Page 69 - Low Temperature Energy Systems with Applications of Renewable Energy

58 Low-Temperature Energy Systems with Applications of Renewable Energy

Equation (2.2) is valid if liquid flowing inside the pipe is turbulent, i.e., Reynolds
number, Re > 3000. The data presented are valid if the polyethylene piping is placed
in the ponds with water currents, and can be used both for extracting heat from the
pond in winter and discharging heat taken from air conditioning systems into the
pond in summer.

2.3.3 Ocean water surface layers
The world’s oceans constitute the largest collector of solar radiation wherein a temper-
ature difference is created between warm surface waters and colder deeper layers. This

difference exceeds 20 C over a kilometer of water depth across large expanses of the
Pacific, Atlantic and Indian oceans; see Fig. 2.8. Thus there is a continuous supply of
ocean thermal energy that can theoretically be transformed into other kinds of energy.
Research has shown the possibility of using such a scheme under conditions of trop-
ical latitudes, where the temperature difference between the surface layers and waters
at 1000 m depth is more than 24 C and changes very little from season to season.

Fig. 2.9 shows a typical scheme for an Ocean Thermal Energy Conversion (OTEC)
power plant. However, commercial use of these systems has yet to be achieved owing
to: (1) the very high cost of capital investments owing to difficult service conditions in
the open sea and energy transfer to the mainland; (2) the enormous size of heat ex-
changers needed to produce meaningful amounts of power; and (3) the necessity of
combating biological over-growth of the heat-exchanger surfaces, as a result of which
50% of gross power can be spent merely to overcome fluid friction in pipelines and
heat exchangers.
Organic working fluids having a low-temperature boiling point, e.g., refrigerants
(ORC) and even water (at a vacuum), may be used for running a power cycle. If water
is used, its boiling point should be decreased to the surface water temperatures by

Fig. 2.8 Temperature difference between ocean surface and depth of 1000 m [OTEC,
Wikipedia; Public domain].