Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: Solar energy is a time dependent, high-temperature radiant energy resource.
The utility of a solar thermal energy system increases if the hot temperature
source is available when it is needed most. This is realized by the thermal
storage of the solar energy. Thermal storage gives greater versatility to a solar
energy system by decoupling the heat source from the heat sink. A large
quantity of energy may be stored during the melting process in a phase change
material (PCM) within a small temperature range. This molten PCM can then
deliver its absorbed heat at a constant temperature in a heating application. In
this study a phase change storage system (PCS) is developed and proposed for
a solar water heating application. This PCS system stores more heat per unit
mass than would be possible with water across the same temperature range.
The heat transfer rate in and out of many PCMs is slow because of the low
thermal conductivity of the PCM. However, heat transfer enhancers (HTE),
such as heat pipes and fins may be added to enhance heat absorption and heat
removal rates. Heat pipes have the inherent capability to transfer heat at high
rates across large distances, even where the temperature difference is small.
In this thesis a description is given of a PCS system consisting of paraffin
wax as the PCM and which uses rectangular heat pipes in conjunction with
aluminium fins to enhance heat transfer. The storage design is modular and
each module has the characteristic that enhanced heat transfer in and out
of the PCM is possible when the module is heated or cooled. It also has the
capability to quickly absorb or alternatively to supply heat at a nearly constant
temperature during the phase change of the module.
A rectangular module was designed and built. The module was then analysed
under controlled heat absorption and heat removal cycles. The heat up experiment
involved an electrical kettle as the hot temperature source. The heat sink
was a mains water heat exchanger. The experimental results were compared
to those of a transient numerical model, which calculates theoretically how the
module will perform thermally under the given test conditions. The numerical
model of the experimental set-up was validated when it was found that the
numerical model results resemble the experimental results. The numerical
model was then adapted to simulate a novel solar water heater (SWH) with
an additional PCS container. The improvement over previous designs is that the additional storage container can be heated to a higher temperature than
the allowable geyser temperature. The system also heats up and cools down
at a faster rate than would be possible without the HTEs. From the numerical
simulation the size and performance of such a system is determined. This
numerical analysis indicated that a phase change storage system in a SWH
application will increase the hot water delivered by a given solar collector
and geyser by increasing the storage capacity and by heating up the geyser
overnight for early morning hot water use. / AFRIKKANSE OPSOMMING: Son energie is ‘n tyd afhanklike, hoë temperatuur radiasie energiebron. Die
bruikbaarheid van ‘n sontermiese energie sisteem verhoog indien die hoë temperatuur
bron beskikbaar is wanneer dit die meeste benodig word. Dit kan
verwesenlik word deur die sonenergie termies te stoor. Termiese storing bied
groter veelsydigheid aan ‘n sontermiese stelsel deur effektief die hittebron te
ontkoppel van die hitte sink. ‘n Groot hoeveelheid energie kan, gedurende
die smeltingsproses in ‘n faseveranderingsmateriaal binne ‘n nou temperatuurband
gestoor word. Hierdie gesmelte materiaal kan weer op sy beurt in
die waterverhittingstoepassing, die geabsorbeerde hitte teen ‘n konstante temperatuur
oordra. In hierdie studie word ‘n sonwaterverwarmer stelsel wat
aangepas is deur ‘n addisionele latente hittestoor daaraan te heg, voorgestel.
Hierdie faseverandering hittestoor kan meer hitte stoor as wat water
in dieselfde temperatuur band sou kon. Die hitteoordrag tempo na en van
baie van die faseveranderingsmateriale (FVM) is egter as gevolg van die lae
termiese geleidingskoëfisient, stadig. Hierdie eienskap kan gelukkig verbeter
word deur hittepype en hitteoordrag verhogings materiaal soos vinne by te
voeg. Hittepype het die inherente eienskap om hitte teen ‘n hoë tempo oor
groot afstande, oor te dra, selfs oor ‘n klein temperatuurverskil.
In hierdie tesis word ‘n ondersoek rakende ‘n faseverandering storingsisteem
wat bestaan uit paraffien was as die FVM en reghoekige hittepype wat te same
met met aluminium finne gebruik word om die hitteoordragtempo te verhoog,
beskryf. Die stoorontwerp is modulêr en elke module het die kenmerk van
hoë hitteoordrag na en van die FVM. Die module het verder ook die eienskap
om vining hitte te absorbeer of hitte af te gee. Dit gebeur teen ‘n konstante
temperatuur gedurende die faseverandering van die FVM.
Presies so ‘n reghoekige module is ontwerp en gebou en onder beheerde hitte
absorbering- en hitte verwyderingsiklusse analiseer. Tydens die verhittings
eksperiment is ‘n elektriese ketel van gebruik gemaak wat gedien het as die
hoë temperatuur bron. Die hitte sink was ‘n hitteruiler wat kraanwater van ‘n
konstante hoogte tenk ontvang het. Die resultate van die volledige toets is met
die resultate van tydafhanklike numeriese model vergelyk. Hierdie numeriese
model bereken teoreties wat die module se storing verrigting onder gegewe
toets omstandighede sal wees. Die numeriese model se resultate het goed vergelyk met die resultate van die eksperimente. Die numeriese model van die
module is toe aangepas om ‘n sonwaterverwarmer met addisionele stoortenk
wat fase verandering materiaal gebruik, te simuleer. Hierdie ontwerp is anders
as vorige ontwerpe in die sin dat hoër temperature as wat die warmwatertoestel
kan hanteer, in die faseverandering storingstenk, bereik kan word. Die sisteem
kan ook as gevolg van die hitteoordrag verhoging materiaal, vinniger verhit
of afkoel en teen ‘n vinniger tempo. Die simulasie van die sonwaterverwarmer
met FVM word gebruik om die grootte en verrigting van die sisteem te bepaal.
Hierdie numeriese model toon aan dat wanneer ‘n addisionele faseverandering
storingstelsel in ‘n sonwaterverwarmer toepassing gebruik word, die warm
water wat die verbruiker uit die sisteem kan verkry, kan verhoog. Die rede
hiervoor is dat meer hitte gestoor kan word, wat beskikbaar gemaak word aan
die warm water tenk.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96140 |
| Date | 12 1900 |
| Creators | Malan, Daniel Johannes |
| Contributors | Dobson, R. T., Stellenbosch University. Faculty of Engineering. Department of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xx, 134 p. : ill. |
| Rights | Stellenbosch University |
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