3413 3413

Source COP =

20 Btu/watt-h 10.750 Btu/watt-h

Case B. A peaking plant (i.e., simple-cycle gas turbine) may have a heat rate in excess of 14,000 Btu/kWh (4.10 kWhh/kWhe) and experience T&D losses of 12% (line losses are generally at their highest in peak conditions), resulting in a total heat rate for a delivered kW of 15,680 Btu/kWh (4.59 kWhh/kWhe). The source COP for the same 20 EER unit would be calculated as:

Power plant heat rate in Btu/watt-h

In SI units, EER would be replaced with watt-h of refrigerant output divided by watt-h of electricity input, and power plant heat rate would be expressed in watt-h heat input divided by watt-h electric output.

Examples

Consider an electric-driven centrifugal chiller rated at 20 EER. The end-use COP would be calculated as:

Source COP =

20 Btu/watt-h

15.680 Btu/watt-h Case C. An efficient baseload (i.e., coal) plant serving an off-peak cooling load may have a heat rate of 8,500 Btu/kWh (2.49 kWhh/kWhe) and experience T&D losses of 3.5% (line losses are generally at their lowest in off-peak conditions), resulting in a total heat rate of 8,798 Btu/kWh (2.58 kWhh/kWhe). The source COP for the same 20 EER unit would be calculated as:

Source COP =

20 Btu/watt-h 8.798 Btu/watt-h

This same chiller unit, when driven by a prime mover, operating on a simple-cycle, with a thermodynamic efficiency of 32% (HHV basis), might have an end-use COP rating of 2.0. In this case, the end-use rating is the same as the source COP rating, since the fuel energy is being consumed on site.

As a basis of comparison, the source COP of the electric-driven unit is computed for three alternative central electric generation systems that could realistically be serving power to this unit.

Case A. An intermediate-load power (i.e., oil-fired steam) plant may have a cooling season heat rate of 10,000 Btu/kWh (2.93 kWhh/kWhe) and experience T&D losses of 7.5%, resulting in a total heat rate of just under 10,750 Btu/kWh (3.15 kWhh/kWhe). The source COP for the 20 EER unit described above would be

In these three examples, the source energy usage was shown to range from 2.5 to 4.5 times that of the end-use energy usage. On the basis of an end-use COP computation, the prime mover-driven unit appears to have grossly inferior thermodynamic performance when compared with the electric-driven unit. However, on the basis of a source COP computation, the two drive options are roughly comparable, varying by degree as opposed to by orders of magnitude. What can be concluded from this analysis of source versus end-use COP for electrically driven equipment is that the source results, or true thermodynamic performance, are quite comparable with on-site fuel- or steam-powered systems. The ensuing chapters in this section apply the standards and concepts presented in this chapter to a variety of cooling, air conditioning, and refrigeration technologies and applications.

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