Wet Region

ENTHALPY, BTU PER LB

Ideal po

Wcompressor Brake power

For electric motor-driven systems, input power depends on motor efficiency as well as refrigeration and compressor efficiency. In English units, input power would be expressed as:

(hpoompeessor * 0.746 kW/hp)

motor /100)

In SI units, this would be expressed as:

For a combustion engine-driven refrigeration system, specific fuel consumption (SFC) of the prime mover would be expressed in English units as:

(hpc compressor x 2,545 Btu/hp-h)

'¡prime mover

In SI units, this would be expressed as:

/prime mover'

It is important to identify the assumptions used in calculating COP and fuel usage. With direct fuel-fired equipment, COP and energy usage may be based on either lower heating value (LHV) or higher heating value (HHV). [Refer to Chapter 5 for detail on heating value of fuel.]

For prime mover-driven refrigeration systems operating on cogeneration cycles, recovered energy must be considered. When heat recovery from a combustion engine is used in a refrigeration system, Equation 33-1 becomes:

£Qp Refrigeration effect Refrigeration effect

Energy — Energy Net energy input input displaced (33-19)

In English units, the net energy input can be expressed as:

Fig. 33-3 Plot of Refrigeration Cycle Phases on p-h Chart. Source: The Trane Company

System Energy Input

In the ideal case of isentropic compression, it is assumed that there is no heat transfer between the vapor being compressed and the surroundings, and that the compression process is frictionless. The actual power that must be applied to the shaft of a mechanical compression system (the brake power) is a function of the compressor isentropic efficiency, which is expressed as:

net input and in SI units as:

kWhnJkWhr = kWhh input/kWhr -

Btu , /ton-h recovered

boiler

kWhh recovered /kWhr boiler

In cogeneration systems using a back-pressure or extraction steam turbine, the energy input is the fuel input to the boiler, and steam at the turbine outlet provides recovered heat. A method of expressing net energy use for a steam turbine is to subtract the total enthalpy (expressed in Btu or kWhr) at the turbine outlet from the total enthalpy at the turbine inlet, and divide by the boiler efficiency. If recovered heat is used to generate additional cooling via an absorption chiller or a bottoming cycle steam turbine, the additional cooling output potential is calculated as:

Additional capacity =

Heat recovered

Energy input requirement

Combining the output of both systems, the energy input rate can be expressed as:

Energy input (33-23)

Total energy= _

input rate (Primary capacity + Additional capacity) COP can then be expressed as:

Combined refrigerant effect

Energy input

Refer to Chapter 2 for a detailed discussion on cogen-eration cycle thermodynamic performance expressions. This includes calculations applicable to quantifying the impact of recovered heat on net and overall system performance.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment