A vast amount of research has been performed towards the goal of under- standing and improving the operation of vapour absorption technologies. This section discusses a selection of research from the eld of vapour absorption that is of relevance to the project at hand.
2.5.1 Simulation Focused Studies
The simulation of vapour absorption systems has been approached through steady state solutions, transient solutions as well as dynamic responses. These various approaches for simulations can provide insight into dierent aspects of a vapour absorption system. In the case of a steady state simulation the resulting eect of altering operating conditions can be assessed. Transient simulations can be used to investigate the start up behaviour and control of a vapour absorption system. Lastly, dynamic simulations can be used to examine the response that the simulation would have to an external disturbance. In addition to the dierent types of simulations, the literature that was found regarding the simulation of a vapour absorption system showed that a number of dierent software packages could be used for the steady state simulations. This section will continue with a short discussion regarding a number of articles that document the simulation of vapour absorption systems.
Herold et al. (1996) shows how the Engineering Equation Solver (EES) can be used to simultaneously solve the heat transfer rates, heat transfer sur- face areas and cycle temperatures from supplied external cycle temperature in combination with the values for the overall heat transfer coe cient values in a steady state scenario. EES is also used in an extension of this example
CHAPTER 2. BACKGROUND LITERATURE AND THEORY 20 to maximize the refrigeration potential that can be obtained from the cycle speci cations.
Grossman and Zaltash (2001) discusses the development and usage of the ABsorption SIMulation program (ABSIM). This simulation program was made for the sole purpose of steady state simulation of vapour absorption cycles and makes use of a graphical user interface that allows a user to con gure the cycle components that are to be simulated. ABSIM is capable of simulating expansive vapour absorption cycles and includes the property functions for a number of vapour absorption working pairs.
Somers et al. (2011) made use of ASPEN Plus, a chemical process package, to simulate both a single eect and a double eect vapour absorption cycle. The results of this simulation compared well with the results obtained from a similar simulation that was performed with EES.
Bakhtiari et al. (2011) developed a model to assess the steady state be- haviour of a single eect water/lithium-bromide cycle. The external streams were used as the user speci cations for this steady state simulation. Results from an experimental apparatus were used for veri cation of the model and the results were favourable.
Kohlenbach and Ziegler (2008) combined the use of a steady state solu- tion with a transient model in order to examine the dynamic response of a water/lithium-bromide cycle. A nite dierence Jacobian method was used to solve the equations via MATLABr.
Kim and Park (2007) implemented dierential equations in order to de- scribe the transient behaviour and dynamic response of a single eect ammo- nia/water absorption chiller. A Runge-Kutta-Merson technique was used to simultaneously solve the equations. The behaviour of the simulated cycle dur- ing transient start-up as well as during a dynamic response to a change in the fuel supply of the cycle was discussed.
2.5.2 Experimental Investigations
The majority of articles that were found during the literature survey that re- ferred to experimental investigations regarding the operation of a vapour ab- sorption system made use of commercially available vapour absorption units as part of a larger installation. A further distinction could be made in or- der to categorize these articles according to similar directions of investigation. Two popular research avenues were encountered, namely the investigation of the steady state performance of a vapour absorption system when the operat- ing conditions were altered and the performance of a solar-driven absorption system. In addition to this, there were a number of articles located that investigated the use of vapour absorption technology in various industrial ap- plications where the heat source for the system was geothermal or from any of a number of sources of process energy.
CHAPTER 2. BACKGROUND LITERATURE AND THEORY 21 Three articles were chosen to be discussed with regards to experimental vapour absorption installations. The rst of which makes use of a commercial vapour absorption system to examine the eect that variations in operating conditions have on the system. Many applications of vapour absorption were found in literature with reference to the source of the thermal energy. However, the decision was made to include a study that examined the operation of a solar activated vapour absorption system. The last article that is included covers the design and construction of a small capacity vapour absorption cycle.
Asdrubali and Grigna ni (2005) made use of a Yazaki water/lithium- bromide absorption chiller as part of an experimental plant. This experimental plant was used to assess the steady state performance of the absorption chiller at various operating conditions.
Agyenim et al. (2010) discusses the use of a small capacity commercial absorption system as the cooling apparatus for a solar powered refrigeration installation. This installation made use of a solar collector to indirectly fuel the absorption chiller. A cold water tank was used as a buer between the chiller outlet stream and the fan coil unit that was used for the space cooling. The operation and performance of the installation is further discussed with the conclusion being made that as far as electrical consumption is concerned the system as a whole compares well with mechanical air conditioning systems.
Kalogirou et al. (2001) discusses the design and construction of a 1 kW water/lithium-bromide vapour absorption chiller. The desired cycle output was used as the design point around which the heat exchangers of the apparatus were sized according to the heat exchanger requirements and the available theory for the heat transfer coe cients. Heat exchangers for the generator, absorber and evaporator were submerged in their respective liquid sections. In addition to this, the length of the generator heat exchanger was varied in order to determine viable lengths of piping for dierent inlet temperatures.
2.5.3 Heat and Mass Transfer
The specialized nature of vapour absorption working uids in combination with the absorption process itself make it necessary to look beyond standard heat transfer text books for relevant information regarding the heat and mass transfer characteristics active in vapour absorption technology.
Charters et al. (1982) discusses the development of an equation that de- scribes pool boiling of water in addition to the evaporation of water from aqueous solutions like water/lithium-bromide. The ndings of this study were only considered to be valid for use with smooth surfaces of platinum, copper and brass.
Varma et al. (1994) reported on the results of an experimental investigation into the heat transfer coe cients during the pool boiling of water/lithium- bromide solution. Stainless steel tubes of three dierent diameters were ori- entated horizontally in the solution. A nding of this study was that the
CHAPTER 2. BACKGROUND LITERATURE AND THEORY 22 solution concentration has a noticeable eect on the value of the heat transfer coe cients.
Táboas et al. (2007) compared the results obtained from the boiling cor- relations for pure and mixed liquids with experimental data for the pooling boiling of ammonia/water. The article presents a new correlation for use with the pool boiling of ammonia/water. This proposed correlation combines the calculation process used in two of the correlations for mixtures.
Fujita and Hihara (2005) presented a model and calculation process for the numerical solution for the heat and mass transfer coe cients in an ab- sorption process. This article compares a few dierent approaches that can be implemented for the analysis of a falling lm absorber and suggests a solution method.