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The effect of different working fluids and internal geometries on the thermal performance of heat pipes in evacuated tube solar collectors
Author(s)
Mukuna, Jean Gad Mubala
Date Issued
2021
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
Evacuated tube heat pipe solar collectors produce hot water more efficiently than the flat plate
type. It is observed from various studies that involve evacuated tube heat pipe solar collector
that its thermal performance depends on various external and internal factors. The internal
factors are primarily related to the performance of the heat pipe. It was assumed that the two
elements that define the efficiency of the heat pipe are the working fluid and the internal
geometry. No inclusive study exists that systematically investigated the combined effect of
working fluid and internal geometry on the thermal performance of heat pipes in evacuated
tube solar collectors.
This investigation consisted of the design and testing of an evacuated tube solar collector
comprising new modified heat pipes. Heat pipes containing inserts with particular profiles were
tested with different working fluids. A multivariate polynomial regression analysis was
conducted to validate the assumption that the merit number of the working fluid and the insert's
surface area affect the efficiency.
The regression analysis that resulted from the assumption that the merit number and the
surface of the insert were the independent variables affecting the heat pipe’s efficiency had to
be rejected due to poor/unacceptably low coefficient of determination (R2) result.
Based on a newly thought assumption, that the boiling point temperature of the working fluid
could be an independent variable affecting the heat pipe’s performance or efficiency, a new
regression analysis produced very acceptable results.
The results have shown that the surface areas of the insert in the heat pipe had an impact on
the efficiency of the solar collector. This is possibly because of the enhancing of the heat
transfer by increased convection between the surfaces of the insert and the vapour of the
working fluid moving from the evaporator to the condenser. For example, with distilled water
as the working fluid, the experiment on the conventional circular heat pipe without an insert
produced an efficiency of 53.3%, while results on the single insert and the S insert show
efficiencies of 59.1% and 64.3% respectively.
A very good correlation with a R2= 0.99 was obtained when expressing the efficiency as a
function of the boiling point of the working fluid for an evacuated tube heat pipe. It transpires
from this correlation that the working fluid with a high boiling number has also a high thermal
efficiency. The comparison between the measured and the predicted results using the final multivariate
polynomial regression (equation 5.2) exhibits excellent accuracy in the prediction of the
performance of an evacuated heat pipe solar collector with R2 = 0.98 and an average error of
1.1%. It may be of assistance in predicting the heat pipe’s efficiency for any untested insert’s
profile, or working fluid’s boiling temperature or a combination of both.
type. It is observed from various studies that involve evacuated tube heat pipe solar collector
that its thermal performance depends on various external and internal factors. The internal
factors are primarily related to the performance of the heat pipe. It was assumed that the two
elements that define the efficiency of the heat pipe are the working fluid and the internal
geometry. No inclusive study exists that systematically investigated the combined effect of
working fluid and internal geometry on the thermal performance of heat pipes in evacuated
tube solar collectors.
This investigation consisted of the design and testing of an evacuated tube solar collector
comprising new modified heat pipes. Heat pipes containing inserts with particular profiles were
tested with different working fluids. A multivariate polynomial regression analysis was
conducted to validate the assumption that the merit number of the working fluid and the insert's
surface area affect the efficiency.
The regression analysis that resulted from the assumption that the merit number and the
surface of the insert were the independent variables affecting the heat pipe’s efficiency had to
be rejected due to poor/unacceptably low coefficient of determination (R2) result.
Based on a newly thought assumption, that the boiling point temperature of the working fluid
could be an independent variable affecting the heat pipe’s performance or efficiency, a new
regression analysis produced very acceptable results.
The results have shown that the surface areas of the insert in the heat pipe had an impact on
the efficiency of the solar collector. This is possibly because of the enhancing of the heat
transfer by increased convection between the surfaces of the insert and the vapour of the
working fluid moving from the evaporator to the condenser. For example, with distilled water
as the working fluid, the experiment on the conventional circular heat pipe without an insert
produced an efficiency of 53.3%, while results on the single insert and the S insert show
efficiencies of 59.1% and 64.3% respectively.
A very good correlation with a R2= 0.99 was obtained when expressing the efficiency as a
function of the boiling point of the working fluid for an evacuated tube heat pipe. It transpires
from this correlation that the working fluid with a high boiling number has also a high thermal
efficiency. The comparison between the measured and the predicted results using the final multivariate
polynomial regression (equation 5.2) exhibits excellent accuracy in the prediction of the
performance of an evacuated heat pipe solar collector with R2 = 0.98 and an average error of
1.1%. It may be of assistance in predicting the heat pipe’s efficiency for any untested insert’s
profile, or working fluid’s boiling temperature or a combination of both.
Additional information
Thesis (DEng (Mechanical Engineering))--Cape Peninsula University of Technology, 2021
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