Civil Engineering Reference
In-Depth Information
made of refl ecting mirrors which will concentrate the Sun light on a power conver-
tor section in the focal point of the dish. Energy transferred to this section usually
will be used to heat up the working fl uid which moves a reciprocating shaft system
that then will be used to start and operate an electricity generator unit.
Power Towers are another type of concentrating solar power systems. In these
systems a large number of fl at mirrors track the Sun movement and focus its light
towards a receiver which is installed on top of a tower. This system like the other
systems in this category uses the Sun energy to warm up the working fl uid in a
receiver. The warmer fl uid then will be used to run a turbine and then generates
electricity by using the mechanical power output of the turbine as the input to an
electricity generator.
As it was mentioned earlier, the main problem with concentrating solar power
systems is it's down time. Down time is the result of lack of sun radiation during the
night and cloudy day times. To overcome this problem storage tanks have been
designed and are used. Storage tanks hold the additional generated energy during
the day times and utilize it during the down times. Three popular types of storage
tanks are two-tank direct, two-tank indirect, and one tank thermo-cline systems.
A two-tank direct storage system is contained of two tanks with high and low
temperatures which utilize the same fl uid as heat exchanger and storage fl uids. Hot
fl uid from thermal collectors arrives in high temperature tank and from there moves
to a heat exchanger that generates steam for running the steam turbine, which pro-
vides the mechanical power required for a generator to generate electricity. Cold
working fl uid from the heat exchanger moves back to the low temperature tank and
from there returns back to the collectors.
The main difference between two-tank direct and two-tank indirect storage
systems are that in the latter system the heat exchange and storage fl uids are two
different fl uids. This usually occurs when the heat exchange fl uid is expensive, and
it is not economical to fi ll both tanks with this fl uid. As a result the system requires
an additional heat exchanger that separates the heat exchange (collector) fl uid from
the storage fl uid. In this confi guration the collector fl uid passes through two heat
exchangers in parallel position. The steam heat exchanger transfers the collector
fl uid heat to steam generating fl uid which as the result will generate electricity after
being changed to steam and going through turbine and electricity generator. The
utilized fl uid in hot and cold storage tanks will be pumped into each other with two
pumps in reverse direction and as it is defi nes by the need for direction of the fl uid
between two tanks. During the day time when hot collector fl uid is available, storage
tank fl uid is pumped from cold tank to hot tank and absorbs heat from collector fl uid
inside the storage heat exchanger and stores this heat in hot tank. During the down
time when sun is not available, hot fl uid is pumped from hot tank to cold tank. The
difference is that in this time collector fl uid absorbs the heat to warm up and further
get utilized in the steam heat exchanger for generating steam and electricity.
One tank thermo-cline system is contained of one tank with three levels of solid
material with high, medium and low temperatures. High-temperature working fl uid
enters to high temperature media on top and leaves from the low temperature media
in the bottom.
