LA ESTRATEGIA - MARCO TEÓRICO Y CONCEPTUAL

2. MARCO DE REFERENCIA

2.2. MARCO TEÓRICO Y CONCEPTUAL

2.2.1. LA ESTRATEGIA

The term low-lift cooling system (LLCS) is used here to describe an integrated system combining radiant cooling, TES, a variable capacity chiller and predictive pre-cooling control to achieve energy efficient cooling. An LLCS achieves energy savings through optimal operation of the chiller throughout a 24 hour period to meet a daily cooling load. The chiller compressor speed, condenser fan speed, and chilled water distribution pump speed can be adjusted and scheduled for each hour of the day to meet the daily cooling load using minimal energy. The use of TES allows the chiller to run at times when it may be more optimal, such as overnight, then store the cooling for use later in the day when it is needed. This research focuses specifically on the use of concrete-core radiant cooling systems, or TABS, or TES.

Armstrong et al [2009a, 2009b] present a detailed description of the component systems, system models, and expected performance of an LLCS that consists of radiant cooling, a variable capacity chiller with variable speed distribution, ideal TES which can be discharged or charged arbitrarily without losses, and optimal predictive control. Computational models are developed in [Armstrong et al 2009a] for a compressor, condenser, liquid evaporator, air-side evaporator, DOAS, radiant cooling sub-system and variable speed transport that are valid under low pressure ratios and low capacity fractions of interest to LLCS. These models are used to compare different combinations of LLCS components to a baseline system comprising VAV distribution with a two speed chiller plant in [Armstrong et al 2009b].

[Armstrong et al 2009b] tested seven combinations of LLCS component systems, including different combinations of systems using variable speed chillers, radiant ceiling panels (RCP) with DOAS, and idealized TES and compared them to a baseline system consisting of a VAV air

handling unit (AHU) with an air-side economizer served by a two speed chiller without TES. The following eight cases were compared:

• VAV system with a two speed chiller (base case system to which others are compared) • VAV system with a variable speed chiller

• VAV system with a two speed chiller and TES • VAV system with a two speed chiller and TES • RCP/DOAS system with a two speed chiller • RCP/DOAS system with a variable speed chiller • RCP/DOAS system with a two speed chiller and TES • RCP/DOAS system with a variable speed chiller and TES

These eight mechanical system cases were simulated in a 20,000 square foot (sqft) medium office building as defined by the Advanced Energy Design Guide (AEDG) [Janargin et al 2006]. A standard performance envelope, glazing, shading, lighting and plug loads were defined based on ASHRAE 90.1 Standard for the Energy Performance of Building excluding Low-Rise Residential [ASHRAE 2004]. Two additional performance levels, a medium performance and a high performance building with increasingly better performance than ASHRAE 90.1 2004 were also simulated. The performance ratings for the different components of the buildings simulated are reproduced from [Armstrong et al 2009b] and shown in Table 1.

Table 1 Building component performance levels used in [Armstrong et al 2009b]

These eight mechanical systems in a medium office building with three levels of performance were simulated in five climate zones: Houston, Memphis, Los Angeles, Baltimore and Chicago. The resulting analysis showed the relative performance of the different combinations of LLCS components by building performance level in a variety of climates. The results, taken from [Armstrong et al 2009b], are shown in Figures 4, 5 and 6.

These results show that LLCS can have major energy savings in a variety of climates and buildings. Large reductions in energy savings occur with the progressive application of LLCS

component systems. In particular the jump between VAV systems and RCP/DOAS systems is pronounced. This is a result of both higher evaporating temperatures and more efficient chiller operation through the RCP as well as more efficient humidity control and reduced fan energy by the DOAS. The addition of TES separately also creates significant energy savings in each system configuration, due to efficient chiller operation at lower condensing temperatures through nighttime operation.

The combination of all of these systems - RCP/DOAS, variable capacity chillers, and TES - leads to the most significant energy savings. For the climates and building types tested in [Armstrong et al 2009b], energy savings for the full LLCS relative to a VAV system with a two speed chiller range from 70 to 74 percent for standard buildings, 46 to 73 percent for medium performance buildings, and 34 to 71 percent for high performance buildings. Most climates had savings in the 60 to 70 percent range. The lower energy savings correspond to the Los Angeles climate, where mild overnight and swing season conditions can lead to extended economizer operation and significant energy savings with a properly operated VAV system.

The Pacific Northwest National Lab (PNNL) conducted a follow up study to the analysis presented in [Armstrong et al 2009b] in which they analyzed the same eight LLCS configurations explained above using 12 modified DOE Benchmark Prototype EnergyPlus input files with two performance levels in 16 different climates. These DOE Benchmarks are representative of a range of commercial building types in the United States from small to large offices, hotels, hospitals, schools, warehouses and others. The 16 locations used in the study were selected to span the climate zones represented in ASHRAE 90.1 [Katipamula et al 2010].

The results from this recent research are similarly promising, if not more promising, than those in [Armstrong et al 2009b]. PNNL estimated that with 100 percent market penetration across 58 percent of newly constructed commercial floor area (where LLCS is applicable) the full LLCS system would save about 72 percent of cooling energy consumption relative to the DOE Benchmark systems. For standard medium office buildings, the average energy savings for the LLCS system relative to the DOE benchmark were 63 percent, while for high performance the average savings were 57 percent. Similar results with significant savings are presented for small offices, large offices, retail, healthcare facilities and other prototypical buildings. An overview of these results from [Katipamula et al 2010], which show the range in percent energy savings of the LLCS system relative to the DOE Benchmark system for a subset of the prototypical building types across all climates are shown in Table 2. The average savings across climates for standard performance buildings of different types are typically 60 to 70 percent, while the average savings for high performance buildings are typically 40 to 60 percent.

34 0 20000 40000 60000 80000 100000 120000

Houston Memphis Los Angeles Baltimore Chicago

A n n u a l e n e rg y u se ( k W h ) VAV w/ 2SD Chiller VAV w/ VSD Chiller VAV w/ 2SD Chiller, TES VAV w/ VSD Chiller, TES RCP/DOAS w 2SD Chiller RCP/DOAS w/ VSD Chiller RCP/DOAS w 2SD Chiller, TES RCP/DOAS w/ VSD Chiller, TES

Figure 4 LLCS configuration energy consumption for a 'standard' performance building in five climates

0 10000 20000 30000 40000 50000 60000 70000

Houston Memphis Los Angeles Baltimore Chicago

35 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Houston Memphis Los Angeles Baltimore Chicago

Figure 6 LLCS configuration energy consumption for a 'high' performance building in five climates

Table 2 LLCS energy savings relative to DOE benchmark by building type across 16 climates

An important comparison to note is the energy savings of the LLCS relative to a VAV system with a variable speed chiller in a medium office building. The VAV system with a variable speed chiller is the most similar system analyzed in [Armstrong et al 2009b, Katipamula et al 2010] to the high efficiency split-system air conditioner (SSAC) used as a baseline system in this research. The simulated energy savings of LLCS relative to a VAV with a variable speed chiller in medium office buildings ranges from 1.4 to 49.7 percent in standard performance buildings, with an average savings of 32 percent. In high performance buildings, energy savings range from -2.4 to 46.8 percent with an average savings of 18.8 percent. In Atlanta, the simulated annual energy savings for a standard performance building were 28.8 percent. These savings will be important to note because the experimental LLCS described in Chapter 6 was tested in Atlanta, with standard efficiency loads.

The potential for cooling energy savings from LLCS is great. However, first cost and practical construction considerations make wide-scale implementation of LLCS in every market a challenge. In addition to energy savings potential, [Katipamula et al 2010] presented economic benefits and barriers to the application of LLCS. Potential benefits include reduced size of HVAC equipment and subsequent reduced first costs, load shifting for reduced demand charges, and better humidity control and comfort performance – benefits previously described. However, a number of barriers limit the applicability of LLCS. The need for collaboration and integrated design between architects and engineers may be problematic. LLCS retrofit applications are limited due to the need to renovate ductwork and add TES. Advanced controls add complexity that not all controls contractors or building operators can support.

[Katipamula et al 2010] investigated the incremental costs and aggregate payback for LLCS systems in the 16 climates studied. For medium office buildings the incremental costs were negative and showed a payback of zero years in all climates. In cooling dominated climates paybacks for schools and large office buildings were in the range of 5 to 10 years, while in mild and heating dominated climates payback periods were greater than 10 years. In part, the cost of LLCS reflects a premium because radiant cooling is an emerging technology and DOAS and RCP systems are not yet produced at scale. However, in the immediate future new medium office buildings, particularly in cooling dominated climates, appear to provide the most cost effective application for LLCS.

The present experimental investgation of LLCS cooling energy savings is largely motivated by the substantial energy savings estimated by the PNNL simulation studies. This research seeks to build on the work of Armstrong et al [2009a, 2009b] and Katipamula et al [2010] to map a low- lift chiller system’s performance, test thermal model identification methods in the context of low-lift cooling, incorporate the behavior of a real TABS (not idealized TES) into a pre-cooling control algorithm, and experimentally test predictive, pre-cooling control of a low-lift radiant cooling system using TABS relative to a conventional variable speed split-system air conditioning system (SSAC). Chapters 3, 4, 5 and 6 will go on to explain the prior research on each of these issues separately, the development of new methods and application of existing methods to LLCS, and an experimental assessment of LLCS sensible cooling savings potential.

Chapter 3 Low lift chiller mapping and modeling

An important premise of low lift cooling is that compressors operate at higher efficiencies, or COPs, when the pressure difference, or lift, across the compressor is small. This is the major source of energy savings. Radiant cooling allows higher evaporating temperatures, and thus higher evaporating pressures, because higher chilled water temperatures are suitable for cooling a radiant concrete floor or radiant panels. TES allows for lower condensing temperatures, and thus lower condensing pressures, through nighttime operation of the chiller to pre-cool the TES or charge TABS. As a result, the difference between condensing and evaporating pressures and temperatures is reduced.

Historically, compressor and chiller efficiency ratings focus on the efficiency of the chiller at a single design load or a small set of part-loads. Because the thermal load on the chiller varies constantly over a year and cooling season, this approach ignores significant savings that could be achieved by operating the chiller at lower speeds and smaller pressure differences. Today, with the decreasing costs of and increasing efficiency and reliability of electrical inverters, or more accurately converters, variable cooling capacity chillers are available in which compressor speeds can modulate to meet loads more efficiently.

This chapter will review the state of knowledge about chiller performance at low lift conditions, present an experimental test stand for evaluating the performance of an air source heat pump with a rotary piston compressor under low lift conditions, and present empirical curve-fit models of the performance of the heat pump.

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