Solar thermal collectors and applications

Renewable energy technologies

Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy These technologies use the sun’s energy and its direct and indirect effects on the earth (solar radiation, wind, falling water and various plants, i. e. biomass), gravitational forces (tides), and the heat of the earth’s core (geothermal) as the resources from which energy is produced. These resources have massive energy potential, however, they are generally diffused and not fully accessible, most of them are intermittent, and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Nowadays, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs, and increasing the reliability and applicability.

A worldwide research and development in the field of renewable energy resources and systems is carried out during the last two decades. Energy conversion systems that are based on renewable energy technologies appeared to be cost effective compared to the projected high cost of oil. Furthermore, renewable energy systems can have a beneficial impact on the environmental, economic, and political issues of the world. At the end of 2001 the total installed capacity of renewable energy systems was equivalent to 9% of the total electricity generation [10]. By applying a renewable energy intensive scenario the global consumption of renewable sources by 2050 would reach 318 exajoules [11].

The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories; energy saving, generation of new working posts and the decrease of environmental pollution.

The energy saving benefit derives from the reduction in consumption of the electricity and/or diesel which are used conventionally to provide energy. This benefit can be directly translated into monetary units according to the corresponding production or avoiding capital expenditure for the purchase of imported fossil fuels.

Another factor which is of considerable importance in many countries is the ability of renewable energy technol­ogies to generate jobs. The penetration of a new technology leads to the development of new production activities contributing to the production, market distribution and operation of the pertinent equipment. Specifically in the case of solar energy collectors job creation mainly relates to the construction and installation of the collectors. The latter is a decentralised process since it requires the installation of equipment in every building or every individual consumer.

The most important benefit of renewable energy systems is the decrease of environmental pollution. This is achieved by the reduction of air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on the public health, agriculture and on ecosystems. It is relatively simple to measure the financial impact of these effects when they relate to tradable goods such as the agricultural crops;

however when it comes to non-tradable goods, like human health and ecosystems, things becomes more complicated. It should be noted that the level of the environmental impact and therefore the social pollution cost largely depends on the geographical location of the emission sources. Contrary to the conventional air pollutants, the social cost of CO2 does not vary with the geographical characteristics of the source as each unit of CO2 contributes equally to the climate change thread and the resulting cost.

In this paper emphasis is given to solar thermal systems. Solar thermal systems are non-polluting and offer significant protection of the environment. The reduction of greenhouse gasses pollution is the main advantage of utilising solar energy. Therefore, solar thermal systems should be employed whenever possible in order to achieve a sustain­able future.

Solar thermal collectors and applications

Collector thermal efficiency

In reality the heat loss coefficient UL in Eqs (2) and (42) is not constant but is a function of collector inlet and ambient temperatures. Therefore: TOC o "1-5" h …

Global climate change

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, …

Limitations of simulations

Simulations are powerful tools for process design offering a number of advantages as outlined in the previous sections. However, there are limits to their use. For example, it is easy …

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