Solar Thermal Power Plant
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28-01-2011, 10:52 AM
Solar Thermal Power Plant
Concept, Design, Simulation and Fabrication
A Project Report of detailed mathematical modeling of Lab Scale Solar Thermal Power Plant of 40 W.
Solar energy is one of the promising source of renewable energy. Among its different methods of tapping solar concentrator is the most viable option. In present study a solar concentrator based upon Direct Steam Generation (DSG) concept is analyzed for Pakistan’s environment. A detailed mathematical model is described for optimum selection of boiler (saturated section), super heater along with their corresponding heat losses. Mathematical calculations were further extended for bare and glass covered tube. Analysis performed on the basis of different Nusselt and Reynolds No. for an open cycle has concluded that glass tube design is more efficient. Comparison of theoretical calculation with experimental setup will be undertaken as the future work of the study.
Solar power is the generation of electricity from sunlight. This can be direct as with photo-voltaic (PV), or indirect as with concentrating solar power (CSP), where the sun's energy is focused to boil water, which is then used to provide power. Concentrated Solar Power (CSP) technology is the one of promising method for tapping solar energy, other than photovoltaic technology. CSP offers a wide variety of methods to generate electricity. However, the Parabolic Trough is the most effective solution to produce power.
3 options were studied, namely as follows
Heat Transfer Fluid (HTF)
Direct Steam Generation (DSG)
Combined Power Cycle
All the technologies were studied in detail with their advantages and disadvantages and it is concluded that DSG is the most viable option. For details the reader may want to refer . This paper consists of detailed of mathematical modeling for the power output of 40 Watts
Having selected steam as a working fluid, its efficiency is then analyzed for both open and closed cycles; with the following conditions assumed
Closed Cycle Open Cycle
Pressure (kPa) 101 101
Pump Inlet Quality 0.1 N/A
Pump Temperature (°C) N/A 25
Degree of Superheat (°C) 15 15
Applying Ideal conditions, we get the following graphs
Figure 1: Ideal Rankine Efficiency comparison for open and closed Cycle systems
It is observed that the closed cycle is more efficient than the open cycle and the efficiency gap widens as we move to higher turbine pressures. However, if operation is carried at low pressure, the efficiency of each cycle is very low; thus, it is not practical to install a plant with low working pressure. However, the mass flow rate required to produce 40 W is quite low and it is also observed that with increasing pressure the mass flow rate of steam further reduces. Therefore, for this power output, finding low flow rate pumps is difficult.
One possible solution is to use an overhead tank that should be at least 2 stories high (6m approx.) from the boiler level in order to get the required pressure and flow rate. Thus, the design pressure is taken as 140kPa. The degree of superheat is also tested at the same pressure however there is no significant difference in overall ideal efficiency of the cycle. Still a 15 degree of superheat is fixed in order to conserve the life of the expander. The expander selected is a steam engine because turbo-machines can’t operate at such low flow rates. The following is the schematic of model plant:
Figure 2: Schematic of Plant for Mathematical Modeling
Coating Selection and Tube Parameters
There are two common solar selective coatings that are used for absorbing maximum possible solar radiation, which are as follows
Of the two, the black nickel electroplating is easily available with a maximum bath length of 1.6m. Therefore, the maximum tube length is constrained by this. The pipe chosen had an average diameter of 1.75in (44.45mm) having a wall thickness of 1.6mm.
Design Calculation and Results
The power out is fixed to 40W and the expander pressure is restricted to 140kPa. It is assumed that the engine isentropic efficiency is 70% where as the pumping efficiency is 80%. The mass flow rate for this cycle under the specified conditions is calculated to be 1 g/s and the total input energy required by the fluid is 2.655kW.
From this, 31.3 W is required to superheat by 15 degrees, while the rest of it is required to produce saturated steam at the flow rate mentioned above.
1. Determination of Surface Temperatures
It is also calculated that the entry length of the flow in the super-heater is greater that total length of the tube itself. Therefore, the heat transfer co-efficient is calculated using the relation of the Nusselt Number  for developing flow against increasing lengths
It is calculated that the heat transfer coefficient decreases marginally where as the surface area of the inside super-heater is increased due to which the overall surface temperature decreased which is calculated using the relation
The result are shown below
Figure 3: Variation of Surface Temperature Againt Superheater Length
In the boiler section the effective heat flux incident on pipe surface that is being transferred to the fluid is calculated using the following relation assuming
which is calculated to be 22.4 kW/m2 and the surface temperature is calculated using the relation:
which is calculated to be 3 degrees higher than the saturation temperature for scored surface and 5.5 degrees higher for extremely polished surface. However, the pipe given is a scored one therefore the corresponding surface temperature is selected.
2. Heat Loss Analysis
Having calculated the surface temperatures, heat losses are then calculated at different wind speeds
2.1. Bare Tube Analysis
Previously, it is calculated that increasing the length of the super-heater, decreases the surface temperature, and thus together the heat losses are decreased. However, on further increasing the length, the surface area factor dominates and the heat losses are again increased.
Figure 4: Total Plant Heat Loss at Different Windspeed with bare tube
As the wind velocity increases, so does the heat loss. One thing to note is that the minimum heat loss for bare tube also varies with the wind speed as shown in the following table:
Wind Speed (m/s) Minimized Heat Loss (kW) Length of Super-heater (m)
0 0.2788 0.22
1 0.4903 0.14
2 0.6665 0.11
3 0.8051 0.0905
4 0.9244 0.0805
5 1.032 0.08
2.2 Glass Analysis
Following assumptions are made in the glass tube analysis:
The glass tube has a diameter of 2in(50.8mm)
There is no vacuum inside the glass tube, air inside is at 1atm
The surface temperature of the glass is equal from inside and outside
The heat that is transmitted by the absorber tube is in the form of convection and radiation and the radiated heat is completely transmitted through the glass.
The convective loss is transferred to the glass where it is conducted and then looses heat to the surrounding atmosphere via convection and radiation
The emissivity of glass is 90%
The glass tube is of the same length as the absorber tube.
The natural convection loss between glass and the tube is calculated using the following relations 
Where Lc stands for characteristic length, Do represents the outside diameter, the subscript gb stands for air between glass and boiler, Ra donates Rayleigh Number where as Pr donates Prandtl number.
The heat loss via radiation between glass and super-heater is calculated by :
Finding the actual heat loss is an iterative procedure because value of glass surface temperature is not known. The iterations were stopped when the Heat Loss by convection between boiler surface and glass equals the total heat loss by glass tube to the atmosphere.
Figure 5: Heat Loss Comparison of Bare Tube Boiler and Boiler Tube with Glass Cover
It is calculated that for a fixed length of super-heater by using glass tube the heat loss is reduced by a considerable level and becomes almost independent of the wind speed where as for the bare tube it increases almost linearly with the wind velocity.
Similar mathematical model is applied to super-heater section against its various lengths. It is calculated that increasing the length of the super-heater did not increase the super-heater heat loss for glass where as it is opposite for bare tube (Figure 6)
Figure 6: Heat Loss of super-heater with and without glass
The results were combined to give the total plant heat loss at constant pressure and as expected the use of glass tube significantly reduced the total plant heat loss and more importantly it is almost independent of wind speed. (Figure 7)
Figure 7: Total Plant Loss with and without glass
3. Area Required
It is calculated that during the average solar heat flux incident at Karachi is 0.456 kW per meter square during the 12 hours of the day, and the results were approximately close to the world solar energy map.
The required area for the parabolic collector is calculated by adding the total heat loss and the heat absorbed by the fluid and dividing it by the absorber efficiency, mirror reflectance efficiency and glass transmittance (if used) which were assumed to be 90%, 90% and 97% respectively. The results for each combination are compared and it is observed that the least area is required when both the super-heater and boiler were covered with glass tube. (Figure 8)
Figure 8: Area required for different combinations of Tube Selections
Although, increasing the length of the super-heater decreases the total area, however, this overshadows the fact that the width of the parabola for the super-heater and boiler would be different for most of the region which will be difficult to control.
In order to calculate the width of parabola, the area of each section is divided by the corresponding length and the results are shown below.
Figure 9: Variation of Parabola Width with different combinations against Super heater Length
As evident from the graph that in order to have a uniform parabola, least width is required with combination of glass super-heater and glass boiler at the length of 0.12m super-heater.
4. Efficiency of plant
The carnot efficiency of the plant is evaluated using the formula:
It is calculated to be 2.36%.
The actual thermal efficiency of the plant is evaluated using general formula of
η_th=W ̇_(net,out)/Q ̇_solar
The results were calculated, as usual, for different length of super-heater and the following results were obtained
Figure 10: Total Plant Efficiency
At our optimized super-heater length, the total plant efficiency is evaluated to be 1.07% with glass and 0.918% without it.
4. Analysis at Different Pressures
The behavior of the optimized design is then analyzed at different pressures and the results showed a significant improvement.
4. 1. Super-heater Surface Temperature.
As the pressure increases, the surface temperature of super-heater is observed to decrease. This is because of the increase in density and the thermal conductivity of steam at higher pressures which increases the heat transfer co-efficient and thus decreases the surface temperature.
Figure 11: Super heater Pipe Surface Temperature against Increasing Pressure
4. 2. Total Heat Loss of the Plant
As the super-heater surface temperature decreases on increasing the pressure, the heat loss decreases initially. On increasing the pressure, the Boiler surface temperature increases due to rise in saturation temperature. This further increases the saturation temperature of the boiler section and in turn increasing the heat loss.
Figure 12: Total Plant Heat Loss with Increasing Pressure
4. 3. Thermal Efficiency of the Plant and the Area required.
The thermal efficiency of the plant increases with increasing pressure.
Figure 13: Plant Efficiency against Increasing Pressure
This leads to reduction in the total area required to produce the same amount of power.
Figure 14: Area Required against Increasing Pressure
4. 4. Mass flow rate
It is observed by increasing pressure, the mass flow rate decreases for the same power output.
Figure 15: Mass flow Rate against Increasing Pressure
Conclusion and Future work:
It is observed that the plant performance improves at higher pressure. For 140kPa, 0.12m is optimized length of super heater, for uniform parabola width for entire boiler and super heater. Heat losses are reduced significantly when application of glass tube. Further losses can be reduced if this tube is evacuated. An experimental setup is being undertaken at Pakistan Navy Engineering College and these mathematical calculations will be validated in future work.
 A PROTOTYPE SOLAR POWER PLANT USING DIRECT STEAM GENERATION (DSG) CONCEPT.
Sulaiman D. Barry, Arsalan Qasim, Saad A. Khan, Syed M. Umair and Shafiq R. Qureshi
 HEAT TRANSFER, YUNUS CENGEL, 2nd Edition
A detailed comaprision of advantages and disadvantages of solar power generation technologies is discussed along with parabolic trough category.
However, the Direct Steam Generation (DSG) is considered to be the most feasible option and hence selected for design calculations and consists.
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29-01-2011, 10:55 AM
your work is very helpful. thank you.
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Joined: Jan 2011
25-09-2011, 09:15 AM
hi Seminar Surveyor,
Thankyou for appreciating. I had quit the work on my research after completing my undergraduation, and have decided to resume my work.
Joined: Jul 2011
26-09-2011, 02:12 PM
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15-10-2011, 08:24 PM
plant solar thermal plant
Joined: Jul 2011
17-10-2011, 10:26 AM
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23-02-2012, 05:08 PM
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