Aspects of laminar free convection from a vertical plate

The thesis is comprised of a number of aspects pertaining to the phenomenon of laminar free convection from a vertical plate. A survey of literature is presented which brings out the fact, that the phenomenon although it has been investigated to a large extent, still gives rise to widespread dispute and uncertainties which need careful examination. An analytical approach is presented showing that the hydrodynamic boundary layer is equal to the thermal boundary layer. In the past it was simply assumed to be so, in order to limit the computations, and the assumption was justified by the excellent agreement between calculations and experimental data. An analysis is presented showing the analogy existing between the Grashof and Reynolds numbers, as it is expected in the case of low velocities and considerable temperature differences, i.e. in free convection. An extensive experimental investigation pertaining to average heat transfer rates from a vertical plate at low Grashof numbers is presented and provides the necessary evidence in a much disputed region. It is shown conclusively that as far as average heat transfer rates are concerned, the agreement with the already accepted relationship can be extended to Grashof numbers as low as 10

Are solar tracking technologies feasible for domestic applications in rural tropical Africa?

That solar tracking improves energy yields from solar harvest systems is not debatable. Nor is the under powering of tropical Africa amidst plenty of energy resources – including solar. This paper presents a review of recent literature on tracking as applied to domestic solar harnessing devices. The purpose is to find basic requirements in design of a suitable solar tracker for the region’s rural homes. It is concluded that Single axis passive trackers possibly will stand better chances of acceptability in the region

Effect of pressure reduction on the free convective heat transfer coefficient

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Finite difference method heat transfer applied to thermally activated building systems

Energy efficient Heating Ventilation Air Conditioning (HVAC) systems are required to off-set the rising cost of electricity and to reduce the amount of natural resources consumed. The implementation of alternative energy saving technologies is often not realized due to various factors hampering the implementation thereof. Thermally Activated Building Systems (TABS) is one of such technologies that utilize the inherent heat storage capacity of a building's concrete structure for comfort heating and cooling purposes. The concrete's heat storage capacity and various other factors need to be accounted for in order to dynamically predict the heat transfer behaviour of such systems. In this study, a commercial software program (MS Excel) was used to develop a two-dimensional FDM heat transfer model capable of dynamically predicting the air and surface temperatures of an enclosure. An experimental model was constructed and used to experimentally validated temperature predictions obtained from the FDM model. The average prediction accuracy of the heat transfer model ranged between -0.4°C and +1.4°C for the enclosure's surface temperatures and varied by +0.7°C for the enclosure's air temperature

Optimizing the performance of the domestic wall mounted space comfort heater

The performance of a wall mounted space comfort heater has been studied with respect to the geometry of its mounting condition. Tests were conducted in a laboratory with the heater positioned at various heights from the floor and the channel that is created by the various gaps with the wall on which the heater was mounted. Tests were also performed with the heater mounted on the wall whose emissivity was adjusted to low, medium and high values as well as placing insulation material on the wall directly behind the heater. The experiments revealed an acceptable geometry of the heater's mounting at least 200mm above the floor, and 50 mm off-set from the wall. The heater's mounting against the wall caused a drop in performance of about 15% of it's maximum “benchmark” performance (with an efficiency of about 41%) achieved when the heater was freely standing on the floor of the laboratory. This efficiency, based on the convective heat transfer generated by the heater's warm/hot surfaces, is relative to the electrical energy input. The heat transfer by radiation from the heater's surface is treated as net loss to the walls of the room/enclosure. The performance of the heater when mounted against the wall improved almost to the benchmark value when the wall behind the heater was made refelective (low emissivity)

The effect of different working fluids and internal geometries on the efficiency of evacuated tube heat pipe solar collectors

In this study, a heat pipe was modified with designed and manufactured inserts of specific profiles in order to investigate the effect of the internal geometries and working fluids on the efficiency of the evacuated tube heat pipe solar collector. The experimental rig was made of a mobile frame, an insulated water tank, a solar simulator and an evacuated tube heat pipe. Using an average irradiance of 700 watts per square meter, the indoor tests were conducted first on a heat pipe without any working fluid (dry mode) and later on the heat pipe containing, in turn, each of the six working fluids for each of the five geometries. Results show that, when inserting different profiles in the heat pipe, there is an enhancement of the heat transfer and hence an increase in the efficiency of the evacuated heat pipe solar collector

The new hydro-mechanical solar tracker: Performance testing with a PV panel

This paper describes work carried out to test the performance of a newly invented solar tracker. It is a gravity driven, bladder-flow controlled, Hooke coupled inclined non polar axis solar tracker. The performance of the tracker when coupled with a PV panel was first modelled in MATLAB® using the Perez anisotropic diffuse radiation and the King cell temperature models. Experiments with two identical 90 Wp panels were done over a 40 day period in outdoor conditions. One PV panel was fixed on optimised slope, the other, was driven by the tracker about a similarly sloped axis. Weather data consisting of total and diffuse radiation, ambient temperature and wind speed was also collected using a Campbell Scientific weather station adjacent the panels. This data was used to simulate performances of the panels in the MATLAB® model. TRNSYS simulations were also done for the two panels. The three sets of results were compared. It was found that the tracked panel yielded 34% more energy than the fixed one and that the experimental results correlated more closely with the MATLAB® models than with TRNSYS ones. Results also indicated that the efficacy of the tracking device could be influenced by the timing of cloudiness during the day and by wind speed

Re-mapping sub-Sahara Africa for equipment selection to photo electrify energy poor homes

This paper provides a missing integrated guide to budding middle class rural sub-Saharan Africa (SSA) homesteads trying to photo-electrify. It first estimates bare minimum requirements for these homes to start emerging from energy poverty. Guidance is given on optimal selection of the most important device for such homes: the light bulb. Along with other essential devices, this gives a daily electrical load of 500 W h, 42 A h at 12 V DC. Building on earlier experimental work on validating TRNSYS in Cape Town, it extends usage of the software to the rest of SSA, aiming to recommend panel and balance of system component sizes to meet the above load all year round. Use is made of panel slopes derived in a related piece of work to formulate an optimisation model for selecting panel–battery–charge controller combinations. A survey of South Africa-made panels and components is done. Then, a method of solving the model is demonstrated by an example in Uganda which selects from the surveyed components to satisfy two alternative technical objectives of ‘least battery storage’ and ‘smallest panel size’. At each of the other 151 stations in SSA, the model is solved only for the first objective. The overall results are then mapped using MATLAB®. It is concluded that from a ‘smallest battery storage’ perspective, usable battery storage capacities in the region range between 70 and 360 A h, with the biggest being in equatorial/tropical rain forest areas of Congo basin and along the mid-western coastal areas. Panel sizes range between 160 and 275 Wp. The dominant recommendation on charge controllers is 15 A

Are solar tracking technologies feasible for domestic applications in rural tropical Africa?

That solar tracking improves energy yields from solar harvest systems is not debatable. Nor is the under powering of tropical Africa amidst plenty of energy resources – including solar. This paper presents a review of recent literature on tracking as applied to domestic solar harnessing devices. The purpose is to find basic requirements in design of a suitable solar tracker for the region’s rural homes. It is concluded that Single axis passive trackers possibly will stand better chances of acceptability in the region