A review of solar thermal technologies

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Abstract

The use of solar energy in recent years has reached a remarkable edge. The continuous research for an alternative power source due to the perceived scarcity of fuel fossils is its driving force. It has become even more popular as the cost of fossil fuel continues to rise. The earth receives in just 1 h, more energy from the sun than what we consume in the whole world for 1 year. Its application was proven to be most economical, as most systems in individual uses requires but a few kilowatt of power. This paper reviews the present day solar thermal technologies. Performance analyses of existing designs (study), mathematical simulation (design) and fabrication of innovative designs with suggested improvements (development) have been discussed in this paper.

Introduction

Of all the renewable sources of energy available, solar thermal energy is the most abundant one and is available in both direct as well as indirect forms. The Sun emits energy at a rate of 3.8 × 1023 kW, of which, approximately 1.8 × 1014 kW is intercepted by the earth, which is located about 150 million km from the sun. About 60% of this amount or 1.08 × 1014 reaches the surface of the earth. The rest is reflected back into space and absorbed by the atmosphere. About 0.1% of this energy, when converted at an efficiency of 10% would generate four times the world's total generating capacity of about 3000 GW. It is also worth noting that the total annual solar radiation falling on the earth is more than 7500 times the world's total annual primary energy consumption of 450 EJ. The annual solar radiation reaching the earth's surface, approximately 3,400,000 EJ, is an order of magnitude greater than all the estimated (discovered and undiscovered) non-renewable energy resources, including fossil fuels and nuclear. However, 80% of the present worldwide energy used is based on fossil fuels. Risks associated with their use are that they are all potentially vulnerable to adverse weather conditions or human acts. World demand for fossil fuels (starting with oil) is expected to exceed annual production, probably within the next two decades. International economic and political crisis and conflicts can also be initiated by shortages of oil or gas. Moreover, burning fossil fuels release harmful emissions such as carbon dioxide, nitrogen oxides, aerosols, etc. which affect the local, regional and global environment.

Section snippets

Study

Chang et al. studied the solar irradiation patterns for the performance testing of thermosyphon solar water heaters and concluded that a distribution factor Ri and its criterion have to be introduced into the CNS test standard to provide reliable test results for solar hot water heaters [1]. Mathioulakis and Belessiotis investigated the performance of a new solar hot water system with an integrated heat-pipe and compared the proposed theoretical model of the collector with the experimental

Study

Schwarzer and Vieira da Silva presented the experimental procedures to be used to calculate the parameters which determine the thermal performance of solar cookers and also suggested a simplified procedure based on energy balance equations for the design of solar cookers [41]. Ekechukwu and Ugwuoke presented the design philosophy, construction and measured performances of a plane-reflector augmented box-type solar-energy cooker which recorded a significant improvement in performance with the

Study

Karsli made a comparative study of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes and a base collector. He concluded that their efficiency depends significantly on the solar radiation and the surface geometry of the collectors and also that at higher reduced temperature parameter, the overall loss is lower [84]. Mahaprata and Imre conducted a parameter sensitivity analysis on a

Study

Karakilcik and Dincer studied both energetic and exergetic performances of a solar pond through efficiency analysis and developed exergy efficiencies for each zone of the solar pond through exergy balance equations [103]. Tamimi and Rawajfeh analyzed the thermal performance of evaporators and their model developed on lumped basis indicates that the efficiency of any solar evaporator is limited by the optical absorptivity of the saline water [104]. Velmurugan and Srithar reviewed various designs

Study

He et al. proposed the solar cooling strategies for tackling long, hot and humid summer which consisted of a multi-functional solar system and a method for indoor ventilation; the design included double walls and a triple roof in order to remove heat by ventilation of the building envelope [121]. Kischkoweit-Lopin studied a huge number of different daylighting systems that allow new and optimized ways of daylight utilization with the consideration that the different systems have to be used in

Study

Grossman describes the current trends in solar powered air-conditioning systems in which he had substantiated that double and triple effect chillers had better COP than the single effect chillers while using LiBr/water as the working fluid and also highlighted the performance of open cycle absorption system and the need to replace it with closed cycle sorption systems [134]. Elsafty and Al-Daini made an economical comparison of solar powered single, double effect vapor absorption

Study

Zhou et al. carried out simulation studies to investigate the performance of a pilot scale solar chimney power plant for different solar intensities by varying collector area and chimney height and the simulated results closely agree with the measured values [148]. von Backstrom et al. investigated the turbine characteristics by developing analytical equations in terms of turbine flow, load coefficient and degree of reaction and their influence on turbine efficiency and proved that if applied

Study

von Backstörm and Fluri conducted a study and developed two analyses for finding the optimal ratio of turbine pressure drop to available pressure drop in a solar chimney power plant for maximum fluid power [155]. Segal and Epstein optimized the important components of a solar central receiver power plant like receiver, heliostat field and power block thus maximizing their efficiency and suggested that beyond a certain limit of the operating temperature the overall system efficiency decreases

Study

Porta et al. found that the overall thermal inertia effects can be adequately represented by a single non-dimensional quantity referred as thermal inertia slenderness ratio. Simulation of the calibrated mathematical model illustrates the influence of thermal inertia on performance [173]. Tiwari et al. reviewed the works on solar distillation, its present status and future perspective and also classified the distillation units [174]. Tzen and Morris presented technologies about renewable energy

Conclusion

A detailed literature survey of major solar thermal technologies comprising of solar water heaters, solar cookers, solar driers, solar ponds, solar architecture, solar conditioning, solar chimneys, solar power plants and solar stills was performed. The review gives a brief overview of the developments in each of these key areas of technology in view to either raise the performance of the presently used system or to build a new system which employs a great deal of innovation and gives better

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    Source: Survey of Energy Resources 2007, World Energy Council.

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