Effect of Temperature on Heat Pump Efficiency in Refrigerators, Air Conditioners, and Water Treatment Plants

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Engineering Research Practice Assignment 3
Proposal Guidelines
Student Name:
Your Name
Student ID Number:
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RESEARCH DETAILS
Research Question:
 How does the temperature affect the efficiency of a heat pump in a refrigerator?
 How does the temperature affect the efficiency of a heat pump in an air conditioner?
 How does the temperature affect the efficiency of a heat pump in a water treatment plant?
Background and Problem Description
A heat pump defines a device used in the transfer of heat from the heat source to a heat sink.
Heat pumps work by transferring thermal energy in the reverse direction of a spontaneous heat
transfer through absorption of the heat from a cold space and then letting it go in a warmer place.
It makes use of external power in accomplishing the transfer of energy task from the source of
heat to heat sink (Bach et al., 2016). The most commonly used heat pump design is composed of
four major components: condenser, evaporator, expensive valve as well as compressor. The
medium of what transfer circulated via such components is called refrigerators.
Mechanical heat pumps make use of the physical features of a volatile evaporating as well as
condensing fluid called refrigerant. The refrigerant is compressed by the heat pump making it
hotter on the part that is to be warmed and the pressure is released to the side in which the heat is
to be absorbed.
The working fluid when in the state of a gas is subjected to pressure and circulated via the
system using a compressor. The vapor that is now very hot and at very high pressure at the
discharge side of the compressor undergoes cooling in a heat exchanger known as a condenser to
the point it is condensed to high pressure and moderate temperature liquid (Fang and Lahdelma,
2015). The condensed refrigerant is then passed through a device that lowers the pressure known
as a metering device which could be an expansion valve, turbine or even a capillary tube. The
refrigerant that is at low pressure thereafter gets into yet another heat exchanger called the
evaporators where there is the absorption of heat and then boiling by the fluid. The refrigerant
then gets back to the compressor and the cycle repeats itself.
It is important the refrigerant gets an adequately high temperature upon compression to release
the heat via the hot heat exchanger (Fikru and Gautier, 2015). At the same time, the fluid has to
get to an adequately low temperature in the cold heat exchanger. To be specific, the difference in
pressure has to be large enough to enable the liquid to condense at the hot side and as well
evaporate in the region of low pressure at the cold side. The greater the difference in
temperature, the greater the pressure difference needed and as a result the higher the required
energy in compressing the fluid. Hence, as for the case of all heat pumps, the CoP is decreased
when the temperature difference is increased.
One of the common benefits that come with heat pumps revolves around their energy efficient
heating method (Jylhä et al., 2015). Ground, as well as air source heat pump efficiency, may go
beyond 300% as they transfer heat as opposed to producing it. Nevertheless, keeping this
efficiency level is of importance to make the use of heat pumps are a worthy investment. The

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efficiency of heat pumps is a factor of how hard they have to work to keep comfortable room
temperature in space where greater efficiency depends on lower flow temperature. This is the
reason for the bigger radiators being more efficient.
The efficiency of heat pumps is a measure of the coefficient of performance that denotes the
extent of efficiency of the heat pump system is able to heat space within the best possible
conditions. Using such a scale, the efficiency of a commercial air source heat pump may be as
high as four while that of the ground source heat pumps may hit 5 (Klein et al., 2017). This
research is aimed at finding the relationship between temperature and the efficiency of a heat
pump in systems including wastewater treatment plant, air conditioner as well as a refrigerator.
The research would then determine the optimum operating temperature for the highest efficiency
of each of the equipment.
Scope of research
The scope of this research problem is as follows:
The work will only involve performing an analysis of the effects of temperature on the efficiency
of a heat pump for the case of the three machines: refrigerator, waste treatment plant, and air
conditioner.
Calculations aimed at determining the efficiency at various temperatures of operation and then
coming up with the optimum operating efficiency temperature for a heat pump
The research and design work is limited to finding the relationship between temperature and
efficiency of operation only and no other design parameters will be taken into consideration
hence will be assumed
The technical bit of the research will be inclusive of hardware systems alongside mathematical
calculations using various statistical analysis tools
Expected Outcomes and benefits
The findings of this research are:
 A mathematical for the optimum temperature for operation of a heat pump for an air
conditioner, a refrigerator, and a waste treatment plant
 The anticipated benefit is to come up with a heat pump that is able to work under
optimum temperatures ensuring maximum energy saving and thus most efficient
Literature Review
Numerous researchers have conducted research in the sector of vapor absorption with the use of
various working pairs with the most common working combinations being LiBr-H2O and NH3-
H2O. A theoretical study was carried out by Alizadeh et al on the optimization as well as the
design of the water-lithium bromide refrigeration cycle. The made a conclusion that a specific
refrigeration capacity higher temperature of the generator results in the high ration of cooling
with smaller heat exchange surface as well as low heat (Lake, Rezaie and Beyerlein, 2017).
There exists a limit factor for water lithium cycles due to the challenge of crystallization. The
analysis of the availability as well as the calculation of irreversibility within a system component
was studied by Anand and Kumar of a single as well as double effect series absorption systems
flow water-lithium bromide. The assumed dimension for result computation was the temperature
of the condenser as well as that of the condenser that was equivalent 140.6⁰C for the case of the
double effect while 87.8⁰C for the case of single effect systems.
The extensive study was carried out by Tyagi on the aqua-ammonia VAR system and generated
the coefficient of performance as well as the rates of mass flow in terms of the operating
parameters including the absorber, generator as well as evaporator temperatures (Levihn, 2017).
He demonstrated CoP, as well as the work, was done was the function of the temperature of the
condenser, absorber, and evaporator alongside being dependent on the features of the binary
solution. The irreversibility in the parts of the aqua-ammonia absorption refrigeration system

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using second law analysis was shown by Ercan and Gogus. The dimensional less exergy of the
individual parts, coefficient of performance, energetic coefficient of performance as well as
circulation of the different temperature of generator, evaporator, and absorber was calculated. A
conclusion was made that the aqua-ammonia system requires a rectifier for high concentrations
of ammonia but would result in extra exergy loss within the system (Love et al., 2017). The
noted that the highest loss in exergy was in the evaporator and followed closely by the absorber.
A conclusion was as well made that dimensionless total exergy loss is a factor of the temperature
of the energy.
A gas-fired and air-cooled LiBr/H2O double effect flow kind of absorption heat pump of 2TR
was investigated by Oh et al that is used in the capacity of an air conditioner. The heat pump
performance of absorption was investigated in the cooling mode via cycle simulation. The
features of the system based on the temperature of the inlet air to the absorber, the concentration
of the working solution, the ratio of the distribution of the solution mass into the first generators
to the total mass of solution from absorber as well as the difference in leaving temperature of the
components of heat exchange. They made a conclusion there is a critical value of the solution
distribution ratio which maximizes the cooling performance of the system. The single effect
water-lithium bromide system was investigated by Aphornratana and Eames through the exergy
analysis approach (Lund et al., 2017). It was demonstrated the irreversibility in the generator
turned out to be the highest followed closely by absorber and then evaporator.
An experimental absorption cooling system controlled by heat produced by solar energy was
developed by Bell et al in which the system components were enclosed in an evacuated glass
cylinder to allow observation of all the processes that were taking place. The thermal
performance of the system was determined through the application of mass as well as energy
mass for every component. Their research was pegged on assumption the working fluids are at
equilibrium as well the temperature of working fluid that left the absorber and generator was
equivalent to the generator and absorber temperature respectively (Lundström and Wallin, 2016).
They made a conclusion that the CoP of the system was a factor of the temperature of the
generator and there exists an optimum value for the temperature of the generator where the CoP
is maximum. They as well made a conclusion that by having the system operate at low
condenser as well as absorber temperatures; it is possible to obtain a satisfactory value of CoP at
a temperature of the generator which may be 68⁰C. The basis vapor absorption refrigeration
system was explained by Horuz who as well conducted a comparative study on the system
depending on ammonia-water as well as water-lithium bromide working pairs. The presentation
of the comparison between the two systems is made with regard to CoP, maximum as well as
minimum pressures as well as the cooling capacity. A conclusion was made that the VAR
system depending on the water-lithium bromide system.
The exergy changes in the solar aided absorption system were studied extensively by Kumar et
al in which they established the increase in first generator heat transfer results in a decrease in
the transfer of heat in the second stage generator (Luo et al., 2016). The rise in the temperature
of the second generator results in a decrease in exergy as well as the transfer of energy in the
condenser. They made a conclusion that the availability at the devices changes with regard to the
device quality. Exergy analysis was conducted by Talbi and Agnew on a single effect absorption
refrigeration cycle using lithium bromide water in the capacity of working fluid pairs. A
computer simulation model was developed depending on the mass and heat balance,
thermodynamic features as well as the equations for heat transfer. The cycle gathers free energy
from diesel engine exhaust. They determined the exergy loss as well as the dimensional less total
exergy loss of every component in which they established the absorber indicated the highest
exergy loss at 59.06% that was closely followed by the generator. They made a conclusion that