Design and optimisation of a solar thermal powered Aqua-Ammonia diffusion-absorption heat pump cycle under seasonal variance for residential use
Abstract
The diffusion-absorption refrigerator was patented by Von Platen and Munters (1926). The
conventionally low coefficient of performance and the high driving temperature have been the
focus of most research since its conception. Previous research pertaining to diffusion-absorption
component modifications, mathematical models, alternative fluid options and transient
experimental simulations are reviewed to determine the drawbacks and performance
improvement opportunities.
In order to apply the diffusion-absorption technology to domestic applications, it is necessary to
increase the coefficient of performance and decrease the required driving temperature. The
design and optimisation discused in this thesis are therefore for the thermodynamic cycle rather
than the diffusion-absorption components. A lower driving temperature will enable the effective
application of solar thermal power since more low-grade heat can be absorbed at lower
temperatures.
A mathematical model was constructed to evaluate the influence of operating parameters on the
diffusion-absorption system performance. This model is validated against experimental results
reported by Ersöz and Najjaran. From the parametric evaluation, opportunities are identified to
increase the system coefficient of performance and lower the driving temperature requirement.
In summary the contribution of the endeavours and outcomes of this thesis are devising methods
to mitigate the negative effects of varying ambient conditions and the optimisation of non-standard
operating parameters.
Methods to enable the realisation of these opportunities were identified and the integration
thereof, into the diffusion-absorption system, is discussed. These methods are afforded by the
residential use of the diffusion-absorption system and will enable supply to both heating and
refrigerating applications.
The main supporting system is the use of a radiant heat rejection with a cold storage system to
improve the condenser and absorber temperatures. This enables the negative influence of interdaily
and seasonal ambient temperature fluctuations to be dampened while the condenser
temperature is decreased. A reduction in condenser temperature results in a lower system
pressure, a lower generator temperature and a higher evaporator heat gain that result in an
increased coefficient of performance.
Consideration is given to the stable continuous operation of the diffusion-absorption system and
the supporting systems necessary to realise this for an inherently intermittent solar supply. The
use of compound parabolic collectors is proposed to absorb solar thermal energy during daytime
while thermal storage systems are proposed to distribute the heat applied to the diffusionabsorption
system. This enables the diffusion-absorption system to be proportionally smaller than
a system operating only during the day.
The strong solution concentration (xss = 0.6171) is optimised to maximise coefficient of
performance with functional constraints specified by the supporting systems and the high and
low temperature yield requirements for domestic appliances as well as bubble pump flow regime
considerations. Refrigeration and heat pump coefficients of performance are respectively
maximised at 0.703 and 1.703 for a generator bubble pump driving temperature of 72.3 °C, while
heating and refrigerating yields can effectively be applied for domestic use. The diffusionabsorption
driving temperature is sufficiently reduced to enable effective solar thermal powering.
A feasibility evaluation was done on the residential application of a solar thermal powered
diffusion-absorption system for a typical family of four. It is reported that a diffusion-absorption
system with a continuous 1102 W heat input (solar thermal) to the generator bubble pump would
be able to supply the essential heating and refrigerating appliances throughout the year. A highlevel
return on investment (ROI = 10.84 years) calculation deemed the proposed system
financially feasible and options to reduce the ROI are discussed.
Novel contributions made in this thesis include the mathematical model constructed in such a way
that fundamental principles and flow regime constraints are included and that the optimisation of
non-standard operating conditions is enabled. The constant condenser outlet temperature
operation of the diffusion-absorption system is the main contribution made in this thesis. This is
enabled through the residential integration and the application of a radiant heat sink system with
cold storage.
The operational stability and performance of the diffusion-absorption system are increased by the
optimisation of identified non-standard operating conditions and the driving temperature is
decreased thereby. This enables the effective solar thermal powering of the diffusion-absorption
system while the solar thermal heat store enables continuous stable 24-hour operation. A
refrigerant expansion tank is thermodynamically designed to maintain the bubble pump and
diffusion-absorption performance irrespective of seasonal fluctuations.
Future recommendations are made for further development of the identified supporting systems.
It is also recommended that detailed component mechanical design is done to build on previous
research and that, provided the necessary funding, the proposed diffusion-absorption system is
constructed and experimentally tested.
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