Roger Moss | University of Warwick (original) (raw)

Papers by Roger Moss

Vacuum flat plate solar thermal collectors offer several advantages over other collectors namely ... more Vacuum flat plate solar thermal collectors offer several advantages over other collectors namely the excellent optical and thermal characteristics they exhibit due to a combination of their wide surface area and high vacuum thermal insulation. These characteristics can offer a variety of applications for industrial process heat as well as for building integration as they are much thinner than conventional collectors making installation possible in limited spaces. However, many technical challenges which need to be addressed to enable wide scale adoption of the technology still remain. This paper will discuss the challenges, expectations and requirements for the flat-plate vacuum solar collector development. In addition, it will provide an overview of work undertaken in Ulster University, Loughborough University, and the University of Warwick on flat-plate vacuum solar thermal collectors. Finally, this paper will present a detailed experimental investigation on the development of a v...

Encapsulating the absorber of a flat plate solar thermal collector in vacuum by an enclosure that... more Encapsulating the absorber of a flat plate solar thermal collector in vacuum by an enclosure that can be evacuated can result in a significant increase in collector performance and achievable operating temperatures. This is a result of the thermal insulation effectiveness of the vacuum layer surrounding the absorber, as less heat is lost during collector operation. This work describes experimental thermal insulation characterization tests of prototype vacuum flat plate solar thermal collectors that demonstrate the improvement in absorber heat loss coefficients. Furthermore, this work describes the selection and sizing of a getter, suitable for maintaining the vacuum inside the enclosure for the lifetime of the collector, which can be activated at low temperatures.

Flat-plate solar thermal collector technology when coupled with vacuum enclosure technology has p... more Flat-plate solar thermal collector technology when coupled with vacuum enclosure technology has potential to supply clean energy efficiently for use in applications including residential water and space heating. This paper focuses on the design of vacuum enclosures for flat-plate solar collectors with specific reference to vacuum enclosures designed for thin micro-channel solar absorber plates (thickness < 10mm). The expectations, requirements and applications of these solar collectors are discussed along with a description of an enclosure concept under consideration. Potential seal materials are identified and their limitations discussed. Finite element modelling results are presented and conclusions made regarding design parameter selection.

In this work a flat evacuated glass enclosure is designed and fabricated and its thermal performa... more In this work a flat evacuated glass enclosure is designed and fabricated and its thermal performance characterized for solar thermal applications. To investigate the effect of the thermal insulation provided by the high vacuum pressure in the enclosure, the heat transmission of the enclosure is determined under both atmospheric and vacuum pressures. The flat evacuated enclosure consists of two glass panes hermetically sealed around their periphery to a stainless steel spacer creating a cavity between the glass panes 15mm wide. An array of stainless steel support spacers are set between the glass panes to prevent the panes from collapsing under the influence of atmospheric pressure.A simple solar absorber is integrated into the enclosure and a novel co-centric port is designed for thermal fluid transfer through the edge spacer to the absorber. The assembly is tested under a solar simulator, and using infrared thermography techniques and thermocouples attached to the enclosure its the...

Solar Energy, 2018

A solar simulator has been designed and built for testing prototype (0.5 × 0.5 m) flat plate ther... more A solar simulator has been designed and built for testing prototype (0.5 × 0.5 m) flat plate thermal collectors. An internally reflecting light tube generates multiple virtual images of the four halogen floodlights to ensure uniform illumination. Ray-tracing simulations were used to choose the tube dimensions and maximum allowable clearance. Illumination measurements agree well with these predictions. The visible & near IR spectrum appears to follow a black body curve. In the absence of a "cold sky" IR filter there is a secondary, long wavelength IR spectral component that causes heating of the cover glass on a solar flat plate collector. The cover glass temperature can be maintained at typical outdoor levels using a cooling fan. The design would be well suited to LED illumination. Simulation of solar collector response to this spectrum shows that an efficiency based on pyranometer readings is approximately 1% higher than would be obtained with an AM1.5 spectrum.

Solar Energy, 2017

Solar thermal collectors for buildings use a heat transfer fluid passing through heat exchange ch... more Solar thermal collectors for buildings use a heat transfer fluid passing through heat exchange channels in the absorber. Flat plate absorbers may pass the fluid through a tube bonded to a thermally conducting plate or achieve lower thermal resistance and pressure drop by using a flooded panel or microchannel design. The pressure drop should be low to minimise power input to the circulating pump. A method is presented for choosing the optimum channel hydraulic diameter subject to geometric similarity and pumping power constraints; this is an important preliminary design choice for any solar collector designer. The choice of pumping power is also illustrated in terms of relative energy source costs. Both microchannel and serpentine tube systems have an optimum passage diameter, albeit for different reasons. Double-pass and flooded panel designs are considered as special microchannel cases. To maintain efficiency, the pumping power per unit area must rise as the passage length increases. Beyond the optimum pumping power the rise in operating cost outweighs the increase in collector efficiency.

Applied Thermal Engineering, 2015

This paper investigates the significance of some micro scaling effects in microchannel absorber p... more This paper investigates the significance of some micro scaling effects in microchannel absorber plates. These plates are to be used in a proposed compact (thin and lightweight) solar thermal flat plate collector (FPC). Forced convection experiments were performed on an instrumented metal plate with micro-channels. Reynolds numbers were in the range 10-100 and fluid inlet temperatures ranged from 5-40 °C. Scaling effects such as viscous dissipation and entrance effects had insignificant impact on the measured average Nusselt number. However, conjugate heat transfer and measurement uncertainties were significant. Conjugate heat transfer was found to reduce the Nusselt number which agrees with the literature, this also resulted in a Peclet number dependent Nusselt number. The local Nusselt number was observed to vary axially despite satisfying the criteria for neglecting entrance effects; this variation increased with the Graetz number. It was observed that the position of the thermocouples can result in an underestimation of the Nusselt number. The results are beneficial for the design and operation of micro-channel absorber plates.

International Journal of Heat and Mass Transfer, 2015

Experimental studies were carried out to investigate the effects of micro-channel geometry on the... more Experimental studies were carried out to investigate the effects of micro-channel geometry on the thermal and hydraulic performance of absorber plates for compact (thin and lightweight) solar thermal collectors. Three plates with channel depths 0.25 mm, 0.5 mm and 1 mm were studied. Each plate had sixty channels which were 270 mm long and 2 mm wide. Experiments were run at typical operating conditions for flat plate solar collectors. The results showed a Reynolds number dependent Nusselt number; this was due to axial thermal conduction. The Nusselt number was observed to increase as the aspect ratio approached unity. Measured friction factors were similar in trend to the predictions for rectangular channels, although the overall rise in fluid temperature resulted in slightly lower friction factors. The plate with 0.25mm deep channels was found to have best thermo-hydraulic performance; thermo-hydraulic performance reduced slightly with increase in hydraulic diameter. The results showed that thermal improvement can be achieved by increasing the fluid velocity, however, pumping the thermal fluid above a pump power per plate area of 0.3 W/m 2 resulted in marginal improvement. The results are beneficial for the design of microchannel absorber plates.

Volume 4: Heat Transfer; Electric Power; Industrial and Cogeneration, 1994

The flow over the high pressure blades of a gas turbine is disturbed by wakes and shock waves fro... more The flow over the high pressure blades of a gas turbine is disturbed by wakes and shock waves from the nozzle guide vanes upstream. These disturbances lead to increased heat transfer to the blade surfaces, the accurate prediction of which is an essential stage in the design process. The Oxford Rotor experiment consists of a highly instrumented 0.5 m diameter shroudless turbine which is supplied with air from a piston tube during the 200 ms run time and simulates realistic engine Mach and Reynolds numbers. Previous experiments have measured blade surface pressures and heat transfer rates, and compared them with similar data from linear cascades. The present work is designed to enable the accuracy of rotation terms in computational fluid dynamics (CFD) calculations to be assessed, by providing heat transfer data from the rotating frame in the absence of wakes. Flow disturbances were avoided by removing the nozzle guide vanes, the correct angle of incidence onto the rotor blades being ...

Experimental Thermal and Fluid Science, 2015

Experimental and theoretical analyses were carried out to investigate the absorber plate temperat... more Experimental and theoretical analyses were carried out to investigate the absorber plate temperature distribution for compact (thin and lightweight) solar thermal collectors. An analytic model combining convective heat transfer with axial conduction in the metal plate was developed. Forced convection experiments were then performed on an instrumented metal plate with micro-channels 0.5 mm × 2 mm × 270 mm long, at various flow rates; the heat transfer fluid was Tyfocor® LS. Reynolds numbers were in the range 10-100 and fluid inlet temperatures ranged from 5-60 °C. The predicted plate temperature profiles from the analytic model were in close agreement with the measured profiles. Thermal entry lengths were found to be significant and resulted in slight variations at the entry portion of the plate at higher flow rates. The model was used to study the effects of varying design/operating parameters and showed that axial conduction can significantly alter the temperature profile in the plate.

Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2001

This paper describes the effect of both an upstream and a downstream vane on the time-resolved su... more This paper describes the effect of both an upstream and a downstream vane on the time-resolved surface pressure field around a high-pressure rotor blade. The geometry of the downstream vane considered is a large low aspect ratio vane located in a swan-necked diffuser duct, similar to those likely to be used in future engine designs. Two test geometries are considered: firstly, a high-pressure turbine stage coupled with a swan-necked diffuser exit duct; secondly, the same high-pressure stage but with a vane located in the downstream duct. Both tests were conducted at engine-representative Mach and Reynolds numbers, and experimental data were acquired using fast response pressure transducers mounted on the mid-height streamline of the high-pressure rotor blades. It is shown that the potential field of the downstream vane causes a relatively large static pressure fluctuation on the late rotor suction surface but that the early suction surface and pressure surface are unaffected by the ...

Measurement Science and Technology, 2000

ABSTRACT

International Journal of Heat and Fluid Flow, 2005

An investigation into the unsteady losses in a high pressure turbine stage has been performed exp... more An investigation into the unsteady losses in a high pressure turbine stage has been performed experimentally at engine-representative conditions. This has been done by making time-resolved measurements of entropy at stage exit over all vane and rotor-relative positions over a wide range of radial height. These measurements provide a unique set of experimental data in that this is the first time that a full-field unsteady entropy survey has been attempted in a turbine facility capable of realistic Reynolds and Mach numbers. In a previous paper [Int. J. Heat Fluid Flow 24 (2003) 698] the exit flow of the rotor at one vane relative phase was reported. This allowed the loss associated with rotor flow features such as wakes, tip flows and secondary flows to be determined. The aim of this paper is to investigate the effects of vane-rotor interaction on both the time-mean and time-resolved performance of the stage. In addition to the experimental results, an unsteady Reynolds-averaged Navier-Stokes prediction of the flow field within the stage has been undertaken. Comparison of the experimental measurements with the prediction showed that time-mean efficiencies differ by 1.2%. The experimental results showed an unsteady change in stage efficiency of 1.2% as the rotor moves relative to the upstream vane. This was much higher than the 0.2% change predicted by the unsteady solver.

Applied Thermal Engineering, 2018

Thermal Science and Engineering Progress

Heat losses from a flat panel solar collector can be significantly reduced by lowering the intern... more Heat losses from a flat panel solar collector can be significantly reduced by lowering the internal pressure to < 0.5 Pa. Compared with conventional collectors, the resulting increase in efficiency is predicted to give a significant gain in annual heat output in the context of a temperate climate with low median irradiance. Two experimental collectors were built and tested to investigate theoretical aspects of evacuated flat plates and develop appropriate technological solutions. One had a metal tray to the rear, the other used two sheets of glass. An array of pillars supports the glass against atmospheric pressure. The experimental procedure was designed to minimise measurement uncertainty. Testing under a solar simulator, with and without a vacuum, showed a sudden drop in heat loss as the pressure was reduced below 0.5 Pa. When evacuated the heat loss coefficient fell from 7.43 to 3.65 W/m 2 K and the efficiency at a nominal test condition of ΔT = 60°C, G = 1000 W/m 2 increased from 36% to 56%. Heat losses from absorber to glass were within 9% of the predicted level. This demonstrates that the heat loss mechanism is well understood.

Solar Energy

Creating a vacuum (< 1 Pa) around a solar absorber in a flat plate solar thermal collector can in... more Creating a vacuum (< 1 Pa) around a solar absorber in a flat plate solar thermal collector can increase efficiency by minimising gaseous conduction and convection between the absorber plate and the glass cover. High performance and architecturally attractive flat plate solar thermal collectors are appealing to building owners and designers for supplying clean and renewable energy cost effectively produced via the façade of the building. This two part paper describes the construction techniques and thermal performance of two vacuum enclosures, fabricated at Ulster University, as prototype components for evacuated flat plate solar collectors. The enclosures were tested at three conditions: 0.0033 Pa, 17 Pa and atmospheric pressure. The first enclosure consisted of two glass panes, sealed to an edge spacer and separated by an array of support pillars on a regular square grid to form a narrow evacuated space. The second enclosure, incorporated an uncooled copper plate to represent a solar thermal absorber. Part 1 of this paper has described the fabrication techniques and compared results from hot-box calorimeter and IR thermography testing of the first enclosure with numerical and analytical predictions. Part 2 describes solar simulator testing of the second enclosure which incorporated an uncooled copper plate. Testing under a solar simulator showed a higher stagnation temperature in the high vacuum test (0.0033 Pa) in comparison with the low vacuum (17 Pa) and atmospheric pressure tests. Curve fitting of a heat transfer model to the transient response data demonstrated that radiation and gas conduction were close to predictions. Simulated results were in close agreement with both the transient response and the steady-state asymptotic plate temperatures.