I want to design a roof top solar thermal system , for which i am planing to use parabolic dish and Stirling engine combination, that provides electrical power and waste heat can be utilized for water heating .
Assuming thermal efficiency of Stirling engine 60%, and electrical efficiency 21%.
if i need 800C at source side of Stirling engine, how big parabolic dish i required.
Please suggest with some explanation on size consideration.
You can estimate it.
The thermal efficiency of the stirling is about 30%. So, the input energy have to be 5kw/0.3=16.7kW
The thermal efficiency of the collector is about0.7, so 16.7/0.7=23.81kW
This is the needed solar energy.
The available solar energy is about 800-1000 W/m^2, you can select 900W/m^2 or 0.9kW/m^2
So the dish area have to be 23.81/0.9=26.46m^2
The aperture is about π*D^2/4=26.46 => D=5.8m^2
So if you select dish aperture diameter close to 6m, you are ok.
You can use the mathematical relationship of thermal efficiency to find the necessary size of your solar collector
Efficiency = mf*Cpf[Tf(outlet)-Tf(inlet)]/[Area*Irradiance]
Dear Mohamed EI,
Here I am looking the find out estimated area of the parabolic dish, which could produce the mentioned temperature at Stirling engine .
Kindly elaborate your answer.
You can estimate it.
The thermal efficiency of the stirling is about 30%. So, the input energy have to be 5kw/0.3=16.7kW
The thermal efficiency of the collector is about0.7, so 16.7/0.7=23.81kW
This is the needed solar energy.
The available solar energy is about 800-1000 W/m^2, you can select 900W/m^2 or 0.9kW/m^2
So the dish area have to be 23.81/0.9=26.46m^2
The aperture is about π*D^2/4=26.46 => D=5.8m^2
So if you select dish aperture diameter close to 6m, you are ok.
Ashutosh,
Evangelos' estimate is an excellent starting point. Extra things to next consider will be;
a) Insulation - having the hot face of the engine at 800°C leads to some not insiginificant losses from plain convection.
b) Depointing - dish tracking will/may be necessary - depending on the power demand curve
c) Shadowing - struts, clouds, buildings all get in the way
d) Dumping the power - whatever power you do *not* extract has to be dissipated from the engine's cold junctions. Ensuring adequate cooling is often a problem for high-temperature Stirling cycles.
Good luck!
Dear James,
I am designing this system for a household of daily ~8 kWh electricity requirement+ required pure water( i am planning to use solar still concept here)+ rest waste heat used for water heat.
If you have time I can share with you, Please share your mail Id.
Dear Ashutosh, I share the opinions of the eminent contributors to your poser. However if the solar radiation at the horizontal surface at the point of location is not directly measured then you may have to use some of the available empirical models to estimate it.
The best Solar Stirling Engines build and tested in USA Solar Power Plants in South part of California (A solar Power plant which produce 700 MW is compound of many units having the diameter of the dish of 11 m, which correspond to cca 100 m2. Each such a module produce 25 kW. If you would use a Stilring engine with those performances you will need an area of the dish in the ration of 25 kw/5kw.=5. Form this ration you need an area of 100 m2/5=20 m2..
Based on tha formula Pi.D2/4=Area. You get
D2= 4.Area/Pi= 4x 20 /3.14= 25.47 which means rafly D=5 m
Of course you have to take into aoount all the sugestions made above bu other exeperts in the field, and the Insolation in your area, and the performances of the stiling engine you want to by. Unfotunately high performane stiling engines are very expensive...
If you choose less expensive engine the Efficiencyof the Stirling engine can fall down dramaticaly, and as a con sequence you will need to increase the area and the diameter of the Parabolic Dish mirror...
Stoian Petrescu
Considering a DIN 900 W/m2, diameter dish 5.2 m, 8 mrad of total optical error, cavity aperture diameter 0.17 m, a global solar to electric efficiency of 0.275 should be possible. This gives a 20 m2 dish area, an incident solar energy of 18.38 kW, and an electric power of 5 kW.
For more details check:
Mathematical model for the study and design of a solar dish collector with cavity receiver for its application in Stirling engines
https://www.researchgate.net/publication/257775037_Mathematical_model_for_the_study_and_design_of_a_solar_dish_collector_with_cavity_receiver_for_its_application_in_Stirling_engines
http://link.springer.com/article/10.1007/s12206-012-0801-0
Best regards
Article Mathematical model for the study and design of a solar dish ...
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