Transmission of heat

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District heating lines

Most heating systems involve the transport of sensible heat in a fluid or gas (such as water or air) through pipes or channels. Examples are the solar heating system illustrated in Fig. 4.21, the heat pump in Fig. 4.104 and the geothermal heating plants in Fig. 4.107. Solar heating systems and heat pumps may be used in a decentralised manner, with an individual system providing heat for a single building, but they may also be used on a community scale, with one installation providing heat for a building block, a factory complex, a whole village or a city of a certain size. Many combined heat and power (CHP) systems involve heat transmission through distances of 10-50 km (decentralised and centralised CHP plants). In some regions, pure heating plants are attached to a district heating grid.

Assuming that a hot fluid or gas is produced at the central conversion plant, the transmission may be accomplished by pumping the fluid or gas through a pipeline to the load points. The pipeline may be placed underground and the tubing insulated in order to reduce conduction and convection of heat away from the pipe. If the temperature of the surrounding medium can be regarded as approximately constant, equal to Tref, the variation in fluid (or gas) temperature Tfluid(x) along the transmission line (with the path-length co-ordinate denoted x) can be evaluated by the same expression (4.98) used in the simple description of a heat exchanger in section 4.2.1. Temperature variations across the inside area of the pipe perpendicular to the stream-wise direction are not considered.

The rate of heat loss from the pipe section between the distances x and (x+dx) is then of the form dE/dx = Jm Cpfluid dTfluid(x)/dx = h' (Tref - Tfluid(x)), (5.1)

where h' for a cylindrical insulated pipe of inner and outer radii r1 and r2 is related to the heat transfer coefficient AFipe (conductivity plus convection terms, cf. (3.34)), by h' = 2n Xppe / log(r2/r1).

Upon integration from the beginning of the transmission pipe, x = x1, (5.1) gives, in analogy to (4.98),

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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