Question

THERMO-SYSTEM ANALYSIS AND DESIGN

2) Draw "Solar Tower" CSP plant with thermal energy storage.

Answer 1

"Soler tower with thermal energy storage"

The power from the sun isintercepted by the earth is approximately 1.8×10^11, which is many thousands of time larger than the present consumption rate on the earth of all commercial energy sources.

Out of the biggest challenges of tbe solar thermal based power plant is the intermittency of the source. Howeer using the highly efficient heat transfer properties of molten salt,this technology facilitates electricity production from weather volatility and,more importantly,it offers the capability to dispatch electricity while solar energy is not available.A
thermal storage system provides an added benefit: allowing the plant to be designed to optimize
the electricity load profile to meet specific market needs. The intermittency can be overcome by
introducing thermal energy storage of molten salt and this can be integrated with the solar thermal
power plants. In this case, excess thermal power is required to be generated at peak sun-shine
hours that is later dispatched during periods of insufficient insolation. Among various thermal
energy storage media available, thermal oils (like Therminol VP-1) and molten salts are used in
commercial applications. Use of thermal oils is restricted due to its high cost, flammability, and
temperature limitation (400°C) for which high thermodynamic efficiency of the power blockcannot be achieved (Flueckiger et al. 2013). On the other hand, molten salt is used in solar power
tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost
medium, its operating temperature is compatible with today?s high-pressure and high-temperature
steam turbines, and it is non-flammable and nontoxic in nature. Characteristics of some of the
commercial salts used in the solar thermal power plants and Therminol VP-1 are given in Table 1
(Kearney et al. 2003). Solar thermal systems using molten salts (40% potassium nitrate,
60% sodium nitrate) as the working fluids are now in operation. Montes et al. (2010) developed a
thermo-fluidynamic model for parabolic trough collectors with various working fluids like oil,
molten salt, or water/steam. The influence of collector length, absorber tube diameter, working
temperature, and pressure has been evaluated on energetic and exergetic performance. The paper
concludes that that direct steam generation is more efficient than oil and molten salt systems.

"Solar tower CSP"

Here CSP indicates concebtrated solar power in which the solar power available is converted in electficity by a concentrator.

Fig. 1 shows the schematic of a solar power towered gas turbine combined plant considered in
the present study. Ambient air at point 1 (300K and 1.01325 bar pressure) enters the compressor
(COMP) and leaves at point 2. Compressed air then enters the solar receiver (R) which raises the
air temperature in two stages until a final temperature of 1000°C (1273 K) is achieved. Two stages
of the solar receiver are: low temperature (LT), and high temperature (HT) modules. In the
receiver cluster the air from the compressor of the gas turbine is heated up to 1000°C by
concentrated solar energy. Fig. 2 shows a schematic of a solar concentrator-receiver system which
traps the incoming solar flux into the working fluid in the receiver. A number of heliostats
concentrate the solar radiation into the solar receivers, which are mounted on top of a tower.
Heated air enters the gas turbine (GT) at point 3 and expands to point 4. The gas turbine module is
similar in construction with the first prototype solar powered gas turbine system, installed during
2002 in the CESA-1 tower facility at Plataforma Solar de Almeria (PSA) in Spain (Heller et al.
2006), but of a larger scale.
A part of the exhaust from GT is passed through a HRSG to produce superheated steam which
in turn will be utilized to run a steam turbine in the bottoming cycle. In the HRSG, air is first used
to raise the temperature of the steam in the superheater (SUP) from point „F? to the point „A?. Then the air is used to evaporate the water from point „E? to „F? in the evaporator (EVAP) and finally, it
is utilized to heat the feed water in the economiser (ECO) i.e., to raise the temperature of the feed
water to saturation temperature. The superheheated steam at point „A? (65 bar pressure and 460°C)
enters into the steam turbine (ST), and after doing work in the steam turbine, steam is exhausted in
the condenser (COND) at point „B? at 0.075 bar pressure. After being condensed in the condenser,
water is pumped from point „C? to point „D? i.e., at the boiler pressure by boiler feed pump (BFP).
The system as well as the programming is developed in such a way that the temperature difference
between the evaporator exit temperature of air (i.e., T6) and the saturation temperature of water
(i.e., TE=TF) does not fall below 15°C for better heat transfer.
The other part of the hot exhaust air from gas turbine is utilized to heat up the molten salt in the
heat exchanger (HX1), and heats up the molten salt during the peak sun-shine hours, coming out
from the cold salt storage tank, which acts as the storage medium. The hot salt coming from HX1
is stored in the hot salt storage tank. While sufficient sunlight is not available, the hot salt from the
hot salt storage tank flows to another heat exchanger HX2, which comprises of economiser,
evaporator and the superheater. HX2 produces superheated steam in the same condition as during
the daytime (i.e., 65 bar, 460°C). The same steam turbine is used to generate the power for 4 hours
at a power level equivalent to that of peak sun- shine hours. The minimum molten salt temperature
at the exit of the economiser is restricted to 290°C to avoid freezing of the molten salt mixture.
The air stream at state points 7 and 8 are mixed in an air mixing chamber. Additional
recoverable heat in the hot exhaust air is further utilized by raising saturated steam at low pressure
(1.01325 bar) for use in a process plant.