are an oft-cited problem. Effective residential-scale ambient-coupled cool storage systems have been demonstrated in climates where ambient night cooling is feasible. Systems based on water, packed bed, intrinsic building thermal capacitance, and a variety of phase change materials with transition points above room temperature (heating) and below room temperature (cooling) have been developed. Storage schemes developed for residential ambient night cooling such as building mass, stratified water tanks, water tubes, phase change material (PCM) building materials, PCM-packed beds, and rock beds all have potential. Although it may have benefits in low-lift dehumidification, for low-lift sensible cooling ice storage is anathema (Balcomb 1983; Buddhi 2000; Hay and Yellott 1969; Karaki 1978; Peck et al. 1979; Perkins 1984; Stoecker et al. 1981; Telkes and Raymond 1949; Turner and Chen 1987).
In commercial buildings, energy savings (not just demand savings) with discrete or intrinsic thermal energy storage and off-the-shelf (in some cases with modified controls) vapor-compression cooling equipment have been successfully demonstrated in a few large (>100,000 [ft.sup.2]) buildings. Large buildings have the advantage that fan speed and static pressure can typically...
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