Problem 4.14 - The BSU Geothermal Heat Pump
System (Wintertime)
Recall Problem
4:13 in which we evaluated the summertime operation of
a geothermal heat pump system modeled after the system currently
under construction at Ball
State University in Indiana. It's purpose is to provide
hot water at 65°C year round together with air-conditioning
during summer and space heating during winter.
In this exercise we wish to evaluate the system
for wintertime operation, in which we wish to provide hot water
at 65°C as well as space heating at above 20°C, while
the outdoor air temperature could be as low as -10°C. In this
operation we introduce two double-port control valves as shown
below, to switch between summertime and wintertime operation,
in which the geothermal heat source now becomes the evaporating
section of the heat pump, and the refrigerant subcooler now heats
the water system used for space heating.
Note again that this system was devised by
us for purposes of this exercise only, and no data about the system
was obtained from BSU. We have used the Refrigerant
R134a, since this is the only refrigerant for which we
have tables available. In fact we had to add new data to our tables,
since with a limit of 1.6 MPa we could not reach the required
temperature of 65°C. Note that the mass flow and actual power
required is not specified, thus this model will represent a system
suitable for any size. All energy results will be in units of
kJ/kg.
Using the conditions shown on the diagram do
the following
- 1) On the P-h diagram provided below
carefully plot the five processes of the heat pump together with
the following constant temperature lines: 65°C (hot water),
13°C (ground loop), and -10°C (outside air).
- 2) Using the R134a
property tables determine the enthalpies at all five
stations and verify and indicate their values on the P-h
diagram.
- 3) Determine the specific work done to drive
the compressor [-43.8
kJ/kg].
- 4) Determine the Coefficient of Performance
of the hot water heating system (COPHW) (defined as
the heat absorbed by the hot water divided by the specific work
done to drive the compressor)[COPHW
= 3.4]
- 5) Determine the Coefficient of Performance
of the space heating water system (COPHP) (defined
as the heat absorbed by the space heating water in subcooling
the refrigerant to 20°C divided by the specific work done
to drive the compressor).[COPHP
= 1.56]
- 6) In the event that no geothermal ground
loop is used to evaporate the refrigerant then the system would
need to be redesigned, reducing inlet to the compressor (1) from
360 kPa to 140kPa, saturated vapor, and the compressor outlet
(2) to 2.0 MPa, 90°C. Carefully plot this new cycle on the
P-h diagram.
- 7) Using the R134a
property tables determine the specific work done to drive
the compressor under the new conditions presented in 6) above
[-71 kJ/kg].
- 8) Using the new conditions presented in
6) above determine the Coefficients of Performance of both the
hot water and space heating systems and discuss the advantages
of using a geothermal ground loop for wintertime operation.[COPHW = 2.3, COPHP
= 0.96]
