Chiller performance ratings typically present the efficiency of a chiller at a very limited set of conditions. The COP of a chiller can vary with many parameters such as compressor speed, condenser fan speed, primary chilled water flowrate, outdoor temperature, chilled water temperature, or the amount of superheat and subcooling. Despite its highly variable nature, chiller performance is generally presented as a single value to enable engineers to compare and specify chillers using a common metric.
COP at design conditions presents only one value of performance, the chiller efficiency at one design load. Another metric, integrated part load value (IPLV), combines chiller performance values at four different operating conditions, with the chiller running at 25, 50, 75 and 100 percent of part load at specified condensing temperatures. The result is a weighted average
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efficiency at four part load conditions. The IPLV rating assumes a chiller will typically run at given part loads a specific amount of time, while in reality a chiller may run at very different part loads and run-times depending on climate, internal loads, mechanical system design, and the attenuation of load peaks by building thermal mass. COP and IPLV, while invaluable to engineers for design and comparison, do not reflect the full range of operating conditions and efficiencies possible with a given chiller.
Dunn et al [2005] found, anecdotally, through monitoring actual system performance in four buildings that the chillers ran on average between 8 and 44 percent of their capacity most of the time. They suggest that this is in part due to over-sizing of equipment and in part due to unoccupied, night-time operation at low loads. On the other hand, Geister and Thompson [2009] found through simulation that many chiller plants, particularly those with multiple chillers, will run at higher part loads and lifts more often than the assumptions used for IPLV, and that actual part-load runtimes will vary significantly by climate. Their conclusion is that an hour by hour simulation with a chiller performance model is the only accurate way to estimate chiller performance and energy savings. The results of these studies and others suggest that design condition COP and IPLV are not enough to predict actual performance of equipment installed in the field.
In an LLCS cooling strategy, variable capacity chillers are employed where part load operation to achieve high efficiency is desirable and nighttime operation is enabled by TES. Understanding the performance of the chiller at a wide range of conditions beyond existing metrics is important for maximizing efficiency and energy savings in LLCS. A performance map or look-up table which specifies system power consumption, COP or electric input ratio (EIR, which is the reciprocal of COP) as a function of condensing temperature, evaporating temperature and cooling capacity can be used to model chiller performance within a predictive TES pre-cooling control algorithm. Surrogate variables might include outdoor temperature and condenser fan speed instead of condensing temperature, chilled water return temperature and chilled water pump speed instead of evaporating temperature, and compressor speed and superheat instead of cooling capacity.
Armstrong et al [2009a] developed a set of physically based models of an LLCS. These were made up of component models of a variable capacity chiller with a variable speed condenser fan and chilled water flow, a radiant cooling distribution system, an idealized TES system, and a DOAS for ventilation and dehumidification. A chiller-radiant subsystem performance map spanning low-lift conditions was created based on computational models of a reciprocating compressor, condenser, liquid evaporator, variable speed pump, variable speed fan and radiant panels. The chiller-radiant subsystem performance map is presented in Figure 7.
The performance map illustrates the efficiency, EIR, of the LLCS chiller-radiant subsystem at a wide range of conditions down to low pressure ratios as a function of outdoor temperature, zone temperature, and cooling capacity. It shows the specific power 1/COP, or EIR, in kilowatts (kW) of electricity per kW of thermal cooling delivered as a function of capacity fraction, which is equivalent to cooling delivered divided by cooling capacity at full-speed, at fixed outdoor
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temperatures Tx(C) and an indoor temperature of 22.2 C (or 72 F). A refrigerant economizer is included in the chiller model. It is evident from Figure 7 that in theory much higher COPs, as high as five to 20 kWe/kWth, are possible at low capacity fraction and low outside temperatures
relative to a typical chiller or heat pump COP of three or four. Enabling chillers too run at these high efficiencies more of the time is the goal of LLCS.
One problem with the model in [Armstrong et al 2009a] for concrete-core radiant cooling applications is that the zone air temperature alone, which is used in the chiller model, is not a sufficient surrogate for the evaporating temperature of the chiller serving the concrete-core. In the case of concrete-core radiant floors, the return water temperature from the pipe embedded in the concrete is the variable of interest for determining chiller efficiency and performance. The return water temperature is related to the slab temperature and cooling rate, and the slab temperature is determined by past cooling rates, slab temperatures, and zone air temperatures. As a consequence, a different performance map is required for chiller- radiant concrete-core systems where current return water temperature is a variable instead of current zone air temperature.
In sections 3.2 and 3.3, a performance map of a heat pump will be measured and mapped in which zone air temperature directly influences the system efficiency. In chapter 6, which describes implementation of an LLCS with a concrete-core radiant floor cooling system, these maps will be modified to represent the performance of an air-cooled chiller in which return water temperature will replace zone air temperature as the evaporator fluid entering temperature of interest. Capacity Fraction (CF) 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
Figure 7 Chiller-radiant subsystem performance map based on first- principles modeling in [Armstrong et al 2009a]
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