SIPHONING THE SUN - SOLAR TOWER POWERS AHEAD K.V ARUN KUMAR M.USHA KIRAN (03671A0356) (03671A0354) (JBIET) (JBIET) [email protected][email protected]ABSTRACT: The word resource finds the meaning as ‘a source of help’. We need help always in some kind, after all no man can live throughout without any help. The greatest help needed by today’s machine man is the ‘energy help’. Till the present day energy is sufficient for his needs. Coming back to present world, there raises a necessity to search for a source, which is omnipresent and omnipotent to save the homosapien. We roamed round the galaxy in search of such an energy, the result of which has led us here. Yes, the same old resource, the solar energy. Solar energy is converted into electrical energy by means of a turbine. This may lookvery confusing now. But we shall see everything soon. Heat from the sun is used to heat air and hence to run a turbine. This concept leads to the unbelievable invention named ‘The solar tower ’. The sol ar chimney power pla nt – conv ert s global irr adi ant into ele ctrici ty . Sin ce chimneys are often associated negatively with exhaust gases, THIS CONCEPT IS ALSO KNOWN AS THE SOLAR TOWER POWER PLANT although it is totally different from the tower conc epts. A solar chimney power plant h as a high ch imney (tower), with a height of up to 1000 meters, and this is surrounded by a large collector roof, up to 3 miles in diameter, that consists of glass or resistive plastic supported on a framework. ENERGY RESOURCES-A threat?
The solar chimney basically operates like a hydroelectric power plant, but instead of
water it uses hot air (Fig. 2).
Beneath a large glass roof air is heated. It enters a vertical tube placed at the center of theroof and creates an up draught there. Inside the tube Kaplan-type wind turbines with
electrical generators are producing electricity. Contrary to the power plants with
reflecting mirrors, the glass roof collector of a solar chimney also operates with overcast
sky, resp. diffuse light - a decisive advantage!
Fig. 2: Principle of the solar chimney: glass roof collector, chimney tube, wind
turbines.
Continuous 24 hours-operations is guaranteed by placing tight water-filled tubes under
the roof. The water heats up during the daytime and emits its heat at night. These tubes
are filled only once, no further water is needed (Fig. 3 and 4).
DAY: NIGHT:
Fig. 3: Principle of heat storage underneath the roof using water-filled black tubes.
The tower is the key to the energy producing concept through the drawing effect it creates
to cause heated air to drive turbines to generate energy. The efficiency of the tower (i.e.the conversion of heat into kinetic energy) is determined by the difference between the
temperature in the collector and the temperature of the environment at the top of the
tower. The relationship between the size and height of the tower directly determines the
power station’s efficiency. The tower needs to be as tall as it is to provide an updraft
sufficient to drive the turbines to achieve proposed outputs. A 1000meter (3000+ feet) tall
and 150 meters (450+ feet) diameter tower will provide preferred efficiency levels and
can be built using conventional construction techniques.
The tower is a simple hollow cylinder structure with a large diameter to ensure stability
from the relationship between the heights to width ratio.
Analysis of the lifespan of a reinforced concrete tower in a dry climate is indicated at
more than 50 years. Carbonization, the usual reason for the deterioration of concrete,
does not take place in low humidity regions .This process is lessened in the absence of
excessive moisture. Temperature differentials produce an updraft velocity of about 15
meters per second and will allow maintenance crews to enter an operating Solar Tower
power station to undertake maintenance without difficulty. This is a distinct advantage
over other power stations that need to be shut down during maintenance periods. Solar
Towers are designed to operate with very little down time.
COLLECTOR:
Hot air is produced by the greenhouse effect in a simple air collector consisting of a
translucent material such as glass or an alternative. The collector surface is raised several
meters above the ground rising to a greater height at the base of the tower. This design
facilitates an environment for air to be directed in vertical movement with minimum
friction loss. The ground and air beneath the collector is heated, the heated air then
moves toward the tower in the center of the collector and then the updraft created by the
cooler air at the top of the tower helps propel the air up and out the tower. The diameter
of the tower for the proposed 200MW output will vary in relation to site requirements,
although generally it is proposed that the collector be approximately five kilometer (3.2
miles) in diameter.
TABLE: - DIMENSIONS OF SOLAR TOWER POWER STATION
TOWER 3000 feet diameter, 400 feet diameter.
CONCRETE 750,000 cubic yards.
COLLECTOR 3.5 miles diameter(glass,polycarbonate,plastic sheet)
TURBINES 32 units x 6.25 Megawatts.
LAND 20.25 square miles(4.5 x 4.5)
CONSTRUCTION 34 months
JOB 2700+(construction),15(ongoing).
OUTPUT 200 MW(200,000 households)
TURBINES:-
With the use of turbines, mechanical output in the form of rotational energy can be
derived from the moving air in the Tower. The turbines are particularly robust and quiet
in operation. Turbines in a Solar Tower do not work with staggered velocity like a free-
running wind energy converter; instead they will use pressure-staged wind turbines,
similar to a hydroelectric power station, where static pressure is converted to rotational
energy using a cased turbine through the installation of a pipe. The energy yield from
this type of cased pressure-staged turbine is about eight times greater than that of a speed-
stepped open-air turbine of the same diameter. The output achieved is proportional to the
product of volume flow per unit and the fall in pressure at the turbine. In order to achievemaximum energy yield, a turbine regulation system is used to maximize production under
all operating conditions. The torque produced on the turbine is the factor that decides the
rate of power produced in the external circuit. This factor can be calculated in-order to
control the torque of the turbine. It depends on the momentum induced in the turbine
designed such that there is maximum velocity. It is seen that sufficient air comes in
proportional to the outgoing air. If the space for inlet is lesser than the required space to
be equal to the outgoing air, then the velocity will decrease because pressure fall will
occur and hence it will be compensated in the form of velocity drop. Even, if the inlet
area is bigger than the calculated area, then more cold air will come in and thus heating of
air decreases which ultimately results in slow down of the velocity of airflow towards the
turbine. Thus CFD studies the various factors that affect the velocity of airflow, helping
the solar chimney arrangement to increase the efficiency as much as possible. Results
from CFD analysis say that air flows through the turbine at a velocity of around 50kms
per second. But still the overall efficiency of the arrangement is only 2-3%.
Would a regenerator improve the efficiency?
“The overall conversion efficiency from solar energy to electricity is 2-3%. Where does the other 97% go?”. There is a temperature drop with altitude of about
10°C for a 100-meter chimney. Large quantities of warm air have to be lifted from the
ground to chimney top. This is gravitational energy loss. The air that leaves the chimney
is above ambient temperature at that altitude. This is thermal energy loss. Ambient air
that is drawn into the collector is warmed and expands with little increase in pressure.
The majority of solar input is lost in the simple expansion of air before it reaches the
turbine. None of this is surrendered to the turbine.
Would a regenerator improve efficiency? A modified solar chimney is proposed where the
paramount consideration is the rigorous elimination of all energy losses. The solar
collector is sealed and double-glazed with low emissive glass. The entire floor area has a
solar absorber. The chimney is well insulated and incorporates a heat exchanger along
most of its length. Warm air rises from the solar collector and drives the turbine. Its
residual energy is then transferred to incoming air in the heat exchanger - such energy
recovery can be over 95% efficient. Incoming air enters well up the chimney at a level ‘h’
from the top, which is needed to drive the system. The incoming air is warmed as it
travels down the chimney. It passes through the turbine and then between the two layers
of glass to ground level, entering the solar collector from underneath the absorber.
