Saffa Riffat

Thermoelectrics: a review of present and potential applications

Thermoelectric devices are solid state devices. They are reliable energy converters and have no noise or vibration as there are no mechanical moving parts. They have small size and are light in weight. As refrigerators, they are friendly to the environment as CFC gas or any other refrigerant gas is not used. Due to these advantages, the thermoelectric devices have found a large range of applications. In this paper, basic knowledge of the thermoelectric devices and an overview of these applications are given. The prospects of the applications of the thermoelectric devices are also discussed.

Development of a prototype low-temperature Rankine cycle electricity generation system

This paper describes the development of a small-scale system designed to generate electricity from low temperature heat (e.g., solar energy). The system operates on the Rankine cycle and uses n-pentane as the working fluid. A prototype system has been designed, constructed and tested. It is capable of delivering 1.5 kW of electricity with a thermal efficiency of 4.3%. Laboratory test results and a cost estimate for the prototype unit are presented in the paper.

A numerical study of solar chimney for natural ventilation of buildings with heat recovery

The performance of a glazed solar chimney for heat recovery in naturally-ventilated buildings was investigated using the CFD technique. The CFD program was validated against experimental data from the literature and good agreement between the prediction and measurement was achieved. The predicted ventilation rate increased with the chimney wall temperature. The effects of solar heat gain and glazing type were investigated. It was shown that in order to maximise the ventilation rate in a cold winter, double or even triple glazing should be used. Installing heat pipes in the chimney for heat recovery not only increased the flow resistance but also decreased the thermal buoyancy effect. To achieve the required air flow rates in naturally-ventilated buildings with heat recovery, use should be made of wind forces.

Development of a solar-powered passive ejector cooling system

This paper describes the development of an ejector refrigeration system that is powered by solar thermal energy. The cooling system contains no active parts and is therefore deemed passive. Water is used as the refrigerant though other natural refrigerants could be used for lower temperature operation. A prototype system was built with a nominal cooling capacity of 7 kW. This system was laboratory tested and then installed in an existing office in Loughborough, UK. The system has operated with a COP of up to 0.3 at this location. The system is also able to provide up to 20 kW of heating to the building during the winter.

A novel thermoelectric refrigeration system employing heat pipes and a phase change material: an experimental investigation

This paper presents results of tests carried out to investigate the potential application of heat pipes and phase change materials for thermoelectric refrigeration. The work involved the design and construction of a thermoelectric refrigeration prototype. The performance of the thermoelectric refrigeration system was investigated for two different configurations. The first configuration employed a conventional heat sink system (bonded fin heat sink) on the cold side of the thermoelectric cells. The other configuration used an encapsulated phase change material in place of the conventional heat sink unit. Both configurations used heat pipe embedded fins as the heat sink on the hot side. Replacement of the conventional heat sink system with an encapsulated phase change material was found to improve the performance of the thermoelectric refrigeration system. In addition, it provided a storage capability that would be particularly useful for handling peak loads and overcoming losses during door openings and power-off periods. Results showed that the heat sink units employing heat pipe embedded fins were well suited to this application. Results also showed the importance of using a heat pipe system between the cold junction of the thermoelectric cells and the cold heat sink in order to prevent reverse heat flow in the event of power failure.

Computational fluid dynamics applied to ejector heat pumps

Computational fluid dynamics is applied to the prediction of ejector performance for heat pumps. It is shown that the performance of an ejector varies with types of refrigerant used. It is also shown that the performance depends on the ejector nozzle type and position.

Pressure loss characteristics of orifice and perforated plates

A study was conducted on the pressure loss characteristics of square-edged orifice and perforated plates. Tests were carried out to determine the pressure loss coefficient for thin plates in a square duct for a range of Reynolds numbers. Computational fluid dynamics (CFD) was used to predict the loss coefficient, and the result was compared with experimental measurement. The effect of plate thickness on the loss coefficient for the orifice plate was studied using CFD.

Natural refrigerants for refrigeration and air-conditioning systems

In the past, several natural working fluids were used in refrigeration and air-conditioning systems. However, the appearance of CFCs caused a drastic reduction in the utilisation of those fluids. Understanding of the environmental damage of CFCs focused attention on alternative refrigerants. From these, the ones that have minimum (or nil) environmental impact are natural ones. This paper presents a review of the application of the main natural refrigerants, for refrigeration and air-conditioning systems, as an alternative to synthetic new refrigerants (HFCs). Recent research and development in this area is analysed and various cycles are considered.

Modelling and measurement of airflow and aerosol particle distribution in a ventilated two-zone chamber

Abstract A CFD analysis of air movement and aerosol particle deposition and distribution in a ventilated two-zone system with a small interzonal opening is presented. The comparisons of average particle concentrations in both zones between computations and experiments are generally satisfactory and acceptable. It is concluded that particle deposition and migration are mainly influenced by the particle properties, the ventilation conditions and the airflow patterns in the two zones. Small particles have a greater influence on the indoor air quality than do the large particles.

Daylight performance of lightpipes

Abstract Increasing the use of natural daylight for lighting purposes in buildings can offer large savings in electricity usage, up to 20–30% of total building energy consumption. One solution is the use of lightpipes that can not only bring light into otherwise inaccessible or dimly lit places, but also improve the internal environment without generating excessive heat. The performance of six light pipes has been monitored in three different areas, a workshop, a residential landing, and a small office. The highest illuminance was 1538 lux, obtained underneath the straight lightpipe on the landing, with an aspect ratio of 2.1. The lowest was 41 lux in the darkest corner of the workshop. The average illuminance for the whole landing was 366 lux and a mean internal to external ratio of 0.48%. The results show that lightpipes are proficient devices for introducing daylight into buildings, the most effective lightpipes being straight, short ones with low aspect ratios; consequently, larger diameter lightpipes would probably be more effective. However, the benefits of lightpipes also include energy savings, user satisfaction and a healthier and improved indoor environment.